Ferroelectric and antiferroelectric switching behaviour in new unsymmetrical bent-core compounds derived from 3-hydroxybenzoic acid

Article abstract of DOI:10.1039/b419404g

Herein we report the synthesis and characterization of several achiral bent-core unsymmetrical compounds exhibiting mesomorphic properties. The lower homologues in two series of compounds show a columnar phase with a rectangular lattice whereas the middle

Full text of DOI:10.1039/b419404g

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www.rsc.org/materials | Journal of Materials Chemistry
Ferroelectric and antiferroelectric switching behaviour in new
unsymmetrical bent-core compounds derived from 3-hydroxybenzoic acid{
H. N. Shreenivasa Murthy and B. K. Sadashiva*
Received 4th January 2005, Accepted 13th April 2005
First published as an Advance Article on the web 27th April 2005
DOI: 10.1039/b419404g
Herein we report the synthesis and characterization of several achiral bent-core unsymmetrical
compounds exhibiting mesomorphic properties. The lower homologues in two series of
compounds show a columnar phase with a rectangular lattice whereas the middle homologues
exhibit a novel columnar phase with an oblique lattice. Interestingly, three of the higher
homologues of the laterally unsubstituted compounds exhibit a smectic phase with ferroelectric
switching characteristics and the analogous fluoro-substituted compounds show a smectic phase
that is composed of antiferroelectric conglomerates.
system6 based on the information that ferroelectric structures
Introduction
require an anticlinic interlayer correlation of the molecules
The occurrence of ‘‘ferro-/antiferro’’-electricity in liquid
crystals was first observed in chiral calamitic systems.1,2
their system. Ferroelectric properties have also been reported
and indeed observed ferroelectric switching characteristics in
However, the discovery of electro-optical switching3 in a
in other chiral bent-core compounds10 as well as in some
mixtures.11 Another significant approach12,13 to decouple the
mesophase exhibited by compounds composed of achiral bent-
core (BC) molecules has aroused considerable interest.
layer interfaces was made by the Halle group using incom-
Chirality in such BC compounds arises due to a unique
patible siloxane units in the terminal position of the BC
combination of polar order (due to a specific arrangement
molecules which provided ferroelectric phases. Very recently,
of molecules along the bent direction) and the tilt of the
homochiral ferroelectric smectic as well as columnar phases
molecules in successive layers.4 There are two possibilities
central unit was observed.14 We have reported earlier the
for the molecules to arrange themselves in such chiral layers.
in BC compounds derived from a 2,7-dihydroxynaphthalene
The molecular arrangement in these chiral layers is such that
properties of symmetrical15 as well as unsymmetrical com-
the polarization (due to the directed organization) can be
influence of a lateral fluorine substituent on the mesomorphic
pounds16 having resorcinol as the central unit. We have shown
parallel or antiparallel in adjacent layers giving rise to ferro-
and antiferro-electric properties respectively. In addition, these
that the position and the number of fluorine substituents
molecules can tilt in syn or anti fashion in successive layers
play a very important role in inducing ferro-/antiferro-electric
properties in five-ring bent-core compounds.16 Recently ferro-
which results in two types of arrangements in both ferro-
(homochiral SmCSPF or racemic SmCAPF) and antiferro-
electric switching properties were also observed in unsubsti-
tuted compounds derived from resorcinol.17
(racemic SmCSPA or homochiral SmCAPA) electric phases. In
the majority of the bent-core compounds investigated so far,
The majority of bent-core compounds which have been
the antiferroelectric properties are more prevalent. This is
reported so far are derived from resorcinol, which is also the
central unit.3,4,7,8,15,16 We reported18 earlier a novel biaxial
not very surprising and has been attributed to the interlayer
fluctuations in successive layers of the antiferroelectric arrange-
smectic A phase for the first time using 3-hydroxybenzoic acid
as the central unit. We also reported19 unusual filamentary
ment of the molecules and is stabilized by entropical gain.
Also, in order to escape from a macroscopic polar order such
growth patterns at the isotropic to SmCPA phase transition
using this central core. These compounds represent18–21 the
an arrangement of molecules is preferred.
The first observation of ferroelectric switching in meso-
phases of BC compounds was made only a few years ago.5,6
The compounds made by the Bordeaux group5 contained
fluorine substituent at the ortho position with respect to the
two terminal n-alkoxy chains. Subsequently, ferroelectric
switching behaviour was observed in different systems.7–9
These systems also contain a fluorine substituent ortho to the
terminal n-alkoxy chains. The Boulder group designed a new
first of several homologous series of compounds containing
substituents such as cyano, nitro, fluoro, chloro or trifluoro-
methyl groups as one of the terminal groups while the other
terminal group is an n-alkoxy chain.
In this paper, the synthesis and mesomorphic properties of
three new homologous series of compounds derived from
3-hydroxybenzoic acid are reported. These are unsymmetrical
compounds having two terminal chains but differing in the
orientation of the ester-linking group (series I). The other two
series of compounds are also unsymmetrical and contain a
lateral fluorine substitutent (series II) and a biphenyl moiety at
one of the terminal positions (series III). The salient features of
the mesomorphic properties observed in these compounds are:
{ Electronic supplementary information (ESI) available: Detailed
synthetic procedures, spectral and analytical data of the intermediates
and the target compounds. See http://www.rsc.org/suppdata/jm/b4/
b419404g/
*sadashiv@rri.res.in
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i) observation of a novel columnar phase with an oblique
lattice for the middle homologues in two series (I and II) of
compounds.
iv) related compounds with a biphenyl moiety in one of the
arms showed only calamitic phases.
ii) observation of ferroelectric switching behaviour in
higher homologues of unsubstituted compounds even though
they do not contain any lateral substituent mentioned earlier
(series I).
Experimental
Materials
The synthesis of these achiral unsymmetrical compounds
was carried out following a route depicted in Scheme 1. The
synthesis of compounds a–f (in Scheme 1) has already been
iii) analogous fluoro-substituted compounds (series II)
showed antiferroelectric conglomerates.
Scheme 1
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reported in an earlier paper.21 The detailed synthetic proce-
dures, spectral and analytical data of the intermediates and the
target compounds are available as ESI.{
Table 2 Transition temperatures (uC) and the associated enthalpies
(kJ mol21) for the compounds of series II
Results and discussion
Textural observations
The transition temperatures and the associated enthalpy
values as determined from thermograms obtained on a
differential scanning calorimeter (DSC, Perkin-Elmer, Model
Pyris 1D) for the compounds of series I to III are summarized
in Tables 1, 2 and 3 respectively.
Compound
n
Cr
?
SmCAPA
BX (Colob
—
)
Colr
I
?
?
?
?
?
?
?
8
6
131.0
47.8
123.0
54.6
116.0
23.6
112.5*
61.6
112.0
80.8
109.0
73.4
108.0
78.7
—
—
—
—
(? 114.0)
12.0
—
9
8
?
(?
?
118.5)
16.0
120.0
17.3
10
11
12
13
14
10
12
14
16
18
?
—
—
—
—
—
Three types of mesophases were observed in the seven
compounds of series I. The optical textures were observed
using a polarizing microscope (Leitz Laborlux 12 POL/
Olympus BX50) provided with a heating stage (Mettler FP 82
HT) and a temperature controller (Mettler FP 5). Compound 1
showed a mosaic texture that is typical for a classical Colr (B1)
phase. The optical texture obtained for the mesophase of
compound 1 on cooling from the isotropic phase is shown in
Fig. 1. On increasing the alkyl chain length, compounds 2–4
were obtained and some unusual textural patterns were
observed for these. The textures obtained on cooling the
isotropic phase of compounds 2 and 3 are shown in Fig. 2(a)
and (b) respectively. Although these highly birefringent
textures are different from those seen normally for a Colr
phase, these are typical features of a columnar phase.
However, very interestingly, the compounds with longer
terminal alkyl chain length (n 5 14, 16, 18) showed optical
textures different from those seen for the lower homologues,
but were all similar. A photomicrograph of a texture obtained
for compound 5 is shown in Fig. 3.
?
?
123.5
18.5
?
?
?
?
126.0
19.7
127.0
19.5
128.0
20.0
—
—
—
?
?
Table 3 Transition temperatures (uC) and the associated enthalpies
(kJ mol21) for the compounds of series II
Compound
n
Cr
?
SmX
SmC
SmA
I
?
?
15
16
a
16
18
141.0*
88.7
140.0*
98.2
?
?
183.0
3.9
180.0
3.1
?
?
197.0a
199.0a
?
?
200.5
10.4
200.0
10.3
?
Table 1 Transition temperatures (uC) and the associated enthalpies
(kJ mol21) for the compounds of series Ia
Enthalpy could not be determined.
The thermal behaviour of the compounds of series II as
determined on DSC and a polarizing microscope is sum-
marized in Table 2. In this series of compounds also three
types of mesophases were observed. Compound 8 showed
Compound
n
Cr
?
SmCSPF
BX (Colob) Colr
I
?
?
?
?
?
?
?
1
2
3
4
5
6
7
a
6
142.0
88.7
124.0*
39.2
102.0
42.2
107.0
49.7
111.0*
96.6
114.0*
104.2
115.5*
111.6
—
—
—
—
—
?
(? 134.0)
15.5
8
?
136.0
18.0
—
—
—
—
—
—
10
12
14
16
18
?
?
137.0
19.7
?
?
138.0
20.8
?
?
?
?
138.5
20.6
138.0
20.7
137.5
20.7
—
—
—
?
?
Key: Cr: crystalline phase; SmCSPF: synclinic ferroelectric phase
with smectic ordering of the molecules; SmCAPA: anticlinic
antiferroelectric phase with smectic ordering of the molecules;
BX(Colob): columnar phase with an oblique lattice; Colr: columnar
phase with a rectangular lattice; SmX: smectic phase with unknown
structure; I: isotropic phase; *: compound has crystal–crystal
transition, enthalpy denoted is the sum of all such transitions.
Fig. 1 An optical photomicrograph of the texture developing on
cooling the isotropic phase of compound 1.
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Fig. 4 An optical photomicrograph obtained for the mesophase of
compound 10.
antiferroelectric SmCP phase. A photomicrograph of the
texture obtained for the mesophase of compound 13 as
observed under a polarizing microscope is shown in Fig. 5(b).
In order to examine the effect of increasing the core-length
on only one side of these systems, we synthesized two com-
pounds having a biphenyl moiety in one of the arms. The
mesomorphic behaviour of these two compounds is quite
interesting and is similar. The clearing temperature of these
Fig. 2 (a) and (b) Optical photomicrographs of the textures develop-
ing on cooling the isotropic phase of compounds 2 and 3 respectively.
Fig. 3 An optical photomicrograph of the texture developing on
cooling the isotropic phase of compound 5.
typical features of a Colr phase, whereas compounds 9–11
showed textures which are slightly different from these. A
photomicrograph obtained for the mesophase of compound 10
is shown in Fig. 4. These textural features also indicate that the
mesophase could be a columnar phase. However, the textural
features obtained for compounds 12–14 are completely
different from those exhibited by the lower homologues.
For example, the optical features of compound 12 showed
banana leaf-like texture and a photomicrograph of this texture
is shown in Fig. 5(a). In contrast, compounds 13 and 14
showed a fringe pattern, which is normally observed for an
Fig. 5 (a) An optical photomicrograph showing banana leaf-like
texture obtained for the mesophase of compound 12. (b) An optical
photomicrograph showing a fringe pattern obtained for the mesophase
of compound 13.
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compounds increased considerably, as also the thermal range
of the mesophases. On cooling from the isotropic phase of
these compounds both homeotropic as well as smooth focal-
conic textures could be observed which are the classical
textures observed for a smectic A phase. On lowering the
one of the arms (by a biphenyl moiety) effectively changes the
bending angle between the arms of the bent-core compounds
and the types of mesophases.
X-Ray diffraction studies
temperature, the homeotropic regions transformed to
a
In order to examine the mesophase structure of all the
compounds X-ray diffraction (XRD) studies (Cu-Ka radiation
from Rigaku Ultrax 18) were carried out. Unless otherwise
mentioned, XRD measurements were carried out on non-
oriented samples. The mesophase of compound 1 gave three
reflections in the small angle region, which could be indexed to
the (11), (02) and (13) planes of a rectangular lattice. Hence
this phase has been characterized as a Colr phase and the
X-ray diffractogram is shown in Fig. 7(a). XRD studies on
powder samples of compounds 2–4 showed many reflections
in the small angle region, which rule out the possibility of a
simple layered structure. The patterns are also different
from what is normally seen for a Colr (B1) phase. The X-ray
diffractogram obtained for compound 2 is shown in Fig. 7(b).
In order to gain more insight into this mesophase structure, an
schlieren texture whereas the focal-conic texture became
broken focal-conic. These textural observations are typical
for a SmA–SmC transition. On lowering the temperature
further the patterns transform to an undefined texture. This
transition is first order and the enthalpy value for the
transition is about 3–4 kJ mol21. The textural changes were
also observed in cells treated for both planar as well as
homeotropic arrangement and the photomicrographs obtained
for compound 16 are shown in Fig. 6(a–f). The lowest tem-
perature phase is smectic and non-switchable (see later for
details). Thus these compounds show polymorphic calamitic
phases whereas the corresponding five-ring compounds with
longer alkyl chain show switchable B phases. From these
examples we can say that increasing the core length on only
Fig. 6 (a–c) Optical photomicrographs obtained for SmA, SmC and SmX phases in cells treated for planar alignment; (d–f) the corresponding
textures obtained in cells treated for homeotropic alignment; compound 16.
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Fig. 9 X-Ray diffractogram obtained for the smectic phase of
compound 6.
Unoriented XRD patterns obtained for the mesophases
of compounds 5–7 showed lamellar periodicity of the
molecules. The d-spacing corresponding to the first reflection
is less than the full molecular length (assuming a fully
extended all-trans conformation of the terminal n-alkoxy
chains) indicating a tilt of the molecules in the smectic layers.
The X-ray diffractogram obtained for compound 6 is shown
in Fig. 9.
Fig. 7 X-Ray angular intensity profiles obtained for (a): Colr phase
of compound 1; (b): BX (Colob) phase of compound 2.
XRD studies have been carried out on the mesophases of
compounds of series II. From the experiments it was observed
that the XRD patterns are similar to those observed for the
compounds of series I. The three reflections obtained in the
small angle region for the mesophase of compound 8 can be
indexed to (11) (02) and (13) planes of a rectangular lattice.
The reflections obtained for the mesophase of compounds
9–11 are comparable to those of compounds 2–4 and hence
designated by the same symbol. The higher homologues
(compounds 12–14) showed lamellar periodicity. Interestingly
the layer spacings obtained for the smectic phases of com-
oriented XRD study of the sample is essential. We have
obtained only a partially oriented pattern for compound 4 and
the diffraction pattern is shown in Fig. 8.
The pattern on the meridian has a lamellar periodicity and
these reflections can be indexed to (10), (20) and (30) planes.
Interestingly, reflections on the equator are also observed.
¯
These reflections could be indexed to the 01 and 01 planes.
Apart from these reflections, two other faint reflections were
¯
obtained which could be indexed to the 11 and 11 planes of an
˚
oblique lattice with the lattice parameters a 5 43.2 A and
˚
pounds (12–14) are less (by 2.5–3 A) than the layer spacings
˚
b 5 66.2 A. The calculated oblique angle is 84u. This partially
obtained for the corresponding compounds (5–7) of series I.
This may be due to different interlayer interactions between
the mesophases of two series of compounds. The d-spacings
obtained for the compounds under investigation are sum-
marized in Table 4. XRD studies have also been carried out
on the mesophases of compound 16. One reflection in the small
oriented pattern indicates a columnar structure with an
oblique lattice for the mesophase. However, this mesophase
does not switch electro-optically and is a novel mesophase
occurring between the rectangular columnar (Colr) phase and
the switchable lamellar (SmCP) phase. Hence this new phase
has been assigned the symbol BX (Colob).
˚
angle region at d 5 53.6 A was obtained at 192 uC and on
lowering the temperature to 160 uC a second order smectic
reflection was obtained without any change in the d-spacing.
The d-spacing corresponding to the first reflection is less than
the full molecular length, which confirms a tilt of molecules
in the smectic strata (calc. tilt angle 47u). The additional
reflection seen in the low temperature phase indicates that on
transition from the SmC to the SmX phase, the molecules are
arranged in well-defined smectic layers. In both SmC and SmX
˚
phases a diffuse wide-angle reflection was observed at 4.7 A. In
the SmX phase no additional reflection was observed in the
wide-angle region, which rules out the possibility of a higher
order smectic phase. An oriented pattern is necessary to arrive
at the precise structure of the SmX phase, but owing to the
high clearing temperatures of the compounds it has not been
possible to get an oriented pattern. Since we are not sure of
the exact structure of this mesophase, the symbol SmX has
been assigned.
Fig. 8 X-Ray diffraction pattern obtained for a sample of compound
4; BX (Colob) phase. $: escaping beam from the experimental setup.
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˚
Table 4 Small angle Bragg reflections (A) and their Miller indices (in parentheses) for the unsymmetrical compounds derived from
3-hydroxybenzoic acid
Lattice parameters
˚
d-Spacings (A) and Miller indices
Compound
a
b
Phase type
Colr
Oblique angle
1
2
4
31.2 (11), 19.8 (02), 12.6 (13)
Calc: 12.8 (13)
¯
62.0 (01), 39.5 (10), 30.9 (11), 27.3 (12), 19.7 (20), 13.5 (30)
Calc: 27.0 (12)
65.8 (01), 43.0 (10), 37.5 (11), 34.1 (11), 21.8 (20), 14.5 (30)
50.7
40.2
43.2
39.6
63.0
66.2
—
BX (Colob
BX (Colob
)
)
79.6
84.0
¯
¯
Calc: 37.8 (11), 34.4 (11)
49.1(01), 24.6(02)
50.8 (01), 25.8 (02), 17.1(03), 13.0 (04)
53.4 (01), 26.6 (02), 17.9 (03), 13.4 (04)
28.9 (11), 19.8 (02), 12.3(13)
Calc: 12.6 (13)
5
6
7
8
—
—
—
42.3
—
—
—
39.6
SmCSPF
SmCSPF
SmCSPF
Colr
—
—
—
—
¯
9
66.0 (01), 41.3 (10), 33.7 (11), 26.5 (12), 20.7 (20), 16.0 (30)
Calc: 26.9 (12)
41.5
42.8
66.2
65.3
BX (Colob
BX (Colob
)
)
85.0
81.0
¯
62.0 (01), 42.3 (10), 33.0 (11), 27.2 (12), 21.2 (20), 14.3 (30)
Calc: 27.0 (12)
11
12
14
16
46.2 (01), 23.1 (02), 15.4 (03)
50.3 (01), 16.8 (03)
53.6 (01)
—
—
—
—
—
—
—
—
SmCAPA
SmCAPA
SmC
—
—
—
—
53.6 (01), 26.7 (02)
SmX
Electro-optical investigations
make an angle with respect to the direction of the polarizer and
analyzer. On reversing the polarity of the applied dc field, the
extinction brushes rotate in a counter-clockwise direction.
On terminating the dc field the positions of the brushes
remain in both the orientations. The bistable states remain
In order to see the effect of electric field on the mesophases of
these compounds, we have carried out detailed electro-optical
investigations. For these experiments ITO coated home made
cells treated with polyimide were used. The sample thickness
was adjusted with suitable Mylar spacers. There was no
response to an applied electric field on the mesophases of
compounds 1–4. In contrast however, interesting electro-
optical effects were observed for the mesophases of com-
pounds 5–7. In general, the switching behaviour of the
mesophases of all the three compounds is similar. Detailed
electro-optical investigations carried out on the mesophase
of compound 5 are described here. A sample of compound 5
was taken in a cell of thickness 10.3 mm and it was cooled
slowly from the isotropic phase under a triangular-wave
electric field. After a threshold voltage (39 Vpp mm21) a
single polarization current peak for each half period was
observed even at low frequencies. This is an indication of a
ferroelectric switching behaviour of the mesophase. The
switching current response trace obtained for compound 5 is
given in Fig. 10(a).
It has been reported22 that a modified triangular-wave
method could be used to check that the field induced polarized
state remains stable even under zero field for a prolonged time
until the sign of the field is reversed. Therefore, we employed
this technique to study the ferroelectric switching in our
compounds. Single polarization current peak observed in this
experiment further supported that the mesophase exhibits
ferroelectric switching characteristics. The switching current
response trace obtained for the mesophase of compound 5 by
employing the modified triangular-wave method is given in
Fig. 10(b).
The bistability of this ferroelectric phase was further
examined by dc field experiments. On applying a dc electric
field (12.3 V mm21) and cooling the isotropic phase of
compound 5, birefringent circular domains were observed.
The feature of these domains is that the extinction brushes
Fig. 10 Switching current response trace obtained for the mesophase
of compound 5 by applying (a) a triangular-wave voltage (480 Vpp
40 Hz,); (b) a modified triangular-wave voltage (420 Vpp, 33 Hz,); cell
thickness 10.3 mm; saturated polarization value #225 nC cm22
,
.
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Fig. 11 Optical photomicrographs obtained under an applied dc
electric field on the mesophase of compound 5; (a) 12.3 V mm21; (b)
0 V; (c) 212.3 V mm21
.
unchanged for several hours. This clearly confirms
a
ferroelectric switching (homochiral SmCSPF) for the meso-
phase of compound. The optical photomicrographs obtained
with and without field are shown in Fig. 11(a)–(c). Thus from
these different techniques we have established the ferroelectric
properties for the higher homologues of series I.
Fig. 12 Switching current response trace obtained for the meso-
phase of compound 13 by applying
(130 Vpp, 40 Hz,); cell thickness 6 mm; saturated polarization value
#105 nC cm22
a triangular-wave voltage
Electro-optical studies were carried out on the mesophases
of compounds of series II. The mesophases of compounds
8–11 did not show any response to an applied electric field at
least up to 30 V mm21. However, those of compounds 12–14
responded to an applied electric field and the effect is more or
less similar in all these three homologues.
.
photomicrographs showing the tristable antiferroelectric
switching behaviour as well as the conglomerates are shown
in Fig. 13(a–c).
We have performed electric field experiments on the
mesophases of compound 16. It is interesting to note that all
the three mesophases (SmA, SmC and SmX) did not show any
For example, a sample of compound 13 was taken in a cell
of uniform thickness of 6 mm, and subjected to a triangular-
wave electric field. On applying a voltage of 130 Vpp at 40 Hz,
two polarization current peaks per half cycle were obtained
indicating the mesophase to be antiferroelectric. The calcu-
lated polarization is #105 nC cm22. The switching current
response trace obtained for the mesophase of compound 13 is
shown in Fig. 12.
response to an applied electric field at least up to 20 V mm21
,
indicating that the mesophases are achiral.
Comparison of the mesomorphic properties of compounds
containing resorcinol and 3-hydroxybenzoic acid as central units
To identify the tilt sense in the antiferroelectric phase of
compound 13, dc field experiments were carried out. The
sample was cooled from the isotropic phase under a dc voltage
of 5.5 V mm21 and colourful circular domains which make an
angle with respect to the polarizer/analyzer direction were
obtained. Interestingly, the brushes in one domain (Domain 1)
rotate in a clockwise direction whereas those in the other
domain (Domain 2) rotate in a counter-clockwise direction.
On reversing the polarity of the applied field these domains
rotate in opposite directions. Thus, domains 1 and 2 are
conglomerates having different tilt and polar directions.
However, on terminating the applied field the extinction
crosses orient along the direction of analyzer. This tristable
switching confirms the antiferroelectric switching behaviour
for this mesophase. The striking feature is the observation of
antiferroelectric conglomerates in this compound. Optical
The mesomorphic properties of the compounds of series I can
be compared with those of the isomeric compounds derived
from resorcinol as the central unit.15 The general structure of
such compounds is shown in structure A. These compounds
showed the well established23 phase sequence Smintercal–Colr–
SmCPA as a function of increase in the terminal alkyl chain
length. The compounds of series I showed Colr–BX (Colob)–
SmCSPF as a function of increasing alkyl chain length. The
structure of this series of compounds differs from those
derived from resorcinol with respect to the orientation of
the ester linking group. As a result of this, the mesophase
sequence completely changes and a novel mesophase has
been obtained. This suggests that the charge distribution
around the molecular structure is an important factor and is
useful in designing new molecular architectures for bent-core
compounds.
Fig. 13 The effect of dc field on the mesophase of compound 13: (a) +5.5 V mm21; (b) 25.5 V mm21; (c) 0 V.
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laterally unsubstituted compounds also exhibit ferroelectric
properties though such compounds are very rare.
Acknowledgements
We thank Dr. V. A. Raghunathan for useful discussion in the
interpretation of the XRD data, Mr. N. Balakrishna Prabhu
and Ms. K. N. Vasudha for technical assistance and the
Sophisticated Instruments Facility, Indian Institute of Science,
Bangalore for recording the NMR spectra.
Similarly the mesophases exhibited by compounds of
series II can be compared with those of the unsymmetrical
compounds derived from resorcinol16 (structure B). The
compounds obtained from resorcinol show Colr and SmCPA
phases, whereas compounds of series II show a Colr–BX
(Colob)–SmCAPA phase sequence as a function of the increase
in the number of carbon atoms in the alkyl chain. A
comparison of these suggests that the compounds derived
from resorcinol have lower transition temperatures than those
derived from 3-hydroxybenzoic acid. The middle homologues
of both the series I and II show a novel columnar oblique
phase. The higher homologues exhibit a switchable homochiral
mesophase (SmCSPF in series I and SmCAPA in series II).
Thus, subtle changes in the chemical structure in these bent-
core compounds result in varying mesomorphic behaviour. It
is appropriate to mention here that it would be interesting to
calculate the charge distribution around the molecule, the
conformation of the molecule as a whole and the bending
angle between the two arms in these closely related chemical
structures, which may throw light on understanding the
structure–property relationships in such bent-core compounds.
H. N. Shreenivasa Murthy and B. K. Sadashiva*
Raman Research Institute, C. V. Raman Avenue, Bangalore-560 080,
Sadashivanagar, India. E-mail: sadashiv@rri.res.in
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Conclusions
In summary, the synthesis of several unsymmetrical achiral BC
compounds derived from 3-hydroxybenzoic acid has been
carried out. The mesophases exhibited by these have been
investigated by a number of different techniques. The lower
homologues (1 and 8) exhibit the classical columnar phase with
a rectangular lattice. The compounds with longer terminal
chain lengths, viz. n 5 8, 10 or 12 carbon atoms, in both the
series show a novel columnar phase with an oblique lattice.
Very interestingly, the mesophase of compounds 5, 6 and 7 is
smectic with ferroelectric properties whereas the analogous
unsymmetrical compounds (12–14) with a fluorine substituent
showed smectic antiferroelectric conglomerates. To the best of
our knowledge this is first report of a columnar phase with an
oblique lattice occurring between a Colr and a polar smectic
phase with ferro-/antiferro-electric properties. The compounds
containing a biphenyl moiety showed only calamitic phases
and no B phase could be observed. It has been shown that
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H. T. Nguyen and M. F. Achard, Liq. Cryst., 2001, 28, 1285.
23 B. K. Sadashiva, V. A. Raghunathan and R. Pratibha,
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2064 | J. Mater. Chem., 2005, 15, 2056–2064
This journal is ß The Royal Society of Chemistry 2005