silver and gold luminescent metallomesogens based on pyrazole ligands

PAPER www.rsc.org/dalton | Dalton Transactions

Silver and gold luminescent metallomesogens based on pyrazole ligands†

Marıa Jose Mayoral,a Paloma Ovejero,a Jose Antonio Campo,a Jose Vicente Heras,a Elena Pinilla,a,b

Marıa Rosario Torres,b Carlos Lodeiro*c and Mercedes Cano*a

Received 20th May 2008, Accepted 12th September 2008First published as an Advance Article on the web 3rd November 2008DOI: 10.1039/b808487d

A series of ionic bis(pyrazole)-silver(I) and -gold(I) complexes [ML2][A] (M = Ag, Au; A = BF4-, PF6

-,NO3

-), prepared by coordination of the mesomorphic L = Hpz2R(n) or non-mesomorphic L = HpzR(n)

pyrazole ligands (Hpz2R(n) = 3,5-bis(4-alkyloxyphenyl)pyrazole; HpzR(n) = 3-(4-alkyloxyphenyl)-pyrazole), has been studied. The complexes exhibit enantiotropic behaviour, showing smectic A (SmA)mesophases. The choice of the ligands allows the achievement of ‘H’ or ‘U’ molecular shapes, whichappear to be responsible for the attainment of liquid crystal mesophases, these not being dependent onthe coordinating or non-coordinating nature of the A counteranions. The new complexes arephotoluminescent both in the solid state and in solution at room temperature. In addition, theluminescent behaviour of selected compounds as a function of the temperature indicates that theluminescence is maintained in the mesophase.

Introduction

Ordered molecular materials are increasingly used in activeelectronic and photonic devices and the supramolecular structuresof liquid crystals can be tailored to improve such devices in order tomake their performances conform to the requirements for practicalapplications.1 New technologies require the use of devices based onphysical properties as light emission or charge transport (displays,solar cells, data treatment and storage, image components), andin this context emitting liquid crystals are useful materials fordisplays applications.2,3

In particular, the interest in liquid crystal materials as a supportof the modern technology of LCDs demands an increase in theirresearch which is specially evident in the cost-saving color LCDsdisplays. Those devices present several limitations such as lowbrightness and energy efficiency due to the use of polarizers andcolor filters which transform a great part of the incident light intothermal energy4 and one way to overcome this shortcoming canbe the use of luminescent liquid crystals.

Although a large number of luminescent liquid crystal or-ganic compounds have been reported, metallomesogens (metal-containing liquid crystals) exhibiting luminescence are scarcelydescribed, and most of them involve lanthanide derivatives.4–8

However, luminescent transition metal-based metallomesogens

aDepartamento de Quımica Inorganica I, Facultad de Ciencias Quımicas,Universidad Complutense, E-28040, Madrid, Spain. E-mail: [email protected] de Difraccion de Rayos-X, Facultad de Ciencias Quımicas,Universidad Complutense, E-28040, Madrid, SpaincREQUIMTE, Departamento de Quımica, Faculdade de Ciencias e Tecnolo-gia, Universidade Nova de Lisboa, 2829-516 Campus da Caparica, MonteDe Caparica, Portugal. E-mail: [email protected]† Electronic supplementary information (ESI) available: Synthesis andcharacterization of the new compounds, including a table containing theanalytical data (Table S1); absorption and normalized fluorescence ofcompounds 1, 5, 13 and 15 (Fig. S1). CCDC reference numbers 688083.For ESI and crystallographic data in CIF or other electronic format seeDOI: 10.1039/b808487d

showing luminescence in the solid state and/or in solu-tion are limited to a few compounds which contain gold,9,10

silver,11 palladium,12 platinum13 or nickel14 centres. In addi-tion, compounds exhibiting luminescence in the mesophase arequite rare and include those showing mesomorphism at roomtemperature.2,15–18 However, it is remarkable to note that in the lastfew years Espinet et al. have reported several stable mesomorphictetrafluorophenyl gold(I) isocyanide complexes which representthe first examples of luminescent metallomesogens displayingstrong luminescence in the mesophase, the solid state and insolution.17

On designing luminescent mesophases, the metallomesogensoffer an interesting opportunity. They combine the liquid crystalproperties with those characteristic of the metal ions that theycontain, therefore typical photoluminescent behaviour of selectedmetal complexes can be used as a support for extending thoseproperties to related metallomesogens.

In this context, looking for new luminescent liquid crystalmaterials, appropriately designed gold(I) and silver(I) complexescan be considered as a good choice. Gold and silver complexesfrequently possess luminescence, this behaviour being usually,but not necessarily, associated with the presence of metal–metalinteractions.9 Then, it is possible to suggest that in the liquidcrystal state the supramolecular ordering of the mesophasescould contribute to overtake metal–metal interactions, and by thisfavouring the luminescence properties.

Previous works from our lab have been driven to estab-lish the mesomorphism of metal complexes based on pyrazoleligands containing long-chained alkyloxyphenyl R substituents(R = C6H4OCnH2n + 1). We have proved that, independent of themesomorphic nature of the 3,5-disubstituted pyrazoles Hpz2R(n)

or the non-mesomorphism of the 3-substituted ligands HpzR(n)

(Hpz2R(n) = 3,5-bis(4-alkyloxyphenyl)pyrazole; HpzR(n) = 3-(4-alkyloxyphenyl)pyrazole), both were able to induce mesomor-phism upon coordination to several metal fragments.19,20 So thecoordination of the non-mesomorphic HpzR(12) ligand to the AuClfragment gave rise to the complex [AuCl(HpzR(12))] which, in

6912 | Dalton Trans., 2008, 6912–6924 This journal is © The Royal Society of Chemistry 2008

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addition to the liquid crystal behaviour, exhibited luminescentproperties.9

On the other hand, we have also recently reported the crys-tal structure of the related luminescent compounds containingthe disubstituted pyrazole with shorter alkyl chains (n = 4),[Ag(Hpz2R(4))2][BF4] and [Au(Hpz2R(4))2][O3S–C6H4–p-CH3], bothexhibiting an unconventional ‘H’ shape (Fig. 1 and 2a),21 which hasbeen found to be consistent with the supramolecular ordering oftheir mesophases.22,23 Then we considered that related compoundswith monosubstituted pyrazoles HpzR(n) should be expected toform ‘U’ shaped molecules, which have also been proved to beuseful for liquid crystal materials (Fig. 2c).24,25

Fig. 1 Molecular structure of [Ag(Hpz2R(4))2][BF4] exhibiting an uncon-ventional ‘H’ shape.21

Following those precedents, in this work we are involved in thesynthesis and study of new complexes of the types [M(HpzR(n))2][A]and [M(Hpz2R(n))2][A] (R = C6H4OCnH2n + 1, M = Ag, Au; A =BF4

-, PF6-, NO3

-) containing mono- and disubstituted pyrazoleligands as well as different counteranions. The selection of theligands was directed to attain potential ‘H’ or ‘U’ molecularshapes as building blocks to achieve the required ordering on

Table 1 Classification of the compounds including the numbering

[Ag(Hpz2R(n))2][A] [Ag(HpzR(n))2][A]

Number n A Type I Number n A Type III

1 12 BF4- IBF4 19 12 BF4

- IIIBF4

2 14 BF4- 20 14 BF4

-

3 16 BF4- 21 16 BF4

-

4 18 BF4- 22 18 BF4

-

5 12 PF6- IPF6 23 4 PF6

- IIIPF6

6 14 PF6- 24 12 PF6

-

7 16 PF6- 25 14 PF6

-

8 18 PF6- 26 16 PF6

-

27 18 PF6-

9 12 NO3- INO3 28 12 NO3

- IIINO3

10 14 NO3- 29 14 NO3

-

11 16 NO3- 30 16 NO3

-

12 18 NO3- 31 18 NO3

-

32 4 PO2F2-

[Au(Hpz2R(n))2][A] [Au(HpzR(n))2][A]

Number n A Type II Number n A Type IV

13 12 BF4- IIBF4 33 12 BF4

- IVBF4

14 14 BF4- 34 14 BF4

-

15 12 PF6- IIPF6 35 12 PF6

- IVPF6

16 14 PF6- 36 14 PF6

-

17 12 NO3- IINO3 37 12 NO3

- IVNO3

18 14 NO3- 38 14 NO3

-

the mesophases (Fig. 2). Different counteranions were consideredto check their influence on the liquid crystal properties. Silverand gold(I) centres were also selected as support for luminescenceproperties. Table 1 depicts the compounds described in this work,including the type and the numbering.

Fig. 2 Schematic representation of the ‘H’ and ‘U’ molecular shape ((a) and (c)) and their potential ordering on the mesophases ((b) and (d)) for[M(Hpz2R(n))2][A] and [M(HpzR(n))2][A] complexes, respectively.

This journal is © The Royal Society of Chemistry 2008 Dalton Trans., 2008, 6912–6924 | 6913

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Experimental

Materials and physical measurements

All commercial reagents were used as supplied. The 3-(4-alkyl-oxyphenyl)-1H-pyrazoles (HpzR(n)) and 3,5-bis(4-alkyloxyphenyl)-1H-pyrazoles (Hpz2R(n)) (R = C6H4OCnH2n + 1; n = 4, 12, 14,16, 18) and the starting Au complex [AuCl(tht)] (tht = tetrahy-drothiophene) were prepared by procedures described in previousworks.19,20,26 Commercial solvents were dried prior to use.

Elemental analyses for carbon, hydrogen and nitrogen werecarried out by the Microanalytical Service of ComplutenseUniversity. IR spectra were recorded on a FTIR Thermo Nicolet200 spectrophotometer with samples as KBr pellets in the 4000–400 cm-1 region: vs (very strong), s (strong), m (medium), w (weak).

1H NMR spectra were performed at room temperature ona Bruker AC200 or on a Bruker DPX-300 spectrophotometers(NMR Service of Complutense University) from solutions inCDCl3. Chemical shifts d are listed relative to Me4Si using thesignal of the deuterated solvent as reference (7.26 ppm), andcoupling constants J are in hertz. Multiplicities are indicated as s(singlet), d (doublet), t (triplet), m (multiplet), br (broad signal).The 1H chemical shifts and coupling constants are accurate to±0.01 ppm and ±0.3 Hz, respectively.

Phase studies were carried out by optical microscopy using anOlympus BX50 microscope equipped with a Linkam THMS 600heating stage. The temperatures were assigned on the basis of opticobservations with polarized light.

Measurements of the transition temperatures were made usinga Perkin Elmer Pyris 1 differential scanning calorimeter with thesample (1–4 mg) sealed hermetically in aluminium pans and witha heating or cooling rate of 5–10 K min-1.

The X-ray diffractograms at variable temperature were recordedon a Panalytical X’Pert PRO MPD diffractometer in a q–qconfiguration equipped with a Anton Paar HTK1200 heating stage(X-Ray Diffraction Service of Complutense University).

Absorption spectra were recorded on a Shimadzu UV-2501PCspectrophotometer and fluorescence emission on a Horiba-Jobin-

Yvon SPEX Fluorolog 3.22 spectrofluorimeter equipped witha ThermoNeslab RTE7 bath. The linearity of the fluorescenceemission vs. concentration was checked in the concentrationrange used (10-4–10-6 M). A correction for the absorbed lightwas performed when necessary. All spectrofluorimetric titrationswere performed as follows: the stock solutions of the ligands(ca. 10-3 M) were prepared by dissolving an appropriate amountof the ligand in a 50 mL volumetric flask and diluting to the markwith chloroform HPLC or UVA-sol grades. Fluorescence spectraof solid samples were recorded on the spectrofluorimeter excitingthe solid compounds at appropriated l (nm).

Preparation of the complexes of the types I and III

To a solution of the corresponding 3,5-bis(4-alkyloxyphenyl)-1H-pyrazole (Hpz2R(n)) or 3-(4-alkyloxyphenyl)-1H-pyrazole (HpzR(n))in dry tetrahydrofurane was added AgA in a 2 : 1 molecular ratiounder dinitrogen atmosphere (Scheme 1). The mixture was stirredfor 24 h in absence of light, and then filtered through Celite. Theclear filtrate was removed in vacuo and the residue was dissolvedin dichloromethane. Addition of hexane gave rise to a colorlesssolid, which was filtered off, washed with hexane and driedin vacuo. Spectroscopic and analytical data are given as ESI.†

Preparation of the complexes of the types II and IV

To a mixture of AgA and [AuCl(tht)] in dry tetrahydrofu-rane was added a solution of the corresponding 3,5-bis(4-alkyloxyphenyl)-1H-pyrazole (Hpz2R(n)) or 3-(4-alkyloxyphenyl)-1H-pyrazole (HpzR(n)) in a 1 : 1:2 molecular ratio under dinitrogenatmosphere (Scheme 1). The mixture was stirred for 24 h inabsence of light, and then filtered through Celite. The clearfiltrate was removed in vacuo and the residue was dissolved indichloromethane. Addition of hexane gave rise to a colorless solid,which was filtered off, washed with hexane and dried in vacuo.Spectroscopic and analytical data are given as ESI.†

Scheme 1


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Table 2 Crystal and refinement data for [Ag(HpzR(4))2][PO2F2] (32)

32

Empirical formula [C26H32N4Ag][PO2F2]Formula weight 641.40Crystal system MonoclinicSpace group C2/ca (A) 25.047(4)b (A) 7.936(1)c (A) 16.167(3)b (◦) 117.768(3)V (A3) 2840.0(8)Z 4T (K) 296(2)F(000) 1312rcalc. (g cm-3) 1.500m (mm-1) 0.817Scan technique w and jData collected (-29,-5, -19) to (29,9,16)q range (◦) 1.84–25.00Reflections collected 10553Independent reflections 2511 (Rint = 0.1584)Completeness to maximum q 100%Data/restraints/parameters 2511/3/160Observed reflections [I > 2s(I)] 1001Ra 0.0652RwF

b 0.2017

a∑

[|F o| - |F c|]/∑

|F o|. b {∑

[w(F o2 - F c

2)2]/∑

[w(F o2)2]}1/2.

X-Ray structure determination of [Ag(HpzR(4))2][PO2F2] (32)

Yellow prismatic single crystals of compound [Ag(HpzR(4))2]-[PO2F2] suitable for X-ray diffraction experiments were success-fully grown by slow diffusion of hexane into a dichloromethanesolution of the parent compound [Ag(HpzR(4))2][PF6] (23). Datacollection was carried out at room temperature on a BrukerSmart CCD diffractometer, with graphite-monochromated Mo-Ka radiation (l = 0. 71073 A), operating at 50 kV and 30 mA. Thedata were collected over a hemisphere of the reciprocal space bycombination of the three exposure sets. Each frame exposure timewas of 10 s, covering 0.3◦ in w. The first 100 frames were recollectedat the end of the data collection to monitor crystal decay. Noappreciable drop in the intensities of standard reflections wasobserved. The cell parameters were determined and refined by aleast-squares fit of all reflections. A summary of the fundamentalcrystal data and refinement data is given in Table 2. The value ofRint for independent reflections is slightly high. The quality of thecrystal, along with its needle-type morphology, have not allowedto get lower Rint. In spite, other agreement parameters are adequateto give a good convergence.

The structure was solved by direct methods and conventionalFourier techniques. The refinement was done by full-matrixleast-squares on F 2.27 Anisotropic parameters were used in thelast cycles of refinement for all non-hydrogen atoms except forC13, C14 and C15, which have been refined isotropically withgeometrical restraints and a variable common carbon–carbondistance. Hydrogen atom bonded to N2 atom has been located ina Fourier synthesis, included and refined as riding on the bondedatom. The remaining hydrogen atoms were included in calculatedpositions and refined as riding on their respective carbon atoms.Final R (Rw) values were 0.065 (0.2017).

The supplementary crystallographic data have been passed tothe Cambridge Crystallographic Data Centre (CCDC depositionnumber 688083). For crystallographic data in CIF or otherelectronic format see DOI: 10.1039/b808487d

Results and discussion

Synthetic studies and structural characterization

The silver(I) compounds [Ag(Hpz2R(n))2][A] (type I) and[Ag(HpzR(n))2][A] (type III) were prepared by reaction of thepyrazole ligands and the corresponding silver salts AgA accordingto Scheme 1. The related gold(I) complexes [Au(Hpz2R(n))2][A] (typeII) and [Au(HpzR(n))2][A] (type IV) were obtained by reactionof [AuCl(tht)] with the selected pyrazole and addition of thecorresponding silver salt AgA (Scheme 1). Table 1 shows theclassification of compounds including the numbering.

All Ag(I) and Au(I) derivatives were isolated as white solids andthey were fully characterized by elemental analysis and IR andNMR spectroscopies (see Experimental section and ESI†). TheAu derivatives containing the longest chains (n = 16, 18) werealways isolated as mixtures from which we were unable to obtainthe pure complexes.

The 1H NMR spectra in CDCl3 solution at room temperatureshow in all cases signals attributed to the pyrazole ligands. Silvercompounds I containing 3,5-disubstituted pyrazoles exhibit oneset of signals each type of protons, indicating the equivalence ofthe two pyrazoles as well as of their substituents. By contrast,the spectra of the related gold compounds II show two signalsof the Ho, Hm and OCH2 protons of the pyrazole substituentsand a single signal of the H4 pyrazolic proton, according withthe unequivalence of the substituents each of the two equivalentpyrazoles.

The above results can be explained in terms of the presenceof a metallotropic equilibrium for the silver derivatives, similarto that previously found for related complexes.21,28 The absenceof NH-pyrazolic signals of these silver compounds, in contrast tothose observed for the gold derivatives in the region of 12–14 ppm,agrees with the metallotropic equilibrium above suggested.

The complexes containing 3-substituted pyrazoles (III, IV) showwell-defined signals each proton, indicating the equivalence of thetwo pyrazole ligands.

Finally it is interesting to note the influence of the counteranionon the chemical shifts. So, it can be observed that, in general terms,the resonances of the compounds carrying the most coordinativecounteranion (NO3

-) appear slightly more downfield shifted thanthose of the compounds containing counteranions with lowcoordinative ability (BF4

-, PF6-).

The infrared spectra in the solid state show the characteristicbands of pyrazole ligands as well as those of the correspondingcounteranions. In particular the (n(C=N) + n(C=C)) and n(NH)absorption bands from the ligands appear at ca. 1600 cm-1 andin the 3200–3400 cm-1 range, respectively. On the other hand, thepresence of the counteranions is also established from the strong–medium broad bands at ca. 850 cm-1 n(PF), 1380 cm-1 n(NO)and 1030 cm-1 n(BF) from the counteranions PF6

-, NO3- and

BF4-, respectively.29 These bands exhibit slight splitting and/or

weak shoulders which can suggest the presence of coordinativeor hydrogen bonding interactions (M ◊ ◊ ◊ X, X ◊ ◊ ◊ H–N; X = F


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Table 3 Selected lengths (A) and angles (◦) for 32

Ag–N1 2.089(7) N1–Ag–N1¢ 177.8(5)N1–N2 1.350(9) N2–N1–Ag 122.0(6)N2–C3 1.36(1) C5–N1–Ag 134.3(7)C3–C4 1.38(1) C5–N1–N2 103.6(7)C4–C5 1.35(1) N1–N2–C3 112.2(7)N1–C5 1.33(1) N2–C3–C4 105.2(7)P–O2 1.420(6) C3–C4–C5 106.0(8)P–F 1.510(5) O2–P–F 108.4(4)N2–H2 1.00 C3–N2–H2 142.4N2 ◊ ◊ ◊ O2 2.702(9) N1–N2–H2 105.2O2 ◊ ◊ ◊ H2 1.72 N2–H2 ◊ ◊ ◊ O2 166.8

Symmetry operation (¢): -x, y, -z + 3/2.

or O) in agreement with the lowering of the symmetry of thefree counteranions. Structural data of the hydrogen-bondinginteractions N–H ◊ ◊ ◊ O observed in 32, as will be described later,support the above suggestion.

Crystal structure of [Ag(HpzR(4))2][PO2F2] (32)

Repeatedly we have tried to grow crystals of the different types ofcompounds described in this work for X-ray purposes, but all at-tempts were unsuccessfully. Then we prepared several homologousderivatives containing shorter chains, which are usually provedto crystallize. So the ionic silver derivative [Ag(HpzR(4))2][PO2F2]bearing the butoxyphenyl group as substituent at the 3-position ofthe pyrazole, and PO2F2

- as counteranion was obtained. The coun-teranion was generated through a hydrolysis process of the PF6

-

anion during the crystallization of the starting [Ag(HpzR(4))2][PF6](23) (Scheme 1), this feature being frequently observed in someexamples described in the literature.30–32 The molecular structureof [Ag(HpzR(4))2][PO2F2] (32) was determined by single X-raydiffraction methods (Fig. 3). The data collection and refinementparameters are detailed in the Experimental section (Table 2) andTable 3 lists selected distances and angles.

Fig. 3 ORTEP plot of [Ag(HpzR(4))2][PO2F2] (32) with 35% probability.Hydrogen atoms, except H2, have been omitted for clarity. Symmetryoperation (-x, y, -z + 3/2).

The compound crystallizes in the monoclinic system, spacegroup C2/c. The asymmetric unit consists of the half of thecationic unit [Ag(HpzR(4))2]+ and of the counteranion PO2F2

-, withthe silver and phosphorus atoms located in an axis. The cationicpart is a two-coordinated silver complex in which the silver centrehas two strongly bonded pyrazoles. The Ag–N distance of 2.089(7)A is similar to those observed in related compounds containingheterocyclic ligands.24,33–35 The N1–Ag–N1¢(-x, y, -z + 3/2) angle

of 177.8(5)◦ agrees with the typical linear coordination around thesilver centre.

The pyrazole ligands exhibit a cis orientation of the NH groups,as deduced by the torsion angle t defined by the four nitrogenatoms of 19(1)◦. From this disposition, the PO2F2

- counteranionis bonded through a moderate hydrogen-bond between eachO2 atom with each NH group of the cationic unit (Table 3),giving rise to an almost planar N2–N1–Ag–N1¢–N2¢–O2¢–P–O2(symmetry operation: -x, y, -z + 3/2) metallocycle with themaximum deviation of 0.32(1) A observed in the O2 atoms. Thecis-molecular cation adopts a special ‘U’ or bowl-like molecularshape (Fig. 3) in which the pyrazole planes are twisted at an angleof 19.6(6)◦, and the benzene plane with its own pyrazole planeforms an angle of 9.8(7)◦. The alkyl chains are almost in the sameplane as the pyrazole rings, with the two arms pointing out in thesame direction.

The cationic entities [Ag(HpzR(4))2]+ are packed in interdigitatedbilayers in which neighbouring cations are oriented with theiralkyl chains alternated (Fig. 4a). In turns, each cation connectswith neighbouring cations in a column along the b axis throughweak coordinative Ag ◊ ◊ ◊ F (x, y - 1, z; -x, y–1, -z + 3/2)interactions of 3.242(6) A involving the fluorine atoms of theneighbouring PO2F2

- counteranion (Fig. 4b). No short contactsbetween columns are observed.

Fig. 4 (a) Packing of the cationic entities of 32 in the bc plane.(b) Molecular chain along the b axis.

Mesomorphic behaviour

The thermal behaviour of all synthesized compounds was estab-lished by using both polarized-light optical microscopy (POM)and differential scanning calorimetry (DSC) techniques. Themesophases were also characterized by small-angle X-ray diffrac-tion (XRD).

All complexes of the series I and II containing Hpz2R(n) presentenantiotropic mesomorphism (Table 4), showing smectic SmAmesophases which were characterized by their focal-conic fan-shaped textures (Fig. 5).

By contrast the complexes of the classes III and IV containingthe non-mesomorphic HpzR(n) ligands are not liquid crystal except[Ag(HpzR(12))2][PF6] (24) and [Au(HpzR(12))2][A] (A = PF6

- (35),NO3

- (37)) (Table 4).DSC thermograms of the complexes I (IBF4, IPF6 and INO3), with

the exception of that of 5, exhibit a similar pattern which is nowdescribed for [Ag(Hpz2R(18))2][BF4] (4).

The first heating cycle shows two endothermic peaks. Thefirst one at 70.0 ◦C (DH = 36.0 kJ mol-1) corresponds to thetransition of a crystal phase to the mesophase and the second


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Table 4 Phase behaviour of new compounds 1–18, 24, 35 and 37determined by optical microscopy and DSC

Complex T/◦C (DH/kJ mol-1)

n Cr→SmA SmA→IL

IBF4 12 1 64 (31.7) 144 (3.1)14 2 69 (32.4) 138 (8.8)16 3 69 (38.2) 132 (8.8)18 4 70 (36.0) 126 (3.8)

IPF6 12 5a

14 6 77 (36.3) 125 (7.8)16 7 79 (13.6) 125 (2.5)18 8 60 (12.3) 104 (3.1)

INO3 12 9 40d 140 (4.7)14 10 40d 132 (1.2)16 11 51 (6.6) 121 (2.9)18 12 68 (11.4) 124 (2.7)

IIBF4 12 13 56d 127b ,d

14 14 61d 138b ,d

IIPF6 12 15 83d 125b ,d

14 16 85d 127b ,d

IINO3 12 17 75d 130b ,d

14 18 99d 134b ,d

IIIPF6 12 24 47 (27.1) 89 (4.8)IVPF6 12 35 — 135c ,d

IVNO3 12 37 — 130c ,d

a Complex 5 exhibits the following phase transitions:

Cr SmC SmA IL116 6.593 29 4 154 3 8. .( ) ( ) ( )æ Ææææ æ Æææ æ Æææb At the beginning of the clearing process. c Monotropic behaviour (35exhibits SmA mesophase at 81 ◦C and 37 exhibits SmA mesophase at120 ◦C). d Temperatures determined by POM.

peak at 126.0 ◦C with a lower enthalpy value of 3.8 kJ mol-1

corresponds to the transition of the mesophase to isotropic liquid.On cooling, from the isotropic liquid, the first exothermic peakat 115.0 ◦C (DH = -3.5 kJ mol-1) is due to the formation of aSmA mesophase in agreement with the typical fan-shaped textureoptically observed. A second exothermic peak at 64.0 ◦C (DH =-15.0 kJ mol-1) is related to the transition of the mesophase to thesolid phase. In the remaining complexes this latter process was notalways detected from their corresponding DSC thermograms dueto the limitant lowest temperature of ca. 50 ◦C of the equipmentused. So, complexes 9 and 10 could be observed as solids at ca.40 ◦C by optical microscopy.

Complex 5 exhibits four endothermic transitions in the firstheating cycle. The first one at 76.4 ◦C (DH = 12.2 kJ mol-1)corresponds to the transition of a crystal phase to another crystalphase, only observed by DSC. The second peak at 93.0 ◦C (DH =29.4 kJ mol-1) is assigned to the transition of the crystal phase toa mesophase identified as the smectic C (SmC) mesophase by itssanded-schlieren-type texture as well as small-angle XRD studies(Table 5). A fan-shaped texture of a smectic A (SmA) mesophaseis clearly observed at 116 ◦C, this temperature being related tothe third peak of the DSC thermogram (DH = 6.5 kJ mol-1), andassociated to a mesophase–mesophase transition. The presenceof this SmA phase at higher temperatures allows us to confirmthe above assignment of the SmC mesophase. The fourth peak at154.0 ◦C with a small enthalpy value (3.8 kJ mol-1) correspondsto the transition of mesophase to isotropic liquid. On cooling twoexothermic peaks at 154 ◦C (DH = -7.9 kJ mol-1) and 80 ◦C(DH = -1.4 kJ mol-1) are related to the isotropic liquid to a SmA

Fig. 5 Microphotographs of the SmA mesophases observed by POM of(a) [Ag(Hpz2R(14))2][PF6] (6) at 111 ◦C, and (b) [Au(Hpz2R(12))2][PF6] (15) at103 ◦C.

mesophase transition and SmA to SmC mesophase transition,respectively. A third peak at 66 ◦C (DH = -11.6 kJ mol-1) agreeswith the crystallisation process.

The gold compounds II and IV with disubstituted and mono-substituted pyrazole ligands exhibit a similar mesomorphism tothat of silver derivatives but also they show two particular features.The first one deals with a higher thermal instability evidenced bya partial decomposition at temperature close to the clearing. Thesecond feature is related to the rate of the transition processes.In particular on heating the clearing process is very slow and thetotal phase transformation from mesophase to isotropic liquidis only reached after increasing the temperature to a value thatdepends of the counteranions. On cooling the mesophase is slowlyformed from the isotropic liquid and in all cases it is maintainedbelow 50 ◦C with no evidence of crystallization. Indeed theviscosity suggests that these systems have formed a smectic glass.In agreement with the above results the processes were not detectedby DSC.

Table 4 contains the results of thermal behaviour of compoundsof classes I–IV obtained by DSC and/or POM.

The influence of the counteranion on the thermal behaviourwas clearly observed for all metallomesogenic derivatives I and IIand several trends can be pointed out. In general terms, for eachchain length the variation of the melting temperatures is in theorder PF6

- > BF4- > NO3

-. However the trend on the clearingtemperatures variation is partially reversed, BF4

- > PF6-. Taking


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Table 5 X-Ray diffraction data for liquid crystalline complexes

Complex T/◦C Position/2q d-spacing/A Miller indices (hkl)

1 115 2.5 34.8 (001)5.0 17.5 (002)7.5 11.7 (003)

19.9 4.5 a

2 115 2.4 37.0 (001)4.9 18.2 (002)7.3 12.1 (003)

21.0 4.1 a

3 76 2.1 42.5 (001)4.1 21.6 (002)6.1 14.5 (003)

20.6 4.3 a

4 80 2.0 44.9 (001)3.9 22.7 (002)5.8 15.2 (003)

19.5 4.5 a

5 110 2.2 39.7 (001)4.4 19.8 (002)6.6 13.3 (003)

19.8 4.2 a

130 2.3 37.5 (001)4.6 18.9 (002)7.1 12.4 (003)

20.0 4.4 a

6 109 2.4 36.5 (001)4.8 18.4 (002)7.0 12.3 (003)

20.2 4.3 a

9 100 2.6 34.5 (001)5.0 17.5 (002)7.6 11.7 (003)

20.1 4.4 a

13 90 2.7 33.0 (001)5.3 16.7 (002)7.9 11.2 (003)

21.4 4.1 a

15 70 2.5 35.9 (001)4.8 18.4 (002)6.9 12.7 (003)

21.3 4.2 a

35 70b 2.1 42.0 (001)4.2 21.0 (002)5.5 13.8 (003)

20.0 4.5 a

37 50b 2.6 34.4 (001)4.2 17.1 (002)5.0 11.1 (003)

20.0 4.1 a

a Halo of the molten alkoxy chains. b On cooling.

into account the volume of the spherical counteranions PF6- >

BF4-,36 it is possible to suggest that the molecular arrangement of

the solid should be more effective for those derivatives containingthe bulkiest PF6

- counteranion, so generating the highest meltingpoints as has been found experimentally. By contrast, the flat NO3

-

group should be less effective than the BF4- and PF6

- and as aconsequence it should produce the lowest melting points. Similar

behaviour has been described for gold(I) and silver(I) isocyanideionic complexes and hexafluorophosphate and tetrafluoroboratepyridine silver derivatives.37,38

On the other hand, the inverse variation of the clearing tendencyof BF4

- and PF6- derivatives could be considered in agreement with

the melting of the anion–cation arrangement in the mesophase. Sothe bulkiest anion should introduce higher difficulties to allow thecation–cation ordering of the fluid phases giving rise to the lowestclearing points.

Thermal and thermodynamics effects of increasing the alkyloxychain length are also studied. So for complexes of the series IBF4

(1–4) and INO3 (9–12) (Table 4), the temperatures of the phasetransition (Cr → M) increase upon increasing the chain length,being the same behaviour suggested for all Au derivatives II (IIBF4,IIPF6, IINO3; 13–18) on the basis of their optical data (Table 4). Thisbehaviour is parallel to that observed on [Ag(R-imidazol)][NO3]and attributed to the melting of alkyl chains.24 Therefore it ispossible to suggest that, mainly in silver compounds I, strong vander Waals interactions between chains should govern the variationof the melting process, being less important the potential loweringon the core–core interactions produced by the increased motionof longer chains.

The inverse variation on the melting temperature observedin IPF6 (5–8) can be attributed to the bulky size of the PF6

-

counteranion, which by increasing the alkyloxy chain lengthshould produce a less favourable packing of the solid reducingthe melting temperatures.

On the other hand, the clearing temperatures of the silvercomplexes I are reduced with the increase of the chain length,which is the inverse of the trend of their counterparts Aucompounds II.

From the above results it is deduced that the silver compounds Iwith BF4

- and NO3- as counteranions present a wider temperature

range of mesophase than those with PF6- following the order

NO3- > BF4

- > PF6-. However for the gold derivatives II, the

increase in the length of the chain (from 12 to 14 carbon atoms)affects in the same way the melting and clearing temperatures andas a consequence the mesophase range follows the general trendin the order BF4

- > PF6- > NO3

-. The above different results inthe mesomorphic behaviour of silver and gold derivatives can beunderstood on the basis of the presence of covalent coordinativeinteractions from the O or F atoms of the counteranions withthe metal centres, which are more common and stronger forsilver(I) than gold(I) derivatives. Evidences of those kinds ofinteractions have been found in several pyrazolyl derivativespreviously described by us.39 Then it is not surprising thatrelated metal–counteranion interactions can be expected in ourcompounds being responsible for the variable stabilization of themesophases experimentally observed.

Mesomorphism of complexes III and IV (Table 4) is restricted tothose bearing 12 carbon atoms on the alkyl chain (24, 35 and 37).Complex 24 shows lower melting and clearing temperatures thanthose of its counterpart with disubstituted pyrazole ligands andthe gold derivatives 35 and 37 exhibit monotropic behaviour. Theabsence of liquid crystal properties of the counterparts containinglonger alkyl chains (n > 12) can be explained as a consequenceof the decrease of the interpenetration of the chains of the ‘U’or ‘bowl’ shaped molecules, and therefore to a decrease in theanisotropy, which renders weaker core–core interactions.


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Selected compounds of the different families were subjected totemperature-dependent power X-ray diffraction (XRD) measure-ments. The XRD pattern shows in all cases three sharp reflectionsin a 1 : 2 : 3 ratio which were indicative of a lamellar structure andthey are indexed as the (0 0 1), (0 0 2) and (0 0 3) reflections,respectively. In addition a broad halo at ca. 4.5 A corresponds tothe liquid-like order of the molten alkylic chains. The results aresummarized in Table 5 and Fig. 6 shows the diffraction patternsof compounds 3 and 13 selected as representative examples.

Fig. 6 X-Ray diffraction pattern of (a) [Ag(Hpz2R(16))2][BF4] (3) at 76 ◦Con heating, and (b) [Au(Hpz2R(12))2][BF4] (13) at 90 ◦C on heating.

The XRD diffractogram of 5 requires a special comment. Asmeasured at 110 ◦C, it shows three peaks in the small-angle regionfrom the (0 0 1), (0 0 2) and (0 0 3) reflections in agreement witha lamellar structure with a layer distance of 39.7 A. In addition, abroad halo at ca. 4.2 A is observed in the wide-angle region, whichis characteristic of the diffraction of the side chains in the liquid-like order. The X-ray diffractogram recorded at 130 ◦C shows asimilar pattern with three sharp small-angle peaks indexed in alamellar mesophase with the Miller indices of (0 0 1), (0 0 2) and(0 0 3) and a layer distance of 37.5 A. XRD analysis allows toidentify these two mesophases as smectic phases being assignedas SmC and SmA mesophases by textural studies. It is importantto note that the layer spacing of the SmA mesophase is smaller

than that of SmC phase, this feature can be due to the loss ofconformational freedom at low temperatures, which causes themore extended chains to overcome the effect of tilting.16,40

The homologous complexes exhibit similar XRD results (Ta-ble 5) showing layer spacing of 34.8 A (1), 37.0 A (2), 42.5 A (3),44.9 A (4), 37.5 A (5), 36.5 A (6), 34.5 A (9), 33.0 A (13) and 35.9 A(15) in the SmA mesophase.

At this point of the discussion and in order to establish potentialsolid–mesophase structure relationships, we think that it could beinteresting to consider the X-ray structure of [Ag(Hpz2R(4))2][BF4](Fig. 1), previously reported by us,21 as a potential model forcompounds I and II. On the basis of a similar ‘H’–shapedmolecular structure for the complexes IBF4 (Table 5) with chainsof n = 12, 14, 16, 18 carbon atoms, we can estimate a molecularlength L of the cationic entity in a full-extended configuration ofca. 43, 48, 54 and 59 A, respectively. Because the d-spacings of thesmectic layer (35–45 A, Table 5) are shorter than L, we can deducethat in the layer the molecules should present interdigitation(Scheme 2), analogously to that established on the crystallinestructure of [Ag(Hpz2R(4))2][BF4] (Fig. 7).21

Scheme 2 Schematic representation of the packing modes of the com-plexes of the types I and II in the SmA mesophases.

Fig. 7 View of the molecular distribution in a layer of the compound[Ag(Hpz2R(4))2][BF4].21

Photophysical studies

The silver(I) and gold(I) compounds based on the 3,5-disubstitutedpyrazole ligands (Hpz2R(n)) 1, 5, 9 (type I) and 13, 15, 17(type II) with n = 12 were considered for photoluminescence


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Table 6 Optical data for compounds 1, 5, 9, 13, 15 and 17 in chloroform solution at room temperature

Compound lmax Abs (nm) e (L mol-1 cm-1) Conc./10-6 M lemis (nm) (lexc = 274 nm) Dl a (nm)

Hpz2R(12) 267 55063 3.24 362b 951 272 71672 2.84 374 1025 272 80126 2.51 380 1089 272 75065 4.45 371 9913 280 77062 2.08 380 10015 276 79566 3.30 371 9517 278 68453 4.90 373 95

a Stocks shift in chloroform, b lexc = 261 nm.

Table 7 Optical data for compounds 1, 5, 9, 13, 15 and 17 as solid samples at room temperature

Compound lmax Exc (nm) lemis (nm) (lexc = 274 nm) lemis (nm) (lexc = 350 nm) Colora Color (lexc = 366 nm UV lamp)

1 272 360 416 White Orange5 272 362 410 White Orange9 272 354 Not emissive White Orange13 280 359 512 White Purple15 276 391 438 White Orange17 278 356 Not emissive White Pale orange

a Solid powder samples.

studies. Related to their counterparts, these compounds exhibitthe largest mesophase range of each counteranion, allowinga better determination of the influence of the anionic groupon the luminescence properties. On the other hand our pre-liminary studies of complexes containing pyrazole-type ligandsrevealed that photophysical characteristics remained constantdespite variations in the chain length of the ligands.9,41 Fromthese considerations, the photophysical properties of the selectedcomplexes [M(Hpz2R(12))2][A], (M = Ag, Au; A = BF4

-, PF6-, NO3

-)were analyzed. All of those derivatives, including the free ligand,are luminescent at room temperature both in solution and in solidstate (Tables 6 and 7). The complexes displaying liquid crystalbehaviour were also proved to be luminescent at the mesophaseby variable temperature luminescence studies.

Solution studies. The UV-Vis absorption spectra of dilutesolutions of all the complexes in CHCl3 exhibit a broad absorptionband in the region of 240–320 nm with the maximum centeredat ca. 280 nm (Table 6). In analogy to the related compoundspreviously published,9 as well as to the free ligand (Table 6),that absorption was associated with the p–p* electronic transitionof the pyrazole groups. The metal complexation of the Hpz2R(12)

ligand induces a slight red-shift in all cases, but no significantdifferences attributed to the influence of the counteranions areobserved (Table 6, Fig. 8 and ESI†).

The emission spectra of complexes in fresh chloroform solutionsshow a broad band between 320–480 nm which is bathochromi-cally shifted related to that of the free ligand and attributedto a ligand-centered transition (Table 6). Again, no appreciable

Fig. 8 Absorption and normalized fluorescence emission spectra of compounds 9 and 17 in fresh chloroform solution at concentrations 4.45 ¥ 10-6 M(9) and 4.90 ¥ 10-6 M (17) (lexc = 274 nm; room temperature).


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Fig. 9 Normalized fluorescence emission spectra of compounds 1, 5, 9, 13, 15 and 17 in solid state (lexc = 274 nm; room temperature).

Fig. 10 Normalized fluorescence emission spectra of compounds 1, 5, 9, 13, 15 and 17 in solid state (lexc = 350 nm; room temperature).

spectral differences ascribed to the counteranions are observed(Fig. 8 and ESI†).

Solid studies. The photophysical properties in the solid statewere also examined in order to establish the influence of the crystalpacking on the luminescence behaviour.

The emission spectra of silver (1, 5, 9) and gold (13, 15, 17)complexes exhibit a similar pattern consistent with two broadbands in the range of ca. 300–800 nm when they are excitedwith l = 274 nm (Table 7). The two bands were observed forall derivatives at ca. 300–400 nm and ca. 600–800 nm, respectively.The first one exhibiting the highest intensity is again attributed toa ligand-centered transition and the latter band of low intensity isassigned to a metal–ligand charge transfer (MLCT)42–46 (Fig. 9).‡

By changing the counteranion, the intensity of the emissionbands decreases in the order PF6

- > BF4- > NO3

- in the silver

‡ The band at ca. 600–800 nm observed in all the complexes could beassigned to a triplet state (centered in the pyrazole unit or in the metal–ligand fragment). In order to shed further light on this point the life time ofthis band, in solid complexes 5 and 15 excited at 274 nm, has been studiedusing a Perkin-Elmer LS45 spectrofluorimeter in phosphorescence mode.Unfortunately, life times obtained gave values below 50 ms; these valuesare in our lamp life time region and this result prevents any assignment.However, the solid state spectrum of the free ligand in the same conditionsdoes not present the band above mentioned (600–800 nm) suggesting tous its assignment to a metal–ligand charge transfer (MLCT).

complexes, and BF4- ~ PF6

- > NO3- in the gold derivatives. This

feature suggests that the presence of coordinative interactionsbetween the metal centre and the counteranions, favoured by theNO3

- group, contributes to modify the luminescence properties.On the other hand the broadness of the luminescence bandsappears to reflect the highly distorted excited states.

Compound 13 exhibits a third band between 420–600 nm.A related emission was not clearly observed in the remainingcompounds, because it should be occluded by the broad andintense band at 300–400 nm. In order to elucidate that option,a new excitation at 350 nm was used. As expected, the emissionappears in the range of ca. 400–600 nm as a broad band (Fig. 10),which could be assigned to a metal-centered emission.

By comparing the above results with those from the solutionspectra, it is evident that both silver and gold derivatives exhibit aslight bathochromic shift of the emission maxima, which is almostunaffected by the change of the metal.

Variable-temperature studies. The luminescent behaviour ofcompounds 1, 5, 9 (type I) and 13, 15, 17 (type II) was studied atdifferent temperatures from the solid state to the isotropic liquid,in order to prove the luminescence at the liquid crystal state.

The emission spectra were recorded in the 300–800 nm range,using a fiber optic system connected to the Horiba-Jobin Yvon-Spex Fluorolog 3.22 spectrofluorometer. The solid samples were


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heated over a hotplate with an external temperature controlprovided with a digital thermo par.

Fig. 11 displays the emission spectra of the 1, 5, 9 and13, 15, 17 complexes as a function of the temperature. Theywere recorded from room temperature until temperatures slightlyhigher than their respective clearing temperatures. As a generaltrend it is observed that the intensity of the luminescencedecreases upon temperature increases and at the clearing tem-perature the emission almost disappears. In spite of the emis-sion quenching of the isotropic liquids, as a consequence of

the increased non-radiative deactivation, a residual emission isgenerally maintained. Indeed, in our compounds, as usual, aremained residual emission is observed at higher temperaturesthan the clearing. From the above results it is also deducedthat the mesophase aggregation does not quench the emission.In fact we can observe that, when the samples overtake themelting temperature generating the mesophase (Table 4), theluminescence is maintained in all cases although with less intensity,following the mentioned variation of the intensity with the tempe-rature.

Fig. 11 Normalized fluorescence emission spectra of compounds 1, 5, 9, 13, 15 and 17 in solid state as a function of the temperature (lexc = 274 nm).The spectra and the temperature list follow the same top to bottom order. The smectic and IL phases are given in all cases.


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On the other hand, the overall pattern of the spectra ismaintained along all of the temperatures used. Therefore theemission maxima in the solid and in the liquid crystal state are thesame (the latter with less intensity) suggesting that the structuralchanges and viscosity of the mesophase do not affect to the excitedstate.

The investigated silver and gold complexes I and II constitutea new contribution to the scarce examples of luminescent metal-lomesogens exhibiting luminescence in the mesophase as well as inthe solid state and in solution. New related compounds based onthese pyrazole ligands will be investigated looking for luminescentmesophases at lower temperatures.

Conclusions

Mesomorphic molecular materials can be used for liquid crystalor OLEDs technologies. To these purposes stability over an ap-propriate temperature range and emission in the visible spectrumare required to perform as liquid crystal OLED materials.

The ionic silver and gold complexes, presented in this work,of the type I [Ag(Hpz2R(n))2][A] and II [Au(Hpz2R(n))2][A] based ondisubstituted pyrazole ligands are new metallomesogens exhibit-ing lamellar phases over a broad of temperature range. The non-conventional ‘H’ molecular shape of these derivatives allows toachieve SmA mesophases, which could be related to the layer-likestructural packing observed in the solid.

By contrast the complexes III and IV, [M(HpzR(n))2][A], bearingthe monosubstituted pyrazole ligands, are not liquid crystals withthe exception of those with n = 12. Rationalizing this differentbehaviour is not a simple matter and an explanation of thoseresults is supported by the following considerations. The ‘U’shaped two-chained molecules of III and IV are arranged in thesolid in a columnar fashion (viewed in Fig. 4), which is differentto that of the layer-like structure found in the four-chained ‘H’shaped molecules of the related compound [Ag(Hpz2R(4))2][BF4].21

Those structural features could be reflected in the difficulty toachieve the required supramolecular ordering of the lamellarmesophases for compounds III and IV. Alternatively, althougha layer-like ordering of the ‘U’ molecules on the mesophase couldalso be considered, as that depicted in Fig. 2d, the increase ofthe length chain above of n = 12 should produce a decreaseon the interdigitation giving rise to a higher random motion ofthe chains. The flexibility of the alkyl chains tends to disrupt thecore–core interactions, this fact being responsible for the lack ofmesomorphism.

The new metallomesogens behave as emitters in the solid stateand in solution. In the liquid crystal state they show a moderateintensity at the same emission maxima to that observed in the solidstate.

The photoluminescence in metallomesogens is generally associ-ated to the excited states created by intermolecular interactions. Inour case the supramolecular organization of ionic compounds inthe mesophase discussed previously depends on the counteranionand it suggests that their stabilization is produced probablywithout core–core interactions in the same way that in the solidstate no metal–metal interactions were found. This fact suggeststhat formation of excimers should not be favourable and this effectcould explain that no spectral changes are observed from the solidto the liquid crystal state.

Acknowledgements

We thank the Ministerio de Educacion y Ciencia of Spain, projectCTQ2006-13344/BQU, and the Comunidad de Madrid (Spain)and Universidad Complutense de Madrid (Spain) project CCG07-UCM/PPQ-2844 for the financial support. We are also indebted toFundacao para a Ciencia e a Tecnologia/FEDER (Portugal/EU)by project PTDC/QUI/66250/2006 FCT-FEDER.

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silver and gold luminescent metallomesogens based on pyrazole ligands

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