COMMUNICATION
A surfactant-encapsulated polyoxometalate complex towards a
thermotropic liquid crystal{
Wen Li,a Weifeng Bu,a Haolong Li,a Lixin Wu*a and Min Lib
Received (in Cambridge, UK) 9th March 2005, Accepted 2nd June 2005
First published as an Advance Article on the web 29th June 2005
DOI: 10.1039/b503550c
added to the solution dropwise. After stirring for 4 h at 40 uC, the
organic phase was separated and evaporated to dryness. The
residue was further dried under vacuum until the weight remained
constant. In contrast to PM-1 which is only soluble in water, the
obtained complex is immiscible in water but readily dissolves in
organic media. This indicates that PM-1 has been successfully
encapsulated. Combining the results of thermogravimetric analysis
(TGA) and elemental analysis, SEC-1 should correspond to the
chemical formula: (L1)13[Tb(SiW11O39)2]?30H2O.
A novel surfactant-encapsulated terbium-substituted hetero-
polyoxotungstate complex [L1]13[Tb(SiW11O39)2]?30H2O
(SEC-1) bearing mesomorphous groups was successfully
prepared by the ionic self-assembling route, exhibiting
characteristic thermotropic liquid-crystalline behavior.
Polyoxometalates (PMs) have received considerable interest in
chemistry and material science because of their potential applica-
tion such as in optical, electric and magnetic fields.1 Recently, an
important progress of PM chemistry is that PMs are enwrapped
with a water-insoluble shell composed of ionic organic molecules
based on cooperatively electrostatic interactions so that the
surfactant-encapsulated clusters (SECs) can be processed con-
veniently into thin films by spin coating, simple casting, and LB
techniques.2 In some cases, the organic surfactant cation not only
plays a structural role but also has a strong effect on the properties
of these hybrids.3 Additional properties of the hybrid materials
based on PM are expected to be obtained by the appropriate
choice of organic components.3c
Differential scanning calorimetry (DSC) curves (Fig. 2) give
phase transitions of L1 and SEC-1 in their first cooling and the
second heating process. Comparing these curves with those of the
first heating run of both virgin samples L1 and SEC-1 (Fig. S2{),
we know that the primary heating run results in the loss of
crystalline water in SEC-1, which is proved by IR and TGA
measurements. Upon cooling, L1 exhibits three phase transitions
at 147, 114 and 82 uC, respectively. Similar phase behavior is found
in SEC-1, which undergoes three exothermic transitions at 180, 113
and 79 uC, respectively. The broad peak of SEC-1 at 180 uC should
correspond to a slow phase transition from isotropic phase to a LC
phase.
Liquid crystals are fascinating organic functional materials
which can automatically self-assemble to various mesophases
through organized packing of organic molecules in a certain
temperature range and provide extra features such as the ability to
respond to external stimuli.4 Therefore, a logical problem is how to
incorporate PMs into liquid crystalline (LC) matrices. Such
hybrids combining the characteristics of LC materials with PMs
are expected to bring exciting synergistic properties. For example,
the optical and magnetic properties of PMs can be incorporated
into LCs. Based on the anisotropic and organized nature of LCs,
the properties of PMs can probably be modified and new
functional materials can be constructed.
LC behaviors of SEC-1 and L1 are also identified by polarizing
optical microscopy (POM) as shown in Fig. 3. On cooling from
isotropic phase, L1 displays clear birefringence at 130 uC (Fig. 3a),
suggesting a characteristic liquid-crystalline phase. On further
cooling, broken texture is observed at 103 uC (Fig. 3b), suggesting
a highly ordered smectic phase. In the case of SEC-1, clearing of
the LC phase is firstly observed at 160 uC, which displays a fine-
grain texture though it is not a typical one (Fig. 3c). When the LC
phase is cooled to below 113 uC, a similar but clear mesophase
texture is seen at 103 uC as shown in Fig. 3d, suggesting another
liquid-crystalline phase. To determine the LC effect of L1, SEC-2
was prepared following the same procedure as for SEC-1, using
dioctadecyldimethylammonium (L2) without azobenzene groups
instead of L1. However, LC behavior was not observed in SEC-2
according to the results of DSC and POM. As a consequence, we
can see that the mesomorphous L1 plays an important role for the
formation of LC in SEC-1.
Motivated by this target, in this communication, we report the
preparation of a surfactant-encapsulated cluster (SEC-1) and its
characteristic thermotropic LC behavior. The azobenzene-contain-
ing surfactant L1,{ as shown in Fig. 1, was prepared according
to modified routes of literature.5–7 The polyoxometalate
K13Tb(SiW11O39)2?15H2O (PM-1) was synthesized by following
a literature procedure.8 SEC-1 was obtained according to a
modified method.2 PM-1 (0.165 g) was dissolved in aqueous
solution (pH 5 5.7) and then 0.283 g of L1 in chloroform was
The mesophases and structures of SEC-1 are further investi-
gated by using X-ray diffraction (XRD), seen in Fig. 4. When
SEC-1 is cooled from isotropic liquid to 150 uC, four equidistant
small angle diffractions are observed apparently (Fig. 4a), which
attribute to a typical layered structure (SmX1) and the layer
spacing is calculated to be 6.50 nm. The diffuse peak at 2h # 20u
(d 5 0.45 nm) indicates a decreasing of the lateral correlation
among neighboring alkyl chains. On further cooling to 103 uC, the
layer spacing increase to 6.80 nm as shown in Fig. 4b, suggesting
aKey Laboratory for Supramolecular Structure and Materials of
Ministry of Education, Jilin University, Changchun, China.
E-mail: wulx@jlu.edu.cn; Fax: +86 431 5193421; Tel: +86 431 5168481
bDepartment of Material Science and Engineering, Jilin University,
Changchun, China
{ Electronic supplementary information (ESI) available: detailed synthesis
10.1039/b503550c
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 3785–3787 | 3785
Li, Wen
