V-Shaped switching and interlayer interactions in ferroelectric liquid crystals

Article abstract of DOI:10.1039/b612065m

Linear electrooptic responses in ferroelectric liquid crystals can be achieved via thresholdless switching. The molecular parameters for the design of ferroelectric liquid crystals that may yield a thresholdless response have not been delineated so far. In this article we explore some of the chemical design features that may be utilised in controlling switching processes, and we develop property-structure correlations in order to achieve materials that exhibit thresholdless behaviour. The Royal Society of Chemistry.

Full text of DOI:10.1039/b612065m

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
V-Shaped switching and interlayer interactions in ferroelectric liquid
crystals
Alexey Petrenko{ and John W. Goodby*
Received 21st August 2006, Accepted 27th October 2006
First published as an Advance Article on the web 17th November 2006
DOI: 10.1039/b612065m
Linear electrooptic responses in ferroelectric liquid crystals can be achieved via thresholdless
switching. The molecular parameters for the design of ferroelectric liquid crystals that may
yield a thresholdless response have not been delineated so far. In this article we explore some
of the chemical design features that may be utilised in controlling switching processes, and we
develop property–structure correlations in order to achieve materials that exhibit thresholdless
behaviour.
produced such a ‘V-shaped’ response was, at the time of the
1. Introduction
discovery, designated as a thresholdless antiferroelectric liquid
Ferroelectric liquid crystals (FLCs) have recently been
crystal response.
commercialised in small area displays, which are typically
The ‘V-shaped descriptor’ subsequently became one of the
used in near eye applications such as in the eyepieces of digital
more controversial terms used in liquid crystals, with debate
cameras. The fast electrooptic responses of ferroelectric liquid
being centred on the very nature of what is ‘ferroelectric or
crystals mean that they are also compatible with direct drive
antiferroelectric’ and the type of mesophase that is involved
on silicon devices (LCOS), and various device configurations
in ‘V-shaped switching’. The main characteristics of the
employed in projection displays. However, there are various
‘V-shaped switching response’ to note are as follows:
drawbacks to using FLCs, most notably the need to utilise
1. The presence of a third state during the dynamic switching
temporal dither to create grey-scale, their weak resistance to
in a planar oriented thin cell. The state occurs at zero field and,
mechanical distortion, and their common inability to form
when the cell is placed between crossed polarizers with the
aligned samples without the formation of defects.
Crossland1 recently reported, without heating or applying
lower polarizer being parallel to the layer normal, almost
complete extinction of light can be achieved.
high voltages, that the CDRR family, the (S,S)-4-[4-[v-
2. The absence or collapse of a ‘ferroelectric or antiferro-
(1,1,3,3,3-pentamethyldisiloxanyl)alkyloxy]benzoyloxy]phenyl
electric’ hysteresis loop, resulting in a narrow shape of the tip
2-fluoro-3-methylpentanoates, can be aligned into monodo-
of the ‘V’ of the electrooptic response (see Fig. 1), indicating
mains, where the monodomains are electro-optically bistable
that switching between the third zero-voltage state and the
(i.e. the buffing alignment direction bisects the two optical
uniform ferroelectric switched states has no threshold.
bistable states). Such stable bistable states, that do not relax
Thus the ‘V-shaped switching response’ appears to be an
into chevron structures, are ideal for device applications. If the
extremely attractive effect, which can produce an electrooptic
states are destroyed, for example by thermal or mechanical
response with grey-scale and high contrast. A device based on
shock, then the bistable monodomain structure can be restored
such an electrooptic was produced3 even though there was no
using only conventional drive voltages, thus the bistability is
complete understanding of the molecular processes at the heart
robust and secure. If these properties can be better understood
of the phenomenon. Nevertheless, if one accepts the current
and reproduced in ferroelectric liquid crystal device mixtures it
view4 that the SmX* phase in the Tokyo mixture is a
would remove possibly the main impediment to the wider
ferroelectric smectic C* phase, a V-shaped form of the
commercial exploitation of FLC devices.
transmittance–voltage dependence at certain frequencies can
Grey-scale operation can be achieved through the utilisation
of ‘V-shaped’ electrooptic switching techniques where the
transmission through the devices varies linearly as a function
of the applied electric field. The ‘V-shaped electrooptic
response’ was first observed in 19962 through the study of
the so-called ‘Tokyo Mixture’ (Fig. 1), which is a formulation
of three different smectic liquid crystals. Under the application
of an electric field the optical response observed, over a broad
temperature range, was found to be hysteresis-free with a
sharp, but linear, change in transmission. The switching which
Fig. 1 Chemical structures and concentration (in weight percent) of
Department of Chemistry, The University of York, York, UK YO10 5DD
{ Current address: Department of Chemistry, The University of Hull,
Hull, UK HU6 7RX.
the components in the Tokyo mixture, which exhibits a ‘V-shaped
electrooptic response’.
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be explained by the formation of twisted states during dynamic
switching in the surface-stabilised geometry. In order to
explain almost complete light extinction when the system
transforms into the twisted state, a model using a polarized-
stabilised kink (PSK) structure was introduced by Clark et al.4
Omitting the details of the phenomenological description of
the polarization stabilised states provided by Clark et al, the
theoretical models explicitly demonstrate that a high magni-
tude of the spontaneous polarization appears to play a crucial
role in the stabilisation and uniformity of the zero field state
which a material adopts in the surface stabilised cell. However,
subsequent studies have shown that this is not necessarily the
case, and most materials can exhibit ‘V-shaped switching’ at
defined frequencies of the applied electric field and at certain
temperatures. Thus we reasoned that details of the molecular
structures and interactions of the materials employed in such
devices are of particular importance. For example, consider
the structure of the surface stabilised ferroelectric device with
bookshelf geometry associated with the layers, which are
shown as dotted lines in Fig. 2. The molecules are shown by
cylinders (aromatic cores) and wiggly lines (aliphatic chains).
The chains overlap at the interfaces between layers, and so
there will be interactions and transmittance of structural
information between the molecules across the layers. Thus, in
addition to interfacial interactions, there will also be interac-
tions and associated correlation lengths between the molecules
and the surfaces (polar interactions), penetration distances
of the surface ordering, and lateral interactions between the
molecules in the bulk.
This technique for controlling mesophase formation and
physical properties has been termed microphase segregation.
The chemically and/or sterically different end groups are
thought to directly affect the interactions between the mole-
cules at the interfaces of the layers in smectic phases, which in
turn affects the way in which the molecules as a whole pack
together and transfer structural information between layers.
Thus, the phase transitions, the smectic polymorphism and
physical properties exhibited by a smectogen are also affected.
Consequently, the chemical nature of the moiety associated
with the terminal position can, in effect, be used to determine
the structure and phase classification of the mesophase
produced. A judicious choice of the terminal moiety, coupled
with a suitable asymmetric centre, can be employed (almost at
will) to create systems that exhibit ferroelectric or antiferro-
electric mesophases. In addition, as the process of microphase
segregation has the effect of suppressing melting points and
introducing glassy phases, wide temperature ranges for the
existence of the liquid crystal state can be induced. This
molecular design technique is relatively unexplored in either
commercial or research liquid crystals. However, it is a new
and powerful design technique that will have many important
implications for the field in years to come.
In this report we describe the mesomorphic properties of a
family of materials where the molecular structure has been
varied systematically to give six family sub-groups A to F
(Fig. 3). The groups have been modified with respect to the
terminal microphase segregating units and lateral polar groups
associated with the central core. We also examine the
electrooptic behaviour of the materials under various fre-
quencies of the applied electric field in order to examine their
‘V-shaped’ responses.
We reasoned that by using microphase segregating techni-
ques we could affect the interfacial interactions, and thereby
also the switching behaviour of a ferroelectric phase in an
applied electric field.
2. Experimental
Materials that exhibit SmC mesophases usually have
molecular structures where there are two aliphatic chains
attached by polar groups to a central aromatic or heteroaro-
matic core, as shown in Fig. 1. Our work,5–8 and that of
Coles et al.,9–12 has shown when the terminal positions in the
aliphatic chains of the molecules are substituted with chemi-
cally and/or sterically different groups to the chains, the type
and nature of the physical properties of the mesophase can be
controlled and melting points can be substantially lowered.
2.1 Syntheses of materials
Full details of the synthetic procedures and spectral and purity
analyses were reported elsewhere.13 However, it should noted
that the materials were purified to a very high level (.99%), in
particular the silyloxy products were purified by preparative
chromatography techniques due to the fact that the starting
materials were often found to be impure with various isomers.
Attachment of the chiral substrates (2-octanol) was achieved
through the use of the Mitsunobu reaction using diethylazo-
dicarboxylate (DEAD) and triphenyl phosphine, which
typically produces products of varying optical purity due to
inversion at the stereogenic centre. Enantiomeric excesses were
determined by chiral HPLC.
Families A, C and E were prepared using essentially the
same synthetic pathway up until the penultimate steps, see
Scheme 1. Thus commercially available 49-hydroxy-4-cyano-
biphenyl, 1, was subjected to hydrolysis using concentrated
sulfuric acid in glacial acetic acid to give 49-hydroxybiphenyl-
4-carboxylic acid 2. The hydroxy group was protected as the
methyl carbonate through reaction with methyl chloroformate
to give 3. Compound 3 was then subjected to esterification
with (R or S)-2-octanol in the presence of DEAD and
triphenyl phosphine to give 4. The Mitsunobu reaction was
favoured over the standard esterification method using
Fig. 2 A ferroelectric smectic liquid crystal device with surface
stabilised geometry and bookshelf alignment of the layers.
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Fig. 3 Target materials studied.
dicyclohexylazodicarboxylate (DCC) and 4-dimethylamino-
pyridine (DMAP) because little deprotection was found to
conditions by using ammonia in ethanol to give 5, which was
a common substrate to the preparations of the three families of
materials. However, the Mitsunobu reaction induces partial
occur. Compound
4 was then deprotected under mild
Scheme 1
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afforded phenol 11. The phenol was benzyl protected to give
12, which was then subjected to lithiation, carbonylation and
deprotection to give 4-hydroxy-3,2-difluorobenzoic acid. The
acid was then esterified with ethanol to give the ethyl ester 15,
which was etherified and saponified to give the alkyl or alkenyl
substituted acids 16. Then as in Scheme 1, esterification with 5
led ultimately to products B1-5, D1-3 and F1-3.
racemisation, preparation of diastereoisomers of compound 5,
and separation by chromatography (chiral HPLC) gave
enantiomeric excesses of the (S)-enantiomers as 0.9 and the
(R)-enantiomers as 0.86 ¡ 0.2.
The acids to be used in the esterifications of 5 were prepared
in similar ways. Ethyl 4-hydroxybenzoate, 6, was etherified
with either an alkyl bromide or a terminally unsaturated
bromide in the presence of potassium carbonate with dry
acetone or butanone as the solvent. After pouring the reaction
mixture into water the various esters (7) were recovered and
purified by recrystallization. The esters were saponified under
acidic conditions to give the acids 8.
2.2 Phase identification by optical and thermal methods
Phase identifications and determination of phase transition
temperatures were carried out by thermal polarized light
microscopy using an Olympus BH-2 polarizing light micro-
scope equipped with a Mettler FP82HT microfurnace in
conjunction with an FP90 Central Processor. Homeotropic
sample preparations suitable for phase characterization were
prepared simply by using very clean glass microscope slides
(washed with acetone). Differential scanning calorimetry
was used to determine enthalpies of transition and to confirm
the phase transition temperatures determined by optical
microscopy. Differential scanning thermograms (scan rate
10 uC min21) were obtained using a Perkin Elmer DSC 7 PC
system operating on Pyris software. The results obtained
were standardized to indium (measured onset 156.68 uC,
DH 28.47 J g21, lit. value 156.60 uC, DH 28.45 J g21).
The individual acids 8 were esterified with compound 5 to
give families A1-3 and C1-3. The unsaturated materials C1-3
were subjected to hydrosilylation with various hydrosiloxanes
in the presence of Karstedt’s catalyst to give products E1-5.
Compounds B1-5, D1-3 and F1-3 were prepared by the
synthetic pathway shown in Scheme 2. The basic difference in
the synthesis with respect to Scheme 1 is the preparation of the
acids (16) used in the final esterification to yield the liquid-
crystalline products. The preparation of 16 utilizes the
lithiation of 1,2-difluorobenzene with butyllithium followed
by treatment with trimethyl borate to give the boronic acid 10.
Oxidation of the boronic acid with hydrogen peroxide
Scheme 2
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2.3 Electrooptical measurements
Initially, an empty cell was placed in a microscope hot-stage
oven and the transmittance values between crossed polarizers
(extinction) and with parallel polarizers (maximum transmit-
tance) were determined. The cell when filled was then placed
again into the hot-stage oven at a set reduced temperature
(typically Tc 2 T = 10 uC), the cell was poled with a DC
voltage and aligned so that the poled state was set to extinction
and the transmittance value was recorded. The material was
then switched, by applying a DC field of opposite polarity, and
the value of transmittance of the poled bright state was
recorded. Comparing the values for the filled cell with those
for the empty cell gave the percentage of transmission.
The cells for the electrical field tests were contained within a
Mettler FP82HT hot-stage oven in conjunction with
Mettler FP90 temperature controller. The applied waveform
was supplied by Hewlett Packard 33120A Arbitrary
a
a
Waveform Generator into a linear 620 amplifier (custom-
built by QinetiQ). The electrical response from the cell was
fed into a nano-current detector (20 kV or 100 kV impedance)
and to a Hewlett Packard 54600B oscilloscope, which was
connected to a PC.
The cells used in the electrical field studies were constructed
from ITO coated glass which had various separations
depending on the experiment performed (i.e. either 2 or 5 mm
cell spacings), with the internal surfaces coated with a
polyimide aligning agent. For the 2 mm cell separations the
materials in their smectic C* phases would be expected to have
their helical structures unwound due to the strengths of the
surface interactions. The cells were filled by capillary action.
Alignment of the materials was achieved by differing
procedures depending on the cell thickness. For the thinner
cells, alignment was achieved by slowly cooling the sample
from the isotropic liquid into the smectic mesophase, at a rate
of 0.1 uC min21 and in the absence of an electric field.
However, in the cases where the materials exhibited both
smectic A and smectic C phases, for thicker cells alignment was
achieved by slowly cooling into the smectic A mesophase from
the isotropic state, and applying a high electric field in the
form of a square wave (typically 16–20 V mm21) at a frequency
of 50 Hz to 1 kHz depending on the material. Once the
material was aligned in the cell, the voltage was removed and
the cell cooled into the smectic C* or CA* mesophases. For
comparison of results, measurements were determined as a
function of the reduced temperature, Tc 2 T, where Tc is the
Curie point.
The determination of tilt angle was achieved by measuring
the optical transmission of the switched states using a photo-
diode (RS303-674; 1 cm2 active area, high speed .50 ns) in an
apparatus custom built by QinetiQ. The equipment was fitted
with a green-eye response filter (Coherent-Ealing, 26-7617-000,
1 in diameter, transmittance 400–700 nm, maximum transmit-
tance 539.5 nm). The transmission values were determined
using a Thurlby Thandar Instruments 1604 Digital Multimeter.
3. Results
3.1 Transition temperatures and mesophase characterization
3.1.1 (R)-1-Methylheptyl 49-(4-n-alkoxybenzoyloxy)biphenyl-
4-carboxylates, A1–3. All of the members of the parent series A
were found to be purely smectogenic with nematic and
frustrated blue and TGB phases being completely suppressed
from their phase sequences. The transition temperatures, along
with their associated enthalpies, are given in Table 1.
The smectic polymorphism was found to be similar to that
of MHPOBC,3 i.e. including SmA*, SmC*, SmCFI1*,
SmCFI2*, SmCA* and SmI* phases. The ferroelectric smectic
C* phase was found to dominate the phase sequences over
broad temperature ranges. It remained enantiotropic regard-
less of the length of the non-chiral alkyl chain and, generally,
the phase stability increased with elongation of the chain at the
expense of the high temperature smectic A* phase. In contrast,
the antiferroelectric smectic C* phase was less pronounced,
and in most cases it remained monotropic. Even when the
antiferroelectric phase appeared on heating, its temperature
range was significantly shorter than during the cooling cycle.
The hysteresis in the phase transition temperatures of the
antiferroelectric phases (on heating and cooling) indicated that
Upon alignment of the liquid crystal, the spontaneous
polarization in the tilted phases was determined, on cooling,
using a triangular waveform current reversal technique. An
AC field in a triangular waveform (typically 10 V mm21 at
30 Hz) was applied to the cell. As the polarization is inverted
by the field, a current pulse was observed. The magnitude of
the spontaneous polarization was determined from the area
under the current-reversal peak.
The light transmission through any specimen cell was deter-
mined using a photodiode in conjunction with a voltmeter.
Table 1 Transition temperatures (uC) and associated enthalpy values (DH/J mol21) for the (R)-1-methylheptyl 49-(4-n-alkoxybenzoyloxy)biphe-
nyl-4-carboxylates, A1 to A3
No.
A1
A2
A3
n
Mp
SmI*
(N
SmCA*
47.1 N)
[3.7]
SmCFI1
*
SmCFI2
*
SmC*
SmA*
Iso Liq Cr
10
12
14
N
N
N
64.5
[24.9]
77.3
[46.5]
55.5
[40.0]
73.0
[—]
N
73.6
[—]
N
78.7
[0.02]
77.2
[0.02]
64
N
N
N
105.4
[0.18]
108.7
[0.28]
109.3
[0.3]
N
N
N
123.6
[5.1]
118.9
[5.0]
115.4
[4.5]
N
N
N
38.1
[0.1]
63.4
[43.9]
29.9
[17.6]
—
(N
69.8 N)
[—]
54.8 N)
[—]
71.2 N)
[—]
(N
37.5
[3.5]
N
56.2
[—]
N
[—]
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schlieren domains which became very mobile with disordered
fluctuations observed throughout the sample.
On cooling the characteristic schlieren pattern gradually
developed over all of the homeotropic region and the
fluctuations diminished (Fig. 5c). At the transition to the
SmCFI2* phase another wave of fluctuations reconstructed
the sample’s appearance, and with a further temperature
decrease the green coloured domain of the antiferroelectric
phase started growing in the homeotropically aligned region of
the specimen (Fig. 5d). The focal-conic fans cleared as a result
of the formation of the alternating molecular tilt in smectic
layers. The direction of the helix that was formed in the
smectic C* phase changed its sign to the opposite hand in the
antiferroelectric phase.
The antiferroelectric properties of the SmCA* phase were
studied in detail through electric field measurements using the
triangular wave method. In the ferroelectric smectic C* phase,
molecular reorientation from one uniform state to another
produced an electric current, which flows between the surfaces
when the external electric field passes a zero point. As a result,
one sharp peak is usually observed in the current–time trace
and the value of the spontaneous polarization was calculated
from the area under this peak in voltage–time coordinates. In
turn, tristate switching in the antiferroelectric phase involves
two independent molecular reorientations giving rise to two
electric currents (Fig. 6b).
Fig. 4 DSC cooling trace for compound A1. Scanning rate
10 uC min21
.
the phase transitions are not conventional liquid crystal phase
changes, which are driven by a change in ‘lattice-type’ of
structure, but rather due to changes in the frustration of the
molecular packing. The SmCA* phase was also found to be
preceded by two modifications of the ferrielectric phase, which
emerge over narrow temperature intervals, thereby preventing
direct SmC*–SmCA* phase transitions. Their appearance is a
direct consequence of the competition between synclinic and
anticlinic ordering.
Thus for compound A2 two switching peaks were clearly
observed in the temperature range of the antiferroelectric
phase (Fig. 6b), thereby confirming that the switching in this
phase proceeds via three states. Thus the switching responses
provide confirmatory evidence for mesophase identification,
which is particularly useful for complex mesophase behaviour
exhibited by some of the materials under investigation.
In general, all three compounds of general structure A
possess quite a high value of the ‘‘saturated’’ spontaneous
polarization, ca. 100 nC cm22 (see results for compound A2)
in the temperature region of the antiferroelectric phase. It can
be seen from Fig. 7 that the magnitude of the spontaneous
polarization grows continuously with decreasing temperature
from the Curie point (SmA*–SmC* transition) and disappears
in the SmI* phase because the electric field was too small to
switch the higher ordered phase. The optical tilt angle has
a different temperature dependence, at the Curie temperature
it has a zero value and it reaches 25u in the antiferroelectric
phase (Fig. 7).
First order phase transitions between the isotropic liquid
and the smectic A* phase, as well as the melting and
recrystallisation transitions, were detected by differential
scanning calorimetry (see Fig. 4 for compound A1). The
SmA*–SmC* and SmCA*–SmI* transitions were also ‘‘visible’’
in the DSC thermograms due to their weak first-order
characteristics. The enthalpy values for the SmA*–SmC*
transitions were approximately ten times smaller than the
typical values of the first-order Iso Liq–SmA phase transition.
For compound A1, it was found that a minor discontinuity
was also obtained at the transitions from the SmC* to the
ferrielectric (FI1) phase, which was classified from the
thermodynamic sequencing of the phases, and the subsequent
SmCA* phase gave rise to small peaks in the DSC curve.
However, in most cases the enthalpies of the second-order
transitions between the smectic C modifications were too small
to measure, and therefore the phase identification was carried
out using polarizing microscopy.
In the microscopy preparations the types of liquid-crystal-
line phases were assigned by comparing their textures to
reference sources. Thus, for compound A3, the smectic A*
phase was characterised via its clear focal-conic domains which
exhibited elliptical and hyperbolic lines of optical discontinuity
and the extinct homeotropic region when the material was
observed between crossed polarizers (see Fig. 5a). At the
transition to the smectic C* phase, the focal-conic fans
appeared broken and in some areas banded as a result of
stabilisation of the molecular tilt and helix formation. At the
same time, the homeotropic region became dark blue (Fig. 5b)
indicating the formation of the helical structure. Small
domains of the schlieren texture were also observed in
uncovered droplets of the material. In the vicinity of the
SmCFI2* phase, the homeotropically aligned region exhibited
3.1.2 (R)-1-Methylheptyl 49-(4-alkoxy-2,3-difluorobenzoyl-
oxy)biphenyl-4-carboxylates, B1–5. The general structure for
compounds
B
includes two ortho-fluorine substituents
attached to the outer ring of the 4-benzoyloxybiphenyl core,
which was used in series A as the parent system for
comparative studies (see Fig. 3). This structural modification
greatly affected the magnitude of the transverse molecular
dipole as a consequence of the introduction of the fluorine
atoms. As predicted in McMillan’s model of the smectic C
phase,14 strengthening the lateral dipole should improve the
stability of the molecular tilt in smectic layers. However, in
series B the stability of the tilted smectic phases was found to
be almost unchanged when the transition temperatures were
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Fig. 5 Textures of compound A3. a) SmA*, 111 uC; b) SmC* phase, 104 uC; c) SmCFI2* phase, 56.5 uC; d) SmCA* phase, 52.2 uC; e) SmI* phase,
34 uC (6100).
compared to those of the type A materials. For instance, the
temperature interval of the SmC* phase for C14 homologues
A3 and B5 is roughly the same and extends over 50 uC
(Table 2).
effect that suppresses the antiferroelectric and frustrated
ferrielectric sub-phases for odd members of the homologue
series. For even numbers the optimal chain length, which
favours the SmCA* phase, is the C12 homologue. Conversely,
the C10 homologue, which exhibits the broadest antiferro-
electric phase in series A, does not possess smectic sub-phases
at all.
Lateral fluorine substituents, however, promote formation
of the TGBA phase, which was observed over a narrow
temperature interval (less than a half of a degree) for all of the
homologues synthesised. At the low temperature end of the
phase sequences, enantiotropic ferrielectric and antiferro-
electric SmC* phases were exhibited by the C12 and C14
homologues (compounds B3 and B5). There is an odd–even
The microscopy studies were carried out as described for
compounds of structure A. It was difficult to observe the
transition from the SmA* to the SmC* phase in the
homeotropic region because the pitch length in the smectic
Fig. 6 The current–time traces recorded for compound A2: a) SmC* phase, 78 uC, electric field 10 Hz, 20 volts peak-to-peak (VPP). b) SmCA*
phase, 68 uC, electric field 10 Hz, 20 VPP.
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covered with fluctuating schlieren domains in the SmCFI2
*
*
phase, which became more fluid and colourful in the SmCFI1
phase. In the SmCA* phase the schlieren texture was replaced
with a network of blue domains.
The compounds were found to possess a large values of the
spontaneous polarization, which in the antiferroelectric phase
reached an apparent value of ca. 160 nC cm22 (compound B2).
This is approximately 30% higher than the corresponding
value determined for the members of series A. The magnitude
of the apparent tilt angle also grew with the reduced tem-
perature and varied between 32u and 33u for the longer chain
homologues.
3.1.3 (S)-1-Methylheptyl 49-(4-alk-v-enyloxybenzoyloxy)bi-
phenyl-4-carboxylates C1–3 and the (S)-1-methylheptyl 49-(4-
alk-v-enyloxy-2,3-difluorobenzoyloxy)biphenyl-4-carboxylates
D1–3. In order to attach silyloxy bulky end groups to systems
such as compounds of structure A and B, first the equivalent
terminal olefins had to be prepared. Thus the (S)-1-methyl-
heptyl 49-(4-alk-v-enyloxybenzoyloxy)biphenyl-4-carboxylates
C1 to C3 and the (S)-1-methylheptyl 49-(4-alk-v-enyloxy-2,3-
difluorobenzoyloxy)biphenyl-4-carboxylates D1 to D3 were
synthesised. However, these intermediates also exhibited
interesting mesomorphic behaviour. The transition tempera-
tures and enthalpies are given together in Tables 3 and 4
respectively.
Fig. 7 The spontaneous polarization and optical tilt angle as a
function of reduced temperature recorded for compound A2. Applied
electric field 20 Hz, 20 VPP.
C* phase was very short, and therefore both phases appeared
optically extinct between crossed polarizers. In free-standing
preparations it was possible to distinguish between textures
of the SmC* and SmA* phases (Fig. 8), as white fingers
associated with the schlieren texture were clearly seen on the
edge of the film in the SmC* phase.
The textures of the ferrielectric and antiferroelectric phases
are shown in Fig. 9 for compound B3. The regions associated
with homeotropic alignment of the smectic A* phase were
In both series of alkenes C and D the phase sequences
appeared less diverse in comparison to the wide variety of the
smectic phases exhibited by the alkyl analogues. Interestingly,
Table 2 Transition temperatures (uC) and associated enthalpy values (DH/kJ mol21) for the (R)-1-methylheptyl 49-(4-alkoxy-2,3-difluoroben-
zoyloxy)biphenyl-4-carboxylates, B1 to B5
No.
B1
B2
B3
B4
B5
n
Mp
SmCA*
SmFI1
—
*
SmFI2
*
SmC*
SmA*
TGBA
Iso
Cr
10
11
12
13
14
N
N
N
N
N
75.5
[31.3]
68.4
[29.4]
52.5
[38.1]
67.2
[48.1]
46.7
[42.6]
—
—
N
—
—
N
N
N
N
N
N
92.9
[0.1]
97.7
[0.27]
99.8
[0.28]
99.71
[0.29]
98.4
N
N
N
N
N
N
N
N
N
N
111.1
[4.2]
108.3
[3.9]
107.3
[4.0]
105.6
[3.9]
N
N
N
N
N
58.2
[28.9]
51.5
[27.1]
44.3
[36.9]
56.3
[45.3]
33.6
[31.1]
—
65.8
[0.01]
N
66.4
[—]
72.2
[—]
—
N
—
—
N
47.1
[—]
—
49.7
[—]
103.9
[4.0]
[0.38]
Fig. 8 Photomicrographs of the free-standing films made for compound B3: a) SmA*, 104.0 uC; b) SmC*, 90.0 uC (6100).
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Fig. 9 Textures for compound B3: a) SmCFI2*, 70 uC; b) SmCFI1*, 66 uC; c) SmCA*, 64 uC (6100).
the presence of the terminal double bond did not appear to
favour formation of the TGB, unlike the saturated analogues,
also, the SmI* phases were not exhibited by these two
homologous series. The SmA* and SmC* phases followed
similar trends to those in series A and B. Namely, the SmA*
phase thermal stability deteriorated with increase of the
alkyl chain length while the SmC* phase gradually became
more stable.
antiferroelectric near the phase transition to the SmCA* phase,
its structure, however, still needs to be clarified.
When observed in the polarizing microscope, the SmCa*
phase showed a characteristic pattern of the focal-conic fans,
which was easily distinguished from the focal-conics of the
SmC* phase (Fig. 10a) through an unusual bright-orange
colour of the edges of the fans. In the SmCA* phase the colour
disappeared giving rise to a focal-conic pattern which was
similar to the texture of the antiferroelectric phase observed
for compounds of structure A, see Fig. 10b. In free-standing
film preparations the schlieren texture of the SmCA* phase
exhibited several two-brush defects that were clearly visible in
the central parts of the film. The appearance of two-brush
dispirations signifies the presence of alternating molecular tilt
direction in the smectic layers and is regarded as a classical
defect characteristically found in schlieren textures of the
SmCA* phase.
In most cases, the antiferroelectric SmCA* phase was
observed only during cooling cycles. The phase was preceded
with the SmCFI1 phase in the C series, which prevented a direct
SmC*–SmCA* phase transition. For the difluorobenzoate
compound D2 the SmC* phase was replaced with a non-
commensurate SmCa* phase, which underwent
a direct
transition to a SmCA* phase without mediation of ferri
phases. This phase appears to exhibit ferroelectric properties at
the higher temperatures in its thermal range and becomes
Table 3 Transition temperatures (uC) and associated enthalpy values (DH/kJ mol21) for the (S)-1-methylheptyl 49-(4-alk-v-enyloxybenzoyl-
oxy)biphenyl-4-carboxylates, C1–C3
No.
C1
C2
C3
n
4
6
8
Mp
SmCA*
SmFI1
*
SmC*
SmA*
Iso
Cr
N
N
N
61.4
[16.2]
61.5
[19.4]
54.1
[33.1]
—
N
—
N
(N
N
55.7
[0.13]
77.9
[0.11]
93.2
[0.28]
N)
N
123.0
[4.6]
119.6
[4.4]
113.4
[4.8]
N
N
N
42.6
23.7
64.8
[—]
47.7
[0.014]
67.6
(N
N)
51.8
[—]
N
N
32.2
[4.74]
Table 4 Transition temperatures (uC) and associated enthalpy values (DH/J mol21) for the (S)-1-methylheptyl 49-(4-alk-v-enyloxy-2,3-
difluorobenzoyloxy)biphenyl-4-carboxylates, D1–D3
No.
D1
D2
D3
n
4
8
9
Mp
SmCA*
SmC*
SmCa*
—
SmA*
Iso Liq
Cr
N
N
N
64.1
[21.0]
72.6
[35.5]
73.4
[31.6]
—
(N
(N
N)
N
58.0
[0.14]
—
N)
N
107
N
N
N
37
[4.04]
104.1
[4.40]
103.5
[4.42]
[16.4]
48.7
[32.6]
54.1
[28.9]
64.6
[—]
75.9
[0.08]
—
84.6
[0.19]
—
N
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Fig. 10 Textures of the smectic phases observed for compound D2: a) SmCa 67 uC; b) SmCA*, 62.7 uC, and for compound C3: c) SmCFI1*,
47.9 uC; d) SmCA*, 45.2 uC (6100).
The texture of the ferrielectric SmCFII* phase for compound
C3 was also composed of focal-conic domains. As shown
in Fig. 10c, the broken fans of the smectic C* phase became
covered with narrow lines indicating a transition to the
SmCFI1*. At the transition to the antiferroelectric phase the
focal-conic defects appeared clearer due to the alternating
layer structure giving an apparent director perpendicular to the
layers (Fig. 10d).
prepared respectively. There were two major consequences of
this structural modification, see Tables 5 and 6. Firstly a
significant depression in phase transition temperatures was
observed for all of the silyloxy homologues synthesised. The
decrease in the isotropization temperature observed for the
siloxane materials was as much as 40 uC to 50 uC when
compared to the alkenes and alkyl analogues respectively
(compare for example E3, C3 and A1). This large drop in
transition temperatures is driven by the siloxane fragments
that group together in the mesomorphic state and, thereby,
increase the relative stability of a layered smectic C* structure
at low temperatures. It also facilitates supercooling, which in
the case of the 2,3-difluorobenzoate compound E1 extends
over 52 uC reaching almost 210 uC before crystallisation/
glassification sets in.
Additional evidence for the properties of the ferroelectric
and antiferroelectric SmC* and SmCA* phases was obtained
through the studies of the electrooptical response of compound
C2. In the smectic C* phase the transmittance as a function of
voltage had the shape of a hysteresis loop. The loop split into
two smaller hystereses in the antiferroelectric smectic CA*
phase indicating the appearance of the third state during the
field-induced switching. The antiferroelectric double hysteresis
could be converted back into the single loop when the
frequency of the applied field became too high and there was
not enough time for relaxation to occur back to the antiferro-
electric state (see Fig. 11).
The second consequence of the inclusion of the siloxane
chain in the molecular structure was that the phase sequences
were reduced to one liquid crystalline phase, which in all
cases was the smectic C* phase. Its stability remained
unaffected by the length of either siloxane or alkyl chain
unless the total number of atoms exceeded 14. In this case,
an increase in both the clearing and melting points ws
observed.
The spontaneous polarizations for compounds D1 to D3
were determined, and the values as a function of the reduced
temperature from the Curie point are shown together in
Fig. 12. Interestingly the values of the spontaneous polariza-
tions for the alkenes C and D were found to be slightly lower
than those determined for compounds A and B.
The antiferroelectric phase was not found for any of the
homologues in series E or F. The odd–even stability effect of
the siloxane chain observed for bimesogen materials15–18
seemed not to be applicable for the mono-substituted siloxane
compounds, synclinic smectic C* ordering was preferred for
both odd and even numbers of the siloxane units in the
terminal chain.
3.1.4 (S)-1-Methylheptyl 49-{4-[v-(1,1,3,3,3-pentamethyl-
disiloxanyl)alkoxy-2,3-difluoro]benzoyloxy}biphenyl-4-carboxy-
lates E1–5 and the (S)-1-methylheptyl 49-{4-[v-(1,1,3,3,3-
pentamethyldisiloxanyl)alkoxy-2,3-difluoro]benzoyloxy}biphe-
nyl-4-carboxylates F1–3. In substituting parent systems C and
D with di- and tri-siloxane units two new series E and F were
The smectic C* phase was identified with the help of a free-
standing preparation. Only four-brush defects were found in
the schlieren texture (in Fig. 13 compare plate a: anticlinic
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Fig. 11 The voltage–optical transmittance (y-axis) curves recorded for compound C2. a) SmC* phase, 88 uC, 2 Hz, 2 VPP; b) SmCA* phase, 44 uC,
20 mHz, 2 VPP; d) SmCA* phase, 44 uC, 2 Hz, 2 VPP. The orientation of the cell was set such that the layer normal of the smectic C* phase was
parallel to the top polarizer. As a result both ferroelectric states had a similar level of transmittance and the hysteresis loops appeared folded into
‘W-shaped’ curves.
Table 5 Transition temperatures (uC) and associated enthalpy values
(DH/kJ mol21) for the (S)-1-methylheptyl 49-{4-[v-(1,1,3,3,3-penta-
methyl)disiloxanyl- and 49-{4-[v-(1,1,3,3,5,5-heptamethyl)trisiloxanyl-
alkoxy]benzoyloxy}biphenyl-4-carboxylates, E1 to E5
No.
E1
E2
E3
E4
E5
n
4
6
4
6
9
m
2
2
1
1
1
Mp
SmC*
Iso Liq
Cr
N
N
N
N
N
32.7
N
N
N
N
N
50.9
[3.1]
69.3
[3.2]
70.3
[5.8]
83.3
[4.2]
91.1
[4.48]
N
N
N
N
N
24
[7.3]
6.3
[11.6]
7.7
[14.6]
12.2
[11.5]
9.9
28.2
[14.4]
32.3
[20.9]
36.4
[17.8]
52.7
[41.8]
Fig. 12 The spontaneous polarization of the ferroelectric phase
plotted as a function of the reduced temperature from the Curie point
for compounds of structure D, at 20 Hz, 2 VPP.
[18.9]
analogues, reaching values of over 200 nC cm22. The
magnitude of spontaneous polarization undergoes a smooth
growth with decrease in temperature, see Fig. 14.
phase for compound C2, and plate b: synclinic phase for
compound E2). Comparison shows that the schlieren texture
(Fig. 13b) possesses only s = 1 singularities, whereas for
parent system (Fig. 13b) s = 1/2 dispirations are also
found. These textures are diagnostic for the formation of
synclinic and anticlinic phases. Broken focal-conic domains
were also observed in the samples prepared in thin electro-
optical cells.
3.2 Electrooptic studies
In order to investigate voltage–transmittance characteristics
as a function of chemical structure, a set of electrooptical
experiments were performed for several of the materials. The
measurements were taken in thin (ca. 2 or 5 mm) planar aligned
cells by applying a triangular electric field and varying the
orientation of the cell between crossed polarizers in transmis-
sion. The transmittance as a function of the electric field was
recorded at different frequencies, voltages and temperatures
for each of the compounds.
In switching studies rather high magnitudes of the optical
tilt angles with respect to the smectic layers were measured.
The experimental measurements found that for all of the
siloxane materials synthesised the tilt angle was ca. 45u.
The spontaneous polarization was found to be almost
two times higher than the corresponding value in the alkene
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Table 6 Transition temperatures (uC) and associated enthalpy
values (DH/J mol21) for the (S)-1-methylheptyl 49-{4-[v-(1,1,3,3,3-
pentamethyldisiloxanyl)alkoxy-2,3-difluoro]benzoyloxy}biphenyl-4-
carboxylates, F1–3
No.
F1
n
1
1
1
m
4
Mp
SmC*
Iso Liq
Cr
N
N
N
43.4
[33.2]
60.3
[23.5]
63.5
[23.4]
N
N
N
63.2
[3.38]
88.7
[3.37]
90.4
[2.45]
N
N
N
28.9
[9.13]
42.6
[20.7]
49.4
[21.3]
F2
8
F3
9
Fig. 14 The spontaneous polarization plotted as a function of the
reduced temperature from the Curie point for compounds of structure
F. Applied field: 20 Hz, 2 VPP.
Firstly, a V-shaped response was simulated for compound
A2, because its rich mesomorphic behaviour is reminiscent of
the phase morphology of the Tokyo mixture, ie the phase
sequence includes SmA*, SmC*, SmCferri* and SmCA* phases.
The spontaneous polarization gradually increases from
30 nC cm22 at the SmA*–SmC* transition and reaches a
value of above 140 nC cm22 near to the transition to the
antiferroelectric phase. This value is fairly similar to the
estimated spontaneous polarization for the phase exhibited by
the Tokyo mixture.2
positive and negative ions in the bulk move in opposite
directions creating the ionic field Eion, which opposes Eext. If
the frequency of the external field is high enough, the charges
do not have time to move far away from each other and
generate a substantial Eion before the external field changes
sign. Thus, a normal hysteresis response is produced as
switching starts at point A and is completed at B (Fig. 16).
When the frequency is low, the mobility of ions increases so
that Eion has a significantly high contribution and, thus a
negative electric field exists when the external field is zero.
Therefore, the switching actually starts at A9 and finishes at B9
(A0 and B0 on the solid curve). If |A0| . |B0| an abnormal
hysteresis takes place, and when |A0| = |B0| a hysteresis-free
V-shape response is produced.
The electrooptical response showed a normal ferroelectric
hysteresis at the frequencies above 5 Hz (Fig. 15a). Decreasing
the frequency resulted in the hysteresis loop shrinking into a
thin line (Fig. 15c). It can be seen in Fig. 15c that the slope of
the line is steep because a sharp change in transmission occurs
between the two switched states. When the orientation of the
cell was changed so the axis of the polarizer was set parallel to
the smectic layer normal a V-shaped response was obtained
(Fig. 15b). The shape of the curve remained unchanged for a
relatively broad range of frequencies.
As shown in Fig. 16, when the V-shaped curve is stabilised,
complete extinction of light was observed at the tip of the V.
The bottom line in Fig. 15b corresponds to zero transmission
when no light passes through the cell. Thus, all the features
of the switching described for the Tokyo mixture can be
simulated in the ferroelectric smectic C* phase. It should be
noted that the voltage–transmission curves depicted in Fig. 15
were recorded at a temperature close to the smectic A*–smectic
C* phase transition. At these temperatures if any type of
randomisation of molecular tilt is induced by the surfaces at
However, below
a certain value the hysteresis loop
reappeared. This effect can be explained in terms of the
distribution of space charge in the surface-stabilised cell.19
When an external field Eext is applied to the liquid crystal cell
the layer spontaneous polarization is aligned along the field
direction, thereby generating opposite surface charges, which
produce an electric field Esur. Since Esur is smaller than Eext
,
Fig. 13 Two- and four-brush defects observed in the schlieren textures: a) compound C2, SmCA* phase, 54.2 uC; b) compound E2, SmC* phase,
72.0 uC (6100).
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Fig. 15 Voltage–transmission curves recorded for compound A2 at various fields, at a temperature of 105 uC (approximately 4 uC below the
SmA* to SmC* phase transition). The relative level of transmission is shown along the y-axis.
second order parameter undergoes gradual growth with the
reduced temperature from the Curie point:
ꢀ ꢁ
1=2
a
1=2
h~
ðTC{TÞ
b
1
P~ P0sin2h
2
where a and b are the coefficients of the first two terms of the
expansion.
The spontaneous polarization of the compound B3 increases
from approximately 76 nC cm22 at 90 uC to 101 nC cm22 at
74 uC. Such an increase in the value of the spontaneous
polarization would seem to be enough to transform a twisted
structure at zero field to a uniform polarization stabilised state
which dominates the material between the two surfaces. As a
result, the level of transmittance drops to zero. The PSK model
may well describe the properties observed, however it should
also be remembered that the materials possess a very short
pitch of the helix in the SmC* phase, and thus helielectric
effects cannot be ruled out.
Fig. 16 Schematic diagram explaining reappearance of hysteresis
loop at low frequency of an external electric field.
the tip of the V, it cannot be a result of the competition
between synclinic and anticlinic ordering as suggested by the
random model. The antiferroelectric and intermediate sub-
phases appear 40 uC below the temperature point where the
V-shaped switching is observed, and therefore such sub-phases
cannot affect the thermodynamic stability of the smectic C*
ordering.
Attempts were also made to simulate V-shape curves of the
voltage–transmittance function in the smectic C* phases of
siloxane materials E4 and E5, see Fig. 18 for compound E5.
These compounds exhibit first-order transitions from the
isotropic liquid to the smectic C* phase. The corresponding
value of the tilt angle in the smectic C* phase, which is just
slightly less than 45u, does not change with temperature. The
spontaneous polarization reaches 110 nC cm22 in the vicinity
of the phase transition, and it monotonically increases with
However, the temperature still appears to be an important
factor in the stabilisation of the intermediary switching state,
as demonstrated for compound B3. A V-shaped curve was
observed in the smectic C* phase using the method described
above for compound A2, but only at higher frequencies (see
Fig. 17). At 90 uC, just a few degrees below the second-order
smectic A*–smectic C* phase transition the dark state does not
correspond to zero transmission. However, the transmittance
level decreases with temperature and at 74 uC complete
extinction is observed.
decreasing temperature to approximately 200 nC cm22
.
In this case, the V-shaped switching was not affected by
temperature, and could be observed throughout the whole
temperature range of the phase, with a small hysteresis being a
function of the frequency of applied field. The V-shaped curve
appears to be turned up side down as the plane of the incoming
light was set parallel to the director in one of the ferroelectric
states. When the molecules are in this state, light travels with a
polarization direction parallel to the principal molecular axis,
If one takes into account the polarization surface kink
(PSK) model,4 the temperature dependence of the transmission
can be explained in terms of the high value of the spontaneous
polarization at lower temperatures. According to the well-
known result related to the Landau expansion for a second
order phase transition20 the spontaneous polarization as a
Fig. 17 Voltage–transmission curves recorded for compound B3 at various electric fields.
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Fig. 18 Compound E5. Photomicrographs show the appearance of the zero-field state (a) and the uniform ferroelectric state (b) during the
switching in a 5 mm cell, and relaxation to a bright state with no field applied (c).
so complete light modulation is achieved. As a result, the cell
appears dark. For the second uniform state the molecules are
switched along the smectic cone to the opposite tilt positions.
The dark texture is observed again, as this time the
polarization plane is perpendicular to the optic axis of the
material. It can be seen in Fig. 18 that the light extinction is not
complete for these states because the tilt angle is slightly less
than 45u
In the configuration described, the third zero-field state
occurs when the molecules reach the central position on the
smectic cone which is at 45u to the plane of incident light. The
photomicrographs taken of compound E5 (Fig. 18) show
the difference in appearance of the uniform dark state and
bright zero field state. The height of the transmission curve
reaches almost the maximum value as shown in Fig. 18. When
the electric field is switched off the material slowly relaxes to
the bright ground state.
the transmission curves recorded for parallel and crossed
polarizers originates from the non-linear polarization
eigenmodes. The results explicitly show that there is no
uniformity achieved in the field-off state. Instead, a twisted
structure seems to be a possibility for these compounds in the
absence of an electric field, because the Mauguin condition is
not fully satisfied as the incoming plane polarized light is not
fully rotated by the twisted structure and emerges elliptically
polarized.
Comparing the results obtained for compounds A2, B3, E4
and E5 shows that in the surface stabilised geometry there is
competition between twisted and uniform, polarization stabi-
lised states. Assuming that the spontaneous polarization plays
an important role in stabilisation of the uniform state there
seems to be a critical value for a particular material above
which a twisted structure transforms into the uniform state.
For example, such transformations occur for compound B3
only at low temperatures where the spontaneous polarization
has a substantially high value. However they do not take
place for compounds E4 and E5 despite the fact that the
spontaneous polarizations are almost double those of com-
pounds A2 and B3. A twisted structure dominates at all values
Similar electrooptic behaviour was observed for compound
E4. It was found that even if the polarizers are set parallel to
each other the transmission value does not reach a minimum
level as indicated by the bottom line in Fig. 19b and remains at
a rather high level. The absence of mirror symmetry between
Fig. 19 Voltage–transmission curves recorded for compound E4 between crossed (a) and parallel (b) polarizers.
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in the polarization–temperature function. Thus, the critical
value of the spontaneous polarization is not universal, rather it
is only one of a number of physical properties of a material
that are important.
from compound A2 is the terminal double bond attached to
the non-chiral chain. The spontaneous polarization is also
roughly the same, approximately 80 nC cm22. The compound
possesses a broad antiferroelectric phase, for which the
electrooptical response was observed and compared to the
results for the ferroelectric phase. In order to include surface
effects in the discussion, the material was investigated when
placed in two planar cells of different thickness (2 and 5 mm).
It can be seen in Fig. 20 that complete extinction of light was
achieved between crossed polarizers, however, between parallel
polarizers the cell transmission reaches only a maximum and
an almost symmetrical inversion of the ‘V-shaped’ response is
observed. It is most likely that a uniform structure is fully
realised here with the plane of polarization of the incoming
light being parallel to the optical axis of the material in the
zero field state.
Any modification to the chemical structure which amplifies
the intermolecular interactions at the same time will also
reduce the chances of the uniform state being stabilized at zero
field. In the case of compound B3, two fluorine atoms laterally
attached to one phenyl fragment of the parent structure A2
serve as additional sites for interactions between neighbouring
molecules within a layer. Similarly, the siloxane chains of
compound E4 greatly affect the interlayer interactions. While
the former can be overcome by a temperature driven increase
in the spontaneous polarization, the latter remains fairly
strong and can not be counteracted by the electrostatic torque
even if the torque is induced by an exceptionally strong
spontaneous polarization.
At 44 uC in the antiferroelectric phase a double hysteresis
loop was observed when the frequency was low enough for the
material to relax into the antiferroelectric ground state. As
This hypothesis is supported by the results obtained for
compound C2. The only difference in its chemical structure
Fig. 20 Voltage–transmission curves for compound C2. Cell thickness is 5 mm unless stated otherwise.
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Fig. 21 Photomicrographs taken for compound C2 while switching under the application of a low-frequency electric field.
shown in Fig. 20, the double hysteresis loop transforms into a
typical ferroelectric hysteresis loop at high frequencies, an
effect which has been described previously for antiferroelectric
liquid crystals.21 For the intermediate frequency range, where
the antiferroelectric state appears transitory during switching,
the optical response curve exhibits features of both ferro-
electric ‘W-shaped’ switching and the double hysteresis loop
of the antiferroelectric phase. By adjusting the frequency, the
‘W-shaped’ curve could be transformed into a ‘V-shaped’ line
at which point the switching process clearly proceeds via four
states. Surprisingly, the uniform state, which was formed at the
tip of the ‘V’ in the ferroelectric phase is, however, still present
in the antiferroelectric phase. As reflected by a voltage–
transmission curve (Fig. 20), the molecular reorientation
from the ferroelectric state occurs via the so-called uniform
‘‘structure’’, where complete extinction of light was observed
and then relaxes back to the antiferroelectric ground state.
In fact it was found that for narrow cell gaps at least, the cell
thickness does not affect either the width of the ‘V-shape’22 nor
the general trends in the material’s response to the electric
field. In both cells a typical ‘W-shaped’ ferroelectric response
was produced at 80 uC when a high frequency field was
applied. This response was converted into a ‘V-shaped’ line at
240 mHz in a 2 mm plate separation and at 2 Hz in a cell with a
gap of 5 mm.
If we consider a hysteresis loop for a standard ferroelectric
material, as shown in Fig. 22, the points where the loop crosses
the electric field axis, i.e. the coercive fields Ec and 2Ec, are
measures of the ease with which the material may be poled.
Where the loop crosses the polarization axes, the remanent
polarizations, Pr and 2Pr, are measures of the degree of the
retention of the poled ferroelectric structure at zero field, i.e. it
provides information about the stability of the structure. For
V-shaped switching, the loop is essentially collapsed, thus the
coercive fields fall to zero and the structure of the mesophase
becomes fairly fluid-like.
A similar situation applies for antiferroelectrics where the
double hysteresis collapses and the threshold field, Etr, which
induces switching between the zero field state and either of the
switched states falls towards zero, as shown in Fig. 23. Thus
V-shaped switching, whether associated with ferroelectric
or antiferroelectric behaviour, requires the organisation of
the molecules to be relatively weak. For example when the
interlayer interactions are weak, as in the case of the bulky
silyloxy materials, V-shaped switching is easily achieved,
whereas where the interlayer interactions are stronger as in
the case of the terminal alkenic materials antiferroelectricity
becomes stabilised and V-shaped switching is unattainable.
The strength of the bulk ferroelectric ordering can be
determined from the in- and out-of-plane correlation lengths
of the molecules with respect to the layers. However, in surface
stabilised devices the strength of the surface interaction and its
penetration into the bulk also becomes important. The weaker
the surface interactions the more likely it will be that V-shaped
switching will occur.
Remarkably, the tip of the V-shaped switching curve still
corresponds to zero field, thereby allowing the speculation
that, at least during the dynamic switching process, the
polarization stabilised uniform structure is thermodynamically
more stable than the antiferroelectric state in surface-stabilised
geometries. The differences in textural appearance of the
antiferroelectric and uniform zero-field states are shown in
Fig. 21.
4. Discussion
From the studies presented it is clear that V-shaped switching
is not only dependent on the spontaneous polarization, but it is
also dependent on the voltage, frequency, molecular design
and the interactions of the molecules with the surfaces within
the cells. With respect to molecular design, polar groups
were incorporated into the cores of the molecules, which
were expected to affect the in-plane correlations, and bulky
terminal groups were incorporated in order to affect the out of
plane correlations.
Fig. 22 Hysteresis loop for a ferroelectric liquid crystal.
J. Mater. Chem., 2007, 17, 766–782 | 781
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Fig. 23 Hysteresis loop for an antiferroelectric liquid crystal.
Wiley-VCH, Weinheim, 1998, vol 2B, pp. 665–691 and references
therein.
Overall decoupling of the layers and the incorporation of
lateral groups which weaken the lateral interactions will
4 P. Rudquist, J. P. F. Lagerwall, M. Buivydas, F. Gouda,
S. T. Lagerwall, N. A. Clark, J. E. Maclennan, R. Shao,
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destabilise the hysteresis loop and support
response to applied electric fields.
a V-shaped
5. Conclusion
5 C. C. Dong, M. Hird, J. W. Goodby, P. Styring and K. J. Toyne,
Ferroelectrics, 1996, 180, 245.
In conclusion for the results obtained on the various materials
studied several observations and property–structure activities
may be drawn:
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7 S. J. Cowling, A. W. Hall, J. W. Goodby, Y. Wang and
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8 A. I. Stipetic, J. W. Goodby, M. Hird, Y. M. Raoul and
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9 P. Lehmann, W. K. Robinson and H. J. Coles, Mol. Cryst. Liq.
Cryst., 1999, 328, 221.
10 W. K. Robinson, P. Lehmann and H. J. Coles, Mol. Cryst. Liq.
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11 B. Musgrave, P. Lehmann and H. J. Coles, Mol. Cryst. Liq. Cryst.,
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12 H. J. Coles, H. Owen, P. Hodge and J. Newton, Liq. Cryst., 1993,
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13 A. S. Peterenko, PhD Thesis, University of Hull, 2002.
14 W. L. McMillan, Phys. Rev. A, 1973, 8, 1921.
15 I. Nishiyama, J. Yamamoto, J. W. Goodby and H. Yokoyama,
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16 I. Nishiyama, J. Yamamoto, J. W. Goodby and H. Yokoyama,
J. Mater. Chem., 2001, 11, 2690–2693.
17 I. Nishiyama, J. Yamamoto, J. W. Goodby and H. Yokoyama,
Ferroelectrics, 2002, 276, 255–265.
18 I. Nishiyama, J. Yamamoto, H. Yokoyama, S. Mery, D. Guillon
and J. W. Goodby, Trans. Mater. Res. Soc. Jpn., 2004, 29,
785–788.
19 A. D. L. Chandani, Y. Cui, S. S. Seomun, Y. Takanishi,
K. Ishikawa and H. Takazoe, Liq. Cryst., 1999, 26, 167.
20 S. T. Lagerwall, in Handbook of Liquid Crystals, ed. D. Demus,
J. Goodby, G. W. Gray, H.-W Speiss and V. Vill, Wiley-VCH,
Weinheim, 1998, vol 2B, p. 552.
1. The V-shape electrooptical response is a property of the
material placed in the surface-stabilised geometry, and can be
observed for switching in the ferroelectric smectic C* phase.
2. At the tip of the V-shaped switching curve, the uniform
polarization stabilised structure is likely to be formed accord-
ing to the mechanism described by models of the polarization
stabilised kink (PSK) and insulating layer structures.
3. The stability of the uniform state is greatly affected by the
chemical structure of the material. Functional groups intro-
duced onto the molecular core and chains may reduce the
polarization effect thereby resulting in domination by the
twisted state. In such cases, the V-shaped switching curve can
still be observed, however, the transmission at the tip of the V
does not become zero. An exception is the twisted structure
formed by high tilt materials.
References
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782 | J. Mater. Chem., 2007, 17, 766–782
This journal is ß The Royal Society of Chemistry 2007