HIGHLIGHT www.rsc.org/materials | Journal of Materials Chemistry

Luminescence of metallomesogens in theliquid crystal stateKoen Binnemans*

DOI: 10.1039/b811373d

Recent progress in the design of low-melting liquid-crystalline metal complexes(metallomesogens) has facilitated the study of the photophysical properties of thesecompounds in the mesophase. Luminescence in the liquid crystal state has been observed formetallomesogens incorporating lanthanide(III), gold(I), silver(I), copper(I) or zinc(II) ions. Analternative approach to liquid-crystalline metal-containing systems is doping a metal complexin a liquid-crystal host. A fascinating property of thesematerials is the ability to observe linearlypolarized emission.

A metal center can add unique magnetic,

spectroscopic or redox properties to

a liquid crystal.1–8 Whereas the first

examples of metal-containing liquid

crystals (metallomesogens) mimicked the

rodlike or disklike shape of the conven-

tional organic liquid crystals, it gradually

became clear that mesomorphism can

also be observed for other coordination

geometries than linear or square-planar

coordination. For most of the metallic

elements, at least one liquid-crystalline

complex has been described in the litera-

ture. However, the high transition

temperatures (>100 �C) and low thermal

stability at these elevated temperatures of

metallomesogens are major drawbacks

that hamper the study of the physical

properties of these materials. It is espe-

Koen Binnemans

Koen Binnem

in chemistry

(Belgium) w

secured a per

in 2002. His

dination chem

ionic liquids.

Katholieke Universiteit Leuven, Department ofChemistry, Celestijnenlaan 200F bus 2404,B-3001Leuven, Belgium. E-mail: Koen.Binnemans@chem.kuleuven.be

448 | J. Mater. Chem., 2009, 19, 448–453

cially difficult to observe light emission

(luminescence) at high temperatures

because of the strong tendency of the

excited states to de-activate via non-

radiative transitions. High-temperature

mesophases of metallomesogens can

often be supercooled to temperatures well

below the melting point, but the viscosity

of the supercooled mesophase increases

with decreasing temperatures until the

mesophase solidifies to a glassy state. The

high viscosity of the supercooled meso-

phases makes the filling of liquid-crystal

cells for luminescence measurements very

difficult, if not impossible. Therefore

most of the luminescence studies on met-

allomesogens that have been performed

so far have been restricted to samples in

the solid state or dissolved in organic

solvents. Luminescence of solid or

solution samples of metallomesogens

containing lanthanide(III),9–17 palladiu-

m(II),18–21 platinum(II),21–24 nickel(II),25

rhenium(I),9 gold(I),26 and gallium(III)27

ans (born in 1970) obtained a PhD degree

from the Catholic University of Leuven

ith Christiane Gorller-Walrand in 1996. He

manent professor position at this university

current research interests include the coor-

istry of rare earths, metallomesogens and

This journ

have been reported. However, careful

design of metallomesogens on the basis of

previously gained experience now enables

the formation of metal complexes that are

liquid crystalline at very moderate

temperatures or even at room tempera-

ture. Thanks to this recent progress,

luminescence studies of metallomesogens

in the liquid crystal state are starting to

appear in the scientific literature. This

highlight gives a short overview of these

recent developments, with an emphasis

on the work performed in the author’s

laboratory.

The lanthanide-containing metal-

lomesogens (lanthanidomesogens) are the

best known class of luminescent metal-

lomesogens.5,6,28 The emission observed

for these materials is metal-centered

between energy levels within the 4f shell of

the trivalent lanthanide ion. An advan-

tage of luminescence by trivalent lantha-

nide ions is that the luminescence spectra

consist of narrow emission lines of a high

coloric purity.29–31 The relative intensity

of the emission lines and their fine struc-

ture is only marginally influenced by the

nature of the ligand in the first coordina-

tion sphere. The excited states have a long

lifetime at room temperature, on the

order of microseconds or even millisec-

onds. The emission wavelength can be

tuned by a proper choice of the lanthanide

ion: red emission by Eu3+, green emission

by Tb3+, blue emission by Tm3+, orange

emission by Sm3+, near-infrared emission

by Nd3+, Er3+ and Yb3+. Unfortunately,

the light absorption by the lanthanide(III)

ions is weak. The molar absorption

coefficient values of the forbidden

intraconfigurational f–f transitions are

al is ª The Royal Society of Chemistry 2009

Fig. 1 Luminescent room-temperature lanthanidomesogens, designed by Piguet and coworkers.35

Fig. 2 Structures of the lanthanide complexes

[Ln(tta)3L2] and [Ln(bta)3L2]. Here, Ln

represent a trivalent lanthanide ion, tta is

2-thenoyltrifluoroacetonate, bta is benzoyltri-

fluoroacetonate, and L is a Schiff base.

Fig. 3 Luminescence spectra of [Eu(tta)3L2]

(grey curve) and [Eu(bta)3L2 ] (black curve) as

a thin film in the mesophase at 25 �C. The

excitation wavelength was 370 nm. All transi-

tions start from the 5D0 level and end at the

different J levels of the 7F term (J ¼ 0–4 in this

spectrum) (adapted from ref. 36).

typically less than 10 L mol�1 cm�1.

Because the photoluminescence intensity

is proportional to the amount of absorbed

light energy, this weak light absorption by

the metal center is a major disadvantage

of luminescent lanthanide compounds.

The problem can—at least partially—be

solved by building a shell of strongly

absorbing organic ligands around the

lanthanide ion. These ligands can act as

an ‘‘antenna’’ for light absorption, in the

sense that they can capture light and

transfer the excitation energy to the

lanthanide ion, where it is converted into

metal-centered luminescence. Depending

upon the match between the energy levels

of the ligands and those of the lanthanide

ion, this energy transfer is more or less

efficient. The organic ligands can also

shield the lanthanide ion from molecules

with high-energy vibrations, like water

molecules. This shielding effect is very

important for the observation of near-

infrared emission by lanthanide ions,

because the radiationless deactivation of

the excited states of the lanthanide ions by

coupling with high energy vibrations

strongly competes with photo-

luminescence.

Pioneering work in the field of lumi-

nescent lanthanidomesogens has been

done by Bunzli and coworkers. In

a seminal paper, they monitored the

luminescence intensity and the excited

state lifetime of a solvated Eu(NO3)3complex of a functionalized diazo-18-

crown-6 as a function of the temperature

to detect phase transitions.32 The inte-

grated intensity of the 5D0 /7F2 transi-

tion Iobs and the Eu(5D0) lifetime tobs were

found to decrease with increasing

temperatures due to more efficient non-

radiative relaxation of the excited state at

higher temperatures. The ln(Iobs/I295K)

versus 1/T curve showed a sigmoidal

shape, with a marked variation at the

melting point, upon heating. However,

the corresponding cooling curve was quite

monotonic. Therefore, the luminescence

measurements allowed the accurate

detection of the transition of the crystal-

line state to the hexagonal columnar

phase during the first heating process. By

monitoring the integrated intensity of the5D4 / 7F5 transition of Tb3+, it was

possible to determine the crystal-to-mes-

ophase transition in a terbium-containing

metallomesogen.33,34 Further lumines-

cence studies on Ln(NO3)3 complexes of

This journal is ª The Royal Society of Chemistry

2,6-bis(1-ethylbenzimidazol-2-yl)pyridine

derivatives synthesized by Piguet and

coworkers showed that the transition of

a crystalline phase to a cubic mesophase

could be detected by measurement of the

luminescence properties during the first

heating process.28 However, the vitrifica-

tion of the cubic mesophase upon cooling

could not be observed. This shows that

this luminescence technique cannot be

applied to the detection of glass transi-

tions. By rational design of mesogenic

liquid, it was possible to obtain liquid

crystalline lanthanide complexes with

melting points between �43 �C and �25�C, and with a mesophase stability range

of more than 250 �C (Fig. 1).35

Yang et al. reported on low-melting

lanthanidomesogens consisting of Lewis-

base adducts of a non-mesomorphic

salicylaldimine Schiff base ligand L

and tris(2,2-thenoyltrifluoroacetonato)

lanthanide(III) or tris(benzoyltrifluoro-

acetonate)lanthanide(III) complexes,

[Ln(tta)3L2] or [Ln(bta)3L2] (Fig. 2).36

These compounds form a smectic A phase

at room temperature. The luminescence

spectra of the Eu(III), Sm(III), Nd(III) and

Er(III) complexes have been measured in

the mesophase (Fig. 3). Of interest is the

fact that near-infrared emission byNd(III)

and Er(III) could be observed. Plots of the

luminescence decay time of the 5D0 level

of [Eu(tta)3L2] or the 4G5/2 level of

[Sm(tta)3L2] as a function of the temper-

ature show a gradual decrease of the

luminescence lifetime with increasing

temperatures. Amarked fall was visible in

these curves at the SmA / I transition

(Fig. 4). The intrinsic quantum yield of

the complexes [Eu(tta)3L2] and [Eu(-

bta)3L2] have been determined by photo-

acoustic methods.37 Although glass

transitions of the vitrified mesophase

could not be detected by luminescence

measurements, photoacoustic spectra

showed this transition clearly.

2009

Very recently, Galyametdinov and

coworkers prepared lanthanidomesogens

that exhibit smectic A and nematic

J. Mater. Chem., 2009, 19, 448–453 | 449

Fig. 4 Luminescence decay time of the 5D0

level of the [Eu(tta)3L2]complex as a function

of the temperature. The luminescence was

monitored at 613 nm (5D0 /7F2 line) and the

excitation wavelength was 370 nm. The

measurements were made during cooling of the

sample. The transition SmA 4 I (clearing

point) can be observed as a jump in the curve at

about 60 �C (adapted from ref. 36).

Fig. 6 Polarization effects in the room-

temperature luminescence spectra of an aligned

supercooled thin film of the europium(III)

complex of the type shown in Fig. 5.38 The

polarizer is either parallel (grey line) or

perpendicular (black line) to the alignment

layers in the liquid-crystal cell.

Fig. 8 Room temperature luminescence

spectrum of [Eu(TTA)3(phen)] in the nematic

liquid crystal host MBBA. The doping

concentration was 4 wt.%. The excitation

wavelength is 396 nm. Adapted from ref. 40.

phases. The ligand was an unusual b-di-

ketone with a cyclohexyl ring (Fig. 5).

When a thin film of the corresponding

europium(III) complex in the nematic

phase was supercooled to a glass phase in

a liquid-crystal cell with alignment layers

for planar alignment, polarized lumines-

cence could be observed for the aligned

samples (Fig. 6).38 The lanthanide

complexes were also well soluble in

nematic liquid-crystal mixtures.

Another approach to obtain lumines-

cent lanthanide-containing liquid crystals

is by dissolving a luminescent lanthanide

complex in a suitable liquid-crystal host

matrix. The advantage of this method is

that the luminescence and mesomorphic

properties can independently be opti-

mized. This allows easier access to

nematic lanthanide-containing liquid-

crystal mixtures. However, one often

faces the poor solubility of the molecular

lanthanide complexes in the liquid-crys-

talline host matrix. The guest–host

concept involving luminescent lanthanide

complexes was first applied by Yu and

Fig. 5 Nematogenic lanthanide complexes.38

450 | J. Mater. Chem., 2009, 19, 448–453

Labes who doped the nematic liquid

crystal 4-n-pentyl-40-cyanobiphenyl

(5CB)with europium(III) 2-thenoyltrifluoro-

acetonate trihydrate [Eu(tta)3$3H2O].39

Binnemans and Moors showed by high-

resolution luminescence spectroscopy

that well-resolved crystal-field fine struc-

tures could be observed for [Eu(tta)3(-

phen)] doped in the liquid-crystal hosts

N-(4-methyloxybenzylidene)-4-butylani-

line (MBBA) and 5CB (Fig. 7).40 The

spectra are more reminiscent of what is

observed for europium(III) ions doped in

crystalline hosts rather than europium(III)

complexes dissolved in organic solvents

(Fig. 8). Binnemans and coworkers

Fig. 7 Structures of the nematic liquid crystal

hostmatricesMBBAand 5CB, and structure of

the europium(III) complex [Eu(tta)3(phen)].

This journ

were the first to observe near-infrared

luminescence from lanthanide-doped

liquid crystal mixtures.41 They studied

the spectroscopic properties of the

lanthanide(III) b-diketonate complexes

[Ln(dbm)3(phen)], where Ln ¼ Nd, Er,

Yb, and dbm is dibenzoylmethane, in the

liquid crystalMBBA. By incorporation of

an erbium(III)-doped nematic liquid crystal

(ErCl3 dissolved in E7) in the pores of

microporous silicon, narrowing of the

erbium(III) emission band in the near-

infrared was observed.42 Luminescent

optically active liquid crystals were

obtained by doping [Eu(tta)3$3H2O] into

a mixture of cholesteryl nonanoate,

cholesteryl tetradecanoate and the ternary

liquid crystal mixture ZLI 1083 from

Merck.43,44 Driesen et al. constructed

a switchable near-infrared emitting liquid-

crystal cell. The authors doped the

lanthanide complexes [Nd(tta)3(phen)] and

[Yb(tta)3(phen)] into the chiral nematic

liquid-crystalmixture ofE7 and cholesteryl

nonanoate.45 However, the contrast ratios

were quite low. The photoluminescence

intensity of europium(III) and terbium(III)

complexes dissolved in nematic liquid

crystal 4-(isothiocyanotophenyl)-1-(trans-

4-hexyl)cyclohexane (6CHBT) strongly

depends on the strength of the applied

electric field.46 This effect was tentatively

assigned to a complex dipolar orientational

mechanism in the liquid crystal matrix.

Driesen and Binnemans prepared glass

dispersed liquid-crystal films doped with

a europium(III) b-diketonate complex.47

The luminescence intensity of the film was

measured as a function of temperature. A

sharp decrease in luminescence intensity

was observed for the transition from

the nematic phase to the isotropic phase.

al is ª The Royal Society of Chemistry 2009

Fig. 11 Luminescent mesomorphic tetrahe-

dral zinc(II) complexes with polycatenar pyr-

azole ligands (top) or bis(pyrazolyl)methane

ligands (bottom).53

This decrease in luminescence intensity at

the clearing point was attributed to less

scattering and thus less efficient use of the

excitation light in the isotropic state.

The liquid-crystal host matrix does not

have to be limited to nematic liquid crys-

tals. Bunzli and coworkers dissolved

different europium(III) salts in the smec-

togenic ionic liquid crystal 1-dodecyl-3-

methylimidazolium chloride, which

exhibits a smectic A phase between �2.8

and 104.4 �C.48 It was observed that

concentrations of europium(III) salts as

high as 10 mol% did not appreciably alter

the liquid-crystalline behavior of the host

matrix. Interestingly, the emission color

of the europium(III)-containing liquid-

crystal mixture could be tuned from blue

(emission color of the host matrix) to red

(emission color of the trivalent europium

ion), depending on the excitation wave-

length and the counter ion of euro-

pium(III). Intense near-infrared emission

could be obtained by doping the b-diket-

onate complexes [Ln(tta)3(phen)] (Ln ¼Nd, Er, Yb) in the same ionic liquid-

crystal host.49

The first examples of d-block metal-

lomesogens luminescing in the liquid

crystal phase were the smectogenic

rodlike gold(I) isocyanide complexes

described by Espinet and coworkers

(Fig. 9).50 The compounds in the solid

state show three intense broad emission

bands. For instance one compound dis-

played emission bands at 384 nm, 490 nm

and 524 nm. The band at 384 nm was

assigned to a fluorescent transition

involving intraligand localized p and p*

orbitals, whereas the phosphorescent

transitions are mainly resulting from in-

tramolecular intraligand transitions.

When the samples were heated, a strong

decrease in luminescence intensity was

observed upon melting from the crystal-

line to the smectic phase, and the lumi-

nescence intensity continued to decrease

until it totally vanished around 130 �C

before reaching the isotropic state.

However, upon cooling the luminescence

transitions re-appeared. The emission

Fig. 9 Luminescent liquid-crystalline gold(I)

isocyanide complexes.50

This journal is ª The Royal Society of Chemistry

bands associated with phosphorescence

were found to weaken more rapidly with

increasing temperature than the bands

associated with fluorescence. Fluoroalkyl

derivatives of crown-ether-isocyanide

gold(I) complexes are luminescent in the

solid state, in the mesophase and in the

isotropic liquid.51 The thermal behavior

of ionic silver(I) complexes with a 2,20-

bipyridine ligand containing the chiral

S-(�)-b-citronellyl group was found to

strongly depend on the counter ion

(Fig. 10).52 Whereas the tetrafluoroborate

and hexafluorophosphate derivatives are

not liquid crystalline, a room-tempera-

ture columnar hexagonal phase with

a columnar helical supramolecular

structure is displayed by the triflate

and dodecylsulfate derivatives. An inter-

esting observation is that the non-

mesomorphic tetrafluoroborate and

hexafluorophosphate samples are not

luminescent at room temperature in the

solid phase, whereas the mesomorphic

triflate and dodecylsulfate compounds

show blue luminescence in the mesophase

at room temperature. The luminescence

spectra exhibit a large Stokes shift

(120–146 nm) and one structureless

emission band. The lifetime of the excited

state was determined to be less than 30 ms.

These observations indicate that the

luminescence is due to excimer emission.

Excimers are formed when a monomeric

molecule is excited to a higher electronic

state by light absorption and when it

subsequently reacts with an unexcited

monomeric molecule. The excimers

of these silver(I)-containing metal-

lomesogens are only formed in the meso-

phase and not in concentrated solutions

in organic solvents. Probably the excimers

arise from the argentophilic interactions

between the silver(I) ions within the

columns of the mesophase.

Fig. 10 Room-temperature chiral silver(I)-cont

2009

Room-temperature tetrahedral zinc(II)

complexes with polycatenar pyrazole or

bis(pyrazolyl)methane ligands show

luminescence in the near ultraviolet and

blue spectral regions (Fig. 11).53 A broad

structureless emission band with a large

Stokes shift was observed. The emission

bands were found to red-shift with

increasing number of alkoxy chains. The

similarity between the absorption and

excitation spectrameans that the emission

is not caused by excimer formation,

notwithstanding the large Stokes shift.

The non-planar shape of the zinc(II)

compounds prevents the formation of

excimers. The luminescence color of

mesomorphic copper(I) metallacrown

complexes at room temperature depends

aining metallomesogens (X ¼ TfO, DOS).52

J. Mater. Chem., 2009, 19, 448–453 | 451

on the cooling rate of the sample from the

molten state.54 Red emission was

observed upon rapid cooling and yellow

emission upon slow cooling. This indi-

cates that kinetic processes (rapid cool-

ing) and thermodynamic processes (slow

cooling) compete with one another in the

self-assembly of the luminescent liquid

crystals. The compounds also show

luminescence in the mesophase. It has

been proposed to prepare rewritable

phosphorescent paper for security appli-

cations on the basis of this type of

metallomesogen. Bruce and coworkers

observed that the luminescence color of

a vitrified mesophase of a liquid-crystal-

line N,C,N-coordinated platinum(II)

complex is different from that observed

for a film of the same compound obtained

by fast cooling from the isotropic phase.55

The samples that were fast cooled from

the liquid-crystal phase displayed mono-

mer emission, whereas the samples fast

cooled from the isotropic state showed

excimer-like emission. Spin-coated thin

films exhibited excimer-like emission,

whereas heat treatment of the sample to

110 �C followed by cooling to room

temperature resulted in a drastic change

of the luminescence color from the red of

the excimer to the yellow of a mixture of

the monomer and the excimer. However,

rubbing of the heat treated film resulted in

a return of the red excimer emission.

Light-emitting metallomesogens can also

be obtained by incorporating an organic

fluorophor in themetal complex. This was

illustrated byGhedini and coworkers who

designed a liquid-crystalline palladium

complex with the fluorescent Nile Red

group.56

The number of studies on luminescence

of metallomesogens in the liquid-crystal

state is still limited, but considerable

progress is to be expected in the near

future. New strategies for the design of

room-temperature mesophases exhibited

by metal complexes will make more types

of metallomesogen available for photo-

physical studies. The possibility of

obtaining polarized emission via these

systems and the use of these compounds

in luminescent devices can be additional

drives for further exploration of this

research field.57 The most promising

metal ions to be included in luminescent

metallomesogens are Eu(III), Tb(III),

Sm(III), Ru(II), Pd(II), Pt(II), Ir(III), Ga(III)

and the non-platinum group d10 systems

452 | J. Mater. Chem., 2009, 19, 448–453

Cu(I), Ag(I), Au(I), Zn(II), and Cd(II).58

This highlight also draws attention to the

fact that most of the emissive excited

states in metallomesogens are not metal-

centered, the luminescence by the triva-

lent lanthanide ions being the exception.

As a consequence, the luminescence

properties of the metallomesogens con-

taining d-block elements are more

strongly affected by the intramolecular

interactions in the mesophase than those

of the lanthanidomesogens. The study of

the luminescence of metallomesogens in

the liquid-crystal state can give valuable

fundamental insight into the photo-

physics of ordered metal-containing

systems. However, the host–guest

approach (doping luminescent metal

complexes in a liquid-crystal matrix)

should not be neglected, because this will

be probably more useful for the develop-

ment of new light-emitting devices.

Acknowledgements

The author acknowledges the F.W.O.-

Flanders (project G.0508.07) and K. U.

Leuven (project GOA 08/05) for financial

support.

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