Clustomesogens: Liquid crystal materials containing transition-metal clusters

Article abstract of DOI:10.1002/anie.201000325

Shining red-NIR light: Bright red-NIR photoluminescent liquid crystals were obtained by grafting six promesogenic units onto an octahedral molybdenum cluster core (see picture). This hybrid self-arranges in a lamellar structure over a wide range of temperatures to give the first clustomesogen. (Figure Presented)

Full text of DOI:10.1002/anie.201000325

Angewandte
Chemie
DOI: 10.1002/anie.201000325
Clustomesogens
Clustomesogens: Liquid Crystal Materials Containing Transition-Metal
Clusters**
Yann Molard,* Frederick Dorson, Viorel Cꢀrcu, Thierry Roisnel, Franck Artzner, and
Stꢁphane Cordier*
Dedicated to Dr. Christiane Perrin
Octahedral metal atom clusters in which metallic atoms are
held together by metal–metal bonds are commonly found in
solid-state compounds prepared by high-temperature syn-
thesis.[1] The metallic octahedron is surrounded by eight face-
the one hand and fluidity on the other hand, to correct
automatically the positioning errors that can occur during the
assembly process. Metal-containing liquid crystals (metal-
lomesogens) are the typical examples in which the unique
properties of anisotropic fluids are combined with the specific
properties of metals (e.g. geometry of coordination, optic,
electronic, magnetic).[8] However, mesomorphic materials
containing covalent metal–metal-bonded entities are rare,
and all examples described up to now, since the pioneer work
of Marchon and co-workers,[9] are based on dinuclear metal–
metal-bonded species.[10] The association of mesomorphism
with the peculiar properties of metallic clusters should lead to
clustomesogens that offer great potential in the design of new
electricity-to-light energy conversion systems, optically based
sensors, and displays.
In the scope of our work dedicated to transition-metal-
cluster based multifunctional materials,[11] we report herein
the elaboration and characterization of liquid-crystalline
materials based on a Mo6 cluster. The synthesis is straightfor-
ward and consists of the one-step reaction of [Mo6Br8F6]2ꢀ
units with carboxylic acid derivatives (Scheme 1), which
results in the in situ exchange of apical Fꢀ by carboxylate
anion along with the formation of HF.
capping and six terminal ligands to form a [M6Qi8Qa ]2ꢀ
6
nanosized unit (Q = chalcogen/halogen, i = inner, a =
apical). Many routes[2] afford soluble discrete [M6Qi8Xa ]2ꢀ
6
units (X = halogen) that exhibit, either in the liquid or solid
state, specific electronic, magnetic, and photophysical proper-
ties related to the number of metallic electrons available for
metal–metal bonds.[3] In particular, they are highly emissive in
the red–NIR region, have photoluminescence quantum yields
of up to 0.23,[3d] display long excited-state lifetimes,[3d,4] and
undergo facile ground- and excited-state electron transfer by
electrogenerated luminescence.[5] Owing to the stronger
i
a
ꢀ
ꢀ
covalent nature of the M Q bond relative to the M X
one, halogen apical atoms can be replaced by inorganic or
organic ligands without any alteration of the (M6Qi8)m+ core,
leading to functional building blocks usable for the design of
supramolecular architectures, polymeric frameworks, or
nanomaterials with unique properties.[6] Although many
examples of hexasubstituted [M6Qi8La ]xꢀ units (L = organic
6
ligand) have been reported,[6d,7] their integration in macro-
scopic devices by a bottom-up approach remains a challenge.
This task requires systems with self-organization abilities on
Owing to the bulkiness of the cluster unit and to its
octahedral coordination,[12] we used a strategy based on the
[*] Dr. Y. Molard, F. Dorson, Dr. T. Roisnel, Dr. S. Cordier
Universitꢀ de Rennes 1
UMR “Sciences Chimiques de Rennes”, UR1-CNRS 6226
Campus de Beaulieu, CS 74205, 35042 Rennes Cedex (France)
Fax: (+33)2-2323-6799
E-mail: yann.molard@univ-rennes1.fr
stephane.cordier@univ-rennes1.fr
Homepage: http://scienceschimiques.univ-rennes1.fr
Dr. V. Cꢁrcu
Department of Inorganic Chemistry, University of Bucharest
Dumbrava Rosie 23, Bucharest 020464, sector 2 (Romania)
Dr. F. Artzner
Universitꢀ de Rennes 1
UMR “Institut de Physiques de Rennes”, UR1-CNRS 6251
Campus de Beaulieu, CS 74205, 35042 Rennes Cedex (France)
[**] We are grateful to UR1 and Fondation Langlois for financial support.
We thank Cristelle Meriadec for X-ray measurements, Dr. V. Marchi-
Artzner for access to the spectrofluorimeter, and Prof. D. Rondeau
for HRMS measurements. Y.M. thanks Dr. M. A. Amela-Cortes for
helpful discussions. This work was performed within the framework
of the PHC Brancusi Program No. 19616UF.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000325.
Scheme 1. Schematic representation of (nBu4N)2[Mo6Br8F6] and the
gallic acid derivatives HL1 and HL2.
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grafting of mesomorphic promoters through a flexible
aliphatic spacer that was successfully applied to obtain
thermotropic mesophases in the case of polyoxometalates,[13]
octahedral coordination complexes,[14] fullerenes,[15] rotax-
anes,[16] catenanes,[17] or bulky lanthanidomesogenes.[18] The
ligand HL2 is based on a gallic acid scaffold and was
synthesized according to reported procedure.[19]
The hybrids were easily and quantitatively obtained by
heating a mixture containing one equivalent of (nBu4N)2-
[Mo6Br8F6][20] and six equivalents of the corresponding gallic
acid derivative. Dissolution of HL1 in THF was necessary
prior heating to obtain a homogeneous medium, while for
HL2 the reaction took place without solvent in the melted
ligand (see the Supporting Information). The grafting of
1
ligands was revealed by H NMR spectroscopy with a small
downfield shift of the singlet corresponding to the aromatic
protons of the ligand gallic moiety and by 19F NMR spectros-
copy with the disappearance of the signal at d = ꢀ203 ppm
corresponding to the starting material (nBu4N)2[Mo6Br8F6]
and the appearance of two signals at d = ꢀ138 and ꢀ151 ppm
corresponding to the formation of HF and nBu4NF, respec-
tively. IR measurements confirmed the grafting by the shift of
the nc=O stretching band from 1714 cmꢀ1 (dimer of the acid) or
1685 cmꢀ1 (monomer) to 1630 cmꢀ1, and the disappearance of
the OH carboxylic acid band at 3300 cmꢀ1 together with the
appearance of the intense and broad band of the nHꢀF
Figure 1. X-ray diffraction pattern of (nBu4N)2[Mo6Br8L2 ]·6HF at 908C
6
obtained on cooling (solid line), and simulated hexagonal pattern
(squares). Inset: Polarized optical micrograph of (nBu4N)2[Mo6Br8L2 ]
6
at 102.68C (see the Supporting Information for colored figure).
smectic B phase (SmB).[23] However, the smectic B focal conic
fan texture is paramorphotic and is only exhibited upon
cooling, when it is inherited from a previous focal conic fan
smectic A or smectic C phase. In our case, no previous smectic
phase could be detected either by DSC or POM. We can
therefore only conclude at this stage that a smectic-type phase
is observed.
vibration at around 3440 cmꢀ1 [21]
Indeed, as already discussed
.
by Clark,[22] Fꢀ reacts like a Brønsted base with carboxylic
acid, leading to the formation of HF and carboxylate. Single
crystals of (nBu4N)2[Mo6Br8L1 ]·xHF·1THF were obtained
6
by gas diffusion of diethyl ether into a THF solution. The
crystal structure of (nBu4N)2[Mo6Br8L1 ]·5HF·1THF deter-
6
mined by single-crystal X-ray diffraction confirmed the
grafting mode of the gallic acid moieties on the cluster
through one oxygen atom of the carboxylate.
Temperature-dependent X-ray diffraction experiments
were carried out to identify the exact nature of the obtained
mesophase. The X-ray diffraction patterns recorded at differ-
ent temperatures within the mesomorphic range are all
qualitatively equivalent and contain two sharp small-angle
reflections characteristic of a layered morphology with a
reciprocal spacing in the 1:2 ratio. Thus, by applying the
Braggꢀs law, for example at 908C (Figure 1), a spacing of
45.5 ꢁ attributed to the interlayer distance was calculated. A
diffuse scattering halo in the wide-angle region centered
around 4.4 ꢁ (h1) and corresponding to the lateral short-
range order of the molten chains and the cyanobiphenyl
moieties confirmed the liquid-crystalline nature of the
mesophase. X-ray diffraction patterns exhibit additional
intense and very broad reflections that can be indexed in a
The thermal and liquid-crystal (LC) properties of
(nBu4N)2[Mo6Br8L2 ]·6HF were investigated by differential
6
scanning calorimetry (DSC), polarized optical microscopy
(POM), and X-ray diffraction. The phase transition and
thermodynamic data are reported in Table 1. The modified
cluster showed broad transition ranges, presumably because
of its high viscosity (see the Supporting Information). The
formation of one LC phase could be detected by DSC ranging
from 22.78C to 103.28C on the second heating cycle. Focal
conic fans textures of smectic-type phase could be obtained by
POM on cooling from the isotropic melt after several hours of
annealing (inset Figure 1). The appearance of transition bars
in the back of some fans, together with their smoothness and
their truncated aspect in some other places, could be
indicative of the formation of a highly ordered hexatic
hexagonal network as (hk0) = (100), (110), (210), (400),
pffiffi pffiffi
corresponding to the reciprocal spacing ratios 1, 3, 7,
pffiffiffiffiffi
16. These reflections traduce the average lateral organiza-
tion of the electron-rich cluster cores and are indicative of a
local hexagonal ordering within the layers.
Pattern refinement of experimental data obtained at 908C
(Figure 1) using the FullProf program[24] is in agreement with
the nature of an hexagonal arrangement of the clusters within
the layers and an average inter-cluster distance of 23 ꢁ.
Decreasing the temperature to 208C induces an increase of
the interlayer distance to 46.7 ꢁ together with a shortening of
the intercluster distance to 21.6 ꢁ.
Table 1: Phase behavior of (nBu4N)2[Mo6Br8L2 ] on the second heating
cycle.
6
Compound
Transition T [8C]
DCp
DH
[kJmolꢀ1 Kꢀ1
]
[kJmolꢀ1
]
(nBu4N)2[Mo6Br8L2 ]
g!Sm X
Sm X!I
22.7
103.2
4.29
–
–
6
32.94
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To understand how the modified clusters are self-assem-
bled, the electronic density profile[25] was calculated from the
intensity of the lamellar Bragg signals at 908C and revealed a
succession of electron-rich (thickness = 14 ꢁ) and electron-
poor (thickness = 31.5 ꢁ) layers corresponding respectively
to the cluster layer and the organic layer (Figure 2). As the
thickness of the organic layer is roughly equal to the total
length of HL2, we may consider that it contains fully
interdigitated ligands. Moreover, the observed local hexago-
nal arrangements of the clusters and the thickness of the
electron rich layers are indicative of clusterꢀs orientation with
their threefold axes perpendicular to the plane of the layers
(Figure 2).[26]
both cases. As depicted in Figure 3, all clusters show a broad
luminescence in the red–NIR area ranging from 570 to more
than 900 nm with a maximum located at 745 nm for (nBu4N)2-
[Mo6Br8F6] and 735 nm for (nBu4N)2[Mo6Br8L2 ].
6
Therefore, we emphasize that the clusters adopt a
dendritic-like configuration with an equal repartition of
nine cyanobiphenyl groups on both of their sides (Figure 2).
In this case, the molecular area per cluster unit was evaluated
at 908C to 460 ꢁ2 from the molecular volume (ca. 21000 ꢁ3 )
,
and the thickness of the layer (45.5 ꢁ). This calculation gives
a molecular area per mesogenic moiety of 51 ꢁ2, which is
more than twice larger than the transverse cross-section of
one cyanobiphenyl groups (22–25 ꢁ2). This result is in favor
of an interdigitation of the cyanobiphenyl moieties from two
adjacent layers giving then an area per mesogenic group of
26 ꢁ2.
Figure 3. Corrected excitation and emission spectra of (nBu4N)2-
[Mo6Q8X6]2ꢀ cluster units (Q = Cl, Br, I; X = F, Cl, Br, I)
are known for their remarkable luminescence properties, such
as high quantum yields and significant microsecond emission
lifetimes.[3d,4] Note that those compounds present also electro-
luminescence properties.[5] Thus, emission measurements
were carried out for all compounds in solution and in the
solid state (on powder for (nBu4N)2[Mo6Br8F6] and on a
sample deposited by spin coating on a silica glass slide for
[Mo6Br8F6] (dashed line) and (nBu4N)2[Mo6Br8L2 ]·6HF (solid line) in
6
the solid state. lexc =450 nm.
This luminescence is obtained for a wide range of
excitation wavelength starting from 300 nm to 550 nm. The
presence of the ligand around the cluster induces slight
changes to its luminescence spectrum, revealing that the
electronic transition responsible of the emission is mainly of
(nBu4N)2[Mo6Br8L2 ]) giving the same emission profile in
6
Figure 2. Representation of the [Mo6Br8L1 ]2ꢀ building block according to single-crystal X-ray diffraction analysis of (nBu4N)2[Mo6Br8L1 ]·xHF·1THF
6
6
(left side) and illustration of the lamellar packing in the smectic X layer of (nBu4N)2[Mo6Br8L2 ]·6HF along the C3 axis of the clusters according to
6
the calculated electronic density profile at 908C (right side). The (nBu4N)+ cations and HF molecules are omitted for clarity.
Angew. Chem. Int. Ed. 2010, 49, 3351 –3355
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3353

Communications
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In summary, we report an original synthesis and character-
ization of the first clustomesogen, that is, a mesomorphic
material containing a transition-metal cluster core. The
[Mo6Br8L2 ]2ꢀ building blocks self-organize in layered Sm
6
phase over a wide temperature range from 228C to 1038C
after the first heating cycle. The luminescence properties of
the native cluster are preserved upon complexation of the
ligand leading to a bright red–NIR luminescent liquid-crystal
material with high potential in the design of red–NIR
signaling or displays. Studies with other kinds of mesogenic
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Experimental Section
Crystal data for C60H66Br8Mo6O30·2(C16H36N)·C4H8O·5(HF): Mr =
3135.07 gmolꢀ1, 0.28 ꢂ 0.22 ꢂ 0.19 mm , triclinic, P1, a = 17.838(3),
3
ꢀ
b = 19.896(4), c = 22.761(4) ꢁ, a = 68.599(9), b = 89.531(10), g =
67.421(9)8,
V= 6862(2) ꢁ3,
Z = 2,
1calcd = 1.517 g.cm3,
m =
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Received: January 19, 2010
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Published online: March 26, 2010
Keywords: cluster compounds · liquid crystals · luminescence ·
.
metallomesogens · organic–inorganic hybrid composites
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