Nematic 2,5-disubstituted thiophenes

Article abstract of DOI:10.1039/b202073b

A large number of new liquid crystals incorporating the 2,5-disubstituted thiophene ring have been prepared and their mesomorphic behaviour studied in order to systematically investigate the correlation between the molecular structure and mesomorphism of thiophene derivatives with different shapes, polarisability and polarity. As a consequence of these investigations we have prepared a new class of liquid crystals incorporating a 2,5-disubstituted thiophene ring and a conjugated trans-carbon-carbon double bond in the terminal chain. These novel thiophene derivatives are the first liquid crystals incorporating a 2,5-disubstituted thiophene ring to exhibit a nematic phase at room temperature. This enables the flexoelectric coefficients of a bent-shaped molecule to be measured directly and at room temperature for the first time to the authors' knowledge. Many of these new thiophenes exhibit a high birefringence and a high nematic clearing point and are of potential use as components of nematic mixtures in LCDs.

Full text of DOI:10.1039/b202073b

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Nematic 2,5-disubstituted thiophenes{
Neil L. Campbell,a Warren L. Duffy,a Gareth I. Thomas,a Janine H. Wild,a
Stephen M. Kelly,b Kevin Bartle,b Mary O’Neill,b Vicky Minterc and Rachel P. Tuffinc
aThe Department of Chemistry. University of Hull, Kingston Upon Hull, UK HU6 7RX.
E-mail: s.m.kelly@hull.ac.uk
bThe Department of Physics. University of Hull, Kingston Upon Hull, UK HU6 7RX
cQinetiq, St. Andrews Road, Malvern, Worcs., UK WR14 3PS
Received 27th February 2002, Accepted 22nd May 2002
First published as an Advance Article on the web 16th July 2002
A large number of new liquid crystals incorporating the 2,5-disubstituted thiophene ring have been prepared
and their mesomorphic behaviour studied in order to systematically investigate the correlation between the
molecular structure and mesomorphism of thiophene derivatives with different shapes, polarisability and
polarity. As a consequence of these investigations we have prepared a new class of liquid crystals incorporating
a 2,5-disubstituted thiophene ring and a conjugated trans-carbon–carbon double bond in the terminal chain.
These novel thiophene derivatives are the first liquid crystals incorporating a 2,5-disubstituted thiophene ring
to exhibit a nematic phase at room temperature. This enables the flexoelectric coefficients of a bent-shaped
molecule to be measured directly and at room temperature for the first time to the authors’ knowledge. Many
of these new thiophenes exhibit a high birefringence and a high nematic clearing point and are of potential use
as components of nematic mixtures in LCDs.
The 2,5-disubstituted thiophene ring is a potentially useful
element of the molecular structure of new nematic liquid
crystals due to a number of factors:1–10 it is aromatic and rigid
in nature; the electron-rich sulfur atom in the thiophene ring
results in a dipole moment perpendicular to the long molecular
axis of the molecule, i.e., apolar derivatives are potentially
of negative dielectric anisotropy; the non-collinear nature of
the 2,5-disubstituted thiophene ring and the lower symmetry
clearing point of the phenyl diester 3 and the nitrogen
heterocycles 4 and 5 are similar. The bonds to the substituents
of these three six-membered aromatic rings are both parallel
and collinear. The 2,5-disubstituted-thiophene derivative
6 with two non-collinear and non-parallel bonds to the
substituents exhibits a clearing point about 100 uC lower
than that of any of the diesters 3–5. The clearing point of the
2,4-disubstituted thiophene 7 is even lower still due to a lower
length-to-breadth ratio. The furan 8 possesses the lowest
clearing point due to the even greater non-linear bond angle.
Therefore, the thiophene ring has generally not been regarded
as a standard molecular building block for liquid crystalline
components of nematic mixtures for LCDs. However, in spite
of this, a significant number of liquid crystals incorporating the
2,5-disubstituted thiophene ring and two other rings have been
prepared.5,11–34 Only a small number of thiophene derivatives
with two-rings in the molecular core have been reported as exhi-
biting a nematic phase, e.g., the 2-n-alkyl-5-(4-cyanophenyl)-
thiophenes exhibit a monotropic nematic phase just above
room temperature.23,24 A diverse range of two-ring thiophenes
with a thioether function joining the flexible terminal aliphatic
chain to the rigid aromatic core have also been prepared.28
Unfortunately, the melting point and clearing point of these
materials are relatively high due, at least in part, to the high
degree of anisotropy of polarisability contributed to by the
thioether function. Against this background it was decided
to synthesise a wide and diverse range of compounds incor-
porating the 2,5-disubstituted thiophene ring in an attempt to
establish the exact combination of structural elements of a two-
ring thiophene required to generate a low-melting, wide-range
enantiotropic nematic phase of low viscosity and high
birefringence of potential use for LCDs.34
generally result in
a low melting point for thiophene
derivatives.11–34 This is due to the large angle between the
thiophene ring and the bonds to the substituents at the 2- and
5-positions, see Table 1 and Fig. 1. However, this molecular
geometry has been found not to be conducive to mesophase
formation in general and nematic phase formation in
particular. This finding is illustrated by the liquid crystalline
transition temperatures, collated in Table 1, for a series of
diesters differing only in the nature of the central ring X,
including one or two thiophene rings.13,22 The collinear nature
of the bonds of the biphenyl central unit to the carboxy
substituents in the four-ring ester 1 results in a large length-
to-breadth ratio of the molecular structure containing four
collinear aromatic rings. The rod-like molecular shape of the
diester 1 also results in a high anisotropy of polarisability of the
delocalised p-electrons in the four conjugated 1,4-disubstituted
phenylene rings. The efficiency of packing of long, thin
calamitic molecules also increases as the length-to-breadth
ratio increases. The presence of strong local dipoles associated
with the carboxy linking groups conjugated with the phenyl
rings may also contribute to the high clearing point of these
esters. The parallel, but not quite collinear, bonds of the
corresponding bithiophene 2 give rise to a much lower clearing
point than that of the biphenyl 1. This is most likely due to the
broadening of the molecular rotation volume caused by the
shape of the two thiophene rings. This results in a lower
effective length-to-breadth ratio for the bithiophene 2. That the
lower clearing point is due to shape rather than the presence of
heteroatoms in the aromatic ring is shown by the fact that the
Synthesis
The known 2-pentylthiophene-5-carboxylic acid, prepared by
a literature method,35 was esterified in the usual way36 using
DCC/DMAP (1,3-dicyclohexylcarbodiimide/4-dimethylamino-
pyridine) with a range of alcohols and phenols to yield the
{Electronic supplementary information (ESI) available: extensive syn-
thetic information. See http://www.rsc.org/suppdata/jm/b2/b202073b/
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DOI: 10.1039/b202073b

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Table 1 Transition temperatures (uC) and the angle of deviation (w) from linearity of the bonds connecting the carboxy substituents to the central
ring X of the diesters 1–83,12
Cr
SmA
N
I
w/u
1
2
3
4
5
6
7
8
.
193
246
209
173
203
164
185
163
.
231
–
.
w400
283
.
0
~ collinear
~ parallel
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
0
| collinear
~ parallel
–
285
0
~ collinear
~ parallel
–
255
0
~ collinear
~ parallel
–
250
0
~ collinear
~ parallel
–
166
32
40
53
| collinear
| parallel
–
[86]a
[62]
| collinear
| parallel
–
| collinear
a[ ] Represents a virtual transition temperature.
esters 9–16 and 21–42. The 4-n-alkylphenols, 4-n-alkoxyphenols,
4-cyanophenol, 1-hydroxy-4-pentylbicyclo[2.2.2]octane were
commercially available. The 4-n-alkyl-4’-hydroxybiphenyls,37
4-(trans-4-n-alkylcyclohexyl)phenol,38 4-(4-pentylbicyclo[2.2.2]-
octyl)phenol,39 trans,trans-4-n-alkyl-4’-hydroxybicyclohexanes,40
4-n-alkyl-4’-hydroxytolanes,41 various halogenated 4-hydroxybi-
phenyls42 were prepared by literature methods or were supplied
by E. Merck. The 4-(5-n-alkylthien-2-yl)phenols required to
prepare the esters 9–13, 17–19, 71–76 and 83–102 and ethers
103–109 were prepared starting from 2-n-alkylthiophenes.
These were brominated using NBS (N-bromosuccinimide)
in a chloroform–acetic acid mixture43 to yield 2-n-alkyl-
5-bromothiophenes. A Suzuki palladium-catalysed aryl–aryl
cross-coupling reaction44,45 with 4-methoxyphenylboronic acid
gave the 2-(4-methoxyphenyl)-5-n-alkylthiophenes.29 Subse-
quent demethylation with boron tribromide46 yielded the
4-(5-n-alkylthien-2-yl)phenols. The 4-(5-heptylthien-2-yl)phenol
was prepared by a modified procedure. A Suzuki aryl–aryl
cross-coupling reaction using 4-methoxyphenylboronic acid
and 2-iodothiophene yielded 2-(4-methoxyphenyl)thiophene,
which was alkylated using BuLi/TMEDA (N,N,N’,N’-tetra-
methylethylenediamine) at 240 uC and 1-bromoheptane to
Fig. 1 Schematic representation of the deviation of the angle between the central ring X of the diesters 1–8 and the bonds to the carboxy group
from 180u.
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yield 2-heptyl-5-(4-methoxyphenyl)thiophene.29 Cleavage of
the methoxy group45 yielded 2-heptyl-5-(4-hydroxyphenyl)-
thiophene. The 2-cyano-5-(4-hydroxyphenyl)thiophene required
for the esters 77–82, was prepared by coupling of 2-bromo-
thiophene-5-carbonitrile with 4-methoxyphenylboronic acid
to give 2-(4-methoxyphenyl)thiophene-5-carbonitrile followed
by removal of the methyl group to give 2-(4-hydroxyphenyl)-
thiophene-5-carbonitrile. The benzoate 20 was prepared
analogously using 4-pentylbenzoic acid and 4-hydroxy-4’-
pentylbiphenyl.37 The bis-2,5-(4-pentylphenyl)thiophene 43
was synthesised from 2,5-dibromothiophene and 4-pentylphenyl-
boronic acid in an aryl–aryl cross-coupling reaction.44,45 The
tolane 44 was prepared by a palladium-catalysed alkynyla-
tion of 1,4-dibromobenzene with commercially available
2-pentyl-5-propynylthiophene. The 2,5-bis[(4-n-alkylphenyl)-
ethynyl]thiophenes 45–49 and 2,5-bis[(4-n-alkoxyphenyl)ethy-
nyl]thiophenes 50–54 were prepared by a palladium-catalysed
cross-coupling reaction of 2,5-dibromothiophene and the
appropriate (4-n-alkylphenyl)acetylenes and (4-n-alkoxyphenyl)-
acetylenes. The two-ring acetylene derivative 55 was prepared
analogously. The esters 56–68 were also prepared via a
palladium-catalysed cross-coupling reaction44,45 using 4-meth-
oxyphenylboronic acid and 2,5-dibromothiophene to produce
2,5-bis(4-methoxyphenyl)thiophene followed by the removal of
the methoxy group using boron tribromide46 and esterifica-
tion36 with DCC and DMAP of the resultant diphenol with
appropriate acids. Alkylation of 2,5-bis(4-hydroxyphenyl)thio-
phene in a Mitsunobu reaction47 yielded the ethers 69 and 70.
The esters 110 and 111 were prepared by esterification of
4-cyano-4’-hydroxybiphenyl and 4-hydroxy-4’-pentylbiphenyl
with (E,E)-2,4-hexadien-1-carboxylic acid.36
Fig. 2 Differential scanning thermogram as a function of tempera-
ture for the first heating and cooling cycle for the ester 93 (scan rate
10 uC min21).
presence of both homeotropic and focal-conic textures,
indicates that the mesophase observed is a calamitic smectic
A phase. The elliptical and hyperbolic lines of optical
discontinuity typical of focal-conic defects were also noted.
These textures classify the mesophase as being smectic A with
a layered structure, where the long axes of the molecules are
on average orthogonal to the layer planes and the in-plane
and out-of-plane positional ordering of the molecules is
short range. The optical texture of the ester 22 develop dark
bars across the backs of the focal conics on cooling from the
smectic A phase, see Table 3. At the same time the homeotropic
areas developed a grey Schlieren texture with only four-point
brushes. This behaviour is typical of the smectic C phase.
Results and discussion
1. Phase characterisation by thermal optical microscopy
The thermotropic mesophases observed for the compounds
shown in Tables 2–11 were investigated between crossed
polarisers using optical microscopy. Only the nematic phase,
the smectic A phase (SmA) and the smectic C phase (SmC)
were observed. The nematic phase was found above the smectic
phase, if present, as expected, e.g., see Tables 3–5 and 10.
Where an enantiotropic nematic phase was formed on melt-
ing a bright, colourful and fluid Schlieren texture with 2-point
and 4-point brushes was observed. A shimmering yellow,
very mobile texture with broad disclination lines was observed
just below the clearing point of the nematic phase. Nematic
droplets were always apparent on cooling the sample of
nematic material from the isotropic liquid formed above the
clearing point. These droplets then coalesced quickly to reform
the nematic Schlieren texture. Optically extinct homeotropic
areas were also often observed. Where an enantiotropic or
a monotropic nematic phase could not be observed directly
an extrapolated value was determined in some cases, e.g., see
Table 2–9. An ideal, linear relationship between the clearing-
point and the mixture composition was assumed across the
phase diagram for mixtures of different composition of the test
material in the commercial nematic mixture E7. Therefore,
linear extrapolation of the transition temperature line to 100%
of the test material gave the value of the virtual, extrapolated
nematic clearing point. The extrapolation was made over as
narrow a composition range as possible in order to be certain
that no significant curvature of the line occurs. The focal conic
(fan) texture was formed on cooling from either the nematic
phase or the isotropic liquid in those compounds, e.g., the
ethers 105–109, see Table 10, which exhibit a smectic A phase.
Optically extinct homeotropic areas were also observed in the
same samples. These areas were optically extinct when viewed
between crossed polarisers, which indicates that the phase is
optically uniaxial. This optical behaviour, i.e., simultaneous
Fig. 3 (a) The energy minimised structure of compound 10 with two
thiophene rings and one phenyl ring. (b) The energy minimised
structure of compound 14 with one thiophene rings and two phenyl
rings. (c) The energy minimised structure of compound 17 with one
thiophene ring and two phenyl rings. (d) The energy minimised
structure of compound 20 with no thiophene rings and three phenyl
rings.
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Table 2 Transition temperatures (uC) of various esters incorporating the 2,5-disubstituted thiophene ring 9–19 as well as the benzoate ester 20
Molecular structure
Cr
N
I
9
.
79
87
.
(72)a
(78)
(76)
(78)
(90)
122
123
169
135
154
180
172
.
10
11
12
13
14
15
16
17
18
19
20
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
88
89
91
97
94
131
118
113
97
97
a( ) Represents a monotropic transition temperature.
2. Phase characterisation by differential scanning calorimetry
derivatives listed in the Tables 2–11 is shown in Fig. 2 for the
ester 93. The enthalpy of transition between the nematic
phase and the isotropic liquid is relatively small (3.4 J g21), as
A typical thermogram obtained by differential scanning
calorimetry of the liquid crystal transitions of the thiophene
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Table 3 Transition temperatures (uC) of a number of diverse 5-pentylthiophenyl-2-carboxylates 21–28
Molecular structure
Cr
SmC
N
I
21
22
23
24
25
26
27
28
.
86
88
–
.
125
135
133
137
127
150
130
129
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
119
.
.
.
.
.
.
.
57
–
100
110
84
–
–
–
74
–
77
–
expected. In comparison the enthalpy of fusion (36.3 J g21
)
cyclohexyl- to a bicyclo[2,2,2]octyl-ring, the clearing point of
each ester increases, but the melting point decreases. This
pattern in the transition temperatures is seen again for the
esters 17–19, where the other thiophene ring of 10 is replaced by
a 1,4-disubstituted phenyl-, cyclohexyl- or a bicyclo[2,2,2]octyl-
ring, see Fig. 3c. Thus, the order of increasing clearing point
of the nematic phase for both series is as shown below, which is
consistent with previous results.48
is much higher. The enthalpy of transition of other thiophene
derivatives from the lamellar smectic A and smectic C phases to
either the relatively disordered nematic phase or the isotropic
liquid is intermediate between these two values. The enthalpy
of transition between the smectic A and the smectic C phase
is small, as expected, as the change from an orthogonal to
a tilted orientation within a lamellar arrangement is small.
These values were determined twice on heating and cooling
cycles on the same sample. However, the values obtained on
separate samples of the same compounds were reproducible
and very little thermal degradation was observed. The base line
of the spectra is relatively flat and sharp transition peaks are
observed. The values for the transition temperatures agree well
(#1–2 uC) with those values determined by DSC.
The effect of the presence of a 2,5-disubstituted thiophene
ring on the clearing point is demonstrated by comparing the
clearing point of ester 10 with two 2,5-disubstituted thiophene
rings with that of 4-pentylphenyl 4’-pentylbiphenyl-4-carboxyl-
ate 20 with phenyl rings in their place.37 The clearing point of
20 is almost 100 uC higher. This is clearly attributable to the
non-linear shape of the thiophene ring and the lower length-to-
breadth ratio of the molecules compared to the more linear
structure of the phenyl ester 20, see Fig. 3d. The anisotropy
of polarisability of the phenyl ester 20 will also be higher than
that of the compounds 14 and 17.
Each compound shown in Table 3 exhibits an enantiotropic
nematic phase. The propyl biphenyl ester 21 has a marginally
higher clearing point than that of the pentyl homologue 14,
see Table 2. However the undecyloxy homologue 22 has a
higher clearing point than those of both alkyl homologues 14
and 21 and also exhibits a smectic C phase. This is due to the
presence of the oxygen atom and the long alkoxy chain.49 The
3. Mesomorphism
The esters 9–13 collated in Table 2 containing two thiophene-
rings and an additional six-membered ring possess a non-linear
molecular shape, see Fig. 3a and exhibit a monotropic nematic
phase. The relatively low length-to-breadth ratio is responsible
for the low clearing points and melting points, which are
relatively independent of the length of the alkyl side chain.
These and the other compounds collated in Table 2 allow a
comparison to be made between a bent-shaped material 10 and
the more linear materials 14–16 with only one thiophene ring
and two six-membered rings in the molecular core, see Fig. 3b.
The more linear shape of 14–16 compared to that of 10 results
in the formation of an enantiotropic, rather than a monotropic,
nematic phase. As the end ring changes from a phenyl- to a
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Table 4 Transition temperatures (uC) of various substituted biphenyl 5-pentylthiophenyl-2-carboxylates 29–36
Molecular structures
Cr
SmC
N
I
29
30
31
32
33
34
35
36
.
57
56
–
–
–
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
75
77
150
190
150
–
84
–
121
113
98
.
132
–
–
.
.
(95)a
90
–
153
a( ) Represents a monotropic transition temperature.
propyl bicyclohexyl ester 23 has a higher clearing point than
that of the propyl biphenyl ester 21 and exhibits a lower melt-
ing point and, therefore, a greater temperature range of the
nematic phase. The clearing point of the pentyl bicyclohexane
ester 24 is also higher than that of the pentyl biphenyl ester 14.
However, the melting points of the pentyl esters are very
similar, unlike those of the propyl homologues, which leads to
both compounds 14 and 24 having a similar nematic phase
temperature range. The pentyl bicyclohexyl ester 24 exhibits a
higher melting and clearing point than those of the analogous
propyl bicyclohexyl ester 23.
Comparison of the transition temperatures of the esters 21,
23 and 25, shown in Table 3, reveal that the replacement of a
1,4-disubstituted ring by a trans-1,4-disubstituted cyclohexane
ring results in a step-wise increase in the clearing point of
the nematic phase. Compounds 26–28 incorporate a carbon–
carbon triple bond as a central linkage between the two phenyl
rings in the molecular core. This results in a higher length-to-
breadth ratio than that of the compounds 14 and 21 without a
central linkage. This results in a higher clearing point for the
tolanes 26 and 27 compared to that of the analogous biphenyls
14 and 21. The melting point is also lower, which results
in broader nematic temperature ranges for the two tolanes.
Therefore the temperature range of the nematic phase
decreases with increasing chain length.
group in the 3 position next to a small fluorine atom in the 4
position of the lateral phenyl ring. The presence of a cyano
group in a lateral position results in the absence of any
mesomorphic behaviour due to steric effects increasing the
intermolecular separation and, therefore, decreasing the forces
of attraction, and also due to the absence of dimerisation
resulting in a low effective length-to-breadth ratio. The pre-
sence of two fluoro substituents in the 3 and 4 positions of
compound 30 induce a low melting point and clearing point
and a narrow nematic range. This is consistent with the smaller
fluorine atom having a much smaller steric effect compared to
that of the cyano group. The presence of a highly polarisable
cyano group with a large permanent dipole moment in
compound 31 as opposed to a fluorine atom in compound 30
in the terminal position results in a compound with higher
melting point and a much higher clearing point than that of
the difluoro compound 30. This is due to the molecular dimer-
isation and a higher degree of molecular polarisation. The
substitution of the fluorine atom of the 3 position in compound
31 by the slightly smaller hydrogen atom, to yield 32 results in
a slightly higher melting point than compound 31. However,
the clearing point is significantly higher as expected due to the
absence of a lateral substituent. The presence of a bromine
atom in the terminal position in compound 33 rather than a
terminal cyano group in material 32 results in a much lower
clearing point and a much higher melting point. This is due to
Compound 29 shown in Table 4 possesses a bulky cyano
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Table 5 Transition temperatures (uC) of the two-ring 5-pentylthiophenyl-2-carboxylates 37–42 and the three-ring thiophene derivatives 43 and 44
Molecular structure
Cr
SmC
N
I
37
38
39
40
41
42
43
44
.
49
93
–
.
(42)a
(87)
–
.
.
.
.
.
.
.
.
–
.
.
.
.
.
.
.
.
v25
v25
v25
52
–
–
–
–
–
–
–
140
111
.
141
.
146
–
–
a( ) Represents a monotropic transition temperature.
the lower dipole moment of the bromine atom. The presence of
a smectic C phase has also been induced.50 The laterally fluoro-
substituted bromo compound 34 is not mesomorphic. The
presence of a fluorine atom next to the carbon–carbon single
bond between the two phenyl rings in compound 34 could have
a much greater influence on the mesomorphic behaviour of
these compounds than the presence of the same substituent
next to the terminal substituent due to a greater degree of
twisting of the adjacent phenyl ring out of the plane of the
substituted phenyl ring. In compounds 35 and 36 a lateral
substituent is positioned on the central phenyl ring of the rigid
core. Compound 36 exhibits a lower clearing point and a
slightly higher melting point than those of the non-laterally
substituted nitrile 32. However, when comparing the effect of
a fluorine atom in the same position on one or other of the
phenyl rings for compounds 31 and 36, respectively, it can be
seen that there is very little dependence of the clearing point on
the position of the lateral substituent. Both compounds exhibit
an enantiotropic nematic phase. When considering compounds
33–35, with a terminal bromine atom, the presence of a lateral
fluorine atom in the other phenyl ring in compound 35 gives
rise to a much lower clearing point than that of the analogous
bromo-substituted compound 33 without a fluorine atom in a
lateral position. Furthermore, the nematic phase is mono-
tropic. Compound 35 has a lower melting point than that of
compound 34 with a fluorine atom in the other ring in a
position meta to the bromine atom.
Based on the interpretation of the thermal data listed in
Tables 2-4, especially the very high clearing point of the
4-cyanophenyl substituted ester 32 in Table 4, various thio-
phene derivatives with effectively two-and-a-half rings 37, 38 or
two-rings 39–42 were synthesised, see Table 5, in an attempt
to produce materials with a nematic phase closer to room
temperature. Compounds 37 and 38 incorporate a naphthalene
ring and exhibit a monotropic nematic phase. Compound 37
with a terminal heptyl chain possesses a much lower clearing
point than that of compound 38 with the polar cyano group.
The naphthalene ester 37 exhibits a much lower clearing point
than that of the analogous tolane 28, see Table 3, with two
phenyl rings joined by a carbon–carbon triple bond in place of
the naphthyl ring, due to the lower length-to-breadth ratio and
anisotropy of polarisability of 37 due to the presence of
the broad naphthalene ring. The two-ring compounds 39–42
containing a 1,4-disubstituted phenyl ring are liquids at room
temperature, and, while compound 42 with a bicyclo[2.2.2]-
octane ring is crystalline at room temperature, none of these
esters is mesomorphic. The non-linear and non-planar confor-
mation of the esters 39–41 is responsible for the absence of
an observable nematic phase for these two-ring aromatic
compounds. While the two-ring phenyl ester 40 is a liquid
at room temperature, the corresponding biphenyl ester 14
with an additional 1,4-disubstituted phenyl ring exhibits an
enantiotropic nematic phase and a high clearing point. This
is clearly due to a more linear and planar molecular shape
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Table 6 Transition temperatures (uC) for the 2,5-bis[(4-n-alkylphenyl)ethynyl]thiophenes 45–49 and the 2,5-bis[(4-n-alkoxyphenyl)ethynyl]thio-
phenes 50–54
R1
R2
Cr
N
I
45
46
47
48
49
50
.
88
72
.
112
81
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
59
94
51
69
42
83
136
174
51
52
53
54
.
.
.
.
95
87
81
79
.
.
.
.
168
139
137
126
.
.
.
.
with a much greater length-to-breadth ratio, and anisotropy
of polarisability, see Fig. 3b. This interpretation appears to
be further confirmed by the fact that the tolane 27, with a
larger length-to-breadth ratio than that of the biphenyl ester
14 due to the presence of an additional carbon–carbon triple
bond as a central linkage in the molecular core, exhibits a
higher nematic clearing point than that of the biphenyl 14.
The thermal data for compounds 43 and 44, shown in
Table 5, show that a thiophene ring in the centre of the mole-
cular core is more conducive to the induction of a nematic
phase. The three aromatic rings making up the central core of
compound 43 all lie in the same plane, see Fig. 4a and 4b. This
may be due to the interaction between the electron rich sulfur
atom and the hydrogen atoms ortho to the bonds between the
thiophene ring and the two phenyl rings (estimated distance
˚
2.7 A). The bond angle of 148u between the two bonds to the
substituent phenyl groups brings the electron rich thiophene
atom and the hydrogen atoms on the phenyl rings ortho to
the bonds close to each other. A dipolar attraction between
them would result in a stabilisation of a planar structure
for the molecular core. This may result in a relatively small
intermolecular distance between adjacent molecules and in a
resultant stabilisation of mesophase behaviour. The non-linear
character of two 2,5-disubstituted thiophene rings in 44 cancels
itself out to some degree, since the bonds to the two terminal
pentyl chains are parallel, if not collinear, assuming a transoid
diposition of the thiophene rings as appears likely, see Fig. 5a
and 5b, cf., compound 2 in Table 1. The thiophene ring and the
terminal alkyl chains are also in the same plane. However, the
tolane 44 is not mesomorphic, whereas the diphenyl thiophene
2 exhibits polymorphism at high temperatures. The two
thiophene rings in the tolane 44 are almost orthogonal to the
central phenyl ring, which is decoupled from them by the
carbon–carbon triple bonds. Therefore, it is the twisted
Fig. 4 (a) The energy minimised structure of compound 43 in the x–y
plane viewed along the z axis. (b) The energy minimised structure of
compound 43 in the x–z plane viewed along the y axis.
J. Mater. Chem., 2002, 12, 2706–2721
2713

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Table 7 Transition temperatures (uC) of the 2,5-bis(4-alkanoyloxyphenyl)thiophenes 56–60, the 2,5-bis[4-(alkenoyloxy)phenyl]thiophenes 61–68
and the 2,5-bis[4-(hexenyloxy)phenyl]thiophenes 69 and 70
R1
R2
Cr
N
I
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
.
202
198
188
185
181
129
184
190
194
148
140
109
130
162
187
.
212
201
199
189
185
233
–
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
210
–
.
.
.
.
v250
240
157
212
–
–
molecular core of the tolane 44, which is responsible for the
absence of an observable liquid crystalline phase.
The tolanes 45–54 with a thiophene ring in the centre of
the molecular core and two carbon–carbon triple bonds were
prepared in order to generate a low melting point due to the
non-planar structure of these materials, see Table 6. The melt-
ing and clearing point of both series decreases systematically
with increasing chain length. This is due to the greater flexi-
bility of longer alkyl chains, which increases the proportion of
non-linear conformations of the terminal aliphatic chain and
the consequent dilution of the attractive van der Waals forces
between the aromatic cores. The melting and clearing points of
the alkoxy substituted tolanes 50–54 are higher than those of
Fig. 5 (a) The energy minimised structure of compound 44 in the x–y
plane viewed along the z axis. (b) The energy minimised structure of
compound 44 in the x–z plane viewed along the y axis.
2714
J. Mater. Chem., 2002, 12, 2706–2721

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Table 8 Transition temperatures (uC) for the 2-(4-[hexanoyloxy]phenyl)-5-propylthiophene 71 and the 2-(4-[hexenoyloxy]phenyl)-5-propyl-
thiophenes 72-75, (4-[(E,E)-hexa-2,4-dienoyloxy]phenyl)-5-pentylthiophene 76, 2-cyano-5-(4-[hexanoyloxy]phenyl)thiophene 77, 2-cyano-5-(4-
[hexenoyloxy]phenyl)thiophenes 78-81 and 2-cyano-5-(4-[(E,E)-hexa-2,4-dienoyloxy]phenyl)thiophene 82
R1
R2
Cr
N
I
71
72
73
74
75
76
77
78
79
80
81
82
C3H7
.
45
16
38
44
40
76
77
83
53
74
70
168
–
.
C3H7
C3H7
C3H7
C3H7
C5H11
CN
.
.
.
.
.
.
.
.
.
.
.
.
42
–
.
.
.
.
.
.
.
.
.
.
.
–
–
.
.
.
.
125
–
CN
(82)a
CN
–
CN
(39)
–
CN
CN
188
a( ) Represents a monotropic transition temperature.
central core and the terminal substituent, rather than between
rings in the core of the molecule, can lead to the formation of a
nematic phase of low viscosity.51,52 The presence of a carbon–
carbon double bond in such a terminal chain is also known to
be conducive to mesophase formation.51,52 Therefore, a series
of 2,5-bis(4-n-alkanoyloxyphenyl)thiophenes 56–60 and related
2,5-bis[4-(alkenoyloxy)phenyl]thiophenes 61–68 were prepared,
see Table 7. The melting and clearing points of the alkanoyloxy
esters 56–60 decrease with increasing chain length as expected.
This can be attributed to the flexibility of the alkyl chain and
its apolar nature. The van der Waals forces between the
relatively rigid aromatic cores are reduced by the presence of
the aliphatic chains. The presence of a trans-carbon–carbon
double bond located in the 2 or 4 position in the hexenoyloxy
chain induces an enantiotropic nematic phase at higher
temperatures for compounds 61 and 63. Whereas the presence
of a cis-carbon–carbon double bond in the 3 position (62) or
the corresponding alkyl-substituted homologues 45–49 in each
case. This is due to the increased van der Waals forces due to
the presence of the two oxygen atoms in conjugation with the
aromatic core and the increase in effective size of the rigid
molecular core compared to that of the corresponding alkyl-
substituted materials with a methylene group in place of the
oxygen atoms. An analogous two-ring material 55 was not
mesomorphic. This may well be due to a non-planar structure,
since both aromatic rings are relatively free to rotate around
the carbon–carbon triple bond, cf. Fig. 5a and 5b.
It is known that an ester group as a linkage between the
J. Mater. Chem., 2002, 12, 2706–2721
2715

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Table 9 Transition temperatures (uC) for the 2-n-alkyl-5-(4-[(E)-alk-2-
enyloxy]phenyl)thiophenes 72 and 83–97
nematic clearing point of the (E)-2-alkenoyloxy-substituted
esters 61 and 65–68 is also much higher than that of the
corresponding alkyl esters 56–60. The combination of these
two factors leads to a wide nematic phase for the (E)-2-
alkenoyloxy-substituted esters 61 and 65–68. Once again as
the chain length of the alkenyl esters increases, the materials
become less rod-like and more flexible, and consequently,
the clearing point decreases. The very high clearing point of
the nematic phase of some of these alkenoyloxy-substituted
esters containing a thiophene ring suggested that equivalent
two-ring esters could also exhibit a nematic above room
temperature.
n
m
Cr
N
I
83
84
72
85
86
87
88
89
90
91
92
93
94
95
96
97
1
2
3
4
5
6
7
8
3
3
3
3
5
5
5
5
3
3
3
3
3
3
3
3
1
2
4
5
1
2
4
5
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
87
43
16
39
47
57
59
65
37
32
17
21
66
64
37
44
–
44
42
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
(37)a
(46)
(42)
(49)
[47]b
47
Only compound 72 of the compounds 71–75 collated in
Table 8 is mesomorphic and exhibits a nematic phase at room
temperature as well as a relatively high clearing point. It is
clear from Fig. 6a and 6b that the phenyl ring in the molecular
core is in the same plane as the thiophene ring. However, the
carbon–carbon double bond and the carboxy group are also
in this same plane. Therefore, compound 72 is a very flat,
planar molecule with nearly all of the carbon and oxygen atoms
making up the molecular skeleton in the same plane. Thus, the
intermolecular distance between adjacent molecules of the ester
72 will be relatively small. The last two units of the alkenoyloxy
chain project out of the rotation volume of the molecular core.
The associated steric effect may be responsible for the relatively
low melting point below room temperature as the symmetry
is lowered. The transition temperatures for the correspond-
ing esters 77–82 with a cyano group in place of the alkyl
group exhibit a greater tendency to form a nematic phase at
higher temperatures due to the high dipole moment and high
anisotropy of polarisability induced by the presence of the
cyano group. Furthermore, a degree of dimer formation, with a
longer length-to-breadth ratio, could be expected. The melting
point and clearing point of the cyano-substituted ester 78 is
much higher than those of the propyl-substituted ester 72. The
high nematic clearing point of the alkenoate 78, with a trans-
carbon–carbon double bond in position 2, could be a result of
the increased conjugation between the double bond and the
carboxyl moiety. However, the isomeric alkenoate 80 with a
trans-carbon–carbon double bond in position 4 also exhibits a
monotropic nematic phase, although at a lower temperature.
Therefore, conjugation alone cannot explain this situation.
However, the presence of a conjugated trans-carbon–carbon
double bond in the 2 position is responsible, at least to a large
extent, for the advantageous liquid crystalline transition tem-
peratures of these esters. This is confirmed by the very high
clearing point of the nematic phase of the diene ester 82 with
two trans-carbon–carbon double bonds in the terminal chain in
positions 2 and 4.
(17)
29
42
–
–
(36)
49
.
.
a( ) Represents a monotropic transition temperature. b[ ] Represents
a virtual transition temperature.
a double bond in the 5 position (64) does not give rise to any
mesomorphic behaviour. The clearing point of the (E)-hexen-2-
yloxy ester 61 with an additional trans-carbon–carbon double
bond in both terminal chains is somewhat higher than that of
the corresponding hexanoyloxy ester 57 with two terminal
aliphatic chains.51–55 It may be due in part to the added rigidity
of the (E)-2-alkenyl chain and a lower number of non-linear
conformations of the chain. However, it may also be due to
the presence of a planar molecular core, if it is assumed that a
degree of hydrogen bonding exists between the ortho-hydrogen
atom on the phenyl rings and the carbonyl function of
the carboxy group. The carbon–carbon double bond is in
the plane of the carboxy group due to delocalisation effects
and conjugation between the p-electrons in both moieties. This
will give rise to an almost planar structure for this molecule.
The lower clearing point of the ethers 69 and 70 than those
of the corresponding esters 61 and 62 could, thus, be explained
by the absence of the carbonyl groups and the associated
stabilisation of planarity. The melting point of the (E)-2-
alkenyl-substituted esters 61 and 65–68 is much lower than that
of the corresponding alkyl esters 56–60, which is unusual. The
The liquid crystal transition temperatures for the 2-n-alkyl-
5-(4-[(E)-hex-2-enoyloxy]phenyl)thiophenes 72 and 83–89 are
listed in Table 9. The clearing point shows a clear odd–even
effect. There are wide differences in the melting points for
individual homologues with no identifiable trend. Only the
propyl homologue 72 exhibits an enantiotropic nematic phase
at room temperature. The homologues 90–93 were prepared in
order to establish the optimum length of the (E)-alk-2-enoyloxy
chain having already determined that the combination of a
propyl homologue with a (E)-hex-2-enoyloxy chain produced
a nematic phase at room temperature, cf. compound 72. Two
further homologues 92 and 93 exhibit a melting point below
room temperature as well as an enantiotropic nematic phase.
Only one homologue 97 of the pentyl homologues 86 and 94–97
possesses an enantiotropic nematic phase. However, its melting
point is relatively high and the nematic temperature range is
narrow.
Fig. 6 (a) The energy minimised structure of compound 72 in the x–y
plane viewed along the z axis; b is a measure of the width of the rotation
volume of the molecular core. (b) The energy minimised structure of
compound 72 in the x–z plane viewed along the y axis; b is a measure of
the width of the rotation volume of the molecular core. The dotted
surface represents electrostatic density.
It was also decided to prepare the (E,E)-2-n-alkyl-5-
(4-[hexa-2,4-dienoyloxy]phenyl)thiophenes 98–102, the 2-(4-
[(E)-hex-2-enyloxy]phenyl)-5-propylthiophene 103 and the
2-n-alkyl-5-(4-[(E,E)-hexa-2,4-dienyloxy]phenyl)thiophenes
2716
J. Mater. Chem., 2002, 12, 2706–2721

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Table 10 Transition temperatures (uC) for the 2-n-alkyl-5-(4-[(E,E)-hexa-2,4-dienoyloxy]phenyl)thiophenes 76 and 98–102, 2-(4-[(E)-hex-2-
enyloxy]phenyl)-5-pentylthiophene 103 and the 2-n-alkyl-5-(4-[(E,E)-hexa-2,4-dienyloxy]phenyl)thiophenes 104–109
R1
R2
Cr
SmA
N
I
98
C3H7
.
75
80
75
79
86
82
82
85
81
98
95
95
86
–
.
136
122
125
116
117
111
–
.
99
C4H9
.
.
.
.
.
.
.
.
.
.
.
.
–
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
76
C5H11
C6H13
C7H15
C8H17
C3H7
–
100
101
102
103
104
105
106
107
108
109
–
–
–
–
C3H7
–
.
.
.
98
C4H9
.
.
.
.
.
92
(97)a
99
101
100
96
C5H11
C6H13
C7H15
C8H17
100
–
–
–
a( ) Represents a monotropic transition temperature.
104–109 shown in Table 10 in an attempt to produce materials
with a nematic phase with a low viscosity, a high clearing
point and a high birefringence.56 All of the diene esters 98–102
prepared exhibit an enantiotropic nematic phase at elevated
temperatures. The thermal data for the diene esters 98–102
shows that the added greater degree of anisotropy of the
polarisability and polarisation attributable to the cyano-group
compared to an alkyl terminal substituent leads to a higher
nematic clearing point. The dienoate ester 98 with two
trans-double bonds in positions 2 and 4 exhibits a higher
clearing point than the corresponding alkenyl esters 72 and
74 with a single carbon–carbon double bond in either position
2 or 4 of the terminal chain, see Table 8. The 2-(4-[(E)-
hex-2-enyloxy]phenyl)-5-propylthiophene 103 does not exhi-
bit an observable liquid crystalline phase due to the high
melting point. However, the first three homologues of the
2-n-alkyl-5-(4-[(E,E)-hexa-2,4-dienyloxy]phenyl)thiophenes 104–
106 with an additional carbon–carbon double bond in position
4 of the terminal chain attached to the phenyl group exhibit
an enantiotropic nematic phase. However, a smectic A phase
is observed from the second homologue 105 of the series. The
temperature for the smectic A to nematic transition increases
J. Mater. Chem., 2002, 12, 2706–2721
2717

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Table 11 Transition temperatures for the (E,E)-hexa-2,4-dienoyloxy-substituted compounds 76, 82, 110 and 111
Structure
Cr
N
I
110
76
.
86
76
.
168
.
.
.
.
.
.
.
.
.
.
125
227
188
111
82
160
168
with chain length and the smectic A phase displaces the nematic
phase from the hexyl homologue 106 onwards.
at least in part, by the lower dielectric anisotropy of the
mixtures at a higher reduced temperature containing the apolar
thiophene derivatives with a diople moment caused by the
polarisable sulfur atom across the long molecular axis.
However, the longer toff time, which is independent of the
dielectric anisotropy, does suggest a higher induced value for
the viscosity, although ton is increased (ca. 50%) more than toff
(ca. 25%). These initial results suggest that some of these
new thiophene derivatives may be of use as components of
nematic mixtures of negative dielectric anisotropy, e.g., for
VAN-LCDs, rather than positive dielectric anisotropy, in order
to increase the birefringence and clearing point of the nematic
phase.
The liquid crystalline transition temperatures of the esters
76, 82, 110 and 111 collated in Table 11 demonstrate once
again that the non collinear angle between the bonds con-
necting the thiophene ring to the rest of the molecule, e.g., in
the esters 76 and 82, leads to a lower clearing point than that
of the analogous phenyl esters, e.g., 110 and 111, with collinear
bonds.
4. Physical properties
Several representative examples of the new compounds have
been evaluated for potential use in nematic mixtures for TN-
LCDs, STN-LCDs and LCDs based on flexoelectricity. A fixed
amount (10 wt%) of two representative examples of the two-
ring thiophenes 82 and 101 was dissolved in a standard nematic
mixture (DOP017) and the physical properties of the two
resultant mixtures assessed and compared to those of DOP-017
the host mixture, see Table 12. The presence of 10 wt% of the
thiophene ester 82 with a trans-carbon–carbon double bond in
the terminal chain leads to a small increase in the refractive
indices and the birefringence of the host mixture. However, the
clearing point of the doped mixture is substantially higher
(16 uC) than that of the host mixture. However, the values for
the birefringence and clearing point of a nematic mixture
containing the diene ester 101 with two trans-carbon–carbon
bonds in the terminal chain are much higher than those of the
first two mixtures. However, the response times, ton and toff
(10–90% transmission) of TN-nematic cells containing the
doped mixtures addressed under identical conditions are also
greater than those of the host mixture. This may be explained,
Provisional evaluation of the flexoelectric coefficients of
these and other liquid crystals with an unusual shape suggest
that a bent shape does not lead automatically to an increase in
the flexoelectric coefficients of a polar host nematic mixture
(E7) as generally postulated.57 The value of the bend and
splay flexoelectric coefficients eb 1 es/K for ester 72 measured in
the pure state at room temperature is lower (0.12 C N21 m21
)
than that of E7 at the same temperature (0.76 C N21 m21).
However, the elastic constant value K of the doped mixtures
may differ to some degree to that of the host mixture, E7. The
value of eb 1 es/K extrapolated for compound 72 from mixtures
with E7 agree well with that measured for the pure compound.
However, the value measured for a mixture of the nematic
mixture E7 doped with 10 wt% of the three-ring thiophene 43,
see Table 5, is almost double (1.44 C N21 m21). This is a large
effect for a 10 wt% concentration of the thiophene 43. These
results are still being evaluated and will be reported in detail
elsewhere.57 However, they do demonstrate the possible use of
Table 12 Transition temperatures, refractive indices and birefringence measured at 25 uC for the nematic host mixture DOP-017 and mixtures
containing 10 wt% of the dopants 82 and 101
Structure
ne
no
Dn
TN–I
DOP-017
110 wt% 82
Host nematic mixture
1.580
1.587
1.485
1.489
0.094
0.099
45.2
51.4
110 wt% 101
1.600
1.490
0.110
59.0
2718
J. Mater. Chem., 2002, 12, 2706–2721

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some thiophene derivatives as dopants to increase the flexo-
electric coefficients of nematic host mixtures for use in LCDs
based on flexoelectricity.58 A large value for the flexoelectric
coefficients could lead to short response times for LCDs based
on the flexoelectricity of nematic mixtures, such as ZBD.58
A patent application has been submitted based on these
preliminary results.59
Synthesis
A short reaction pathway to a typical liquid crystalline
phenylthiophene derivative 72 is described below in order to
illustrate the spectral data obtained for the final compounds
9–111 and the reaction intermediates leading to them. Exten-
sive synthetic information is included in the Electronic
Supplementary Information.{
2-Bromo-5-propylthiophene. A solution of 2-propylthio-
phene (6.00 g, 0.0476 mol) and NBS (8.47 g, 0.0476 mol) in
a 1 : 1 mixture of chloroform/acetic acid (50 cm3) was stirred
and heated under reflux for 30 min. Water (100 cm3) and ether
(100 cm3) were added to the cooled solution. The organic layer
was separated off and washed with 10% sodium hydroxide
solution (1 6 100 cm3) then brine (2 6 50 cm3) and dried
(MgSO4). After filtration, the filtrate was concentrated under
reduced pressure. The crude product was purified by distilla-
tion under reduced pressure to yield 8.82 g (89.9%) of the
desired product. Boiling point. /uC: 98 @ 20 mmHg. 1H NMR
(CDCl3) dH: 0.96 (t, J ~ 7.6, 3H), 1.65 (sext, J ~ 7.6, 2H), 2.71
(td, J ~ 7.6, J ~ 1.0, 2H), 6.52 (dt, J ~ 1.0, J ~ 3.7, 1H), 6.83
(d, J ~ 3.7, 1H). IR nmax/cm21: 2967, 2935, 2875, 1448, 1047,
963, 789. MS m/z: 206, 204 (M1), 177, 175, 96. GLC 98.3%.
Conclusions
The combination of molecular elements required to induce
an enantiotropic phase at room temperature for an organic
compound incorporating a 2,5-disubstituted thiophene ring has
been identified. This is the first time a room temperature
nematic phase has been reported for thiophene derivatives.
The combination of these molecular elements also contribute
towards generating a high birefringence, a high nematic clear-
ing point as well as a relatively low viscosity for these materials.
The new thiophene compounds can be used to induce a high
birefringence in nematic mixtures for LCDs. The compounds
synthesised may be potentially useful components of nematic
mixtures of negative, rather than positive, dielectric anisotropy
for use in LCDs, such as VAN-LCDs, or LCDs based on
flexoelectric effects, such as ZBD.
2-(4-Methoxyphenyl)-5-propylthiophene. Tetrakis(triphenyl-
phosphine)palladium(0) (2.31 g, 0.0020 mol) was added to
a stirred solution of 2-bromo-5-propylthiophene (8.86 g,
0.0430 mol) and 4-methoxyphenylene boronic acid (7.90 g,
0.0520 mol) in a mixture of 20% sodium carbonate solution
(50 cm3) and 1,4-dioxane (100 cm3). The reaction mixture was
heated under reflux for 5 h. The cooled reaction mixture was
diluted with ether (200 cm3) and water (200 cm3). The aqueous
layer was washed with ether (2 6 50 cm3) and the combined
organic extracts were washed with a 10% HCl solution (1 6
100 cm3) then with brine (2 6 100 cm3) and dried (MgSO4).
After filtration, the solvent was removed under reduced pres-
sure and the residue purified by column chromatography on
silica gel using a 1 : 1 hexane/dichloromethane mixture as the
eluent followed by recrystallisation from hexane to yield 5.80 g
Experimental
Techniques
The structures of intermediates and final products were con-
firmed by proton (1H) nuclear magnetic resonance (NMR)
spectroscopy (JEOL JMN-GX270 FT nuclear resonance
spectrometer), infrared (IR) spectroscopy (Perkin-Elmer 783
infrared spectrophotometer) and mass spectrometry (MS)
(Finnegan MAT 1020 automated GC/MS). Reaction progress
and product purity was checked using a CHROMPACK CP
9001 capillary gas chromatograph fitted with a 10 m CP-SIL
5CB (0.12 mm, 0.25 mm) capillary column. All of the final
products were more than 99.5% pure by GLC. Transition tem-
peratures were determined using an Olympus BH-2 polarising
light microscope together with a Mettler FP52 heating stage
and a Mettler FP5 temperature control unit. The analysis
of transition temperatures and enthalpies was carried out by
a Perkin-Elmer DSC7-PC differential scanning calorimeter.
Molecular modelling was carried out using ChemDraw Ultra 6,
CambridgeSoft, USA, and MM1 energy minimisation. Simula-
tions using either the TITAN (a proprietary Merck program)
or Cerius 260 molecular modelling packages gave essentially the
same results. The disposition of the thiophene and phenyl rings
in the compounds 10, 14, 17, 20, 43, 44 and 72 shown in
Fig. 3–5, was chosen in each case to give the most linear
structure. This is based upon the assumption that the preferred
molecular conformation will be that giving rise to the highest
length-to-breadth ratio and greatest anisotropy of polarisa-
bility consistent with current understanding of the nematic
phase of thermotropic, calamitc liquid crystals. However, there
is a high degree of free rotation about the inter-annular bonds
and, as a consequence, a distribution of molecular conforma-
tions will be present. The out-of-plane nature of the rings
in Fig. 3a–d and 5a, 5b are clearly a consequence of this.
Therefore, an assumption of a cisoid or transoid disposition
is irrelevant. The chemical bonds in the compounds shown in
Fig. 4a, 4b define the molecular geometry of the aromatic core
and, therefore, no assumptions need to be made. The terminal
aliphatic chains are assumed to adopt an all-trans, antiperi-
planar conformation.
1
(58.1%) of the desired product. Melting point /uC: 41–42. H
NMR (CDCl3) dH: 1.00 (t, J ~ 7.6, 3H), 1.72 (sext, J ~ 7.6,
2H), 2.78 (td, J ~ 7.6, J ~ 1.0, 2H), 3.82 (s, 3H), 6.71 (dt, J ~
1.0, J ~ 3.7, 1H), 6.89 (d, J ~ 9.0, 2H), 7.00 (d, J ~ 3.7, 1H),
7.48 (d, J ~ 9.0, 2H). IR nmax/cm21: 2961, 2874, 1551, 1469,
1290, 1255, 1181, 1034, 829, 793. MS m/z: 232 (M1), 203, 188.
GLC 99.8%.
4-(5-Propylthien-2-yl)phenol
A 1.0 M solution of boron tribromide in dichloromethane
(50.0 cm3, 0.0500 mol) was added dropwise to a cooled (0 uC)
stirred solution of 2-(4-methoxyphenyl)-5-propylthiophene
(5.80 g, 0.0250 mol) in dichloromethane (100 cm3). The
reaction mixture was stirred at room temperature overnight, ,
then poured onto an ice/water mixture (200 g) and stirred
(30 min). The organic layer was separated and washed with
brine (2 6 100 cm3) and then dried (MgSO4). The product
was purified by column chromatography on silica gel using
dichloromethane as the eluent, followed by recrystallisation
from ethanol to yield 4.55 g (83.5%) of the desired product.
The product was used without further purification. Melting
point. /uC: 92–94. 1H NMR (CDCl3) dH: 0.99 (t, J ~ 7.6, 3H),
1.71 (sext, J ~ 7.6, 2H), 2.78 (td, J ~ 7.6, J ~ 1.0, 2H), 4.85 (s,
1H), 6.71 (dt, J ~ 1.0, J ~ 3.7, 1H), 6.81 (d, J ~ 9.0, 2H), 6.99
(d, J ~ 3.7, 1H), 7.43 (d, J ~ 9.0, 2H). IR nmax/cm21: 3354,
2961, 2872, 1606, 1515, 1444, 1381, 1258, 1108, 952, 829, 795.
MS m/z: 218 (M1), 203, 189. GLC 99.7%.
J. Mater. Chem., 2002, 12, 2706–2721
2719

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A solution of N,N’-dicyclohexylcarbodiimide (0.47 g, 0.0023 mol)
in dichloromethane (20 cm3) was added to a solution of
(E)-hex-2-enoic acid (0.26 g, 0.0023 mol), 4-(2-propylthiophen-
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methane mixture as the eluent and recrystallisation from
ethanol to yield 0.15 g (20.8%) of the desired product, see
Tables 8 and 9 for the transition temperatures of this ester. 1H
NMR (CDCl3) dH: 0.98 (t, Jab ~ 7.6, 3H), 1.00 (t, Jml ~ 7.6,
3H), 1.55 (sext, Jba ~ 7.6, Jbc ~ 7.6, 2H), 1.73 (sext, Jlk ~ 7.3,
Jlm ~ 7.6, 2H), 2.27 (dt, Jkl ~ 7.3, Jkj ~ 1.7, 2H), 2.79 (td, Jcb ~
7.6, Jcd ~ 1.0, 2H), 6.02 (dt, Jji ~ 15.9, Jjk ~ 1.7, 1H), 6.73
(dt, Jdc ~ 1.0, Jde ~ 3.4, 1H), 7.07 (d, Jed ~ 3.4, 1H), 7.10 (d,
29 D. J. Byron, L. Komitov, A. S. Matharu, I. McSherry and
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J
fg ~ 9.0, 2H), 7.18 (d, Jij ~ 15.8, 1H), 7.55 (d, Jgf ~ 9.0, 2H).
IR nmax/cm21: 2967, 2935, 2876, 1742, 1654, 1511, 1208, 1154,
1116, 979, 800. MS m/z: 314 (M1), 218 (C13H14OS1), 189
(C9H12OS1), 97 (C6H9O1). GLC 99.9%.
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33 M. Funashi and J.-I. Hanna, Mol. Cryst. Liq. Cryst., 2001, 368,
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Acknowledgements
The technical staff in the Department of Chemistry at the
University of Hull are thanked for provision of analytical
services. DERA (Malvern) is thanked for financial support of
postgraduate studentships (W. L. D, N. L. C and J. H. W.).
The EPSRC is thanked for financial support of postgraduate
studentships (K. B. and G. I. T.) and of an Advanced
Fellowship (S. M. K.). E Merck are thanked for the generous
provision of some reaction intermediates.
This paper is reproduced with the permission of Her
Britannic Majesty’s Stationary Office.
40 R. Eidenschink, Mol. Cryst. Liq. Cryst., 1985, 113, 57;
R. Eidenschink, M. Ro¨mer, G. Weber, G. W. Gray and K. J.
Toyne, Ger. Pat., 3,321,373, 1983.
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49 W. L. McMillan, Phys. Rev. A, 1974, 8, 1921.
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