Molecular control of macroscopic cubic, columnar, and lamellar organizations in luminescent lanthanide-containing thermotropic liquid crystals

Article abstract of DOI:10.1021/ja0446101

The connection of lipophilic gallic acid derivatives at the 5,5′- or 6,6′-positions of the rigid 2,6-bis(1-ethyl-benzimidazol-2-yl)pyridine core provides two pro-mesogenic tridentate ligands L10 and L12, whose molecular shapes, anisometries, and directional intermolecular π-stacking can be tuned. X-ray diffraction data in the crystalline state, combined with solution ¹H NMR measurements, show that complexation with trivalent lanthanides, Ln(III), produces the neutral hemi-disklike complexes [Ln(Li)(NO₃)₃] (i = 10, 12), which dimerize to give the rodlike bimetallic complexes [Ln₂(Li)₂(NO ₃)₆] at lower temperature. The relevant thermodynamic parameters for the latter process depend on the nature of the ligand, the size of the metal ion, and the strength of the intermolecular interactions involved in the condensed phase. These three-dimensional models obtained for the complexes in the crystals and in solution are eventually confronted with small-angle XRD profiles recorded in the intermediate thermotropic liquid crystalline phase, in which the rigidity of the packed polyaromatic cores is maintained, while the alkyl chains are molten. According to the specific geometries and nuclearities of the molecular complexes, three types of mesophases (lamellar, columnar, and cubic) can be induced, which provides a direct correlation between the microscopic arrangements and the macroscopic ordering in lanthanide-containing metallomesogens.

Full text of DOI:10.1021/ja0446101

Published on Web 12/30/2004
Molecular Control of Macroscopic Cubic, Columnar, and
Lamellar Organizations in Luminescent
Lanthanide-Containing Thermotropic Liquid Crystals
Emmanuel Terazzi,† Ste´phane Torelli,† Ge´rald Bernardinelli,‡ Jean-Pierre Rivera,†
Jean-Marc Be´nech,† Cyril Bourgogne,§ Bertrand Donnio,§ Daniel Guillon,§
Daniel Imbert, Jean-Claude G. Bu¨nzli, Andre´ Pinto,| Damien Jeannerat,| and
Claude Piguet*,†
Contribution from the Department of Inorganic, Analytical and Applied Chemistry, and
Department of Organic Chemistry, UniVersity of GeneVa, 30 quai E. Ansermet,
CH-1211 GeneVa 4, Switzerland, Laboratory of X-ray Crystallography, UniVersity of GeneVa,
24 quai E. Ansermet, CH-1211 GeneVa 4, Switzerland, Ecole Polytechnique Fe´de´rale de
Lausanne, Laboratory of Lanthanide Supramolecular Chemistry, BCH 1402,
CH-1015 Lausanne, Switzerland, and Institut de Physique et Chimie des Mate´riaux
de Strasbourg-IPCMS, Groupe des Mate´riaux Organiques, 23 rue du Loess, B.P. 43,
F-67034 Strasbourg Cedex 2, France
Received September 6, 2004; E-mail: Claude.Piguet@chiam.unige.ch
Abstract: The connection of lipophilic gallic acid derivatives at the 5,5′- or 6,6′-positions of the rigid 2,6-
bis(1-ethyl-benzimidazol-2-yl)pyridine core provides two pro-mesogenic tridentate ligands L10 and L12,
whose molecular shapes, anisometries, and directional intermolecular π-stacking can be tuned. X-ray
diffraction data in the crystalline state, combined with solution 1H NMR measurements, show that
complexation with trivalent lanthanides, Ln(III), produces the neutral hemi-disklike complexes [Ln(Li)(NO3)3]
(i ) 10, 12), which dimerize to give the rodlike bimetallic complexes [Ln2(Li)2(NO3)6] at lower temperature.
The relevant thermodynamic parameters for the latter process depend on the nature of the ligand, the size
of the metal ion, and the strength of the intermolecular interactions involved in the condensed phase. These
three-dimensional models obtained for the complexes in the crystals and in solution are eventually confronted
with small-angle XRD profiles recorded in the intermediate thermotropic liquid crystalline phase, in which
the rigidity of the packed polyaromatic cores is maintained, while the alkyl chains are molten. According to
the specific geometries and nuclearities of the molecular complexes, three types of mesophases (lamellar,
columnar, and cubic) can be induced, which provides a direct correlation between the microscopic
arrangements and the macroscopic ordering in lanthanide-containing metallomesogens.
enthalpy and small melting entropy)3 is thus required for the
generation of a mesophase occurring between the crystalline
Introduction
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 mol-
ecules.1 A thermally induced microsegregation between the
flexible alkyl chains (large melting entropy and small melting
enthalpy)2 and the rigid organometallic core (large melting
and isotropic liquid states.1 Lipophilic complexes possessing
rigid rodlike cores (i.e., in which the axial component is larger
than the radial component) thus produce mesophases with
nematic (orientational order without positional order) or smectic
(orientational order with one-dimensional positional order)
organization, while those with rigid disklike cores (i.e., in which
the radial component is larger than the axial component) lead
to nematic and columnar mesophases (two-dimensional arrange-
ments of the columns). Specific molecular shapes mainly result
from the global anisometry of the ligands, but the stereochemical
preferences of the metal ions also contribute to the three-di-
mensional architectures.1 Except for the special case of ferro-
cene, in which “10-coordinated” Fe(II) is sandwiched between
two planar aromatic ligands,1c metal ions favoring tetrahedral
or octahedral coordination have been ignored for a long time
in this field because of their considerable three-dimensional
† Department of Inorganic, Analytical and Applied Chemistry, Uni-
versity of Geneva.
‡ Laboratory of X-ray Crystallography, University of Geneva.
§ Institut de Physique et Chimie des Mate´riaux de Strasbourg-IPCMS.
Ecole Polytechnique Fe´de´rale de Lausanne.
| Department of Organic Chemistry, University of Geneva.
(1) (a) Serrano, J. L. Metallomesogens, Synthesis, Properties and Applications;
VCH: Weinheim, Germany, 1996. (b) Donnio, B.; Guillon, D.; Deschenaux,
R.; Bruce, D. W. In ComprehensiVe Coordination Chemistry; McCleverty,
J. A., Meyer, T. J., Eds.; Elsevier: Oxford, U.K., 2003; Vol. 7, Chapter
79, pp 357-627. (c) Deschenaux, R.; Goodby, J. W. In Ferrocene:
Homogeneous Catalysis, Organic Synthesis, Material Sciences; Togni, A.,
Hayashi, T., Eds.; VCH: Weinheim, Germany, 1995; Chapter 9, pp 471-
495.
(2) Williams, D. H.; O’Brien, D. P.; Bardsley, B. J. Am. Chem. Soc. 2001,
123, 737-738.
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9
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Macroscopic Cubic, Columnar, and Lamellar Organizations
A R T I C L E S
Chart 1
expansions, which do not match the above-mentioned criteria.
It is only during the past decade that six-coordinate Mn(I) and
Re(I) centers have been successfully coordinated to rodlike
bidentate carbene-imine4 or diimine5 ligands to produce nematic
or smectic mesophases. Interestingly, columnar mesomorphism
can be alternatively induced when a sufficient number (m )
12-18) of lipophilic alkyl residues are bound to the three-
dimensional pseudo-octahedral core, as in [M(â-diketonate)3]
complexes (M ) Cr, Mn, Fe).6 This suggests that the peculiar
decoration of the central core with lipophilic residues (i.e., the
geometry of the interface) may overcome structural limitations
evidenced by the shape of the rigid core.7 The formation of
thermotropic cubic phases with polycatenar metallomesogens
is a remarkable application of this concept, and their formation
can be tentatively rationalized, either by an undulation of layers
originally found in smectic phases8 or by the oscillation of
columns in columnar mesophases.9 This opens perspectives for
the introduction of spherical and bulky trivalent lanthanide ions,
Ln(III), into thermotropic mesophases because the latter ions
prefer high coordination numbers and display no pronounced
stereochemical preference.10 A logical approach thus aims at
forcing Ln(III) to enter a cavity embedded into ligands display-
ing rodlike or disklike anisometries.11 For instance, Galyamet-
dinov, Binnemans, and co-workers have intensively investigated
the cylindrical [Ln(L1)3X3] complexes in which Ln(III) is
coordinated by three monodentate zwitterionic Schiff bases L1
(Chart 1) and by the three counteranions (X ) nitrate, chloride,
alkyl sulfate).12 Smectogenic behavior results in agreement with
the global rodlike shape of these complexes.11,12 On the other
hand, when rigid macrocyclic porphyrin or phthalocyanine
platforms such as L2 react with Ln(III) to give the neutral tabular
sandwich complexes [Ln(L2-2H)2], columnar mesomorphism
is observed along the complete lanthanide series.13 However,
(4) (a) Bruce, D. W.; Liu, X. H. Liq. Cryst. 1995, 18, 165-173. (b) Liu, X.-
H.; Abser, M. N.; Bruce, D. W. J. Organomet. Chem. 1998, 551, 271-
280.
(5) Rowe, K. E.; Bruce, D. W. J. Chem. Soc., Dalton Trans. 1996, 3913-
organization in mesophases is not strictly bipolarized between
columnar and lamellar organization, and the same rigid core
may lead to both types of mesophases assuming a judicious
curvature of the interface between the incompatible parts of the
molecule (alkyl chains vs rigid core). For instance, the original
smectic mesophase (SmA) observed for [Ln(L1)3X3] (lamellar
organization) is transformed into a columnar hexagonal ar-
rangement (Colh) in [Ln(L3)2(L3-H)(CF3SO3)2] when one
additional lateral alkyl chain is connected.14
Inspired by these results, we have developed simple semirigid
aromatic tridentate ligands, in which rodlike or bent geometries
depend on the position at which the lipophilic residues are
connected. Moreover, these two basic shapes interconvert upon
complexation to Ln(NO3)3, which offers interesting perspectives
for the exploration of the correlations between molecular
geometries and macroscopic order (Figure 1).15-17
3915.
(6) (a) Zheng, H.; Swager, T. M. J. Am. Chem. Soc. 1994, 116, 761-762. (b)
Swager, T. M.; Zheng, H. Mol Cryst. Liq. Cryst. 1995, 260, 301-307.
(7) (a) Morrone, S.; Bruce, D. W.; Guillon, D. Inorg. Chem. 1996, 35, 7041-
7048. (b) Ziessel, R.; Douce, L.; El-ghayoury, A.; Harriman, A.; Skoulios,
A. Angew. Chem., Int. Ed. 2000, 39, 1489-1493. (c) Date, R. W.; Fernandez
Iglesias, E.; Rowe, K. E.; Elliott, J. M.; Bruce, D. W. J. Chem. Soc., Dalton
Trans. 2003, 1914-1931.
(8) (a) Bruce, D. W. Acc. Chem. Res. 2000, 33, 831-840. (b) Diele, S. Curr.
Opin. Colloid Interface Sci. 2002, 7, 333-342.
(9) (a) Donnio, B.; Heinrich, B.; Gulik-Krzywicki, T.; Delacroix, H.; Guillon,
D.; Bruce, D. W. Chem. Mater. 1997, 9, 2951-2965. (b) Donnio, B.; Bruce,
D. W.; Delacroix, H.; Gulik-Krzywicki, T. Liq. Cryst. 1997, 23, 147-
153. (c) Donnio, B.; Rowe, K. E.; Roll, C. P.; Bruce, D. W. Mol. Cryst.
Liq. Cryst. 1999, 332, 383-390.
(10) Kaltsoyannis, N.; Scott, P. The f-Elements; Oxford University Press: New
York, 1999.
(11) For a recent comprehensive review, see: Binnemans, K.; Go¨rller-Walrand,
C. Chem. ReV. 2002, 102, 2303-2346.
(12) (a) Galyametdinov, Y.; Athanassopoulou, M. A.; Griesar, K.; Kharitonova,
O.; Soto Bustamante, E. A.; Tinchurina, L.; Ovchinnikov, I.; Haase, W.
Chem. Mater. 1996, 8, 922-926. (b) Binnemans, K.; Galymetdinov, Y.
G.; Collinson, S. R.; Bruce, D. W. J. Mater. Chem. 1998, 8, 1551-1553.
(c) Binnemans, K.; Galyametdinov, Y. G.; Van Deun, R.; Bruce, D. W.;
Collinson, S. R.; Polishchuk, A. P.; Bikchantaev, I.; Haase, W.; Prosvirin,
A. V.; Tinchurina, L.; Litvinov, U.; Gubajdullin, A.; Rakhmatullin, A.;
Uytterhoeven, K.; Van Meervelt, L. J. Am. Chem. Soc. 2000, 122, 4335-
4344. (d) van Deun, R.; Binnemans, K. Liq. Cryst. 2001, 28, 621-627. (e)
Galyametdinov, Y. G.; Haase, W.; Malykhina, L.; Prosvirin, A.; Bikchan-
taev, I.; Rakhmatullin, A.; Binnemans, K. Chem.-Eur. J. 2001, 7, 99-
105. (f) Binnemans, K.; Moors, D.; Parac-Vogt, T. N.; van Deun, R.; Hinz-
Hu¨bner, D.; Meyer, G. Liq. Cryst. 2002, 29, 1209-1216.
Detailed investigations of the thermal behavior of the non-
coordinated trans-trans ligands L4-L9 demonstrate that (1)
(14) Martin, F.; Collinson, S. R.; Bruce, D. W. Liq. Cryst. 2000, 27, 859-863.
(15) Nozary, H.; Piguet, C.; Tissot, P.; Bernardinelli, G.; Bu¨nzli, J.-C.
G.; Deschenaux, R.; Guillon, D. J. Am. Chem. Soc. 1998, 120, 12274-
12288.
(13) (a) Piechocki, C.; Simon, J.; Andre´, J. J.; Guillon, D.; Petit, P.; Skoulios,
A.; Weber, P. Chem. Phys. Lett. 1985, 122, 124-128. (b) Miwa, H.;
Kobayashi, N.; Ban, K.; Ohta, K. Bull. Chem. Soc. Jpn. 1999, 72, 2719-
2728. (c) Binnemans, K.; Sleven, J.; De Feyter, S.; De Schryver, F. C.;
Donnio, B.; Guillon, D. Chem. Mater. 2003, 15, 3930-3938.
(16) Nozary, H.; Piguet, C.; Rivera, J.-P.; Tissot, P.; Bernardinelli, G.; Vulliermet,
N.; Weber, J.; Bu¨nzli, J.-C. G. Inorg. Chem. 2000, 39, 5286-5298.
(17) Nozary, H.; Piguet, C.; Rivera, J.-P.; Tissot, P.; Morgantini, P.-Y.; Weber,
J.; Bernardinelli, G.; Bu¨nzli, J.-C. G.; Deschenaux, R.; Donnio, B.; Guillon,
D. Chem. Mater. 2002, 14, 1075-1090.
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A R T I C L E S
Terazzi et al.
Figure 1. Conformational changes (trans-trans f cis-cis) occurring upon complexation to Ln(III) and associated molecular anisometries observed for (a)
5,5′-disubstituted ligands (L10) and (b) 6,6′-disubstituted ligands (L12).
two-atom spacers separating the tridentate binding unit from
the peripheral semiflexible alkoxyaryl groups are required for
inducing mesomorphism (Figure 2a),15 (2) the nature of the two-
atom spacers influences mesomorphism (Figure 2b),15 (3) the
number of intercalated 4-hydroxybenzoic groups, together with
the length of the alkyl chains, affects the transition temperatures
and the mesophase stabilities (Figure 2c),15,17 and (4) the rodlike
ligands provide smectic mesophases, while the bent geometry
favors columnar mesomorphism (Figure 2d).16,17
In this article, we report on (i) the characterization of the
mesogenic behavior of the bent 5,5′-disubstituted hexacatenar
complexes [Ln(L10C12)(NO3)3] along the complete lanthanide
series (Ln ) La-Lu), (ii) the comparison of this mesomorphism
with that of the isomeric rodlike 6,6′-disubstituted hexacatenar
complexes [Ln(L12C12)(NO3)3] (Scheme 1), and (iii) the ratio-
nalization of the molecular structural changes responsible for
the switch between lamellar, columnar, and cubic mesophases.
Since the exploitation of the lanthanide-centered luminescence
in pure20 or doped21 liquid crystals is attractive for improving
the brightness and angular view in liquid crystal display
(LCD),21c the photophysical properties of the europium-contain-
ing mesophases have been investigated, particularly with the
aim of probing the crystalline to liquid crystalline transitions.22
However, the flat interfaces between the rigid core and the
flexible chains in L5-L9 cannot overcome the considerable
perturbation of the intermolecular interactions produced by the
coordination of the bulky Ln(NO3)3 units. As a consequence,
no mesophase could be detected for the complexes [Ln(Li)-
(NO3)3] (i ) 5-9).15-17 The connection of four supplementary
divergent dodecyloxy chains in L10C12 (Scheme 1) significantly
increases the volume of the chains without affecting the volume
of the rigid core. This mechanism produces a significant
Results and Discussion
Syntheses of the Ligands L10Cn, L12Cn (n ) 0, 1, 2, 3, 4,
8, 12), and Their Nitrato Complexes with Zn(II) and
Ln(III). The ligands L10Cn and L12Cn are obtained in one step
(yields ) 50-70%) via the coupling of 2,6-bis(5-hydroxy-1-
ethylbenzimidazol-2-yl)pyridine (3)15 or 2,6-bis(6-hydroxy-1-
ethylbenzimidazol-2-yl)pyridine (4),16 respectively, with the
adequate gallic acid derivatives 2Cn, or with benzoic acid, in
the presence of 1-[3-(dimethylamino)propyl]-3ethylcarbodiimide
curvature of the interfaces in the phasmidic ligand L10C12
,
which is responsible for the formation of columnar mesomor-
phism (Cr 25°C8 Colh 61°C8 I).18 Preliminary studies showed
that the hexagonal columnar mesomorphism is maintained
in the associated lutetium complex [Lu(L10C12)(NO3)3]
(Cr 160°C8 CoIh 223°C8 dec), while the first lanthanide-contain-
ing cubic mesophase was evidenced in [Eu(L10C12)(NO3)3]
(18) Terazzi, E.; Be´nech, J.-M.; Rivera, J.-P.; Bernardinelli, G.; Donnio, B.;
Guillon, D.; Piguet, C. J. Chem. Soc., Dalton Trans. 2003, 769-772.
(19) Morale, F.; Date, R. W.; Guillon, D.; Bruce, D. W.; Finn, R. L.; Wilson,
C.; Blake, A. J.; Schro¨der, M.; Donnio, B. Chem.-Eur. J. 2003, 9, 2484-
2501.
(Cr 140°C8 Cub ∼200°C8 dec).18 Although this remarkable ampli-
fication of the minor contraction of the nine-coordinate ionic
radius on going from Eu(III) to Lu(III) escaped rationalization
in the preliminary communication,18 a recent report on the
closely related tridentate Schiff base L11 (Chart 1), which
induces columnar mesophases (Colr, Colh, and Colo) in [M(L11)-
Cl2] complexes (M ) Mn, Co, Ni, Zn), confirms the beneficial
effect provided by curved hexacatenar interfaces for the genera-
tion of mesomorphism with large metallo-organic cores.19
(20) (a) Binnemans, K.; Malykhina, L.; Mironov, V. S.; Haase, W.; Driesen,
K.; van Deun, R.; Fluyt, L.; Go¨rller-Walrand, C.; Galyametdinov, Y. G.
ChemPhysChem 2001, 680-683. (b) Binnemans, K.; Lodewyckx, K.;
Parac-Vogt, T. N.; van Deun, R.; Goderis, B.; Tinant, B.; Van Hecke, K.;
Van Meervelt, L. Eur. J. Inorg. Chem. 2003, 3028-3033.
(21) (a) Binnemans, K.; Moors, D. J. Mater. Chem. 2002, 12, 3374-3376. (b)
van Deun, R.; De Fre´, B.; Moors, D.; Binnemans, K. J. Mater. Chem. 2003,
13, 1520-1522.
(22) Suarez, S.; Mamula, O.; Imbert, D.; Piguet, C.; Bu¨nzli, J.-C. G. Chem.
Commun. 2003, 1226-1227.
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Macroscopic Cubic, Columnar, and Lamellar Organizations
A R T I C L E S
Figure 2. Mesogenic behavior of the tridentate ligands L4-L9. Illustration of the influence of (a) the length of the spacer, (b) the nature of the spacer, (c)
the number of intercalated 4-hydroxy benzoic acid units and the length of the alkyl chains, and (d) the interconversion between rodlike and bent geometries
(Cr: crystal, SmA: Smectic A, SmC: Smectic C, Colr: rectangular columnar, N: nematic, I: isotropic liquid).15-17
(EDCI) and 4-(dimethylamino)pyridine (DMAP, Scheme 1). The
1H NMR spectra confirm dynamically averaged C2V sym-
metries on the NMR time scale for the ligands, while the lack
of NOE effect between the ethyl residues and the protons
connected at the meta positions of the pyridine ring agrees
with the trans-trans arrangement of the 2,6-bis(benzimi-
dazole)pyridine core previously established for L4-L9.15-17
Reactions of L10Cn, L12Cn, with Zn(NO3)2‚6H2O and Ln(NO3)3‚
xH2O (Ln ) La-Lu, x ) 1-4) in acetonitrile/dichlorometh-
ane produce good yields (80-90%) of the complexes
[Zn(L10Cn)(NO3)2], [Ln(L10Cn)(NO3)3], and [Ln(L12Cn)(NO3)3]
(n ) 0, 1, 2, 3, 4, 8, 12; Ln ) La-Lu, except Pm) con-
taining variable amounts of cocrystallized solvent molecules
(Table S1).
The 35-40 cm-1 blue shift observed for the CdC aromatic
stretching frequency upon complexation is characteristic of
meridional tricoordination of the bis(benzimidazole)pyridine
units to the metal,15-17,23 while the negligible variation of the
stretching vibration of the carbonyl groups (maximum red
shift e 9 cm-1) confirms their lack of interaction with the
metal ions. Vibrational spectroscopy is also well-suited for
investigating the binding mode of the nitrate anions.24 The
large separations detected between the antisymmetric (νa) and
the symmetric (νs) stretching vibrations of the NO2 group (∆ν
(23) Petoud, S.; Bu¨nzli, J.-C. G.; Schenk, K. J.; Piguet, C. Inorg. Chem. 1997,
36, 1345-1353.
(24) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination
Compounds, 5th ed.; John Wiley & Sons: New York, 1997; Part B, pp
87-89.
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A R T I C L E S
Terazzi et al.
Scheme 1. Synthesis and Nomenclature of the Ligands L10Cn, L12Cn and their Complexes [Ln(LiCn)(NO3)3]
) νa - νs ) 178-225 cm-1, Table S2) point to bidentate
coordination in all lanthanide complexes.24 For the zinc
complexes [Zn(L10Cn)(NO3)2], the large number of intense
bands in the 1000-1600 cm-1 domain prevents a reliable
assignment of ν(NO2) stretching vibrations, and no conclusion
regarding the nitrate binding mode can be drawn. Attempts to
crystallize complexes with lipophilic alkoxy chains (n ) 2-12)
failed, but X-ray quality crystals could be obtained with the
less lipophilic ligands having n ) 0, 1. Diffusion of volatile
ethers (i.e., diethyl-, disopropyl-, or tert-butylmethyl ether) into
concentrated polar organic solutions of the ligand, or of the
complexes, gives fragile prisms for L10C1 (5), [Zn(L10C1)-
(NO3)2]‚3DMF (6), [La(L10C0)(NO3)3(C3H6O)] (7), [Eu(L10C0)-
(NO3)3(CH3CN)]‚2CH3CN (8), [Yb(L10C0)(NO3)3]‚CH3CN (9),
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Macroscopic Cubic, Columnar, and Lamellar Organizations
A R T I C L E S
Figure 3. Perspective view of L10C1 with ellipsoids represented at the
40% probability level and numbering scheme.
[Eu2(L12C0)2(NO3)6]‚3CH3CN‚0.5H2O (10), and [Lu(L12C0)-
(NO3)3]‚1.5C3H6O (11).
Solid-State Structures. The organization in liquid crystalline
phases is reminiscent of that observed in the isotropic liquid
(weak intermolecular interactions and no structural correlation)
and that evidenced in the crystal (strong intermolecular interac-
tions and long-range structural correlation). We can therefore
reasonably use X-ray diffraction data collected on single crystals
for modeling the molecular structure of the rigid cores in
metallomesogens because their geometry and macroscopic
organization are poorly affected by the melting process, mainly
due to the disorganization of the terminal aliphatic chains.
Ligand L10C1 (5) and Complex [Zn(L10C1)(NO3)2]‚3DMF
(6): trans-trans versus cis-cis Conformations. In agreement
with NOE measurements in solution, the aromatic 2,6-bis-
(benzimidazol-2-yl)pyridine unit of the noncoordinated ligand
L10C1 adopts the usual trans-trans conformation in the solid
state (i.e., both N2 and N4 atoms adopt a trans conformation
relative to N1, Figure 3). This conformation provides an
approximate rodlike arrangement of the rigid core, and the
considerable interplanar angles between the gallic ester and the
benzimidazole ring to which it is connected (63.9° and 68.0°,
Table S3) eventually induce a crosshatched organization of the
successive aromatic units in L10C1, as previously reported for
L8 (Figure 3).17
The ligand L10C1 is thus considered as a central “planar”
tridentate aromatic core (pyridine-benzimidazole interplanar
angles: 30.4° and 39.4°, Table S3), flanked with two appended
“perpendicular” gallic ester groups. Comparison with the crystal
structure of the higher homologue L10C4 shows only minor
structural variations between the tridentate units (Figure S1),18
which implies that the length of the flexible alkyl chains has
no significant effect on the rigid core. This crosshatched
arrangement provides two privileged pseudo-orthogonal direc-
tions, along which intermolecular π-staking between the rigid
cores may develop in the crystal. Three such interactions are
indeed detected in L10C1; two involve molecules related by
inversion centers (gallic ester/gallic ester, d ) 3.67 Å, and
benzimidazole/benzimidazole, d ) 3.56 Å), while the third one
occurs between one benzimidazole ring and one gallic ester
residue belonging to two different molecules related by a mirror
plane (d ) 3.34 Å, Figure S2).
Figure 4. Perspective view of [Zn(L10C1)(NO3)2] in the crystal of 6 with
ellipsoids represented at the 40% probability level and numbering scheme.
The nonbonded DMF molecules have been omitted for clarity.
S5), as previously reported for [Zn(L11)Cl2]19 and [Zn(L13)-
(NO3)2] (Figure S3).18
Again, the gallic esters in 6 show large interplanar angles
with the benzimidazole rings to which they are connected (56.1°
and 89.6°, Table S4), thus favoring two pseudo-orthogonal
directions for intermolecular π-stacking interactions, which are
observed in the crystal. (Figure S4).
We conclude that the free L10Cn ligands adopt the trans-
trans conformation leading to rodlike anisometry (Figure 3),
while their tricoordination to a metal ion produces the expected
conformational change providing a bent arrangement in the
complexes (Figure 4). Moreover, the crosshatched organization
of the aromatic sidearms is not affected by the complexation
process. The latter structural feature allows weak intermolecular
interactions to develop along two privileged pseudo-orthogonal
directions in the crystals.
Bent Complexes with L10C0 (7-9). Crystals 7-9 contain
neutral monometallic complexes [La(L10C0)(NO3)3(C3H6O)],
[Eu(L10C0)(NO3)3(CH3CN)], and [Yb(L10C0)(NO3)3], together
with interstitial solvent molecules. Complexes 7 and 8 can be
considered as being isostructural, while 9 possesses a different
structure. The cis-cis conformation of the roughly planar
tricoordinated aromatic ligand L10C0 (pyridine-benzimidazole
interplanar angles: 5.7-30.4°, average 13(9)°, Tables S6-S8)
is similar to that found in the zinc complex 6, while the geometry
and coordination of the three bound bidentate nitrate anions are
comparable in the three complexes 7-9 (Figure 5).
Upon complexation to Zn(NO3)2 in 6, the three nitrogen atoms
N1, N2, and N4 of the almost planar tridentate ligand adopt a
cis-cis conformation (pyridine-benzimidazole interplanar angles
) 10.1-12.0°, Table S4). In this case, the connection of the
gallic acid residues at the 5,5′-positions produces an approximate
bent geometry for the coordinated ligand. One oxygen atom of
each monodentate nitrate anion completes a distorted trigonal
bipyramidal (tbp) environment around Zn(II), in which the two
N(bzim) (N2, N4) occupy the apical positions (Figure 4, Table
The contraction of the lanthanide ionic radii, in going from
La to Yb, is responsible for the removal of the extra solvent
molecule bound in the first coordination sphere of the large
10-coordinate ions in 7 (La) and 8 (Eu), leading to a nine-
coordinate ion in 9 (Yb, Figure 5), but the common LnN3O6
cores are almost superimposable (Figure S5). This behavior
parallels a similar decrease in the coordination number when
9
J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005 893

A R T I C L E S
Terazzi et al.
CN)10
calculation of standard experimental ionic radii R
)
La
1.264 Å (expected 1.27 Å),26 RECuN)10 ) 1.142 Å (expected 1.17
CN)9
Å),25 and R
) 1.046 Å (expected 1.042 Å),26 by using
Yb
Shannon’s definition with r(N) ) 1.46 Å26 and r(O) ) 1.31
Å.27 As previously noticed for [Zn(L10C1)(NO3)2] (6), the gallic
esters in 7-9 display significant torsions with respect to the
benzimidazole rings to which they are connected (18.5-83.3°,
average 55(23)°, Tables S6-S8). This mechanism provides the
usual crosshatched arrangement of the semirigid aromatic units
along the bent strand coordinated to Ln(NO3)3 (Figure 5). As a
consequence, gallic ester/gallic ester and benzimidazole/benz-
imidazole π-stacking interactions systematically occur in the
crystals of 7-9 (Figures S6-S8).
Rodlike Complexes with L12C0 (10-11). As previously
established for the complexes with L10C0, the decrease of the
coordination number from CN ) 10 in [Eu2(L12C0)2(NO3)6]
(10) to CN ) 9 in [Lu(L12C0)(NO3)3] (11) is ascribed to the
contraction of the lanthanide radii (Figure 6). The tricoordinated
6,6′-disubstituted rodlike ligand L12C0 in 10 and 11 adopts the
expected cis-cis conformation, which is compatible with a close
approach of two monometallic building blocks in the crystal
structures. In 10, the trend of Eu(III) to reach 10-coordinate
environments allows the formation of two intermetallic oxygen
bridges involving two oxygen atoms of the equatorial nitrate
anions (O1c and O1d, Figure 6a). The resulting Eu-Obridging
bond distances (2.51-2.63 Å, average 2.57(6) Å, Table S10)
are marginally longer than the Eu-Ononbridging bonds
(2.44-2.60 Å, average 2.50(6) Å, Table S10), and 10 can be
best described as a neutral pseudo-centrosymmetric bimetallic
complex [Eu2(L12C0)2(NO3)6] with a relatively short interme-
tallic distance Eu‚‚‚Eu ) 4.5186(10) Å.
The Eu-N bonds in the bimetallic complex [Eu2(L12C0)2-
(NO3)6] (average Eu-Npy ) 2.63(1) Å, average Eu-Nbenzimid
) 2.54(2) Å, Table S10) are comparable with those found in
the bent monometallic analogue [Eu(L10C0)(NO3)3(CH3CN)]
(8: Eu-Npy ) 2.617(2) Å, average Eu-Nbenzimid ) 2.51(2) Å,
Table S9), which points to negligible interstrand repulsions in
the dimer, as further supported by the calculated 10-coordinate
CN)10
Eu
ionic radii in 10 (R
) 1.18 Å, expected 1.17 Å).25,26 As
usual, the tridentate bis(benzimidazole)pyridine units are roughly
planar (interplanar angles 15.3-27.5°), while the terminal gallic
esters exhibit significant torsions (16.7-87.8°, average 61(31)°,
Table S11). Interestingly, the dimeric complex displays no
significant intermolecular interaction because the gallic esters
and the benzimidazole rings are less accessible in these rodlike
molecules (Figure S9).
In the related monometallic Lu complex 11, the smaller Lu-
(III) ion is nine-coordinated, and no Lu-Obridging bond is
detected (Figure 6b). However, two slightly different complexes
are observed in the asymmetric unit that adopt a quasi-
centrosymmetric arrangement reminiscent of that found in
[Eu2(L12C0)2(NO3)6]. The nonbonded Lu‚‚‚Lu distance in 11
(6.6446(4) Å) is obviously longer than the related intramolecular
intermetallic separation in the Eu dimer, but it is short enough
to confirm that the coordinated rodlike ligands are compatible
with a close approach of the monometallic building blocks. The
lack of propensity of Lu(III) to expand its coordination number
Figure 5. Perspective views of (a) [La(L10C0)(NO3)3(C3H6O)] in the crystal
of 7, (b) [Eu(L10C0)(NO3)3(CH3CN)] in the crystal of 8, and (c) [Yb(L10C0)-
(NO3)3] in the crystal of 9 with ellipsoids represented at the 40% probability
level and numbering schemes. The nonbonded solvent molecules have been
omitted for clarity.
going from [Eu(L13)(NO3)3(CH3OH)] (10-coordinated)25 to
[Lu(L15)(NO3)3] (nine-coordinated).15 The lack of strong steric
constraints in complexes 7-9 is testified by standard Ln-O
and Ln-N bond distances (Table S9), which allows the
(26) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767.
(27) Bu¨nzli, J.-C. G.; Klein, B.; Chapuis, G.; Schenk, K. J. Inorg. Chem. 1982,
21, 808-812.
(25) Piguet, C.; Williams, A. F.; Bernardinelli, G.; Moret, E.; Bu¨nzli, J.-C. G.
HelV. Chim. Acta 1992, 75, 1697-1717.
9
894 J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005

Macroscopic Cubic, Columnar, and Lamellar Organizations
A R T I C L E S
esters displaying significant torsions (54.4-78.4°, Table S12).
Since the central aromatic core is less embedded in [Lu(L12C0)-
(NO3)3] (Figure 6b) than in [Eu2(L12C0)2(NO3)6] (Figure 6a),
the pyridine-benzimidazole units of neighboring complexes
related by inversion centers are involved in head-to-tail π-stack-
ing intermolecular interactions (dpy-benzimid ) 3.11-3.72 Å,
Figure S10).
From this study, we conclude that (1) the tricoordinated
cis-cis ligand strands L10C0 and L12C0 control the global
anisometry of the complexes by adopting respectively bent and
rodlike crosshatched arrangements, (2) the smaller lanthanides
(Yb, Lu) display nine-coordination with three bidentate nitrate
anions completing the coordination sphere, while the larger
lanthanides (La, Eu) expand their coordination number to 10
via either fixation of an extra solvent molecule (bent complexes)
or dimerization of two monometallic building blocks sharing
two nitrate anions (rodlike complexes), and (3) efficient inter-
molecular π-stacking interactions in monometallic complexes
occur along two pseudo-orthogonal directions defined by the
orientation of the coordinated tridentate binding unit and one
of the gallic esters. These points are crucial for further inter-
pretation of the intermolecular interactions occurring between
rigid cores in the liquid crystalline state (vide infra).
Solution Structure of the Nitrato Complexes [Ln(L12C0)-
(NO3)3], [Ln(L12C12)(NO3)3], and [Ln(L10C12)(NO3)3] (Ln )
La, Eu, Lu). Since intermolecular interactions are weaker in
liquid crystals than in crystals, some flexibility, reminiscent of
molecules in solution, must be combined with solid-state struc-
tural models for providing a reliable picture of the molecular
1
and macroscopic organizations evidenced in mesophases. H
NMR spectroscopy is particularly efficient for addressing
1
solution structures of lanthanide complexes.28 The H NMR
spectrum of the nonlipophilic diamagnetic complex [Lu(L12C0)-
(NO3)3] in CD3CN/(CD3)2CO (4:1) shows the 10 signals
expected for an average C2V symmetry on the NMR time scale
(Figure S11a), obtained by a minor relaxation of the monome-
tallic molecular structure evidenced for 11 in the solid state
(Figure 6b). The significant downfield complexation shift ob-
served for H1 upon coordination to Lu(III) (∆δ ) 0.73 ppm,
Table 1) is diagnostic for the coordination of the central pyri-
dine ring,15-17,29 while the detection of NOE effects between
H9‚‚‚H2 and H10‚‚‚H2 confirms the cis-cis conformation of
the tridentate binding unit resulting from the concomitant
coordination of the benzimidazole sidearms. In conclusion, the
meridional tricoordination of L12C0 to Lu(III) established in the
solid state (Figure 6b) is maintained in solution.
Although we expect no signal in mass spectrometry for
neutral complexes, some residual ESI-MS peaks are detected
when a solution of the Lu complex 11 in acetonitrile/acetone is
sprayed. The most intense peak occurring at m/z ) 964.0 can
be assigned to [Lu(L12C0)(NO3)2(C3H6O)]+ in which one nitrate
has been replaced by an acetone molecule. The minor peaks
observed at m/z ) 1513.1 ([Lu(L12C0)2(NO3)2]+) and m/z )
Figure 6. Perspective views of (a) [Eu2(L12C0)2(NO3)6] in the crystal of
10 and (b) [Lu(L12C0)(NO3)3] in the asymmetric unit of the crystal of 11
with numbering scheme. Ellipsoids are represented at the 40% probability,
and the nonbonded solvent molecules have been omitted for clarity.
(28) (a) Peters, J. A.; Huskens, J.; Raber, D. J. Prog. Nucl. Magn. Reson.
Spectrosc. 1996, 28, 283-350. (b) Forsberg, J. H. In Handbook on the
Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., Eyring,
L., Eds.; Elsevier: Amsterdam, 1996; Vol. 23, Chapter 153, pp 1-68. (c)
Piguet, C.; Geraldes, C. F. G. C. In Handbook on the Physics and Chemistry
of Rare Earths; Gschneidner, K. A., Jr., Bu¨nzli, J.-C. G., Pecharsky, V.
K., Eds.; Elsevier Science: Amsterdam, 2003; Vol. 33, pp 353-463.
(29) Lavallee, D. K.; Baughman, M. D.; Phillips, M. P. J. Am. Chem. Soc. 1977,
99, 718-724.
explains its reluctance to form the bridged dimer. The calculated
CN)9
ionic radii R
) 1.032 Å exactly match that expected for
Lu
nine-coordinate Lu(III),26 in agreement with standard Lu-O and
Lu-N bond lengths (Table S10). Again, the ligand adopts the
usual crosshatched configuration with a roughly planar tridentate
binding unit (interplanar angles 19.4-20.3°), and terminal gallic
9
J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005 895

A R T I C L E S
Terazzi et al.
Table 1. 1H NMR Chemical Shifts for L12C0 and [Ln(L12C0)(NO3)3] in CD3CN/(CD3)2CO (4:1)‚(Numbering Scheme in Figure S11)
H1
H2
H3
H4
H5
H6
H7
H8
H9
H10
L12C0
8.05
8.60
7.11
8.78
8.33
8.37
6.41
8.76
7.35
7.60
8.07
7.93
7.17
6.92
9.43
7.53
7.87
7.34
16.56
8.29
8.22
8.40-8.80
8.76
7.49
7.61
4.76
4.70
5.82
5.05
1.34
1.72
2.21
1.80
[La(L12C0)(NO3)3]
[Eu(L12C0)(NO3)3]
[Lu(L12C0)(NO3)3]
7.50-7.80
7.87-7.97
7.65
7.5-7.8
7.87-7.97
7.78
8.24
1874.0 ([Lu2(L12C0)2(NO3)5]+) suggest that a dimerization
process, leading to [Lu2(L12C0)2(NO3)6], occurs in solution.
However, the exclusive detection of the monometallic C2V
symmetrical complex by 1H NMR (Figure S11a) indicates that
such an equilibrium can be neglected for Lu, within the limit
of sensitivity of this technique (g2%). This approximation does
not hold for the corresponding La and Eu complexes which
ligand L12C12 (MML12 ) 1713 g/mol). Application of eq 2
C12
C12
C12
C12
with νjL12 ≈ νjLu-L12 leads to MMLuL12 ≈ 2800(300) g/mol,
which compares reasonably well with 2073 g/mol calculated
for the monometallic nitrato complex. We cannot expect a much
better agreement because the hypothesis that both the flexible
ligand and the semirigid complex behave as spheres in solution
is too approximate. Recently, a detailed investigation of the
formation of almost identical diastereoselective ion pairs in
organic solution showed that their diffusion coefficients differ
by 6%.32 A parallel spectrophotometric titration of L10C12 with
Lu(NO3)3‚3H2O in CH2Cl2/CH3CN (1:1) shows a single end
point for the ratio L10C12/Ln ) 1.0 (Figure S13). The
1
generate two series of H NMR signals (Figures S11b,c). In
the latter cases, the residual ESI-MS signals show strong peaks
arising from the bimetallic dimers [Ln2(L12C0)2(NO3)5]+ (Eu:
m/z ) 1828.1; La: m/z ) 1802.2), while only faint peaks can
be assigned to the monometallic complexes [Ln(L12C0)(NO3)2-
(C3H6O)]+ (Eu: m/z ) 942.6; La: m/z ) 928.1). A combination
of these observations with the dual isolation of the monometallic
complex 11 for Lu (Figure 6b) and the bimetallic bridged dimer
10 for Eu (Figure 6a) suggests that the dimerization process
experimental data can be satisfyingly fitted with nonlinear least-
squares techniques33 to equilibrium 3 (log(â ) ) 6.1(2)),
which confirms that (i) no significant (<1%) decomplexation
occurs for the complexes at millimolar concentration, and (ii)
dimerization is negligible for Ln ) Lu.
Lu
11
(eq 1) becomes sizable for large Ln(III) ions, that is, the
Ln-L12C0
associated thermodynamic constants Kd
decrease in going
L10C12 + Ln(NO3)3 h [Ln(L10C12)(NO3)3] âLn (3)
from La to Lu.
11
Ln-LiCn
2[Ln(LiCn)(NO3)3] h [Ln2(LiCn)2(NO3)6] Kd
(1)
These data, combined with the low molecular mass found above
for the Lu complex, indicate that [Lu(L12C12)(NO3)3] exists in
solution as a neutral monometallic complex with an idealized
This mechanism is further supported by NMR diffusion
measurements30 performed on the analogous lipophilic com-
C2V symmetry, reminiscent of that found in the solid state (Figure
1
plexes [Ln(L12C12)(NO3)3] in CD2Cl2. Their H NMR spectra
6b). Further DOSY experiments performed on [Eu(L12C12)-
(NO3)3] in CD2Cl2 demonstrate that the first series of peaks
(series A in Figure S14) corresponds to a single species with
DA ) 5.7(4) × 10-10 m2‚s-1, while the second series (B) is
assigned to a different species that diffuses slightly slower: DB
) 4.6(2) × 10-10 m2‚s-1. The ratio DA/DB ) 1.24(2) translates
into MMB/MMA ) 1.9(1) (eq 2), which confirms that B is a
dimer. Finally, the diffusion coefficient obtained for complex
A (DA ) 5.7(4) × 10-10 m2‚s-1) matches that previously found
closely resemble those recorded for [Ln(L12C0)(NO3)3], which
implies the existence of a single C2V symmetrical complex for
Lu, while mixtures of two complexes in variable ratios are
detected for La and Eu (Figure S12). Assuming that (i) the
Einstein-Smoluchowski-Stokes auto-diffusion theory holds for
our lipophilic complexes and (ii) the molecules in solution can
be roughly modeled by spheres possessing an hydrodynamic
radius rh, the auto-diffusion coefficient Dm at a fixed temperature
in CD2Cl2 is proportional to (νjm‚MMm)-1/3, where MMm stands
for the molecular mass of the molecule and νjm stands for its
specific partial volume.30 The determination of Dm by diffusional
NMR for shape-similar complexes and/or ligands is thus an
efficient tool for deducing the molecular mass of an unknown
species (x) in solution, when a reference compound (r) is
measured under the same conditions (eq 2).31
for the monometallic complex [Lu(L12C12)(NO3)3] (DLu-L12
C12
) 5.5(1) × 10-10 m2‚s-1). We conclude that the Eu com-
plex exists in solution as a mixture of the monometallic unit
[Eu(L12C12)(NO3)3] (species A) and the bimetallic dimer
[Eu2(L12C12)2(NO3)6] (species B).
Variable-temperature NMR data in CD2Cl2 (273-313K) show
that the [Eu(L12C12)(NO3)3]/[Eu2(L12C12)2(NO3)6] ratio smoothly
3
increases from 1.12 (273 K) to 2.88 (313 K, Figure S15a).
Dx
Dr
νjr‚MMr
νj ‚MM
Eu-L12C12
Calculation of Kd
(equilibrium 1) for each temperature,
)
(2)
x
x
x
followed by application of van’t Hoff equation (eq 4) allows
Eu-L12C12
the estimation of ∆Hd
∆Sd
) -25(2) kJ‚mol-1 and
For the rodlike complex [Lu(L12C12)(NO3)3], which exists as a
Eu-L12C12
) -47(5) J‚mol-1‚K-1 (Figure S15b).
single species in CD2Cl2, NMR-DOSY experiments give
DLu-L12 ) 5.5(1) × 10-10 m2‚s-1, which can be compared
C12
Eu-LiC12
∆Hd
with DL12 ) 6.5(1) × 10-10 m2‚s-1 measured for the free
C12
Eu-LiC12
Eu-LiC12
-R ln(Kd
) )
- ∆Sd
(4)
T
(30) (a) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1986, 19, 1-45. (b)
Waldeck, A. R.; Kuchel, P. W.; Lennon, A. J.; Chapman, B. E. Prog. Nucl.
Magn. Reson. Spectrosc. 1997, 30, 39-68. (c) Pregosin, P. S.; Martinez-
Viviente, E.; Anil Kumar, P. G. J. Chem. Soc., Dalton Trans. 2003, 4007-
4014.
(31) Greenwald, M.; Wessely, D.; Goldberg, I.; Cohen, Y. New J. Chem. 1999,
337-344.
(32) Martinez-Viviente, E.; Pregosin, P. S.; Vial, L.; Herse, C.; Lacour, J.
Chem.-Eur. J. 2004, 10, 2912-2918.
(33) (a) Gampp, H.; Maeder, M.; Meyer, C. J.; Zuberbu¨hler, A. Talanta 1985,
32, 1133-1139. (b) Gampp, H.; Maeder, M.; Meyer, C. J.; Zuberbu¨hler,
A. Talanta 1986, 33, 943-951.
9
896 J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005

Macroscopic Cubic, Columnar, and Lamellar Organizations
A R T I C L E S
Table 2. Phase-Transition Temperatures and Enthalpy and
These parameters are typical for a dimerization process occur-
ring in a noncoordinating solvent for Ln(III), such as CD2Cl2,
because the formation of the dimer is enthalpically driven
(formation of two additional Eu-O bonds without breaking
Eu-solvent bonds), but entropically limited (decrease of the
translational entropy when two monomers dimerize). The
formation of the dimer in CD2Cl2 is thus favored (i) at low-
temperature and (ii) for 10-coordinate lanthanides as found in
[Eu2(L12C12)2(NO3)6] and [La2(L12C12)2(NO3)6] (Figure S12).
However, this trend is reversed in coordinating solvent because
(i) the competition for the 10-coordinated site strongly reduces
the enthalpic contribution and (ii) the release of two solvent
molecules upon dimerization inverts the entropic trend. The
latter points explain the difficulty in crystallizing the dimer out
of acetonitrile solutions at room temperature, even for large
Ln(III) ions.
Entropy Changes for the Ligands L10Cn (n ) 1, 2, 3, 4, 8, 12) and
L12C12
1
1
1
compound
transitiona
T/
°
C
∆H/kJ
‚
mol-
∆S/J‚ ‚
mol- K-
L10C1
Cr1 f Cr2
Cr2 f I
92
130
65
b
b
b
b
b
b
b
b
L10C2
Cr1 f Cr2
Cr2 f Cr3
Cr3 f Cr4
Cr4 f Cr5
Cr5 f I
Cr1 f Cr2
Cr2 f I
Cr1 f Cr2
Cr2 f I
95
110
144
175
50
83
87
118
16
0
25
61
-5.6c
7.8
12.1
b
-15c
19
27
b
L10C3
L10C4
b
b
-21.8c
21.8
b
-61c
56
b
L10C8
Cr f I
L10C12
Cr1 f Cr2
Cr2f Colh
Colh f I
Cr f I
b
b
3.3
31.5
b
b
10
95
L12C12
58
The related isomeric bent complexes [Ln(L10C12)(NO3)3] (Ln
1
a Cr ) crystal, Colh ) hexagonal columnar phase, I ) isotropic fluid.
First-order transition temperatures are given as the onset of the peak
observed during the second heating processes (Seiko DSC 220C differential
scanning calorimeter, 5 °C‚min-1, under N2); the liquid crystalline phases
were identified from their optical textures and from SA-XRD studies. b Glass
transition determined by polarizing light microscopy (PLM). c Cold crystal-
lization.
) La, Eu, Lu) display H NMR spectra strictly analogous to
those discussed above for [Ln(L12C12)(NO3)3], with the exclu-
sive formation of the monometallic complex [Lu(L10C12)(NO3)3]
in CD2Cl2, while a mixture of monometallic and bimetallic
dimeric complexes are observed with the larger lanthanides (Ln
) La, Eu). NMR DOSY experiments performed on the
[Eu(L10C12)(NO3)3]/[Eu2(L10C12)2(NO3)6] mixture indicate the
existence of only two complexes in solution with DA ) 4.5(1)
× 10-10 m2‚s-1 and DB ) 3.5(4) × 10-10 m2‚s-1 (Figure S16a).
The ratio DA/DB ) 1.29(2) translates into MMB/MMA ) 2.1(1)
(eq 2), which confirms the dimer/monomer relationship between
collapse of the peaks characteristic of the crystalline state, to
give three small angle reflections in the mesophase indexed as
(hk) ) (10), (11), (20) (squared spacing ratios h2 + k2 + hk )
1, 3, 4), confirming the formation of a hexagonal columnar
arrangement (Colh, planar groups p6 or p6mm). The unit cell
parameter varies from a ) 41.1 Å (30 °C) to a ) 42.6 Å (60
°C, average 41.9 Å), thus leading to a cross-sectional area S )
1517 Å2 (Table S13). Comparison with the estimated total length
of the ligand in its fully extended conformation (d ) 57 Å)
implies significant interdigitation and/or folding of the molten
chains. The broad diffraction peak centered around 4.5 Å can
be assigned to the molten alkyl chains,9 but no specific peak
arising from a regular packing of the molecules along the
columnar axis is observed. To reconcile (i) the well-established
crosshatched conformation of the rigid core, (ii) the pseudo-
orthogonality of the intermolecular π-stacking found in the
crystal structures of L10Cn, and (iii) the formation of a columnar
mesophase, we propose that the planes of the gallic ester residues
are oriented orthogonal to the column axis, while the planes of
the tridentate binding units are parallel (Figure S18). According
to the crystal structures of L10Cn, the minimum thickness of a
single slice is approximately h ≈ 6 Å, and the associated vol-
ume of the unit cell can be estimated: V ) S‚h ) 9102 Å3.
Taking into account a realistic density of d ≈ 1 g‚cm-3 in
the mesophase,18 we calculate that the number of molecules
per unit cell amounts to Z ) 3.2 ≈ 3 (eq 5, NAv is
Avogadro’s number, MMm is the molecular weight of the
molecule in g‚mol-1, and V is the volume of the unit cell
in Å3).18
these complexes. Application of the van’t Hoff eq 4 to the
Eu-L10C12
VT-NMR data recorded in CD2Cl2 gives ∆Hd
)
Eu-L10C12
-54(4) kJ‚mol-1 and ∆Sd
) -162(16) J‚mol-1‚K-1
(equilibrium 1), which points to a sizable enthalpic driving force
for the formation of the dimer (Figure S16b). From this set of
NMR data, we conclude that the bent (i ) 10) or rodlike (i )
12) monometallic molecular structures found for the complexes
[Ln(LiC0)(NO3)3] in the solid state (Ln ) La, Eu, Yb, Lu) are
maintained in solution along the complete lanthanide series.
However, dimeric bimetallic complexes similar to the one
evidenced in the crystal structure of [Eu2(L12C0)2(NO3)6]
systematically coexist in solution with the monometallic building
block (eq 1). For large Ln(III), the dimer becomes the major
product in noncoordinating solvents at low temperature.
Organization of the ligands L10C12 and L12C12 and of
their Nitrato Lanthanide Complexes in the Liquid Crystal-
line State. The connection of six lipophilic alkoxy chains to
the tridentate rigid cores in L10C12 and L12C12 favors a thermally
driven microsegregation process resulting from the huge entropic
release accompanying the melting of the flexible chains.2 The
detailed investigation of L10Cn (n ) 1, 2, 3, 4, 8, 12) shows
that n ) 12 corresponds to the minimum length for which a
well-defined two-step melting process is observed Cr 25°C8
Colh 61°C8 I (Table 2).
Polarized light microscopy (PLM) of the L10C12 mesophase
displays a fan texture with cylindrical domains and with a
homeotropic zone, which altogether are typical of a hexagonal
columnar mesophase (Colh, Figure S17). The transition from
the crystal into the columnar mesophase (Cr f Colh) cannot
be reliably detected by differential scanning calorimetry (DSC)
(i.e., a weak drift of the baseline is detected), but small-angle
X-ray diffraction profiles (SA-XRD) unambiguously show the
Z ) (d‚NAv‚V‚10-24)/MMm
(5)
Three ligands are on average thus packed within each unit cell
with a head-to-tail arrangement of the π-stacked tridentate
binding units (Figure S18). This produces a rough disklike
arrangement of the six gallic esters groups compatible with
columnar organization. The irregular and partial interdigitation
9
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A R T I C L E S
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Table 3. Phase-Transition Temperatures for the Complexes [Ln(L10C12)(NO3)3] and [Ln(L12C12)(NO3)3]
compound
transitiona
T/
°
C
compound
transitiona
T/°C
[Pr(L10C12)(NO3)3]
g f M
125b
175b
249
[Pr(L12C12)(NO3)3]
g f M
100b
140b
190
M f Cub
Cub f Dec
g f M
M f Cub
Cub f Dec
g f Cub
M f Cub
Cub f Dec
g f M
M f Cub
Cub f Dec
g f M
M f Cub
Cub f Dec
g f M
M f Cub
Cub f Dec
g f M
M f Cub
Cub f Dec
g f Cub
Cub f Dec
g f M
M f Dec
g f Cub
Cub f Dec
g f Colh
Colh f Cub
Cub f Dec
g f M
M f Dec
g f Colh
Colh f Dec
g f Colh
Colh f I
[Nd(L10C12)(NO3)3]
[Sm(L10C12)(NO3)3]
[Eu(L10C12)(NO3)3]
[Gd(L10C12)(NO3)3]
70b
[Nd(L12C12)(NO3)3]
[Sm(L12C12)(NO3)3]
[Eu(L12C12)(NO3)3]
[Gd(L12C12)(NO3)3]
120b
160b
180
140b
211
130b
180
160b
180b
190
Cub f Dec
g f Cub
Cub f Dec
140b
>200
120b
180b
190
g f Cub
Cub f Dec
145b
>200
100b
160b
190
[Tb(L10C12)(NO3)3]
[Dy(L10C12)(NO3)3]
[Ho(L10C12)(NO3)3]
[Er(L10C12)(NO3)3]
g f Cub
Cub f Dec
g f Cub
Cub f Dec
g f Cub
Cub f Dec
g f Colh
155b
205
[Tb(L12C12)(NO3)3]
[Dy(L12C12)(NO3)3]
[Ho(L12C12)(NO3)3]
[Er(L12C12)(NO3)3]
160b
171
155b
212
90b
180
155b
217
160b
175
160b
221
150b
180b
185
Colh f Dec
[Tm(L10C12)(NO3)3]
[Yb(L10C12)(NO3)3]
[Lu(L10C12)(NO3)3]
g f Colh
Colh f Dec
g f Colh
Colh f Dec
g f Colh
Colh f Dec
160b
221
[Tm(L12C12)(NO3)3]
[Yb(L12C12)(NO3)3]
[Lu(L12C12)(NO3)3]
160b
180
160b
223
175b
195
160b
223
155b
190
I f Dec
195
a Cr ) crystal, Colh ) hexagonal columnar phase, Cub ) cubic phase, g ) glass, M ) lamello-columnar phase, I ) isotropic fluid, Dec ) decomposition.
Transition temperatures are obtained by PLM, DSC and SA-XRD measurements and given for the second heating processes; the liquid crystalline phases
were identified from their optical textures and from SA-XRD studies. b Glass transition determined by PLM.
of the tridentate binding units, belonging to different unit cells,
along the columnar axis may explain the lack of well-defined
structural correlations between the layers. Interestingly, the
isomeric bent ligand L12C12 shows no mesomorphism and
isotropizes at 58 °C (Table 2). This behavior is reminiscent of
the well-established phenomenological reluctance of bent
derivatives possessing a 2,6-bis(benzimidazole)pyridine cores
for producing thermotropic liquid crystalline phases (only L9
has been shown to produce a Colr mesophase over 8 °C).17 In
this context, it is remarkable that the complexes [Ln(L10C12)-
(NO3)3] (Ln ) Pr-Lu),34 in which the tricoordinated cis-cis
ligands are indeed bent, display liquid crystalline behavior at
high temperature (Table 3). A thorough combination of ther-
mogravimetric (TG), DSC, and IR analyses shows that (i) the
interstitial water molecules (Table S1) are lost around 60-70
°C before the melting processes, (ii) these complexes are stable
over several heating-cooling cycles provided that the isotro-
pization temperature is not reached, (iii) the melting processes
systematically correspond to glass transitions (Table 3), and (iv)
the isotropization process is accompanied by a fast exothermic
decomposition resulting from the oxidation of the organic matrix
by nitrate anions at high temperature in the isotropic liquid.15-17
The phase transitions accompanying the melting processes can
be unambiguously detected by PLM and further confirmed by
SA-XRD measurements (Table S13). For the smallest lan-
thanides (Ln ) Er-Lu), the birefringent textures (Figure S19a)
observed for the monometallic complexes [Ln(L10C12)(NO3)3]
in the mesophases (160-223 °C, Table 3), combined with the
detection of up to seven fine reflections in the SA-XRD patterns
(Figure 7a, squared spacing ratios h2 + k2 + hk ) 1, 3, 4, 7, 9,
12, 13 with (hk) ) (10), (11), (20), (21), (30), (22), (31)) indicate
the formation of a well-correlated hexagonal columnar arrange-
ment (Colh, plane group p6 or p6mm).9,18
The mathematical analysis leads to a rather short intercolumn
separation in the range a ) 31.2-31.9 Å, with associated cross-
sectional areas of S ) 842-876 Å2 (Table S13). The observation
of a single diffuse band at 4.5 Å (Figure 7a), characteristic of
the molten alkyl chains, suggests a poor structural correlation
between the slices along the columns, as previously discussed
for the free ligand. Taking a typical stacking period of
approximately h ≈ 4.5 Å between two packed tridentate
aromatic binding units within the column (i.e., the associated
broad peak could be masked by the one of the molten chains)18
and a density of d ) 1 g‚cm-3 in the Colh mesophases, we
calculate Z ) 1.1 ≈ 1 molecule per unit cell (eq 5). Therefore,
a single elliptic hemi-disklike complex [Ln(L10C12)(NO3)3] (Ln
) Er-Lu) occupies the cross section of one column, or two
complexes occupy a columnar section 9 Å high (Figure 8a).
This assumption has been checked by a molecular dynamics
simulation of a hexagonal cell containing two molecules dis-
posed in head-to-tail orientation and that are 9 Å in height, the
other dimensions being set from the experimental cross section
of the column. The calculation shows that the space is well filled
with the aliphatic chains, with a density of 0.92, and that the
molecules adopt a tilted position from the columnar axis. From
the crystal structure of [Yb(L10C0)(NO3)3] (9), one can reason-
ably estimate that the length of the coordinated bent ligand
(34) The complexes [Ln(LiC12)(NO3)3] (i ) 10, 12; Ln ) La, Ce) display thermal
behavior close to that described for Ln ) Pr and Nd, but their reproducibility
is doubtful and these complexes are not further considered.
9
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Macroscopic Cubic, Columnar, and Lamellar Organizations
A R T I C L E S
of the molten chains and a tilt of the slices within the column.
Since the coordination of the bulky Ln(NO3)3 metallic unit
restricts intermolecular π-stacking between the coordinated
tridentate units to a partial overlap between the central pyridine
rings, we propose that the complexes in one column adopt head-
to-tail orientations for ensuring (i) weak intracolumnar packing
and (ii) pseudo-circular spreading of the flexible chains (Figure
8b),35 as previously reported for [M(L11)Cl2] (M ) Mn, Co,
Ni, Zn).19
For the midrange lanthanides, homeotropic textures are
observed by PLM for the complexes [Ln(L10C12)(NO3)3] (Ln
) Sm-Ho) in the temperature range 130-212 °C (Table 3).
Application of pressure on the sample induces bright ortho-
rhombic flashes in the texture, which suggests the formation of
a viscous cubic mesophase (Figure S19b).8,9 This is confirmed
by SA-XRD profiles displaying a set of up to nine fine
reflections for which the reciprocal spacings are in the ratio
2
2
2
x
x
x
x
x
x
x
x
x
9
h +k +l ) 1, 2, 3, 4, 5, 6, 7, 8, and
(Figure 7b) compatible with a body-centered cubic lattice (a )
37.3-38.3 Å, V ) 51951-56244 Å3, Table S13) since the
x
presence of the reflection corresponding to the ratio
7
permits the immediate elimination of any primitive (P-Bravais
type) and face-centered (F-Bravais type) space groups. For a
body-centered cubic lattice (I-type), the considered sequence
x
x
x
x
x
x
x
x
x
in this case is: 2, 4, 6, 8, 10, 12, 14, 16, 18,
and the reflections can be indexed as (110), (200), (211), (220),
(310), (222), (321), (400), and (411/330). Among the 10 cubic-
centered groups, the four space groups Ia3h, I4132, I4h3d, and
Ia3hd can be disregarded because of the presence of forbidden
reflections not allowed by the general conditions.36 We are left
with six equiprobable groups (they all have the same general
conditions of extinction), which are I23, I213, Im3h, I432, I4h3m,
and Im3hm. The use of eq 5 with d ≈ 1 gives Z ) 16 complexes
per unit cell, which is in line with the 16 general or special
Wykoff positions found in the space group Im3hm.18 Thus, by
analogy with all known cubic space groups discovered in
thermotropic liquid crystals,8b,37 the most probable one, and the
one we will consider thereafter, is the Im3hm space group (No.
229). It is, however, impossible at this stage to discriminate
between a micellar (Figure S20a) or a bicontinuous (Figure
S20b,c) structure for this type of cubic phase. From a molecular
point of view, the larger metallic cores provided by the midrange
lanthanides in [Ln(L10C12)(NO3)3] (Ln ) Sm-Ho) are still
compatible with the exclusive formation of monometallic com-
plexes (i.e., the crystal structure of [Eu(L10C0)(NO3)3(CH3CN)]
1
is monomeric (8), and the H NMR data show [Eu(L10C12)-
(NO3)3] to be the major species in solution). However, the
intracolumnar interactions involving the coordinated tridentate
binding units are significantly weakened by the increasing size
of the metallic Ln(NO3)3 cores (Figure 8b). As a consequence,
the flexibility within each column increases, which eventually
allows larger oscillations (undulating columnar cores model).9a
Figure 7. SA-XRD profiles in the liquid crystalline phases and associated
indexations for (a) [Lu(L10C12)(NO3)3] at 200 °C, (b) [Dy(L10C12)(NO3)3]
at 200 °C, and (c) [Nd(L10C12)(NO3)3] at 80 °C.
(35) (a) Lai, C. K.; Serrette, A. G.; Swager, T. M. J. Am. Chem. Soc. 1992,
114, 7948-7949. (b) Zheng, H.; Lai, C. K.; Swager, T. M. Chem. Mater.
1994, 6, 101-103. (c) Coco, S.; Diez-Esposito, F.; Espinet, P.; Fernandez-
Mayordomo, C.; Martin-Alvarez, J. M.; Levelut, A.-M. Chem. Mater. 1998,
10, 3666-3671.
(36) International Tables for Crystallography, 4th ed.; Hahn, T., Ed.; The
International Union of Crystallography by Kluwer Academic Publishers:
Dordrecht, The Netherlands, Boston, London, 1995; Vol. A.
(37) Diele, S.; Go¨ring, P. In Handbook of Liquid Crystals; Demus, D., Goodby,
J. W., Gray, G. W., Spiess, H. W., Vill, V., Eds.; Wiley-VCH: Weinheim,
Germany, 1998; Vol. 2B, Chapter XIII, pp 887-900.
strand in its extended form in [Ln(L10C12)(NO3)3] amounts to
L ≈ 45 Å, while the perpendicular expansion of the complex
is smaller (about 35 Å, Figure 8a).18 The long axis of the ellipse
(45 Å) is significantly longer than the intercolumnar separation
(32 Å), which implies a significant interdigitation and/or folding
9
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A R T I C L E S
Terazzi et al.
Figure 8. Molecular modeling of [Ln(LiC12)(NO3)3] (i ) 10, Ln ) Er-Lu, and i ) 12, Ln ) Yb-Lu) in the Colh mesophase. Schematic representations
of (a) the unit cells, (b) the head-to-tail packing of two adjacent cells within a column, and (c) the arrangement of the columns within the hexagonal lattice.
This mechanism, combined with the occurrence of competitive
intercolumnar interactions involving the peripheral gallic esters,
produces point dislocations that may be responsible for the
formation of the cubic mesophase (Figure S20).8b
units [Ln(L10C12)(NO3)3] (Figure 8) to give the rodlike bimetal-
lic complexes [Ln2(L10C12)2(NO3)6] (Figure 9). Since the
locations of the 12 alkyl chains are not strictly perpendicular
or parallel to the axis of the cylinder, a columnar packing is
destabilized, and a less organized lamellar arrangement can be
postulated. Such a lamellar organization is supported by the
calculation of the molecular areas S ) 75-85 Å2 (Table S13),
which correspond to approximately three times the cross section
of one chain for a monomer,38 in agreement with the cylindrical
arrangement proposed in Figure 9. Moreover, the average
interlayer separation amounts to ∼44-50 Å, which compares
well with the total length of the coordinated bent ligand L10C12
in its elongated form (d ) 45 Å).18 Finally, some residual
The thermal behavior of [Ln(L10C12)(NO3)3] with the larger
lanthanides Ln ) Pr and Nd is quite different. The melting
process leads to a birefringent mesophase (temperature range
70-175 °C, Table 3 and Figure S19c), which displays one sharp
reflection at d ) 44.0-50.2 Å and two broad reflections at
8.5-8.8 and 4.5 Å (molten alkyl chains, Figure 7c). At higher
temperatures, this mesophase is transformed into the Cub phase
previously detected with the analogous midrange lanthanides
(a ) 38.2 Å, Table S13). According to the crystal structure of
[Eu2(L12C0)2(NO3)6] (10), we expect that the dimerization
process, occurring for large Ln(III), assembles two monometallic
(38) Doolitle, K. J. J. Appl. Phys. 1951, 22, 1471-1479.
9
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Macroscopic Cubic, Columnar, and Lamellar Organizations
A R T I C L E S
Chart 2
behavior of the isomeric complexes [Ln(L12C12)(NO3)3]. Again,
thorough combination of TG, DSC, and IR analyses similarly
shows that (i) the interstitial water molecules are lost prior to
the melting processes, (ii) the complexes are stable over several
heating-cooling cycles, provided that the isotropization tem-
perature is not reached, and (iii) the melting processes system-
atically correspond to glass transitions (Table 3). We note that
the decomposition occurs at lower temperatures (∆T ) 30-40
°C) when compared with the analogous complexes [Ln(L10C12)-
(NO3)3] (Table 3). The PLM and SA-XRD studies show the
formation of very similar organizations in the mesophases with
a smooth evolution from hexagonal columnar (Colh) for the
smallest Ln(III) (Lu, Yb, Figure 8 and Table 3) toward a cubic
phase (Cub) for midrange Ln(III) (Er-Tb, Figure S20 and Table
3), and finally lamello-columnar (LCol) followed by Cub phases
for the larger Ln(III) (Pr-Gd, Figure 9 and Table 3). A
remarkable difference concerns the detection of the lamello-
columnar mesophases along a considerable part of the lanthanide
series for [Ln(L12C12)(NO3)3] (Pr-Tm), whereas this arrange-
ment is only found for Pr and Nd in [Ln(L10C12)(NO3)3] (Table
S14). This can be ascribed to the larger propensity of
[Ln(L12C12)(NO3)3] to dimerize because the approach of two
rodlike complexes is more favorable. This is shown by the
thermodynamic data obtained for the complexes [Eu(LiC12)-
(NO3)3] (i ) 10, 12) in CD2Cl2. From the collected enthalpic
and entropic contributions for these complexes, the critical
temperatures for which KLdn-Li ) 1 (equilibrium 1) amount to
Eu-L10C12
Eu-L10C12
Tc ) ∆Hd
∆Hd
/∆Sd
) 334(39) K and Tc
)
Eu-L12C12
Eu-L12C12
/∆Sd
) 542(96) K. In other words, Tc is
the temperature at which each compound exists as a 1:1 mixture
of monomer ([Ln(LiC12)(NO3)3]) and dimer ([Ln2(LiC12)2-
(NO3)6]) in solution, when the total concentration of ligand
amounts to 3 mol‚dm-3. Above this temperature, the mono-
metallic complex dominates, and we conclude that the prob-
ability to isolate the dimer by crystallization at room temperature
is larger with L12C12, in agreement with the rather systematic
detection of lamello-columnar phases at “low” temperatures for
[Ln(L12C12)(NO3)3] (Table S14). Please note that the density
of d ≈ 1 g‚cm-3 considered in the mesophases approximately
corresponds to a concentration of 0.5 mol‚dm-3 in [Ln(LiC12)-
(NO3)3], which is of comparable magnitude as that used for the
interpretation of Tc.
Figure 9. Molecular modeling of [Ln2(LiC12)2(NO3)6] (i ) 10, Ln )
Pr-Nd and i ) 12, Ln ) Pr-Tm) in the lamello-columnar mesophase.
Schematic representations of (a) the rodlike bimetallic complexes and (b)
the macroscopic ordering in the 3D-lattice.
positional ordering within each layer, reminiscent from a
columnar arrangement, is compatible with the broad peak
detected around 8.5-8.8 Å (Figure 9). This postulated lamello-
columnar arrangement (LCol) is only found for the larger
Ln(III) because of their higher propensity to form dimers at
low temperature. If the large negative entropy of the dimeriza-
tion process, demonstrated in absence of solvation by 1H NMR
in CD2Cl2, also holds in the liquid crystalline phase, dissociation
into monometallic complexes is expected at higher temperature
with the formation of the cubic phase, a sequence which is
indeed systematically observed experimentally.
Detecting Phase Transitions in Cubic and Columnar
Liquid Crystals by Lanthanide-Centered Luminescence. The
strong red europium-centered photoluminescence evidenced in
the nonlipophilic model complexes [Eu(Li)(NO3)3] (i ) 13, 14,
Chart 2)23,25 can be exploited for extracting thermal and
Our interpretation of the dependence of the mesomorphism
on the lanthanide size is further supported by the thermal
9
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A R T I C L E S
Terazzi et al.
structural information on the melting process.22,39,40 As a first
step toward this goal, the bent complex [Eu(L10C12)(NO3)3] has
been investigated at 10 and 295 K with high-resolution laser-
excited luminescence to probe the metal ion environment. We
focus here on the three following points that are crucial in the
field of luminescent liquid crystals and phase transitions. (1)
7
The excitation profile of the nondegenerate Eu(5D0 r F0)
transition in [Eu(L10C12)(NO3)3] shows one narrow symmetrical
component at both 10 K (17 215 cm-1, full width at half-height,
fwhh ) 3.0 cm-1) and 295 K (17 226 cm-1, fwhh ) 7.8 cm-1
,
Figure S21a), which is diagnostic for the existence of a single
metallic environment in the microcrystalline sample, very similar
to that found in the monometallic nine-coordinate model
complex [Eu(L14)(NO3)3] (17 226 cm-1 at 295 K),23 for which
the X-ray crystal structure has been solved.25 (2) Upon selective
excitation, the europium-centered emission spectrum is domi-
nated by the hypersensitive electric dipolar 5D0 f 7F2 transition,
while the magnetic dipolar transition 5D0 f 7F1 is split into 2J
+ 1 ) 3 components (Figure S21b). These features imply a
low symmetry around Eu(III) in [Eu(L10C12)(NO3)3], as found
in the crystal structure of the complex [Eu(L14)(NO3)3].25 (3)
The Eu(5D0) luminescence decay is a single exponential, and
the corresponding lifetime amounts to τ ) 1.29-1.25 ms
between 10 and 295 K. This minor temperature dependence
points to an efficient protection of the metallic center from
coupling with high-energy oscillators. Moreover, the close match
with τ ) 1.15-1.11 ms reported for [Eu(L14)(NO3)3] (between
4 and 295 K)25 eventually demonstrates that a very similar nine-
coordinate environment holds in [Eu(L10C12)(NO3)3], which
contrasts with the 10-coordinate site exhibited in the less-
protected [Eu(L10C0)(NO3)3(CH3CN)] complex (8). From a
theoretical point of view, the temperature-dependent deactivation
processes should result in an overall decrease of the emission
intensity (Iobs) and associated lifetime (τobs), varying as the
exponential function y ) y0‚exp(-C/RT).41 Therefore, plots of
ln(Iobs/I295K) or ln(τobs/τ295K) versus the inverse of the absolute
temperature (T-1) should be linear as long as no first-order phase
transition occurs.39,40 However, sigmoid variation is seen for
both logarithmic plots, when [Eu(L10C12)(NO3)3] enters its cubic
mesophase during the first heating process (Figure 10). Since
the contribution due to the concomitant change in the refractive
index is weak compared with the large coupling between the
Figure 10. (a) DSC traces and (b) plots of ln(Iobs/I295K) (red) and ln(τobs
295K) (blue) versus the inverse of the absolute temperature (T-1) obtained
during the first heating/cooling cycle in [Eu(L10C12)(NO3)3].
/
τ
from linearity for the plots of ln(Iobs/I295K) or ln(τobs/τ295K) versus
-1, as it is highlighted in Figure 10b during the first cooling
T
process. The minor residual curvature can be assigned to the
faint nonlinear contribution of the refractive index.40 In other
words, the different macroscopic organizations characterizing
the original microcrystals and the Cub mesophase can be probed
by luminescence thanks to the associated minor structural
variations induced at the molecular level. However, glass
transitions are not accessible. Unfortunately, the signals observed
for the europium-centered emission at high temperature are too
broad to allow a detailed crystal-field analysis of the geometrical
changes in the coordination sphere induced by the phase
transition.
This sensitive luminescent technique has been further ex-
ploited for probing the Cr f Colh transitions occurring with
small lanthanides. The doped complex [Lu0.95Eu0.05(L10C12)-
5
phonon density of states and the DJ electronic states induced
in the less ordered mesophase,39 the sigmoid curve can be traced
back to minor structural changes induced by the Cr f Cub
melting process. A mathematical analysis gives T ) 413(2) K
(i.e., 140(2) °C) for the transition temperature, which exactly
matches that found by PLM and SA-XRD (140 °C, Table 3).
Interestingly, the first heating process displays a first-order
transition Cr f Cub (Figure 10a), which strongly depends on
the conditions used during the original isolation of the complex
in the solid state. The following cooling process and subsequent
heating/cooling cycles show that the organization in the Cub
mesophase is supercooled in the final glassy state (Figure 10a).
Since no significant structural variation occurs during the
subsequent phase transition, we do not expect major deviations
(NO3)3] displays the same melting process (g 160°C8 Colh) as
that found in the pure Lu complex (Table 3). The emission
spectrum (Figure S22) and the Eu(5D0) lifetime in the solid state
(τ10-295K ) 1.29-1.22 ms) are typical of the low-symmetrical
nine-coordinate metallic center found in [Eu(L10C12)(NO3)3].
Plots of ln(Iobs/I295K) or ln(τobs/τ295K) versus T-1 indeed show
sigmoid curves during the first heating, with T ) 161(2) °C in
very good agreement with PLM measurements (Figure S23).
Again, the cooling process and subsequent heating/cooling
cycles only display the glass transition that escapes detection
by DSC and luminescence.
Conclusion
(39) Suarez, S.; Imbert, D.; Gumy, F.; Piguet, C.; Bu¨nzli, J.-C. G. Chem. Mater.
2004, 16, 3257-3266.
(40) Driesen, K.; Binnemans, K. Liq. Cryst. 2004, 31, 601-605.
(41) (a) Kleinerman, M.; Choi, S.-I. J. Chem. Phys. 1968, 49, 3901-3910. (b)
Hu¨fner, S. Optical Spectra of Transparent Rare Earth Compounds;
Academic Press: New York, 1978.
The ligands L10Cn and L12Cn are ideally suited for exploring
the consequences of specific molecular shapes and charge
distributions on the intermolecular interactions responsible for
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Macroscopic Cubic, Columnar, and Lamellar Organizations
A R T I C L E S
the macroscopic organization. In these systems, (i) the global
anisometry (rodlike versus bent) of the ligand can be modulated
by the judicious connection of the semirigid lipophilic sidearms
at the 5,5′- or 6,6′-positions of the benzimidazole rings, (ii) the
rodlike h bent interconversion process can be induced by simple
complexation to metal ions, (iii) the carboxylate spacers fix the
two gallic esters residues roughly perpendicular to the tridentate
binding unit, thus providing two predictable directions for
intermolecular π-stacking, and (iv) the gallic esters possess three
divergent lipophilic alkoxy chains, which increases the volume
ratio V(alkyl chain)/V(rigid core) and favors thermotropic
mesomorphism.7-9 This versatility is completed by the minor,
but significant, structural changes associated with the lanthanide
contraction. For the lighter (i.e., the larger) lanthanides, the trend
for 10-coordination favors the formation of the bimetallic rodlike
nitrato-bridged dimers [Ln2(LiCn)2(NO3)6], but dissociation
yielding the monometallic hemi-disklike complexes [Ln(LiCn)-
(NO3)3] with the smaller Ln(III) dominates the solid state and
solution structures. This crucial variation of the microscopic
arrangement, combined with the tuning of the specific anisom-
etry of the ligand LiC12 (i ) 10, 12), provides a complete library
of molecular shapes. Moreover, the predetermined orthogonal
directions for intermolecular π-stacking allows a rational
amplification of the molecular organization to program mac-
roscopic order in the liquid crystalline phases. For the smaller
Ln(III), the hemi-disklike monometallic complexes [Ln(LiC12)-
(NO3)3] are compact enough to induce intermolecular pyridine-
pyridine interactions between head-to-tail tridentate cores, which
are responsible for the formation of columns in hexagonal
lattices. When the ionic radius increases in the monometallic
complexes [Ln(LiC12)(NO3)3] (i.e., for midrange Ln(III)), the
intracolumnar interactions weaken. Orthogonal intercolumnar
packing occurring between the peripheral gallic ester residues
competes with intracolumnar packing, thus leading to the
oscillation of the columns. Dislocation results, and a body-
centered cubic phase Cub is formed.8,9 Finally, the existence of
rodlike dimers [Ln2(LiCn)2(NO3)6] with the larger Ln(III) re-
moves strong intermolecular π-stacking, and a residual lamellar
ordering is detected. In the absence of coordinating solvent, the
considerable entropic cost of the dimerization process is
expected to control the free energy at high temperature, and
the lamello-columnar phase (induced by the dimeric bimetallic
building blocks) is indeed transformed upon heating into the
cubic phase (induced by the monometallic building blocks). The
concomitant detection of anomalous luminescence behavior
occurring during first-order phase transitions is also encouraging
for implementing new optical functions in liquid crystalline
displays.21c,42
Acknowledgment. We thank H. Lartigue, S. Schweizer, B.
Bocquet, and F. Gumy for their technical assistance. Financial
support from the Swiss National Science Foundation, National
Research Program 47 “Supramolecular Functional Materials”
is gratefully acknowledged.
Supporting Information Available: The Experimental Sec-
tion and the associated analytical data. Additional tables (Tables
S1-S15) and figures (Figures S1-S24) for structural and
spectroscopic analyses and crystallographic data for compounds
5-11 in CIF formats. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA0446101
(42) (a) Weder, C.; Sarwa, C.; Montali, A.; Bastiaansen, C.; Smith, P. Science
1998, 279, 835-837. (b) Sato, Y.; Takahashi, N.; Sato, S. Jpn. J. Appl.
Phys. 1998, 37, L129-L130. (c) Yamaguchi, R.; Ito, Y.; Sato, Y.; Sato, S.
Mol. Cryst. Liq. Cryst. A 1999, 331, 2417-2422.
9
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