A chiral electrooptic response in a racemic liquid crystal

Article abstract of DOI:10.1039/b416884b

Over recent years, little attention has been paid to the electrooptic properties of racemic modifications of smectic liquid crystals, however, in this communication we report on a chiral response of a racemic modification to an applied electrical field wh

Full text of DOI:10.1039/b416884b

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A chiral electrooptic response in a racemic liquid crystal
Stephen J. Cowling,a Alan W. Hallb and John W. Goodby*b
Received (in Cambridge, UK) 3rd November 2004, Accepted 23rd December 2004
First published as an Advance Article on the web 28th January 2005
DOI: 10.1039/b416884b
Although we prepared examples of material 11 in its
Over recent years, little attention has been paid to the
electrooptic properties of racemic modifications of smectic
liquid crystals, however, in this communication we report on a
chiral response of a racemic modification to an applied
electrical field while in its synclinic smectic C phase.
enantiomeric R- and S-forms, we specifically focus in this article
on its racemic modification and describe its synthesis, chemical
analysis and physical properties.
In order to ensure that the racemic form (ee of zero) of 11 was
obtained we decided to start its synthesis with the reduction of
2-octanone, 1, with sodium borohydride, see Scheme 1 below. The
synthesised 2-octanol, 2, was first subjected to analysis using NMR
and chiral GC, and then used immediately in subsequent
transformations, thereby reducing any possibility that it could be
contaminated or subjected to microbial attack. Chiral GC was
found to separate the enantiomers to give symmetrical peaks,
however, these could not be resolved to the baseline.
In a seminal review of 1977, Meyer demonstrated when the
symmetry of the ‘synclinic’ smectic C liquid crystal phase is
reduced from C2h to C2, via the incorporation of stereogenic
centres into the structures of the constituent rod-like molecules of
the mesophase, extrinsic ferroelectric properties can arise.1
Ferroelectric behaviour was experimentally found the year before
in the now classical material DOBMBC.2 The discovery of
ferroelectricity in liquid crystals led directly to the invention of a
wide variety of electrooptic devices for use as light valves, gates
and displays.3 In recent years surface stabilised bistable devices
(SSFLCDs) have been commercialised in reflective mode microdis-
plays for use in the eyepieces of digital cameras and camcorders,4
and monostable and ‘V-shaped’ switching mode devices have been
utilised in prototype fast-switching displays (sub-millisecond)
operating with grey-scale.5
Conversely, in 1972 McMillan6 theorised that various forms of
the synclinic smectic C phase could be ferroelectric in the absence
of molecular chirality, however, such behaviour has never been
observed in practice. More recently, a similar theoretical model for
ferroelectricity in non-chiral materials was proposed by Tredgold
for molecules with kinked molecular shapes associated with ester
or imine linking groups.7 For these structures he postulated that
the dipoles associated with such linkages become spontaneously
aligned to give a ferroelectric phase without the need for chirality.
However, like McMillan, Tredgold’s theoretical models have
never been realised under experimental conditions. However, in
recent years banana-like liquid crystals have been shown to exhibit
ferroelectricity in domains, without the presence of molecular
chirality.8
Phenol 6 was prepared by standard methods starting from
4-hydroxybenzoic acid, 3. The hydroxy group was first protected
as the carbonate to give 4, this intermediate was esterified with the
racemic form of 2-octanol in the presence of diisopropyl
azodicarboxylate (DIAD), triphenylphosphine and THF to give
5. Compound 5 was deprotected using a mixture of ammonia and
ethanol to generate the phenol 6. The optical purity of the phenol
was determined using chiral GC analysis before it was used
further. In this case the phenol was able to be resolved to the
baseline, and within the limits of experimental error the
enaniomeric excess was found to be zero.
49-(11-Hydroxyundecyloxy)biphenyl-4-carboxylic acid, 9, was
prepared starting from 4-cyano-49-hydroxybiphenyl, 7, which was
first alkylated with 11-bromoundecan-1-ol in the presence of
potassium carbonate in butanone to give 8, followed by base
hydrolysis of the nitrile to give 9.
Acid 9 and phenol 6 were esterified in the presence of 1-(3-
dimethylaminopropyl)-3-ethyl
carbodiimide
hydrochloride
(EDAC) and 4-(dimethylamino)pyridine to give 10. Under these
conditions 10 is formed without any self-esterification of 9. The
last step in the pathway was the esterification, under the same
conditions, for 10 with cyclohexylacetic acid to give the racemic
modification 11.
In the following we focus our attention on one particular
material, compound 11, based on the classical material
MHPOBC.9 We took the generic structure of the MHPOBC
family of materials, and modified it such that the non-branched
aliphatic chain was substituted with a terminal carbocyclic ring –
cyclohexane being selected because of its non-polarity, steric bulk
and compatibility with aliphatic chains. In this case we expected
that the bulky nature of the terminal group would cause disruption
of the packing of the tails of the molecules at the layer interfaces,
thereby possibly weakening the inter-layer interactions, and
allowing us control over properties such as mesophase formation
and tilt angle.
The chemical and structural analyses of the racemic modifica-
tion of 4-(1-methylheptyloxycarbonyl)phenyl 49-[11-(2-cyclohexyl-
acetoxy)undecyloxy]biphenyl-4-carboxylate, 11, gave the following
results:
Transition temperatures: K 58.4 SmC 77.1 SmA 83.0 uC Iso Liq
Enthalpies of transition: 73.6, 0.4, 2.84 J g21
1H NMR (CDCl3) d: 0.86–1.01 (5H, m), 1.20–1.40 (24H, m),
1.47 (2H, quint), 1.53–1.87 (14H, m), 2.17 (2H, d), 4.02 (2H, t),
4.05 (2H, t), 5.16 (1H, sext), 7.00 (2H, d), 7.31 (2H, d), 7.60 (2H,
d), 7.70 (2H, d), 8.13 (2H, d), 8.23 (2H, d) ppm.
IR (KBr) nmax: 1262, 1601, 1720, 1738, 2848, 2922 cm21
.
EA: Calc.: C, 76.18; H, 8.70. Found: C, 76.31; H, 8.97%).
OR: [a] 5 0.00u; 0.0025 g ml21 in CHCl3.
23
D
*jwg500@york.ac.uk
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Scheme 1
Initial mesophase classification was achieved using polarized-
light optical microscopy (POM). Fig. 1 shows the homeotropic
and focal-conic forms of the smectic A phase (a) and the Schlieren
and broken focal-conic textures of the smectic C phase of racemate
(b). The Schlieren texture is typical of an achiral smectic C phase,
which shows no colour change upon rotation of the analyzer
indicating that the phase has no helical structure.
X-Ray diffraction studies on aligned and unaligned specimens
of the racemic modification were performed as a function of
temperature. In all cases the X-ray patterns obtained were for
layered phases with disorganized arrangements of their constituent
molecules. For example, the results obtained for the wide angle
region showed broad diffuse peaks related to the lateral positional
order between the molecules. In the narrower angle region, strong
reflections were seen which were related to the correlations
between the layers. For all of the mesophases the layers were found
to be uncorrelated with diffuse packing of the molecules. The layer
spacings were, however, at higher q values in the tilted smectic C
phase, than in the smectic A phase.
˚
Fig. 2 The layer spacing (A) for 11 as a function of temperature (uC).
perpendicular to the surfaces of the cell. For chiral materials, when
the associated spontaneous polarization couples to the DC field
two switched states are obtained, one with the polarization
pointing up, the other with it pointing down. Switching occurs as
shown by the movement of the molecules about a cone of
degenerate energy. Thus for a positive DC bias one state is formed,
and for a negative bias the other state is obtained. For a racemate
an equivalent mixture of switched states would be expected to be
obtained, provided that enantioselective separation occurs.
Approximately 50% transmission under crossed polars thus would
be expected.
The layer spacing as a function of temperature is shown in
Fig. 2. The smectic A layer spacing was found to be approximately
˚
42 A. As the temperature is lowered further the transition to the
smectic C phase is marked by a rapid decrease in the layer spacing
over a temperature range of approximately 5 uC, upon which the
layer spacing becomes temperature independent. This behavior is
typical of a smectic C phase formed from a smectic A phase via a
second-order phase transition, with the layer spacing falling and
eventually levelling off with the reduced temperature.
Thus, electrical field studies were performed on the racemate
using a variety of wave forms and ITO coated cells of different
alignment agents (polyimide, nylon etc.), orientations of the
buffing directions, (parallel- and anti-parallel rubbed) and cell
spacings (1.5 and 5 mm). Remarkably, switching behaviour typical
of ferroelectricity, rather than those associated with dielectric or
flexoelectric effects, was observed for the achiral synclinic smectic
C phase of the racemate.
A schematic of the type of cell construction used for electrical
field investigations of tilted smectic liquid crystals is shown in
Fig. 3. The molecules are arranged in layers, where the layers are
The racemate was found to be relatively sensitive to the nature
and quality of the aligning agent and the origins of the cells, with
the best results being obtained in cells obtained from the
University of Chalmers (Sweden). For the bright extreme switched
state, 95% transmission was obtained relative to the transmission
through an empty cell (i.e. to act as a baseline for the contrast
ratio). For the fully switched dark state transmission was close to
that for an empty cell observed between crossed polars. Similar
results were obtained in thin cells (1.5 mm) and thick cells (5 mm)
alike, see Fig. 4. In this figure the switched states are shown upon
reversal of the DC field. The bright state corresponds to an
Fig. 1 (a) The homeotropic and focal-conic textures of the SmA phase,
and (b) the Schlieren and broken focal-conic textures of the SmC phase of
racemate 11.
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Fig. 3 A schematic representation of the two switched states for a ferroelectric liquid crystal in a device constructed of ITO coated glass surfaces coated
with suitable homogeneous aligning agent. The spontaneous polarization, which is associated with molecular chirality, couples to the DC field.
Fig. 4 The switched states for the racemic material 11 in a cell with a
1.5 mm spacing and parallel rubbed polyimide alignment coatings. Left
side .90% transmission, right side less than 5% transmission.
approximate value of 95% transmission, and the dark state less
than 5% transmission. However, the value of the ‘‘induced
spontaneous polarization’’ was found to be zero, but the sign was
determined to be positive (‘‘Ps+’’).10 In addition, in thicker cells the
switching profiles gave V-shaped optical/transmission and elec-
trical responses. As a result, this gives us the possibility of creating
devices with gray-scale operation, and without the need for chiral
materials which are often expensive to produce.11
Comparison of the results obtained for the determination of the
spontaneous polarization for the racemate with those obtained for
the enantiomers, shows that the enantiomers typically reached
maximum values of approximately 80–100 nC cm22 at reduced
temperatures of 50u from the Curie point, see Fig. 5(a). The slight
differences between the R- and S-enantiomers arises from slight
differences in optical purity. This result confirms that the racemate
is indeed racemic as no polarization could be determined, and the
behaviour of each enantiomer is typical of a conventional
ferroelectric liquid crystal. Interestingly the switched tilt angle of
the racemate (23u) was considerably lower than the values found
for either of the enantiomers (36u), see Fig. 5(b).
These results appear to be quite remarkable in that we have an
apparent chiral response to an electric field applied to a racemic
modification. Such a response may be due to amplification by the
liquid crystalline media. This outcome suggests that either our
conventional understanding of ferroelectric behaviour in chiral
smectic C* phases requires modifying, and there is a greater
dependency on flexoelectric and dielectric effects, or that parity at
the macroscopic level has been broken in liquid crystals in a way
that we as yet do not understand. Similar observations, and
comments, have also been made in the study of absolute
asymmetric synthesis.12
Fig. 5 (a) The polarization (nC cm22) and (b) the tilt angle (u) of the R-,
S- and racemic forms of 11, as a function of the reduced temperature.
Stephen J. Cowling,a Alan W. Hallb and John W. Goodby*b
aKingston Chemicals Ltd, Cottingham Road, Hull, UK, HU6 7RX
bLiquid Crystal Group, Department of Chemistry, University of York,
York, UK YO10 5DD. E-mail: jwg500@york.ac.uk
Notes and references
1 R. B. Meyer, Mol. Cryst. Liq. Cryst., 1977, 40, 33.
2 R. B. Meyer, L. Lie´bert, L. Strezlecki and P. Keller, J. Phys. Lett.
(Paris), 1976, 36, L69.
3 N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 1980, 36, 899.
4 www.Displaytech.com, see, for example, JVC to use Displaytech’s
Technology, The Daily Camera, February 13, 2001.
5 Y. Miyazaki, H. Furue, T. Takahashi, M. Shikada and S. Kobayashi,
Mol. Cryst. Liq. Cryst., 2001, 364, 491.
6 W. L. McMillan, Phys. Rev, A, 1973, 8, 1921.
7 R. H. Tredgold, J. Phys. D, 1990, 23, 119.
8 D. M. Walba, E. Korblova, R. Shao, J. E. Maclennan, D. R. Link,
M. A. Glaser and N. A. Clark, Science, 2000, 288, 2181.
9 A. D. L. Chandani, E. Goreka, J. Lee, Y. Ouchi, H. Takezoe and
A. Fukuda, Jpn. J. Appl. Phys., 1989, 28, L1265.
10 J. W. Goodby, E. Chin, T. M. Leslie, J. M. Geary and J. S. Patel, J. Am.
Chem. Soc., 1986, 108, 4729.
11 S. J. Cowling, J. W. Goodby and A. W. Hall, Int. Pat. Appl., PCT/
GB2004/003573 filed 19.08.2003.
12 B. L. Feringa and R. A. van Delden, Angew. Chem., Int. Ed., 1999, 38,
3418.
The authors wish to thank EPSRC and Kingston Chemicals
Ltd. for financial support, and Dr P Rudquist of Chalmers
University, Sweden for supplying us with device cells and for many
useful discussions.
1548 | Chem. Commun., 2005, 1546–1548
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