Direct transition from a nematic phase to a polar biaxial smectic a phase in a homologous series of unsymmetrically substituted bent-core compounds

Article abstract of DOI:10.1039/b309262c

The synthesis and characterization of twenty seven new compounds belonging to three homologous series of compounds derived from 3-hydroxybenzoic acid are reported. One of the terminal positions of the constituent molecules of these compounds is substitute

Full text of DOI:10.1039/b309262c

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A R T I C L E
Direct transition from a nematic phase to a polar biaxial smectic
A phase in a homologous series of unsymmetrically substituted
bent-core compounds{
R. Amaranatha Reddy and B. K. Sadashiva*
Raman Research Institute, C. V. Raman Avenue, Sadashivanagar, Bangalore, 560 080,
India. E-mail: sadashiv@rri.res.in
Received 4th August 2003, Accepted 13th November 2003
First published as an Advance Article on the web 19th December 2003
The synthesis and characterization of twenty seven new compounds belonging to three homologous series of
compounds derived from 3-hydroxybenzoic acid are reported. One of the terminal positions of the constituent
molecules of these compounds is substituted with a strongly polar cyano group while the other end contains an
n-alkyl chain attached to a biphenyl moiety. Twenty two of these compounds show the polar partial bilayer
biaxial smectic A phase which is antiferroelectric in nature. In addition, a direct transition from the nematic
phase to the polar partial bilayer biaxial smectic A phase which has not been seen before is also reported. The
mesophases have been characterized using polarized light optical microscopy, differential scanning calorimetry,
X-ray diffraction studies and electro-optical experiments.
in Fig. 1. If the molecules are orthogonal with respect to the
Introduction
layer planes, the achiral mesophase is SmAP (polar smectic A)
due to the polarization along the bent direction while in the
chiral SmCP phase, the molecules are tilted. We examined some
binary mixtures of a compound exhibiting the B2 phase with
one composed of rod-like molecules exhibiting the bilayer
smectic A (SmA2) phase which resulted in a very interesting
phase diagram.7,8 At certain concentrations of the bent-core
molecules (4–13 mol%), a biaxial smectic A2 (SmA2b) phase
was induced in which the BC molecules underwent an
orientational transition as the temperature was lowered from
The theoretical prediction of the existence of the biaxial smectic
A phase was made by de Gennes1 nearly three decades ago. The
biaxial smectic A phase has also been called the CM phase
(where M stands for McMillan) and Brand et al.,2 have
discussed theoretically its physical properties. However, the
first experimental evidence for the occurrence of the biaxial
smectic A phase was claimed in a binary mixture of liquid
crystalline side chain polymer laterally substituted with
naphthalene groups and a monomeric material.3 The binary
phase diagram indicates that both the polymer and the
monomer are completely miscible throughout the concentra-
tion range. Mixtures with up to 50 mol% of the monomeric
liquid crystal show a nematic phase and the lower temperature
phase has been claimed to be a biaxial smectic A phase. The
biaxial smectic A phase has also been observed in a binary
mixture of a metallomesogen and 2,4,7-trinitrofluorenone
(TNF).4 In this case, the strong face to face interactions
between TNF and the metallomesogen reduces molecular
rotation around the long molecular axis, which leads to
biaxiality. In addition, there is also a claim for the observation
of a biaxial smectic A phase in a low molecular mass liquid
crystalline material composed of boomerang-shaped mole-
cules.5 The conoscopic experiments reveal the uniaxial inter-
ference pattern for the smectic A phase in the absence of an
electric field. As pointed out by Hegmann et al.,4 in the above
two cases3,5 the textural features of non-aligned samples
resemble the uniaxial smectic A phase and unambiguous proof
for the existence of the biaxial smectic A phase has not been
provided.
The discovery6 of a electro-optically switchable mesophase
in compounds composed of bent-core (BC) molecules, has
opened a new sub-field of liquid crystals. Such compounds
offer many exciting possibilities for new mesophases with
interesting physical properties. Due to the specific shape of the
molecules, unlike in calamitic liquid crystals, the in-layer
properties can be varied (m is non-equivalent to 2m) as shown
Fig. 1 A schematic representation of long molecular axis (n), layer
normal (k), c vector (projection of n), in-plane director (m) and the
in-layer polarization (P) for the chiral and achiral layers containing
biaxial BC molecules which induce a polar packing within the layers
resulting in a polarization along the bent direction.
{ Electronic supplementary information (ESI) available: physical and
spectral data obtained for the compounds of series 1j, 1k and 1l. See
http://www.rsc.org/suppdata/jm/b3/b309262c/
3 1 0
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T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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the uniaxial smectic A2 phase. Perhaps, this represents the first
experimental evidence for the existence of a biaxial smectic A
phase in low molecular weight compounds, although in a
mixture. Recently, we have reported9,10 the synthesis of several
compounds composed of highly polar unsymmetrically sub-
stituted BC molecules which exhibit the partial bilayer biaxial
smectic A (SmAdb) phase. All these compounds contain a
highly polar cyano group at one terminal position of the
constituent molecules which overlap in an antiparallel
orientation. In fact, these compounds exhibit a direct transition
from the uniaxial smectic Ad phase to a biaxial smectic Ad
phase on lowering the temperature. We had reported that the
SmAdb phase is apolar in both longitudinal and transverse
directions. We now show that at fairly high electric fields and at
high frequencies, the SmAdb phase is actually antiferroelectric
in nature and it is designated as a SmAdPA phase. In view of the
new experimental evidence, the quartet structure proposed
earlier10 does not hold good. Here, we present the synthesis and
characterization of three new homologous series of unsymme-
trical compounds derived from 3-hydroxybenzoic acid. Many
of these exhibit a direct transition from the nematic phase to
the polar partial bilayer biaxial smectic A (SmAdPA) phase
which represents the first example of such a transition in pure
compounds. It is appropriate to mention here that recently
Prehm et al.11 have observed the biaxial smectic A phase in
some rod-like bolaamphiphiles carrying a long semiperfluori-
nated chain. In addition, unambiguous experimental evidence
for an achiral biaxial smectic A phase exhibiting an antiferro-
electric switching behaviour has been provided by Eremin
et al.12
synthesis and characterization of compound 1j10, its spectral
and analytical data are given below.
4-Cyanophenyl-4-{3-[4-(4-n-decylbiphenyl-4-
carbonyloxy)benzoyloxy)benzoyloxy} benzoate, 1j10.
A mixture of 4-cyanophenyl-4-[3-(4-hydroxybenzoyloxy)ben-
zoyloxy]benzoate10 (0.2 g, 0.42 mmol) and 4-n-decylbiphenyl-
4-carboxylic acid (0.142 g, 0.42 mmol) was suspended in
anhydrous chloroform (10 ml). To this reaction mixture was
added N,N-dicyclohexylcarbodiimide, (0.1 g, 0.48 mmol) and a
catalytic amount of 4-(N,N-dimethylamino)pyridine and the
mixture was stirred at room temperature for 15 hours. The
precipitated dicyclohexylurea was filtered off and chloroform
(50 ml) was added to the filtrate. This organic solution was
washed with 2% aqueous ice-cold sodium hydroxide solution
(3 6 60 ml) and finally washed with water (4 6 75 ml) and
dried over anhydrous sodium sulfate. The residue obtained
after removal of solvent was chromatographed on silica gel
using chloroform as an eluent. Removal of solvent from the
eluate afforded a white material which was crystallized from
butan-2-one, yield 58%; mp 169 uC; 1H NMR (CDCl3,
400 MHz) d (ppm): 8.27–8.03 (m, 7H, Ar-H), 7.7–7.48 (m,
3
9H, Ar-H), 7.38–7.19 (m, 8H, Ar-H), 2.62–2.58 (t, J 7.68 Hz,
2H, Ar-CH2-), 1.59–1.57 (quintet, 3J 6.86 Hz, 2H, Ar-CH2–
CH2-), 1.57-1.21 (m, 14H, 7 6 -CH2-), 0.83–0.8 (t, 3J 6.68 Hz,
3H, 1 6 -CH3); IR (KBr) nmax: 2922, 2852, 2243, 1736, 1605,
1275, 1080 cm21; C51H45NO8 requires, C, 76.58; H, 5.67; N,
1.75%; found: C, 76.68; H, 5.56; N, 1.73%.
In general, the chemical structure of all the compounds was
confirmed by 1H NMR spectroscopy (Bruker AMX400
spectrometer) with 1% tetramethylsilane in deuteriochloroform
as an internal standard, infrared spectral data (Shimadzu
FTIR-8400 spectrophotometer) and elemental analysis (Carlo-
Erba 1106 analyzer). The thermal behaviour was investigated
using a polarized light optical microscope (Leitz Laborlux 12
POL/Olympus BX 50) equipped with a heating stage and a
controller (Mettler FP52 and FP5 respectively) and also from
thermograms recorded on a differential scanning calorimeter
(Perkin-Elmer Model, Pyris 1D). The enthalpy values of
various transitions were also determined using the latter
method. The calorimeter was calibrated using pure indium as
a standard. X-Ray diffraction measurements on non-oriented
samples were carried out using Cu-Ka radiation from a rotating
anode generator (Rigaku Ultrax 18) with a flat graphite crystal
monochromator. The diffraction patterns were recorded on
an image plate (Marresearch). The samples were taken in
Lindemann capillaries and the sample temperature in each case
was controlled to within ¡0.1 uC. The conoscopic experiments
and birefringence measurements were carried out using a
microscope (Leitz Ortholux – II POL BK) equipped with a
The general molecular structure of the six-ring compounds
under investigation is shown below. All the twenty seven
compounds are esters and twenty two of them exhibit the
SmAdPA phase.
Experimental
Materials
The unsymmetrically substituted strongly polar BC com-
pounds belonging to the three different homologous series 1j,
1k and 1l were synthesized following a procedure described
earlier,10 except that in the final step, 4-n-alkylbiphenyl-4-
carboxylic acids were used; the synthetic pathway is shown in
Scheme 1. 3-Hydroxybenzoic acid and 4-cyanophenol were
procured commercially and used without further purification.
2-Fluoro-4-hydroxybenzoic acid and 3-fluoro-4-hydroxybenzoic
acid were prepared following procedures described in the litera-
ture.13 4-Benzyloxybenzoic acid, 2-fluoro-4-benzyloxybenzoic
acid and 3-fluoro-4-benzyloxybenzoic acid were synthesized
following procedures described by us.14 The procedure for the
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Scheme 1 General synthetic pathway used for the preparation of unsymmetrically substituted banana-shaped mesogens. Reagents and conditions:
(i) DCC, cat. DMAP, dry CHCl3, room temp., 15 h, 81%; (ii) cat 5% Pd–C, H2, 1,4-dioxane, 55 uC, 75%; (iii) 3-benzyloxybenzoic acid, DCC, cat.
DMAP, dry CHCl3, room temp., 15 h, 72%; (iv) cat 5% Pd–C, H2, 1,4-dioxane, 55 uC, 73%; (v) 4-benzyloxybenzoic acid, DCC, cat. DMAP, dry
CHCl3, room temp., 20 h, 68%; (vi) cat. 5% Pd–C, H2, 1,4-dioxane, 55 uC, 80%; (vii) 4-n-alkylbiphenyl-4-carboxylic acids, DCC, cat. DMAP, dry
CHCl3, room temp., 15 h, 50–60%.
heating stage and a controller (Mettler FP 82 and FP 80HT
respectively).
defects, which are the characteristic features of the uniaxial
nematic phase could be seen. Similar textural features could
also be seen for compounds 1j8 to 1j11 indicating a uniaxial
nematic phase. The clearing transition enthalpy value for this
phase is in the range of 0.4 to 0.6 kJ mol21. Compounds 1j8
and 1j9 show an additional monotropic phase. On slow
cooling a homeotropically aligned nematic phase of compound
1j9, a schlieren texture with 2- and 4-brush defects were
obtained indicating the biaxiality of the lower temperature
phase. The presence of the 2-brush defects in the lower
temperature phase indicates that the mesophase is polar and is
also a distinguishing feature of the biaxial smectic Ad phase.10
This mesophase shows an antiferroelectric switching beha-
viour, which will be described later. The sample was also
Results and discussion
The phase transition temperatures and the associated enthalpy
values for the 27 highly polar unsymmetrically substituted
compounds belonging to the three different homologous series
1j, 1k and 1l are summarized in Tables 1, 2 and 3 respectively.
All nine compounds of the 1j series are mesomorphic. On slow
cooling a thin film of the isotropic liquid of compound 1j6
under a polarizing microscope, birefringent droplets appear
which vanish subsequently to give rise to a homeotropic
texture. However, in some regions of the slide 2- and 4-brush
3 1 2
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Table 1 Transition temperatures (uC) and enthalpies (kJ mol21) for compounds of series 1ja
Compound
1j6
n
Cr
?
SmAdPA
SmAd
N
I
6
8
9
156.0
46.7
170.0
50.2
160.0
39.2
169.0
56.8
169.0
44.7
158.0
53.6
157.0
58.5
156.0
38.4
—
?
—
—
—
?
?
204.0
0.48
195.0
0.45
189.8
0.43
189.0
0.47
187.5
0.57
?
?
?
?
?
?
?
?
?
1j8
?
(151.5)b
?
?
1j9
?
?
(158.0)
0.48
171.3
0.15
176.5
0.13
181.0
0.25
184.3
0.16
185.0
0.1
184.0
0.09
1j10
1j11
1j12
1j14
1j16
1j18
10
11
12
14
16
18
?
?
174.8
0.2
185.3
0.4
192.0
2.9
199.2
4.76
204.5
5.8
206.7
6.6
?
?
?
?
?
?
?
?
—
—
—
—
?
?
?
?
?
?
?
154.0
65.4
?
?
a key for all three Tables 1–3: Cr:crystalline phase; SmAd:partial bilayer uniaxial smectic A phase; SmAdPA:partial bilayer biaxial antiferroelec-
tric smectic A phase; SmX:unidentified antiferroelectric smectic phase. N:nematic phase; I:isotropic phase. ? phase exists; — phase does not
exist; temperature in parentheses indicate monotropic transitions. b enthalpy could not be determined as the sample crystallizes immediately.
Table 2 Transition temperatures (uC) and enthalpies (kJ mol21) for compounds of series 1k
Compound
1k6
n
I
?
?
?
?
?
?
?
?
?
Cr
?
SmAdPA
SmAd
N
6
8
9
175.5
57.8
155.0
53.1
160.0
52.3
161.5
48.2
161.5
58.9
160.0
45.6
159.5
42.8
158.0
48.9
—
—
?
—
—
—
?
?
197.2
0.42
184.9
0.4
183.0
0.4
179.8
0.38
180.2
0.57
1k8
?
?
?
1k9
?
(152.5)
0.5
162.5
0.2
168.0
0.18
172.5
0.1
176.8
0.14
178.2
0.06
179.2
0.05
1k10
1k11
1k12
1k14
1k16
1k18
10
11
12
14
16
18
?
?
166.0
0.1
177.0
0.3
183.0
2.7
191.0
4.54
197.0
5.8
202.0
6.6
?
?
?
?
?
?
?
?
—
—
—
—
?
?
?
?
?
?
?
157.5
49.9
?
?
examined in a homogeneously aligned cell. A photomicrograph
showing the transition from the nematic phase to the polar
biaxial smectic Ad phase and the texture of the polar biaxial
smectic Ad phase are shown in Fig. 2. A fan-shaped texture
with sharp lines over the fans excludes the possibility of a SmC
phase which normally exhibits a broken fan-shaped texture. As
Table 3 Transition temperatures (uC) and enthalpies (kJ mol21) for compounds of series 1l
Compound
1l6
n
Cr
?
SmX
—
—
—
—
—
—
—
?
SmAdPA
SmAd
N
I
?
?
?
?
?
?
?
?
?
6
8
9
159.5
54.0
157.0
40.2
154.0
35.9
158.0
40.2
158.0
37.4
154.5
38.7
153.0
43.4
148.5
47.0
—
—
?
—
—
—
?
?
197.5
0.59
186.4
0.47
183.1
0.35
179.8
0.69
179.8
0.58
1l8
?
?
?
1l9
?
(149.0)
0.23
162.0
0.13
167.0
0.13
170.3
0.11
174.0
0.1
176.2
0.17
176.5
0.12
1l10
1l11
1l12
1l14
1l16
1l18
10
11
12
14
16
18
?
?
166.9
0.1
177.6
0.48
182.0
3.15
189.5
4.8
195.0
5.8
199.5
?
?
?
?
?
?
?
?
—
—
—
—
?
?
?
?
160.0
0.34
166.0
?
?
?
149.0
44.5
?
?
?
0.1
6.36
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Fig. 4 Plot of the transition temperatures as a function of the number
of carbon atoms in the n-alkyl chain for homologues of series 1j.
phase is induced, and these two phases exist even for a
compound (1j18) with an n-octadecyl chain. While compounds
1j10 and 1j11 are trimesomorphic, on ascending the homo-
logous series, the uniaxial nematic phase gets eliminated. It is
also interesting to note that the thermal range of the SmAdPA
phase increases from 2.3u for compound 1j10 to 30 uC for
compound 1j18 on ascending the series. A plot of the transition
temperatures as a function of the terminal n-alkyl chain for
series 1j is shown in Fig. 4.
As can be seen in Table 2, the lower homologues of series 1k
viz. compounds 1k6 and 1k8 exhibit only a uniaxial nematic
phase, while compound 1k9 shows a direct transition from the
nematic phase to a polar biaxial smectic Ad phase. Compounds
1k10 and 1k11 show nematic, uniaxial smectic Ad and polar
biaxial smectic Ad phases on cooling the isotropic phase.
However, as observed in series 1j, the higher homologues
(1k12, 1k14, 1k16 and 1k18) exhibit SmAd and SmAdPA
phases. The textural features of the three mesophases as well as
the enthalpy values for the different transitions of this series of
compounds are similar to those obtained for the compounds of
series 1j. The conoscopic patterns obtained in the uniaxial and
biaxial smectic Ad phases for compound 1k12 are shown in
Fig. 5. In this series also only two homologues are trimeso-
morphic and the thermal range of the SmAdPA phase increases
on increasing the terminal chain length. The introduction of a
fluorine substituent ortho to the carbonyl group suppresses the
clearing temperatures while the melting point increases for
most of the homologues. A plot of the transition temperatures
as a function of the terminal n-alkyl chain length for series 1k is
shown in Fig. 6 and one can see that the like transition points
fall on smooth curves.
Fig. 2 Photomicrographs showing (a) a transition from a nematic
phase to a polar biaxial smectic Ad phase and (b) a texture of the polar
biaxial smectic Ad phase obtained in a homogeneously aligned cell for
compound 1j9.
Fig. 3 Photomicrographs of the conoscopic patterns obtained from a
homeotropically aligned sample of compound 1j9; (i) a uniaxial pattern
in the nematic phase at 162 uC, and (ii) a biaxial pattern in the SmAdPA
phase at 156 uC.
we shall see later, the XRD measurements indicate that the
lower temperature phase of compound 1j9 has a partial bilayer
structure. The confirmation of the existence of the biaxial
smectic Ad phase was obtained by conoscopic experiments. The
uniaxial interference pattern in the nematic phase splits as the
temperature is lowered and the patterns obtained in the two
phases are shown in Fig. 3. On lowering the temperature
slightly (e.g. 0.2 uC) the isogyres move away from the field of
view. These observations clearly indicate that the lower
temperature phase is in fact a polar biaxial smectic Ad phase.
The enthalpy value of this transition is in the range of 0.2 to
0.5 kJ mol21. Perhaps, this is the first example of a compound
showing a direct transition from the uniaxial nematic phase to a
polar partial bilayer biaxial smectic A (SmAdPA) phase. It has
been predicted theoretically16 that, if the lower temperature
SmA phase is biaxial then the high temperature nematic phase
is supposed to be biaxial. Contrary to this, we observe only a
uniaxial nematic to a biaxial SmA phase transition.
In the case of compounds of series 1l whose transition
Fig. 5 Photomicrographs of the conoscopic patterns obtained for a
homeotropically aligned sample of compound 1k12, (i) the uniaxial
smectic Ad phase at 175 uC, and (ii) the polar biaxial smectic Ad phase
at 171 uC.
On ascending the homologous series, the polar biaxial
smectic Ad phase gets stabilized and the uniaxial smectic Ad
3 1 4
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Fig. 8 Photomicrograph of a schlieren texture obtained in the polar
biaxial smectic Ad phase exhibited by compound 1l11 at 160 uC.
0.3 kJ mol21. The 2- and 4-brush defects observed in the higher
temperature SmAdPA phase is retained in this lower tempera-
ture phase also. This mesophase has been designated as SmX
phase since its structure is not clearly understood. In series 1l,
only one compound, 1l9 showed a direct transition from the
uniaxial N phase to the SmAdPA phase though the latter is
monotropic. The enthalpy value obtained for the SmAd-N
phase transition varied from 0.1 to 0.48 kJ mol21 and for the
Fig. 6 Plot of the transition temperatures as a function of the number
of carbon atoms in the n-alkyl chain for homologues of series 1k.
SmAd–I phase transition, it was 3.15 to 6.36 kJ mol21
.
However, for the SmAdPA phase to SmAd phase transition, the
enthalpy value was fairly low and was of the order of 0.1 to
0.23 kJ mol21. A plot of the transition temperatures versus the
number of carbon atoms in the n-alkyl chain for the
homologues of series 1l is shown in Fig. 10.
XRD experiments were performed following a procedure
described previously.9,10 Eight different compounds belonging
to the three homologous series were examined. The non-
oriented samples, in the smectic phase showed a sharp
reflection in the small angle region corresponding to the
layer spacing. The second order reflection obtained was found
to be somewhat weak. In addition, a diffuse wide-angle
Fig. 7 Photomicrographs of the texture showing (a) a transition from
a nematic phase to a uniaxial smectic Ad phase, (b) a uniaxial smectic
Ad phase and (c) a polar biaxial smectic Ad phase in a homogeneously
aligned cell of a sample of compound 1l11.
temperatures and associated enthalpy values are given in
Table 3, the mesomorphic behaviour is similar to that observed
for series 1j and 1k except that compounds 1l16 and 1l18 show
an additional mesophase below the SmAdPA phase.
A photomicrograph showing a phase transition from the
uniaxial nematic phase to the SmAd phase for compound 1l11
is shown in Fig. 7(a). Photomicrographs of the textures
observed for the fully formed SmAd and SmAdPA phases for
the same compound are shown in Fig. 7(b) and Fig. 7(c)
respectively. A schlieren texture is obtained when the homeo-
tropically aligned SmAd phase is cooled to the SmAdPA phase
and a typical texture observed for compound 1l11 is shown in
Fig. 8. Interestingly, compounds 1l16 and 1l18 show trimeso-
morphism and the lowest temperature phase was not observed
in series 1j and 1k. On slow cooling a sample of the
homeotropically aligned SmAd phase of compound 1l16 to a
temperature of 175 uC, a schlieren texture with both 2- and
4-brush defects could be obtained indicating that the phase is a
polar biaxial smectic Ad phase as observed for the lower
homologues 1l9 to 1l14. A photomicrograph of the texture
obtained for the SmAdPA phase of compound 1l16 is shown in
Fig. 9(a). On cooling this SmAdPA phase to a temperature of
160 uC, a transition takes place and a further decrease in the
temperature increases the birefringence of the mesophase and a
photomicrograph of this texture is shown in Fig. 9(b). Similar
mesomorphic behaviour was observed for compound 1l18
also. The enthalpy of this transition is of the order of 0.1 to
Fig. 9 Photomicrographs obtained for compound 1l16, (a) schlieren
texture in the SmAdPA phase and (b) more birefringent schlieren
texture in the SmX phase.
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spacing could be observed from X-ray experiments, while
cooling the SmAdPA to the SmX phase obtained for
compounds 1l16 and 1l18. Interestingly, both these mesophases
switch electro-optically and hence we have designated them as
SmAdPA and SmX phases respectively. A space filling model of
an antiparallel arrangement of strongly polar molecules used to
measure the length of the partial bilayer structure is shown in
Fig. 12. Similar molecular conformations have been considered
by Eremin et al.12 and Schroder et al.15 for the polar biaxial
smectic A phases in compounds containing two terminal
chains. However, in the system studied by us, the molecules
contain a strongly polar terminal cyano group which facilitates
an antiparallel arrangement of the molecules resulting in a
partial bilayer structure. For example, a compound containing
˚
an n-undecyl chain would give a value of 57 A (l, as shown
in Fig. 12). As can be seen in Table 4, we found a value of
˚
54.4 A for compound 1l11. Since there can be interlayer
interactions (and/or gauche conformation of chains), the
experimentally observed values are slightly lower.
Fig. 10 Plot of the transition temperatures as a function of the number
of carbon atoms in the n-alkyl chain for homologues of series 1l.
Electro-optical switching behaviour has been examined for
several compounds of the three homologous series of com-
pounds. As an example, the switching behaviour of compound
1l18 is described here. A cell having a thickness of 18 mm was
constructed for aligning the sample homogeneously. The
isotropic liquid of this compound was cooled slowly under a
triangular-wave electric field of about ¡280 V at a frequency of
1 kHz until the smectic Ad phase was obtained. No polarization
current peak/s could be observed in this high temperature
uniaxial smectic Ad phase. However, on cooling the sample to a
temperature of about 174 uC (the temperature was found to
decrease by about 2u due to the dissipation caused by ion flow),
two polarization current peaks slowly appeared for each half
period indicating an antiferroelectric ground state structure for
the lower temperature biaxial smectic Ad phase. The electro-
optical current response trace obtained in this phase is shown
in Fig. 13. The current was measured across 1 kV resistance
Fig. 11 Intensity profile of the X-ray diffraction pattern in the
SmAdPA phase of compound 1l16 obtained at 166 uC as a function
of the Bragg angle h.
˚
scattering with a maximum at 4.7 A was obtained indicating a
liquid-like in-plane order. As described earlier,9,10 the layer
spacing, d was found to be temperature independent in both the
smectic phases. The X-ray angular intensity profile obtained in
the SmAdPA phase of compound 1l16 is shown in Fig. 11. The
layer spacings d, obtained from XRD studies, for the smectic
phases of eight compounds investigated and the calculated
molecular length L in the most extended form with an all trans
conformation of the n-alkyl chain are given in Table 4. It can be
seen that the layer spacing d, is significantly larger than the
calculated molecular length. This provides evidence for a
partial bilayer structure of the smectic layers, in both the higher
and lower temperature mesophases. No change in the layer
Fig. 12 A space filling model of the antiparallel arrangement of
strongly polar molecules used to measure the length of the partial
bilayer structure.
Table 4 The layer spacings d obtained from XRD studies, for the
smectic phases of several compounds investigated and the molecular
length L, measured in the most extended form with an all trans
conformation of the chain
˚
d/A
˚
L/A
Compound
First order
Second order
1j9
48.6
51.3
58.8
52.0
57.8
54.4
58.8
61.7
24.3
—
29.4
—
28.9
—
29.4
30.8
43.3
45.4
51.1
45.4
51.1
45.4
51.1
53.5
1j11
1j16
1k11
1k16
1l11
1l16
1l18
Fig. 13 Switching current response obtained by applying a triangular-
wave voltage of about ¡280 V at 1 kHz in the SmAdPA phase of
compound 1l18 at 170 uC; polarization value P # 140 nC cm22; cell
thickness, 18 mm.
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and the polarization value determined is about 140 nC cm22
.
No optical switching could be observed due to the absence of
layer chirality and only a focal-conic texture was observed
which was also independent of the polarity of the applied
field.12 The additional sharp lines obtained in the focal-conic
texture of the SmAdPA phase disappeared on application of an
electric field. On turning off the electric field, no striped pattern
along the smectic layers could be seen indicating the absence of
the tilt of the BC molecules. If the molecules were to be tilted in
the mesophase, then we should have seen optical switching as
observed for the B2 phase. In addition, we do see strong
dynamic fluctuations in this phase as observed by Eremin
et al.12 and Schroder et al.15 and such fluctuations are not seen
in the tilted smectic phases. This supports the orthogonal
arrangement of molecules in the mesophase. On lowering the
temperature further into the SmX phase, the two polarization
current peaks were retained indicating the antiferroelectric
nature of the mesophase but the saturated polarization value
increased to 250 nC cm22. A trace of the current response
obtained in this SmX phase is shown in Fig. 14. However, from
the available data, it is difficult to speculate the exact nature of
the lower temperature SmX phase. An increase in the
birefringence and also the polarization value, suggests a tilt
of the molecules in this mesophase, which cannot be detected
from XRD studies. As pointed out by Brand et al.,16 SmA
phase to SmC phase transition can be divided into two steps.
First there could be a smectic A to CM phase transition during
which a preferred direction in the layers arise, which still could
maintain the orthogonal arrangement of the molecules. In the
second step, during the CM to smectic C phase transition, a tilt
arises leading to the biaxiality in the tilted SmC phase.
Fig. 15 Switching current response obtained in the SmAdPA phase at
132 uC exhibited by compound 1g16 by applying a triangular-wave
voltage ¡190 V at 500 Hz (threshold ¡125 V); polarization value,
P # 125 nC cm22; cell thickness, 9.4 mm.
It is necessary for us to point out the following concerning
the biaxial smectic Ad phase. We had reported10 earlier, that
the partial bilayer biaxial smectic A phase is apolar in both
longitudinal and transverse directions and had proposed a
quartet structure for the arrangement of these highly polar
unsymmetrical bent-core molecules. In the light of null
transmission ellipsometry experiments19 on one of the com-
pounds viz. 1g14 studied earlier, and also electro-optical
experiments described below for another homologue (1g16),
the proposed quartet structure is not applicable to these
systems. The switching behaviour of the mesophases of
compound 1g16 was re-examined as follows. On cooling the
isotropic liquid of this compound in a homogeneously aligned
cell (cell thickness, 9.4 mm), under a triangular-wave electric
field of about ¡150 V and a frequency of 500 Hz, a focal-conic
texture was obtained. No polarization current peak/s could be
observed in this higher temperature SmAd phase. On lowering
the temperature, a phase transition occurred to a SmAdb phase
(according to the earlier nomenclature) and the growth of two
polarization current peaks for each half period could be clearly
Fig. 16 Optical photomicrograph obtained under the same condi-
tions given in Fig. 15 for the same compound.
seen. The current was measured across 1 kV resistance. The
saturated polarization value calculated at ¡190 V (threshold,
¡125 V at 500 Hz) is about 125 nC cm22. The electro-optical
response trace obtained is shown in Fig. 15. The optical
photomicrograph obtained under these conditions is shown in
Fig. 16. These experimental observations reveal that the
molecular bow plane is orthogonal to the layer planes and
the polarization is antiparallel in adjacent layers. It is likely that
the molecules form a polar non-tilted smectic layer which is due
to the strong anisotropic interactions caused by the very
specific shape of the molecules. The energetically more
probable structure is antiferroelectric ordering of the dipoles
in adjacent layers which agree well with the observed optical
textures, which show both 2- and 4-brush defects. The 2-brush
defects in this case are dispirations,17,18 which are combinations
of half strength disclinations which are defect patterns in the
in-plane director, and screw dislocations. We have given a
fairly detailed account of these in our earlier studies9,10 on two
other similar systems. Based on these experimental observa-
tions, a molecular model has been proposed for the SmAdPA
phase and is shown in Fig. 17. It should be mentioned here that
though we had considered this model earlier, it was ruled out
based on the observations made under those experimental
conditions. The above model is similar to the one proposed by
Eremin et al.12 for a polar biaxial smectic A phase.
We have also carried out quantitative measurements of the
in-layer birefringence (Dm) in the SmAdPA and SmX phases of
compound 1l18 using a quarter wave plate as a compensator at
˚
l 5893 A. For this purpose, well aligned samples were obtained
as follows. A cell was constructed in which one of the plates is
conducting (ITO coated) having a gap of 85 mm that is etched
out. The other plate is an ordinary glass slide and both of them
are pretreated with octadecyltriethoxysilane (ODSE) for
obtaining the homeotropic alignment. The cell thickness was
Fig. 14 Switching current response obtained by applying a triangular-
wave voltage of about ¡ 280 V at 1 kHz in the SmX phase of
compound 1l18 at 152 uC; polarization value P # 250 nC cm22; cell
thickness, 18 mm.
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from the SmAdPA to the SmX phase. However, this cannot rule
out the possibility of the tilt of BC molecules with respect to the
layer normal which coincides with optic axis in this arrange-
ment. Perhaps, the local heating problem of the sample by the
continuous application of electric field has an influence on the
measured values.
Some general observations can be made concerning the
mesomorphic properties of these three homologous series of
compounds. The introduction of fluorine at ortho position
(series 1k) reduces the clearing temperatures by about 10 uC
while the melting points are increased marginally. The
mesophase thermal ranges of both the SmAd and SmAdPA
phases are reduced as a result of ortho-fluoro substitution.
However, in the case of meta-fluorine substituted compounds
(series 1l), both melting as well as clearing temperatures are
reduced by about 5–10 uC. Also, an additional unidentified
antiferroelectric SmX phase has been observed for higher
homologues. The clearing temperature is highest for n ~ 18 in
all three series of compounds. This is in complete contrast to
what is normally observed for the SmA phase in a homologous
series of calamitic liquid crystals where this temperature
decreases on ascending a series. A cursory look at the plots
shown in Fig. 4,6 and 10 clearly shows that the clearing
temperature curve for the SmAd phase rises and goes above the
falling N A I curve. This is again unusual and not observed in
calamitic liquid crystals.
Fig. 17 A schematic representation of a polar packing of molecular
pairs within the layer with an antiferroelectric ordering between
successive layers.
Conclusions
Three new homologous series of unsymmetrically substituted
compounds composed of bent-core molecules containing a
highly polar cyano terminal substituent have been synthesized.
XRD studies show that the both the smectic A phases have
partial bilayer structure as also the unidentified SmX phase.
The lower temperature partial bilayer biaxial smectic A phase
and the SmX phases are antiferroelectric in nature. A model
has been proposed for the polar packing of molecular pairs
within the layer with an antiferroelectric ordering between
successive layers. Out of the 27 compounds investigated, 22
show the partial bilayer polar biaxial smectic A (SmAdPA)
phase and 4 of them show a direct transition from the nematic
phase, which has been observed for the first time.
adjusted to be 37.3 mm using appropriate spacers. The oven
temperature was maintained to an accuracy of about 5 mK. A
square-wave electric field of about 0.68 V mm21 at a frequency
of 10 kHz was applied between the electrodes to obtain an
aligned sample. The optical measurements were carried out
under a continuous application of electric field by slow cooling
the sample from the uniaxial to the biaxial smectic Ad phase
(Tub) and to the SmX phase until the sample crystallizes. A plot
of in-layer birefringence (Dm) as a function of temperature
(Tub 2 T) is shown in Fig. 18. The in-layer birefringence
continuously increases on lowering the temperature in the
SmAdPA and SmX phases. This value increases from 0.007
(at Tub transition) to 0.011 in the SmAdPA phase and from
0.0114 to 0.02 in the SmX phase. Though the DSC thermogram
shows a weakly first order phase transition, no significant jump
in the in-layer birefringence could be observed at the transition
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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