Polyphilic multicomponent dimers with perfluorinated cores

Article abstract of DOI:10.1080/15421400490435026

The linking of segments, which have a tendency to phase separate at suitable temperatures, by covalent bonding has been employed in the synthesis of inorganic-organic hybrids. This approach can be utilised to tailor the stability range and the properties (eg viscosity) of liquid crystal materials. Research into such materials has focused strongly on the combination of calamitic moieties and silicones, however the introduction of a third phase separating segment, of which perfluorinated carbon chains are a good example, is an attractive area of research. In this paper we will report the synthesis and characterisation of materials incorporating a range of calamitic mesogens. These are attached either terminally or laterally to flexible linear chains containing alkyl and siloxane units fused to a central perfluorinated chain. The phase behaviour of the investigated materials is characterised by nematic and smectic phases with transition temperatures close to ambient. The size of the different linear units was varied systematically in order to investigate their contribution to the mesomorphic phase behaviour. The liquid-crystalline properties, investigated by optical polarising microscopy, differential scanning calorimetry and X-ray diffraction will be discussed.

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Polyphilic Multicomponent
Dimers with Perfluorinated
Cores
a
a
a
K. J. Shepperson , T. Meyer & G. H. Mehl
a
Department of Chemistry , University of Hull ,
Hull, UK
Published online: 18 Oct 2010.
To cite this article: K. J. Shepperson , T. Meyer & G. H. Mehl (2004) Polyphilic
Multicomponent Dimers with Perfluorinated Cores, Molecular Crystals and Liquid
Crystals, 411:1, 185-191, DOI: 10.1080/15421400490435026
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Mol. Cryst. Liq. Cryst., Vol. 411, pp. 185=[1227]–191=[1233], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490435026
POLYPHILIC MULTICOMPONENT DIMERS WITH
PERFLUORINATED CORES
K. J. Shepperson, T. Meyer, and G. H. Mehl
Department of Chemistry, University of Hull, Hull, HU6 7RX, UK
The linking of segments, which have a tendency to phase separate at suitable
temperatures, by covalent bonding has been employed in the synthesis of inor-
ganic-organic hybrids. This approach can be utilised to tailor the stability
range and the properties (eg viscosity) of liquid crystal materials. Research
into such materials has focused strongly on the combination of calamitic moi-
eties and silicones, however the introduction of a third phase separating seg-
ment, of which perfluorinated carbon chains are a good example, is an
attractive area of research. In this paper we will report the synthesis and
characterisation of materials incorporating a range of calamitic mesogens.
These are attached either terminally or laterally to flexible linear chains con-
taining alkyl and siloxane units fused to a central perfluorinated chain. The
phase behaviour of the investigated materials is characterised by nematic
and smectic phases with transition temperatures close to ambient. The size
of the different linear units was varied systematically in order to investigate
their contribution to the mesomorphic phase behaviour. The liquid-crystalline
properties, investigated by optical polarising microscopy, differential scan-
ning calorimetry and X-ray diffraction will be discussed.
Keywords: liquid crystal; micro-scgregation; perfluorinated; self-organisation; siloxane
INTRODUCTION
The influence of molecular geometry upon liquid crystalline phase behav-
iour is well understood and has been investigated thoroughly [1,2]. Less
attention has been paid in the past to the question of miscibility between
covalently bonded groups in systems that can lead to microphase separ-
ation [3]. In recent years this question has been addressed with studies
of siloxane and perfluorinated groups linked to conventional mesogens
We thank Paul Kouwer for the XRD studies. We also thank Quinetiq and the EU for their
financial support of this project.
Address correspondence to G. H. Mehl, Department of Chemistry, University of Hull, Hull,
HU6 7RX UK
185=[1227]

1
86=[1228]
K. J. Shepperson et al.
[4,5,6]. Research on polyphilic materials that combine both perfluorinated 30
and siloxane moieties in the same system is thus attractive.
For the investigation of materials containing a range of microphase
separating groups on the liquid crystalline phase behaviour; perfluori-
nated groups and siloxane moieties were selected in conjunction with
conventional calamitic mesogens. In order to examine linear dimeric 35
materials the cyanobiphenyl group 1 was selected, as it allows a compari-
son with a large number of related systems. The cyanobiphenyl group was
attached to a hydrocarbon chain of five or eleven methylene units with an
ether linkage as shown for molecules 2 and 3 depicted in Scheme 1.
The mesogens were bonded to a disiloxane, trisiloxane or tetrasiloxane 40
moiety in a hydrosylilation reaction leading to materials 4, 5, 6, 7 and 8
in a convergent synthesis. A perfluorinated diol containing either eight,
compound 9, or twelve difluoromethane units, material 10, were chosen
SCHEME 1 Synthesis of linear polyphilic systems.

Polyphilic Multicomponent Dimers with Perfluorinated Cores 187=[1229]
and both alcohol functions were connected by an esterification reaction
using either pentenoic or undecenoic acid to give the central perfluorinated
cores 11, 12, 13, and 14. The mesogenic groups 4, 5, 6, 7 and 8 were sub-
sequently linked with the siloxane unit to the central perfluorinated core
via a hydrosilylation reaction leading to the final products 15, 16, 17, 18,
1
2
9, 20, 21, 22, 23, 24, and 25, whose properties are listed in Figures 1,
, and 3.
In order to evaluate the influence of a modulation in the architecture
of the mesogens a laterally functionalised mesogen 26 was connected to
the perfluoralkyl cores 11, 12, 13 and 14 leading to the target com-
pounds 27, 28, 29 and 30 shown in Scheme 2 and where the properties
are listed (Fig. 4) [7,8]. The mesogen selected is characterised consisting
of four aromatic rings in the central core with terminal alkoxy chains of
eight methylene units, and a short spacer consisting of a tetramethyldisi-
loxane unit and five methylene groups linked to the aromatic core via an
ether linkage. This compound was used previously for the investigation of
nematic multipodes and dendrimers [8].
SCHEME 2 Synthesis of laterally connected dimers.

1
88=[1230]
K. J. Shepperson et al.
RESULTS
The materials of an overall linear shape with a perfluoralkyl core where
x¼10 CF units showed a smectic A phase, which increases in stability
2
as the alkyl spacer lengths n and m were increased from five to eleven
units. A highly ordered smectic phase was observed for compounds 17
and 18, which was dependent on the length of the spacer between the
biphenyl and the siloxane moiety.
With a shorter perfluorinated core of six CF units an increase of the
2
transition temperatures of the smectic A phase can be observed, which is
less stable than for the materials with a longer central perfluorinated link-
ing group. Additionally the highly ordered smectic X phase is only observed
for material 21.
The size of the siloxane group, z, was increased in order to observe the
effect on liquid crystalline behaviour in these materials. When comparing
the effect of siloxane and perfluorinated groups on the liquid crystalline
phase behaviour, it is noticeable that the enlargement of siloxane moiety
is considerably more effective in lowering the transition temperatures than
a decrease in the size of the perfluorinated unit. This is shown by material
2
3 (Figure 3).
FIGURE 1 Linear materials with a central perfluorinated core of 10 CF units.
2
2
FIGURE 2 Linear materials with a central perfluorinated core of six CF units.

Polyphilic Multicomponent Dimers with Perfluorinated Cores 189=[1231]
FIGURE 3 Linear materials with an enlarged siloxane unit.
FIGURE 4 Lateral materials with a central perfluorinated core of six or ten CF2
units.
The transition temperatures of the side-on mesogens was lowered by
attachment to the perfluorinated siloxane core and increased by the
lengthening of the alkyl spacer m. The effect on the liquid crystalline
behaviour of altering the spacer and core in the lateral materials is less
dramatic than observed in the linear materials.
CONCLUSIONS
A series of polyphilic dimeric materials were synthesised and their phase
behaviour examined. The central perfluorinated chain proved to be a suit-
able tool for lowering the transition temperatures of smectic A and nematic
phases of conventional mesogenic groups. With a suitable length of alkyl
spacer in the appropriate position the materials also exhibited a highly
ordered smectic X phase. The effect of this core on a number of architec-
tures proved to be quite strong for the linear materials. For the laterally
connected dimers the lowering of transition temperatures is much less
noticeable. These results are in accordance with earlier research, where

1
90=[1232]
K. J. Shepperson et al.
it was observed that for suitabley slecleted laterally attached systems there
is little effect on the liquid crystalline behaviour when they are attached to
central core units.
EXPERIMENTAL
The synthetic procedure for compound 15 is given as an example of the
synthesis of the dimeric materials.
0
Preparation of 4 -Pent-4-enyloxy-biphenyl-4-carbonitrile (2)
5
-bromo-1-pentene (7 g, 0.0469 mol) was added to 1 (9.16 g, 0.0469 mol) in
butanone (150 ml), potassium iodide (1.00 g) and potassium carbonate
25.88 g, 0.187 mol). The reaction mixture was heated under reflux for
4 hours. The inorganic materials were removed by filtration and the sol-
(
2
vent removed in vacuo. The crude product was recrystallised twice from
methanol to yield yellow=white crystals. Yield: 83%, 10.27 g. Transition
1
Temperatures: K 83.8 N 89.0 I. NMR H: 1.95 (2H, qu), 2.3 (2H, q), 4.0
(
1
2H, t), 5.05 (2H, m), 5.9 (1H, m). IR: 3414, 2932, 2227, 1641, 1603,
531, 1470, 1184. Mass Spec: 69, 140, 151, 166, 195, 263.
0
Preparation 4 -[5-(1,1,3,3-Tetramethyl-disiloxanyl)-pentyloxy]-
biphenyl-4-Carbonitrile (4)
2
(
0
(5 g, 0.0189 mol) was mixed with THF (20 ml) and Karstedt’s catalyst
50 ml) added then this was added to 1,1,3,3-tetramethyl disiloxane (25 g,
.189 mol) and THF (20 ml). The reaction mixture was left to stir over-
night. The solvent was then removed in vacuo. The crude product was
purified by column chromatography (silica, DCM, hexane 3:2) to yield a
white room temperature liquid crystalline material. Yield: 41%, 3.09 g.
1
Transitions: K -14.1 SmA 46.1 I. NMR H: 0.0 (8H, m), 0.1 (4H, m), 0.5
(2H, t), 1.4 (4H, m), 1.8 (2H, qu), 4.0 (2H, t), 4.6 (1H, m), 6.9 (2H, d),
1
3
7
1
.5 (2H, d), 7.6 (4H, q). NMR C: 18, 23, 53 (5C), 59, 110, 117, 119,
24,125 147,155.
Preparation of Pent-4-enoic acid 1H, 1H, 12H, 12H perfluro-
pent-4-enoyloxy-dodecyl ester (11)
9
(1 g, 0.00177 mol) was added to 4-pentenoic acid (0.45 g, 0.00453 mol)
with DMAP (0.29 g, 0.00226 mol) and PTSA (0.43 g, 0.00226 mol), this
was dissolved in DCM (50 ml). After stirring for 5 mins DIC (1.38 g,
0
.0113 mol) was added to the solution and the reaction mixture left to stir

Polyphilic Multicomponent Dimers with Perfluorinated Cores 191=[1233]
for 24 hrs. The reaction mixture was filtered to remove the DCU by-product
and the solvent removed in vacuo. The crude product was purified by col-
umn chromatography (DCM, Silica) to yield a white crystalline solid. Yield:
0
.90 g, 70%. Transition Temperatures: K 53.1 I
H NMR: 2.4 (2H, q), 2.55 (2H, t), 4.6 (2H, t), 5.05 (2H, t), 5.9 (1H, m).
Mass spec: 83, 94,164, 564, 601, 643, 684, 700, 726.
Preparation of (15)
1
1 (0.1 g, 0.190 mmol) was dissolved in THF (5 ml) and Karstedt’s catalyst
(20 ml) added. The solution had air bubbled through it for 30 secs then it
was left to stir for 40 mins. 4 (0.22 g, 0.570 mmol) was added and the mix-
ture left to stir overnight. The solvent was removed in vacuo and the crude
product purified by column chromatography (silica, DCM=Hexane 2:3) to
give a clear liquid.
Yield: 0.08 g, 29%. Transition Temperatures: K 10.2 SmA 12.3 I.
1
NMR H: 0.0 (8H, m), 0.1 (4H, m), 0.5 (4H, t), 1.4 (8H, m), 1.8 (2H, qu),
m
4
2
.0 (2H, t), 4.5 (2H, t), 6.9 (2H, d), 7.5 (2H, d), 7.6 (4H, q). IR (KBr) max:
931, 2862, 2231, 1766, 1618, 1499, 1474, 1257.
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