metallomesogens

Metallomesogens Synthesis, Properties, and Applications

Edited by Jos6 Luis Serrano

VCH Weinheim New York Base1 Cambridge Tokyo

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Metallomesogens

Edited by Jose Luis Serrano

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0 VCH Verlagsgesellschaft mbH, D-6945 1 Weinheim (Federal Republic of Germany), 1996

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ISBN 3-527-29296-9

Metallomesogens Synthesis, Properties, and Applications

Edited by Jos6 Luis Serrano

VCH Weinheim New York Base1 Cambridge Tokyo

Prof. Dr. JosC Luis Serrano Quimica Organica Instituto de Ciencia de Materiales de Aragon Facultad de Ciencias-I. C. M. A. Universidad de Zaragoza-C. S. I. C. 50009 Zaragoza Spain

produced. Nevertheless, editor, authors and publisher do not war- rant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may in- advertently be inaccurate.

Published jointly by VCH Verlagsgesellschaft mbH, Weinheim (Federal Republic of Germany) VCH Publishers, Inc., New York, NY (USA)

Editorial Directors: Dr. Peter Gregory, Dr. Ute Anton Production Manager: Dip1.-Ing. (FH) Hans JBrg Maier

Library of Congress Card No. applied for.

A catalogue record for this book is available from the British Library.

Deutsche Bibliothek Cataloguing-in-Publication Data: Metallomesogens : synthesis, properties, and applications / ed. by Jose Luis Serrano. - Weinheim ; New York ; Base1 ; Cambridge ; Tokyo : VCH, 1996

NE: Serrano, JosC Luis [Hrsg.] ISBN 3-527-29296-9

0 VCH Verlagsgesellschaft mbH, D-69451 Weinheim (Federal Republic of Germany), 1996 Printed on acid-free and chlorine-free paper. All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated into a machine-readable language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Typesetting: K t V Fotosatz GmbH, D-64743 Beerfelden. Printing: betz-druck gmbh, D-64291 Darmstadt. Bookbinding: Wilhelm Osswald & Co, Wallgasse 6, D-67433 Neustadt. Printed in the Federal Republic of Germany.

To our families, for the time we neglected them, and especially to Pablo, whose loss saddened us all, and to Manuel, whose arrival restored our happiness.

Biography

Jose' Luis Serrano studied chemistry at the University of Zaragoza, Spain, where he received his Diploma and Ph. D. for work in the field of liquid crystals in the group of Professor Enrique Melendez. In 1985 he obtained the position of professor titular of organic chemistry at the University of Zaragoza. Since 1985 he has been at the In- stitute of Materials Science of Aragon (ICMA) and was Vice-Director of the institute from 1987 to 1991. His research interests include ferroelectric liquid crystals and metallomesogens of both low and high molecular weight.

Some years ago, after attending a lecture on the structural variants of organic mesogens, a Spanish physicist described the field of liquid crystals as being similar to a “treatise on entomology”. Indeed, the review article by Dietrich Demus [I], written on the occasion of the centenary of liquid crystal chemistry, supports this observation, since terms such as “discotic skeleton”, “calamitic”, “phasmidic”, “spinal” and “the anatomy of the liquid crystal” are frequently used. Most of the compounds described in this paper consist of a relatively small selection of elements, such as carbon, oxygen, nitrogen, phosphorus and sulfur together with the halogens. And yet, although their coordination possibilities are somewhat limited, an enormous number of liquid crystalline compounds has been reported [2,3].

However, just when the number of new syntheses of new structures seemed likely to be exhausted, a new class of compounds with remarkable potential has emerged in recent years: metal-containing liquid crystals or “metallomesogens”. These materials open the door to a rich variety of new geometrical shapes including square- planar, octahedral, square-pyramidal, sandwich and lantern structures which are, in many cases, unobtainable in purely organic compounds. Thus, a new generation of molecular shapes has appeared, as exemplified by open-book-shaped [4], brick-like [ 5 ] , shish-kebab-like [6] and worm-like [7] compounds, as well as by a large number of compounds whose shapes are reminiscent of capital letters of the Latin alphabet; for example C, D, H, I, K, 0, P, T, U, V, X, and Z.

The field of liquid crystals, as are all areas of materials science, is multidisciplin- ary. Chemists, physicists and engineers all have an interest in mesomorphic compounds, and frequently their priorities tend not to overlap. In general, the synthesis of new liquid crystalline materials and the study of their structure-property relationship has been the main objective of chemists. In contrast, physicists tend to have quite different interests. Unlike chemists, physicists are not interested in the properties of a series of compounds, but instead prefer to study a sample of a single compound which is both stable and of potential value. This discrepancy between the requirements of chemists and physicists has also affected the field of metallomeso-

VIII Preface

gens and, in consequence, an abundance of new materials has been synthesized, but the characterization of their physical properties has lagged far behind. This is probably the most important problem in metallomesogen research. Only by placing a greater emphasis on the physical characterization of these materials will we be able to talk about the real possibilities of these compounds rather than just their potential. Liquid crystal research in general must face this problem. Although thousands of mesogens have been reported, physical properties have been measured of only hundreds of compounds and fewer than a hundred materials have found applications in technical devices.

In contrast, far fewer metallomesogens have been synthesized and yet, although this field is still in its infancy, considerable advances in the understanding of their physical properties have been made, as highlighted by the following reports on:

0 Novel types of ionic thermotropic liquid crystals [8] and covalent soaps [9] of

0 Self-assembling mesomorphic coordination complexes [ 10, I 11. 0 One-dimensional conducting metallomesogens [ 121. 0 New liquid crystals showing ferroelectric behavior: square pyramidal oxovana-

dium complexes showing a unidimensional arrangement possessing supramolecular domains of polarization [13]. Molecules with a chiral mesogenic core [14] and nondiscotic compounds bearing multiple chiral tails [ 151.

0 Paramagnetic liquid crystals showing parallel or perpendicular orientation in magnetic fields [ 161.

0 Induction of mesophases by means of charge-transfer complexation [17]. 0 Improvement in the processing of high performance aromatic liquid crystal

silver derivatives.

polymers by metal complexation [ 181.

To date, several reviews have been published on lyotropic and thermotropic low molecular weight metallomesogens [ 191 and more specifically on calamitic [20], discotic [21] and polymeric metallomesogenic materials [22]. These reviews, on the whole, tend to give a descriptive appraisal of the structure-property relationships in metallomesogens. In this book, we wish to present a general overview of metallomesogens up to the first half of 1994, which will be helpful to all people working in the field as well as to those who have a general interest in this subject. Thus, we have chosen to describe four different aspects of these compounds, namely materials, synthesis, structural characterization methods, and physical properties. Al- though the subject of metallomesogens is a specialized area of liquid crystals, we have tried to present the material in a self-explanatory way. We have therefore included a general background on the basic concepts of liquid crystals with particular emphasis on mesophase nomenclature and phase classification. In each chapter we have also given a brief explanation of the more significant points discussed. The layout of the book is as follows:

Chapter 1 presents a short introduction to liquid crystals and, in particular, to metallomesogens. Chapters 2 - 5 focus on metallomesogens with an emphasis on

Preface IX

liquid crystal structure-property relationships. Chapters 6 and 7 outline the synthetic approaches to the preparation of metallomesogens. Chapters 8 and 9 mainly describe the structural characterization methods which are particularly important in the field of metallomesogens: electron paramagnetic resonance (EPR) and diffraction techniques. Other methods, such as polarizing optical microscopy, differential scanning calorimetry (DSC), infrared, and NMR spectroscopy, are commonly used for the study of liquid crystals in general. Chapters 10 and 11 highlight the physical properties of metal-containing compounds. Chapter 10 is devoted exclusively to magnetic properties and reflects the considerable amount of research interest in this subject. All other physical properties studied so far are presented in Chapter 11. Finally, Chapter 12 considers the more fundamental aspects of metallomesogens and attempts to predict future developments in this field.

The authors all belong to the Materials Science Institute of Aragon (ICMA). This research center unites research workers from two different institutions, the University of Zaragoza and the Consejo Superior de Investigaciones Cientificas (CSIC). Since its creation in 1985, one of the lines of research at the ICMA has been devoted to liquid crystals. The three main areas of investigation are metallomesogens, ferroelectric liquid crystals and liquid crystalline polymers. All of the authors are involved in one or more of these areas.

A number of people assisted in the preparation of this book. The authors would like to thank Dr. D. Broer, Prof. Dr. V. Orera and Prof. Dr. A.M. Levelut for their meticulous and painstaking revision of some of the chapters in this book. We are indebted to Dr. N. Thompson for his excellent correction of the manuscript (style and in some cases contents). We would also like to thank the rest of the people of the Zaragoza Liquid Crystals Group who have tolerated our hysterical moments and forgave us for devoting less of our time to them. Finally we are grateful to the various editors and workers of VCH (especially Dr. P. Gregory, Dr. U. Anton) for their kind guidance and patience throughout the preparation of this book.

Zaragoza, August 1995 J.L. Serrano

References

[ I ] D. Demus, Liq. Cryst. 1989, 5, 75-1 10. [2] a) Flussige Kristalle in Tabellen I (Eds.: D. Demus, H. Demus, H. Zaschke) VEB Deutscher

Verlag fur Grundstoffindustrie, Leipzig, 1974; b) Flussige Kristalle in Tabellen II (Eds.: D. Demus, H. Zaschke) VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig, 1984; c) Handbook of Liquid Crystals (Eds.: H. Kelker and R. Hatz) Verlag Chemie, Weinheim, 1980; d) P. Ekwall in Advances in Liquid Crystals (Ed.: G. H. Brown) Academic Press, New York, 1975, Vol. 1, pp. 1 - 152; e) G. W. Gray in Advances in Liquid Crystals (Ed.: G. H. Brown) Academic Press, New York, 1976, Vol. 2, pp. 1-72; f) S. Chandrasekar

X Preface

in Advances in Liquid Crystals (Ed.: G.M. Brown) Academic Press, New York, 1982,

[3] a) Liquid Crystal Polymers I, II and III (Adv. in Polymer Science Vols. 59-61) (Eds.: M. Gordon, N. A. Plate) Springer-Verlag, Berlin, 1984; b) E. T. Samulski and D. B. DuPre in Advances in Liquid Crystals (Ed.: G.M. Brown) Academic Press, New York, 1979, Vol. 4, pp. 121 - 145; c) H. Ringsdorf, B. Schlarb, J. Venzmer, Angew. Chem. Int. Ed. Eng. 1988, 27, 113- 158; d) L.R. Dix, Trends Polym. Sci. 1993, I , 25-30.

[4] J. Barbera, P. Espinet, E. Lalinde, M. Marcos, J.L. Serrano, Liq. Cryst. 1987, 2,

[5] a) Y. G. Galyametdinov, G. I. Ivanova, I. V. Ovchinnikov, Zhurnal Ohshchei Khimie 1984,

[6] M. Hanack, M. Lang, Adv. Muter. 1994, 6, 819-833. [7] G. S. Attard, R. H. Templer, J. Muter. Chem. 1993, 3, 207-213. [8] D.W. Bruce, D.A. Dummur, E. Lalinde, P.M. Maitlis, P. Styring, Nature, 1986, 323,

[9] M. J. Baena, P. Espinet, M.C. Lequerica, A.M. Levelut, J. Am. Chem. SOC. 1992, 114,

[ 101 H. Abied, D. Guillon, A. Skoulios, H. Dexpert, A.M. Giroud-Godquin, J. C. Marchon,

[ I I] R. Atencio, J. Barbera, C. Cativiela, F. J. Lahoz, J. L. Serrano, M.M. Zurbano, J. Am.

[12] Z. Belarbi, C. Sirlin, J. Simon, J. J. Andre, J. Phys. Chem. 1989, 93, 8105 - 81 10. [I31 a) A. G. Serrette, P. J. Carroll, T. M. Swager, J. Am. Chem. SOC. 1992, 114, 1887- 1889;

[14] P. Espinet, J. Etxeberria, M. Marcos, J. Perez, A. Remon, J. L. Serrano, Angew. Chem.

[I51 M. J. Baena, J. Barbera, P. Espinet, A. Ezcurra, M. B. Ros, J. L. Serrano, J. Am. Chem.

[ 161 a) M. Marcos, J. L. Serrano, Adv. Muter. 1991, 5, 256- 257; b) E. Campillos, M. Marcos, J. L. Serrano, J. Barbera, P. J. Alonso, J. I. Martinez, Chem. Mater. 1993, 5 , 151 8 - 1525.

[ 171 D. Singer, A. Liebmann, K. Praefcke, J. H. Wendorff, Liq. Cryst. 1993, 14, 785 - 794. [18] A. A. Dembek, R. R. Burch, A. E. Feiring, J. Am. Chem. SOC. 1993, 115, 2087 -2089. [19] a) A.M. Giroud-Godquin, P.M. Maitlis, Angew. Chem. Int. Ed. Eng. 1991, 30,

375 -402; b) P. Espinet, A. Esteruelas, L.A. Oro, J. L. Serrano, E. Sola, Coord. Chem. Rev. 1992, 117, 21 5-274; c) D. W. Bruce in Inorganic Materials (Eds.: D. W. Bruce, D. O’Hare) Wiley and Sons, Chichester, 1992, Chap. 8, 407-490.

Vol. 5, pp. 47-78.

833 - 842.

54, 2796; b) A. Roviello, A. Sirigu, P. Iannelli, A. Immirzi, Liq. Cryst. 1988, 3 115.

791 -792.

4182-4185.

J. PhyS, 1988, 49, 345-352.

Chem. SOC. 1994, 116, 11 558 - 11 559.

b) A. Serrette, T. M. Swager, J. Am. Chem. SOC. 1993, 115, 8879-8880.

Int. Ed. Eng. 1989, 28, 1065-1066.

SOC. 1994, 116, 1899- 1906.

[20] S.A. Hudson, P.M. Maitlis, Chem. Rev. 1993, 93, 861 -885. [21] K. Ohta, I. Yamamoto, J. Synth. Org. Chem. Jpn. 1991, 49, 486-496. [22] L. Oriol, J. L. Serrano, Adv. Muter. 1995, 7, 348-369.

Contents

Biography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

List of Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jose' Luis Serrano

1.1 General Concepts: Metallomesogens . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 General Concepts: Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.3.1 Calamitic Mesophases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Discotic Mesophases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Lyotropic Mesophases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.3 Mesophases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Part A Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 Low Molecular Weight Lyotropic Metallomesogens . . . . . . . . . . . . .

2.1 Micellar Lyotropic Metallomesogens . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Chromonic Metallomesogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Teresa Sierra

2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VI

VII

XIX

1

1 4 9

11 14 15 20

23

29

29 34 40 41

XI1 Contents

3 Low Molecular Weight Calamitic Metallomesogens . . . . . . . . . . . . . .

3.1 Metal-Organic Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Metal-Organic Liquid Crystals with Monodentate Ligands . . . . . . . 3.1 . 1 . 1 Nitrile Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 . 1 . 2 Pyridine Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 . 1 . 3 Amine Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 . 1 . 4 Thiolate Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2 Metal-Organic Liquid Crystals with Bidentate Ligands . . . . . . . . . . 3.1 -2.1 2-Substituted Pyrrole Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2.2 Salicylideneamine Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2.3 Enaminoketone Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2.4 Aroylhydrazine Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2.5 P-Diketone Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2.6 Salicylaldehyde Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2.7 Alkylcarboxylate Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2.8 Monothio-/3-Diketone Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2.9 Dithiolene Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2.10 Dithiobenzoate Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2.1 1 Dithiocarbamate Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2.12 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3 Metal-Organic Liquid Crystals with Tetradentate Ligands . . . . . . . . 3.1.3.1 Annelide Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3.2 Porphyrin Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3.3 Salicylidenediamine Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3.4 2,2 '-Bipyridine Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3.5 1,10-Diaza-4,7,13,1 6-Tetrathiacyclooctadecane Derivatives . . . . . . . . . 3.1.3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Organometallic Liquid Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Organometallic Liquid Crystals with Monodentate Ligands . . . . . . 3.2.1.1 Alkynyl Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1.2 Isonitrile Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Organometallic Liquid Crystals with Bidentate Ligands . . . . . . . . . . 3.2.2.1 Dinuclear ortho-Palladated Complexes . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2.2 Mononuclear ortho-Metallated Complexes .....................

Jose' Luis Serrano and Teresa Sierra

3.2.2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Organometallic Liquid Crystals with Metal-n Bonds . . . . . . . . . . . . 3.2.3.1 Metallocene Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3.2 Butadiene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

43

43 43 44 46 51 51 52 53 54 54 71 73 74 81 81 83 83 84 88 89 90 90 90 91 92 93 94 94 95 96 97

100 100 100 111 116 117 117 122 122 123

Contents XI11

4

4.1 4.1.1 4.1.1.1 4.1.1.2

4 . 1 . 2 4.1.3 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.2 4.3.3 4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.4.6 4.4.7 4.4.8 4.5

4.5.1 4.5.2 4.6 4.6.1 4.6.2 4.7 4.7.1 4.7.2 4.8

Low Molecular Weight Discotic Metallomesogens . . . . . . . . . . . . . . . 131 Joaquin Barbera

Mononuclear Metal Complexes with Bidentate Ligands . . . . . . . . . . 132

Complexes with one Aliphatic Ring at each Phenyl Ring . . . . . . . . 133 Complexes with two or more Aliphatic Chains at each Phenyl Ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

P-Diketonate Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

Dithiolene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 1. 2.Dioxime Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

Carboxylate Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Dithiocarboxylate Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 Benzalimine Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 Metal Complexes with Tridentate Ligands . . . . . . . . . . . . . . . . . . . . . . 1.4. 7-Triazacyclononane Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Pyridinediyl-2. 6.dimethanol Complexes ........................ 156 1.3. 5-Triketonate Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Mononuclear Metal Complexes with Tetradentate Ligands . . . . . . . 160 Phthalocyanine Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 Porphyrin Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 Tetraazaporphyrin Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 Tetrapyrazinoporphyrazine Complexes . . . . . . . . . . . . . . . . . . . . . . . . . 175 1.4.8. 1 I-Tetraazacyclotetradecane Complexes .................... 176 Bis(salicy1idene)diimine Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 P-Diketonate Schiff Base Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . 178 Calixarene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 1.3.5. 7-Tetraketonate Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Dibenzaldiimine Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 Metal Complexes with Hexadentate Ligands .................... 1.4.7.10.13.1 6.Hexaazacyclooctadecane Complexes . . . . . . . . . . . . . . . 183 1.3. 5Triketonate Schiff Base Complexes . . . . . . . . . . . . . . . . . . . . . . . 184 Metal Complexes with Octadentate Ligands .................... 186 Bisphthalocyanine Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 1.3.5. 7.Tetraketonate Schiff Base Complexes .................... 187 Cyclopentadiene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

Dinuclear Metal Complexes with Bidentate Ligands . . . . . . . . . . . . . 145

155

Dinuclear and Tetranuclear Metal Complexes with Tetradentate

183

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188

5 Metallomesogenic Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

5.2 Lyotropic Metal-Containing Liquid Crystal Polymers . . . . . . . . . . . . 195

Luis Oriol

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

XIV Contentss

5.2.1 5.2.2 5.3 5.3.1 5.3.1.1 5.3.1.2 5.3.2 5.3.2.1 5.3.2.2 5.3.3 5.4 5.4.1 5.4.1 . 1 5.4.1.2 5.4.2 5.5 5.6

Main-Chain Lyotropic Metal-Containing Liquid Crystal Polymers . Side-Chain Lyotropic Metal-Containing Liquid Crystal Polymers . . Thermotropic Metal-Containing Liquid Crystal Polymers Calamitic Main-Chain Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Alternative Designs of Main-Chain Polymers . . . . . . . . . . . . . . . . . . . Calamitic Side-Chain Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Alternative Designs of Side-Chain Polymers .................... Columnar Thermotropic Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crosslinked Metallomesogenic Polymers ....................... Metal as the Crosslinking Agent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal-Modified Main-Chain Polymers . . . . . . . . . . . . . . . . . . . . . . . . . Metal-Modified Side-Chain Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . Reactive Metal-Containing Liquid Crystals .....................

Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . .

Main-Chain Polymers Based on Metallomesogenic Units . . . . . . . . .

Side-Chain Polymers Based on Metallomesogenic Units . . . . . . . . .

Borderline Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Part B Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6 Design and Synthesis of Low Molecular Weight Metallomesogens . Mercedes Marcos

6.1 6.2

6.2.1 6.2.2 6.2.3 6.2.4 6.2.5 6.2.6 6.3

6.3.1 6.3 . 1 . 1 6.3.1.2 6.3.1.3 6.3.1.4 6.3.2

6.3.2.1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal-Organic and Organometallic Liquid Crystals with Monodentate Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitrile Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pyridine Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isonitrile Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alkynyl Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alkylamine Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thiolate Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal-Organic Liquid Crystals with Bidentate and Tetradentate Non-Cyclic Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation of [M.(N202) ].TYpe Metallomesogens Schiff Base Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enaminoketone Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aroylhydrazine Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6’.Diamino.2.2’.bipyridine and 2-Aminopyridine Derivatives . . . . .

P-Diketone Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

Preparation of [M.02]..[M.0,].. and [M.O, ].Type Metallomesogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

195 20 1 202 202 202 208 210 210 210 214 216 216 216 218 223 225 227 228

233

235

235

236 237 238 240 241 242 243

243 243 243 247 247 249

250 250

Contents XV

6.3.2.2 6.3.3 6.3.3.1 6.3.4 6.3.4.1 6.3.4.2 6.3.4.3 6.3.4.4 6.3.5 6.3.5.1 6.3.5.2 6.4 6.4.1 6.4.2 6.4.2.1 6.4.2.2 6.4.3 6.4.3.1 6.4.3.2 6.4.3.3 6.4.4 6.5 6.5.1 6.5.2 6.5.2.1 6.5.2.2 6.5.2.3 6.5.2.4 6.5.2.5 6.5.3 6.5.3.1 6.5.3.2 6.5.3.3 6.6 6.6.1 6.6.2 6.6.2.1 6.6.2.2 6.6.2.3

Carboxylate Derivatives ...................................... 253 Preparation of [M-(02S2)]-Type Metallomesogens . . . . . . . . . . . . . . . 255 Monothio-P-diketone Derivatives .............................. 255 Preparation of [M-S4]-Type Metallomesogens . . . . . . . . . . . . . . . . . . . 256 Dithiolene Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 Dithiocarboxylate Derivatives ................................. 259 Alkylxanthato Derivatives .................................... 261 Dithiocarbamato Derivatives ................................. 261 Preparation of [M-N4]-Type Metallomesogens . . . . . . . . . . . . . . . . . . . 262 2-Phenylazopyrrole and 2-Phenylazomethinopyrrole Derivatives . . . 262 Glyoximate Derivatives ...................................... 263 Metal-Organic Liquid Crystals with Macrocyclic Ligands . . . . . . . . 263 Preparation of Cyclic Diazatetrathiaether Derivatives . . . . . . . . . . . . 264 Preparation of Azacyclam Metallomesogens .................... 265 1.4.7.Trisubstituted.1.4. 7.triazacyclononane Derivatives . . . . . . . . . . . 265 Preparation of Metallomesogens from Other Aliphatic Azacyclams 266 Preparation of Porphyrin Metallomesogens .................... 266 Octasubstituted Porphyrin Derivatives ......................... 266 Tetrasubstituted Porphyrin Derivatives ......................... 269 Disubstituted Porphyrin Derivatives ........................... 270 Preparation of Phthalocyanine Metallomesogens . . . . . . . . . . . . . . . . 270 Organometallic Liquid Crystals with Bidentate Ligands . . . . . . . . . . 272 Synthesis of the Ligands ..................................... 274 Preparation of ortho-Metallated Dinuclear Complexes . . . . . . . . . . . 274 Azobenzene Derivatives ...................................... 275 Azoxybenzene Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 Schiff Base Derivatives ...................................... 276 Azine Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 Pyrimidine Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 Preparation of ortho-Metallated Mononuclear Complexes . . . . . . . . 281 ortho-Palladated Mononuclear Complexes ..................... 281 ortho-Metallated Mercury Complexes .......................... 285 ortho-Metallated Manganese and Rhenium Complexes . . . . . . . . . . . 285 Organometallic Liquid Crystals with Metal-n Bonds . . . . . . . . . . . . 286 Preparation of Dieneiron Tricarbonyl Complexes . . . . . . . . . . . . . . . . 286 Preparation of Metallocenes ................................. 287 Ferrocene Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 Ferrocenophane Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 Preparation of Ruthenocene Derivatives ....................... 293 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294

XVI Contents

7 Synthetic Strategies for Metallomesogenic Polymers . . . . . . . . . . . . .

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2

Polymer-Forming Step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 Metal-Poly(yne) Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Spinal Columnar Metallomesogenic Polymers . . . . . . . . . . . . . . . . . .

Bridging Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Polymerization Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Metal Modification of Preformed Organic Polymers . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Luis Oriol

Synthetic Strategies for the Incorporation of the Metal in the

7.2.3

7.2.4

Metallomesogenic Polymers Obtained Using Metal Salts Plus

Metallomesogenic Polymers Obtained by Conventional Organic

Part C Structural Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8 X-Ray Studies of Metallomesogens . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.1 X-Ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.1 Basic Concepts in X-Ray Diffraction of Liquid Crystals . . . . . . . . . 8.1.2 X-Ray Diffraction Studies of Metallomesogens . . . . . . . . . . . . . . . . . 8.2 X-Ray Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Basic Concepts in EXAFS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 EXAFS Studies of Metallomesogens . . . . . . . . . . . . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Joaquin Barbera

9 Electron Paramagnetic Resonance of Paramagnetic Metallomesogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pablo J. Alonso

9.1 9.2 9.3 9.3.1 9.3.2 9.3.3 9.3.4 9.3.5 9.4

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Basic Concepts of EPR Spectroscopy Summary of the Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . “&Type Macrocycles and Polyamine Ligands . . . . . . . . . . . . . . . . . . P-Diketonate Complexes ..................................... Schiff Base Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Monomeric Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymeric Liquid Crystals Containing Paramagnetic Metals Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . .

301

301

302 302 302

311

312 315 319

323

325

326 326 330 343 343 344 346

349

349 350 354 354 357 359 373 375 380 381

Contents XVII

Part D Physical Properties and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . 386

10

10.1 10.2 10.3 10.4 10.5 10.6 10.7

Magnetic Properties of Metallomesogens ...................... 387 Pablo j : Alonso

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 Basic Concepts of Magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388 Measurements on Dinuclear PMLC . . . . . . . . . . . . . . . . . . . . . . . . . . . 394 Measurements on Mononuclear PMLC . . . . . . . . . . . . . . . . . . . . . . . . 398 Measurements on Metallomesogenic Polymers . . . . . . . . . . . . . . . . . . 402 Mesophase Order and Magnetic Susceptibility . . . . . . . . . . . . . . . . . . 403 Orientation of PMLC by Magnetic Fields . . . . . . . . . . . . . . . . . . . . . . 412 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417

11 Other Physical Properties and Possible Applications of Metallomesogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 M . Blanca Ros

11.1 11.2 11.2.1 1 1.2.2 11.2.3 1 1.2.4 1 1.2.5 11.2.6 11.3 11.3.1 11 -3.2 11.3.3 11.4 11.4.1 1 1.4.2

General Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 Birefringence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 Biaxiality .................................................. 426 Dichroism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428 Thermochromism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434 Nonlinear Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439 Photoeffects: Energy Migration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442 Electrical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443 Electrical Conductivity and Redox Properties . . . . . . . . . . . . . . . . . . . 445 Dielectric Behavior .......................................... 454 Ferroelectricity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458 Other Properties and Borderline Cases . . . . . . . . . . . . . . . . . . . . . . . . 466 Rheological Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466 Ion Transport and Permeation Properties ...................... 471 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473

12 Concluding Remarks ........................................ 481 Jose' Luis Serrano and M . Blanca Ros

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485

List of Contributors

Pablo J. Alonso Instituto de Ciencia de Materiales de Aragon Facultad de Ciencias Universidad de Zaragoza-C. S. I. C. 50009 Zaragoza, Spain

Joaquin Barbera Quimica Organica Facultad de Ciencias-I. C. M. A. Universidad de Zaragoza-C. S. I. C. 50009 Zaragoza, Spain

Mercedes Marcos Instituto de Ciencia de Materiales de Aragon Facultad de Ciencias Universidad de Zaragoza-C. S. I. C. 50009 Zaragoza, Spain

Luis Oriol Quimica Organica Escuela Universitaria Politecnica de Huesca-I. C. M. A. Universidad de Zaragoza-C. S. I. C. Ctra. Zaragoza, s/n 22071 Huesca, Spain

Blanca Ros Quimica Organica Facultad de Ciencias-I. C. M. A. Universidad de Zaragoza-C. S. I. C. 50009 Zaragoza, Spain

JosC Luis Serrano Quimica Organica Facultad de Ciencias-I. C. M. A. Universidad de Zaragoza-C. S. I. C. 50009 Zaragoza, Spain

Teresa Sierra Instituto de Ciencia de Materiales de Arag6n Facultad de Ciencias Universidad de Zaragoza-C. S. I. C. 50009 Zaragoza, Spain

1 Introduction

Jose' Luis Serrano

1.1 General Concepts: Metallomesogens

The study of metallomesogens, the colloquial and accepted name for metal-containing liquid crystals, is a relatively new area in the field of liquid crystals.

If we do not consider the papers related to alkali and alkaline earth metal soaps, only a few papers dealing with this subject have been published in the literature up to 1986. However, since then a significant increase in the number of reports has occurred (Fig. 1-1).

The field of metallomesogens is considered to be a young branch of the one hundred-year-old liquid crystal science. However, the first account of these materials is as

year

Figure 1-1. Publications on Metallomesogens (excluding alkali and alkaline-earth soaps).

2 J. L. Serrano

old as the history of mesogenic compounds itself. In the middle of the 19th century, a number of soaps, such as the ones mentioned above, were reported to exhibit double refraction phenomena in aqueous solution [ I ] . Several of these soaps are alkali metal salts of naturally occuring fatty acids, and they behave as lyotropic metallomesogens. It should be noted that these studies were undertaken forty years before Reinitzer’s work on cholesterol derivatives which showed “double melting points” and interesting “color effects”. Reinitzer’s paper is accepted to be the earliest manuscript on liquid crystals [2]. Later, he also reported on the appearance of similar phenomena for the silver salt of cholesteric acid [3]. This could be the first reference to a liquid crystal containing a transition metal. However, it is broadly accepted that the first paper dealing with metallomesogens appeared in 1910. Vorlander described the thermotropic properties of alkali metal carboxylates which, in some cases, exhibit lamellar phases upon melting [4]. Once again the soaps!

During the next three decades a number of authors [5] further studied the phases observed in soap solutions. However, as far as metallomesogens are concerned, it was not until 1960 that this field of research was significantly stimulated, mainly by Skoulios and Luzzati [6]. In fact, amphiphilic compounds were studied from the basis of a sub-discipline in its own right, which a large number of metallomesogens be- longs [7].

In contrast, only one paper dealing with covalent metallomesogens was published in the same period of time. Once again, it was Vorlander who described the smectic arrangement in some diary1 mercury Schiff base derivatives in 1923 (Fig. 1-2a) [8]. In 1957 Graham et al. reported a liquid crystal phase in ferrocenecarboxaldehyde, but

Figure 1-2.

I Introduction 3

later studies carried out by Verbit et al. proved that this phase is in fact a plastic crystal [9]. Until the 1970s no more covalent metallomesogens were described.

In 1971 Young et al. [lo] published an often neglected paper, describing the mesogenic properties of several compounds containing group IV elements. In this work, the authors describe the mesomorphism behavior of two Schiff base derivatives of tin and germanium which show two unidentified smectic phases (Fig. 1-2b). These results had previously been presented at the Third International Liquid Crystal Conference in Berlin one year earlier.

In 1976 MalthCte et al. reported smectic order for a number of ferrocene derivatives (Fig. 1-2c) [ I 11.

As can be seen in Fig. 1-2, the first three covalent metallomesogens described were curiously all organometallic Schiff base derivatives.

The first report on covalent liquid crystalline coordination complexes appeared in 1977. Giroud et al. described the mesomorphic properties of several nickel and palladium dithiolene derivatives (Fig. 1-3) [ 121.

Figure 1-3.

Later, papers describing metallomesogens were sporadically published every year until the “explosion” in metallomesogen research started in 1986. This marked increase in research on metallomesogens gives rise to an important question: Why this sudden interest in these materials?

An overview of the publications concerning metal-containing liquid crystals reveals that the majority of authors, using quite similar words, enclose in their texts an attractive and promising leitmotif “. . . liquid crystalline materials in which a metal atom is incorporated into the molecular skeleton are of interest because such new materials are expected to have not only the intrinsic properties of organic mesogens but also the unique properties based on metal atoms.. .“.

The possibility of combining the properties of liquid crystals (fluidity, ease of processability, one- or two-dimensional order, etc.) with the properties associated with metal atoms (color, paramagnetism, a electron-rich environment, etc.) is probably the main origin of the progress in this field.

Metallomesogens are certainly a singular example of symbiosis in materials science. Compounds showing interesting magnetic [ 131 (paramagnetic liquid crystals, control of the molecular orientation in a magnetic field), electrical [I41 (one-dimensional conductors), optical [ 151 (strong birefringence, dichroism, nonlinear optical behaviour) and electro-optical [ 161 (photoelectric behavior, ferroelectric electro-optic responses) properties have already been obtained.

4 J.L. Serrano

Likewise, for those fascinated by “chemical architecture” and the relationship between the molecular structure and the physical properties of compounds, the study of metallomesogens has opened up remarkable possibilities to achieve not just new, but unprecedented and more easily accessible liquid crystalline molecular structures than the ones provided by classical organic molecules.

In addition, and perhaps more interestingly, the possibility of tuning the physical (mesomorphic, electrical, optical, magnetic, etc.) properties of metallomesogens is significantly extended, since both the organic ligand and the metal center can be var- ied. A particular ligand can be complexed to numerous different metal atoms which differ in nature or oxidation state. Alternatively, the well-documented effects of structural modifications found for organic liquid crystals can also be applied to the organic ligand in a coordination complex.

Some of the more important results in these areas of research have already been mentioned in the preface, but many others will be highlighted in the following chapters.

1.2 General Concepts: Liquid Crystals

As explained in the preface, our intention was to write a self-contained book. For this reason we have included the following section which is devoted to the relevant basic concepts in liquid crystals science. For those who wish to gain a deeper insight into this field, a number of excellent books and monographs exist which cover the general topic [I71 or more specific areas [lS].

In the field of liquid crystals, molecular structure and molecular order play a fundamental role and, although the terminology which has evolved is derived from a number of sources, classical Greek has provided the basis for the terms used (e.g. mesophase, calamitic, smectic, enantiotropic, lyotropic, thermotropic). As a consequence, many researchers not directly involved in this field may find the jargon somewhat intimidating. However, it is sufficient to familiarize oneself with a few relatively simple terms in order to fully understand the general principles which apply to liquid crystals.

Liquid crystals (or mesogens) are materials which exhibit liquid crystalline behavior (or mesomorphism). This behavior appears under given conditions, when phases occur in which the molecular order is intermediate between that of an ordered solid crystal and a disordered liquid or solution; these intermediate phases are called mesophases. Liquid crystals have been defined as “orientationally ordered” liquids or “positionally disordered” crystals [ 191 and combine properties of both the crystalline state (e.g. optical and electrical anisotropy) and the liquid state (molecular mobility and fluidity). There are two different ways in which a mesophase can be formed, and these give rise to the main classes of liquid crystals:

a) Mesophases can be formed by pure compounds (or mixtures of compounds) by the influence of temperature. In this case, the liquid crystal is termed thermotropic.

I Introduction 5

When the thermotropic mesophase appears both in the heating and the cooling process (i.e. when it is thermodynamically stable) it is called enantiotropic. Thermody- namically unstable mesophases, which only appear in the cooling process due to the hysteresis in the crystallization point, are referred to as monotropic.

b) Mesophases can also be observed as a result of certain species (e.g. amphiphiles) forming anisotropic aggregates in the presence of a solvent (usally water). These mixtures are known as lyotropic liquid crystals. The occurrence of a particular lyotropic mesophase depends on the temperature and the constitution of the mixture.

A large number of compounds of both low and high molecular weight (polymers) have been described as thermotropic or lyotropic liquid crystals, and some of them exhibit both types of behavior (amphotropic liquid crystals). Thermotropic liquid crystals have gained a relevant place in the field of materials science, whereas lyotropic liquid crystals are of fundamental interest in life science. Both kinds of self- organizing systems play an important role in supramolecular chemistry [20].

The intermolecular forces responsible for the molecular arrangement in liquid crystals are essentially the same as those predominant in molecular solids. However, only molecules containing certain structural elements exhibit liquid crystalline behavior. Mesogenic molecules need to meet a series of structural and electronic criteria [21] so that a satisfactory packing of molecules is achieved which allows appropriate interactions between neighboring molecules. The existence of a permanent dipole moment, its magnitude, or the anisotropy of the molecular polarizability can be determining factors in the promotion of liquid crystallinity.

Thermotropic liquid crystals are classically divided into two main groups depending on their structural features; calamitic (rod-like) (Fig. 1-4a) and discotic (disc-like)

a

Figure 1-4.

6 J.L. Serrano

Figure 1-5.

b

(Fig. 1-5a). In both cases, the molecules can be described as cylinders with a high degree of structural anisotropy. Calamitic compounds have a structure in which the axial component is larger than the radial components (Fig. 1-4 b). On the other hand, discotic compounds, as the name implies, are disc-like, therefore the radial components are larger than the axial component (Fig. 1-5 b). Using these theoretical models, the phase transitions shown by a large number of both low and high molecular weight mesogens have been successfully explained. In recent years however, an increasing number of new mesogenic compounds which cannot be described by either model has been reported, including molecules combining both calamitic and discotic shapes (phasmidic compounds), calamitic and discotic twins and compounds with very large lateral substituents. New theoretical models have been proposed in order to explain the mesogenic behavior of these materials [22], for example, a brick-like (sanidic) geometry can be proposed for some cases (Fig. 1-6).

In a similar way, lyotropic liquid crystals [23] can be described by a relatively simple model. In compounds which form lyotropic mesophases, the molecules usualIy possess the amphiphilic character typical of compounds with surface active properties, and consist of a polar head group and one or more apolar, aliphatic chains. The polar head group can be formed by either ionic moiety (cationic, anionic, or zwit- terionic) or by one or more nonionic groups which have strong dipole moments capable of interaction with polar solvents. A representative example is sodium stearate (a soap), which forms a lyotropic mesophase in aqueous solutions (Fig. 1-7).

As in the field of thermotropic liquid crystals, a significant number of lyotropic structures which differ from this typical model have been discovered in recent years.

1 Introduction 7

Figure 1-6.

Figure 1-7.

For example, chromonic lyotropic disc-shaped or lath-shaped molecules which, by means of x-interactions, self-organize into columnar mesophases in the presence of a solvent. Most of these molecules do not even incorporate the polar groups which are present in amphiphiles with the more conventional soap structure [24] (Fig. 1-8, see p. 14). Despite these recent departures from convention, the vast majority of thermotropic mesogens described are covalent materials, whereas many lyotropic liquid crystals are ionic.

The preceeding discussion refers to low molecular weight compounds, but the conclusions regarding the structure-property relationship, which arose from the systematic investigation of organic liquid crystals molecules can also be applied to macromolecules. Thus, a wide variety of liquid crystalline polymers (LCPs) has been reported. Some of them are commercially available due to their technological advantages with respect to thermoplastic materials for engineering applications. The molecu-

8 J.L. Serrano

Figure 1-8.

lar architecture of liquid crystalline macromolecules basically depends on the way in which the mesogenic units (either calamitic or discotic) are incorporated into the polymeric chain. The most important types of liquid crystalline polymers are shown schematically in Fig. 1-9. As regards the polymer structure, an initial classification can be established between one-dimensional and crosslinked polymers. The one-dimensional polymers can be classified according to the way in which mesogenic units are introduced into the polymeric backbone. Hence, the two principal types are main-chain LCPs (where mesogenic units are introduced as constituents of the polymer chains) and side-chain LCPs (where mesogenic units are introduced as branches of the polymeric chains). Clearly, there are numerous possibilities for the design of these types of polymers depending on the kind of mesogenic unit introduced. Poly- mers can be lyotropic or thermotropic, calamitic or discotic or, alternatively, intermediate types (combined LCP) of polymeric structure in which mesogenic units are present both in the main backbone and as side-chains.

The possibility of crosslinking polymer chains further increases the number of polymeric liquid Crystalline systems accessible. Thus, liquid crystalline elastomers (materials with a low degree of crosslinking) or anisotropic networks (materials with a high degree of crosslinking) are the latest contributions in this field.

As a consequence of the ever increasing scope of the study of liquid crystals it is becoming increasingly difficult to make a simple classification of mesomorphic materials. Molecular structures which cannot be described by the proposed models are reported more and more frequently. One important contribution to this phenomenon

I Introduction

IAIN-CHAIN POLYMERS SIDE-CHAIN POLYMERS

9

U ANISOTROPIC NETWORK

U LIQUID CRYSTALLINE

ELASTOMERS

LIQUID CRYSTALLINE POLYMERIC SYSTEMS

'\ T CROSSLINKED POLYMERS

/' ONE-DIMENSIONAL POLYMERS

PRINCIPAL TYPES OF ARRANGEMENTS I SLIGHTLY CROSSLINKED HIGHLY CROSSLINKED

COMBINED ARRANGEMENTS

shape of mesogens

Figure 1-9.

c

is, without doubt, the investigation of metallomesogens. Given the large number of possibilities for different coordination geometries in metal-containing systems, new compounds with structures and molecular shapes considerably different from conventional organic systems have appeared.

1.3 Mesophases

The three basic types of mesogens described above (calamitic, discotic, amphiphilic) are associated with three types of mesophases in which the supramolecular geometry determines the resulting molecular arrangement.

This type of classification is valuable for the sake of convenience, but it is necessary to point out the dichotomy, discussed above, that exists between the mesophase type and the molecular structure. Many compounds whose structures are rather different from rod-like are capable of showing calamitic phases [25] (Fig. 1-10). Con- versely, compounds with an elongated molecular shape can give rise to columnar

10 J L . Serrano

Figure 1-10. Nematic and smectic mesophases.

Figure 1-11. Columnar mesophases.

I Introduction 11

a b

Figure 1-12. a) Nematic and smectic mesophases; b) discotic nematic mesophases.

mesophases usually associated with disc-shaped molecules [26] (Fig. 1-1 1). This phenomenon is very commonly observed for metallomesogens, where molecules with the same mesogenic core unit can exhibit calamitic or discotic phases depending on the number and position of aliphatic chains [27] (Fig. 1-12).

1.3.1 Calamitic Mesophases

There are two types of calamitic mesophase: the nematic and the smectic (or lamellar) mesophases [28,29].

The least ordered mesophase is the nematic phase (N). In this phase, the molecules align parallel in a preferred direction which is called the director (2). The molecules can move within the nematic phase and are able to rotate around the molecular long axis. In spite of this freedom of movement the molecules are, on average, aligned in one direction (Fig. 1-13 a), that is they possess orientational, but no positional order.

Smectic mesophases (S) show a higher degree of order than the nematic phase. The molecules are organized into layers. Within a layer the molecules tend to align parallel to each other. A number of smectic phases exist which differ in the degree of order present both within and between the layers. Each smectic modification is denoted by a letter, for example the smectic A phase (in which the molecules are aligned parallel to the layer normal without having positional order within the layer). Smectic phases are often represented by the letter S with the corresponding subscript letter (e.g. the SA phase). The normal to the layers can be aligned parallel to the director (as in the smectic A phase) or tilted (as in the smectic C phase) (Fi.g 1-13b). SA and

12 J. L. Serrano

Figure 1-13. Schematic representation of calamitic mesophases: a) N, nematic; b) S,, smectic A; c) S,, smectic C; d) S,, smectic B; e) S,, smectic G.

Sc phases are the least ordered smectic phases and are also the most commonly observed. Due to the molecular mobility inherent in these phases and their low viscosities, they are called fluid mesophases. In addition, the smectic B, the smectic F and the smectic I hexatic phases show intralayer positional order as well as interlayer

1 Introduction 13

bond orientational order (e.g. the molecules in the hexatic SB phase adopt a hexagonal arrangement). Other types of smectic phases show three-dimensional order, restricted mobility and higher viscosities. In these phases, known as crystal smectic phases (denoted by the letters B, E, G, H, J, K), the molecules show intralayer as well as interlayer positional order [29].

In cases where the molecules are chiral, the structure of fluid mesophases can have an additional property. Chirality in nematogenic molecules can cause a twist in the director alignment, giving rise to the chiral nematic (N*) or Cholesteric (Ch) phase. In the chiral nematic phase the director has a helical shape. The structure of this phase is caused by molecules aligning parallel to the director at any position of it, as schematically illustrated in Fig. 1-14a (although only layers are represented in this illustration, a chiral nematic does not form a layered structure). The distance necessary for the director to describe a full turn of the helix is called the pitch of the helix.

N'

(a)

Figure 1-14. Schematic representation (cholesteric); b) S& chiral smectic C.

SC'

(b)

of two chiral mesophases: a) N*, chiral nematic

In the chiral smectic C* phase the helical alignment is caused by the tilt plane of the angle, which changes its direction from layer to layer thus forming a helix (Fig. 1-14b). Chiral nematic and smectic phases show optical activity, and the smectic C* phase can additionally give rise to ferro-, ferri-, or antiferroelectric properties.

14 J L . Serrano

1.3.2 Discotic Mesophases [30]

Since the discovery of the first discotic mesogen in 1977 [30] the term discotic mesophases has appeared in the literature. The term discotic mesophase can, however, be misleading in terms of the shape of the molecules which form the phase. In the same way that non-calamitic molecules can form mesophases which appear typically calamitic, some molecules which are not disc-shaped are capable of forming discotic phases. When the term discotic is applied to mesophases, it does not neces- sarily imply that the molecules have the geometric shape of a disc.

Three different classes of discotic mesophase have been defined: nematic, columnar and lamellar. In the discotic nematic phase (ND), the arrangement of the disc- like molecules is similar to that in the nematic phase formed by calamitic molecules (Fig. 1-25a) in columnar mesophases, the molecules tend to stack in columns which could give rise to a different type of arrangement. In the nematic columnar phase (N,) the columnar superstructure acts like the rod-like molecules in the nematic calamitic phase (Fig. 1-l5b). In other cases, the columns are parallel to one another and form a periodic two-dimensional array. Thus hexagonal (Dh), rectangular (D,) and tetragonal (Dtet) columnar discotic phases have been described (Fig. 1-1 5 c- e). In addition, and similarly to calamitic mesophases, orthogonal (Fig. 1 - 15 c, e) and tilted (Fig. 1-1 5 c- e) columnar mesophases are possible.

The lamellar discotic phase (DL) is defined as a smectic-like organization of molecules. It has been suggested that the molecules are tilted with respect to the layers, but this point does not appear to have been established unequivocally (Fig. 1-16) [31].

Dh Dr Dtet

(C) (a (e)

Figure 1-15. Schematic representation of five discotic phases: a) N,, nematic discotic; b) N,, columnar nematic; c) D,, discotic hexagonal; d) D,, discotic rectangular, e) D,,,, discotic tetragonal.

1 Introduction 15

Figure 1-16. Schematic representation of the structure proposed for the discotic lamellar (DL) phase.

1.3.3 Lyotropic Mesophases

In lyotropic phases, a new parameter must be considered: the solvent. As a result, not only the temperature, but also the number of components in the solution and their concentrations are decisive factors for the appearance of these mesophases. Lyotropic liquid crystals can be divided into two classes: the first group is made up of discoid molecules which give rise to phases for which the name chromonic was proposed. The second type of lyotropic mesophase is formed by amphiphilic molecules. Since all amphiphiles are capable of forming micellar aggregates, this class of lyotropic liquid crystals will in the following be referred to as micellar lyotropic mesogens, even if some of these lyotropic phases do not consist of micelles. In discoid systems the shape of the molecules themselves is directly responsible for the supramolecular order in a similar way to the thermotropic discotic mesophases described previously. As in thermotropic discotic systems, it is not a prerequisite that the molecules have a disc-like shape in order to form a chromonic phase. In lyotropic systems the discotic nematic (ND), columnar nematic (N,) and hexagonal (H) (also named middle M) mesophases have all been described. Lyotropic discotic phases are isostructural with their thermotropic analogs (Fig. 1-17, see p. 16).

In amphiphilic lyotropic systems, anisotropic supramolecular aggregation is responsible for mesophase formation. Depending on the nature of the amphiphile and its concentration, different types of aggregate are formed which define the type of arrangement in the mesophase. Three main types of micelle can be defined: plate- shaped (or discotic), columnar and spherical. In addition, depending on the nature and concentration of the solvent, each of these micellar structures can be formed with either the polar headgroup (normal topology phase) or the apolar hydrocarbon tail (reverse topology phase) on the outer surface of the micelle (Fig. 1-18, see p. 17). The type of micelle formed is indicated by a subscript 1 (normal phase) or 2 (reverse phase) alongside the letter which designates the type of lyotropic phase formed.

16 J.L. Serrano

a

Hexagonal Columnar Columnar Nematic

Figure 1-17. Schematic representation of the three discotic lyotropic mesophases (chromonic).

Each of the aggregated micelles can give rise to distinct types of order [33]. Mesophases formed by discotic micelles can be classified as either nematic discotic (ND, or ND,), similar to discotic phases described previously, or lamellar (L) phases. The lamellar mesophase, also called the neat or gel-like (G) phase is frequently observed in binary amphiphile/water systems and is isostructural with the SA phase (Fig. 1-1 9a). Phases formed by rod-like micelles can be classified as micellar nematic (Ncl or Nc2) or hexagonal (Hcr or HC2) columnar. The hexagonal phase is also re-

I Introduction 17

Spheric Normal Micelle Spheric Reverse Micelle

Figure 1-18. Schematic drawing of the spheric micelles showing the two possible molecular orientations (normal or reverse) depending on the nature and the concentration of the solvent.

ferred to as the middle phase (M) (mesomorphous fibrous hexagonal solution phase) which involves cylindrical micelles arranged in a hexagonal array similar to that observed in discotic hexagonal mesophases (Fig. 1-19b, see p. 18).

Finally, the phase formed by spherical micelles is classified as the cubic micellar phase (I, or Iz), also known as the viscous optically isotropic phase (V), which consists of spherical micelles packed in a body centered cubic lattice (Fig. 1-19c).

Also, a nonmicellar cubic phase formed by amphiphilic and non-amphiphilic molecules has been described. This is a bicontinuous phase with symmetry Ia3d [34].

The most important types of mesophase which have been mentioned in this introduction are summarizad in Table 1-1 using the nomenclature employed in this book. In the liquid crystal literature, however, different nomenclature has been used on occasion. Although many of the calamitic, discotic and lyotropic mesophases have closely related structures, in some cases the nomenclature is very different and can lead to confusion. It is appropriate now to undertake a revision of the concepts used in the nomenclature of the various mesomorphic states and to put forward some suggestions for the unification of this nomenclature.

A number of suggestions are given in Table 1-1 with this aim in mind. It would be more convenient in the unification of the nomenclature to give priority to the symmetry of the phase rather than to the molecular geometry of the component molecules. The use of the letter H to denote hexagonal, for example, could be applied without distinction to thermotropic discotic, chromonic like lyotropic and micellar lyotropic phases. The various subscript letters could then be used to distinguish between these classes of phase, for example Hc (hexagonal columnar belonging to either thermotropic or chromonic lyotropic mesogens) and Hcl or HC2 (micellar lyotropic). The subscript 1 and 2 are already used to indicate a micellar lyotropic phase (normal and reverse topology). The subscript letter D indicates that the mole-

18 J L . Serrano

Micelle

Disc-Micelle Spherical Micelle Rod-Micelle

Micellar Discotic Nematic

I Micellar Columnar Nematic

Micellar Cubic Lamellar (C)

(a)

Micellar Columnar Hexagonal

(b) Figure 1-19. Schematic representation of micellar mesophases.

cules or aggregates involved have a planar structure whose axis of orientation ( z ) within respect to the phase is orthogonal (or almost orthogonal) with the plane of the molecule or the micelle (x-y plane) (Fig. 1-20).

The subscript C indicates that a columnar superstructure (formed by either stacked disc-like molecules or amphiphilic molecules arranged in a rod-shaped micelle)

I Introduction 19

Table 1-1. Classification of Mesophases

Thermotropic Lyotropic

Calamitic Discotic Discoid Disc- Rod- Spherical- Non- amphiphiles micelles micelles micelles micellar (C hromonic-like) amphiphiles

J+ [Tcl Cub. IaJd[Ia] I , ( v , ) ~ Cub. Ia3d[Ialb

I2 (V2) a

The symbols without brackets or parenthesis correspond to those used in this book. a) Other nomenclature is used by some authors; b) Proposed nomenclature which could unify all isostructural phases. N nematic, N * chiral nematic, Ch cholesteric, S smectic, S * chiral smectic, N, discotic nematic, N, columnar nematic, D, discotic lamellar, L, lamellar discotic, D, discotic hexagonal, H, hexagonal columnar, D, discotic rectangular, R, rectangular columnar, D, discotic tetragonal, T, tetragonal columnar, H hexagonal, M middle, N,, micellar discotic nematic, N,), micellar reverse discotic nematic, L lamellar, G (gel-like or neat), N,, micellar columnar nematic, N,, micellar reverse columnar nematic, H,, micellar hexagonal columnar, Hc, micellar reverse hexagonal columnar, MI middle, M, middle reverse, 1, micellar cubic isotropic, I, micellar cubic reverse isotropic, V, viscous optically isotropic cubic, V, viscous optically cubic reverse, Cub. IaSd: Ia cubic phase with symmetry IaSd.

t 2 :

Figure 1-20.

20 J. L. Serrano

gives rise to the liquid crystalline order. Irrorder to simplify the nomenclature system it would not be necessary to introduce any new symbols. It is possible to differentiate between isostructural thermotropic discotic and chromonic lyotropic phases in that lyotropic phases always appear in the presence of a specified solvent or solvents. The suggestions made regarding nomenclature arise from the necessity of writing this introduction, however “scholars have the field of liquid crystals so that they may put forward their opinion regarding this subject”.

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1 Introduction 21

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Mol. Cryst. Liq. Cryst. 1986, 140, 131 -152; b) H. Sakashita, A. Nishitani, Y. Sumiya, H. Terauchi, K. Ohta, I. Yamamoto, Mol. Cryst. Liq. Cryst. 1988, 163, 211-219.

[32] N. Boden, R. J. Bushby, L. Ferris, C. Hardy, F. Sixe, Liq. Cryst. 1986, 2, 1109- 1125. [33] P. A. Winsor, in Liquid Crystals and Plastic Crystals (Eds.: in G. W Gray and I? A . Win-

sor), Vol. I , Ellis Horwood, Chichester, 1974, p. 199. [34] a) V. Luzzati, P. Mariani, H. Delacroix, Makromol. Chem. Macromol. Symp. 1988, 15,

1 - 17; b) J. Charvolin, J.-F. Sadoc, J. Phys. Chem. 1988, 92, 5787-5792; c) M. Clerc, E. Dubois-Violette, J. Phys. ZZ France 1994, 4, 275 - 286; d) D. W. Bruce, B. Donnis, S. A. Hudson, A. M. Levelut, S. Megtert, D. Petermann, M. Veber, J. Phys. II France 1995, 5, 289 - 302.

Part A. Materials

In this first section, metallomesogens described in the literature up to the first half of 1994 are discussed. Special attention is paid to the structure-mesogenic property relationship and to the establishment of correlations between different types of compounds. Other relevant properties will also be indicated where appropriate.

This section has been divided into four chapters. The first three chapters are devoted to low molecular weight (LMW) compounds, whereas the fourth chapter deals with metal-containing liquid crystalline polymers. This division has been made on account of the significant differences between these two groups in both synthesis and properties. Further subdivisions have been made within the two groups regarding the nature of the mesophases formed: lyotropic or thermotropic.

In the case of polymeric materials, all compounds are discussed in one chapter (Chap. 5 ) because of the relatively low number of compounds described to date. In contrast, the large number of LMW metallomesogens necessitates the division of the discussion into three different chapters. Chap. 2 is focused on LMW lyotropic metallomesogens, whereas Chaps. 3 and 4 are devoted to thermotropic metallomesogens which show calamitic and discotic mesophases respectively. This second subdivision of thermotropic metallomesogens has been made due to the wide variety of compounds to be discussed, and also due to the generally different molecular geometries of compounds showing calamitic and discotic mesophases. As explained above, a large number of important exceptions from this general behavior have been found in the field of metallomesogens.

It is further noteworthy that in metallomesogens a significant number of amphotropic liquid crystals (showing both lyotropic and thermotropic properties) exists. Naturally, these materials have been included in both corresponding chapters.

Taking into account the electronic character of the complexes, two different types of compound can be distinguished: neutral and ionic.

In neutral complexes the metal is very often joined to an organic ligand, which normally leads to a system with a molecular geometry very similar to that of organic liquid crystals. For this reason, the ligand is in many cases referred to as the pro-

24 Part A . Materials

mesogenic part of the molecule. The metal atom can form a covalent bond with either a carbon atom or a heteroatom, in both cases we can speak of coordination complexes. However, in order to distinguish between these two types of compounds, we have decided to adopt the term suggested by R. Dagani [l], and complexes in which the metal atom is bonded directly to a carbon atom are organometallic, whereas the term metal-organic complexes is used when the metal is bonded to atoms other than carbon.

In ionic compounds, two parts of the molecule need to be considered: the promesogenic unit and the non-promesogenic counterion. The promesogenic group is mainly responsible for the molecular order observed in the mesophase. The (nonmesogenic) counterion is normally a small mobile molecule which is situated near to the charge on of the promesogenic unit. However, in some cases the counterion plays an important role in the formation of mesophase (for example lauryl sulfate derivatives). The metal center can be situated either in the promesogenic unit or act as counterion. Strictly speaking, both types of compounds are metallomesogens and would normally be considered in a book such as this. However, as explained above (see Preface), lyotropic metallomesogens are the first examples of metal-containing liquid crystals described in the literature. Most of these compounds are anionic, surface active compounds formed mainly by alkali and alkaline earth metal salts of different organic acids carrying long linear or branched alkyl chains. Many of these compounds also exhibit thermotropic behavior and arc thus amphotropic. The large number of papers published dealing with different aspects of these materials indicates that a discussion of their properties is beyond the scope of this book. The study of these materials constitutes a sub-discipline in its own right within the field of liquid crystal science. A number of excellent reviews [2] and monographs [3] devoted to the study of the mesomorphic states and other properties of these compounds have been published. For this reason, in each of the chapters dealing with low molecular weight mesogens in which these compounds appear, only a brief comment on their features is made. However, the properties of structurally similar metallomesogens containing other metals from groups I and I1 or those bearing hydrophobic groups which differ from the typical aliphatic chains, will be covered.

The section of the book describing materials has been split into four chapters which are represented in Scheme 1.

Low Molecular Weight Lyotropic Metallomesogens (Chapter 2)

Chapter 2 has been divided in two sections depending of the nature of the mesophase: discotic (also named chromonics) or micellar.

Low Molecular Weight Calamitic Metallomesogens (Chapter 3)

Chapter 3 includes a discussion of thermotropic compounds which show calamitic mesophases. The main division in this chapter has been made according to the type of bonds involved in the metal complexation: organometallic systems (M-C or

Part A. Materials 25

METALLOMESOGENIC MATERIALS

(Chapter 5)

LOW MOLECULAR WEIGHT METALLOMESOGENS

/ \ A ONE-DIMENSIONAL CROSSLINKED

THERMOTROPICS LYOTROPICS THERMOTROPICS

CALAMITICS DISCOTICS

(Chapter 3)

DISCOIDS

(chromonics)

Scheme 1.

M--71 bonds) and metal-organic systems (M-heteroatom bonds). The discussion of metal-organic systems includes liquid crstalline, ionic alkyl carboxylates with metal- containing counterions.

In some cases, this division is a little ambiguous because two or more different organic ligands are joined to the metal. In these examples, the complexes have been classified according to the ligand with the dominating promesogenic character. In this way comparative studies with other families of metallomesogens are more convenient. Alkene-stilbazoleplatinum complexes have been classified as metal-organic monodentate complexes due to the priority given to the promesogenic stilbazole [4] (Fig. 1 a, see p. 26). In a similar way, ferrocene-iminocopper derivatives have been classified as metal-organic systems because the ferrocene unit can be considered as a terminal substituent on the iminocopper complex [ 5 ] (Fig. 1 b). In contrast, the asymmetric azomethine/bdiketonate mononuclear palladium complexes have been classified as organometallic derivatives because the parent complexes are dinuclear azomethine complexes [6] (Fig. 1 c).

In each subsection, the complexes have been divided according to the denticity of the ligand which normally influences the final geometry of the complexes.

Low Molecular Weight Discotic Metallomesogens (Chapter 4)

In this chapter thermotropic compounds forming discotic mesophases are included. The division of this chapter has been made according to the denticity of the ligands

26 Part A. Materials

l a

l b

l c Figure 1.

only, due to the fact that very few examples of organometallic discotic metallomesogens have been described to date.

Metallomesogenic Polymers (Chapter 5)

The chapter, devoted to high molecular weight metallomesogens or metallomesogenic polymers, considers the molecular architecture of the polymeric systems. Thus,

Part A . Materials 27

these polymers have been divided into two main groups: one-dimensional and crosslinked polymers. In the first group, we consider all macromolecular design in which polymeric chains, linear or branched, are extended in one direction. These compounds may be further classified according to the type of mesogenic behavior (lyotropic or thermotropic) and to the arrangement of the mesogenic units within the polymer structure. According to this last criterion, two types of polymer structure are distinguishable: main-chain and side-chain polymers.

Finally, a subsection is devoted to the study of crosslinked polymers and covers both slightly crosslinked and highly crosslinked polymers. In these cases, polymer chains form in two- or three-dimensional networks. Clearly, these highly organized structures do not exhibit lyotropic or thermotropic mesogenic properties, but they can be considered to be liquid crystalline materials as their organization resembles the liquid crystalline phase in which reactive monomers have been polymerized.

References

[l] R. Dagani, Chem. Eng. News 1994, 72, 31 -40. [2] a) P. A. Winsor in Liquid Crystals and Plastic Crystals (Eds.: G. W. Gray, P. A. Winsor),

Ellis Horwood, Chichester, 1974,5, 225 -287; b) P. Ekwall in Advanced in Liquid Crystals (Ed.: G. H. Brown) Academic Press, New York, 1975, Vol. I , pp. 1 - 152.

[3] a) A. Skoulios, V. Luzzati, Nature, Lond. 1959, 183, 1310-1312; b) A. Skoulios, Adv. in Colloid and Interface Science, 1967, 1 -79; c) N. Boden, R. J. Bushby, L. Ferris, C. Hardy, F. Sixe, Liq. Cryst. 1986, 2, 1109- 1125.

[4] J. P. Rourke, F.P. Fanizzi, N. J. S. Salt, D. W. Bruce, D.A. Dunmur, P.M. Maitliss, J . Chem. SOC., Chem. Commun. 1990, 229-231.

[ 5 ] Y.G. Galyarnetdinov, O.N. Kadkin, I.V. Ovchinnikov, Bull. Russ. Acad. Sci., Div. Chem. Sci. 1992, 41, 316-319.

[6] M. J. Baena, P. Espinet, M.B. Ros, J.L. Serrano, A. Ezcurra, Angew. Chem. Znt. Ed. Eng. 1993, 32, 1203 - 1205.

2 Low Molecular Weight Lyotropic Metallomesogens

Teresa Sierra

In this chapter, compounds which have been described as forming mesophases in the presence of a solvent (polar or apolar) and which contain a metal atom (ranging from alkali to transition metals) will be considered. Since a discussion of the lyotropic behavior of alkali and alkaline earth metal carboxylates would be too extensive, and given the large number of studies published on them, only a few representative examples will be discussed to remind us that they do constitute an important area in the field of lyotropic metallomesogens. On the other hand, special attention will be paid to compounds containing transition metal atoms or hydrophobic chains which differ from the typical alkyl chains.

In order to clarify the discussion, a division has been made based on the type of molecular shape and type of aggregation observed. In this way, conventional surfactants which organize into micelles (in the following referred to as micellar lyotropic metallomesogens) and disc- or lath-like molecules that align due to n-interaction between the polyaromatic central nuclei (chromonic metallomesogens) have been treated separately. In the case of micellar systems, a supramolecular aggregate (i.e. micelle) assembles into a liquid crystal. In disc- or lath-like systems, the mesogenic unit is not an aggregate, but the molecule itself.

The nomenclature used throughout this chapter has been explained in Chapter 1 devoted to the basic concepts of liquid crystals.

2.1 Micellar Lyotropic Metallomesogens

The very first surfactants which can be considered to be metal-containing lyotropic liquid crystals are the alkali metal soaps [I] . In the presence of water, hexagonal (HI) and lamellar (L) phases are mainly formed as a function of the concentration

30 i? Sierra

and the temperature [2]. Aqueous sodium laurate, for instance, shows a transition from a lamellar mesophase (L) to a hexagonal mesophase (HI) at a concentration of 59 wt.070 at 100°C. Other alkali metal soaps, such as potassium myristate, show an additional cubic phase (II): L 66 I , 59 H, wt.070 surfactant). A number of intermediate mesophases have been observed in this type of system, but many of these have not been definitively identified. In general, and from a point of view of the structure-mesogenic property relationship, replacing one cation by another while maintaining identical hydrophobic units has a perceptible influence on the generation of the aggregate. It has been established that for potassium, sodium, and lithium salts of a given carboxylic acid a decrease in the critical micellar concentration (CMC) is observed. This decrease also correlates with a change in the concentration necessary for the formation of the hexagonal phase [3]. Ternary systems such as alkali soap/fatty acid/water, alkali soap/alkyl alcohol/water and alkali soaphon- polar organic solvent/water have been extensively studied [2,4].

Investigation of structurally modified alkali metal carboxylates such as caesium pentafluorooctanoate (CsPFO) led to the discovery of a nematic phase in lyotropic systems [ 5 -71. The CsPFO/water system forms bilayer aggregates and, over a wide of concentration range (0.1 1 - 0.43 volume fraction of CsPFO) and temperature range (from 1 1 to 77 "C), discoid micelles organize into a nematic (N,) phase between an isotropic micellar solution at higher temperatures and a lamellar phase at lower temperatures [6]. In contrast, LiPFO/water systems preferentially aggregate in- to cylindrical micelles which assemble into a hexagonal columnar phase [7]. This different behavior has been explained as a consequence of the rigidity and hydrophobicity of the fluorocarbon chain which, combined with the low hydration energy of the cesium ion, promotes the stability of bilayer aggregates as opposed to the cylindrical aggregates present in the LiPFO/water system in which the lithium ion has a higher hydration energy.

As a continuation of the general subject of metal salts of organic acids, derivatives of dodecylbenzenesulfonic acid have been reported to form a lyotropic, lamellar liquid crystalline phases [8]. Alkaline earth metals such as magnesium(I1) and barium@), transition metals such as manganese(u), cobalt@), nickel@), copper(II), chromium(rrr), iron(rrr), and other metals such as aluminium(II1) form dodecyl- benzenesulfonates which show a lamellar liquid crystal phase (L) with water. Some of the compounds show an additional lamellar phase, named the transition phase (T), at higher surfactant concentrations (a large excess over the CMC).

Metal-organic complexes of transition metals offer the possibility of obtaining amphiphilic molecules with a polar head group and a hydrophobic part which, imitating the structure of a classical soap, may give rise to ordered aggregates in the presence of a polar solvent.

Consequently, the annelide molecule has an amphiphilic-like structure and, in addition, it can coordinate to transition metals (Fig. 2-1) [9, 101. A lyotropic mesophase is observed in the cobalt(IIr)annelide/water system over a range of concentrations (0.83 -0.17 wt.070). The system is lamellar at room temperature. The transition temperature to an isotropic solution is 32°C. X-ray data provide evidence for a

Figure 2-1. complex.

2 Low Molecular Weight Lyotropic Metallomesogens 31

The cobalt(rii)/annelide L CI

monolayer-like structure which is schematically represented in Fig. 2-2. The structure in this system differs from those typically found in bilayer-forming soaps.

In the search for metal-organic structure elements for amphiphilic molecules to form lyotropic liquid crystalline phases, new complexes of iron(I1) have been described in which the metal is located in the polar head group and a long alkylamine ligand acts as the hydrophobic part (Fig. 2-3) [I I ] . Although these compounds are quite unstable in water, all the complexes show a hexagonal mesophase (HI) in this solvent.

Figure 2-2. Schematic representation of the lamellar lyotropic phase of the system Co(iii)annelide/water as determined by X- ray studies. Adapted from reference 9.

88

32 7: Sierra

1 M = Li, Na, K, 0.5 Ca

Figure 2-3. The aquatetra(cyano)dodecylaminoferrate( 11) complexes.

In spite of its instability to hydrolysis, lyotropic properties have been observed for the bis(ethylenediamine)lauratecolbalt(m) complex (Fig. 2-4) [ 121, which shows a hexagonal phase (HI) with water at room temperature. Another related complex exhibiting a hexagonal phase (H,) with water, as studied by the Lawrence penetration experiment [ 1 31, is a chromium(rr1) complex derived from salicylaldehyde (Fig. 2-5).

2 +

Figure 2-4. The bis(ethylenediamine)lauratecobalt(rrI) hexafluorophosphate complex.

r

Figure 2-5. The bis(ethylenediamine)dodecyloxysalicylaldehydechromium(~~ I) hexafluorophosphate complex.

Further examples of compounds in which metal complexes form the polar head group of an amphiphilic molecule are based on bipyridine ruthenium(r1) o r rhodium(rr1) complexes (Fig. 2-6). Both types of compounds incorporate either one long alkyl chain (R, = CH,, R2 = C12H25, C19H39, C21H43, C31H63) or two long alkyl chains (R, = R2 = C19H39 and R, = C12H25, R2 = C19H39). The two types of derivative mainly show cubic (I1) and hexagonal (HI) phases respectively, before an isotropic solution is formed in water. As an example, the ruthenium(r1) derivative in which Rl = CH3 and R2 = C3,H63 shows a cubic phase apparently stable between 0


n +

M n GI- RIJ(II), n=2

Rh(lll), n=3

Figure 2-6. Surfactant tris(bipyridine) derivatives of ruthenium(I1) and rhodium(rI1).

and 100°C. In addition, a hexagonal (HI) phase is formed at 98"C, between the cubic and solid phases [14, 151.

The most recent reports dealing with lyotropic metal-organic liquid crystals are based on oxovanadium(1v) [ 161 and silver(1) [ 171 amphiphilic complexes.

Oxovanadium(1v) based surfactants are represented in Fig. 2-7. Those complexes which have two alkyl tails (R, = R2 = C18H37 and R, = R2 = C10H2,, respectively) show a lamellar mesophase in the binary system with water. The compound with octadecyl chains shows the lamellar phase at a concentration of 12 wt.Yo and temperatures above 38 "C. The didecyl-substituted derivative shows the mesophase at ambient temperature throughout a concentration range between 14 and 50 wt.070. The lamellar mesophase of this compound exists at concentrations as low as 6wt.Yo if a cosurfactant, such as decanol, is added to the binary system. The compounds which have only one hydrophobic tail (R, = CH3, R2 7 C,2H25 and R, = CH3, R2 = C16H33) do not show lyotropic mesomorphism in binary systems with water. However, ternary mixtures with decanol display mesomorphism. Both lamellar and hexagonal (H,) mesophases are observed for the compound with the dodecyl chain depending on the composition.

Figure 2-7. Oxovanadium-based surfactants with tetraanionic diamidate-diphenolate ligands.

Binary mixtures of the (diazatetrathiamacrocycle)silver(I) complex represented in Fig. 2-8 and acetonitrile show lyotropic behavior at room temperature over a concentration range of 15 - 35 wt.% acetonitrile. The existence of an intralayer modulation in the lyotropic lamellar mesophase (Fig. 2-9) is noteworthy. The thermotropic behavior of this complex will be discussed in the next chapter.

34 7: Sierra

n l +

Figure 2-8. The [ 1 ,l0-bis-(4-chlorobenzoyl)-l,lO-diaza-4,7,13,16-tetrathiacyclooctadecane]silver(1) triflate complex.

Silver(1) cationic complex of 1,l O-bis(4-chlorobenzoyI)-I , I 0-diaza-4.7,13,16-tetrathiacyclooctadecane

0 CF3SOJ'

Figure2-9. Model for the structure of the modulated lamellar mesophase of the [l,lO-bis- (4-chlorobenzoyl)-l,10-diaza-4,7,13,16-tetrathiacyclooctadecane]silver(1) triflate complex [ 171.

2.2 Chromonic Metallomesogens

The antiasthma drug disodium chromoglycate (Fig. 2- 10) can be considered to be the first lyotropic metallomesogen recognized as showing columnar mesophases in water [I 8, 191. In fact, it constitutes the origin of the generic name of this type of lyotropic

Figure 2-10. The antiasthma drug diso- Na' 'OOC COO Na' dium chromoglycate.


. . imidoyl)xanthone-2-carboxylate, an anti-asthma drug. C6H13

mesogen organization: that is hexagonal (H) and nematic (N,) mesophases formed by columns which exist as a result of .n-interaction between the planar aromatic molecules. Other drugs, such as sodium 5-hexyl-7-(S-methylsulfonimidoyl) xanthone-2-carboxylate (Fig. 2- 1 I), display similar behavior in water [20].

In a similar manner to the drugs discussed above, a number of azo dyes (e.g. sodium salts of sulfonic acids) have been reported to form chromonic-type columnar hexagonal (H) and columnar nematic (N,) phases in water. Two examples of compounds which exhibit this type of behavior are Sirius Supra Brown RLL and Acid Red (Fig. 2-12) [21]. The phase sequence observed with increasing temperature in a system of 16% Sirius Supra Brown RLL in water follows the sequence H (r.t.)+ H + N, +Nc+N, + I + H + I + I.

Na+ -03S

Acid Red

Figure 2-12. The azo dyes Sirius Supra Brown RLL and Acid Red.

A model has been proposed for this particular hexagonal mesophase in which planar molecules are stacked in columns that lie in a herringbone array within a water continuum. The long axes of the columns are arranged in a hexagonal lattice (Fig. 2-1 3).

More recently, a number of disc-like molecules showing thermotropic discotic mesomorphism have been reported to show additional lyotropic mesogenic behavior in the presence of a suitable solvent. In systems where the discoid molecule is substituted with polar peripheral substituents (e.g., COOH, COO-Na' , S03H,

36 T Sierra

M X substitution

C U ~ + -COOH 4

Cu2+ -COONa+ 4

Cu2+ -COOH 4 , 5

C U ~ + -COOH 3

Co2+ -COOH 4

Zn2+ -COOH 4

Zn2+ -COOH 435

SO; Na + ) polar solvents are appropriate to general lyotropic mesophases. Apolar solvents are suitable for the generation of liquid crystalline phases of molecules which have paraffinic tails surrounding the central polyaromatic core.

As far as polar systems are concerned, transition metal complexes of phthalocyanines and porphyrins are the main representatives. Indeed, a copper(r1) complex of sodium tetracarboxylatophthalocyanine shows a lyotropic mesophase in the presence of aqueous ammonia [22]. In general, the ability of transition metal complexes of phthalocyanine with peripheral carboxy groups to form lyotropic phases depends on the metal. The inclusion of divalent transition metals, such as copper(II), zinc@) and cobalt(I1) (Fig. 2-14) leads to the appearance of H and N, mesophases in water ammonia mixtures, whereas aluminum(II1) complexes are not liquid crystalline. This finding is probably due to the formation of an oxydimer (phthalocyanine/Al-0-AVphthalocyanine) [23, 241. In analogy, copper(I1) and nickel@) complexes of (sulfophenyl) porphyrin (Fig. 2-15) show H and Nc mesophases in a water ammonia mixture [24, 251. Once again, the formation of lyotropic mesophases with trivalent metals is prevented due to the additional coordination mentioned above. Systems with different polarity are also known to form lyotropic mesophases. Porphyrins substituted with a long alkyl chain (Fig. 2-16) show lyotropic mesomorphism in mixtures with paraffins or alkyl halides of the same chain length as the

X

X

X


Figure 2-15. Copper(I1) and nickel@) complexes of tetra(4-sulfopheny1)porphyrin derivatives.

Figure 2-16. Copper(l1) complexes of (4-n-alkyloxyphenyl)-tri(4-tolyl)porphyrins.

12,22

lateral substituent attached to the porphyrin. The mesophase, which is yet to be identified, shows birefringence with a schlieren texture [26].

Dinuclear copper(r1) and rhodium(I1) soaps (Fig. 2-17) both show a thermotropic hexagonal columnar mesophase between 100 and 120°C [27]. However, when a hydrocarbon solvent is added in fractions above SOwtPlo, a texture typical of a nematic mesophase is observed [28]. It seems, therefore, that the addition of a paraffinic solvent to a hexagonal columnar array of a discotic, thermotropic transition metal complex reduces the interaction between columns. A breakdown of the lateral order occurs, and the formation of a lyotropic columnar nematic (N,) phase results (Fig. 2-1 8). The possibility of obtaining a lyotropic cholesteric phase has also been suggested, namely, when a chiral solvent, such as (+)camphene, is a constituent of the lyotropic complex/solvent system.

Other disc-like molecules bearing peripheral alkyl tails with structures completely different from those of the carboxylates discussed above, can exhibit lyotropic mesomorphism in a similar way. The only requirement for the formation of the lyotropic phase is the addition of a solvent which diminishes intercolumnar interaction in the discotic thermotropic mesophase. Recently, tetrapalladium organyls with chloro,

38 7: Sierra

Figure 2-17. Schematic representation of the columnar arrangement of dinuclear copper(1r) and rhodium(i1) carboxylates.

Figure 2-18. Representation of the columnar hexagonal arrangement of metallic carboxylates in the absence of solvent (slightly interdigitated paraffin chains) and the lyotropic columnar nematic mesophase (disentangled paraffin chains). Adapted from Ref. 28.

bromo, iodo, thiocyanato or azide bridges (Fig. 2-19) have been reported to form lyotropic mesophases in apolar solvents such as heptane, pentadecane, or eicosane [29].

At high concentrations of the chloro-bridged complex a mesophase similar to the thermotropic discotic oblique columnar phase (DobJ is observed. Once again, addition of paraffinic solvents to the systems decreases the lateral interaction between the columns, leading to the formation of less ordered mesophases. On dilution, two fluid mesophases of the nematic type have been identified. It has been proposed that their structure is likely to be columnar. To support the columnar model, experiments


X = CI, Br, I, SCN, N3 0 0 OR OR

Figure 2-19. General structure of tetrapalladium(1I)imino complexes.

on the intercalation of an electron donor into this binary system have been carried out. It is known that charge-transfer interactions between disc-shaped electron donors and an acceptor, such as TNF (2,4,7-trinitrofluorenone), favor the formation of columns based on intercalated TNF molecules, resulting in stabilization or induction of thermotropic mesophases [30]. In the same way, ternary systems (tetrapalladium complex/TNF/solvent) display a lyotropic nematic phase corresponding to a columnar arrangement. On the basis of this experiment, cholesteric properties have been generated in ternary systems by intercalating a chiral electron-acceptor such as ( - )-TAPA (i.e., (-)-2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxyl)propionic acid) (Fig. 2-20). Columnar aggregates may also be assumed to be the basis of this “chromonic N, phase” [31].

An undoubtedly novel structure for lyotropic liquid crystals is that represented by the chiral dinuclear chromium(II1) complex Na[cr,(~-tart,H) (phen),] (tart = tar-

O2N NO2

02N*N02

N,O TAPA

Figure 2-20. (-)-2-(2,4,5,7-tetranitro-9-fluorenylidene- aminooxy)propionic acid [( -)-TAPA, (-)-2)].

+OH

0

40 7: Sierra

Figure 2-21. The chiral dinuclear chromium(r1I) complex, Na[Cr,(L-tart,H) (phen),].

trate) depicted in Fig. 2-21 [32]. It has neither a conventional amphiphilic structure (polar head and hydrophobic tail), which would lead to the formation of micellar lyotropic phases, nor a planar aromatic core with peripheral solubilizing substituents, which would make it a chromonic metallomesogen. Nevertheless, the system Na [Crz (L-tartzH) (phen)J/water forms a lyotropic mesophase at concentrations above 0.006 moll- ' at 20 "C. The phase is birefringent, and 23Na NMR experiments indicate the presence of sodium ions in an anisotropic environment (as determined by the observation of a quadrupolar splitting). The mesophase has not yet been identified, but a lamellar array seems likely. This lyotropic phase would consist of monolayers of metal complex ions separated by layers of water.

2.3 Summary

All micellar lyotropic metallomesogens covered in this chapter are ionic, and furthermore, most of them are salts of organic acids with alkali, alkaline earth and transition metal counterions. Only a limited number of complexes containing the metal in the promesogenic part of the molecule have been reported to organize into micellar aggregates. The metal can be located either in the cationic (Co"', Cr"', Ru", Rh"' and Ag' complexes) or in the anionic (Fe" and VO" complexes) part of the amphiphilic salt. Many of these are unstable to hydrolysis which is their common limitation.

In recent years, a growing number of disc-like molecules have proved to possess a suitable structure for the formation of chromonic mesophases in the presence of a solvent. Neutral compounds have been described which are derived from carboxylates, phthalocyanines, porphyrins or tetrapalladium imino complexes. The possibility of the formation of a charge transfer system with tetrapalladium complexes must also be highlighted. In addition, such systems could show a cholesteric lyotropic mesophase if the electron acceptor is chiral, for example tetrapalladium complex/hydrocarbon solvent/chiral electron acceptor [( -)-TAPA].

Within this group of chromonic lyotropic compounds, ionic and covalent complexes can be found. Most of the ionic compounds belong to the group of alkali metal salts of organic acids (drugs, dyes, salts derived from porphyrin and phthalocyanine), in which the anionic part arranges as a result of n-interaction to form col-


umnar hexagonal or columnar nematic phases. Covalent complexes which display chromonic phases consist of disc- or lath-like molecules containing a transition metal. In this case axial interaction give rise to a columnar organization. As a general remark, lyotropic behavior can be observed for many thermotropic discotic metallomesogens either by adding an apolar solvent or by replacing the lipophilic chains by polar groups and then adding a polar solvent.

References

[I] A.E. Skoulios, Adv. in Colloid and Interface Science 1967, I , 79-110. [2] P. A. Winsor in Liquid Crystals and Plastic Crystals (Eds.: G. W. Gray, P. A. Winsor),

[3] N. V. Usoltseva, Izv. A N SSSR, Ser. Phyzich. 1989, 53, 1992. [4] P. Ekwall, Colloid Polym. Sci. 1988, 266, 1150- 1161, and references therein. [5] N. Boden, R. J. Bushby, L. Ferris, C. Hardy, F. Sixl, Liq. Cryst. 1986, 1, 109- 125. [6] N. Boden, P. H. Jackson, K. McMullen, M.C. Holmes, Chem. Phys. Letters 1979, 65,

[7] P.G. Morris, P. Mansfield, G. J.T. Tiddy, Faraday Symp. Chem. SOC. 1979, 13, 38-48. [8] D. Tezak, S. Popovic, S. Heimer, F. Strajnar, Progr. Colloid Polym. Sci. 1989, 79,

[9] D. Markovitsi, A. Mathis, J. Simon, J. C. Wittman, J. Le Moigne, Mol. Crysr. Liq. Cryst.

Ellis Horwood, Chichester, 1974, Chap. 5 , 225 -287, and references therein.

476 - 479.

293 - 296.

Letters 1980, 64, 121 - 125. [lo] D. Markovitsi, R. Knoesel, J. Simon, Nouv. J . Chim. 1982, 6, 531 -537. [11] D. W. Bruce, D. A. Dunmur, P. M. Maitlis, J. M. Watkins, 0. J. T. Tiddy, Liq. Cryst. 1992,

[I21 D.W. Bruce, I.R. Denby, G. J.T. Tiddy, J.M. Watkins, J . Muter. Chem. 1993, 3,

[ 131 A. S. C. Lawrence, M. P. McDonald, Liquid Crystals (Eds.: G. H. Brown, G. J. Dienes and

[I41 D.W. Bruce, J.D. Holbrey, A.R. Tajbakhsh, G.J.T. Tiddy, J . Mater. Chem. 1993, 3,

[ 151 D. W. Bruce, J. D. Holbrey, G. J. T. Tiddy, ISh International Liquid Crystal Conference,

[16] S. S. Zhu, T. M. Swager, Adv. Muter. 1995, 7, 280-283. [I71 F. Neve, M. Ghedini, G. DeMunno, A.M. Levelut, Chem. Muter. 1995, 7, 688-693. [18] J. S. G. Cox, G. D. Woodard, W. C. McCrone, J Pharm. Sci. 1971, 60, 1458- 1465. [I91 N. H. Hartshorne, G. D. Woodard, Mol. Cryst. Liq. Cryst. 1973, 23, 343-368. [20] T.K. Attwood, J.E. Lydon, Mol. Cryst. Liq. Cryst. 1984, 108, 349-357. [21] T.K. Attwood, J.E. Lydon, F. Jones, Liq. Cryst. 1986, 1, 499-507. [22] S. Gaspard, A. Hochapfel, R. Viovy, C. R. Acad. Sc. Paris 1979, C289, 387 - 390. [23] N. Usoltseva, V. V. Bykova, N. M. Kormilitsyn, G. A. Ananieva, V. E. Maizlish, I1 Nuovo

[24] N. Usoltseva, V.V. Bykova, Mol. Cryst. Liq. Cryst. 1992, 215, 89-100. [25] V. Bykova, N. Usoltseva, G. Ananjeva, A. Semeikin, T. Karmanova, 15Ih International

[26] S. Gaspard, P. Maillard, J. Billard, Mol. Cryst. Liq. Cryst. 1985, 123, 369-375.

11, 127- 133.

911 -916.

M. M. Labes). Gordon and Breach, London, 1966, 1 - 19.

905 - 906.

Budapest (Hungary), 1994.

Cimento 1990, 120, 1237- 1242.

Liquid Crystal Conference, Budapest (Hungary), 1994.

42 7: Sierra

[27] a) A.M. Godquin-Giroud, J.C. Marchon, D. Guillon, A. Skoulios, J. Phys. Lett. 1984, 45, L681 -L684; b) A.M. Giroud-Godquin, J. C. Marchon, D. Guillon, A. Skoulios, J. Phys. Chem. 1986,90,5502- 5503; c) J. C. Marchon, P. Maldivi, A. M. Giroud-Godquin, D. Guillon, A. Skoulios. D.P. Strommen, Phil. Trans. R. SOC. Lond. A 1990, 330,

[28] M. Ibn-Elhaj, D. Guillon, A. Skoulios, A.M. Giroud-Godquin, J. C. Marchon, J. Phys.

[29] N. Usoltseva, K. Praefcke, D. Singer, B. Gundogan, Liq. Cryst. 1994, 16, 601 -616. [30] a) K. Praefcke, D. Singer, B. Kohne, M. Ebert, A. Liebmann, J.H. Wendorff, Liq. Cryst.

1991, 10, 147- 159; b) H. Bengs, 0. Karthaus, H. Ringsdorf, C. Baehr, M. Ebert, J.H. Wendorff, Liq. Cryst. 1991, 10, 161 - 168; c) K. Praefcke, D. Singer, M. Langner, B. Kohne, M. Ebert, A. Liebmann, J.H. Wendorff, Mol. Cryst. Liq. Cryst. 1992, 215, 121 - 126; d) D. Singer, A. Liebmann, K. Praefcke, J. H. Wendorff, Liq. Cryst. 1993, 14, 785-794; e) K. Praefcke, D. Singer, A. Eckert, Liq. Cryst. 1994, 16, 53-65.

109- 116.

II France 1992, 2, 21 97 - 2206.

[31] N. Usoltseva, K. Praefcke, D. Singer, B. Giindogan, Liq. Cryst. 1994, 16, 617-623. [32] N. Koine, M. Iida, T. Sakai, N. Sakagami, S. Kaizaki, J. Chem. SOC. Chem. Commun.

1992, 1714-1716.

3 Low Molecular Weight Calamitic Metallomesogens

Jose' Luis Serrano and Teresa Sierra

3.1 Metal-Organic Liquid Crystals

In this section liquid crystalline complexes containing a metal atom bonded to the organic ligands via heteroatoms will be discussed. The section has been divided into three subsections dealing with monodentate Iigands, bidentate ligands, and tetradentate ligands, respectively. Within each subsection the compounds have been further classified according to the nature of the atom or atoms coordinated to the metal center: nitrogen, oxygen, or sulfur.

3.1.1 Metal-Organic Liquid Crystals with Monodentate Ligands

In 1986, a new type of metal-containing liquid crystal, formed by complexation of a transition metal with terminally functionalized organic ligands (4-cyanobiphenyl and stilbazole derivatives), was reported [I]. So far, only two types of metal-ligand bonds have been described for metallomesogens containing a monodentate ligand: M-N (in nitrile, pyridine and amine derivatives) and M-S (in thiolate complexes).

Nitrile and pyridine ligands are usually mesogenic or promesogenic compounds, and the metal atom acts either as a linking group between two ligands, effectively doubling length of the molecule (in Pd", Pt", and Ag' derivatives), or as a polar terminal substituent complexed to only one ligand (in Rh', Ir' and Au' complexes). In the latter case, the metal center causes only a slight perturbation of the rod-like structure of the ligand and the intermolecular forces are relatively unaffected. Thus the liquid crystal character in the complex is usually retained. In complexes derived from two monodentate ligands, two coordination geometries are possible: linear or square-planar. It seems reasonable to assume that a square-planar geometry could hinder the appearance of liquid crystallinity, but usually the lateral groups coordinated to the metal are small and do not significantly increase the L / D (length to width)

44 J. L. Serrano and T Sierra

ratio. Therefore, these complexes behave as typical organic calamitic liquid crystals and exhibit nematic or smectic mesophases.

It is worth emphasizing the simple structures of amine and thiolate complexes, which, although containing no aromatic rings, but only alkyl groups lead to mesomorphic properties. The higher mobility and flexibility of these structures make it possible to obtain very different ordering in addition to nematic and smectic mesophases. Lamellar, cubic or hexagonal-micellar arrangements have been identified. At the molecular level, structures which differ from the elongated calamitic structure can be obtained (U-shaped, trigonal).

It should be noted that most of the ionic thermotropic metallomesogens described in the literature so far are metal-organic liquid crystals, with monodentate ligands and they are mainly silver(1) complexes of ligands derived from pyridines or amines are known. Only a few examples of copper complexes have been described. In all cases, the metal atom is a constituent of the cationic moiety responsible for the molecular ordering within the mesophase. So far, there has not been an example described in which the metal center is in a promesogenic anionic part of an ionic complex.

3.1.1.1 Nitrile Ligands

Platinum and Palladium

Platinum(I1) and palladium(I1) both coordinate with nitrile groups to give bis(nitri1e) complexes with a trans-square-planar geometry [2 - 41 (Fig. 3- I ) .

Figure 3-1. General struc- - CI ture of trans-square-pla-

61 complexes. R CORE - ) - c = N - ~ - N = c + ~ ~ R UNIT nar bis(nitrile)metal(rI)

Bis(4-alkoxy-4'-cyanobiphenyl)platinum(11) complexes give rise to smectic A and nematic mesophases. In contrast, analogous palladium(r1) complexes do not show nematic mesophases, but a smectic C phase appears, which was not present in the uncomplexed ligand. The transition temperatures of palladium(1r) compounds are significantly lower than those of the corresponding platinum(I1) complexes, and both kinds have significantly higher transition temperatures than the corresponding ligands (Table 3-1).

Crystal structures determined for platinum(I1) and palladium(r1) complexes of this type show that they are isomorphous and isostructural. It seems that no structural reason exists for the different behavior of such similar compounds. To date, no explanation has been put forward to account for this phenomenon. However, the answer may lie in the electronic configurations of the two metals. The higher polarizability of the platinum ion may lead to stronger molecular interactions, and hence to higher melting temperatures and more stable mesophases.

3 Low Molecular Weight Culamitic Metullomesogens 45

Table 3-1. Mesomorphic properties of bis(nitrile)platinum(II) and -palladium(rr) complexes.

1 LIGAND R M Mesomorphic Properties -1

64 77 80

167 187 209

119 122 146 . 165 180

' 120 (. 89)

When the alkoxy group in the ligand is replaced by an alkyl group, nematic mesomorphism is favored. Once again, a clear difference between the behavior of platinum@) and palladium(I1) complexes is found; whereas platinum complexes show enantiotropic mesophases, a monotropic nematic phase only is observed in palladium complexes (see Table 3-1 for an example).

Alkylcyanobicyclohexyl ligands have also been coordinated to palladium(1r) [2]. A monotropic nematic mesophase is observed for the two derivatives reported,

3-1). The monotropic nematic phase has been stabilized by means of mixtures, which are not strictly binary mixtures, but a disproportionation reaction takes place:

PdC12(C3H7-C6H,oC6Hlo-CN)2] and [PdCl,(CSHii-C,H,oC6Hio-CN),] (Table

Rhodium

When rhodium(1) is used as the metal center, mono(carbonitri1e) complexes can be prepared such as ~ is (RhCl(C0)~L) (Fig. 3-2) [I]. In spite of their relatively low melting points, these complexes decompose at their clearing temperatures.

Figure 3-2. K (51 S?) 79 N 81 I (dec)

46 J.L. Serrano and 7: Sierra

3.1.1.2 Pyridine Ligands

One of the problems associated with materials of the type discussed in the previous section is the relatively labile metal-nitrile linkage which limits the stability of the complexes. As pyridine-metal bonds are generally more stable, an extensive study has been carried out on complexes which contain mesogenic alkoxystilbazole ligands (Fig. 3-3).

Silver

The reaction of stilbazole ligands with a variety of silver(1) salts gives rise to linear ionic complexes (Fig. 3-3) [5]. Non-mesogenic (X- = BF;) and mesogenic (X- = C,2H2,0SO<) anions were first used as counterions. The tetrafluoroborate derivatives with long alkoxy tails show smectic A and smectic C phases at high temperatures, where decomposition occurs, and the compounds are moisture sensitive. In contrast, much lower transition temperatures and richer liquid crystalline polymorphism were found for the lauryl sulfate derivatives [6]. Besides smectic phases (S, and SA), the compounds in the series with short terminal chains show nematic behavior, which makes them the first examples of ionic nematogenic metallomesogens. Ionic amphiphiles (i.e. carboxylates, ammonium salts) are known to exhibit lamellar smectic-like phases when heated. The ocurrence of a nematic phase in these systems is particularly surprising. This behavior signifies that anisotropic attractive and repulsive interactions can overcome the isotropic coulombic forces which would preferentially lead to a lamellar organization.

The appearance of a viscous, optically isotropic mesophase upon heating the smectic C phase of these silver(1) dodecylsulfate complexes is also noteworthy. X-ray studies carried out on these structures proved that this isotropic mesophase to be cubic [7]. The existence of the cubic phase is strongly dependent on the length of the alkyl chain in the anion. For example, when octylsulfate is present a cubic phase is not observed [S].

For the nematic and smectic A phases exhibited by alkoxystilbazole silver(1) complexes a great tendency to align homeotropically is observed. Some interdigitation is deduced for the smectic A phase, which must be responsible for the high tilt angle measured in the smectic C phase, for example, 46" for bis[(nonyloxy)stilbazole]silver(1) dodecylsulfate. Further support for the existence of interdigitation may arise from the observation that there is no odd-even effect for the materials which show a smectic A phase. This means that the influence of the terminal carbon atoms of the alkoxy tails is greatly reduced [7].

3 Low Molecular Weight Calamitic Metallomesogens 47

The effect of different and smaller anions, such as nitrate and triflate have also been studied [9]. For derivatives with triflate anion nematic phases are observed for complexes derived from stilbazoles with short alkoxy tails. This phase is not observed for the analogous nitrate salts which, even with the shortest chains, display smectic behavior; when n = 2 (Fig. 3-3) a strongly homeotropic smectic A phase forms on melting the crystal, and an additional smectic C phase appears when n 2 5. Little difference between the melting and clearing temperatures of the derivatives containing these two anions was found.

Nonmesogenic pyridine ligands, namely (pyridylmethy1ene)aniline and phenyl- pyridinecarboxylate derivatives, have also been complexed with the silver(1) cation [lo]. Several counterions (BF, , CF3S0,, NO, and PF;) have been employed, allowing an extensive comparative study. The nematic phase is not found as commonly as it was in the stilbazole derivatives discussed above. Only the (pyridylmethy1ene)- aniline tetrafluoroborate complex containing ethoxy chains shows a nematic phase, the reason for this behavior possibly being the increased planarity of the stilbazole ligands. Decomposition is observed for most of these complexes once they reach the isotropic state.

In general, tetrafluoroborate complexes show the best mesomorphic properties. However, the role of the counterion in determining the mesophase structure seems to strongly depend on the ligand being used (Table3-2). In compounds with the same anion, the ester derivatives exhibit better liquid crystalline properties than the analogous imine derivatives. Phenyl pyridylcarboxylate derivatives show wider phases as well as a richer polymorphism than the corresponding imine complexes. A comparison can also be made between stilbazole and imine derivatives in terms of their melting and clearing points (Table 3-3). The use of either nitrate or triflate anions gives rise to different effects for each type of ligand. For imine derivatives,

Table 3-2. Mesomorphic properties of bis(pyridylmethy1ene)- and bis(pyridylcarboxylate)silver(I) complexes.

103.4

155.0


-CH=N- NO$

CF3SO3-

-CH=CH- Nos-

CF3SOj

Table 3-3. Mesomorphic properties bis(pyridylmethy1ene) and bis- (stilbazole)silver(I) complexes with different counteranions.

84.2 154 186(dec)

153.7 173.4

152.0 221.0 250.0

158.0 165.0 181.0 250.0

A X- Mesomorphic Properties

K SG sc SA I

the small planar nitrate anion leads to low melting points, wide mesophases and a rich polymorphism. Higher melting temperatures and lower clearing points are found when the larger triflate anion is used. In contrast, little difference is observed in melting and clearing temperatures between nitrates and triflates with stilbazoles as ligands [9].

The central core of the ligand in stilbazole complexes was modififed by introduction of lateral fluoro-substituents into silver(1) complexes with triflate and dodecylsulfate counterions [ I I].

Depending on the position of the fluoro substituent, the mesomorphic behavior is affected in a different way. When the fluoro substituents occupy position 3 of the aromatic rings (Fig. 3-4) nematic mesophase are formed less frequently. Melting points are decreased, and the smectic C phases are destabilized, resulting in a pronounced widening of the smectic A phases. In contrast to unsubstituted stilbazole complexes, interdigitation of the alkoxy chains does not occur in this phase as shown

Figure 3-4.


by X-ray studies. In the case of dodecylsulfate (DOS) derivatives the cubic phase disappears completely. When the fluoro substituent occupies position 2, different effects are observed. The clearing points are significantly depressed and melting points are also decreased, but to less extent. The smectic C phase is stabilized at the expense of the orthogonal smectic A phase. The cubic phase of DOS complexes still exists, but it is now observed between a nematic and a smectic A phase.

Rhodium and Iridium

Coordination of the pyridine ligands described previously has also been carried out with different metals. Bis(pyridylmethylene)anilinerhodium(I) complexes have been prepared, but were found to be non-liquid crystalline materials with melting points above 300 "C [12]. However, liquid crystalline iridium(1) and rhodium(1) mono- (pyridylmethy1ene)aniline cis-complexes have been reported (Fig. 3-5) [ 12, 131 which have a similar structure to the nitrile complexes described above (see Fig. 3-2). These complexes exhibit relatively low melting temperatures. Furthermore, they represent an important achievement because liquid crystal materials have been obtained from nonmesogenic ligands. The groups [IrCl(CO),] and [RhCl(CO),] turn out to successfully induce nematic mesomorphism when acting as polar terminal groups. The iridium group equals the nitrile group in stabilizing the nematic phase while the rhodium group is significantly more effective. Mesogenic alkoxystilbazole ligands have also been coordinated to rhodium(1) and iridium(1) [14]. Wider smectic A phase ranges have been observed in comparison with analogous (pyridylmethy1ene)aniline complexes (Table3-4), and the melting points are similar to those of the uncomplexed ligands.

Figure 3-5. M = Rh(l), Ir(l)

hn+lCnO h N - ; O c 0 I I

Figure 3-6. CI

Lateral fluoro substitution in iridium(1) monostilbazole complexes (Fig. 3-6) [15, 161 has a strong destabilizing effect on mesophases, giving rise to monotropic behavior in 2-fluorinated systems and destruction of liquid crystal properties when the fluoro substituent occupies 3-position.


H17C80-@H=N<N

Table 3-4. Mesomorphic properties of bis(stilbazole)rhodium(i) and iridium(1) complexes.

Rh

LIGAND M Mesomorphic Propelties

K Sn N I

79 p 77) 87

05 123 130 . 92 113 120

In order to investigate the concept that non-symmetrical molecules should give rise to low-melting metallomesogens because of a less efficient packing in the crystal, non-centrosymmetric platinum(rr) complexes have been studied (Fig. 3-7) [ 17, 181. The use of an inert metal such as platinum avoids the risk of disproportionation, which can occur in non-centrosymmetric complexes, and thus stable alkenestilbazole platinum complexes have been prepared. Most of these complexes show a smectic A phase which, in some cases can be supercooled to as low as - 30 "C for several hours.

Gold

Gold(1) has also been coordinated with an alkoxystilbazole ligand [ l ] to obtain a mesogenic complex (Fig. 3-8) which shows an unidentified mesophase between 120 O and 200 "C before decomposition occurs.

K 120 M 200 I(dec) Figure 3-8.


3.1.1.3 Amine Ligands

Ionic complexes of copper(r1) with long primary or secondary amines, ([Cu (RNH,),] or [Cu(R2NH),] (N03)2) exhibit a mesophase which is attributed to the melting of the long aliphatic chains [19]. The authors suggest a smectic- like arrangement, but this is yet to be confirmed.

Ionic amine complexes of silver(1) have also been reported [20] to show lamellar phases consisting of layers of U-shaped molecules which are comparable to classical lyotropic mesophases (Fig. 3-9).

Ag' Figure 3-9. Schematic representation of the lamellar mesophase of U-shaped amino silver(1) complexes. - l? -CnH2n*lNH2

Bis(n-alkylammonium)tetrabromozincates [(C,H2, + ,NH3)2ZnBr4] [2 11, in which the organic ligand is not coordinated with the metal, exhibit several transitions upon heating: two high entropy solid-solid transitions and a transition into a smectic mesophase. X-ray studies of the high temperature solid polymorph show a layered structure in which ionic regions, still crystalline, are sandwiched between hydrocarbon regions, which are in a liquid-like state. This observation is consistent with a smectic mesophase of a rod-like structure. Indeed, the appearance of a smectic phase before the isotropic liquid state is not unexpected.

3.1.1.4 Thiolate Ligands

Silver thilolates (AgSR) with alkyl tails ranging from n-butyl to n-decyl display thermotropic liquid crystalline behavior consisting of lamellar (SA), optically isotropic

52 I L . Serrano and 7: Sierra

SOLID LAMELLAR PHASE (SA)

CUBIC PHASE SR below Plane rrrr. SR above plane

Figure 3-10. Schematic representation of the molecular arrangements in the solid and the mesophases (lamellar and micellar) of silver

MICELLAR PHASE thiolates. Adapted from reference (HEXAGONAL COLUMNAR) 22.

cubic and hexagonal-micellar phases (Fig. 3-10) [22] . In spite of the noncalamitic molecular structure of the compounds, the appearance of the lamellar phase is not unexpected taking into account the crystal organization of these complexes. In the crystal, silver atoms, in a quasi-hexagonal arrangement, are connected by bridging p3-thiolate groups, which extend perpendicular to the central plane on both sides (see Fig. 3-10). This arrangement is the most efficient in terms of molecular packing when the chains melt (as long as the chains are relatively short), thus giving rise to a smectic A phase on heating. On further heating, the compounds assume an intermediate optically isotropic cubic phase structure, and the transition from a p3-bridged to a ,u2-bridged state results in a hexagonal-micellar arrangement. This corresponds to a change from trigonal to linear coordination of the amines to the silver atoms. In compounds containing alkyl groups longer than ten carbon atoms, only the hexagonal-micellar mesophase is observed on heating the crystals.

The first metallomesogens derived from copper(1) are thiolate derivatives [22] which show a behavior similar to that of silver(1) thiolates.

3.1.1.5 Summary

The most relevant characteristics of monodentate metallomesogens are shown in Ta- ble 3-5. Metallomesogens derived from monodentate ligands with calamitic structure proved to behave similarly to calamatic organic liquid crystals. General structural considerations of the mesogenic properties of organic systems (terminal chain length, lateral substituents, central bridging units, etc.) can also be applied to


Table 3-5. A pyridinoplatinum(i1) complex (see refs. 14, 15) containing two different organic ligands. In this table only the promesogenic one, i.e. the pyridine derivative, is considered.

M-X bond

M-N

-

Pyridine

Amine

W ) 1 covalent square planar

Ir(l) 1 covalent square planar

AU(l) 1 covalent linear

Ag(l) 2 ionic linear Pt(ll)# 1 covalent square planar

Ag(U 2 ionic linear Cu(ll) 4,2 ionic square planar Pd(ll) 4 covalent square planar

M-S Thiolate Ag(l) 1 covalent trigonal I linear Cu(l) covalent

monodentate metallomesogenic compounds. Indeed, discussion in this section has been mainly focused on these.

In covalent complexes, the incorporation of the metal leads to either metal-metal or metal- heteroatom intermolecular interactions, causing two effects. Firstly, it may promote mesomorphism, allowing the use of nonmesogenic ligands. Secondly, complexes from mesogenic ligands usually show transition temperatures which are much higher than those of the corresponding ligands and, consequently, decomposition often takes place. The design of unsymmetric metallomesogens has been put forward as a suitable solution to the problem of high transition temperatures.

In ionic complexes, the counterion plays an important role in determining mesomorphic properties and, in relation with the nature of the ligand structure, it influences the type and range of mesophases.

3.1.2 Metal-Organic Liquid Crystals with Bidentate Ligands

Metal derivatives of bidentate organic ligands constitute the largest group within metal-containing liquid crystals. Of particular importance are salicylidenamine complexes because of the enormous variety of molecular structures which have been thoroughly investigated. Results from these studies have allowed the relationships be-

54 J. L. Serrano and 7: Sierra

tween molecular structure and mesomorphic behavior to be clarified in more detail than for any other group of metallomesogens.

Another important group within bidentate-ligand complexes are 8-diketone derivatives, which have played an important role in studying both calamitic and discotic metallomesogens.

For the purpose of lucidity, metallomesogens derived from bidentate ligands have been divived into five sections, classified according to the ligand donor atoms involved in the coordination sphere of the metal.

1. N,N: 2-substituted pyrrole derivatives (Sec. 3.1.2.1) 2. N,O: salicylideneamine (Sec. 3.1.2.2), enaminoketone (sec. 3.1.2.3) and

aroylhydrazine derivatives (Sec. 3.1.2.4) 3. 0,O: 8-diketone (Sec. 3.1.2.5), ehydroxybenzaldehyde (Sec. 3.1.2.6) and

alkanoate (Sec. 3.1.2.7) derivatives 4. 0,s: monothio-8-diketone derivatives (Sec. 3.1.2.8) 5 . S,S: dithiolene (Sec. 3.1.2.9), dithiobenzoate (Sec. 3.1.2.10) and dithiocarbamate

(Sec. 3.1.2.1 1) derivatives Bidentate ligands have mostly been attached to transiton metals, although a few

examples of metals from groups 111 (thallium(1)) and IV (lead(r1)) have also been reported.

3.1.2.1 2-Substituted Pyrrole Derivatives

Nickel(i1) and copper(I1) have been coordinated to 2-substituted pyrrole derivatives to yield ML2 bidentate complexes [23]. Nematogenic nickel(1r) complexes have been obtained from ligands derived from 2-phenylazopyrrole and 2-phenylazomethinepyrrole, which show nematic phase behavior in their own right. Likewise, a bis(2-phenylazomethinepyrrole)copper(1i) complex has been described which exhibits a nematic phase (Table 3.6). However, bis(2-phenylazopyrrole)copper(11) complexes undergo an irreversible rearrangement on heating before liquid crystalline properties can be observed. The reason for this is the formation of a nonmesogenic binuclear complex via the exchange of one of the chelating centers: the pyrrole nitrogen now acts as an electron donor.

3.1.2.2 Salicylideneamine Derivatives

Metal complexes derived from Schiff bases have been known since the nineteenth century [24]. In particular, salicylideneamine derivatives have played an important role in coordination chemistry [25]. Many different metals can bind ligands of this type and, to date, copper(ii), nickel(ii), vanadium(iv), palladium(II), platinum(II), iron(lII), rhodium(1) and iridium(1) salicylideneamine derivatives have been reported to show liquid crystalline properties. The compounds most commonly reported are


N - w N - @ X & , Ni

N a O C & , Ni

CH a O C & , , Ni

CH -0C6H13 Cu

Table 3-6. Mesomorphic properties of nickel(1r) and copper@) complexes of 2-substituted pyrrole-derived ligands.

K N I

207.7 221.9

173.7 200.7

221.9 205.0

129.8 168.3

copper(II), palladium(r1) and oxovanadium(1v) complexes. In contrast, only a few examples of rhodium(I), iridium(1) and platinum(I1) complexes have been described.

In spite of their structural similarity, examples of calamitic ketoneimine metallomesogens have not been reported to date. This is probably due to an unfavorable steric effect caused by the presence of an alkyl group attached to the carbon atom of the imine linkage, which could disrupt the molecular ordering necessary to yield a liquid crystalline arrangement.

Numerous different metals have been complexed with the same salicylideneamine ligands. Therefore, the discussion of this group of metallomesogens has been organized on the basis of the structure of ligands and not the kind of metal being center of the complex. The discussion is divided into six different sections depending on the number and position of the aromatic rings contained in the ligands (see scheme

Scheme 3-la: Ovchinnikov and coworkers were first to exploit the coordination possibilities of salicylideneanilines and, in 1984, they reported the first imino-derived metallomesogens (Fig. 3-1 I ) [26]. All these materials are smectogens, the only exception being the compund with m = 1, which is not liquid crystalline.

At first sight, these complexes could be expected to show discotic mesomorphism due to their rather broad molecular structure, that is, “brick-like” shape with a square-planar geometry of the metal center. However, observation of their textures by polarizing optical microscopy and X-ray studies show that these compounds are actually calamitic mesogens. Moreover, the considerable degree of rotation of the

3-1, p. 56).

56 J. L. Serrano and 'I: Sierra

a) RQCOR N-phenyl-salicylaldirnine derivatives

0.i: M = Cu(ll), VO(IV), Pd(ll) 4.

b) RQ<o,Q,. Salicylidene-polyaromatic amine derivatives

0.i: M = Cu(ll), Pd(ll), Pt(ll) A

x = 40- N-phenyl-4-benzoyloxysalicylaldirn ine

c, R G + x . o Q { G R derivatives

N-phenyl-4-benzyloxysalicylaldimine derivatives

M = Cu(ll), Ni(ll), VO(IV)

AM: X = -Clip-

2

d) R ~ ; Q : '" - R' N-alkyl-4-benzoyloxysalicylaldimine derivatives

0.i: M = Cu(ll). Ni(ll), Pd(ll) /:

"a:Qo;.' N-alkyl-5-benzoyloxysalicylaldimine derivatives

+c=N- M = CU(l1) H R '

1) Polynuclear salicylideneamine derivatives

Scheme 3-1. Types of salicylideneamine derivatives treated in Sec. 3. I .2.2.

1

Figure 3-11.

benzene rings with respect to the salicylaldiminatocopper groups, which leads to an increase in the thickness of the molecular cores, appears to be strongly obstructing a discotic stacking [27].


As was first observed by Ovchinnikov [26], and later confirmed by means of X-ray diffraction, N-(alkoxyphenyl)-4-alkoxysalicylaldimines mainly show smectic mesomorphism when coordinated to copper(I1) (Table 3-7) [28 - 321. When both chains in the ligand are short the compounds are not liquid crystalline. If one of both tails is elongated a smectic A phase is observed. In addition, a smectic C phase is found for cases where both alkoxy tails are long. As depicted in Table 3-7, similar behavior to that typical of organic liquid crystals is exhibited by these compounds when terminal chains are lengthened.

Table 3-7. Mesomorphic properties of bis [N-(4-alkoxyphenyl)-4-alkoxysalicylaldiminato] copper(1r) complexes.

169.7

164.6

' 177.7 ' 161.2

Palladium(@ has also been incorporated as a metal center into similar imine complexes. Smectic mesomorphism is also predominant in these systems. This fact can be accounted for by considering the crystal structure of the copper(I1) and palladium(1r) complexes, which shows the presence of parallel layers of molecules. These layers are of comparable thickness to the interlayer smectic ordering after heating [27, 331. Higher phase transition temperatures and increased thermal stability have been found for all palladium(I1) derivatives when compared to their copper(r1) analog (see Table 3-8). When the ether linkages between the lateral chains and the mesogenic core are substituted by ester groups, a similar effect is observed [31]. Enantiotropic smectic phases are exhibited by copper(I1) (mostly Sc) and palladium(I1) (S, and S,)

58 J.L. Serrano and T Sierra

Table 3-8. Mesomorphic properties of bis[N-(4-alkoxypheny1)-4-alk- oxysalicylaldirninato] and bis[N-(4-alkanoyloxyphenyl)-4-alkanoyl- oxysalicylaldiminato]copper(~~) and -palladium(II) complexes.

Mesomorphic Properties M A n m

K sc SA I

Cu - 7 6 150 155 168

Pd - 7 6 171 201 ' 215

Cu -CO- 7 7 193 207 . Pd -CO- 7 7 238 247 250

complexes, further, the transition temperatures are found to be higher than those of the analogous alkoxy derivatives (Table 3-8).

Schiff base copper(I1) complexes containing alkylanilines have also attracted considerable attention regarding the subject of structure-property relationship. Once again, smectic mesomorphism is mainly observed, and smectic A phases are especially common (Table 3-9). In general, more ordered smectic mesophases are obtained compared with those formed by the free ligands. Indeed, some of the ligands show nematic behavior which completely disappears after complexation [32, 34, 351. The smectic C phase, commonly observed in alkoxyaniline complexes, does not often occur in analogous alkyl systems unless very long alkylchains are present [32]. This is the case even if the free ligand shows a smectic C phase. When oxovanadium(1v) is complexed with alkylaniline Schiff bases [36], a smectic A phase is mainly formed by derivatives with longer alkyl tails. The oxovanadium(1v) complexes, in which longer substituents are necessary before mesophases occur, have higher transition temperatures than the copper@) analog (Table3-9). These findings can be accounted for by a stronger intermolecular interactions caused by the rigid molecular core of by the oxovanadium(1v) coordination sphere and the four aromatic rings.

As far as the structure of the smectic A phase of these complexes is concerned, X- ray studies have demonstrated the existence of a marked interdigitation between layers [37]. In addition to the interdigitation, the chains situated on the same side must be partially folded in order to effectively fill the space between the molecules [38]. Metal complexation is expected to cause a large increase in the molecular biaxiality, therefore efforts have been made to elucidate whether or not the smectic phases of these copper complexes are biaxial. Conoscopic investigations on the copper(r1) complex (n = 12,


Table 3-9. Mesomorphic properties of bis[N-(4-alkylphenyl)-4-alkoxysalicylaldiminato]copper(~~) and -oxovanadium(1v) complexes.

i r n M

2 2 cu 2 2 vo 2 4 c u

2 4 vo 2 6 Cu

2 6 VO

2 8 cu 2 8 vo 0 6 Cu

0 14 Cu

Mesomorphic Properties

K s1 s2 SA I

' 128 p 128)

156

90 SE 98 SB 114 140

165 (- 156)

119 140

153 160

121 136

150 (Sc 142) 161

131 145 9

98 Sc 105 124

rn = 4 in Table 3-9) revealed that the smectic A phase is uniaxial [39]. However according to X-ray measurements, a short range correlation between metal centers exists and so the molecular packing can be described in terms of pairs of molecules in a side-by-side arrangement [40]. The results of X-ray and EXAFS studies on the smectic B phase suggest the presence of a local biaxial ordering in this phase [41]. Nevertheless, these conclusions are not valid for the description of the smectic A phase.

Differences in the mesogenic properties of the various metal derivatives can be explained in terms of the type of coordination of the metal. Molecular packing in the mesophase can be affected by the presence of two different conformations of the copper(I1) derivative: a square-planar geometry and a distorted tetrahedral geometry as deduced from crystallographic data [27]. In contrast, palladium(I1) complexes only form a square-planar conformation [33] which should enhance intermolecular interactions, and hence mesophase stability. Similarly, molecular interaction is favored in oxovanadium(1v) complexes, which have a square-pyramidal geometry and a V=O bond at the apex of the pyramid. The same types of mesophases (mainly smectic) have been found for derivatives of all three metals. In contrast,


tetrahedrally coordinated cobalt(I1), zinc(l1) and nickel(r1) derivatives are not mesomorphic [42].

With respect to the resulting materials properties, an interesting modification of these imine ligands is the incorporation of a stereogenic center in the molecule. Examples which incorporate L-butyl lactate as a chiral moiety and copper(1r) or oxovanadium(1v) as the metal center have been reported [43]. These complexes are the first paramagnetic metallomesogens to show ferroelectric properties. Another approach was based on systems with the same chiral tail but with palladium(r1) as the metal center [44]. Studies on different structural modifications of the ligands revealed a strong dependence of the mesomorphic properties on the distance between the stereogenic carbon and the rigid core. The further away the branching is situated from the core, the greater the stability of the mesophases. Only cinnamate derivatives show a thermodynamically stable ferroelectric chiral smectic C ( S E ) phase (Table 3-10). Ghedini and coworkers have also described palladium(I1) complexes incorporating a variety of chiral alkoxyanilines [45]. Only the citronellyl derivative shows the potentially ferroelectric SE phase in the cooling process.

Scheme 3-1 b: Recently, a new type of alkyl-substituted aromatic amine, namely the 4-(4'-alkylphenyl)anilines, have been incorporated into imine ligands [46,47]. Their complexes with copper(II), oxovanadium(Iv), palladium(r1) and platinum(l1) metals have been described and, in general, show smectic C and smectic A mesophases at temperatures higher than those of the analogous alkylaniline derivatives (Table 3-1 1). It is noteworthy that this work includes the first report on platinum(1r) complexes of Schiff base ligands.

Table 3-10. Mesomorphic properties of copper(II), oxovanadium(Iv), and palladium(i1) complexes with chiral Schiff base ligands.

H

M X Mesomorphic Properties

K SC' M I

c u -CH=CH-COO- 109.2 120 SA 160.1

vo -CH=CH-COO- 125.0 137 SA 177.0

Pd -CH=CH-COO- 132.2 156 SA 183.4

Pd -coo- 123.6 114) Ch 126.5

Pd -0- 108.8 (. 33.5 SA 73.2) *


Table 3-11. Mesomorphic properties of bis[N-4'-alkylbiphen- yl)-4-decyloxysalicylaldiminato]copper(11), -palladium(II), -oxovanadium(rv), and -platinum(iI) complexes.

n M Mesomorphic Properties

K sc SA I

12 cu 201 229 243.5

12 Pd 197 257 268

10 vo 182 241 262

10 Pt 218 272.5 204.5

Only smectic mesomorphism is observed in this type of complex regardless of the metal and even when aromatic amines without an alkyl tail are present in the ligand (Fig. 3- 12). The corresponding ligands, however, display both nematic and smectic mesophases [48].

Figure 3-12.

Scheme 3-1 c: Considerable research interest has been focused on salicylideneaniline complexes which incorporate a benzoate group in the para-position of the salicylidene moiety. The general formula of these compounds is shown in Figure 3-13 and several research groups have reported novel metal-containing liquid crystals


Figure 3-13.

which have one feature in common: Unlike the p-alkoxysalicylideneaniline complexes discussed earlier, which mainly show smectic mesomorphism, 4-(benzoyloxy)salicylideneaniline complexes [49] exhibit no smectic A, but a nematic phase before clearing to the isotropic liquid [32].

Alkoxyanilines have been incorporated into many of these imine complexes (Ta- ble 3-1 2). For the copper(1r) complexes, the shortest alkoxyaniline derivatives (nz = 1

Table 3-12. Mesomorphic properties of bis{N-(4al koxyphenyl)-[4-(4alk- oxybenzoyloxy)]salicylaldiminato~copper(~~) and -palladium(rr) complexes.

ko I_\ H

c

1 1 181.0 185.0 268.0

10 10 163.6 247.9 248.5

199.3 9 207.4 280.9

161.1 227.0 239.7 247.1

230.4

10 2

10 10

10 2 Ni I 219.5

10 2 FeCl 201.7 252.7

10 10 180.0 190.2 210.1


- O C H ~ ~ 0 C l o H 2 ,

-0CioHzi

- O O C ~ O C , , H , ,

or 2) only display a nematic phase unless the alkoxy tail in the benzoyloxy group is long (n 2 11). In these cases a smectic C phase exists below the nematic phase [50]. NickelfII) complexes, which are not mesomorphic when the ligand is derived from 4-alkoxysalicylideneaniline, have been found to form a nematic phase when an alkoxyaniline with a chain length of up to four carbon atoms is incorporated into the ligand [49]. Mesomorphic bis[N-(4-alkoxyphenyl)-[4-(benzoyloxy)salicylaldiminato]oxovanadium(~~) [ 5 I ] and ironchloride(rr1) [52] complexes have also been described. The geometry of the oxovanadium compounds is square-pyramidal and for the iron(l11) derivatives, an intermediate between square-pyramidal (with the chlorine atom standing apical) and trigonal bipyramidal (with the two nitrogens apical) [53]. The similarity between these two structures gives rise to a comparable molecular arrangement in the solid and leads to similar melting temperatures (see Table 3-12). These compounds show nematic and smectic C phases, as do the copper(rr) complexes. However, there is also an additional ordered birefringent phase (Scryst) in the oxovanadium derivatives. In general, the mesomorphic range decreases in the order Cu > VO > Fe.

If the extended conjugation caused by the ester linkage is interrupted by replacing this moiety by a benzyloxy group [54], smectic phases, similar to those exhibited by bis[(N-alkoxyphenyl)salicylaldimine]copper(~~) complexes (S, and SA phases), are observed. Consequently, the 4-alkoxybenzyloxy group promotes smectogenic behavior in a similar way to a very long terminal alkoxy tail (Table 3-13).

Compared with the alkoxyaniline derivatives (Table 3-14), the incorporation of 4-alkylanilines into 4-(benzoyloxy)salicylideneaniline complexes has a significant effect on mesomorphic properties [55]. For copper(I1) complexes no smectic C phase is observed when an alkylaniline, such as n-butylaniline instead of an alkoxyaniline

K sc M I

160.3 180 SA 225.8

127.8 SA 161.2 - 166.8 233.9 N 255.7

Table 3-13. Mesomorphic properties of copper(I1) complexes with different 4-substituted salicylaldehyde-derived ligands.


cu

Ni

Pd

Table 3-14. Mesomorphic properties of bis(N-(4-butylphenyl)- [4-(4-hexadecyloxybenzoyloxy)] salicylaldiminato)copper(~~), -nickel(II), and -palladium(II) complexes.

K S N I

158 233

166 . 129 167 222

H33C160

is present in the ligand. In nickel(I1) complexes, however, mesomorphic behavior disappears completely when the alkoxyaniline group is replaced by an alkylaniline. Palladium(r1) has also been investigated as the metal center with these ligands, and in this case, an unidentified smectic phase is formed below the nematic phase.

Polar substituents in the mesogenic core of the complexes derived from 4-(benzoyloxy)salicylaldimines have been studied in terms of their effect on mesogenic properties. Fluorosubstituents in the aniline ring lead to the retention of nematic mesomorphism usual for compounds with this structure. However, if a cyano group with its stronger dipole moment is present, only a smectic A phase is observed (Fig. 3-14) [56].

H21ClO0

X = Z = F Y = H K1 162K2171 N2051

X = Z = H; Y = F

X = Z = H; Y = CN

X = Y = H; 2 =CN

K1 81 K2 166 N+K3 172 N 196 I

K 165 SA 205 I

K1 176 K, 186 K, 194 5 ~ 2 7 6 I(deC) Figure 3-14.


Table 3-15. Mesomorphic properties of laterally halogen substituted bis(N- (4-butylphenyl)-[4-(4-hexadecyloxybenzoyloxy) ]salicylaldiminatojcopper(~~) complexes.

H33C160

Substitution

3 5

H H

CI H

H CI

Br H

H Br

CI CI

CI Br

Br CI

Br Br

. OC 1 gH 33

-- Mesomorphic Properties

K N I

158 233

148 196

158 183

174 179

125 165

144 (- 137)

142 (. 126)

146 (. 107)

151 (* 107)

Lateral substitution by halogens in the salicylaldehyde ring has also been investigated (Table 3-1 5 ) . Monosubstitution in positions 3 or 5 causes a broadening of the molecule which has a less dramatic effect on the mesomorphism of the complex than it does on that of the free ligands [57]. Destabilization of the mesophases is more drastic in the case of 3,5-disubstitution. However, a monotropic nematic phase always exists even in the extreme case of 3,5-dibromo substitution complex [58].

Alkoxy tails have recently been attached as lateral substituents to the benzene ring bearing the ester function in this type of Schiff base ligand [59]. In general, the mesomorphism is retained, indicating that the lateral tail is oriented along the molecular axis. Substitution in the 3-position causes favoring smectic mesophases, whereas substitution in the 2-position promotes nematic behavior.

Scheme 3-Id: Incorporation of a benzoate ring into the mesogenic core allows the use of alkylamines without the loss of mesomorphic properties. Moreover, enantiotropic nematic behavior has been reported for bis [N-alkyl[4-(4-decyloxybenzoy1- oxy)salicylaldiminato]nickel(~~) complexes with alkylamines with an alkyl chain length of ten carbon atoms (Table 3-16). A monotropic smectic C phase is also dis-


Table 3-16. Mesomorphic properties of bis(N-alkyl-[4-(4-alkoxybenzoyloxy)]salicylaldiminato)nickel(~~) and -copper(rr) complexes.

n m M Mesomorphic Properties

K SC N I

10 1 Ni 241.5 206.3 213.9)

10 2 183.1 k 175.0)

10 6 129.9 168.0

10 10 131.9 154.6

6 1 c u

10 1

14 1

14 10

10

10

2

6

10 10 1

9 211.7 266.1

173.9 178.2 * 224.0

167.8 204.2

107.4 (. 97.0) 124.0

164.2 170.7

100.6 143.3

116.3 9 134.0

played by the methylamine homolog [60]. Copper(r1) complexes with same structure show similar phase behavior but have lower melting temperatures (Table 3-1 6). A systematic study of the mesogenic properties of the copper(r1) complexes has been carried out by of varying both the length of the alkoxy tail in the benzoate group (n in Table 3-16) and the alkylamine chain length (m in Table 3-16) [61, 621. The occurence of the nematic phase is generally predominant within the whole range of compounds, except for those which contain long alkoxy substituents (n L 10 when m = 1, or m 2 10 when n = 14) the latter favoring the smectic C phase.

The dependence of the stability of the nematic phase on the metal is illustrated by the representative data collected in Table 3-17. Only a nematic phase is displayed by the propylamine derivatives of copper(rr) [60- 631, nickel(r1) [60, 641, and oxovanadium(1v) [64] complexes regardless the length of the tail alykl chain attached to the benzoyloxy ring. Nematic thermal stability follows the order NiLz > CuLz > VOL,.

3 Low Molecular Weight Culumitic Metallomesogens 67

Table 3-17. Mesomorphic properties of bis(N-propyl-[4-(4-decyloxybenzoyloxy)]salicylaldiminato)nickel(~~), -copper(ri), and -oxovanadium(1v) complexes.

0

134 171

vo * 128 145

This order appears to correspond to that of the planarity of the structure. This sequence is reverse to that found when alkylanilines are present in the ligand [36].

X-ray studies on these complexes have demonstrated a relationship between the crystal packing and the general predominance of nematic phases over smectic phases [65]. The crystal packing of complexes derived from 4-(benzoyloxy)salicylaldimine does not consist of layers of parallel molecules (see section a), but appears to be an introductory state of the actual nematic liquid crystal phase which is subsequently formed. Exploitation of this strong tendency to exhibit the nematic phase means that a cholesteric phase can be quite easily obtained if chiral alkylamines, such as 2-methylbutylamine, are used in the ligand [66]. Indeed, copper(I1) and nickel@) complexes of such chiral Schiff bases show blue phases in addition to a cholesteric phase. If the stereogenic center is in the 0-carbon of the amino group [67], e.g. 2-octylamine, destabilization of the cholesteric phase occurs.

Within this group, only one complex displaying a smectic A phase has been described, and this compound has iron(m) as the metal center [68] (Fig. 3-15a). The geometrical environment of the iron(II1) center is square-pyramidal, similar to that in oxovanadium(1v) Schiff base complexes. Interestingly, this complex easily undergoes chemical transformation to form a dinuclear iron(rI1) complex which also shows mesomorphic behavior [69] (Fig. 3-1 5 b).

Scheme 3-1 e: The molecular shape of 4-(benzoyloxy)salicylaldimine complexes, and hence their mesogenic properties, can be drastically modified by altering the position of susbstitution in the aldehyde ring [70,71]. If complexes are derived from the more commonly used 2,4-dihydroxybenzaldehyde, the molecule can be envisaged as two

68 A t . Serrano and I: Sierra

\ / 0 K 1 0 5 S ~ 1 1 5 N 1 5 9 I

p-0 t H 25 c l 2 -)bo)-(=J-c,oH2,1*

0

Figure 3-15. Synthesis of a dinuclear p-0x0 bridged iron(rr1) complex from an iminoiron(I11) complex [FeL,Cl] (L = N-dodecyl-4-(4-decylbenzoyloxy)-salicylaldim~ne).

b

1 Figure 3-16.

rod-like structures joined by a central group (Fig. 3-16). In contrast, if complexes are derived from 2,5-dihydroxybenzaldehyde, the molecule resembles a rod-like structure bearing two lateral substituents which correspond to the amine parts (Fig. 3-1 7). Whereas smectic C and nematic phases were identified for the copper(I1) complexes


Figure 3-17.

represented in Fig. 3-16, a different type of mesomorphic behavior is displayed by those represented in Fig. 3-1 7. Compounds derived from methylamine show smectic C and nematic phases. Intermediate lengths of N-alkyl groups (with the exception of butyl- and pentylamines) promote nematic behavior. Mesophases are not observed for compounds derived from butyl- and pentylamines, alkylamines longer than the nonyl- amine or p-substituted anilines (for representative examples see Table 3-1 8, p. 70).

Scheme 3-l$ Within the group of imine complexes discussed in this section, two particular classes of compounds deserve particular attention. The first one represents the first heteronuclear metallomesogen [72]. The molecule consists of a 4-(ben- zoy1oxy)salicylideneaniline promesogenic ligand coordinated to copper(I1). Its unusual characteristic is the presence of a ferrocene terminus substituting thepara-position of the aniline ring (Fig. 3-18). The ferrocene group occupies a cleft in the molec-

Figure 3-18. n=12 K214N2231

70 J L. Serrano and 7: Sierra

Table 3-18. Mesomorphic properties of Schiff base COpper(1J) complexes derived from 2,4-dihydroxybenzaldehyde and 2,5-dihydroxybenzaldehyde.

R substitution

-CH3 4

4

-CH3 5

-C10H21 5

~ O C 1 o H * 1 5


K sc N I

173.9 178.2 224.0

115.7 134.0

120.6 S 163.6 247.9 248.5

107.4 237.5 265.7

103.9 . 59.2 K2 119.0 k 119.2

ular structure and does hence not hinder the arrangement of the molecules in the mesophase. The complexes with n = 10 and 12 show nematic phases. Melting temperatures are much higher, and nematic phase are significantly narrowed compared with those observed for the corresponding alkyl or alkoxyaniline derivatives.

The second remarkable structure is the only example of a monovalent metal coordinated to a salicylaldimine ligand [73]. The resulting dinuclear complex consists of two dicarbonylmetal(1) (rhodium or iridium) moieties coordinated to a trans- 1,4-diaminocyclohexyl Schiff base ligand (Fig. 3-1 9). Both complexes show smectic

M = R h K 1 4 1 S ~ 1 4 5 1

M = Ir K, 120 K2 142 SA 169 I Figure 3-19.

3 Low Molecular Weight Calarnitic Metallomesogens 71

A phases, the iridium(1) derivative having a significantly wider liquid crystalline temperature range.

3.1.2.3 Enaminoketone Derivatives

Copper and Palladium Enaminoketones are capable of N,O-coordinating to transiton metals in a similar way to salicylideneamine ligands. Square-planar complexes of copper(I1) and palladium(I1) with appropriate ligands bearing this coordinating group are obtained which show liquid crystalline properties [74,75]. With this in mind, several promesogenic compounds with a rod-like shape have been synthesized. The compound with the simplest structure (Fig. 3-20a) is not liquid crystalline. However, when a 4-substituted benzene ring is linked to the nitrogen atom (Fig. 3-20b)

X-Q-Y

0- ‘M.

Y 4 3 X -

Cu(ll)

Figure 3-20. Summary of enaminoketone-metal@) complexes.


Table 3-19. Mesomorphic properties of enaminoketonatocopper(l1) and -palladium(II) complexes.

I I

mesomorphism is observed, including nematic, smectic A and smectic C phases at high temperatures (Table 3-19). When a 4-substituted benzene ring is present in the amine while the carbon atom only bears an alkyl chain, a smectic A phase is still observed (Fig. 3-2Oc).

Elongation of the rigid molecular core results in broadening of the mesophases. The incorporation of a 1,4-disubstituted trans-cyclohexane ring, rather than a benzene ring, was chosen to decrease the melting temperatures. Both cyclohexyl com-

Table 3-20. Mesomorphic properties of enaminoketonatocopper@) complexes.

* 235 *

* 160 *

(. 114) * 1375

118 1 * 1036 * 1112 - 1253 - 1258 -


pounds (Fig. 3-20 d and e), are liquid crystalline. However, enaminoketonecopper(I1) complexes derived from alkylamines (Fig. 3-20d) show an enantiotropic nematic phase which is destabilized as the alkyl chain length increases (Tabie 3-20) [76,77]. In contrast, nematic and smectic A phases are stabilized, under the same conditions, in complexes whose structure is derived from 4-(4-alkylcyclohexyl)aniline (Fig. 3-20e) [75]. The reason for this may lie in the fact that N-alkyl chains in the alkylamine derivatives would act as lateral substituents thus not being oriented along the molecular long axis, which is most probably due to the nonplanar environment of the nitrogen atom.

In general, a comparison in phase behavior between the complexes and the free enaminoketone ligands shows that the smectic polymorphism present in the ligands is replaced by more disordered mesophases in the complexes. The copper(I1) complexes shown in Fig. 3-20b show smectic C, smectic A, and nematic phases whereas the free ligand shows more ordered smectic phases (SG and S,) [78]. In a similar way, smectic H, smectic C, smectic A (bilayer), and nematic phases observed for the free ligand shown in Fig. 3-20d are replaced by smectic C, smectic A and nematic phases upon complexation [77].

3.1.2.4 Aroylhydrazine Derivatives

Aroylhydrazinatonickel(r1) and -copper(II) complexes have recently been reported as a new class of metallomesogen (Fig. 3-21) [79].

Nickel Bis(alkoxyaroylhydrazinato)nickel(rI) complexes show a stable nematic mesophase when relatively short alkyl chains are present (n = 4-6), nematic and smectic C phases for intermediate chain lengths (n = 8 - 10) and only a smectic C phase for the longest chains studied (n 2 12) (Table 3-21). Results of X-ray studies of these alkoxy complexes suggest a molecular separation in the smectic layer by twice the molecular width. Additionally, in bis(alkylaroylhydrazinato)nickel(II) complexes the absence of the outboard dipole associated with the ether linkage of the alkoxy tails gives rise to the appearance of a smectic A phase only [80]. An explanation for both phenomena may lie in the association between the alkoxy oxygen atom and the N- methylidene moiety in the central core.

14

Table 3-21. Mesomorphic properties of aroylhydrazinatonickel(I1) complexes.

J. L. Serrano and I: Sierra


. Copper

(Alkoxyaroylhydrazinato)copper(II) complexes have also been prepared, and these compounds are far less thermally stable than the nickel(rr) analog. The copper(r1) complexes show an unidentified mesophase and decompose a few degrees above this phase transition.

3.1.2.5 P-Diketone Derivatives

Copper

Square-planar 8-diketonatocopper(r1) complexes exhibit either calamitic or discotic mesomorphism depending on subtle differences in the molecular structure. The first examples of copper(I1) 8-diketonates described as liquid crystalline (Fig. 3-22) [81] show very ordered discotic phases with a lamellar organization [82].

K 85.5 DL 128.5 I Figure 3-22.


n = 1 K 194 (N 178.5) I

Figure 3-23. n = 2 K 173 (N 158.7) I

Elongation of the molecule in one direction is achieved by introducing biphenyl units [83, 841 (Fig. 3-23), generating a molecule with a greater L / D (length to width) ratio. In the elongated species a monotropic nematic phase is observed. The molecule can such be regarded as a "rod-like" structure with two lateral substituents. Moreover, Chandrasekhar pointed out the biaxial character of this nematic phase obtaining evidence from conoscopic studies and X-ray data of the three mean directions of the molecule [85].

As a continuation of the discussion of structural differences giving rise to either calamitic or discotic mesophases, the behavior of copper(r1) complexes with 1 -(4-alkoxybiphenyl)-3-alkylpropane-l,3-dione ligands provides an even more striking example of subtle changes leading to phases of a different nature. The complexes and their phase transition temperatures are given in Table 3-22. The presence of a methyl group as the lateral substituent in the complex (rn = 1, Table 3-22) [86] leads to the formation of a discotic rectangular ordered phase (Dro). However, only a nematic phase is observed as the alkyl group becomes longer and bulkier. Ethyl and propyl groups both give rise to the formation of an enantiotropic nematic phase, whereas a monotropic nematic phase is detected upon rapid cooling of the butyl derivative [87].

Ohta accounts for this phenomenon in terms of the steric packing requirements of the lateral alkyl substituents (Fig. 3-24) [88]. The relatively small methyl group would allow the formation of dimers with an overall discotic shape suitable to form a D,, phase. However, the presence of an ethyl group should already produce a greater steric hindrance which precludes the formation of dimers, thus leading to the appearance of a calamitic nematic phase. The same reasoning can also be applied to the longer alkyl substituents which obstruct intermolecular interactions to a great extent, hence only a monotropic nematic phase is observed for the butyl derivative. Conoscopic observation of these compounds [88] confirmed that the nematic phase is uniaxial, in contrast to the biaxial character proposed for the complexes prepared by Chandrasekhar (Fig. 3-23) [85].

76

Table 3-22. Mesomorphic properties of [ 1 -(Cdodecylbiphen- yl)-3-alkylpropane-1,3-dionato]copper(11) complexes.

J. L. Serrano and I: Sierra

n Mesomorphic Properties

K mesophase I

1 135.1 Dro 208.8 *(dec)

2 171.9 N 183.6

3 159.9 N 160.0

4 153.3 (N)#

# Monotropic nematic phase only detected on rapid cooling.

Concerning the relationship between molecular structure and calamitic-thermotropic behavior the following considerations should be taken into account.

(a) 4-alkylcyclohexylphenylalkyl analogs of the complexes reported by Chandrase- khar have been described by Haase et al. [89] (Table3-23, see p.78). These compounds show monotropic nematic behavior. X-ray measurements have also been carried out.

(b) Only monotropic nematic behavior has been found for the p-diketonatocopper@) complexes reviewed so far. However, stabilization of the nematic phase has been achieved by broadening the molecule to result in complexes with the structure shown in Fig. 3-25 [90]. These complexes exhibit enantiotropic mesophases exclusively.

(c) Smectic mesomorphism, which is commonly found for the uncomplexed p- diketone ligands [88, 901, is not generally observed in the complexes unless polar substituents are introduced into the molecule. The promotion of smectic character is more pronounced in complexes containing ligands with four rings, i.e., A = Ph (Table 3-24, see p. 79) [91].

Lateral molecular interaction appears to be favored due to the polarity of these substituents which generate a lateral dipole pointing away from the molecular core. To confirm this hypothesis, fluoro substituted ligands with the fluoro substituent in different positions have been investigated (Table 3-25, see p. 80) [92]. On moving the fluoro substituents from the 4'- to the 3'- or the 2-position, a dipole still exists in the direction away from the molecular center. Smectic mesomorphism is observed, but


m = l Dro

m = 2 , 3 enantiotropic N

m = 4 monotropic N

Figure 3-24. Model proposed for the chain length effect of n-alkyl groups on the mesomorphism in bis[ 1 -(Cdodecyloxybiphenyl)-3-n-alkyl- 1,3-dionato]copper(11) complexes. Adapted from reference 88.

as the dipole gets closer to the molecular center, its stability diminishes. When the fluoro substituent is situated in the 2‘-position, smectic mesophases are not observed. In this case the lateral dipole points towards the center of the molecule.

A number of different P-diketone and P-dialdehyde complexes were investigated in order to evaluate whether the bis@-diketonato)copper(II) group can act like a biphenyl group (see figure in Table 3-26) [93]. The study shows that only the compound which contains a benzene ring in the ligand is liquid crystalline. This result implies that the chelate ring system resembles the character of a single benzene ring

18 .I L. Serrano and 7: Sierra

Table 3-23. Mesomorphic properties of bis- 1 -[4-(4-pentylcyclohexyl)phenyl]-3-alkylpro-

pane-I ,3-dionato)copper(11) complexes.

H11C5

Mesomor hic Properties 11 1 8 1 135 (. 129)

R R'

0.. / o F".. 0 0

R { R ringA: , 0 Figure 3-25.

rather than that of a biphenyl group. Indeed, a slight similarity exists between the mesophase temperature range of the copper(1r) compound and that of the terphenyl analog, although the nature of the mesophases are different (Table 3-26, see p. 80).

Compounds of the type described above in which R = methyl are not liquid crystalline. However, increasing the length of the molecule by introducing promesogenic units leads to the formation of a nematic phase even when R = methyl (Fig. 3-26) [94].


Table 3-24. Mesomorphic properties of b-diketonatocopper(I1) complexes with polar terminal groups.

X A

F -

Br -

X 3 -

CN -

F Ph

Br Ph

;F3 Ph

CN Ph


K sc SA N I

193 p 175)

212 p 182 183)

188 p 131 132)

230 203)

155 (. 122) 250 . 150 (. 141) 260 . 228 (. 203) 265

259 273

Figure 3-26. K1 186.4 K2 221.8 N 225.9 I(dec)

80

Table 3-25. Mesomorphic properties of P-diketonatocopper(I1) complexes with different positions of fluoro substitution.

J L. Serrano and i? Sierra

F-substitution Mesomorphic Properties

K sc SA N I

4' 155 (. 122) 250 . 3' 176 209 . 2' 180 165)

2 170 150) 179 188

Table 3-26. Mesomorphic properties of 2-substituted propane- 1,3-dione derived copper(I1) complexes.


8

9

10

3.1.2.6 Salicylaldehyde Derivatives

109.4 196.1 254.2 263.9

131.8 194.5 247.0 256.8

70.8 192.3 242.1 250.7

Copper 5-Substituted salicylaldehydes have a molecular structure which is suitable for the design of calamitic liquid crystalline metal complexes. A smectic C phase has been identified in complexes derived from 2,5-dihydroxybenzaldehyde (Table 3-27) [95]. These compounds have a very large L/D ratio that favors molecular interaction and, as a consequence, very high melting and clearing temperatures are observed. This leads to thermal decomposition of these complexes just above their clearing temperature.

In contrast, a number of 4-substituted salicylaldehyde complexes have been reported which do not show liquid crystalline behavior at all [70].

Table 3-27. Mesomorphic properties of bis[5-(4-alkoxybenzoyloxy)salicylaldehydato]copper(~~) complexes.

n Mesomorphic Properties t Ki K2 K3 sc

3.1.2.7 Alkylcarboxylate Derivatives

Alkali and AIkaline-Earth Metals Lamellar phases have been found to be formed by anhydrous alkali and alkaline- earth soaps [96]. As an example, the sodium soaps [96b] show a lamellar crystal organization at temperatures below 100 “C which is converted into a ribbon-like organization upon heating. These ribbons contain the polar groups in a quasi-crystalline form dispersed in a liquid matrix comprised of the molten hydrocarbon chains. Fur- ther heating gives rise to fusion of the polar groups and thus a labile lamellar organization is formed in which the fused state of the chains is maintained with an essentially crystalline character of the polar groups. In addition, discotic behavior has been found for some alkali carboxylates, namely sodium laurate, and some potassium and rubidium salts. Lithium, potassium, rubidium and cesium salts behave

82 J, L. Serrano and i? Sierra

similar to the sodium soaps. For alkaline-earth metal soaps, lamellar crystalline phases have also been found which undergo phase transition into a discotic organization at higher temperatures.

Lead

L,ead(II) forms soaps with long chain carboxylic acids which show smectic behavior in addition to a highly ordered lamellar phase at lower temperature [97]. This observation contradicts that of Sime and Adeosun [98] who identified a smectic G phase and a cubic isomorphous phase for these lead(I1) soaps. Both mesophases, smectic and lamellar, are exhibited by the shorter members of the series (from hexanoate to dodecanoate), but with longer alkanoate tails only the more ordered phase is observed. A degree of controversy exists regarding the structural assignment of the smectic phase found. From X-ray studies [99] of lead@) bis(decanoate), Ellis deduced a reduction in the layer spacing from about 30.9 A for the unheated salt (lamellar crystalline) to about 29.8 A in the first crystalline phase and to 22.9 A in the smectic phase. This reduction, although large for a smectic A phase, is not impossible. Fur- thermore, the smectic phase was found to be weakly biaxial by the authors who confirmed it to be a smectic C phase [K, 86 Kz 98.4 Sc 1 1 1.2 I]. In contrast, Bazuin et al. [ 1001 observed a homeotropic texture for the smectic phase which can only correspond to the orthogonal smectic A phase in which the alkyl chains are completely disordered and in which the remaining ionic interaction still allows the formation of a two-dimensional layered structure. The phase sequence would then be: K85 Mcryst 96 S, 113 I.

(CH3(CH,),-C=C-CsC-(CH,),COO- Figure 3-27.

More recent studies on lead(I1) soaps include modification of the alkyl chains by incorporating acetylene units (Fig. 3-27) [ lol l . A glassy state along with unidentified highly ordered smectic phases with clearing temperatures comparable to those of lead@) alkanoates were found in these lead@) alkadiyonates, which are precursors of poly(diacety1enes).

Thallium

Thalliurn(1) alkanoates have also been found to be liquid crystalline. Thallium(1) decanoate exhibits five solid (crystalline) phases and a smectic A phase before the isotropic liquid state is reached [K, -41 K2 15.6 K3 33.8 K4 54.4 K5 132 SA 211 I] [ 1021. Thallium(1) salts derived from branched alkylcarboxylic acids also show stable lamellar phases [ 1031.


3.1.2.8 Monothio-P-diketone Derivatives

Nickel Monothio-P-diketones form very stable square-planar nickel@) chelates, which makes them suitable ligands for the formation of metallomesogens [104]. A cis-configuration of the ligands is proposed for bis[2-(4-substituted-phenyl)-3-mercaptopropenato]nickel(~) complexes (see Table 3-28), which have a linear molecular structure with a calamitic shape. All of the complexes synthesized (R = CnHZn+, (n =

2,3,6), O-CnH2,+ (n = 3 -6)) are mesogenic with wide nematic temperature ranges (Table 3-28).

Table 3-28. Mesomorphic properties of monothio-/3-diketonatocopper(Ir) complexes.

122 224 *(dec)

114 173

155 232

134 227

113 231 9

129 0 711 . 3.1.2.9 Dithiolene Derivatives

Nickel and Platinum In 1977, when only two rod-like metallomesogens had been described [105, 1061, Giroud and Mueller-Westerhoff envisaged a planar terphenyl-like structure in 1,2-dithiolene nickel complexes which would make them suitable for obtaining mesogenic properties. This attempt led to the first mesogenic dithiolatonickel(I1) complexes which show a smectic C phase [107]. Later, platinum(I1) complexes of the same ligands were reported to be liquid crystalline [108], showing nematic and smectic C mesophases. Subsequent systematic studies of dithiolene complexes were con-


4 Ni

7

8

10

Table 3-29. Mesomorphic properties of bis( f ,2-dithiolato)- nickel(rr), platinum(lr), and palladium(l1) complexes.

if

K S N I

117 175 '(dec) - 109 184 *(d@

106.5 189 * ( d 4

103 189 *(W

4 Pd

10

cerned with three transition metal centers: nickel(II), platinum(II), and palladium(r1) [109, 1101 (see representative examples in Table 3-29). The first example of a nematogenic nickel(I1) metallomesogen ever reported is the complex substituted with terminal butyl chains. For compounds with longer tails, smectic mesomorphism is observed. All the nickel@) and platinum(I1) complexes display enantiotropic mesophases which are nematic for the shortest tails and smectic for the longest alkyl substituents. However, palladium(r1) complexes are not liquid crystalline. It is known that palladium(l1) and platinum(r1) dithiolene complexes tend to dimerize [ I l l ] . In the palladium(1r) complexes the tendency to form dimers with high dissociation energy dominates. Even upon heating into the isotropic the dimer does not dissociate into the monomer, thus preventing the formation of a liquid crystal. In nickel(r1) and platinum(1r) complexes the dispersion forces are more dominant, and dissociation into the monomer is possible at the temperature of the mesophase.

205

207 *(dec)

3.1.2.10 Dithiobenzoate Derivatives

Nickel

The use of the thiocarboxylate as a coordinating group for metal-containing liquid crystals was first studied by Ohta who employed alkoxythiocarboxylate (xanthate)


Figure 3-28. K 81 (S 79.4) I

ligands [112]. Coordinating these to nickel(I1) results in materials which exhibit a monotropic smectic phase (Fig. 3-28).

When a 4-substituted thiobenzoate is used as a ligand [113], enantiotropic SH and Sc mesophases are observed when n = 8 (Fig. 3-29a). However, this complex exhibits a unique phase behavior when heated to temperatures between 230 and 285 "C. At these temperatures the smectic rod-like molecule (blue) rearranges into a bent shape via an intermolecular reaction (Fig. 3-29 b). The resulting nematic appears red. This process can be reversed by sulfur abstraction using triphenyl phosphine to yield the dithiobenzoate complex.

K 155 SH 189 Sc 230 I(dec)

Figure 3-29. K1 89 K2 128 N 200 I

Palladium and Zinc Besides to nickel@), 4-alkoxydithiobenzoate ligands have also been coordinated to palladium(I1) and zinc@), yielding isostructural complexes with similar liquid crystalline properties (Table 3-30, see p. 86).

A nematic phase is observed in the complexes with the shortest tails, and a smectic C phase is formed when terminal tails are lengthened. A more ordered smectic phase is present below the smectic C phase in the nickel@) and palladium(I1) complexes substituted with decyloxy terminal chains. This phase has been identified as smectic H for the nickel@) compound. In the homologous series studied, both melting and clearing temperatures follow the sequence Pd > Ni > Zn. Single crystal studies [ 1 141 have revealed an approximate square-planar coordination geometry for the palladi-

86

Table 3-30. Mesomorphic properties of bis(4-alkoxydithio- benzoato)nickel(ii), -palladium(ii), and -zinc(ri) complexes.

.I L. Serrano and T Sierra

n M Mesomorphic Properties K S sc N I

5 Ni 229 238 9

6 211 235

10 145 174 230

5 Pd 246 297 320

6 232 307 318

10 114 201 312 . 5 Zn 172 201

6 155 194

10 136 164 182

Figure 3-30. Molecular structure of [Pd(%odtb),]. Adapted from reference 114.

um(I1) center in bis(4-n-octyloxydithiobenzoato)palladium(11) (Pd(&odtb),) which is slightly distorted towards a square-pyramidal structure (Fig. 3-30).

In contrast, the analogous zinc@) complex, [Zn2(8-odtb)4], forms centrosymmetric dimers with the zinc(i1) center in a pentacoordinated environment with an approximately trigonal pyramidal geometry. In solution in organic solvents, however, these dithiobenzoate zinc(r1) complexes exist as monomers. EXAFS studies [ 1 151 have demonstrated that zinc(i1) complexes still retain their dimeric structure in the mesophase. This observation makes them interesting from a structural point of view, since they are different from most of the known mesogenic complexes in exhibiting a relatively open central core (Fig. 3-31). In contrast to the zinc(I1) complexes,


Figure 3-31. Molecular structure of [Zn2(8-odtb),l. Adapted from reference 114.

EXAFS experiments showed that the palladium(I1) complexes form a dimeric structure in the crystal which disappears completely in the smectic C phase.

X-ray studies of the smectic C phases have shown large tilt angles for both palladium@) and nickel@) complexes. Such high tilt angles are unlikely to be observed for a normal Sc phase and hence some degree of interdigitation can be assumed.

Modification of the mesogenic core of these thiobenzoate complexes has been undertaken by laterally fluorinating the phenyl rings [ 1 161. Bis(4-alkoxy-3-fluorodi- thiobenzoate)nickel(rr) and -palladium(II) complexes show smectic C phases even for the shortest alkoxy chain lengths. The smectic crystal phase formed by non- fluorinated complexes is not detected in the fluoro derivatives. Zinc(I1) complexes are liquid crystalline but the mesophase (M) is unidentified (Table 3-31).

If n-alkoxy substituents are replaced by branched chains (Fig. 3-32) [I 171, melting and clearing temperatures are decreased. Likewise, the phase behavior is strongly affected. Only if the branching site is in the 2-position of the alkoxy tail mesomorphism is observed. In this case a smectic C phase is formed before decomposition occurs.

Table 3-31. Mesomorphic properties of bis(4-butyloxy-3- fluorodithiobenzoato)nickel(II), -palladium(II), and -zinc(II) complexes.


-C6H13 Pd

-CilH17 Pd

K 140 Sc 162 (dec)

K sc N I

232.1 252

200.9 237.3

Gold

Figure 3-32.

Dithiobenzoatogc.-[Irr) complexes [4, 1 I] (Fig. 3-33) have been reported to show a smectic A phase at temperatures above 100°C, but they decompose upon clearing into the isotropic or even at lower temperature. When X = methyl the melting points are lowered, but the thermal stability of the mesophases is also decreased.

HZ"+IC"O 5 b A < 1 s X = CI. Br, CH, Figure 3-33.

3.1.2.11 Dithiocarbamate Derivatives

Dithiocarbarnate complexes derived from piperazine with nickel(rr), palladium(rr), or copper(r1) as metal centers have recently been described [I 18, 1191, and found to exhibit smectic and nematic mesophases (Table 3-32). When alkoxyphenyl groups are

Table 3-32. Mesomorphic properties of bis(dithiocarbamato)palladium(II), -nickel(@, and -copper(u) complexes.


attached to the nitrogen in order to achieve a more rigid structure, a smectic C phase appears, which is significantly less stable in the copper(I1) complexes. In the palladium@) complexes, flexible N-alkyl chains suffice to give rise to nematic mesomorphism. Zinc@) complexes have been prepared, but they have not been found to be liquid crystalline.

3.1.2.12 Summary

If special emphasis had to be placed on a very important class of compounds discussed in this section, salicylideneamine and P-diketone derivatives would be the main candidates.

A large number of papers concerning the structure property relationship of salicylaldimine derivatives have been published. In spite of the fact that most of these compounds have a broad brick-like shape with four peripheral chains, only calamitic mesophases have been identified. Smectic mesomorphism undoubtedly predom- inates. However, if the molecular polarizability is increased along the molecular long axis, nematic behavior is favored. More ordered mesophases are generated as the terminal chains are lengthened, a phenomenon commonly found in organic liquid crystals. An important structural aspect of these mesophases is that interdigitation occurs in most of the lamellar phases.

The environment around the metal center can be either tetrahedral, square-pyramidal or square-planar in these complexes. However, it has been widely demonstrated that square-planar or square-pyramidal coordinations lead to the appearance of liquid crystalline properties, whereas the tetrahedral geometry usually prevents mesophase formation.

Within this class of complexes many paramagnetic metals (copper(II), vanadium(Iv), iron(II1)) have been introduced as centers, and so significant studies regarding their magnetic properties have been carried out which will be discussed in Chap. 10. A number of ferroelectric derivatives have also been described among this class of materials.

P-Diketonatocopper(I1) complexes also deserve special attention with respect to the continuous search for a biaxial nematic mesophase. An important and ongoing discussion of this phenomenon still remains unsettled, and conclusive data have yet to be obtained.

Another interesting structural aspect of these complexes lies in the possibility of obtaining calamitic and discotic mesophases from one mesogen. A P-diketonate core surrounded by four peripheral chains may show calamitic and discotic behavior depending on minor, but influential, modification of its molecular structure. The requirements for obtaining calamitic or discotic mesophases have been discussed in this chapter and will also be covered in the next chapter which is focused on noncalamitic metallomesogens.


3.1.3 Metal-Organic Liquid Crystals with Tetradentate Ligands

A small number of ligands exist which, when coordinated to a metal via four donor atoms, still enable a rod-like molecular shape to be formed. These compounds lead to the appearance of calamitic mesophases and will be discussed in this section.

The discussion is divided into three subsections, classified by the heteroatoms coordinated to the metal center as follows:

1. N4: annelide (Sect. 3.1.3.1) and porphyrin derivatives (Sect. 3.1.3.2) 2. N202: bis(salicy1idene)ethylenediamine (Sect. 3.1.3.3) and 6,6'-bis(acylamino)-

%,a'-bipyridine derivatives (Sect. 3.1.3.4) 3. S4: 1,10-diaza-4,7,13,16-tetrathiacyclooctadecane derivatives (Sect. 3.1.3.5).

3.1.3.1 Annelide Derivatives

When copper(i1) is coordinated with annelide ligands a variety of molecular assemblies can be obtained including thermotropic ionic liquid crystals (Fig. 3-34). Upon heating the solid phase (assigned as lamellar, L , ) of the annelidekopper chloride complex [120], two lamellar mesophases (L, and L3) have been identified before clearing into the isotropic at 107 "C.

2+

2cr

L, -113 L2 87 L3 107 I Figure 3-34.

3.1.3.2 Porphyrin Derivatives

Porphyrin complexes are typical structures used for the generation of discotic liquid crystals but have also proved to be good candidates for calamitic mesomorphism

K 140 SF 207 SE 224 SB 234 I Figure 3-35.


[121]. Indeed, smectic and nematic mesophases can be observed for zinc(x1) complexes of $1 5-disubstituted porphyrins (Fig. 3-35). These structures have been suggested as possible candidates for the observation of a biaxial nematic phase [122].

3.1.3.3 Salicylidenediamine Derivatives

Nickel and Copper Coordination of bis(salicy1idene)ethylendiamine to metals such as copper(I1) or nickel(1r) leads to complexes with a square-planar geometry that permits all ligands to lie in 1 O planarity. If substituents in the salicylidene rings occupy positions 5 and 5‘, a rod-like molecular shape is produced resulting in liquid crystalline materials. Accordingly, N,N’-bis( 5-alkoxysalicylidene)ethylenediaminocopper(11) and -nickel@) complexes 1123, 1241 show enantiotropic phases which are mostly smectic. In most cases a smectic A phase is found upon cooling the isotropic liquid, and one or two crystal smectic phases are observed before crystallization (Table 3-33). Discrepancies are found between the thermoanalytic data reported in the two references cited above. The temperatures reported by Shaffer [I231 are clearly lower than those given by Paschke 11241. This is certainly due to different synthetic methods applied to obtain the complexes. N,N’-bis(5-alkylsalicyljdene)ethylenediamine derivatives have also been studied,

which has caused some controversy (Table 3-33). While Paschke reported a behavior

Table 3-33. Mesomorphic properties of [bis(salicylidene)ethy- lenediarninato]nickel(~~) and -copper(Ir) complexes.

R M Mesomorphic Properties

K s1 s2 SA I

-0C7H15 # Ni 76 109 192 296

-0C7H15 9 Ni 116 201

-0C7H15 CU . 257 270

-0C7H15 9 CU 102 235 . -C6H13# cu 252 282

-C6H13 Ni 84 * 118 245 310

-CsH13 Ni 126 SE 226 258

Data obtained by (# ) Paschke et al. [124, 1251 ( Q ) Shaffer et al. (1231. (n) Ohta et al. [126].


similar to that of the alkoxy derivatives [I251 (i.e., smectic ordered mesophases and smectic A phases at higher temperatures), Ohta [ 1261 observed clearing temperatures lower by about 50 "C, and found a smectic E phase below the smectic A phase. This smectic E to smectic A transition is accompanied by a reversible dissociation of dimers in nickel(I1) complexes which accounts for the observed increase in the interlayer distances. In general, nickel(r1) complexes are much more stable than their copper(i1) analogs. This trend could be due to the strong axial interactions present in salicylaldiminatocopper(rr) complexes [ 1271 which also explains their stability. Upon transition into the smectic A phase, the possibility of such stabilization is lost.

X-ray studies of a nickel(I1) complex show a square-planar geometry of the metal center. The two terminal alkoxy tails are in an all-trans conformation and are oriented in such a way that the molecules have the highest possible elongation which compensates the banana shape of the central core (Fig. 3-36) [125].

Figure 3-36. Molecular structure of the [bis(5-hexyloxysalicylidene)ethylenediami~ato]nickel@) complex. Adapted from reference 125.

Vanadium

Square-pyramidal oxovanadium(1v) complexes of bis(salicy1idene)diamine ligands (Fig. 3-37) have been reported [ 1281 to show a unidirectional (head-to-tail) smectic (crystalline and smectic A type [ 1291) arrangement exhibiting supramolecular domains of polarization that could lead to ferroelectric properties.

3.1.3.4 2,2'-Bipyridine Derivatives

In analogy to the bis(salicy1idene)ethylenediamine complexes described above, N20,-type coordination can also occur with 6,6'-bis(acylamino)-2,2'-bipyridines. Complexation with transition metals (Pd", Ni", Co" or Cu") yields complexes with a square-planar geometry of the coordination site [ 1301. Palladium(I1) complexes

3 Low Molecular Weight Calamitic Metallonresogens 93

Table 3-34. Mesomorphic properties of 6,6'-bis(acylamino)-2,2'-bipyridine- copper(I1) complexes.

n Mesomorphic Properties

K SC I

12 78 132

14 88 130

16 92 128

18 97 123

20 9 101 122

are for certain not mesomorphic, and some doubt still exists about the phase behavior of nickel(I1) and cobalt(r1) analogs. In contrast, copper(r1) complexes display a smectic C phase when the alkyl chain length is between 12 and 20 carbon atoms (Table 3-34) [131].

3.1.3.5 1,10-Diaza-4,7,13,16-Tetrathiacyclooctadecane Derivatives

Coordination of copper(1) with a nonmesogenic ligand, such as the bis[4-(n-al- kyloxy)benzamide] derivative of 1,l 0-diaza-4,7,13,16-tetrathiacyclooctadecane [ 1321, can induce smectic mesomorphism. Thus a novel example of an ionic complex, [CU(L) PF6] displaying calamitic thermotropic liquid crystalline properties has been obtained (Fig. 3-38).

A tetrahedral environment for the copper(1) ion has been confirmed by X-ray crystallography data in contrast to the square-planar geometry assigned to the cor-

Figure 3-38. X = PFQ, BFi


responding palladium(rl) complex ([P~(L)] [(BF4)J, in which mesomorphic behavior has been observed but not conclusively characterized [ 1331. Tetrahedral coordination in other types of metallomesogens appears to prevent the formation of mesophases. The formation of mesophases in these tetrahedral complexes was first believed to be based on a monomeric structure of the copper(r) complex cation. The number and position of the aliphatic chains in these systems cause a overall rod-like shape in spite of the tetrahedral geometry of the metal center. However, recent X-ray crystallography studies on isoelectronic silver(1) complexes which dislay similar mesogenic behavior to the copper(1) analogs, suggest that the amphiphilic character of the cationic complexes is actually responsible for mesophase formation through coulombic attractive forces [134]. Indeed, a bilayer organization has been proposed which is based on U-shaped molecules in a similar way to what has already been reported for aminosilver(1) complexes [20] (see Fig. 3-9).

3.1.3.6 Summary

Most of the structures included in this section have also provided the basis for designing noncalamitic metallomesogens, for example, salicylidenediamine, porphyrin and azamacrocycle derivatives (see Chap. 4). However, this section again demonstrates that the formation of calamitic and discotic mesophases is dependent on small structural differences in the ligand (see P-diketon derivatives in Sect. 3.1.2.5). It is not only the rigid core which affects the formation of a particular type of liquid crystalline phase, but also the number of peripheral chains.

Both ionic and covalent metallomesogens occur within the group of complexes derived from tetradentate ligands. Annelide and azamacrocycle derivatives are further examples of ionic complexes which form the promesogenic units of a calamitic metal-containing liquid crystal.

A third important issue concerning these complexes arises from the square-pyramidal (ethylenediaminato)oxovanadium(Iv) complexes which assemble into a special unidirectional arrangement within the mesophase. This arrangement is responsible for the observation of ferroelectric properties. Further studies of this phenomenon have been carried out on polysubstituted salicylidenediamines which form discotic liquid crystalline phases. These will be discussed in detail in Chap. 4.

3.2 Organometallic Liquid Crystals

The following section is concerned with liquid crystalline complexes in which the metal atom is coordinated by a covalently bound organic ligand via either a carbon atom or a n-system. Both monodentate (phenyl, alkynyl, isonitrile) and bidentate (azo, imine, azine and azoxy) ligands can be C-coordinated to a metal, therefore, the


discussion is divided into two subsections. Furthermore, bidentate ligands can give rise to two different kinds of complexes: dinuclear and mononuclear orthometallated compounds. As far as x-coordinated complexes are concerned, the largest group consists of liquid crystals derived from metallocenes. Examples of butadiene complexes have been included into a further section.

3.2.1 Organometallic Liquid Crystals with Monodentate Ligands

This group includes the first thermotropic liquid crystals, covalently incorporating a metal into their structure. In 1923, Vorlander described symmetrical linear mercury(I1) derivatives which show smectic behavior (Fig. 3-39a) [105]. A related asymmetric linear mercury(I1) complex exhibiting smectic phases was also reported (Fig. 3-39b) [105].

Figure 3-39. b

In spite of these encouraging results, almost fifty years passed before the second paper dealing with this type of liquid crystals appeared. In 1971, Young et al. [135] reported the mesomorphic properties of some organometallic Schiff base derivatives based on the group IV metals germanium(Iv), tin(1v) and lead(1v) (Fig. 3-40). Liquid crystalline character was observed for the germanium and tin derivatives. Two different smectic phases, S, and S2, were found which are enantiotropic for the tin

M = Sn(lV)

M = Ge(lV)

K 165 S, 171 S1 173 I

K 176 (S, 169 S, 175) I Figure 3-40.


12 8

14 8

12 5

12 6

12 10

12 12

12 14

12 16

complex and monotropic for the germanium complex. The authors proposed two different packing models to explain this behavior.

In addition to the phenyl-type ligands described above, two other types of monodentate ligand have been C-coordinated to transition metals: alkynyl and isonitrile ligands. Most of the complexes described are symmetric and contain palladium(I1) or platinum(I1) as the metal center. There are also a few examples of unsymmetric organometallic complexes derived from gold(1).

K SA N I

174.9 189.6

173.7 (. 171.3) 183.7

170.3 199.8

173.6 196.6

173.9 (. 170.3) 183.0

173.7 177.3 179.7

173.6 178.7 . 170.4 9 176.0 .

3.2.1.1 Alkynyl Derivatives

In comparison with metal-alkyl or metal-alkenyl a-bonds, the metal-alkynyl linkage is significantly more stable to moisture and air. Bis(alkynyl)platinum(n) metallomesogens have been described (Table 3-35) [I 36, 1371 which show nematic behavior in addition to a smectic A phase, the latter appearing in compounds with the longest alkoxy tails ( ( n +m) 2 22). Complexes of this type have not been studied in depth, but it seems that more than two aromatic rings are needed to obtain liquid crystalline materials.

A surprising aspect in these systems is the appearance of liquid crystallinity in compounds with lateral substituents as large as trimethylphosphine (Table 3-35) and triethylphosphine (Table 3-36) [138]. The extended n-conjugation in such a rigid rod- like structure seems to be mainly responsible of the presence of liquid crystal properties. This advantage overcomes the destabilizing effect of bulky lateral substituents. Indeed, an increase in the length of the molecule (four aromatic rings) leads to wider

Table 3-35. Mesomorphic properties of di(arylethyny1)- bis(trimethylphosphine)platinum(lr) complexes.


-0OC- -coo-

Table 3-36. Mesomorphic properties of di(arylethyny1)bi.s- (triethylphosphine)platinum(lr) complexes.

K N I

134 156

PEt3

H,&O ~ X - @ C - f + C i C ~ Y ~ O C 8 H , , - PEt,

-0oc- -COO-

-coo- -COO-

* 124 * 142 *

114 156

mesophases in triethylphosphine derivatives, which became even larger if the molecule is made unsymmetric by use of two different alkynyl ligands [138, 1391.

3.2.1.2 Isonitrile Derivatives

Free isonitrile compounds, especially aromatic isonitriles, are chemically reactive due to the presence of the carbon in the isocyano group, which can be considered a carbene analog. Coordination to a metal is a means of stabilizating the isocyano group.

Palladium and Platinum Square-planar palladium(I1) and platinum(I1) complexes derived from 4-isonitrilephenyl, 4-alkoxy benzoate, 4-alkoxyphenyl, 4-isonitrilebenzoate and 4-isonitrile- 4'-alkoxybiphenyl have been reported to display liquid crystalline behavior (Fig. 3-41) [140, 1411. Smectic C and nematic phases (Table3-37) are obtained. Very similar phase behavior is observed for complexes of both metals. Loss of weight was observed by thermogravimetric analysis up to 280°C for platinum complexes and 220 "C for the palladium analogues, indicating thermal decomposition. Never- theless, the isonitrile complexes are thermally more stable than their corresponding nitrile derivatives.

Figure 3-41.

X = I , Br, CI Y = -, coo, ooc M = Pd(ll). Pt(ll)

98

Table 3-37. Mesomorphic properties of bis(isonitri1e)pal- ladiurn(i1) and -platinurn(lt) complexes.

J.L. Serrano and 7: Sierra

X = l Pt

~ ~

LIGAND M Mesomorphic Properties

K S c N I

125 150 199

H 2 5 C , 2 0 W N C Pd ( * 134 143 205

H 1 7 C , 0 a O O C e N C I Pd 186 218 266

H l 7 C ~ O ~ C 0 O ~ N C Pd 1. 157 241

-H

-0CH3

-0C3H7

'OC6H13

X = l PI I* 163 230

K sc SA N I

172 9 227 270 * ( d 4

157 190

120 (. 94 99)

87 (. 81)

X = l l Pt I * 196 222 265

Gold

Gold(1) coordinates to an isonitrile group to yield linear complexes containing only one ligand. Complexes derived from nematogenic 4-alkoxyphenyl 4-isonitrilebenzoate ligands [I421 have been reported. They show mainly smectic mesophases with high transition temperatures (Table 3-38). In an attempt to disturb the strong intermolecular interaction in order to obtain metallomesogens with low melting temperatures, laterally substituted nonmesogenic isonitrile ligands have been successfully

Table 3-38. Mesomorphic properties of isonitrilechloro- gold([) complexes.

H2.C100 ~ O O C ~ N ~ C - A U - C I

R

3 Low Molecular Weight Culamitic Metallomesogens 99

employed [142]. Thus gold(1) complexes with lower transition temperatures and smectic A and/or nematic mesophases (Table 3-38) were generated.

Even nonmesogenic 4-alkoxy-4'-isonitrilebenzene ligands [ 1431 can be employed for the preparation of liquid crystalline complexes (displaying a smectic A phase) with gold(1). An increase in the Au-X dipole ( I<Br<Cl) leads to an increase in both melting and clearing points and is responsible for the trend in the resulting mesomorphic properties (Table 3-39). In contrast, (4-alkoxy-4'-isonitrilebiphenyl)- gold(1) complexes show the reverse behavior, with melting points increasing in the order C1< Br < I. This order corresponds to a significant increase in the polarizability of the system due to the presence of the organic biphenyl moiety [144].

Table 3-39. Mesomorphic properties of isonitrile- halogold(1) complexes.

n X Mesomorphic Properties

K SA I

1 CI 123 174

1 Br 104 144

1 I 100

2 CI 132 294 *(dec)

2 Br 145 290 *(dec)

2 I 167 250 '(dec)

Unsymmetric gold(1) complexes bearing both a nonmesogenic alkynyl ligand and a nonmesogenic isonitrile ligand have been reported to display smectic A mesophases (Fig. 3-42) [ 1451. However, unlike isonitrilephenyl gold(1) complexes, they decompose upon clearing into the isotropic. This behavior could be due to the presence of the alkynyl-gold(1) bond instead of an isonitrile-gold(1) bond.

R = H, n = 12 K 101.8 SA 121.0 I(dec)

Figure 3-42. R = OC10H21, n = 12 K1 73.1 K2 140.4 SA 152.5 I(dec)


3.2.1.3 Summary

As can be seen from the compounds discussed in this section the majority of monodentate organometallic liquid crystals described to date contain palladium(I1) or platinum(1r) as the metal centers. Both metals show square-planar geometry with the two promesogenic core units in a trans conformation. In this way, a centrosymmetric elongated dimer is obtained which promotes the formation of liquid- crystalline phases. In complexes which contain two neutral isonitrile ligands, the two other positions around the divalent metal center are occupied by halogen atoms in order to maintain neutrality. In contrast, in alkynyl complexes in which the ligands themselves compensate the charge of the metal, these two extra positions are occupied by neutral ligands (i.e., trialkylphosphines).

Unsymmetric linear complexes in this group are mainly represented by isonitrile- gold(1) complexes which contain only one ligand. In most of the examples, the metal-halogen group acts as a terminal substituent which promotes molecular interaction responsible for the appearance of smectic phases in a similar way to that described for iridium@) and rhodium(1) metal-organic derivatives (see Sect. 3.1 .I). These strong molecular interactions can be weakened by the introduction of lateral alkoxy substituents without the loss of the liquid crystalline character.

3.2.2 Organometallic Liquid Crystals with Bidentate Ligands

This class of compound includes ortho-metallated derivatives of palladium(II), mercury(u), manganese(1) and rhenium(1). Dinuclear and mononuclear ortho-palladated complexes constitute one of the most widely studied and important groups in the field of metallomesogens. They are comparable to bidentate salicylaldimine derivatives in metal-organic liquid crystals.

The first examples of ferroelectric and cholesteric organometallic complexes originate from this group. In addition, a number of novel and interesting structures, such as the “open-book” shape, have been reported for these materials.

With regard to metals other than those mentioned above, only a few mononuclear ortho-metallated complexes incorporating mercury(II), manganese(1) and rhenium(1) have been described to date.

3.2.2.1 Dinuclear ortho-palladated Complexes

Dinuclear ortho-palladated complexes always consist of two ligands joined by a central bridge. These complexes can therefore be described as having H-shape. It is possible to structurally modify two parts of the molecule; the vertical beams or the horizontal link of the H. On the one hand, different promesogenic core units can be used as the vertical moiety of the H-shaped molecule, and the possibilities include azobenzene, imine, azine and phenylpyrimidine ligands. On the other hand, the hori-


zontal part can also be modified by changing the intermetallic bridges. Halogens, thiocyanate and alkylcarboxylates have all been employed in this way.

a) Azo Ligands In 1982, Ghedini et al. described dinuclear azopalladium(1r) complexes as the first examples of ortho-metallated mesogens [ 1461. Ortho-metallation has been shown to occur in the more electron-rich ring, therefore, the general structure of these complexes is that illustrated in Table 3-40. All of the complexes show nematic behavior, and in each case a homeotropic texture is formed spontaneously. As usual, the metal complexes have higher transition temperatures than the corresponding free ligands.

Table 3-40. Mesomorphic properties of dinuclear ortho-palladated azobenzene complexes.

- R R' Mesomorphic Properties

K SE SA N I

-OC2H5 -0OCC4Hg 212 215

-C2H5 -0OCCsH11 210 (- 190 200) 225

-C2ti5 -0OCC6H13 190 205

-C2H5 -OOC(CH2)3CH=CH2 165 185

The influence of the nature of the halo-bridge on mesogenic properties was studied by the same research group [I471 (Table3-41). All three complexes are nematogenic and, in addition, smectic mesomorphism is observed for the bulkier bromo- or iodo-bridges.

The most extensive study of azopalladium(I1) complexes is concerned with p - alkyl-p'-alkoxyazobenzene ligands [ 148 - 1501. Almost statistical metallation in both


Table 3-41. Mesomorphic properties of dinuclear ortho-palladated 4-ethyl-4'-heptanoyloxyazobenzene complexes with chloro, bromo and iodo bridges.

I OOCCBH1, Y

X Mesomorphic Properties

K SB SA N I

1 f 1 (. 205) : 215 11: 220 225 230

benzene rings was detected by 'H NMR spectroscopy. Mixtures of isomeric complexes are always obtained, except for complexes with the ligand of n = 1 and m = 7 (figure in Table 3-42, see p. 103), in which metallation takes place in the alkoxy- benzene ring only (i.e., isomer b in Table 3-42).

In general, a significant stabilization of the nematic phase is observed for the complexes compared with the free ligands, the latter being either not liquid crystalline at all (n = 1, m = 1) or displaying a thermodynamically unstable nematic phase (n = 1, m = 2, 7 or n = 2, m = 12). Additional smectic mesophases are observed for

OCn%n+l

Figure 3-43. v

"HZn+i


Table 3-42. Mesomorphic properties of dinuclear ortho-palladated 4-alkyl-4-'-alkoxyazobenzene complexes.

r i m Mesomorphic Properties

K sc SA N I

1 1 229 254

1 2 214 235

1 7 175 194

1 12 150.4 (. 141) 165 174

2 12 162.5 (. 140) 169.1

complexes with the longest alkyl chains. Moreover, complexes from nonmesogenic mono-p-substituted ligands (Fig. 3-43) show monotropic smectic behavior for alkyl chain lengths of M = 10, 12, 14 [149].

The behavior of the complex in which M = 1 and m = 12 is also worth mentioning (see figure in Table 3-42). This complex shows a monotropic smectic A and smectic C phase which both are not displayed by the free ligand. On freezing, this compound forms a glassy smectic phase whereas the corresponding ligand crystallizes from the nematic phase. A study of the conoscopic figure of the nematic phase reveal that the arms of the cross split slightly into isogyres on rotation of the sample manner, indicating a weak biaxial character in the complex [ 15 I]. SANS and SAXS experiments [ 1521 have shown an significant decrease of the molecular length at the solid to mesophase transition. Several reasons have been put forward to explain this phenomenon. Among them, the authors suggest an almost entire chain fusion which provides a space-filling arrangement within the mesophase.

The modification of the core unit in these complexes has been investigated by Hoshino et al. [153]. The structures studied are represented in Table3-43. Lateral substituents, which cause dramatic reductions in mesophase stability in the ligands due to steric effects, destabilize that of the complexes to a less extent. The side-chains

104 J. L. Serrano and i? Sierra

Table 3-43. Mesomorphic properties of dinuclear orrho-palladated 2-alkoxycarbonyl-4-(4-methoxybenzoyloxy)-4'-ethoxy azobenzene complexes.

n LIGAND COMPLEX

K N I K N I

1 149 194 265 275 *(dec

4 111 122 211 266 *(dec

8 * 7 5 * 0 1 - 135 183 *

are able to fill the gaps of the molecular core, hence the nematic phases are considerably wider than those observed for the ligands.

Structural modification of the lateral chains have been carried out by means of incorporating chiral alkoxy substituents (Fig. 3-44) [154]. Either of the 1,4-disub-

R' = -OCH2CH2'CHCH,CH2CH=CCH3 , n = 10 K 150.0 (SC' 147.0) I

AH3 AH3

R' = -'CHCH~CH~CHZCH~CH~CH~ , n = 10 K 118.9 (Sc* 113.4) I I

CH3 Figure 3-44.


stituted benzene rings can undergo ortho-palladation. A potentially ferroelectric S z phase is found for ( S ) - ( - )-P-citronellol derivatives, whereas the free ligand only forms a cholesteric phase (n = 7, 10). For (I?)-( -)-2-octanol derivatives, monotropic mesomorphic phases are observed even when the corresponding ligands are not liquid crystalline. As was found for the achiral complexes of this type, the lamellar spacings revealed by X-ray measurements in the smectic A phase are shorter than predicted by calculations. It is therefore probably that either separate melting or interdigitation of the alkyl chains occurs in the smectic A phase.

b) Imine Ligands The first dinuclear imino palladium(r1) complexes described [ 1551 are complexes of Schiff bases derived from benzaldehyde or acetophenone with long alkyl or alkoxy tails on both sides of the ligand (Fig. 3-45).

R

All these complexes, except those with acetato bridges, show an enantiotropic smectic A phase. An additional smectic C phase is observed in the chloro-bridged complexes derived from p-decyloxybenzaldehyde. When shorter tails are used (e.g., 4-methoxybenzylidene-4’-butylaniline (MBBA) complexes) a nematic phase appears above the smectic A phase of the chloro-bridged complex [156]. Melting points of the acetato- and thiocyanato-bridged compounds are higher than those of the halo- bridged complexes. In contrast, clearing temperatures show the reverse trend, giving rise to the widest mesophase for the halo-bridged compounds. The structure of the acetato-bridged complexes is unique because the molecules adopt an “open-book” molecular shape (Fig. 3-46a) [ 1571. In contrast, the thiocyanato-bridging group forces the molecule into a completely coplanar geometry of the two square-planar palladium centers (Fig. 3-46b) [158]. Chloro and bromo bridges can yield either coplanar or


a b C

Figure 3-46. a) Nonplanar open book structure of the acetato-bridged complex. b) Planar structure of the thiocyanato-bridged complex. c) The planar structure of the chloro-bridged complex.

bent molecules, but in general an average planar conformation is suggested to exist in solution or in the melt due to a lack of steric strain (Fig. 3-46c) [159].

Nevertheless, X-ray crystallographic studies [ 1561 of the complex derived from MBBA show that the chloro-bridged complex is not planar but the two square planes form a Cl . . .Cl dihedral angle of 141.3' with a Pd-Pd separation of 3.326 A (Fig. 3-47).

Figure 3-47. Molecular structure of the cyclopalladated dimer [(L)PdCl], (HL = N-(4-methoxybenzylidene)-4'-butylaniline). Adapted from reference 156.

'H NMR spectroscopy [I601 revealed a trans-geometry for both the nonplanar acetato-bridged complexes and the planar chloro- and bromo-bridged analogues. The nonplanar acetato-bridged compound should occur as two enantiomers which are indistinguishable by 'H NMR. The thiocyanato-bridged complexes consist of a mixture of isomers due to the different possibilities of coordination of the two unsymmetric thiocyanato groups.

Structural modification related to the terminal tails has been carried out on both acetato- and chloro-bridged complexes. Studies performed on chloro-bridged com-


plexes derived from benzilideneanilines with a polar group either in position 4 or 4' [161, 1621 have revealed the appearance of a smectic A phase even if the free Iigands are nematogenic. The exception of this trend is the compound with a cyano group which only shows a nematic mesophase. This behavior is observed regardless of the position of the polar group in either the aldehyde or the aniline ring. The results are similar to those obtained for alkyl-substituted imines, which indicates that it is the central core which is mainly responsible for the mesomorphic properties. The polar group plays only a secondary role which affects the transition temperatures. A destabilizing effect on mesophases is observed in compounds containing an electron- withdrawing group (NO2, CN) in each aniline ring. This can be attributed to electronic repulsions which can be cancelled out when the polar group is in the aldehyde ring of the molecule.

Another modification to the lateral tails, with the aim of achieving a potentially ferroelectric S z phase, consists in the introduction of chirality (Fig. 3-48) [163]. Once again the chloro-bridged complexes were chosen for preliminary studies, because they exhibit a rich mesomorphism and are most likely to form wide smectic C phases [ 1551. Complexes containing two chiral alkoxy substituents (2-octyloxy) show an enantiotropic ferroelectric S z phase. The complex containing four chiral tails only shows a monotropic S z phase. In each case the chiral smectic C phase is preceded by a smectic A phase when cooling the isotropic liquid.

Remarkably, the S z phase is more stable by about 20°C when the chiral tail is situated in the aniline ring, which has a higher mobility. The layer spacing of the smectic A phase found by X-ray measurements appears to be shorter than the theoretical value estimated using Dreiding models. However, the difference is small enough to rule out any significant interdigitation of the molecules.

K1 115.2 K2 127.9 Sc' 155.8 SA 219.9 I

Figure 3-48.

R"'" 0

0Y CsH13

K 132.3 Sc' 175.7 SA 230.1 I

7sH13

h H 1 3

K 113.7 (Sc'88.3) S~230.1 I

108 1 L. Serrano and T Sierra

The acetato-bridged complexes discussed above are not liquid crystalline [ 1551. However, the introduction of other carboxylato-bridges, such as CH,-CHCl-COO- (Fig. 3-49a), leads to the formation of a smectic A phase [I641 despite the fact that these bridges are bulkier than the acetato group. The reason for this must be the dipole introduced by the carbon-chlorine bond, which should cause increased intermolecular interaction. Replacement of one of these a-chloro- carboxylato bridges by a thiolato group [ 1641 (Fig. 3-49 b) yields a cis-palladium(I1) complex which shows a cholesteric phase.

oC&3

a K 152 SA 186 I b K 140 Ch 157 I Figure 3-49.

c) Azine Ligands

When Schiff bases are coordinated to palladium(rr), dinuclear acetato-bridged palladium complexes are obtained, which are not liquid crystalline. However, smectic C and/or nematic phases are observed when benzalazine derivatives are employed as ligands [165]. 'H NMR data indicate the existence of two possible isomers, trans and cis in an approximate ratio of 3 : 1. In spite of their open-book shape, these acetato-bridged compounds are liquid crystalline. This could be due to the presence of the extra carbon-nitrogen double bond which could increase the anisotropy of the polarizability, and/or the decrease in the melting points caused by the existence of cidtruns mixtures. The chloro-, bromo-, and thiocyanato-bridged benzalazine complexes exhibit wider mesophases, greater thermal stability, and lower melting points than the corresponding acetato-bridged species (Table 3-44, see p. 109).

A complete study has been performed which deals with a homologous series of carboxylato(C,H2,+,COO-)-bridged complexes (Fig. 3-50) [ 1661.

All the complexes show at least a nematic phase except those with the shortest alkylcarboxylato bridges. A smectic C phase appears for the compounds with the shortest carboxylate chains (n = 0-3) and also with the longest ones (n = 10, 11, 13, 15, 17). Carboxylates of intermediate length, however, seem to destabilize smectic mesophases. When short carboxylate chains are present (Fig. 3-51 a) the lateral

3 Low Molecular Weight Calamitic Metullomesogens 109

Table 3-44. Mesomorphic properties of dinuclear ortho-palladated 4,4’-didecyloxybenzalazine complexes wtih acetato, chloro, bromo, and thiocyanato bridges.

X Mesomorphic Properties

. K i K2 sc I

CH3C02 131.8 151.1

CI 102.0 226.0

Br 118.0 249.0

SCN 113.8 123.8 218.6

substitution does not dramatically affect the length-to-width ratio (L/D) . However, when the number of carbon atoms is increased (Fig. 3-51 b), this ratio decreases and the alkyl groups disturb the lateral packing. As the chains become longer (Fig. 3-51 c), they stretch out and align along the long molecular axis and do not significantly increase the molecular width (0) of the molecule in an unfavorable way.

As was already observed for imine complexes, the open book-shaped palladium(t1) complexes are actually racemic mixtures due to the fact that inversion of the “book” is impossible. If a chiral center is incorporated into the molecule, an optically active mixture can be obtained [167]. Thus, by using (R)-2-chloropropionic acid as a carboxylato bridge, a mixture of trans-AR,R (MOio), truns-AR,R (34%), and cis-R,R (32%) isomers is obtained which shows a SE phase. This mixture is the first example of an organometallic liquid crystal to display ferroelectric behavior (Fig. 3-52).

A similar study has been published by Zhang et al. [168, 1691 who investigated imino, azo, and azine ligands, and used (S)-( -)-CH3-CHCl- COO- and (S)-( -)- (CH3)2-CH-CHCl-COO- as the bridging groups of the dinuclear complexes. A chiral smectic C phase is also displayed by most of these complexes.

1 10 .l L. Serrano and 7: Sierra

T ("C)

150-

140-

130-

120-

110-

100-

90-

I I I l l I I I 1 I I I I I I

0 1 2 3 4 5 6 7 8 9 1 0 1 1 13 15 17 n

Figure 3-50. Transition temperatures as a function of the length of the carboxylate-bridge in the ortho-palladated dimers [(L)Pd@-0,C-C,H,,+ ,)I2 (HL = 4,4'-didecyloxy-2,2'-dihydroxybenzalazine). (# ) When n = 0, the complex decomposes just after the transition into the smectic C phase. Adapted from reference 166.

b C

Figure 3-51. Molecular model showing the arrangement of the carboxylate chains in the complexes [(L)Pd@-0,C-C,H2,+ , ) I 2 (HL = 4,4'-didecyloxy-2,2'-dihydroxybenzalazine). Adapted from reference 166.

3 Low Molecular Weight Calamitic Metallomesogens 11 1

Figure 3-52.

d) Pyrimidine Ligands

%I

Phenylpyrimidine compounds have been employed as C,N-donor ligands to give dinuclear ortho-palladated complexes with good thermal stabilities [ 1701. The two phenylpyrimidine moieties are situated in a trans geometry. Acetato-bridged complexes are not liquid crystalline, whereas the presence of a halogen bridge gives rise to quasi-planar structures (dihedral angle of 120"- 140') which promote smectic mesomorphism (Table 3-45, see p. 112) [171].

3.2.2.2 Mononuclear ortho-Metallated Complexes

Palladium

First attempts to obtain mononuclear ortho-palladated metallomesogens were already carried out when dinuclear halo-bridged palladium complexes were first reported [147]. Neutral ligands, such as pyridine, quinoline or aniline, are able to cleave the halo bridge and coordinate to one of the palladium atoms. Liquid crystalline properties were observed for pyridine (N,N-cis) and quinoline (N,N-trans) complexes (Table 3-46, see p. 112).

Since then, with the main objective of obtaining metallomesogens with low melting temperatures, unsymmetric ortho-palladated compounds have been described with either j?-diketone or N,O-chelating ligands.

1 12 J# L. Serruno and 7: Sierra

Table 3-45. Mesomorphic properties of dinuclear ortho-palladated 5-hexyl-2-(4-undecyloxyphenyl)pyrri- midine complexes with acetato, chloro, bromo, and iodo bridges.

X - CH$O;,

CI

Br

I


K1 K;, SA 1

139.6

100 177.2 218.8

112 171.3 202.1 *

109.7 148.6 192.9

Table 3-46. Mesomorphic properties of mononuclear orfho-palladated 4-ethoxy-4'-heptanoyloxy azobenzene complexes with neutral N-donor ligands.


Dinuclear azo- and azomethine-ortho-palladated complexes have been cleaved by a ligand derived from acetylacetone. The promotion of nematic mesomorphism is generally observed after the change from a dinuclear to a mononuclear complex. This observation is more noticeable in the case of azomethine complexes [I721 which rarely show a nematic phase in the dinuclear derivative (Fig. 3-53). A possible explanation for this finding is that the unsymmetric P-shaped mononuclear complex disrupts the molecular arrangements more than the H-shaped dinuclear molecules thus favoring less ordered mesophases.

( h H 1 3

Q OCsH13

Figure 3-53. K1 74.1 Kz 146.1 (Sc 140) SA 266.2 I(dec) K 84 SA 118.5 N 124.8 I

The formation of a smectic C phase has been achieved by using a broader &diketone ligand substituted with phenyl rings (Fig. 3-54) [173]. In this way, a ferroelectric S z phase has been obtained for mononuclear ortho-palladated azomethine complexes. This phase occurs at significantly lower temperatures than that of the parent dinuclear complex [ 1 631.

CsH13

Figure 3-54. K 114.4 (Sc’ 110.8) SA 118.4 I

1 14 J L. Serrano and i? Sierra

With the aim of obtaining an S z phase at relatively low temperatures, an N,O-chelating ligand (L-alanine) has been incorporated into azo, imine and azine derivatives (Fig. 3-55) [174]. Other, more bulky amino acids do not cause formation of a chiral smectic C phase but only a smectic A phase is generated.

Q 0C14H29

K 81.5 Sc' 174.0 I

Figure 3-55.

Chiral mononuclear palladium(I1) complexes containing both an azo and an acetylacetone ligand have been described and show nematic and smectic A mesophases [1751.

Liquid crystalline 4,4'-dialkoxyazoxybenzene compounds have attracted much attention as ligands for unsymmetric mononuclear ortho-palladated complexes in connection with P-diketone, Schiff base or azobenzene ligands. With P-diketones (Fig. 3-56a), only nematic mesomorphism has been observed [176]. In contrast, with

a

OCGH13

Q

K 90 N 105 I

Figure 3-56.

b C

R R

R = -Cali17

R=R(-)-P-OCTYL K 1 1 3 S ~ * 1 3 2 . 3 1 R=-C&I17 K128.8(Sc91.3N 111.3) I

K 128 SA 144 I R = -CZHS K 113.4 (N 102.4) I


a Schiff base as the second ligand (Fig. 3-56b), smectic mesomorphism appears to be more favored [177]. Furthermore, the incorporation of chiral tails in the Schiff base ligand leads to the formation of a more ordered smectic H mesophase [45]. Monotropic nematic behavior is the most commonly observed mesomorphic property for complexes incorporating an azobenzene ligand (Fig. 3-56c) [178]. A thermodynamically unstable smectic C phase is displayed by the complex derived from 2-hydroxy-(4-n-dodecyloxy)-4'-n-octylazobenzene.

Dinuclear @-chloro pyrimidine)palIadium(rr) complexes can be converted into mononuclear palladium(r1) complexes [179] in a similar way to the compounds discussed above. Cleavage of the bridge has been carried out with complexes containing ligands derived from acetylacetone (Fig. 3-57 a) and also neutral dinitrogen ligands (Fig. 3-57 b) such as phenanthroline. The phenanthroline derivative represents another example of the relatively uncommon ionic thermotropic liquid crystal complexes.

y d 1 3

OC11 H23

K 83 (SA 68) I

Figure 3-57. a

p H 1 3

OC11H23 -

K 146 N 158 I

b

+ B F ~ '

Manganese and Rhenium

Recently, new mononuclear ortho-metallated complexes with metals other than palladium(I1) have been reported. One of these species is the first liquid crystalline compound containing manganese(1) [ 1 SO], and consists of an organometallic imine complex with an octahedral coordination (Fig. 3-58). A Schiff base, containing four

K 154 N 190 I

Figure 3-58. (free ligand: K~ 63 K~ I 16 sG 124 S, 202 N 298 I )

1 16 J. L. Serrano and i? Sierra

aromatic rings which shows rich mesomorphism in its own right, is coordinated to the manganese(1) tetracarbonyl group. The resulting complexes have lower transition temperatures than the free ligand and only show nematic behavior. The smectic mesomorphism present in the ligand disappears due to the destabilization caused by the bulky lateral manganese group. Analogous rhenium(1) complexes have been also prepared, and these show enantiotropic nematic phases. The rhenium complexes have lower melting points and more stable mesophases than their manganese analogs [181].

Mercury

Mercury(I1) has also been reported as a metal center in ortho-metallated liquid crystals through coordination by a chiral azoxy ligand (Fig. 3-59) [182]. The geometry of coordination is linear, with the second valence occupied by a chloride ion. The 1 : 1 mixture of isomers, which results from metallation occurring in both benzene rings, shows a potentially ferroelectric S z phase within a wide temperature range around room temperature.

n = 6 K ~ 1 5 Sc' 58.2 I

n = 10 K c15 Sc' 63.5 I Figure 3-59.

3.2.2.3 Summary

It is clear from this section that the central bridge plays an important role in the geometry, hence the mesomorphic properties of dinuclear ortho-palladated complexes. Halogeno and thiocyanato bridges cause planar molecular structures which promote mesomorphism. In contrast, alkylcarboxylate bridges force the molecule to adopt an open-book shape which, although not effective in generating mesophases, does not prevent mesophase formation in several cases. It is noteworthy that the open- book structure is chiral, and when a chiral alkylcarboxylate bridge is present (e.g.,


2-chloropropionate) an optically active diastereomeric mixture is obtained which exhibits a ferroelectric SE phase. Indeed this mixture is the first example of a ferroelectric metallomesogen reported in the literature. Replacing one of the alkyl carboxylate bridges with a thiolate bridge leads to the formation of a chiral nematic phase. This is the first example of a cholesteric phase in organometallic liquid crystals. As research in this field progressed, a number of organometallic ferroelectric liquid crystals with a planar structure have been reported. These compounds are chloro-bridged palladium(I1) complexes with either one or two chiral substituents attached to the imine ligands. The number and position of the chiral chains strongly influence the ferroelectric properties.

Mononuclear palladium(I1) complexes are unsymmetric derivatives which are usually obtained from the corresponding dinuclear precursors. From a schematic point of view, one of the ligands, which constitutes the vertical beams of the H- shaped dinuclear complex, is substituted by a different coordinating group (p- diketone, Schiff base, bipyridine or other N,O-chelate). The main consequence of this change is a significant decrease in the transition temperatures observed in the mononuclear complex compared with those of the dinuclear complex. The reason for the decrease is mainly the reduced symmetry in the mononuclear derivative.

Within the scope of mononuclear complexes, a number of mercury(II), manganese(1) and rhenium@) ortho-metallated complexes have also been described. In these cases, the metal group acts as a lateral substituent but appears to be partially shielded by the ligand. In spite of the octahedral coordination of manganese(1) and rhenium(I), in which four positions are occupied by carbonyl groups, these complexes are still liquid crystalline.

3.2.3 Organometallic Liquid Crystals with Metal- 7c Bonds

The great stability of the ferrocene system led to its incorporation into metal-containing liquid crystals. First attempts to introduce the ferrocene unit into mesogenic materials were carried out many years ago, but only recently a large number of ferrocene derivatives which are liquid crystalline have been described. The largest number of examples within the group of organometallic liquid crystals, in which the metal is bonded to a n-system in the organic ligand, contains the ferrocene moiety. Other metal-n bonded complexes which incorporate a ruthenocene unit or a butadiene iron group have been reported in the literature.

3.2.3.1 Metallocene Derivatives

In 1957, ferrocenecarboxaldehyde was reported to show a liquid crystal phase between 45 and 124.5 "C (Fig. 3-60a) [183]. Almost twenty years later, studies by Verbit and Halbert showed that this compound is not a liquid crystal but a plastic crystal [184]. These authors carried out the first attempts to obtain liquid crystal properties in ferrocene derivatives by the synthesis of imino derivatives of ferrocenecarbox-

1 18 J L. Serrano and 7: Sierra

K 45.0 Plastic Crystal Phase 124.5 I

C

n=10 K143N1591 Figure 3-60.

aldehyde (Fig. 3-60b). None of these compounds turned out to be mesomorphic. One year later, however, liquid crystalline ferrocene derivatives showing a nematic phase were described by MalthCte et al. (Fig. 3-6Oc) [106]. Surprisingly, 4-benzoyloxy benzylideneaniline is able to tolerate a large terminal group without loosing its mesomorphic properties.

Since then, several studies have been published aimed at obtaining liquid crystalline ferrocene derivatives. Singh et al. proposed the idea that 1,l'-disubstituted ferrocene units could serve as the basis for a mesogenic unit [185] by imparting both rigidity and collinearity to the system when additional substituents are present which are also collinear and rigid. A number of compounds reported in order to confirm this proposal show a thermodynamically unstable smectic phase (Fig. 3-61).

Figure 3-61.

However, phase stability is only increased if the molecule containing the ferrocene unit is lengthened by promesogenic groups [ I 86- 1891. Accordingly, the first enantiotropic liquid crystalline t,1 '-disubstituted ferrocenes are derived from 4-alkoxy- 4'-cyanobiphenyl (Fig. 3-62).


I - - \ - n = 6 K 109.5 SC 118.2 I -(CH*)".O \ / \

wCN n = 8 K 102.5 S i 108.5 I

Figure 3-62. & - ( c H 2 ) " . 0 ~ C N n = 10 K 104.5 Sc 109.3 I

X-ray crystallography studies of these 1 ,I '-disubstituted ferrocene molecules have demonstrated that they exist in the fully extended "S" geometry [190]. The distance between both cyclopentadiene rings is too large to allow a favorable length-to-width (L/D) ratio unless extended premesogenic substituents are introduced. A comparative study [ 1911 between ferrocenyl derivatives and analogous benzene and trans- cyclohexyl derivatives (Fig. 3-63) has confirmed the idea that the presence of a step- like structure will cause a lack of planarity. It has also been suggested that the rigidity is also reduced by the ferrocene unit due to the relatively low energy barrier for rotation of the cyclopentadienyl rings. Neither of these two factors occurs in benzene or cyclohexyl derivatives, therefore, the clearing temperatures in ferrocene derivatives are significantly lower.

H13c6 4 K 107 (N 63) I

13c6 c02 0 c02- OC8H 17

K 138 Sc 176 N 280 I

Figure 3-63. K98 Sc 180 N 283 I

Further studies on 1 ,I '-disubstituted ferrocene derivatives were focused on promesogenic substituents derived from phenyl benzoates. The influence of the orientation of the ester linkages in both units (Fig. 3-64) [I921 showed that a combination of both electron delocalization (electrostatic interactions) and rotational motion (rigidity of the organic rod) are probable origins of the liquid crystalline behavior. For promesogenic units derived from hydroquinone (Fig. 3-64 a), monotropic nematic behavior is observed for compounds with intermediate length terminal chains. However, homologs with long terminal chains are not liquid crystalline. In contrast, when the substituents attached to the ferrocene unit are derived from p-hydroxybenzoic acid (Fig. 3-64 b), nematic and smectic A mesophases are stabilized. This trend is the same as that observed for organic liquid crystals [193].

120 J. L. Serrano and ?: Sierra

n = 6 K172(N153)1

n=10 K1701

a

Figure 3-64.

Unsymmetric substitution of the ferrocene unit in positions 1 and 1' has led to a decrease of melting points and, more interestingly, for the first time to the appearance of smectic A and smectic C mesophases in ferrocene containing liquid crystals (Fig. 3-65) [194].

K 132 Sc 148 SA 153 I Figure 3-65.

Only one metal other than iron has been coordinated to cyclopentadienyl rings. 1 ,l'-Disubstituted ruthenocenes have been described to show similar phase behavior to their iron(I1) analogs, but the phases occur within smaller temperature ranges (Fig. 3-66) [ 1951.

K 167 SA 169 I Figure 3-66.


An accomplishment to the 1 , I ’-disubstituted ferrocenes was the development of 1,3-disubstituted ferrocenes (Fig. 3-67 a). These compounds are liquid crystalline with a stronger tendency to form the nematic phase than that of the 1,l’-disubstituted ferrocenes [ 1881 (Fig. 3-67 b).

K 172 N 206 I a

Figure 3-67. b

However, 1,3-disubstituted derivatives always need reasonably long promesogenic units as substituents attached to the ferrocene unit in order to exhibit mesophases. Short substituents do not overcome the separating effect of the ferrocenyl group, making the intermolecular interactions too weak to generate molecular ordering in the mesophase [ 1961. Smectic mesomorphism is favored when promesogenic units derived from p-hydroxybenzoic acid are introduced into the molecule (Fig. 3-68) [ 1971 which corresponds to the observation made for the 1 ,l’-disubstituted derivatives.

Figure 3-68. K 1 9 0 S ~ 2 2 8 1

Unsymmetrically 1,3-disubstituted ferrocenes have attracted particular attention because chirality can be introduced into the molecule [198]. As in the corresponding 1,l’-disubstituted systems, broad mesogenic ranges and enantiotropic mesophases are observed (Fig. 3-69). If these compounds could be prepared in their optically active form, 1,3-disubstituted ferrocenes would be promising candidates for ferroelectric liquid crystals [ 1991. However, their physical investigation has not been published yet.

122 1 L. Serrano and T Sierra

3.2.3.2 Butadiene Complexes

Liquid crystalline butadiene iron@) tricarbonyl complexes have been reported [200], in which the butadiene group is located either in a terminal position (Fig. 3-70a), or it constitutes part of the mesogenic core (Fig. 3-70 b). If these complexes had two different substituents, they should, in a similar way to the 1,3-disubstituted ferrocenes, be chiral and potentially ferroelectric. However, thermal racemization has been observed above a certain temperature [201].

3.2.3.3 Summary

Initially, ferrocene was regarded as a bulky terminal group to introduce the properties of the metal center into the molecule. Therefore, a long premesogenic unit must be used in order to obtain liquid crystalline materials. More recently, the ferrocene unit has rather been considered a pivot around which two premesogenic units in the 1- and 1’-positions can partially rotate. However, X-ray studies show that only the fully extended “S’tgeometry is present in these molecules. As a consequence, ferrocene acts as a step in a calamitic structure allowing the formation of mesophases at relatively low temperatures. The appearance of stable mesophases turns out to be more favored when the system is 1,3-disubstituted. In this case the ferrocene group is effectively a lateral substituent whose steric effect can be compensated by introducing long promesogenic units as substituents.

The possibility of obtaining a noncentrosymmetric molecular ordering from chiral ferrocene and butadiene derivatives open a new access towards ferroelectric properties


in metal-containing liquid crystals. A significant effort is required in the future to prepare optically active ferrocene compounds and to evaluate their ferroelectric properties.

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4 Low Molecular Weight Discotic Metallomesogens

Joaquin Barbera

This chapter deals with metal-containing thermotropic liquid crystals showing mesophases other than classical calamitic nematic and smectic. It does not take into consideration the molecular shape. In fact, most of this substantial group of metallomesogens are disc-shaped. However, several series of compounds with other molecular geometries are also covered which show mesophases typical of disc-like mesogens. Thus, metal-containing compounds displaying the following types of mesophase are mentioned: columnar (Dh, D,, etc.), discotic nematic (ND, also called lenticular), nematic columnar (N,) and discotic lamellar (DL).

Discotic mesophases formed by hexaalkanoyloxybenzenes were first reported by Chandrasekhar and coworkers in 1977 [l]. These molecules have a rigid planar central core surrounded by flexible aliphatic chains. X-ray diffraction revealed that their mesophases show a columnar structure in which the disc-shaped molecules stack along the column axis. Since then, hundreds of liquid crystalline compounds have been described to form columnar mesophases (denoted generically by the letter D). These phases have been classified into different types [2]. This classification arises from the nature of intracolumnar stacking, which can be long-range (ordered) or short-range (disordered), and from the intracolumnar order. The column axes form a two-dimensional lattice which can be hexagonal, rectangular, etc. The nature of the order in the mesophase is denoted by subscript letters (for example, D,, signifies a disordered hexagonal columnar phase and D,, an ordered rectangular columnar phase). Although the flat disc is the most common shape, columnar mesophases have also been reported for molecules having a non-planar core, described as pyramidal, conical or bowl-like. In addition to the columnar mesophases, discotic compounds can also show the discotic nematic mesophase (denoted N,) which is characterized, as in calamitic nematics, by the random positional arrangement of the molecules and the presence of orientational order only. Recently, a new mesophase type, called nematic columnar (denoted Nc), has been reported [3, 41 in which columns of stacked discs adopt a nematic arrangement rather than a two-dimensional assembly.

132 J. Barbera

The introduction of metal ions into mesogenic molecules has brought about a considerable expansion in the field of discotic liquid crystals, because the coordination geometry of many metals (in particular square-planar and, to a lesser extent, square- pyramidal coordination) causes the molecule to adopt an approximately planar structure. Indeed, the application of transition metal coordination chemistry in the design of liquid crystals has led to the discovery of a large number of novel compounds capable of exhibiting noncalamitic mesomorphism, and some of these compounds have shapes which are not disc-like. Notwithstanding the diversity of molecular geometries that fulfill the requirements to exhibit discotic mesomorphism, generally speaking, molecules derived from noncalamitic metallomesogens (as well as noncalamitic organic mesogens) consist of a central core surrounded by a number (four or more) of flexible groups. In certain cases, individual molecules do not match these characteristic, but they are capable of aggregating into supramolecular units which fulfill the requirements. Although, in many cases, the molecules (or the aggregates) do not contain a flat core, all of them have in common the existence of one easily identifiable short molecular axis. In the mesophases the molecules correlate their short axes in a preferred orientation, as happens with the long axes in calamitic mesophases.

In this chapter, as in the chapter devoted to calamitic metallomesogens, the compounds have been classified by the type of ligand. The discussion has been divided into several parts, according to the ligand denticity (bidentate, tridentate, etc.) and the type of complex (mononuclear, dinuclear, etc.), and further sub-divided into sections devoted to each particular family of ligand.

4.1 Mononuclear Metal Complexes with Bidentate Ligands

4.1.1 P-Diketonate Complexes

/3-Diketonate complexes have been the subject of extensive (and sometimes controversial) work in the field of metallomesogens, with the scope of the research concerning calamitic as well as non-calamitic systems. The mesomorphic properties of this kind of complex change drastically depending on the number of aliphatic chains in the phenyl rings. For this reason, the discussion of mesomorphism in noncalamitic P-diketonate complexes has been divided into two subsections. One subsection is devoted to those compounds containing only one chain in each phenyl ring, which mostly exhibit lamellar phases, and the other is focused on those compounds containing two or more chains in each phenyl ring, which mostly form columnar phases.

4 Low Molecular Weight Discotic Metallomesogens 133

4.1.1.1

P-diketonates were the first disc-like complexes reported to exhibit mesomorphism in the pure state. Their liquid crystalline properties were described for the first time in 1981 by Giroud and Billard, who described a series of P-diketonate complexes containing four aromatic rings symmetrically (n = m) or unsymmetrically (n # m) substituted with various alkyl chains (Table 4-1) [5]. The only nickel complex prepared (M = Ni) is nonmesomorphic, whereas the three copper complexes (M = Cu) show a well organized mesophase, as indicated by the large clearing molar enthalpies. On the basis of preliminary X-ray studies by Levelut [2 b], it was proposed that this phase is lamellar. The layered nature of this phase was confirmed by NMR studies carried out later by Ribeiro and Giroud [6]. The NMR measurements reveal that in these complexes the aliphatic chains adopt a preferred orientation parallel to the magnetic field direction, as the case for calamitic nematic and smectic systems, instead of a random distribution around the core, as it occurs in columnar phases.

Ohta and coworkers studied an extensive series of P-diketonate copper(I1) complexes, analogous to those of Giroud and Billard, symmetrically substituted with four alkyl chains (Table 4-2, R = R' = CnH2n+l, n = 0-12) [7]. They reported mesomorphism for all the homologs in which n 2 4 , although the nature of mesophase was not elucidated. The clearing enthalpies range between 25 and 36 KJ mol-'. On the basis of the existence of a eutectic point in the binary phase dia-

Complexes with one Aliphatic Chain at Each Phenyl Ring

Table 4-1. Mesomorphic properties of tetraalkyl bis( l ,3-diphenyl-I ,3-propanedionate) complexes.

cu 10 10 K 85.5 Ma 128.5 I

c u 12 12 K 95 Ma 113 I

a Lamellar phase

134 J. Barbera

Table 4-2. Mesomorphic properties of tetrasubstituted bis( 1,3-di- ~henyl-l,3-propanedionato)copper(11) complexes.

0. ,o o'cu. 0

grams, the authors concluded that these phases are true liquid crystal phases and not crystals. However, this conclusion has been controversial, as the miscibility test is considered a criterion for isomorphism but not for mesomorphism.

The analogous series of copper(I1) complexes bearing alkoxy groups (Table 4-2, M = Cu, R = R = CnHznt ,0, n = 1 - 12, 14) was reported to display mesomorphism when n 2 3, with the mesophase being extremely viscous [8, 91. The unsym- metrical copper complexes (M = Cu, R = CnHZn+,O, R' = CmHZm+,O, n # m ) are not mesomorphic [lo] (see Table 4-2). The high viscosity, along with the large clearing enthalpies (44-85 K J mol-'), suggest that these phases are highly ordered. X- ray diffraction experiments revealed reflections characteristic of a lamellar phase, in which the molecules are tilted in each layer by a constant angle irrespective of the chain length [8]. However, despite the tilted and layered structure, this phase was found to be immiscible with the smectic C phase of a standard material, and therefore it was thought to be a new type of mesophase, for which the term discotic lamellar and the symbol DL were proposed. However, whether this phase is a true liquid crystal or a crystalline phase remains unclear.

A number of analogous copper complexes bearing octylthio or octylsulfonyl groups (Table 4-2; R = R' = C8H,,S; R = C8Hi7S, R' = C8Hl,SO2) [ I I ] are not mesogenic.


Unsymmetric copper(I1) complexes of /I-diketones bearing one alkyl and one alkoxy chain in each ligand (Table 4-2, R = CnHZn+], R' = C,H2,+10, n = 8, 9, 10) [I21 all exhibit a mesophase. The mesophase of each compound is totally miscible with that of the other complexes in the series and also with the DL phase shown by the tetraalkoxy complexes [8]. However, the clearing enthalpies in these alkyl- alkoxy-substituted complexes are smaller than those in the symmetrical analogs, lying in the range of 18 -29 KJ-mol- ' , suggesting a lower degree of order. The clearing temperatures of the unsymmetrically substituted complexes are similar to those found for the compounds containing four alkyl chains. They are about 30- 35 "C lower than those of the compounds containing four alkoxy chains (see Table 4-2).

The crystal structure of another complex in this series (M = Cu, R = C7H15, R' = C ~ H I ~ O ) , as well as those of several complexes belonging to the tetraalkyl and tetraalkoxy series with copper(II), palladium(I1) and nickel@) as metal centers (Fig. 4-I), were studied by single-crystal X-ray techniques in a series of reports by Usha et al. [13]. In all cases, the structure corresponds to that of a lamellar crystal in which the molecules form layers, and the layers stack along one of the crystallographic axes. No differences were found in the coordination geometry of the three metals investigated (square-planar), despite the fact that the copper and palladium complexes were reported to be mesomorphic whereas the nickel complex was not. The aliphatic chains adopt the fully extended conformation and align almost parallel with respect to each other.

M = CU , R = CeH17-

M = CU , R = CeH170-

M = Pd , R = CaHj,-

M = Pd, R = CloH2,-

"vR \ n \

M = Ni , R = C8Hj7- Figure 4-1. R R

Discotic mesomorphism has been also reported in /3-diketonate oxovanadium(1v) complexes, the coordination of which is square-pyramidal (Fig. 4-2, n = 6- 10, 12, 14, rn = 6- 10, 12, 14, n = or # nz) [lo, 141. One of the main points of interest in oxovanadium complexes is the possibility of forming columnar structures as a result of the vanadium-oxygen bond aligning along a common axis. Indeed, this phenomenon was observed for other liquid crystalline oxovanadium complexes by Swager et al. [15]. Although it was initially thought that the /3-diketonate complexes might be polymeric due to axial intermolecular ( * - V=O. - .V=O. - ) interaction, this possibility was ruled out by the study of the V=O stretching frequency by IR spectroscopy and by magnetic measurements, both indicating a monomeric structure in the crystalline phase. The mesomorphic properties of the oxovanadium complexes are not as

136 J. Barbera

are not as pronounced as those of the copper analogs, and they only exhibit monotropic mesomorphism. The optical textures are characteristic of ordered hexagonal columnar phases (Dho), in contrast to those of the copper analogs which show a discotic lamellar (DL) phase [8 - 101. The isotropic-mesophase transition temperatures in the vanadyl complexes are about 25 - 30 "C lower than in the analogous copper complexes with alkoxy chains of the same length (see Table 4-3). The D,, phase was found to be totally miscible with the hexogonal columnar mesophase of a dicopper(I1) tetraalkanoate [16] used as a reference. Although the X-ray experiments confirmed the hexagonal structure of the phase, considerable three-dimensional order was detected in the diffraction patterns, suggesting that the phase should be regarded as a "disordered crystal phase". In agreement with this, the clearing enthalpies were found to be large (44-60 KJ mol-').

Table 4-3. Mesomorphic properties of bis[ 1,3-bis(4'- alkoxyphenyl)-1,3-propanedionate] complexes.

Mesomorphic Properties I . " I I f li 1 K 37 DL 169 I

K 87 DL 163 I

K 164 (Dho 141)' I

a Monotropic transition


The mesomorphic behavior of P-diketonatopalladium complexes (Table 4-3, M = Pd, n = 7 - 10, 12, 14) was more difficult to study because of their lability [9, lo]. Mesomorphism was observed when n~ 10. In spite of the similar molecular structures, the textures of these palladium complexes are different from those of the copper analogs, and the mesophases of the two series are not miscible in a contact preparation [9]. These differences were supported by X-ray diffraction, which showed that the structures of the mesophases are lamellar in both cases, but the packing is different for each kind of metal complex.

Up to this point, all the complexes reviewed in this section have been derived from aromatic P-diketone ligands containing two benzene rings and one aliphatic chain in each ring. Ohta and coworkers investigated a different approach, in which one of the rings of the P-diketone ligand is replaced by an alkyl group and an additional benzene ring is attached to the other end of the ligand (Fig. 4-3) [17]. These copper@) complexes are not expected to be discotic at first sight, as the molecules have an almost rod-like shape. Surprisingly, the complexes in which R' = methyl show a discotic mesophase, whereas those in which R' = ethyl, propyl, butyl, octyl or dodecyl show a calamitic nematic mesophase. On the basis of X-ray studies of the methyl derivatives the authors proposed that the complex molecules form dimers which stack into columns, and the columns form a rectangular two-dimensional lattice (Dro mesophase). The dimers contain four aliphatic chains around the dimeric central core and therefore fulfill the requirement to form columnar mesophases (Fig. 4-4).

0. ,0

R " B , ' R

R = CnHZn+IO n = 8-12, 16, 18 R' = CH3

n = 4,8 ,12 R'= C4H9

n = 8 R' = C B H I ~

n=12 R' = C12H25

Figure 4-3. R = C,H2,+r n = 16 R' = CH3

138 J. Barbera

Figure 4-4. Schematic illustration of the dimeric structure proposed for bis(4'-alkoxybi- phenylbutane- 1,3-dionato)copper(11) complexes in the D,, mesophase.

4.1.1.2 Complexes with two or more Aliphatic Chains at each Phenyl Ring

None of the P-diketonate complexes considered previously in this chapter, containing four benzene rings and one aliphatic chain in each ring, show a two-dimensional columnar mesophase. This phenomenon is accounted for by the empirical fact that six or more substituents are usually needed to obtain columnar mesophases. In the case of the oxovanadium complexes discussed previously the liquid crystal nature of the columnar phase has not been confirmed.

With the aim of obtaining discotic complexes capable of displaying columnar structures, a new approach was tried in which each benzene ring was substituted with two alkoxy chains in the 3- and 4-positions, thus giving a total number of eight substituents in the complex molecule (Table 4-4, M = Cu, n = or # rn) [lo, 18, 191. The copper complexes in which n = rn = 7, 8,9, 11 as well as the unsymmetric analog with n = 9 and rn = 11 are liquid crystalline. On the other hand, the octyl analog (alkyl instead of alkoxy substituents) is not mesomorphic [19]. The mesophase was identified as disordered hexagonal columnar by X-ray diffraction and by miscibility experiments with a triphenylene ester as a reference [20]. This assignment was also supported by the small clearing enthalpies: 2.0-2.6 KJ mol-'. In all cases, the mesomorphic ranges are rather narrow (< 17 "C). The melting temperatures are much higher and the clearing temperatures very much lower in these copper complexes than in the copper analogs with four alkoxy chains [8]. A change in the positions of the alkoxy groups from the 3- and 4- to the 3- and 5-positions of the benzene rings led to the loss of mesomorphism. When the central copper(I1) atom was replaced by nickel(r1) (M = Ni, n = in = 9) the liquid crystal properties were also de- stroyed [18b] (see Table 4-4).

The appearance of columnar mesomorphism in the P-diketonatocopper(r1) complexes containing eight substituents encouraged other research groups to prepare b- diketonate complexes derived from other metals.

Swager and co-workers investigated the influence of the number and the length of the side chains on the mesomorphism in oxovanadium P-diketonate complexes (Table 4-4, M = VO, n = rn = 4, 5 , 8, 10) [21]. The emphasis of their work focused on the possibility of obtaining ferroelectric discotic systems as a result of the formation of linear chain structures ( a -V=O. .V=O- - ) previously reported by the same group for other liquid crystalline oxovanadium complexes [ 151. These 8-diketonate complexes, however, were shown by IR spectroscopy to be monomeric in all phases. The octyloxy (n = rn = 8) and decyloxy (n = rn = 10) derivatives exhibit a complex meso-


Table 4-4. Mesomorphic properties of ocataalkoxy-substituted bis( 1,3-diphenyI- 1,3-propanedionate) complexes.

M n rn

cu 8 8

cu 9 9

cu 9 1 1

Ni 9 9

vo 8 8

vo 10 10

Pd 8 8

Pd 10 10

~~~ ~



morphic behavior with very limited mesophase stability. Indeed, only “frozen” mesophases could be obtained by supercooling the isotropic liquid. They were identified by X-ray diffraction as Dhd and Dhd phases. Both Dhd and Dho phases have been detected in both compounds ( n = rn = 8 or lo), but their appearance is highly dependent on the thermal history of the sample. However, the lack of fluidity of the mesophase and the high transition enthalpies observed when cooling from the isotropic into the D,, phase (16 KJ mol-’ for the octyloxy and 30 KJ mol-’ for the decyloxy derivative) indicate that they are not conventional columnar mesophases. The temperature data corresponding to the first heating-cooling cycle are shown in Table 4-4. The octyloxy derivative was also studied by Maitlis et al., but they described it as nonmesomorphic [lo]. The transition temperatures of all the oxovanadium complexes with eight side chains were considerably decreased in comparison with the oxovanadium complexes with only four peripheral chains [lo, 141. In an analog containing ten dodecyloxy chains (Fig. 4-5 a, n = 12), the presence of two additional alkoxy groups stabilized the mesophase, and thus an enantiotropic Dhd phase was observed between 24 and 122°C [21]. This example represents the widest

140 J. Barbera

Figure 4-5.

columnar mesophase reported for P-diketonatometal complexes. In contrast with the octasubstituted analogs, this complex possesses a - . V - 0 . * . V - 0 . * - linear chain structure in the solid state, as indicated by IR spectroscopy and by the yellow color characteristic of linear chain formation. IR studies also indicate the possibility of weak chain formation in the mesophase.

Oxovanadium complexes of B-diketones containing only one benzene ring and two alkoxy chains per ligand (Fig. 4-5b, R = CH3, n = 8, 10; R = CF,, n = 4, 5, 6, 10) are not mesomorphic [21].

Maitlis studied palladium(r1) complexes derived from P-diketone ligands with a total number of eight chains (Table 4-4, M = Pd, n = m = 6 , 8, 10) [lo]. These compounds are thermally stable, and exhibit decreased transition temperatures in comparison with the derivatives containing four chains described in the same paper (see Sec. 4.1.1.1). The compounds in which n = 10 form a mesophase between 93 and 97 "C which was tentatively identified as discotic nematic or columnar nematic.

Serrano and coworkers prepared a series of thalliumfr) complexes bearing a varying number of decyloxy groups in different positions (Table 4-5). They constitute a rare example of discotic metallomesogens containing a monovalent metal ion [22]. The derivative which contains one alkoxy group per benzene ring (compound 1) is not liquid crystalline. However, when the compound contains two or three alkoxy groups per ring (compounds 2 and 4) a monotropic Dhd mesophase is observed. These molecules have a half-disc shape, and therefore the columns must be formed


Table 4-5. Mesomorphic properties of alkoxy-substituted 1,3-diphenyl- 1,3-propanedionatothaIlium(1) complexes

:ompound Ar Ar‘ Mesomorphic Properties


by dimers. This suggestion is consistent with the intercolumnar distance measured by X-ray diffraction. The complex substituted in the 3,S-positions (compound 3) instead of the 3,4-positions is not mesomorphic, which is a similar trend to that observed by Giroud for the copper analogs [18b]. The dimeric nature of the thallium(1) complexes was confirmed by a single-crystal X-ray analysis carried out on a nonmesomorphic short-chain member of the series [23]. In the solid state, dimerization occurs as a result of T1-T1 bonding interaction reinforced by axial T1-0 interaction between neighboring dimers (Fig. 4-6). These axial interaction stabilize the stacking of the dimeric disc-shaped cores to give a columnar structure.

Mesomorphism has also been investigated in P-diketonate complexes derived from metals with an octahedral coordination geometry: iron(III), manganese(1rr) and chromium(Ir1) (Table 4-6) [24]. These compounds contain three P-diketone ligand units, each substituted with two, four, five or six dodecyloxy chains, thus giving complexes with a total number (N) of six, twelve, fifteen or eighteen side chains. In spite of their non-planar structure these complexes, except for that with N = 6, exhibit mesomorphism. The complexes are liquid crystalline at room temperature, and their mesophases were identified as disordered hexagonal columnar and disordered rectangular columnar. The mesophases were not miscible in contact preparations between any of these octahedral complexes and the square-planar bis( /3-diketonato)copper

142 J. Barbera

WC TI --- TI : 3.747 A

. . . I

Figure 4-6. Dimeric nature of thallium(i1) complexes. a) dimer; b) stacking of the dimers to give a columnar structure. (Adapted from reference 23).

complex derived from the same ligand. The lack of miscibility is probably caused by the very different molecular shape, which produces a different packing arrangement within the columns. The names octahedral hexagonal (0,) and octahedral rectangular (0,) were proposed by the authors for the columnar mesophases of these non-disc-like complexes.

4.1.2 Dithiolene Complexes

The first disc-like dithiolene complex described as liquid crystalline was reported independently by Veber et al. [25] and by Ohta et al. [26] (Table 4-7, R = C,H2,+,0; n = 9, 11, 12; R' = H). The two research groups initially reported that these nickel complexes showed an unidentified mesophase. Later, Ohta assigned this phase as discotic lamellar (DL) [27]. However, the mesophase-isotropic transition enthalpies are very high, approximately twice those of the crystal-mesophase transition. This


Table 4-6. Mesomorphic properties of alkoxy-substituted tris(l,3-diphenyl- 1,3-propanedionate) complexes.

0

Ar

Ar'

N M Ar Ar'

Ar

Ar'

Ar Ar' Mesomorphic Properties a

a o h : different hexagonal columnar phases. Or: rectangular columnar phase.

suggests that these phases are not true liquid crystals. Indeed, an X-ray diffraction study of the complexes, carried out subsequently by Veber et al. in collaboration with Levelut, showed unambiguously that the so-called discotic mesophases of the dithiolenenickel(I1) complexes were, in fact, crystalline phases [28].

Interest in dithiolene nickel complexes arises from their good n-acceptor properties, which could be applied in one-dimensional organic conductors. However, for such applications the disc-like molecules must stack in columns. With this aim in mind, Ohta synthesized a series of dithiolene nickel complexes susbstituted with eight peripheral chains (Table 4-7, R = R' = CnH2n+10, n = 1 - 12) [29, 301. These complexes turned out to be efficient x-acceptors, comparable to haloanils. Enantio-

144 J Barbera

Table 4-7. Mesomorphic properties of substituted bis( 1,2- diphenylethane-1,2-dithiolato)nickel(11) complexes. -

R R' Mesomorphic Properties ___

K 124 D? 166 I

K 137 (D 119)a I

K 81 Dhd 118 I

K 72 Dhd 108 I

Non-mesomorphic

a Monotropic transition to an unidentified mesophase

tropic Dhd mesomorphism was observed in all the derivatives in which n = 5 - 12. The derivatives in which n = 2-4 show a monotropic discotic, the nature of which could not be assigned due to rapid crystallization. On the other hand, the analogous nickel complexes containing eight alkyl chains instead of alkoxy chains (Table 4-7, R = R' = CnHZn+,, n = 6, 8) are not mesomorphic, but isotropic liquids at room temperature [30].

4.1.3 1,2-Dioxime Complexes

Ohta and coworkers prepared a series of discotic nickel(1r) and palladium(1r) complexes derived from di(3,4-dialkoxyphenyl)ethane- 1,2-dioxime ligands (Table 4-8; M = Ni, II = 4, 8, 12; M = Pd, n = 1 - 12) [31, 321. It was found that the three nickel derivatives and most of the palladium analogs (n = 2 - 12) exhibit disordered hexagonal columnar (Dhd) mesophases. Some compounds are mesomorphic at room temperature and show an extremely wide mesomorphic range (> 250 "C in some cases). The clearing temperatures are much higher, and the mesomorphic ranges much broader in the dioximate complexes than in the P-diketonate and dithiolene complexes (see Sec. 4.1.1 and 4.1.2). It is interesting to note that, in addition to mesomo-


Table 4-8. Mesomorphic properties of bis[ 1,2-bis(3',4'- dial koxypheny1)ethane- 1,2-dioximate] complexes.


Ni 4

Ni 8

Ni 12

Pd 4

Pd 8

Pd 12

phic properties, the dioximate nickel and palladium complexes also exhibit thermochromism and solvatochromism.

4.2 Dinuclear Metal Complexes with Bidentate Ligands

4.2.1 Carboxylate Complexes

Metal carboxylates were the first metal-containing liquid crystals, and they were described as early as 1910 when Vorlander observed that alkali metal carboxylates (soaps) formed lamellar phases in the anhydrous state [33]. Since 1959 Skoulios et al. have studied the carboxylates described by Vorlander and other alkali, alkaline earth metal and cadmium soaps [34]. X-ray diffraction revealed that these compounds exhibit a rich structural polymorphism: lamellar, ribbon-like, and cylindrical or columnar phases were observed. In addition to the thermotropic mesomorphism, the alkali metal soaps also show lyotropic behavior (see Chap. 2).

Mesomorphic dinuclear transition metal carboxylate complexes were reported in 1964 by Grant 1351. He observed that the copper(I1) alkanoates (Table 4-9, M = Cu)

146 J. Barbera

Table 4-9. Mesomorphic properties of dinuclear tetraalkanoate complexes.

M n

c u

c u

c u

c u

Rha

Rha

Rub

Rub

Rub

MoC

MoC

CrC

CrC

WC

3

7

11

19

7

11

7

11

19

7

11

7

11

7


K 195 Dr ~ 2 0 0 dec.

K 85 Dh >200 dec.

K 107 Dh >200 dec.

K 118 Dh >ZOO dec.

K 95 Dh >200 dec.

K 100 Dh >200 dec.

K 103 Dh dec.

K 99 Dh dec.

K 104 Dh dec.

K 100 Dh 147 I

K 111 I

K 99 Dh >300 dec.

K 98 Dh >320 dec.

K 90 I

I Complex containing a metal-metal single bond. Complex containing a metal-metal double bond.

CComplex containing a metal-metal quadruple bond.

are liquid crystalline, but their mesophases were not characterized. It was not until 1984 that this family of compounds were studied thoroughly by Giroud and Mar- chon, and the structure of their mesophases elucidated by Guillon and Skoulios by means of X-ray diffraction [16, 36, 371. Copper(I1) complexes of carboxylic acids were known to exist in the solid state as dimeric molecules in which two copper atoms are held in close proximity to each other by four carboxylate groups, leading


to a core with a “lantern” shape [38]. The linear chain copper(I1) alkanoates investigated (Table 4-9, M = Cu, n = 2-23) show mesomorphism when n 2 3 . The X-ray patterns of the mesophase are characteristic of a hexagonal columnar mesophase with liquid-like disorder within the columns. Thus, this series provided the first example of a mesogen with only four peripheral chains to show a thermotropic hexagonal columnar mesophase. The clearing transition occurs above 200°C and is difficult to detect due to thermal decomposition of the samples at those temperatures.

The linear chain complex in which n = 3 (copper butyrate) is the shortest member of the series to show mesomorphism. However, the X-ray patterns indicate that the columnar mesophase has a rectangular symmetry (D, phase). The difference in behavior with respect to the the other members of the series might be due to the short length of the alkyl chains, which are unable to fill the intercolumnar space as effectively as in the case of longer chains. Indeed studies of the dynamic behavior of several members of the series by quasielastic neutron scattering experiments showed that in the mesophase the aliphatic chains are completely disordered except for the first four carbon atoms [39]. Therefore it appears that this part of the carbon skeleton cannot melt into a liquid-like disordered state.

The structural model proposed was determined more accurately by EXAFS spectroscopy [40]. These studies revealed that the molecules have a dinuclear core which stacks into rows of dicopper tetracarboxylate units surrounded by the aliphatic chains (see Fig. 4-7). The copper atoms exhibit intradimer square-planar coordination with four oxygen atoms and interdimer axial ligation with an oxygen atom of the neighboring molecule.

Subsequently, Giroud and coworkers synthesized and studied the mesomorphic behavior of a series of analogous dirhodium(r1) [41] and diruthenium(I1) [42,43] tetracarboxylates (Table 4-9; M = Rh, n = 7, 11; M = Ru, n = 3 - 11, 13, 15, 17, 19). The X-ray patterns recorded in the mesophase show reflections characteristic of a two-dimensional hexagonal array of columns, similar to that found in the hexagonal columnar mesophase of the isostructural copper alkanoates [16, 36, 371. Thus, for the three series of linear-chain dinuclear alkanoate complexes investigated, the same type of mesophase was observed irrespective of the nature of the metal.

EXAFS spectroscopy on rhodium heptanoate ( n = 6) showed that the structure of the polar columnar core in the mesomorphic state is very similar to that described for their copper analogs: the columns are formed by stacked dirhodium tetracarboxylate units bonded by intermolecular Rh. * -0 apical ligation (see Fig. 4-7) [44]. The existence of a covalent bond between the two rhodium atoms in each individual complex was clearly confirmed by the value of the intramolecular metal-metal distance (2.34 A), which is considerably shorter than the intramolecular distance between the nonbonded metal atoms in the copper carboxylates (2.62 A). The rhodium alkanoates are, therefore, the first reported examples of metal-metal bonded columnar mesogens.

The investigation of linear-chain tetraalkanoates described above was extended to other copper and ruthenium carboxylates containing branched and unsaturated chains (Table 4-10) [37, 43, 45-47]. Branching or incorporation of double bonds

148 J. Barbera'

0 Metal atoms

0 Oxygenatoms

I Figure 4-7. Arrangement of alkanoate complexes within the columns in the hexagonal columnar mesophase.

into the side chains depresses the melting temperatures compared with those of the saturated straight chain complexes. Most of the compounds containing modified chains are mesomorphic and show a hexagonal columnar mesophase, except for the complex containing triple bonds [C12H2,C=C-C=C(CH2)8C02]4C~2 [46], which displays a lamellar mesophase. From X-ray diffraction measurements, it was concluded that the mesophase of this compound has a biaxial lamellar structure denoted DL. It was shown that this complex could be processed in the mesomorphic state into highly ordered fibers of good optical quality, which maintained the same structure as the unprocessed phase. Samples of the complex were polymerized at room temperature by UV radiation with no detectable disruption in the crystal ultra- structure.

A branched-chain complex, [(C8H,7)2CHC02]4C~2, was complexed with the bidentate ligands pyrazine (see Fig. 4-8) and 4,4'-dipyridyl [45]. The adducts exhibit mesomorphism at room temperature, in contrast to similar complexes of straight chain copper [45] and ruthenium [43] alkanoates, where the liquid crystal behavior was lost upon complexation. The mesophase of the adducts is not hexagonal; however, its structure could not be clearly elucidated by X-ray diffraction.


Table 4-10. Mesomorphic properties of dinuclear branched and unsaturated tetracarboxylate complexes.

M R

CBHl7\ c u ,CH-

CBH17


a Complex containing a metal-metal double bond.

Figure 4-8. Probable structure of the pyrazine ad- duct of a branched tetraalkanoato dicopper(r1) complex (R = (C,H,,),CH). n

150 J. Barbera

Giroud et al. subsequently synthesized dinuclear mixed-valence ruthenium(r1)- ruthenium(1rr) carboxylates with the general formula Ru2(C,H2,+ 1C02)4X (X = C1, C,H2,+,COz, n = 8, 11) [48]. The two pentacarboxylate derivatives (X = CnH2,+ 1C02) exhibit columnar mesomorphism. By X-ray diffraction the phase structure was determined to be hexagonal for the laurate complex (n = 11) and rectangular (slightly distorted pseudo-hexagonal) for the pelargonate derivative (n = 8). The intercolumnar distances are only slightly larger than in the Ru(II)- RU(II) tetracarboxylates, and this suggests that the fifth chain is located inside the column, thus not significantly affecting the intercolumnar spacing and presumably leading to an increase of the stacking periodicity.

Chisholm and co-workers prepared the first examples of liquid crystalline compounds containing metal-metal quadruple bonds (Table 4-9, M = Mo, n = 3 - 11, 17) [49, 501. These molybdenum(I1) alkanoates exhibit disordered hexagonal columnar mesomorphism when n = 4 to 10. In contrast with the copper, rhodium and ruthenium analogs, the molybdenum carboxylates do not decompose before clearing, thus allowing the mesophase-isotropic transition temperatures and enthalpies to be determined. The clearing enthalpies ranged from 2.5 to 0.8 KJ mol-’, which is consistent with a Dhd-I transition. Similar quadruple-bonded chromium(1r) and tungsten@) complexes were studied in order to make a comparison of the mesomorphic properties (Table 4-9; M = Cr, n = 7, 11, 17; M = W, n = 7). The tungsten(I1)-containing compounds are not liquid crystalline. The related chromium(rr) compounds, on the other hand, show mesomorphic properties. Several branched-chain, perfluorinated-chain and aromatic molybdenum(r1) and chromium(l1) carboxylates were also investigated, and some of them are liquid crystalline [50]. Branching in the chains depresses the melting points, whereas the presence of perfluoroalkyl chains and p-alkyl or p-alkoxy substituted aromatic rings increases them.

A series of dirhodium(1r) tetrabenzoates substituted with a varying number of alkoxy chains were prepared by Serrano and co-workers (Table 4-1 1, M = Rh) [51]. These molecules contain four aromatic rings attached to the dinuclear carboxylate unit, leading to a larger central core. These complexes were shown to display columnar mesophases, the type of which depends on the number of peripheral chains. The tetraalkoxy derivatives (R = R’ = H, n = 10- 14) show a columnar mesophase with rectangular symmetry and regular intracolumnar stacking. X-ray diffraction experiments revealed that the aliphatic chains adopt a preferred orientation along one axis instead of surrounding the central core. In contrast, a smectic mesophase has been described in an analogous copper complex (Table 4-11, M = Cu) [44]. The octasubstituted complex (R = C10H210, R’ = H) shows a viscous phase at room temperature, assigned as rectangular columnar, and a hexagonal columnar mesophase at higher temperature. Finally, the derivative containing twelve decyloxy chains (R = R’ = C,oH210) exhibits a crystalline phase at room temperature and a hexagonal columnar mesophase at higher temperatures. The results of these studies were rationalized as follows: The larger size of the rigid core in the benzoate derivatives in comparison with that in alkanoates precludes the formation of the hexagonal mesophase when only four paraffinic chains are present (R = R’ = H). The small

4 Low Molecular Weight Discotic Metallomesogens 15 1

Table 4-11. Mesomorphic properties of dinuclear alkoxy-substituted benzoate complexes.

1 0

R

7H2n+1

R' R' R'

M n R R'

2ua 8 H H

Rh 8 H H

Rh 10 H H

Rh 14 H H

Rh 10 C1oHzi0- H

Rh 10 C1oHziO- CioHzi0-


K 175 S (dec.)

K 231 I

K 156 Dro 203 I

K 94 Dro 189 I

Dr 138 Dhd 235 I

K 11 5 Dhd 202 I

a No Cu-Cu bond.

number of chains is not enough to fill space around the larger core efficiently, and therefore the chains tend to align in a preferred direction, leading to a rectangular mesophase (Fig. 4-9a). On the other hand, in compounds with the same large core, the hexagonal mesophase is stabilized in the presence of twelve chains, which have sufficient volume to effectively fill space in a hexagonal lattice (R = R' = CloH210) (Fig. 4-9 b). An intermediate situation occurs in complexes containing eight chains (R = C,oH2,0, R' = H). This complex adopts a rectangular structure at low temperatures (Fig. 4-9 c). At higher temperatures, conformational disorder increases, allowing space-filling arrangements of the lateral alkyl chains around the core and thus resulting in a transition from a rectangular to a hexagonal phase (Fig. 4-9d). It was concluded that the main factor govering the symmetry of columnar mesophases is the ratio of the volume occupied by the rigid core to the volume filled by the flexible peripheral substituents of the molecules.

152 J. Barbera

................... xz %&#= ...................

(b) (d)

Figure 4-9. Schematic representation of the arrangement of the central cores and the peripheral chains in alkoxy-substituted tetrabenzoatodirhodium(I1) complexes: a) complexes with four peripheral chains in the D,, mesophase; b) complex with twelve peripheral chains in the D,, mesophase; c) complex with eight peripheral chains in the D, mesophase; d) complex with eight peripheral chains in the D,, mesophase.

4.2.2 Dithiocarboxylate Complexes

In 1988, Ohta and coworkers reported that alkanedithiolate nickel(I1) complexes containing linear alkyl chains, (Table 4-1 2, M = Ni, R = CnH2,,+ 1, n = 5 - 12) exhibit monotropic mesophase [52]. The derivatives in which n = 2-4 are not liquid crystalline. Optical microscopy, X-ray diffraction and IR spectroscopy showed that the mesomorphic phase is lamellar. However, the question as to whether this phase is discotic lamellar or classical smectic could not be answered because the complexes decompose when heated into the mesophase.

An analogous branched-chain complex was reported to exhibit an enantiotropic hexagonal columnar mesophase in addition to a monotropic discotic lamellar (DL) phase (Table 4-12, M = Ni, R = C4H9(C2H,)CH) [53]. However, the high viscosity of the columnar phase and the clearing enthalpy (39.8 KJ mol-'), too large for a Dhd-I transition, raise some doubts about whether it is a true liquid crystal or a crystalline phase.

Mesomorphic behavior was not observed by Chisholm and coworkers in molybdenum(I1) alkyldithiocarboxylates (Table 4-12, M = Mo, R = CnH2n+l, n = 3-7) [50].


Mo C7H15-

Table 4-12. Mesomorphic properties of dinuclear tetrakis(al kanedithiocarboxylate) complexes.

K 180 I


K 82 (Ma 55)b I

K 91 (Ma 68)b I

The absence of mesomorphism in these compounds, despite them having a molecular structure similar to the carboxylates, was interpreted as a result of weaker axial donor interaction in the case of sulfur as opposed to oxygen.

4.2.3 Benzalimine Complexes

The first well-characterized organometaliic (i.e. containing carbon-metal bonds), disc-like liquid crystals were described in 1992 by Praefcke and coworkers [54]. These authors found mesomorphism for two series of ortho-palladated benzalimine dinuclear and tetranuclear complexes. One of these series is derived from a bidentate monoimine ligand and contains two palladium atoms and eight flexible side chains (Table 4-13, M = Pd, X = OAc, C1, Br, I, SCN). The other series is derived from a tetradentate bisimine ligand and will be dealt with in Sec. 4.5.2.

The halogeno- and thiocyanato-bridged dinuclear complexes (Table 4-1 3, X = C1, Br, I, SCN) exhibit a monotropic discotic nematic (N,) mesophase, whereas the nonplanar acetato-bridged member (X = OAc) is not liquid crystalline. Mesomor- phism does not occur in similar nonplanar acetato-bridged ortho-palladated imine complexes having calamitic structures [55]. In this work, the number of peripheral chains once again appears to be crucial in determining the type of mesomorphism observed: the ortho-palladated imine complexes containing two chains per ligand exhibit smectic mesophases (see Chap. 3, Fig. 3-45) [55 a]. On the other hand, the pres-

154 J . Barbera

Pd SCN

Pt CI

Table 4-13. Mesomorphic properties of dinuclear substituted bis[N- (4'-alkylphenyl)-2,3,4-trialkoxybenzaIirnine] complexes.

K 96 (No 50)b I

K 92 (ND 54)b I

ic Properties

Charge transfer complexesa

No mesophase

Dho

Dho

Dho? ND

ND

Dho

a Results based on contact preparations and on mixtures with TNF. b Monotropic transition.

ence of two additional chains in the ligand produces a more disc-like molecular shape, thus leading to the formation of the discotic nematic mesophase.

These dinuclear palladium organyls form charge transfer complexes on doping with strong electron acceptors such as 2,4,7-trinitrofluorenone (TNF) [54, 561. The induced mesophases are ordered hexagonal columnar (Dho) when X = chloride and bromide, nematic (ND) when X = thiocyanate and both types when X = iodide (see Table 4-1 3). The structures of the mesophases were confirmed by X-ray diffraction, and this seems to be the first report of stabilization of an ND phase by charge transfer complexation. For the columnar mesophase an intercalation model was proposed in which the columnar structure is induced by charge transfer interaction between disc-shaped donor molecules and strong electron acceptors.

An analogous platinurn(I1) complex (Table 4-13, M = Pt, X = C1) [57] shows a monotropic mesophase which was identified as discotic nematic, based on miscibility


studies with the analogous palladium complex. Induction of a mesophase, tentatively identified as Dho, was observed in mixtures of this platinum mesogen with TNF.

4.3 Metal Complexes with Tridentate Ligands

4.3.1 1,4,7-Triazacyclononane Complexes

Although several types of disc-like metallomesogen derived from nitrogen donor macrocycles have been described, most of the promesogenic ligands used are tetradentate (see Sec. 4.4.1 -4.4.5). In 1992, Lattermann and coworkers reported a new type of metallomesogen derived from the tridentate ligand 1,4,7-triazacyclononane (Table 4-14) [58, 591. The four compounds investigated display a disordered rectangular columnar mesophase. UV, NMR and IR spectroscopy and mass spectrometry showed the monofacial nature of these complexes due to the small diameter of the 1,4,7-triazacyclonane ligand. The molecular core has a pyramidal or conical shape and, in the cases of the chromium, molybdenum and tungsten derivatives, it was sug-

Table 4-14. Mesomorphic properties of 1,4,7-tris(3',4'-di- decyloxybenzoy1)- 1,4,7-triazacyclononane complexes.

I

M I Mesomorphic Properties I

156 J. Barbera

gested that the carbonyl ligands create a dipole along the threefold molecular axis (Fig. 4-10). Thus, stacking of these molecules should give rise to a permanent polarization, which could give rise to ferroelectric or antiferroelectric behavior.

4.3.2 Pyridinediyl-2,6-Dimethanol Complexes

(Pyridinediyl-2,6-dimethanolato)dioxomolybdenum complexes were synthesized by Serrette and Swager with the aim of obtaining polymeric assemblies capable of forming columnar suprastructures (Table 4-1 5, n = 8, 10, 12, 14, 16) [60]. The compounds in which n = 10- 16 exhibit disordered hexagonal columnar mesophases. IR spectroscopy showed that the Mooz units form a polymeric (. *Mo=O- * .Mo=O* -), structure in the mesophase and in the crystal, while the molecules exist as monomers in the isotropic liquid. From X-ray and IR data a columnar structure was proposed

Table 4-15. Mesomorphic properties of [4-(3',4',5'-trialk- oxybenzyloxy)pyridinediyl-2,6-dimethanolato] dioxomolybdenum(v1) complexes.

I Mesomorphic Properties r n 0 K 135 I

10 K 106 Dhd 131 I I 16 K 96 DM 141 I


Figure 4-11. Probable arrangement of the polar assembly of [4-(trialkoxybenzy1oxy)pyridinediyl-2,6-dimethanolato]dioxomolybdenum(v1) complexes in the hexagonal columnar mesophase.

for this kind of non-disc-like “tapered” mesogen in which the molybdenum head groups are situated near the center of the column. The ( a .M=O- * *M=O. a),, chain aligns along the column axis, and the molecules extend radially from this axis (see Fig. 4-1 1).

4.3.3 1,3,5Triketonate Complexes

Swager and coworkers synthesized two series of dinuclear copper(r1) complexes derived from tridentate 1,3,5-triketone ligands (Tables 4-16, 4-17, n = 5 - 8 , 10, 12, 14, 16) [61, 621. Both types of complex show columnar mesophases which are stable over a wide temperature range, although the second series of complexes (Table 4-17) is not disc-shaped.

Complexes of the first series, carrying twelve alkoxy chains (Table 4-16, X = Y = C,H,,+fO), form disordered tilted rectangular columnar (Drd) mesophases when n = 5 - 10 and disordered hexagonal (Dhd) mesophases when n = 12 - 16. The mesophase exhibited by the second series (Table 4-17) was identifed as hexagonal columnar with liquid-like (i.e. short-range) order in the columns (Dhd). X-ray diffraction supports the existence of a columnar suprastructure (Fig. 4-12a) in these non- disc-like complexes which is similar to that of the complexes depicted in Table 4-16 (Fig. 4-12b). The appearance of mesomorphism in the two series of complexes is

Table 4-16. Mesomorphic properties of alkoxy-substituted bis(l,5- diphenyl- 1,3,5-pentanetrionato)dicopper( 11) complexes.

X

n X Y Mesomorphic Properties

K 68 D d 181 I

K 73 Dhd 140 1

K 120.5 Dhd 192 I

K 151.5 Dhd 231 I

Table 4-17. Mesomorphic properties of alkoxy-substituted bis( 1-phenyl- 1,3,S-hexanetrionato)dicopper(n) complexes.

n X Y Mesomorphic Properties

10 C10H210- CH3 K 102 Dhd 220 I

16 Ci6H=O- CH3 K 91.5 DM 178 I

10 C10H210- CF3 K 156 Dm 221.5 I

10 H CH3 K 159 I


(b) Figure 4-12. Schematic representation of the disc-shaped structures formed by alkoxy-substituted triketonate copper(I1) complexes in the D,, mesophase: a) in (l-phenyl,l,3,5-hexanetrionato)dicopper(rI) complexes (Table 4-1 7) a 180 O rotation of nearest neighbors produces a disc shape; b) (1,5-diphenyl-l,3,5-pentanetrionato)dicopper(11) complexes (Table 4-16) approximate a disc shape in unimolecular form.

very sensitive to substitution. In 3,4-dialkoxyphenyl-substituted derivatives (X = H) the mesophase type changes, or mesomorphism is less pronounced compared with the 3,4,5-trialkoxyphenyl derivatives (X = C,H,,+,O) of the same chain length.

160 J Barbera

4.4 Mononuclear Metal Complexes with Tetradentate Ligands

4.4.1 Phthalocyanine Complexes

Phthalocyanine-metal complexes constitute the most widely studied subject in the field of disc-like metallomesogens. Although lyotropic mesomorphism had been reported in 1979 in aqueous solutions of the sodium salt of phthalocyanine- 4,4’4”,4”’-tetracarboxylic acid [63], the first thermotropic compound of this class was reported by Simon and co-workers in 1982 [64]. The interest of these authors in this kind of material arose from its potential as a one-dimensional conductor, in which the conducting chains would be formed by the columns of the discotic mesophase. Moreover, phthalocyanines are able to form stable complexes with a wide variety of metal ions.

Metal-containing and metal-free phthalocyanine derivatives, substituted with eight dodecyloxymethyl side chains, denoted as (C,20CH2)8PcM (Table 4-18, M = H,, Cu, Zn, Mn, n = 12) [64, 651, exhibit exceptionally wide mesophases. The small-angle X-ray diffraction patterns indicate a two-dimensional hexagonal structure with an almost constant lattice parameter (intercolumar distance) of 31 A regardless of the central atom.

Lead(I1) and tin(I1) ions cannot be accommodated by the phthalocyanine cavity and form out-of-plane complexes [66]. Simon et al. showed that alkoxymethyl substituted phthalocyaninato lead (11) complexes (Table 4-18, M = Pb, n = 8, 12, IS) form a hexagonal columnar mesophase, which is stable at room temperature when n = 8 and 12 [67]. Thus, the presence of the lead ion dramatically lowers the phase transition temperatures of liquid crystalline phthalocyanines. X-ray diffraction revealed an intercolumar reflection corresponding to a distance of 31 ..k for the compound with n = 12, which is in perfect agreement with the intercolumnar spacing found for the other phthalocyanine complexes containing the same side chain [65]. In addition, a broad peak observed at 7.4 A in the diffractogram of the lead complex was interpreted as due to the presence of pairs of molecules coupled in an antiferroelectric way. The analogous tin@) complex is not stable; in the presence of air it is rapidly transformed into the dihydroxotin(1v) compound (Table 4-18, M = Sn(OH)2, n = 12) [67]. This oxidized compound was obtained in a pure state from the tin@) complex by reaction with hydrogen peroxide [68]. It displays a rectangular columnar mesophase as well as another phase at higher temperature which is probably associated with the loss of water. Finally, in the isotropic liquid polymerization takes place which affords a polymeric liquid crystalline material. When comparing the transition temperatures of the different phthalocyanine metal complexes, the order Pb < Mn < Cu < Sn(OH)2 < Zn ZE H2 is observed for the melting and Sn(OH)* < Pb < H2 < Mn < Cu = Zn for the clearing points.

Unsubstituted bis(phthalocyaninato)lutetium(iv) was one of the first known examples of an intrinsic molecular semiconductor [69]. With the aim of producing col-


Table 4-18. Mesomorphic properties of 2,3,9,10,16,17,23,24- octakis(dodecyloxymethy1)phthalocyanines.

c u 12

Zn 12

12 I Mn


umnar liquid crystals with one-dimensional semiconducting properties, Simon et al. synthesized this kind of sandwich complex carrying long alkyl chains [70]. They prepared three alkoxymethyl-substituted lutetium derivatives, denoted as [(C,0CH2)8Pc]2Lu, (Table 4-19, R = C,H2,+10CH2, n = 8, 12, 18), and the corresponding oxidized compounds [(C,0CH2)8Pc]2Lu + SbCl, . AH of these complexes, apart from the neutral complex in which n = 8, are mesomorphic at relatively low temperatures. X-ray experiments indicated a disordered hexagonal columnar structure with an average intracolumnar stacking distance of 7.3 A , corresponding to twice the thickness of the phthalocyanine unit.

An alkoxy substituted derivative, denoted as [(C,20)8Pc]2Lu, (Table 4-1 9, R = C12H250) and its oxidized analog [(C120)8P~]2L~+BF; were also prepared [71]. In the mesophase, these systems exhibit the characteristic diffraction peaks of an ordered hexagonal columnar structure. In the same paper, a similar result was described for a lithium analog (C120)8PcLi which was also studied for its potential conducting properties. For these dodecyloxy-substituted compounds the intercolum-

162 J . Barbera

Table 4-19. Mesomorphic properties of octasubstituted bis- (phthalocyaninato)lutetium(lv) and-lutetium(v) complexes.

nar distance is 34.6-34.9 A. Since a value of around 31 A had been found for (C,20CH2)8PcH2 and its metal complexes [65], it was concluded that when the paraffinic side chain is linked to the core by an oxygen atom (alkoxy chain) two effects are observed: (i) ordered columnar mesophases are obtained; (ii) the intercolumnar distance is significantly larger than in the alkoxymethyl derivatives.

A similar result was found later for a phthalocyaninatoplatinum(i1) complex substituted with eight dodecyloxy groups (Table 4-20, M=Pt, R=C1,HZ5) [72]. This compound forms a hexagonal columnar mesophase with an intracolumnar period of 3.29 A and a hexagonal lattice constant of 34.3 A . The value of 3.29 A for the intracolumnar periodicity was later explained by other authors on the basis of EXAFS studies, assuming a oxidation state higher than + 2 for the platinum ion [73]. The unusually short Pt. * *Pt distance was attributed to attractive overlap of d,2 platinum orbitals.

Two independent research groups prepared a series of alkoxy-substituted copper phthalocyanine complexes and their corresponding dihydrogen derivatives (Ta- ble4-20, M=Cu, H,; R=CnH2n+I, n = 6- 12, 18) [74, 751. The results obtained are in reasonable agreement with the results found by Simon and co-workers for other alkoxy-substituted phthalocyanines, and all the compounds in this series display an ordered hexagonal columnar (Dho) mesophase. The distance between columns measured for the compound in whith n = 12 is 35.6 A, which is in fair agreement with those found for dodecyloxy-substituted phthalocyanine complexes of other metals


Table 4-20. Mesomorphic properties of 2,3,9,10,16,17,23,24-octaalkoxyphthalocyanines.


K 94 Dho >345 I

K 83 D b 309 I

K 170 Dtet 223 ND 270 I

K 112 D b >345 I

K 95 D b >345 I

K 204 Dteta 242 ND 290 I

K 77 Dho >350 dec.

K <-lo0 D d 205 I C4H9\

,CH-CHZ- I Pt C2H5

a Distorted square columnar mesophase.

[71,72] (see Table 4-21). These data further confirm the crucial role played by the linkage between the side-chains and the disc-like core. From X-ray studies and MM2P calculations (Allinger Molecular Mechanics Program) [74 a], the authors concluded that the ether linkage must be essentially coplanar with the aromatic ring, whereas the alkoxymethyl chain stands at an angle of 35" with respect to the aromatic plane. This difference accounts not only for the smaller intercolumnar distance measured, but also for the disorder within the columns of the alkoxymethyl derivatives.

A number of dihydrogen and copper homologs, unsymmetrically substituted with different alkoxy groups, were found to be mesomorphic, but their liquid crystal

164 J. Barbera

Table 4-21. X-ray diffraction data for the hexagonal columnar mesophases of 2,3,9,10,16,17,23,24-octakis(dodecyloxymethyl)- and octakis- (dodecy1oxy)phthalocyanines.

~~

Compound Phase Lattice Compound Phase Lattice

phases could not be identified [75]. In contrast, the compounds containing branched alkoxy groups (Table 4-20, M=H2, Cu, Pt, Pb; R=C4H,(C2H5)CHCH2) [75,76] show a square columnar (Dtet) mesophase except for the platinum complex, for which an oblique columnar mesophase structure (Doh) was proposed. A slight deformation of the tetragonal lattice was found for the copper complex which is probably due to a tilting of the macrocyclic plane with respect to the columnar axis.

In order to further understand the effect of the side-chains on the mesomorphic properties, Simon and co-workers prepared a series of phthalocyanine derivatives bearing alkyl substituents (Table 4-22, M=H2, Cu, Ni; R=CnH2n+,, n = 5, 6, 8, 10) [77]. Liquid crystalline phases were found for all the compounds. For the copper and nickel complexes, the transition temperatures were higher than those of the metal- free phthalocyanines. The structure determined for the mesophase by X-ray diffraction corresponds to a hexagonal packing of columns, the only exception being the mesophase of the copper complex with n = 5 , which has a rectangular structure (Drd). Branching of the chains close to the core supresses mesomorphism; thus the metal-free phthalocyanine and the nickel complexes with R=C4H9(C2H5)CHCH2 do not display liquid crystalline properties.

The analogous alkyl-substituted lutetium(1v) complexes (Table 4-19, R=C,H2,+ , n = 8, 12, 18) show a narrow Dhd phase (< 13 "C) [78]. Additionally, at lower temperatures a disordered oblique columnar (Dob,J phase is observed for the compound with n = 12 and a lamellar (DL) phase for n = 18. The transition from the low-temperature to the high-temperature mesophases takes place via the isotropic liquid.

The effect of the lateral substituent on mesomorphism was also investigated by Cho and Lim, who compared the mesomorphic behavior of phthalocyanines con-


Table 4-22. Mesomorphic properties of 2,3,9,10,16,17,23,24-octaalkylphthalocyanines.

M R

C4H9\ HP ,CH-CH,-

C2H5

C4%\ Ni ,CH-CH,-

C2H5


K 250 DH 323 I

K 124 Dhd 186 I

K 267 I

K 260 Dd 342 I

K 81 DM 180 I

K 190 DH 373 I

K 293 I

taining different types of peripheral substituent (Table 4-23, M=H2, Cu, R= C12H25SCH2, C I ~ H ~ ~ S ( C H ~ ) ~ O C H ~ , C12H250, C,2H250CH(CH3)CH,OCH2) [79]. All the derivatives are liquid crystalline, their mesophases being tentatively identified by their optical textures as Drd for the dodecylthiomethyl derivatives (M=H2, Cu, R=C12H25SCH2), and Dho for the 4-dodecylthio-2-oxabutyl and dodecyloxy derivatives (M=H,, Cu, R=C12H25S(CH2)20CH2, C12H25O). Branch- ing in the chain causes a decrease in the melting point, thus resulting in liquid crystallinity at room temperature. For the metal-free compound containing chiral branched chains (M = H2, R= C, 2H250C* H(CH3)CH20CH2), a texture characteristic of a cholesteric mesophase was observed, from which it was concluded that this compund is the first example of a pure cholesteric discogen based on phthalocyanine.

166 J Barbera

Table 4-23. Mesomorphic properties of 2,3,9,10,16,17,23,24-0~- tasubstituted phthalocyaninatocopper(I1) complexes.

R R

a Unidentified mesophase


However, the mesophase of the copper analog was not identified. The transition temperatures are higher in the copper complexes than in the free ligands. In Table 4-23, the phase transitions of an alkoxymethyl- and an alkyl-substituted analog have also been included for comparison.

Simon et al. reported mesomorphic behavior for a phthalocyanine copper complex substituted with crown ether groups and for the corresponding metal-free derivative (Fig.4-13, M=Cu, H2) [80]. The copper complex was obtained from its synthesis in a state which can be considered either as a highly disordered solid or a highly ordered liquid crystal with a two-dimensional square lattice (a = b = 20.8 A). The molecules stack with the crown-ether moieties in a staggered conformation (Fig, 4-14), forming ion channels with an intercrown distance of 8.2 A, which corresponds to the distance between equally oriented planes. Similar results were found for the metal-free compound. The existence of a square columnar structure in these compounds stands in contrast to the hexagonal structure found for


Figure 4-13. Crown-ether substituted phthalocyanine copper complex.

b

Figure 4-14. Schematic illustration of the I packing of phthalocyanines substituted with crown ether groups in the D,,, mesophase. Succesive planes are staggered and the periodicity along the column axis becomes 8.2 A.

a

most of the phthalocyanine derivatives substituted with hydrocarbon chains. This was explained by assuming that in the latter case, the paraffinic chains form a quasi- liquid medium around the central macrocycle; therefore the columns are almost perfectly cylindrical and allow hexagonal packing. On the other hand, the crown ether moieties cannot fill the voids in the same way, and a square column packing symmetry is favored.

Other metals (or semimetals) which have been complexed with mesogenic phthalocyanine ligands are cobalt and germanium. Hanack and co-workers prepared phthalocyaninatocobalt(II), -cobalt(IIr), -nickel(rI) and -lead@) derivatives substituted with eight octyloxymethyl groups [81]: (C80CH2)8PcCo, Na[(CxOCH2)xPcCo(CN)2], (C80CH2)8P~Ni, and (C80CH2)8PcPb. Upon heating, all of these compounds transform from the solid state into a highly viscous birefrin-

168 J. Barbera'

gent phase, the structure of which was not determined. The ionic cobalt(rr1) complex was polycondensed to give a mesomorphic polymer.

A germanium(1v) phthalocyanine derivative substituted with eight ester-type peripheral chains, (C,202C)8PcGe(OH)2, was synthesized by Dulog and Gittinger [82]. This compound displays a disordered hexagonal columnar (Dhd) mesophase between -20 and 171 "C. At this temperature, polymerization occurs to yield the corresponding poly(germoxane). Disorder in the columns was expected in the monomer because of the presence of hydroxyi groups standing perpendicular with respect to the molecular plane.

Cook and coworkers used a different approach to obtain mesomorphic phthalocyanine complexes by introducing the paraffinic chains into the nonperipheral 1, 4, 8, 11, 15, 18, 22 and 25-positions (Table 4-24; M = H2, Cu, Zn, Ni; R = CnHZn+, , n = 4- 10) [83, 841. All the compounds with n s 6 for M = H2, Cu, Ni and with

Table 4-24. Mesomorphic properties of 1,4,8,11,15,18,22,25-octasubstituted phthalocyanines.

M R

a Monotropic transition b Unidentified mesophase


n 2 5 for M = Zn show a hexagonal columnar (Dhd) mesophase. Several members of the series are polymorphic and exhibit one or two additional mesophases with hexagonal (Dhd) or rectangular (Drd) symmetry at lower temperatures. The transition temperatures vary in the order Ni G H2<Cu<Zn for the clearing points and Ni<H,<Cu<Zn for the melting points. It is noticeable that the analogous phthalocyanines substituted with alkoxy instead of alkyl groups, (Table 4-24, M = H2, R = C,H2,+10) are not mesomorphic [83a]. However the analogs bearing alkoxymethyl groups (Table 4-24, M = HZ, Cu, Zn, R = C7H150CH2) do show liquid crystalline behavior [85].

Later, Cook and coworkers investigated a number of metal-free and copper-containing phthalocyanine dimers and trimers functionalized in the nonperipheral 1, 4, 8, 11, 15, 18, 22, 25-positions with alkyl substitutents and oxalate-derived spacers (Tables 4-25, 4-26) [86]. All the compounds in the two series are mesomorphic at room temperature. In addition to a Dhd mesophase, the dimers and the metal-free trimer exhibit a rectangular columnar phase at lower temperatures. Compared with the monomeric phthalocyanines bearing alkyl substituents in the same positions [83 b] (see Table 4-24), the dimers and trimers have lower melting and higher clearing points. Thus, by linking the phthalocyanine macrocycles, stabilization of the mesophase was achieved, and the phase width was substantially increased. It is interesting to note that the hexagonal lattice parameter measured by X-ray diffraction was practically constant (a = 23 A ) for the monomer, dimer and the trimer containing octyl groups. This means that the building blocks of the columns are single macrocyclic units in all cases.

Table 4-25. Mesomorphic properties of 1,4,8,11 , I 5,18,22,25-octasubstituted phthalocyanine dimers.

0

-C-

Mesomorphic Properties , Dr 135 Dhd 180 1

cu Dr 161 Dhd 254 I

170 J Barbera'

Table 4-26. Mesomorphic properties of 1,4,8,11,15,18,22,25-octasubstituted phthalocyanine trimers.

M Mesomorphic Properties

H P

cu Dhd 252 I

Dr ~ 8 0 DM 177 I

Ohta, in collaboration with Simon, studied the effect of appending the phthalocyanine macrocycle with eight p-alkoxyphenyl groups (Table 4-27, M = H,, Cu, n = 8, 10, 12, 18) [87]. The steric hindrance caused by these bulky groups precludes a planar conformation of the peripheral benzene rings. Hexagonal columnar (Dhd) mesomorphism was observed for the compound in which M = H2 and rectangular columnar (Drd) mesomorphism for the derivative with M = Cu. In the Drd mesophase of the copper complexes, the molecules are tilted by an angle of 35.7 O with respect to the column axis, and this tilt leads to a shift of the discs by about 2.5 A. This phenomenon has been attributed to the tendency of the copper center to have axially coordinated ligands.

The sandwich-type bis(phthalocyaninato)lutetium(Iv) complex bearing p-octade- cyloxyphenyl groups displays a tetragonal columnar mesophase at room temperature and a hexagonal columnar (Dhd) phase at higher temperatures (Table 4-27, M = Lu, n = 18) [88]. The tendency to form a two-dimensional tetragonal arrangement of columns has also been observed for phthalocyanines and tetrapyrazinopor- phyrazines containing sterically hindered crown ether and branched-chain substituents [75, 76, 80, 891. The Dtet,d to Dhd phase transition in the lutetium(1v) complex was explained as follows. In the square lattice of the Dtet,d phase there is no rotation of the phthalocyanine core. When this mesophase is heated, the thermal mobility of


Table 4-27. Mesomorphic properties of 2,3,9,10,16,17,23,24-octakis(4’-alkoxyphenyl)phthalocyanines.

a Sandwich-type bis(phthalocyaninato)lutetium(IV) complex (see Table 4-19)

the alkyl chains increases and this causes the packing of the columns to loosen; the phthalocyanine units begin to rotate, resulting in the formation of a hexagonal lattice.

4.4.2 Porphyrin Complexes

The first mesogenic metalloporphyrins were described in 1987 by Gregg et al. (Table 4-28; M = HZ, Zn; R = C,H2,+,02C, n = 4, 6, 8) [90]. The porphyrin macrocycle differs from phthalocyanine in that it lacks the fused benzene rings and the meso-nitrogen atoms are replaced by meso-carbon atoms. The smaller size of the disc-like core was expected to lead to more fluid phases. However, against this expectation, the mesophases are very viscous and were found to exhibit textures characteristic of highly ordered discotic phases. The structures of these phases were not determined. The mesophase-mesophase and, in particular, the mesophase-isotropic transition

172 J. Barbera

Table 4-28. Mesomorphic properties of 2,3,7,8,12,13,17,18- octasubstituted porphyrins.

M R

H2

Zn

H2

Zn

c u

Pd

Cd

Zn

Mesomorphic Properties a

K 96 D1 99 D2 166 I

K 91 D1 101 D2 201 I

K 84 D 89 I

K 107 D 162 I

K 84 D 132 I

K 89 D 123 I

K 103 D 136 I

K 169 I

a D, D1, D2: unidentified mesophases

temperatures are higher for the zinc-containing derivatives than for the metal-free compounds.

The same authors also investigated the mesomorphic behavior of an analogous series of porphyrin octaethers (Table 4-28; M = H2, Zn; R = CnH2n+10CH2, n = 4, 6, 8, lo), which differs from the ester series in the linking group between the side chains and the central core [91]. For comparison, the analogous octyloxyethyl-substituted copper(rr), palladium(I1) and cadmium(I1) complexes were prepared (Table 4-28; M = Cu, Pd, Cd; R = C8HI70CH2). A relatively fluid liquid crystal phase, which was not identified, was observed in the metal-free porphyrin octaether with n = 8 and in all corresponding metal complexes. When R = C8H,70CH2 the melting points vary in the order H2 = C u < P d < C d < Z n while the clearing points follow the sequence H2 < Pd < Cu < Cd < Zn. When comparing the liquid crystal phase width, the order is H2 (5 "C) < Cd (33 "C) = Pd (34 "C) < Cu (48 "C) < Zn (55 "C). In the octyloxyethyl-substituted zinc complex (M = Zn, R = C8H,70CH2, the introduction of an electron acceptor (CN or NO2) into one of the meso-carbon atoms


depresses the phase transition temperatures. Surprisingly, in the octaesters, reversing the direction of the ester linkage from R = C,H2,+102C to R = C,H2,+IC02CH2 suppressed the liquid crystalline properties (Table 4-28).

Substitution of the porphyrin macrocycle with only four chains reduced their tendency to form liquid crystal phases. Thus, tetraphenylporphyrins substituted with ether [92] or ester [93] groups in the para-positions of the benzene rings appear to be nonmesomorphic (Table 4-29, M=H2, Co, Zn, R=CIOH2,0; M=H2, Zn, Cu, R=C16H3302C). However, tetraphenylporphyrins substituted with alkyl groups in the para positions of the benzene rings were reported to be mesomorphic by Shimizu and coworkers (Table 4-29, M = H2, Co, Ni, Cu, Zn, Pd, Al(OH), R = CI2H24 [94, 9.51. All the compounds investigated in this series show a lamellar discotic (DJ phase [94], except for the hydroxoaluminum derivative which exhibits a disordered hexagonal (Dhd) phase [95]. The hydroxoaluminium derivative was the first reported aluminum-containing discotic mesogen, and represents a rare example of a liquid crystalline compound with four lateral alkyl chains to display hexagonal columnar mesomorphism. Up to then, the only compounds containing four chains reported to show Dh mesomorphism were dinuclear tetraalkanoates (see Sect. 4.2.1) and tetrasubstituted vanadyl P-diketonates (see Sect. 4.1.1 [ 141). The hydroxoalumnium complex undergoes p-0x0 dimerization above the clearing point, resulting in a

Table 4-29. Mesomorphic properties of tetrasubstituted 5,10,15,20-tetraphenylporphyrin complexes.

I R R

R R

M R Mesomorphic Properties

Zn C 1 OH 21 0- K 170 I

Zn c 16H dbc- K 138 I

Zn c 1 ZH 25- K 37 DL' 52 DL 220 I

AI(0H) C12H25- K 84 Dhd 150 I

174 J; Barbera

change in the mesomorphic behavior. The dimeric complex forms lamellar mesophases, in which additional columnar order is observed.

4.4.3 Tetraazaporphyrin Complexes

The tetraazaporphyrin macrocycle differs from porphyrine in that it contains four rneso-nitrogen atoms. Mesomorphism was reported independently by two groups in a series of octa(alky1thio)-substituted tetraazaporphyrins (Table 4-30, M = H2, Zn, Ni, Cu, Co, n = 4, 6-10, 12) [96, 971. The interest in alkylthio substituents lies in the polarizability of the sulfur atom, which can increase the intermolecular interac-

Table 4-30. Mesomorphic properties of 2,3,7,8,12,13,17,18-octa(alkylthio)-5,10,15,20-tetraazaporphyrins.

Mesomorphic Properties I M " I H2 6

H2 10

Ni 6

NI 10

co 6

c o 10

c u 6

cu 10

Zn 6

Zn 10

K 78 Da 92 I

K 81 I

K 75 Da 154 I

K 76 Dhd 90 I

K 45 Da 244 I

K 62 Dhd 177 I

K 80 Da 190 I

K 69 Dhd 122 I

K 46 Da 168 I

K 54 Dhd 87 I

a Undetermined non-hexagonal columnar mesophase


tion. All metal complexes were found to be mesomorphic, whereas the metal-free ligands show lower tendency toward mesomorphism. A similar trend was observed by Gregg et al. in porphyrin octaethers [91] (see Sec. 4.4.2). The conclusion drawn from this was that in porphyrin-type derivatives the metal plays an important role for mesophase formation. The complexes display disordered columnar mesophases, the structure of which is independent of the nature of the metal. X-ray diffraction data and optical microscopy indicate that the columns adopt a hexagonal packing when n 2 7 and a nonhexagonal packing, the exact nature of which could not be determined, when n s 6 [97]. Once again, the stronger tendency to form hexagonal phases upon increasing the volume of the paraffinic part of the molecule is demonstrated.

4.4.4 Tetrapyrazinoporphyrazine Complexes

Mesogenic octa(dodecy1)tetrapyrazinoporphyrazine and its copper(Ir), nickel(I1) and cobalt(I1) complexes were described by Ohta and coworkers (Table 4-31, M = HZ,

Table 4-31. Mesomorphic properties of tetrakis(5,6-dialkylpyra- zino[2,3-b])porphyrazines.

N' N

R R H Mesomorphic Properties

K 11 8 Dhd 238 dec.

K 11 4 Drd 288 dec.

K 1 1 8 Drd 264 dec.

K 74 Drd 255 dec.

176 J. Barberd

Cu, Ni, Co, R = CI2H2J [76,98]. These macrocyclic compounds are phthalocyanine analogs with nitrogen atoms replacing eight of the carbon atoms in the peripheral aromatic rings. The metal-free ligand forms a disordered hexagonal columnar (Dhd) mesophase, like the majority of the octasubstituted phthalocyanine derivatives (see Sec. 4.4.1). However, upon metal coordination, a change occurs in the symmetry of the mesophase. Indeed, the three metal complexes form a disordered rectangular (Drd) phase. The mesomorphic phases are wide, and the compounds decompose before clearing.

A branched-chain tetrapyrazinoporphyrazine and its copper complex were also prepared M=H,, Cu, R=C,H9(CH3CH2)CHCH2) [76]. Branching in the lateral substituents decreases the transition temperatures; thus these compounds clear without decomposition. The metal-free derivative exhibits a tetragonal columnar liquid crystal phase and, at lower temperatures, another mesophase of probably rectangular symmetry. The copper complex forms a mesophase which could not be identified. It seems that branched or sterically hindered chains favor a two-dimensional square packing of columns (Dtet phase), as was also observed for similar phthalocyanines [80, 88, 891.

4.4.5 1,4,8,1l-Tetraazacyclotetradecane Complexes

In 1991, Ringsdorf and coworkers described liquid crystalline properties for a tetrasubstituted copper complex of 1,4,8,11 -tetraazacyclotetradecane (Fig. 4-1 5, M = Cu(NO&, R = H) [4]. The mesophase is stable from 18°C up to decomposition of the compound above 160°C. However, the nature of the mesophase was not determined. On the other hand, the analogous octasubstituted copper complex (Fig. 4-15, M = Cu(NO&, R = CIoH2,O) and the two precursor cyclam ligands are not mesomorphic.

R R

\ d R Figure 4-15.

4.4.6 Bis(salicy1idene)diimine Complexes

In 1993, Swager and coworkers reported the mesomorphic properties of a series of oxovanadium(rv) complexes of dimeric Schiff bases (Fig. 4-1 6) [99]. Interestingly, IR spectroscopy revealed that some of these compounds form polymeric linear chain

4 Low Molecular Weight Discotic Metallomesogens 1 I7

Figure 4-16.

Series 1

0 0

Series 2

Series 3

R = -CHZ-CH2-

structures ( - . V = O - - - V = O . -),. The same chain formation had been found by the same authors in a series of rodlike oxovanadium Schiff base complexes [15]. The authors proposed that this kind of intermolecular dative coordination in liquid crystalline materials should create non-centrosymmetrical assemblies with second-order non-linear optical and ferroelectric properties. Most members of this series are mesomorphic, and in some cases the liquid crystal phases are stable at room temperature (Table 4-32). Some of the compounds are polymesomorphic and exhibit a number of transitions between Dhd and Dho mesophases, between Dhd and Drd mesophases, and even between different Dhd structures. At first sight, columnar

178 J. Barbera

SERlESa R

1 XH2CHzCH2-

2 -CH2CHzCH2-

3 -CH2CHzCHz-

Mesomorphic Properties b

Dho 119.5 Dhd3 156 Dhdz 168 Dhdl 175 I

K 62 Drdl 107 Dhd 170 I

Dho 36 DM 97.5 Drd 151 I

a See Figure 4 16 Dhdi , Dhd2, Dhd3: different disordered hexagonal columnar mesophases

mesomorphism would not be expected in series 1 and 2 (Fig. 4-16) because the molecules are not disc-shaped. In fact, the formation of columnar mesophases by these non-disc-like derivatives is due to the presence of the correlated structures schematically illustrated in Fig. 4-17.

4.4.7 P-Diketonate Schiff Base Complexes

The use of complementary molecular shapes to generate correlated structures which have the desired overall disc-shape was exploited to generate mesomorphism in a series of P-diketonate Schiff base complexes (Table 4-33, M = Ni, Cu, Pd, VO, n = 10, 12, 16) [IOO]. It was found that the square-planar-coordinated complexes (M = Ni, Cu, Pd) display a disordered hexagonal columnar (Dhd) mesophase at relatively low temperatures. On the other hand, the square-pyramidal coordinatated oxovanadium complexes do not form liquid crystal phases. The mesophase-isotropic transition temperatures increase in the order Ni<Cu<Pd. In the X-ray patterns, a halo at 7.2 A , which is twice the intercore distance (3.6 A), was assigned to a doubling of the periodicity along the column axis. Thus, diffraction studies confirmed that the molecules are correlated in an antiparallel way, and they provided evidence for an antiphase structure for the columnar mesophase (see Fig. 4-1 8).

4.4.8 Calixarene Complexes

Oxotungsten(v1) calix[4]arene complexes (Fig. 4-19, R=H, C12H250) were prepared with the aim of developing new materials with ferroelectric and non-linear optical properties [ 1011. The bowl-shape of these noncentrosymnietric molecules was predicted to lead to columnar phases with polar order. The mesophases of the two compounds investigated in the series were observed only in the first heating process,


Molecular shape Disc shape

Series 1

Series 2

Figure 4-17. Schematic representation of the disc-shaped structures formed by [bis(salicylidene)diiminato]oxovanadium(~v) complexes in the columnar mesophases (see Fig. 4- 16). A disc shape is generated by 180" rotation of nearest neighbors in series 1 and 90" rotation of nearest neighbors in series 2.

Figure 4-18. Schematic representation of the disc-shaped structures formed by 1 -(3,4,5-trialk- oxypheny1)-I ,3-butanedionate Schiff base complexes in the D,, mesophase. 180 O rotation of nearest neighbors produces a disc shape.

180 J. Barherd

Table 4-33. Mesomorphic properties of bis[ I -(3',4',5'- trialkoxypheny1)t ,3-butanedione] Schiff base complexes.

R = H I C12H25O- Figure 4-19.

which is either due to partial decomposition or to a conformational change in the isotropic state. Although the mesophases were identified as columnar judging from their textures, their structure could not be assigned by X-ray diffraction. For the compounds with R = H, the discrete clearing enthalpy value (6.3 KJ mol-') sug-


SERlESa X n

gests that the columnar phase is disordered, whereas when R=C,2H250 the higher clearing enthalpy (21.3 KJ mol-') and the microscopic textures indicate that the columnar phase is ordered hexagonal (Dho). Host-guest complexation with Lewis bases such as dimethylformamide or pyridine suppresses mesomorphism. The destabilizing effect of the occupation of the molecular cavity suggests that, in the columnar mesophases, the bowl-shaped cores adopt a head-to-tail arrangement in which the oxotungsten group protrudes into the cavity of the neighboring molecule.


4.5 Dinuclear and Tetranuclear Metal Complexes with Tetradentate Ligands

4.5.1 1,3,5,7-Tetraketonate Complexes

Liquid crystalline 1,3,5,7-tetraketonate dicopper complexes were reported by Swager and coworkers in 1994 (Fig. 4-20) [62]. The compounds substituted with twelve (Series 1) or six chains (Series 2, X = C,,H,,O) exhibit disordered hexagonal columnar (Dhd) mesophases. The derivative with only four chains (series 2, X = H) is not mesomorphic (Table 4-34). The fact that compounds in series 1 and 2 show the same type of mesophase and the close similarity in the hexagonal lattice constant measured for each series (a = 34.0 and 33.6 A respectively, when n = 12) indicate that the non-disc-shaped molecules in Series 2 adopt a correlated organization, similar to that observed in analogous triketonate complexes [61, 621 (see Sec. 4.3.3, Fig. 4-12). Neighboring molecules must be rotated by 90" with respect to each other, thus projecting a circular shape along the column axis. The need for complementary molecular shapes was confirmed by the immiscibility of compounds from series 1 and 2. Intermolecular dative associations are thought to be responsible for the correlated arrangement.

182 J. Barbera

n = 6,10,12 Series I

X

x

Series 2 X = H , C12H250- Figure 4-20.

4.5.2 Dibenzaldiimine Complexes

Praefcke and coworkers prepared a series of tetranuclear organometallic compounds derived from two types of bisimine ligand (Table 4-35). They contain a tetrapallado- macroheterocycle in their central core [54, 1021. All of the compounds show enantiotropic columnar mesophases. The planar members of the series (X = C1, Br, I, SCN) show a columnar mesophase, probably with a disordered oblique structure

The phase behavior of the two acetato-bridged complexes is more complicated; the structure of their mesophases could not be established and the phase transitions are not reversible.

All of these tetranuclear palladomesogens form charge transfer complexes with strong electron acceptors. Induction of hexagonal columnar mesophases was detected in mixtures of the planarly bridged compounds with TNE Finally, these tetrapalladium compounds also show lyotropic mesomorphism in apolar organic solvents [103]. Nematic lytotropic mesophases were observed in the presence of heptane, pentadecane and eicosane.


Table 4-35. Mesomorphic properties of bis[bis(2,3,4-trialkoxy- benzal)diimine] tetrapalladium(11) complexes.

yc1 PH 25 ?C1ZH25

I'

u x 1,4-Phenylene CI

1.4-Phenylene Br

1,4-Phenylene I

1,4-Phenylene SCN

I 4,4'-Stilbenylene CI


K 71 D0b.d 301 dec.

K 69 D0b.d 279 dec.

K 62 D&.d 265 1

K 50 D0b.d 290 dec.

K 125 D&.d 242 Ma 260 dec


4.6 Metal Complexes with Hexadentate Ligands

4.6.1 1,4,7,10,13,16-Hexaazacyclooctadecane Complexes

The mesogenic properties of cobalt(I1) and nickel@) 1,4,7,10,13,16-cyclooctadecane complexes were described by Ringsdorf and coworkers in 1991 (Table 4-36, M = CO(NO~)~ , Ni(N03)2) [4]. In spite of the fact that the metal-free precursor ligand is not mesomorphic, the nickel and cobalt complexes display liquid crystalline properties. Optical microscopy and X-ray diffraction studies suggest a discotic nematic order for the cobalt complex with an additional columnar ordering of the

184 J. Barbera

Table 4-36. Mesomorphic properties of 1,4,7,10,13,16-hexa- kis(4'-alkoxybenzyl)-1,4,7,10,13,16-hexaazacyclooctadecane complexes.

C14H290

M Mesomorphic Properties

Co(N03)2

Ni(NOd2

K 30 Nc 60 I

K 29 Ma 95 I


azamacrocycles. This structure corresponds to a novel type of mesophase which the authors called nematic columnar (Nc).

4.6.2 1,3,5-Wketonste Schiff Base Complexes

In 1992, Swager described mesomorphism in several series of homodinuclear and heterodinuclear 1,3,5-triketonate Schiff base complexes (Tables 4-37, 4-38) [61, 621. Homodinuclear compounds depicted in Table 4-37 containing six lateral subsituents (X=C,H2,+10, n = 6 , 7, 8, 10, 12, 14, 16) and short diamine spacers (R= CH2CH2, CH2CH2CH2) display a disordered hexagonal columnar (Dhd) mesophase, whereas the complex containing only four chains (R = CH2CH2; X = H) and those containing sterically hindered diamine linkages (R = cis-I ,2-cyclohexylidene, CH2C(CH3),CH2; X = C,H2,+,0, n = 12) are not mesomorphic. As far as the heterodinuclear compounds in Table 4-38 are concerned (n = 24, 16), only those in which M = Cu, M' = Ni, and M = Pd, M' = Ni are mesomorphic; the liquid crystal phase in these cases has also been assigned Dhd. These were the first heteronuclear metallomesogens reported to form columnar phases. The occurence of mesomor-


Table 4-37. Mesomorphic properties of bis[ 1 -(3’,4’,5‘-trialkoxyphenyl)-l,3,5-hexanetrione] Schiff base dicopper(I1) complexes.

X


K 81 Dhd 248 I

CHzCHzCHz- CnH~n+10- K 64 Dhd 200 I

Table 4-38. Mesomorphic properties of heteronuclear bis[ I-(3’,4‘,5‘- trialkoxyphenyl)l-3,5-hexanetrione) Schiff base complexes.

OCnHzn+l


K 97 Dhd 226 I

Pd 14 K 105 Dhd 232 1

186 J. Barbera

phism in these two series of semi-disc-shaped molecules was explained by a correlated structure in which neighboring molecules are rotated by 180 " with respect to each other, in a similar manner to the model proposed for D-diketonate Schiff base complexes [IOO] (see Sec. 4.4.7, Fig. 4-18). This model was supported by a single-crystal X-ray analysis of one of the homodinuclear complexes (Table 4-37, R= CH2CH2CH2, n = 6) , for which an antiparallel dimeric arrangement via intermolecular Cu. - .O dative associations was found.

4.7 Metal Complexes with Octadentate Ligands

4.7.1 Bisphthalocyanine Complexes

A dodecasubstituted planar bisphthalocyanine derivative and its dinuclear copper complex (Fig. 4-21, M = HZ, Cu) exhibits a liquid crystal phase between room temperature and 300 "C [104]. For the mesophase of the metal-free ligand, X-ray diffraction indicates an orthorhombic (D,) structure at room temperature and a tetragonal (D,J structure at 200 "C. The nature of the liquid crystal phase of the copper complex was not determined. The protonated form of the uncomplexed compound shows a smectic mesophase with nematic order within the layers. In comparison with the monomeric dihydrogen and copper phthalocyanines substituted with the same 2-ethylhexyloxy group [75,76] (see Sec. 4.4.1, Table 4-20), the bisphthalocyanines show significantly lower melting points, higher clearing points and considerably broader mesophases.

R = CH3(CH*),-?H-CH,-

Figure 4-21. CHzCH2


4.7.2 1,3,5,7-Tetraketonate Schiff Base Complexes

Swager and coworkers investigated a hexasubstituted Schiff base derived from one of the 1,3,5,7-tetraketonate complexes discussed in Sec. 4.5.1 (Fig. 4-22) [62]. As observed for the related hexasubstituted triketonate (Sec. 4.3.3), tetraketonate (Section 4.5.1) and triketonate Schiff base complexes (Sec. 4.6.2), this dinuclear copper(I1) complex forms a disordered hexagonal columnar mesophase in spite of its semidisc shape. The proposed structure consists of a correlated columnar stacking in which neighboring molecules are rotated by 180 ", thus giving rise to an antiparallel organization (see Fig. 4-18). The close similarity between the measured intercolumnar spacing for this hexasubstituted Schiff base complex (a = 34.0 A) and for the analogous dodecasubstituted tetraketonates (a = 33.6 A ) and hexasubstituted tetraketonates (a = 33.5 A) having the same chain length (see Sect. 4.5.1) supports the presence of correlated structures in the mesophases. Indeed, in the proposed organization the projected section of the rigid core and the surrounding paraffinic chains is very similar regardless of whether the molecule has an inherent disc-shape or the disc-shape is generated by a correlated dimer.

I

Figure 4-22. OC1ZH25

4.8 Cyclopentadiene Complexes

Disc-like metallocenes were described in 1991 by Schumann, who prepared two pen- tadienyl thallium(1) complexes containing five phenyl rings substituted in the para positions with ester-type chains (Fig. 4-23, n = 2, 5 ) [105]. The ethyl derivative (n = 2) melts at 250°C directly into the isotropic liquid, but the pentyl derivative (n = 5 ) melts at 180°C into a viscous oil. Although this oily melt was thought to be a mesophase, its nature was not investigated.

188 J. Barbera

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SOC. Chem. Commun. 1987, 1086 - 1088; b) A. S. Cherodian, A. N. Davies, R. M. Rich- ardson, M. J. Cook, N. B. McKeown, A. J. Thornson, J. Feijoo, G. Ungar, K. J. Harrison, Mol. Cryst. Liq. Cryst. 1991, 196, 103 - 114.

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5 Metallomesogenic Polymers

Luis Oriol

5.1 Introduction

Metallomesogenic polymers or metal-containing liquid crystalline polymers (abbre- viated MLCPs) constitute one of the most recent contributions to research on metal- organic polymers, which are designed in an attempt to combine the advantages of both their organic and inorganic constituents [I]. The variety of readily available metals incorporated into these polymers introduces new physical properties and, consequently, new applications in the field of polymeric materials traditionally based on a few elements (C, H, N, 0, halogens, S, P). In recent years some noteworthy contributions to the field of materials science can be mentioned [2]. A number of metal- organic polymers have been reported as molecular ferromagnets, synthetic metal conductors, materials for nonlinear optics, ferroelectric materials, as well as metallomesogenic polymers. Interest in these last materials arises from the possibility of combining the promising properties of metallomesogens with the processability of polymers.

The structural design of metallomesogenic polymers is essentially similar to that for conventional organic LCPs: metallomesogenic cores incorporated in the main- chain or side-chain of a polymeric structure. All the structural possibilities which have been reported for liquid crystal polymers (LCPs) can be designed by using suitable metallomesogen monomers as schematically represented in Fig. 5-1.

The synthetic strategies used to obtain metallomesogenic polymers can generally be classified into two groups depending on whether the metal atoms are introduced during the polymer synthesis (Strategy 1) or by metal modification of a previously synthesized organic polymer (Strategy 2). The synthetic difficulties encountered in obtaining unsymmetrically substituted metal complexes as monomers limit the design and synthesis of some types of polymeric structures, especially side-chain polymers. Furthermore, there are numerous examples of crosslinked metal-containing LCPs since the metal modification of preformed polymeric chains usually leads to

194 L. Oriol

2

ONE-DIMENSIONAL MLCPS e LYOTROPIC MLCPS THERMOTROPIC MLCPs

MAIN-CHAIN (e.9.)

M W [ (rod-like mesogens) (disc-like mesogens) 12

SIDE-CHAIN (e.g.)

CROSSLINKED MLCPS (e.g.)

n Figure 5-1. Classification of metallomesogenic polymers (M LC Ps) .

complexation of repeat units of different chains. In other words, the metal acts as a crosslinking agent.

Despite the novelty of this field of materials chemistry, examples of the main types of liquid crystalline behavior have been reported. However, the future goals require an effort on the physical characterization of these emerging materials. After obtaining a wide background knowledge of the structure-physical property relationships, a careful selection of the properties of metal and liquid crystalline states will lead to the optimum design of the required multifunctional material.

Owing to the variety of systems to be treated in this chapter, they have been classified in terms of their mesogenic behavior (lyotropic or thermotropic) and chemical structure. One-dimensional metallomesogenic polymers exhibiting lyotropic or thermotropic properties will be treated in different sections. A special section is devoted

5 Metallomesogenic Polymers 195

to crosslinked metallomesogenic polymers. In addition, main-chain and side-chain polymeric designs will be treated in separate subsections.

5.2 Lyotropic Metal-Containing Liquid Crystal Polymers

Lyotropic metallomesogenic polymers were among the first examples of metallomesogens to be described. We can classify the reported examples of these polymers into two groups, depending on whether the metallomesogenic unit is introduced into the main-chain or the side-chain.

5.2.1 Main-Chain Lyotropic Metal-Containing Liquid Crystal Polymers

The first examples of MLCPs were lyotropic systems obtained from the rod-like conjugated platinum polymers reported by Hagihara and co-workers in 1978 [3]. A wide variety of conjugated metal-poly(yne) polymers having lyotropic mesogenic properties have since been described by this research group. Table 5-1 shows some representative poly(yne) polymers with a molecular design based on ethynyl and phenyl units and containing different transition metals, whose liquid crystalline properties have been reported [3 - 81. Spectral analyses and viscosity measurements indicated that these polymers have a rod-like structure [3 a] due to the rigid linearity of the carbon- carbon triple bonds, the p-phenylene rings and the square-planar trans-configuration of the metal which was confirmed by 31P-NMR [4]. Tri-n-butylphosphine (PBu,) is used as a ligand to stabilize the metal-carbon o-bond. In addition, the favorable interactions between the metal d-orbital and the n:-orbital of the ethynyl carbon (dn: - pn * overlap) stabilizes these polymeric acetylide complexes [5].

Structural studies based on the properties of dilute solutions (viscosity and sedi- mentation velocity measurements) of polymer 1 (M = Pt) in n-heptane were reported by Motowoka et al. [9]. The results suggest that the polymer can be represented as a “worm-like chain” with appreciable flexibility. These polymers are generally soluble in a variety of organic solvents such as dichloromethane, tetrahydrofuran, or trichloroethylene. The high solubility is attributed to the n-butyl substituents attached to the phosphorus atom, which decrease intermolecular attractions and facili- tate the polymeric chain-solvent interactions.

The rigid, rod-like polymeric structure and the high solubility of these systems both favor the appearance of liquid crystalline properties in concentrated solutions. Most of the polymers shown in Table 5-1 exhibit lyotropic mesogenic behavior. There are, however, some exceptions. Nickel polymers of type 1 and 2 do not form lyotropic liquid crystalline phases due to their low molecular weight and low solubility [6],

Oriol 196 I _

I1 d

II II II II II d d d d d

II II II II

E E E Z II I1 II II I /

E E Z E E

I I I I

0

0


a consequence of the low stability of the nickel-carbon and nickel-phosphorous bonds. Dissociation of the tri-n-butylphosphine ligands takes place, which lowers the solubility. In contrast, polymers that contain two kinds of metal atoms (nickel and platinum or palladium) are more stable and soluble, and form anisotropic phases in trichloroethylene or tetrahydrofuran (polymers 6 and 7 with M’ = Ni). Liquid crystalline properties were not observed in polymer 2 with M = Pd, which exhibited a low solubility. To enhance the solubility, alkyl groups were introduced into the phenyl ring. As a result, polymers 3 with M = Pd and R = methyl or ethyl exhibit lyotropic liquid crystalline behavior.

The common liquid crystalline phase of these metal-containing poly(yne) polymers appears at a concentration above a critical value, which depends on the molecular weight. The anisotropic phase was characterized as nematic, based on the textures observed by polarizing optical microscopy [3, 61. In addition, rheological measurements showed an abrupt change in viscosity at the transition from the isotropic to the anisotropic phase above the critical concentration, as can be expected from nematic ordering [3].

Recently, Abe and coworkers [lo] reported studies of the phase behavior and molecular ordering of a deuterium labeled platinum poly(yne) polymer by 2H NMR spectroscopy (see Fig. 5-2). The experimental results were also compared with those of theoretical calculations and complement the structural conclusions previously obtained for these polymers [9].

L PBu3 f ~c .c - -p1 -c .c -c . c

PBu3 D D

Figure 5-2. Deuterium labeled platinum poly(yne) polymer prepared and studied by Abe and coworkers [lo].

8

Several new series of similar materials have been synthesized by taking into account the molecular design of the aforementioned lyotropic metal-containing poly(yne) polymers. The first example is a family of cationic organopalladium polymeric complexes, shown in Fig. 5-3 [ 1 I]. Only for polymers 9 a and 9 d the formation of anisotropic nematic solutions in dichloromethane was reported.

The second examples are new lyotropic poly(yne) polymers which contain disilane, disiloxane and phosphine groups in addition to transition metals (Fig. 5-4) [12]. These polymers may be of interest in terms of the physical properties derived from the dx (metal)-pn (acetylenic carbon)-empty d orbital (Si or P) conjugation. The transition metals selected were Pt, Pd and Ni. 31P NMR spectra were consistent with an all-trans configuration at the M(II) centres. Therefore, concentrated solutions of these polymers can form lyotropic liquid crystals. Thus, solutions of silicon-contain-

198 L. Oriol

PBUJ P B u ~

CI-Pd-CnC-Y-CIC-Pd-CI + I I PBu, P B u ~

KPF, (excess)

acetone I 9

Y Z Nomenclature

Qa -CH&Hz- Qb

-CH=CH- 9c

9d

99 Figure 5-3. Ionic organopalladium polymeric complexes with lyotropic liquid crystalline properties [ I I].

# - H3C

ing polymers shown in Fig. 5-4 (except 10d) in dichloromethane or 1,2-dichloro- ethane form an anisotropic phase above a critical concentration.

Metal complexation can be used as a means of solubilizing and processing rigid- chain polymers which were hitherto difficult to process. In this way, metal modified polymers form liquid crystalline solutions and fibers, films or coatings can be processed from them. Jenekhe and coworkers introduced this approach to process polymers [ 131. Many heat-resistant or high-temperature polymers such as poly(p-phenylene-2,6-benzobisthiazole) can be solubilized in aprotic organic solvents (e.g. nitro- alkanes) by reversible electron donor -acceptor complex formation at specific heteroatom donor sites with Lewis acids (e.g. AlCl, or GaCl,). The complexation of polymeric rigid chains decreases the intermolecular attraction and allows their solvation. A nematic liquid crystalline phase was observed for these systems. An important stage in the processing is the decomplexation or regeneration of the pristine polymers by using a nonsolvent.

An interesting modification of this approach has recently been described by Dembek and coworkers [ 141, who have synthesized metallomesogenic aramids which are soluble in organic solvents, by using organometallic y6-coordinated diamine monomers. The bulky q6-organometallic substituents increases the solubility of the


10

M Y Nomenclature

y e y e

I I Me Me

y e y e Pt -si-o-si- 1 Ob

I I Me Me

Me y e Pd -Si-O-Si- 1 oc

I I Me Me

-Si-Si- 1 Oa Pt

Pd -PPh - 1 Od

y e y e

I I Figure 5-4. Poly(yne) polymers containing transition metals, disilane, disiloxane and Me Me

y e y e phosphine groups in the main chain [12]. Ni -si-O-si- 1 Of

I 1 All polymers, except for 10d, display lyotropic liquid crystalline behavior. Me Me

-Si-Si- 1 Oe Ni

poly(pphenyleneterephtha1amide) chains while maintaining their rigid-rod character (see Fig. 5-5 a). As a consequence, solutions of this polymer in N,N-dimethylacetamide (DMAc) are liquid crystalline when the concentration of the polymer is greater than 4-6% wt. The observed mesophase was found to be nematic. Even alternating copolymers, formed by complexing only half of the diamine units, are soluble in organic solvents (Fig. 5-5 b). Furthermore, the metalloaramids can be isolated or processed into strong air-stable yellow films.

The modification of the chromium ligands allows the control of the persistence length to diameter ratio, and therefore the solubility properties. Polymer 11, in which chromium tricarbonyl was substituted by trimethylphosphine chromium dicarbonyl forms liquid crystalline solutions at higher critical concentrations. The potential utility of this synthetic approach arises from the reversibility of the complexation, which allows the processing of poly(pphenyleneterephtha1amide) in organic solvents.

Lyotropic properties have also been observed in metal-containing systems based on inorganic polymers. The first antecedents can be found in a natural product which consists of a clay fraction of Japanese soil named imogolite. The lyotropic properties of imogolite and its blends with poly(viny1 alcohol) and hydroxypropyl cellulose in acetic acid solutions have been studied in depth [15].

200 L. Oriol

12

Figure 5-5. Soluble transition metal, n complexed a) polyamides and b) copolyamides (rnetalloararnids) [ 141.

Recently, Davidson et al. reported nematic ordering in colloidal solutions of LiMo3Se3 in N-methylformamide [ 161. The molecular organization in the lyotropic nematic phase arises from the parallel orientation of polymeric chains with respect to a common direction (the nematic director). This arrangement is based on a staggered stacking of Mo,Se; units which form a molecular wire. These negatively

t nematic director

Molecular wire based on

the staggered stacking of n Mo3Se; units

a

Figure 5-6. Molecular wires based on n Mo3Se, units in the lyotropic nematic phase. Adapted from reference 16 b.


charged needles are surrounded by N-methylformamide-coordinated Li' cations to ensure electrical neutrality (Fig. 5-6). These works open new possibilities for the investigation of new metal-containing liquid crystalline systems derived from inorganic polymers.

5.2.2 Side-Chain Lyotropic Metal-Containing Liquid Crystal Polymers

The reported examples of side-chain lyotropic systems deal with metallophthalocyanine derivatives of poly(y-benzyl-L-glutamate) (Fig. 5-7) [ 171. It is well known that this polymer, which has a helical conformation, forms lyotropic mesophases in concentrated solutions [ 181. Metallophthalocyanine units, as dye components of the liquid crystalline system, were fixed onto the side-chain of poly(y-benzyl-L-glutamate) by means of a Friedel-Crafts reaction. The contents of metallophthalocyanine rings introduced was less than 3 mol% in order to avoid the formation of insoluble and crosslinked products. At lower contents, liquid crystalline solutions in common organic solvents were observed, as in the case of the parent polymer.

t HN-CH -CO f; t HN-YH -CO *-x I

Figure 5-7. Poly(y-benzyl- L-g1utamate)s with low metallophthalocyanine ring content. Crosslinked polymers are obtained as byproducts [17].

HOOC'

13

y 2

FH2

? c=o I

6 M = Fe(lll)

M = Co(ll)

M = Cu(ll)

M = Ni(ll)

202 L. Oriol

5.3 Thermotropic Metal-Containing Liquid Crystal Polymers

The structural design of thermotropic metal-containing liquid crystal polymers is based on the extensive knowledge of thermotropic metallomesogenic structures of low molecular weight. This section deals with noncrosslinked thermotropic metallomesogenic polymers, which can be classified into two groups according to the kind of metallomesogenic unit used in the molecular design; calamitic (rod-like) and columnar (all the examples are concerned with disc-like structures) thermotropic MLCPs. Furthermore, it is possible to subdivide these two groups, depending on whether the mesogenic units are located in the main-chain or attached to the side- chain.

5.3.1 Calamitic Main-Chain Polymers

Most of the developments of noncrosslinked thermotropic metallomesogenic polymers concern semiflexible main-chain polymers. The reason for this is the ease of design of symmetrically substituted metal complex monomers. Apart from the polymers based on metallomesogenic units, it is possible to introduce metal atoms into a liquid crystalline polymer structure, by incorporating them not into the mesogenic core, but into the flexible spacer.

5.3.1.1 Main-Chain Polymers Based on Metallomesogeaic Units

Salicylaldimine metal complexes, especially copper(u) complexes, have been used as monomers by different polymer research groups. Indeed, structure-mesogenic property relationships can be found from the study of polymeric systems based on the square-planar geometry of liquid crystalline copper(r1) salicylaldiminate complexes which have been reported. Most of the work concerns complexes derived from 2,4-dihydroxybenzaldehyde (see Fig. 5-8). These complexes have a stepped structure [ 191

Figure 5-8. Metal complex monomers derived from 2,4-dihydroxybenzaldehyde with crankshaft geometry.


which is appropriate for obtaining liquid crystallinity through their incorporation into a main-chain polymer. The square planar geometry imparts an overall rod-like shape to the metallomesogenic unit, which is similar to a crankshaft monomer because of the disruption of the linearity caused by the stepped structure. Organic crankshaft monomers have been widely used for decreasing the transition temperatures of LCPs without alteration of the mesogenic properties [20]. Moreover, this metallomesogenic unit allows the modification of both the imine and the p-hydroxy groups.

Sirigu and coworkers first synthesized a series of homo and copolymers based on copper@) salicylaldiminates, which were obtained by copper complexation of monomers incorporating two bidentate groups (see Fig. 5-9) [21]. These polymers display a monotropic mesophase tentatively identified as smectic A for all composi- tions. However, one of the disadvantages inherent in this synthetic approach is the low degree of polymerization that can be obtained.

As an alternative, Serrano and coworkers proposed the synthesis of metallomesogenic polymers by a conventional polymerization method using suitably functionalized bis(salicylaldiminato)copper(II) complexes [22]. Using this approach several different series of polyesters have been synthesized (Fig. 5-10) [23].

HO

H15C70 ~ C H = N ~ O - R I O N = H C ~ O C ~ H ~ ~

H I & ~ O ~ C H = N ~ \ / 0-R2 0 \ / N=HC & \ / OC7Hi5

HO OH

OH

0 - R i

1 -x

14

R1 = -(CH2)12-0- x = O , 0.25.0.50.0.75, 1

R2 = -(CH*-CH2-0-)3-

Figure 5-9. A series of homo and copolymers derived from a bis(salicylaldiminato)copper(II) mesogenic unit [21].

204 L. Oriol

Nomenclature m R References

15 2 -CnHZn+l (n = 5, 10) W b I

17 12 -CnHPn+l (n = 4-1 3) ~ 3 1

16 10 -CnHZn+j (n = 1,4-10, 12, 14, 16) [22a, 22bj

20 10 O-CnHzn+l (n = 5, 10) W b I

Figure 5-10. Thermotropic metallomesogenic homopolyesters based on bis(salicy1aldiminato- copper(n) complexes.

Figs. 5-1 1, 5-12 and 5-13 show the mesogenic properties of these polymers, which mainly depend on the number of aromatic rings in the metallomesogenic units. Four aromatic rings are required in the monomer before mesogenic properties are obtained. The length of the flexible spacer, which decouples the motions of the rigid mesogenic core, is also a condition for mesophase formation. Polymer 15, which has a short flexible spacer, is not liquid crystalline and exhibits low thermal stability, which was attributed to the high concentration of metallomesogenic units [22 b]. Only polymer 18, which contains a very bulky N-aryl substituent, shows a nematic phase before decomposition. Polymers 16 and 17 differ only in the length of the flexible spacer (m = 10 or 12 respectively), but there are some differences in their properties. Fig. 5-1 1 shows the dependence of transition temperatures and mesogenic behavior of polymers 16 on the length ( n ) of the N-alkyl groups. Polymers with n > 4 exhibit a mesophase before decomposition occurs. The nature of the mesophase observed is directly related to the length of the lateral alkyl groups. An enantiotropic nematic phase is observed for polymers with n = 5 - 10, which becomes monotropic when n> 10. Polymers 17, containing a longer flexible spacer (rn = 12), also show mesomorphic behavior, but in these cases the mesomorphic are monotropic (Fig. 5-12). The enantiotropic phase exhibited by polymers with n = 7- 10 are replaced by


280

260

240

220

i? 200 i=

180

160

140

Figure 5-11. Transition temperatures of

’ If ’

4 6 8 10 12 14 16

n

N-alkyl polyesters 16 (A) melting transition, (0) - nematic-isotropic phase transition, (*) a peak corresponding to a nematic-isotropic phase transition was not observed in the DSC traces.

a monotropic one after moderately annealing the samples [23]. In the case of polymers 17, in which n = 10 or 1 I , evidence of a monotropic tilted smectic phase was found in addition to a nematic phase.

When N-aryl substituents are introduced instead of N-alkyl substituents (polymers 18, 19 and ZO), an increase in the chain rigidity is caused. As a consequence, phase transitions occur at higher temperatures, and mesophases are only observed over a short temperature range before decomposition at temperatures close to 300 “C (Fig. 5-13).

In conclusion, an optimal structural design of these polymers involves a rod-like metallomesogenic unit containing at least four aromatic rings, lateral groups which decrease intermolecular attractions and long flexible spacers which “dilute” the rigid units.

A different series of metal-containing copolymers (21), based on a linear Schiff base copper(I1) complex derived from 2,5-dihydroxybenzaldehyde, was reported by Stupp and coworkers [24]. The copolymers were synthesized by a transesterification reaction of a random liquid crystalline terpolymer [25] (Fig. 5-14a) with a functionalized tetradentate copper(I1) complex (Fig. 5- 14 b). The organometallic unit was

206 L. Oriof

260

240

220

1 a0

160 4 I

4 6 8 10 12

n

Figure 5-12. Transition temperatures of N-alkyl polyesters 17 ( A ) melting transition, (0) nematic-isotropic phase transition.

Polymer m x Transition temperatures (“C)

10 2 *CSH13 dec

2 GOH21 K 274 N dec

19 10 -C6H13 K 270 N 288 dec

10 -c 1 OH2 1 K 257 N 265 I

20 10 -0-C5Hl K 289 N dec

10 -0-CiOH21 K 262 N 271 I

Figure 5-13. Transition temperatures of N-aryl polyesters 18 - 20.


(a)

- 0 0 0 - - 0 O c o - -OC-(CH*)s-CO -

21

Figure 5-14. a) Structural units of the organic terpolymer precursor of metallomesogenic copolymers 21 obtained by transesterification with b) a functionalized bis(salicyla1diminato)- copper(I1) complex [24].

incorporated in concentrations ranging from 5 to 20 mol% without disrupting the liquid crystallinity. However, some doubts arise regarding the location of the organometallic units within the terpolymer after its modification. Indeed, an incomplete transesterification reaction was suggested by the authors.

Ferrocene derivatives, because of their synthetic versatility, are very attractive mesogenic units to be included in a polymeric chain in order to obtain metallomesogenic polymers. Furthermore, ferrocene derivatives have other interesting properties [26], such as UV-stabilization, smoke and soot retardation, and, in addition, they introduce the structural irregularity necessary to decrease phase transition temperatures. This interesting background led to the synthesis of a series of ferrocene- containing copolyesters by Lenz and coworkers (see Fig. 5-15) [27]. All samples show a nematic phase, except for the polyester with x = 0, which decomposes before melting.

22

Figure 5-15. Ferrocene-based metallomesogenic copolymers (0 < x < 1) [27].

208 L. Oriol

Ferrocene units can be reversibly oxidized, leading to redox active ionomers, as have been reported by Zentel. These redox active systems have mainly been studied in side-chain LCPs. However, these authors have also presented the first results on a main-chain liquid crystalline copolymer based on a ferrocene unit (see Fig. 5-16) 1281.

23

K 78 SB 106 SA 114 I

Figure 5-16. Redox active ferrocene-based main-chain liquid crystal polymer [28].

Hanabusa and coworkers [29] have synthesized a series of calamitic liquid crystalline polymers incorporating P-diketonatocopper(I1) complexes as the metallomesogenic units (see Fig. 5-17). A stable mesophase is obtained by selecting a flexible spacer of adequate length. The introduction of lateral alkyl chains assists in avoiding thermal decomposition. The transition temperatures of these polymers are depressed by copolymerization using two different flexible spacers.

5.3.1.2 Alternative Designs of Main-Chain Polymers

A different approach to obtain thermotropic main-chain MLCPs is the introduction of metal atoms not into the mesogenic core (metallomesogenic units), but into the flexible spacer which connects the organic mesogenic units. Kuschel and coworkers [30] inserted tin atoms into the flexible spacer of the backbone of both main- chain and side-chain LCPs. The aim of the work was to introduce centers as markers (atoms with unusual scattering properties) in the liquid crystalline phase. Bis(3-hy- droxypropy1)dimethyltin [3 11 was selected as a difunctionalized monomer for polymerization. In this section only the results obtained on the main-chain LCPs, which are shown in Fig. 5-18, will be discussed.

Only polymer 27 shows liquid crystalline behavior which is due to the stronger anisotropic interactions of aromatic triads [32]. If we compare the thermal properties of tin-containing polyesters with those of conventional liquid crystalline polyesters with the same premesogenic core as polymers 25 and 26 and a decamethylene spacer [33], the disappearance of liquid crystallinity upon incorporation of a dimethyltin


24

Nomenclature m n Transition temperatures ("C)

24a 6 3 K 252 LC dec

24b 12 1 K 253 LC dec

24c 12 3 K 202 SA 225 I

24d 12 5 K 192 SA 225 I

24e 12 7 K 230 SA 234 I

Figure 5-17. Chemical structure and transition temperatures of homopolymers based on copper@) P-diketonate metallomesogenic units [29].

Transition Nomenclature U m temperatures ("C)

1 951 1 10 K971 25

26

27 e C = N o N = C e 10 K 87 s1 123 52 129 S3 141 Sc 183 I

Figure 5-18. Structural modifications and transition temperatures of main-chain polymers based on tin-organic flexible spacers [30].

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moiety into the spacer is evident. This behavior is related to the obstruction of lateral packing and consequently reduction of intermolecular attraction associated with the branching of the flexible spacer. The influence of the geometry of the spacer on the transition temperatures and the mesomorphic behavior in thermotropic LCPs has been widely reported, particularly the replacement of the -CH2 by a -CR2 unit [34]. The more pronounced effect observed for the organotin polyesters can be attributed to the size of the tin atom. The differences in solid state properties between polyesters with m = 1 (amorphous), and rn = 10 (crystalline) seem surprising and might be justified by the same arguments. Long flexible polymethylene spacers separate the amorphous regions associated mainly with bulky tin flexible spacers and also the crystalline regions associated with rigid core units.

5.3.2 Calamitic Side-Chain Polymers

Very few articles have been published to date regarding thermotropic MLCPs based on a side-chain molecular design. One of the reasons for this lack of references is the difficulty in synthesizing unsymmetric reactive metal complexes which allow the synthesis of side-chain polymers. Indeed, the vast majority of low molecular weight metallomesogens are symmetric metal complexes. Furthermore, it is necessary to carefully select the method of polymerization. The introduction of metal centers into previously synthesized SCLCPs containing coordination sites may lead to partially crosslinked materials. Consequently, this synthetic method is not an adequate strategy to obtain one-dimensional side-chain metallomesogenic polymers.

5.3.2.1 Side-Chain Polymers Based on Metallomesogenic Units

Strictly speaking, the first cases of calamitic metallomesogenic polymers which are thermotropic side-chain polymeric systems, are the ferrocene-containing polysiloxanes shown in Fig.5-19 [35]. These polymers were synthesized by grafting poly- (methylhydrosiloxane) (PMHS) or a copolymer, PMHS-poly(dimethylsi1oxane) with unsymmetrically substituted mesogenic ferrocenes bearing a reactive group at the end of an aliphatic chain (see Fig. 5-20).

The polymers retain the mesogenic properties of their corresponding ferrocene monomers, but show wider mesophases. The smectic phases were characterized by X-ray diffraction. The authors claim that polymer 29 is the first example of a chirul metallomesogenic polymer, due to the planar chirality of the unsymmetrically 1,3-disubstituted monomer. However, data about the chirality of the monomer were not further discussed.

5.3.2.2 Alternative Designs of Side-Chain Polymers

Kuschel and coworkers [30] used the bis(3-hydroxypropy1)dimethyltin monomer mentioned previously to synthesize the side-chain polymers shown in Fig. 5-21. From

5 Metallomesogenic Polymers 2 1 1

-coo +coo ~ O C 1 * H 3 ,

H~c=CH- (CH&O - @ c o o ~ o o c ~ - -

30

K 124 Sc 131 SA 141 I

31

K 168 SA 201 I

Figure 5-20. Unsymmetrically substituted reactive metallomesogens [35].

the point of view of the structural design, these polymers may be considered as “combined” metal-containing liquid crystalline polymers since they contain metals in the polymeric backbone and organic mesogenic units in the side-chain. Polymer 33 was prepared as a reference model. The flexible tin spacer causes some modifications in the properties. The tin-containing polymer is amorphous and melts into

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32

Polymer

33

Transition temperatures ("C)

32 g55Sj69S79Sclll I

33 KE8 SJ 101 Sc 122 N 127 I

Figure 5-21. Transition temperatures of a side-chain polymer based on a tin-organic flexible spacer (32) and the corresponding organic side-chain LCP (33) [30].

smectic phases similar to those formed by polymer 33, but at lower temperatures. However, a nematic phase, which is present in polymer 33, is not observed for polymer 32.

Attractive copolymeric systems incorporating side organic mesogenic groups and side nonmesogenic ferrocene groups have been described by Zentel and coworkers (Fig. 5-22) [36]. The interest in these materials lies in the possibility of achieving liquid crystalline elastomers via reversible redox formation of ionomers. Ferrocene groups are chemically stable in both the reduced and the oxidized state (ferrocenium ions). This concept is an interesting approach to ionomers which are usually obtained by neutralization of polymeric acids.

The oxidation of the copolymers was carried out with copper(I1) perchlorate [36a] or I,4-benzoquinone/H2SO4 [36 b, 36~1. The latter oxidizing agent leads to liquid crystalline ionomers with higher thermal stabilities since the perchlorate counterion is a strong oxidizing agent, Table 5-2 shows the mesogenic behavior of some examples of copolymers, both in their reduced and oxidized states. The reduced non-

5 Metallomesogenic Polymers 2 13

REDUCED STATE OXIDIZED STATE

-+ CH -COO-(CH&-O I +

(x ranges from 0 to aprox. 0.1)

CH - C O O - C H ~ - C H ~ ~ I + 45

34 R=-CN

35 R=-OCHB CH - C O O - C H ~ - C H , ~ I

45 l'*

+

Figure 5-22. Reversible redox formation of ionomers from side-chain copolymers based on ferrocene units [36].

Table 5.2. Mesomorphic properties of the reduced state of side-chain liquid crystal polymers containing ferrocence units.

Poly- R 070 Ferro- Counter- Transition temperatures ("C) Refer- mer cene a ion ences

Reduced state Oxidized state

34a -CN 0 g 2 4 N I13 I 1364

[36 a1

-

34b -CN 9.4 c10, g 30 N 94 I g 37 N 95 I [36 a1

35a -OCH3 0 - 35b -OCH3 10.4 ClO, g 26 S, 66 N 96 I g 36 N 97 I dec [36a] 35c -OCH3 10.2 1/2SO:- g32SA68N1001 g35SA74N1061 [36b,c]

g 33 S, 97 N 120 I

a Ferrocence content determined by potentiometric titration; Counterion in the oxidized state.

mesogenic ferrocene comonomer has a minor influence on the liquid crystalline behavior. However, oxidation introduced some modifications, especially in polymers 35, depending on the oxidizing agent and the final counterions. In the case of perchlorate, the smectic A phase is suppressed and the polymer shows a low thermal

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stability. On the other hand, polymer 35c in the oxidized state (SO:- as counterion) shows high thermal stability and similar liquid crystalline behavior to the parent polymer.

Dynamic mechanical measurements of the LC ionomers proved the formation of ionic aggregates which act as ionic crosslinking sites. Clusters of ionic aggregates were also observed by SAXS measurements. The same authors also studied blends of polymer 35c (as a redox active LC ionomer) and an amorphous ionomer (sulfonated polystyrene) [36 b, 36~1.

5.3.3 Columnar Thermotropic Polymers

Phthalocyanine-based polymers have been reported as interesting electric conductors [37] The preparation of octaalkyl-substituted phthalocyanine metal complexes, which form columnar liquid crystalline phases over extended temperature ranges [38], led to the design of thermotropic LCPs based on the structure design shown in Fig. 5-23 [40-471. The term spinal columnar liquid crystals was proposed [39] for

Nomenclature M X R1, R2 References

36 Sn 0 Rj = R2 = - C H ~ - O - C I ~ H ~ ~ [391

37 Si 0 R1= R2 = CH2-0-Cj2H25 [401

30 Si 0 R1 = R2 = -O-CnH2,+1 [41-441

39 Si 0 R1 = -0-CHB [43-451

R2 = -0-CnH17

40 Ge 0 R1 = R2 = -COO-C12H25 [461

41 CO CN R1 = R2 = -CH2-O-CaH,, [471

Figure 5-23. Thermotropic spinal columnar polymers based on phthalocyanines.


such polymers. However, most of the examples are polysiloxanes and not truly metallomesogenic polymers.

A different approach to fix the liquid crystalline order of phthalocyanine-metal complexes by polymerization was undertaken by Drenth and coworkers, who synthesized side-chain polymers from unsymmetric, metal-free or metal-containing phthalocyanine monomers [48]. These monomers contain reactive acrylate or meth- acrylate groups which polymerize by a free radical mechanism in benzene solutions. The copper polyacrylate shown in Fig. 5-24 was synthesized in this way. Unlike the monomer, which shows a transition from the crystalline phase into the Dho (discotic hexagonal ordered) mesophase, the polymer is not liquid crystalline. However, X-ray measurements indicate that its structure in the solid state corresponds to a columnar Dho arrangement. As a consequence, this polymer may be of interest as a one-dimensional conductor.

42

Figure 5-24. Side-chain polyacrylate derived from a metallomesogenic phthalocyanine copper@) complex [48].

Metalloporphyrins also show columnar mesophases, and it is possible to insert second and third row transition metals into these systems. Therefore, they are good candidates to give electrically conductive polymers, as proposed by Collman and coworkers [49].

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5.4 Crosslinked Metallomesogenic Polymers

Liquid crystalline elastomers, or anisotropic networks, are among the most interesting examples of crosslinked LC polymers. Liquid crystalline elastomers have very interesting physical properties which arise from the possibility of generating perfectly aligned LC monodomains by applying mechanical fields. For example, a mechanical strain signal can be transformed into an optical signal or electric response (piezoelectric effect) [50]. Ordered polymeric networks obtained by in-situ photopolymerization of reactive low molecular weight liquid crystals [51] have proved to be very attractive as optical devices with permanent properties.

Crosslinked metallomesogenic polymers can be synthesized by two different approaches according to the nature of the crosslinking agent: a) Metal atoms can act as the crosslinking agent. This possibility involves a preformed organic polymer, containing ligand groups which are subject to metal modification. b) Mesogenic metal complexes carrying reactive groups (acrylates, methacrylates, etc.) which allow crosslinking.

5.4.1 Metal as the Crosslinking Agent

The examples reported to date can be classified depending on whether the starting polymer contains the coordination sites in the polymeric chain or in the side groups.

5.4.1.1 Metal-Modified Main-Chain Polymers

There are not many examples of main-chain LCPs containing ligand groups attached to the mesogenic core and which are subsequently modified by metal complexation. Hanabusa and coworkers first reported a series of homo- and copolyesters, containing bipyridinediyl units, which form ionic complexes with iron(r1) and copper(r1) by reaction of metal salts with a solution of the parent polymer (see Fig. 5-25) [52]. EPR measurements on the copper(I1)-modified polymers indicated that an octahedral coordination geometry is the most probable. Polymers with low metal content exhibit smectic or nematic mesohases. However, on increasing the metal content, the liquid crystalline properties are suppressed due to the reduced mobility of the bipyridinediyl mesogenic units caused by metal crosslinks.

Similar conclusions were obtained from a different series of polymers containing P-diketone ligand groups (see Fig. 5-26). Only polymers 45 in which x = 0.8 and x = 0.5 were metal modified due to the lack of solubility of polymer 45 with x = 0. Copper(I1) and nickel@) were used as modifiers [53]. A square-planar coordination is present in the case of copper(r1) but no data regarding the coordination sphere were nickel@) polymers. The modified polymers show nematic behavior, as do the parent systems.


t { O - C H ~ ~ \ ” “ / CH2-OOC-(CH2),-CO

43 n=10,11

f OC-@OO-(CH,&o-Oi 4 O C ~ C O O - C H 2 ~ C H 2 - 0 \ ” \ N / &-x

44 x = 0.9,0.8

M = Cu(ll), Fe(ll)

Figure 5-25. Crosslinking of main-chain LCPs based on metal complexed, mesogenic bipyridine cores [52].

Cu(ll) or Ni(ll)

45 X = 1, 0.8, 0.5, 0

Figure 5-26. Series of thermotropic polyesters containing 8-diketone ligand groups [53].

Main-chain polyazomethines, the repeat units of which are derived from 2,4-dihydroxybenzaldehyde, exhibit liquid crystallinity depending of the central bridge [54]. These polymers can be easily modified by metal complexation. Fig. 5-27 shows the structure of homo- and copolyazomethines which have been modified

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with copper(rr), vanadium(1v) (V02+) , and iron(111) [ 5 5 ] . The mesomorphic properties were retained in modified polymers with a metal content lower than about 30% (molar proportion of metal ions with respect to repeat units). However, the modification obtained by the introduction of the metals is strongly dependent on the central linking groups. The flexible ethylene linkage gives rise to intra-chain coordination, whereas rigid aromatic cores favor crosslinking. In the case of the copolymers, EPR spectroscopy shows that complexation only significantly affects the ethylene units.

Nomenclature -EZk

Figure 5-27. Selected polyazomethines for metal modification [551.

50 (1 : 1)

5.4.1.2 Metal-Modified Side-Chain Polymers

The metal complexation of side-chain liquid crystalline polymers bearing ligand groups generally yields crosslinked materials whose transition temperatures and mesomorphic behavior depend on the degree of crosslinking. The mobility of the chain segments is reduced at the crosslinking sites, affecting the mobility of the adjacent mesogenic groups. As a consequence, when high metal contents are introduced, nonfusible materials are obtained. Slightly crosslinked materials with elastic properties (LC elastomers) can also be obtained by adequate selection of the organic structures and at low metal contents. LC elastomers have thus been obtained via metal-complexation of a side-chain LCP bearing p-cyanobiphenyl mesogenic units


51

52 n=6. 12

Figure 5-28. Crosslinking of side-chain LCPs bearing p-cyanobiphenyl mesogenic units via platinum complexation [56].

(Fig. 5-28) [56]. However, studies of the elastic properties are not reported. Cross- links are formed by a trans platinum(I1) complex, which is synthesized by a ligand exchange reaction. In the cases of polymers with lower metal contents, the appearance of a mesophase of the same nature as that in the parent polymer was confirmed by X-ray diffraction and optical microscopy.

The same authors reported the modification of a polyacrylate based on P-diketone mesogenic units [57] (see Fig. 5-29) which was modified by complexation with copper@), nickel@) and cobalt(II1) (the polymer was modified using CoC12, but the absence of an EPR signal indicated that oxidation of Co(rr) to CO(III) had occurred). In these polymers, the chance to retain the mesomorphic properties (Smectic A phase) depends on the nature of the metal introduced. Copper(I1) stabilizes the mesophase. At relatively high metal contents ( > 20%), however, nickel(I1) and colbat(I11) have a negative effect on the liquid crystallinity of the parent polymer 53, despite the fact that the nickel@) complex has a square planar structure like the copper(I1) complex.

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COO-(CH&O

V 53 Cu(ll), Ni(ll), Co(lll)

% rnol Metal complexed units Transition temperatures ("C)

_ _ _ g 15 SA 170 I

Cu(ll) 7.6 g 15 SA 170 I

__-_

Cu(ll) 50.4 g 16SA240dec

Ni(ll) 11.1 g 12 SA 163 I

Co(lll) 16.6 g 21 SA 154 I

Figure 5-29. Transition temperatures of metal-modified side-chain LCPs containing P-diketone units [57].

A similar synthetic approach was also used by Zhang and coworkers for modifying the P-diketone-based polysiloxanes shown in Fig. 5-30 [ 5 8 ] . Unlike the aforementioned polymers, the authors suggest that metal complexation occurs between two adjacent ligands on the same polymeric backbone. This conclusion was only supported by the fact that metal modified polymers were soluble in common organic solvents. However, no high percentages of metal ions were introduced. In any case, a strong modification of the thermal properties was observed on metal complexation, which led to an increase in mesophase width. The authors suggest a fixation of the ordered macromolecular arrangement of the liquid crystalline state by complexation. Typical textures were not observed by optical microscopy. The possibility of a discotic arrangement of intrachain copper(i1) P-diketonate complexes was proposed, although no evidence for this order was given. Furthermore, complexation using palladium exerts a more marked effect on the retention of the mesogenic behavior. An alternative explanation for the results observed could be given by taking into account previous experience on LC elastomers. In the cases where crosslinking units are mesogenic cores, stabilization of the mesophase formed by the noncrosslinked parent polymer is observed [~OC].

The same research group has described a series of palladium coordinated side- chain LCPs based on imine or azo derivative side mesogenic units (see Fig. 5-31) [59]. An intrachain coordination was also proposed to account for the high solubility of the polymers with low metal content (<65% molar ratio). The parent polymers


X Transition temperatures ("C)

54 Metal complexation u 4 Ji'. .I;- - _ _ --[ -F" .-;$ -x

(S;H2)11 (S;H2)11 ( y b ) 1 1

0 0 0 !$ ( 0 0 :.: )

\ \ \

Figure 5-30. LC polysiloxanes OC1ZH25 oC12H25 0C12H25

containing 8-diketone units as 55

M = Cu; x = apr. 0.7

M = Pd; x = apr. 0.7

K 74 LC 125 I

K 106 S 205 dec

ligand groups (structure of the modified polymer proposed by the author) [58].

show a smectic phase which is modified by complexation, depending on the metal and ligand bridges as follows:

(1) The acetato-bridged polymers show a smectic phase (the nature of the smectic phase was not assigned) for low palladium contents, but are nematogenic when the Pd content is higher than 27 percent.

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(2) In the cases of the chloro-bridged polymers, an increase in the mesophase range is observed upon complexation. The mesophase observed was assigned as discotic nematic on the basis of optical textures.

The adverse effect of the acetate bridge on the smectic mesophase stability was explained in terms of its open-book configuration [60] which disrupts the ordered arrangement of the macromolecules.

% mol Polymer x Ligand bridges (Y) complexed units Transition temperatures ("C)

g 90 S 116 I

K 81 S 118 N 138 I

-- 56 CH

57 N

58 CH CH&OO 11 g 93 S 116 I

CHs-COO 27 g 8 5 N l l l I

_ _

CI 27 K82ND215dec

59 N CI 30 K 97 Ni 135 N2 200 dec

Figure 5-31. LC polysiloxanes containing imine and azo units as ligand groups (structure of the modified polymer proposed by the author) [59].

It should be noted that due to the incomplete characterization of these polymers, these conclusions should be treated with caution. Confirmation of the structure of some mesophases proposed is required, as well as more detailed studies of the modification caused by the introduction of metals. The solubility of the modified polymers does not constitute sufficient evidence that non-crosslinked polymers are obtained.

Serrano and coworkers synthesized side-chain LCPs, based on salicylaldimine mesogenic cores, which are readily complexed with copper(I1) ions (see Fig. 5-32)


60 n=6 , 10

Crosslinked polymers 61

% mol Polymer n complexed units Transition temperatures ("C)

K 114 sx 155 SA 172 N 179 I

K 72 Sc 91 SA 159 I

_ _ 60 6

60 10

61 6 10 K 131 LC 178 I

_ _

61 10 10 K 78 Sc 174 I

61 10 20 K80Sc1831

61 10 65 K 80 Sc 201 I

Figure 5-32. LC polyacrylates based on salicylaldimine as ligand groups [61].

[61]. From premiliminary results, it can be concluded that copper(r1) crosslinking stabilizes the smectic C phase of the parent polymer 60 (n = 10).

5.4.2 Reactive Metal-Containing Liquid Crystals

The design and synthesis of low molecular weight metal-containing liquid crystals containing reactive groups allows crosslinked polymers to be obtained. By selecting a suitable method of polymerization, highly ordered polymeric networks can be formed. In search of supramolecular structures as low dimensional conductors, Drenth and coworkers [62] designed octa-n-alkoxy substituted phthalocyanines and

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62

X = H or CH3

M = HH (metal free), Cu, Zn, Co, Ni, Pb

Figure 5-33. Reactive metallomesogens based on metallophthalocyanines (only metal free and copper(1r) monomers were polymerized) [62].

their metal complex derivatives carrying functional groups at the ends of the aliphatic chains (Fig. 5-33). The aim was to fix the columnar organization of a discotic mesophase (especially Dho) by polymerization.

Attempts to carry out the photoinitiated polymerization in the ordered mesophase were unsuccessful. For this reason, thermal-bulk polymerization at the mesophase temperature, using AIBN as the initiator, was used to obtain the polymeric networks. Metal-free acrylates or methacrylates 62 and their copper(l1) complexes were used as monomers. The materials obtained did not reveal any phase transition in the DSC measurements, thus suggesting the formation of a crosslinked network. Further- more, the columnar structure present in the monomer is maintained in the polymer without change, as demonstrated by X-ray measurements. The conductivity of the polymer was shown to be higher (about 2 orders of magnitude) than that of the corresponding monomer.

Attard and coworkers have also reported [63] the topochemical photopolymerization of a diacetylenic dinuclear copper(I1) complex (Fig. 5-34). This complex exhibits an unusual lamellar discotic phase the viscoelastic properties of which easily allow the processing of highly ordered crystalline fibers. The exceptional orientation of these fibers can be locked into a polymeric network by UV irradiation without disrupting the superstructure of the fiber. Application of these highly ordered poly- diacetylene networks as nonlinear optical media has been suggested.

Finally, the polymerization of a solution of a copper(I1) P-diketonate reactive complex (Fig, 5-35) by Hanabusa an co-workers should be mentioned [57b]. How-


63

Phase transitions ("(2): K 85-89 D 210 dec

Figure 5-34. Reactive metallomesogens based on a copper(I1) carboxylate [63].

64

Figure 5-35. Copper(1r) P-diketonate diacrylate [57 b].

ever, the experimental description of the polymerization, and the thermal properties of the supposed network indicate the formation of oligomers.

5.5 Borderline Cases

Metallomesogenic polymers give rise to new possibilities in the relatively well-established field of liquid crystalline polymers. However, the limits of this recently emerging line of research are not completely defined. Thus, there are some examples of liquid crystalline polymeric materials into which metals are introduced in order to achieve specific properties. Consequently, the metal can be considered as an additive rather than a structural component of a metallomesogenic unit. Some selected examples will be discussed in this section.

From these borderline cases, LCPs containing crown ethers or oligooxyethylene segments in their structure deserve to be mentioned since interesting properties are

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SELF-REGULATION OF MESOMORPHIC BEHAVIOR MESOGENIC

RECOGNITION

U 65

Figure 5-36. Self-regulation of mesomorphic behavior of LCPs containing crown ethers via molecular recognition [64].

achieved upon metal complexation. As an example, Percec and coworkers [64] described the ability to self-regulate the mesomorphic behavior of the side-chain LCPs shown in Fig. 5-36 by molecular recognition of alkali ions.

Kimura and coworkers [65] have designed ion-conducting materials based on side- chain LCPs containing crown ethers and photosensitive azobenzene mesogenic units. In these “smart” materials, the ionic conduction can be switched by UV irradiation. The photoinduced ionic conductivity switching is suggested to be based on the local disturbance of the smectic order due to photochemically induced cis-trans isomerization of the azobenzene units. As a consequence, there is an alteration of the migration behavior of ions between the crownether hopping sites, the latter being closer to one another in the smectic arrangement (see Fig. 5-37). Similar structural designs have been applied to the synthesis of mesomorphic polyelectrolytes [66] based on host -guest systems with oligooxyethylenic spacers instead of crown ethers.

A number of examples of linear polyethyleneimines reported by Ringsdorf and coworkers, which are N-substituted with mesogenic cores, can also be considered as borderline materials [67]. The complexation of the polymeric backbones of these polymers with copper(@ enhances the stability of their layered structures. Conse- quently, metals can be employed as additives to stabilize the smectic order of Langmuir-Blodgett (LB) mono- and multilayers obtained from these polymers.


SITES FOR HOPPING OF

ION-CONDUCTING CARRIERS 0 -

66 PHOTOSENSITIVE

UNIT -

UV light

-~ 6

LOCAL DISTURBANCE OF THE SMECTIC ORDER

Figure 5-37. Photoresponsive ion-conducting behavior of LC polysiloxanes carrying crown ether-substituted azobenzene units [65].

These representative examples underline the diffuse nature of the “borderline”, and also the interdependence between organic LCPs and metallomesogenic polymers.

5.6 Concluding Remarks

Because of the relatively recent developments in the field of metallomesogenic polymeric systems, initial endeavors have targeted the design and synthesis of materials, mainly based on the molecular engineering of organic LCPs. However, deeper studies are required in order to establish correlations between mesogenic activity and molecular structure, as well as to understand the numerous geometric possibilities introduced by the metal complexes. Efforts to design alternative structures which differ from organic LCPs are especially important in the case of side-chain polymers, which require the synthesis of unsymmetrically substituted metal-containing monomers. The synthesis of ordered ultrastructures containing metal centers is also an important goal which can be achieved by in-situ polymerization of reactive metallo-

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mesogens. By this method, liquid crystalline monodomains of metallomesogens can be frozen to yield an anisotropic network. The introduction of chirality, based on the metal complex geometry, is also an interesting goal to be achieved.

However, one of the main challenges for materials scientists working in the field of metallomesogenic polymers is the thorough investigation of the physical properties, which can open an access to new applications of liquid crystalline polymers.

5.7 References

[I] a) OrganometaNic Polymers (Eds: C. E. Carraher, C. U. Pittman, J. E. Sheats), Academic Press, New York, 1978; b) C.E. Carraher, J. Chem. Ed. 1981, 58, 921 -934; c) Metal- Containing Polymeric Systems (Eds: J. E. Sheats, C. E. Carraher, C.U. Pittman), Plenum Press, New York, 1985: d) J. E. Mark, H. R. Allcock, R. West, Inorganic Polymers, Pren- tice Hall, Englewood Cliffs, 1992; e) H.R. Allcock, Adv. Muter. 1994, 6, 106-115.

[2] See for instance: C. T. Chen, K. S. Suslick, Coord. Chem. Rev. 1993, 128, 293 - 322 and references therein.

[3] a) S. Takahashi, M. Kariya, T. Yatake, K. Sonogashira, N. Hagihara, Macromolecules 1978, 11, 1063-1066; b) S. Takahashi, E. Murata, M. Kariya, K. Sonogashira, N. Hagihara, Macromolecules 1979, 12, 1016- 1018.

[4] N. Hagihara, K. Sonogashira, S. Takahashi, Adv. Polym. Sci. 1981, 41, 149-179. [5] M. L. H. Green in OrganometaNic Compounds, (Eds: G. E. Coat, M. L. H. Green, K.

(61 S. Takahashi, H. Morimoto, Y. Takai, K. Sonogashira, N. Hagihara, Mol. Cryst. Liq.

[7] S. Takahashi, Y. Takai, H. Morimoto, K. Sonogashira, .l Chem. Soc., Chem. Commun.

[8] S. Takahashi, Y. Takai, H. Morimoto, K. Sonogashira, N. Hagihara, Mol. Cryst. Liq.

[9] M. Motowoka, T. Norisuye, A. Teramoto, H. Fujita, Polym. J. 1979, 11, 665-670.

Wade), 3 I d ed., Vol. 2, Methuen, London, 1968, p. 203.

Cryst. 1981, 72, 101-105.

1984, 3-5.

Cryst. 1982, 82, 139-143.

[lo] a) A. Abe, S. Tabata, N. Kimura, Polym. J . 1991, 23, 69-72; b) A. Abe, N. Kimura, S.

[I 1 3 K. Onitsuka, H. Ogawa, T. Joh, S. Takahashi, Chem. Lett. 1988, 1855- 1856. [I21 S. Kotani, K. Shiina, K. Sonogashira, Appl. Organomet. Chem. 1991, 5, 417-425. [13] a) S.A. Jenekhe, P.O. Johnson, A.K. Agrawal, Polym. Muter. Sci. Eng. 1989, 60,

404-409; b) S. A. Jenekhe, P. 0. Johnson, A. K. Agrawal, Macromolecules 1989, 22, 3216-3222; c) S. A. Jenekhe, P.O. Johnson, Macromolecules 1990, 23, 4419-4429; d) M.F. Roberts, S.A. Jenekhe, Polym. Commun. 1990, 31, 215-217; e) M. F. Roberts, S.A. Jenekhe, Chem. Muter. 1990,2, 629-631; f) C. J. Yang, S. A. Jenekhe, Chem. Mat- er. 1991, 3, 878-887; g) S.A. Jenekhe, C. J. Yang, Chem. Muter. 1991, 3, 985-987.

[I41 a) A.A. Dembek, R.R. Burch, A.E. Feiring, Polyrn. Prep. 1993, 34, 172- 173; b) A.A. Dembek, R. R. Burch, A. E. Feiring, J. Am. Chern. Soc. 1993, 115, 2087-2089; c) A. A. Dembek, P.J. Fagan, M. Marsi, Macromolecules 1993,26, 2992-2994; d) A. A. Dembek, R. R. Burch, A. E. Feiring, Macrornol. Symp. 1994, 77, 303 - 313.

Tabata, Macromolecules 1991, 24, 6238 - 6243.


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Crystal Conference, Budapest, Hungary, July 1994.

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230 L. Oriol

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Chem. 1989, 190, 2727 - 2745. [49] J. P. Collman, M. B. Zisk, W. A. Little in Organic Superconductivity (Eds: V. Z. Kresin,

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[51] see for instance: a) D. J. Broer, J. Boven, G.N. Mol, G. Challa, Mukromol. Chem. 1989, 190, 2255 -2268; b) D. J. Broer, I. Heynderickx, Macromolecules 1990, 23, 2474-2477; c) I. Heynderickx, D. J. Broer, Mol. Cryst. Liq. Cryst. 1991,203, 1 13 - 126; d) M. H. Litt, W. Whang, K. Yen, X. Qian, J. Polym. Sci. Polym. Chem. 1993, 31, 183-191.

[52 ] K. Hanabusa, J. Higashi, T. Koyama, H. Shirai, N. Hojo, A. Kurose, Mukromol. Chem.

[53] K. Hanabusa, Y. Tanimura, T. Suzuki, T. Koyama, H. Shirai, Makromol. Chem. 1991,

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27, 1869- 1874; b) P. J. Alonso, J.1. Martinez, L. Oriol, M. Piiiol, J. L. Serrano, Adv. Mater. 1994, 6, 663-667.

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[60] P. Espinet, J. Etxebarria, M. Marcos, J. PCrez, A. Remon, J. L. Serrano, Angew. Chem. Znt. Ed. Engl. 1989, 28, 1065-1066.

[61] a) E. Campillos, PhD Thesis, University of Zaragoza, 1993; b) P. J. Alonso, E. Cam- pillos, M. Marcos, J. I. Martinez, L. Oriol, M. Pifiol, J. L. Serrano, Poster presented at the I S h International Liquid Crystal Conference, Budapest, Hungary, July 1994.

[62] J.F. van der Pol, E. Neeleman, J. C. van Miltenburg, J.W. Zwikker, R. J.M. Nolte, W. Drenth, Macromolecules 1990, 23, 155- 162.

[63] G.S. Attard, R.H. Templer, J . Muter. Chem. 1993, 3, 207-213. [64] see for instance: a) V. Percec, G. Johansson, R. Rodenhouse, Macromolecules 1992, 25,

2563-2565; b) V. Percec, G. Johansson, J . Muter. Chem. 1993, 3, 83-96; c) V. Percec, J. Heck, G. Johansson, D. Tomazos, M. Kawasumi, G. Ungar, J . Macromol. Sci. 1994, A31, 1031-1070; d) V. Percec, J. Heck, G. Johansson, D. Tomazos, M. Kawasumi, P. Chu, G. Ungar, J . Macromol. Sci. 1994, A31, 1719-1758.

193, 2149-2161.

[65] H. Tokuhisa, M. Yokoyama, K. Kimura, Macromolecules 1994, 27, 1842- 1846. [66] see for instance: a) C. J. Hsieh, G.H. Hsiue, C.S. Hsu, Makromol. Chem. 1990,

191, 2195-2203; b) V. Percec, D. Tomazos, J. Muter. Chem. 1993, 3, 633-642 and

[67] R. Festag, H. Ringsdorf, M. Seitz, J. H. Wendorff, Poster presented at the lSh Znterna- 643 - 650.

tional Liquid Crystal Conference, Budapest, Hungary, July 1994.

Part B. Synthesis

From a chemical point of view, metallomesogens can be considered a “meeting point” for inorganic and organic chemists working in the field of’ liquid crystals. Therefore, the design and synthesis of metallomesogens involves strategies different from those used in conventional organic liquid crystal synthesis. The versatility of organic chemistry, which allows the preparation of a wide variety of ligands, combined with the possibilities of coordination chemistry, opens access to a wide range of metal-containing mesogenic structures.

Chapters 6 and 7 are devoted to the preparation of low and high molecular weight metallomesogens. They are intended to be a guide and source of inspiration for the design of metal-containing liquid crystals by helping inorganic chemists in the selection of organic ligands and assisting organic chemists in the choice of metal complexes with an adequate geometry. The main synthetic routes leading to low molecular weight materials are reviewed in Chapter 6 , which covers both the synthesis of ligands and the coordination of metals. The molecular engineering of metallomesogenic polymers is discussed in Chapter 7, in which particular attention is paid to the different possibilities of incorporating metallomesogenic units into polymers.

6 Design and Synthesis of Low Molecular Weight Metallomesogens

Mercedes Marcos

6.1 Introduction

Metallomesogens consist of a metal ion (the electron acceptor) and one or more donor ligands. The liquid crystalline properties of a metallomesogen depend on the nature of both the metal ion and the ligands. The variety of metal ions which can be incorporated into metallomesogens is considerable, but the variety of ligands available is virtually unlimited due to the deep knowledge in the field of organic chemistry available for their synthesis. Many types of ligand are known, and the mesogenic properties of their metal complexes have been investigated. As a result of this information, the possibility now exists for the design of new ligands. The application of these could lead to metal complexes with extraordinary and even predic- table properties.

The design of a ligand for a metallomesogen involves the consideration of the stability and geometry of the resulting metal complexes. Some ligands are, of course, easier to synthesize than others, and a useful design should bear in mind to provide a facile synthetic accessibility. In this context, a number of reaction types have been shown to be especially useful for ligand synthesis, although, in principle, all organic reactions are applicable.

The synthesis of ligands and complexes will be dealt with in this chapter, and the most relevant methods will be illustrated using selected examples. The discussion is concerned with monodentate, bidentate or polydentate ligands containing nitrogen, oxygen, or sulfur donor atoms. Aspects of the design and synthesis of ligands which form 0- or x-carbon-metal bonds in their complexes will also be dealt with. The discussion in this chapter is divided into sections according to the nature of the ligand rather than the nature of the complexes formed.

236 M. Marcos

6.2 Metall-Organic and Organometallic Liquid Crystals with Monodentate Ligands

Metallomesogens with monodentate ligands can be synthesized by the reaction of a commercially available or previously prepared metal complex with a promesogenic ligand (Scheme 6-1).

cJs-[MCI(CO)~RCN] M=Rh CJS-[MX(CO)~RP~] M=Rh,lr

trans-[MC12(RCN),] M=Pd,Pt [AS(RPYM+X'

Metal -- [MXz(CNR)2] M=Pd, Pt [AgSRl - RSH i I Intermediate RNC [AuCI(CNR)]

Scheme 6-1.

The appropriate choice of metal allows the synthesis of liquid crystalline complexes containing either one or two ligands. For example, rhodium(1) or iridium(1) form metallomesogens containing only one ligand where the metal group takes a terminal position within the molecule. The use of metals such as palladium(II), platinum(@, or some silver(1) complexes gives rise to a calamitic molecular structure containing two ligands. Here, the metal acts as a linking unit between the two organic ligands.

These materials have the particularity that the ligands do not undergo chemical modification prior to coordination.

The introduction of the metal atom in the examples discussed above causes increased phase transition temperatures and, in certain cases, can even give rise to liquid crystalline properties by coordination of nonmesogenic ligands to the metal. This is the result of an increase in the molecular polarizability without increasing the molecular width.

These complexes can be either covalent (Rh, Ir, Pd, Pt complexes) or ionic in character (some Ag or Cu complexes). In the case of ionic systems, the counter ion plays an important role for the mesomorphic properties of the complexes and must be chosen carefully.

6 Design and Synthesis of Low Molecular Weight Metallomesogens 237

6.2.1 Nitrile Derivatives

Synthesis of the Ligands

The ligands used in the synthesis of liquid crystalline nitrile derivatives are either commercially available or can easily be obtained by Williamson etherification of 4-hydroxy-4-cyanobiphenyl (Scheme 6-2).

Rhodium Derivatives

The reaction between a liquid crystalline 4-alkoxy-4'-cyanobiphenyl ligand and the complex [Rh,Cl,(CO),] proceeds by cleaving the chloro bridges of the dinuclear complex and subsequent figuration (Scheme 6-3).

Scheme 6-3.

formation of the mononuclear species [I , 21 with cis-con-

oc, /'I\ $0 hexane F' oc' 'cI/ 'CO co

2 R-CN + Rh Rh 2 OC-Rh-NC-R

Palladium and Platinum Derivatives

The reaction between a 4-alkoxy-4'-cyanobiphenyl ligand and the complexes [M(C6H5CN),Cl2] leads to liquid crystalline complexes with the formula trans- [MC12L2], where M = palladium(I1) or platinum(l1) (Scheme 6-4) [ I , 21. Platinum(rr) complexes can also be obtained by reaction of the ligand with [PtC121,] [3].

.. -,. J 1 or toluene

Scheme 6-4. M = Pd, Pt

238 M, Marcos

6.2.2 Pyridine Derivatives

Synthesis of the Ligands 4-Alkoxy-4’-stilbazoles [4] have been synthesized by Heck reaction [5] of 4-alkoxy- iodobenzene with 4-vinylpyridine using a palladium acetate catalyst and triethylamine in acetonitrile as solvent (Scheme 6-5).

Scheme 6-5. n

4‘-n-Alkoxy-N-(4-pyridylmethylene)anilines are formed by condensation of 4-pyri- dinecarboxaldehyde with a 4-n-alkoxyaniline in dry ethanol with small amount of acetic acid as catalyst (Scheme 6-6) [6].

Scheme 6-6.

4‘-Alkoxyphenyl-4-pyridinecarboxylates are prepared by reaction of a 4-alkoxyphenol and 4-picolinic acid chloride with triethylamine in dichloromethane (Scheme 6-7) [6] .

Scheme 6-7,

Rhodium and Iridium Derivatives Rhodium(1) and iridium(1) pyridine complexes are obtained by reacting dinuclear complexes with the general formula [MX(COD)]2 (COD = 1,5-cyclooctadiene), with promesogenic pyridine ligands [L,: alkoxystilbazole, or 4’-n-alkoxy-N-(4-pyridyl-

6 Design and Synthesis of Low Molecu lar Weight Metallomesogens

M = Rh(l), Ir(l)

X = CI or Br

Scheme 6-8.

239

oc, ,L"

oc' 'x M

L2

methy1ene)aniline) in a 1 : 2 molar ratio. Square-planar complexes with the general formula [MX(COD)L, ] are formed according to Scheme 6-8. When carbon monoxide is bubbled through a solution of the complex [MX(COD)L,] in dichloromethane, the coordinated olefin is rearranged, and a c~s-[MX(CO)~L,] complex is formed (M = Rh', Ir', and X = C1 or Br) [7-91.

Platinum Derivatives

The unsymmetric metallomesogens trans-(z;l'-alkene) (4'-alkoxy-4-stilbazole)di- chloroplatinum(I1) are synthesized by the method shown in Scheme 6-9, by replacing of ethene in complex (2) (obtained from Zeise's salt (1) by reaction with the alkoxystilbazole (L)) by another alkene (Scheme 6-9) [lo].

Silver Derivatives

Silver(1) salts can be obtained [3, 6, 1 I ] by addition of the ligand L, (L, = alkoxystilbazole, L2 = 4'-n-alkoxy-N-(4-pyridylmethylene)aniline or L3 = 4'-alkoxyphenyl- 4-pyridinecarboxylate) to a solution of a silver([) salt (AgX, X = BF,, CF,SO,, NO;, PF, , DOS (dodecylsulfate)) in ethanol. The ionic complexes with the general formula [Ag(L,)2] + X - are obtained as precipitates. Analytical and spectroscopic data confirm the presence of two ligands in accordance with the stoichiometry and structure shown in Scheme 6-10.

240 M. Marcos

0 0 AQX + 2Ln [ L-Ag-Ln]X

Scheme 6-10.

6.2.3 Isonitrile Derivatives

Synthesis of the Ligands Isonitrile derivatives are prepared according to Scheme 6-1 1 . The appropriately substituted nitro compound is first reduced to the amine. Formylation of the amino group using formic acid and acetic acid anhydride, followed by dehydration in the presence of phosphoryl chloride and diisopropylamine yields the ligand [ 121.

(CH3C0)20 * R-NH-CHO - R-NC HP, Pd/C(B%)

EtOH/CH&OOEt, r.t. HCOOH. 50°C - HZO t R-NH2

Scheme 6-11. R-NOZ

Platinum and Palladium Derivatives Transition metal isonitrile complexes are generally prepared by ligand exchange reactions [12] which are illustrated in Scheme 6-12 (see p. 241). Platinum(1r) complexes are obtained by reaction of PtXz (COD) (X = CI, Br or I) with two equivalents of an isonitrile ligand in chloroform. The palladium(r1) analogs were prepared by reacting palladium chloride with two equivalents of an isonitrile in the presence of excess KX (X = Br or I ) (6 equivalents) in acetone or, alternatively, by reaction of PdC12(CH3CN), with an isonitrile in acetone.

Gold Derivatives In contrast to the platinum(r1) and palladium(r1) compounds discussed above, isonitrile gold(1) complexes contain only one organic ligand coordinated to the metal center and have the general formula AuCl(CNR). The gold(1) complexes 3 are prepared by a ligand exchange reaction (see Scheme 6-13, p. 241) of a mesogenic isonitrile (2) and AuCl(S(CH,),) (1) [13].


lNc-R CHC13

Scheme 6-12.

acetone I PdC12 H,C-CN-Pd-NC -CH3

6.2.4 Alkynyl Derivatives

Synthesis of the Ligands 1 -Alkynes are synthesized according to Scheme 6-14 by a coupling reaction between an aryl halide and trimethylsilylacetylene using a homogeneous catalyst consisting of palladium(I1) and copper(I1) complexes. Subsequent desilylation using tetra- n-butylammonium fluoride yields the 1 -alkynyl ligand [ 141.

Scheme 6-14.

n-Bu4NF

(3) THF, -78"C* R/eCrCH

242 M. Marcos

Platinurn Derivatives Only platinum(1r) alkynyl derivatives have been reported so far. Platinum(I1) alkynyl derivatives bearing trimethylphosphine or triethylphosphine as ligands have been synthesized (Scheme 6-15), and both are liquid crystalline [14]. The method of synthesis led to the preparation of symmetrically or unsymmetrically substituted complexes.

Scheme 6-15. R' =or+R2

6.2.5 Alkylamine Derivatives

Silver Derivatives Alkylamines form cationic liquid crystalline complexes when coordinated to silver(1). The stoichiometric reaction (Scheme 6-1 6) between alkylamines and soluble silver salts in acetonitrile yields salts with the formula [Ag(H2NC,H2,+ l)]X (X = NO,, BF,, CH,CO,) [15].

X = NOi, BF;, CH3COY Scheme 6-16.

Copper Derivatives Alkylaminocopper(ii) complexes are prepared by reaction of copper nitrate with long-chain primary (such as octyl-, decyl-, dodecyl- and octadecylamine) or secondary (didodecylamine) amines (Scheme 6-1 7).

L' acetonitrile I

C U ( N O ~ ) ~ ~ H ~ O + 4 H2NCnH2n+l - L' -c,"- L'

L' L'


Analysis has shown that primary amines form tetracoordinated copper(I1) complexes, whereas the complexes formed with secondary amines show a metal to ligand ratio of 1 : 2 [16].

6.2.6 Thiolate Derivatives

Silver Derivatives

A series of liquid crystalline aliphatic primary silver(1) thiolates (AgSC,H2, + ,), has been synthesized according to Scheme 6-1 8 [ 171.

The synthesis and handling of the compounds was carried out, where possible, excluding light as a precaution against photodecomposition. However, the isolated compounds proved to be photostable. Preparation of the same complexes employing different stoichiometries and reactant concentrations revealed that these conditions have a significant effect on the transition temperatures of the complexes formed [17].

6.3 Metal-Organic Liquid Crystals with Bidentate and Tetradentate Non-Cyclic Ligands

In this section, the preparation of metal chelate complexes derived from bidentate or tetradentate ligands will be described. The variety of different ligands and complexes discussed in this section is summarized in Scheme 6-19, see p. 244.

6.3.1 Preparation of [M-(N,O,)]-Type Metallomesogens

6.3.1.1 Schiff Base Derivatives

A) Bidentate Schiff Base Ligands

The method of synthesis for bidentate Schiff bases depends on the nature of the substituents attached to the ligand. In general, the ligands are prepared by condensation of the appropriate aldehyde with a substituted aniline in ethanol in the presence of small amounts of glacial acetic acid. The complexes can be obtained by either of the two synthetic routes shown in Scheme 6-20 [18-261.

244 M. Marcos

f

P N -

I ~ - 4 : ~ Or R 4 - Metallic SNa intermediate s, .s

n=1 ,2 R-(,M.)--R *

n = 1 , 2 , 4

U

n = 1,2,3 Scheme 6-19.

Route a shows the synthesis of the Schiff base ligand as described above, followed by the reaction of a metal ion with the Schiff base in the presence of a base. These reactions are usually carried out in homogeneous alcoholic or aqueous alcoholic solution in the presence of a base such as acetate or triethylamine. This method is useful for the synthesis of N-arylsalicylaldimines but occasionally fails for N-alkyl complexes due to hydrolysis of the Schiff base [IS].

Route b represents an alternative approach. In this method a bis(salicyla1de- hydato)-metal complex is formed, and the precipitated salt is reacted with a primary amine [19]. This procedure, originally discovered by Schiff, is very convenient and therefore the most important general preparative method for this type of compound. However, Route a is most commonly used in the synthesis of metallomesogens [20 - 261, because it allows comparisons to be made between the mesomorphic properties of the complexes and those of the metal-free ligands.

The intermediate salicylaldehyde complexes formed in Route b have a rod-like structure when an alkoxybenzoate group is attached to the 5-position of the salicylaldehyde ring and therefore are liquid crystalline [27], In contrast, when the alkoxybenzoate substituent is situated in the 4-position of the salicylaldehyde ring,


p = position 4 or 5 R'= alkoxy, HZn+,CnCOO, H 2 n + l C n O O C 0 0

R3 = H, CH3

M = Cu(ll), Ni(ll), Pd(ll), VO(IV), Zn(ll), Co(ll), FeCI(II1) Scheme 6-20. A=AcO, Sodz.

none of the bis(salicylaldehydato)copper(II) complexes described exhibits mesomorphic properties [ 191.

The geometry of the complexes depends on both the metal and the ligand, and it can range from square-planar to tetrahedral. Square-planar and square-pyramidal geometries give rise to liquid crystallinity, whereas complexes with a tetrahedral geometry are generally not mesomorphic.

The number of liquid crystalline complexes incorporating Schiff base ligands is large due to the variety of substituents (R', R2 and R3 in Scheme 6-20) that can be attached to the ligand [ 18 - 261.

2-Hydroxyazobenzenes form metal complexes in a similar way to salicylideneanilines. They have been used as ligands in an attempt to generate mesogenic materials [28, 291. However, the complexes obtained are not liquid crystalline.

B) Tetradentate Schiff Base Ligands Tetradentate Schiff base ligands (4) (Scheme 6-21) are prepared by condensation of 5-alkoxy-2-hydroxybenzaldehyde (2) (synthesized from 4-alkoxyphenol(l) according to the method of Casiraghi et al. [30]), with a diamine (3). The copper(II), nickel(I1)

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[31] and oxovanadium(1v) [32] Schiff base complexes (5) have been synthesized by coordination of the metal with the diimine ligand (4) according to Scheme 6-21.

CHO

NiCI2.6H20 I or (VO)S045H20 01 CU(OAC)~.H~O

M=Ni(II), Cu(ll), VO(IV)

= C,HI (salen) C~H; isalp) ’ CH2C(CH3)2CH2 (Me2salpn) Scheme 6-21.

Calamitic metallomesogens containing two dicarbonyl-rhodium(1) or iridium(1) moieties bound to a tetradentate Schiff base ligand have been synthesized modifying the established coupling reaction between 2,4-dihydroxybenzaldehyde and the appropriate diamine [33]. The synthesis of these materials is shown in Scheme 6-22.

The bis(dicarbonyl)rhodium(I) complexes are prepared by reaction of the ligand (H2L,) with [ Rh2(C0)4C12] in methanol/tetrahydrofuran in the presence of triethylamine.

The iridium(1) complexes [IIr (C0>2]L,] are prepared in two steps, by first reacting the ligand (H2L,) with [Ir2(COD),C12] to give the bis(cyclooctadiene)iridium(r) complexes [(Ir(COD)]L,]. The cyclooctadiene is then replaced by reaction with carbon monoxide (1 atm, 20°C). Only the complexes containing a trans-cyclohexyl linking unit (L, in Scheme 6-22) are liquid crystalline.

Complexes of Schiff base ligands derived from 1,3-diketones have also been investigated [34]. The palladium(II), nickel(Ii), and copper(i1) complexes all have a square-planar geometry, the vanadyl derivative is square-pyramidal. The synthesis is carried out as illustrated in Scheme 6-23 by reacting the 8-diketone first with ethylenediamine and subsequently with the metal salt.


C,0H21 polyeth Br/KHC03 yleneglycol-dioxane CIOHZIOQCHO

HoQcHo OH OH

H

H A

[Rh2(C0)4CI,]/ MeOH-THF, Et3N or [Ir2(COD),CI2]/ CO

M = Rh(l). Ir(l) R=-CH2CH2-, 0 .

Scheme 6-22. L1 L2 4

6.3.1.2 Enaminoketone Derivatives

Enaminoketones are highly promising Iigands because of their rod-like molecular shape and their ability to form stable transition metal complexes. Enaminoketone copper(I1) complexes [35] are synthesized following the route shown in Scheme 6-24, see p. 248.

Methylketone 1 is prepared from the salt 2 by a formylation reaction. Liquid crystalline ligands (HL) were obtained when a methanolic solution of 2 was treated with the appropriate amine (3). The copper complexes (CuL,) are prepared by reaction of the ligands with copper(I1) acetate.

6.3.1.3 Aroylhydrazine Derivatives

Aroylhydrazones (A, Fig. 6-1) can coordinate to a divalent metal ion either from the enol form (B) or the ketone forms (C and D).

Sacconi [36] showed that the tendency of the ligand to react with nickel(I1) in the enol form becomes more pronounced as the conjugation with the group R in the hydrazine moiety increases. Aryl substituents favor the enol tautomer of such ligands.

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Ro / + H~NCH~CH~NHZ

ethanol

O 0 1

MA.nH20 1 M = Pd(ll), Ni(ll), Cu(ll). VO(IV) A = AcO-, Sod’

R O G / \ R O R

RO OR RO OR Scheme 6-23.

MeOH, reflux CU(OAC)~.~H~O

CUL2 Scheme 6-24.

The structure of the complexes formed is strongly dependent on the coordination ability of the counterion of the metal salt. For instance, aroylhydrazones (A in Fig. 6-1) react with nickel(1r) acetate yielding the corresponding bis(aroy1hydra- zinato)nickel(Ir) complexes (B) via deprotonation at the P-nitrogen atom. In con-


O-H I

0 II

R - C - N H - N = C H - R ' 3 R-C = N-N=cH-R'

Keto Enol

A

Figure 6-1. 0 C D

trast, the reaction of (A) with nickel@) chloride gives the dichlorobis(aroy1hydra- zone)nicke1(11) (D see Fig. 6-1). These octahedral complexes undergo dehydrohalogenation to give the square-planar neutral complexes upon treatment with alcoholic potassium hydroxide. In order to maintain a planar central core in the molecule and to minimize an increase in molecular width, both factors that can pro- foundly affect the mesogenic behaviour of molecules, the complexes bis[N-methylidene-(4'-n-alkoxy)benzoylhydrazinato]nickel(~i) or copper(I1) were synthesized as outlined in Scheme 6-25 [37].

KOH/EtOH

reflux 2.5 h EtO-C '-OH + RBr -

H

2) M(02CCH&.nH20 EtOH, reflux

RO

M = Ni(ll), n = 4

M = Cu(ll). n = 2

Scheme 6-25. R' = , H2,+1CnO 0

H,NNH,.HpO EtOH, reflux 1

6.3.1.4 6,6'-Diamino-2,2'-bipyridine and 2-Aminopyridine Derivatives

6,6'-Diamino-2,2'-bipyridine and 2-aminopyridine derivatives have been recently reported in the literature [38] as novel N202-type ligands to give square-planar metal-

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lomesogens. The ligands were synthesized by reaction of 6,6’-diamino-2,2’-bipyridine or 2-aminopyridine with the appropriately suhstituted acid chloride. The 6,6‘- bis(acylamino)-2,2’-bipyridines act as tetradentate ligands, coordinating to the metal in a square-planar geometry via the nitrogen atoms belonging to the pyridine rings and the oxygen atoms of the amide groups. Dissociation of amide protons upon complex formation may afford complexes analogous to N, N’-di(salicy1idene)- ethylenediamine (salen) complexes (Sect. 6.2.1.1 B). These compounds are expected to be more stable than the corresponding salen complexes, especially in acidic solution, because the easily hydrolyzable Schiff base moieties of the salen complexes are replaced by the more stable bipyridine moiety. 2-Acylaminopyridine derivatives can act as bidentate ligands via the nitrogen atom in the ring and the oxygen atom of the amide group. They form square-planar complexes. In this case the ligand to metal ratio is 2: 1.

Copper(rI), nickel(Ir), cobalt(I1) and palladium(I1) complexes of 2-acylaminopyri- dines were synthesized using copper nitrate, nickel nitrate, cobalt acetate, and lithium tetrachloropalladate in methanol or ethanol according to Scheme 6-26.

Scheme 6-26.

H R A N-C‘

M = Cu(ll). Ni(ll), Co(ll), Pd(ll) H

Li[PdCI,] MeOH

-

6.3.2 Preparation of [M-02]-, [M-041-, and [M-O6]-Qpe Metallomesogens

6.3.2.1 j?-Diketone Derivatives

There are two types of P-diketone used in this kind of complex, 1,3-disubstituted propane-1,3-diones and 3-substituted pentane-2,4-diones. The synthetic routes to the two types of ligand differ significantly.

The 1,3-disubstituted propane-I ,3-dione ligands are prepared, as shown in Scheme 6-27, by reaction of a methyl ketone (2) and the appropriate ester (1) with


sodium hydride [39, 401. Depending on the nature of the substituents (R' and R2), the transition metal P-diketonates exhibit either discotic or calamatic mesophases.

In most cases the methyl ketone and ester are not commercially available and must be synthesized as required. A wide variety of different substituents R' and R2 has been incorporated into b-diketone ligands [41- 521. Compounds with different numbers of alkyl or alkoxy chains in different positions of benzene rings have been prepared (Scheme 6-27) in addition to the ligands depicted in Scheme 6-28.

Scheme 6-28.

M = Cu, X = CI- or AcO- n=2 M = Pd, X = CI- or AcO-

= VO, X = S04'-, n = 5 .R3

R' = or # R2= -CnHPn+,. -R3, e R 4 ,

The other class of P-diketone ligand, 3-substituted pentane-2,4-diones, lead to rod- like complexes that exhibit calamitic mesophases [53, 541. The preparation of these systems is outlined in Scheme 6-29.

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SI H d L O

H2SOdCH3COOH 1 NaOHIH2SO4

1) CZHSMgCH(COOCpH& ethedHCl

2) AcOH/H,SO,/NaOH I POCI, DMF/HCON(CHs)p 1

(Ck+C0)20

p-toluensulfonic acid BF3, H+

R’ = H, CH3

R

Scheme 6-29.

Thallium(1) P-diketonate complexes of propane- 1,3-dione ligands have been synthesized [55] by reaction of the P-diketone with thallium(r) ethoxide in toluene or petroleum ether (Scheme 6-27).

Most of the work concerning liquid crystalline P-diketone-metal complexes has been carried out employing 1,3-disubstituted propane-I ,3-diones. The general method of synthesis of P-diketone-metal complexes is shown in Scheme 6-28 [39-52, 56-60]. The synthetic route leading to these compounds allows the preparation of symmetrically or unsymmetrically substituted compounds.

The metal centers of these complexes have a square-planar geometry, but the structure of the whole molecule cannot be considered planar, because the aromatic rings are twisted with respect to the metal coordination plane.


Figure 6-2.

M = Fe, Mn, Cr

Alkoxyphenyl-substituted 1,3-diketones have been reacted with iron trichloride under a variety of conditions. Complexes which do not contain chlorine and show C : H : Fe ratios consistent with a tris(diketonate) complex are obtained (Fig. 6-2). The geometry of this complex is octahedral. Manganese(m) and chromium(m) complexes of this type have also been investigated. In spite of their non-planar structure, which causes a reduction in the attractive dipolar forces, all of these complexes exhibit mesomorphism [61].

Copper(@ complexes derived from P-dialdehydes and P-diketones with the general structure L2Cu have been prepared by the synthetic route illustrated in Scheme 6-29 [53].

An alternative approach to 3-substituted pentane-2,4-diones has been carried out by Serrano et al. [54] by reaction of acetylacetone with sulfuryl chloride in toluene (Scheme 6-30, see p. 254). 3-Substituted pentane-2,4-diones were also synthesized from thallium(1) salts of B-diketones by C-alkylation, followed by coordination to the metal atom to give the P-diketone-metal complexes [54]. However, liquid crystals were not obtained by this route.

6.3.2.2 Carboxylates Derivatives

Metal carboxylates are synthesized, depending on the metal, either by direct reaction of the acid with a metal salt or by a ligand exchange reaction of sodium carboxylates with a metal salt. The complexes are either ionic, such as sodium, lead(II), and thallium(1) carboxylates, or covalent such as copper(II), rhodium@) or ruthenium@) carboxylates.

The sodium soaps are obtained from fatty acids by neutralization with a dilute solution of sodium methanolate [62] (Scheme 6-3 1, see p. 254).

Cadmium(r1) dicarboxylates are synthesized by ligand exchange of a sodium alkanoate with cadmium chloride [63], while lead@) carboxylates are obtained by the direct reaction of alkanoic acids with lead@) nitrate [64] (Scheme 6-3 1).

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Cu(ACO)p2H@

Ethanol

Scheme 6-30.

COP + H20 + 2 H2,+&,,COOTlt 1-1 ?* (H2,,,1CnC00)2Pb

1 CH30-Na+ 2 H~,,,C,COONa+ + CHJOH

1 CdC12

(Hzn+1CnCO0)2Cd +NaCI

Scheme 6-31.


The formation of thallium(1) alkanoates is achieved by reacting carboxylic acids with a slight excess of thallium(1) carbonate in anhydrous methanol [65] (Scheme 6-3 1).

The reaction between copper(I1) sulfate or acetate and the sodium salt of a carboxylic acid in ethanol leads to the formation of the corresponding copper(I1) alkanoate whose structure consists of a homobimetallic complex with bridging carboxylate units (Scheme 6-32) [66]. Branched-chain copper(I1) carboxylates have also been synthesized [67]. The possibility of these complexes existing as oligomeric or polymeric species is discussed.

0 n- d-5 A P

Molybdenum(I), chromium(l), tungsten(II), ruthenium(1) and rhodium(1) carboxylates can be synthesized by the reaction of the metal hexacarbonyls with the appropriate carboxylic acid using diglyme as solvent [68] (Scheme 6-33).

M = Rh(ll), R ~ ( l l ) - R = n-alkyl or OOC, ,H2,+ ,

[M(CO),] + 4 RCOOH

M = Mo(ll), Cr(ll), W(II), Rh(ll), Ru(ll) R = n-alkyl

Scheme 6-33.

Rhodium(I1) and ruthenium(I1) analogs of the copper(I1) carboxylates are also obtained by exchange of the four acetate ligands in Rh2(CH3C00)4 or Ru2- (CH3C00)4 by carboxylates (Scheme 6-33) [69- 711.

6.3.3 Preparation of [M-(02S,)]-Type Metallomesogens

6.3.3.1 Monothio-P-diketone Derivatives

Monothio-/?-diketones form very stable square-planar nickel(r1) chelates [72]. The synthetic route leading to these compounds is shown in Scheme 6-34.

The 4-substituted phenylmonothiomalondialdehydes are rather unstable, therefore they are converted into the corresponding nickel@) chelates without being isolated.

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Na2S.9H20

,c, C H

R Scheme 6-34. H

In analogy to other OS-chelates, a cis-configuration of the ligands has been proposed for the di[2-(4-substituted phenyl)-3-mercaptopropenato]nickel(11) complexes, as shown in Scheme3-34 [72].

6.3.4 Preparation of [M-S&Type Metallomesogens

6.3.4.1 Dithiolene Derivatives

The first systematic study of metallomesogens containing d-block elements was carried out by Giroud and Muller-Westerhoff, who prepared dithiolene complexes of nickel(ii), palladium(rI), and platinum(T1) [73].

The main series, investigated with two p-alkylphenyl substituents, were synthesized by the route outlined in Scheme 6-35 (complexes A).

The synthesis was carried out by acetylation of a I-phenylalkane (1) at -20 "C in dichloromethane in the presence of aluminium chloride to give the p-alkylaceto- phenone (2). These compounds were subsequently monobrominated using bromine in glacial acetic acid at 60 "C to afford the I-@-bromoacetylpheny1)alkane (3). Reac- tion of (3) with potassium ethyl xanthate in ethanol or acetonitrile yielded the xanthate (4), which was cyclized using hydrogen bromide in acetic acid to form the dithiocarbonate (5). This species is cleaved with sodium methoxide in methanol to give the styryl dithiolates (6), which react with the appropriate metal halide or tetrahalometallates to initially form the dianions of complexes A. The neutral complexes are obtained by oxidation of the dianions with air or iodine.

n-Acceptor discogens are desirable as discotic columnar mesogens because of their potential application as one-dimensional conductors. One of the ways to obtain n- acceptor discogens is to introduce a number of alkyl or alkoxy chains into bis(dithi0-


lato)nickel(u) complexes. In order to synthesize such bis(dithiolato)nickel(II) complexes, the corresponding benzoin, benzil or acyloin must be prepared as precursors [74] (Scheme 6-36, see p. 258).

The unsubstituted complex, bis( 1,2-diphenylethane-l,2-dithiolato)nickel(11), has been obtained from benzoin. However, benzoins substituted with long alkyl chains cannot be prepared using this method, and only the dimethyl, dimethoxy and tetramethoxy benzoin derivatives have been synthesized.

The limitation of the benzoin method can, to a certain extent, be overcome by using the corresponding benzil systems as precursors (Scheme 6-36). By this route, tetraalkyl- or tetraalkoxybis(dithiolato)nickel(rI) complexes can be prepared.

The benzil method did, however, prove unsuccessful in the synthesis of octaalkyl- and octaalkoxy-substituted bis(dithiolato)nickel(II) complexes. The octaalkyl complexes were synthesized using the acyloin condensation (Scheme 6-36), a reaction generally used for aliphatic esters rather than aryl esters.

The synthesis of the corresponding tetraalkyl-substituted acyloins, used as precursors for the octaalkyl complexes, was carried out as illustrated in Scheme 6-37, see p. 258. Even by this acyloin method, the preparation of tetraalkoxy-substituted benzoins was unsuccessful.

An alternative attempt was reported by Wenz, involving a synthetic route leading to 3,3’,4,4‘-tetrapentyloxybenzil [75] (Scheme 6-38, see p. 259). As a result, the synthesis of 3,3’,4,4‘-tetradecyloxybenzil and its corresponding, liquid crystalline bis(dithiolato)nickel(II) complex was achieved for the first time.

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[-method) R-CHO R-COOEt [iii)y-melhod)

R’ KCN\ Ly% R‘

R = a R 2 R-C-CH--R R = O R 2

R’ = H, R~ = O C H ~

6 t)H R’ = R2 = H R‘ = H. R2 = CH3

R’ = R2 = OCH3

R’ = R2 = alkyl, reaction R’ = R’ = alkoxy, no reaction

‘1 p4s10

2) NiCI,.6H20 J

l ) p4s10

2) NiCI,.GH,O

R-C-C-R

6 6 SeOp

R-CH= CH-R

1) MgnHF 2) CICH=CHCI R ’ R2 = alkyl, alkoxy

R’ = H, alkyl, alkoxy

R1 = H, reaction

alkoxy, no reaction Scheme 6-36. R’ = R2=alkylor

t R-Br

NaOBr I

R R R 1 R = Alkyl 1

0 OH Scheme 6-37.


Scheme 6-38.

6.3.4.2 Dithiocarboxylate Derivatives

A) Metal n-Alkyldithiocarboxylates Discotic liquid crystalline dinuclear complexes of nickel@) with alkyldithiocarbox- ylate (CnH2n+lCS2) ligands [76] are prepared, following the synthetic route shown in Scheme 6-39, by treating an alkylbromide with magnesium and carbon disulfide [77].

In this nickel complex the ligands are coordinated in a weakly distorted square- pyramidal geometry. The dimeric units are organized in layers which are slightly displaced with respect to each other, due to intermolecular Ni-S interaction between neighboring units [78 ] .

Scheme 6-39. I

B) Metal Alkoxydithiobenzoates The 4-alkoxydithiobenzoate ligands (n-odtb, where n indicates the number of carbon atoms in the alkoxy chain) are synthesized according to Scheme 6-40. Due to the rela-

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X = CI. Br

M = Ni, Pd, Zn

Scheme 6-40.

vely low stability of the free acids, the ligands are isolated and stored as the sodium salts 1791.

Dithiobenzoic acids are not liquid crystalline as a result of the weakness of intermolecular hydrogen bonding, which precludes the formation of stable dimers. This behavior is in contradiction to that of the alkoxybenzoic acids which are dimeric and do form mesophases.

Gold(m) complexes with the formula [AuX,(n-odtb)], where X = C1 or Br, are prepared according to Scheme 6-40 a by reaction of [AuCl(tht)] (tht = tetrahydro- thiophene) with the sodium salt of the acid. By this, dimeric complexes [((n-odtb)AuJ2] are formed. These dimers are then oxidised in the presence of elemental halogen to give the mononuclear gold complexes in good yield.

Palladium(II), nickel@), and zinc(I1) complexes containing two dithiobenzoate ligands are prepared according to Scheme 6-40 b.

The palladium(r1) complexes [Pd(n-~dtb)~] are synthesized by stirring an excess of the sodium salt of the acid in water with sodium tetrachloropalladate.

These complexes are obtained as powders, deep-red for shorter (n < 8) and green for longer alkoxy chain lengths. The authors tentatively assign the green color of the complex to a dimeric structure (Fig. 6-3a). The red complex is believed to have two forms, consisting of monomeric and dimeric palladium units (Fig. 6-3 a and b).

The nickel@) complexes [Ni(n-~dtb)~] are prepared by dropwise addition of an aqueous solution of nickel chloride to an aqueous solution containing an excess of the sodium salt of (n-odtb) in an inert atmosphere. These complexes have a planar geometry, as confirmed by 'H NMR spectroscopy, indicating that the complexes are diamagnetic.

The zinc(]]) complexes are obtained by reaction of zinc acetate with Na(n-odtb) in dilute acetic acid. Osmometric molecular weight measurements in toluene and chloroform show them to be monomeric. However, single-crystal X-ray analysis of


RO' w a

Figure 6-3. b

the butoxy- and octyloxy-derivatives proved the existence of dimers. This signifies that these complexes are monomeric in solution and dimeric in the solid state.

When the alkoxydithiobenzoates carry lateral fluoro substituents [ S O ] the preparation of the complexes can be carried out by an alternative method. In this case of synthesis involves the direct reaction of two equivalents of the acid with a metal chloride (Ni, Pd) or an acetate (Zn) in ethanol, as for the n-alkyldithiocarboxylates (Scheme 6-39). This route has two advantages in that the synthesis of the sodium salt is not required and that only a stoichiometric ligand : metal ratio is required.

6.3.4.3 Alkylxanthato Derivatives

The synthesis of bis(n-alkylxanthato)nickel(II) complexes [76] is carried out according to the route illustrated in Scheme 6-41. The appropriately substituted alcohol is reacted with carbon disulfide under basic conditions, and the salt formed in this step is converted directly into the nickel@) complex.

s s

s s

1)KOH NiCI,.6H20 I \ / \ ROH- ROCSSK EtOH + RO-C,(,Ni,),C-OR

Scheme 6-41.

6.3.4.4 Dithiocarbamato Derivatives

Dithiocarbamato-metal complexes are a well-known group of coordination compounds which, with the appropriate chemical modifications, represent a new type of liquid crystalline material [81]. The synthetic route leading to these derivatives is shown in Scheme 6-42.

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or H20

1)HCI aq. reflux 2)NaOH, CS$ HzO I NaZ[PdCI,J or

NiCIz.6Hz0 or CuSO4.3H20 or ZnCI,

R O O -

M = Ni(ll), Pd(ll), Cu(ll), Zn(ll) Scheme 6-42.

The piperazine derivatives used as precursors are prepared by either 0- or N- alkylation. The complexes were obtained either by a metathesis reaction of the isolated sodium salts of the precursors with metal salts, or by reaction of the amines with potassium hydroxide and carbon disulfide followed by an in situ reaction with the metal salt.

The copper(Ir), palladium(rr) and nickel(r1) compounds are expected to have a molecular structure involving tetragonal association of square-planar bischelates, whereas the zinc@) complex would have a dimeric, bridged tetragonal form [81].

6.3.5 Preparation of [M-N4]-Type Metallomesogens

6.3.5.1 2-Phenylazopyrrole and 2-Phenylazomethinopyrrole Derivatives

2-Phenylazopyrrole- and 2-phenylazomethinopyrrole-metal complexes have been reported in the literature [82] as a novel class of liquid crystalline complexes with a trans-square-planar structure (Fig. 6-4).

The ligands are synthesized by condensation of 2-pyrrolcarboxaldehyde with an appropriately p-substituted aniline.

Figure 6-4.


Nickel(I1) and copper(I1) complexes were obtained by reaction of the ligand with nickel acetate tetrahydrate or copper acetate dihydrate in methanol in a ligand : salt ratio of 2 : 1.

6.3.5.2 Glyoximate Derivatives

Bis [ 1,2-di(3’,4’-di-n-alkoxyphenyl)et hane- 1,2-dioximato] metallomesogenic complexes are prepared following the synthetic route outlined in Scheme 6-43. The reaction of the a-diketone derivative (for the synthesis see Sec. 6.2.4.1) with hydrox- ylamine hydrochloride in ethanol, followed by addition of an ethanolic solution of metal salt. Subsequent neutralization yields the desired complexes [83].

OR

1) NH20H.HCVKOH 2) WCtp or NiCI2.6H20/ AcOH

OR I

OR 6 R

Scheme 6-43. M = Pd(ll), Ni(ll)

6.4 Metal-Organic Liquid Crystals with Macrocyclic Ligands

In most cases, the preparation of macrocyclic metallomesogens involves the initial preparation of the macrocyclic ligand followed by its reaction with a metal salt. However, for particular ligands such as phthalocyanines, the separate synthesis of the ligand is not a prerequisite, and the complex can be formed by a template synthesis in a single step. In this case the metal ion selectively promotes the synthesis of the macrocycle.

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6.4.1 Preparation of Cyclic Diazatetrathiaether Derivatives

A series of bis[4-(n-alkoxy)benzamide] derivatives (2 in Scheme 6-44) of 1,1 O-diaza- 4,7,13,16-tetrathiacyclooctadecane (1) and the corresponding cationic copper(1) [84] and silver(1) [85] complexes were reported.

M = Cu(l), Ag(l) 4 u J

(3) X- = CF3SO$, P F i

Scheme 6-44.

The bis(amide) derivatives (2) are obtained by acylation of 1 with acyl chlorides. In fairly good yields, white, air-stable solids are formed. Copper(1) complexes 3 are prepared by reaction of 2 with an equimolar amount of [Cu(CH3CN),][PF6] in acetonitrile. The initial copper complex is synthesized by reaction of copper(1) oxide with hexafluoroplatinic acid in acetonitrile [86]. In principle, the N2S, donor series 2 could give rise to hexacoordinated complexes upon coordination of metal ions. However, given the low donor ability of amide nitrogen atoms, tetracoordination by sulfur donors, represented by structure 3 in Scheme 6-44, is more likely. The nature of the macrocyclic cavity of 2 is such that either a square-planar or a tetrahedral coordination geometry can be accommodated. A tetrahedral geometry has been suggested by the authors for 3, based on crystallographic data for complexes of the type [Cu(L)][PF,], which contain a ligand with a donor environment very similar to that of 2, and show a copper(1) ion bound to a distorted tetrahedral array of sulfur donors [87].

Silver(1) derivatives of 1,l O-bis[4-(dodecyloxy)benzoyl]-l,1 O-diaza-4,7,13,16-tetrathiacyclooctadecane were synthesized in a dry nitrogen atmosphere excluding light. Reaction of ligand (2) with silver salts AgX (X = CF3S03, PF6) in a dichloromethane/acetonitrile mixture afforded colorless solutions from which white, air- and light-stable complexes were isolated in high yields [ 8 5 ] .


6.4.2 Preparation of Azacyclam Metallomesogens

6.4.2.1 1,4,7-Trisubstituted-1,4,7-triazacyclononane Derivatives

Nickel Derivatives A liquid crystalline nickel@) complex with the tridentate ligand 1,4,7-triazacyclononane ([9]aneN3) has been synthesized as outlined in Scheme 6-45 [88].

OR‘

Scheme 6-45.

The nonmesomorphic cyclic amine 2 was obtained by reduction of the liquid crystalline cyclic compound 1. The reduction was achieved using BH3 .THF complex. The metal complex is obtained by adding a solution of an equimolar amount of nickel dinitrate hexahydrate in anhydrous THF to a solution of compound 2 in the same solvent.

Two main differences are apparent between the IR spectra of the complex and the ligand (2). Firstly, the small absorption band due to the azacyclic N-C-H bonding at 2790 cm-’, which can be found in the spectrum of 2 and in that of unsubstituted [9]aneN,, is apparently shifted towards higher wave numbers upon complex formation, that is the band can no longer be observed beside the strong absorptions of the methylene groups of the complex. This phenomenon is typical for metal complexes which contain nitrogen as ligand atoms. Secondly, the band at 1385 cm-’ and the small bands at 1300, 802, 742 and 642 cm-’ indicate the presence of a nitrate group in the product 3. The presence of uncomplexed nickel nitrate as an impurity can be ruled out to be responsible for these absorption bands, because this salt is insoluble in hexane at room temperature and would therefore be removed during the purification of the nickel complex 3 [88].

Chromium, Molybdenum and Thngsten Derivatives Complexes containing chromium(m), molybdenum(rI1) and tungsten(I1r) have been prepared by the reaction of the 1,4,7-trisubstituted-I ,4,7-triazacyclononane with

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metal hexacarbonyls in anhydrous dimethylformamide (DMF) under a nitrogen atmosphere [89] (Scheme 6-46).

The 1,4,7-trisubstituted-1,4,7-triazacyclononane ligand and the three carbonyl groups are coordinated to the metal center in an octahedral geometry as illustrated in Scheme 6-46.

L OR'

CH2A

M - Cr(lll), Mo(lll), W(lI1)

Scheme 6-46.

6.4.2.2 Preparation of Metallomesogens from Other Aliphatic Azacyclams

The induction of columnar mesophases by molecular recognition has been achieved by complexation of transition metal ions with substituted azamacrocycles [90]. Col- umnar mesophases are not observed for the free amines. The synthesis of these systems is shown in Scheme 6-47, see p. 267.

6.4.3 Preparation of Porphyrin Metallamesogens

The porphyrin nucleus (Fig. 6-5) consists of four pyrrole rings linked by methylene bridges. These compounds are aromatic systems with 18 n-electrons originating from the nine conjugated double bonds in a planar ring. The porphyrin nucleus allows complexation of numerous metals including magnesium, iron, zinc, nickel, cobalt, copper, and silver resulting in metal complexes with the general core structure shown in Fig. 6-5.

In terms of liquid crystallinity, it is important to state the number of peripheral alkyl chains required for mesophase formation of porphyrin complexes.

Liquid crystalline metal-porphyrin complexes have been described with two, four and eight alkyl chains. The method of synthesis of the metal-free porphyrins is different depending on the number of chains, but all of the methods use a pyrrole derivative as the precursor.

6.4.3.1 Octasubstituted Porphyrin Derivatives

Octaester-substituted metal-porphyrin complexes have been prepared using a particularly simple and practical method which is outlined in Scheme 6-48, see p. 268.


R = ~ O C " H 2 " + , Scheme 6-47.

Figure 6-5.

First, the porphyrinooctaacetic acid (6) is synthesized in several steps starting from the readily available dimethyl 1,3-acetonedicarboxylate (1) and acetylacetone (2) [91] (Scheme 6-48). The octaester is then formed by warming a solution of 6 in the appropriate alcohol in the presence of sulfuric acid to 50°C (48 h). Finally, treatment of the free base (7) with a metal salt in boiling chloroform/methanol gives the metal complex (8) [92].

Metal octakis(alky1thio)tetraazaporphyrin complexes containing eight sulfur atoms directly bound to the porphyrin core have been reported. These compounds are liquid crystalline [93]. The complexes were synthesized as shown in Scheme 6-49, see p. 269.

Sodium cis- 1,2-dicyano- 1,2-ethylenedithiolate (1) reacts with 1.6 equivalents of bromooctane in DMF at room temperature to give compound 2. The magnesium porphyrazine (MgP) is prepared by reaction of 2 with magnesium in propanol [94]. The free base (H2P) is obtained by treating the magnesium derivative with a 5 % solution of trifluoroacetic acid in dichloromethane. Treatment of the free base with the appropriate metal acetate in chlorobenzene results in the formation of the

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H3C02C- f C0zCH3 H3C 1) RONO/HCI

CH3 2) Zn + AcOH

(1)

, (3) CH3

H

(5)

HOOC-H2CHCH2-COOH -

CHz-COOH

HOOC-CHp CHz-COOH

HOOC-CHI, ‘CH~-COOH

(4)

HZS04 -F PH~(CH~COOR)B

(7) ROH

/ Zn(OAc),

CHzCl2/MeOH

i (6) ROOC-HzC CH2-COOR

ROOC-CHp ROOC-CH2 QcH2-cooR \ I CHz-COOR

\ I ROOC-CH2 CH,-COOR

Scheme 6-48. (8)

metalloporphyrazine complexes. In the case of the cobalt(i1) derivative, the synthesis and purification must be carried out under a nitrogen atmosphere in order to avoid oxidation which leads to contamination with cobalt(Ii1).


M(OAc)2.nH,O

C~HSCI 4

reflux

Trifluoroacetic acid CH2C12 1

(HP)

6.4.3.2 Tetrasubstituted Porphyrin Derivatives

One of the commonly used methods for the synthesis of tetrasubstituted porphyrins (Scheme 6-50) involves the reaction of a p-substituted benzaldehyde (1) with pyrrole (2) in propionic acid. The tetrasubstituted porphyrin [H,P(OR),] is treated with the

Q H

-

RO

Scheme 6-50.

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metal acetate or chloride in chloroform/methanol to give the metal complexes [MP(OR)4] (M = Co, Zn, Cu, Ni, Pd or AlOH) [95].

6.4.3.3 Disubstituted Porphyrin Derivatives

The synthesis of liquid crystalline disubstituted porphyrinatozinc(l1)) complexes has been described [96]. The preparation of 5,15-bis(4'-alkoxyphenyl)porphyrinatozinc(I1) complexes (Scheme 6-5 1) involves the reaction between activated BF,-Et,O, 4-alkoxybenzaldehyde (1) and dipyrrylmethane (2) in 1 : 1 methanol/dichloromethane excluding oxygen, at temperatures below 50 "C, and with aid of ultrasound. Oxidation of the free base with tetrachlorobenzoquinone (chloroanil) and subsequent complexation with zinc acetate gives the disubstituted prophyrinatozinc(I1) ZnP(OR),.

.OR

p"

2) Zn(OAc), ZnP(OR)2

HzP(ORh RO' Scheme 6-51.

6.4.4 Preparation of Phthalocyanine Metallomesogens

Phthalocyanine and metallophthalocyanines hiive been stimulating the field of metallomesogen research for many years because they exhibit properties that are interesting for applications in materials science. Two detailed reviews on metallophthalocyanines have been published recently by Simon [97] and Hanack [98]. Further- more, several methods of synthesis of liquid crystalline metallophthalocyanines have been reported [99 - 1081.

The structure of the phthalocyanine core (Fig. 6-6) consists of a planar ring system formed by four isoindole units, connected by bridging nitrogen atoms. The molecule contains two imine nitrogen atoms which can be coordinated to a variety of metals such as copper(ii), nickel@), lead(II), cobalt(II), tin@), zinc(1r) and luthenium(I1).

Metallophthalocyanine complexes can be synthesized by the following general methods (Scheme 6-52): (a) Passing ammonia through melted phthalic anhydride or phthalimide in the presence of a metal salt. (b) Heating o-cyanobenzamides or


Figure 6-6. H ~ P c , M = H,

aCN CN

MorMX2 or

Scheme 6-52.

1MX2 HB03

0s + H2N-CO-NH2

0

phthalonitriles with metals or metal salts. (c) Reaction of phthalic anhydride or its amide with urea and a metal salt, preferably in the presence of a boric acid type catalyst. (d) Reaction of the dianion of a substituted phthalocyanine with a metal salt.

The phthalocyanine core must be substituted with long peripheral chains in order to generate liquid crystallinity. The starting materials used in the synthesis of phthalocyanines depend on the nature of the peripheral chains required (for example, alkyl, alkoxy, alkoxymethyl). The main starting materials used in the synthesis of

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ortho-xylene Catechol 4-methoxybenzaldehyde 2,5-dialkylfuran Figure 6-7.

phthalocyanines with 4,5-disubstitution (see Fig. 6-7) are ortho-xylene [99, 1001, catechol [ 101 3, and 4-methoxybenzaldehyde [ 1021. 2,5-Dialkylfurans [ 1031 are used to obtain 3,6-dialkylsubstituted phthalocyanines.

The starting materials discussed above are subsequently converted into a suitable derivative which can be condensed to give either the appropriate phthalocyanine ring or to directly afford the metallophthalocyanine complex.

Starting from a diketone derivative which is condensed with diaminomaleonitrile to give the disubstituted dicarbonitrile, the octa(dodecy1)tetrapyrazinoporphyrazine (C,2P~H2) and its copper(I1) complex (C12PzCu) can be synthesized [108].

As a representative example, the synthetic route leading to a liquid crystalline, octasubstituted phthalocyaninato metal complexes (Cu", Pb", Ni", Co") was first proposed by Hanack et al. (Scheme 6-53, see p. 273) [99, 1001.

Bromination of ortho-xylene (1) yields the 1,2-dibromo-4,5-dimethylbenzene (2) the methyl groups of which are then brominated to afford 4,5-bis(bromomethyl)-1,2- dibrornobenzene (3). Treatment of compound 3 with sodium alkoxides in the corresponding alcohols or with a phenoxide in ethanol results in the formation of 43- bis(alkoxymethy1)- or 4,5-bis(phenoxyrnethyl)- 1,2-dibrornobenzenes (4) respectively. These compounds are converted into the corresponding ortho-dinitriles (5) by reaction with a small excess of copper(1) cyanide in dirnethylformamide. When the reaction of the dibromo compound with copper(1) cyanide is carried out in high concentrations using a large excess of copper cyanide, the copper(I1) phthalocyanines (ROCH& PcCu (7) are obtained directly. The lead(r1) and cobalt(i1) complexes (ROCH,),PcPb (8) and (ROCH2)8PcCo (9) are obtained by reaction of the corresponding ortho-dinitrile 5 with lead@) oxide or cobalt(i1) chloride, respectively, in dry ethylene glycol. The nickel@) complexes (ROCH2),PcNi (10) are synthesized from 1,3-diimino-5,6-bis(alkoxyrnethyl)- 1,3-dihyclroisoindole derivatives (6) which are obtained in nearly quantitative yield from ( 5 ) by bubbling ammonia through a methanolic solution of 6.

6.5 Organometallic Liquid Crystals with Bidentate Ligands

The ortho-metallation process is an aromatic substitution which occurs in two steps. At first, coordination of the nitrogen atom in the ligand to the metal takes place. The second step involves the attack of the metal in the ortho-position of the aromatic ring of the ligand [109].


CHp-0-R CHz-0-R

, (5 )

RONa

(3) ROH. 6OoC *Br

(4)

large excess in conc. solution 1 CuCN

A-O-H&

M = Cu(ll), Pb(ll), Co(ll), Ni(ll)

(7) (8) (9) (10) R-0-HpC CHp-O-R

Scheme 6-53. (7-10)

Certain requirements must be met by the ligand, whose donor atom is nitrogen, to form an ortho-metallated complex [l lo]: (a) The nitrogen atom must be tertiary. (b) The ligand must be possible to form a planar, five-membered ring containing the metal. (c) The metal must displace a proton from an aromatic carbon atom. The carbon atom must not be highly deactivated regarding electrophilic attack.

The formation of a structure containing a five-membered ring is favored, even if the ligand has more than one position at which cyclization can occur.

All studies of mesogenic ortho-metallated complexes reported to data wertfocused on the N-donor ligands shown in Fig. 6-8: azobenzene, azoxybenzenes, Schiff bases, azines, and pyrimidines. All liquid crystalline ortho-metallated complexes described to date are derived from palladium(@, with the exception of a few examples of mesogenic mononuclear ortho-metallated complexes of mercury(I1) [ 1 1 11, manganese(1) or rhenium(1) [112, 1131.

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Figure 6-8.

6.5.1 Synthesis of the Ligands

The ligands used for ortho-metallation reactions are classical organic compounds (azobenzenes, azoxybenzenes, imines, azines or pyrimidines), most of which have been described as organic liquid crystals [114, 1151.

Typical procedures for the synthesis of the ligands are outlined in Scheme 6-54 [ I 15 - 1171. Azobenzene ligands are prepared by reaction of the diazonium salt of the substituted aniline with a phenol [I 16, 1181. Azoxybenzene ligands are synthesized by oxidation of azobenzenes with hydrogen peroxide [116]. Imine ligands are prepared by condensation of a benzaldehyde with the appropriate amine [I 161. Azines are prepared by reaction of the aldehyde with hydrazine hydrate [115]. The synthesis of pyrimidine ligands makes use of an amidine hydrochloride and a derivative of a 8-dicarbonyl compound [ 1 171.

6.5.2 Preparation of ortho-Metallated Dinuclear Complexes

ortho-Metallated dinuclear complexes with the general formula [M2(Lj2p-X2] (X = OAc or CI) are prepared by cyclometallation of the ligands (HL) with a metal salt or a previously synthesized metal complex. These complexes can also be used as precursors for new dinuclear complexes with different central bridging units (X = Br, I, SCN) by metathesis reaction.


6.5.2.1 Azobenzene Derivatives

Dimeric complexes with chloride bridges are obtained in two ways by metallation of substituted azobenzenes (Scheme 6-55). When the ligand contains a different substituent in each of the two benzene rings, the product is a mixture of six isomers. This mixture arises by virtue of the fact that ortho-metallation can occur in either of the aromatic rings. The six isomers are shown in Scheme 6-55. They can be divided into cis- and trans-isomers regarding the arrangement of the donor atoms around the central unit. The cis- and trans-isomers have three possible distributions of the terminal chains each.

When the azobenzene ligand is unsymmetrically substituted, ortho-metallation preferentially occurs in the more electron-rich ring [ 1101. This fact, together with the general tendency of this type of complex to form the more symmetric trans-isomers, indicates that single isomers can be formed in some reactions [ I 191. However, characterization of the complexes by 'H NMR spectroscopy shows that they generally consist of a mixture of isomers the complete characterization of which is difficult [ 120, 1211.

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R3 I R' R3

(3 2 kN

I

/ [ route a )

route b) *

R,

R = or z R'

Scheme 6-55.

CI [ 0 C ~ N - P c l - N : C ~ ] I I

CI Ethanol

6.5.2.2 Azoxybenzene Derivatives

Treatment of an azoxybenzene with (Pd2(PhCN)2C12] in refluxing ethanol affords the chloro-bridged dinuclear complex in good yield [122]. The synthetic route is similar to that leading to the azo complexes (see Scheme 6-55).

Azoxybenzene is less reactive than azobenzene, therefore, it could be classified as a poorer donor ligand. As mentioned previously, the cyclopalladation reaction can be considered as an electrophilic attack by the palladium center at the organic molecule. In this light the high reactivity of p-alkoxyazoxybenzene can probably be attributed to the presence of the alkoxy substituent, which is a strongly activating group for electrophilic substitution.

6.5.2.3 Schiff Base Derivatives

Complexes with similar structures to those of azobenzene complexes can be prepared using Schiff bases as ligands. Scheme 6-56 shows the synthesis of some derivatives with different bridging units. The ortho-palladated ring system is obtained by reaction of the imine with Pd3(0Ac), under reflux conditions in glacial acetic acid, affording the dinuclear complexes with acetate bridges. Dinuclear complexes with chloro, bromo, iodo, or thiocyanato bridges can also be obtained [123,124].

In complexes of this type, two isomers are possible, cis or trans, depending on the arrangement of the two Schiff base ligands in the dimer. Structural studies [125]


2 Pd3(OAC)6 *

AcOH, reflux

I /HCI MeOH

Scheme 6-56. fi show that only the trans isomer is formed. 'H NMR spectra reveal that the complexes with chloro, bromo, and thiocyanato bridges are planar. The thiocyanato- bridged complexes exist as a mixture of isomers arising from the relative disposition of the unsymmetric bridging ligands (Fig. 6-9, see p. 278).

The acetato-bridged complexes have a rigid structure in the form of an open book in which the coordination plane of each palladium center forms a small dihedral angle. These compounds must exsist as a racemic mixture of the D and L enantiomers according to their geometric structure.

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OR ""0 RO

0 R

Figure 6-9.

RO Ro6

RoQ RO

OR

HL' HL2 Figure 6-10.

Imines derived from 2,3,4-trialkoxybenzaldehyde also form ortho-metallated complexes [ 1241. The imine ligand HL' (Fig. 6-1 0) is prepared by p-toluenesulfonic acid- catalyzed condensation of commercially available 4-hexylaniline with 2,3,4-tri(hexyl- 0xy)benzaldehyde in toluene. This ligand reacts with palladium acetate (Pd3 (OAc),) in glacial acetic acid resulting in the formation of the acetato-bridged dinuclear palladium(r1) complex. The complexes [Pd2Lb-X2], in which X = C1, Br, I and SCN, can subsequently be prepared by exchange reaction [123].

The application of the ortho-palladation reaction to a bisimine HL2 (Fig. 6-10) led to the formation of a tetranuclear palladium complex. The preparation of the halogeno- and thiocyanato-bridged dodecaethers is carried out by exchange reaction using procedures similar to that described for the dinuclear palladium complexes. The structures postulated for this novel series of cyclopalladated compounds, each containing four cyclometalla rings and twelve flexible side chains, are in agreement

6 Design and Synthesis of LOW Molecular Weight Metallomesogens 279

RO

Figure 6-11. OR OR

with the results of IR and 'H NMR studies. The structure of the chloro-bridged complex is shown in Fig. 6-1 1 . Osmometric molecular weight determination indicates the nonoligomeric character of these mesomorphic palladium(~~) complexes 11241.

Treatment of chloro-bridged imine complexes with the chiral salt ( R )- KO2CC*HC1CH3 in dichloromethane yields complex 2 (Scheme 6-57), which upon further treatment with mercaptans (CnHZn+ ,HS) (Pd: RSH = 2: 1) produces the mixed-bridge compounds (3 in Scheme 6-57) [126]. 'H NMR spectroscopy indicates thata unique cis arrangement of the two imine moieties is present in the complex. In contrast, the chloro- and carboxylato-bridged complexes all show the trans structure.

(2)

Scheme 6-57. (3)

6.5.2.4 Azine Derivatives

Another type of ortho-metallated complex is obtained from azines. ortho-Metalla- tion in only one ring is achieved by reaction of the azine with Pd3(0Ac)6 under

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reflux conditions in glacial acetic acid to afford the dinuclear complexes with acetate bridges [127]. Metathesis reaction of this type of complex results in the formation of dinuclear complexes with chloro, bromo, or thiocyanato bridges in a similar manner to that described for imines.

The ‘H NMR spectra of these complexes [I281 show that only the trans isomer is formed in bromo- and chloro-bridged complexes, whereas both cis and trans isomers are obtained in a 3 : 2 ratio when the bridge is thiocyanate.

The liquid crystalline acetato complexes have been subject of special attention due to their unique structure. As mentioned above, the synthetic method used in the preparation of the complexes is not enantioselective. A mixture of cis and trans isomers (30% and 70% respectively) is obtained. In addition, the trans isomer, with an “open-back” structure, consists of a racemic mixture of two enantiomers. How- ever, when chiral carboxylates are used as the central bridging units, a mixture of three diastereoisomers is formed. A material has been synthesized (Scheme 6-58) using azine ligands (R=OC,,H2,) and chiral bridges ((R)-O,C-C*HClCH,) [129]. The product of the reaction is composed of a mixture of trans-A, R, R (34Vo) trans- A , R , R (34%) and cis-R,R (32%) diastereoisomers, all of which are optically active [129].

b

Q (1)

Scheme 6-58.

h CH3

transA.R,R + trans-A.R,R 68%

Complexes containing different achiral linear carboxylato bridges have been also synthesized by treatment of the chloro-bridged complex with an excess of the sodium salt of the appropriate acid [ 1301. The synthetic route is similar to that described for chiral carboxylato-bridged complexes (Scheme 6-58), The materials obtained are a mixture of cis and trans isomers.


L =

6.5.2.5 Pyrimidine Derivatives

The mesogenic compound 5-( 1 -hexyl)-2-{[4'-( 1 -undecyloxy)phenyl])pyrimidine reacts with the complex [Pd2(PhCN)2C12] in ethanolic solution to give the chloro-bridged dinuclear cyclopalladated complex in a similar manner to that described for the azo complexes (Scheme 6-55 b) [ 13 I].

The chloro-bridged dinuclear cyclopalladated pyrimidine complex is used as a starting material in reactions with either monodentate or chelating ligands [ 1271.

6.5.3 Preparation of ortho-Metallated Mononuclear Complexes

6.5.3.1 ortho-Palladated Mononuclear Complexes

ortho-Metallated complexes tend to have a high reactivity due to the nature of the bonds in the complex. The o-metal-carbon bond is relatively strong, but palladium -halogen bonds are less stable. Dinuclear palladium(I1) complexes with halogeno bridges react with monodentate or polydentate ligands to give neutral or cationic mononuclear complexes (Scheme 6-59).

pyridine aniline quinoline or triphenylphosphine

Scheme 6-59.

) = N

N

OH C-$'HR

X = C H , N

OR

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A) Reaction with Monodentate Ligands

Breaking of the chloro bridge in dinuclear palladium(I1) complexes [Pd2(Ln)2p-C12] with monodentate ligands (L) such as pyridine, quinoline, aniline and triphenylphosphine (Scheme 6-59 a) gives rise to mononuclear complexes whose spectroscopic data indicate that the new ligand is in a trans-arrangement with respect to the ortho- metallated carbon atom [ 13 1, 1321.

B) Reaction with Monoanionic Chelating Ligands

These reactions involve cleavage of the halogeno bridge under displacement of the chloride ions from the chloro-bridged dinuclear palladium(I1) complex.

The reaction between 8-hydroxyquinolinate anion and chloro-bridged dinuclear palladium(I1) complexes derived from substituted pyrimidine has been reported. The product of this reaction is a neutral complex (Scheme 6-59 b). An X-ray diffraction study on the complex shows that the Pd-C and Pd-0 bonds are in a cis arrangement (13 11.

Mononuclear palladium(I1) complexes derived from azo, imine or pyrimidine ligands have also been prepared by cleaving the bridging groups of the dinuclear complexes [PdzLy-Clz] with potassium acetylacetonate and silver nitrate (1 : 1 molar ratio) [131], thallium(1) acetylacetonate (Tl(acac)) [133, 1341, or other thallium(1) 1,3-disubstituted P-diketonates [ 1351 (Scheme 6-60).

B) T;$' 0

R i

Scheme 6-60.


The dinuclear complexes obtained from azo derivatives exist as an equimolar mixture of isomers, which results from a nonselective palladation of the benzene rings which bear different terminal alkoxy chains (RO or R O groups). The mononuclear complexes derived from these mixtures also exist as a 1 : 1 mixture of isomers.

Mononuclear palladium(1r) complexes derived from azoxy and salicylaldimine ligands (3 in Scheme 6-61) have been prepared from dichloro-bridged dinuclear complexes by cleavage of the chloride bridge with silver tetrafluoroborate in acetonitrile to give the complex [Pd@-alkoxyazoxybenzene) (MeCN)J BF4 (2), which is subsequently reacted with a Schiff base (Scheme 6-61, see p. 284).

Complex 3 should be formed as a mixture of two isomers which differ in the arrangement of donor atoms around the metal center. The structures of the cis and trans isomers are shown in Fig. 6-12 (I and 11, respectively). Indeed, analysis of the 'H NMR spectra shows that both isomers are present in all products in a cis to trans ratio of 1 : 5 [136a].

Figure 6-12. I

The intermediate compound [Pd @-alkoxyaz~xybenzene)(MeCN)~] BF, also reacts with azobenzenes to give liquid crystalline-mixed complexes [(Azoxy)Pd(Azo)] [29].

ortho-Palladated mononuclear complexes, derived from azine, azo and imine ligands, have been reported. Complexes containing an L-amino acid have been prepared by using the L-amino acid as a cleaving agent for the chloro bridges of the dinuclear complexes (Scheme 6-62) [ 1 371.

Based on IR data, the authors propose structure 2 for these complexes. The v ~ - ~ absorption band of the complexes is found at 3200-3100cm-', compared to 3500-3300cm-' for the free amino group.

C) Reaction with Neutral Chelating Ligands

Mesogenic mononuclear palladium(1r) complexes of pyrimidine have been described [131], in which the bridge is cleaved by reaction with a neutral nitrogen-containing chelating ligand. This process involves two steps. First, the chloro bridge is allowed to react with silver tetrafluoroborate in acetonitrile. Under such conditions, the silver ion acts as a chloride scavenger and, in the second step, the solvated species [Pd(L)(MeCN)J BF4 undergoes subsequent addition of 2,2'-bipyridine (or substituted derivatives) to give the [Pd(L)(NN)]BF, complexes. The synthetic route is similar to that outlined in Scheme 6-61.

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I 0

I

I 0 I 1 I


6.5.3.2 ortho-Metallated Mercury Complexes

ortho-Metallated mercury complexes [HgL"Cl] (n = 6, 10) derived from azoxybenzene have been prepared [ 1 I 11 by reacting mercury acetate with the ligand (HL) according to Scheme 6-63.

Elemental analyses and spectroscopic data (IR, 'H and 13C NMR) indicate that a 1 : 1 mixture of complexes A and B (Scheme 6-63) are formed by a nonselective metallation of the benzene rings.

6.5.3.3 ortho-Metallated Manganese and Rhenium Complexes

Octahedral ortho-metallated manganese(1) and rhenium(1) complexes derived from imines have been synthesized [ 1 12, 1 131 by reaction of the complex [MCH3(CO),] with a Schiff base ligand in toluene according to Scheme 6-64.

R

Q

Scheme 6-64.

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6.6 Organometallic Liquid Crystals with Metal- n Bond

6.6.1 Preparation of Dieneiron lkicarbonyl Complexes

The general procedure for the synthesis of dieneiron(I1) tricarbonyl complexes consists of a direct reaction between the diene with either iron@) pentacarbonyl or triiron(1r) dodecacarbonyl in an iron: diene molar ratio of 1 : 1 [138, 1391.

Butadieneiron(I1) tricarbonyl liquid crystal complexes with structures A and B (Fig. 6-13) have been reported in the literature [ 1401.

H 3 C n C 0 2 0 Z = N G 0 2 C G U oc*F?-co

co Complex A

complex B Figure 6-13.

The synthesis of these complexes is significantly different in comparison to that described previously in this chapter because the metal complex is prepared in the first step, and is subsequently modified by the introduction of the promesogenic groups. Depending on the functional groups present in the dieneiron(I1) tricarbonyl unit, the metal-containing unit may be either a terminal group (Fig. 6-13, structure A) or part of the central core of the molecule (Fig. 6-1 3, structure B). These complexes are accessible in an optically active form when different substituents are attached to the dieneiron(I1) tricarbonyl unit. However, thermal racemization has been observed [141] (Fig. 6-14). This racernization signifies that a temperature limit exists for the use of these complexes as chiral liquid crystals.

The synthetic route for compound A (Fig. 6-13) as a representative example is outlined in Scheme 6-65.


di-n-butyl ether H2S04

Scheme 6-65.

6.6.2 Preparation of Metallocenes

Metallocenes, such as dicyclopentadienyl-metal rr-complexes, are of general interest not only because of their aromaticity, manifested in their reactivity towards electrophilic substitution [142, 1431, but also from a theoretical point of view. For the chemist, metallocenes are fascinating because of their interesting molecular geometry, which offers many new possibilities for the synthesis of novel mesogenic systems.

The liquid crystalline metallocene derivatives described to date are all derived from ferrocene [144- 1591, with the exception of one series of 1,l'-disubstituted ruthenocene liquid crystals [ 1 601.

6.6.2.1 Ferrocene Derivatives

Ferrocene is a sandwich compound consisting of two cyclopentadienyl rings and one iron atom as center. Ferrocenes are chemically and thermally stable aromatic species. One of the most characteristic reactions of ferrocene is the electrophilic substitution reaction. Using this type of reaction, many different liquid crystalline ferrocene derivatives have been synthesized.

Starting from ferrocene or some commercially available ferrocene derivatives (Fig. 6-15), 1 -substituted, 1 , I '-disubstituted, and 1,3-disubstituted ferrocene liquid crystals have been prepared.

In I-substituted ferrocene compounds the ferrocenyl unit acts as a terminal group, whereas in 1 , I '-disubstituted and 1,3-disubstituted ferrocene derivatives it constitutes the central core.

Ferrocene U Ferrocene Ferrocene Ferrocene Figure 6-15. carboxaldehyde carboxylic acid dicarboxylic acid

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1 ,l’-disubstituted ferrocenes exist in the fully extended S-shaped geometry [ 1611 where the ferrocene unit introduces a “step” into the structure. In contrast, 1,3-disubstituted ferrocene derivatives contain the bulky ferrocene unit in the center and the two substituents, when arranged collinearly, lead to liquid crystalline behavior.

1,3-Disubstituted ferrocene derivatives, which contain different groups (R, and R,) in the same ring, are members of the symmetry point group C1. Such compounds are chiral and may therefore be separated into enantiomers [ 1621. Newman type projections of 1,3-disubstituted and 1,l‘-disubstituted ferrocene are shown in Fig. 6- 1 6.

Chiral achiral Figure 6-16.

A) Monosubstituted Ferrocene Derivatives Monosubstituted ferrocene derivatives, as mentioned previously, contain the ferrocenyl unit as a terminal group. An example of the synthesis of monosubstituted ferrocene derivatives is shown in Scheme 6-66.

Fo

-c AgN03 KOH I coo^

6 Design and Synthesis of Low Molecu[ar Weight Metallomesogens 289

The complexes are synthesized by reacting equimolar amounts of methyl 4-ferro- cenylbenzoate (prepared from ferrocene and methyl p-aminobenzoate via the diazonium salt) with a phenol derivative [146], or the silver salt of the corresponding 4-ferrocenylbenzoic acid with an iodide [ 1451.

B) 1,l'-Disubstituted Ferrocene Derivatives

1 ,I1-Disubstituted ferrocene-containing liquid crystals are obtained by condensing a 1 ,I1-ferrocenediacid chloride or a 1,l'-ferrocenedicarboxylic acid with various phenols or alcohols [147- 150, 153, 1541. They are also prepared from ferrocene by Friedel-Crafts acylation [151].

Different synthetic routes used to prepare symmetrically 1 ,l'-disubstituted ferrocene-containing liquid crystals are outlined in Scheme 6-67.

Scheme 6-67.

CH3COCI -COCH3

AICI, H C O C d CHpCI,

Br(CH2),.,COCI AICI,, Zn. CHzCI,

NaOCl 1 H20

1 I

H O O R

e c o c l NaH

DMF PhCH,N(CH3)3CI

ClOC-&

290 M. Marcos

The 1,l'-diacid chloride is obtained from the reaction of commercially available 1 ,l'-ferrocenedicarboxylic acid [ 1501 with oxalyl chloride in dichloromethane and pyridine. In an alternative approach, ferrocene is first converted into 1,1 '-diacetylferrocene and then oxidised with sodium hypochlorite to give the dicarboxylic acid [163].

The diacylation of ferrocene with aliphatic w-bromo carboxylic acid chlorides and aluminum chloride was initially marred by extensive oxidation of the ferrocene due to the presence of either aluminum chloride or the acid chloride itself. This is a typical problem encountered whenever strong electrophiles are added to the easily oxidisible ferrocene. This recurring problem was overcome by carrying out the reaction in the presence of a large excess of granular zinc. The use of zinc to avoid the oxidation of ferrocene could also be useful in other similar electrophilic substitution reactions involving ferrocene, where the electrophilic reagent can also act as an ox- idising agent.

Reduction of the carbonyl groups in this case required a careful choice of reducing agent to prevent potential reductive debromination. The use of triethylsilane in the presence of trifluoroacetic acid [I641 (which has been employed previously to reduce other ferrocene derivatives [I 521) resulted in the complete recovery of starting material. The reduction of the carbonyl group was achieved in quantitative yields using a mixture of lithium aluminum hydride/aluminum chloride as the reducing agent. The low solubility of the 1 ,I'-bis(w-bromoalky1)-1 ,l'-ferrocene derivatives in ethanol required a different etherification procedure carried out in nonaqueous conditions. The etherification was carried out using DMF/NaH with a catalytic amount of benzyl trimethylammonium chloride and the appropriately substituted phenol [ 15 I] .

Unsymmetrically 1,1 '-disubstituted ferrocene-containing liquid crystals have also been synthesized [152, 1531 following the synthetic routes shown in Scheme 6-68.

Unsymmetrically 1 ,l'-disubstituted ferrocene derivatives are obtained by treatment of the 1,l'-ferrocene diacid chloride with benzyl alcohol to give the monoprotected acid. This monoacid is converted into the acid chloride and then esterified with a phenol. Removal of the benzyl protective group, transformation into the acid chloride and, finally, reaction with a different phenol gives the unsymmetrically substituted product [ 1531 (Scheme 6-68, Route a). LJnsymmetric 1 -alkyl- 1 '-substituted ferrocenes have been prepared using Route b in Scheme 6-68. Friedel-Crafts reaction between ferrocene and diphenylcarbomoyl chloride, followed by Friedel- Crafts acylation with an acid chloride gives the 1, l '-disubstituted ferrocene intermediate. The deactivating effect of the substituent introduced in the first Friedel-Crafts reaction directs the second substituent into the unsubstituted ring. Subsequent selective reduction of the carbonyl group to give the alkyl substituent was achieved using triethylsilane in trifluoroacetic acid. Hydrolysis of the amide with KOH gave the 1 -alkylferrocene-1 '-carboxylic acid. The esters were prepared by reacting an appropriately substituted phenol with 1-alkylferrocene-1 '-carboxylic acid and dicyclohexylcarbodiimide (DCC) with 4-(N-pyrrolidino)pyridine (PPY) as a catalyst [I521 (Scheme 6-68, Route b).


Fc-CO-N

AICb RCOCl

Rj-Fc-COOH

J N x + P P Y

e C 0 &

R, 4 R, = H or alkyl

Scheme 6-68. R2 # alkyl

CH2CI2, Et3N reflux

c l c o c o c l CH2CI2

1 HOOC-Fc-COOBn 1 E ~ ~ N

reflux

CIOC-Fc-COOEn

EtOH /CH2CIz H2/Pd-C

R , O O O C - F C - C O O H I I c l c o c o c l

C) 1,3-Disubstituted Ferrocene Derivatives 1,3-Disubstituted ferrocene-containing liquid crystals are prepared by reaction of the ferrocene 1,3-diacid chloride with phenol derivatives such as 4-hydroxyphenyl 4-alkoxybenzoates, 4-alkoxyphenyl 4-hydroxybenzoates, or 4-alkoxyphenols. The synthesis is carried out under reflux conditions in anhydrous dichloromethane in the presence of triethylamine and a catalytic amount of 4-(N-pyrrolidino)pyridine [ 1561 (Scheme 6-69).

292 M. Marcos

1) aq. NaOH/CI

2) H,NCN EtOH/CH2Clp (U1)

CHpCI? I Mno2 t

H~COC-COCHS

a) KOH b) HCI

1) c l c o c o c l CH2C12, Et3N

2) H O O R 2

I H O O C ~ C O O H

CH2Clp/Et3N

Scheme 6-69.

The 1,3-ferrocene diacid chloride is prepared as shown in Scheme 6-69. Acetyla- tion of ethylferrocene with acetic acid anhydride and boron trifluoride etherate gives 1 -acetyl-3-ethylferrocene. The product is free from contamination by 1 ’-ethyl- and 2-ethyl- 1 -acetylferrocenes. Oxidation with activated manganese dioxide in dichloromethane results in the formation of 1,3-diacetylferrocene. After that, quantitative conversion into the 1,3-bis(methoxycarbonyl)ferrocene is carried out [ 1651. Hydrolysis of the diester with a strong base gives 1,3-ferrocenedicarboxylic acid which is then converted into the diacid chloride using oxalylchloride. The 1,3-diesters are prepared by reacting an appropriately substituted phenol with the diacid chloride.

6.6.2.2. Ferrocenophane Derivatives

A bridged 3,3’-disubstituted ferrocene showing liquid crystal properties has been described [166]. Complexes of this type have a fixed U-shaped (cis) structure. Their synthesis is carried out starting from ferrocene. Acylation with acetyl chloride followed by Willgerodt-Kindler reaction (employing sulfur and morpholine) gives the 1,l’-ferrocenediacetic acid [ 1671 which is converted into dimethyl-I ,l’-ferrocene- diacetate using diazomethane [ 1671. The dimeihyl ester of ferrocene-1 ,1 ‘-diacetic acid is treated with sodium triphenylmethyl in anhydrous ether to yield 1 ,l’-(a-carbo- methoxy-P-ketotrimethylene)ferrocene, which is directly hydrolyzed and decarbox- ylated in glacial acetic acid with hydrochloric acid. By this procedure 1 ,l’-(a-ketotri- methy1ene)ferrocene is formed [168]. The a-keto group is reduced to give 1,l’-trimethyleneferrocene [ 1691 which is diacylated by Friedel- Crafts acetylation using an


excess of acetyl chloride/aluminum chloride [ 1701. The procedure described by Rinehart et al. [ 1711 was used to convert the 3,3'-diacetyl-l,l'-trimethyleneferrocene into the dicarboxylic acid [172]. Esterification of this dicarboxylic acid with the appropriate lithium phenolate affords the desired products [ 1661.

The synthetic route to obtain the target complexes is shown in Scheme 6-70.

NaC(Ph),

ether -

COOCH3

1) NaOH 2) HCI

e C H 2 C O O C H 3 C H ~ N ~ -CH,COOH Fe -

-CH~COOCH~ &-CH2COOH

H"

1) c l c o c o c l

2 ) H O G A , Bu"Li, THF

R-OOC

Scheme 6-70. R-OOC

6.6.2.3 Ruthenocene Derivatives

CH3C\0

1,l'-Disubstituted ruthenocenes (Fig. 6-17) are prepared by esterification of 1 ,I)-ruthenocene diacid chloride (by a similar method to that used for 1,l'-ferrocene diacid chloride) with the appropriate phenol [160].

294 M. Marcos

Figure 6-17.

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[I271 R. H . Ceder, J. Sales, X. Solans, M. Font-Altaha, J. Chem. Soc., Dalfon Trans. 1986,

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[I331 M. J. Baena, P. Espinet, M. B. Ros, J. L. Serrano, Angew. Chem. Int. Ed. Engl. 1991,

[134] M. Ghedini, D. Pucci, E. Cesarotti, 0. Francescangeli, R. Bartolino, Liq. Cryst. 1994,

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[138] R. Petit, G. F. Emerson, Adv. Organometal. Chem. 1964, 1-46. [139] M. Cais, N. Maoz, J. Organometal. Chem. 1966, 5, 370-383. [I401 L. Ziminski, J. Malthete, L Chem. SOC., Chem. Commun. 1990, 1495-1496.

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York, 1989, pp. 946-953.

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833 - 842.

327 - 341.

Engl. 1993, 32, 1201 - 1203.

1351-1358.

555 - 560.

Int. Ed. Engl. 1989, 28, 1065- 1066.

Organometallics 1990, 9, 2028 -2033.

269-275.

30, 711-712.

16, 373-380.

1992, 4, 1119-1123.

Cryst. 1993, 231, 191 - 198.


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[I481 P. Singh, M.D. Rausch, R. W. Lenz, Liq. Cryst. 1991, 9, 19-26. [I491 J. Bhatt, B.M. Fung, K.M. Nicholas, J . Organomet. Chem. 1991, 413, 263-268. [150] K. P. Reddy, T. L. Brown, Liq. Cryst. 1992, 12, 369-376. [I511 J. Bhatt, B.M. Fung, K.M. Nicholas, Liq. Cryst. 1992, 12, 263-272. [152] N. J. Thompson, J. W. Goodby, K. J. Toyne, Liq. Cryst. 1993, 13, 381 -402. [I531 R. Deschenaux, M. Rama, J. Santiago, Tetrahedron Lett. 1993, 34, 3293-3296. [ 1541 R. Deschenaux, J. L. Marendaz, J. Santiago, Helv. Chim. Acta 1993, 76, 865 - 876. [155] R. Deschenaux, J.L. Marendaz, J . Chem. SOC., Chem. Commun. 1991, 909-910. [156] R. Deschenaux, I. Kosztics, J. L. Marendaz, H. Stoeckli-Evans, Chimia 1993, 47,

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1. 1226 - 1229.

7 Synthetic Strategies for Metallomesogenic Polymers

Luis Oriol

7.1 Introduction

There are numerous synthetic strategies that offer the possibility of introducing metal atoms or ions into the structure of polymeric materials [I]. Due to the relatively recent development of metallomesogenic polymers, however, only a few synthetic routes have been explored to date. Nevertheless, the reported examples cover a wide range of metal-containing polymers including heterochain polymers of a-bonded transition metals, polymeric metal complexes, and cofacially stacked polymeric macrocycles. From a synthetic point of view, the methods for obtaining this diverse range of materials can be classified into two types depending on the step in which metal entities are incorporated into the polymeric system.

a) Metal atoms incorporated in the polymer-forming step Metal poly(yne) polymers, phthalocyanine-metal complex polymers and some types of coordination polymers, are synthesized using strategies in which metal atoms are introduced either by the reaction of a bifunctional organic monomer with a metal salt to form the polymer or by polymerization of monomers which already contain a metal center. Since the majority of liquid crystalline phthalocyanine polymers are related to metalloid derivatives, in particular silicon and germanium, a survey of their synthesis will also be included.

b) Metal modifcation of a previously preformed organic polymer In this case, the molecular structure of the main-chain or side-chain polymer must contain functional groups which allow the complexation or anchoring of metal atoms.

Special emphasis will be put on the synthetic routes for obtaining metallomesogenic polymers by methods other than the classical polymerizations used in

302 L. Oriol

the preparation of condensation or addition organic polymers [2] (e.g. metal poly- (yne) polymers and spinal columnar metallomesogenic polymers). In the cases of materials obtained by conventional methods, only a brief note will be made since the polymerization methods do not differ greatly from those of analogous organic polymers. Furthermore, a detailed discussion will not be made regarding the preparation of metal-containing promesogenic monomers, as their synthesis is similar to equivalent low-molecular weight metallomesogens covered in Chap. 6.

7.2 Synthetic Strategies for the Incorporation of the Metal in the Polymer-Forming Step

There are several different synthetic strategies which allow the use of metal-containing systems as monomers in polymerization reactions. Generally speaking, the examples reported may be divided into two groups; condensation or addition polymers according to the classification proposed by Carothers [3]. However, in order to better understand the different families of metallomesogenic polymers, they will be discussed in different subsections depending on their structure. The vast majority of examples reported can be classified as condensation polymers, although the experimental polymerization conditions strongly depend on the type of metallomesogenic polymer formed.

7.2.1 Metal-Poly(yne) Polymers

The examples of lyotropic poly(yne) polymers reported to date essentially correspond to the systems synthesized by Hagihara, Tktkahashi and coworkers. These authors have reported many low- and high-molecular weight metallomesogens which contain a a-alkynyl-metal bond. Transition metal alkynyls are usually prepared by reaction of metal halides with alkynylation reagents such as lithium, magnesium or copper alkynyl compounds [4]. However, these methods could not be employed for the preparation of a number of metallomesogens [ 5 ] , therefore, several new methods were developed and modified for the synthesis of polymers.

a) Dehydrohalogenation

The synthesis of lyotropic poly(yne) polymers by dehydrohalogenation is accomplished using a catalytic amount of a copper(1) halide (CuX) in an amine solvent in the presence of an acid acceptor. The polymers obtained using this method are actually condensation polymers because of the elimination of hydrogen halide. The monomers used are trans-[P (n-Bu)&MC12 and dialkynyl compounds. A number of representative examples are shown in Fig. 7-1.

7 Synthetic Strategies for Metallomesogenic Polymers 303

'I' - - arnine, N2

L = P(mBu)3 X = CI, Br. I

e.g. [rll (dug)

M Y Reaction conditions Yield (THF, 25°C) i w (GPC) References

pt Cul. NHEtp, reflux, 24h 85% 0.98 31 000 [61

Pd CuCP(n-Bu), (1:4), NHEtp, RT, 3h 98% 0.46 22000 [71

Figure 7-1. Synthesis of lyotropic metal poly(yne) polymers by dehydrohalogenation.

The reactions are carried out in an inert atmosphere (or vacuum) as this avoids oxidation since the copper(1) halide/amine system may cause oxidative coupling of terminal acetylenes [6]. The addition of a catalytic amount of copper(1) halide greatly accelerates the polycondensation reaction [7]. Although copper(1) chloride, bromide and iodide are equally effective, copper(1) iodide is generally chosen due to its air stability and ease of handling [7]. The basicity of the amine has a strong effect on the degree of polymerization. Higher molecular weight polymers are obtained using strongly basic amines (diethylamine and piperidine give optimum results) [7,8]. The polycondensation proceeds smoothly in diethylamine at room temperature when M = palladium, or under reflux conditions when M = platinum, to give high-molecular weight polymers. Furthermore, the addition of a free phosphine to the system [triphenylphosphine or tri(4-methyl-pheny1)phosphine in a ratio of 4 : 1 with respect to copper(^)] greatly influences the molecular weight of palladium polymers, probably due to the fact that a free phosphine prevents dissociation of the phosphine ligand from the Pd(PR3)2 moiety [8]. In the synthesis of the polymers shown in Fig. 7-2, gaseous butadiyne was first converted into the corresponding solid metal monomer complex, which can be handled more easily, and subsequently reacted with the metal halide [9].

The design of arrangements of metal atoms at regular and alternating distances in the polymer backbone can be achieved by using an appropriate chloro-alkynyl metal complex monomer instead of t rans- [P(n-B~)~]~MCl~ [8, 101. The synthesis of metal poly(yne) polymers that contain an alternate arrangement of platinum and palladium atoms in the main-chain has also been described [I l l . Two examples of these types of polymeric chain designs are shown in Fig. 7-3.

304 L. Oriol

L I

CI-M-CI

L L = P(/FBu)J

+

Pt reflux, 24h 96% 0.9-1.3 -7oooo [6,81

Pd RT, 3h 93% 0.43 6300 [71

Figure 7-2. Synthesis of lyotropic metal poly(yne) polymers derived from butadiyne by dehydrohalogenation.

L L a) I I

CI-Pt-C=C-CeC-~-CI I

L L i L = P(/FBU)J + Cul (cat.)

L NHEt2, RT

L = P(+Bu)j

Figure 7-3. Dehydrohalogenation synthesis of lyotropic metal poly(yne) polymers with a) alternating distances between metal atoms; and b) regular arrangements of metal atoms.

b) Oxidative coupling

The polymerization of acetylenic compounds by an oxidative coupling reaction is often employed to prepare conjugated linear polyrners as well as binuclear transition metal complexes [5 , 121. This reaction usually proceeds in high yields and is therefore useful in the synthesis of metal poly(yne) polymers [13]. The main advantage of this


method is that no stoichiometric restrictions are inherent and, consequently, a high degree of polymerization can be achieved. A number of representative examples which have been synthesized using Hay's reagent as the oxidant (copper(1) chloride-oxygen with N,N,N,N'-tetramethylethylenediamine (TMEDA) in excess) is shown in Fig. 7-4. The choice of solvent is very important in order to avoid the pre- cipitation of polymeric chains and it was found that dichloromethane gave the best results. The reaction is carried out under a nitrogen atmosphere and the degree of polymerization strongly depends on the reaction time.

ff 95% 5.01 95000

Pd 60% 1.02 26000

97% low - ff solubility

Figure 7-4. Synthesis of lyotropic metal poly(yne) polymers by oxidative coupling [ 131.

Lyotropic nickel-poly(yne) polymers cannot be obtained using either of the methods described above because of the instability of the nickel halide (method a) and the instability of nickel complexes to Hay's reagent (method b). As an alternative, nickel polymers were synthesized using the method described below.

c) Alkynyl ligand exchange

Nickel-containing poly(yne) polymers can be synthesized by a copper(1) iodide-amine catalyzed alkynyl exchange reaction [14]. The addition of small amounts of tributyl phosphine is essential to prevent the dissociation of the phosphine ligand, which could cause decomposition of the polymer. Examples of this type of polymer are shown in Fig. 7-5.

The synthetic possibilities offered by these three methods allows the design of poly(yne) polymers containing transition metals (group 10 metals) as well as disilane, disiloxane and phosphine groups (selected examples are given in Fig. 7-6) [15].

306 L. Oriol

HCPC-Ni-CICH + + HCIC-Y-CICH Cul - P(PBu)~ (cat.) ii-CEC-Y-C.C f + 2 H C I C H I NHEt2, reflux, NZ, 6h L

e.g. Irll (dug)

Y Yield (THF, 25°C) gw (GPC)

+ 95% 0.20 10000

0.19 13000 k L t -c=c-lycIc-

-CEC-Ni-csc- 85%

95% 0.15 11000

L

Figure 7-5. Synthesis of lyotropic nickel poly(yne) polymers by alkynyl ligand exchange [ 141.

* t a) Cl-Y-Cl t + HCS-Y-CSCH Cul (cat.) +~-c.c-y-czc

L amine, Ar

L

L = P(PBu)~ -

e.g. M Y Reaction conditions Yield Mn (GPC)

FH3 $343

AH3 CH3

7H3 7H3

pt -Si-0-Si- Piperidine, reflux, 6 h 70% 17000 I

Pd -Si-0-Si- NHEt2, RT, 6h 60% 14000 I I CHj CH3

t t Cul (cat) i +$-c.c-y-c.c I L L

yield: 45%, E n = 16000

b) HC=C-Ni-C=CH + HC=C-Y-C=CH NHEt2. Ar, reflux, 7 h

F H 3 fH3 Y: -Si-0-Si-

I I CH, CH3

Figure 7-6. Synthesis of lyotropic nickel poly(yne) polymers containing disilane, disiloxane and phosphine groups [ 151.


The potential applications that can be derived from the electronic structure of these metal-poly(yne) polymers stimulated the search for new synthetic routes. Plat- inum, palladium, and nickel (group 10 metals) poly(yne) polymers can be obtained in high yields using bistrimethyltin alkynyl derivatives as monomers, which react with ML2X2 as outlined in Fig. 7-7 [16]. This synthetic strategy has also been extended to new poly(yne) polymers containing iron [17], ruthenium [18], rhodium [19], platinum [20], and cobalt [21], because it avoids the use of amine solvents, which many transition metals are unstable to.

BuLi 2 Me3SnCI HCtC-Ph-CICH - LiCIC-Ph-CICLi -Me3Sn--C~C-Ph-CX+SnMe3

POLYMERIZATION:

f L I

Me3Sn-CEC-Ph-CEC-SnMe3 + X-M-X - : X= halogen (CI) M= Pt. Pd, Ni i w = 70000

L= e.g. P(~-BU)~, A s ( r r B ~ ) ~

Figure 7-7. Alternative synthesis of metal poly(yne) polymers using bistrimethyltin alkynyl derivatives as monomers (TMS = trimethylsilyl).

7.2.2 Spinal Columnar Metallomesogenic Polymers

The reported examples of spinal columnar metallomesogenic polymers, as defined by Simon and coworkers [22], mainly deal with cofacially stacked polymeric macrocycles based on phthalocyaninato-metal complexes. The design and synthesis of this kind of material have been stimulated by its promising applications as new electronic materials [23]. Three different structural arrangements of cofacial stacking can be proposed depending on the nature of the linkages forming the polymeric backbone, and these are represented in Fig. 7-8. The synthetic strategy used to obtain the polymer clearly depends on the structural design required, and numerous examples have been described which fall into the following categories [24]: (a) spinal polymeric chains formed by covalent bonds. (b) spinal polymeric chains formed by covalent -coordinate bonds. (c) spinal polymeric chains formed by coordinate bonds.

In the field of liquid crystalline materials, the structures investigated so far are mainly related to the first two types (Fig. 7-8a and 7-8b) and, in particular, oxo- bridged complexes. Some of the synthetic approaches are described below. However, most of the examples reported involve metalloid (Si or Ge) complexes.

The conventional procedure for the synthesis of oxo-bridged phthalocyanine polymers is the uncatalyzed thermal bulk polycondensation of dihydroxy monomers

308 L. Oriol

e.g. M = metalloids (Si, Ge), Sn

L = -0-, -C IC-

e.g. M = Fe(ll), Co(ll), Ru(ll)

L = NAN W I N x N Figure 7-8. Structural designs of cofacially stacked polymeric macrocycles.

[25]. Unsubstituted monomeric dihydroxysilicon phthalocyanine yields a reasonably high degree of polymerization (dp 2 100) at high reaction temperatures (above 400 "C). However, in the case of substituted monomers, which promote thermotropic liquid crystalline behavior, low reaction temperatures are required in order to avoid the thermal decomposition of peripheral substituents. Simon and coworkers reported the synthesis of tin [26] or silicon [27] oxo-bridged spinal columnar short-chain oligomers obtained at temperatures lower than 200 "C. Other authors have reported the synthesis of similar polysiloxanes [28] or polygermoxanes [29] with a high degree of polymerization by thermal polycondensation of dihydroxy monomers at 200 "C in the liquid crystalline phase (Fig. 7-9). In each case, a degree of polymerization of about 100 was found.

Wegner and coworkers have developed alternative new synthetic methods. Poly- siloxanes were synthesized from unsubstituted phthalocyanine monomers by solution catalyzed polycondensation using temperature stable solvents and small amounts of a metal chloride as a dehydrating agent [30]. The best results were obtained using refluxing tributylamine (reaction temperature = 21 7 " C ) and refluxing quinoline (reaction temperature = 237 "C) with cadmium chloride, calcium chloride or iron trichloride as catalysts. However, the reaction rate of the octasubstituted monomer was very low, yielding moderately high molecular weight polymers only at long reaction times (several weeks). Although the mechanism of this reaction is unknown, the


HO OH * *

M

d k

R References

Sn C1ZH25G 25

Si c1 ZH250- 26

Si CiIH1P- 27

Ge C12H2OOC- 28

Figure 7-9. 0x0-bridged phthalocyaninato polymers synthesized by thermal bulk polycondensation.

reason for this low reactivity seems to be the steric hindrance of the reactive centers [31]. One possibility for a much faster and more quantitative polycondensation is the use of better leaving groups by functionalization of the hydroxyl groups (see Fig. 7-10, p. 310). The trifluoroacetate silylester is an activated monomer that can be condensed at temperatures up to 200°C (bulk polycondensation) due to the easy cleavage of the ester bonds. A catalyst is not required in this step and the resulting polymers have higher degrees of polymerization than any sample obtained from the dihdroxy monomers. However, the isolation and purification of the trifluoroacetate silylester monomer is difficult, and the presence of triflouroacetic acid, formed in the reaction, causes depolymerization.

The same authors reported a third method of polymerization, which gives high molecular weights (Fig. 7-1 1) [32]. In this case, a dichloro-monomer is condensed in solution at relatively low temperatures (about 100 "C) using halogenophilic condensation agents.

Metallomesogenic polymers with a backbone composed of covalent-coordinate bonds can also be obtained by polycondensation methods, as exemplified in Fig. 7-12 (see p. 311) for polymers synthesized by Hanack and coworkers [33].

310 L. Oriol

catalyst (e.g. Fe CI,)

solvent (e.g. toluene)

Figure 7-10. Alternative synthetic routes using dihydroxysiliconphthalocyanine monomers.

halogenophilic condensalion agents (1)

solvent

(1): AgSO3CF3. TIS03CF3, (CU(CH~CN)~]SOJCF~

Solvent: e.g. toluene, 1-chloronaphlhalene

Reaction temperature: 100°C

Figure 7-1 1. Pol ycondensation of dichlorosiliconphtha1ocyanine monomers.

Figure 7-12. Synthesis of phthalocyanine polymers with a backbone composed of covalent- coordinate bonds.

7 Synthetic Strategies for Metallomesogenic Polymers 3 1 1

7.2.3 Metallomesogenic Polymers Obtained using Metal Salts plus Bridging Ligands

Coordination polymers, in which monomer units consist of organic ligands coordinated by metal ions, can be obtained by three general methods [34]: (a) a multiva- lent ligand that can simultaneously attach two metal ions (b) polymerization of metal complexes containing reactive organic groups. (c) the reaction of a purely organic polymer, containing the ligand groups, with metal ions.

In the first two methods, the metal atoms are introduced into the polymer structure during the polymer formation step. The metal is either introduced as a reactive monomer (method a), or it is already present in the organic functionalized monomer (method b). In this sub-section, a number of examples of metallomesogenic polymers obtained by method a will be discussed.

The disconnection approach to metallomesogenic polymers by method a (see Fig. 5-13) suggests the use of monomers which consist of organic ligands which can simultaneously coordinate to two metal ions in order to form a polymeric chain. Tak- ing into account the structural requirements of metallomesogenic polymers (rigid cores, generally with calamitic or discotic shapes, connected by flexible spacers), the synthesis of this monomer is usually difficult. Furthermore, the coordination geometry of the selected metal must not cause any strong perturbation of the optimum geometry of the mesogenic unit. For this reason the square planar geometry is usually

DISCONNECTION U H15cflQ<No H e;?Y

0-R - OC7H15 [ref. 361

H' OH L3

U H 2 m + l C m ~ O O C --@-(C,H&O G C O O ~ m H 2 m + l [ref. 371

Figure 7-13. Disconnection approach to metallomesogenic polymers: metal salts plus bridging ligands.

0

312 L. Oriol

preferred and copper(I1) is the most frequently used metal. One of the main disadvantages of this method is that only low degrees of polymerization are generally achieved [35]. The monomer ligands used in the synthesis of homo and copolymers of two series of copper(r1) metallomesogenic polymers, based on salicylaldimine [36] and /3-diketone [37] ligands, are shown in Fig. 7-13. In both cases, polymers were obtained by reaction of the bifunctionalized ligands (two bidentate coordination centers) with a stoichiometric amount of a copper(1r) salt in 1,4-dioxane solution. Mo- lecular weight data were not reported and, in the case of the salycylaldiminato polymers, depolymerization by dissolution was detected.

7.2.4 Metallomesogenic Polymers Obtained by Conventional Organic Polymerization Methods

A different strategy for the incorporation of metal atoms into the structure of liquid crystalline polymeric chains is the preparation of metal complexes that contain reactive organic groups. These compounds can be used as monomers for the synthesis of polymers using synthetic methods typical of organic polymers. Tables 7-1 and 7-2 contain literature examples of metallomesogenic polymers synthesized by synthetic methods commonly used for condensation polymers (Table 7-1) [38 -421 or addition polymers (Table 7-2) [43 - 461.

The majority of the condensation polymers described to date are polyesters which have been synthesized in a variety of ways. Polyesters derived from bis(2,4-dihydroxy- salicylaldiminato)copper(rI) complexes have been the most commonly studied systems so far. They can be synthesized by interfacial polymerization [38]. Attempts to synthesize these systems by direct polycondensation or low-temperature solution polycondensation using pyridine as the solvent were not successful [38c]. The electron-withdrawing character of the complex monomer, combined with the steric hindrance of the lateral imine groups, decrease the reactivity of the phenol groups. In fact, no polymers with a very high molecular weight were obtained according to viscosimetry data. Other different series of metallomesogenic polyesters obtained by low-temperature solution polycondensation [39] and melt polycondensation [41,42] have also been reported.

Optimum results were obtained in the case of polyamides synthesized by low temperature solution polycondensation [40]. The solubility of the organometallic polyamides in the reaction solvent gives rise to polymers with higher molecular weights than those obtained in the synthesis of poly(p-phenylene terephthalamide).

Only a few examples have been reported concerning addition polymers (Table 7-2). Acrylates are commonly used as the reactive group and their reactivity is affected by the metal complex structure. In the case of ferrocene derivatives [43] the radical-initi- ated polymerization (azobisisobutyronitrile (AIBN) as initiator) in toluene yields copolymers whose copolymer composition, investigated by 'H NMR spectroscopy and potentiometric titration, is in good agreement with the monomer ratios em-

7 Synthetic Strategies for Metallomesogenic Polymers 3 13

Table 7-1. Metallomesogenic Condensation Polymers.

METHOD OF POLYMERIZATION MONOMERS AND REMARKS REFERENCES

CIoc-x-COCI 1

TERPOLYMER

INTERFACIAL POLYCONDENSATION [381 (phase transfer catalyzsd) main-chain homo and copolymers (molecular weights depending on R and X. In general oligorners are obtained)

see reference [38] for X and R

LOW-TEMPERATURE SOLUTION POLYCONDENSATION main-chain copolymers

1401 LOW TEMPERATURE SOLUTION POLYCONDENSATION main-chain, rigid homo and copolymers (high molecular weights, around 78000)

MELT TRANSESTERIFICATION (of a 1411 preformed organic liquid crystal terpolymer) main-chain copolymers

MELT POLYCONDENSATION 1421 main-chain or side-chain homopolymers [zn (GPC) ranging: 6600-18000]

see reference (4.21 for groups R and

Tabl

e 7-

2. M

etal

lom

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Add

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n P

olym

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~

ME

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MO

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INIT

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P

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[fin

(G

PC

) ran

ging

: 141

00-4

6000

]

side

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poty

mer

s 9

RA

DIC

AL-

INIT

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D S

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N

[441

P

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(AIB

N, t

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ne)

net

wo

k

THE

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AL

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tem

p.)

netw

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phot

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was

un

succ

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pera

ture

, 254

nm


ployed. However, in the case of the P-diketonatocopper(I1) complex, the properties of the resulting polymeric network seems to indicate a low reactivity [44]. Similar reactivity problems were encountered in the multifunctionalized copper(I1) phthalocyanine shown in Table 7-2. This compound did not react by a photoinitiated mechanism, probably due to the high photoabsorption of the phthalocyanine core [45].

Fibers of diacetylenic copper(I1) carboxylates can be polymerized by irradiation (A = 254 nm) for up to 6 h at room temperature. Under these conditions a complete topochemical polymerization occurs [46].

7.3 Metal Modification of Preformed Organic Polymers

The other approach to the design and preparation of liquid crystalline polymers incorporating metal atoms involves the synthesis of a suitably functionalized organic polymer followed by the subsequent incorporation of the metal centers. A number of chelating groups that have been introduced into the mesogenic units of main- chain or side-chain liquid crystal polymers are collected in Table 7-3 147 - 541 along with the metal ions used in the subsequent metal complexation. Metal complexation introduces changes which may depend on the reaction conditions such as solvent, temperature and, most importantly, on the concentration ratio of the two reactants. Generally, complexation is carried out in polymer solutions and, depending on the metal ratio used, the modified polymer may also be soluble, which allows the introduction of higher metal contents. With polymers of a poor solubility, a finely dispersed polymer suspension has been used and good yields have been obtained [48]. The metal content may be determined by different techniques such as UV-VIS spectroscopy, atomic absorption spectroscopy or inductively-coupled plasma atomic emission spectroscopy. The random reactivity of metal ions (Cu(rr), Pd(II), Ni(1I) and Fe(II1) are the most commonly used) leads to the coordination of chelating groups from different polymeric chains. Consequently, crosslinked materials are usually obtained by this method, as illustrated in Fig. 7-14.

As an alternative to metal complexation of polymeric ligands, metal entities may be incorporated into a polymer by branching organic reactions (graft polymers) using metal complexes with organic functional groups. Depending on the functionalization of the branching agent, networks (multifunctionalized reactants) or side-chain polymers (monofunctionalized reactants) can be obtained.

As an example, the Friedel -Crafts reaction of poly(y-benzyl-L-glutamate) with acid chlorides derived from metallophthalocyanines leads to branched and crosslinked materials (Fig. 7-15) [%]. Polymers containing more than 3 mol% of metallophthalocyanine entities are insoluble.

The grafting of monofunctionalized metallomesogenic reactants onto polymeric backbones allows the synthesis of side-chain metallomesogenic polymers. Ferrocene- containing side-chain polysiloxanes can be synthesized in this way by the hydroxyla-

316 L. Oriol

Table 7-3. Metal Modification of Preformed Liquid Crystal Polymers Containing Ligand Groups.

LIGAND GROUPS TYPE OF PRISTINE POLYMER METAL REFERENCES

-@: N-

OH c;S

Main-chain polymers

Main-chain polymers

Side-chain polymers

Main-chain polymers

Side-chain polymers

Sidechain polymers

Sidechain polymers

Side-chain polymers

Cu(ll), Fe(ll1)

Cu(ll), Ni(l1)

Cu(ll), Ni(ll), Co(lll), Pd(ll)

Pt(ll)

Pd(ll)

Pd(ll)

tion reaction of polyhydrogenosiloxanes with terminal vinyl functionalized ferrocenes [56] (see Fig. 7-16, p. 319). The reaction conditions used are similar to conventional hydroxylation reactions using a platinum(l1) complex as catalyst.


M

I - I Mesogenic unit containing ligand groups

1"

Figure 7-14. Schematic drawing of crosslinked materials usually obtained by metal modification of: a) preformed organic main-chain polymers or; b) side-chain polymers.

318 L. Oriol

,COCl

+

COCl

AIC13 ClOC

t HN-CH -CO 4 I FH2

y 2

F=O

?

co

Q (+ crosslinked materials) HOE'

Figure 7-15. Branching of poly(y-benzyl-L-glutamate) with metal phthalocyanines by the Fiedel-Crafts reaction [ 5 5 ] .


1 ,l '- or 1,3-disubstituted ferrocene

Figure 7-16. Grafting of polysiloxanes with terminal vinyl mesogenic ferrocene derivatives (the rectangle represents a promesogenic unit derived from a linearly disubstituted phenyl benzoate) [56].

References

D. Wohrle in Handbook of Polymer Synthesis, Part B (Ed: H. R. Kricheldorf), Marcel Dekker, New York, 1992, Chapter 18. see for instance: a) G. Odian Principles of Polymerization, 2nd ed., Wiley, New York, 1981; b) Handbook of Polymer Synthesis, Part B (Ed: H. R. Kricheldorf), Dekker, New York, 1992. W.H. Carothers, J . Am. Chem. SOC. 1929, 51, 2548-2559. a) M. L. H. Green in Organometallic Compounds 3rd ed. Vol. 2, (Eds: G. E. Coat, M. L. H. Green, K. Wade), Methuen, London, 1968, p. 203; b) 0. M. A. Salah, M. I. Bruce, J . Chem.

see for instance: S. Takahashi, T. Kaharu, Mem. Znst. Sci. Ind. Res., Osaka Univ. 1992,

T. F. Rutledge, Acetylenes and Allenes, Reinhold, New York, 1969, p. 403. S. Takahashi, M. Kariya, T. Yatake, K. Sonogashira, N. Hagihara, Macromolecules 1978,

S. Takahashi, H. Morimoto, E. Murata, S. Kataoka, K. Sonogashira, N. Hagihara, J . Polym. Sci. Polym. Chem. 1982, 20, 565-573. a) K. Sonogashira, Y. Fujikura, T. Yatake, N. Toyoshima, S. Takahashi, N. Hagihara, J. Organomet. Chem. 1978, 145, 101 - 108; b) K. Sonogashira, S. Takahashi, N. Hagihara, Macromolecules 1977, 10, 879- 880.

SOC. 1974, 2302 - 2304.

49, 47-56.

11, 1063 - 1066.

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[lo] S. Takahashi, Y. Ohyama, E. Murata, K. Sonogashira, N. Hagihara, J. Polym. Sci.

[ l I ] K. Sonogashira, S. Kataoka, S. Thkahashi, N. Hagihara, J. Organomet. Chem. 1978, 160,

[12] a) P. J. Kim, H. Masai, K. Sonogashira, N. Hagihara, Inorg. Nucl. Chem. Lett. 1970, 6, 181-185; b) Y. Fujikura, K. Sonogashira, N. Hagihara, Chem. Lett. 1975, 1067.

[13] S. Takahashi, E. Murata, K. Sonogashira, N. Hagihara, J . Polym. Sci. Polym. Chem.

[14] K. Sonogashira, K. Ohga, S. Thkahashi, N. Hagihara, J. Organomet. Chem. 1980, 188,

[15] a) S. Kotani, K. Shiina, K. Sonogashira, Appl. Organomet. Chem. 1991, 5, 417-425; b) T. Matsumoto, S. Kotani, K. Shiina, K. Sonogashira, Appl. Organomet. Chem. 1993, 7,

[I61 see for instance: a) A.E. Dray, F, Wittmann, R.H. Friend, A.M Donald, M.S. Khan, J. Lewis, B. F. G. Johnson, Synth. Met. 1991, 41-43, 871 -874; b) B. F. G. Johnson, A. K. Kakkar, M. S. Khan, J. Lewis, A.E. Dray, R. H. Friend, F. Wittmann, J. Muter. Chem.

[I71 B. F. G. Johnson, A. K. Kakkar, M. S. Khan, J. Lewis, J. Organomet. Chem. 1991, 409,

[I81 S. J. Davies, B. F. G. Johnson, J. Lewis, P. R. Raithby, J. Organomet. Chem. 1991, 414,

[19] a) S. J. Davies, B. F.G. Johnson, M. S. Khan, J. Lewis, J. Chem. SOC., Chem. Commun. 1991, 187- 188; b) M. S. Khan, S.J. Davies, A.K. Kakkar, D. Schwartz, B. Lin, B. F. G. Johnson, J. Lewis, J. Organomet. Chem. 1992, 424, 87-97.

[20] J. Lewis, M. S. Khan, A. K. Kakkar, B.F.G. Johnson, T. B. Marder, H. B. Fyfe, F. Witt- mann, R. H. Friend, A. E. Dray, J. Organomet. Chem. 1992, 425, 165- 176.

[21] M.S. Khan, N.A. Pasha, A.K. Kakkar, P.R. Raithby, J. Lewis, K. Fuhrmann, R.H. Friend, J. Muter. Chem. 1992, 2, 759-760.

[22] C. Sirlin, L. Bosio, J. Simon, J. Chem. SOC., Chem. Commun. 1987, 236-237. [23] see for instance: a) C. W. Dirk, E.A. Mintz, K.F. Schoch, T.J. Marks in Advances in

Organometallic and Znorganic Polymer Science (Eds: C. E. Carraher, J. E. Sheats, C. U. Pittmann) Dekker, New York, 1986, p. 275; b) T. J. Marks, Science 1985, 227, 881 - 889; c) J. Simon, P. Bassoul in Phthalocyanines. Properties and Applications Vol. 2 (Eds: C.C. Leznoff, A.B.P. Lever) VCH, New York, 1993, Chapter 6.

[24] a) D. Wohrle in Handbook of Polymer Synthesis, Part B (Ed: H. R. Kricheldorf), Dekker, New York, 1992, p. 1175; b) M. Hanack, M. Lang, Adv. Muter. 1994, 6, 819-833.

[25] R.D. Joyner, M.E. Kenney, Inorg. Chem. 1962, 1, 717-718. [26] C. Sirlin, L. Bosio, J. Simon, J. Chem. SOC., Chem. Commun. 1987, 379-380. [27] C. Sirlin, L. Bosio, J. Simon, J. Chem. SOC., Chem. Commun. 1988, 236-237. [28] P.G. Schouten, J.M. Warman, M.P. de Haas, J.F. van der Pol, J. W. Zwikker, J. Am.

[29] L. Dulog, A. Gittinger, S. Roth, T. Wegner, Makromol. Chem. 1993, 194, 493 - 500. [30] E. Orthmann, G. Wegner, Makromol. Chem., Rapid Commun. 1986, 7, 243-247. [31 ] T. Sauer, G. Wegner, Makromol. Chem., Macromol. Symp. 1989, 24, 303 - 309. [32] a) W. Caseri, T. Sauer, G. Wegner, Makromol. Chem., Rapid Commun. 1988, 9,

651 -657; b) T. Sauer, G. Wegner, Macromolecules 1991, 24, 2240-2252. [33] a) J. Metz, M. Hanack, J. Am. Chem. SOC. 1983,105, 828-830; b) M. Hanack, A. Beck,

H. Lehmann, Synthesis 1987, 703 -705.

Polym. Chem. 1980, 18, 349-353.

319- 327.

1980, 18, 661 -669.

237-243.

61 3 - 621.

1991, I, 485-486.

C12 - C 14.

C51 -C53.

Chem. SOC. 1992, 114, 9028-9034.


[34] see for instance: a) J. C. Bailar in Organometallic Polymers (Eds: C. E. Carraher, C. U. Pittman, J.E. Sheats), Academic Press, New York, 1978, p. 313; b) C. E. Carraher, .l Chem. Ed. 1981, 58, 921-934; c) D. Wohrle in Handbook of Polymer Synthesis, Part B (Ed: H.R. Kricheldorf), Dekker, New York, 1992, p. 1161.

[35] R.D. Archer, Coord. Chem. Rev. 1993, 128, 49-68. [36] C. Carfagna, U. Caruso, A. Roviello, A. Sirigu, Makromol. Chem., Rapid Commun.

[37] K. Hanabusa, T. Isogai, T. Koyama, H. Shirai, Makromol. Chem. 1993, 194, 197-210. [38] a) M. Marcos, L. Oriol, J. L. Serrano, P. J. Alonso, J. A. PuCrtolas, Macromolecules 1990,

23, 5187-5191; b) U. Caruso, A. Roviello, A. Sirigu, Macromolecules 1991, 24, 2606- 2609; c) M. Marcos, L. Oriol, J. L. Serrano, Macromolecules 1992, 25, 5362-5368.

1987, 8, 345-351.

[39] P. Singh, M. D. Rausch, R. W. Lenz, Polym. Bull. 1989, 22, 247 -252. [40] a) A. A. Dembek, R. R. Burch, A. E. Feiring, J Amer. Chem. SOC. 1993,115,2087 - 2089;

b) A.A. Dembek, R. R. Burch, A.E. Feiring, Polyrn. Prep. 1993, 34, 172- 173; c) A.A. Dembek, R.R. Burch, A.E. Feiring, Macromol. Symp. 1994, 77, 303-313.

[41] J.S. Moore, S.I. Stupp, Polym. Bull. 1988, 19, 251-256. [42] J. Lindau, H. Fischer, U. Rotz, K. Jurkschat, F. Kuschel, Makromol. Chem., Rapid Com-

[43] a) A. Wiesemann, R. Zentel, T. Pakula, Polymer 1992, 33, 5315-5320; b) A. Wiese-

[44] K. Hanabusa, T. Suzuki, T. Koyama, H. Shirai, Makromol. Chem. 1992, 193,

[45] J.F. van der Pol, E. NeeIeman, J.C. van Miltenburg, J.W. Zwikker, R. J.M. Nolte,

[46] G.S. Attard, R.H. Templer, J Muter. Chem. 1993, 3, 207-213. [47] K. Hanabusa, J. Higashi, T. Koyama, H. Shirai, Makromol. Chem. 1989, 190, 1 - 8. [48] a) L. Oriol, P. J. Alonso, J. I. Martinez, M. Piiiol, J. L. Serrano, Macromolecules 1994,

27, 1869- 1874; b) P. J. Alonso, J.I. Martinez, L. Oriol, M. Pifiol, J.L. Serrano, Adv. Muter. 1994, 6, 663 - 667.

[49] P. J. Alonso, E. Campillos, M. Marcos, J. I. Martinez, L. Oriol, M. Pifiol, J. L. Serrano, Poster presented at the 15th International Liquid Crystal Conference, Budapest, Hun- gary, July 1994.

[50] K. Hanabusa, Y. Tanimura, T. Suzuki, T. Koyama, H. Shirai, Makromol. Chem. 1991,

[51] a) K. Hanabusa, T. Suzuki, T. Koyama, H. Shirai, A. Kurose, Polym. J 1990, 22, 183 - 186; b) F. Wu, R. Zhang, Y. Jiang, Chin. J Polym. Sci. 1991, 9, 71 -78; c) K. Hanabusa, T. Suzuki, T. Koyama, H. Shirai, Makromol. Chem. 1992, 193, 2149-2161; d) Z. Zhou, D. Dai, R. Zhang, Chin. J Polym. Sci. 1992, 10, 70-74.

[52] a) K. Hanabusa, T. Suzuki, T. Koyama, H. Shirai, N. Hojo, A. Kurose, Makromol. Chem. 1990, 191, 489-496; b) K. Hanabusa, T. Suzuki, T. Koyma, H. Shirai, N. Hojo, J Macromol. Sci. Chem. 1990, A27(9-II), 1379- 1387.

[53] a) G. Chen, R. Zhang, Chin. .I Polym. Sci. 1991, 9, 339-346; b) G. Chen, P. Xie, R. Zhang, Mol. Cryst. Liq. Cryst. 1993, 225, 373-381.

[54] S . Zhang, S. Bi, P. Xie, R. Zhang, Chin. .l Polyrn. Sci. 1992, 10, 281 -286. [55 ] K. Hanabusa, C. Kobayashi, T. Koyama, E. Masuda, H. Shirai, Y. Kondo, K. Takemoto,

[56] R. Deschenaux, I. Kosztics, U. Scholten, D. Guillon, M. Ibn-Elhaj, .l Muter. Chem. 1994,

mun. 1991, 12, 477-482.

mann, R. Zentel, Liq. Crysf. 1993, 14, 1925 - 1934.

2149-2161.

W. Drenth, Macromolecules 1990, 23, 155 - 162.

192, 233 - 244.

E. Iizuka, N. Hojo, Makromol. Chem. 1986, 187, 753-761.

4, 1351-1352.

Part C. Structural Characterization

From a general point of view, the liquid crystalline properties of metallomesogens (mesophase types, transition temperatures and transition enthalpies) are investigated using the same experimental techniques as those usual for classical mesogens: polarizing optical microscopy, miscibility studies and differential scanning calorimetry (DSC). Furthermore, a more detailed characterization of the mesophase structures can be performed applying more specialized methods such as X-ray diffraction, neutron scattering, electron diffraction, and various spectroscopic techniques. A description of these techniques is beyond the scope of this book and they have been covered in detail in many review articles and books [i]. To readers who are not familiar with the techniques mentioned above it is recommended to study those reviews.

In the field of metallomesogens, the presence of the metal center makes it possible to apply methods of structure determination, which cannot be applied to organic mesogens to the same extent. n?lo types of technique have proved particularly useful for the determination of the structure of the metal-containing liquid crystals:

Firstly, the experimental methods that make use of X-rays, namely X-ray diffraction and EXAFS, allow the investigation of the local arrangement of the molecules in the mesophase. X-ray diffraction has also been extensively applied to the study of classical liquid crystals. However, this technique has proved to be especially useful when heavy atoms, such as metals, are present in the molecule. On the other hand, EXAFS spectroscopy provides information about the environment of heavy atoms, and therefore it can be applied to the investigation of liquid crystals only when their molecules contain metals.

Secondly, many metal ions are paramagnetic, and their presence in the mesogenic molecules enables them to be studied by electronic paramagnetic resonance (EPR) spectroscopy.

Part C of this book is devoted to these two types of experimental technique, with emphasis on studying the unique properties arising from the presence of the metal. Chapter 8 deals with X-ray techniques (X-ray diffraction and extended x-ray absorp-

324 Part C Structural Characterization of Metallomesogens

tion fine structure (EXAFS)), and Chapter 9 is focused on electron paramagnetic resonance (EPR) spectroscopy.

References

[l] a) D. Demus, L. Richter, Textures of Liquid Crystals, Verlag Chemie, Weinheim, 1978; b) G. W. Gray, J. W. G. Goodby, Smectic Liquid Crystals, Leonard Hill, Glasgow, 1984; c) Liquid Crystals and Plastic Crystals (Eds. G. W. Gray, P. A. Winsor), Vol. 2, Ellis Hor- wood, Chichester, 1974; d) H. Kelker, R. Hatz, Handbook of Liquid Crystals, VCH, Weinheim, 1980.

8 X-Ray Studies of Metallomesogens

Joaquin Barbera

X-ray techniques are a powerful tool in the investigation of structures at a molecular or even atomic level, because the X-ray wavelength has the same order of magnitude as intermolecular and interatomic distances. These methods have proved very useful in the determination of the molecular arrangement in both mesophases and solid state. Several types of information can be obtained from X-ray data: molecular conformation, interatomic distances, direction of orientation of the molecular axes, degree of orientational order, existence of short-range or long-range positional order, intermolecular distances, interlayer distance in lamellar phases, correlation length, etc.

As far as metal-containing liquid crystals are concerned, the presence of the metal center makes these complexes particularly suitable for X-ray studies. For several reasons this method of investigation is invaluable for gaining deeper insight into the structural properties of metallomesogens.

0 New geometries are generated, which in many cases are different from classical geometries of purely organic liquid crystals. This can lead to new types of molecular packing.

0 Even when the molecular geometry does not differ significantly from classical rod-like or disc-like, the mesophases are sometimes difficult to identify by microscopy alone because optical textures and viscosity are often different from those observed for organic liquid crystals. Therefore, the application of X-ray techniques is of great interest in order to unambiguously assign the type of liquid crystal phase.

0 Molecular shape and, as a consequence, the mesomorphic properties are influenced by the coordination geometry around the metal center. The type of coordination can be determined by X-ray methods.

0 The possibility of intramolecular and intermolecular coordinating interactions between two metal centers, or between a metal center and an atom in the ligand, gives rise to new possible kinds of attractive forces that may in turn lead to novel mesomorphic structures.

326 J Barbera

0 The scattering power of atoms is proportional to the atomic number 2, as X-rays are scattered largely by the atomic electrons. Metal atoms contain more electrons than atoms typically present in organic molecules and therefore metal-containing molecules are stronger X-ray scatterers. In particular, the presence of the metal increases the structure factor at wide angles and reinforces the intensity of diffraction peaks over the whole angular range.

0 The presence of heavy metals in the molecular structure also makes the use of X-ray spectroscopic techniques (Extended X-ray absorption fine structure, EX- AFS and X-ray absorption near-edge structure, XANES) possible, allowing the determination of the local environment of the metal.

On the basis of the points mentioned above, two main types of X-ray methods have been applied to metal-containing liquid crystals, namely X-ray diffraction and EXAFS. Therefore, the discussion will deal with these two techniques of structural analysis. The aim of this chapter is not to give a general view over X-ray studies performed on metallomesogens, but to discuss representative examples of the application of X-ray techniques to the investigation of metal-containing liquid crystals. The examples have been chosen on the basis of their relevance in illustrating the influence of the metal on the mesomorphic structure, with special emphasis on the structural peculiarities which originate from the presence of the metal center. The discussion will mainly concern the investigation of the structure of the liquid crystal state; nevertheless, where appropriate, the structure of the solid state will also be discussed.

8.1 X-Ray Diffraction

8.1.1 Basic Concepts in X-Ray Diffraction of Liquid Crystals

Diffraction techniques have traditionally been very useful in the study of both metal- containing and metal-free liquid crystals. Among them, X-ray diffraction is the most widely used method [I]. Other techniques, such as neutron and electron diffraction, have been extensively used for the investigation of metal-free liquid crystals, but very rarely for the study of metal-containing mesogens [2].

The X-ray diffraction pattern of a liquid crystal phase affords several types of information depending on the angular region investigated. In general, two regions are examined in the pattern:

(1) The small-angle maxima are due to intermolecular interferences along the director in the case of elongated molecules (known traditionally as calamitic mesogens) or along a direction perpendicular to the director in the case of flattened molecules (known traditionally as discotic mesogens), and correspond to long distances (tens of Angstroms). Periodic distances d i n the structure, such as the interlayer spacing, are calculated from these maxima by applying Bragg’s law (Eq. 8.1),

8 X-Ray Studies of Metallomesogens 321

nL = 2 d sin 8 (8.1)

where n is an integer, L the X-ray wavelength, and 8 the diffraction angle. From Bragg's law, it follows that there is a reciprocal relationship between the separation of the planes d i n real space and the diffraction angle 6' in the X-ray pattern. There- fore, the larger the distance d, the smaller the angle 8. In general, the d value calculated from Bragg's law corresponds roughly to the molecular length in calamitic compounds and to the molecular diameter in discotic compounds. For a nematic phase there is no periodic structure and the positional order is only short range. Hence the scattered X-ray intensity is a continuous function in reciprocal space. On the other hand, for smectic and columnar mesophases, in which the structure consists of a periodic arrangement of layers or columns, respectively, a certain number of peaks are observed in the scattered intensity. Therefore, the small-angle maxima are diffuse for nematic phases and sharp, that is Bragg peaks, for smectic and columnar phases. In columnar phases the spacing ratio of the small-angle maxima are characteristic of the type of columnar packing. Thus, for example, ratios of 1 : l6: fi: fi: fi.. . indicate a hexagonal columnar phase. The symmetry of the two-dimensional columnar lattice must be taken into account in order to determine the lattice constant. In the case of a hexagonal symmetry, the lattice constant a can be calculated from the first peak, which usually corresponds to the (1 0 0) reflection. The distance d, deduced from this peak by Bragg's law, is the separation between planes of columns and not the separation between nearest neighboring columns (see Fig. 8-1). The hexagonal lattice constant a is calculated from Eq. 8.2,

(8.2) a = dkos 30" = d 2 4 6

where d is calculated using Bragg's law (Eq. 8.1).

Figure 8-1. Schematic view of a hexagonal columnar mesophase along the column axis.

328 J Barbera

(2) The wide-angle maxima are due to intermolecular interferences in the direction perpendicular to the director in the case of calamitics or along the director in the case of discotics, and correspond to short distances (typically between 3 and 6 A). In general, these distances correspond roughly to the molecular width in calamitic compounds and to the molecular thickness in discotic compounds. The wide-angle maxima are diffuse for those mesophases in which the molecular packing perpendicular to the director is liquid-like, such as nematic, smectic A and C phases. On the other hand, the wide-angle peaks are sharp for smectic phases other than A and C, in which there is two-dimensional order within the layer. In the case of columnar phases, the wide-angle peaks are more or less sharp depending on the extent to which the order extends along the column. In all cases, the peak width is reciprocally related to the correlation length {. This magnitude can range from a few Angstroms (corresponding to two or three molecules) to several hundreds of Angstroms.

Intermolecular distances can be calculated from the positions of the peak maxima using Bragg’s law. However, for the smectic phases with hexagonal packing of the molecular long axis within the layers, the lateral distance D between close neighbor molecules is calculated from the measured distance L by means of Eq. (8.3), which is similar to Eq. (8.2).

D = L/COS 30 ’ = L 2 4 6 (8.3)

Some authors have proposed that, for smectic A and C phases with liquid-like packing within the layers, the mean intermolecular distance D must be estimated from a modification of Bragg’s law (Eq. (8.4)) [I h].

1.117A = 2 D s i n 8 (8.4)

For highly ordered mesophases, the number and spacing ratio of the wide-angle maxima are characteristic of the type of mesophase. For instance, a smectic E mesophase yields a series of sharp maxima that can be indexed according to an orthorhombic (two-dimensional rectangular) lattice, characteristic of the in-plane order. In a similar way, the smectic G and H mesophases give diffraction maxima characteristic of a monoclinic (in-plane tilted rectangular) lattice.

The simplest way to study the various diffraction maxima is to employ a powder sample, which consists of a polydomain with random director orientation. Each diffraction maximum will be averaged to give a ring centered around the incident beam. In this case, the X-ray pattern appears as a set of inner and outer concentric circles, that can be either sharp rings or diffuse haloes (Fig. 8-2a). Such powder patterns are intrinsically one-dimensional as the scattered intensity varies only with the diffraction angle, not with the direction. Thus, the positions of the peaks are completely specified by the radius of each ring, which in turn is directly related to the diffraction angle. In this case, a detector which gives a radial plot of the scattered intensity completely describes the diffraction pattern (Fig. 8-2b).


...

... J

I I I I I I I I

2 4 6 0 10 12 14 Theta (degrees)

Meridian

- - Equator I) Figure 8-2. Typical X-ray diffraction patterns of a smectic A mesophase. a) The pattern of an unoriented sample (powder pattern) consists of one inner sharp ring and one outer diffuse ring; b) the detector produces a radial plot of the scattered intensity which corresponds to the superimposed rectangle in (a); c) the pattern of an oriented smectic A phase consists of a pair of inner meridional sharp peaks and a pair of outer equatorial diffuse bands.

Diffraction patterns recorded on aligned samples provide more precise information about the local arrangement of the molecules. Alignment is usually achieved by the application of electric or magnetic fields to the fluid mesophase or by mechanical shearing on a surface when the mesophase is viscous. In the case of polymers, oriented fibers can be easily obtained by drawing from the mesomorphic state. In the

330 J. Barbera

X-ray pattern of an aligned sample, each ring splits into two arcs or crescents whose angular extension depends on the degree of orientational order (Fig. 8-2c). Thus, one kind of information that can be obtained from this type of pattern is an estimation of the degree of orientation. The higher the degree of orientational order of the molecules, the more the scattered intensity is concentrated. This information is especially useful in the case of mesomorphic polymers in order to evaluate the quality of oriented fibers. Usually, in X-ray patterns from oriented samples, the region of the pattern parallel to the direction of alignment of the molecular axes is called the meridian or meridional direction, whereas the region of the pattern perpendicular to the direction of alignment is called the equator or equatorial plane (Fig. 8-2c). Thus, the diffraction maxima situated along the meridian are due to interferences along the director, and this means that for calamitic mesogens the meridional maxima appear at small angles (long distances), while for discotics they appear at wide angles (short distances). On the other hand, the equatorial maxima correspond to lateral intermolecular interferences, and thus they appear at wide angles (short distances) for calamatic mesogens and at small angles (long distances) for discotics. Typical X-ray patterns from a nonoriented (a and b) and from an oriented (c) smectic A mesophase are depicted in Fig. 8-2. In the oriented pattern the meridional peaks correspond to the interlayer spacing and appear along the normal to the layers, whereas the equatorial crescents correspond to in-layer interferences.

The director in magnetically aligned mesophases can adopt two kinds of orientation: parallel or perpendicular to the magnetic field. This information can be deduced from the distribution of the meridional and equatorial maxima on the diffraction pattern in relation to the magnetic field direction. If the orientation is parallel, the anisotropy of the magnetic susceptibility is positive, whereas if the orientation is perpendicular, the anisotropy of the magnetic susceptibility is negative.

8.1.2 X-Ray Diffraction Studies of Metallomesogens

When a metal center is present in the liquid crystal molecules, the structure of the mesophase is expected to have some peculiarities, which will be apparent in the X-ray diffraction pattern. One of these characteristics is the appearance of several orders of reflection in the layer planes of smectic phases. This phenomenon occurs in both low molecular weight [3 - 61 and high molecular weight [7] metallomesogens. On the other hand, one single peak is generally observed for classical smectic phases of organic liquid crystals because the higher harmonics are very weak. This is due to molecular motions (thermal fluctuations of conformation and position) which blur the structure and, as a consequence, the electronic density profile normal to the layer becomes sinusoidal. For metal-containing mesogens, the electron-rich metal atom gives rise to a peak in the electron density profile, and the effect of blurring the structure is thus reduced. In this case the diffraction pattern consists of a set (usually two, three or four) of equally spaced Bragg spots at small angles, which appear along the meridional direction in oriented samples.

8 X-Ray Studies of Metallomesogens 33 1

Another interesting peculiarity arises from the special geometry of certain metal- containing molecules. This phenomenon is remarkable in the field of calamitics, where many metallomesogens have been reported which have a shape which differs from that of classical rod-like mesogens. These molecular shapes can be described as brick-like, open-book-like (or roof-like), or in the form of letters of the alphabet: H, P, S , U, etc.

The influence of the molecular architecture on the molecular organization in the mesophase has been extensively studied by Levelut in collaboration with several research groups using X-ray diffraction [8]. During the progress of their work, several phenomena not found in organic mesogens have been observed. In many systems, the biaxial nature of the molecule imparts some kind of local order due to the fact that, in addition to the orientation of the long axis characteristic of all the calamitic mesophases, there is a short-range orientation of the short axes. This causes some sort of local nematic order in a plane perpendicular to the director. This phenomenon has been found in the smectic A mesophase of certain mononuclear salicylaldiminocopper complexes [3] and in the nematic and smectic C mesophases of dinuclear ortho-palladated complexes [2d, 91. In the X-ray diffraction patterns obtained from magnetically aligned mesophases of these complexes, a halo at medium angles centered in the equatorial region has been found corresponding to a lateral distance of 8-9 A, which corresponds roughly to the molecular width. This halo appears in addition to the small-angle maxima along the meridian and the diffuse crescents on the equator, which are generally found in classical mesophases. The medium-angle halo was interpreted as being due to intermolecular interferences between neighboring molecules coupled side-by-side in pairs or even in ribbons. The origin of this phenomenon has been assigned to the anisotropic geometry of the brick-shaped (or in some cases roof-shaped) molecules, which should make free rotation as in classical disordered mesophases difficult. Furthermore, the X-ray patterns of magnetically aligned samples of dinuclear ortho-palladated complexes show medium angle diffuse scattering centered on the meridian. This corresponds to a distance of 9- 10 A, providing evidence of an additional correlation in a direction parallel to the director [9]. This means that an organization appears in which neighboring bimetallic cores are located on each side of the median plane of the layer, at a distance of about 4.5 - 5 A from this plane. As a consequence, neighboring molecules are mutually shifted with respect to each other by about 9- 10 A along a direction parallel to the director. Although the origin of this phenomenon remains unclear, it must somehow be related to the tendency of the molecules to attain the most efficient packing possible in the mesophase. In Fig. 8-3 the two kinds of correlation described above are schematically depicted for roof-shaped ortho-palladated azine complexes.

A side-by-side coupling of brick-shaped molecules could eventually lead to biaxial nematic or smectic A mesophases. The X-ray diffraction patterns provide evidence that this kind of molecular arrangement occurs at a local level. However, it is difficult to assess if the mesophase as a whole is uniaxial or biaxial on the basis of X-ray diffraction only. The only case in which a biaxial thermotropic nematic mesophase

332

9-10 A

L Barbera

\

\

/

/

\

\

/

/

Figure 8-3. Schematic drawing of the coupling a) perpendicular and b) parallel to the director proposed for roof-shaped dinuclear orfho-palladated complexes.

has been claimed was reported for a P-diketonatocopper(r1) complex [lo]. The X-ray pattern of the nematic mesophase in this compound shows three diffuse bands, corresponding to spacings characteristic of the three principal axes of the molecule: a maximum at 3 1 A along the meridional axis, corresponding to the molecular length, and two maxima at 24 A and 4.7 A on the equatorial plane, corresponding to the molecular width and thickness, respectively. This is the X-ray pattern that would be expected from an orthorhombic fluid. Although this pattern does not prove the existence of long-range biaxiality, the biaxial character of the mesophase was supported by conoscopic investigation. However, more detailed studies are needed in order to unambiguously classify the structure of this mesophase.

Another consequence of the peculiar geometry of many metal-containing liquid crystals is related to the apparent molecular length and to the smectic layer thickness revealed by X-ray diffraction. These distances are calculated from the small-angle reflections in the region of the meridian. In particular, the apparent molecular length is an important parameter in all mesophases because it can be related to the global molecular shape. In general, for the majority of metal-free liquid crystals, the measured apparent length l is smaller than the molecular length estimated from molecular models assuming a fully extended conformation of the aliphatic chains. This phenomenon is especially noticeable for mesogens containing long chains, and it has been accounted for by chain melting, that is conformational disorder of the hydrocarbon chains in the mesophase [I I ] (statistical disorder of the molecular axis also reduces the effective length). Metal-containing liquid crystals with the classical rod-


like geometry show the same general behavior as metal-free mesogens. For other geometries, the packing requirements lead to peculiar arrangements that modify the mesophase parameters. The most characteristic example in this field is the investigation of several series of salicylaldimine complexes, whose molecules contain two ligands which are joined by the metal and are shifted with respect to each other along their long axes, giving rise to a molecular shape which resembles a distorted H (Fig. 8-4). The H-structure contains four branches of unequal length, which are symmetric with respect to the metal center, the latter being located on the horizontal bar of the H. Therefore, the apparent molecular length measured experimentally has to be compared to two lengths: that of the ligand, that is one side of the H, and that of the complex, which is roughly equal to twice the length of the longer branch of the H.

R

R

/ M\

i

I Figure 8-4. Molecular structure and simplified geometrical representation of salicylaldimine complexes.

Several series of this kind of metallomesogen containing different terminal chains and metal centers have been studied by means of X-ray diffraction both on unaligned [12- 141 and magnetically aligned [3, 4, 15, 161 samples. From the X-ray patterns, the layer thickness of the smectic phases and of the smectic fluctuations in the nematic phase can be evaluated. In the case of orthogonal mesophases (i.e. the long molecular axes are on average perpendicular to the layer planes), this magnitude coincides with the apparent molecular length. However, in the case of tilted smectic phases or skewed cybotactic (i.e. containing smectic C fluctuations) nematic phases, the layer

334 J. Barbera‘

thickness is always smaller than the measured apparent length. The layer thickness can be calculated directly from the X-ray patterns of both aligned and unaligned samples by applying Bragg’s law (Eq. 8.1) to the inner diffraction peak. However, the apparent length in tilted mesophases can only be evaluated from the X-ray patterns of well oriented samples, in which the small-angle maxima lying in the meridional direction split into four points. This corresponds to a situation in which all molecular axes are oriented in the same direction and the smectic layers arrange themselves at a constant angle (equal to the tilt angle) with respect to this direction. The layer normal forms a revolution cone whose axis is parallel to the molecular director, resulting in a cylindrical symmetry around the director. The consequence of this is that each peak splits into two maxima displaced on either side of the meridian by the tilt angle. This structural situation and the corresponding four-point pattern are schematically depicted in Fig. 8-5. This phenomenon can also occur locally in skewed cybotactic nematic mesophases, which contain localized fluctuating regions of smectic C-like order. Interestingly, this situation has not only been observed for many low molecular weight salicylaldimine complexes [ 15,161 but also for the analogous high molecular weight copper(I1) complexes in their nematic mesophase [ 17, 181.

For the H-shaped salicylaldimine complexes described above, which form nematic, smectic A and smectic C phases, a large amount of data has been collected regarding the apparent molecular length 1 and layer thickness d (either in the smectic mesophases or in the layer fluctuations of the nematic phases). Representative values are collected in Table 8-1. The most interesting data concern the comparison between apparent length I, measured from the X-ray patterns, and molecular length estimated from molecular models for the ligand and the complex when the aliphatic chains are in the fully extended conformation. From these values a dual regime was found regardless of the metal, which depends on the substituents R and R’. For most compounds, except compound 1 which contains a short R group (R’ = CH3), the apparent length 1 was considerably shorter than the estimated length of the complex (distance between the two furthest ends of the longer branches of the H) and is comparable with the estimated length of the ligand. In some cases, the apparent length of the pure ligand in the nematic mesophase was measured experimentally, and the value obtained is close to the apparent length measured in the mesophase of the corresponding complexes [12, 15, 161. On the other hand, surprisingly, the apparent length 1 measured for the compound with a short R’ group (R’ = CH,) is smaller than that for complexes with long R’ groups (R’ = C6H4-C4H9, C6H4-OC5H,, , CIOH2,), and its value lies between the estimated length of the complex and that of the ligand. This phenomenon was explained by taking into account the peculiarities of the molecular shape and the requirements for an efficient packing (Fig. 8-6). For long R’ groups (compounds 2 to 8, Table 8-1) a structural model was proposed in which molecules belonging to neighboring layers interdigitate in order to fill space effectively (Fig. 8-6a). This type of packing is a consequence of the peculiar structure of the mesogenic molecule which consists of two rod-like ligands linked together by the metal and shifted with respect to each other in the direction of the rod axis. Fig. 8-6a shows that the apparent length (distance between molecular centers along

8 X-Ray Studies of Metallomesogens

Meridian

; a :’

335

Equator

D

Figure 8-5. Schematic representation of the four-point diffraction pattern of an aligned smectic C phase. The pattern (a) corresponds to a structural situation (b) in which the molecular axes are oriented in a single direction and the normals to the layers form a revolution cone around the director axis.

the director) corresponds roughly to the length of the ligand. In some cases the experimental value of I is even smaller than the ligand length estimated from models, and this is most probably due to chain melting. The tendency of this kind of molecule to interdigitate has been confirmed by single-crystal X-ray analysis [ 191. For molecules with a short R’ group, the shape of the complex is very different, as R’ behaves as a lateral substituent on the otherwise rod-shaped complex. This leads to a packing similar to that of classical calamitic mesogens; thus the apparent length 1 corresponds more to that estimated for the complex (Fig. 8-6 b). The

336 J Barbera

Table 8-1. Structural characteristics of the mesophases of salicylaldiminato complexes.

3 Ni Cloy1O G C O O -

-CH3

-C10H21

G o H z 1 +

410H21 +

N

N

N

N

N

SA

SA

SA

39 43.5

27.5 36.5

27.5 38

29 37

29 32.5

37 37

31 31

29 29

26 30 53

41 40.5 53

44 40.5 53

38 40.5 53

28 33 43

0 40.5 53

0 34 45

0 34 45 a vo c12H250- e c 4 H 9 +

d : Layer periodicity in the smectic phase or in the smectic fluctuations in the nematic phase; I : apparent molecular length measured on X-ray patterns; a: tilt angle in the smectic phase or in the smectic fluctuations.

molten state of the chains, however, accounts for the fact that the experimental value of 1 is smaller than that estimated from molecular models for compound 1 (Table 8-1).

The magnetic properties of the metal influence the magnetic behaviour of the whole molecule, which in turn determines the magnetic behaviour of the liquid crystalline bulk material. Information about this aspect can be provided by X-ray diffraction. In particular, the X-ray diffraction study of magnetically oriented


complex length

I ligand length

complex length

Figure 8-6. Schematic illustration of the molecular packing in the mesophase proposed for salicylaldiminato complexes. a) Molecules with a distorted H-shape adopt an interdigitated structure; b) molecules with a more rod-like shape adopt a classical end-to-end structure.

mesophases makes it possible to determine the sign of the anisotropy of the magnetic susceptibility AX. This magnitude is given by Eq. (8.5):

Ax = xi1 -XI ( 8 . 5 )

where xll and xl are the magnetic susceptibility component parallel and perpendicular to the molecular long axis. If xII >xI, the anisotropy of the magnetic susceptibility is positive and, as a result, the director is oriented parallel to the applied magnetic field. However, if xi1 <xI, the anisotropy of the magnetic susceptibility is negative and the director arranges itself in a plane perpendicular to the applied field. In the latter case a polydomain is obtained where different microdomains are oriented randomly in all directions contained in the aforementioned plane. Orientation of all the domains in a single direction is achieved by applying the magnetic field in two perpendicular directions. In this way, all the domains are aligned in the direction perpendicular to both magnetic field directions. The sign of AX can be easily determined from the distribution of the diffraction maxima on the X-ray diffraction pattern in relation to the applied magnetic field.

338 J. Barbera

Two terms contribute to the anisotropy of the magnetic susceptibility (Eq. 8.6): the contribution of the paramagnetic entity (Axpar), due mainly to the metal, and the anisotropic diamagnetic susceptibility (Axdia), due mainly to the organic ligand.

A compromise between these two contributions is responsible for the actual orientation of the mesophase director in the presence of a magnetic field [15]. For bis(salicy1aldiminato) complexes (Table %I) , it has been observed that for the same ligand, the copper(l1) complexes have a greater tendency to align perpendicular to the magnetic field than the nickel(i1) or oxovanadium(rv) complexes. This is due to the strong anisotropic paramagnetic contribution of the copper center to the molecular magnetic susceptibility, as determined by EPR [4, 20, 211. On the other hand, EPR studies have shown that the nickel derivatives are diamagnetic [21]. Although the oxovanadium complexes are paramagnetic [4, 20, 221, the situation in which the V=O bond is perpendicular to the magnetic field is favored in this case. Thus, the mesophases of the nickel and oxovanadium complexes orientate with the director parallel to the magnetic field in all cases studied (compounds 3, 4 and 8, Table 8-1). As far as the copper(l1) complexes are concerned, the nature of the ligand strongly influences the molecular orientation. Thus, for ligands containing only two benzene rings, the paramagnetic contribution from the metal is predominant and AX is negative (compounds 1,2 and 7). However, in ligands containing three benzene rings the diamagnetic contribution is increased, and it overcomes the paramagnetic entity. Consequently, A x is positive (compounds 5 and 6). Interestingly, a positive sign of AX has been found for an isomer of compound 1 in which the ester substituent does not occupy the 4-position of the salicylidene ring, but the 5-position (compound 9, Fig. 8-7) [ I 51. The different magnetic orientation of these two complexes, which may seem surprising at first sight, results from geometric effects. From molecular models it has been found that the four benzene rings are practically coaxial in compound 9, which is not the case for compound 1. As a consequence, the molecules are more rod-like, which seems to favor an orientation of the director parallel to the magnetic field.

Another property influenced by the presence of a metal center is the intermolecular force present in the mesomorphic state. Traditionally, van der Waals forces have been considered as being mainly responsible for mesomorphism. In some cases, dipole-dipole interactions, ionic forces or hydrogen bonding are also involved. Although in many metallomesogens the molecular interactions are essentially of the same kind as in classical mesogens, it is true that the presence of the metal leads to the possibility of intermolecular interactions as a result of coordination with a neighboring molecule. Several series of non-disc-like molecules display columnar mesomorphism due to dimerization, or to some other kind of intermolecular correlation through interactions that involve the metal atom. Single-crystal X-ray analysis revealed that the occurrence of columnar stacking in /3-diketonatothallium(1) complexes takes place due to dimerization of the semidisc-shaped molecules via intermolecular


Figure 8-7.

Compound 1

coo

@ ) J - C H s C L 0

CH3-N 'LQ ooc,

\

OCIOHZl Compound 9

TI -T1 interactions (Fig. 8-8 a) [23,24]. In addition, T1- 0 axial ligation stabilizes the stacking of the disc-shaped dimers and promotes the columnar structure (Fig. 8-8b).

Intermolecular associations are also thought to be responsible for the occurrence of columnar mesomorphism in several series of diketonate, triketonate, tetraketonate complexes and their corresponding Schiff base derivatives. A correlated arrangement has been proposed on the basis of X-ray diffraction patterns in order to explain the occurrence of columnar mesomorphism in some non-disc-like triketonate, tetraketonate and Schiff base complexes [25 -281. The measured intercolumnar spacings supported the existence of a columnar superstructure in these complexes which projects a disc shape along the column axis. Additional support for this structural model was found in the case of P-diketonate Schiff base complexes containing different metals, in which a halo at 7.2 A, corresponding to twice the inter-core correlation distance (3.6 A), was found in the X-ray patterns [28]. This halo was assigned to the doubling of the periodicity along the column axis and corresponds to the distance between two consecutive, equally oriented molecules (Fig. 8-9).

340 J Barbera

“VC TI ---TI : 3. 747 A

Figure 8-8. Crystal structure of bis- 1,3-(4‘-methoxypheny1)- 1,3-propane- dionatothallium(1). a) View of the dimer in the direction of the stacking axis; b) view of the stacked central cores along a direction

C

- perpendicular to the stacking axis (adapted from [24]).

The existence of axial intermolecular coordinating interactions has been found by X-ray diffraction [29] and by EXAFS [30,31] in the crystal and the columnar phases of alkanoate complexes also. These lantern-shaped complexes are arranged into rows of molecules which are generated by polymeric metal-oxygen interactions. The EXAFS studies on this series will be discussed in Sect. 8.2.2. A transition from the columnar phase to a nematic mesophase has been observed in this kind of complex by mixing with hydrocarbon solvents above a certain solvent concentration [32]. X-ray diffraction revealed that the constituent units of this type of mesophase are not individual molecules but one-dimensional supramolecular assemblies consisting of columns of molecules which align parallel to the nematic director, and the long- range lateral positional order characteristic of the columnar mesophase is not present.


Figure 8-9. Schematic representation of the dimerization of fl-diketonate Schiff base copper(r1) complexes. Dimerization of the semidisc-shaped individual molecules generates disc-shaped dimers capable of forming columnar mesophases.

This is apparent in the X-ray patterns in which a diffuse ring appears in the small- angle region, consistent with the lack of long-range positional order, instead of the set of sharp peaks typical of columnar structures.

The influence of coordinating interactions on the mesomorphic properties has been clearly revealed for a series of silver thiolates in which a structural change from a k[AgSR] layer structure to a cyclic (AgSR)* structure causes a transition from a lamellar to a micellar mesophase [6]. By means of X-ray analysis, the solid phase of this series of compounds had previously been found to consist of a layered structure containing a central plane of trigonal-planar coordinated silver atoms in a quasi-hexagonal arrangement, connected by bridging p 3-SR groups extending perpendicular to the central slab on both sides (Fig. 8-10) [33]. On melting, some of these compounds undergo a transition into a smectic A mesophase [6] for which X- ray diffraction studies revealed that the aliphatic chains are in a molten state and the polar units are packed in a similar way to that found in the solid. At higher tempera-

342 J . Barbera

SOLID LAMELLAR PHASE (SA)

Figure 8-10, Schematic representation of the different structures adopted by

MICELLAR PHASE silver thiolates, AgSC,H,,+, (adapted (HEXAGONAL COLUMNAR) from [6]).

ture, a micellar mesophase is formed, which has been characterized as hexagonal columnar. The transition takes place via an optically isotropic phase, probably possessing a cubic structure. For the members of the series with longer chains ( n r 12, Fig. 8-10), the SA mesophase is not observed and the micellar mesophase is, formed directly from the crystalline phase. The X-ray data are consistent with the micelles being cyclic (AgSR), structures with p2-bridged metal units (linearly coordinated silver). The chains are situated in the outer part of the cycle, and the resulting disc- like units pack with a stacking period of 5 A and an intercolumnar separation of 34.1 A. The structural changes are depicted in Fig. 8-10.

In some cases, intermolecular interactions arise from electronic effects caused by the peculiar shape of the molecule. Simon, in collaboration with Skoulios and Guillon, has performed extensive X-ray studies of phthalocyanine liquid crystals [34]. Some of these complexes, containing certain divalent metals (Pb, Sn), have a large dipole moment perpendicular to the rnacrocycle plane, because the bulky metal ions cannot be accommodated within the phthalocyanine cavity and form out-of- plane complexes. X-ray diffraction experiments revealed that these complexes form columnar mesophases [35]. The X-ray patterns of the lead complexes, show a broad maximum at 7.4 A in addition to the small-angle peaks due of the intercolumnar arrangement. This value corresponds roughly to twice the thickness of one phthalocyanine unit, and it was interpreted as being due to intracolumnar order arising from the presence of pairs of molecules arranged in an antiferroelectric way.

Mesomorphic structures can be induced by charge transfer interactions between disc-shaped donor molecules and strong electron acceptors. This phenomenon has been proposed to explain the mesomorphic properties of mixtures of some palladium [36] and platinum [37] organyls with 2,4,7-trinitrofluorenone (TNF). X-ray diffraction experiments showed that the structures of the mesophases are hexagonal colum-


nar and nematic (ND). This seems to be the first report of stabilization of a discotic nematic mesophase by charge transfer.

8.2 X-Ray Absorption Spectroscopy

8.2.1 Basic Concepts in EXAFS

Extended X-ray absorption fine structure (EXAFS) is the most common spectroscopic X-ray absorption technique [38] and represents one of the most suitable methods for the structural and electronic investigation of metallomesogens. Indeed, this technique can provide precise information on the local environment of the metal center both in crystalline and mesomorphic phases.

EXAFS analysis involves the measurement of the oscillations in X-ray absorbance which take place on the high photon energy side of the absorption edges of an ele- ment. This phenomenon arises from interference between the outgoing photoelec- tron ejected by absorption of the X-ray photon and that part of its wave function which is back scattered by the surrounding atoms. These oscillations therefore contain information on the local structure of the absorbing atom and, in particular, the number and nature of the neighboring atoms and the distances between the absorbing atom and these neighbors. This information cannot be provided by diffraction studies of noncrystalline materials such as glasses, powders, liquid crystals, etc. In- deed, a major advantage of the technique is that it does not rely on long range crystallographic order. Another advantage of the use of EXAFS for studying metal- containing liquid crystals is that the heavy metals are strongly backscattering. This means that the technique is highly sensitive for metal-containing systems.

The resulting spectrum contains a series of peaks which represent the oscillations of the X-ray absorption coefficient ~ ( k ) as a function of the photon energy (Fig. 8-1 1 a). The frequency and the amplitude of this oscillating modulation depend on the distance between the absorber and the neighbor atom and on the atomic number of the atomic species that backscatters the outgoing wave, respectively. The signals are a sum of sinusoidal oscillations arising from the different coordination shells around the absorbing atom. The next step in the data analysis is the Fourier transformation of the EXAFS modulation into a pseudoradial distribution function, which contains peaks that are assigned to the different atoms surrounding the absorbing atom (Fig. 8-1 1 b). The contributions of the different coordination shells can be separated and each of them studied independently. The isolation of the single shell data can be performed by inverse transformation of the peaks identified in the Fourier-transformed data. The inverse Fourier-transformed function is finally analysed by comparison of the amplitudes and phases with those of model compounds containing the same type of atoms. A common alternative approach is the use of numerical fitting procedures in which the spectrum is compared with the

344 J. Barbera

2 6 10 14

k (A-')

400 P2

0 3 6 9

R (A,

Figure 8-11. a) K-edge rhodium EXAFS absorption spectrum of rhodium(r1) heptanoate in the crystalline phase at 20°C; b) pseudoradial atomic distributions around the rhodium atoms (adapted from [31]).

theoretical spectrum calculated from the general EXAFS theory. In these calculations the theoretical parameters are allowed to vary in order to minimize the reliabili- ty factor, so that the simulated spectrum fits the experimental spectrum as closely as possible.

8.2.2 EXAFS Studies of Metallomesogens

EXAFS is a very useful tool to elucidate structural details of the crystalline and liquid crystalline phases of some metallomesogens. This technique has been used to elucidate the type of packing adopted by the polar cores of dicopper(r1) [30] and dirhodium(l1) [3 I] tetraalkanoates. From these studies, it has been confirmed that the molecules have a lantern-shaped dinuclear core which stacks in both the crystal and the liquid crystal state into rows of dimetal tetracarboxylate units. This is in fair agreement with the results of single-crystal X-ray diffraction analyses of members of


the copper alkanoate series containing short chains reported previously [39]. The copper atoms have an intradimer square-planar coordination with four oxygen atoms and interdimer axial ligation with an oxygen atom of the neighboring molecule. This ligation is thought to be a major factor contributing to the cohesion within the columns. In the structural model proposed for the mesophase on the basis of the EXAFS studies, the Cu-Cu axis of each dimer is parallel to the column axis and the dimers are successively offset on either side of this axis (Fig. 8-12). The structure of the polar cores of rhodium alkanoates in the mesomorphic state is very similar to that found for their copper analogs. The existence of a covalent bond between the two rhodium atoms in each individual complex was clearly established by the value of the intramolecular metal-metal distance measured by EXAFS (2.34 A), which is shorter than the intramolecular distance between the nonbonded metal atoms in the copper carboxylates (2.62 A). Rhodium alkanoates are the first examples of metal-metal bonded columnar mesogens reported. Binary mixtures of rhodium and copper alkanoates with similar chain lengths form homogeneous hexagonal columnar mesophases, which were shown by EXAFS to consist of randomly distributed pure copper and pure rhodium columns [31].

n

Figure 8-12. Schematic representation of the stacking of the polar cores proposed for the hexagonal columnar mesophase of dinuclear tetraalkanoates.

Metal atoms

0 Oxygenatoms

346 J. Barbera

EXAFS has been used to prove the nonplanar, concave configuration of mesomorphic octaalkoxyphthalocyaninatoplatinum in its crystalline phase [40]. The concave shape was attributed to attractive overlap of d: Pt orbitals, which in turn could explain the unusually short Pt - * .Pt distance of 3.29 A observed by other authors for the hexagonal columnar mesophase of the same complex [41]. Moreover, the related technique XANES (X-ray absorption near-edge structure) suggests that the platinum ion assumes an oxidation state higher than + 2 [40].

EXAFS studies on a liquid crystalline salicylaldiminocopper complex provide evidence of copper-copper correlations at distances corresponding to both the molecular width and thickness [42]. Indeed, EXAFS analyses performed in both the solid and in the low-temperature smectic phase show a correlation at 3.85 A, a distance that roughly corresponds to the molecular thickness. This result was not unexpected given the X-ray diffraction studies on the smectic A mesophase of this kind of compound, for which a side-by-side correlation between metal centers had been found at about 8.5 A, a distance roughly consistent with the molecular width [3]. These results, together with previous single-crystal X-ray diffraction measurements [19], suggest a local biaxial smectic structure in which the molecular rotations around the longest molecular axis are more hindered than in usual smectics.

References

[ l ] For reviews on the application of X-ray diffraction techniques to liquid crystals see: a) J. Falgueirettes, P. Delord in Liquid Crystals and Plastic Crystals (Eds.: G. W. Gray, P.A. Winsor), Vol. 2, Ellis Horwood, Chichester, 1974, pp. 62-79; b) K. Fontell in Liquid Crystals and Plastic Crystals (Eds.: G. W. Gray, P.A. Winsor), Vol. 2, Ellis Horwood, Chichester, 1974, pp. 80- 109; c) A. De Vries, Pramana, Suppl. No. 1, 1975, 93 - 113; d) I. Chistyakov in Advances in Liquid Crystals (Ed.: G.H. Brown), Vol. 1, Academic Press, New York, 1975, pp. 143 - 168; e) L. V. Azaroff, Mol. Cryst. Liq. Cryst. 1980, 60, 73 - 98; f) H. Kelker, R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim, 1980, pp. 221-242; g) A.M. Levelut, J. Chim. Physique 1983, 80, 149-161; h) A. De Vries, Mol. Cryst. Liq. Cryst. 1985, 131, 125 - 145; i) L. V. Azaroff, Mol. Cryst. Liq. Cryst. 1987, 145, 31 -58; j) P. S. Pershan, Structure of Liquid Crystal Phases, World Scientific, Singapore, 1988.

[2] a) A. M. Giroud-Godquin, P. Maldivi, J. C. Marchon, M. Bee, L. Carpentier, Molecular Physics 1989, 68, 1353-1365; b) L. Carpentier, M. Bee, A.M. Giroud-Godquin, P. Maldivi, J. C. Marchon, Molecular Physics 1989, 68, 1367- 1378; c) R. Bartolino, G. Coddens, F. Rustichelli, M.C. Pagnotta, C. Versace, M. Ghedini, F. Neve, Mol. Cryst. Liq. Cryst. 1992,221, 101 - 108; d) V. Formoso, M. C. Pagnotta, P. Mariani, M. Ghedhi, F. Neve, R. Bartolino, M. More, G. Pepy, Liq. Cryst. 1992, /I, 639-654.

[3] A.M. Levelut, M. Ghedini, R. Bartolino, F. P. Nicoletta, F. Rustichelli, J. Phys. France

[4] E. Campillos, M. Marcos, J. L. Serrano, J. Barbera, P. J. Alonso, J. I. Martinez, Chem.

[ 5 ] T. Kuboki, K. Araki, M. Yamada, S. Shiraishi, Bull, Chem. SOC. Jpn. 1994, 67, 948-955.

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[8] For a review on this work see: A.M. Levelut, MoI. Cryst. Liq. Cryst. 1992, 215, 31 -46. [9] P. Espinet, J. Pkrez, M. Marcos, M.B. Ros, J.L. Serrano, J. Barbera, A.M. Levelut,

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[15] J. Barbera, A.M. Levelut, M. Marcos, P. Romero, J.L. Serrano, Liq. Cryst. 1991, 10,

[I61 B. Borchers, W. Haase, Mol. Cryst. Liq. Cryst. 1991, 209, 319-328. [ 171 U. Caruso, A. Roviello, A. Sirigu, Macromolecules 1991, 24, 2606 - 2609. [ 181 P. J. Alonso, J. A. Puertolas, P. Davidson, B. Martinez, J. I. Martinez, L. Oriol, J. L. Ser-

1191 S. Armentano, G. De Munno, M. Ghedini, S . Morrone, Inorg. Chim. Acta 1993, 210,

[20] M. Marcos, J. L. Serrano, Adv. Mater. 1991, 3, 256-257. [21] a) M. Marcos, P. Romero, J.L. Serrano, J . Chem. SOC., Chem. Commun. 1989,

1641 -1643; b) M. Marcos, P. Romero, J.L. Serrano, Chem. Muter. 1990, 2, 495-498. [22] P. J. Alonso, M.L. Sanjuan, P. Romero, M. Marcos, J. L. Serrano, J . Phys. Condens.

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[25] A.G. Serrette, P. J. Carroll, T.M. Swager, J . Am. Chem. SOC. 1992, 114, 1887- 1889. [26] C. K. Lai, A. G. Serrette, T. M. Swager, J. Am. Chem. SOC. 1992, 114, 7948-7949. [27] A. G. Serrette, C. K. Lai, T. M. Swager, Chem. Mater. 1994, 6, 2252-2268. [28] H. Zheng, C.K. Lai, T.M. Swager, Chem. Mater. 1994, 6, 101 - 103. [29] H. Abied, D. Guillon, A. Skoulios, P. Weber, A.M. Giroud-Godquin, J.C. Marchon,

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348 J. Barbera'

[33] I.G. Dance, K.J. Fisher, R.M.H. Banda, M.L. Scudder, fnorg. Chem. 1991, 30,

[34] For a review on phthalocyanine-based liquid crystals see: J. Simon, P. Bassoul in Phthalocyanines (Eds.: C. C. Leznoff, A. B. P. Lever), Vol. 2, Verlag Chemie, Weinheim,

183-187.

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[37] K. Praefcke, B. Bilgin, J. Pickardt, M. Borowski, Chem. Ber. 1994, 127, 1543- 1545. [3R] For reviews on basic principles of EXAFS and other X-ray absorption spectroscopic tech-

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1994, 785-787.

1990, 12, 629-633.

9 Electron Paramagnetic Resonance of Paramagnetic Metallomesogens

Pablo J. Alonso

9.1 Introduction

For many years Electron Paramagnetic Resonance (EPR) spectroscopy has been a useful tool for the investigation of conventional liquid crystals. Organic liquid crystalline materials are commonly diamagnetic, therefore spin probes must be dissolved in the sample to be studied. EPR spectra are recorded and analyzed as a function of temperature through the various phases [ I , 21. Because of the orientational order in mesophases, some anisotropic terms in the spin Hamiltonian are not completely averaged. Therefore, the spectra of the paramagnetic probes obtained from liquid crystalline phases differ from those of isotropic liquids. These anisotropies, as well as their thermal evolution, provide information about mesophases [3 - 81. Glarum and Marshall [3,4], as well as Nordio et al. [5 , 61 developed the first theoretical model for calculating EPR line shapes obtained from partially oriented systems within the fast motion limit. This simple theory allows to obtain information about the order in the phase. In addition, it explains any unusual dependence of the width of the hyperfine components on the nuclear quantum number by the reduction of the secular and pseudosecular relaxation mechanisms caused by molecular ordering [4].

A general version of the early theory for the slow motion region has been developed by Polnaszek, Bruno and Freed [7]. It is noteworthy that in all these models it was assumed that the molecules are cylindrical (rod-like) and the phases uniaxial. Consequently, a molecular potential depending only on the angle between the molecular axis and the director was used to describe the molecular motion. Despite the fact that additional low symmetry effects are neglected in this approach, the results obtained are adequate for the conventional organic calamitic liquid crystals. Another important defect in this theory was pointed out by Luckhurst and Yeates [8]. They argue that in all these experiments the order and dynamics of the probe molecules are measured, which can differ significantly from those of the host molecules.

350 €!.I. Alonso

Since the discovery of new families of metal-containing liquid crystals [9- 131, with some of these molecules being paramagnetic (paramagnetic liquid crystals, PMLC), EPR spectroscopy once again appeared to be a valuable technique for studying structural aspects of these interesting materials without the problems associated with the use of structurally unrelated probes. In particular, EPR provides information about: (i) structure of the mesogenic molecules and its relationship with mesophase order; (ii) molecular dynamics in the mesophase; (iii) magnetic interactions between molecules and also between molecules and external magnetic fields; (iv) structural and dynamic aspects of liquid crystal polymers.

For the above purposes, the EPR spectra of the compounds are measured in different mesophases as a function of temperature and metal concentration. Special attention is paid to the evolution of the electronic Zeeman and hyperfine spin Hamiltoni- an terms as well as to the lineshapes.

As the samples are generally powders or liquids, the interpretation of the EPR spectra is somewhat complicated in comparison with those obtained from single crystals. In the fluid phases, care has to be taken in order to avoid possible domain alignment due to the external magnetic field.

In spite of the potential advantages of this technique, the number of EPR studies on PMLC is small. EPR spectroscopy is applied only to copper(rr) complexes and even less frequently to oxovanadium mesogenic materials. The variety of structures studied is also limited and includes complexes of P-diketones, salicylaldiminates and salicylaldehydes, together with some copper complexes where the metal is coordinated by four nitrogen atoms (phthalocyanines and annelides). In all of the materials mentioned above, the metal is tetracoordinated and has a planar geometry. Further- more, the EPR spectra of a number of paramagnetic liquid crystal polymers have been recorded. The aim of this chapter is to give a critical view over the results published in the literature to date as well as to provide an outlook on possible future developments.

9.2 Basic Concepts of EPR Spectroscopy

At first, a short introduction to the basic concepts of EPR spectroscopy will be given. The reader with a deeper interest in this topic is advised to consult some more specialized books, for example [14- 181. As mentioned above, EPR studies of PMLC are limited to copper(r1) and oxovanadium compounds. The outermost electronic configuration for these ions is 3d9 and 3d', respectively, with 2D as the ground term of the free ion. In a molecule, the fivefold orbital degeneration is removed, and the ground state is an orbital singlet. Isolated molecules can therefore be described as an S = 1/2 system. The interaction of the unpaired electron with the magnetic field is described by the Zeeman interaction Rz, involving the g-tensor. In the principal

9 Electron Paramagnetic Resonance of Paramagnetic Metallomesogens 35 1

axes frame of the g-tensor this interaction is described by the following spin Hamilto- nian (SH):

where pB is the Bohr magneton, S the electronic spin (S = 1/2 for both copper and oxovanadium compounds) and g,, g,, and g, the principal values of the g-tensor. If the point symmetry is axial, two of the three principal values of the g-tensor are equal (8, = gy) and the notation g, = g, = g, and g, = gI1 is commonly used. Mix- ing of the ground state with the excited states as a result of second order spin-orbit interaction is responsible for both the anisotropy and the shift of the g-tensor from the free electron value.

Some isotopes have a nuclear spin: vanadium occurs as an isotope with almost 100% natural abundance ("V, I = 7/2), whereas copper has two isotopes with I = 3/2 (63Cu and "Cu). Their nuclear moments are very similar (63gN/65gN = 0.94). In this situation the magnetic interaction between the unpaired electron and the nuclear spin has to be considered in order to describe the EPR spectra. This is done by adding the hyperfine (hf) contribution to Eq. 9.1. The hyperfine contribution is described by tensorial coupling between the electronic and nuclear spins. In its principal axes the expression is:

.?& = A , S, I, + A y Sy Zy +A, S, I, (9.2)

where I is the nuclear spin and A,, A, and A, are the principal values of the hyperfine tensor. If the point symmetry is axial, again two of the three principal values of the hyperfine tensor are equal (A, = A,,) and the notation A, = A, = A I and A , = A l l is used.

For an isolated molecule the position of the resonance line depends on the orientation of the magnetic field relative to the principal axes of the paramagnetic entity due to the anisotropic terms in the spin Hamiltonian. In a single crystal, the resonance line position changes as the magnetic field orientation varies with respect to the crystal axes, because only a limited number of different molecular orientations are present. The situation is rather different for an amorphous or a polycrystalline powder sample. In these cases all the possible molecular orientations are equally probable and a superposition of the spectra for all orientations occurs. A number of singularities at the resonance fields also appear in the spectrum which correspond to the principal values of the interaction tensors, and so the principal values of the coupling tensor can be obtained [15, 16, 191. In a liquid crystal, partially oriented samples can be obtained by different methods and anisotropic spectra are then recorded. The anisotropic spectra always appear complex due to the partial disorder of the liquid crystal. A special treatment is necessary to analyze the angular evolution of the EPR spectra in these compounds. This has been achieved by Ovchinnikov and coworkers for film samples in the smectic phase which were homeotropically aligned [20,21].

352 i?J. Alonso

As said before, analysis of the spectra obtained from isolated paramagnetic entities, especially the g and hf tensor values obtained, provides information about the electronic structure of the paramagnetic metal in the mesogenic molecules and also structural information about the metal-ligand bond. However, EPR spectra can be markedly modified due to the two possible dynamic contributions: motion of the paramagnetic molecules in the fluid phases and exchange interaction among the magnetic entities, both of which have to be taken into account in the case of PMLC. The basis of the theory for systems with exchange interaction and motional effects is well established, and a summary of this theory is given by Abragam [22] and Car- rington and McLachlan [14]. Both effects are influential in EPR spectroscopy of PMLC and their investigation provides valuable information about the dynamics and structures of mesophases. These effects are briefly discussed below.

Firstly, the exchange interaction will be considered. The influence of strong exchange on the EPR spectrum is to induce, in all cases, a collapse of the hyperfine splitting to zero and, if the g-tensor of the individual magnetic entities involved are equivalent (the same g-tensor being when the principal axes are parallel), a narrowing of the lines. If the magnetic entities are not equivalent (e.g. the presence of different paramagnetic molecules or if their g-principal axes are not parallel) the averaging of the g-tensor is accompanied by a less efficient narrowing effect. A qualitative view onto these effects follows. An EPR transition can be envisaged as a microwave-induced spin inversion. In the case of noninteracting spins, the inverted spin is localized on a particular site (see Fig. 9-1 a), but if the spins interact during the inversion process, the spin occurs at different sites (Fig. 9-1 b). Since the hyperfine structure is a consequence of the superposition of the applied magnetic field and the field due to the nuclear dipole, different hyperfine lines correspond to the different orientations of the nuclear dipole (MI = -Z, . . . , Z) all of which have the same population, I f the spin is spread out among different nuclei it experiences an averaged hyperfine field equal to zero. That explains the collapse of the hyperfine structure as a result

llllllll I 4 b)

Figure 9-1. Diagram representing exchange effects on the EPR spectra. a) Spin system without interaction; b) system of spins coupled by exchange interaction.

9 Electron Paramagnetic Resonance of Paramagnetic Metallomesogens 353

of exchange. The narrowing of the lines can be understood in the same way, because the dipolar broadening arising from the dipole-dipole interaction (having a random distribution throughout the spin sites) is averaged to zero when the spin is delocalized. The g-tensor accounts for the dependence of the Zeeman interaction on the orientation of the magnetic field relative to the crystal field and, therefore, its averaging is again a consequence of the spin delocalization in the case of interacting spins.

The second effect to be discussed is that of molecular motion. This is the blurring of some anisotropies of the g- and hyperfine tensors and, in the fast motion limit, it also induces a narrowing of the EPR lines. Indeed, if the motion is not restricted and the paramagnetic entity can reach any orientation in space, the spectrum appears to be isotropic with a g-factor and hyperfine coupling constant equal to a third of the trace of the corresponding static tensors.

For sake of simplicity, a system will be considered in which the g- and hyperfine tensors are coaxial; that is both have a common principal axis. With this restriction, any qualitative factor is not lost, but the corresponding equations are simplified. In the laboratory frame (xL, yL, zL) the spin Hamiltonian is given by

where

g = D'

(9.3)

D is the orthogonal matrix relating the laboratory axes to the principal axes (x, y, z ) :

If the paramagnetic entity moves, the matrix D and the g- and hyperfine tensors in the laboratory system are time dependent, the spin Hamiltonian

(9.6)

can be separated into two components: the static component, &, and the dynami- cal component, X ' ( t ) , by introducing the time-averaged tensors, (g$ and (A,), as

354 RJ. Alonso

Thus the static Hamiltonian results:

and the time-dependent contribution that accounts for both the collapse and broadening of the lines is

In the fast motion limit, the line positions are given by the static part of the Hamil- tonian (Eq. (9.8), and from these we can obtain the mean values of the QaiQaj

products which, in principle, can be related to the order parameters [23,24].

9.3 Summary of the Experimental Results

In this section a summary of the experimental EPR results on paramagnetic metal- organic liquid crystals is given.

9.3.1 [N4]-qpe Macrocycles and Polyamine Ligands

Markovitsi et al. I251 reported an EPR study of an amphiphilic copper(i1) complex containing an annelide ligand (see Fig. 9-2).

H2 H2 H2 /'-'\

c 18H 37-CH ' cu/

/ H2

,C - N NH-(CH 2CH 20) s-CH 3

Figure 9-2. Formula of the annelide

Markovitsi et al. Adapted from \C-NH/ \N--(CH~CH 2 q 5 - ~ ~ copper(I1) complex studied by

'c-c Hz H2 P I .


This compound produces homogeneous organic solutions, but in water it exists in a micellar form and melts in three stages, each of which is a lamellar phase: C- (108 K) -+ L1 -(328 K) -+ L2 -(358 K) + L, -(383 K) -+ I. EPR spectra were recorded for the various phases. In homogeneous organic solutions the spectra are typical of those obtained from copper(r1) in a square-planar coordination geometry, but with SH parameters that depend on the solvent. For example in toluene gI1 = 2.303, g, = 2.064, Al l = 567 MHz and A, = 107 MHz whereas in methanol gll = 2.183, g, = 2.075, A l l = 498 MHz and A , = 58 MHz. These changes are attributed to the modification of the copper environment by solvation.

In the case of a micellar solution in water, the EPR spectrum corresponds to that of a nonoriented axial paramagnetic entity without any resolved hyperfine structure. The lines are broader for the micellar aggregates than in homogeneous organic solutions with the same concentration. The authors suggest that the broader lines could be due to a stronger dipolar interaction favored by the high local concentration of copper(I1) ions on the surface of the micelle. However, they do not offer any explanation for the disappearance of the hyperfine structure.

The spectra of the different lamellar phases have been measured as a function of temperature. In the crystalline (C) phase and in the lamellar phases (Ll, L2, L3) the spectra are always typical of that obtained from a randomly oriented powder of an S = 1/2 system in an axial environment. In the isotropic phase the spectrum only consists of an asymmetric single line having a shape intermediate between Gaussian and Lorentzian. The hyperfine structure is not resolved in any of the phases. The values obtained for the g-factors are given in Table 9-1.

Table 9-1. Spin Hamiltonian parameters in the different phases of the Cu" annelide complex shown in Fig. 9-2. Adapted from [25].

2.203 2.202 2.101

2.061 2.066 2.080

2.108 2.111 2.114 2.008

The observation of AM = f 2 transitions (half-field lines) in the spectra recorded at low temperature (30 K) indicates the existence of some exchange interaction between the copper(I1) ions and suggests the presence of magnetic pairs.

Another example of a mesogenic system incorporating the magnetic copper(I1) ion tetracoordinated by nitrogen is the phthalocyaninatocopper complex shown in Fig. 9-3.

Andrt et al. [26] studied the EPR spectra of octakis[(dodecyloxy)methyl]- phthalocyaninato copper(I1) (C,2PcCu) (where R = CH2-0-C,2H25) both in con-

356 P.l Alonso

R R

@&/-++ R Figure 9-3. Formula of the metal-free

phthalocyanine studied by Andre et al. I \

R

R R Adapted from [26].

centrated and dilute solutions in the corresponding metal-free phthalocyanine (C,,PcH) and in the diamagnetic zinc compound (C12PcZn). The metal-free phthalocyanine forms a discotic phase (D), as does the ClzPcCu complex. The phase sequences for these compounds are:

C ~ ~ P C H C-(353 K) -+ D-(543 K) --f I

C~ZPCCU C-(333 K) + D-(583 K) -+ I

The room temperature spectra of mixtures of the CI2PcCu complex in both the diamagnetic compounds CI2PcH and ClzPcZn were measured. The spectra can be described by an axial spin Hamiltonian including the Zeeman and the hyperfine interaction with the copper nucleus and the hyperfine interaction with the nitrogen ligands (I4N, I = 1, 99.63% natural abundance). The SH parameters obtained from the analysis of the spectra are collected in Table 9-2.

Table 9-2. Spin Hamiltonian parameters (at room temperature) of Cu" in the C,,PcCu phthalocyanine diluted in C,,PcH and C,,PcZn. Adapted from [26].

C , ,PcH C,,PcZn

2.160 k 0.002 2.045 kO.002

631 ? 3 112 *2 49 -+ 1 43 + 1

2.166 + 0.002 2.052 f 0.002

645 + 3 200 + 2 50 k1 42 & 1

9 Efectron Paramagnetic Resonance of Paramagneiic Metallomesogens 357

The EPR spectrum of a mixture of C,2PcCu and ClzPcH was also measured as a function of temperature (10-400 K). The loss of the nitrogen hyperfine structure in the discotic phase was noted by the authors, but they did not give any explanation for this behavior.

The EPR spectra of concentrated pure C,2PcCu were also recorded at different temperatures between 10 and 600 K. In this case the spectra differ from those observed for diluted samples and consist of a broad structureless line centered at g = 2.05, whose width changes with temperature. In the solid phase the width of the line decreases as the temperature rises, but at the melting transition a broadening of the line is observed. Above this point, the linewidth does not change upon increasing the temperature. The monotone thermal variation of the line width in the solid phase is associated with a decrease in the dipolar interaction as a consequence of thermal expansion. The broadening of the line during the C + D transition is tentatively explained as being due to modification of the exchange interaction as a result of molecular rearrangement at the phase transition.

9.3.2 P-Diketonate Complexes

EPR studies of several /3-diketonatocopper(I1) complexes have also been reported. The general structure of these compounds is shown in Fig. 9-4, the compounds studied, along with their phase types and transition temperatures, are given in Table 9-3.

Table 9-3. bis(&diketonato)copper(II) studied by EPR spectroscopy. The structure of the compounds is given in Fig. 9-4. Ri = C,H4 - RJ.

R; R; Phase (T, in K) Reference

C (350) D, (389) D, (414) I

C, (294) C2 (358) D (448) I

27, 28, 29

27, 28, 29

PI - CsH,, -CsHn P2 - CIOH21 - C,OH2, 29

P4 - @ - CIOH2, - OCZH, C (460) I (440) N m 30 P5 - @ - ClOH2, - OC,H7 C (457) I(427) Nm 30

- OCBH,, P3 -OCSH17

C: solid crystalline phase, D: discotic phase, N: nematic phase, I: isotropic phase, "': monotropic phase

358 P.J. Alonso

While the symmetric complexes (R, = R2) show discotic behavior, the unsymmetric compounds each exhibit a monotropic nematic phase.

The first EPR investigation of discotic copper(I1) P-diketonates was performed by Eatsman et al. [27], who investigated the spectra of compounds p, and p3 (see Table 9-3). Marked differences between the spectra of the two compounds were found. In the case of compound PI, the authors analyzed the EPR spectrum of a single crystal in the solid low temperature-ordered phase, as well as its thermal evolution through the different phases. In each case no hyperfine structure was observed, which is a strong indication for the existence of some degree of exchange interaction between the copper ions. The spectra are interpreted as being caused by copper(11) in a square-planar environment. In addition, the dependence of the halfwidth on the orientation of the magnetic field follows a law given by 13 cos2 8- 1 I 4'3 (6' is the angle of magnetic field with respect to the symmetry axis of the copper complex), indicating that the exchange interaction is one-dimensional. This one-dimensional character is interpreted as a consequence of the columnar stacking in the crystalline state.

Examination of the spectra as a function of temperature through the discotic phases reveals only minor changes at the phase transitions. At the C + D1 transition, some changes in the resonance line shape are detected: although the linewidth remains anisotropic, a clear broadening is observed in the narrowest resonance line (for 0 = 54 "). At the D, + D2 transition, only a slight broadening of the line (about 10%) is observed, together with a decrease in the dependence of the linewidth on orientation of the magnetic field. Although these changes have not been extensively investigated, they suggest that exchange interactions still play a significant role in the discotic phase.

A brief description of the EPR spectrum of compound p3 is also given in this pioneering work [27]. The authors remark that the spectrum does not correspond to a system in which exchange interactions are present, and they indicate that a complex spectrum, consisting of 18 lines (14 mT apart), is observed in the 0 = 54" single-crystal spectrum, while the resonance for 8 = 0" consists of a Gaussian single line. The structure of the 8 = 54" spectrum changes at the C1 + C2 and C2 + D transitions without any broadening of the line. In the discotic phase, a spectrum with resolved splitting is observed. SH parameters were not given in this work.

Later, Bose and Sadashiva [28, 291 repeated the EPR studies of these compounds and their work also included a studies, of compound b2. They observed a strong similarity between the spectra of compounds PI and p2. Both compounds give rise to strongly exchange-narrowed asymmetric spectra described by an axial g-tensor ( g - 2.243, g, = 2.069 for P, and gli = 2.28, g, = 2.06 for P2) but without any hy- / I ~

perfine structure. On the other hand, compound & shows EPR spectrum typical of an orthorhombic structure at room temperature (C, phase) with g, = 2.55, gu = 2.06 and g, = 1.78, as well as a resolved, complex hyperfine structure. When EPR spectra are recorded as a function of temperature, changes are observed. In the mesophase, a significant modification of the line structure is observed, but the spectrum remains orthorhombic (g, = 2.46, g,, = 2.06 and g, = 1.90 at 369 K), as in the anisotropic solid. According to the authors, this indicates a lamellar structure. In the isotropic


phase, an isotropic structureless line at g = 2.06 is observed. A detailed study of the structure found for the solid was undertaken by measuring the EPR spectra at two different frequencies (X- and Q-band). This experiment allowed Bose and Sadashiva [29] to interpret this spectrum as being due to forbidden AMI = f 1 transitions that are observed together with the allowed AM, = 0 transitions, which become partially allowed as a result of copper quadrupolar interaction.

The authors therefore concluded that in these copper(I1) compounds the discotic mesophase is structurally similar to a crystal and different from the mesophases formed by purely organic discotic systems. In addition, the differences in behavior of compounds p1 and p2 in comparison with compound p3 can be understood in terms of their crystal structures. In p1 and p2, one-dimensional S = 1/2 Heisenberg antiferromagnetic interaction is favored, because copper(I1) is located at an inversion center of the unimolecular unit cell. In contrast, for compound p3 the repeat unit of the bimolecular unit cell is a centrosymmetric molecular pair. The strong quadrupolar effect observed is associated with distortions occurring in the dimer.

The EPR spectra of the calamitic copper(r1) diketonates (p4 and ps) have been reported by Chandrasekhar et al. [30]. The spectra of both compounds are alike and show similar thermal behavior. In all the phases (C, I and N), and in both compounds, the spectra are interpreted as being due to axial copper(r1) ions, with gIl = 2.261 and g, = 2.062 in the nematic phase. The observation of a clearly resolved hyperfine structure in each spectrum rules out the existence of exchange interaction, which contradicts the interpretation given by the author for the thermal evolution of the magnetic susceptibility. The observation of an anisotropic EPR spectrum, even in the isotropic liquid phase (I), indicates high viscosity, and values higher than about lo-' s for the rotational correlation time are proposed. This phenomenon is associated with the existence of strong near-neighbor correlations as evidence being provided for by dielectric studies [30]. For more comments on this point see Sects. 10-6 and 10-7.

9.3.3 Schiff Base Derivatives

A lot of attention has been paid to EPR studies of mesogenic oxovanadium and copper complexes with bidentate Schiff base ligands. The general structure of these materials is depicted in Fig. 9-5.

R2

Figure 9-5. General formula of metal complexes with Schiff base ligands.

360 l ?J . AIonso

Table 9-4. Copper and oxovanadium complexes of Schiff base derivatives studied by EPR spectroscopy. q3 = (C,H,). R, and R, see Fig. 9-5.

M Reference

CU f31-381 VO [39-411

S3M(n) -0 -CnHz,, t I - 4 - F Cu [33]

S4M(n,m) -OOC-q3-0C,H,tl -CrnH~rn+~ Cu [41]

S5M ( n , m ) - OOC - @ - OC,H,, I -@-OCrnHi2m+iI Cu 41 VO [42, 431

SGM ( n , m ) - 0- CH2- @-OC,H, 1 - 4 - OCrnHiZrn+ 1 I Cu [41, 451 V 0 [45]

VO [42-441

S7M (n) - OOC - @ - OC,H,+ 1 - q3 - OCH2 - C*HC1- CH, CU [4G]

All the compounds studied (see Table9-4) are described as calamitic, and their mesophases are of the nematic (N) or smectic (S,,Sc, . . .) types.

The earlier EPR studies on Schiff base derivatives were performed by Ovchinnikov et al. [31-331 on compounds of the S2Cu(n, m) family, which were studied in the solid state using single crystals, polycrystalline samples, and frozen solutions in toluene, and in the different mesophases using both nonoriented and homeotropically aligned samples. A wide range of experimental data has been collected for S2Cu (7,l) [3 1,321, and these results can easily be extended to the other compounds [33]. In the following, the work of Ovchinnikov's group on S2Cu(7,1) will be discussed. Compound S2Cu (7,l) has a transition from the crystal to a smectic phase at 406 K which clears into the isotropic liquid at 413 K.

The EPR results on single-crystal samples are interpreted on the basis of the crystal structure of the complex [32, 331. Single crystals of S2Cu(7,1) are obtained from solutions in chloroform/acetone, the unit cell (monoclinic with a = 43.662 A, b = 10.267 A , c = 18.506 A and p = 107.29") contains eight molecules, four of which have a square-planar coordination and the others with tetrahedral coordination. A layered structure resembling that of a smectic phase is found. Although two different types of copper(r1) complex are present, the EPR spectrum consists of a single line for all orientations of the magnetic field. The principal values of the g- tensor are gx = 2.0590, gy = 2.0985 and g, = 2.2080 (g = 2.12). A hyperfine structure was not observed in any case, and the linewidth was found to be anisotropic. If 6 is the angle between the magnetic field and the normal to the molecular layer, the linewidth is proportional to (3 cos2 8- 1)'. This value is in agreement with the existence of a spin diffusion mechanism due to two-dimensional exchange interaction


between the two types of molecules. Taking into account the crystal structure, the authors calculated the SH parameters to be gll = 2.190, g, = 2.049 (g = 2.090) for the planar complex and gll = 2.298, g, = 2.068 (g = 2.144) for the tetrahedral one.

When polycrystalline samples are studied, different spectra are obtained depending on the synthetic procedure. Crystallization from solution yields a black powder whose spectrum is characterized by g, = 2.060, gv = 2.103 and g, = 2.21 1 which is due to copper monomers. If the solid state is achieved by cooling the sample from the melt, a green solid is obtained, and the spectrum corresponds to dimers with anisotropic exchange coupling. After subsequent heating, a third polycrystalline phase (brown solid) is obtained whose EPR spectrum consists of a single line at g = 2.085.

The study of the EPR spectra of S2Cu(7,1) in the different phases was also performed by Ovchinnikov et al. [32]. In the isotropic phase the EPR spectrum consists of a single isotropic line at g = 2.089 which is associated with the existence of both exchange interaction and motion of the molecules. At the isotropic-to-smectic phase transition the spectrum changes and two lines appear; one symmetric line at g = 2.124 and the other, an unsymmetric signal, at g = 2.049. Their relative intensities depend on the cooling rate with the g = 2.124 line being more intense when the cooling rate is lower.

This investigation was extended by Ovchinnikov’s group [33, 341 to other compounds of the S2Cu(n, m) family (with n = 7 and rn = 1 - 10, 12) as well as to S3Cu(n) (with n = 7). In all cases, the most intriguing fact is the appearance of two different paramagnetic entities in the mesophase; that is the presence of two different copper(I1) complexes. The shape of the unsymmetric line observed at higher field and its g-value (about 2.05) led the authors to assign it to the perpendicular component of the EPR signal of a copper complex with a planar or almost planar configuration. On the other hand, the symmetric shape of the second line (at g = 2.12) was related to averaging of the anisotropy of the g-tensor of the second type of complex present (tetrahedrally coordinated complex) as result of either rapid rotation of the molecules or exchange interaction. Molecular rotation can be ruled out since the spectrum does not change when the sample is frozen in liquid nitrogen.

In order to explain these results the Russian group [33] proposed the following model. In the solid, two types of molecule (square-planar and tetrahedrally coordinated molecules) coexist. The square-planar molecules are mesomorphic, but the tetrahedral ones are not. During the transition into the mesophase the tetrahedral molecules are distorted while the mesomorphic molecules remain planar. On cooling from the isotropic phase (in which all the molecules are planar) the sample becomes nonuniform and some of the planar molecules are significantly distorted. In the mesophase, the distorted and planar molecules cannot coexist as in the single crystal and they form separate regions of exchange coupled structurally similar molecules. Planar molecules are responsible for the formation of the mesophase with their long axes being ordered within the structure of the phase. This explains why the g, value in the mesophase is close to the g, value observed for the single crystal. It has been tentatively proposed that the structural organization of the tetra-

362 l?J. Alonso

hedral molecules is likely to consist of isotropic droplets of a low viscosity liquid in the high viscosity mesophase formed by the planar molecules. Although it is well known that copper(r1) complexes have a reasonable plasticity, this model is quite complicated and includes too many ad hoc assumptions. In a later section, EPR spectra of copper(n) Schiff base complexes showing exchange interaction will be discussed in which the different EPR features in the mesophase are explained using a simpler model.

Other interesting results presented by Ovchinnikov et al. concern the use of EPR measurements on oriented paramagnetic liquid crystals in order to obtain structural information about the mesophases [20, 21, 35, 361. The most straightforward way to do this is the investigation of a single domain sample. Since it is rather difficult to prepare single domain samples from these compounds, homeotropically oriented samples in a flat cell were used to measure the EPR spectra in the smectic phase. The thin sample is a multidomain with a strong uniaxial orientation normal to the layer. The other domain axes randomly distributed within the layer plane. If intermolecular exchange interaction takes place, each domain produces only one exchange-narrowed line. Thus the sample exhibits a broad nonuniform glass-l ike spectrum. In the following, a single domain will be considered. In order to calculate the line position, a laboratory (X, I: Z ) coordinate system is introduced in a way that Z is normal to the cell plates and the magnetic field B lies in the Y Z plane (a, is the angle of magnetic field with respect to the Z axis). The principal axes of the domain g-tensor are (x,y, z ) and both systems are related by the Euler angles ( y , 8,@). The effective g-value is:

g 2 ( ( p ) = Pcos2 a,+Q sin2 a,+R sin 2a,

P = g: cos2 e + ( g ; cos2 y + g f sin2 y ) sin2 o with

(9.1 1)

(9.12)

Q = (g: sin2 ty+g; cos2 y ) sin2 + + k: sin2 e+(g; cos2 ty + g ; sin2 y ) cos2 0 ) cos2 @ + +(g2~g; )s inycosWsin2#cos8 (9.1 3)

R = {(g; cos2 ty+g2sin2 i y - g t ) cos o cos G+(g:-g;) sin ty cos y sin@) sin 8

(9.14)

For a given sample y and 0 are the same for all the domains but the angle @ varies randomly. Due to this randomness the spectrum resembles that of a glass, but the position and number of features depend on the orientation of the sample in the magnetic field. These features are determined from the solution 8g /aa , = 0. Numerical calculations for concrete values of the g-factor have been performed by Ovchinnikov et al. [ 3 6 ] . They indicate that Eq. (9.1 1 ) generally gives up to four singularities for the corresponding EPR spectrum. On rotation of the sample, the positions of these singularities are modified and may collapse and achieve their extreme position.


Hence, the principal values (gx, gy, g,) and the orientational parameters (y, 0) determining the orientation of the magnetic axes of the domain relative to the normal of the flat sample can be obtained from measurements of the EPR spectrum as a function of p.

Thus, the determination of the molecular structure of the mesophase on the basis of the De Vries classification of the smectic phases [47] can be achieved as follows. For uniaxial (SA, SB and S,) phases, in which the director points perpendicular to the molecular layer, two singularities are expected, whereas three singularities must be observed in the case of monoclinic symmetry (tilted smectic phases). The case of four spectral features appearing (that follows from Eq. (9.1 1) for a general case) is not included in the De Vries classification system. Four singularities have actually been found for S2Cu (7,9) samples [36]. This must be due to pseudosmectic arrangement with low symmetry. It is interpreted in terms of the geometric form of the copper(I1) Schiff base complex. The coordination environment of the metal ion forms a step with respect to the other planar sections of the molecule, and the absence of unit cell symmetry due to extended molecular packing is the reason for the decrease in the phase symmetry. A further degree of order around the short molecular axis within the smectic layer was implicitly assumed by Ovchnnikov et al. [36] and from the analysis of the four component spectrum, information about this orientation could have been achieved.

This kind of investigation, however, was not pursued by the authors, although more recent studies indicate the existence of such additional order. These studies are based on the modification of the effective g-value throughout the mesophases of several exchange coupled copper-containing liquid crystals [37] as well as on the thermal evolution of the effective hyper fine parameter of some oxovanadium mesogenic complexes [40, 411. This point will be discussed in more detail at a later stage.

EPR investigations of copper and oxovanadium Schiff base derivatives have also been carried out by Ghedini et al. at the University of Calabria (Italy) [38, 391 and at the Institute of Materials Science at Zaragoza (Spain) [37, 42, 43, 45, 461.

The Italian group studied a series of SICu(12, m ) compounds (see Table 9-4) with m = 1-4, 6, 8, S2Cu(12,6) [38], as well as the oxovanadium analogs S1V0(12,m), m = 1-4, 6, 8 and S2V0(12,6) [39]. After the thermal characterization and X-ray diffraction investigation of the compounds was completed, the authors analyzed the EPR spectra both in the solid phase (crystal polymorphism is observed depending on the thermal history of the sample) and in the mesophases.

All the spectra of the copper compounds are described by an SH including only an orthorhombic electronic Zeeman term, characterized by the three principal values (gx, gu, g,) and an anisotropic linewidth. The presence of the anisotropic linewidth, together with the absence of a hyperfine structure, is a strong indication for the existence of intermolecular exchange interaction. In Table 9-5 a the principal values of the g-tensor obtained by Ghedini et al. [38] for these copper compounds in the crystalline state are presented.

The same type of EPR signal is also found for the mesophases, which in each compound are smectic. A detailed description of the phase sequences is given in [38]. The principal g-values for these spectra are collected in Table 9-5 b.

364 f ? J . Alonso

Table 9-5a. CulI g-factors in the compounds studied by Ghedini et al. [38] in the crystalline phase.

Compound gx gY gz E

SlCu(l2,l) s 1 c u (1 2,2) Sl c u (12,3) S 1 Cu (1 2,4) * 93 Vo

7 yo

SICu(12,6)* 38% 62%

S1 Cu( 12,8) S2Cu(12,6)

2.065 2.050 2.047 2.047 2.047

2.042 2.045

2.050 2.046

2. I30 2.103 2.204 2.060 2.110

2.059 2.109

2.065 2.080

2.167 2.176 2.175 2.225 2.220

2.230 2.155

2.230 2.210

2.120 2.110 2.109 2.1 1 1 2.126

2.110 2.103

2.115 2.112

= (S,I + 2&7,)/2 * Two different copper spectra are observed with the proportions indicated in the table.

Table 9-5 b. Cu" g-factors in the mesomorphic compounds studied by Ghedini el al. [38] measured in the mesophases.

Compound Phase gx gY gz &!

s1 c u (1 2,2) s* 2.054 2.086 2.152 2.097 s 1 c u ( 12,3) s A 2.055 2.110 2.130 2.098 s 1 c u (12,4) s* 2.055 2.110 2.145 2.103

SB 2.050 2.090 2.190 2.110 SE 2.054 2.090 2.190 2.111

S ICu(12,6) s* 2.059 2.115 2.132 2.102 S2Cu (1 2,6) S A 2.059 2.120 2.134 2.104

In spite of the practically axial (square-planar) environment of the copper(I1) ion in the isolated molecule, it is worth noting that all the spectra show an orthorhombic symmetry within the mesophase. At first sight, this could be interpreted as a consequence of intermolecular averaging caused by exchange interaction adopting different orientations for the gI1 axis and for one of the two g, axes. The other g, axis remains parallel for all interacting molecules. However, the authors claim that a molecular packing arrangement corresponding to such intermolecular interaction is not consistent with the one proposed on the basis of X-ray diffraction data obtained from both the solid and the smectic phases [38]. The authors therefore suggested that the orthorhombic g-symmetry observed must be interpreted in terms of intramolecular anisotropic averaging, assuming that molecules can rotate freely about


their long axis. Thus, gll exchange rapidly with the g , axis in the perpendicular direction while the other g , direction remains unperturbed. In order to explain the absence of a hyperfine structure, a magnetic exchange interaction between the moving copper centers has to be assumed. This motion can also explain the more pronounced averaging effect observed in the mesophase in comparison with that found for the crystalline phase, as follows from the g-tensor anisotropies displayed in Tables 9-5 a and 9-5 b.

The oxovanadium analogs were studied by EPR spectroscopy in their different phases by Ghedini et al. [39]. Only compounds SlVO(12,m) with m = 4, 6, 8 and S2VO (12,6) show smectic phases (S, phase, compounds SlVO (12,8) and S2V0(12,6) additionally show an Sc phase). The authors did not give any numerical data for the EPR spectra except for compound SlVO(12,4). This material shows the following phase sequence: C , - (253 K)+C2 - (438 K)+I - (429 K)+SA -

The spectrum measured in the thermodynamically unstable crystalline phase (C,) shows a complex structure that is tentatively attributed to the hyperfine interaction of the 51V nucleus, together with a superhyperfine interaction with the neighboring vanadyl center. On the other hand, the spectrum of the stable C2 phase consists of an exchange-narrowed structureless line at g = 1.990+0.005. Further heating of the sample induces new changes and, in the isotropic phase, the spectrum broadens and shows a partially resolved hyperfine structure. This spectrum is not that of an isotropic, and it is interpreted as being due to a vanadyl moiety in a highly viscous liquid. The smectic A phase is reached by subsequent cooling of the sample (monotropic S, phase). In the smectic A phase the spectrum is similar to that observed in the isotropic, but the lines are broader.

No attempt was made to obtain SH parameters by fitting the spectra. Instead, the authors focused their attention on a discussion of the modifications observed in the spectra at the crystal-crystal transitions. The observation of either exchange-narrowed spectra or spectra due to isolated oxovanadium moieties is interpreted as being due to changes in the correlation length between the metal centers, as observed by means of X-ray diffraction experiments, which induce some modifications in the exchange pathways. On the other hand, the authors do not take into account the orientation of the domains (in the fluid phase) induced by the magnetic field. As will be discussed later, this phenomenon is not unusual for oxovanadium metallomesogens. Thus, the interpretations put forward concerning the spectra in the isotropic and the smectic phases are not unequivocally supported.

Alonso et al. [37,40] studied the EPR spectra of a pair of representative compounds of the S2 family: S2Cu(lO,5) and S2V0(10,5). In contrast to the investigations of the Italian group, these studies focus on the modifications in the EPR spectra obtained from the mesophases in order to gain information about the local structure and molecular dynamics of these high-temperature phases. The dilution of S2VO (10,5) by the homologous diamagnetic mesogen S2Pd(lO, 5) was also studied [40]. The three compounds show a very similar phase sequence (see Table 9-6).

(405 K)+C2.

366 P J Alonso

Table 9-6. Phase structures and transition temperatures (in Kelvin) of SZM(10,5) (M = Cu, VO and Pd). Adapted from [41].

Phase S2CU(lO, 5 ) S2V0(10,5) S2Pd(lO,5)

Heating c + s,; 398 S,+SA 403 SA+I 435

1+SA 435 SA*SC 403

Cooling s, --t C' 378

C'+C 373

41 9 430 453

43 8 430

388

428 458 478

47s 453

393

While S2Cu (10,5) shows an exchange-averaged spectrum indicated by the collapse of the hyperfine structure, this effect is negligible for S2V0(10,5) and the signal in all the phases has a well resolved structure due to hyperfine interaction with the "V nucleus. The dipole-dipole interaction, which broadens the lines in the pure samples, is avoided by studying the EPR spectrum of a mixture of S2V0(10,5) with S2Pd(10,5). In addition to the magnetic exchange, the effects of molecular motion on the spectra in the fluid phases have to be considered in all cases. By comparing the results for the three systems, information on the dynamics of the mesomorphic molecules and on the local order in the smectic planes is obtained. Both are closely related with the prismatic shape of these molecules.

Firstly, the results on the copper compound will be considered [37]. The X-band EPR spectra of a S2Cu(lO, 5) sample measured in the stable solid phase (Fig. 9-6a), in the smectic phase (Fig. 9-6b) (the same spectra are obtained from both the SA and S, phase), in the isotropic phase (Fig. 9-6c) and in a frozen solid preserving the smectic texture as deduced from optical microscopy (Fig. 9-6d) are given in Fig. 9-6. The two spectra recorded at room temperature can be described using an almost axial spin Hamiltonian, the g-tensor anisotropy being lower in the frozen smectic phase than in the stable solid. This is more clearly seen in the Q-band spectra where the orthorhombic components of the g-tensor are resolved. The spectra of a frozen solution of S2Cu(lO, 5) in toluene is also described. In this case, the hyperfine structure due to the copper nucleus is resolved. Measurements of the EPR spectra of S2Cu(lO, 5) at different temperatures were also reported. The spectra do not change at temperatures lower than that of the crystalline-smectic C transition. The spectrum measured in the isotropic phase (see Fig. 9-6c) consists of a broad (AH,,= 10 mT) signal at g-2.11. In the smectic phase, the line shape is similar to that observed for the isotropic liquid but it contains some asymmetry which broadens the signal towards the low-field side. The SH parameters corresponding to S2Cu(lO, 5) in its different phases are given in Table 9-7.


I I

MAGNETIC FIELD ( T )

Figure 9-6. X-band EPR spectra of an S2Cu (10,5) sample measured in the various phases: a) as-received sample measured at room temperature, b) smectic phase, c) isotropic liquid, d) sample frozen to room temperature from the isotropic liquid. Adapted from [37] .

Table 9-7. Spin Hamiltonian parameters corresponding to the EPR signal of S2Cu(lO, 5) in its different phases (from [37]) . The accuracy is estimated to be about kO.01 for g values and about 5 MHz for the hf constant.

Phase gx gY gz E A 1 1 iMH4

Stable solid 2.04 2.08 2.21 2.11 - - 2.10 Smectic -

Isotropic - - - 2.11 Frozen toluene sol. 2.04 2.05 2.24 2.11 485

The g-factor data of the stable solid and of the frozen toluene solution indicate that a partial averaging of the g-tensor, generating a shift of the parallel g-factor (g, value), takes place in crystalline S2Cu (10,5) because the molecules are not coaxial. A comparison of the spectra obtained from the stable solid and the frozen smectic samples shows that this effect is more pronounced in the frozen sample. This is inter-

368 l?J. Alonso

preted as a consequence of the freedom of rotation around the long axis of the molecule in the mesophase which results in a distribution of the orientations of the short axis (related to gz and gr values) when the mesophase is frozen. Exchange interaction is also present in the smectic phase (as deduced from the lack of hyperfine structure), and if the molecules were free to rotate around the long axis an axial spectrum (with effective g-values of g, ~ 2 . 1 4 and gI1 ~ 2 . 0 8 ) would be observed. Since this is not the case, it follows that some restricted motion of the molecule around its long axis takes place in the mesophase. This restricted motion maintains some short range order as a consequence of locally correlated motion.

It is worth mentioning that this interpretation stands in contrast to the interpretation of EPR results on some compounds of the S1 and S2 families given by Ghedini et al. [38]. These authors based their interpretation on the existence of some molecular motions even in the solid phases, In as far as their EPR spectra do not show any resolved hyperfine structure, their results can also be interpreted in the way as those presented above [37, 451.

In order to get more information about this type of motion, a detailed investigation of the thermal evolution of the EPR spectra of homologous S2VO (10,5) has recently been performed 140, 411. As mentioned previously, the effect of exchange interactions can be neglected for the oxovanadium compound, because the hyper fine structure is clearly shown at any given temperature. The EPR spectra of S2V0(10,5), measured at different temperatures, are shown in Fig. 9-7. The low-temperature spectrum resembles that of a frozen toluene solution except for the linewidth. It can be

I 1

0.23 0.31 0.39 0 MAGNETIC FIELD (T)

Figure 9-7. X-band EPR spectrum of a S2VO (1 0,5) sample measured at

17 different temperatures. Adapted from v11.


t

described with an SH that includes the contributions given by Eqs. (9.1) and (9.2) using the following parameter values:

gii = 1.947k0.005 All = 500k5 MHz

g, = 1.983k0.005 A , = 175k5MHz

When the temperature is increased, a reduction in the parallel hyperfine splitting occurs without a significant broadening of the signal being observed. Both the effective hyperfine splitting constant and the linewidth (defined as the second derivative peak-to-peak distance) are displayed in Fig. 9-8. Since the phase transition temperatures show hysteresis, data from both the heating and cooling cycles are included. This thermal evolution is interpreted as a consequence of the partial averaging caused by molecular motion. To analyze this effect, the authors [40,41] start off from the Polnazsek, Bruno and Freed (PBF) theory [7], which provides an extension of the results of Freed and coworkers [48] regarding an anisotropic liquid consisting of rod-like particles. The PBF theory describes the evolution of the EPR signal over the whole range of motional frequencies. It was modified in order to account for the motion modes of prismatic molecules. By comparing the thermal evolution of the EPR signal with the results of numerical simulations, the authors [40,41] conclude that in the smectic phases the motion mode around the long axis is a restricted libra-

500 14

h t N

v 444 1 .O

*o 0

$ 6

4 4

71 rn R d 71

D ii

rn

310 P J . Alonso

tion of small angular amplitude. These results are in agreement with the existence of considerably high in-plane local order in the orientation of the short molecular axes in the mesophase, as the measurements of the strongly magnetically exchanged S2Cu (1 0,5) compound suggest. This kind of local in-plane order has also been suggested for similar liquid crystalline copper compounds by means of X-ray diffraction measurements [49].

Similar results were found for S6M(10,rn) (rn = 1-10, M = Cu, VO) compounds which form smectic mesophases [41, 451. The spectra are independent of the length of the aliphatic chain (from rn = 1 to rn = lo), but significant differences are observed depending on the metal. In the room temperature solid phase, all copper compounds show a practically axial spectrum with an exchange averaging that an- nihilates the hyperfine structure. The g-factors are gll = 2.235k0.005 and g, = 2.050*0.005. These values are in agreement with the data obtained for isolated molecules in a frozen toluene solution. While the spectra measured in the smectic phase consist of a broad symmetric line, the spectra measured in the frozen smectic phase at room temperature show a marked asymmetry. This is interpreted as a reduction of the anisotropy of the effective g-tensor as a consequence of the loss of parallelism of the interacting molecules.

In the case of oxovanadium S6V0(10, rn) compounds the "V hyperfine structure is observed at any given temperature [45]. In this case the parallel hyperfine splitting constant also decreases as the temperature increases, but it remains far from that corresponding to the value obtained for the isotropic liquid. Although a detailed analysis of these facts has not been undertaken, such behavior can be qualitatively interpreted as being similar to that of S2V0(10,5), as discussed above.

A brief description of the EPR spectra of some copper and oxovanadium compounds of the 54 and S5 families has been also given in the literature. The investigation of the oxovanadium complexes S4VO (1 0,5) and S5VO (1 0,5) was published some time ago [42,43] while some preliminary results on the analogous copper compounds have been reported recently [41].

S4V0(10,5) shows a nematic phase with the following phase sequence for an as- received sample C-(382 K)+N-(404 K)+I. On cooling the sample, only the I+N phase transition (at 404 K) is observed and a solid with a nematic texture (a nematic glass) is obtained at lower temperatures. Subsequent heating induces a cold crystallization at about 353 K and then the previous C-(382 K)-+N-(404 K)+I phase transition sequence is observed. On heating, SSVO(10,5) shows the phase sequence C-(447 K)-*SCrYFt -(498 K)-+N-(539 K)+I, but upon cooling a hysteresis of about 20 degrees is observed for the N+Scryst and Scryst+C phase transitions.

The EPR spectra of both compounds have been measured at different temperatures in their different phases. At room temperature, the spectrum obtained from the stable solid phase of S4V0(10,5) is a typical axial spectrum for a square-planarly coordinated oxovanadium. The spectrum can be described by a standard SH with the following parameter values:


811 = 1.945t0.005 A 1 , = 485 f 5 MHz

g, = 1.980t0.005 A , = 180k5MHz

The EPR spectrum of the stable solid S5V0(10,5) can be interpreted in the same way, with the SH parameters being:

811 = 1.940t0.005 A 1 1 = 500k5 MHz

g, = 1.970t0.005 A , = 190f5MHz

In the case of compound S4V0(10,5), modifications in the spectrum are not observed in either the solid or the smectic phase, but some changes are detected when the sample reaches the isotropic phase. The EPR spectra of such a sample measured at two different temperatures in the isotropic phase (namely 413 K and 423 K) are shown in Fig.9-9. At lower temperature (413 K), the spectrum only shows the perpendicular component (Fig. 9-9 a) whereas at 423 K some traces of the parallel components are observed (Fig. 9-9 b). The intensity of the parallel spectrum is much lower than that obtained from the solid phase. In both cases, the spectra of the

Figure 9-9. EPR spectrum of a S4V0(10,5) sample measured a) in the isotropic phase at 413 K and b) .24 .2a .32 3 6 .40

at 423 K. Adapted from [43]. MAGNETIC FIELD (T)

312 R J Alonso

isotropic liquid phase are understood as a consequence of the magnetic field-induced orientation of the molecules with their long axis parallel to the measuring magnetic field. This implies that some extended domains of interacting molecules with their axes correlated remain in the isotropic phase, as discussed in Sect. 10-7. The results shown in Fig. 9-9 suggest some local anisotropic molecular organization in the macroscopically isotropic liquid. Similar behavior is found for S5VO (1 0,5).

The main difference between the two oxovanadium compounds is that only in S4V0(10,5) is it possible to freeze the molecular orientation by cooling into the glassy state at room temperature in a magnetic field, whereas this is not possible in the case of S5VO (10,5). The preferential molecular orientation of the S4VO (10,5) is erased by heating the sample to 353 K (the cold crystallization transition temperature). The macroscopic alignment observed at room temperature is associated with S4V0(10,5) forms a nematic glass.

The analogous copper compounds have recently been studied by recording their EPR spectra at different temperatures [41]. The phase sequence of these materials is similar to that of the oxovanadium analogs. In the case of the stable solid, the EPR spectra correspond to a practically axial S = 1/2 system with gll = 2.23 and g, = 2.05 for S4Cu(lO,5) and an orthorhombic S = 1/2 system with g, = 2.05, g, = 2.09 and g, = 2.19 for S5Cu(lO,5). In the first case there is some indication for a weak exchange that partially averages the hyperfine structure. In the second case, the absence of any hyperfine structure and the narrowing of the lines indicate that the exchange interaction is stronger. The spectra of both copper compounds have also been recorded as a function of temperature. In the smectic phase the exchange interaction is still present, thus inducing an averaging of the spectra along with a collapse of the hyperfine structure. This effect disappears in the nematic phase. This disappearance is associated with the loss of correlation among the metal centers due to translational freedom in the nematic phase.

As a last example of paramagnetic metal derivatives with Schiff base ligands a comment will be made on the recent work published by Alonso et al. [46] on the EPR spectra of a chiral metallomesogen, S7(10), of the S7 type shown in n b l e 9-4; Bis(N- [4”- [(2S)-2-chloro-propoxy]phenyl-4-(4’-n-decyloxybenzoyloxy)salicyladimino~copper@). This investigation is the first to date on a chiral paramagnetic smectic C material. It is of special interest because it provides information about the molecular arrangement in the various liquid crystalline phases. The phase sequence of the compound is: Cf’-(407 K)+Cf-(433K)+C-(455 K)+Sc*-(495 K)-+SA-(528K) +Ch-(533 K)+I. The C”-C+C phase transitions are not observed after the first heating cycle, and the thermodynamically stable solid phase is the C phase.

The EPR spectra of S7(10) were recorded as a function of temperature up to 503 K, and changes in the EPR spectra have been found at each of the phase transitions except for Sc.+SA. The spectra measured in the different phases are shown in Fig.9-10. It is noteworthy that after cooling the sample to room temperature, the spectrum is the same as that of the stable phase C . In all cases, the EPR signal corresponds to an S = 1/2 spin in an orthorhombic symmetry without any resolved hyperfine structure. The absence of the hyperfine structure is due to strong exchange


Figure 9-10. X-band EPR spectra of a freshly prepared S7Cu (10) sample measured at a) room temperature, b) 413 K, c) 438 K, and d) 473 K, respectively; e) room temperature spectrum of a sample which was previously

290 310 330 350

heated to 483 K. Adapted from [46]. 0 (mT)

Table 9-8. Effective g-factor for Cu" in SCu(7) measured in the different phases. The accuracy is estimated to be about f 0.01. Adapted from [46].

2.03 2.04 2.04 - 2.08 2.08 2.09 -

gx gY gz 2.24 2.21 2.19 g 2.12 2.1 1 2.11 2.09

-

= (gx + gy + &)/3

interaction among the paramagnetic copper centers. Measurements at room temperature in all the crystal phases (C", C' and C) were also performed at the Q-band. The principal g-tensor values are collected in Table 9-8.

Information about changes in the molecular packing in the pre-smectic phases can be obtained from the changes in the effective g-factor in the crystal-crystal phase transition. Such modifications in the solid state phases are not commonly observed for other metal-containing compounds and could be related to the asymmetry of the chiral molecules.

9.3.4 Other Monomeric Compounds

Campillos et al. [50] have reported the thermal evolution of the EPR spectrum of bis[5-(4-tetra-decyloxybenzoyloxy)salicylaldehyde]copper(11) (Fig. 9-1 1). This com-

314 P L Alonso

14H29O

OCi4H29

Figure 9-1 1. Formula of the bis[5-(4-tetradecyloxybenzoyloxy)salicylaldehyde]copper(r~) complex.

pound shows the following phase sequence: C , - (406 K) -+ Cz - (45 1 K) -+ C3 -

At room temperature, the spectrum is typical of an S = 1/2 entity in an axial environment. It can be described by a Zeeman Hamiltonian characterized by gil = 2.28k0.02 and g, = 2.08k0.02. The spectrum does not show any resolved hyperfine structure, and this is associated with a strong exchange interaction among copper centers. Changes in the spectrum are not detected up to 493 K. In the smectic C phase a broadening of the lines together with a small decrease in the g-anisotropy is observed. The structure of the molecules in this compound is similar to that of the Schiff base complexes, therefore, this behavior can be interpreted in the same way as before. In the smectic phase, the exchange interaction still operates due to some correlation among the paramagnetic centers within the smectic planes. In this fluid phase, however, some motion around the molecular long axis occurs, and this is responsible for the partial averaging of the g-tensor leading the lower apparent anisotropy of the signal. The spatial impediment to molecular rotation due to the molecular shape could, in this case, be the reason for the averaging not being total. However, the possibility of partial alignment of the smectic domains by the measuring magnetic field, as observed for the oxovanadium compounds S4VO(1O75) and S5V0(10,5), cannot be completely ruled out. The smectic phase order cannot be frozen at room temperature, so this point can not be further elucidated.

Recently, interesting results have been found for a new copper thiocarbamate complex [41, 511. This compound shows smectic phases upon cooling to temperatures below 520 K. The molecular shape appears more cylindrical in comparison with the compounds discussed above. In the crystalline state, the EPR spectrum shows an orthorhombic (almost axial) symmetry and the hyperfine structure is collapsed. The HS parameters are g, = 2.03+0.01, gu = 2.02+.0.01 and g, = 2,09+-0.01, typical of copper, coordinated by four sulfur atoms [52, 531. Changes in the spectrum are found in the smectic phases, While the hyperfine structure remains collapsed, in- dicting the exchange interaction is still operating in the mesophases, the Zeeman contribution can be described by an axial g-tensor (g;, = 2.03lt0.01 and g ; = 2.05k0.01). Because of the cylindrical shape of the molecules, they are free to rotate around their long axis, therefore, exchange interaction averages two of the principal axes of the molecular g-tensor yielding an effective g-tensor with gii = gu

(500 K)+S,-(516 K)- i I.


and g; = (gx+gz)/2. In addition, the cylindrical symmetry of the molecules allows the determination of the thermal evolution of the Maier-Saupe parameter from the EPR data.

9.3.5 Polymeric Liquid Crystals Containing Paramagnetic Metals

Following the description of EPR experiments on low molecular weight metal-organic liquid crystals, it is now appropriate to consider the results available on polymeric materials. Metal-containing mesomorphic polymers have received increasing attention in recent years because of their interesting properties, which result from the presence of the metal. Three general strategies can be applied in the synthesis of this type of material.

Method 1: Metal complexes with functional groups in their periphery which can be polymerized.

Method 2: Polymer formation through coordination reactions to form polymeric metal complexes.

Method 3: Coordination of a metal to a preformed polymer which incorporates chelating groups.

In spite of the growing interest in metal-containing polymers, EPR studies on these materials are rare. Only a few studies performed by Hanabusa and co-workers [54-571 as well as by Alonso et al. [41, 58-63].

The first paper dealing with this topic was published by Hanabusa et al. [54] in 1989. The authors studied the EPR spectra of several liquid crystalline polyesters which contain mesogenic bipyridinediyl units. Copper(I1) was incorporated into a hexacoordinated complex with six nitrogen atoms of three bipyridinediyl groups. The thermal evolution of the EPR signal of copper(I1) was studied, and it was found that the anisotropy increases with increasing temperature. The signal is isotropic (g= 2.1 1) up to 373 K (solid phase), whereas at higher temperatures it becomes axial: g l l = 2.19, g, = 2.05 at 398 K (liquid crystalline phase) and gl l = 2.22, g, = 2.05 at 433 K (isotropic liquid). This thermal evolution is associated with the distortion of the octahedral copper complex, which becomes elongated at the transition from the crystalline state to the liquid crystal phase. In the isotropic liquid this distortion is more pronounced.

EPR studies on paramagnetic metal-containing polymers prepared by Method 3 have also been performed on some main-chain polyesters [55], side-chain polyacrylates [41, 56, 631 and some polyazomethines [60, 611. In all cases the magnetic metal was copper(I1) except in the polyazomethines where oxovanadium and iron@) were also introduced.

Several liquid crystalline main-chain polyesters were prepared by Hanabusa et al. [55]. These compounds consist of homo- and copolymers that contain a P-diketone unit, which can coordinate to a metal as a bidentate ligand. A certain degree of crosslinking inevitably takes place and the polymers lose their linear molecular structure. Despite crosslinking, incorporation of the metal yields a thermotropic liquid

376 RJ, Alonso

crystalline polyester and fibers can be drawn from the melt. An EPR study on these fibers has been reported in [ 5 5 ] . It was found that the spectrum depends on the orientation of the magnetic field relative to the fiber axis. The authors conclude that the fiber axis lies in the copper(i1) coordination plane.

Hanabusa et al. [56] also synthesized a copper-containing side-chain polyacrylate by complexation of a metal to the P-diketone group. The EPR signal due to the copper@) entity is reported and it is attributed to the presence of a square-planar complex.

Recently a new type of side-chain polyacrylate has been synthesized in Zaragoza [62, 631. In these polymers, the chelating group is the aldimine group and different copper(i1)-containing polymers with a copper content up to about 35% have been prepared. While the metal-free material shows smectic A and C phases, the copper- containing polymers only exhibit the smectic C phase. It is noteworthy that the melting temperature remains practically unchanged with increasing copper concentration, but the smectic C - isotropic transition temperature increases with increasing metal content.

The EPR signal due to the copper(i1) ion has been recorded as a function of the copper content. For low metal concentrations (less than 10- 15%) the spectra correspond to isolated planar copper complexes, whereas for higher metal concentrations some changes are found that are associated with the existence of exchange interaction. The thermal evolution of the EPR signal has also been monitored as a function of temperature between room temperature and the isotropic liquid phase transition [41, 631. The most characteristic feature of the spectra is the monotone decrease of the hyperfine splitting together with a shift of the parallel feature of the spectrum towards low g-values. This behavior can be explained by motion of the copper complexes in the fast limit. The motion of the copper(I1) centers is restricted by the impediment imposed by the polymer structure, as indicated by the fact that the effective value of the parallel SH constant is by far different from that expected for a free rotation situation.

The synthesis and characterization of several series of hydroxy-functionalized liquid crystalline polyazomethines have been performed by Serrano and coworkers 1641. The hydroxy groups are introduced into the ortho-position of the azomethine group and lead to the formation of strong rings with the nitrogen atoms as a result of hydrogen bonding. The rings provide exceptionally good sites for the capturing of a variety of transition metals by complexation. As an extension of this work, the EPR spectra of some copper-containing polyazomethines have been studied recently in Zaragoza [60]. The compounds selected for the study were three hydroxy-functionalized polyazomethines: two homopolymers derived from ethylenediamine (ED) and 1 ,Snaphthalenediamine (NP) and a copolymer from ethylenediamine and 1,5-naphthalenediamine (CEN). These polymers were chosen because of the structural differences in the central core of the repeat unit. Two possible situations may now occur: the metal can either be coordinated to two different repeat units acting as bidentate ligands, giving a crosslinked material, or the coordination of the metal takes places with a tetradentate repeat unit if bending of the polymeric chain is possi-


ble. Intrachain coordination can only take place in the case of repeat units derived from ethylenediamine due to the rigid structure of the N-arylsalicylaldimine ligands. The ED polymer is not liquid crystalline, but both the NP and CEN polymers exhibit nematic behavior over a wide temperature range (92 and 84 K, respectively). Metal was introduced into two further polymers, which are nonmesomorphic, in order to prove the conclusions drawn from the investigation of the three polyazomethines mentioned above: a homopolyazomethine derived from 1 ,4-phenylenediamine (PH) and the corresponding copolymer (1 : 1) derived from ethylenediamine and 1,4-phenylenediamine (CEP).

A detailed study of the EPR signal as a function of the metal content has been performed on copper-containing polymers [60]. In all cases, the signal is interpreted as being due to a planar copper complex having a practically axial symmetry, and the hyperfine structure is clearly resolved in the parallel component of the spectrum. For the five polymers studied, the EPR signal is independent of the copper concentration and only broadening of the signal is observed as the copper content increases. The broadening is a consequence of more extended dipole-dipole interaction. The EPR signals measured for the three homopolymers are different. This difference is clearly visible in the case of ED as compared with NP and P H homopolymers. For both CEN and CEP, the EPR signal is the same as that observed for the ED homopolymers. However, the lines observed in the copolymers are narrower than those found for the spectra of the ED homopolymer with the same copper content.

These results are summarized in Fig. 9-12 and lead to the following conclusions: copper ions can be introduced into hydroxy-functionalized polyazomethines with repeat unit derived from a salicylaldimine. Depending on the structure of this repeat unit, the metal gives rise to crosslinking (NP and PH homopolymers) or bending (ED homopolymer and CEN and CEP copolymers) of the polymeric chains. The EPR measurements provide valuable information about the affinity of the copper to the different repeat units. Furthermore, the spectra contain information about microstructure of the original polymers. The EPR spectra of copper(rr), introduced in- to ED, NP, and P H homopolymers allow to distinguish between the intermolecular crosslinking site and the tetradentate intrachain site for the metal ion. While the former coordination occurs in the aromatic diamine polymers (NP and PH), the latter situation occurs in the case of the ED homopolymers. A comparison between the EPR spectra recorded from the copolymers with those obtained from the homopolymers reveals that copper(i1) occupies a tetradentate intrachain site in copolymers and the same chelate core is formed as in the ED homopolymer. Evidence for crosslinking is not found. Analysis of the linewidth evolution as a function of copper content provides useful information about the structure of the copolymers. Consequent- ly, a tendency to alternate the repeating units is strongly suggested [60].

This work has recently been extended to oxovanadium and iron(rr1) chelated polyazomethines [61]. No changes are found in the EPR spectra of the various polymers containing iron(rII), which is easily understood because of the spherical symmetry of this 3d5 ion. However, differences are observed in the spectra of the

378 P J Alonso

2.23 t

0.26 0.3 0.34 0

I

MAGNETIC FIELD (T)

2.25

01

2.17 0 5 1 0 15 20 25

% cu

m m 0

7 ' 14 -

0 5 10 15 2 0

% cu

18

Figure 9-12. Top: room temperature X-band EPR spectra of the PH, NP, ED homopolymers and CEN and CEP copolymers containing 5% copper. Bottom: dependence of g,, and parallel hyperfine splitting on the copper content (mol Yo) for the five types of polymers. PH: full circles; NP: full triangles, EP: full squares; CEN: open circles; CEP: open squares. Note the similarity of the copolymer spectra to that obtained for the ED homopolymer. Adapted from [60].

oxovanadium derivatives of different polyazomethines. They are in agreement with the previous interpretation given for the copper-containing polymers.

Only one EPR study on metal-containing mesomorphic polymers prepared by a coordination reaction has been undertaken. This work was performed by Hanabusa et al. [57], who report on the formation of some thermotropic liquid crystalline


polymers linked via bis(P-diketonato)copper(II) complexes. A number of different mesophases were detected in the different polymers (melting points at about 473 K), and some of them have been identified as smectic A. Fibers can be prepared from the melt. The EPR spectra of all the compounds studied by Hanabusa et al. [57] reveal a square-planar complex of copper(I1) (gli = 2.26, g, 5: 2.06 and A = 540 MHz, the hyperfine structure in the perpendicular feature is not resolved). The spectra were also recorded from fibers, with the magnetic field either parallel or perpendicular to the fiber axis. While the spectrum measured in the parallel orientation only shows a narrow signal at g = 2.06 (the perpendicular signal), the spectrum measured with the magnetic field perpendicular to the fiber axis shows both the parallel and perpendicular features of the spectrum. From this, the authors concluded that the copper coordination plane in the fibers is oriented in such a way that it contains the fiber axis.

EPR studies of metal-containing liquid crystal polymers prepared by polymerization of preformed metallomesogens (Method 1) have been reported by the Zaragoza research group [58,59]. Copper polymers with the structure shown in Fig. 9-13 with n = 5, designated as [N5Cu][PIO], and with n = 7, [N7Cu][P10], have been studied.

r

L

Figure 9-13. Chemical structure of [NnCu][PIO] polymers. Adapted from [58].

In a preliminary study [%I, the EPR spectrum of a nonoriented polymer [NSCu] [Pl 01 was measured as a function of temperature between room temperature to 5 18 K. This temperature range covers the crystalline, nematic and isotropic phases. The polymer was also studied after cooling the isotropic liquid to room temperature. While the EPR spectrum of a freshly prepared sample (measured at room temperature) consists of a simple signal indicating a square-planar copper(I1) complex (gI1 = 2.20, g, 5: 2.01 and A II = 525 MHz), the EPR spectrum measured in the fluid phase only shows a broad isotropic single line (g = 2.08) without any resolved structure. On the other hand, the EPR spectrum of a sample that has been cooled down to room temperature from its isotropic phase is more complicated. In addition to the

380 I? J. Alonso

contribution of square-planar copper(II), the spectrum also shows an isotropic signal at g = 2.08. The ratio of the intensities of both signals depends on the cooling rate. This is related to the appearance of a new endothermic peak, detected in the DSC traces of samples cooled from the isotropic phase, and it is tentatively associated with another crystal form in which Cu-Cu interactions take place between different metallomesogen units.

Recently, a more detailed EPR study on the polymer [N7Cu][P10] was presented in connection with X-ray and magnetic susceptibility measurements [59]. Three types of powder samples were studied at room temperature: polymers as obtained (type I), samples annealed at temperatures close to the melting temperature in order to increase the crystallinity (type 11) and samples obtained by quenching the nematic melt (type 111). The EPR spectra of the three types of sample correspond to a square- planar copper complex and can be described using the same SH parameters. How- ever, while the spectra of semicrystalline samples (types I and 11) are identical, type I11 samples show a smaller linewidth. In all cases the hyperfine structure can be resolved at least in the parallel feature, and the splitting is typical of isolated paramagnetic copper(r1) entities, The possible influence of exchange interaction is ruled out and the differences in the linewidth are therefore related to dipolar broadening of the signal. The difference found for the linewidth in the type I11 sample, as compared with types I and 11, is rationalized by taking into account that the Cu-Cu mean distance is greater in the nematic phase than in the semicrystalline samples as a result of translational freedom.

Fibers of polymer [N7Cu][P10] can be obtained from the melt and EPR measurements on fibers have been performed [59]. The evolution of the spectrum with respect to the orientation of the magnetic field relative to the fiber axis has been measured. The results enabled the authors to obtain structural information about the orientation of the metal in the polymer chain, indicating that the normal to the copper coordination plane is randomly perpendicular to the fiber axis.

It can be seen that although relatively scarce, EPR measurements in paramagnetic metal-containing liquid crystal polymers provide useful structural information about this type of material.

9.4 Conclusion

In spite of the information which can be gained from EPR spectroscopic studies on PMLC, there are very few such studies on these materials and, in general, only a partial study in each particular case is given. No comprehensive investigation of any system has yet been presented. On many occasions, EPR studies focus only on a specific aspect: characterization of the molecular structure around the metal, evidence of a weak intermolecular magnetic interaction, the monitoring of a phase transition, etc. These seem to be a common feature of the physical studies on metal-organic liquid crystals.


The first attempt to undertake a systematic study of PMLC by using EPR spectroscopy was made by Ovchinnikov’s group [31-331. Their efforts to classify the mesophases of PMLC using EPR spectroscopy [20, 21, 35, 361 is worth emphasizing, but after preliminary studies they did not pursue this work.

However, these limited studies have proved useful in providing valuable structural information, especially in the case of polymeric systems. The use of EPR spectroscopy to study the disposition of the metallomesogenic units in a fiber drawn from a nematic melt [57, 591 and the study of the structure of copolyazomethines [60, 611 are some representative examples.

A number of papers that combine EPR studies with other structural techniques (X-ray diffraction, etc.) have been published in recent years. The first such paper was published by Ghedini et al. [38]. In order to make the EPR data compatible with the X-ray diffraction data, some ad hoc hypotheses concerning molecular motion modes were included, but the authors maintained the conventional classification of phases formed by rod-like molecule.

More recently, an alternative explanation has been proposed by Alonso and coworkers [37, 40, 411. It is based on the existence of some additional local order in the mesophase besides that characterizing the phase. The possibility of this being bulk order has also been pointed out. The existence of this in-plane order is closely related to the particular molecular motion modes due to the brick-like shape of the molecules, which is far different from either the conventional rod-like or disc-like geometries. It is suggested that a new scheme of classification for the mesophases of these metallomesogenic materials has to be adopted; a suggestion made by Ovchinnikov et al. [36] some time ago.

More work regarding PMLC with unconventional molecular shapes needs to be done, with a particular emphasis on the study of dynamic aspects of EPR spectra, which are closely related to the local order in the different phases. This kind of investigation has to be combined with other structural techniques in order to relate the local order with the order of the bulk material. It is worth mentioning that studying the anisotropy of the magnetic susceptibility is also closely related to the existence of an additional order beyond that of the classical mesophases. This point will be discussed in Chap. 10.

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[55] K. Hanabusa, Y. Tanimura, T. Suzuki, T. Koyoma, H. Shirai, Makromol. Chem. 1991,

[56] K. Hanabusa, T. Suzuki, T. Koyoma, H. Shirai, Makromol. Chem. 1992, 193,

[57] K. Hanabusa, T. Isogai, T. Koyoma, H. Shirai, Makromol. Chem. 1993, 194, 197-210. [58] M. Marcos, L. Oriol, J. L. Serrano, P. J. Alonso, J. A. Puertolas, Macromolecules 1990,

[59] P. J. Alonso, J.A. Puertolas, P. Davidson, B. Martinez, J.I. Martinez, L. Oriol, J.L. Ser-

[60] L. Oriol, P. J. Alonso, J. I. Martinez, M. Piiiol, J. L. Serrano, Macromolecules 1994, 27,

[61] P. J. Alonso, J.I. Martinez, L. Oriol, M. Piiiol, J.L. Serrano, Adv. Mat. 1994, 6,

[62] E. Campillos, Doctoral dissertation, University of Zaragoza, Zaragoza, Spain, 1993. [63] P. J. Alonso, E. Campillos, M. Marcos, J. I. Martinez, L. Oriol, M. Piiiol, J.L. Serrano,

[64] J. Barbera, L. Oriol, J. L. Serrano, Liq. Cryst. 1992, 12, 37-47.

teschi, C. Zanchini, Chem. Mater: 1993, 5, 876-882.

gens, Pefiiscola, Spain, 1993.

859- 861.

Matter 1990, 2, 9173-9182.

30, 3091 - 3096.

Mat. 1993, 5, 1518-1525.

667 - 670.

1989, 50, 113-119.

zers, E. J. Reijerse, J. Schmidt) 1989, North-Holland, Amsterdam, p. 114- 126.

1989, 190, 1-8.

192, 233 - 244.

2 149- 2161.

23, 5187-5191.

rano, Macromolecules 1993, 26, 4304 - 4309.

1869- 1874.

663 - 667.

International Liquid Crystal Conference, Budapest, Hungary, 1994.

Physical Properties and Applications

The following chapters are not intended to systematically review the physical properties and potential applications of metallomesogens known up to now, but they outline the most significant contributions made to this field.

These chapters also have a twofold purpose. For those working on the physical characterization of liquid crystals, a recent account of the most relevant physical properties of metallomesogens is provided. In addition, for those who are not familiar with the physical behavior of metallomesogens, an introduction into the basic aspects of some of ther physical properties is given. For a more thorough description of these properties, the readers are advised to make themselves acquainted with more specialized texts devoted to the physical behavior of liquid crystals [ 1 - 91.

Since research on metallomesogens began, one of the most attractive goals was the creation of paramagnetic fluids, and perhaps it is due to this aim that the physical characterization of these materials has been carried out more thoroughly. In consequence, the magnetic properties of metallomesogens will be discussed in a separate chapter, while other physical properties will be treated together in the last chapter of this book.

References

[l] Liquid Crystals and Plastic Crystals (Eds.: G. W. Gray, P. A. Winsor), Ellis Horwood,

[2] Advances in Liquid Crystals, (Ed.: G. H. Brown), Academic Press, New York, 1975, 1976,

[3] Handbook of Liquid Crystals (Eds.: H. Kelker and R. Hatz), VCH, Weinheim, 1980. [4] W. H. de Jeu, Physical Properties ofLiquid Crystalline Materials, Gordon and Breach Sci-

[5 ] Polymer Liquid Crystals, (Ed.: A. Ciferri, W. R. Kirgbaum, R. B. Meyer), Acedemic Press,

Chichester, 1974, Vols. 1 and 2.

1978, 1979, 1982, 1983, VO~S. 1-6.

ence Publishers, New York, 1980.

New York, 1982.

386 Part D Physical Properties and Applications

[6] Recent Advances in Liquid Crystalline Polymers (Ed.: L. L. Chapoy), Elsevier Applied

[7] Thermotropic Liquid Crystals, (Ed.: 0. W. Gray), John Wiley & Sons, Chichester, 1987. [8] G. Vertogen, W. H. de Jeu, Thermotropic Liquid Crystals, Fundamentals, Springer-Verlag,

[9] P.G. de Gennes, J. Prost, The Physics of Liquid Crystals, Oxford University Press,

Science Publishers, London, 1985.

Berlin, 1988.

Oxford, 1993.

10 Magnetic Properties of Metallomesogens

Pablo J. Alonso

10.1 Introduction

Magnetism results from the interaction of electrons with a magnetic field. It arises from the alignment of the spin and orbital magnetic moments of the electrons in a magnetic field or from their Larmor motion caused by the magnetic field. The latter effect, called diamagnetism, is present in all materials, whereas the first phenomenon only appears in systems with unpaired electrons (paramagnetism, ferromagnetism, antiferromagnetism, etc.). The interest in studying magnetic properties is twofold. Firstly, the application of materials with specific magnetic properties in diverse areas is growing every day and secondly, the study of magnetic properties provides a wide range of information about the electronic properties and the structure of the materials (for instance, see [I -41).

Recently, the investigation of the magnetic properties of liquid crystals (LCs) has become a goal for many researchers. Of particular interest has been the study of the relationship between the anisotropy of the magnetic susceptibility and the microscopic structure of liquid crystals [5 - 71. Conventional organic liquid crystals are diamagnetic and the main contribution to the susceptibility anisotropy arises from aromatic rings. In fact, in a magnetic field the benzene molecule tends to orientate in a way that the field is in the plane of the six-membered ring. Intermolecular interactions responsible for the structure of mesophases produce domains with a macroscopic anisotropic magnetic susceptibility that can be aligned by a moderate magnetic field. For instance, the domains in a mesophase composed of many calamitic molecules can be oriented with the director parallel to the applied field. This capability can be used to prepare aligned samples for different applications as well as to control the structural orientation of mesomorphic units in polymers.

During the last two decades, along with the advent of metal-organic compounds with mesomorphic properties [8 - 121, new aspects regarding magnetism in liquid crystals have appeared. In a number of these compounds, the incorporated metals

388 f ? J . Alonso

possess unpaired electrons which produce a paramagnetic response due to their permanent electronic moments in addition to the diamagnetic contribution described above. New possibilities for the potential application of these materials, based on their magnetic properties, are now accessible and the possibility of some degree of collective magnetic order (ferro- and antiferromagnetism) has been theoretically proposed [13, 141. All metallomesogens with unpaired electrons studied to date behave as paramagnetic systems and they are therefore named paramagnetic liquid crystals (PMLC).

In PMLC the molecular paramagnetic response is generally anisotropic, and as in conventional liquid crystals, this property is transferred onto the mesophase domains. In some cases the anisotropic paramagnetic contribution to the magnetic susceptibility is in competition with the diamagnetic contribution. This competition can lead to orientation of the mesophase domains in a magnetic field which are different from those of similar diamagnetic compounds [ 15 -211.

Since the magnetic properties of nonparamagnetic metal-organic mesogens are basically the same as those of widely studied organic liquid crystals, only PMLC will be considered here. The chapter is organized as follows: after a brief revision of the basic concepts of magnetism (Sec. 10-2) a review of the magnetic measurements on PMLC is given starting with dinuclear PMLC (Sec. 10-3). In Sec. 10-4, results concerning mononuclear PMLC are reported, and the descriptive part ends with a discussion of the limited magnetic data available on polymeric PMLC (Sec. 10-5). Sec. 10-6 is devoted to the analysis of the relationship between mesophase order and magnetic properties. Finally, the important point of the orientation of the new PMLC in magnetic fields is discussed in Sec. 10-7.

10.2 Basic Concepts of Magnetism

When a sample is placed in a homogeneous magnetic field H i t acquires a magnetization M (A4 = m/ K where rn is the magnetic moment of the sample and Vits volume). The magnetic susceptibility x is defined by the expression:

SM= x6H (10.1)

The change in the magnetization SM is produced by a change in the magnetic field 6H. In general, is a second rank symmetric tensor which, in an appropriate reference frame (its principal axes), has a diagonal expression which is characterized by its principal values (xa, a = X J Z ) . For an isotropic sample, x is a scalar, x, whereas in samples with axial symmetry two of the three principal values are degenerate and it is usual to denote them by xII = xz and xI = x x = x y , with z being the distinguished axis.

I0 Magnetic Properties of Metallomesogens 389

In general, over a large magnetic field range, x is independent of H , and:

M = x H (1 0.2)

As discussed previously, two contributions to the total susceptibility, the diamagnetic Xd and the paramagnetic xp, are expected. The paramagnetic contribution will be discussed in the following pages.

When a sample is perturbed by an external magnetic field H, its energy per unit volume E is modified and the magnetization M is related to this energy by:

which in quantum mechanics formalism becomes:

where p is the density operator and Ythe system Hamiltonian. For noninteracting molecules, the Hamiltonian Yis considered for an isolated molecule in a magnetic field H and the former expression takes the form:

N Tr (eCflkT]

M = Tr (( - VH)Ze- (10.5)

where N is the number of paramagnetic entities per unit volume. The simplest case in paramagnetism is an ensemble of molecules with a degenerate

ground state in the absence of any applied magnetic field, which is well separated in energy (compared with k T ) from the excited states. The magnetic behavior of the system can be described by introducing an effective spin S, with 2S+ 1 equal to the ground state degeneration. So the Hamiltonian for a molecule in the presence of a magnetic field H only including the Zeeman contribution is:

pB being the Bohr magneton bg = 13.996 GHz/T) and g the g-tensor of the paramagnetic entity, which is a second rank symmetric tensor (whose principal values are g,,g,,,g,). Firstly, an ensemble of identical molecules with the same orientation with respect to the magnetic field will be considered. The exponential in Eq. (10.5) can be developed in powers of T - ' , and in the high temperature limit ( IpsgHI 4 k T) the linear response is given by keeping terms up to T - ' :

390 P J . Alonso

in components:

Then the paramagnetic susceptibility tensor is given by:

where Tr (S,S, , ) = (1/3)S(S+ 1)(2S+1)~3~,. Eq. (10.9) is the well known Curie law for the magnetic susceptibility. Since the principal axes of the g and g 2 tensors coincide, they also constitute the principal axis of the paramagnetic contribution to the magnetic susceptibility. Thus its principal values are:

(10.10)

Next, the case of identical paramagnetic molecules with different orientations in the sample will be considered. The orthogonal transformation which relates the molecular axes to the sample axes is n and the paramagnetic susceptibility tensor of the sample is:

(10.1 1)

where (. . .>* means averaging over the different molecular orientations. It is worth noting that Tr x = x,+xv + xz, and so for an isotropic sample (like a powder or polydomain sample) the measured susceptibility x is:

Np;g2S(S+ 1 ) - c -- 1

3 3k T T x = - Tr (x) =

where

1 N p k g 2 S ( S + 1) g2=-(gz+g$+gl) and C = 3 3k

Often the Curie constant, C, is expressed as:

(1 0.1 2)

(10.13)

( 1 0.1 4)

10 Magnetic Properties of Metatlomesogens 391

where p = g vS(S+ 1) is the effective number of Bohr magnetons 0, = 1.732 for the free electron).

In the case of a sample with different noninteracting paramagnetic entities, the total susceptibility is obtained by adding Eq. (10.11) over all particles. For an isotropic sample, an expression similar to Eq. (10.12) is obtained for the magnetic susceptibility, but with a constant C given by:

P i c = - c N;g,"S,(S, + 1 ) 3k i

(10.1 5 )

The sum includes all species i (with a volume concentration N,). The temperature-independent paramagnetism (TIP) contribution to the paramag-

netic susceptibility will now be considered [2,4]. This contribution arises from mixing non thermally populated excited states with the ground state. The TIP contribution only becomes significant at sufficiently high temperatures (usually higher than 80 K) because then the Curie contribution significantly decreases.

Throughout this introduction it has been assumed that the magnetic entities are isolated and magnetic interactions between them do not occur. If an exchange interaction does exist the Curie law (Eq. (10.12)) describing the magnetic behavior of the sample in the paramagnetic region is replaced by the Curie-Weiss law:

( 1 0.1 6 )

where 0 > 0 indicates ferromagnetic and e< 0 antiferromagnetic coupling. A plot of x-' versus T gives a straight line from which the sign and the value of B can be ex- trapolated from the intercept with the temperature axis. At this point it is interesting to note that deviations from the Curie law (Eq. (10.12)) could be due to causes other than an intermolecular exchange interaction. One alternative cause could be the existence of a zero field, the splitting of which produces a breakdown of the ground state degeneration in the absence of magnetic field. This phenomenon occurs in systems with S? 1 and it is due to the mixing of the ground state with some excited states through spin-orbit coupling. Its effect on the thermal evolution of the averaged paramagnetic susceptibility x is quite similar to that of an antiferromagnetic interaction [2].

One interesting situation often found in molecular magnetism occurs when two different magnetic entities of the same molecule are strongly coupled via a diamagnetic ligand which transmits the magnetic interaction between them. These systems are known as magnetic dimers and usually are phenomenologically described by a coupling between the local spins given by the scalar Heisenberg-Dirac-Van Vleck (HDVV) Hamiltonian [22,23]:

(10.17)

392 RJ. AIonso

where a positive value of J means ferromagnetic coupling and a negative J signifies antiferromagnetic coupling. The Hamiltonian of the dimer in the presence of a magnetic field H is:

<P= pBSlg, H + , I J ~ , S ~ ~ ~ H - ~ J S ~ S ~ = pgSgH-J[S2-Sf-S$)

+ + B ( S , -S2)(91 -g2)H (10.18)

where S = Sl +S2 is the total spin and g = ( g l +g2)/2 is the averaged g-tensor of the dimer. The local spins, S, and S2, are no longer good quantum numbers. In the simplest case of magnetically equivalent ions (gl = g2) the total spin S is thus a good quantum number with eigenvalues ranging from S1 + S 2 to IS, -S21 and the Hamiltonian (Eq. (10.1 8)) is:

Sr= pgSgH-J(S2-Sf-Si] (10.19)

In the absence of a magnetic field the (2S1 + 1) x (2S2 + 1) degeneration is removed by the exchange interaction and a structure of SI + S 2 - IS, -S2 1 + 1 multiplets la- belled by the quantum number S (each of them 2S+1 times degenerated) is found. Their energies are:

dJS] = -J((S+ 1)S-(S1+ 1)Sl - ( S 2 + 1)SZ) (10.20)

If the Zeeman term is smaller in comparison with the exchange contribution, then the paramagnetic response of the system of noninteracting dimers is identical to that of a set of S spin multiplets. Taking into account the thermal population of S multiplets according to the Boltzmann statistic and the multiplet multiplicities, Eq. (10.9) is modified to give the Bleaney-Browers law:

where N is the number of dimers per unit volume. In the particular case of an isotropic sample consisting of dimers of identical ions with a spin S, = S2 = 1/2, Eq. (1 0.21) is reduced to:

(10.22)

This well-known equation was first derived by Bleaney and Browers [24].

10 Magnetic Properties of Metallomesogens 393

It is interesting to note that in the case of nonidentical magnetic ions, although g1 # g2, the Hamiltonian (Eq. (1 0.19)) is still a good first order approximation because the last term in Eq. (10.18) does not have matrix elements within each multiplet [23]. If l pB(g , -g2 )Hl is small compared with the multiplet separation (given by J ) , Eq. (10.22) gives a good estimation for the susceptibility of the dimers.

Finally, one must consider the situation in which some small magnetic interaction between dimers exists. These cases can be considered phenomenologically by modifying Eq. (10.21), as was done with the Curie law to obtain the Curie-Weiss expression. The paramagnetic susceptibility tensor is:

s. +s.

where 0 accounts for the magnetic interactions between dimers. When a magnetic molecular material undergoes a phase transition that modifies

the molecular packing, as happens in liquid crystals, the intermolecular magnetic interaction may also change. Even if the structure of the molecules does not change during the molecular rearrangement, a discontinuity in the magnetic susceptibility at the transition temperature Tt takes place. The change in the magnetic susceptibility Ax can be calculated from Eq. (10.16) for monomers, or from Eq. (10.23) in the case of dimers, to obtain:

(1 0.24)

where A0 is the change in the Curie-Weiss constant as a result of the transition, assuming that Oe Tt.

Up to now the discussion has been limited to the linear response region ( I p B g . H ( ekT). If this is not the case, however, the expansion leading to Eq. (10.7) is no longer valid and saturation effects may take place. In such a situation, and for an isotropic noninteracting spin ensemble, the magnetization is given by:

where Bs@) is the Brillouin function defined as:

(10.25)

(1 0.26)

394 P J . Alonso

At sufficiently high fields and at sufficiently low temperatures y-+ m and Bs(y)+ 1. Thus, the magnetization reaches its saturation value M, = Ng,uBS.

In the case where intermolecular interaction are also present, and within the Weiss mean field approximation, Eq. (10.25) is modified to give:

(10.27)

with 0 = Cy, whose asymptotic behavior for T-, M gives the Curie-Weiss law. In the case of S = 1/2 it takes the form:

M = N,usgS tanh (1 0.28)

10.3 Measurements on Dinuclear PMLC

Giroud-Godquin et al. [25] were the first to report the measurement of magnetic susceptibility as a function of temperature in the different phases of binuclear paramagnetic metallomesogens. These authors reported the evolution of x ( T ) for a number of thermotropic binuclear copper(I1) carboxylates which show discotic columnar phases. The molecular structure of these complexes is given in Fig. 10-1.

Two types of carboxylates have been studied to date: type D1 [R = (CH2),1-2-CH3, n = 12, 181 and type D2 [R = CH2-CH-(C9H&]. Both D1 and D2 show a crystal ~ discotic phase (C-D) transition at the tempertures given in Table 10-1.

Figure 10-1. Molecular structure of the dinuclear copper carboxylates studied by Giroud-Godquin et al. Adapted from [25] .


Table 10-1. Temperature of the C-+D transition for the dinuclear copper compounds studied by Giroud-Godquin et al. (Adapted from [25]).

~

D1

D2

(n = 12) dodecanoate (n = 18) octadecanoate C U [ O ~ C C H ~ - CH- (C,H,,),l,

380 3 90 348

The magnetic susceptibility x ( T ) changes abruptly at the C-t D transition temperature, and is 5 - 6 % lower in the discotic phase than in the crystalline state. In both phases the magnetic susceptibility has a contribution due to the intradimeric exchange interaction between the two copper atoms. The magnetic susceptibility can be described by an isotropic HDVV spin Hamiltonian (Eq. (10.17)). Evidence of interdimeric interaction is not observed. The thermal evolution of x ( T ) can be explained by the following equation:

(1 0.29)

where Xdjm corresponds to the dimeric contribution given by Eq. (10.22). zmon, given by Eq. (10.12), is due to a monomeric impurity which is present in a proportion x. The last term accounts for the TIP contribution, but the authors do not give any value for this. By fitting Eq. (10.29) to the experimental results, the authors found the values collected in Table 10.2. The low temperature evolution of x (T) in samples that have been quenched in liquid nitrogen after previously being heated to the discotic phase has also been measured. Their behavior has been explained using Eq. (10.29), and the values of the parameters in this case are closer to those found for the discotic phase than to those found in the crystalline phase. This change in behavior can be reversed by leaving the sample at room temperature for a few weeks.

Table 10-2. Parameters for the paramagnetic contribution to x M ( T ) in the dinuclear copper compounds studied by Giroud-Godquin et al. (Adapted from [25]).

Compound Phase g J (cm-') x p (VO)

D1 n = 12 C (T<380 K) D (T>380K) quenched quenched +aged

n = 18 C (T<390 K) quenched

D2 C (T< 348 K) D (T> 348 K)

2.17 2.16 2.25 2.19

2.05 2.16

2.12 2.15

- 146 - 156 - 163 - 150

- 139 - 156

153 166

0.7 0.7 0.4 0.5

0.3 0.9

0.0 0.0

396 P J AIonso

The changes found in the magnetic parameters, g-factor and intramolecular exchange constant J, are interpreted by the authors [25] as a consequence of a molecular modification triggered by the change in the molecular packing at the phase transition. Quenching of the discotic phase to room temperature explains the similarity between the data found for g and J in the discotic liquid crystalline and in the quenched sample. The changes found in the susceptibility measured some weeks after quenching are explained as a consequence of further crystallization when the sample is aged at room temperature for some time.

Haase and co-workers [26] also investigated the thermal evolution of x for some binuclear copper(I1) compounds of the D 1 family (with n = 8 and 16) as well as that of previously studied copper(I1) dodecanoate (n = 12). Although they found a temperature dependence similar to that reported by Giroud-Godquin [25] (see above) for all the compounds, they realized that fitting the data to Eq. (10.29) results in a high variation of the magnetic parameters; the g-factor changes from 2.12 to 2.21 (n = 8) and from 2.00 to 2.16 (n = 16), while the exchange interaction parameter J varies from -153cm-' to -180cm-' ( n = 8 ) and from -120cm-' to -172cm-' (n = 16) when the sample undergoes transition from the crystalline to the discotic phase. This high variation would imply an unexpectedly strong modification of the dinuclear complex during the phase transition. In order to overcome this difficulty, Haase and coworkers [26] proposed an alternative model in which a weak intermolecular magnetic interaction is allowed and the paramagnetic susceptibility is fitted by Eq. (10.23) with S, = S2 = 112. The dinuclear moiety does not change its structure during the phase transition, but the different molecular packing at the phase transition modifies the weak intermolecular exchange interaction, so only a change in the Weiss constant 0 is allowed at the C+D transition. In this situation, taking into account that Tc+D%O, the discontinuity in the susceptibility at Tc-D is given by Eq. (10.24) where Ax = xD-xC. Finally, Haase and coworkers [26] put forward an additional hypothesis. They assumed that the intermolecular exchange is effectively zero in the crystalline phase ( O , = O ) whereas it takes a finite value (0, = At?) in the disscotic phase due to the liquid crystalline order. A value of 0, = - 5 K is obtained for both n = 8 and n = 16, indicating a weak antiferromagnetic coupling between the dinuclear discotic molecules in the columnar mesophase.

This model has recently been supported by EXAFS measurements on Cu2@-02CC,H2,+,) with n = 6 , 7, 12, 18, and 22, which showed that the bond lengths in the molecular core remain unchanged upon transition from the solid to the mesophase (see review by Giroud-Godquin and Maitlis [8] and references therein). The absence of any interdimeric magnetic interaction in the crystalline state and its sudden appearance in the less ordered discotic phase is difficult to explain. One alternative could be the existence of a ferromagnetic coupling between dimers which becomes weaker upon melting the crystalline solid into the less ordered discotic phase. Indeed, such weakening of the magnetic interaction in the S+N transition has been confirmed by EPR measurements for some mononuclear copper(r1)- containing liquid crystals. Since no EPR measurements on the dinuclear D 1-type

I0 Magnetic Properties of Metatlomesogens 3 97

copper(I1) compounds have been carried out so far this important point could not be further clarified.

Recently, Galyametdinov et al. [27] performed magnetic susceptibility measurements on the dinuclear iron(II1) compound shown in Fig. 10-2. The phase sequence of this compound is: C-(378 K)+SA -(388 K)-tN-(432 K)+I.

Figure 10-2. Dinuclear p-oxo-bridged iron(I1r) compound studied by Galyametdinov et al. Adapted from [27].

The magnetic susceptibility was measured as a function of temperature between 4.2 and 420 K and, after subtraction of the diamagnetic contribution, the paramagnetic contribution was fitted to Eq. (10.29). The contribution of the intramolecular iron(m) dimers, calculated using the HDVV Hamiltonian (Eq. (10.17)) for S1 = S2 = 5/2 is:

with y = J / ( k T ) C 2e2y + 1 Oe6Y + 28 + 60e20y + 1 10e30Y

T 1 + 3 e2Y + 5 e6Y + 7 e'2y + 9e20Y + 1 1 e30Y Xdim = -

(10.30)

After correcting the values of the monomer contribution ( ~ 2 . 6 % ) and of the temperature-independent paramagnetism contribution (TIP = 400 lop6 cm3 mol), the best fit parameters (g = 2.00, a typical value for Fe"' (d5%), J = -91.8 cm-') were found. The negative sign of J indicates an antiferromagnetic coupling between the two iron atoms in the dinuclear complex via the p-0x0 bridge. In that case, a drastic variation in the susceptibility is found neither at the C+S, phase transition nor at the SA+N transition. An increase in ~(7') observed at temperatures above 395 K is explained by partial decomposition of the compound which is also observed by polarizing microscopy.

398 PJ. Alonso

10.4 Measurements on Mononuclear PMLC

In spite of the large number of known mononuclear PMLC [8-121, magnetic susceptibility data obtained from these materials are very scarce and restricted to some copper and oxovanadium derivatives of fi-diketones and Schiff bases.

Copper(rr) bis(P-diketonates) have the general formula depicted in Fig. 10-3. Magnetic susceptibility measurements are available for three of these compounds (see Table 10-3, p. 399).

Figure 10-3. Molecular structure for the div-diketonato)cop- R I per(@ complexes.

Giroud-Godquin et al. [28] published the first magnetic susceptibility data obtained from a liquid crystalline copper(r1) compound (DK 1 in Table 10-3). The magnetic susceptibility was measured for the bulk material by the Faraday method (alongside the direction of the applied magnetic field). It was found that the paramagnetic contribution of copper(r1) is partially compensated by the diamagnetic contribution due to the molecular structure in such a way that the total susceptibility changes from positive values at room temperature to negative values in the isotropic phase. The thermal evolution of x was measured while cooling from the isotropic phase. A positive discontinuity of x ( A x = 0.3 x cm3 g- ') is found at the 1 4 D h transition kD>,<xr) whereas a decrease in x is observed at the Dh+C transition. A more detailed analysis of these data is not available to date.

Haase et al. [26] measured the thermal evolution of the magnetic susceptibility of compound DK2 (Table 10-3) using the Faraday method. For this compound, the total magnetic susceptibility is positive over the whole temperature range (300-475 K). After correction for the diamagnetic contribution of the molecular skeleton (estimated to be x d = - 7 4 6 . 6 ~ cm3 mol-I), the paramagnetic contribution was fitted by a Curie law (Eq. (10.12)) to give a magnetic moment of p = 1.801 k0.006 (g = 2.02) over the whole temperature range. This value of p appears to be too small. It was found that the C+D+I phase transitions did not influence the magnetic susceptibility. Since alignment effects induced by the magnetic field in the X-ray diffraction experiments were not observed, the authors suggest that there is still the possibility of antiferromagnetic coupling effects compensating the diamagnetic anisotropy. This kind of coupling was previously observed by Eatsman et al. [30] in the EPR spectra of a compound with the same structure and with R = phenyl-C8HI,.


Table 10-3. Bis@diketonato)copper(II) on which magnetic susceptibility measurements have been carried out.

Rl R2 Phases References

R, = R2

335 K 440 K K - D, -1 DK2 G O C I r , H * I

DK 3 $ \ 0

H3C I

K 461 K 452 K*

-I -N

* Monompic transition

Chandrasekhar et al. [29] studied the thermal evolution of the magnetic susceptibility of compound DK3 (see Table 10-3). It shows calamitic behavior and the nematic phase is reached by cooling the isotropic phase below the melting point (monotropic phase). The authors verified that the anisotropy of the magnetic susceptibility is positive by observing the Fredericksz transition: the molecules in the nematic phase are mainly oriented parallel to the magnetic field. They therefore concluded that the measured x value in the nematic phase corresponds to xl l ( I I stands for the director direction) in an aligned sample. However, the authors assume that orientation effects are not present in the isotropic phase, so the values obtained at high temperature should be due to the isotropic susceptibility. By subtracting the diamagnetic contribution, the paramagnetic contribution is obtained. In both the nematic and the isotropic phase the thermal evolution of x can be fitted by the Curie law (Eq. (10.12)). Although discontinuities are not found, the Curie constant in the isotropic phase is two times as high as that found for the nematic phase. The differences in the Curie constant have been tentatively assigned to the existence of antiparallel correlations in the nematic phase, but this interpretation does not agree with the EPR data obtained by the same authors [29]. Indeed, the observation of a clearly resolved hyperfine structure in the EPR spectrum in the nematic phase is

400 F?J Alonso

strong evidence against the existence of such an interaction (see Chap. 9). The change in the temperature dependence of the susceptibility in the isotropic could be due to orientation effects in this phase, as have been observed for other metallomesogens [31,33].

Another family of mononuclear metallomesogens, copper and vanadyl derivatives of Schiff bases, has been the subject of magnetic susceptibility measurements. The compounds have the general structure shown in Fig. 10-4. All of these materials have been described as calamitic, and their mesophases have been assigned in the conventional way. In Table 10-4 the phase transition temperatures of the compounds are given.

Figure 10-4. General structure of the metal complexes of Schiff base derivatives.

Magnetic susceptibility studies of compound BSI Cu were reported by Haase et al. [26]. The authors measured the thermal evolution of x through the crystalline and nematic phases and found a jump at the C+N transition kN>xC). The sign of the jump is opposite to that found for the dinuclear copper compounds. In the crystalline phase, the thermal evolution can be fitted by the Curie law given a magnetic moment per atom of p = 1.797 (g = 2.01), but in the nematic phase the thermal evolution of x shows a more marked decrease in the magnetic susceptibility than in the low temperature crystal phase. An explanation for this behavior is not given.

A similar study was performed by Borchers and Haase [33] on BS2Cu, a mixture of two copper mesogens (see Table 10-4, p. 401). As in the investigation of BSICu, the magnetic susceptibility in the crystalline state is fitted by the Curie law with p = 1.878 (g = 2.1 1). Again, a jump in x is detected at the C+N transition kN>xC). Taking into account the anisotropy of the magnetic susceptibility (AX) and estimating the paramagnetic contribution from measurements on a single crystal of a related Schiff base complex [35] as well as from EPR data of similar mesomorphic copper compounds [36], the authors calculated the Maier-Saupe order parameter ( S = 0.6) at T = 456 K on the basis that both the molecules and the nematic phase have uniaxial symmetry. This point will be discussed later (see Sect. 10-6).

Recently, Campillos [34] studied the thermal evolution of the magnetic susceptibility of compounds BS3Cu and BS3VO. For both materials the temperature dependence of x can be fitted by the Curie law (Eq. (10.12)) in a temperature range between 4 and 300 K with a mean square g-value of 2.1 1 and 1.98 for copper and oxyvanadyl compounds respectively. Although x is very low at high temperatures, its


Table 10-4. Liquid crystal-metal complex derivatives of Schiff bases on which magnetic susceptibility measurements have been reported. R, and R, are given in Fig. 10-4.

M R, R2 Phases References

BS 1Cu

BS2Cu

BS 3Cu

BS 3VO

c u

c u

c u

vo

Q 9 %HI3

I 0

thermal evolution has been measured for the oxovanadium complex above 300 K up to the isotropic phase. A number of interesting features is observed: changes are not detected in the magnetic susceptibility at the phase transitions and in the solid C1 and Cz phases x decreases with increasing temperature. A monotone increase in x is observed in the smectic C phase and even in the isotropic state. In order to explain this behavior, the possibility of a molecular orientation induced by the magnetic field has been proposed.

402 l ? J AIonso

10.5 Measurements on Metallomesogenic Polymers

One type of metal-containing liquid crystalline polymer is derived from salicylaldimines [37,38]. These materials are of interest because the incorporation of magnetic moieties allows control over the molecular orientation of the polymers in magnetic fields. In addition, there is the possibility of observing magnetic order in these systems [39], but EPR results [38, 401 indicate that this phenomenon is very unlikely because of the low degree of the intermetal exchange interaction in metallomesogenic polymers. In spite of their potential interest, very few magnetic measurements on polymers based on metallomesogens have been made. Recently [38], the preliminary magnetic characterization of a nematic polyester derived from a metallomesogenic copper(r1) moiety (represented as P1Cu) was reported. The chemical structure of this compound is shown in Fig. 10-5.

a- O1('H3)-O 0

0 3:

Figure 10-5. Structure of the PlCu polymer unit. Adapted from [38].

In order to obtain information about the microstructure of this polymer and its influence on the magnetic properties, three types of sample were studied: type I (PI Cu-I) is the polycrystalline polymer as obtained by synthesis, type I1 (PICu-11) is a sample annealed at temperatures close to the melting transition (188"C, the treatment increases the crystallinity of the sample). Type Ill (PICu-111) is a frozen nematic phase obtained by quenching the nematic melt. The magnetic susceptibility was measured for all the samples as a function of temperature in the range between 5 K and room temperature, and the data were fitted to the following equation:

C X m =xo+- T- e (10.31)

which corresponds to a superposition of a background susceptibility x0 and a Curie-Weiss contribution with the values for xo, C and 8 given in Table 10-5.


Table 10-5. Values for the parameters obtained by fitting Eq. (10.31) to the magnetic susceptibility data of the PlCu polymers. (Adapted from [38]).

Type 1 Type I1 Type I11

C (emu K mol-I) 0.43 0.43 0.44 0 (K) - 0.28 - 0.25 - 0.34 xo (emu mol- I) x lo4 - 9.4 + 2.6 - 8.3

The effective copper(I1) moments, as determined from the constant C, range between 1.85 and 1 . 8 0 ~ ~ (g-values between 2.13 and 2.17) and are in agreement with the EPR data. However, the prediction of a weak antiferromagnetic interaction (8 = -0.3 K) is not confirmed by EPR measurements [38], although a large degree of uncertainty is inherent in the determination of 8. The most intriguing phenomenon is the reversed sign of the background susceptibility xo in the annealed sample as compared with that of the untreated and frozen nematic samples. In order to carry out an analysis of xo, the simultaneous presence of two contributions must be considered; the diamagnetic contribution related to the organic ligand and the TIP associated with the copper(r1) ion. Since EPR data indicate that the copper(I1) environment does not change upon thermal treatment, the only way to obtain a positive value of xo is to allow the diamagnetic term to decrease as a result of annealing. This behavior was also observed for other copper(I1) polymers, where it was found that low molecular weight polymers are diamagnetic whereas high molecular weight polymers are paramagnetic [41]. The change of xo is therefore interpreted as a consequence of the straightening of the polymeric chain caused by annealing.

Magnetization measurements as a function of the applied magnetic field were carried out with these polymers (PICu) at low temperatures (2.5 and 5 K) far from the linear response region. In all cases, the evolution predicted by Eq. 10.28 can be fitted to the data to obtain a Weiss mean field constant y which is in agreement with the Curie constant and 8 values given in Table 10-5.

10.6 Mesophase Order and Magnetic Susceptibility

This section is devoted to studies of the magnetic susceptibility of metal-containing liquid crystals in relation to the degree of order in their mesophases. In particular, the peculiarities found in the alignment of domains by the application of a moderate dc magnetic field will be discussed. While conventional organic liquid crystals show general orientational behavior (for instance, most calamitic liquid crystals align with the director parallel to the magnetic field), PMLC do not show a general response [15-211. This peculiarity must be associated with the two main contributions to the

404 PJ AIonso

molecular magnetic susceptibility: the diamagnetic contribution due to the organic skeleton and the paramagnetic contribution associated with the metal center. The latter contribution, depending on the metal, can increase or decrease depending on the degree of order in the phase.

Firstly, the concept of order in mesophases will be analyzed based on the ideas developed by de Gennes [5 ] and Vertogen and de Jeu [7], who distinguished between the microscopic and the macroscopic approximation of this concept.

Microscopic Approximation

This approximation is essentially valid for rigid molecules. A reference frame associated with the molecule (x’,yM,zM) and a reference frame associated with the phase domain (xD,y , z ) are defined. The orientation of a molecule is given by the Euler angles I2(@, 8, w ) chosen according to the Goldstein convention [42] (see Fig. 10-6).

D D

This yields the expression:

(10.32)

Figure 10-6. Orientation of the molecular axes with respect to the domain axes given by the Euler angles.


with

n(@,6, W ) =

c o s @ c o s 1 + ~ - s i n @ s i n ~ c o s 8 s i n @ c o s y / + c o s @ s i n ~ c o s 8 s i n v s i n 8

-cos @ sin ty-sin @ cos w cos 8 -sin @ sin cy+cos @ cos v / cos 0 cos ty sin 8

sin @ sin 8 -cos @ sin 8 cos 0

(10.33)

The order of the phase is characterized by a functionf(s2) = f(@, 8, w ) , which describes the probability density of finding a molecule with an orientation given by Q(@, 8, w) . The functionf(s2) can be expanded as a function of the Wigner matrices [43] as:

(10.34)

and the coefficients ahM, are given by:

These coefficients define the function f ( s 2 ) and are the microscopic order parameters.

Macroscopic Approximation The microscopic approach gives rise to some difficulties. Molecules are generally not rigid and, in addition, it is difficult to relate the order parameters ahM' to macroscopic measurements. It is therefore useful to introduce a definition of order based on measurable macroscopic properties. These kinds of order parameter are related to some tensorial properties. In particular, if A is a second-rank tensorial macroscopic property (dielectric permittivity, magnetic susceptibility, etc.) then the second- rank tensorial order parameter Q is defined by the relation:

( 1 0.3 6 )

Q equals zero for an isotropic system. The constant G is arbitrarily chosen, often in such a way that in a totally ordered phase (a single crystal) Q, = 1.

In particular within the scope of studying magnetic properties, the magnetic susceptibility x D (associated with a domain) will be used as the second-rank tensor to define the order parameter Q. This order parameter will now be introduced and

406 €?J. Alonso

its relationship with the magnetic measurements in the mesophase analyzed. Liquid crystals in general, and among them metal-organic liquid crystals, consist of molecules whose magnetic interactions can be considered weak (which is supported by magnetic measurements). The magnetic susceptibility of a domain is simply obtained by adding up the contributions of all molecules taking into account their relative orientations. If x is the molecular magnetic susceptibility, the magnetic susceptibility of the domain (xD) is given by:

where 0 indicates averaging over all molecular orientations defined by a($, 8, v) . In components:

(10.38)

When xMkD) is the isotropic part of the magnetic susceptibility tensor of a molecule (a domain), it follows that:

X D = X M - X - (1 0.3 9)

The anisotropic part of the magnetic susceptibility tensor of the domain which gives information about the order is:

(10.40) D M x k p = X a p - X d a p = C q i j , a p X i j i j

where the fourth-rank tensor q is defined by:

with the following properties:

(10.41)

(10.42)

Until now, the choice of neither molecular nor domain axes has been discussed and Eq. (10.41) is independent of such axes, emphasizing the tensorial character of q. With the aim of finding a minimal set of order parameters, it is possible to ade- quately select both molecular and domain axes. If the molecular axes coincide with the principal axes of the magnetic susceptibility k y = x y d i j ) :

x h p = C q i j , a p x y = C q i i , a p X Y (10.43) ij i


and if we choose the principal axes of the macroscopic susceptibility tensor khp = x&aap) for the domain axes, the macroscopic susceptibility tensor is defined by:

x&= C q i i , a a X M = C QiaXM i j i

(1 0.44)

with Qia describing the order of the phase, and which are defined by the parameters:

Q. ia = 4.. ri,aa = (QiaQia) (10.45)

which with Eq. (1 0.42) is verified:

C Qia = C Qia = 0 i a

(1 0.46)

The nine Qia values can thus be expressed, in the general case, as a function of four independent parameters. One definition for these independent parameters is given by Straley [44], who defines them as the averaging of the following four functions of the Euler angles:

1

2 F ,= - (3cos20- l ) ; Fz=s in20cos2@ ; F,=sin20cos2ty

(1 0.47) 1

2 F4 = - (1 +cos2 0) cos 2@ cos 2 I,-cos O sin 2@ sin 2 ty

and they are:

S is the order parameter introduced by Maier and Saupe [45] to describe uniaxial phases of uniaxial molecules (N and SA phases of rod-like molecules), T and S describe the order of uniaxial molecules in biaxial phases (Sc phase of rod-like molecules) and T coincides with (2/3)D; D was introduced by Alben et al. [46] for this kind of biaxial phase. The two parameters S and T are the only ones needed to describe systems of uniaxial molecules. The two other parameters U and V are necessary when considering biaxial molecules. For instance, U describes the angular tendency of the molecular orientation with respect to the molecular axis.

The order parameters Qia can be expressed as a function of the Straley parameters as follows:

408 P.I Alonso

1 1 1 1 1 1 1 1 1 1

6 4 4 2 6 4 4 2 3 2

1 1 1 1 1 1 1 1 1 1

Q,, = - S + - T+ - U+ - V Qxy = - S - - T+ - U- - V Q,, = - - S - - U

Q = - S + - T - - u - - V Q y y = - S - - T - - u + - V Q = - - s + - U " 6 4 4 2 6 4 4 2 y z 3 2

zx 3 2 3 2 3

2 Q,, = - S (10.49)

1 1 1 1 Q = - - S - - T Qzy= - - S + - T

The principal values of the macroscopic susceptibility tensor are then expressed as:

(10.50)

To describe the anisotropy of the tensor A, it is important to define the following quantities:

with the inverse relation:

1 1 1 1 2

3 2 3 2 3 A , = A - - A A + - S A ; A y = A - - A A - 6 A ; A , = A + - A A

(1 0.52)

The molecular values of x M , A x M , 6xM and those of the domain, x D , AxD, S x D , are related by:

U 2

X D = X M ; A \ x ~ = s A x ~ - - ~ x ~ ; 6 x D = - T A X ~ - V ~ X ~ (10.53)

It follows that the anisotropy of the susceptibility of the domain is not only due to the molecular anisotropy ( A x M , 6xM), but is also a consequence of the domain order. Moreover, in a general case, it is not possible to obtain information about the order of the phase directly from the susceptibility measurements without using some additional hypothesis. This point will now be discussed for some specific cases by us-


ing simple models. It is worth noting that in an isotropic system ( S = T = U = V = 0) the macroscopic susceptibility is isotropic regardless of the molecular anisotropy.

If conventional calamitic organic liquid crystals are considered, their molecules are assumed to be uniaxial (LI = V = 0) and the principal axes of the magnetic susceptibility of the molecule (and of the domain) coincide with the geometrical axes of the molecule (and of the domain). Thus, for the conventional uniaxial phases:

A X ~ = X ~ ; - X ~ = S A X ~ (10.54)

and for the smectic C phase it can be shown that:

(10.55) 6 x D = x,D-x,D = - T A X M

In a calamitic organic LC, the magnetic susceptibility has diamagnetic character when A x M > O and it is expected to be independent of temperature. From Eq. (10.54) it follows:

X D - X S ( T ) = XI1 - x

(1 0.56)

where x y is obtained from measurements on a single crystal. The thermal evolution of x t gives a value for the thermal evolution of the order parameter S. The conventional organic calamitic liquid crystal is aligned in a sufficiently strong magnetic field with the domain axis in the direction of the magnetic field, so the measurement of the longitudinal magnetic susceptibility directly gives x f ; . From E q . (10.55)) it follows that:

(1 0.57)

so, measurement of the transverse magnetic susceptibility gives information about the thermal evolution of the order parameter T = (F,) in the smectic C phase of a conventional calamitic organic liquid crystal.

This type of analysis is widely used in the field of calamitic organic liquid crystals [5,7], but it presents some difficulties when applied to the magnetic susceptibility of PMLCs. In this case the magnetic susceptibility consists of two contributions: the diamagnetic contribution associated with the organic skeleton and the paramagnetic contribution due to the paramagnetic metal. In general, the principal axis of each of these two contributions do not coincide [31, 47-49]. It is noteworthy that there are only few references in which detailed studies of the thermal evolution of the magnetic susceptibility are reported for paramagnetic metal-organic liquid crystals [29,33].

410 P J . Alonso

In spite of these difficulties, Eq. (10.54) has been used to describe the thermal evolution of the diamagnetic susceptibility of a domain. Chandrasehkar et al. [29] studied the thermal evolution of a bis(JI-diketonato)copper(II) complex (DK 3 shown in Table 10-3), which exhibits a monotropic nematic phase. By using Eq. (10.56) and the values of the order parameter S obtained by applying the Maier-Saupe theory, the authors estimated the diamagnetic contribution and then calculated the thermal evolution of the paramagnetic contribution in the nematic and isotropic phases. In both phases, the paramagnetic contribution follows the Curie law, but the magnetic moment in the isotropic phase was found to be twice as high as that measured in the nematic phase. As mentioned previously, a plausible explanation for this behavior is not given, but it is noteworthy that the g-factor values obtained from susceptibility measurements are g = 2.0 (nematic phase) and g = 2.8 (isotropic phase), with the latter value in strong disagreement with EPR data (gli = 2.26, g , = 2.06, g = 2.13).

Borchers and Haase [33] analyzed the magnetic susceptibility of a mixture of copper Schiff base derivatives (see Fig. 10-4) in which R, = OOC-@-O-C6HI3 and R2 = C,H9 or 0-CH, (Table 10-4). In this case, the anisotropy of the molecular magnetic susceptibility is obtained from measurements on a single crystal, while the anisotropy of the paramagnetic molecular contribution is obtained from EPR data. In this way, a value of 830 .6 at 456 K (TC-.,N = 452 K) was estimated.

To analyze these results it is necessary to refine the molecular model. In a large number of compounds studied, the environment of the metal is approximately square-planar, with the molecular long axis within the coordination plane. However, it follows from EPR measurements (see for example [31, 32, 361) that the paramagnetic properties associated with the metal are practically axial (the axis being normal to the coordination plane). A simple but more realistic model to describe the molecular susceptibility is that depicted in Fig. 10-7, in which the diamagnetic and the paramagnetic contributions to the molecular susceptibility are assumed to be coaxial.

If AX,, = xp 1 1 - xp I is the anisotropy of the paramagnetic molecular susceptibility in the molecular reference frame given in Fig. 10-7, the molecular paramagnetic contribution is given by:

(1 0.58)

and (see Eq. (10.52)) in the same axes the diamagnetic contribution is:

10 Magnetic Properties of Metallomesogens 41 1

Figure 10-7. A more realistic model for describing the magnetic properties of PMLCs.

(10.59)

Thus, for the total molecular magnetic susceptibility we obtain:

(1 0.60)

From Eq.(10.53) the magnetic susceptibility of a domain as a function of the

1

2 X M = X d + X p ; AxM = Axd--hxp ; 6xM = 6xd+Axp

order parameters is given by:

412 l?.J Alonso

X D =XM=Xd+Xp

1 1

2 2 AxD = Sh~d-- ( (S+U)hx~-- U8xd (10.61)

Borchers and Haase [33] performed their calculation using Eq. (10.61) under the assumption of an axial system of axial molecules ( T = U = I/ = 0), so:

A x D = S A x d - - A x p (1 0.62)

This hypothesis has been commonly accepted, but some data have recently appeared which indicate that this axiality simplification is not accurate [31, 32, 491. The calculation of the magnetic susceptibility of a domain is closely related to the orientation of a domain in a dc magnetic field, which will be discussed in the following section.

10.7 Orientation of PMLC by Magnetic Fields

It is a well-known fact that a liquid crystal can be macroscopically aligned by applying a moderate magnetic field. Consequently, the measured magnetic susceptibilities correspond to a particular principal value of xD. This phenomenon, widely studied for conventional organic liquid crystals, is described as a coupling between the magnetic field and the director [ 5 -71, and it is a consequence of a cooperative effect of molecular orientation in a domain (the molecules have orientational order as a result of anisotropic interactions associated with the order of the mesophase) and by the anisotropy of the magnetic susceptibility of the domain. If no such interaction between the molecules existed, no orientational order would be observed. This important point will now be illustrated numerically. The diamagnetic contribution of the molecular magnetic anisotropy to the overall anisotropy is mainly provided by aromatic rings. The susceptibility of the benzene molecule is xI1 = -91 x 10- cm3 mol-' and x L = -37x 10-%m3 mol-' (where I I and I stand for the directions parallel and perpendicular with respect to the six-fold molecular symmetry axis). The difference in energy between a parallel and a perpendicular orientation (the latter being the most favorable) in a magnetic field of 1 T is (in temperature units) about 5 x 1 O P 5 K. In the case of a mesogenic molecule with four or six aromatic rings, even in the most favorable arrangement, this energy of anisotropy is 0; = K at most. In consequence, for a typical mesophase temperature (in


the 300 - 500 K range), the ratio between the probabilities for a parallel and a perpendicular orientation (to the magnetic field) of an isolated molecule differs from unity by a factor of less than The isolated molecules are not oriented by an external magnetic field, thus, has been pointed out by Dekker [6] and de Gennes [5], molecular orientation in mesophases by application of a magnetic field is a cooperative phenomenon among = lo8 or more molecules in a domain. The orientation in the mesophase is thus driven by the anisotropy of the domain magnetic susceptibility (Eq. (1 0.53)) and, consequently, by the order parameter.

In most calamitic organic liquid crystals A x M > 0 and, from Eq. (10.54), A x D > 0 for the conventional (N, SA, Sc . . .) phases, so the domains usually orientate with the director parallel to the magnetic field. The behavior observed in experimental studies of metallomesogens is more complex and situations were found where the director orientates either parallel or perpendicular to the magnetic field [ 15 -211. A clear explanation for this behavior has not been given until now.

It is worth noting that in the case of nonparamagnetic metal-organic liquid crystals, such as some nickel@) complexes [16, 191, the behavior is the same as that found in organic liquid crystals. The difficulty arises when PMLC are considered. Here, it is necessary to consider the contribution due to the paramagnetism of the metal, besides the diamagnetic Eontribution to the anisotropy. The contribution of the metal, in a great number of cases, can be directly estimated from EPR measurements. For a typical temperature of 350K, the paramagnetic contribution to the energy of anisotropy per molecule (in temperature units) is given by:

O&(K) = 1 . 5 9 ~ 10-4(gT, -g:) (10.63)

For the most common compounds studied values are found to be 10-4K for copper(I1) (gli -2.25, g, =2.05), and - 2 x lop5 K for oxovanadium (gll - 1.95, g, = 1.98). In these compounds, the orientation in a magnetic field also has to be a cooperative phenomenon in a domain containing at least lo8 molecules. It is noticeable that the signs of the anisotropy in copper (AxL>O) and oxovanadium (Ax; c 0) compounds are opposite.

In order to discuss the behavior of PMLCs, the families shown in Table 10-6 (see p. 414) will now be considered. All these groups contain compounds with conventional calamitic phases whose orientations in a magnetic field have been studied (see Table 10-7, p. 415). While all oxovanadium systems orientate with the director parallel to the magnetic field, some copper compounds orientate with the director parallel (Ax > 0) and others with the director perpendicular (Ax < 0) to the magnetic field (see Table 10-6).

The orientation of the domains in the external magnetic field depends on the relative diamagnetic and paramagnetic contributions to the anisotropy of susceptibility, and some general conclusions can be drawn from the results given above:

i) All oxovanadium compounds orientate with their director parallel to the magnetic field. Since AxY<O from Eq. (10.61) if follows that:

41 4 I? J. Alonso

Table 10-6. Some types of metallomesogens studied.

Anisotropy for M = Cu

1

I1

111

1v

V

RO

OR Q-CI 0

Ax<O

A x > O

(10.64)

and, in a uniaxial calamitic phase (U = 0 and S > 0), a value for AxD > 0 is obtained in all cases.


Table 10-7. References for copper and oxovanadium compounds of the types collected in Table 10-6.

Type Metal References

I c u c u

I1 c u c u c u vo vo

111 c u IV c u

c u vo vo vo vo

V c u c u

R' = CnHZni- I R' = C,Hl,

n = 1-10 n = 5 , 10 n = 1, 5, 10 n = l n = 5 , 10

n = l

R, = OCH,

R, = OCH, R, = OC,H5 R, = OC5Hll

R, = OCIOHZi

R, = OC,OH,, R, = CnHZn+ I n = 2 , 5 R, = CnH,n+l n = I - to

ii) For copper(I1) compounds the orientation depends on the number of aromatic rings in the molecule (4 or 6). When four rings are present in the molecule, it also depends on the type of molecular structure (type I11 versus types I and 11). For Cu" AX: > 0 and from Eq. (1 0.61) we obtain:

For a uniaxial calamitic phase (U=O and S>O) it follows that:

(1 0.65)

(10.66)

The behavior of the compounds containing six aromatic rings per molecule (types IV and V) can be understood as a consequence of the high value of the diamagnetic contribution to the anisotropy of the susceptibility in comparison with the paramagnetic contribution. However, the differences in the behavior of compounds of types I and I1 compared with those of type 111 cannot be explained using this simple model.

416 l?J Alonso

Figure 10-8. Two models for the molecular structure of the compounds collected in Table 10-8 sug-

a b gested by Marcos et al. Adapted from [17].

In order to account for these differences it is necessary to consider the molecular structure of the mesogenic compounds in more detail. Marcos et al. [17] dia- grammatically represented the complexes collected in Table 10-6 and classified them according to the two diagrams shown in Fig. 10-8.

Compounds of type I1 present a more linear structure which is represented in Fig. 10-8a. These systems can be described as a structure formed by a long calamitic mesogenic unit with two lateral substituents. On the other hand, the molecular structure of compounds of types I, 11, IV and V are represented in Fig. 10-8b. These systems consist of two rod-like mesogenic units laterally bound by the central metal core. The shape of such molecules differs from that of calamitic molecules and it can be envisaged more accurately as an orthorhombic prism. The mesophases formed by this type of compound cannot be described as classical calamitic uniaxial phases (i.e. the order parameter U can take values different from zero). Indeed, X-ray diffraction measurements on compounds of this type [15, 531 indicate the existence of some correlation in the Cu - Cu distance, which strongly suggests that the packing of the mesogenic molecules is laterally ordered. In addition, EPR measurements [32, 491 on copper(I1) compounds also indicate a certain degree of order of the molecular axes is the smectic phases.

Bearing these ideas in mind, the difference in the orientation in a magnetic field between type I11 compounds as opposed to types 1 and I1 can be analyzed using Eqs. (10.65) and (10.66). In type I11 compounds the anisotropy of the magnetic susceptibility is given by Eq. (10.66), whereas for types I and I1 there is an additional negative contribution:

(1 0.67) U 2

- - (AX? + SX;)

which must be added as a consequence of both the molecular structure and the more ordered molecular packing in the mesophase. This contribution is responsible for the negative sign of the anisotropy of the magnetic susceptibility (see Table 10-6). In compounds of type IV and V, these effects are compensated by the large number of

I0 Magnetic Properties of Metallomesogens 41 I

aromatic rings. In any case, the additional ordering induces a diminution in the total anisotropy of the magnetic susceptibility. For instance, a unique orientation in a magnetic field of the type IV copper compound in which R, = CIOH2, and R2 = 0-CI0H2, has not been observed [19].

Finally, the possibility of local molecular order in the isotropic phase [49], supported by recent EPR measurements, must be considered. According to the previous discussion, this result implies the existence of small domains (with a number of molecules higher than lo8) with order parameters different from zero (see Eq. (10.61)). This would indicate the existence of some complex molecular organization even in a macroscopically isotropic phase. It would also explain the discrepancies found by Chandrasekhar et al. [29] in the magnetic susceptibility measurements of compound DK3 discussed previously.

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[3] A. H. Morrish, The Physical Principles of Magnetism, John Wiley, New York, 1980. [4] R. L. Carlin, Magnetochemistry, Springer Verlag, Berlin, 1986. [5] P. G. de Gennes, The Physics of Liquid Crystals, Clarendon, Oxford, 1974. [6] A. J. Dekker, Liquid Crystal Physics I. Lecture Notes, University of Groningen, Gron-

[7] G. Vertogen, W. H. de Jeu, Thermotropic Liquid Crystals, Fundamentals, Springer

[8] A.M. Giroud-Godquin, P.M. Maitlis, Angew. Chem. Int. Ed. Engl. 1991, 30, 375-402. [9] P. Espinet, M.A. Esteruelas, L.A. Oro, J. L. Serrano, E. Sola, Coord. Chem. Rev. 1992,

[lo] D. W. Bruce, InorganicMaterials (Eds.: D. W. Bruce, D. O’Hare), Wiley, New York, 1992,

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[I61 M. Marcos, P. Romero, J. L. Serrano, J. Chem. SOC., Chem. Commun. 1989, 1641 - 1643. [I71 M. Marcos, P. Romero, J. L. Serrano, J. Barbera, A.M. Levelut, Liq. Cryst. 1990, 7,

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119- 126.

418 €?I Alonso

[22] J. Owen, E. A. Harris, Electron Paramagnetic Resonance (Ed.: J. G. Geschwind),

[23] A. Bencini, D. Gatteschi, EPR of Exchange Coupled Systems, Springer Verlag, Berlin,

[24] B. Bleaney, K.D. Browers, Proc. Roy. SOC. 1952, A214, 451 -465. [25] A.M. Giroud-Godquin, J. M. Latour, J. C. Marchon, Znorg. Chem. 1985,25, 4452-4454. [26] W. Haase, S. Gehring, B. Borchers, Mat. Res. SOC. Symp. Proc. 1990, Vol. 175. [27] Y. Galyametdinov, G. Ivanova, K. Griesar, A. Prosvirin, I. Ovchinnikov, W. Haase, Adv.

[28] A.M. Giroud-Godquin, G. Sigaud, M. F. Achard, F. Hardouin, J Phys. Lett. 1984, 45,

[29] S. Chandrasekhar, B.K. Sadashiva, B.S. Srikanta, Mol. Cryst. Liq. Cryst. 1987, 151,

[30] M. P. Eastman, M. L. Horng, B. Freiha, K. W. Sheu, Liq. Cryst. 1987, 2, 223 -228. [31] P. J. Alonso, M.L. Sanjuan, P. Romero, M. Marcos, J. L. Serrano, J Phys. Condens.

[32] J. 1. Martinez, Doctoral dissertation, University of Zaragoza, Zaragoza, Spain, 1994. [33] B. Borchers, W. Haase, Mol. Cryst. Liq. Cryst. 1991, 209, 319-328. [34] E. Campillos, Docioral dissertation, University of Zaragoza, Zaragoza, Spain, 1993. [35] B.N. Figgis, M. Gerloch, J. Lewis, R.C. Slade, J Chem. SOC. (A) 1968, 2028-2038. [36] I. G. Bichantaev, Yu. G. Galyametdinov, I. V. Ovchinnikov, J . Struct. Chem. 1987, 28,

[37] C. Carfagna, U. Caruso, A. Roviello, A. Sirigu, Makromol. Chem., Rapid Commun.

[38] P. J. Alonso, J.A. Pukrtolas, P. Davidson, B. Martinez, J. I. Martinez, L. Oriol, J. L. Ser-

[39] J. S. Moore, S. I. Stupp, Polymer Bull. 1988, 19, 251 -256. [40] M. Marcos, L. Oriol, J. L. Serrano, P. J. Alonso, J. A. Puertolas, Macromolecules 1990,

[41] F. Zuo, 1. Yu, M. B. Salamon, X. Hong, S.I. Stupp, J . Appl. Phys. 1991, 69, 7951-7953. [42] H. Goldstein, Classical Mechanisms, Addison-Wiley, Reading, Massachusetts, 1959,

[43] A. R. Edmonds, Angular Momentum in Quantum Mechanics, Princeton University

[44] J. P. Straley, Phys. Rev. 1974, A 10, 1881 - 1887. [45] W. Maier, A. Saupe, Z. Natur$ 1959, 14a, 882-889. [46] R. Alben, J. R. McColl, C.S. Shih, Sol. Stat. Commun. 1972, 11, 1081 - 1084. [47] W. Haase, B. Borchers, Magnetic Molecular Crystals (Eds.: D. Gatteschi, 0. Kahr, J. S.

Miller, E Palacio), NATO AS1 Series E Vol. 198. Kluwer Academic Publishers, London,

[48] I. G. Bichantaev, R. M. Galimov, I. V. Ovchinnikov, Theor. Exp. Chem. 1988, 24, 360- 364. [49] P. J. Alonso, M. Marcos, J. I. Martinez, V.M. Orera, M. L. Sanjuan, J.L. Serrano, Liq.

[50] M. Marcos, P. Romero, J. L. Serrano, C. Bueno, J.A. Cabeza, L.A. Oro, Mol. Cryst. Liq.

[5 11 P. Romero, Doctoral dissertation, University of Zaragoza, Zaragoza, Spain, 1992. [52] M. Marcos, P. Romero, J. Serrano, Chem. Mat. 1990, 2, 495-498. [53] E. Campillos, M. Marcos, J. L. Serrano, J. Barbera, P. J. Alonso, J.I. Martinez, Chem.

Plenum, New York, 1972.

1990.

Mat. 1992, 4, 739-741.

L 387 -L392.

93 - 107.

Mutter 1990, 2, 9173-9182.

685 -691.

1987, 8, 345-351.

rano, Macromolecules 1993, 26, 4304-4309.

23, 5187-5191.

Chap. 5 .

Press, Princeton, New Jersey, 1968.

1991, pp. 245-253.

Cryst. 1993, 13, 585-596.

Cryst. 1989, 167, 123- 134.

Mat. 1993, 5, 1518- 1525.

11 Other Physical Properties and Possible Applications of Metallomesogens

M. Blanca Ros

In this chapter, different physical properties of metal-containing liquid crystals will be discussed. To begin with, a number of general aspects regarding metallomesogens will be covered, followed by a treatment of optical and electric behavior. Along with the magnetic properties, these two types of physical response have so far been the most widely explored in metal-containing liquid crystals. Finally, other relevant properties and applications studied in this kind of material will be discussed.

Although they do not constitute true metallomesogens, a number of materials in which metals and liquid crystal order are involved have been reported in the literature. In these cases, the metal atoms act more as an “additive”, but the influence of the metal on the physical properties of the new materials, compared with those of the metal-free analogs, provides such benefits that they deserve being mentioned in this chapter. A number of these borderline materials will be covered throughout this chapter.

Information is given about the physical behavior and characterization methods used for both low and high molecular weight thermotropic and lyotropic metallomesogens. Due to the fact that many of the physical properties considered in this chapter are closely related to potential applications, some practical possibilities for metallomesogens are also indicated where appropriate.

In contrast, a discussion of results obtained from more common spectroscopic studies of the mesomorphic state (IR, NMR, etc.) [I], which have mainly provided structural data, is not included.

11.1 General Aspects

In the previous chapters, the most important and fundamental physical property of metallomesogens, which is the raison d’etre of this branch of liquid crystal science, has been discussed in detail, and that is the formation of mesophases.

420 M.B. Ros

In a similar way to organic liquid crystals, physical parameters such as melting and clearing points, temperature and concentration range of the mesomorphic states, transition enthalpy and entropy changes, or identification of the mesophase formed are the most basic features of metallomesogens to be determined. For this purpose, the use of well known techniques such as polarizing optical microscopy, X-ray diffractometry and calorimetry is indispensable. A number of these fundamental parameters in metallomesogens will be briefly discussed in the following para- graphs.

Generally speaking, the majority of thermotropic metallomesogens exhibit melting points of around 100°C or higher. While in organic liquid crystals the objective is often to increase the intermolecular interactions in order to generate mesomorphic order, the problem in metallomesogens is to prevent these interactions from being so strong that they lead to undesirably high melting temperatures. Fortunately, as has been seen in previous chapters, the skills of the designers of metallomesogens are producing significant progress in this direction.

The metal atom makes an important contribution to molecular interactions in the solid state via its d-orbitals and, fortunately, in the liquid crystal arrangement, too. Thus, metallomesogens have been described which exhibit mesophases over a temperature range of 100- 150°C (or even larger) before clearing to the isotropic liquid. Examples have been reported which clearly demonstrate the role of the metal in stabilizing mesophases [2]. Nevertheless, the high transition temperatures often affect the stability of the material, revealing one of the main drawbacks in some of these compounds. The thermal instability inherent in a number of thermotropic materials is comparable to the chemical instability observed in many lyotropic systems.

In order to be useful as advanced materials, metallomesogens must have reasonable thermal stability and also be inert. Since complexes all have a metal center which can act as a reactive site towards moisture or air, inertness and stability are subjects which need to be addressed. At this stage, it should be pointed out that remarkable advances in terms of the stability and temperature range of mesophases have been achieved in recent years. This fact suggests that metallomesogens could surely match the performance of organic liquid crystal materials.

Metal-containing liquid crystals have offered the opportunity to detect and study a curious thermodynamic phenomenon, the so-called “multiple melting” [3] and “multiple clearing” behavior [4]. These are kinetic effects based on the existence of metastable phases which melt (or clear) at lower temperatures than the thermodynamically stable phases. Multiple melting behavior has been reported with increasing frequency and has been extensively studied by Ohta et al. [3a-d].

As far as other thermal properties are concerned, it can be seen from the data available that, in general, the phase transitions in metallomesogens involve rather similar enthalpic trends and magnitudes [ 1 b, 51 to the ones determined for the same phase transitions in organic liquid crystals. In fact, these values have been used by some authors to support, or draw conclusions about, the mesophase assignment [6]. Nevertheless, few comparative studies and relevant discussions about the enthalpy

I1 Other Physical Properties and Possible Applications of Metallomesogens 42 1

and entropy changes [7] or the order parameter [7 c, 7 e, 7 f, 81 in metallomesogens and their parent ligands have been reported to date. Therefore, conclusions regarding these trends are difficult to draw.

The optical textures of these new materials mainly resemble those already observed for calamitic or columnar mesophases produced by organic liquid crystals. However, in their routine characterization, it can easily be seen that metal-containing systems are generally more viscous than purely organic mesophases. This is a rather qualitative consideration and quantitative measurements have scarcely been performed.

The high viscosities of metallomesogens are certainly a limitation for fast-switching device applications. However, the lower molecular mobility is probably responsible for quite a common phenomenon observed in these compounds. A number of publications on metallomesogens report that many of these compounds are able to preserve the macroscopic mesophase structure on solidification by freezing [7 c, 91, and interestingly, more and more glass transitions in low molecular weight metal-containing liquid crystals are being detected [7 f, 101. Consequently, it is possible to obtain frozen liquid crystalline order below the melting temperature, which increases the range of control over molecular order. These results “open new and unexplored possibilities for these low molecular weight materials in fields such is nonlinear optics or recording techniques in which glassy states are used” [ I l l .

Liquid crystalline order is mainly based on weak van der Waals interactions that involve polarizable electron density. It is well known that metal atoms have a high density of polarizable electrons. Due to this polarizability, the introduction of metal centers has become established in the field of liquid crystals as a means to increase the overall molecular polarizability. Furthermore, this presence of metal centers will not only influence the mesogenic character, but also other important liquid crystal properties such as birefringence or dielectric constants.

Quantitative data regarding the positive contribution of metals to a higher polarizability in metallomesogens can be given by the mean polarizability (a) and the polarizability anisotropy ( A a ) of such structures. While the mean molecular polarizability results from the contribution of the electronic, ionic and dipolar polarizations, Aa arises from the differences between the longitudinal and transverse molecular polarizabilities [ 121.

In Table 1 1 - 1 these parameters are shown for a number of iridium, palladium, platinum [I 31, silver [14], and copper [7 fl metallomesogens, whose structures are shown in Fig. 1 1 - 1 (p. 423). For the sake of comparison, the data corresponding to the parent organic ligands are also shown. These data were determined by refractive index studies, using isotropic liquid solutions for 0 [I31 and in the mesophase either pure samples [7fl or mixtures [13] for Aa.

If we compare the polarizabilities of these metallomesogens with the values of conventional organic liquid crystals, such as their parent ligands, we can deduce that the inclusion of a metal into a liquid crystalline system does lead to an increase in the overall polarizability. However, other structural effects must also be taken into account.

For materials of type I (IrLCl(CO),), the complexation of one ligand to a small metal-containing moiety gives rise to a significant enhancement of both mean

P

N

N

Tab

le 1

1-1.

Pol

ariz

abil

ity

data

mea

sure

d fo

r a

num

ber

of m

etal

lom

esog

ens

(Fig

. 1 1

-2)

and

thei

r co

rres

pond

ing

liga

nds

(L).

Com

poun

d M

etal

R

a

n a

10-4

0 h

a x

R

ef.

% Ia

ir

n-

CnH

Tn+

l -C,H

,-C

H=

CH

- 5

59

26

5

42

10

13

PJ

Ir

n-C

,H,,

+ - C

,H, - C

H =

CH

-

7 6

2k

6

59

i 12

13

a

R

[J-'

C2 m

2]

[J-'

C2

m-'

1

Ib

Ic

ir

n-C

nHzn

t I -C

GH

,-C

H=

CH

- 9

60

k6

4

9i 1

0 13

II

a [X

: O

Tfl

b

Ag

n-C

,H,,

1 - C

6H4 - C

H =

CH

-

6 8

2k

8

d 14

Il

b [

X:

OT

fl

Ag

n-C

nHzn

+l -C

,jH

,-C

H=

CH

- 8

92

k9

d

14

IIc

[X:

DO

S]'

Ag

n-C

,Hz,

+

I - C

6H4 - C

H =

CH

-

10

14

k1

4

d

14

LI

and

LII

5,

7,

9 3

5*4

e

28

+6

' 14

I1

1 a

Pd

n-C

SH, 1

- C

6H4 - C

6H4 -

82

+8

d

13

111 b

P

t n-

C,H

, I - C

6H4 - C

,H4 -

90

f 10

d

13

I11 c

P

d n-

C,H

,, -C

,HIo

-C,H

,-

86

k9

3

9i8

13

I1

1 d

Pd

~-C

SH

1 1

- C

6H I o

- C

6H I o

-

79

+8

3

0+

6

13

LII

Ia a

nd L

IIlb

37

.5

19.4

13

L

IIIC

36

.2

16.0

13

L

IIId

35

.5

12.6

13

IV

c

u

n-C

6H13

-

7 13

5 k 5

- 5

9.1

7f

LIV

(H)

58.5

f 0.

5 -2

9.8

7f

(a)

- C

&-

= 1

,4-d

isub

stit

uted

be

nzen

e, - C

6Hl0

- =

1,4-disubstituted-trans-cyclohexyl, (

b)

OT

f:

CF,

SO;

, (c

) D

OS:

n-

C

,,H,,O

SO;,

(d)

Dat

a no

t av

aila

ble,

(e)

Ave

rage

dat

a of

lig

ands

wit

h n

= 5

, 7,9

.

11 Other Physical Properties and Possible Applications of Metallomesogens 423

R <N -[: CO Rc 0 0 * 3 R \ /

\ /N-Ag-N

I II

111 IV

Figure 11-1. Chemical structure of the metallomesogens whose polarizability parameters are reported in Table 11-1.

polarizability and polarizability anisotropy, with values more than twice as high as those determined for the ligands. In the symmetric compounds (type I1 (AgL2X), 111 (ML,Cl,) and IV CuL2) which contain two ligands, the magnitude of d and Act are also around twice the value of those for the ligand, but this indicates an additive behavior rather than an increase due to complexation.

The high polarizabilities found in complexes of type I have been used to explain the significant stabilization of their liquid crystalline order (higher mesophase-isotropic liquid transition temperatures) in comparison with the ligands; both ligand and complexes have similar melting points [15]. Despite the lack of similar measurements for related complexes containing 4-pyridylbenzylideneanilines as the ligand (Ligand: NC5H, - C H = N - C6H4 - OC,H2,+ ,, complexes: [MLCl(CO),]), the mesomorphism of their iridium and rhodium derivatives can be explained in a similar manner. Interestingly, in this case, the ligands do not show liquid crystalline behavior [161.

11.2 Optical Properties

The interaction between liquid crystals and light is one of the most interesting, and certainly the most beautiful, feature of the mesomorphic state. How light is affected by liquid crystals is also the basis for most applications of thermotropic materials. In the following section, the most outstanding optical properties of metallomesogens explored to date will be considered.

424 M.B. Ros

11.2.1 Birefringence

Birefringence is related to the polarizability, discussed previously, and also to the anisotropic nature of the molecules.

It is well known that when light passes through an anisotropic uniaxial mesomorphic medium, the beam is split into two perpendicularly polarized waves which prop- agate at different velocities (v). This phenomenon is caused by the fact that the medium possesses two principal refractive indices, denoted n, and no (also called nll and n I respectively) according to whether the plane of polarization of the light is parallel or perpendicular to the optic axis (n = d v , c : velocity of light in a vacuum). In the case of uniaxial liquid crystal phases, the optical axis is indicated by the director.

This phenomenon is most obvious when a liquid crystal is placed between the crossed polarizers of a microscope, which allows their basic characterization by means of the textures observed.

The difference between the two principal refractive indices is termed the birefringence (An = n, ~ no). In practice, if n,> no, An it said to be positive and, in the opposite case, it is negative. The majority of organic liquid crystals have a positive birefringence and, as an example, technologically important nematic liquid crystals have a positive An of around 0.1 -0.2. However, for chiral nematic phases and discotic materials the birefringence is negative [ 171.

The refractive indices in uniaxial organic liquid crystals are dependent on the wavelength of the incident light and the temperature and they have been proved to be pri- marily governed by the constituents of the liquid crystal. For visible light, no (nl) in thermotropic phases is close to 1.50, and it is normally not strongly dependent on the molecular constituents. The value of no decreases as the wavelength of the light increases and also increases slightly with temperature; more pronounced changes occur only near the phase transition. On the other hand, in these materials n, (rill) is very dependent on the molecular constituents; it varies from 1.5 for a saturated compound to around 1.9 for highly conjugated systems. As the temperature rises, or the wavelength of the light increases, this refractive index decreases steadily [ 181.

Very similar trends and magnitudes have been observed for the refractive indices of the metallomesogens studied so far.

Positive birefringence values in the range of 0.10-0.40 have been estimated for a number of materials (Fig. 11-2) in the nematic phase. The compounds were studied by a variety of techniques [7 e, 7 f, 19, 201, using a He-Ne laser light source (623 nm) with the materials contained in aligned cells (director parallel to the glass surfaces) [21]. In some cases, materials with significantly high optical anisotropies at low temperatures were found [ 19 b]. In addition, an odd-even effect, similar to that reported for organic liquid crystals, was observed in the only homologous series studied to date [19b].

The effect of introducing a metal atom clearly depends on the structural characteristics of the new complex. Indeed, complexes have been reported with birefringence values similar to [7e, 7f, 19b] (see Fig. 11-3, p. 426) or higher [19b] than those of the parent ligands, but examples have also been found with lower values [20].


R : C6H13O-CsHqCH=CH- R : CH,O-C6H4-CH=CH-

no: 1.55 n, : 1.77 ref. 19b no: 1.54 n,: 1.75 ref. 19b

An : 0.18 (TC - T : 7 "C) An : 0.21 (TC - T : 24 "C)

R : CmH2m+10-C6H4-C6H4- ref. 19b

[m=6] no: 1.61 n,: 1.81

An : 0.20 (TC - T : 18 "C)

[ mixture 1: l : l m = 2,4, 6 ]

no : 1.63 n, : 1.96

( room temp.) An : 0.33 (Tc - T : 64 "C) An : 0.4

nP u v-6' '13

A

1 1 OCsH13

Figure 11-2. Refractive indices and birefringence data measured for some metallomesogens in the liquid crystalline phase (T,: temperature of the isotropic liquid-nematic mesophase transition, - C,H, - = 1,4-disubstituted benzene, - C,H,o - = 1 ,Cdisubstituted-trans-cyclohexyl).

At this point, it is worth mentioning that, close to an absorption band, the refractive index shows anomalous behavior [22]. As a consequence, the birefringence increases significantly approaching an absorption in the UV or visible zone range of light. Therefore, colored materials such as many of the ones cited above, and most other metallomesogens, generally show high birefringence.

However, many optical and electrooptical applications use light in the visible part of the spectrum. This means that the absorption bands of metallomesogens should be below 400 nm. For this reason, it is important to choose the most suitable organic ligands and metal centers which give rise to colorless complexes. This should help to obtain materials with potential applications in the fields of polarizers and retardation optics, or polymer-dispersed liquid crystals where a high An gives rise to effective scattering. However, research in this direction has not been reported so far.

The birefringence of a material, at a particular temperature, is related to the anisotropy of polarizability, the molar density and the degree of molecular order. In short, this property, as for any other macroscopic properties, depends on the order parameter S ( A n a s ) [18]. As a result, in addition to their significance for applications, refractive index evaluations can provide useful information about the structure of the

426 M.B. Ros

1.70

I 1.65

n

1.60

1.55

C& G * W p - c , H , , 0-p.0

C ~ H I ~ - N a >C,H,

-

-

-

-

I I I 50 100 150

liquid crystal phases at both the bulk and the molecular levels and this has also been discussed in context with metallomesogens [7 fl.

11.2.2 Biaxiality

The above discussion concerned uniaxial materials in which there is only one optical axis along which a plane polarized light wave travels without its state of polarization being changed. However, the search for biaxial liquid crystals, that is compounds with two optical axes such as the gypsum crystal, has been an attractive challenge for a long time. Indeed, the search for biaxial nematic compounds has been considered as “The Holy Grail” in the field of liquid crystals.

The inherent interest in materials with two optical axes, which mainly concerns thermotropic systems, is twofold. Academic interest arises because several theoretical

I1 Other Physical Properties and Possible Applications of Metallomesogens 427

and computational approaches have predicted both its existence and its transitional parameters [231. The practical interest in these materials stems from the possibility of switching the molecules around not one, but two different inertial axes, which should have an advantageous effect on the response times.

Curiously, the first low molecular weight biaxial nematic liquid crystal described was a metal-based lyotropic system (potassium laurate/ 1 -decanol/D,O) [24]. In the late 1980s, Chandrasekhar reported the first thermotropic biaxial nematic liquid crystal, which has the structure shown in Fig. 11-4 [25a].

K ( 168.5 "C Nbiaxial ) 186.5 "C I

Figure 11-4. Chemical structure of the first thermotropic biaxial nematic metallomesogen.

The structural design of this compound supported the idea that in order to achieve thermotropic biaxial mesogens, the features of both rod-like and disc-like molecules should be combined. To this aim, the structural varieties possible in metallomesogens offered an attractive approach.

The nematic phase of the P-diketonatocopper(r1) derivatives and their binary mixtures [25 b] was studied by conoscopy, using samples which were homeotropically aligned in cells (director perpendicular to the glass surface [21]) by application of an ac electric field. On switching from the orthoscopic to the conoscopic condition, the biaxiality was visible in both pure and mixed samples. The cross produced in the extinction position split into two characteristic hyperbolic isogyres upon rotation of the microscope stage [25, 261. The authors were able also to demonstrate a reversible uniaxial-biaxial nematic phase transition (Nu - Nb).

Since then, other nematic metallomesogens with structures similar to those reported by Chandrasekhar have been prepared, but they were either shown to be uniaxial [27] or remained unstudied.

Other examples of conoscopic characterization have been reported in other, structurally different, metallomesogens. Ghedini et al. [7 c] published photomicrographs

428 M.B. Ros

showing a very slightly split cross in the nematic phase of a dinuclear azopalladi- urn@) complex. In addition, based on calorimetric and order parameter studies, these authors claim the biaxial character of the mesophase. The metallomesogen described above has a value of ASN-, = 0.35 J K-' and a major order parameter (P2) = 0.2 near the transition to the isotropic liquid. Both data are lower than those found in the corresponding ligand (3.5 J K - ' and 0.32 respectively) and are close to the values theoretically predicted for biaxial molecules [23 b, 261.

Likewise, Pyzuk et al. [7e] commented on the biaxial birefringence observed in the nematic phase of enaminoketone-copper complexes, but the degree of biaxiality was considered negligible as it is lower than 0.003.

Along with optical observations, other experimental techniques are used to explore the biaxial orientational order of mesophases: X-ray diffraction, NMR and EPR spectroscopy [28]. Based on some of these techniques, the occurrence of biaxial order in a number of mesophases of metallomesogens has been reported [7c, 291. How- ever, as in the cases described previously, the significant data regarding the biaxial nature of the phase have not been determined and, in a number of cases, only local order has been described [29b-d].

Different research groups, working in both fields of organic liquid crystals and metallomesogens have been devoting a great deal of effort to the search for a biaxial nematic material. However, this biaxial phase remains elusive and it appears that metallomesogens have not, as yet, lived up to their promise.

To date, irrespective of the chemical nature of the compounds, there have been few accounts of low molar mass thermotropic biaxial nematic liquid crystals [30]. Fur- thermore, some controversy exists regarding the proper method of characterization of some systems [31].

11.2.3 Dichroism

From the point of view of optical properties, one of the attractive characteristics obtained by the introduction of a metal atom into liquid crystals is the possibility of obtaining colored materials. But why color in liquid crystals? The answer to this question lies in the necessity for color in a number of applications of liquid crystals: passive blocking filters, laser addressed devices, polarizers based on dichroic effects or the utility of thermochromism. Likewise, many host-guest devices (displays, shut- ters, modulators) use dyes [21].

A dye is a substance that absorbs light of a certain wavelength, causing the light reflected from, or transmitted through the dye to appear colored. Some dye molecules absorb light of a certain wavelength more efficiently when the light is polarised along one axis of the molecule, and these systems are said to be dichroic. Dichroic materials are the most suitable to be used in some types of liquid crystal display. By applying an electric field to a mixture of a liquid crystal and a dye, both the liquid crystal and the dye molecules are reoriented (see Fig. 11-5) [21,32]. High contrast


FUFST-HOST SYSTEM WITHOUT FlFl D

DYE MOLECULE LIQUID CRYSTAL (HOST) /

deeply coloured

GUEST-HOST SYSTEM WITH FIELD

hv -* 000

(white light) Oo0O oooo -

h-

Figure 11-5. Dye-liquid crystal interactions in a host-guest nematic display. (Adapted from reference [32]).

ratios are obtained in these displays when the dye molecules have high order parameters within the mesophase matrix.

To be a good candidate as a dye, certain criteria must be met: high extinction in the desired wavelength region, dichroism, solubility in the host, chemical and thermal stability and a high order parameter in the liquid crystal phase. In comparison with the organic dyes usually employed (nonmesomorphic azo and anthraquinone derivatives) [33], inherently colored metallomesogens would provide higher solubili-

430 M.B. Ros

ties in the host, as they are liquid crystalline in their own right. They should lead to an increase in the absorbance of the mixture without affecting the transition temperatures. Moreover, they should also orient significantly better within the mesophase, resulting in high order parameters. Finally, the elongated structure of many metallomesogens should give rise to a positive dichroism.

With this aim in mind, researchers were prompted to explore the dichroic characteristics of metallomesogens, namely some dithio complexes of nickel, palladium, and zinc [34,35]. The chemical structures and the most relevant properties of thew metallomesogens as dyes are shown in Fig. 11-6 and Table 11 -2.

M : Ni, Pd, Zn

VI

Figure 11-6. Chemical structure of dichroic metallomesogens whose properties are given in Table 11-2.

The dichroic properties of dyes are usually described by two parameters: the optical order parameter and the dichroic contrast ratio. The optical order parameter (So,,) is defined as

where A I I and A I are the absorbances when the polarisation direction of the incident light is parallel and perpendicular to the alignment direction respectively. The dichroic contrast ratio (Rd) (also termed as CR or R,,,) is defined as the ratio of these absorbances, Rd = AII/A I [21, 361.

The physical characterization of the metallomesogens studied has been undertaken in homogeneously aligned samples in the nematic phase. Spectra were recorded using UV-Vis or UV-Vis-NIR spectrophotometers fitted with a polarizer parallel or perpendicular to the direction of alignment. Commercially available nematic liquid crystals (KIS and E7 from BDH, MBBA from Aldrich and ZLI2830 from Merck), which do not absorb light in the wavelength range studied, were used as host systems.

Tab

le 1

1-2.

Lin

ear

dich

roic

pro

pert

ies

of a

num

ber

of m

etal

lom

esog

ens

(Fig

. 11

-6)

and

orga

nic

dyes

.

Hos

t a

A,,, in

hos

t S

op

b

Rd

C (T

) nm

("

C)

Sol

ub.

in h

ost

Ref

. vo

w/w

V-N

i 86

0 (2

8 x

103)

e

VI-

Ni-C

,g

375

(1 2

7 x

1 03

)

VI-

Zn-

C,g

37

2 (6

6 x

lo3)

' V

I-Pd

-C,g

43

2 (5

3 x

1 03

)

595

(IS

X 10

3)

335

(55

x 10

3)

D16

k

586

(1

2~

10

3)

D2

' 47

0 (

30

~

lo3)

K15

M

BB

A

E7

E

7

ZL

I 283

0 E

7

E7

E

7

869

887

380'

59

7'

377'

44

0 ' 59

6 49

6

0.57

f - 0.6

- 0

.7

0.5 - 0

.55

0.7 - 0

.8

0.63

0.

75

4.97

(20

) f 5.

1 -6

.0

(30)

7.

0-

10.1

(30

) 4.

2 - 4

.7 (

30)

8.7 - 1

2.8

(30)

6.6

10

< 10

34

<

10

34

< 0.

2 35

<

0.2

35

< 0.

3 35

<

0.2

35

2.2

33 a

f 33

a

(a)

Com

mer

cial

ly a

vaila

ble

nem

atic

liq

uid

crys

tals

. (b

) S

op: (

A ,, -A

L)/

(A

+2

A,)

(m

axim

um v

alue

s);

A:

abso

rban

ces.

(c

) R

,:

AII

/AL

. (d)

Tem

pera

ture

at

whi

ch t

he m

easu

rem

ent

has

been

mad

e. (

e) I

n he

xane

sol

utio

n. (

f) D

ata

not

avai

labl

e. (

g) n

: nu

mbe

r of

car

bon

atom

s of

the

ter

min

al a

lkox

y ch

ain.

For

Ni,

n =

5 -9

; fo

r P

d,

n =

6-

10 a

nd f

or Z

n, n

= 4

- 10

. (h

) D

ata

for

com

plex

w

ith n

= 9

. (i

) R

epre

sent

ativ

e da

ta f

or a

ll c

ompl

exes

of

the

seri

es. (j) D

ata

for

com

plex

in

whi

ch n

= 6

. (k

) A

nthr

aqui

none

dic

hroi

c dy

e. (

1) A

zo d

ichr

oic

dye.

e L

432 M.B. Ros

As can be seen from Table 1 1-2, all the metallomesogens studied exhibit high light absorption in liquid crystalline solution, and the values are even larger than those observed for common organic dyes such as D16 and D2 (anthraquinone and azo derivatives, both dichroic dyes commercially available from Merck Ltd.). Interesting- ly, compound V-Ni absorbs in the near infrared region [34] while the rest of the compounds show temperature-dependent dichroism in the UV-Vis range [35].

The linear dichroic parameters of these metallomesogens are outstanding. The So, values obtained in these mixtures are always positive and very high, close to the values provided by common organic dyes. The estimated So, values for the dithiobenzoate-palladium complexes (structures of type VI in Fig. 11-6) are particularly impressive. In the range of the charge-transfer absorption (432 nm), s,, values up to 0.8 have been measured for the hexyloxyderivative (n = 6) [35]. This high value indicates that the alignment of these molecules in the liquid crystal host is efficient, probably due to their mesomorphic nature. Absorption spectra for the charge- transfer band of the hexyloxy-palladium complex [VI-Pd-Cd are reproduced in Fig. 1 1-7, and a considerable dichroism that varies with temperature can be observed.

400 500

wavelength (nm)

Figure 11-7. Absorption spectra of the complex VI-Pd-C, dissolved in E7. (Adapted from [35]).


The variation of the order parameter with temperature for the blends containing the palladium dye is shown in Fig. 11-8.

As far as the contrast between parallel and perpendicular absorption is concerned, all the metal-containing systems studied show values in the same range as those of organic dyes, making them suitable for host-guest displays even at low concentrations. For example, when a nickel complex of type VI is used as dye, the cell has been described as purple in the absence of an electric field, but when a field is applied, it appears colorless [37].

Figure 11-8. Order parameters as a function of temperature for VI-Pd-C, dissolved in E7. Values for n = 6 (O), n = 7 (+), n = 8 (e), n = 9 (0) and n = 10 (W). (Adapted from [35]).

However, in general, the stability of these mixtures is low. The solutes precipitate out of solution when the samples are left for some period of time, a month in the case of mixtures of complexes of type V with K15 [34], but overnight in the case of the dithiobenzoates (type VI) [35].

Nevertheless, as proof that the interest in this type of compound for use in display devices and thermal recording media still exists, the dithiobenzoate derivatives with nickel and palladium as metal center have been patented by a Japanese company [38]. Furthermore, some of these complexes also exhibit a smectic C phase [39],

434 M.B. Ros

therefore they have been suggested as suitable materials for host-guest type ferroelectric displays [35]. This possibility, however, has yet to be investigated.

11.2.4 Thermochromism

Thermochromism is a well-known and useful property frequently observed in many different types of material [40].

The change of color with temperature is certainly a phenomenon which is of interest in the field of liquid crystals, and many applications are based on this property; thermometers (fever indicators, gadgets, design applications, etc.) or warning signals (e.g., on heaters).

While this type of color change has rarely been observed for nonchiral organic liquid crystals [41], a number of papers have reported thermochromism in nonchiral metal-containing liquid crystals [3d, 15, 16, 27b, 42, 431, however, sometimes in a rather anecdotal fashion [15,16].

The pioneering studies in this area can probably be attributed to Ohta et al., who published photomicrographs demonstrating the thermochromism of a number of palladium(I1) and nickel(@ bis(0ctasubstituted) diphenylglyoximato complexes which form columnar mesophases [42]. These compounds change from red to yellow with increasing temperature due to the blue shift of characteristic absorption bands.

One of these bands is the d - p band associated with the nd,2 - (n + l )p , transition, that is also correlated with the tendency of these molecules to stack in a columnar arrangement. For example, in nickel complexes this absorption changes from 481 nm at room temperature to 424 nm at 245 “C (see Fig. 11-9) [42a]. In addition, the intensities of the absorbances also change with increasing temperature.

The significant energy shift associated with the d-p band has been attributed to an increase in the interdisc distance within the characteristic columnar packing of the mesophase (Dhd) with increasing temperature. Due to the fact that the polarization of this band is parallel to the z-axis of the complex, that is parallel to the columns, the variation in intensity is considered to be a consequence of the gradual change in the columnar arrangement parallel to the UV-Vis light, as illustrated in Fig. 11-10 (p. 436).

Changes in the aggregation of the molecules can also be achieved by changes in the pressure. With this idea in mind, the pressure dependence of this absorption band was also studied for the nickel complexes described above. The dependence is in good agreement with the theory, and a red shift is observed on increasing the pressure [42a]. In fact, nonsubstituted bis(glyoximato)metal(Ir) complexes of nickel, palladium, and platinum have been reported by these authors to have an application as “pressure indicators” [42a].

Nevertheless, absorbance shifts of this type and their relationship with columnar stacking are not confined to glyoximato derivatives. Indeed, such modifications are well known to occur for the characteristic Q-band of phthalocyanines and spectro-

I1 Other Physical Properties and Possible Applications of Metallomesogens 43 5

4

2

1

Figure 11-9. Electronic spectra of a thermochromic discotic

K r.t. Dhdt 2 1 1 O C DMP 232OC I 1

U nickel complex over a range of temperatures. (Adapted from 300 400 500 600 700 800

wavelength (nm) [ 4 w .

scopic data on metal-containing phthalocyanine liquid crystals have also been reported [43].

Interestingly, a different type of thermochromism, of chemical origin, has been observed for nonchiral metallomesogens [3d, 27bl.

For example, a calamitic nickel dithiobenzoate derivative [3d] is blue in the smectic phases, but the color changes to red on the transition into the nematic phase. Howev- er, as the authors and other researchers [39] have proved, this change originates from an intermolecular reaction. On heating, two molecules of the metallomesogen react with each other to give two new molecular structures, accompanied, on further heating, by degradation. Before this degradation takes place, the process is chemically reversible.

The most common thermochromic liquid crystals are those which form a chiral phase, mainly a cholesteric mesophase [44]. In these materials there is twofold optical activity: a) molecular optical activity due to the presence of a chiral chemical

436 M.B. Ros

UV-Visible light

Heat >

j

Figure 11-10. Schematic models of the columnar arrangements in films of Pd complexes for electronic absorption spectra. (Adapted from [42b]).

structure and b) macromolecular optical activity associated with the helical structure of the director. Cholesteric media rotate the plane of polarized light but, more interestingly, a unique feature of the helical structure is observed when the optical wavelength of the material (A’ = rzp) is equal to that of the incident light 1 ( A = np where n is the refractive index of the material and p is the pitch of the helix, both n and p are temperature dependent). In this case, the light is selectively reflected and hence, on changing the temperature, the wavelength of the reflected light is also changed. This phenomenon is easily observed as a change of color occurs when the reflected light is in the visible range of the spectrum. Both phenomena, selective reflection of light and thermochromic behavior of this phase form the basis of many applications 1441.

To date, several cholesteric metallomesogens have been reported but very few data have been reported regarding their physical properties [4a, 45, 461.

The first papers to report physical data on cholesteric mesophases involving metal atoms do not concern true metallomesogens. A chiral nematic phase was induced in mixtures by dissolving a number of chiral D3-symmetry complexes in an organic


nematic liquid crystal [46a]. The purpose of this experiment was to measure the twisting power of these complexes. Depending on factors such as the metal complex, the nematic host, the temperature and the concentration, different magnitudes of pitch and trends for the change in pitch (in the range of 10- 100 pm) were determined.

Shinkai et al. studied systems which are more closely related to metal-containing liquid crystals [46b]. They showed that the inclusion of different alkali metal cations in liquid crystalline steroid-substituted crown ethers did not affect the mesomorphism. But interestingly, depending on the alkali cation and its concentration, the pitch of the cholesteric phase varies (Fig. 11-1 1). Due to the nature of the organic matrix, these pitch changes take place in the visible region of light.

The description of the behavior in these cholesteric materials has not been extended by the authors to include a thermochromic analysis, but it constitutes another

Figure 11-11. Plot of wavelength of maximum reflection versus [MSCN]/[crown ether]. Data measured spectrophotometrical- ly at 27°C. (A, = np, n = mean index of reflection, p = helical pitch). (Adapted from [46b]).

5oa

4811 h

E v

460

440

0 0.02 0.05 0.1 0

[ M%CN ] / [ liq.cryst. ]

438 M.B. Ros

example of color changes. This transformation of a chemical signal to a physical one was proposed as promising in host-guest sensor systems.

The first, and so far the only, attempt to characterize the chirality and the thermochromism of a cholesteric metallomesogen in its mesophase was performed by Espinet et al. on a number of chiral liquid crystalline orlho-palladated derivatives [4a]. Although these complexes exhibit a cholesteric phase in their own right, the authors were forced to determine their chiral properties in induced cholesteric mixtures because of orientational problems with the pure compounds.

By using the Grandjean-Can0 method [47] and the commercial nematic host RO- TN 404 (available from Hofmann-La Roche), pitches between -100 and 200 pm were measured (see Fig. 11-12).

The cholesteric blends were reported to show a divergent temperature dependence of the helical pitch as well as an inversion of the helical twist sense with temperature. A right-handed helix was found at low temperatures, whereas a helix with the opposite twist sense was observed at higher temperatures.

The helical twist changes sense at around 90- 100 "C depending on the molecular length of the chiral component. This type of behavior has also been observed for

?C6H13 oCEH13

I 100

G

oCEH13 OC6H13

-1 00 I ' I . I

-200 I I 1 I I

10 30 50 70 90 110

T("C) - Figure 11-12. Temperature dependence of the cholesteric pitch p for a mixture of 21.2 wt% of the Pd(r1)-complex in the commercially available nematic liquid crystal RO-TN404. (Adapted from [4a]).


structurally quite different organic mesogens, but conclusive explanations regarding the origin of this phenomenon or the chemical structure dependence have yet to be proposed.

11.2.5 Nonlinear Optical Properties

The application of an electric field (E) on a material influences the distribution of charges in atoms and molecules. Consequently, it becomes microscopically and macroscopically polarized. The microscopic (p) and macroscopic ( P ) polarizations can be represented respectively as

p=po+aE+pEE+yEEE+ . . .

P = P~+x"'E+x'~ 'EE+x'~ 'EEE+. . and

The microscopic parameters (p and po) are the induced and intrinsic dipole moments of the molecules (or atoms), a is the so-called polarizability and p, y, etc. are the first, second and higher order hyperpolarizabilities. At the macroscopic level, P is the volume polarization or dipole density, Po is the intrinsic polarization of the material and x ( ~ ) are the susceptibility coefficients that are tensors of the order (i+ 1).

The nonlinear optical phenomenon can be explained if one considers the way in which light, in terms of its electric field (E) , interacts with matter. If the first two terms on the right hand sides of the above equations are sufficient to describe the practical situation, the variations in the field (E) and polarization are linearly related, and well-known effects such as absorption, reflection or refraction occur.

However, when an intense electric field is associated with the incident light (lasers), the coefficients of the terms beyond the first two become significant and these terms do not vary linearly with E. In this case, nonlinear polarization of the molecules (material) takes place. As a consequence of this phenomenon, the incident light is also altered and a variety in nonlinear optical (NLO) effects may occur: frequency variations, changes of the propagation characteristics of the light, etc. [48].

The appearance of different NLO effects, termed the second, third and higher order effects, is highly dependent on the magnitudes of the coefficients in the nonlinear terms. The structural requirements of the materials for the generation of the various NLO effects at both molecular and bulk level are also different [48].

Nonlinear optical behavior in liquid crystals has been known for a long time [49]. As a result of their unique and complex physical structures, liquid crystals are optically highly nonlinear materials in that their physical properties are easily perturbed by an electromagnetic field. In addition, mesomorphic materials have been proved to be very suitable media to achieve non-centrosymmetric molecular arrangements for second order phenomena (x(2) dependent), either in their own right [50] or by electric or magnetic field poling [48 b, 50al.

440 M.B. Ros

In recent years, coordination complexes have achieved prolific entrance into the field of liquid crystals and they possess promising possibilities as regards nonlinear optics [51]. Metal atoms have electronegativities which differ from those of atoms which constitute organic systems (carbon, nitrogen, oxygen, etc.), and this fact influences the polarizability of a system which contains a metal center. In addition, there is the possibility of metal-ligand and ligand-metal charge transfer, and metal centers can also act as effective donor or acceptor groups depending on their oxidation state. All the properties described above could enhance the two requirements for the nonlinear interactions between light and the material in question; the easy and strong polarizability of the material. In addition, macroscopic effects must also be considered, such as chirality with the origin in the metal center and molecular flexibility.

Consequently, the combination of the properties associated with the presence of an organometallic or metal-organic moiety, together with the liquid crystalline order does indeed make metallomesogens very attractive materials for photonic studies. So far, we can only talk in terms of a future that appears bright as the possibilities are, as yet, relatively unexplored.

In 1992, Ghedini et al. reported nonlinear optical responses for a liquid crystalline palladium azoxy derivative (see structure in Fig. 11-2) [52].

In the nematic phase, they observed self-focusing and self-phase modulation phenomena [48 a, 49a]. Both phenomena are called “self-action” effects since the nonlinear response of the material affects the incident beam through an intensity dependent refractive index. As the self- focusing effect is concerned, the refractive index of the NLO medium is larger in the center of the beam than that at its edges. Consequently, the material acts as a positive lens and focuses the beam. In the case of the phase modulation effect, the result is broadening of the frequency conformation.

The authors indicated the thermal origin of these third order effects &(3) dependent), but also the extremely low values of optical intensities needed in comparison with those usually required to achieve similar effects in organic nematic liquid crystals.

Metallomesogens which show a chiral smectic C phase possess the structural requirements needed for second-order nonlinear phenomena as the smectic C* phase gives rise to noncentrosymmetric molecular order at a macroscopic level. Bearing this in mind, the NLO possibilities of several ferroelectric metallomesogens have been explored. Very recently, second harmonic generation in imine P-diketonate complexes of palladium and platinum (Fig. 11-13) have been reported [53].

pm V - ‘ have been determined 10°C below the transition into the smectic C* phase at phase matching conditions. These results provide support for the possibilities of this type of material for NLO applications.

Nevertheless, as happens with organic liquid crystals, there are several problems which must be overcome if metallomesogens are to translate their potential into application. For example, the optical attenuation due to scattering related to the density

For these compounds, effective coefficients (&) in the range of


C 101 "C SmC* 109°C SmA 124 "C I

ooO::; 0.03

> 0.025

& 0.02

4 0.015 0.01

'0 5 1 0 1 5 20 25 30

1, - T ("C)

Figure 11-13. Second-harmonic generation parameters of a ferroelectric metallomesogen in the chiral smectic C phase as a function of temperature. (Tc: temperature of the transition to the chiral smectic C phase on cooling).

and order fluctuations must be reduced. In addition, depending on the application, colorless NLO materials might also be needed. While the color is not a serious limitation for electrooptic responses, for the more interesting frequency doubling, low attenuation at 400nm (blue laser) is required [48a,49a].

In this context, it should be mentioned that the NLO possibilities of other metallomesogens have also been measured off-resonance by the conventional methods. However, measurements have been made using solutions rather than bulk materials.

The second-order hyperpolarizabilities (p) of some thermotropic rhodium(1) and iridium(1) stilbazole derivatives were found to be around 24 x esu (data evaluated by EFISH, A = 1.907 pm) [54]. These values represent considerable nonlinearities which are in the range of many metal complexes or organic compounds [48,51].

As far as third-order nonlinearities are concerned, the high nonlinear responses of platinum and palladium alkynyl polymers [51 b, 551, which form lyotropic phases in chlorinated solvents [56], should be mentioned. Third-harmonic generation (043 w ) and four-wave mixing (20, - ~ 2 4 0 3 ) experiments in solution, not in the lyotropic phase, revealed values of y of around esu [51 b]. Likewise, the third-order parameters for thermotropic zinc, copper, nickel, oxyvanadium, and cobalt tetraphenylporphyrins [57] and a copper phthalocyanine derivative [58] have been reported.

values in the range of 1.5 to 6 . 0 ~ 10" esu have been determined for the porphyrin derivatives in benzene solutions [57]. On the other hand, a microscopic hyperpolarizability ( y ) of up to esu was found for the phthalocyanine derivative [58] by using different methods of measurement, namely third harmonic generation (THG) and electric field-induced second-harmonic generation (EFISH).

The characterization of the hyperpolarizabilities of many other metallomesogens is necessary, but, more interestingly, the exploration of the properties which may

By using the degenerate four-wave mixing (DFWM) method,

442 M.B. Ros

arise from their liquid crystalline arrangement is a matter in which many things remain to be discovered.

11.2.6 Photoeffects: Energy Migration

The nature of the molecular stacking in some columnar mesophase of metallomesogens, which is characterized by a quasi-one-dimensional order of closely packed chromophores, has made these materials very promising candidates for studying different photoeffects.

The absorption of photons by a material produces either the formation of excited states or the liberation of electrons, both of which result in a variety of valuable effects [59]. The exciton (a mobile but localized nonconducting excited state) may revert to the ground state by a number of different mechanisms (see Fig. 11-14). Depending on the nature of the relaxation mechanism, fluorescence, phos- phorescence, energy migration or photochemical reaction may result. In addition, the absorption can generate electron-hole pairs which can give rise to photoconductivity, photovoltaic or photomagnetic effects. All of these phenomena undoubtedly have a practical significance in areas which include molecular electronics, Xerox copiers, laser addressed displays, optical fibers, matrix addressing of flat panel displays, conversion of solar energy, etc. [60].

In this section, the phenomenon of energy migration will be discussed. Properties related to the electrical conductivity will be covered in the subsequent section.

As far as energy migration is concerned, studies have only been carried out on mesomorphic phthalocyanine derivatives [43 c, 611. The most relevant examples are phthalocyaninatozinc(I1) complexes [61 c]. In the mesophase, the rigid part of the

n - n 0 0 0 J

w > z m 4

INTERSYSTEM CROSSING

EXCiTON TRANSFER

- } v L

Figure 11-14. Energy level diagram showing transitions induced by light absorption and deacti- vation pathways for a molecule. So = ground state, S, = first singlet excited state, T, = first triplet excited state, VL = vibrational level. (Adapted from [~OC]).

I I Other Physical Properties and Possible Applications of Metallomesogens 443

molecule is surrounded by flexible hydrocarbon chains which promotes the formation of segregated columns. The intercolumnar distance is much larger ( - 20 - 40 A ) than the intracolumnar stacking periodicity ( - 3 -4 A), and this means that energy migration should be one-dimensional (Fig. 11-15).

Figure 11-15. Schematic representation of the absorption of photons in columnar liq-

MIGRATION

uid crystal phases. I

By using nanosecond absorption spectroscopy, laser-induced (Nd-YAG) triplet excitons were studied in both the solid and the mesomorphic state [61 c]. In addition to a specific discussion regarding fluorescence data and kinetic decay models, the authors drew the following main conclusions: i) triplet states are formed by both the solid and the mesophase; ii) the energy migration has a unidirectional character, iii) the photonic migration is more efficient in the liquid crystal arrangement than in the solid and finally, as regards the influence of the metal atom, iv) the photoeffect is improved in the metal complex in comparison with that observed for the corresponding metal-free phthalocyanine. These conclusions are illustrated by the data collected in Table 11-3 (p. 444).

11.3 Electrical Properties

To date, two main electrical properties have been studied in the area of metallomesogens: those related to their capability to act as electrically conducting materials and those based on their dielectric behavior.

Tab

le 1

1-3.

Mig

ratio

n en

ergy

par

amet

ers

of m

esom

orph

ic p

htha

locy

anin

e de

riva

tives

. P

P

P

A

Pha

se t

rans

itio

ns

T("

C)

~O

"-

X~

~~

10

4*,y

rrb

10

-3N

C

Ld

(pm

) se

(ps)

1

05

-Df(

cm2

s-')

H,

K 7

9 D

hd

260

I 20

4.

0 8.

0 2.

5 0

.9-

1.1

40

1.6 - 2

.5

85

4.5

9.0

2.2

0.8 - 1

.O

7.4

8-

14

Zn

K78

Dh

,305

1 20

3.

6 7.

1 2.

8 1.

0 45

1.

5 90

2.

6 5.

1 3.

8 1

.4

0.4

170

(a)

Tri

plet

exc

iton

mol

ar f

ract

ion.

(b)

Tra

p m

olar

fra

ctio

n. (

c) A

vera

ge n

umbe

r of

mol

ecul

es in

the

col

umn

betw

een

two

trap

s. (

d)

Mea

n co

lum

n le

ngth

bet

wee

n tw

o tr

aps.

(e)

Exc

iton

hop

ping

tim

e fr

om s

hort

-tim

e de

cay

curv

es.

(f)

Exc

iton

diff

usio

n co

effi

cien

t.

!a R


11.3.1 Electrical Conductivity and Redox Properties

The very first papers dealing with electrical conductivity in thermotropic metallomesogens describe studies of the properties of molten carboxylates. Cation mobility was proposed as the origin of the conductivity [62]. More recent research regarding this subject is focused on Molecular Electronics [63]. The challenge in this area is to reduce the size of functional electronic elements to molecular dimensions, with the ultimate aim of achieving miniaturization.

Scientists from many disciplines are now combining their expertise to study new materials, which differ from the traditional inorganic conductors and exhibit remarkable conductivity properties. At present, there are several classes of compounds which are known to behave as semiconductors, molecular metals or superconductors: charge-transfer and ionic radical salts, polymers, macrocycles or fullerenes [64]. Sur- prisingly, many examples are organic in nature [64] but, in most of these groups, both organometallic and metal-organic compounds are also present [59,64].

For those readers who are not familiar with this field, the question will arise why or how such materials can transport electrons. The basic concepts of the band model provide the key to this question [64b,65]. The conduction properties result from the electronic structure of the energy levels in the condensed phase.

When two atomic orbitals combine, the resulting molecular orbital has two distinct energy levels, as represented by the diatomic molecule in Fig. 11-16A. In a similar way, three energy levels are apparent in triatomic systems. However, when a large number of atoms or molecules are brought together to form ordered chains or macrostructures (such as columnar mesophases), an energy band will form if there is a sufficient overlap of the atomic or molecular orbitals. These may merge to form a supermolecular orbital.

Nevertheless, in order to produce metallic behavior the electronic population of these energy bands is of crucial importance. Conductors are associated with partially filled bands in which it is possible for a large number of electrons to move easily into infinitesimally higher energy levels within the band. Higher energy gaps between the occupied and empty states gives rise to semiconducting or insulating behavior. The distinction between the last two possibilities is only a question of orders of magnitude of the conductivity cr (unit: S cm-'), and the insulating state is defined rather arbitrarily (see Fig. 11-16B). On the other hand, in metallic compounds the conductivity increases as the temperature decreases, whereas in semiconductors this conductivity decreases with decreasing temperature.

The electric charge transport in low-dimensional materials [66] composed of molecules M is determined by different factors: the generation of free charges, the concentration of charge carriers (n) and the mobility of the charges (p):

generation of transoort of - carriers carriers

MMMMM - M-MMMMM - MM-MMM (n 1 oc)

446 M.B. Ros

(A)

ATOMIC DIATOMIC TRlATOMlC POLYATOMIC ORBITAL MOLECULE MOLECULE STACKING

f E,c2eV

VB CB I] Z VB W

VB

INSULATOR SEMICONDUCTOR CONDUCTOR

S : lo-''- 10' S.cm-' (r.t.1 O C lo-'oS.cm-' (r.t.1 s r 1o0S.cm-' (r.t.)

Figure 11-16. Band model for organic insulators, semiconductors and conducting materials.

The free charge carriers may be created either directly from the molecules (M) or by the introduction of dopants into the system. The first case shows intrinsic conductivity and both electrons and holes act as carriers:

MMMMM Ft M-M'MMM carrier: h + or e-


In the second case the conductivity is induced by a dopant:

MMMIAM P MMM'IAM (carrier: h') p-type

MMMIDM P MMM-I&M (carrier: e - ) n-type

Nonmesomorphic metallomacrocycles such as porphyrin and phthalocyanine derivatives have been widely studied as quasi-one-dimensional conductors [67]. They possess electronic and morphological characteristics, in addition to a stacked packing in the solid state, which promote semiconducting and conducting properties. The metal centers and the delocalized rr-systems may interact and create pathways for change transport.

A similar molecular arrangement can be provided by columnar mesophases, and a number of laterally substituted porphyrin and phthalocyanine derivatives form suitable liquid crystal phases. Consequently several authors have expressed an interest in the possibility of obtaining conducting properties in metallomesogens [6 a, 68, 691.

The original work on conducting columnar metal-containing liquid crystals was performed by Giroud-Godquin [6a]. A Pdiketonatocopper complex was shown to exhibit conductivities, probably not intrinsic, on the order of S cm-' in a Dh

The majority of papers describing electrical conductivity in discotic metallomesogens deal with phthalocyanines which contain copper, lutetium, or lithium as metal center [68, 691. In these materials the conductivity arises from the intrinsic thermal or photochemical generation of charge carriers. Data such as the thermal activation energy for this conduction process or the charge carrier concentration may be estimated from the redox potentials of the compounds determined in solution. The average size of a single domain in the mesophase has been estimated to be in the range of 1 vm and the column length in the order of 500 A [68 b]. Complex impedance measurements performed on pressed pellets have revealed some outstanding results.

Firstly, the macroscopic conductivities, which depend on the frequency ranges used in the experiments, give relatively high values in the range of lo-'- lo-" S cm-' in the mesophase (ac conductivity). These values increase with increasing temperature. These metal-containing phthalocyanines act as semiconductors.

The second aspect worth commenting on is the increase in conductivity of the mesophase in comparison with that of the solid state by up to two orders of magnitude (see Fig. 11-17) [68b,c]. This behavior has been attributed to a higher mobility of the carriers and a more efficient orbital overlap in the mesophase. From X-ray studies the presence of tilted columns in the columnar phase has been deduced [69a]. In contrast, other results have recently been reported for several metal-free mesomorphic phthalocyanines, concerning the dependence of the conductivity on the nature of the condensed phase [70].

Thirdly, intracolumnar hopping processes involving metal-containing aromatic cores can be considered as the main cause of the electrical conductivity in these mo-

' mesophase.

448 M.B. Ros

I I I I I I I I >

log f

-2 0 2 4

Figure 11-17. Dielectric data of the lutetium (circles) and lithium (squares) alkyloxyphthalo- cyanine complexes in the solid (300 K, in white) and liquid crystalline (375 K, in black) phases. (Adapted from [68b]).

lecular materials at high frequencies (> 1 0-2 - lo5 Hz). This intracolumnar nature of the conductivity indicates an effective intercolumnar insulation due to the presence of the peripheral alkyl chains in the mesogenic structures.

The phthalocyanines described above display higher conductivities in the liquid crystalline phase at temperatures higher than 80 “C. However, for broader applications it was thought necessary to extend the conductivity range to much lower temperatures. With this aim in mind, the approach of preserving the mesophase order by polymerization was investigated by Nolte et al. [69b, 69~1. This “freezing” process should also allow better control over the stacking arrangement.

One example of this approach involves the polymeric copper(I1) “network” reported by the authors (see Fig. 11-18) [69c]. The conductivity of the polymeric bulk material in which the D,, structure is retained, was shown to be two orders of magnitude higher than that for the corresponding unpolymerized phthalocyanine (1 x IO-’S cm-’ versus 7x 10-loS cm-’ at 175 “C). Similar trends have been observed for other phthalocyanine polymers (e.g., skewer structures), despite the fact that they do not exhibit mesophase transitions but form crystalline D,, structures at room temperature [71].

A well-known method to increase the conductive properties of a material is the addition of suitable dopants. This approach has also been applied to phthalocyanine derivatives [69]. Thus, by using iodine (Iz) as a dopant, increases in (r by a factor of up to lo4 have been observed in low and high molecular weight materials. In- terestingly, the increase in 0 caused by doping metal-containing systems is much lower than the factor of lo8 observed for the parent free-metal phthalocyanines [69b, 721.


\ ) c A A

A : -COO-(CHZ)ja - 0 0

THREE-DIMENSIONAL NETWORK above plane

below plane

Polymerization

I

I (T (1 75 "C) : 1 x 1 0-5 S.m-' (T (1 75 "C) [60-70% l2 ] : 5 x l o 3 S.m*'

Figure 11-18. Schematic structure and conducting properties of a phthalocyanine metallomesogen and its corresponding network obtained by polymerization.

While the materials mentioned above can be classified as p-doped one-dimensional semiconductors, in 1994 the first observation of an n-type discotic liquid crystalline semiconductor was reported [73]. Potassium metal, an electron donor, was used as dopant to obtain a new n-doped conducting liquid crystal.

The study was carried out on an organic discotic material doped with potassium (6Yo). This material exhibits a Dh mesophase, for which a conductivity in the direction parallel to the columns of 2 . 9 ~ S cm-' and remarkable carrier mobilities were determined.

These results show that both n- and p-type semiconductors can be achieved in metal-containing mesophases.

Also related with doping processes are the results reported for crown ether-substituted phthalocyaninatocopper complexes [69 b, 741. Although none of the materials exhibits liquid crystalline phases, a substantial increase in the electrical conductivity was observed when the material was complexed with an alkali metal ion in the crown ether ring. The magnitude of the increase is related to the size of the alkali cation.

As indicated above, the materials studied are not liquid crystalline, but their significance justifies their discussion within this chapter. These results are especially rele-

450 M.B. Ros

RO OR

Q co "7

, I

Figure 11-19. Schematic representation of the multiwired molecular cable proposed for the self-assembly of a mesomorphic crown ether phthalocyanine. (Adapted from [75]).

vant given that Nolte et al. [75] have recently reported a novel crown ether molecule which does form a hexagonal columnar phase (see Fig. 11-19).

This liquid crystalline crown ether does not contain a metal center. However, metals could easily be introduced into either the phthalocyanine core or the crown ether rings or, alternatively, by skewer polymer formation. It is only a matter of time that a deeper understanding of the role of the mesophase and metal on conducting properties is obtained. This is only the first step on the road leading to the intriguing prospect of constructing a multiwired molecular cable. Indeed, fibers consisting of bundles of single parallel strands have been observed in such metal-free materials by transmission electron microscopy.

So far the discussion in this section has been devoted to metallomacrocycles. How- ever, other types of molecular metals are known and these will be considered in the following pages. An alternative approach to conducting metallomesogens involves charge-transfer salts, although this line of investigation remains relatively unexplored. In this case, a partial charge transfer from a donor molecule to an acceptor molecule within a segregated stacking arrangement is required to generate electrical conductivity [59,64]. The first report of work in this context is attributed to Mueller- Westerhoff who reported a number of results about dithiene derivatives (Metal: Ni, Pd, Pt) [76] (see Fig. 11-20). These metallomesogens are structurally analogous to

11 Other Physical Properties and Possible Applications of Metallomesogens 45 1

Mesomorphic electron acceptors Nonmesomorphic electron acceptors

C4H9

c4HQ+ M : Ni, Pd, Pt

[M(bdt),] complexes

[M(dmit),] complexes

Figure 11-20. Metallomesogenic dithiene derivatives used as electron acceptors for conducting charge-transfer-salt formation and some structurally related constituents of well-known conducting and superconducting salts. (bdt: bisdithiolene; dmit: 1,3-dithio-2-thione-4,5-di- thiolate).

[M(bdt)2] and related to [M(dmit)J, compounds which have both been extensively and successfully used for the preparation of remarkable molecular metals or superconductors [64a, 65 b].

Based on their weak electron acceptor character [M = Ni, n = 5, E,, = 0.06 V, Eo2 = -0.76 V], the mesomorphic dithiene complexes were mixed with different proportions of a liquid crystalline TTF-derivative as the donor component (TTF tetrathiafulvalene). Despite the fact that liquid crystalline order remained in the mixtures, the donor -acceptor charge transfer was negligible, so conducting behavior is not expected. Since this first attempt, other relevant studies regarding this subject have not been reported [77].

The strategy to obtain conductivity in polymeric systems is also poorly documented. Indeed, this approach is only represented by the lyotropic polyynes mentioned above [78]. The conductivity of these systems is reported to be surprisingly poor. The role of the mesophase order on conductivity remains uninvestigated.

Whatever approach is undertaken to obtain conducting metallomesogens, the presence of suitable donor or acceptor molecules is of fundamental importance. Fortu- nately, the wide range of possibilities offered by different metals is of invaluable help.

With this aim in mind, the design of new structures (mainly discotic) and the investigation of their redox properties seem to be pursued with increasing intensity [79]. Indeed, the relatively recent entry of voltammetric methods into materials science appears to be very promising for metallomesogens, since different kinds of sample and viable applications can be investigated [80].

452 M.B. Ros

As a simple example of potential application, the electrochromic properties of one particular type of metallomesogen is worth mentioning. These materials undergo reversible electric field-induced reactions (oxidation or reduction) which are accompanied by color changes. This fascinating behavior has been described for mesomorphic lutetium-phthalocyanine complexes in solution but, once again, data regarding these phenomena in the mesophase are not yet available [4b].

Tho significant pieces of work deserve discussion in context with the redox behavior of metallomesogens: Zentel et al. [81] used an approach to polymeric crosslinking which involves the exploitation of ionic interactions between metal atoms within the polymer matrix. Via redox reaction of ferrocene units the degree of crosslinking can be tuned and a novel reversible formation of elastomeric materials has been described (Fig. 11-21). Praefcke et al. suggested another interesting potential of metallomesogens arising from their donor/acceptor characteristics: the control over supramolecular order by charge-transfer interaction [82, 831.

By mixing disc-shaped dinuclear palladium [82] or platinum [83] mesogens with 2,4,7-trinitrofluorenone (TNF) (electron acceptor), the authors stabilized the ND phase of metallomesogens and furthermore, they induced Dho phases. They proposed a donor-acceptor intercalation model to explain the induction of these types of columnar order (Fig. 11-22, see p. 454). However, no clear explanation regarding the question whether these effects are due to a real charge transfer or, as is more likely, to geometric changes has been given.

In spite of the accounts described so far, it should not be concluded that these electrical phenomena are restricted to thermotropic materials only. Electrical conductivity in lyotropic metallomesogens has been extensively studied [84). The conductivity exhibited by these systems is associated with inorganic ion migration and it is orientationally dependent on the nature of the various lyotropic mesophases. Moreover, electrooptic effects (turbidity changes) have been observed in some of these systems.

In addition, charge-transfer interactions in lyotropic systems can induce mesophase order [85] and, interestingly, even a cholesteric phase has been induced [85 b].

As discussed in the previous section (p. 442), a number of materials are capable of generating electron-hole pairs by photon absorption. If the electrons and holes can be separated by means of an external electric field, photoconductivity occurs, that is light-induced enhancement of the electrical conductivity of a material. Alter- natively, if the separation occurs due to differences in the mobility and diffusivity of the electrons and holes, as with metal-semiconductor junctions or n-p semiconductor junctions, a voltage difference is generated (photovoltaic effect) [59].

These types of behavior, which have been widely studied for organic materials and organometallic or macrocyclic compounds, have also been envisaged for metallomesogens. Gregg et al. reported photovoltaic effects in symmetric cells filled with liquid crystalline magnesium, cadmium, palladium, or zinc porphyrin complexes [2 c, 861. The authors did not study the mesophase itself, but rather the ability of the metallomesogen to promote macroscopic order which, on cooling, provided polycrystalline films or cylindrically ordered samples.


I Blend 111 (50% whn) Blend IV (%% w/w)

gl 306 K SrnA 344 K N 372 K I g1 310 K SmA 352 K N 382 K I

$- + - C H 2 - C H 2 q

g, 308 K SmA 347 K N 379 K I Polymer 1 b

Polymer 2b

S O ~ N ~

Figure 11-21. Approach to liquid crystalline elastomers by ionomeric interactions in metallomesogenic materials.

The best results were obtained from the zinc complexes. The occurrence of a substantial and stable photovoltaic effect was found, which is comparable with that of some of the better organic solar cells [2c].

Under short circuit conditions, under white light illumination of 150 mW cm-2, the photocurrent densities use) ranged between 0.2 and 0.4 mA cm-2 for 1.5 - 3 pm

454 M.B. Ros

RO

OR

RO

OR

U 0 C I R 0

mesomorphic electron donor (ND) (-1 Mixture

S ND and Dho induced mesophases

0 2 N

(TNF) non-mesomorphic electron acceptor - Figure 11-22. Induced liquid crystalline phases by charge-transfer interactions involving metallomesogens.

thick cells. The cells were prepared by letting the fluid compound penetrate between two slides by means of capillary forces. The open circuit voltages were in the range of - 150 to -400 mV. The rise and decay times of the photocurrent were less than 10ms. The photocurrent increased linearly with the intensity of the incident light ( I , ) at all wavelengths up to Io>10'5 photons s- ' cm-2 and was three orders of magnitude higher than that of the corresponding metal-free material.

The authors claim this to be the first unambiguous example of a photovoltaic cell controlled entirely by interfacial kinetics. The charge separation was not produced by a gradient of electrochemical potentials, but was based on the different kinetics of hole versus electron injection at the illuminated interface. The former was estimated to be about 20 times more likely than the latter.

11.3.2 Dielectric Behavior

Dielectric studies involve the response of matter to the application of an electric field as a consequence of the material being polarizable. Experimentally, these studies are carried out by filling a capacitor with the material to be investigated and monitoring

I 1 Other Physical Properties and Possible Applications of Metallomesogens 455

how the capacitance increases upon application of an electric field from a value ( C ) to a value ( E C ) , where E is the dielectric permittivity of the material.

Both parameters are related to the average polarization induced by the electric field E. The average polarization is a parameter arising not only from the electronic and the atomic polarization but also, to a great extent, from the anisotropic orientational polarization of the liquid crystal molecules. Consequently, it is not difficult to understand how these materials behave.

If the molecular long axis tends to align in the direction of the electric field then, by a cooperative effect, the domains orientate with the director in the same direction. If the molecules tend to align with their long axis perpendicular to E, the director will be reorientated perpendicular to the field. The strength of the electric field necessary to move the liquid crystal molecules is relatively low since the director is usually free to align in any direction. Indeed, the dielectric permittivity of a liquid crystal mainly depends on the temperature and the nature of the mesophase.

The information obtained from dielectric measurements on liquid crystalline materials is divided into two categories [87]. On the one hand, measurements performed in static fields provide values of the dielectric permittivity ( E ) parallel and perpendicular to the director and thus reveal the anisotropy of the permittivity AE = E~~ - E , (positive or negative). The potential application of liquid crystals in displays depends on the magnitude and sign of this value.

On the other hand, it is possible to study the behavior of the material in electric field of variable frequency. This technique is called dielectric spectroscopy and provides valuable information about the phases and molecular dynamics of the liquid crystal [88]. This approach can provide information about the movements (or modes) of the molecules upon application of an electric field, assuming that these modes involve changes in the dipole moment. Depending on the type of alignment of the material and the frequencies used, different molecular modes can be analysed from the variation of E with the electric field frequency: noncollective modes (10' - I O l 3 Hz), such as molecular rotation around the short or the long molecular axis or intramolecular rotation around single bonds, and collective modes (< lo6 Hz).

Dielectric spectroscopy can also reveal the relaxation times (relaxation frequencies) of these orientations. After the field is removed, the orientation polarization decays within a characteristic time 5, which is termed the relaxation time. The process of (re)orientation of the permanent dipole moments caused by the field changes re- quires a defined period of time. In alternating fields this leads to a time delay between the average orientation of the dipole moments and the field, a phenomenon which is more noticeable at frequencies of the order of T-'. At much higher frequencies the orientation polarization can no longer follow the variation of the field. Within the relaxation regime there is a phase difference between the applied field and the induced polarization, which causes a dissipation of energy in form of heat which is absorbed by the dielectric medium. Conventionally, this dielectric absorption is accounted for by considering a complex dielectric constant E * written as E * = E ' - j & " .

456 M.B. Ros

The magnitudes of E ’ and E ” are considered as a function of the electric field and temperature during dielectric relaxation measurements. If the experimental values E ‘ and E” can be described by a single relaxation time model (Debye model) [89], the graphic plot of E ‘ versus E ” is a semicircle centered on the E ‘ axis (Cole-Cole plot).

Additionally, from the plot of In 7 versus reciprocal temperature, the activation energy of the relaxation process (EA) can be calculated.

As far as metallomesogens are concerned, basic dielectric spectroscopy studies have mainly been reported on salicylaldiminate derivatives.

Perez Jubindo et al. [90] have analysed the dielectric behavior of an oxovanadium complex at different temperatures using a range of frequencies from 10’ to lo9 Hz. In homogeneously aligned samples they observed a thermally activated relaxation at medium frequencies (around 1 O6 Hz) in the isotropic liquid, the smectic A phase and the smectic C phase (see Fig. 11-23). From these measurements an activation energy of about 70 kJ mol-‘ was calculated in the isotropic liquid and 89 kJ mol-’ in the two smectic mesophases.

1 I I I I I 1

of

I#

&

0.4

0.2

0.0 2 3 4 5 6 7 8 9

log (freq [Hzl) Figure 11-23. Real ( E ’ ) and imaginary ( E ” ) parts of the dielectric permittivity versus the loga- rithm of the electric field frequency at 145°C in the S, phase. (0) Experimental points, (El) losses after the subtraction of the electrode polarization contribution, ( 0 ) after the subtraction of the electrode polarization contribution and dc conductivity contribution; continuous line is the fitting of ( 0 ) data to the Havriliak-Negami equation. (Adapted from [90]).

I 1 Other Physical Properties and Possible Applications of Metallomesogens

\

/

P Pt \

Y

flip-flop - Y

457

/ \

Figure 11-24. Representation of the molecular structure showing the direction and movement of the net dipolar moment.

Based on the alignment conditions and the molecular structure, the authors have assigned this mode to the situation where the dipole moment is transverse to the long molecular axis p t ( p r = 0) (see Fig. 11-24). In combination with the results from EPR studies [91] (see Chap. lo), the authors tentatively propose a thermally activated cooperative flip-flop process between potential wells.

As pointed out by the same authors, this kind of relaxation in calamitic organic liquid crystals is attributed to free or hindered rotation around the molecular long axis. The rotation occurs at frequencies of 1 GHz and with an activation energy of 30-60 kJ mol-' in organic liquid crystals. However, for these brick-shaped molecules the frequencies involved are two orders of magnitude lower. Slower movement occurs and larger activation energies are required for these motions in metallomesogens.

Dielectric measurements have also been performed on main-chain copper(I1) metallomesogenic homopolymers [92]. The characterization of one semicrystalline polymer with a relatively low molecular weight was performed using electric fields with frequencies ranging from 0.1 Hz to 10 kHz. These studies have revealed several relaxation processes. A p-relaxation and an a-relaxation in addition to a melting transition and a cold crystallization. While the a-mode corresponds to the dynamics of the glass transition with a typical correlation length of several monomeric units,

458 M.B. Ros

0

0 - (CH2)lO

P-relaxation

EA : 14 - 15 kcal. mol-’

Figure 11-25. Chemical structure of a metallomesogenic polymer studied by dielectric spectroscopy for which p-relaxation has been proposed.

the stronger &relaxation is present below Tg and has been attributed to the more local motions of the 4-hydroxybenzoate units (Fig. 1 1-25).

11.3.3 Ferroelectricity

Dielectric spectroscopy is of particular interest in the field of ferroelectric liquid crystals. It allows the study of the two most characteristic collective modes of the chiral smectic C phase: the soft mode, associated with tilt angle fluctuations, and the Goldstone mode, which describes the motions of the molecular director at constant tilt angle (see Fig. 11-26) [88c]. The latter motion relaxes at lower frequencies, typically in the range of 10- 100 Hz for organic ferroelectric liquid crystals.

GOLDSTONE MODE SOFT MODE

Figure 11-26. Schematic representation of the two most characteristic molecular movements exhibited by ferroelectric liquid crystals.


m

K 81.3OC SmX* 144°C SmC* 151.8"C Ch 162 I

12

10 E'

8

6

4

2

0

Figure 11-27. Three-dimensional spectrum of E' versus electric field frequency and temperature of the ferroelectric metallomesogen shown at the top of the figure. (Adapted from [94]).

Studies of this kind on ferroelectric metallomesogens have detected the Goldstone modes at frequencies lower than 100 Hz (Fig. 11-27) [93-951. However, so far, soft modes have not been reported for the ferroelectric smectic C* phases of metallomesogens. This is either because it has not been investigated or it has not been detected even after the suppression of the Goldstone mode by the application of a dc field [94].

Ferroelectric materials are distinguished from ordinary dielectric media because they have very large permittivities and offer the possibility of retaining some residual electrical polarization after removing the field (see Fig. 1 1-28).

The investigation of ferroelectric liquid crystals (FLC) is an active area of research [96]. The materials are characterized by the presence of smectic C order involving chiral molecules with strong transverse dipole moments. Accordingly, this type of material gives rise to polar order with a C2 symmetry. The chiral smectic C meso-

460 M.B. Ros

polarization

Figure 11-28. Schematic representation of the hysteresis loop characteristic of ferroelectric materials, along with the influence of the electric field on polarization and alignment of the dipoles.

phase (Sc *) is helical in nature, which implies a cancellation of all dipole moments in the bulk material. However, under external constraint of surface or electrical conditions, the helical structure can be removed (Fig. 11-29). In such cases, it is possible to prepare well-aligned films with a net polarization, called the spontaneous polarization (P,) .

FLCs combine two properties: the existence of a net polarization and the subtle molecular reorientation necessary in order to obtain optical contrast. Due to this, it is possible to switch these materials by the application of an electric field and the switching times are much faster (microseconds) than nematic liquid crystals (milliseconds) (see Fig. 11-30), This rapid electrooptic response is the basis for their technological application in display devices [96].

The ferroelectric phenomenon in metal-containing liquid crystals has been investigated because of the properties which may arise from the presence of metal atoms. Metal atoms could provide polarizability, polarity and, interestingly, paramagnetic centers which would increase the possibilities for the study of ferroelectric be- h avi or.

I I Other Physical Properties and Possible Applications of Metallomesogens

- - P

-

electric field or

p,t& surface conditions :

46 1

-

ch

HELICOIDAL STRUCTURE UNWOUND STRUCTURE

Figure 11-29. Molecular and dipolar orientation in the chiral smectic C phase before and after the unwinding of its characteristic helical structure.

NEMATIC DISPLAY FLC DISPLAY

Figure 11-30. Reorientation of a molecule in displays based on nematic and chiral smectic C phases.

Since the report of the first ferroelectric metallomesogen [97], for which only switching characteristics could be estimated, spontaneous polarizations of up to 200 nC cm-2 have been achieved [93, 94, 97- 1011. The relevant data for a number of representative examples of ferroelectric metallomesogens (Fig. 1 1-3 1) are given in

R'

R3

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Fig

ure 11-31. C

hem

ical

str

uctu

re o

f th

e fe

rroe

lect

ric

met

allo

mes

ogen

s w

hose

pro

pert

ies

are

give

n in

Tab

le 1

1-4

.


Table 11-4. These data will be used to illustrate a number of points in the following discussion.

Similar trends to those observed for organic FLC appear to be applicable to ferro electric metallomesogens. Thus, the structural characteristics of the organic ligand dominate the magnitude and sign of the spontaneous polarization (P,).

As far as the influence of the chiral substituent on the P, is concerned, it can be said that its chemical structure (Table 11-4, entries 9- 11) imparts different dipole moments to the macroscopic polarization. The position of the chiral moiety within the central core (Table 11-4, entries 1 -2, 4-6) affects its free rotation thus influenc- ing the dipolar coupling. The number of chiral groups (Table 11 -4, entries 1, 2 vs. 3; and 4-6 vs. 7) is also an important factor and evidently increases not only the dipole density, but also the viscosity of the material. As far as the molecular length is concerned, this controls the entire molecular disposition within the layers and, hence, the orientation of the dipoles (see Fig. 11-32, p. 465).

It appears that, for a given structure, the nature of the metal center does not significantly influence the ferroelectric properties (Table 11-4, entries 9, 12, 13), however, further studies are necessary in this area.

From the data available now, it seems the complexation of chiral organic structures to metal centers can lead to three possible effects. The magnitude of the spontaneous polarization can be either increased, decreased or remain unaltered with respect to that of the uncomplexed ligands. The structure of the metallomesogen as a whole seems to be the key issue. In contrast, complexation usually leads to more viscous materials and, hence, slower response times in the switching process. Experimentally, this higher viscosity has necessitated the use of electric fields with lower frequencies (5 -40 Hz) and higher voltages (40- 1000 V) than the ones used for the study of common organic FLCs.

The response times observed for metallomesogens are in the range of milliseconds as opposed to microsecond switching in low molecular weight organic FLCs. Although relatively slow, these switching times are similar to those recorded from FLC polymers. This is easily understandable if one considers the molecular structure of these metallomesogens. The molecules have a length to width ratio which is significantly smaller than that of a typical organic FLC. Upon switching the electric field, they must rotate in a cooperative process, describing a cone angIe without moving their center of mass.

The important parameters switching time and viscosity represent a serious draw- back for the use of metallomesogens in conventional fast response FLC displays. However, the structural characteristics, the possibilities for molecular design, and the tendency to freeze the smectic C* order on cooling make metallomesogens suitable and attractive compounds not only for academic studies, but also for new applications for FLCs.

It is worth noting that paramagnetic copper and oxovanadium complexes are not only interesting from the point of view of FLC, but also for applications in which a combination of ferroelectric and magnetic orientation is used. Likewise, the novel possibilities for ferroelectric materials in NLO should be mentioned. Polar order in

P

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Tab

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b'igure 11-32. Temperature dependence of the polarization of ferroelectric metallomesogens as 'I function of the achiral chain length. (Adapted from [loo]).

c~ noncentrosymmetric arrangement is inherent in these systems [50b], and the first ic\ults in this area have been discussed in a previous section (p. 440) [53]. In addition, Iic known pyro- and piezoelectric responses [96a] which occur in smectic C* materi- tls remain unexplored so far as metallomesogens are concerned.

At this stage, it is appropriate to describe novel and less common approaches to I crroelectric liquid crystalline materials. Several authors have prepared metallo- inesogens in which the center of chirality is the metal atom [103], and in a number

( I these systems, namely ferrocene complexes [I03 b], a smectic C phase has been I -*ported. Alternatively, there is the attractive ferroelectric order approximation de- \:iibed by Swager et al. based on results obtained from monooxo complexes [104]. I lie authors suggest these compounds to be dipolar switches involving the M=O bond, resembling the switching phenomena observed for inorganic ferroelectric salts ( L cc Fig. 11-33) [105].

* P

I :tire 11-33. Swager's approach to new I I ,)electric metallomesogens.

-f P

466 M.B. Ros

Chiral materials are racemic mixtures which have not yet been resolved into optical isomers in the field of liquid crystals, although a number of approaches to nonmesomorphic ferrocene derivatives have been reported [106]. The 0x0 compounds described above have shown some responses consistent with ferroelectric behavior [107]. However, in order to establish the real possibilities of both types of materials further experimental evidence is needed.

11.4 Other Properties and Borderline Cases

As far as the physical behavior of metallomesogens is concerned] magnetic, optical and electrical properties have been the main aspects studied so far. However, other approaches to the physical characterization have been tentatively explored. There are many compounds which, although not strictly being metallomesogens, are interesting materials in which liquid crystalline order is combined with the presence of metal atoms. A number of such as examples have already been mentioned in previous sections of this chapter.

Both the areas will be discussed in the following section.

11.4.1 Rheological Properties

When the phenomenon of liquid crystallinity is described, the liquid-like fluidity of the material is always an outstanding feature. Within the mesophase, the molecules can be moved by the influence of external forces such as electric or magnetic fields or mechanical stress. Indeed, as we have seen, this property is the basis of the important practical applications of this unique state of matter; displays, commuting devices, processibility of lyotropic polymers, etc. Furthermore, these properties are also fundamental for many experimental methods that allow the investigation of mesomorphic order.

The nature of these responses to external fields actually depends on the viscoelastic properties of the material. The literature on liquid crystals covers a wide variety of theoretical and experimental studies [lo81 of the rheological properties of organic liquid crystals, and in order to characterize the mesophases both traditional and new techniques are being used.

For the nematic phase, the elastic theory is the most established. From different studies it has been concluded that deformations in liquid crystals can be described in terms of three basic types of modification: splay, twist and bend (see Fig. 11-34) These basic deformations involve director orientation changes in only two direction! but combinations of these basic deformations must be used to describe more corn plicated situations.


SPLAY TWIST BEND

K11 K22 K33

Figure 11-34. Schematic representation of the three basic deformations in liquid crystals and their corresponding coefficients.

The three basic operations in liquid crystals are mediated in the potential energy of the system by three coefficients: K, (splay), K22 (twist) and K33 (bend) which are termed the elastic constants. They should be experimentally determined in order to describe the material, since their magnitude (in dyne) is closely related to the type of mesophase. While their absolute values decrease as the temperature increases, the ratios between the constants are nearly independent of temperature.

As far as the fluidity of the mesophase is concerned, the flow basically depends on the angle that the director makes with the flow direction and also on the velocity gradient along that direction. Therefore, viscosity is more complex than other liquid crystal properties and, for example, the nematic phase is characterized by five independent coefficients. Three of the coefficients are classic shear viscosities ql , q2 and q3 corresponding to different orientations of the director with respect to the shear field (Fig. 11-35). Another coefficient, denoted as q1,2 describes an intermediate geometry and the remaining coefficient ( y ) is related to the rotation about the molecular center of mass.

If we restrict ourselves to the three simple deformations, we can consider ylsplaY,

qtwist and qbend as the corresponding experimental viscosities and Kjj/q as their

/

J n

771 r12 773

Figure 11-35. Schematic representation of the basic flows of the molecules in the liquid crystalline phase and their corresponding parameters.

468 M.B. Ros

viscoelastic ratio. These parameters can be measured by using a geometrically suitable experimental setup.

Recently, Versace et al. [ 1091 reported the viscoelastic properties of metallomesogens for the first time. They measured Kll/qsplay, K22/qtwist and K33/qbend of two palladium complexes that differ structurally in their length/width (L/ W ) ratio and the data obtained were compared with those corresponding to the uncomplexed ligand (Fig. 11-36).

oCsH13

Azpac Azpac2

Figure 11-36. Chemical structure of the metallomesogens for which the viscoelastic parameters have been measured (see Fig. 11-37).

Of the different techniques suitable for these studies the authors chose light scattering. Since the polarizability of a liquid crystal medium is anisotropic, the director fluctuations give rise to fluctuations in the optical dielectric permittivity, and thus to light scattering.

Measurements in the nematic phase resulted in the following main conclusions: (i) K , and K,, of both metallomesogens are very similar, but smaller than those of the ligand. (ii) For metallomesogens, the smaller the L/Wratio the larger Kzz, while the twist elastic constant is about the same for both the ligand and the complexes. (iii) The viscoelastic ratio K/q obtained for metallomesogens (values in the range of 1 0 ~ 7 - 1 0 ~ 6 c m 2 s ) are one order of magnitude lower than those estimated for the uncomplexed ligand (Fig. 1 1-37). The lower viscoelastic constants have been attributed to a large increase in the mean viscosity for the metal-containing materials as a consequence of the presence of metal atoms.

In addition, evidence of flexoelectric effects have also been established for one of these compounds (Azpac) [I 101. This effect is related to the coupling of an electric field with the gradient of the director field in the nematic phase. For these metallomesogens this property is found to be similar to that found for calamitic organic nematic phases.

When polymers are considered instead of low molecular weight compounds, the study of the viscoelastic behavior of these materials is preferably accomplished by


1 o4

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t

1 0 5

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107

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10 20 30 40 50

Tc- T ("C)

I

€ I

I I

10 20

Tc- T ("0)

Figure 11-37. Viscoelastic ratios of both the metallomesogen Azpac and the uncomplexed organic ligand (azoxy derivative) in the mesophase represented in terms of the reduced temperature. (Adapted from [ 1091).

dynamic measurements. For this purpose, dynamic mechanical spectroscopy has proved as the probably most versatile and reliable technique for the characterization of the dynamic behavior of materials [lll].

The physical basis of this kind of study is very similar to that of the dielectric spectroscopy mentioned previously. Instead of applying an electric field that causes a polarization in the material, a small cyclic deformation is induced in the material by a cyclic mechanical stress field. Due to the viscoelastic character of the polymer, if the stress frequency is very high or the temperature is low, the molecular chains cannot relax, giving rise to a phase difference between the applied stress and the deformation. This fact allows the detection of molecular changes that cannot be observed by other techniques.

The dynamic mechanical properties of viscoelastic materials are described by a complex modulus: E* = E'+jE". Other authors use the torsional or shear modulus G instead of the tensile modulus E depending on the type of deformation applied [ I 1 I] . E' (the elastic modulus) is related to the energy that could be recovered after the deformation, whereas E" (the dissipative component) is related to the dissipated energy which cannot be recovered. The ratio E"/E' = tan 6 is called the loss factor. A material in which E" = 0 is completely elastic and it will return to its original state

470 M.B. Ros

with complete energy retention. In contrast, when E' = 0, the material will be very viscous and in its deformation all the energy will be lost due to friction.

The graphic representation of these properties is quite similar to the ones described for dielectric permittivities and, from these data, the activation energy of the deformation can be estimated. For polymers, both the dielectric and dynamic mechanical methods of characterization are complementary.

The viscoelastic behavior of metallomesogenic polymers has been analysed by different authors. In one such study, carried out by Puirtolas et al. [92] on a copper(rr) main-chain polymer, these measurements revealed the relaxations a and p, activation energies, and the particular phenomenology, which is dependent on the thermal history of the sample. These parameters were in good agreement with those measured by dielectric analysis (p. 457).

Likewise, dynamic mechanical characterization has been used to determine the elastomeric properties of ionomeric materials based on ferrocene polymers reported by Zentel et al. (see Fig. 11-21) [81].

Reduced polymer samples (pure or biphasic blends) were found to always show a continuous increase in tan 6 with increasing temperature, which indicates a noncrosslinked nature of the polymer. However, this phenomenon does not occur in the corresponding oxidized polymer. The value of tan 6 is nearly independent of temperature for pure oxidized samples and even more clearly so for binary mixtures. This trend is indicative of slightly crosslinked polymers such as the original elastomeric polystyrene (see Fig. 11-38, polymer 2b). These results proved the existence of ionic aggregates in these systems.

Apart from the accounts described above, other significant mechanical properties have not been evaluated for metal-containing liquid crystalline materials to date.

Metallomesogenic polymers are very versatile and promising materials which could combine their intrinsic optical, electrical and magnetic properties with the significant advantage of ease of processibility. The inherent liquid crystal order provides a suitable molecular array from which fibers or thin films can be prepared. Us- ing samples prepared such, physical properties and possibilities are analysed [8 b, 86,112,113].

At this stage the outstanding results regarding a new approach to improve the polymer properties and the processibility of a high performance organic polymer must be mentioned. Via reversible complex formation, organic rigid rod polymeric structures become soluble in common aprotic solvents. More interestingly, these new materials form lyotropic phases [113]. From these media, high quality films and fibers are being successfully processed with promising possibilities even for large-scale processes.

One example to illustrate this method is the case of poly(pphenyleneterephtha1- amide) (PPTA), which forms fibers of high strength and high modulus such as Kevlar. This polymer is technologically processed from concentrated sulfuric acid at elevated temperatures. However, Dembek et al. [113d] have reported that the polymer PPTA-Cr(C0)3 in N,N-dimethylacetamide forms a lyotropic mesophase similar to that formed by PFTA in sulfuric acid. The metalloaramid can be processed

11 Other Physical Properties and Possible Applications of Metallomesogens 47 1

lo9-

lo8 - 10'-

106-

lo5-

lo4-

7 lo2 Q

lo\ 10'

A

A '. polymer2b

I a

- 13.0

r+ - 11.0 5 0 2

- 9.0

- 7.0

- 5.0

- 3.0

- 1.0

1

20 60 100 140 180 220

T ( " C ) Figure 11-38. Dynamic mechanical behavior of Blend IV (triangles) (see Fig. 11-21) and its components: polymer l b (squares) and polymer 2b (circles). Temperature dependence of the elastic modulus (G') and loss factor (tan 6 ) measured at a frequency of 10 rad s- ' . (Adapted from [81b]).

from the organic medium and then decomplexed to yield the processed PPTA. Never- theless, neither modulus nor strength parameters have been published and so direct comparisons cannot be made.

11.4.2 Ion Transport and Permeation Properties

In the previous sections it was shown that metallomesogens, mainly phthalocyanine derivatives, have been proposed as materials for the transport of both photons or electrons due to their stacking order. Based on similar considerations, some authors have also proposed these materials as being suitable for ion transport.

By means of a suitable molecular design, metal-containing discotic structures with this property have been obtained. These compounds, which are 15-crown-5 ether macrocycles (n : 1) (see Fig. 1 1 -39), have been reported to form mesophases in which channels are formed along the columnar axis [I 141.

Several authors have proposed applications for these materials including information storage and transmission. However, as yet, experimental data regarding their transport properties have not been reported.

In contrast, the transport properties of the 18-crown parent system have been studied. These compounds, which are curiously nonmesomorphic [74], exhibit a

412 M.B. Ros

Figure 11-39. Molecular structure of benzo crown ether phthalocyanines and the ion channel organization proposed within the mesophase or solid state.

selective and efficient ion transport of metal cations through the assembled channels in the solid state.

Ion transport properties are not restricted to disc-like structures. Polymer/liquid crystal composites, which are not strictly metallomesogens (i.e., borderline materials), have been investigated as suitable candidates for practical ion permeation.

Shinkai et al. [ I 151 have reported that mixtures consisting of polycarbonate, an ordinary nematic organic liquid crystal and a crown ether derivative act as rapid and selective cation membranes. Moreover, the transport through these ternary compo- site membranes is directly affected by the molecular motion of the liquid crystal phase.

The so-called “liquid membrane in a polymer clothing” seems to be suitable as a good thermocontrolled system for the transport of some cations: migration of cations below the melting point does not occur, but very efficient transport can be at- tained in the liquid crystal phase.

Further studies involving polymer/mesomorphic crown ether membranes [ 1 151 also showed a selective ion permeation (see Fig. 11-40), which demonstrates that the self-assembling nature of liquid crystals is useful for the organization of ionophoric crown ether stacks.

In an attempt to provide new synthetic membranes, the authors considered that this kind of material possesses biomimetic properties with respect to the phase transition phenomena.

In this context, the ionic-conductivity switching reported by Tokuhisa et al. [116] in crown ether substituted azobenzene polysiloxanes is also noteworthy. The alkali metal ion transport is dramatically decreased by irradiation of UV light and restored


(3) CH3(CH2)7CH=CH(CH2)7COO-cholesterol I

J Pelpienem - 1 I 3 for Na’

Pelprenem - 1 1 3 and / Pelprenem I 1 2 for K+

I I I I I I

15 25 35 45 55 65 T (“C)

Figure 11-40. Plots of permeation coefficient PGt versus transport temperature. Pelprene is a polymeric matrix in which the liquid crystal is embedded. (Adapted from [115c]).

by exposure to visible light as a result of cis-trans isomerization of the azo group (see Chap. 5) .

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12 Concluding Remarks

Jose Luis Serruno and M. Bluncu Ros

In 1990, Professor G. Whitesides [ I ] was reflecting on the future of chemistry when he coined a sentence describing the history of liquid crystals very well: “The study of liquid crystals advances on two feet: one is utility, one curiosity”. Ever since the practical possibilities of liquid crystalline materials were realized, curiosity and utility have gone hand in hand such that liquid crystals now contribute significantly to the field of materials science and to our everyday life.

The field of metallomesogens, although no longer in its infancy, has become a little “lame” despite the proliferation of new and fascinating materials. The curiosity for new compounds and structures has been the main driving force behind the progress in this area, but to date very few practical applications have been realized for these materials.

Chapters 8- 11 in this book gave a thorough description of the physical behavior of metallomesogens known at present. Such materials exhibit a wide variety of physical characteristics which ranges from very simple to more sophisticated effects. Many of the properties are similar to those observed for organic liquid crystals, but others, for instance certain magnetic responses, are exclusively found for materials containing metal atoms. In consequence, many researchers in the metallomesogen field, including the authors of this book, are of the opinion that this area has great potential. Thus, we agree with the words of A.M. Giroud-Godquin and P. Maitlis published in the first review of metallomesogens [2]: “We suspect that the best application has not yet been thought of.”

In general, the characterization of the physical properties of thermotropic metallomesogens has been carried out more thoroughly than that of lyotropic metal- containing compounds. In this book the interesting and promising practical possibilities for thermotropic metallomesogens in the fields of photonics, electronics, op- tronics, magneto-materials and life science have been highlighted. However, the great interest currently being shown in surfactants [3] indicates that the future of metallolyotropic systems appears to be bright. Additionally, photochemical [4] and chemical [ 5 ] properties, antiasthmatic activity, which might be extended to other

482 J, L. Serrano and M. B. Ros

pharmaceutical applications [6 ] , hygrometric devices or humidity sensors [7], magnetic materials [8], etc., are all either directly or indirectly related to the formation or avoidance of lyotropic mesophases.

The favorable properties of these materials are not only attractive from a practical point of view but they are also of fundamental interest. However, despite the diverse physical properties of metal-containing liquid crystals discovered to date, such behavior is only known for individual examples, therefore new chemical structures should be considered for study.

Without question, to turn the undoubted practical possibilities of these compounds into reality, an in-depth physical study of these materials must be extended to encompass a wider variety of structures. It is only in this way that the potential of these materials can be assessed and progress towards a more rational structural designs can be achieved. In the authors’ view, this is the challenge that metallomesogen researchers must meet during the years to come.

One of the aims of these concluding remarks is to motivate those working on materials characterization to advance in this direction. Likewise, the authors encourage those currently involved in the synthesis of metallomesogens or those who, after reading this book, have become attracted to this field, to characterize their materials in more detail. With this approach a greater appreciation of metal-containing liquid crystals will be achieved. Clearly this will entail knocking on several doors. In the first instance physicists and engineers will need to be persuaded to characterize the physical properties and to carry out tests regarding the technical application of these materials. Although this will be difficult it will be worth it! Herein lies the problem of interdisciplinary collaboration, however, one which is eventually particularly fruitful.

Progress in the field of metallomesogens will not be difficult nor time consuming. Fortunately, this area can take advantage of the already considerable understanding of the properties of organic liquid crystals. Owing to this headstart, the field of metallomesogens, the youngest member of the liquid crystal family, wilt grow quick- ly and develop in its own right.

Recent developments: This book covers the field of metallomesogens up to June, 1994, with a few articles published afterwards. In the period between the writing of the original manuscript and the correction of the printed proofs, a number of new significant advances have been made in this field. As an example of the vitality of and future prospects for this subject, three examples dealing with three different aspects of metallomesogens, i.e. synthesis, structural characterization and physical properties should be highlighted. Thus Bunz et al. have published the first thermotropic organometallic polymer [9], Marchon et al. have proved the dynamic exchange of axial ligation in the columnar phase of dirhodium tetracarbonyl derivatives by means of I3C NMR spectroscopy [lo] and Ghedini et al. have reported the first chiral metallomesogen which exhibits an electroclinic effect [ 1 11. As can be seen, the field of metallomesogens continues to enjoy a healthy growth.

12 Concluding Remarks 483

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[5] a) T. Saji, K. Hoshino, S. Aoyagui, J. Am. Chem. SOC. 1985, 107, 6865-6868; b) T. Saji, K. Hoshino, S. Aoyagui, M. Goto, J . Am. Chem. SOC. 1991,113, 450-456; c) S. Muiioz, G. W. Gokel, 2 Am. Chem. SOC. 1993, 115, 4899-4900.

1993, 3, 905-910.

[6] T.K. Attwood, J.E. Lydon, Mol. Cryst. Liq. Cryst. 1984, 108, 349-357. [7] S. B. Elliot, U. S. Patent US 5,354,496 (CAS 122:22766u). [8] S.S. Zhu, T.M. Swager, Adv. Muter. 1995, 7, 280-284. [9] a) M. Altmann, U.H.F. Bunz, Angew. Chem. Znt. Ed. Eng. 1995, 34, 569-571;

b) M. Altmann, V. Enkelmann, G. Lieser, U. H. F. Bunz, Adv. Muter. 1995, 7, 726-728. [lo] M. Bardet, P. Maldivi, A.M. Giroud-Godquin, J.C. Marchon, Langmuir 1995, 11,

[ I l l M. Ghedini, D. Pucci, N. Scaramuzza, L. Komitov, S.T. Lagerwall, Adv. Muter. 1995, 2306-231 1.

7, 659-662.

Index

acceptor see also redox properties 440 acetate

bridge 105 ff, 111, 153, 182, 221 counterion 242

acetylacetone coligand 1 13 ff acidred 35 activation energy 447,456 f, 470 2-acylaminopyridine

complexes see cobalt, copper, nickel,

synthesis of 250 palladium

addition polymer 302,312,3 14 f alignment 432,455,451

by electric field 427 by magnetic field see also orientation by magnetic field 329 ff, 351, 398 f, 403,412 ff

homeotropic 351,360, 362,427 homogeneous 424,456

carboxylate 2, 6,29 f, 81 f, 145,253 ff crown-ether

alkali metal

chromism 437 f electrical conductivity 449 ion transport 471 ff

molecular recognition 226 pentafluorooctanoate 30 phthalocyanine, electrical conductivity 447 f

alkaline-earth metal carboxylate 2, 29, 81 f, 145 dodecylbenzenesulfonate 30 porphyrin photovoltaic effect 452

alkanoate see carboxylate alkene coligand 50,239 alkyn yl

complexes see platinum polymer see poly(yne) polymer synthesis of 241

dodecylbenzenesulfonate 30 phthalocyanine 36 polymer complexation (reversible) 198 porphyrin 173 f, 269 f

zinc

aluminium

mine complex see copper, iron, silver,

amino acid coligand 114 aniline coligand 11 1 annelide see cobalt, copper antiasthma drug 34 f, 481 antiferromagnetism 387, 391 ff, 396,

apparent molecular length 332 ff aramid see chromium aroy 1 hydrazine

398,403

complexes see copper, nickel synthesis of 249

averaging of the EPR spectrum

486 Index

by exchange 352 ff by motion 353

averaging of the g-factor 352 f, 367 azide bridge 38 azine

complexes see palladium synthesis of 274

complexes see palladium polymer see palladium synthesis of 274

complexes see mercury, palladium synthesis of 274

azo

azoxy

barium see alkali-earth metal benzalimine see benzylideneamine,

bis( benzy1idene)diamine bcnzoatc see carboxylate benzylideneamine

complexes see manganese, palladium,

synthesis of 274

carboxylate complexes 82, 148 P-diketone copper 7 5 3 3 1 f, 427 f enaminoketone copper 428 ortho-palladated

complexes 103, 33 I , 428 porphyrin zinc 91 salicylideneamine copper 58 f, 33 1

bilayer organization 30, 5 1 f, 94, 341 2,2’-bipyridine

coligand 115 complexes see rhodium, ruthenium polymer see copper, iron

azoxy /I-diketone palladium 425 enaminoketone copper 425 f nitrile palladium 425 stilbazole complexes 425

complexes see cobalt, copper, nickel,

platinum, rhenium

biaxiality 426 f

birefringence 424

6,6’-bis(acylamino)-2,2’-bipyridine

palladium

synthesis of 250 bis(benzy1idene)diamine see palladium bis(ethy1enediamine)dodecyloxysalicylalde-

bis(ethy1enediamine)laurate see cobalt bisphthalocyanine see copper bis( salicy1idene)diamine

hyde see chromium

complexes see copper, iridium, nickel, rhodium, vanadium

synthesis of 245 f bis(sa1icylidene)diiminc see

bis(salicy1idene)diamine Bleaney-Browers law 392 blue phase 67 Boltzmann statistic 392 Bragg’s law 326 ff branched chains 147 ff, 164 f, 170, 176,

Brillouin function 393 butadiene

complexes see iron synthesis of 286

186

caesium see alkali metal cadmium

carboxylate 145,253 porphyrin 172

photovoltaic effect 452 calcium see alkaline-earth calixarene see tungsten camphene 37 carbox ylate

bridge 108 ff, 280 complexes see alkali metal, alkaline-earth

metal, cadmium, chromium, copper, lead, molybdenum, rhodium, ruthenium, thallium, tungsten

carrier mobility 445 ff charge carrier 445 ff charge transfer 440,450 ff

benzylideneamine complexes 154,342 f,

bis(benzy1idene)diamine palladium 39, 452

182,342 f

Index 487

dithiolene complexes 450 f salts 445,450 f

chiral polymer 210 chirality 13, 60,67, 104, 107, 109, 113 f,

chromonic 15,29,34 chromium

aramid I99 f, 313 bis(ethylenediamine)dodecyloxy salicylal-

carboxylate 150, 255 P-diketone 141 f, 253 dodecylbenzenesulfonate 30 phenanthroline tartrate 39 f polymer complexation (reversible) 198 ff,

1,4,7-triazacyclononane 155 f, 265 f CMC see critical micellar concentration cobalt

121 f, 210,228,435 ff, 459 f, 465 f

dehyde 32

470 f

2-acylaminopyridine 250 annelide 30 6,6’-bis(acylamino)-2,2’-bipyridine 92 f,

bis(ethy1enediamine)laurate 32 cyclobutadiene-c yclopentadiene

P-diketone polymer 219,220,316 dodecylbenzenesulfonate 30 1,4,7,10,13,16-hexaazacyclooctadecane

183 f, 266 phthalocyanine 36, 167 f, 270 ff phthalocyanine polymer 201,2 14, 3 1 1,

poly(yne) polymer 307 porphyrin 173,267 ff

reactive phthalocyanine 224 salicylideneamine 60,243 ff tetraazaporphyrin 174 f, 267 f tetrapyrazinoporphyrazine 175 f

color see also dichroism, electrochromism, and thermochromism 425,441 collapse of the hyperfine structure 352 condensation polymer 302, 3 12, 3 13

250

polymer 482

318

NLO 441

conoscopic studies 58,75, 103, 332,427 f copper

2-acylaminopyridine 250 amine 51,242 annelide 90

EPR 350,354f aroyl hydrazine 74, 247 ff bipyridine polymer 2 16 f, 3 16

6,6’ -bi s( ac ylamino)-2,2' -bipyridine 92 f,

bis(salicy1idene)diamine 9 1 f, 246 bisphthalocyanine 186 carboxylate 37 f, 145 ff, 253, 255, 340 f

EPR 375

250

EXAFS 147,340,344 f magnetic susceptibility 394 ff neutron scattering 147

carboxylate

tetrathiacyclooctadecane 93 f, 264

biaxiality 331 f, 427 electrical conductivity 447 EPR 357ff magnetic susceptibility 398 ff NMR 133

P-diketone polymer 208,209,3 1 1 ff crosslinked 216 f, 219 ff, 316 EPR 375f,379

P-diketone Schiff base 178, 339 dithiocarbamate 88 f, 261 f

dodecylbenzenesulfonate 30 enaminoketone 71 ff, 247

diacetylenic complex see copper, reactive

1, I O-diaza-4,7,13,16-

P-diketone 74 f, 133 ff, 137 f, 251 ff, 332

EPR 374

biaxiality 428 birefringence 425 polarizability 421 ff

36, 160, 162 ff, 168 ff, 270 ff electrical conductivity 447 ff EPR 355ff NLO 441

phthalocyanine see also bisphthalocyanine

phthalocyanine polymer 20 1 , 2 15,3 18

488 Index

electrical conductivity 448 f cyclopentadiene iron 1 19 polyethyleneimines 226 1 ,lO-diaza-4,7,13,16- porphyrin 36 f, 172 f, 269 f tetrathiacyclooctadecane silver 94

NLO 441 @-diketone complexes 135, 141,338 pyrrole 54,262 f dithiocarboxylate complexes 86 f reactive carboxylate 224 f, 3 14 f salicylideneamine copper 335,346 reactive P-diketone 224 f, 3 14 f thiolate silver 341 reactive phthalocyanine 224, 314 f, 448 f 1,3,5-triketone Schiff base copper 186 salicylaldehyde 8 1,244 f cubic phase 17,30,32,46, 52, 342

salicylideneamine 54 ff, 243 ff, 331, 333 ff Curie law 390 ff, 398 f, 400 EPR 373f Curie constant 390,399

dielectric studies 456 f Curie temperature 39 I ff, 402 ff EPR 338,359 ff Curie-Weiss law 391 ff, 402 f EXAFS 59,346 cybotactic 333 f ferroelectricity 462 ff cyclobutadiene see cobalt magnetic properties 400 f, 41 0,414 ff cyclopentadiene see iron, ruthenium,

salicylideneamine ferrocene 69 thallium salicylideneamine polymer 202 ff, 3 1 1 ff,

334 density operator 389 crosslinked polyazomethines 217 f, 222 f, diamagnetic material 349, 356,365

316 diamagnetism 387 dielectric studies 456 f diamidate-diphenolate see vanadium EPR 376 ff, 402 f 1 ,lO-diaza-4,7,13,16- viscoelasticity 470 tetrathiacyclooctadecane

1,4,8,1 l-tetraazacyclotetradecane 176,266 complexes see copper, palladium, silver tetraazaporphyrin 174 f, 267 f 1,3,5,7-tetraketone 181,339 1,3,5,7-tetraketone Schiff base 187,339 tetrapyrazinoporphyrazine 175 f thiolate 52 1,3,5-triketone 157 ff, 339 1,3,5-triketone Schiff base 184 ff, 339

length 328,365,416 time 359

correlation

critical micellar concentration 30 crosslinked polymer 8, 193 ff, 201, 210,

crown ether see also alkali metals 166 f,

crystal structure

216 ff, 316 ff, 452 f, 470 f

170, 225 ff, 450

benzylideneamine palladium 106 bis( salicy1idene)diamine nickel 92 carboxylate copper 146, 340

synthesis of 264 dibenzaldiimine see bis(benzy1idene)diamine dichroic contrast ratio 430 ff dichroism 428 f

dithiolene nickel 430 ff dithiocarboxylate complexes 430 ff

behavior 454 ff permittivity 455 ff spectroscopy 455 ff, 469 f

dielectric

cyclopentadiene-iron polymer 470 f salicylideneamine complexes 456 f, 459 salicylideneamine-copper polymer 457 f

diimine see bis(benzylidene)diamine, bis( salicylidene)diamine, /3-diketone Schiff

base, 1,3,5,7-tetraketone Schiff base, I ,3,5-triketone Schiff base

P-di ketone

complexes see chromium, copper, iron, manganese, nickel, palladium, thallium, vanadium

palladium polymer see cobalt, copper, nickel,

synthesis of 25 1 f P-diketone Schiff base

complexes see copper, nickel, palladium,

synthesis of 246 dinuclear complexes 67 f, 100 ff, 145 ff,

157,181, 184 ff, 224 dioxime

complexes see nickel, palladium synthesis of 263

vanadium

4,4’-dipyridyl coligand 148 disodium chromoglycate 34 dithiobenzoate see dithiocarboxylate dithiocarboxylate

complexes see gold, molybdenum, nickel, palladium, zinc

synthesis of 259 f

complexes see copper, nickel, palladium,

synthesis of 261 f

complexes see nickel, palladium, platinum synthesis of 256 f

chromium, cobalt, copper, iron, magnesium, manganese

dodecylsulfate counterion 46, 239 donor see also redox properties 440 DOS see dodecylsulfate counterion dye 201,428 ff dynamic mechanical spectroscopy 2 14,

dynamic modulus 469 ff

dithiocarbamate

zinc

dithiolene

dodecylbenzenesulfonate see aluminium,

469 ff

elastic constants 467 f deformations 466 f

Index 489

470 f

charge-transfer complexes 450 f crown-ether alkali metals 449 P-diketone copper 447 dithiolene complexes 450 f metallomacrocycles 445 ff one-dimensional 160,447 ff phthalocyanine complexes 447 ff polymer 214 f, 223,448 f, 451

electrical conductivity 445 ff

electrical properties 3,443 ff electrical susceptibility 439 ff electron paramagnetic resonance (EPR)

annelide copper 350,354 f bipyridine-copper polymer 21 6, 375 P-diketone copper 357 ff P-diketone polymers 218 ff, 357 f, 379 dithiocarbamate copper 374 fiber 376,379 f line narrowing 352 f, 358 line shape 349 phthalocyanine complexes 355 ff salicylaldehyde copper 373 ff salicylideneamine complexes 338, 359 ff,

salicylideneamine polymers 376 ff, 402 f spectroscopy 349 f

electrochromism 452 phthalocyanine lutetium 452

electroclinic effect salicylideneamine palladium 482

enaminoketone complexes see copper, palladium synthesis of 247

one-dimensional 443 phthalocyanine complexes 442

363 ff

energy migration 442

EPR see electron paramagnetic resonance Euler angles 362, 404,407 EXAFS see extended X-ray absorption fine structure exchange interaction 352, 358, 361, 365 f,

370 ff, 391 ff, 482 elastomers 8,212, 216,218, 220,452 f, exciton 442 ff

490 Index

extended X-ray absorption fine structure 343 f carboxylate complexes 147,340, 344 f dithiocarboxylate complexes 86 f, 346 phthalocyanine platinum 162, 346 salicylideneamine copper 59, 346

Faraday method 398 fast motion limit 349, 354, 376 ferrocene see iron cyclopentadiene ferrocenophane 292 f ferroelectric

aLine palladium 461 ff benzylideneamine palladium 462 f benzylideneamine P-di ketone complexes 462 f liquid crystals 13,440,458 ff salicylideneamine complexes 462 f, 482

ferromagnetism 387, 391 fiber 148, 198,224, 315,450,470

EPR 376,379f films I98 f, 452, 470 flexoelastic effect

azoxy P-diketone palladium 468 f fluoro substituent 48 f, 64,76 ff, 87 Fredericksr; transition 399

gallium

germanium polymer complexation (reversible) 198

phcnyl 3,95 phthalocyanine I68 phthalocyanine polymer 214, 301, 307, 309

glass transition 42 I , 457 f, 470 glyoxime see dioxime gold

isonitrile 98 f, 240 dithiocarhoxylate 88, 260 stilbazole 50

Goldstone mode 458 f graft polymer 3 I6 grafting 2 10, 3 17, 3 I9 g-tensor 350 ff, 362, 389 ff

averaging 352, 367 principal values 350 ff, 360, 362 ff, 372 ff

halogen bridge 37 f, 101 f, 105 ff, 108, 111, 153 f, 182,222

coligand 44 f ,49 f, 97 ff, 150 substituent 65

Hamiltonian 349 ff, 389 half field EPR transition 355 HDVV Hamiltonian see Heisenberg-Dirac-van Vleck Hamiltonian

Heisenberg interaction see Hcisenberg-Dirac-van Vleck Hamiltonian helical

391 f, 395,397

mesophase 435 ff, 459 f pitch 436 ff twist 438 f

heteronuclear complex 69, 184 hexaalkanoyloxybenzene 13 1 I ,4,7,10,13,16-hexaa~acyclooctadecane

complexes see cobalt, nickel hexachloroantimonate counterion 161 hexafluorophosphate counterion 32,47, 93,239

complexation 18 1 systems 428 ff, 437 f

host-gucst

hydroxo coligand 160, 168, 173 2-hydroxyazobenzene see palladium

azoxy-2-hydrox y azobenzene hyperfine

interaction 349 ff, 358 principal values 35 1 ff

hyperpolarizability 439 ff

imine see benzylideneamine, bis(benzylidene)diamine, bis( salicylidene)diamine, P-diketone Schiff base, salicylideneamine, 1,3,5,7-tetraketone Schiff base,

1,3,5-triketone Schiff base imine polymer see palladium

Index 491

infrared spectroscopy 41 9 interdigitation 46, 87, 107, 334 f ion transport see alkali metal inorganic polymer 199, 201 ionic conductivity 226 f, 445, 452 ionomers see iron cyclopentadiene,

IR see infrared spectroscopy iridium

polymer

bis(salicy1idene)diamine 70 f, 246 f pyridylmethyleneaniline 49,238 f, 423 stilbazole 49,238 f

birefringence 425 NLO 441 polarizability 421 ff

iron amine 31 bipyridine polymer 216 f, 316 butadiene 122, 286 cyclopentadiene 2 f, 117 ff, 287 ff, 465 f cyclopentadiene-copper salicylideneamine

cyclopentadiene polymer 207, 2 10 f, 3 13, 317,319

69

dielectric behavior 452 f ionomers 208,212 ff, 314 f, 452 f, 470 f viscoelasticity 470 f

P-diketone 141 f, 253 dodecylbenzenesulfonate 30 phthalocyanine polymer 201,3 18 poly(yne) polymer 307 salicylideneamine 62 f, 67,243 ff

magnetic susceptibility 396 ff salicylideneamine polymer 21 8, 3 16

EPR 378f isonitrile

complexes see gold, palladium, platinum synthesis of 240

Langmuir-Blodgett 226 lead

carboxylate 82, 253 phenyl 95

reactive phthalocyanine 224 linear coordination 46,50 ff, 1 16, 342 linear response approximation 389, 393 lithium see also alkali metal

phthalocyanine 16 1 f

local order 331, 368, 370 lutetium

electrical conductivity 447 f

phthalocyanine 160 ff, 164, 170 f, 270 f electrical conductivity 447 f electrochromism 452

magnesium see also alkaline-earth metal

magnetic properties 3, 387 ff magnetic

porphyrin photovoltaic effect 452

dimer see magnetic pair interaction 350, 391 moment 387 f multiplet 39 1 order 388 pair 346,355,359,391 ff susceptibility

and mesophase order 388,403 ff anisotropy of the 330, 337 f, 387 f,

benzene ring 387,412 diamagnetic contribution 388,397 f,

molecular 411 paramagnetic contribution 338,388 ff principal values 390 ff, 408 zero field contribution 391

398 f, 400,406 ff

403 ff

magnetization 388 f, 393

Maier-Saupe order parameter 400, 407 ff,

Maier-Saupe theory 410 manganese

measurements 403

410

benzylideneamine 1 15 f, 285 P-diketone 141 f, 253 dodecylbenzenesulfonate 30 phthalocyanine 160

phthalocyanine 160, 164, 167, 270 ff, 342 mercury

492 Index

azoxy 116,285 phenyl 2,95

interaction 140 f, 147, 156, 176 f, 186, 338 ff, 345 f

metal-heteroatom intermolecular

ortho-metallization 100 ff, 153 f, 182, 33 1 metal-metal intermolecular interaction

141,338 f, 346 metal-metal intramolecular bond 147,

150,345 metallocene see also cyclopentadiene

1 17 ff, 187,287,465 f micelle 15 f, 18 f, 29, 341 f, 355 molecular

dynamics 365 magnetic susceptibility 403 ff mechanics calculations 163 motion 349, 353,361, 366,374,455 ff plasticity 362 recognition 226,266 rotation 36 1,455 ff shape

brick-like 6, 55, 331,366, 369, 381,416 h-shape 100,333 lantem-shape 146 f, 340,344 open-book shape 105, 109,222,331 prismatic see brick-like p-shape I13 roof-shape see open-book shape sanidic see brick-like s-geometry 119 u-shape 51, 94

molybdenum carboxylate 150, 255 dithiocarboxylate 152 f pyridinediyl-2,6-dimethanol 156 f 1,4,7-triazacyclononane 155 f, 265 f

monothio-b-diketone complexes see nickel synthesis of 255 f

nematic columnar phase 14, 35 ff, 13 1 f, 184

networks 8, 216, 223 ff, 228, 315, 317, 448 f

neutron scattering 103, 147, 326 nickel

2-acylaminopyridine 250 aroylhydrazine 73 f, 247 ff 6,6’-bis(acylamino)-2,2’-bipyridine 92 f, 250 bis(salicy1idene)diamine 91 f, 246 P-diketone 133, 135, 138 P-diketone polymer 21 6 f, 2 19 f, 3 1 6 P-diketone Schiff base 178, 339 dioxime 144 f, 263

dithiocarboxylate 84 ff, 152, 259 ff thermochromism 434 ff

dichroism 430 ff thermochromism 435

dithiocarbamate 88 f, 261 f dithiolene 3, 83 f, 142 ff, 256 f

dichroism 430 ff electrical conductivity 450 f

dodecylbenzenesulfonate 30 1,4,7,10,13,16-hexaazacyclooctadecane

183 f, 266 monothio-P-diketone 83,255 f phthalocyanine 164, 167 ff, 270 ff phthalocyanine polymer 20 1, 3 18 poly(yne) polymer 195 ff, 305 ff porphyrin 36 f, 173,269 f

pyrrole 54,262 f reactive phthalocyanine 224 salicylideneamine 60,62 ff, 243 ff, 333 ff

tetraazaporphyrin 174 f, 267 f tetrapyrazinoporphyrazine 175 f 1,4,7-triazacyclononane 155 f, 265 1,3,5-triketone Schiff base 184 ff, 339 xanthate 84, 261

239,242

complexes see palladium, platinum,

NLO 441

EPR 338

nitrate counterion 47, 51, 155, 176, 183 f,

nitrile

rhodium

Index 493

polymer see platinum synthesis of 237

NLO see nonlinear optical properties NMR see nuclear magnetic resonance nonlinear optical properties 193,224,439

azoxy 1 P-diketone palladium 440 benzylideneamine I P-diketone complexes

effective coefficient 440 f phthalocyanine copper 441 poly(yne) polymers 441 porphyrine complexes 441 second order effects 439 ff stilbazole complexes 441 third order effects 439 ff

nuclear magnetic resonance 102, 106 f, 133, 195, 197,419,428,482

nuclear spin 351 f

440 f

octahedral coordination 1 15, 14 1, 2 16 octylsulfate counterion 46 optical properties 3,423 ff, 460 order

and magnetic susceptibility 403 ff macroscopic approximation 405 f magnetic 388 microscopic approximation 404 f parameter 354,421, 425,428 f, 433

Maier-Saupe 400, 407 ff, 410 minimal set 406 optical 430 ff Straley 407 tensorial 405 f

orientation by magnetic field 329 ff, 336 ff, 351, 365, 372, 374, 396, 398, 401, 403,412 ff

oxovanadium see vanadium

palladium 2-acylaminopyridine 250 azine 108 ff, 279 f, 331

ferroelectricity 461 f azine /a-amino acid 1’14, 283 azo 101 ff, 275,331

biaxiality 428 neutron scattering 103 NMR 102

azo I a-amino acid 114,283 azo I aniline 11 1,282 azo I P-diketone 1 14,282 azo polymer 220 ff, 316 azo I pyridine 11 1, 282 azo I quinoline 11 1,282 azoxy 276 azoxy lP-diketone 114

birefringence 424 f flexoelectric effect 468 f NLO 440 viscoelasticity 468 f

azoxy I 2-hydroxyazobenzene 114 f, 283 azoxy I salicylideneamine 114 f, 283 benzylideneamine see also

bis(benzy1idene)diamine 105 ff, 153 f, 182,276 ff, 342 f

charge transfer 154,452 ferroelectricity 462 ff NMR 106 thermochromism 438 f

benzylideneamine I a-amino acid 1 14,283 benzylideneamine ID-diketone 11 3,282

ferroelectricity 462 f NLO 440

6,6’-bis(acylamino)-2,2’-bipyridine 92, 250

bis(benzy1idene)diamine 37 ff, 182, 342 1,10-diaza-4,7,13,16-tetrathiacycloocta-

P-diketone 135, 137, 140,251 P-diketone polymer 220 f, 3 16 P-diketone Schiff base 178, 339 dioxime 144 f, 263

dithiocarboxylate 85 ff, 260 f

decane 93 f

thermochromism 434

dichroism 430 ff EXAFS 87,346

dithiocarbamate 88 f, 261 f dithiolene 3, 84, 256 f

electrical conductivity 450 f

494 Index

enaminoketone 71 f imine polymer 220 ff, 3 I6 isonitrile 97 f, 240 nitrile 44 f, 237

birefringence 425 polarizability 42 1 ff

poly(yne) polymer 196 ff, 303 ff cationic 197 f electrical conductivity 45 1 NLO 441

photovoltaic effcct 452 porphyrin 173,269 f

pyrimidine I 1 1, 28 1 pyrimidine / 2,2'-bipyridine I 15 pyrimidine /P-diketonc 1 15, 282 salicylidcneamine 57 ff, 243 ff

electroclinic effect 482 ferroelectricity 462 ff, 482

1,3,5-trikctone Schiff base 184 ff, 339

liquid crystal 349 ff, 394 ff liquid crystal polymer 375 ff, 402 ff probe 349 susceptibility tensor 388 ff, 394, 396 ff,

paramagnetic

404,410 ff paramagnetic entity

paramagnetism 387 motion 352 f, 360, 368, 374

temperature independent 39 1,395,397, 403

PBF theory see Polnasek, Bruno and Freed theory pentafluorooctanoate see alkali metal,

caesium, lithium permeation 471 ff phenanthroline bee chromium phenyl see germanium, lead, mercury, tin phen y lp yridinecarbox ylatc

complcxcs SCP silver synthesis of 238

phosphine coligand 96 f,195 ff, 242, 303, 305 ff

photochemical properties 435, 481 photopolymerization

in-situ 216,224, 227 topochemical 224,3 14,3 I5

photoeffects 442 ff, 4.52 ff photosensitive 226 f photovoltaic effect 442,452 ff

porphyrin complexes 452 ff phthalocyanine

complexes see aluminium, cobalt, copper, germanium, lead, lithium, lutetium, manganese, nickel, platinum, tin, zinc

synthesis of 270 ff phthalocyanine dimer and trimer 169 phthalocyanine polymer see cobalt,

platinum copper, germanium, iron, nickel, silicon, tin

alkynyl 96 f, 242 benzylideneamine 154 f

benzylideneamine / P-diketonc charge transfer 452

ferroelectricity 440 NLO 440f

dithiolene 83 f, 256 f electrical conductivity 450 f

isonitrile 97 f, 240 nitrile 44 f, 237

polarizabilily 421 ff nitrile polymer 2 19, 3 16 phthalocyanine 162, 164

poly(yne) polymcr 19.5 ff, 199, 303 ff

salicylideneamine 60 stilbazole 49 f, 239

polarizability 439,421 ff enaminoketone copper 421 f nitrile complexes 421 ff stilbazole complcxes 422 f

369

375

315,318

EXAFS 162,346

NLO 441

Polnasek, Bruno and Freed theory 349,

polyacrylate 213,215, 219 f, 223, 314,

polyamide see also aramid 200 f, 3 13,

Index 495

polyazomethine 217 f polycondensation see polymerization polyester 203 ff, 212, 21 6 f, 312 f polygermoxane see also germanium 308 polymer processibility 470 f polymerization 148, 160, 168

in-situ see photopolymerization interfacial 3 12 f melt polycondensation 31 3, 315 solution

polyaddition 314 f polycondensation 308 ff, 312 f, 315

polyaddition 224, 3 14 polycondensation 307 ff

thermal bulk

topochemical see photopolymerization polymethacrylate 226, 3 14 polysiloxane see also silicon 2 10 f, 2 15,

219 ff, 227,308,317,319 poly(yne) polymer see cobalt, iron, nickel,

palladium, platinum, rhodium, ruthenium, s i 1 icon NMR 195,197

porphyrazine see tetraazaporhyrin porphyrin

complexes see aluminium, cadmium, cobalt, copper, nickel, palladium, vanadium, zinc

synthesis of 266 ff potassium see also alkali metal

n-type dopant 449 pyrazine coligand 148 pyridylmethyleneaniline

complexes see iridium, rhodium, silver synthesis of 238

pyridine coligand 11 1 pyridinediyl-2,6-dimethanol see

pyrimidine molybdenum

complexes see palladium synthesis of 274

complexes see copper, nickel synthesis of 262

pyrrole

quinoline coligand 11 1

reactive metallomesogens 224 f, 227 carboxylate see copper P-diketone see copper phthalocyanine see cobalt, copper, lead,

nickel, zinc redox properties 445,450 ff refractive index 421, 424 ff, 436,440 relaxation 442, 455 ff

frequency 455 ff time 455 ff, 460

response time 427,461 ff rhenium

benzylidenamine 1 16,285 rheological properties 466 f rhodium

2,2’-bipyridine 32 f bis(salicy1idene)diamine 70 f, 246 f carboxylate 37 f, 147, 150 f, 253 ff, 340 f,

482 EXAFS 147,340,344 f

nitrile 45, 237 poly(yne) polymer 307 pyridylmethyleneaniline 49, 238 f, 423 stilbazole 49,238 f

NLO 441 ribbon-like organization 81 ruthenium

2,2’-bipyridine 32 f carboxylate 147 ff, 253 ff cyclopentadiene 120, 293 poly(yne) polymer 307

ruthenocene see ruthenium cyclopen tadiene

salicylaldehyde see copper salicylaldimine see salicylideneamine salicylaldimine polymer see

salicy Iideneamine polymer salicylideneamine see also

bis( salicy1idene)diamine complexes see cobalt, copper, iron, nickel,

palladium, platinum, vanadium, zinc

496 Index

synthesis of 243 f salicylideneamine polymer see copper, iron, vanadium saturation effect in magnetism 393,403 Schiff base see benzylideneamine, bis(benzyIidene)diamine, bis(salicy1i-

deneldiamine, P-diketone Schiff babe, salicylideneamine, 1,3,5,7-tetraketone Schiff base, 1,3,5-triketone Schiff base

self-organizing structures see also molecular recognition 7,472 f complementary molecular shapes 137,

140 f, 157, 178, 181, 186 f, 338 f semiconductor

n-doped 447 ff p-doped 447 ff

SH see spin Hamiltonian side-by-side coupling 33 1,346 silicon

phthalocyanine polymer 214,301,307 ff,

poly(yne) polymer 197, 199

amitie 51, 242 cholesteric acid salt 2 1,10-diaza-4,7,13,16-tetrathiacyclooclade-

cane 33 f, 94,264 phenylpyridine carboxylate 47 f, 239 f pyridylmethylene aniline 47 f, 239 f stilbazole 46 ff, 239 f

birefringence 425 polarizability 42 1 ff

thiolate 51,243, 341 f

448

silver

single domain EPR spectrum 362 Sirius Supra Brown RLL 35 soap see carboxylate sodium see alkali metal soft mode 458 f spectroscopic studies see also dielectric

spectroscopy, dynamic mechanical

spin Hamiltonian 349, 351 ff, 366,374 dynamic contribution 352 ff exchange interaction 352 f hyperfine contribution 351 static contribution 353 f

spontaneous polarization 460 ff square-pyramidal coordination 59,67,86,

135, 178 stilbazole

complexes see gold, iridium, platinum, rhodium, silver

synthesis of 238

(-)-TAPA see (-)-2-(2,4,5,7-tetranitro-c)-fluorenylidene- aminooxy)propio nic acid

tartrate see chromium 1,4,8,11-tetraazacyclotetradecane

complexes see copper synthesis of 266

complexes see cobalt, copper, nickel, zinc synthesis of 267 f

tetrabromozincate 5 1 tetrafluoroborate counterion 46,93 f, 1 15,

tetrahedral coordination 60, 93 1,3,5,7-tetraketone see copper 1,3,5,7-tetraketone Schiff base see copper (-)-2-(2,4,5,7-tetranitro-9-fluorenylidene-

tetranuclear complexes 182 tetrapalladium organyl 37 ff, 182, 342 tetrapyrazinoporphyrazine see cobalt,

thallium

te traazaporphyrin

161,239,242

aminooxy)propionic acid 39

copper, nickel

carboxylate 82,253 ff cyclopentadiene 187 P-diketone 140 f, 252 f, 338 f

thermochromism 434 spectroscopy, IR, NMR, UV-vis 443,435 benzylideneamine palladium 438 f

spinal columnar polymer 2 14,2 16, 2 18,

spin diffusion 360 phthalocyanine complexes 434 f

dioxime complexes 434 ff 302,307 f, 448 dithiocarboxylate nickel 435

Index 491

thiocyanate bridge38, 105 f, 108, 153, 182 thiolate

bridge 108 complexes see copper, silver

independent

phenyl 3,95 phthalocyanine 160,270 f phthalocynanine polymer 214,308 f polyester 208 ff, 3 13

TNF see 2,4,7-trinitrofluorenone 1,4,7-triazacyclononane

TIP see paramagnetism, temperature

tin

complexes see chromium, molybdenum, nickel, tungsten synthesis of 265 f

counterion triflate counterion see trifluorosulfate

trifluorosulfate counterion 47, 239 trigonal-planar coordination 52, 341 trigonal-pyramidal coordination 86 1,3,5-triketone see copper 1,3,5-triketone Schiff base see copper,

2,4,7-trinitrofluorenone 39, 154 f, 182,

tungsten

nickel, palladium

342,452

calixarene 178 ff carboxylate 150, 255 1,4,7-triazacyclononane 155 f, 265 f

ultraviolet-visible spectroscopy 4 19,

UV-vis see ultraviolet-visible 430 ff

spectroscopy

vanadium bis(salicy1idene)diamine 92, 176 ff, 246 diamidate-diphenolate 33 P-diketone 135 f, 138 ff, 251 f B-diketone Schiff base 178 porphyrin NLO 441 salicylideneamine 58 ff, 243 ff, 333 ff

dielectric studies 456 ff

EPR 338,363 ff ferroelectricity 462 ff magnetic susceptibility 400 f, 414 f

salycilydeneamine polymer 218, 3 16 EPR 378

viscosity 42 1, 463 ff, 467 ff coefficients 467f

viscoelasticity 466 ff azoxy lb-diketone palladium 468 f coefficients 467 f cyclopentadiene-iron polymer 470 f ratio 468 f salic ylideneaminexopper polymer 470

Weiss mean field approximation 394 Wigner matrices 405

XANES see X-ray absorption near-edge

xanthate structure)

complexes see nickel synthesis of 261

X-ray absorption near-edge structure 346 X-ray diffraction 326 ff

amine zinc 51 annelide cobalt 30 aroylhydrazine nickel 73 azine palladium 33 1 azo palladium 103, 331 benzylideneamine complexes 106 f, 342 f bis(benzy1idene)diamine palladium 342 f bis(sa1icylidene)diamine nickel 92 carboxylate complexes 82, 146 f, 150,

cyclopentadiene iron 119 1,10-diaza-4,7,13,16-tetrathiacyclo-

octadecane silver 94 /3-diketone complexes 135, 137, 141,332,

338 f, 398 /3-diketone Schiff base complexes 178,

339 dithiolene nickel 143 phthalocyanine complexes 160 ff, 169,

340 f

342

498 Index

salicylideneamine complexes 59, 33 1,

salicylideneamine-copper polymer 334,

1,3,5,7-tetraketone copper 339 I ,3,5,7-tetraketone Schiff base copper 339 thiolate silver 341 f 1,3,5-triketone copper 339 1 A5-triketone Schiff base complexes

333 ff, 346,363 f, 370,416

380 f

186,339

Zeeman interaction 350 ff, 356,363, 389 zero field contribution 391 zinc

amine 51 dithiocarbamate 89, 261 f dithiocarboxylate 85 ff, 260 f

dichroisrn 430 ff EXAFS 86,346 NLO 441

phthalocyanine 36, 160,168 f, 270 f energy migration 442 ff EPR 356f

porphyrin 90 f, 171 ff, 266 ff photovoltaic effect 452 ff

reactive phthalocyanine 224 salicylidenearnine 60, 243 ff tetraazaporphyrin 174 f, 267 f

metallomesogens

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