Liquid crystal properties resulting from synergetic effects between non-mesogenic organic molecules and a one nanometre sized octahedral transition metal cluster

Article abstract of DOI:10.1039/c0cc05012a

This work presents the synthesis and studies of a red-NIR luminescent liquid crystal compound based on an octahedral metallic cluster core orthogonally bounded to six non-mesogenic organic ligands. It evidences synergetic effects between the organic and i

Full text of DOI:10.1039/c0cc05012a

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Liquid crystal properties resulting from synergetic effects between
non-mesogenic organic molecules and a one nanometre sized octahedral
transition metal clusterw
Ana S. Mocanu,ab Maria Amela-Cortes,a Yann Molard,*a Viorel Cırcu*b and
a
´
Stephane Cordier
Received 17th November 2010, Accepted 10th December 2010
DOI: 10.1039/c0cc05012a
This work presents the synthesis and studies of a red-NIR
luminescent liquid crystal compound based on an octahedral
metallic cluster core orthogonally bounded to six non-mesogenic
organic ligands. It evidences synergetic effects between the organic
and inorganic parts of the hybrid, resulting in the generation of
liquid crystal properties on cooling from the isotropic melt.
hexafluorinated cluster. We showed that a smectic phase could be
induced by grafting mesomorphic units onto the cluster through
a flexible aliphatic spacer. Our aim in this work is to understand
whether the cluster core, despite its bulkiness and its octahedral
coordination sphere, can induce the formation of liquid crystal
phases when non-mesomorphic ligands are grafted onto it. Such
behaviour has already been reported for metallomesogens
containing mononuclear complexes.11 We present herein, the
synthesis, characterisation and mesogenic properties of a Mo6
cluster substituted by six non-mesomorphic ligands based on a
gallic acid scaffold modified with three oxydecamethyleneoxy-
biphenyl arms (HL, Scheme 1). The biphenyl unit (BP) is widely
used in the construction of main-chain or side-chain polymers
with liquid crystal properties, or able to control the liquid crystals
surface alignment. It is used mainly as a core unit and, in few
Luminescent liquid crystals have a great potential to be used in
electro-optical devices and, besides the purely organic liquid
crystalline systems investigated so far,1 recent results showed
that metallomesogens2 (liquid crystalline materials incorporating
metal ions) are very good candidates for such multifunctional
materials.3 In this perspective, the incorporation of octahedral
transition metal clusters into liquid crystals (LC) is expected to
lead to very interesting emissive properties taking into account
that these clusters (Mo6, Re6 and W6) are highly emissive in the
red-NIR area displaying long excited state lifetimes with photo-
luminescence quantum yields up to 0.23.4,5 Moreover, the
emissive excited state of cluster-based compounds is mainly metal
centered5 and therefore neither the coordination of promesogenic
ligands onto the metallic octahedron nor intermolecular inter-
actions should affect significantly their emission properties. Let
us recall that octahedral M6 cluster building blocks, obtained
via high temperature solid state synthesis,6 can be introduced
either directly7 or after functionalisation8,9 in supramolecular
assemblies or materials, without significant alterations of their
intrinsic properties (luminescence, redox). We recently reported
the first example of a clustomesogen consisting of a Mo6X8
cluster core modified with six gallate derivatives containing three
long alkyl chains terminated by cyanobiphenyl (CNBP) units.10
This compound was obtained together with 6 HF molecules by
heating a mixture containing the corresponding gallic acid and a
a
´
Universite de Rennes 1, UMR ‘‘Sciences Chimiques de Rennes’’,
UR1-CNRS 6226, Equipe Chimie du Solide et Materiaux,
´
Campus de Beaulieu, CS 74205, 35042 Rennes Cedex, France.
E-mail: yann.molard@univ-rennes1.fr; Fax: +33 223 236 799
b Department of Inorganic Chemistry, University of Bucharest,
Dumbrava Rosie 23, Bucharest 020464, sector 2, Romania.
E-mail: viorel_carcu@yahoo.com
w Electronic supplementary information (ESI) available: Experimental
section; NMR, IR, UV-Vis spectra; POM pictures; DSC thermo-
gramms; X-ray diffraction patterns. See DOI: 10.1039/c0cc05012a
Scheme 1 Representation of the (nBu4N)2[Mo6Br8F6] cluster precursor
and the HL organic ligand used in this work.
c
2056 Chem. Commun., 2011, 47, 2056–2058
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cases, as a mesogenic terminal group linked through a spacer to
the polymer backbone.12 In our case however, Differential
Scanning Calorimetry (DSC) and Polarised Optical Microscopy
(POM) experiments showed that HL does not have liquid crystal
properties.
HL was obtained in three steps following reported procedures
for similar compounds.10 The grafting procedure of HL on the
cluster core was slightly modified compared to that reported
earlier10 in order to eliminate the HF by-product (see ESIw).
Indeed, the use of triethylamine during the reaction, combined
to a washing of the organic phase with an aqueous solution,
allows the removal of HF. 19F NMR experiments revealed the
1
loss of all fluorine atoms, H NMR showed a small downfield
shift of the singlet signal corresponding to the aromatic proton
of the gallic moiety of HL from 7.34 ppm to 7.37 ppm;
IR spectroscopy confirmed the monodentate ligation of the
carboxylate through the creation of a M–O single bond with
the shift of the nCQO stretching band from 1687 cmꢀ1 for the
free acid to 1630 cmꢀ1 for the hybrid.9,10,13 The stoichiometry
was further confirmed by elemental analysis (see ESIw).
Fig.
1
X-Ray diffraction pattern of (nBu4N)2[Mo6Br8L6] at
70 1C obtained on cooling from the isotropic (the arrows indicate
the position of the reflections relative to the clusters organisation
within the layers) liquid. Inset: polarized optical micrograph of
(nBu4N)2[Mo6Br8L6] at 82 1C.
The liquid crystal properties of (nBu4N)2[Mo6Br8L6] were
investigated by POM, DSC and X-ray diffraction. The thermal
data are summarized in Table 1. The assignment of the observed
liquid crystalline phase to a layered SmA phase was done in a
first attempt by temperature dependent POM observations
where a typical fan-shaped texture with several homeotropic
regions developed on cooling from the isotropic liquid
(inset Fig. 1). The modified cluster melts straight to the isotropic
liquid. On cooling at 10 1C minꢀ1, three exothermic peaks were
present in the DSC thermogram (see ESIw). The first one, a
relatively sharp peak at 80 1C, corresponds to the transition
from the isotropic liquid to a liquid crystal phase, while the two
others correspond to successive transitions to crystalline phases
at 67 1C and 51 1C. The enthalpy change of the I–SmA
transition is 57.9 kJ molꢀ1 which is a very high value when
compared to the normal range of such transition.14 However,
this class of compounds may be regarded in the light of first
generation dendrimers containing a Mo6 cluster rigid core.
Therefore, if the enthalpy change is divided by the number of
mesogenic groups (18), it yields to a value of 3.3 kJ molꢀ1 per
mesogenic unit which perfectly matches the range of I–SmA
transitions.15 Such approach was successfully employed in the
case of fullerene15,16 or silsesquioxane17 containing liquid crystal
dendrimers.18 The second heating cycle shows one exothermic
peak between two endothermic peaks which is indicative of a
double melting behaviour (see ESIw).19
To gain more insights regarding the arrangement of the
molecules in the mesophase, temperature-dependent X-ray
diffraction measurements were carried out (Fig. 1). At 70 1C,
the X-ray diffraction pattern of the mesophase contains a single
and broad peak in the low angle region, assigned to the (001)
diffraction plane and corresponding to a lamellar structure with
an inter-layer distance of 35.7 A. The fact that this signal is
much broader than its equivalent in the X-ray pattern of its
CNBP–Mo6 analogue is probably due to the ability of CNBP
mesogenic groups for better defining the lamellar nature of the
mesophase. Indeed, it is frequently observed that CNBP units
will lead to layered phases when grafted as side chains onto a
polymer or as terminally appended units in dendrimers.18 The
diffuse scattering halo in the wide angle region centered around
4.5 A (h in Fig. 1) corresponds to the lateral short range order of
the molten chains and the biphenyl moieties, confirming the
liquid crystalline nature of the mesophase. Besides these two
signals, additional very broad and intense reflections are
observed. Although the appearance of such reflections may
indicate some kind of local ordering of the clusters within the
layers, the reflections broadness excludes any clear geometrical
interpretation. However, by analogy with previously reported
results,10 one might expect an orientation of the modified
clusters with their C3 axis perpendicular to the plane of the
layers and an equal repartition of the organic ligands on both
sides of the metallic core. So far, if we compare the results
obtained for (nBu4N)2[Mo6Br8L6] and for its CNBP–Mo6
analogue,10 we can notice that the lack of cyano groups
has a dramatic impact on the mesomorphic behaviour of the
obtained hybrid: (i) the destabilisation of the mesophase, (ii) a
significant narrowing of the mesomorphic temperature range
from approximately 83 1C for the CNBP analogue to 17 1C for
(nBu4N)2[Mo6Br8L6] and, (iii) a one nanometre shortening of
Table 1 Thermal data for HL and (nBu4N)2[Mo6Br8L6] obtained by
DSC at 10 1C minꢀ1
Compound
HL
Transition
Tc/1C
DH/kJ molꢀ1
Cr–Cr0
Cr0–I
I–Cr
Cr–I
Cr–I
(I–SmA)
(SmA–Cr)
(Cr–Cr0)
101
118
85
119.2
55.1
76.6
109a,b
82
78.8
(nBu4N)2[Mo6Br8L6]
280.6
(57.9)
(22.6)
(34.4)
(80)
(67)
(51)
a
b
Data taken from the second heating cycle. On the second heating
cycle there still can be seen a small peak with Tmax = 120 1C that
corresponds to the melting of the Cr0 form (see Fig. S7 in the ESIw),
c
information that is supported by POM observations. These values
represent Tonset taken from DSC measurements.
c
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the lamellar interlayer distance which, according to molecular
modelling (by means of Hyperchem software), may correspond
to a higher interdigitation of the BP moieties. This last point
is corroborated with the X-ray diffraction patterns obtained
below 70 1C.
2 D. W. Bruce, D. A. Dunmur, E. Lalinde, P. M. Maitlis and
P. Styring, Nature, 1986, 323, 791–792; B. Donnio, D. Guillon,
R. Deschenaux and D. W. Bruce, in Comprehensive Coordination
Chemistry II: From Biology to Nanotechnology, ed. J. A. Mc
Cleverty, J. J. Meyer, M. Fujita and A. Powell, Elsevier, Oxford,
2003, pp. 357–627.
3 T. J. Bunning, L. V. Natarajan, V. Tondiglia, R. L. Sutherland,
D. L. Vezie and W. W. Adams, Polymer, 1995, 36, 2699–2708;
T. Kato, in Molecular Self-Assembly, 2000, pp. 95–146;
L. Cunha-Silva, S. Lima, D. Ananias, P. Silva, L. Mafra,
L. D. Carlos, M. Pillinger, A. A. Valente, F. A. A. Paz and
J. Rocha, J. Mater. Chem., 2009, 19, 2618–2632; A. B. Gaspar,
M. Seredyuk and R. Gutlich, Coord. Chem. Rev., 2009, 253,
2399–2413.
4 A. W. Maverick and H. B. Gray, J. Am. Chem. Soc., 1981, 103,
1298–1300; D. G. Nocera and H. B. Gray, J. Am. Chem. Soc.,
1984, 106, 824–825.
5 A. W. Maverick, J. S. Najdzionek, D. MacKenzie, D. G. Nocera
and H. B. Gray, J. Am. Chem. Soc., 1983, 105, 1878–1882.
6 K. Kirakci, S. Cordier and C. Perrin, Z. Anorg. Allg. Chem., 2005,
631, 411–416.
7 F. Grasset, F. Dorson, S. Cordier, Y. Molard, C. Perrin, A.-M. Marie,
T. Sasaki, H. Haneda, Y. Bando and M. Mortier, Adv. Mater., 2008,
20, 143–148; F. Grasset, F. Dorson, Y. Molard, S. Cordier,
V. Demange, C. Perrin, V. Marchi-Artzner and H. Haneda, Chem.
Commun., 2008, 4729–4731; F. Grasset, Y. Molard, S. Cordier,
F. Dorson, M. Mortier, C. Perrin, M. Guilloux-Viry, T. Sasaki and
H. Haneda, Adv. Mater., 2008, 20, 1710–1715.
8 G. Prabusankar, Y. Molard, S. Cordier, S. Golhen, Y. Le Gal,
C. Perrin, L. Ouahab, S. Kahlal and J. F. Halet, Eur. J. Inorg.
Chem., 2009, 2153–2161; S. Ababou-Girard, S. Cordier, B. Fabre,
Y. Molard and C. Perrin, ChemPhysChem, 2007, 8, 2086–2090;
B. Fabre, S. Cordier, Y. Molard, C. Perrin, S. Ababou-Girard and
C. Godet, J. Phys. Chem. C, 2009, 113, 17437–17446.
9 Y. Molard, F. Dorson, K. A. Brylev, M. A. Shestopalov,
Y. Le Gal, S. Cordier, Y. V. Mironov, N. Kitamura and
C. Perrin, Chem.–Eur. J., 2010, 16, 5613–5619.
10 Y. Molard, F. Dorson, V. Circu, T. Roisnel, F. Artzner and
S. Cordier, Angew. Chem., Int. Ed., 2010, 49, 3351–3355.
11 B. Donnio, B. Heinrich, T. Gulik-Krzywicki, H. Delacroix,
D. Guillon and D. W. Bruce, Chem. Mater., 1997, 9, 2951–2965;
B. Donnio and D. W. Bruce, New J. Chem., 1999, 23, 275–286.
12 S. I. Kim, M. Ree, T. J. Shin and J. C. Jung, J. Polym. Sci., Part A:
Polym. Chem., 1999, 37, 2909–2921; T. Ranganathan, C. Ramesh
and A. Kumar, J. Polym. Sci., Part A: Polym. Chem., 2004, 42,
2734–2746; G. D. Liu, J. G. Gao, L. L. Song, W. J. Hou and
L. C. Zhang, Macromol. Chem. Phys., 2006, 207, 2222–2231;
X. Y. Liu, W. Xu, G. D. Ye and Y. Gu, Polym. Eng. Sci., 2006,
46, 123–128; M. Li, H. Lai, B. X. Chen, X. Y. Liu and Y. Gu, Liq.
Cryst., 2010, 37, 149–158.
Indeed, lowering the temperature to 60 1C (first crystalline
phase) is traduced on the X-ray diffraction pattern by an
increase of the interlayer distance and by the appearance of a
signal located at 3.18 A relative to p–p stacking interactions of
the biphenyl moieties (see ESIw). Let us note that, such impact
on mesogenic properties by removal of CN groups has been
previously reported for amphiphilic liquid crystalline oligomers
based on semiperfluorinated ester derivatives of gallic acid.20
As the emissive excited state of Mo6 clusters is mainly metal
centered,21 the coordination of organic ligands onto the six
metal atoms should not induce significant changes in the
emission properties of the cluster core. Therefore, emission
measurements were carried out for (nBu4N)2[Mo6Br8L6] on a
sample deposited by spin coating on a silicon wafer, heated at
90 1C and cooled to room temperature. (nBu4N)2[Mo6Br8L6]
shows the same broad emission, from 550 to nearly 900 nm with
a maximum located at 740 nm than its fluoride precursor, for a
wide range of excitation wavelength starting from 300 nm to
550 nm (see ESIw). This similar behaviour upon light excitation
is of significant importance because it shows that different types
of ligands can be used to modify the liquid crystalline properties
of the clustomesogens without altering their luminescence
properties. Yet, it gives them a major advantage on metallo-
mesogens containing d-block elements for which, most of the
emissive excited states are not metal centred and thus, for which
the emission is strongly affected by the nature of the ligands and
the intramolecular interactions occurring in the mesophase.22
In conclusion, we have shown in this work that, despite its
octahedral coordination sphere and its bulkiness, a one nano-
metre sized octahedral transition metal cluster can promote
the formation of a liquid crystal phase when it is bounded to
six non-liquid crystal organic ligands. The intrinsic lumines-
cence properties of the cluster core are not significantly
modified by the grafting of the organic ligands and are not
influenced by intermolecular interactions existing either in the
mesomorphic or crystalline phase. These two important results
open new and fascinating perspectives in the design of other
type of clustomesogen for which these evidenced synergetic
effects between the metallic core and the organic bounded
ligands will play an essential role in the behaviour of the
resulting organic–inorganic hybrid nanomaterial.
13 F. Dorson, Y. Molard, S. Cordier, B. Fabre, O. Efremova,
D. Rondeau, Y. Mironov, V. Circu, N. Naumov and C. Perrin,
Dalton Trans., 2009, 1297–1299.
14 S. Singh, Phys. Rep., 2000, 324, 108–269.
15 B. Dardel, D. Guillon, B. Heinrich and R. Deschenaux, J. Mater.
Chem., 2001, 11, 2814–2831.
16 T. Chuard, R. Deschenaux, A. Hirsch and H. Schonberger, Chem.
Commun., 1999, 2103–2104; R. Deschenaux, B. Donnio and
D. Guillon, New J. Chem., 2007, 31, 1064–1073.
This work was done in the frame of the French-Romanian
PHC Brancusi program 2009–2010/19616UF. The authors
thank UR1, Region Bretagne (CREATE program) and
Fondation Langlois for financial support. V.C. and A.S.M.
thank CNCSIS-UEFISCSU (project PNII Idei ID_954) for
funding. Y.M. wish to thank Dr F. Artzner and C. Meriadec
to provide access to SAXS measurements and Dr V. Marchi-
Artzner for access to spectrofluorimeter.
17 G. H. Mehl and I. M. Saez, Appl. Organomet. Chem., 1999, 13,
261–272.
18 B. Donnio and D. Guillon, Adv. Polym. Sci., 2006, 201, 45–155;
I. M. Saez and J. W. Goodby, Structure and Bonding, 2008, 128,
1–62.
19 K. Ohta, H. Muroki, K. I. Hatada, I. Yamamoto and
K. Matsuzaki, Mol. Cryst. Liq. Cryst., 1985, 130, 249–263;
K. Ohta, H. Muroki, K. I. Hatada, A. Takagi, H. Ema,
I. Yamamoto and K. Matsuzaki, Mol. Cryst. Liq. Cryst., 1986,
140, 163–177; K. Ohta, H. Ema, H. Muroki, I. Yamamoto and
K. Matsuzaki, Mol. Cryst. Liq. Cryst., 1987, 147, 61–78.
20 A. Yamaguchi and A. Yoshizawa, Mol. Cryst. Liq. Cryst., 2007,
479, 1219–1227.
Notes and references
´
1 R. Gimenez, M. Pinol and J. L. Serrano, Chem. Mater., 2004, 16,
1377–1383; Y. Sagara, S. Yamane, T. Mutai, K. Araki and
T. Kato, Adv. Funct. Mater., 2009, 19, 1869–1875.
21 H. Honda, T. Noro, K. Tanaka and E. Miyoshi, J. Chem. Phys.,
2001, 114, 10791–10797.
22 K. Binnemans, J. Mater. Chem., 2009, 19, 448–453.
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2058 Chem. Commun., 2011, 47, 2056–2058
This journal is The Royal Society of Chemistry 2011