The nematic discotic phase in materials containing a siloxane core

Article abstract of DOI:10.1080/15421400490435404

The synthesis of a series of novel multipodes based on disc-shaped mesogens with a siloxane core is described. Their phase behaviour is characterised, and compared to the liquid crystalline properties of the discotic molecules without the siloxane moietie

Full text of DOI:10.1080/15421400490435404

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The Nematic Discotic Phase in
Materials Containing a Siloxane
Core
a
a
a
Paul Kouwer , Wolter Jager , Wim Mijs , Stephen
a
b
b
Picken , Keith Shepperson & Georg Mehl
a
Polymer Materials and Engineering, Delft University
of Technology, Delft, BL, The Netherlands
b
Department of Chemistry, University of Hull, Hull,
United Kingdom
Published online: 18 Oct 2010.
To cite this article: Paul Kouwer , Wolter Jager , Wim Mijs , Stephen Picken ,
Keith Shepperson & Georg Mehl (2004) The Nematic Discotic Phase in Materials
Containing a Siloxane Core, Molecular Crystals and Liquid Crystals, 411:1, 377-385,
DOI: 10.1080/15421400490435404
To link to this article: http://dx.doi.org/10.1080/15421400490435404
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Mol. Cryst. Liq. Cryst., Vol. 411, pp. 377=[1419]–385=[1427], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490435404
THE NEMATIC DISCOTIC PHASE IN MATERIALS
CONTAINING A SILOXANE CORE
Paul H. J. Kouwer, Wolter F. Jager, Wim J. Mijs, and Stephen J. Picken
Delft University of Technology, Polymer Materials and Engineering
Julianalaan 136, 2628 BL Delft, The Netherlands
ꢀ
Keith J. Shepperson and Georg H. Mehl
University of Hull, Department of Chemistry, Cottingham Road,
Hull, HU6 7RX, United Kingdom
The synthesis of a series of novel multipodes based on disc-shaped mesogens
with a siloxane core is described. Their phase behaviour is characterised
and compared to the liquid crystalline properties of the discotic molecules
without the siloxane moieties. We observed that incorporation of the siloxane
group results in a strong decrease of all transition temperatures, resulting
for one series in (close to) room temperature nematic mesophases. In a second
series all liquid crystalline properties were suppressed. Introduction of
nematic phases was accomplished via the formation of charge transfer
complexes.
Keywords: charge transfer complexes; discotic; liquid crystals; mesomorphic properties;
nematic; siloxane
INTRODUCTION
Discotic liquid crystalline materials were first discovered twenty-five years
ago. Among the many architectural motifs that have been published since
then, surprisingly few display the nematic (N or N ) mesophase [1].
D
Disc-shaped mesogens that show a nematic phase, usually features this
phase at high temperatures. These high temperatures have been reduced
by means of asymmetrical substitution of the mesogen [2]. Attachment of
the mesogen to a side chain polymer [3] and application of charge transfer
complexes [4].
We would like to thank the Ramsay Memorial Fellowship Trust (PK) and QinetiQ–Malvern,
UK (KJS) for financial support.
ꢀ
Corresponding author. Fax: þ 44 1482 466411, E-mail: g.h.mehl@hull.ac.uk
377=[1419]

3
78=[1420]
P. H. J. Kouwer et al.
Multipodes, wherein several mesogens are hold by one single core, offer
the advantages of low molar mass systems (pure, monodisperse molecules)
and polymers (suppression of crystallisation). The use of siloxanes as core
material is appealing, since these materials show a substantial suppression
of crystallisation [5a]. Moreover, they tend to (micro-)phase segregate,
which could lead to new, fascinating mesophases [5b,c].
In this paper, we present the synthesis and liquid crystalline properties
of the first discotic multi- and monopodes, based on a siloxane core. Two
different mesogens were applied in this study, A1 and B1, both showing
nematic discotic mesophases. Charge transfer complexes, resulting from
the non-covalent interactions between the mesogens and TNF are used
to modify the phase behaviour of monomers and dimers.
RESULTS AND DISCUSSION
Synthesis
The synthesis of hydroxyl functionalised mesogen A1 shown in Figure 1
and the corresponding propionate ester A2 has been discussed before
[
3a]. The adipic ester dimer was prepared by a straightforward esterifi-
cation reaction between the mesogen (small excess) and the adipoyl chlor-
ide, see Scheme 1. Subsequent purification by column chromatography
yielded the dimer A3 in a good yield.
Siloxane moieties incorporate into the spacer by means of high yielding
hydrosilylation reactions, requiring a slight extension of the spacer, see
Scheme 2. Pentenoic acid was protected with hexamethyldisilazane,
preventing the acid to complex with the platinum in Karstedt’s catalyst.
The protected compound was then attached to the siloxane group via
the hydrosilylation reaction. After deprotection of the acid under basic
conditions, coupling reactions under standard esterification conditions
with A1 or B1 yielded the siloxane multipodes. It is emphasised that the
synthesis was performed in this order. Extensive testing showed that the
internal triple bonds of the mesogens A1 and B1 are too susceptible to
carry out the hydrosilylation reaction with the siloxane group.
Phase Behaviour
The phase behaviour of the methyl-substituted mesogens (series A) is
summarised in Table 1. Mesogens A1–A3 behave qualitatively as was
expected: For A2 the transition temperatures drop, due to the presence
of the longer spacer and the absence of the stabilising hydrogen bonds,
compared to A1. Linking two mesogens together as in A3 results in a rise
in the transition temperatures, because of the reduced mobility of the

The Nematic Discotic Phase in Siloxane Core
379=[1421]
SCHEME 1 Synthesis of the short-spacer dimer and the corresponding monomer.
a) Propionyl chloride, triethylamine, 4-(dimethylamino)pyridine, CH Cl . (b)
Adipoyl chloride, 4-(dimethylamino)pyridine, triethylamine, CH Cl .
(
2
2
2
2
mesogens. The liquid crystalline properties of the mesogens with the
siloxane groups are fully suppressed. Earlier, we described the nearly linear
dependence of the clearing temperature TNI with the fraction of rigid meso-
gen in the material. Obviously, this fraction in the siloxane compounds
strongly reduced. The crystallisation temperature, however, is much less
dependent on the rigid mesogen fraction, which resulted in loss of the
liquid crystalline properties.
Similar characteristics are observed in the phase behaviour of the
hexyloxy-substituted mesogens (series B), which are summarised in
Table 2. The clearing temperatures of the mesogens with the siloxane
groups are strongly reduced with respect to mesogen B1. However, in this
series, the melting temperatures have decreased considerably as well,
resulting in stable nematic phases at low temperatures. In fact, we believe

3
80=[1422]
P. H. J. Kouwer et al.
SCHEME 2 Synthesis of the multipodes with siloxane cores. (a) Hexamethyl-
silazane, THF. (b) Pentamethylsiloxane (monomer) or tetramethylsiloxane (dimer)
or tetrakis(dimethylsiloxy)silane (tetramer), Karstedt’s catalyst, toluene. (c) A1 or
2 2
B1, diisopropylcarbodiimide, 4-(dimethylamino)pyridine, CH Cl .
that B2 is the second material showing a nematic discotic phase at room
temperature [6]. Again the dimer B3 shows some stabilisation of the meso-
phase, compared to B2. The reduction of the clearing temperature of B4
can be explained by realising that the central siloxane group is larger in
the case of the tetramer.
TABLE 1 Phase Behaviour of the Methyl-Substituted Mesogens
a,b
Mesogen
Phase behaviour
A1
A2
A3
A4
A5
K
K
K
K
K
174
155
169
135
142
N
N
N
226
196
209
I
I
I
I
I
a
K ¼ Crystalline, N ¼ Nematic discotic, I ¼ Isotropic.
b
ꢁ
Phase transitions in C.

The Nematic Discotic Phase in Siloxane Core
381=[1423]
TABLE 2 Phase Behaviour of the Hexyloxy-Substituted Mesogens
a,b
Mesogen
Phase behaviour
B1
B2
B3
B4
K
K
71
11
34
26
N
N
N
N
117
I
I
I
I
24
55
45
K
K
1
1
K
K
2
38
34
2
a
K ¼ Crystalline, N ¼ Nematic discotic, I ¼ Isotropic.
b
ꢁ
Phase transitions in C.
X-ray Diffraction
Various samples have been subjected to X-ray diffraction measurements.
ꢁ
The results for B3 are shown in Figure 2. At 50 C, a characteristic pattern
for a nematic phase is observed: The diffuse reflection at small diffraction
˚
angles (d ¼ 22.5 A), corresponds to the lateral disc-disc distance, while the
˚
diffuse reflection at larger angles (d ¼ 4–4.5 A) matches sum of the p-p dis-
tance between two mesogens and the characteristic alkyl-alkyl distances.
˚
the reflection of the siloxane groups (d ¼ 7.8 A) is observed as a shoulder
in the latter reflection, indication that in the N phase microphase separ-
D
ation occurs at least to some extent.
Dimer B3 shows an extra phase between the crystalline and the nematic
discotic phase, which could not be identified by its optical texture. How-
ever, the presence of a large number of reflections in the small and wide
angle region indicates that this phase can be identified as a second crystal-
line phase.
Charge Transfer Complexes
Specific interactions, such as charge transfer (CT) interactions [4a] have
shown to introduce liquid crystalline properties in non-mesogenic materials
FIGURE 1 Structure of the investigated mesogens.

3
82=[1424]
P. H. J. Kouwer et al.
ꢁ
FIGURE 2 X-ray diffraction pattern of the siloxane dimer B3: (a) at 50 C (ND
ꢁ
phase) and (b) at 28 C (Col =K phase).
X
2
[
4,7]. We have studied the results of these non-covalent interactions be-
tween the methyl-substituted mesogens (series A) and the electron de-
ficient 2,4,7-trinitro-9-fluoreneone (TNF). At equimolar ratios, all CT
complexes show a N mesophase, see Table 3. This result is particularly
D
interesting for the complexes with siloxane mesogenics (in which all liquid
crystalline properties have disappeared), since in these materials the meso-
phase is induced by mixing two crystalline compounds.
The combination of DSC studies of various complexes and carefully
prepared contact samples between the methyl-substituted mesogens and
TNF allowed us to construct schematic phase diagrams, see Figures 3
and 4. Addition up to 50 mol-% of TNF [8] to mesogens A2 or A3 hardly
effects the clearing temperature of these materials. The melting tempera-
ture of the crystalline phase is slightly suppressed – resulting in wider
TABLE 3 Phase Behaviour of the Charge Transfer Complexes of A2–A5 with TNF
a,b
Complex
Phase behaviour
TNF
K
K
K
K
K
176
106
135
123
127
I
I
I
I
I
A2:TNF
A3:TNF
A4:TNF
A5:TFN
N
N
N
N
182
193
162
163
a
K ¼ crystalline, N ¼ nematic discotic, I ¼ isotropic.
b
ꢁ
Phase transitions in C.

The Nematic Discotic Phase in Siloxane Core
383=[1425]
FIGURE 3 Schematic phase diagrams of TNF with the short-spacer dimer A3
right) and the corresponding monomer A2 (left).
(
mesophase – and biphasic areas are observed at low TNF fractions.
Complexes at exact equimolar concentrations do not show any phase
separation during microscopy experiments and the transitions observed
in DSC experiments are sharp and well defined. Addition of more TNF
results in a strong destabilisation of the nematic phase and complexes
with more than 75 mol-% TNF immediately macrophase seperate into
fractions of pure TNF and complexes containing about 75 mol-% TNF.
The sketched phase diagrams of the siloxane multipodes with TNF look
rather similar. However, in these complexes the nematic phase is only
observed at TNF concentrations between 25 to 75 mol-%. Less than
2
5 mol-% or more than 75 mol-% of TNF again leads to macroscopic
demixing, even in the isotropic phase. It is interesting to note that the max-
ima in the clearing temperatures of both complexes occur at equimolar
FIGURE 4 Schematic phase diagrams of TNF with the siloxane dimer A5 (right)
and the corresponding monomer A4 (left).

3
84=[1426]
P. H. J. Kouwer et al.
ratios, indicating an optimal 1:1 interaction between the mesogenic
moieties and the TNF molecules. Optimal properties at 1:1 ratios have
been reported for other charge transfer complexes before, but remark-
ably, in other cases the best properties were obtained from deviating
ratios [7b,c].
CONCLUSIONS
We have synthesised the first series of multipodes consisting of a siloxane
core and discotic mesogens. The siloxane group lowers all transition tem-
peratures, in particular the clearing temperatures. This resulted in a total
shift of the nematic phase to lower temperatures for the hexyloxy-substi-
tuted mesogens (series B). To the best of our knowledge, mesogen B2 is
the second mesogen that displays a nematic discotic phase at room tem-
perature. For the methyl-substituted mesogens (series A) introduction of
the siloxane groups resulted in the loss of all liquid crystalline properties.
Addition of TNF resulted in the formation of charge transfer complexes,
which gave rise to reintroduction of liquid crystalline properties. The best
properties were observed at 1:1 donor-acceptor complexes. We believe that
the application of siloxanes is a step forward towards room temperature ne-
matic discotic materials, provided that the melting temperature of the crys-
talline phase can be suppressed equally effective.
REFERENCES AND NOTES
[
1] (a) Cammidge, A. N. & Bushby, R. J. (1998). In: Handbook of Liquid Crystals, Demus, D.,
Goodby, J. W., Gray, G. W., Spiess, H. W., & Vill, V. (Eds.), V. Wiley VCH: New York, Vol.
2B, 693.
(
b) Praefcke, K. (2001). In: Physical Properties of Liquid Crystals: Nematics, Dunmar,
D. A., Fukuda, A., & Luckhurst, G. R. (Eds.), INSPEC, London, 17.
[
2] (a) Janietz, D., Praefcke, K., & Singer, D. (1993). Liq. Cryst ., 13, 247.
b) Kumar, S. & Varshney, S. K. (2000). Angew. Chem. Int. ed. Engl ., 39, 3140.
3] (a) Kouwer, P. H. J., Jager, W. F., Mijs, W. J., & Picken, S. J. (2000). Macromolecules, 33,
336.
b) Favre-Nicolin, C. D. & Lub, J. (1996). Macromolecules, 29, 6143.
(
[
4
(
[
4] (a) Complexes of an electron-rich donor with an electron rich acceptor, showing an
induced absorption band are often referred to as charge transfer complexes. However,
it should be stressed that the attractive interaction between these molecules is rather
a combination of Van der Waals, quadrupolar and charge transfer interactions. For the
sake of clarity, in this contribution the name charge transfer complex is used to
describe the total sum of interactions.
(
b) Kouwer, P. H. J., Van den Berg, O., Jager, W. F., Mijs, W. J., & Picken, S. J. (2002).
Macromolecules, 35, 2576.
[
5] (a) Els a¨ sser, R., Mehl, G. H., Goodby, J. W., & Veith, M. (2001). Angew. Chem. Int. Ed.
Engl ., 40, 2588.

The Nematic Discotic Phase in Siloxane Core
385=[1427]
(
b) Kreuder, W. & Ringsdorf, H. (1990). Makromol. Chem ., 102, 199.
c) Tschierske, C. (1998). J. Mater. Chem, 8, 1485.
6] Kumar, S. & Varshney, S. K. (2000). Angew. Chem. Int. ed. Engl ., 39, 3140.
(
[
[
7] (a) Ringsdorf, H., W u¨ stefeld, R., Zerta, E., Ebert, M., & Wendorff, J. H. (1989). Angew.
Chem. Int. ed. Engl ., 28, 914.
(
b) Ringsdorf, H. & W u¨ stefeld, R. (1990). Phil. Trans. R. Soc. Lond A ., 330, 95.
c) Praefcke, K. & Holbrey, J. D. (1996). J. Inclusion Phenom., 24, 19.
8] In all complexes the discussed ratios are the molar ratios of the mesogenic groups and
(
[
TNF, rather than the actual molecular ratios.