Interfacial layer interactions: Their effects on synclinic and anticlinic smectic mesophase behaviour in liquid crystals

Article abstract of DOI:10.1039/b610154b

Through the use of bulky cyclic terminal groups, anticlinic smectic C phases have been observed in 2-methylbutyl materials based on the MHPOBC motif. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b610154b

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Interfacial layer interactions: their effects on synclinic and anticlinic
smectic mesophase behaviour in liquid crystals{
Stephen J. Cowling* and John W. Goodby
Received (in Cambridge, UK) 17th July 2006, Accepted 1st September 2006
First published as an Advance Article on the web 18th September 2006
DOI: 10.1039/b610154b
weaken the interlayer interactions and induce TGB phases. We
have also demonstrated that it is possible to generate high tilt
anticlinic phases via the inclusion of polar fluorinated end groups,
and we have shown how the inclusion of a single hydrogen atom in
a fluorinated chain causes extreme disruption to the mesophase
formation11 compared to their purely perfluorinated analogues.
In this article we demonstrate that through the inclusion of an
ether-linked cyclobutane or oxetane ring at the terminal position of
a terminal chain, see Fig. 2, it is possible to promote the formation
of anticlinic behaviour in compounds containing the weakly polar
and less sterically hindered chiral 2-methylbutyl moiety. We also
show that there is a marked odd/even effect which either promotes
synclinic or anticlinic behaviour. As a basis of reference it was also
necessary to prepare conventional materials of equivalent structure
and chain length to those with terminal ring structures. For this
purpose compounds of series 1 were prepared. In all of the series
presented we also incorporated lateral fluoro substituents in the
core in order to moderate the lateral molecular interactions.
The synthetic route for the preparation of compounds from
series 1 is shown in Scheme 1 and the synthetic route for the
preparation of compounds from series 2 and 3 is shown in
Scheme 2. For the sake of brevity, the full synthetic details are
given in the ESI.{
Compound 1 was alkylated using an alkyl bromide with
potassium carbonate as the base in butanone. The methyl
protecting group was removed by hydrolysis to give carboxylic
acid 3. The final step to give compounds 1.1–1.6 was an
esterification with compound 4 using dicyclohexylcarbodiimide
(DCC) and N, N9-dimethylaminopyridine (DMAP). The synthesis
of compound 4 has been reported previously.12 The synthesis of
compounds 2.1–2.6 required the preparation of 1-methylcyclobu-
tanemethanol (7). This was achieved by derivatisation using
methyl iodide and LDA to give 6 followed by reduction of the
carboxylic acid using lithium aluminium hydride to give alcohol 7.
From this point, the preparation of compounds 2.1–2.6 and 3.1–
3.6 followed the same synthetic pathway. Compounds 9 and 10
Through the use of bulky cyclic terminal groups, anticlinic
smectic C phases have been observed in 2-methylbutyl materials
based on the MHPOBC motif.
Since the first discovery by Meyer et al.1 of a liquid crystal which
exhibited ferroelectric behaviour where the lath-like molecules are
tilted in the same direction in layers (synclinic), research into
ferroelectric liquid crystals has predominantly concentrated on
investigations involving changes to the central core geometries of
mesogenic materials or by varying the length of the terminal
aliphatic or chiral aliphatic chain (tail), resulting in a number of
detailed reviews of ferroelectric LCs being reported.2 Classical
investigations of ferroelectric LCs have led to a number of
interesting phenomena being discovered including spontaneous
polarisation inversion3 as a function of temperature and the
discovery of novel mesophases such as the Twist Grain Boundary
(TGB) phase,4 and antiferroelectric and ferrielectric subphases of
the smectic C phase.5 Extensive research in the area of ferro- and
anti-ferroelectric LCs has provided for a number of property–
structure–activity relationships to be developed for materials based
on the central core structures, shown in Fig. 1.
Typically these cores were substituted with chiral end-groups
such as 1-methylalkyl6 and 1-trifluoromethylalkyl.7 In some cases a
methoxy or an ethoxy group was located at the terminal position of
the chiral tail.8 The 1-methylheptyl substituted systems predomi-
nantly exhibit antiferroelectric (anticlinic) phases, with many
materials also exhibiting novel ‘‘V’’-shaped switching in electrooptic
experiments, which have been used in prototype display devices.
There have not been many studies which examine the effects of
the interfaces between the smectic layers. In order for the
formation of various types of smectic phase behaviour there has
to be a transfer of structural information9 across the interlayer
interface between the diffuse smectic layers. In a recent publica-
tion10 we demonstrated that through the inclusion of bulky
alicyclic rings at the ends of the molecules it was possible to
Fig. 1 General structure of ferroelectric/antiferroelectric liquid crystals.
Department of Chemistry, University of York, Heslington Road, York,
UK YO10 5DD. E-mail: sc520@york.ac.uk; jwg500@york.ac.uk;
Fax: +44 (0)1904 432592; Tel: +44 (0)1904 432539
{ Electronic supplementary information (ESI) available: Full synthetic
details for the synthesis of compounds of series 1, 2 and 3. See DOI:
10.1039/b610154b
Fig. 2 Structures of compounds of series 1–3.
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Table 1 Structure and transition temperature of series 1
Compd
n
x
y
Transition temperaturesa/uC
1.1
1.2
1.3
1.4
1.5
1.6
a
14
14
14
15
15
15
H
H
F
H
H
F
H
F
H
H
F
K 82.2 C* 127.4 A 168.2 Iso
K 60.7 C* 123.7 A 165.8 Iso
K 72.7 C* 99.9 A 139.3 Iso
K 73.7 C* 127.6 A 167.3 Iso
K 53.1 C* 122.5 A 164.8 Iso
K 71.0 C* 98.1 A 138.5 Iso
H
In the table A is smectic A, C* is smectic C*.
Scheme 1 Preparation of alkoxy compounds of series 1.
Table 2 Structure and transition temperature of series 2 and 3
Compd
n
x
y
Transition temperaturesa/uC
2.1
2.2
2.3
2.4
2.5
2.6
10
10
10
11
11
11
H
H
F
H
H
F
H
F
H
H
F
K 32.8 C* 125.7 A 146.7 Iso
K 41.8 C* 123.9 A 147.6 Iso
K 36.1 C* 101.5 A 115.7 Iso
K 59.4 CA* 88.9 C* 124.0 A 146.8 Iso
K 38.2 CA* 89.8 C* 119.3 A 145.7 Iso
K 68.1 C* 101.2 A 117.1 Iso
H
Compd
n
X
y
Transition temperaturesa/uC
3.1
3.2
3.3
3.4
3.5
3.6
a
10
10
10
11
11
11
H
H
F
H
H
F
H
F
H
H
F
K 25.6 C* 101.3 A 135.3 Iso
K 220.1 C* 98.9 A 143.7 Iso
K 31.3 C* 54.7 A 108.0 BP 111.2 Iso
K 52.0 CA* 70.0 C* 104.5 A 130.5 Iso
K 32.4 CA* 65.0 C* 112.3 A 124.9 Iso
K 73.4 C* 74.8 A 102.6 Iso
Scheme 2 Preparation of compounds of series 2 and 3.
H
In the table A is smectic A, C* is smectic C* and CA* is
antiferroelectric smectic C*.
were prepared by reacting compound 7 or 8 with the appropriate
dibromoalkane. The final synthetic steps were identical to the
preparation of compound 1.1–1.6.
The compounds were all characterised by polarised optical
microscopy, differential scanning calorimetry and electrooptical
analysis.10 The transition temperatures for compounds of series 1
are given in Table 1 and the transition temperatures for
compounds of series 2 and 3 are given in Table 2.
required. Examination of cyclobutane (series 2) and oxetane
(series 3) reveals much more varied behaviour. For the compounds
with a decylmethylene spacer (2.1–2.3 and 3.1–3.3) the mesophase
behaviour is similar to that of the alkoxy compounds in series 1.
However, by POM an antiferroelectric phase was also observed in
2.4, 2.5, 3.4 and 3.5. Confirmation was obtained via electrooptic
studies. Thus, each material was flow-filled into 4 mm antiparallel
buffed polyimide cells (Linkam) and the direction and magnitude
of spontaneous polarisation was determined using a current
reversal technique. Compounds 2.6 and 3.6 do not show any
evidence of anticlinic ordering and there was a reduction in
mesophase stability, which is associated with the position of the
fluoro substituent. The interaction of the fluoro substituent with
the adjacent carbonyl effectively causes an internal rotation of the
terminal phenyl ring which in turn reduces the ability of the
Compounds of series 1, which are of conventional structure, all
exhibit smectic A and smectic C* phases. There is clearly a strong
effect of the position of the lateral fluoro substituent on mesophase
stability where transition temperatures are significantly lower when
the fluoro substituent is in the 2-position, compounds 1.3 and 1.6.
The combination of the 2-methylbutyl and the long alkoxy chains
at the smectic layer interfaces strongly promote synclinic
behaviour, which is expected from the current structure–property
relationships based on the fact that for anticlinic behaviour a
terminal branched chain, typically a 1-methylheptyl unit is
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Fig. 3 Spontaneous polarisation as a function of reduced temperature
for compounds 1.1, 2.1, 3.1, 1.4, 2.4 and 3.4.
Fig. 4 Binary phase diagram of 3.4 with MHPOBC.
LCDs is the need for pitch compensation. Although these
materials exhibit extremely high switching thresholds, a 2-methyl-
butyl group provides a longer pitch than a 1-methylheptyl.
Most antiferroelectric liquid crystals have very short pitches,
typically ,2 mm and these new materials, due to their miscibility,
may provide a way to provide pitch compensation in AFLC
mixture to facilitate alignment in future devices.
molecules to pack efficiently laterally for mesophase formation.
This change in core conformation is sufficient to prevent the
formation of anticlinic organisation in these homologues.
Fig. 3 shows the spontaneous polarisation as a function of
reduced temperature for compounds 1.1, 2.1, 3.1, 1.4, 2.4 and 3.4.
It was clear that the alkoxy compounds from series 1 switched
solely in a ferroelectric manner, giving rise to a single current peak
which is associated with a ferroelectric response. Identical
behaviour was also observed for 2.1 and 3.1. Most interestingly,
compounds 2.4 and 3.4 began switching in a ferroelectric mode but
when they approached the transition to the anticlinic phase, the
spontaneous polarisation fell rapidly towards zero. A threshold
voltage was observed which was too large for further switching to
occur. This behaviour indicated that the transition to the
antiferroelectric state results in an apparent threshold to switching
indicating strong interlayer interactions. The direction of polarisa-
tion for all of the materials reported was negative and there was no
evidence to suggest that the sign of polarisation for compounds
2.4, 2.5, 3.4 and 3.5 was undergoing polarisation inversion.
To confirm if this mesophase was an antiferroelectric phase,
mixture studies with 1-methylheptyloxycarbonylphenyl octyloxy-
biphenyl carboxylate (MHPOBC) were carried out. The materials
were found to be completely miscible with MHPOBC throughout
the concentration range. Fig. 4 shows a binary phase diagram for
3.4 mixed with MHPOBC. Clearly the smectic A–isotropic liquid,
smectic C*–smectic A and anticlinic smectic C–smectic C* show
linear behaviour over the whole mixture range. Only the
ferrielectric smectic Cc phase and the hexatic stacked smectic I
phase are suppressed at approximately 60 wt% 3.4 in MHPOBC.
The bulky cyclic group does not support the formation of a higher
order smectic phase and to date none have been observed with this
type of end group. This provides further evidence that this type of
group may interfere with the organisation of the molecules at the
interface via reduction of the interlayer interactions through steric
crowding.
In conclusion we have demonstrated that it is possible to
investigate the layer packing and interlayer interactions in lamellar
smectic phases via the use of electrical field studies. Property–
structure correlations obtained from such studies should be useful
in the design of future materials for display and projector devices
where grey scale and rapid response times are required.
This work is published with the permission of the Controller of
Her Britannic Majesty’s Stationery Office.
Notes and references
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It has been shown that the inclusion of a small bulky group at
the terminus of an alkyl chain in a typical ferroelectric liquid
crystal can alter the interactions at the smectic layer interfaces
sufficiently to promote anticlinic phase behaviour. Indeed, one of
the major problems for commercialisation of antiferroelectric (AF)
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Chem. Commun., 2006, 4107–4109 | 4109