Nematic tolanes and acetylenes

Article abstract of DOI:10.1039/b209045g

The first liquid crystals incorporating a non-conjugated carbon-carbon triple bond in the terminal chain to exhibit a nematic phase above the melting point are reported. A variety of compounds incorporating a carbon-carbon triple bond in the terminal chain or between two phenyl rings have been synthesised as part of an investigation of the effects of the shape, conformation and rigidity of the terminal chains of liquid crystals on their mesophase behaviour. A series of related tolanes incorporating an alkenyloxy chain with an additional carbon-carbon double bond or a simple alkoxy chain as a terminal substituent also were prepared for comparison purposes.

Full text of DOI:10.1039/b209045g

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Nematic tolanes and acetylenes
Graham W. Skelton,a Dewen Dong,a Rachel P. Tuffinb and Stephen M. Kelly*a
aThe Department of Chemistry, University of Hull, Kingston-upon-Hull, UK HU6 7RX.
E-mail: s.m.kelly@hull.ac.uk
bQinetiq, St. Andrews Road, Malvern, Worcestershire, UK WR14 3PS
Received 16th September 2002, Accepted 9th January 2003
First published as an Advance Article on the web 4th February 2003
The first liquid crystals incorporating a non-conjugated carbon–carbon triple bond in the terminal chain to
exhibit a nematic phase above the melting point are reported. A variety of compounds incorporating a carbon–
carbon triple bond in the terminal chain or between two phenyl rings have been synthesised as part of an
investigation of the effects of the shape, conformation and rigidity of the terminal chains of liquid crystals on
their mesophase behaviour. A series of related tolanes incorporating an alkenyloxy chain with an additional
carbon–carbon double bond or a simple alkoxy chain as a terminal substituent also were prepared for
comparison purposes.
It became clear very early on in liquid crystal research that at
least one of the terminal substituents attached to the molecular
core had to be a flexible aliphatic chain in order to dilute the
central organic core and thereby induce a relatively low melting
point, or at least a melting point low enough for a liquid crys-
talline phase to be observed.1–10 The dispersion forces between
the molecular cores of many liquid crystals, especially those
containing two or more aromatic rings, would lead to high
melting points in the absence of at least one terminal aliphatic
substituent. It is for this reason that most early synthetic liquid
crystals possessed two terminal alkyl or alkoxy chains.1–10
Most mesomorphic compounds incorporating two short termi-
nal aliphatic chains usually exhibit a nematic phase. However,
most liquid crystalline materials with long terminal chains, e.g.
7–12 methylene units, often exhibit a smectic phase, sometimes
in addition to a nematic phase, especially in the absence of
lateral substituents. The viscosity of the nematic phase also
increases as a homologous series is ascended. Therefore, in
order to promote the formation of a nematic phase—rather
than a smectic phase—with a low viscosity, short alkyl chains
are often incorporated into liquid crystals designed for LCDs
based on the nematic phase.
The presence of a carbon–carbon double bond in a terminal
chain has been shown to strongly influence the mesomorphism
of particular compounds and their physical properties. It was
found that the presence of a trans carbon–carbon double bond
separated from the molecular core by an odd number of carbon
atoms in the terminal alkenyl chain exhibit a high nematic
clearing point.11–15 The corresponding compounds containing
the cis isomers of the same chains, i.e. with the carbon–carbon
double bond in the same position, but with the opposite con-
figuration, exhibit very low (usually extrapolated) nematic
clearing points. This big difference in clearing points may be
attributed to the non-linear conformation of an alkenyl chain
containing a cis carbon–carbon double bond in these positions.
It is has been postulated that either a linear or zig-zag fully
extended conformation of the terminal aliphatic chain pro-
motes nematic phase formation by maintaining the rod-like
shape of the molecule or, more exactly, of its rotation volume.
This results in a high length-to-breadth ratio, a high anisotropy
of molecular polarisability and a small intermolecular distance
between parallel molecules, which leads to an optimum balance
between attractive dispersion forces and steric and electronic
repulsive forces. The high clearing point of some alkenyl com-
pounds is consistent with this interpretation in that the greater
rigidity of the alkenyl chain results in fewer non-linear
conformations. A higher degree of molecular polarisability
due to the p-electrons in the carbon–carbon bond may also
contribute to a higher clearing point.11–15
The influence of a carbon–carbon triple bond located in the
terminal chain on the mesomorphic behaviour of liquid crystals
has been studied to a much lesser degree.16–18 In most reported
cases, the triple bond is attached directly to the molecular core.
Conjugated triple bonds give rise to high transition tempera-
tures due to a combination of a high length-to-breadth ratio
and a high degree of anisotropy of polarisability due to the
extended conjugation.16–18 Unfortunately, the thermal and UV
stability of conjugated acetylenes has been found to be insuf-
ficient to permit their use in LCDs. Non-conjugated triple
bonds attached to the core of a molecule also usually give rise
to high melting points and low clearing points.16–18 In con-
trast to alkenyl-substituted liquid crystals, there are almost
no references18 to liquid crystals with a carbon–carbon triple
bond located within the terminal chain itself, i.e. not attached
directly to the molecular core. A series of model compounds
has now been synthesised to extend these studies. Tolanes19,20
have been chosen as model compounds due to their high
tendency for nematic phase formation, absence of polymorph-
ism, ease of synthesis and almost rigid, linear molecular struc-
ture with very little conformational deviation. A propyl chain
was chosen as a fixed terminal substituent to minimise the effect
of the conformation of this chain as another variable para-
meter. The only dipole moment is associated with the oxygen
atom. Moreover, the oxygen atom is conjugated with the aro-
matic ring and contributes to the high clearing points of the
nematic phases often observed for tolanes. There are no func-
tional groups or atoms with a strong permanent dipole, such as
a cyano group, and, therefore, no interdigitation of neighbour-
ing molecules is expected. Thus, it was hoped that the tolanes
chosen would represent a simple system for the study of the
effects, especially conformational effects, of the presence of
a carbon–carbon triple bond in the terminal chain of a com-
pound on its mesomorphic behaviour.
Results and discussion
Synthesis
The 4-n-alkyl-4’-hydroxytolanes were prepared by Sonogashira
cross-coupling of commercially available (4-n-alkylphenyl)-
acetylenes and 4-iodophenol using a palladium catalyst.21 The
ethers 1–14 were prepared via Williamson ether alkylation of
450
J. Mater. Chem., 2003, 13, 450–457
This journal is # The Royal Society of Chemistry 2003
DOI: 10.1039/b209045g

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Table 1 Transition temperatures (uC) for the 4-n-alkyl-4’-alkoxytolanes (1–14)
n
m
Cr
N
I
Ref.
1
2
3
4
5
6
7
8
3
3
3
3
3
3
3
5
5
5
5
5
5
5
1
2
3
4
5
6
7
1
2
3
4
5
6
7
.
.
.
.
.
.
.
.
.
.
.
.
.
.
61
89
83
73
57
56
48
47
62
55
64
51
48
45
.
.
.
.
.
.
.
.
.
.
.
.
.
.
66
96
.
.
.
.
.
.
.
.
.
.
.
.
.
.
20
20
20
(75)a
84
70
76
70.5
58
89
72
79
70
20
19
20
20
20
20
19,20
9
10
11
12
13
14
77
74
aMonotropic transition temperature.
the resultant 4-n-alkyl-4’-hydroxytolanes with commercially
available 1-bromoalkanes. The Mitsonubu alkylation reaction22
was used to prepare the respective alkenyloxy tolanes 15–18
and hexa-2,4-dienyloxytolane 19 from the corresponding
alcohols. The alkynyloxy tolanes 20–37 were prepared by
Williamson ether alkylation of the 4-n-alkyl-4’-hydroxytolanes
and the appropriate tosylates produced in the normal way
from the commercially available acetylenic alcohols and
4-methylbenzenesulfonyl chloride.
Phase characterisation by thermal optical microscopy
The thermotropic mesophases observed for the compounds
shown in Tables 1–3 were investigated between crossed polari-
sers using optical microscopy. The only phase observed was the
nematic phase. Nematic droplets were observed on cooling
from the isotropic liquid to form the Schlieren texture with two
and four-point brushes characteristic of the nematic phase,
along with optically extinct homeotropic areas. As the sample
was cooled further, the texture often formed more optically
extinct homeotropic areas, which indicates that the phase is
optically uniaxial. The birefringent and homeotropic areas
flashed brightly on mechanical disturbance. This behaviour
and the simultaneous presence of both the homeotropic and the
Schlieren texture confirms that the mesophase observed is
indeed a nematic phase. The compounds prepared with a
carbon–carbon triple bond within the terminal chain did not
exhibit an observable nematic phase despite substantial super-
cooling below the melting point for most homologues.
Table 2 Transition temperatures (uC) for the alkenyl ethers (15–18),
diene ether (19) and the acetylenes (20–28)
–OR
Cr
N
I
15 (E)
16 (Z)
17 (E)
18 (Z)
–OCH2CHLCHC3H7
–OC2H4CHLCHC2H5
–OC3H6CHLCHCH3
–OC4H8CHLCH2
.
.
.
.
.
.
.
.
.
.
.
.
.
.
77
50
76
49
92
97
60
52
42
76
72
56
56
64
.
.
.
.
.
84
(49)a
86
61
141
—
.
.
.
.
.
.
.
.
.
.
.
.
.
.
19 (E,E) –OCH2CHLCHCHLCHCH3
20
21
22
23
24
25
26
27
28
–OCH2CMCCH3
–OCH2CMCC3H7
–OCH2CMCC4H9
–OCH2CMCC5H11
–OC2H4CMCH
—
—
—
Phase characterisation by differential scanning calorimetry
.
(47)a
—
–OC2H4CMCCH3
–OC2H4CMCC2H5
–OC3H6CMCH
The values for the transition temperatures were confirmed by
differential scanning calorimetry (DSC). Good agreement
(y1–2 uC) with the values determined by optical microscopy
were obtained. A typical thermogram obtained by DSC of the
liquid crystal transitions of the mesomorphic tolanes and
—
57
.
.
–OC4H8CMCH
(59)a
aMonotropic transition temperature.
Table 3 Transition temperatures (uC) for the tolanes (29–37)
–OR
Cr
N
I
29
30
31
32
33
34
35
36
37
–OCH2CMCCH3
–OCH2CMCC3H7
–OCH2CMCC4H9
–OCH2CMCC5H11
–OC2H4CMCH
.
.
.
.
.
.
.
.
.
100
59
39
46
67
72
56
56
64
—
—
—
.
.
.
.
.
.
.
.
.
—
.
(64)a
—
–OC2H4CMCCH3
–OC2H4CMCC2H5
–OC3H6CMCH
—
57
.
.
–OC4H8CMCH
(60)a
aMonotropic transition temperature.
J. Mater. Chem., 2003, 13, 450–457
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Fig. 1 Differential scanning thermogram as a function of tempera-
ture for the first heating and cooling cycle for the tolane 15 (scan rate
10 uC min21).
acetylenes listed in Tables 1–3 is shown in Fig. 1 for the tolane
15. The enthalpy of transition between the nematic phase and
the isotropic liquid is relatively small (3.8 J g21), as expected. In
comparison, the enthalpy of fusion (91.2 J g21) of the transition
from the crystalline state to form the nematic phase is much
higher. These values were determined twice on heating and
cooling cycles on the same sample. The values obtained on
separate samples of the same compounds were reproducible
and very little thermal degradation was observed. The baseline
of the spectrum is relatively flat and sharp transition peaks are
observed. Both transitions are first order as expected. A degree
of supercooling below the melting point is observed on the
cooling cycle. The energy of crystallisation is much lower than
that of melting, which suggests the formation of a different
crystal form on cooling. This is confirmed by the appearance of
an exothermic peak at 35 uC due to a crystal–crystal transition,
and the same peak on the second heating cycle as on the first.
Fig. 3 Plot of the transition temperatures of the 4-n-alkoxy-4’-
pentyltolanes (8–14) against the number of the carbon atoms (m) in
the alkoxy chain.
The melting point decreases with increasing chain length due to a
higher degree of flexibility of the terminal chain and a higher
number of non-linear conformations. The van der Waals forces
of attraction between the aromatic cores are also diluted by
longer chains. The same trends in the transition temperatures are
observed for the corresponding 4-n-alkoxy-4’-pentyltolanes
(8–14) as for compounds 1–7 (see Fig. 3), although all the
homologues 8–14 exhibit an enantiotropic nematic phase. The
melting point (av. 53 uC) and the clearing point (av. 74 uC) of the
pentyl homologues 8–14 are lower on average than those (av. 67
and 77 uC, respectively) of the propyl homologues 1–7. However,
the difference in the average melting points is greater than that in
the average clearing points and so the temperature range for the
nematic phase is broader for the pentyl homologues.
Mesomorphism
The chemical bonds in compound 4, shown in Fig. 4, define
the molecular geometry of the aromatic core and, therefore, no
assumptions about the shape of the core need to be made: the
1,4-disubstituted phenyl rings and the conjugated carbon–
carbon triple bond situated between the phenyl rings of the
tolanes automatically generate a linear structure. However, there
is a high degree of free rotation about the interannular bonds
and, as a consequence, the phenyl rings adopt an out-of-plane,
orthogonal arrangement to each other. It is reasonable to
assume that the terminal propyl chain will adopt an all-trans,
antiperiplanar (zig-zag) conformation, as any other confor-
mation would be strongly non-linear and include eclipsed
The thermal data for two short homologous series of 4-n-alkyl-
4’-alkoxytolanes (1–14) are collated in Table 1 and plotted
against the number of carbon atoms (m) in the terminal alkoxy
chain in Fig. 2 and 3. All the homologues of the 4-n-alkoxy-4’-
propyltolanes (1–7) exhibit a nematic phase, although the
nematic phase for compound 3 is monotropic, i.e. formed on
cooling below the melting point. An odd–even effect is observed
for the nematic clearing point, as expected (Fig. 2); see below.
Fig. 2 Plot of the transition temperatures of the 4-n-alkoxy-4’-
propyltolanes (1–7) against the number of the carbon atoms (m) in
the alkoxy chain.
Fig. 4 The energy-minimised structure of compound 4 with an all-trans,
antiperiplanar conformation of the hexyloxy chain (a) in the x–y plane
viewed along the z axis and (b) in the x–z plane viewed along the y axis.
452
J. Mater. Chem., 2003, 13, 450–457

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carbon–hydrogen bonds involving the second and third carbon
atoms in the chain. This assumption would not have to be valid
for longer alkyl chains. Therefore, the only real degree of
molecular flexibility is located in the second terminal (alkoxy)
chain. An all-trans conformation of the terminal hexyloxy
chain of the ether 4 is shown in Fig. 4(a) and (b) (from different
perspectives) and a corresponding alternating cis–trans con-
formation is shown in Fig. 5(a) and (b). Molecular modelling
packages only really find the lowest energy conformation of an
alkyl chain as submitted, i.e. fully-extended or linear. Fig. 4
shows the fully-extended terminal hexyloxy chain to be planar
and Fig. 5 shows an out-of-plane bond between the second and
third carbon atoms in the alternating linear (cis–trans) confor-
mation of the hexyloxy chain. However, the latter conforma-
tion would still give rise to a more symmetrical rotation volume
and a higher length-to-breadth ratio, plus a greater degree of
molecular polarisability (see also Table 4, where these two
extreme conformations of the terminal hexyloxy chain are
shown schematically). On this basis, the assumption of an
extended all-trans conformation appears unreasonable,
although it has been used to explain the alternation in the
clearing point of homologous series, such as that shown in
Fig. 2 and 3.2–4,23–25 It was postulated that the alternation is
due to alternating effects on the anisotropy of polarisablity on
ascending a homologous series as the polarisability increases on
passing from an odd number of carbon atoms in an alkoxy
chain to an even number.23–25 The axial polarisability is
increased about twice as much as the polarisability in an
orthogonal direction on passing from an even to an odd alkoxy
The trans double bond in 15 and 17 leads to higher clearing
points than that of the hexyloxy tolane 4, without a double
bond in the chain; the double bonds in cis and terminal
positions in 16 and 18, respectively, give rise to a lower clearing
point. This may be explained by assuming an all-trans, anti-
periplanar conformation of the terminal chain (see Table 4).
The shape of the chains in 16 and 18 distorts the ideal rotation
volume, whereas the conformation of the chains in 15 and 17
remains within it. The added rigidity of these alkenyloxy chains
compared to that of the hexyloxy-substituted tolane 4 will also
contribute to a higher clearing point. A higher degree of polar-
isability due to the presence of the sp2-hybridised p-bonds may
also contribute. This interpretation seems to be confirmed by
the very high clearing point of the nematic phase formed by the
(E,E)-hexa-2,4-dienyloxy-substituted tolane 19 with two con-
jugated trans carbon–carbon double bonds in the terminal
chain. If the terminal chain of the tolanes 15–19 were to adopt
the corresponding alternating cis–trans conformation, shown
in Table 4, then a higher clearing point than that of the ana-
logous hexyloxy-substituted tolane 4 would be expected for all
of them: the shape of the chain is identical, but the chain would
adopt fewer non-linear conformations due to the added rigidity
caused by the carbon–carbon double bond and the anisotropy
of molecular polarisability should be higher due to the
p-electrons in the double bond. This is seen not to be the case.
The situation is even more complex for the analogous tolanes
20–28, with carbon–carbon triple bonds in their terminal
chains (see Table 2). Only the tolanes 24, 27 and 28, with
carbon–carbon triple bonds in terminal positions, exhibit an
observable nematic phase. This can be explained if the same
assumption of an all-trans, antiperiplanar conformation of the
terminal chain is made for compounds 20–28, incorporating a
triple bond, as that made for the analogous materials 15–19,
incorporating a double bond. The shape of the terminal chain
of 20–23, 25 and 26, with a triple bond contained within the
chain, does not fit completely within the rotation volume of this
chain conformation (see Table 4 for 21 and 26). This is con-
sistent with the non-mesomorphic nature of these materials.
However, the chain of 21 would fit within the rotation volume
of an alternating (cis–trans) conformation of the chain and,
consequently, a high clearing point would be expected. This is
clearly not so. The chain of 26 fits within neither rotation
volume and this compound is not mesomorphic. However, the
triple bonds in terminal positions of 24, 27 and 28 only extends
to a small degree from either of these ideal shapes. Therefore, a
lower clearing point would be expected for these tolanes com-
pared to that of the hexyloxy-substituted tolane 4 and, indeed,
this is observed to be the case for all the acetylenes studied with
a carbon–carbon triple bond in the terminal positions of the
chains. The same trends are observed for the corresponding
pentyl homologues 29–37, with carbon–carbon triple bonds in
their terminal chains (see Table 3), i.e. only 33, 36 and 37, with
triple bonds in terminal positions, exhibit an observable nema-
tic phase. Therefore, the correlation between the position of the
carbon–carbon triple bond in the terminal chain and the
observed liquid crystalline behaviour described above may be
generally valid. In this case, only acetylenes with the triple bond
in a terminal position will show a pronounced tendency for
mesophase formation. However, the clearing point will be at
least marginally lower than that of an analogous compound
with a saturated ethyl group (C2H5) in place of the acetylene
moiety. Any other position of the triple bond in the chain, not
immediately adjacent to or conjugated with the molecular core,
will lead to very low clearing points or to the complete absence
of any observable mesomorphic behaviour.
chain. As
a consequence, the anisotropy of molecular
polarisability is greater for homologues with even alkoxy
chains, e.g. ethoxy or butoxy, and the plot of the clearing points
lies above that of the corresponding homologues with an odd
alkoxy chain, e.g. propoxy and pentoxy (see Fig. 2 and 3).
However, this simple rationale does not explain the more
complex plots of clearing point against chain length observed
for many different classes of liquid crystals.1–10 Therefore, it
seems reasonable to assume that a combination of these two
extreme conformations, as well intermediate conformations, is
most probably present in the nematic phase. This can be
attributed to thermal fluctations and molecular rotation with a
statistical preference for an alternating cis–trans conformation,
probably due to space filling and entropy requirements, in spite
of the partially or fully eclipsed, anticlinic carbon–carbon
bonds present in this conformation. However, it is also highly
likely that some of the population of chains will also be in
extended the all-trans conformation; see below.
The position and configuration of the trans and cis carbon–
carbon double bonds of the alkenyloxy-substituted tolanes
15–18 were chosen on the basis of previous studies11–15 to give
rise to the highest clearing point for a nematic phase. A clear
alternation in the clearing point is seen on ascending the series.
Physical properties
Fig. 5 The energy-minimised structure of compound 4 with an alternating
trans–cis conformation of the hexyloxy chain (a) in the x–y plane viewed
along the z axis and (b) in the x–z plane viewed along the y axis.
Several representative examples of the new compounds have
been evaluated as potential components of nematic mixtures
J. Mater. Chem., 2003, 13, 450–457
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Table 4 Two possible notations that model the conformation of the terminal chain (incorporating six carbon atoms) of the 4-substituted-4’-
propyltolanes (4, 15–19, 21, 26 and 28)
TN–I prediction
All-trans conformation
Alternating cis–trans conformation
All-trans
cis–trans
TN–I/uC
6
Parity
Parity
76
15
Higher
Higher
84
49
86
Lower
Higher
Higher
Higher
16
17
Highest
Highest
Higher
Lowest
141
19
21
26
Low
v60
v56
Lowest
Lower
Lower
59
28
for use in LCDs. A fixed amount (10 wt%) of the acetylene 28
was dissolved in a standard nematic mixture (ZLI 3086) from
E. Merck. The presence of 10 wt% of 28, with a carbon–carbon
triple bond in the terminal position of the flexible aliphatic
chain, leads to a corresponding increase (y10%) in the
refractive indices and the birefringence of the host mixture.
However, the clearing point of the doped mixture is the same as
that of the host mixture. The birefringence of this guest–host
mixture is plotted against temperature in Fig. 6. The plot shows
a non-linear dependence of the birefringence on temperature,
454
J. Mater. Chem., 2003, 13, 450–457

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simple presumption that a high length-to-breadth ratio and
degree of molecular polarisability, due to a long, thin rotation
volume around the molecular long axis, automatically gives rise
to a higher clearing point for the nematic phase does not appear
to be valid. This is clearly the case for these simple tolanes with
alkoxy, alkenyloxy and acetylene terminal chains, where dipole–
dipole interactions due to non-conjugated heteroatoms, which
are known to lead to low clearing points8–10, or other strong
dipole–dipole or dipole-induced interactions, are absent.
Experimental
Techniques
The structures of intermediates and final products were con-
1
firmed by H NMR (JOEL JMN-GX270 FT) and IR (Perkin
Elmer 783) spectroscopy, and mass spectrometry (MS; Finnegan
MAT 1020 automated GC/MS). Reaction progress and product
purity wer 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 temperatures were deter-
mined 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 tempera-
tures and enthalpies was carried out using a Perkin-Elmer
DSC7-PC differential scanning calorimeter. Molecular model-
ling was carried out using ChemDraw Ultra 626 and MM1
energy minimisation. Simulations using the Cerius 227 molecular
modelling packages gave essentially the same results.
Fig. 6 Plot against temperature of the birefringence of compound 28
(10%) in a host nematic mixture.
which reflects the usual increase in the order parameter below
the clearing point with decreasing temperature. A similar
increase in the birefringence of this host mixture was obtained
using equivalent amounts of the corresponding alkenyl
derivatives, e.g. 17. Initial investigations of the photochemical
stability of 28 (Heraeus Instruments Suntest CPS1) indicated
that such compounds with non-conjugated carbon–carbon
triple bond in the terminal chain could be sufficiently stable for
use in LCDs. This is consistent with the absorption spectrum of
this kind of compound, cf. the UV/vis spectrum of a solution of
the corresponding pentyl homologue 37 (10 wt%) in ethanol
(Fig. 7). There is no absorption above 330 nm, which is below
the cut-off frequency of a standard LCD with glass substrates
and polarisers. These initial results suggest that liquid crystals
with a carbon–carbon triple bond in a terminal position of the
flexible aliphatic chain may be of potential use as components
of nematic mixtures of LCDs. However, further tests, including
switching studies of nematic mixtures and more stringent
stability tests, are required to confirm these findings.
Synthesis
A short reaction pathway to several typical liquid crystalline
tolanes (4, 15–20) is described below in order to illustrate the
spectral data obtained for the final compounds in Tables 1–3
and the reaction intermediates leading to them.
4-Hydroxy-4’-propyltolane. 4-Propylphenylacetylene (5.00 g,
34.72 mmol) was added dropwise at room temperature to a
mixture of 4-iodophenol (7.28 g, 33.67 mmol), Pd(PPh3)2Cl2
(0.46 g, 0.66 mmol), copper iodide (0.25 g, 1.32 mmol), tri-
ethylamine (5.00 g, 49.60 mmol) and tetrahydrofuran (40 cm3).
The mixture was stirred for 24 h at room temperature, then
filtered and the filtrate evaporated down under reduced pressure.
The crude product was purified by column chromatography on
silica gel using a 1 : 1 (v/v) mixture of light petroleum (b.p. 40–
60 uC) and dichloromethane as eluent, and recrystallised from
hexane to yield the desired phenol (4.42 g, 57%). Purity 96.2%
(GLC). 1H NMR (CDCl3): dH 0.94 (t, 3H), 1.65 (sext, 2H), 2.59
(t, 2H), 4.96 (s, 1H), 6.81 (dt, J ~ 8 Hz, 2H), 7.14 (dt, J ~ 8 Hz,
2H), 7.41 (dt, J ~ 2 Hz, 2H), 7.44 (dt, J ~ 2 Hz, 2H). IR (KBr)
Conclusions
The results of this study appear to be consistent with the
empirical interpretation of Gray2–4 of the nematic state, which
presupposes that molecules responsible for the formation of the
nematic phase are dynamic entities, which must consist of a
fairly rigid molecular core and at least one terminal chain in an
all-trans, antiperiplanar conformation, with a subtle balance
between flexibility and rigidity. The carbon–carbon double bond
in a limited number of positions and configurations in the
terminal chain consistent with this conformation seems to
embody this compromise. The carbon–carbon triple bond does
not. Perhaps a degree of fluidity of the chain is necessary to
satisfy a requirement for space filling between molecules. The
n
max/cm21: 3500, 2960, 1600, 1520, 1450, 1250, 1100, 840. MS:
m/z 236 (M1), 207, 178.
4-Hexyloxy-4’-propyltolane (6). A mixture of 1-bromohexane
(0.19 g, 1.17 mmol), 4-hydroxy-4’-propyltolane (0.25 g,
1.06 mmol), potassium carbonate (0.58 g, 4.24 mmol) and
butanone (40 cm3) was heated at 80 uC overnight. The cooled
reaction mixture was filtered and the filtrate evaporated down
under reduced pressure. The crude product was purified by
column chromatography on silica gel using a 95 : 5 (v/v) mix-
ture of light petroleum (b.p. 40–60 uC) and ethyl acetate as
eluent, and recrystallised from ethanol to yield the desired ether
(0.18 g, 52%). Purity 99.7% (GLC). 1H NMR (CDCl3): dH 0.90
(t, 3H), 0.94 (t, 3H), 1.34 (m, 4H), 1.45 (sext, 2H), 1.64 (quint,
2H), 1.78 (quint, 2H), 2.59 (t, 2H), 3.96 (t, 2H), 6.86 (dt, J ~
8 Hz, 2H), 7.14 (dt, J ~ 8 Hz, 2H), 7.42 (dt, J ~ 8 Hz, 2H), 7.45
Fig. 7 UV/vis spectrum of a solution of compound 37 (10%) in ethanol.
J. Mater. Chem., 2003, 13, 450–457
455

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(dt, J ~ 8 Hz, 2H). IR (KBr) nmax/cm21: 2930 (C–H stretch),
1604 (CLC stretch), 1515 (CLC stretch), 1248 (C–O stretch),
1077 (C–H in-plane def. 1,4-disub.), 840 (C–H out-of-plane def.
1,4-disub.). MS: m/z 320 (M1), 236, 207.
(CLC stretch), 1515 (CLC stretch), 1243 (C–O stretch), 1175
(C–H in-plane def. 1,4-disub.), 993 (C–H in-plane def. 1,4-
disub.), 835 (C–H out-of-plane def. 1,4-disub.). MS: m/z 316
(M1), 236, 207.
4-[(E)-2-Hexenyloxy]-4’-propyltolane (15). A solution of
triphenylphosphine (0.61 g, 2.33 mmol), (E)-2-hexen-1-ol (0.23 g,
2.33 mmol), 4-hydroxy-4’-propyltolane (0.50 g, 2.12 mmol),
diethyldiazodicarboxylate (0.40 g, 2.33 mmol) and tetrahydro-
furan (40 cm3) was prepared at 0 uC. The reaction mixture was
stirred at room temperature overnight. The solvent was removed
under reduced pressure and the crude product was purified by
column chromatography on silica gel using a 90 : 10 (v/v)
mixture of light petroleum (b.p. 40–60 uC) and ethyl acetate as
eluent. The crude product was recrystallised from ethanol to
2-Butynyl toluene 4-sulfonic acid ester. A solution of
p-toluenesulfonyl chloride (3.48 g, 18.2 mmol) in pyridine
(1.92 g, 24.3 mmol) and chloroform (40 cm3) was added drop-
wise to a solution of 2-butyn-1-ol (0.85 g, 12.1 mmol) in
chloroform (40 cm3) at 0 uC. The reaction mixture was main-
tained at 0 uC overnight. After stirring for 1 h at room
temperature, diethyl ether (20 cm3) and water (30 cm3) were
then added. The organic layer was separated off and the
aqueous layer extracted into dichloromethane. The combined
organic layers were washed with water (1 6 100 cm3), dried
(MgSO4), filtered and the filtrate evaporated down under
reduced pressure. The crude product was purified by column
chromatography on silica gel using a 90 : 10 (v/v) mixture of
light petroleum (b.p. 40–60 uC) and ethyl acetate as eluent to
yield a clear liquid (1.22 g, 45%), which was used without
1
yield the desired ether (0.18 g, 26%). Purity 99.8% (GLC). H
NMR (CDCl3): dH 0.91 (t, 3H), 0.94 (t, 3H), 1.45 (sext, 2H), 1.65
(sext, 2H), 2.07 (m ,2H), 2.59 (t, 2H), 4.49 (d, 2H), 5.71 (m, 1H),
5.87 (m, 1H), 6.87 (dt, J ~ 8 Hz, 2H), 7.14 (dt, J ~ 8 Hz, 2H),
7.41 (dt, J ~ 8 Hz, 2H), 7.44 (dt, J ~ 8 Hz, 2H). IR (KBr) nmax
/
cm21: 2926 (C–H stretch), 1604 (CLC stretch), 1514 (CLC
stretch), 1284 (C–H out-of-plane def. trans-CLC), 1239 (C–O
stretch), 1175 (C–H in-plane def. 1,4-disub.), 1003 (C–H in-plane
def. 1,4-disub.), 968 (C–H out-of-plane def. trans-CLC), 837 (C–H
out-of-plane def. 1,4-disub.). MS: m/z 318 (M1), 236, 207.
1
further purification. Purity 97.3% (GLC). H NMR (CDCl3):
dH 1.88 (t, 3H), 2.50 (s, 3H), 4.68 (quart, 2H), 7.43 (d, 2H), 7.82
(d, 2H). IR (KBr) nmax/cm21: 2930, 1600, 1320, 1270, 1130,
1050, 960, 810. MS: m/z 244 (M1), 155, 91.
4-(2-Butynyloxy)-4’-propyltolane (20). A mixture of 2-buty-
nyl toluene 4-sulfonic acid ester (0.19 g, 1.17 mmol), 4-hydroxy-
4’-propyltolane (0.25 g, 1.06 mmol), potassium carbonate (0.58 g,
4.24 mmol) and butanone (40 cm3) was heated at 80 uC over-
night. The cooled reaction mixture was filtered and the filtrate
evaporated down under reduced pressure. The crude product
was purified by column chromatography on silica gel using a
95 : 5 (v/v) mixture of light petroleum (b.p. 40–60 uC) and ethyl
acetate as eluent, and recrystallised from ethanol to yield the
desired ether (0.17 g, 50%). Purity 99.8% (GLC). 1H NMR
(CDCl3): dH 0.95 (t, 3H), 1.65 (sext, 2H), 1.88 (t, J ~ 2 Hz, 3H),
2.58 (t, 2H), 4.68 (quart, J ~ 2 Hz, 2H), 6.95 (dt, J ~ 8 Hz,
2H), 7.14 (dt, J ~ 8 Hz, 2H), 7.41 (dt, J ~ 8 Hz, 2H), 7.45 (dt,
J ~ 8 Hz, 2H). IR (KBr) nmax/cm21: 2929 (C–H stretch), 1602
(CLC stretch), 1507 (CLC stretch), 1240 (C–O stretch), 1003
(C–H in-plane def. 1,4-disub.), 833 (C–H out-of-plane def. 1,
4-disub.). MS: m/z 288 (M1), 235, 206.
4-[(Z)-3-Hexenyloxy]-4’-propyltolane (16). Synthesised as
for 15, using (Z)-3-hexen-1-ol (0.23 g, 2.33 mmol). Yield
0.22 g, 33%. Purity 99.9% (GLC). 1H NMR (CDCl3): dH
0.94 (t, 3H), 0.99 (t, 3H), 1.64 (sext, 2H), 2.10 (m, 2H), 2.56 (m,
4H), 3.97 (t, 2H), 5.42 (m, 1H), 5.55 (m, 1H), 6.85 (dt, J ~ 8 Hz,
2H), 7.14 (dt, J ~ 8 Hz, 2H), 7.42 (dt, J ~ 8 Hz, 2H), 7.44 (dt,
J ~ 8 Hz, 2H). IR (KBr) nmax/cm21: 2934 (C–H stretch),
1605 (CLC stretch), 1508 (CLC stretch), 1247 (C–O stretch),
1176 (C–H in-plane def. 1,4-disub.), 1029 (C–H in-plane def.
1,4-disub.), 835 (C–H out-of-plane def. 1,4-disub.). MS: m/z
318 (M1), 235, 206.
4-[(E)-4-Hexenyloxy]-4’-propyltolane (17). Synthesised as
for 15, using (E)-4-hexen-1-ol (0.23 g, 2.33 mmol). Yield
0.28 g, 41%. Purity 99.8% (GLC).1H NMR (CDCl3): dH 0.94 (t,
3H), 1.65 (m, 5H), 1.84 (m, 2H), 2.16 (m, 2H), 2.59 (t, 2H), 3.97
(t, 2H), 5.48 (m, 1H), 5.55 (m, 1H), 6.85 (dt, J ~ 8 Hz, 2H),
7.14 (dt, J ~ 8 Hz, 2H), 7.41 (dt, J ~ 8 Hz, 2H), 7.45 (dt, J ~
8 Hz, 2H). IR (KBr) nmax/cm21: 2940 (C–H stretch), 1604 (CLC
stretch), 1513 (CLC stretch), 1285 (C–H out-of-plane def. trans-
CLC), 1249 (C–O stretch), 1119 (C–H in-plane def. 1,4-disub.),
1048 (C–H in-plane def. 1,4-disub.), 966 (C–H out-of-plane def.
trans-CLC), 836 (C–H out-of-plane def. 1,4-disub.). MS: m/z
318 (M1), 236, 207.
Acknowledgements
The technical staff in the Department of Chemistry at the
University of Hull are thanked for provision of analytical
services. Qinetiq (Malvern) is thanked for financial support of a
postgraduate studentship (G. W. S.). The EPSRC is thanked
for financial support of an Advanced Fellowship (S. M. K.).
This paper is reproduced with the permission of Her
Britannic Majesty’s Stationary Office.
4-(5-Hexenyloxy)-4’-propyltolane (18). Synthesised as for 15,
using 5-hexen-1-ol (0.23 g, 2.33 mmol). Yield 0.25 g, 37%.
Purity 99.9% (GLC).1H NMR (CDCl3): dH 0.94 (t, 3H), 1.64
(m, 4H), 1.81 (m, 2H), 2.13 (m, 2H), 2.59 (t, 2H), 3.98 (t, 2H),
5.02 (m, 2H), 5.84 (m, 1H), 5.83 (dd J ~ 18, 1 Hz, 1H), 6.85 (dt,
J ~ 8 Hz, 2H), 7.14 (dt, J ~ 8 Hz, 2H), 7.41 (dt, J ~ 8 Hz, 2H),
7.44 (dt, J ~ 8 Hz, 2H). IR (KBr) nmax/cm21: 2942 (C–H
stretch), 1610 (CLC stretch), 1518 (CLC stretch), 1247 (C–O
stretch), 1172 (C–H in-plane def. 1,4-disub.), 1021 (C–H in-
plane def. 1,4-disub.), 833 (C–H out-of-plane def. 1,4-disub.).
MS: m/z 318 (M1), 236, 207.
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