Liquid crystalline properties of unsymmetrical bent-core compounds containing chiral moieties

Article abstract of DOI:10.1039/b419479a

The synthesis, characterization and mesomorphic properties of new unsymmetrical bent-core compounds containing chiral moieties are reported. These compounds have been derived from either 3-hydroxybenzoic acid or 2,7-dihydroxynaphthalene as the central unit. The chiral group is incorporated either within the bent-core structure as a linking group or is appended in the terminal position. The influence of the position of the chiral moiety on the mesomorphic properties has been investigated. The bent-core compounds containing a chiral group (octan-2-ol) in the terminal chain exhibit a direct transition from chiral calamitic phases such as SmC*, SmC*_γ to a polar banana phase with a two-dimensional lattice (Col_obP _A*) on decreasing temperature. In order to prove this phase sequence, the corresponding compound with a racemic octan-2-ol group at the terminal chain has been synthesized. The preliminary observations and investigations are presented for comparison with the chiral compound. Possible explanations for this unusual phase sequence and the models for the Col _obP_A* (SmC_sP_A*) phase are also discussed on the basis of X-ray and electric field experiments. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b419479a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Liquid crystalline properties of unsymmetrical bent-core compounds
containing chiral moieties{
R. Amaranatha Reddy,{a B. K. Sadashiva*a and U. Baumeisterb
Received 4th January 2005, Accepted 16th June 2005
First published as an Advance Article on the web 6th July 2005
DOI: 10.1039/b419479a
The synthesis, characterization and mesomorphic properties of new unsymmetrical bent-core
compounds containing chiral moieties are reported. These compounds have been derived
from either 3-hydroxybenzoic acid or 2,7-dihydroxynaphthalene as the central unit. The
chiral group is incorporated either within the bent-core structure as a linking group or is
appended in the terminal position. The influence of the position of the chiral moiety on the
mesomorphic properties has been investigated. The bent-core compounds containing a chiral
group (octan-2-ol) in the terminal chain exhibit a direct transition from chiral calamitic
phases such as SmC*, SmC*c to a polar banana phase with a two-dimensional lattice (ColobPA*)
on decreasing temperature. In order to prove this phase sequence, the corresponding
compound with a racemic octan-2-ol group at the terminal chain has been synthesized.
The preliminary observations and investigations are presented for comparison with the
chiral compound. Possible explanations for this unusual phase sequence and the models for
˜
the ColobPA* (SmCSPA*) phase are also discussed on the basis of X-ray and electric field
experiments.
polar switching in the mesophase of compounds composed of
1. Introduction
achiral banana-shaped molecules. Immediately thereafter,
careful experiments on this mesophase were carried out by
Link et al.8 who established that the phase is actually
antiferroelectric with possible chiral (SmCAPA) and racemic
(SmCSPA) structures. However, it was also possible to obtain a
homogeneously chiral state by adding a chiral dopant to this
mesophase.8 In order to investigate the influence of a chiral
group in banana-shaped mesogens, Nakata et al.9 studied the
chiral analogues of bent-core molecules. The ferroelectric
Molecular chirality is an important factor in liquid crystals,
which manifests in the form of helical ordering of the
constituent molecules in the mesophase. The cholesteric phase1
and the other blue phases (BP)2,3 with helical structure and the
discovery of ferro- and antiferro-electricity in chiral calamitic
compounds stimulated the growth of the study of liquid
crystals due to their potential practical applications. After a
century of the discovery of the first liquid crystal, a new
chiral phase namely the helical SmA* phase (twist grain
boundary smectic A phase) was discovered.4,5 Generally, com-
pounds exhibiting the antiferroelectric smectic C* (SmC*A)
phase show subphases such as the SmC*a and the ferrielectric
(SmC*c) phases. For a clear understanding of the correlation
between the molecular structure and the appearance of these
phases and phase sequences, a number of chiral rod-like
compounds have been synthesized and investigated by several
groups.6
phase obtained had
a racemic ground state structure
(SmCAPF*) though the compound was composed of chiral
molecules. This observation violates a rule that the handedness
of chiral structure in fluid phases is determined by the
molecular chirality. A brilliant approach by Walba et al.10
was based on the knowledge that the ferroelectric structure
requires an anticlinic interlayer correlation in bent-core
molecules and they chose racemic (¡) octan-2-ol to satisfy
this requirement (known to exhibit SmCa structure in calamitic
compounds) and reported a new variant of ferroelectric
switching mesophase (USmCSPF) in banana liquid crystals.
In addition, several other ferro- and antiferro-electric switch-
ing columnar phases made up of achiral bent-core molecules
were also reported.11 Remarkably, most of the antiferroelectric
switching columnar phases show special switching behaviour,
that is, the polarization reversal always takes place around the
long axis during the switching process (so-called switching of
chirality)11b–d and not as a collective rotation around a tilt
cone, as usually observed in the tilted polar smectic phases. We
have investigated the mesomorphic properties of a few bent-
core compounds containing chiral moieties, the corresponding
racemic compound in a special case for comparison and the
preliminary results obtained are presented here.
Until recently, it was thought to be a general rule that in
order to produce chiral or polar structures in fluid phases it is
necessary to incorporate chirality at the molecular level.
However, in 1996, Niori et al.7 demonstrated a ferroelectric
aRaman Research Institute, C. V. Raman Avenue, Sadashivanagar,
Bangalore-560 080, India. E-mail: sadashiv@rri.res.in
bInstitute of Physical Chemistry, Martin-Luther-University,
Mu¨hlforte-1, D-06108 Halle, Germany
{ Electronic supplementary information (ESI) available: Synthetic
procedures and analytical data for all the compounds discussed here,
textural photographs of SmCs phase under electric field. See http://
dx.doi.org/10.1039/b419479a
{ Present address: Institute of Organic Chemistry, Martin-
Luther University, Kurt-Mothes-Str.2, 06120 Halle, Germany;
E-mail: amaranatha.reddy@chemie.uni-halle.de
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Fig. 1. This texture is characteristic of a blue phase and has
been characterized as such. It has a short range of about
1.2 uC. On cooling this phase, the cholesteric phase with oily
streaks appears, as shown in Fig. 2, and the background
becomes blue. On increasing or decreasing the temperature the
back ground blue colour changes. Since these two transitions
could not be resolved clearly on a DSC thermogram, the
enthalpy given is the sum of I to BP to N* phase transitions,
which is about 1.25 kJ mol21. The thermal range of the
cholesteric phase was about 32 uC and no other phase
transition could be seen.
2. Results and discussion
In the case of compounds derived from 2,7-dihydroxy-
naphthalene, the chiral group is present within the bent-core
structure as a linking group. For the compounds synthesized
using 3- hydroxybenzoic acid, the chiral group is present as
part of the terminal n-alkyl chain. The mesophases obtained
were investigated by the classical techniques of polarized light
microscopy, differential scanning calorimetry (DSC), X-ray
diffraction (XRD), and electro-optical investigations. The
molecular structure, transition temperatures and the asso-
ciated enthalpies are given for each of the compounds studied.
2.2. Mesomorphic properties of compound B1
2.1. Mesomorphic properties of compound A1
The molecular structure of this compound is similar to that of
compound A1 except that the terminal 4-n-tetradecyloxy-
benzoate moiety is replaced by 4-n-octadecyloxycinnamate.
This amounts to extending the length of the arms. Thus,
compound B1 exhibits two additional phases which are,
On cooling the isotropic liquid of compound A1 sandwiched
between a glass slide and a cover slip and viewed under a
polarizing microscope,
a colourful platelet texture was
obtained. A photomicrograph of this texture is shown in
Fig. 1 Optical photomicrograph of the blue phase of compound A1
at 162.5 uC.
Fig. 2 Optical photomicrograph of the cholesteric phase of com-
pound A1 at 152 uC.
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a-methyl group on the transition temperature is quite drastic
and is similar to that observed in calamitic systems.12 It
has already been reported13–19 that compounds derived from
2,7-dihydroxynaphthalene show banana phases. However,
none of the compounds described in this manuscript contain-
ing a 2,7-dihydroxynaphthalene central unit exhibit a banana
phase. This is due to the fact that the lactate bridging group,
perhaps, renders the system linear and hence only calamitic
chiral mesophases are obtained.
2.4. Mesomorphic properties of compounds of series I
We have examined the mesomorphic properties of some
compounds derived from 3-hydroxybenzoic acid containing a
terminal chiral chain. The transition temperatures and the
associated enthalpies for these four compounds D1–D4 are
collected in Table 1. None of these compounds exhibits a blue
phase. When a thin film of the isotropic liquid of compound
D1 was cooled slowly, a planar texture typical of a cholesteric
phase was observed. Sometimes a schlieren texture could also
be obtained. On cooling the sample further, multiple domains
began to develop which transformed to a homeotropic texture
as the temperature was lowered. This homeotropic texture
existed over a short range of temperature and changed to
a pseudo-homeotropic texture. When this was sheared, a
schlieren texture was obtained, which indicated that this phase
is a chiral smectic C (SmC*) phase. Interestingly when the
sample was heated from the homeotropically aligned SmA
phase, straight filaments could be seen to grow which finally
coalesced to an undefined texture indicating the presence of a
TGBA phase. This mesophase behaviour is similar to that
observed for compound B1. When a sample of compound D1
was observed between glass plates treated for homogeneous
alignment, the smectic C* phase appeared with partially
aligned banded focal-conic texture on cooling the focal-conic
texture of the SmA phase. This indicates again the chiral
nature of the smectic C phase. The ferroelectic behaviour of
this phase was confirmed by electro-optical studies. The higher
homologues viz. compounds D2 and D3 show a TGBA phase
on cooling the isotropic liquid and SmA, SmC* phases appear
with decreasing temperature. A photomicrograph of the
appearance of the TGBA phase from the isotropic phase of
compound D3 is shown in Fig. 4. On increasing chain length
further as in compound D4, only TGBA and SmC* phases
Fig. 3 Optical photomicrograph showing a filamentary texture of the
TGBA phase originating from the homeotropic regions of the SmA
phase of compound B1.
however, monotropic. As observed for compound A1, this
compound also exhibits an enantiotropic blue phase over a
short temperature range and a cholesteric phase over a thermal
range of 34 uC. On cooling the cholesteric phase to a tem-
perature of 134 uC, under a polarizing microscope, the field
of view becomes dark and stays like that until the sample
crystallizes. However, before crystallization occurs, if the
sample is heated slowly, then filaments can be seen to grow
from the dark regions and a photomicrograph of this
filamentary texture is shown in Fig. 3. This behaviour is
characteristic of the twist grain boundary smectic A (TGBA)
phase growing from the homeotropic regions of the smectic
A (SmA) phase. The thermal range of this TGBA phase is only
1 uC. The SmA phase is highly metastable and invariably the
TGBA phase crystallizes directly.
2.3. Mesomorphic properties of compounds C1 and C2
These two compounds are similar to compound B1 except
that 4-n-alkoxy-a-methylcinnamic acids are used in the final
step of the synthesis. 4-n-Tridecyloxy-a-methylcinnamic acid
and 4-n-octadecyloxy-a-methylcinnamic acid were used to get
compounds C1 and C2 respectively. Compound C1 exhibits
both blue and cholesteric phases, while in compound C2 a
monotropic TGBA phase is observed in addition. As can be
seen, both the melting and clearing temperatures are reduced
and the TGBA phase is still monotropic. Thus, the effect of the
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Table 1 Transition temperatures (uC) and enthalpies (italics; kJ mol21) for the compounds of series I (abbreviations: N*: Cholesteric phase;
TGBA: Twist Grain Boundary Smectic A phase; SmA: Smectic A phase; SmC*: Chiral smectic C phase; Cr: Crystalline phase; I: Isotropic liquid)
Compound
n
Cr
SmC*
SmA
TGBA
N*
N
I
N
N
N
N
D1
D2
D3
D4
a
10
11
12
14
N
N
N
N
117.0
22.3
120.0
25.0
122.5
26.2
122.5
26.9
b
N
N
N
N
124.5a
126.9a
127.9a
129.8a
N
125.0a
N
N
N
N
125.7
0.55
127.9
2.3
126.0
1.9
N
127.2b
0.92
—
—
—
N
128.3a
128.6c
4.0
—
130.0d
4.4
c
Enthalpy could not be determined. Enthalpy denoted is the sum of both SmC* to SmA and SmA to TGBA phase transitions. Enthalpy
d
denoted is the sum of SmC* to SmA, SmA to TGBA and TGBA to isotropic phase transitions. Enthalpy denoted is the sum of both SmC* to
TGBA and TGBA to isotropic phase transitions.
˚
spacing of 46.3 A. The estimated tilt angle by assuming a bent-
were observed. A photomicrograph showing the transition
from the TGBA to the SmC* phase on cooling a sample of
compound D4 is shown in Fig. 5. The powder X-ray
diffraction measurements indicated only a layered structure
for the SmC* phase of compound D4 with a first order layer
core structure of the compound amounts to about 25u which
is consistent with the tilt angle observed from electro-optical
investigations (#22u). The mesophase behaviour of the
investigated compound clearly indicates that the bent-core
structure has no influence though it exists. This can be
explained by assuming a free rotation of the aromatic bent-
core structure throughout the temperature ranges of the meso-
phases. However, in the chiral smectic C phase, the symmetry
is low enough to promote the chirality-induced improper
ferroelectricity, and does not really depend on the dipole–
dipole interactions.
Electro-optical investigations were carried out in the SmC*
phase as follows. A sample of compound D4 in the isotropic
phase was placed into a cell which was constructed for planar
alignment of the mesophase and having a thickness of about
14 mm. By applying a triangular-wave electric field of about
¡85 V and at a frequency of 40 Hz, a single polarization
current peak could be observed for each half period indicating
a ferroelectric switching behaviour for the mesophase. The
calculated polarization value was about 23 nC cm22 which is
rather small when compared with the polarization value
observed for the mesophase of an achiral bent-core com-
pound.20 The occurrence of a high polarization value in bent-
core compounds is due to the long-range correlation of the
packing-induced dipoles along the bent direction for steric
reasons. The current response trace obtained in the SmC*
phase of compound D4 is shown in Fig. 6.
Fig. 4 Optical photomicrograph showing the appearance of the
TGBA phase on cooling the isotropic liquid of compound D3.
2.5. Mesomorphic properties of the chiral compound E1
The molecular structure of this compound (Fig. 7) is similar to
the compounds of series Dn but the terminal n-alkyl chain is
replaced by an n-alkoxy chain. Only one compound was
prepared for comparison. The four mesophases exhibited by
this compound were examined under a polarizing microscope.
When a thin film of the isotropic phase of this compound was
cooled, a schlieren texture was obtained which excludes the
Fig. 5 Optical photomicrograph showing the transition from the
TGBA phase to the SmC* phase for compound D4.
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Fig. 6 Switching current response obtained by applying a triangular-
wave voltage of about ¡85 V and at a frequency of 40 Hz in the SmC*
phase of compound D4 at 124 uC. Cell thickness, 14 mm; polarization
value, PS # 23 nC cm22
.
Fig. 8 Optical photomicrograph obtained in the SmC*c phase of
compound E1 at 125 uC.
possibility of a normal SmA phase and at first sight this is
indicative of a SmC*a phase. Since this mesophase exists over a
narrow thermal range, it has not been possible to characterize
completely and hence has been designated as an X1 phase. On
lowering the temperature, a pseudo-homeotropic texture was
obtained. The optical textures, X-ray and electro-optical
investigations on this mesophase are identical to the SmC*
phase of compound D4 and as we shall see later for more
evidence this phase has been characterized as a chiral
ferroelectric smectic C phase. On viewing the sample between
untreated glass plates (5 mm), this mesophase appears pseudo-
homeotropic which indicates that the helical pitch must be
much smaller than the thickness of the cell. Regardless of
sample rotation no colour change could be observed which
suggests that the helical axis is perpendicular to the substrate.
On decreasing the temperature further to 140 uC, the texture
changed to a weakly birefringent phase. The characteristic
feature of this phase is that the texture showed strong
fluctuations even at a constant temperature and this is
typically observed for a ferrielectric (SmC*c) mesophase. A
photomicrograph of this strongly fluctuating texture obtained
is shown in Fig. 8. Sometimes these optical textures are
reminiscent of the features of a polar SmA phase made up of
bent-core molecules,21 but the aligned X-ray diffraction
pattern and further the optical switching exclude such a
possibility. On lowering the temperature to 119 uC, a phase
transition took place, which was accompanied by a drastic
change in the texture. The textural features coupled with high
viscosity observed for this mesophase were completely
different from those observed in all the mesophases of other
compounds and are actually indicative of a columnar phase.
As we shall see later, based on aligned X-ray pattern and
electric field experiments, this mesophase has been designated
˜
as a ColobPA* phase (more precisely, SmCSPA*; a modulated
SmCP phase).
When a sample of compound E1 was cooled from the
isotropic phase in a cell treated for homogeneous alignment
(6 mm), initially a fan-shaped texture was obtained for the
highest temperature X1 phase (Fig. 9a). At the transition to
the next lower temperature phase, a kind of striped pattern
perpendicular to the layer normal appeared. These features
completely changed on lowering the temperature when a
phase transition to the ferrielectric phase takes place. On
decreasing the temperature further, a columnar phase with
more birefringent colours is obtained. The optical photo-
micrographs obtained for all these phases and phase transi-
tions are shown in Fig. 9.
The phase sequence of compound E1 was further confirmed
by XRD and electro-optical studies. The well-aligned sample
(the X-ray beam was directed parallel to the surface aligned
sample) showed the following XRD pattern. Since the X1
phase has a thermal range of only 0.7 uC, it has not been
possible to obtain accurate XRD data. In the SmC* phase, in
Fig. 7 The molecular structure and the phase sequence of compound E1. The corresponding phase transition enthalpy values (kJ mol21) are given
below the transition temperatures (uC). X1: unidentified mesophase; SmC*: chiral ferroelectric smectic C phase; SmC*c: chiral ferrielectric smectic
˜
C phase; ColobPA* (SmCSPA*): antiferroelectric polar switching columnar (modulated) banana phase with an oblique cell; * indicates only the
molecular chirality which is independent of mesophase chirality.
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Fig. 10 X-Ray diffraction patterns of the aligned sample of com-
pound E1: (a) SmC* phase at 144 uC, (b) SmC*c phase at 130 uC, (c)
ColobPA* phase at 116 uC and (d) is the small angle region of (c) with
Miller indices.
angle reflections with their higher order and a sharper diffuse
pattern in the wide-angle region indicate the compact packing
of bent-core molecules in the modulated phase as typical for
bent-core molecules to obtain the polar order (see Fig. 11a).
Finally, electro-optical studies were carried out on these
mesophases to complement the above observations. Com-
pound E1 was placed in a cell, which was constructed for
planar alignment of the sample. The sample was cooled under
a triangular-wave electric field, and in the thermal range of the
SmC* phase a single polarization current peak was obtained.
The calculated polarization (PS) value was about 25 nC cm22
and a current vs. time response trace obtained is shown in
Fig. 12. Although bistable switching could be clearly seen
under a polarizing microscope, on terminating the applied
field, the texture relaxes such that it exhibits an equidistant
striped pattern perpendicular to the layer normal and is shown
in Fig. 13c. On application of an electric field exactly at the
threshold voltage these striped pattern disappear and simulta-
neously a polarization current peak appears in the triangular-
wave and on removal of electric field recovers this striped
pattern. These observations suggests the helical pitch and the
helical axis is parallel to the substrate which can be unwound
and rewound on application or removal of electric field,
respectively. This also explains the observation of almost dark
textures in a homeotropically aligned cell in which the helical
axis coincides with one of the optical axes (see for example,
Fig. 5). This can also be realized by the relaxation of extinction
crosses along the directions of the crossed polarizers on
removal of electric field as shown in Fig. 13c. Moreover, the
rewinding/unwinding switching process is visible only at low
frequencies (,0.2 Hz) and at higher frequencies (.0.2 Hz)
a direct bistable switching process takes place. Thus, this
ferroelectric phase has a synclinic and synpolar structure. On
Fig. 9 Optical photomicrographs obtained in
a homogeneously
aligned cell (6 mm) for the mesophases of compound E1: (a) X1 phase
growing from the isotropic liquid at 147 uC, (b) transition to the SmC*
phase at 146.4 uC, (c) SmC* phase at 144 uC, (d) transition to the
SmC*c phase at 140 uC, (e) SmC*c phase at 130 uC and (f) transition to
the ColobPA* phase at 118 uC.
the small angle region, the first order reflection was obtained
˚
with d 5 46.9 A. The crescent-like diffuse pattern in the wide-
˚
angle region at 4.7 A indicates a liquid-like multi-domain
alignment from tilted organization of molecules (see
a
Fig. 10a). In order to estimate the corresponding small tilt
angle,22 we performed an x-scan profile along the wide-angle
region which was fitted by two Gaussian functions and the
resultant tilt angle obtained was about 13–14u. A similar XRD
pattern in the SmC*c phase is observed (Fig. 10b) with the first
˚
order layer spacing d 5 47.8 A. This unusual increase of layer
˚
thickness (#0.9 A) can be explained by stiffening of the
aliphatic chains on decreasing temperature. Remarkably,
the spot-like layer reflections in the small angle region and
the outer diffuse scattering on the equator obtained in both
ferroelectric and ferrielectric phases are reminiscent of the
XRD pattern normally observed for rod-like molecules which
is consistent with the phase assignment such as SmC* and
SmC*c. However, the aligned XRD pattern of the low
temperature phase (Fig. 10c,d) clearly indicates a modulated
structure with an oblique lattice having the lattice parameters
˚
˚
a 5 61.2 A, b 5 49.3 A and b 5 112.2u. The presence of the
preferred orientation direction of the wide-angle region
(intensity of the diffuse pattern on the top-right and bottom-
left positions decreases, see Fig. 10c) clearly indicates a
synclinic tilt (y22u, measured from the oriented pattern) of
the molecules in adjacent columns. The more intense small
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Fig. 11 (a) Observed phase transition from chiral calamitic phases to B-phase on decreasing temperature for chiral compound E1, this is due to an
interplay between the packing-induced dipoles and the chirality-induced dipole coupled with the temperature dependent rotational barrier around
the molecular long axis of the bent-core. (b) As expected, two different switching mechanisms were observed for calamitic chiral phases and the
polar B-phases. In the classical chiral phases, switching around a tilt cone reverses both tilt and polarization directions which retains the
macroscopic chirality. However, in AF B-phases, the collective rotation of molecules around the long axis reverses only the polarization direction
and retains the tilt direction, which changes the sense of macroscopic chirality during the switching process. Different colours of molecules in the
fragments (modulated phase) indicate opposite chirality, which is independent of the molecular chirality; a and b are the lattice parameters for the
modulated phase.
lowering the temperature to 130 uC under the same conditions,
the polarization value increased marginally in the SmC*c
phase and was about 40 nC cm22. Surprisingly, at this
transition to the ferrielectric phase only a broadening of the
ferroelectric polarization peak which is obtained in the SmC*
phase could be observed. It has not been possible to resolve
this polarization peak in order to realize the ferrielectric nature
of the mesophase, which indicates a direct switching between
the two ferroelectric states without relaxing to the ferrielectric
ground state.23 This means the field-induced ferroelectric
state at the threshold field is further stabilized by strong
surface interactions. The polarization value reported here was
Fig. 12 Switching current response obtained by applying a triangular-
obtained in the field-induced ferroelectric state of a triangular-
wave voltage. The optical observations under a polarizing
microscope indicated the homochiral switching (switching
wave voltage of about ¡195 V and at a frequency of 300 Hz in the
SmC* phase of compound E1 at 143 uC. Cell thickness, 8.9 mm;
polarization value, PS # 25 nC cm22
.
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Fig. 13 Switching behaviour of chiral smectic C phase on application
of electric field (cell thickness, 6 mm): (a) unwound ferroelectric state at
+30 V; (b) unwound ferroelectric state at 230 V; (c) rewound ground
state ferroelectric phase on removal of applied field; compound E1.
between bright and dark domains in which the molecules are
oriented in different directions with respect to the polarisers) as
shown in Fig. 14. During the switching process, the polariza-
tion reversal takes place around a tilt cone of the molecules as
expected in chiral smectic phases (see Fig. 11b) which is
indicated by the textural changes from bright to dark and vice
versa. This switching behaviour is retained even in the high
temperature SmC* phase indicating the homochiral nature of
the chiral smectic C phase. However, on decreasing the
temperature, in the modulated phase, the polarization value
increased drastically to about 250 nC cm22. The two polariza-
tion current peaks for each half period of the triangular-wave
(Fig. 15a) clearly confirm the antiferroelectric ground state
structure for this phase. Furthermore, no optical switching
could be observed during the switching process (see Fig. 15b),
the field-on and off states of optical textures are identical
except for a small change in colour. This means that the
position of the molecules remains unchanged during the
Fig. 14 Electro-optical switching in homochiral domains (synclinic
tilt of molecules in the adjacent layers) observed for the SmC*c phase
of compound E1 at 130 uC (6 mm); (a) +30 V; (b) 230 V and (c) 0 V.
Polarization reversal takes place around the tilt cone of the molecules.
switching process, which suggests that the polarization reversal
takes place around the long axes of the molecules, the so called
the switching of chirality, and is typical for antiferroelectric
switching columnar banana phases (see Fig. 11b).11b–d This can
be understood by a restricted rotation around a tilt cone which
is due to the boundaries between the ribbons/clusters with
alternating polarization direction. Switching around a tilt cone
in the columnar phase would require a complete reorganiza-
tion of the ground state structure, which is more complicated
in the viscous compact medium. Because of the synclinic tilt of
molecules in adjacent clusters as indicated by X-ray diffraction
studies with an antiferroelectric organization, the ground state
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from linear to bent structure on decreasing temperature and
instead it may be due to an increase of tilt angle and
with different interactions between the columns as they are
structurally different phases. The calculated molecular length
with a bent structure (bending angle #140u, which is just at the
upper limit for the mesophases of bent-core compounds)
having an all-trans conformation of chains varies from 52 to
˚
57 A depending on the chain orientation with the arms of the
core part. Therefore, these observations suggest that the bent-
core structure exists in all the three phases investigated and this
special phase sequence can be explained in a better way on the
basis of a temperature dependent rotational barrier around
the aromatic bent-core (see Fig. 11a). At high temperature, the
thermal molecular motion is greater such that the molecules
can still rotate around the aromatic part (low value of
rotational barrier due to the relatively larger bending angle
#140u), though they are hindered to some extent. The
transverse chirality-induced dipoles on the end chains are
considered to be aligned only statistically which is responsible
for the occurrence of calamitic chiral phases, and therefore the
local molecular dipoles are least important in this case (see
Fig. 11a). On decreasing temperature, the rotational barrier of
the aromatic part increases, that is, the free rotation around
the long axis is more hindered,21 which leads to the long-range
steric packing of adjacent molecules directed along the bent
direction responsible for the occurrence of a high polarization
value (10 times higher when compared with classical chiral
phases), thereby exhibiting the properties of polar banana
phases (see Fig. 11a). Therefore, in the lower temperature
polar banana phase, the chirality-induced dipole is least
important and the packing-induced dipoles are dominant due
to sterically packed aromatic moieties with long-range
correlation of these dipoles. The phase sequences in polar
liquid crystals are influenced to some extent by the complex
interplay between chirality-induced dipoles (due to chiral
group) or packing-induced dipoles and the steric interactions
at the interlayer interfaces. Smaller polarization or dipole
values within the layers can be compensated by the formation
of helical structures (e.g. SmC*) at high temperatures.
However, on lowering the temperature, the packing-induced
larger local dipoles prefer only an antiferroelectric organiza-
tion. Because of the strong competition between polar order
and steric factors, the polarized layers that are probably
induced initially could be fragmented simultaneously into
clusters, which are responsible for the formation of modulated
phase at lower temperature. Because of the tilted organization
of molecules in the modulated phase, the polarization direction
must be perpendicular to the direction of the 2D modulation
based on symmetry considerations.24 On the basis of distinct
features observed between high and lower temperature phases
using optical textures, XRD and electric field experiments, we
conclude that compound E1 indeed shows a phase transition
from the classical chiral phases to a polar banana type
modulated phase. Such behaviour was first observed by
Lagerwall et al.25 for a bent mesogenic dimer which is linear
in shape at high temperature and bent at low temperature
thereby exhibiting a direct transition from a classical chiral
phase to a banana phase on decreasing temperature. The phase
sequence of compound E1 is similar to a phase sequence
Fig. 15 (a) Switching current response obtained by applying
a
triangular-wave voltage of about ¡100 V and a frequency of 300 Hz
in the ColobPA* phase of compound E1 at 115 uC; cell thickness, 6 mm;
polarization value, PS # 250 nC cm22; (b) optical photomicrograph
obtained in this mesophase under a triangular-wave electric field
(¡100 V and a frequency of 50 Hz, cell thickness: 6 mm), which is
independent of the presence or absence of electric field.
structure can be described as a racemic state (SmCSPA-type) in
which every alternating cluster changes its chirality sense (see
Fig. 11a, with different shades of molecules). On application of
an electric field a homogeneous chiral FE state (SmCSPF-type)
is induced and on reversing the sign of the applied
field it switches to the other homogeneous chiral FE state
(SmCSPF-type) with opposite chiral sense. Therefore, the
switching process takes place between two homochiral
ferroelectric states via the racemic antiferroelectric ground
state.11b–d In view of the fact that this mesophase exhibits a
columnar structure (oblique cell) composed of chiral mole-
˜
cules, it has been designated as a ColobPA* (SmCSPA*) phase.
It should be noted that at high temperature chiral phases
could be obtained with characteristic lower polarization value
as observed for calamitic ferroelectric and ferrielectric phases
which are very much comparable with the ferroelectric phase
obtained for the corresponding alkyl derivatives (Dn series)
by optical textures, X-ray and electric field experiments. On
decreasing temperature, in the columnar phase, the corres-
˚
ponding (and comparable) d-spacing which is about 45.8 A
˚
(47.8 A is the corresponding value in the SmC*c phase at high
˚
temperature). The reduction of the layer spacing by about 2 A
in the columnar phase does not really explain the transition
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Fig. 16 The molecular structure together with the phase sequence of compound F1. The corresponding phase transition enthalpy values (kJ mol21
are given below the transition temperatures (uC). X2: unidentified mesophase; SmCa: smectic C phase with anticlinic tilt; SmCs:smectic C phase
)
˜
with synclinic tilt; ColobPA(SmCSPA): antiferroelectric polar switching columnar (modulated) banana phase with an oblique cell.
observed for many other achiral bent-core molecules with a
relatively larger bending angle between 130–140u which show
a direct transition from classical calamitic phases (SmA, SmC)
to a polar banana phase (SmCP) with decreasing tempera-
ture.22 In this phase sequence for some homologues,22 no
change in the layer spacing was observed at the transition from
the calamitic to the polar banana phase. The presently
investigated compound induces the classical chiral phases
(e.g. SmC*) at high temperature due to the fact that the
molecules are chiral but there is no influence on the banana
phase formed at low temperature as indicated by a racemic
ground state structure. On the basis of experimental observa-
tions, two possible models can be discussed for the modulated
retains the schlieren texture but with thermal fluctuations and
a photomicrograph of this is shown in Fig. 17a. The transition
to the lower temperature phase (indicated as SmCs) was
accompanied by a clear change in the optical texture. At the
transition the schlieren texture shows strong fluctuations
and the brush defects disappear on decreasing temperature
(Fig. 17b). On reducing the temperature further, a banana
phase was obtained with similar optical texture as seen for the
corresponding chiral compound E1. In a cell treated for
homogeneous alignment, the X2 phase shows a fan-shaped
texture and the lower temperature SmCa phase shows a broken
fan-shaped texture with an irregular striped pattern which is
˜
ColobPA* (SmCSPA*) phase. One simple way to generate the
modulated phase from the smectic phase11c,26 is as shown in
Fig. 11a, by introducing the fragmentation into a layer
structure.11c This can still retain the polarization direction in
adjacent clusters and alternate only in adjacent fragmented
layers. The other possible model, in which the fragmentation
of layers takes place, is followed by a change in the
polarization direction. Although the latter one is a more
complicated phase transition, the high enthalpy value obtained
for this transition supports such a structural change. This
means the polarization direction alternates in the direction of
either of the lattice parameters a or b.26 In most cases, it is not
possible to distinguish these models from XRD measurements
where the electron density of the liquid-like clusters is the same
whether the molecules point up or down along the column. It
is also more complex from the electric field experiments as it is
not possible to correlate directly the field-induced structure
with the virgin structure.
In order to confirm the above-mentioned phase sequence,
we have synthesized the corresponding bent-core compound
using racemic octan-2-ol. The preliminary observations and
investigations are described below as a comparison for the
chiral compound.
2.6. Mesomorphic properties of the racemic compound F1
(Fig. 16)
(a) Optical textures. The sample sandwiched between
untreated glass plates was cooled from the isotropic state
and viewed under crossed polarisers. The highest temperature
X2 phase shows a schlieren texture with 2- and 4-brush defects.
On cooling the X2 phase, the SmCa phase was obtained which
Fig. 17 Optical photomicrographs obtained for compound F1; (a)
schlieren texture of SmCa phase at 144 uC; (b) SmCs phase at 130 uC.
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dominant in the SmCs phase. The birefringence of the optical
texture also increases at this phase transition. The lowest
temperature banana phase shows textures similar to that
shown in Fig. 9f.
to straightening of the aliphatic chains. Recently, Thisayukta
and Samulski27 have shown that independent of the chiral
group, even branching of the aliphatic chain at the a-carbon
atom favours the anticlinic organization of the molecules at
the interlayer interfaces.
(b) X-Ray investigations. X-Ray diffraction measurements
were carried out on both powder and surface aligned samples.
The powder X-ray diffraction pattern in the SmCa and SmCs
phases proves the layer structure with the corresponding layer
On reducing temperature to the banana phase, the non-
symmetric intensity in the wide-angle region (with respect to
the equator) is retained indicating the synclinic tilt of the
molecules even in this phase, though the pattern in the small
angle region is completely changed. The additional reflections
are indicative of a columnar phase and can be indexed to an
oblique cell as shown in Fig. 18c, with the lattice parameters
˚
spacings of 46.3 A and 47 A respectively. A diffuse peak in the
˚
˚
wide-angle region in both the phases at 4.7 A suggests fluidity
of these. The aligned X-ray diffraction pattern in both the
SmC phases showed spot-like reflections in the small angle
region as discussed for the corresponding chiral phases which
indicates similarity with those phases. Remarkably, the high
temperature SmC phase shows the symmetric intensity on both
sides of the equator (see Fig. 18a) which suggests an anticlinic
organization of molecules in adjacent layers and hence it has
been tentatively designated as a SmCa phase. Furthermore, the
weakly birefringent optical textures with 2- and 4-brush defects
support this phase assignment. This means the corresponding
mesophase (SmC* phase) of the chiral compound maintains a
uniform tilt of the molecules while in the racemic compound
the tilt alternates in adjacent layers. On decreasing the
temperature, in the lower temperature SmC phase, the non-
symmetric intensity on both sides of the equator (the intensity
at the top-right and bottom-left positions clearly decreases)
indicates the synclinic organization of molecules in adjacent
layers (see Fig. 18b). The increase of birefringence of the
optical textures at this phase transition is a further indication
for this organization. As discussed by Walba et al.,10 the
presence of a racemic octan-2-ol group in the end chain at the
a-carbon atom promotes the anticlinic tilt at the interlayer
interfaces for rod-like molecules (which is synclinic for bent-
core molecules). However, on decreasing temperature the
occurrence of a synclinic structure (SmCs phase) could be
explained on the basis of increased interlayer interactions due
˚
˚
a 5 30.4 A, b 5 45.2 A and b 5 104.4u. The corresponding
˚
layer spacing in the columnar phase is 43.8 A which is smaller
in comparison with the layer spacing of the high temperature
˚
SmCs phase (47 A). In this case there may be an indication for
the decrease of bending angle in the columnar banana phase,
but this may also be due to other possibilities as discussed for
the chiral compound which cannot be excluded. If the SmCs
phase is expected as the high temperature phase with respect to
the banana phase, then it is more reasonable to understand a
fragmentation in the SmCs phase to generate a SmCSPA-like
modulated structure on decreasing temperature. It is more
complicated to induce fragmentation in the SmCa phase
because of the weak interlayer fluctuations.
(c) Electro-optical studies. The lowest temperature banana
phase shows an antiferroelectric switching behaviour and the
typical current response curve obtained is shown in Fig. 19 and
no optical switching was observed. These observations are
consistent with those obtained for the corresponding banana
phase of the chiral compound. On increasing the temperature,
in the SmCs phase, only dielectric switching could be detected
(see Fig. S1, ESI{) and this can be clearly distinguished from
the polar switching observed in the SmC*c phase. A tiny
current peak is observed in this phase under the triangular-
wave which is retained even in the high temperature SmCa
phase and the isotropic phase, indicating that it is due to ionic
Fig. 18 X-Ray diffraction patterns of an oriented sample of
compound F1: (a) SmCa phase at 144 uC (tilt angle # 14u), (b)
SmCs phase at 130 uC (tilt angle # 15u), (c) ColobPA phase at 115 uC
with Miller indices (tilt angle # 18–20u).
Fig. 19 Switching current response obtained by applying a triangular-
wave voltage of about ¡75 V and a frequency of 25 Hz in the ColobPA
phase of compound F1 at 114 uC. Cell thickness, 6 mm; polarization
value # 285 nC cm22
.
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motion. No polarization switching or dielectric switching
could be seen in the SmCa phase. In this phase, the extinction
crosses coincide with the optical axis, and are independent of
field reversal.
column, 3.9 mm 6 300 mm, Waters Associates Inc.) and using
1% ethyl acetate in dichloromethane as the eluent.
4.2. Synthesis
The central cores used were 2,7-dihydroxynaphthalene and
3-hydroxybenzoic acid while the chiral materials used were
(R)-(2)-octan-2-ol and (S)-(2)-ethyl lactate and were all com-
mercial compounds. The synthesis of 3-fluoro-4-benzyloxy-
benzoic acid and 4-n-alkoxybenzoic acids were carried out as
described in our earlier publication.28 E-4-n-Alkoxycinnamic
acids and E-4-n-alkoxy-a-methylcinnamic acids were
prepared as described earlier.29 4-n-Alkylbiphenyl-4-carboxylic
acids were prepared as described in the literature.30 4-n-
3. Conclusions
An attempt has been made to understand the mesomorphic
behaviour of some unsymmetrical chiral bent-core com-
pounds. These chiral compounds were synthesized using
optically active lactic acid and octan-2-ol. Though many
compounds exhibited calamitic mesophases, at least one
showed a direct transition from a classical chiral phase to
an antiferroelectric polar banana phase with decreasing
temperature. In order to prove this phase sequence, we have
also synthesised the corresponding bent-core compound using
racemic octan-2-ol. Indeed, this compound shows a phase
sequence of a classical SmC phase to a B-phase. This supports
our observation of a transition from classical chiral phases to a
B-phase in the chiral compound, that is, the introduction of
a chiral group makes the SmC phase chiral and it has no
influence on the B-phase as expected.
Dodecyloxybiphenyl-4-carboxylic
acid
was
prepared
following
a
procedure described in the literature.31,32
3-Benzyloxybenzoic acid was prepared as reported earlier.33
7-Benzyloxy-2-hydroxynaphthalene 1 was obtained by react-
ing 1 : 1 equivalents of 2,7-dihydroxynaphthalene and benzyl
chloride in the presence of anhydrous potassium carbonate in
4. Experimental
4.1. Measurements
The mesophase behaviour was observed under an optical
polarized light microscope (Leitz Laborlux 12 POL/Olympus
BX 50) provided with a heating stage and a central processor
(FP 82 HT and FP 90 respectively). A differential scanning
calorimeter (Perkin-Elmer, Model Pyris 1D) was used to
determine the transition temperatures and the associated
enthalpies of the mesophases under investigation. The
powder X-ray diffraction patterns in the mesophases
obtained were recorded using a 4 kW rotating anode X-ray
source (Rigaku Ultrax 18) with graphite crystal mono-
˚
chromated Cu-Ka radiation (1.54 A). The diffraction patterns
of the samples were collected on an imaging plate
(Marresearch). The X-ray diffraction patterns of the aligned
samples (free drop on a glass plate, surface aligned) were
taken by a 2D-detector HI star (Siemens, Germany) using
Ni-monochromated Cu-Ka radiation. The polarization mea-
surements were carried out using a standard triangular-wave
method. The triangular-wave voltages were generated from
a waveform generator (Wavetek, Model 39), which was
amplified using an amplifier (TREK MODEL 601B-3). The
current response traces were recorded using a Tektronix
Oscilloscope (TDS 220).
In order to confirm the chemical structure of the compounds
synthesized, 1H NMR spectra were recorded on a spectrometer
(Bruker AMX 400, tetramethylsilane as an internal reference)
and the infrared spectra were obtained on a Shimadzu
spectrophotometer (FT IR-8400). The elemental analysis was
carried out using an analyzer (Carlo-Erba 1106). All the target
materials as well as the intermediate compounds were purified
by column chromatography (silica gel, 60–120 mesh) and
were crystallized using suitable analytical quality solvents. The
purity of all these compounds was confirmed using normal
phase high performance liquid chromatography (m-Porasil
Scheme 1 Synthetic pathway used to obtain the unsymmetrical chiral
mesogens from 2,7-dihydroxynaphthalene. Reagents and conditions: (i)
4-n-tetradecyloxybenzoic acid, DCC, DMAP, dry CH2Cl2; (ii) E-4-
n-octadecyloxycinnamic acid, DCC, DMAP, dry CH2Cl2 and (iii) E-4-
n-tridecyloxy- or E-4-n-octadecyloxy-a-methylcinnamic acid, DCC,
DMAP, dry CH2Cl2.
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butan-2-one.
(R)-(+)-2-[4-(Benzyloxy)phenyloxy]propionic
subjected to hydrogenolysis to obtain the bis-phenol 7.
Compounds A1, B1, C1 and C2 were obtained by esterifying
the bis-phenol 7 with an appropriate carboxylic acid as shown
in Scheme 1.
acid 3 was prepared by Mitsunobu reaction34 of 4-benzyloxy-
phenol with (S)-(2)-ethyl lactate in the presence of triphenyl-
phosphine and diethylazodicarboxylate followed by hydrolysis
of the resulting product using aqueous alkali.35 Compound 3
was esterified with 1 in the presence of DCC, DMAP and
dry CH2Cl2 and the material 4 thus obtained was subjected
to hydrogenolysis to yield bis-phenol 5. This was further
esterified using two equivalents of 3-fluoro-4-benzyloxy-
benzoic acid to yield dibenzyloxy ester 6, which was again
(S)-(+)-4-(1-Methylheptyloxy)benzoic acid 9 was prepared
by etherification reaction of methyl-4-hydroxybenzoate with
(R)-(2)-octan-2-ol following a procedure already described36
and the resulting compound 8 obtained was hydrolyzed using
aqueous alkali. The acid 9 so obtained was esterified using
4-benzyloxyphenol and the benzyloxy ester 10 thus obtained
was subjected to hydrogenolysis to give the phenol 11. This
was again esterified using 3-benzyloxybenzoic acid which
resulted in the diester 12. Hydrogenolysis, followed by
reesterification of the phenol 13 thus obtained, with 4-benzyl-
oxybenzoic acid resulted in the triester benzyl ether 14.
Catalytic hydrogenolysis of the benzyl group of compound
14 resulted in the required phenol 15. This was again esterified
as shown (Scheme 2) using the appropriate 4-n-alkylbiphenyl-
4-carboxylic acids or 4-n-dodecyloxybiphenyl-4-carboxylic
acid to obtain the target compounds (D1–D4 and E1). The
corresponding racemic compound F1 was synthesized using
a racemic octan-2-ol following a procedure described for
compound E1.
Acknowledgements
The authors thank Ms. K. N. Vasudha for technical support
and the Sophisticated Instruments Facility, Indian Institute of
Science, Bangalore for recording the NMR spectra.
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