Encoding calamitic mesomorphism in thermotropic lanthanidomesogens

Article abstract of DOI:10.1039/b605253c

Peripheral cyanobiphenyl dendrimers impose a microphase organization compatible with smectic mesomorphism, in which the bulky nine-coordinate lanthanide core is located between the decoupled mesogenic sublayers made up of parallel cyanobiphenyl groups. Th

Full text of DOI:10.1039/b605253c

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Encoding calamitic mesomorphism in thermotropic
lanthanidomesogens{
Emmanuel Terazzi,a Bernard Bocquet,a Ste´phane Campidelli,b Bertrand Donnio,c Daniel Guillon,c
Robert Deschenaux*b and Claude Piguet*a
Received (in Cambridge, UK) 12th April 2006, Accepted 11th May 2006
First published as an Advance Article on the web 6th June 2006
DOI: 10.1039/b605253c
as observed for [Ln(L1)(NO3)3] (Fig. 1).6 An alternative strategy
Peripheral cyanobiphenyl dendrimers impose a microphase
was reported for the design of [60]fullerene-containing liquid
crystals, in which the bulky C60 unit is embedded within a flexible
part connecting two peripheral mesogenic dendritic cyanobiphenyl
cores in L2 (Fig. 1).7 Since the intermolecular interactions
responsible for the residual organization of the rigid cores of L2
in the mesophase mainly result from the parallel arrangement of
the mesogenic cyanobiphenyl groups, a smectic A phase was
obtained. Based on this study, one can envisage replacing C60 with
other bulky edifices, such as lanthanide cores. Following this
reasoning, we aim at forcing lanthanidomesogens to form fluid
thermotropic smectic and nematic phases by using the low-
generation dendritic complexes [Ln(Li)(NO3)3] (i = 3, 4), in which
a central tridentate 2,6-bis(benzimidazole)pyridine unit replaces the
malonate spacer found in L2 (Fig. 1). The recent report of nematic
mesomorphism around 85–90 uC for the related, but unsymme-
trical complexes [Ln(L5)(diketonate)3], in which cyanobiphenyl
groups are decoupled from the bulky lanthanide core, is an
encouraging pioneering step towards the controlled formation of
fluid layered organisation with luminescent lanthanides (Fig. 1).8
organization compatible with smectic mesomorphism, in which
the bulky nine-coordinate lanthanide core is located between
the decoupled mesogenic sublayers made up of parallel
cyanobiphenyl groups.
The design of thermotropic metallomesogens (metal-containing
liquid crystals)1 follows similar rules as those applied for organic
compounds, and a common strategy consists of designing
anisometric molecules possessing at least two incompatible parts,
which are separated by a molecular interface. The incompatible
parts generally correspond to polarizable rigid cores and poorly
polarizable flexible chains, the latter being located at the periphery
of the molecule.2 The existence of these two spatially separated
portions is responsible for the generation of two successive phase
transitions corresponding (i) to the melting of the aliphatic chains,
and the concomitant formation of the liquid-crystalline phase,
followed by (ii) the ultimate melting of residual packed rigid cores,
which leads to the nematic phase or to the isotropic liquid.2 For
trivalent lanthanide metal ions, LnIII, the considerable size of their
high-coordination spheres limits the efficiency of the microsegrega-
tion processes operating between the polarizable bulky metallo-
organic core and the appended flexible paraffinic chains, and the
occurrence of thermotropic lanthanide-containing liquid crystals
(lanthanidomesogens) remains rare.3 Two main strategies have
been considered for restoring the minimal electronic and structural
incompatibilities required for an efficient microsegregation: (1) the
LnIII ions are embedded into aromatic polarizable cavities
produced by several wrapped ligands, which globally display
rodlike or disklike shapes (Fig. S1, ESI{),3–5 or (2) the
coordination of LnIII to pro-mesogenic polycatenar ligands
possessing curved molecular interfaces and large melting entropies
helps to induce thermotropic mesomorphism at ‘low’ temperature,
aDepartment of Inorganic, Analytical and Applied Chemistry, University
of Geneva, 30 quai E. Ansermet, CH-1211 Geneva 4, Switzerland.
E-mail: Claude.Piguet@chiam.unige.ch.; Fax: +4122 379 6830;
Tel: +4122 379 6034
bInstitut de Chimie, Universite´ de Neuchaˆtel, Av. de Bellevaux 51, CP
158, CH-2009 Neuchaˆtel, Switzerland.
E-mail: robert.deschenaux@unine.ch
cInstitut 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
{ Electronic supplementary information (ESI) available: Experimental for
the syntheses of L3–L4 and of their complexes. Tables of elemental
analyses, 1H NMR shifts, and indexation of SA-XRD profiles. Figures
reporting 1H NMR spectra, van’t Hoff plot, DSC traces and SA-XRD
profiles. See DOI: 10.1039/b605253c
Fig. 1 Molecular structures of L27 and of the complexes
[Ln(L1)(NO3)3],6b [Ln(Li)(NO3)3] (i = 3, 4), and [Ln(L5)(diketonate)3].8
2922 | Chem. Commun., 2006, 2922–2924
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in agreement with the dimerization process shown in equilibrium
(1)6b (MM stands for the molecular mass of the complexes, v and
¯
A
v are the specific partial volumes).11
¯
B
MMB/MMA = (D /D )3?(v /v )
(2)
¯
¯
A
A
B
B
1
Integration of the variable-temperature H NMR data shows
the ratio [Eu(L4)(NO3)3]/[Eu2(L4)2(NO3)6] to smoothly increase
from 5.9 at 298 K, to 20.0 at 318 K in CD2Cl2. Calculation of
KEdu;L4 (equilibrium (1)) at each temperature gives a linear van’t
Scheme 1 Reagents: (i) CH2Cl2, DCC, 4-DMAP, 4-carboxybenzalde-
hyde, rt, 16 h, 33% (ii) THF, H2O, NaClO2, H2NSO3H, rt, 2 h, 97% (iii)
CH2Cl2, EDCI, 4-DMAP, 12 h, reflux, 77% (iv) THF, H2O, NaClO2,
H2NSO3H, rt, 2 h, 100% (v) CH2Cl2, EDCI, 4-DMAP, 12 h, reflux, 70%.
Hoff plot, from which the thermodynamic parameters DHdEu;L4
=
255(4) kJ mol21 and DSdEu;L4 = 2166(11) J mol21 K21 can be
determined (Fig. S3, ESI{). These parameters are typical for a
dimerization process occurring in a non-coordinating solvent for
LnIII complexes (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
The syntheses of the ligands L3 and L4 are shown in Scheme 1.
Esterification of 17 with 4-carboxybenzaldehyde gives 2, which is
oxidized into the corresponding carboxylic acid 3. A second
esterification allows its coupling to the tridentate binding unit 410
to give L3.{ Ligand L4 was prepared in a similar way from the
aldehyde derivative 5.9 The 1H NMR spectra of L3 and L4
confirm dynamically-averaged C2v symmetries on the NMR time
scale, 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 usual trans–trans arrangement of the non-coor-
dinated 2,6-bis(benzimidazole)pyridine core (Figure S2a, ESI{).6b
Lanthanide complexes with formula [Ln(Li)(NO3)3]?xH2O (i =
3, 4; ESI, Table S1{) are obtained by mixing stoichiometric
amounts of the hydrated lanthanide salt Ln(NO3)3?nH2O and Li in
dichloromethane–acetonitrile (1 : 1). The presence of co-crystal-
lized water molecules is confirmed by thermogravimetric (TG)
measurements, which show dehydration of the complexes during
the first heating process in the 25–100 uC range. In solution, the
complexation of L4 can be followed by 1H NMR and the
diagnostic complexation shifts observed for the aromatic protons
in the diamagnetic [Lu(L4)(NO3)3] complex (Fig. S2b and Table
S2, ESI{) points to the dynamically-average C2v meridional tri-
coordination of the 2,6-bis(benzimidazol-2-yl)pyridine to the metal
ion, as previously demonstrated for [Lu(L1)(NO3)3].6b The
detection of NOE effects between the ethyl residues and the
protons connected at the meta-positions of the pyridine ring
confirms the cis–cis conformation of the coordinated tridentate
dimerize). These values compare well with DHdEu;L1
=
254(4) kJ mol21 and DSdEu;L1 = 2162(16) J mol21 K21 previously
reported for the nine-coordinated monomer [Eu(L1)(NO3)3],6b and
this allows us to conclude that EuIII is similarly nine-coordinated
by the tridentate aromatic receptor ligand and three bidentate
nitrates in [Eu(L4)(NO3)3] and in [Eu(L1)(NO3)3].
The thermal and liquid-crystalline properties of L3 and L4 were
investigated by differential scanning calorimetry (DSC) and
polarized transmitted light microscopy (PLM, Table 1). The
nature of the phases was established from their optical textures
(PLM), which display the typical Schlieren’s texture of fluid
nematic (N) phases. This strongly contrasts with the poorly fluid
hexagonal columnar mesophase (Colh) evidenced for the parent
hexacatenar ligand L1, which confirms the beneficial effect of the
peripheral cyanobiphenyl groups for inducing calamitic meso-
morphism. Moreover, the reduced transition temperatures
observed on going from L3 to L4 illustrate the favorable
dendrimeric effect occurring in this system.7,9 Examination of the
Table 1 Phase-transition temperatures and enthalpy and entropy
changes for L3 and L4, and for the complexes [Ln(L4)(NO3)3] (Ln =
Lu, Tb, Gd and Eu).
DH/
DS/
1
Transitiona
T/uC kJ mol21 J mol21 K21
receptor. The H NMR spectra of the paramagnetic complexes
Compound
[Eu(L4)(NO3)3] are different, not only because of the additional
lanthanide-induced shifts (LIS) and lanthanide-induced relaxation
(LIR) effects produced by the Eu-centred unpaired electrons, but
also because each broadened resonance is split as the result of the
dimerization process previously established for the closely related
non-dendritic complex [Eu(L1)(NO3)3] (equilibrium (1) and
Fig. S2c, ESI{).6b
L3
L4
G A N
N A Ib
G A N
N A I
82
324
52
238
100
150
170
—
—
—
11.0
—
—
—
—
—
21.5
—
—
—
6.9
—
32.3
—
19.8
—
24.2
[Lu(L4)(NO3)3] G A SmA
SmA A PCrc
PCr A SmA
SmA A I/Dec. 200
3.3
—
15.0
—
9.1
—
11.1
[Tb(L4)(NO3)3] G A SmA
90
2[Eu(L4)(NO3)3] P [Eu2(L4)2(NO3)6] KdEu;L4
(1)
SmA A I/Dec. 190
[Gd(L4)(NO3)3] G A SmA
85
SmA A I/Dec. 188
[Eu(L4)(NO3)3] G A SmA
80
A DOSY experiment performed on [Eu(L4)(NO3)3] (CD2Cl2,
298 K) shows that a first series of peaks (series A in Fig. S2c ESI{)
corresponds to a single species with an auto-diffusion coefficient
DA = 2.7(1) 6 10210 m2 s21, while the second series of peaks
(series B in Fig. S2c{) is assigned to a different species which
diffuses slightly slower DB = 2.2(2) 6 10210 m2 s21. The ratio DA/
DB = 1.2(1) translates into MMB/MMA = 1.8(2) by using eqn. (2),
SmA A I/Dec. 186
G = glass, SmA = smectic A phase, N = nematic phase, I =
a
isotropic fluid, Dec. = decomposition; temperatures are given as the
onset of the peak observed during heating processes; the liquid-
crystalline phases were identified from their optical textures and
from XRD studies. Too weak to be detected by DSC. Re-entrant
partially crystallized phase (PCr).
b
c
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two parallel cyanobiphenyl segments corresponding to the termini
of the extended rodlike complex (each segment occupies an
2
average surface of 22 A in a SmA phase).12 Taking an
˚
approximate, but realistic molecular density of d = 1 g cm23 in
the mesophase, we eventually estimated the molecular volume of
the complex according to: V = MMcomplex6 l 6 10224/d 6 Nav,
(MMcomplex, molecular weight of the complex, Nav, Avogadro’s
number, and l, a temperature corrector, see ESI{) and then
13
calculated the molecular area A = V/d001
.
We systematically
2
˚
found that A $ 46.0–60 A (Table S3, ESI{), in agreement with an
orthogonal arrangement of the terminal groups with respect to the
layers, and thus to a SmA type of phase, reminiscent of that
observed for some main-chain LC dendrimers (Fig. 2b).13
In conclusion, the typical columnar and cubic mesomorphism
evidenced for the hexacatenar complexes [Ln(L1)(NO3)3]6b is
replaced by smectogenic organization with the dendritic analogues
[Ln(L4)(NO3)3]. These results are in line with previous observa-
tions reported for fullerene-containing liquid-crystalline dendri-
mers,7,9,14 and larger decoupling between the mesogenic
cyanobiphenyl groups and the polarizable lanthanide core,
produced by the stepwise increase of the dendrimer generation,
is a promising tool for further reducing melting temperatures, a
crucial point for designing fluid calamitic lanthanidomesogens.
Fig. 2 (a) Simple molecular model for [Ln(L4)(NO3)3] (Ln = Lu, Tb, Gd
and Eu) in the unit cell. (b) Arrangement of [Ln(L4)(NO3)3] in the SmA
mesophase.
complexes by PLM indicates that only the complexes with L4 are
mesogenic. Because of their reduced trend towards dimerization,6b
only [Ln(L4)(NO3)3] with metals of the second part of the
lanthanide series (Ln = Eu, Gd, Tb and Lu) have been investigated
by DSC (Fig. S4, ESI{) and small angle X-ray diffraction (SA-
XRD) (Table S3 and Fig. S5, ESI {). The complexes are mesogenic
over a large temperature range (Table 1), and PLM observations
show fluid and birefringent textures. The phase transformation
from the solid to the fluid phases usually corresponds to glass
transitions (faint bend on the DSC traces), for which the
temperatures have been determined by PLM. The isotropizations
correspond to irreversible first-order phase transitions, because the
complexes rapidly decompose in the liquid state at high
temperature. Consequently, natural textures cannot form and the
organization within the phases are deduced from small angle X-ray
diffraction measurements collected during the first heating process.
The X-ray diffraction patterns recorded for the mesogenic
complexes [Ln(L4)(NO3)3] (Ln = Eu, Gd and Tb) are similar.
At 80–100 uC, two low-angle reflections start to develop, and
reach maximum intensity at 160 uC, before breaking down at 180–
200 uC. At 160 uC, two small-angle reflections in the ratio 1 : 2
were obtained, characteristic of a lamellar organization. They were
Notes and references
{ Experimental procedures and characterization processes are reported in
the ESI.{
1 B. Donnio, D. Guillon, D. W. Bruce and R. Deschenaux, in
Comprehensive Coordination Chemistry II: From Biology to
Nanotechnology, ed. J. A. McCleverty and T. J. Meyer, Elsevier,
Oxford, UK, 2003, vol. 7, ch. 7.9, pp. 357–627.
2 E. Terazzi, S. Suarez, S. Torelli, O. Mamula, H. Nozary, D. Imbert,
J.-P. Rivera, E. Guillet, J.-M. Benech, G. Bernardinelli, R. Scopelliti,
B. Donnio, D. Guillon, J.-C. G. Bu¨nzli and C. Piguet, Adv. Funct.
Mater., 2006, 157 and references therein.
3 K. Binnemans and C. Go¨rller-Walrand, Chem. Rev., 2002, 102, 2303.
4 (a) Y. G. Galyametdinov, G. I. Ivanova, A. V. Prosvirin and O. Kadkin,
Russ. Chem. Bull., 1994, 43, 938; (b) K. Binnemans, Y. G.
Galyametdinov, R. V. Deun, D. W. Bruce, S. R. Collinson, A. P.
Polishchuk, I. Bikchantaev, W. Haase, A. V. Prosvirin, L. Tinchurina,
I. Litvinov, A. Gubajdullin, A. Rakhmatullin, K. Uytterhoeven and
L. V. Meervelt, J. Am. Chem. Soc., 2000, 122, 4335.
5 K. Binnemans, J. Sleven, S. D. Feyter, F. C. D. Schryver, B. Donnio
and D. Guillon, Chem. Mater., 2003, 15, 3930 and references therein.
6 (a) K. Binnemans, K. Lodewyckx, B. Donnio and D. Guillon, Chem.–
Eur. J., 2002, 8, 1101; (b) E. Terazzi, S. Torelli, G. Bernardinelli,
J.-P. Rivera, J.-M. Benech, C. Bourgogne, B. Donnio, D. Guillon,
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indexed as the (00l) reflections of a smectic phase with d001
˚
$
˚
120 A and d002 $ 60 A. (Table S3 and Fig. S5, ESI{). The
˚
additional diffuse band measured at larger angle (d $ 4.5 A)
corresponds to the aliphatic molten chains. The behavior of
[Lu(L4)(NO3)3] is slightly different. Although the smectic phase
starts to develop at 100 uC, an unspecified partially crystallized
phase develops between 140 and 160 uC during the first heating
event, which eventually restores the smectic phase at 190 uC.6b The
large interlayer distances, assigned to the first harmonic of a
˚
smectic phase (d001 $ 120 A, Table S2, ESI{), agrees with the
˚
approximate 119 A length estimated by a simple molecular
modeling for a single complex [Ln(L4)(NO3)3] in its extended
conformation (Fig. 2a). In order to verify that the extended
molecular conformation prevails in the mesophase, we determined
the molecular area of such an arrangement in the smectic layers,
2
and compared it to 44 A , which accounts for the cross-section of
14 B. Donnio and D. Guillon, Adv. Polym. Sci., 2006, DOI: 10.1007/
12_079.
˚
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This journal is ß The Royal Society of Chemistry 2006