Introducing Bulky Functional Lanthanide Cores into Thermotropic Metallomesogens: A Bottom-Up...

DOI: 10.1002/adfm.200500195

Introducing Bulky Functional LanthanideCores into Thermotropic Metallomesogens:A Bottom–Up Approach**

By Emmanuel Terazzi, Stéphane Suarez, Stéphane Torelli,Homayoun Nozary, Daniel Imbert, Olimpia Mamula,Jean-Pierre Rivera, Erwann Guillet, Jean-Marc Bénech,Gérald Bernardinelli, Rosario Scopelliti, Bertrand Donnio,Daniel Guillon, Jean-Claude G. Bünzli,* and Claude Piguet*

1. Lanthanides and ThermotropicMetallomesogens: A Tricky Link

The design of thermotropic metallomesogens, that is, metal-containing liquid crystals, obeys the same rules as those estab-lished for the preparation of mesophases with organic mole-cules.[1] Roughly speaking, these rules claim that good molecu-lar candidates for producing thermotropic mesophases shouldpossess polarizable rigid cores displaying large anisometries(i.e., rod-like when the axial component is larger than the ra-dial component or disk-like for the reverse situation), to whicha set of flexible and poorly polarizable hydrocarbon chains areconnected, either to the termini (rod-like) or to the periphery(disk-like). According to their high polarizability, the metalions are usually incorporated within the core of the mesogeniccomplexes.[2] There is no strict correlation between the metalcoordination numbers and the global anisometries of the rigidcores of the mesogenic complexes. For instance, rod-like com-plexes leading to nematic and smectic mesophases may result

Adv. Funct. Mater. 2006, 16, 157–168 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 157

Basic thermodynamic considerations are used to rationalize the formation of thermotropic mesophases incorporat-ing aromatic ligands for trivalent lanthanide metal ions (Ln). Standard flat and bent molecular interfaces, separatingthe central binding unit from the lateral, flexible alkoxy chains, provide programmed lamellar and columnar organi-zation in the mesophases, which are removed upon complexation to Ln(NO3)3 and Ln(CF3CO2)3. Only signifi-cantly curved aromatic/aliphatic interfaces, found in polycatenar ligands, are able to overcome the considerable per-turbations of the intermolecular interactions produced by the introduction of the bulky metallic core. A richmesomorphism results, which can be tuned by a judicious control of the metallic coordination sphere. The exploita-tion of specific, metal-centered luminescence for probing phase transitions and microscopic environments in meso-phases is also discussed, as is the use of ionic liquids for producing tunable luminescent mesophases.

–[*] Prof. C. Piguet, E. Terazzi, Dr. S. Torelli, Dr. H. Nozary,

Dr. J.-P. Rivera, Dr. J.-M. BénechDepartment of Inorganic, Analytical and Applied ChemistryUniversity of Geneva30 quai E. Ansermet, CH-1211 Geneva 4 (Switzerland)E-mail: [email protected]. J.-C. G. Bünzli, Dr. S. Suarez, Dr. D. Imbert, Dr. O. Mamula,E. Guillet, Dr. R. ScopellitiLaboratory of Lanthanide Supramolecular ChemistryÉcole Polytechnique Fédérale de Lausanne (EPFL)BCH 1402, CH-1015 Lausanne (Switzerland)E-mail: [email protected]. G. BernardinelliLaboratory of X-ray CrystallographyUniversity of Geneva24 quai E. Ansermet, CH-1211 Geneva 4 (Switzerland)Dr. B. Donnio, Dr. D. GuillonInstitut de Physique et Chimie des Matériaux de Strasbourg-IPCMSGroupe des Matériaux Organiques23 rue du Loess, B.P. 43, F-67034 Strasbourg Cedex 2 (France)

[**] Financial support from the Swiss National Science Foundation Na-tional Research Program 47 “Supramolecular Functional Materials”is gratefully acknowledged.

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from linear, two-coordinate metals (CuI, AgI, AuI, HgII)[1,2] orbidimensional, four-coordinate tetragonal RhI and IrI complex-es, in which the polar terminal [M(CO)2Cl] groups are con-nected to a promesogenic, rod-like ligand.[1a,3] Alternatively,thermotropic, disk-like complexes providing columnar meso-phases can be obtained with four-coordinate metals (CuII, PdII,PtII) displaying tetragonal coordination spheres,[1,2] as well aswith linear two-coordinate AgI complexes bound to divergentpoly(alkoxy)stilbazole ligands.[1,4] Moreover, the need for com-plexation processes resisting entropy-driven dissociation occur-ring at high temperature in mesophases (i.e., in the absence ofa solvent) directed efforts toward the incorporation of d-blockions exhibiting large enthalpic contributions to the dativebonds. Conversely, trivalent lanthanides, e.g., LnIII, present co-ordination behaviors dominated by entropy-driven complexa-tion processes, which are prone to invert at high tempera-ture,[5,6] and their preference for large coordination numbers,without pronounced geometrical requirements, strongly limitsthe straightforward preparation of rod-like or disk-like com-plexes.[7] Therefore, thermotropic lanthanide-containing metal-lomesogens remained exotic for a long time, and the only logi-cal approach to their synthesis aimed at forcing LnIII to entercavities embedded into rigid polyaromatic, rod-like, semi-disk-like or disk-like ligands.[7] For instance, Galyametdinov, Binne-mans, and co-workers have intensively investigated the cylin-drical [Ln(L1)3X3] complexes, in which LnIII is coordinatedby three wrapped monodentate zwitterionic Schiff bases L1(Scheme 1) and by three counterions (X = nitrate, chloride,

alkylsulfate).[8] The lanthanide LnX3 core is deeply embeddedwithin the three aromatic binding units and smectic behaviorresults, in agreement with the global rod-like shape of thesecomplexes.[7,8] On the other hand, when rigid macrocyclic por-phyrin or phthalocyanine platforms, such as L2, react with LnIII

to give the neutral disk-like tabular sandwich complexes[Ln(L2–2H)2], columnar mesomorphism is observed along thecomplete lanthanide series.[9] However, organization in meso-phases is not strictly bipolarized between columnar andlamellar organization, and the same rigid core may lead toboth types of mesophases assuming a judicious curvature ofthe interface between the incompatible parts of the mole-cule (alkyl chains versus rigid core).[10] For instance, theinitial smectic mesophase (smectic A, SmA) observed for[Ln(L1)3X3] (lamellar organization) is transformed into acolumnar hexagonal arrangement (Colh) in [Ln(L3)2(L3–H)(CF3SO3)2] when one additional lateral alkyl chain is con-nected.[11] This trend is further confirmed for the hexacatenarligand L4, which possesses three diverging alkoxy chainsconnected to appended gallic acid groups and forms tabularcomplexes {[(L4–2H)M]2Ln(NO3)2} (M = NiII, CuII) displayinghexagonal columnar mesomorphism (melting temperatureTm = 59–90 °C).[12]

These observations suggest that the deleterious enthalpicperturbation of the intermolecular electrostatic interactions, in-duced by the complexation of the bulky lanthanide core, maybe limited when the metal is either masked within a “cocoon”of wrapped aromatic strands, as in [Ln(L1)3X3], or cleared

158 www.afm-journal.de © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2006, 16, 157–168

Jean-Claude Bünzli is a physical-inorganic and analytical chemist by training and an active re-searcher in the field of coordination and supramolecular chemistry of lanthanide ions. He earned adegree in Chemical Engineering in 1968 and a Ph.D. in 1971 from the École Polytechnique Fédér-ale de Lausanne (EPFL). He spent two years at the University of British Columbia as a teachingpostdoctoral fellow (photoelectron spectroscopy) and one year at the Swiss Federal Institute ofTechnology in Zürich (physical organic chemistry). He was appointed assistant-professor at theUniversity of Lausanne in 1974 and promoted to full professor of inorganic and analytical chemis-try in 1980; he was Dean of the Faculty of Sciences (1990–1991) and an elected Vice-Rector (1991–1995), in charge of student affairs and of research programs in biomedical sciences. He was ap-pointed at the EPFL in 2001. His research focuses mainly on the relationship between luminescentproperties of lanthanide ions and the structure of the edifices in which these ions are embedded, onthe use of lanthanide ions as luminescent probes, and on the design of self-assembled buildingblocks for the synthesis of materials with predetermined photophysical and magnetic properties.

Claude Piguet studied chemistry at the University of Geneva and received his M.S. degree in Chem-istry in 1986 and his Ph.D. with felicitations in 1989 in the field of biomimetic copper-dioxygencomplexes. After postdoctoral work in the groups of Professors J.-M. Lehn, A. F. Williams, andJ.-C. G. Bünzli, he initiated research projects in the field of supramolecular chemistry of lanthanideions. In 1995 he received the Werner Medal of the New Swiss Chemical Society for these contribu-tions and the Werner grant. He was an Assistant Professor at the University of Geneva from 1995to 1998 and was appointed Full Professor of Inorganic Chemistry in 1999. His research interestsinclude the design of discrete multimetallic supramolecular complexes fitted with d-block and/orf-block metal ions, programming of novel electronic functions depending on intermetallic commu-nications, molecular tuning of local dielectric constants, unravelling of the thermodynamic originof multicomponent self-assembly processes, preparation of lanthanide-containing metallomeso-gens, and the development of simple paramagnetic NMR methods for addressing solution struc-tures.

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E. Terazzi et al./Functional Lanthanide Cores in Thermotropic Metallomesogens

from its counterions, as in [Ln(L2–2H)2]. Moreover, the in-creased curvature of the molecular interface in polycatenar sys-tems favors the formation of the mesophase, with a preferencefor columnar and cubic supramolecular organizations. Thesevarious aspects originate from a common basic thermodynamicdescription of the melting processes, which is rarely explicitlydeveloped for rationalizing phase transitions.[13]

2. Some Thermodynamic Origins of ThermotropicLiquid Crystals: Application to Mesogenic Ligandsfor Lanthanide Metal Ions

Let us recall that the thermodynamic definition of the stan-dard Tm of a solid is the temperature at which the chemicalpotentials of the pure solid and the pure liquid are equalunder a fixed external pressure.[14] In common chemical terms,the free-energy change of the melting process is given by

DGm = DHm–TmDSm = Gl–Gs = 0 (DHm and DSm are the meltingenthalpy and entropy, respectively; Gl: free energy of the liquidstate; Gs: free energy of the solid state), and the coexistence ofthe two phases only results from the additional mixing entropy.We can thus easily deduce that Tm = DHm/DSm, which is a valu-able relationship for programming the formation of thermotro-pic mesophases, that is, phases occurring from the melting ofthe solid but separated from the isotropic liquid by a secondmelting process. In most liquid crystals, such double-meltingprocesses can arise from the presence of two incompatibleparts in the molecule,[15] which results from the connection ofone or more long, flexible, and poorly polarizable saturated hy-drocarbon chains to rigid, highly polarizable polyaromaticcores.[16] In simple cases, the minimum energy in the crystallinephase corresponds to a microsegregated organization, whichmaximizes the intermolecular multipolar electrostatic interac-tions between the polarizable aromatic cores, while the fullextended hydrocarbon chains fill the voids, as illustrated inFigure 1a for the rod-like ligand L5.[17] Assuming a simplisticthermodynamic model, the first phase transition occurringupon heating, often termed the melting process, can be as-signed to the melting of the packed hydrocarbon chainsTm = DHchains

m /DSchainsm because the breaking of the moderate in-

termolecular interactions (DHchainsm > 0) is largely compensated

by the large entropic increase (TmDSchainsm >> 0) resulting from

the relaxation of the vibrational and rotationaldegrees of freedom (Fig. 1b). The second phase transition, theso-called clearing process, occurs at a higher temperatureTc = DHcores

m /DScoresm = DHc/DSc because the remaining intermo-

lecular cohesive interactions mainly involve packed aromaticcores (DHcores

m ), which have to be overcome by the relativelysmall entropic contribution (TDScores

m ). The clearing processeventually leads to the isotropic liquid (Fig. 1c). Obviously,these two successive melting processes are strongly correlatedin real systems, and the molten alkyl chains still undergo someintermolecular cohesive interactions in the mesophase, whichmust be removed when entering the isotropic liquid, while alarge part of the intermolecular interactions involving the aro-matic cores are already destroyed when entering the liquid-crystalline (LC) state. Consequently, the molecules in LCphases display significant motions, which are responsible formacroscopic fluidity, and the enthalpic and entropic contribu-tions to the melting process are generally larger than thoserecorded for the subsequent clearing process (as illustrated inFig. 1). Although this approach of strict decoupling of the twomelting processes is inadequate for quantitatively unravellingphase transitions and for rationalizing sophisticated behaviors,such as the formation of monotropic mesophases, we believethat the rather naive consideration of a mesophase as made upof packed clusters of semi-organized rigid aromatic cores dis-tributed in a continuum of molten alkyl chains remains a usefulguideline for synthetic chemists, who aim to tune transitiontemperatures and macroscopic organizations in thermotropicmetal-containing liquid crystals.

The thermal behaviors of ligands L5–L12 (see Figs. 2,3) illus-trate some of the points of our simplified approach. For ligandL6, oxidation of the ether spacers into ester bridges has a dra-

Adv. Funct. Mater. 2006, 16, 157–168 © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.afm-journal.de 159

N

OH

CmH2m+1

H2n+1CnO

N

OH

C12H25

O

O

H25C12O

H25C12O

OCnH2n+1

OCnH2n+1

HN

N

NH2n+1CnO

H2n+1CnO

NH

N

N

H2n+1CnO OCnH2n+1

N

H2n+1CnO OCnH2n+1

N

L1

L2

L3

L4

N N

OH HO OO

O

OC14H29

OC14H29H29C14O 29C14O

H29C14O

OC14H29H

O

Scheme 1. Chemical structures of ligands L1–L4.

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matic structural effect on the orientation of the two appendedbenzoic acids, which now adopt perpendicular arrangementswith respect to the planar central tridentate binding unit. Thiseffect is further confirmed with the extended ligand L7, wherethe number of appended aromatic groups is increased.[18] A dif-ferent sequence of mesophases is observed with an originalsmectic organization (one degree of orientational order andpartial positional order), which is transformed into nematic or-ganization (one degree of orientational order) prior to isotropi-zation (Fig. 2a).[17,18] The Tm and Tc temperatures for L5 andL6 are comparable as a result of their rather similar shapes andpolarizabilities. (Tc(L5) < Tc(L6) is tentatively assigned to theincreased polarity of the carboxylate spacers). The correspond-ing parameters for L7 are significantly larger because of itsoverall higher polarizability. In this context, the loss of the rigidcentral aromatic core in L8[19] is expected to significantly re-duce DHcores

m , and we indeed measure only DHc = 9 kJ mol–1,which is responsible for the dramatic decrease in Tc (Fig. 2c).For L9, the introduction of flexible, three-atom spacers reducesthe size of the rigid aromatic core, which decreases DHcores

m . Tm

and Tc thus merge to give a single melting process, and meso-morphism is removed (Fig. 2c).[17]

Besides DHcoresm , which can be independently tuned by judi-

cious design of the molecular structure of the rigid aromaticcore, DSchains

m is the second parameter amenable to chemicaltuning, via the connection of an increased number of flexiblechains to the central rigid core.[20] The resulting so-called poly-catenar ligands L10 (tetracatenar),[21] L11 (hexacatenar),[21]

and L12 (hexacatenar)[22] systematically display a lower Tm

than their two-chain analogues L6 (DTm = 106 °C) and L8(DTm = 107 °C), diagnostic for a significant increase of DSchains

m

(i.e., a measure of the efficiency of filling the space adequatelyin the crystal, Fig. 3a). However, the parallel reduction of Tc,observed in going from L6 to L12 (DTc = 132 °C), confirms thatthis extra entropic contribution affects both melting and clear-ing processes, which may limit the domain of existence of themesophases for these systems. We thus conclude that, from achemical point of view, the domain of existence of a thermotro-pic liquid-crystal phase is maximum when: i) the rigid aromaticcore displays considerable intermolecular interactions (DHcores

m

is large); ii) many hydrocarbon chains are attached to the rigidcore (DSchains

m is large); and iii) the enthalpic and entropic con-tributions to the melting and clearing processes are poorly cor-related. This latter point is rarely observed in real systems, andsome compromise has to be found for inducing thermotropicmesophases, as illustrated in Figures 2,3 for ligands L5–L12.

3. Molecular Interfaces and MacroscopicOrganization in Thermotropic Liquid Crystals

The ligands L5–L8 (Fig. 2) exhibit rod-like anisometries,with their flexible alkoxy chains connected to one or two circu-lar faces of the molecular cylinders (i.e., parallel to the longmolecular axis). This arrangement produces a flat interface be-tween the two incompatible parts of the molecules and, afterentering the thermotropic phase at Tm, the disordered molten


N N

N NN

OOO

O

a) Solid

b) Mesophase

c) Liquid

kJ/mol5.27m =H∆

KJ/mol66m ⋅=S∆C144m °=T

kJ/mol2.18c =H∆

KJ/mol39c ⋅=S∆

C193c °=T

58 Å

51 Å

L5

N

NN

NN

O

O

O

O

N

NN

NN

O

O

O

O

N

NN

NN

O

O

O

O

N

NN

NN

O

O

O

O

N

NN

NN

O

O

O

O

N

NN

NN

O

O

O

O

N

NN

NN

O

O

O

O

N

NN

NN

O

O

O

O

N

NN

NN

O

O

O

O

N

NN

NN

O

O

O

O

N

NN

NN

O

O

O

O

N

NN

NN

O

O

O

O

Figure 1. The double melting process evidenced for ligand L5. a) X-raycrystal structure in the solid state. b) Organization in the smectic A(SmA) mesophase deduced from small-angle X-ray scattering (SAXS)patterns. c) Chemical structure of L5. The selected conformation aims toillustrate the large number of degrees of freedom of the ligand in theisotropic liquid.

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hydrocarbon chains prevent intermolecular packing involvingthe latter terminal surfaces. Consequently, only intermolecularinteractions perpendicular to the molecular axis may develop,and nematic and smectic mesomorphism occurs, which is char-acterized by molecules displaying lateral cohesion (Fig. 4a).When these peripheral interactions remain weak, one degreeof orientational order is induced, which corresponds to thenematic organization (N), the lowest level of macroscopic or-ganization. Conversely, when the lateral interactions betweenthe cylindrical rigid cores become considerable, the orienta-tional order is completed with a partial positional ordering,and layered smectic organizations result (SmX).[1,20] Ligand L6

illustrates these predictions, since the connection of two dode-cyloxy chains to the termini of the rod-like core produces smec-tic behavior at low temperature (smectic C, SmC, and SmA).Interestingly, the increased Brownian motion induced at highertemperature further limits lateral interactions between themolecules in the smectic phases (partial positional + one degreeof orientational order) and may eventually result in transfor-mation into a nematic phase (one degree of orientational or-der) prior to isotropization.[17] When more than two flexiblechains are connected to each end of the cylindrical core, theresulting bent interface between the polarizable parts and thechains allows intermolecular interactions to operate specifical-


b)

L9

ICr C107 → °

c)

O

O

O

O

O

O

O OC12H25

O

OH25C12O

O

N

N

NN

N

O

O

OC12H25

O

O

H25C12O

O

O

H25C12ON

N

NN

NO

O

O

O

O

O

OC12H25

a)

X = H2: L5

N

N

NN

NO

X

H25C12O

O

X

OC12H25

ISmACr C193C144 → → °°

X = O: L6 INSmASmCCr C262C223C217C131 → → → → °°°°

L7 INSmXCr C322C280C062 → → → °°°

L8 ISmACr C153C144 → → °°

Figure 2. Mesomorphic behaviors of the mesogenic ligands L5–L9. Illustration of the influence of a) the nature of the spacer, b) the number of interca-lated 4-hydroxy benzoic acid units, and c) the rigidity of the central aromatic core (Cr: crystal, SmC: smectic C, SmX: undetermined smectic phase, N:nematic, I: isotropic liquid) [17–19].

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ly along a single, preferred direction, which coincides with theaxis of the cylinder (Fig. 4b).[23] Consequently, the moleculespack on top of each other and form columns possessing at leastone degree of orientational order (nematic columnar, NCol),which can be completed with additional degrees of positionalorders when intercolumnar interactions become sizeable(hexagonal (Colh), rectangular (Colr), or oblique (Colo) colum-nar).[1,20] The Colh reported for the hexacatenar ligand L12 can

thus be simply assigned to the two-dimensionallycurved interface produced by the divergent con-nection of the dodecyloxy chains to the terminalgallic acid residues (Fig. 3a), a well-describedbehavior in polycatenar systems.[4,23,24] It is, how-ever, worth noting that further bending of the rig-id cores in the V-shaped ligands L13 and L14 isdeleterious for mesomorphism, and a Colr meso-phase is only observed for L14 over 8° (Fig. 3b).Finally, a three-dimensional arrangement of theflexible alkyl chains around an approximatelyspherical rigid core obviously prevents any pre-ferred directions for intermolecular interactions,and the microsegregation process provides cubicmesophases (Fig. 4c).[24]

4. Introducing Bulky Lanthanide Coresinto Thermotropic Liquid Crystals

In order to control the constitution and the ge-ometry of the coordination sphere of trivalentlanthanides, a prerequisite for inducing predeter-mined electronic, optical, and magnetic functions,we selected Ln(NO3)3 and Ln(CF3CO2)3 metalliccores, since six positions are blocked in a con-trolled fashion by the three bidentate counter-an-ions. Therefore, their reactions with the triden-tate binding units of the ligands L5–L7 and L12–L14 are expected to give well-defined, nine-coor-dinate complexes. This assumption is confirmedby the molecular structures of the non-lipophilicmodel complexes [Ln(L15)(NO3)3] (Fig. 5a) and[Ln(L16)(CF3CO2)3]n (n = 1,2; Fig. 5b).[5,17,18,25]

Three crucial structural changes occur upon com-plexation of the tridentate binding unit to LnX3

(X = NO3, CF3CO2). Firstly, the original transconformations of the N-benzimidazole rings withrespect to the central pyridine ring in the triden-tate unit, characteristic of the free ligands, trans-forms into the alternative cis–cis arrangement inthe resulting complexes. Consequently, the globalrod-like and bent shapes of the ligands inter-convert upon complexation (Fig. 5), which dra-matically alters the geometry of the molecularinterface in the mesogenic ligands L5–L7 andL12–L14. Secondly, two bound bidentate anionsare located on the opposite sides of the aromatic

plane defined by the coordinated tridentate binding unit in thecomplexes (Fig. 5). The latter effect is expected to stronglydisturb the intermolecular interactions between the rigid coresand to reduce the molecular anisotropy. This translates intoa considerable reduction of DHcores

m , and a concomitant de-crease in Tc, Tc = DHcores

m /DScoresm . The domain of existence of

the mesophase thus shrinks, and we did not observe LC behav-ior for the complexes [Ln(Lk)(NO3)3] (k = 5,7,8,14).[17,18,25,26]


R = H: L10 ICr C85 →→→→ °

R = OC10H23: L11 ICr C37 →→→→ °

L12 IColCr C61h

C25 →→→→ →→→→ °°

L14 IColCr C203r

C195 →→→→ →→→→ °°

L13 ICr C58 →→→→ °

N

N

NN

NO

O

H25C12O

O

O

OC12H25

H25C12O

H25C12O

OC12H25

OC12H25

O

N

O

O

O

N

O

OO

O

R

OC10H23

OC10H23

R

H23C10O

H23C10O

N

N

NN

N

O

O

O

H25C12O

H25C12O

OC12H25 H25C12O OC12H25

OC12H25

N

N

NN

N

O

O

O

O

O

H25C12O OC12H25

a)

b)

O

O

O

O

Figure 3. Mesomorphic behaviors of the polycatenar ligands L10–L14. Illustration of theinfluence of a) the number of alkoxy chains and b) the interconversion between rod-likeand bent geometries (Cr: crystal, Colh: hexagonal columnar, Colr: rectangular columnar,I: isotropic liquid) [18,21,22].

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Thirdly, CF3CO2– and, to a lesser extent, NO3

– show sometendency to act as bridging ligands. This is illustratedfor [Ln(L16)(CF3CO2)3]n, which crystallizes i) as a mono-mer when water molecules coordinate to the metal in[Lu(L16)(CF3CO2)3(OH2)] and prevent dimerization, butii) as a dimer [Ln(L16)(CF3CO2)3]2 under strictly anhydrousconditions (Fig. 5b). However, the related lipophilic H-shapedcomplex [Ln(L14)(CF3CO2)3]2 also escapes mesomorphism.[18]

It is only when three divergent alkoxy chains are attached toeach termini of the aromatic cores in L12 and L13 that a dou-ble melting process can be detected for the lanthanide com-plexes [Ln(Lk)(NO3)3] (k = 12,13). The existence of the meso-phases, developing at rather low temperature (70–220 °C), isstrongly indebted to the increased entropy of melting of theflexible chains DSchains

m , associated with the extremely high or-dering required for the packing of six alkoxy chains per mole-cule in the solid state. Interestingly, the macroscopic organiza-tion depends on the size of the lanthanide metal ion, and aminor relative total contraction of 15 %, on going from LaIII toLuIII, is sufficient to span all three possible organizations foundin thermotropic mesophases: lamellar, columnar, and cubic(Fig. 6).[5,22] A combination of differential scanning calorimetry(DSC), polarized light microscopy (PLM), and small angleX-ray scattering (SAXS) shows that the hemidisk complexes[Ln(L12)(NO3)3] exhibit Colh mesophases for small LnIII cores(Ln = Er–Lu), body-centered cubic organization (Cub, Im3̄m)for mid-range LnIII (Ln = Sm–Ho) cores, and a sequence oflamello-columnar (Lcol) and body-centered cubic mesophasesfor large LnIII (Ln = La–Nd, Fig. 6) cores.[5] Very similar be-haviors are observed for the related hexacatenar complexes[Ln(L13)(NO3)3], except for the Lcol phase, which is formed atlow temperature along the major part of the lanthanide series(Ln = La–Ho), thus pointing to its improved stabilization for li-gands adopting rod-like geometrical arrangements upon coor-dination.[5]

For the smaller LnIII cores, the complexes[Ln(Lk)(NO3)3] (k = 12, 13) exist as monomericbuilding blocks in the solid state, as shown for thenon-lipophilic model compound [Ln(L15)(NO3)3]in Figure 5a,[5] and in a non-coordinating solvent,as ascertained by their diffusion coefficients mea-sured in CD2Cl2.[5] Therefore, we can safely con-clude that similar monomeric building blocksexist in the mesophase. According to the two-di-mensional bent curvature of the molecular inter-face displayed in [Ln(Lk)(NO3)3] (k = 12,13; seeFig. 5a for a structural model), we expect thatcolumnar organization should result if the pre-ferred direction for intermolecular packing occur-ring perpendicular to the coordinated tridentatebinding unit is not too disturbed by the size of themetallic core (Fig. 4). These assumptions are ver-ified for the smaller LnIII cores, and an efficienthead-to-tail packing of the bound tridentate bind-ing units along the columns is responsible for thecolumnar mesomorphism. The weak intercolum-nar interactions, responsible for the positional or-

der in the Colh mesophase, are assigned to residual interaro-matic interactions involving pairs of appended gallic acids(Fig. 6).[22] When the size of the metal increases for mid-rangeLnIII cores, the intracolumnar packing along the columnar axisis severely reduced, while the intercolumnar interactions in-volving the orthogonal gallic acids pairs are not affected.Competition between intra- and intercolumnar interactions op-erates, and either “oscillations”[27] or puckering[28] of the col-umns is the likely result. In the former situation, the resultingformation of dislocation points eventually leads to the isotropicgrowth of the columns in the three Cartesian directions ofspace, and thus a bicontinuous structure (Fig. 6).[29] In the lat-ter case, however, the cylindrical rods are pinched off, and dis-crete micelles form; the three-dimensional, close packing ofthese soft micelles, driven by the maximal occupation of theavailable space, leads to a micellar cubic phase. Direct evi-dence for the structure of the bicontinuous Im3̄m phase is stillscant. Moreover, detailed structural modeling of the complexes[Ln(Lk)(NO3)3] (k = 12,13) in the Im3̄m Cub mesophase doesnot allow unambiguous differentiation between micellar orbicontinuous descriptions for the macroscopic organization(Fig. 6).[5] An intermediate interpretation with a maximum ofthe electronic density located around the special positions ofthe body-centered network, together with residual electronicdensities in between, can be reasonably proposed (perhaps par-tially fused spheroid micelles). For large LnIII cores, dimeriza-tion, 2[Ln(Lk)(NO3)3]� [Ln(Lk)(NO3)3]2 occurs at roomtemperature in the solid state and in solution, because the en-thalpic trend for improving the coordination number to ten forlarge LnIII ions overcomes the considerable entropic cost of thedimerization process.[5] We can thus safely assume that the firstmesophase formed for large lanthanides upon heating indeedcontains [Ln(Lk)(NO3)3]2 dimers, which can be structurallymodeled from the crystal structure of [Eu(L16)(NO3)3]2,[5] acomplex very similar to that shown in Figure 5b for


2 privileged directions for

intermolecular interactions.

Perpendicular to the

molecular axis.

a) Flat interface

Smectic

mesomorphism

b) 2D-curved interface c) 3D-curved interface

Rigid

coreRigid

core

Rigid

core

1 privileged direction for


Parallel to the

molecular axis.

No privileged direction for


Columnar

mesomorphism

Cubic

mesomorphism

Main

molecular

axes

Figure 4. Correlations between the geometry of the molecular interfaces separating theincompatible parts of the molecule and the resulting macroscopic organizations.

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[Lu2(L16)2(CF3CO2)6], except for the replacement of CF3CO2–

with NO3–. The complete dimer can be included within a cylin-

der 45 Å long and 9 Å in diameter.[5] These geometrical char-acteristics perfectly match the macroscopic organization de-duced by SAXS measurements in the Lcol mesophase; a strongsupport for assigning its origin to the existence of rod-like di-meric building blocks (Fig. 6). However, we predict, and ob-serve, that there exists a critical temperature Td for the dimeri-zation process, at which the favorable enthalpic contributionDHd, brought by the formation of the two additional Ln–O(nitrate) bonds in the dimer, is overcome by the entropic costTDSd (i.e., Td = DHd/DSd). The monomer is thus restored in themesophase, which induces a phase transition from the Lcol or-ganization, typical for dimeric building blocks, toward the Cuborganization, typical for monomeric building blocks (Fig. 6).This interpretation is further corroborated by the far more fre-quent observation of this phase sequence for the hexacatenarcomplexes [Ln(L13)(NO3)3]2, which can be assigned to signifi-cantly reduced steric constraints occurring upon the dimeriza-tion of two parallel ligand strands, instead of interpenetratedstrands as found in [Ln(L12)(NO3)3]2.[5]

We eventually concluded that the increased curvature of themolecular interface in polycatenar ligands is a prerequisite forinducing a double-melting process, when bulky metallic coresare introduced within pseudoplanar binding units. Minor struc-tural changes, brought about by the specific arrangement of thecoordination sphere, further drastically modulate intermolecu-lar interactions and consequently macroscopic organization.The formation of lanthanide-containing mesophases with themacrocyclic 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane, theso-called (2,2) platform, benefits from the same design with theconnection of two (L10) or three (L11) alkoxy chains to eacharomatic side arm (Fig. 3). Although the free ligands are non-mesomorphic, the rigidification of the central core induced bythe introduction of Ln(NO3)3 in the macrocyclic cavity favorsmicrosegregation processes, and the expected Colh mesophaseswere detected in the 58–200 °C range.[21] Such rigidificationprocesses are well-documented, and large macrocyclic poly-amines, substituted with alkoxybenzoyl groups, have been re-ported by Lehn et al. to form columnar phases. The mesophasestability was found to decrease with increasing size of thecrowns, which was attributed to the growing conformationalflexibility of the molecule, and thus to the quite substantial de-viation from a disk-like structure. A means to improve themesophase stability consisted of stiffening the azacrown coremoieties through metal coordination.[30] Similarly, several newnon-mesomorphic azacrowns [18]aneN6 (1,4,7,10,13,16-hexa-azacyclooctadecane) were prepared by Ringsdorf and co-work-ers. Cobalt and nickel complexes of [18]aneN6 exhibited meso-phases. In this case, the conformational rigidification of thesystem appeared to be essential for mesomorphism induc-tion.[31] Related observations have been reported for othermacrocycles, such as [18]aneN2S4 (1,10-diaza-4,7,13,16-tetra-thiocyclooctadecane), [14]aneS4 (1,4,8,11-tetrathiocyclotetra-decane), substituted 1,4,7-triazacyclononane, and azo-substi-tuted calix[4]arene.[32]


a) 5,5’-disubstituted

L15

[Yb(L15)(NO3)3]

N

N

NN

NO

O

O

O

Yb (NO3)3

N

N

NN

N

O

O

O

b) 6,6’-disubstituted

L16

[Lu(L16)(CF3CO2)3]2

Lu (CF3CO2)3

CH3CN/H2O CH3CN

[Lu(L16)(CF3CO2)3(OH2)]

O

Figure 5. Trans–trans to cis–cis conformational change occurring uponcomplexation of a) the rod-like (i.e., 5,5′-disubstituted) ligand L15 toLn(NO3)3, thus giving bent complexes, and b) the bent (i.e., 6,6′-disubsti-tuted) ligand L16 to Ln(CF3CO2)3, thus giving rod-like complexes. Thecrystal structures are those of [Yb(L15)(NO3)3], [Lu(L16)(CF3CO2)3(OH2)]and [Lu(L16)(CF3CO2)3]2.

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5. Exploiting New Optical Properties in LanthanideMetallomesogens

Insertion of LnIII ions into molecular edifices results in elec-tronic levels being split by (weak) ligand-field effects, and tran-sitions between these sublevels are strictly governed by sym-metry selection rules: A transition is allowed if Cop ∈ Ci × Cf,that is, if the irreducible representation of the electric or mag-netic dipolar operator is contained in the products of the irre-ducible representations of the initial and final wavefunctions.Therefore, the analysis of high-resolution luminescence spectraalong these lines allows the determination of the ligand-fieldsplitting of the levels and of the site symmetry of the LnIII ion.The EuIII ion is a particularly powerful luminescent probe,since its major emitting state is the unsplit level 5D0, which sim-plifies the analysis,[33] and we have taken advantage of thision to investigate its microenvironment in mesophases and tomonitor the changes induced by the Cr-to-LC transitions (andvice versa). In fact, both the luminescence intensity (I) andlifetime of the excited state (s) diminish with increasingtemperature because non-radiative de-excitation involves tem-perature-dependent vibrational and/or electronic processes.The expected variation with temperature, T, is y = y0e–C/RT,[34]

so that monotonic ln(yobs/y295K) curves are expected (y = I or s)in the absence of a phase transition. When the material underinvestigation undergoes such a phase transition, the refractiveindex is known to display a sigmoidal variation. From a theo-retical point of view, the emission intensity of LnIII ionsis expressed by Equation 1

I�J� J ′� � 64p4�m3

3h�2J � 1�n�n2 � 2�2

9DED � n3DMD

� �(1)

where I(J,J′) [s–1] represents the probability of spontaneousemission; is the average energy of the transition [cm–1];h is Planck’s constant, 6.63 × 10–27 erg s (1 erg = 1 g cm2 s–2);

(2J + 1) is the degeneracy of the initial state (1 for Eu(5D0));DED and DMD (in esu2 cm2) are the contributions from theelectric and magnetic dipole operators, respectively; and n isthe refractive index of the medium.[35] Therefore, S-shapedvariation in I is expected over the phase transition. In the caseof EuIII, the transition most sensitive to changes in the environ-ment of the metal ion is the hypersensitive 5D0 → 7F2 transition,which is purely electric dipole in nature; as a consequence, thesecond term of Equation 1 vanishes and grouping all the con-stants (cst) yields the simplified Equation 2

I�0� 2� � n�n2�2�2

9× DED × cst (2)

Experimental data on [Eu(NO3)3L10]·0.25H2O are reportedin Figure 7. The compound displays a phase transition to a Colhphase (as proved by PLM and SAXS measurements) at 86 °C,inducing the expected S-shape variation in refractive index.The change in luminescence intensity also shows such a sigmoi-dal curve, most of the observed difference being genuine to thephase transition since the contribution from the refractive in-dex term in Equation 2 only amounts to, at most, 15 % of thetotal variation. Moreover, the same experiment conducted on anon-mesomorphic reference compound [Eu(NO3)3L10a], inwhich the C10 alkyl chains are replaced by methyl groups, pre-sents a smooth variation of the luminescence intensity versustemperature. Finally, the concomitant S-shape change in thelifetime of the 5D0 level fully confirms the sensitivity of the lu-minescence parameters to the changes in the metal microenvir-onments during the melting process. The changes are revers-ible, and several heating–cooling cycles always generate thesame patterns. A similar effect has also been observed for[Tb(NO3)3L10], as far as the intensity of the 5D4 → 7F5 transi-tion is concerned.[36] Sigmoidal variation is seen for both loga-rithmic plots (intensity and lifetime) when [Eu(NO3)3L12]enters its cubic mesophase during the first heating process;however, the following cooling process (and subsequent heat-


Ln=La-Nd

Ln=Sm-Ho

[Ln(L12)(NO3)3]

Lamellar

70-120ºC

Ln=Er-Lu

130-220ºC

130-200ºC

Columnar

Cubic

micellar bicontinuous

130-180ºC

Figure 6. Mesomorphic behavior of the hemidisk complexes [Ln(L12)(NO3)3].

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ing–cooling cycles) show that the Cub mesophase is super-cooled in the final glassy state. As a result, no significant struc-tural variation occurs during these subsequent phase transi-tions, and both logarithmic plots become monotonic withtemperature.[5] Consequently, luminescence intensity and/orlifetime switching can be used to signal Cr–LC transitions inEuIII and TbIII mesomorphic compounds, but glass transitionsare not accessible by this detection technique.

Room-temperature ionic liquids (RTILs) are usually com-posed of a bulky organic cation and an inorganic anion and arepresently receiving a wealth of attention because of their lowvolatility and potential as solvents for various “green” chemicalprocesses. We have taken advantage of the mesogenic proper-ties of an ionic liquid presenting LC behavior at room tempera-ture, 1-dodecyl-3-methylimidazolium chloride ([C12-mim]Cl).This salt forms lamellar, sheet-like arrays in the crystallinephase and an enantiotropic smectic LC phase at temperatureshigher than 0–20 °C, depending on its water content. Introduc-ing lanthanide salts (up to 10 mol-%) into this ionic liquid doesnot significantly alter its LC properties. We have used the EuIII

luminescence to probe the coordination sphere of the metalion in the RTIL, demonstrating, for instance, the formation ofpolychloro species with low symmetry derived from an idea-lized cubic symmetry. The intrinsic quantum yield (i.e., thequantum yield obtained upon metal excitation) of theEuIII-centered luminescence is large, ranging from 52–61 % de-pending on the anion. Consequently, the emission color of the

EuIII-containing materials can be tuned from blue to red byvarying the counterion and the excitation wavelength (Fig. 8),that is, by tuning the mix between the blue RTIL emission andthe red, metal-centered luminescence. This creates interestingperspectives for the design of environmentally friendly lumi-nescent devices.[37]

6. Summary and Outlook

A principal conclusion of this work concerns the deleteriouseffect of introducing bulky lanthanide cores into thermotropicmesophases. Intermolecular packing interactions responsiblefor the cohesion of the rigid cores become so small that usuallyboth melting processes coincide, and mesomorphism is lost.Although two original strategies based on i) masking theLn(NO3)3 core by using three wrapped aromatic strands in[Ln(L1)3X3][6,8,11] or ii) expulsing counterions in [Ln(L2–2H)2][9] successfully produced thermotropic mesophases, thedesign of polycatenar ligands possessing curved molecular in-terfaces appears as a complementary and versatile tool forforcing microsegregation processes to occur in these sys-tems.[5,12,21,22] The bent interfaces obviously favor columnar andcubic organizations (Fig. 4), which are generally associatedwith high viscosity, a severe drawback when considering poten-tial applications of the metal-centered properties in fast-re-orienting, switchable LC displays (LCDs).[7] However, the pre-cise control of the molecular arrangement within the buildingblock offers an attractive possibility for modifying the finalmacroscopic organization, provided that the amplification pro-cess is rationally controlled. The complete exploration of thethree types of thermotropic LC mesophases with the samecomplexes [Ln(Lk)(NO3)3] (k = 12,13), while only varying thesize of the central metal ion, gave strong support for this as-sumption. From a functional point of view, the exploitation ofthe Ln-centered luminescence in mesophases is still in its in-


250

µµW

Ln

(τΤ

/τ 2

95)

2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4

n

1.48

1.50

1.52

1.54

1.56

1.58

-2.0

-1.5

-1.0

-0.5

0.0

Ln

(IT

/ I 2

95)

2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4

103 / T

0.0

-0.1

-0.2

-0.3

Figure 7. Cr-to-LC phase transition in [Eu(NO3)3L10]·0.25H2O, as charac-terized by various techniques. a) DSC heating (solid line) and cooling(dotted line) curves and refractive-index change (�). b) Luminescence in-tensity variation (�) compared to the one of a model compound withoutmesomorphic properties (�; contributions from the n(n2+2)2/9 term(Equation 2) is shown on top, �), and lifetime change of the Eu(5D0) ex-cited state (�).

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.1 0.2 0.3 0.4 0.5 0.6

y

x

Figure 8. Part of the trichromatic diagram displaying the position of[C12-mim]Cl and EuIII-containing samples (5 mol-%) as a function of thenature of the anion (×: RTIL; �: chloride; �: nitrate; �: trifluoromethane-sulfonate; �: perchlorate) and the excitation wavelength (grey: 274 nm;violet: 285 nm; blue: 334 nm; green: 344 nm; yellow: 353 nm; orange:361 nm; red: 393 nm). Reprinted with permission from [36b]; copyright2004, American Chemical Society.

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fancy,[38] and a definitive choice, motivated by reliable scientificcriteria (efficiency, quality of the colors, tuning, switching), be-tween pure lanthanide metallomesogens and Ln-doped liquidcrystals cannot be made at present. For instance, the use ofhigh-resolution emission of EuIII and TbIII centers in thermo-tropic liquid crystals and in ionic liquids offers a novel, fasci-nating technique for probing microscopic environments in softmatter. Finally, extension to heteropolymetallic f–f systems,bringing concomitant, but different emission colors, could offera practical solution for increasing brightness and view angles inLCDs.

Received: April 6, 2005Final version: August 15, 2005

Published online: December 8, 2005

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