Nematic silsesquioxanes - Towards nanocrystals dispersed in a nematic liquid crystal matrix

Article abstract of DOI:10.1039/b001951h

The reaction of suitable organic groups in combination with well defined inorganic silsequioxane cores leads to covalently bound organic-inorganic hybrid materials, which exhibit nematic and smectic C phase behaviour close to room temperature.

Full text of DOI:10.1039/b001951h

Nematic silsesquioxanes—towards nanocrystals dispersed in a nematic liquid
crystal matrix
Ralf Elsäßer,a Georg H. Mehl,*a John W. Goodbya and Demetri J. Photinosb
a Department of Chemistry, Hull University, Hull, UK HU6 7RX. E-mail: G.H.Mehl@chem.hull.ac.uk
b Department of Physics, University of Patras, Patras, Greece
Received (in Oxford, UK) 10th March 2000, Accepted 4th April 2000
The reaction of suitable organic groups in combination with
well defined inorganic silsequioxane cores leads to cova-
lently bound organic–inorganic hybrid materials, which
exhibit nematic and smectic C phase behaiour close to room
temperature.
obtained by esterification of 2 with 3, the product of the
esterification of octyloxybenzoic acid with one of the phenol
groups of 4,4A-biphenol. The attachment of a 1,1,3,3-tetra-
methyldisiloxane group B to 4 in a hydrosilylation reaction
using Karstedt’s catalyst leads to the functionalised monomer
5.
Much attention has been paid to the investigation of nanoscopic
organic–inorganic hybrids which are amorphous, crystalline or
which exhibit bicontinuos, columnar or lamellar morpholo-
gies.1–4 This makes the investigation of systems where a
covalently attached inorganic core is dispersed homogeneously
in a nematic matrix a fascinating proposition.
Our approach is based on molecular designed LC materials
where it has been established that the linkage of polyphilic
groups to nematogenic molecules leads to layered structures.5,6
This requires a systematic approach to arrive at the targeted
nematic materials based on crystalline silsesquioxane cores A,
shown in Fig. 1 as a model system for a monodisperse inorganic
crystal of defined stereochemistry. The linkage of the liquid-
crystalline rod-shaped mesogenic moieties to the inorganic
cores is effected by spacers consisting of organic methylene
groups bonded via 1,1,3,3-tetramethyldisiloxane groups to the
core. Eight mesogenic groups are appended from the cuboid
core leading to a molecular structure, where in the LC phase the
nematic director field is responsible for the deformation of the
spatially isotropic molecules to assemble into the nematic phase
structure.
The structurally related mesogen 6 containing only three
aromatic rings was synthesised as published earlier and
hydrosilylated as described for 4 leading to the material 7.2
In order to assess whether the phase behaviour is determined
by microphase separation or by the size of the relatively bulky
methyl groups of the siloxane units, 4 was reacted with
1,1,3,3,3-pentamethyldisiloxane C leading to material 8, an
anologue to compound 5. The reaction of 6 with C in a similar
manner yields compound 9, structurally similar to material 6.
In order to assure the formation of liquid-crystalline phases
above ambient temperature aromatic core structures of the
mesogenic units were selected containing three and four
aromatic rings, linked by C–C single bonds or ester groups
shown in Scheme 1. In order to assure a suitable stability range
of the liquid-crystalline phase terminal alkyl chains of eight or
eleven methylene groups were selected. The lateral attachment
of the mesogens to the central core was chosen to favour side-on
interactions of the mesogens following an established concept.7
A spacer of five methylene groups and a siloxane unit was used
to obtain a suitable degree of decoupling of the rigid aromatic
mesogens and the silsesquioxane crystalline core.
The synthesis of the functionalised monomer 5 is shown in
Scheme 1. Alkylation of 2,4-dihydroxymethylbenzoate 1 with
1-bromooctane in the p-position of the aromatic ring and
subsequently with 5-bromopent-1-ene in the o-position fol-
lowed by removal of the protective methyl group leads to the
intermediate 2. Compound 4 containing four aromatic rings was
Scheme 1 Reagents and conditions: i, K2CO3, KI, butanone, 1-bro-
mooctane; ii, K2CO3, KI, butanone, 5-bromopent-1-ene; iii, NaOH–MeOH,
HCl–H2O; iv, DCC–DMAP, 3; v, 1,1,3,3-tetramethyldisiloxane, Karstedt’s
catalyst.
Fig. 1 Structures of the siloxane and silsequioxane cores.
DOI: 10.1039/b001951h
Chem. Commun., 2000, 851–852
This journal is © The Royal Society of Chemistry 2000
851

Table 1 Transition temperatures as determined by DSC
of the organic and siloxane groups. The increase in the melting
points for 8 (76.6 °C cf. 67.1 °C for 5) and 9 (24.8 °C cf. 18 °C
for 7) can be attributed to the more bulky siloxane groups,
leading to more stable crystalline phases.
Compound
Structure
Phase transitions/°C
4
5
6
4
4-B
6
cr 99.4 N 173.2 Iso
cr 67.1 [SmC 51.3] N 123.8 Iso
cr 53.8 N 72.7 Iso
The material 10 containing an inorganic core has an
isotropisation temperature of 145.8 °C, a rise by 22 °C
compared to the monomer 5, indicating that the incorporation of
a suitably functionalised inorganic core can promote the
stability of the mesomorpic state. The increase of the stability
range of the smectic C phase by 60 °C to 111.3 °C observable
for material 10 is remarkable. This is accompanied by a fall of
the crystallisation temperature to 59.5 °C of this structure.
Additionally a monotropic highly ordered LC phase of
undetermined structure was observed below 33.1 °C.
A special feature of the cuboid material 11, where the
inorganic core is decorated with eight mesogens containing
three aromatic is the absence of a crystalline state, only a low
glass transition temperature of 219.3 °C could be observed.
The material clears from the nematic phase to the isotropic state
at 50.5 °C and shows an underlying smectic phase (smectic X)
at 37.6 °C, confirming thus the versatility of the selected
systematic approach, geared towards low temperature organic–
inorganic anisotropic fluids incorporating nanocrystalline
cores.
7
6-B
cr 18.0 N 39.1 Iso
8
9
4-C
6-C
cr 76.6 [SmC 58.2] N 122.0 Iso
cr 24.8 N 39.5 Iso
10
11
[4-B]8-A
[6-B]8-A
cr 59.5 [SmX 33.1] SmC 111.3 N 145.8 Iso
Tg 219.3 SmX 37.6 N 50.5 Iso
The organic–inorganic hybrid materials containing a central
silsesquioxane core were obtained by reacting octavinylsil-
sesquioxane A with 5 in a hydrosilylation reaction leading to
material 10. The bond formation could be monitored by the
occurrence of the signals for the ethylene bridge in the 1H NMR
spectrum and unambiguously in this context by the appearance
of peaks at d 4.1 and 9.8 in the 13C NMR spectra. The fusion of
the inorganic core C in a similar manner with the functionalised
mesogenic side-chain 7 containing three aromatic rings resulted
in compound 11. The materials were purified by column
chromatography, indicating that the incorporation of siloxane
groups in the spacer alters the solubility of these materials, when
compared to related structures.8,9 The eluent for material 11 was
dichloromethane–hexane with the mixture gradient changed
from 1+1 to 4+1 during the elution.
We acknowledge the EC for funding in the framework of the
TMR network ‘Molecular Design of Functional Liquid Crys-
tals’ and thank the members of the network for the many helpful
discussions.
The transition temperatures of the liquid-crystalline materials
4–11 are listed in Table 1. All of these materials exhibit a
nematic phase as the highest stable liquid-crystalline phase,
characterised by a typical schlieren texture when observed
using optical polarising microscopy.
For the four-ring system 4 the side-on attachment of the
microphase seperating tetramethylsiloxane group leads to
a marked decrease of the isotropisation temperature from
173.2 °C for 4 to 123.8 °C for compound 5, a fall of 49.4 °C.
For the three-ring system 6 with an isotropisation temperature
of 72.7 °C fusion to be tetramethylsiloxane group, leading to
compound 7, results in a decrease of the clearing temperature to
39.1 °C, a fall of 33.6 °C.
This modification of the mesogenic moieties by siloxane
groups is additionally accompanied by a strong fall in the
melting temperatures to 67.1 °C for 5 (99.4 °C for 4) and to
18.0 °C for 7 (53.8 °C for 6) when compared to the starting
materials. An interesting feature is the occurrence of a low
temperature smectic C phase characterised by broken focal
conics and a schlieren texture for material 5, a feature very
unusual in rod shaped materials with a lateral side-chain.10–12
The structurally related materials containing a pentame-
thyldisiloxane endgroup, 8 and 9, have similar isotropisation
temperatures as observed for 5 and 7 containing tetra-
methyldisolaxane groups, indicating that the liquid-crystal
phase behaviour is governed mainly by microphase separation
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Chem. Commun., 2000, 851–852