Biaxial smectic A phase in homologous series of compounds composed of highly polar unsymmetrically substituted bent-core molecules

Article abstract of DOI:10.1039/b109546c

Several compounds belonging to three new series of five-ring banana-shaped esters, which are unsymmetrically substituted with respect to the central phenyl ring, have been synthesised. One of the arms of these bent-core molecules has been terminally subst

Full text of DOI:10.1039/b109546c

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Biaxial smectic A phase in homologous series of compounds
composed of highly polar unsymmetrically substituted bent-core
molecules
B. K. Sadashiva,* R. Amaranatha Reddy, R. Pratibha and N. V. Madhusudana
Raman Research Institute, C.V. Raman Avenue, Bangalore 560 080, India.
E-mail: sadashiv@rri.res.in
Received 19th October 2001, Accepted 25th January 2002
First published as an Advance Article on the web 6th March 2002
Several compounds belonging to three new series of five-ring banana-shaped esters, which are unsymmetrically
substituted with respect to the central phenyl ring, have been synthesised. One of the arms of these bent-core
molecules has been terminally substituted with a highly polar cyano group while the other end contains an
alkoxy chain. The mesophases exhibited by these compounds have been characterised using a combination of
optical polarising microscopy, differential scanning calorimetry and X-ray diffraction studies. These studies
indicate that most of the compounds show two smectic A mesophases which have a partial bilayer ordering.
Conoscopic experiments clearly reveal that the lower temperature smectic A phase is biaxial in nature. A
structural model has been proposed for the lower temperature biaxial smectic A phase. It is argued that
quartets of molecules, which are conducive to the formation of the biaxial phase, can form in the layers. The
in-layer birefringence of the biaxial smectic A phase has also been measured as a function of temperature for
one of the compounds.
Recently, we have reported20 the synthesis of a compound
Introduction
composed of highly polar unsymmetrically substituted BC
molecules. The unsymmetrical molecule contains an n-alkoxy
chain in one of the arms of the bent-core, while the other arm is
terminally substituted with a highly polar cyano group. This
The credit for the preparation of the first compound with a bent
molecular shape and exhibiting mesomorphic properties has
been attributed1,2 to Vorlander. However, it was Niori et al.3
who showed that achiral compounds composed of banana-
shaped or bow-shaped molecules exhibit a smectic phase with
polarized layers. A little later Link et al.4 demonstrated that the
chirality arising in these smectic layers is due to the tilting of
the molecules about the ‘‘arrow’’ directions of the bows, thus
breaking the achiral symmetry of the layers. Since then, there
has been intense research activity in this field.5 In the last few
years several hundred compounds composed of bent-core (BC)
molecules have been synthesised5–14 with a view to under-
standing the relationship between molecular structure and the
mesophases exhibited by such compounds. At least five
different B (banana) types of liquid crystals have been clearly
identified in pure compounds and some new variants have been
added recently.10 In the electrically switchable B2 phase, the
bent-cores pack along a common axis to give rise to an electric
polarisation of the layer. Also, the molecular plane tilts with
respect to the layer-normal to produce a chiral layer, though
the molecules themselves are usually achiral.4 We reported
earlier that binary mixtures of a compound exhibiting the B2
phase with one composed of lath-like molecules exhibiting
the bilayer smectic A phase showed a very interesting phase
diagram.15,16 At certain concentrations of the bent-core
molecules a biaxial smectic A2 (SmA2b) phase was obtained
in which the BC molecules had undergone an orientational
transition as the temperature was lowered from the uniaxial
smectic A2 phase. The biaxial smectic A phase has also been
observed in a binary mixture of a metallomesogen and 2,4,7-
trinitrofluorenone.17 There have been claims for the observa-
tion of the biaxial smectic A phase in a binary mixture of
polymeric and monomeric materials18 and in an oxadiazole
compound.19 However, in the latter two cases18,19 unam-
biguous proof for the existence of the biaxial smectic A phase
has not been provided.17
compound exhibits a partial bilayer biaxial smectic A (SmAdb
)
phase in which the bent cores of two neighbouring molecules
overlap in an antiparallel orientation as in the case of highly
polar lath-like molecules.21,22 In fact, this represents the first
example of a single compound to undergo the biaxial smectic
A to uniaxial smectic A transition. In this paper we report
the synthesis and characterisation of compounds composed
of highly polar unsymmetrically substituted BC molecules
belonging to three different homologous series. These five-ring
compounds are esters having the general molecular structure I.
Of the sixteen compounds reported here, fourteen exhibit the
biaxial smectic A (SmAdb) phase. Based on the experimental
observations, a structural model has been proposed for this
SmAdb phase.
Experimental
General
In general, the intermediate and final compounds were puri-
fied by column chromatography on silica gel (ACME, 60–
120 mesh) using appropriate solvent systems. The purity of all
the compounds was checked by thin layer chromatography
(Merck Kieselgel 60F254 precoated plates) and by normal phase
high performance liquid chromatography (m porasil column,
3.9 mm 6 300 mm, Waters Associates Inc.) using 1% ethyl
acetate in dichloromethane as the eluent. The yields of the
compounds were in the range of 60–80%.
The chemical structure of all the compounds was confirmed
1
by using a combination of H as well as 13C nuclear magnetic
resonance spectroscopy (Bruker AMX 400 spectrometer)
with 1% tetramethylsilane in deuteriochloroform as an inter-
nal standard, infrared spectroscopy (Shimadzu FTIR-8400
DOI: 10.1039/b109546c
J. Mater. Chem., 2002, 12, 943–950
This journal is # The Royal Society of Chemistry 2002
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spectrophotometer) and elemental analysis (Carlo-Erba 1106
analyser). The phase behaviour of all the compounds was
4’-Cyanophenyl 4-hydroxybenzoate, 1b. Compound 1a (11.0 g)
was dissolved in 1,4-dioxane (100 ml) and 5% Pd/C catalyst
(2.2 g) was added. The mixture was stirred at 55 uC in an
examined by using
a polarised light microscope (Leitz
Laborlux 12 POL/Olympus BX50) 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 enthalpies
for various transitions were also determined using the latter
method. The calorimeter was calibrated using pure indium as
a standard. X-Ray diffraction studies 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 collected on an image plate
(Marresearch). Unoriented samples were taken in Lindemann
capillaries and the sample temperature in each case was
controlled to within ¡0.1 uC. Monodomain samples were also
investigated, and will be described later. The conoscopic
observations were made using a microscope (Leitz Ortholux-II
POL BK) equipped with a heating stage and controller (Mettler
FP82 and FP80HT respectively).
Synthesis
The unsymmetrically substituted bent-core molecules 1g,
1h and 1i were synthesised following the general synthetic
pathway shown in Fig. 1. 3-Hydroxybenzoic acid and
4-cyanophenol were obtained commercially and used without
further purification. 2-Fluoro-4-hydroxybenzoic acid and
3-fluoro-4-hydroxybenzoic acid were prepared following pro-
cedures described in the literature.23 4-Benzyloxybenzoic acid,
2-fluoro-4-benzyloxybenzoic acid and 3-fluoro-4-benzyloxy-
benzoic acid were prepared according to procedures used by us
previously.24 The detailed procedure used for the synthesis and
characterisation of the compounds of series 1g is given below.
4’-Cyanophenyl 4-benzyloxybenzoate, 1a. 4-Cyanophenol, 1
(5 g, 42 mmol) and 4-benzyloxybenzoic acid, 2 (9.58 g,
42 mmol) were dissolved in dry chloroform (100 ml). After
the addition of N,N’-dicyclohexylcarbodiimide (DCC), (10 g,
48 mmol) and a catalytic amount of 4-(N,N-dimethylamino)-
pyridine (DMAP), the mixture was stirred at room temperature
for about 15 hours. The dicyclohexylurea which precipitated
was filtered off and the filtrate diluted with chloroform. This
solution was washed with 2% aqueous acetic acid solution
(3 6 100 ml) and 5% ice cold sodium hydroxide solution (3 6
100 ml) and finally washed with water and dried over
anhydrous sodium sulfate. The crude residue obtained was
chromatographed on silica gel using chloroform as an eluent.
Removal of solvent from the eluate afforded a white material
which was crystallised from a mixture of chloroform and
acetonitrile, yield 11.2 g (81%); mp 179.5–180 uC; 1H NMR
(CDCl3, 400 MHz) d (ppm): 8.14 (d, J 8.8 Hz, 2H, Ar-H),
7.72 (d, J 8.7 Hz, 2H, Ar-H), 7.4–7.33 (m, 7H, Ar-H), 7.07 (d,
J 8.8 Hz, 2H, Ar-H), 5.17 (s, 2H, ArCH2O-); IR (KBr) nmax
:
3099, 3064, 2231, 1728, 1685, 1604, 1508, 1225, 1163,
1070 cm21; C21H15NO3 requires C, 76.58; H, 4.59; N, 4.25%;
found: C, 77.0; H, 4.53; N, 4.20%.
Fig. 1 General synthetic pathway used for the preparation of the
compounds of series 1g, 1h and 1i.
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atmosphere of hydrogen till the required quantity of hydrogen
was absorbed. The mixture was filtered whilst hot and the solvent
removed under reduced pressure. The residue was chromato-
graphed on silica gel and eluted with a mixture of 5% acetone
in chloroform. Removal of solvent from the eluate gave a pro-
duct which was crystallised from a mixture of 1,4-dioxane and
petroleum ether (bp 60–80 uC), yield 6 g (75%); mp 205–206 uC;
1H NMR (acetone-d6, 400 MHz) d (ppm): 8.1 (d, J 7.6 Hz, 2H,
Ar-H), 7.73 (d, J 7.7 Hz, 2H, Ar-H), 7.35 (d, J 7.6 Hz, 2H, Ar-H),
Ar-OCH2-CH2), 1.50–1.26 (m, 22H, 11 6 CH2), 0.89–0.86 (t, J
7.0 Hz, 3H, 1 6 CH3); 13C NMR (CDCl3, 400 MHz) d (ppm):
164.2, 163.8, 163.5, 163.4, 155.7, 155.4, 154.2, 151.1, 133.7,
132.4, 132.0, 131.8, 130.5, 129.8, 127.7, 127.5, 126.3, 123.6,
122.8, 122.1, 120.9, 118.1, 114.4, 109.9, 68.4, 31.8, 29.6, 29.5,
29.3, 29.0, 25.9, 22.6, 14.0; IR (KBr) nmax: 2916, 2850, 2243,
1736, 1602, 1508, 1278, 1255, 1220, 1166, 1078 cm21
;
C49H49NO9 requires C, 73.94; H, 6.21; N, 1.76%; found: C,
73.65; H, 6.27; N, 1.44%.
6.94 (d, J 7.7 Hz, 2H, Ar-H), 5.73 (s, 1H, Ar-OH); IR (KBr) nmax
:
3352, 3103, 3060, 2237, 1733, 1685, 1612, 1595, 1496, 1271,
1068 cm21; C14H9NO3 requires C, 70.29; H, 3.79; N, 5.85%;
found: C, 70.29; H, 3.7; N, 6.05%.
Results and discussion
The phase transition temperatures and the associated enthal-
pies for the highly polar unsymmetrically substituted com-
pounds belonging to the three different homologous series, viz.,
1g, 1h and 1i are summarised in Tables 1–3 respectively. The
compounds of series 1g are not laterally substituted while in
series 1h and 1i, a fluoro substituent is introduced in the ortho
and meta positions respectively with respect to the carboxylate
4-Cyanophenyl 4’-(3@-benzyloxybenzoyloxy)benzoate, 1c. Syn-
thesised as described for the preparation of compound 1a.
Quantities: compound 1b (5.5 g, 23 mmol), 3-benzyloxybenzoic
acid (5.25 g, 23 mmol), DCC (5.6 g, 27 mmol), DMAP (cat.
quantity), dry chloroform (75 ml); yield 7.3 g (72%); mp 146–
147 uC; 1H NMR (CDCl3, 400 MHz), d (ppm): 8.28 (d, J 8.5 Hz,
2H, Ar-H), 7.84–7.74 (m, 4H,Ar-H), 7.47–7.26 (m, 11H, Ar-H),
5.16 (s, 2H, ArCH2O-); IR (KBr) nmax: 3100, 3066, 2227, 1726,
1602, 1581, 1508, 1485, 1417, 1270, 1016 cm21; C28H19NO3
requires C, 74.82; H, 4.26; N, 3.11%; found: C, 74.53; H, 4.20;
N, 3.12%.
Table 1 Transition temperatures (uC) and enthalpies (kJ mol21) for
compounds of series 1ga
Compound
1g12
n
Cr
SmAdb
SmAd
I
.
.
.
.
.
.
12
13
14
15
16
18
.
.
.
.
.
.
129.5
41.11
129.0
80.86b
129.5
77.28b
129.5
89.07b
130.0
86.67b
129.5
60.57b
.
.
.
.
.
.
130.5
0.17
135.0
0.09
138.0
0.13
140.2
0.09
141.9
0.08
143.2
0.11
.
.
.
.
.
.
130.8
3.48
135.5
4.24
140.2
4.81
144.0
5.27
147.5
5.79
153.5
6.17
4-Cyanophenyl 4’-(3@-hydroxybenzoyloxy)benzoate, 1d. Syn-
thesised as described for the preparation of compound 1b.
Quantities: compound 1c (7 g), 5% Pd/C (1.4 g), 1,4-dioxane
(100 ml); yield 4.28 g (75%); mp 211–213 uC; 1H NMR
(acetone-d6) d (ppm): 8.31 (d, J 8.8 Hz, 2H, Ar-H), 7.94 (d, J
8.8 Hz, 2H, Ar-H), 7.7–7.2 (m, 8H, Ar-H), 2.94 (s, 1H, Ar-OH);
IR (KBr) nmax: 3298, 3105, 3068, 2245, 1730 1600, 1589, 1508,
1487, 1270, 1170, 1058 cm21; C21H13NO5 requires C, 70.19; H,
3.64; N, 3.89%; found: C, 69.89; H, 3.53; N, 4.09%.
1g13
1g14
1g15
1g16
1g18
akey for all three Tables 1–3. Cr: crystalline; SmAdb: partial bilayer
biaxial smectic A phase; SmAd: partial bilayer uniaxial smectic A
phase; I: isotropic. bHas crystal-crystal transitions; the enthalpy
denoted is the sum of all the transitions. Temperatures in parenth-
eses indicate monotropic transitions.
4-Cyanophenyl 4’-[3@-(4--benzyloxybenzoyloxy)benzoyloxy]-
benzoate, 1e. Synthesised as described for the preparation of
compound 1a. Quantities: compound 1d (6.5 g, 18 mmol),
4-benzyloxybenzoic acid (4.13 g, 18 mmol), DCC (4.5 g,
22 mmol), DMAP (cat. quantity), dry chloroform (75 ml); yield
7.2 g (70%); mp 167–168 uC; 1H NMR (CDCl3) d (ppm):
8.28 (d, J 8.4 Hz, 2H, Ar-H), 8.18–8.06 (m, 4H, Ar-H), 7.75 (d,
J8.3 Hz, 2H, Ar-H), 7.62–7.34 (m, 11H, Ar-H), 7.08 (d, J
Table 2 Transition temperatures (uC) and enthalpies (kJ mol21) for
compounds of series 1h
8.6 Hz, 2H, Ar-H), 5.17 (s, 2H, ArCH2O-); IR (KBr) nmax
:
Compound
1h12
n
Cr
SmAdb
SmAd
I
.
.
.
.
3101, 3072, 2231, 1740, 1730, 1602, 1502, 1444, 1250,
1056 cm21; C35H23NO7 requires C, 73.8; H, 4.07; N, 2.46%;
found: C, 74.15; H, 3.97; N, 2.32%.
12
14
16
18
.
.
.
.
130.0
66.79
132.0
68.32
131.5
60.30
131.0
60.26
—
—
(.
—
(.
.
1h14
130.5)
4.19
137.5
4.97
143.5
5.95
4-Cyanophenyl 4’-[3@-(4--hydroxybenzoyloxy)benzoyloxy]-
benzoate, 1f. Synthesised as described for the preparation of
compound 1b. Quantities: compound 1e (7 g), 5% Pd/C (1.4 g),
1,4-dioxane (75 ml); yield 4.2 g (71%); mp 206–207 uC; 1H
NMR (acetone-d6) d (ppm): 8.32 (d, J 8.6 Hz, 2H, Ar-H), 8.15–
8.09 (m, 4H, Ar-H), 7.94 (d, J 8.5Hz, 2H, Ar-H), 7.76–7.6 (m,
6H, Ar-H), 7.03 (d, J 8.6 Hz, 2H, Ar-H), 2.82 (s, 1H, Ar-OH);
IR (KBr) nmax: 3385, 3101, 3066, 2237, 1738, 1600, 1500, 1413,
1250, 1163, 1060 cm21; C28H17NO7 requires C, 70.14; H, 3.57;
N, 2.92%; found: C, 69.8; H, 3.59; N, 2.60%.
1h16
130.0)
0.12
132.5
0.06
1h18
.
.
Table 3 Transition temperatures (uC) and enthalpies (kJ mol21) for
compounds of series 1i
Compound
1i12
n
Cr
SmAdb
SmAd
I
.
.
.
.
.
.
12
13
14
15
16
18
.
.
.
.
.
.
119.5
80.68
120.0
82.75
120.5
85.46
121.0
72.64
121.0
67.15
120.0
44.8
(.
.
119.2)
0.019
123.4
0.017
126.2
0.044
128.0
0.09
129.0
0.08
130.0
0.11
.
.
.
.
.
.
120.0
3.86
125.5
4.22
130.0
4.77
133.8
4.95
137.0
5.47
142.3
5.85
4-Cyanophenyl 4’-{3@-[4--(4@@-n-alkoxybenzoyloxy)benzoyl-
oxy]benzoyloxy}benzoate, 1g. Synthesised as described for
the preparation of compound 1a. Quantities: compound 1f
(0.2 g, 0.42 mmol), 4-n-tetradecyloxybenzoic acid (0.14 g,
0.42 mmol), DCC (0.1 g, 0.48 mmol), DMAP (cat. quantity),
1i13
1i14
.
1i15
.
1
dry chloroform (10 ml); yield 0.2 g (60%); H NMR (CDCl3,
1i16
.
400 MHz) d (ppm): 8.31–8.27 (m, 4H, Ar-H), 8.16–8.08 (m, 4H,
Ar-H), 7.77–7.74 (d, J 8.5 Hz, 2H, Ar-H), 7.65–7.54 (m, 2H,
Ar-H), 7.43–7.37 (m, 6H, Ar-H), 6.99 (d, J 8.9 Hz, 2H, Ar-H),
4.05 (t, J 6.6 Hz, 2H, Ar-OCH2), 1.83 (quin, J 7.9 Hz, 2H,
1i18
.
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Fig. 2 Photomicrograph of the optical texture of the SmAdb phase of
compound 1g14 at 135 uC (crossed polarisers), Magnification 6250.
Fig. 4 Differential scanning calorimetric scans at a rate of 5 uC min21
for compound 1g16: a) heating cycle, b) cooling cycle. The insert shows
the SmAdbASmAd phase transition in a magnified scale.
group of the middle phenyl ring of the arm containing the
n-alkoxy chain. Each compound is designated by a number
which represents the series and the number of carbon atoms
in the alkoxy chain (e.g. 1i12 means the compound containing
the n-dodecyloxy chain in series 1i). All the six compounds
of series 1g (Table 1) are dimesomorphic and enantiotropic.
On cooling the isotropic liquid of a sample (compounds 1g12
to 1g18) sandwiched between a glass slide and a coverslip and
viewed under a polarising microscope a homeotropic texture
is obtained. However, in some regions a focal-conic texture
could also be seen. This mesophase has been identified as
uniaxial smectic A phase. As the temperature is lowered
further, a phase transition takes place in which the homeo-
tropic regions exhibit a schlieren texture with both two-brush
and four-brush defects. The presence of the two-brush defects
in this lower temperature mesophase may be due to one of
the three structural possibilities as discussed in our earlier
communication.20 As we have argued in that paper, the
lower temperature phase corresponds to the biaxial smectic A
(SmAb) phase. As pointed out by Brand et al.25 a distinguishing
feature of the biaxial smectic A phase is the occurrence of
¡½ strength defects, unlike in the smectic C phase with tilted
molecules which allows only disclinations of strength ¡1. A
typical texture of the lower temperature mesophase is shown in
Fig. 2. We can indeed see in Fig. 2 disclinations of strength ¡½
as well as those with strength ¡1. The latter could be expected
to split to form the lower strength defects to reduce energy.
However, this would require a large rearrangement of the
director field. The two structures would be separated by a
potential barrier which appears to lead to the metastability
of the disclinations of strength ¡1 in the systems studied by us.
Strong fluctuations in intensity are seen in this phase as
observed by us previously20 and this phase has been identified
as biaxial smectic A phase. Sometimes this mesophase exhibits
a striped texture as shown in Fig. 3 and an investigation into
the origin of this feature is underway, the results of which will
Fig. 5 Plot of the transition temperatures versus the number of carbon
atoms in the n-alkyloxy chain for homologues of series 1g.
be published elsewhere. A differential scanning calorimetric
(DSC) thermogram obtained for compound 1g16 is shown in
Fig. 4 and all the transitions can be seen clearly. While the
clearing point enthalpy ranges from about 3.5 to 6 kJ mol21
,
that for the biaxial smectic A to uniaxial smectic A phase
transition is quite low and is of the order of 0.1 kJ mol21. A
plot of the transition temperatures against the number of
carbon atoms in the alkoxy chain for this series is shown in
Fig. 5. One can see smooth curve relationships for like transi-
tions and both the mesophase–mesophase and mesophase–
isotropic transition point curves rise gradually in ascending the
series while the melting points remain more or less constant.
Only four compounds were synthesised in series 1h as the
compound 1h12 is non-mesomorphic and compound 1h14
does not exhibit the biaxial smectic A phase. Though com-
pounds 1h16 and 1h18 do exhibit the biaxial smectic A phase,
the former is monotropic. The characteristic textural features
observed in series 1g were observed for these two compounds
as well. The six homologues of series 1i shown in Table 3
do exhibit both the uniaxial as well as the biaxial smectic A
phases. Again the textural characteristics and the DSC data
are similar to those observed for homologues of series 1g. A
plot of the transition temperatures versus the number of carbon
atoms in the alkoxy chain for the series 1i is shown in Fig. 6.
Again, a rising trend is obtained for like transitions and the
melting points lie practically on a horizontal line on ascending
the series. It is interesting to note that the near constancy of
Fig. 3 Photomicrograph of the striped pattern obtained for compound
1g14 at 132 uC (crossed polarisers), Magnification 6250.
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Fig. 6 Plot of the transition temperatures versus the number of carbon
atoms in the n-alkyloxy chain for homologues of series 1i.
the melting points in each of the three series of compounds
is rather unusual. A comparison of the transition tempera-
tures of the three series of compounds 1g, 1h and 1i reveal
the following features. The introduction of an ortho-fluoro
substituent (series 1h) lowers the clearing temperatures by
about 10 uC while the melting points are increased marginally.
Also, the compound with n-dodecyloxy chain (1h12) is
rendered non-mesogenic. However, in the case of meta-fluoro
substituted compounds (series 1i), both the melting and
clearing points are reduced by about 10 uC. This suggests
that the position of the fluoro substituent (dipolar interactions)
has an effect on the stability of the mesophase in such bent-core
compounds.
In order to confirm the optical biaxiality of the lower
temperature smectic A phase, we have carried out conoscopic
experiments. For this purpose, well aligned samples have to
be used and we have achieved the same in the following way.
A cell is constructed in which one of the glass plates has an
ITO conducting coating with a gap of 1 mm that is etched out.
The other plate is an ordinary glass slide and both are
pretreated with octadecyltriethoxysilane (ODSE) which facili-
tates a homeotropic alignment of the smectic A liquid crystal.
The thickness of the sample (usually y30 mm) is controlled by
using appropriate spacers and measured using an interference
technique before filling the sample. For compound 1g15, the
conoscopic observations between crossed polarisers which are
set at 45u to the direction of the electric field (178 V, 10 kHz)
show the uniaxial interference pattern in the smectic A phase
(140.5 uC) and as the temperature is lowered to 139.5 uC this
splits to give the biaxial pattern. On further lowering the
temperature (139.2 uC) the separation between the isogyres
continuously increases. On reducing the temperature (139.0 uC)
further the isogyres go out of the field of view. All the three
conoscopic patterns are shown in Fig. 7.
The switching behaviour of the mesophase was examined as
follows. A sample (compound 1i14) was taken in a cell similar
to the one used for conoscopic observations. The cell had a gap
of 80 mm between the electrodes. As before, an electric field
aligned the biaxial director. Careful observations under the
action of a square wave voltage in the frequency range of 1
to 10 Hz did not reveal any switching in the sample upto
an electric field of 3 V mm21. Further, observations using a
half-wave compensator clearly showed that the field-induced
alignment was such that the molecular plane was orthogonal
to the electric field. The optical anisotropy in the layer plane
did not vary with field upto 3 V mm21. We tried to get an
aligned planar sample by using ITO plates coated with silicon
monoxide at an oblique angle. As the sample was cooled
at 0.5 uC min21 across the isotropic to uniaxial smectic A
Fig. 7 Photographs of the conoscopic patterns from a homeotropically
aligned sample of compound 1g15: a) the uniaxial smectic Ad phase at
T ~ 140.5 uC, b) biaxial smectic Ad phase at T ~ 139.5 uC, c) biaxial
smectic Ad phase at T ~ 139.2 uC.
transition temperature, batonnets were seen to grow and merge
to give a focal-conic (FC) texture. In some regions the FC
domains were quite large, and remained so as the sample was
cooled to the biaxial smectic phase. The application of a low
frequency (1 to 5 Hz) square wave voltage across a sample,
which was 5.8 mm thick did not show any switching behaviour
in the biaxial phase. However, for voltages above y15 V, there
was electrohydrodynamic motion which did not change when
the sample was heated to the uniaxial smectic A phase.
X-Ray diffraction studies were carried out on ten com-
pounds. The detailed experimental procedure used has already
been described in a recent communication.20 For all the
samples, in the small angle region, a sharp reflection corres-
ponding to the smectic layer spacing could be seen. The second
order reflection was found to be rather weak. The values
obtained for both the first order as well as the second order
reflections and the calculated molecular length L in their most
extended form with an all trans conformation of the alkoxy
chain is given in Table 4. In addition, the X-ray diffraction
pattern showed a diffuse scattering in the wide-angle region
˚
with a peak around 4.6 A indicative of a liquid-like in-plane
order. A significant point to note is that the d values are almost
J. Mater. Chem., 2002, 12, 943–950
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Table 4 The layer spacings d, of the smectic phases of the ten
compounds investigated and the molecular length L, in the most
extended form with an all trans conformation of the alkoxy chain
compound 1g14, this would imply that each tetradecyl chain
˚
contributes about 11 A to the layer thickness. The fully
˚
stretched all trans chain length is y17 A, but as the lowest
temperature at which the SmAdb phase occurs is above 118 uC
or so, the chain can be expected to have considerable
conformational flexibility. The effective length of the chain is
then smaller than that of a fully stretched conformation,
˚
d/A
˚
L/A
Compound
First order
Second order
1g13
1g14
1g16
1g18
1h16
1h18
1i13
1i14
1i16
1i18
50.8
52.3
54.0
55.7
53.0
54.8
52.4
53.9
55.5
56.6
25.4
26.1
27.0
27.9
26.5
27.4
26.2
27.0
27.7
28.3
47.9
49.2
51.9
54.5
51.9
54.5
47.9
49.2
51.9
54.5
˚
probably by 1 or 2 A. Further, the chain is attached to the arm
of a bent aromatic core which makes an angle of 30u with the
bow-string axis. The chain-axis makes an angle of y36u with
the axis of the arm of the core to which it is attached, and the
effective contribution of chain to the layer thickness depends
on the angle between the chain axis and the plane of the
aromatic core. In the usual BC molecules in which both ends
have alkyl chains, packing efficiency would favour the chains to
be in the plane defined by the central core. In the SmAd and
SmAdb structures, the number of chains in the aliphatic parts of
the layer is only half the number of aromatic cores in the
densely packed central part. This in turn gives rise to a relative
orientational freedom for the chains to increase the entropy.
˚
temperature independent (within experimental error of ¡0.2 A)
in the entire range of both smectic phases. It can be clearly seen
that the layer spacing d, in most of the cases is significantly
larger than the calculated molecular length. This evidently
implies a partial bilayer structure of the smectic layers in both
the higher and lower temperature smectic phases and hence
we have designated them as SmAd and SmAdb phases.
However, it is seen that for very long alkyl chains such as
n-octadecyloxy chain, the d values are closer to those of L.
This perhaps implies that the long chains adopt gauche
conformations for which there is sufficient space to fill. One
can also see from this table that the d values increase as the
terminal alkyl chain length is increased. We have also con-
ducted X-ray diffraction experiments using samples mounted
on a horizontal glass plate pretreated with ODSE. In the
SmAdb phase, we see equatorial spots corresponding to the
layer order and the diffused scattering is now confined to arcs
centered at an angle to the horizontal plane. The biaxial
director is not aligned, but we may note the two spacings in the
layer will not be very different for BC molecules unlike in the
biaxial phase made of board-like molecules.17 Hegmann et al.17
have observed additional reflections in a monodomain sample
for a mixture composed of a metallomesogen and trinitro-
fluorenone in the lower temperature biaxial smectic A phase.
From the molecular structure of these compounds (structure
I), it is clear that all the four ester groups in the aromatic core
have dipole moments whose long axis components are parallel
to that of the cyano end group. In the case of compounds of
series 1h and 1i, the C–F bond on one of the phenyl rings has a
dipole moment which is almost exactly orthogonal to the long
axis of the molecule. The dipolar interactions between two
neighbouring molecules in all these cases favour an antiparallel
configuration, and in view of the distribution of dipoles in the
core, a complete overlap between the aromatic cores can be
expected. The mutual configuration of a molecular pair can
then be expected to be as shown in Fig. 8 which maximises
dispersion interaction and allows for a better packing of the
˚
The final effect is that the chain contributes about 11 A to the
effective thickness of the partial bilayer.
In order to understand the optical properties of the medium
under an electric field, two possible structures can be
considered. (i) Each layer has a polar order of the BC
molecular pairs. As the arrow-axis aligns orthogonal to the
electric field, the successive layers could have antiferroelectric
order as shown schematically in Fig. 9. However, it can be
noted that the in-plane polarization of the layers of 1gn
compounds would be quite large in view of the contribution
from the strongly polar cyano end groups of both molecules in
the pair. Hence we could expect that a relatively low electric
field would have produced an induced ferroelectric order, and
under the reversal of a field of y3 V mm21, a switching in the
induced ferroelectric state. As a switching is not seen, nor
indeed even a change in the lateral optical anisotropy even at
3 V mm21 the model sketched in Fig. 9 can safely be ruled out.
(ii) It is clear from Fig. 8 that the BC nature of the molecules
leads to a fairly large lateral component of the cyano dipole
moment unlike in the case of rod-like molecules. The dipolar
interaction between the lateral components would favour the
formation of a pair of molecular pairs with their arrow axes
being antiparallel. Thus, we can expect that four molecules
form a quartet (see Fig. 10) whose aromatic region is biaxial
but apolar in both longitudinal and transverse directions. The
orientational freedom of the chains can facilitate a relatively
free rotation of the quartets about their long axes, giving rise
to the uniaxial SmAd phase. As the temperature is lowered,
the core region undergoes a rotational freezing giving rise to
the biaxial SmAdb phase, with an apolar in-layer director
corresponding to the arrow axes.
We have made quantitative measurements of the in-layer
birefringence (Dm) of the SmAdb phase of compound 1i14 at
˚
l 5893 A by using a quarter wave plate as a compensator.
˚
molecules. The length of the molecular core is y32 A along the
‘bow string axis’ between the nitrogen atom of the cyano group
and the oxygen atom of the alkoxy chain. In the case of
The oven temperature is maintained to an accuracy of about
5 mK. However, as a strong electric field (0.4 V mm21) is
applied to get an aligned sample, the temperature of the latter
Fig. 8 The antiparallel configuration of a pair of molecules showing the complete overlap of the aromatic cores.
J. Mater. Chem., 2002, 12, 943–950
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Fig. 10 Schematic diagram of the proposed quartet structure of the
highly polar BC molecules which is apolar in both longitudinal and
transverse directions and gives rise to the partial bilayer smectic liquid
crystal. The pair of molecules whose cores are shown in full lines are
in the plane of the paper and those with the dotted lines are below that
plane.
In these compounds, the BC molecules orientate with their
arrow axes in the layer-plane.
Conclusions
We have synthesised sixteen compounds belonging to three
homologous series of unsymmetrically substituted compounds
composed of bent-core molecules which contain a highly polar
cyano group at one terminal position. Fourteen of these com-
pounds exhibit the SmAdb phase. The mesophase–mesophase
and mesophase–isotropic transition points follow smooth
curves as functions of the alkoxy chain length. However, the
melting points are about the same for all the homologues in
each of the three series which is somewhat unusual. X-Ray
diffraction studies show that both the smectic A phases are
partially bilayer in nature. We have proposed a structural
model for the lower temperature mesophase (SmAdb) in which
four molecules form a quartet whose aromatic region is biaxial
but apolar in both longitudinal and transverse directions.
Fig. 9 Schematic representation of a polar packing of molecular pairs
within the layer with an antiferroelectric ordering between successive
layers.
is found to be increased by about 1 uC, due to dissipation
caused by ion flow. Hence the optical measurements were
made on cooling the sample under a continuous application of
the field and the temperatures indicated in Fig. 11 correspond
to those of the oven. Though the DSC thermograms indicate
a very weak first order transition between SmAd and SmAdb
phases (with DH y400 J mol21), the local heating problem
mentioned above prevents us from measuring any small jump
in Dm at the transition point. Dm increases continuously as the
temperature is lowered in the SmAdb phase. The data can
be fitted to an equation of the form Dm ~ A(Tub — T)b giving
an index b ~ 0.5 (Fig. 11). The local heating problem perhaps
has an influence on the measured value.
As a result of these investigations, we believe that the
occurrence of both uniaxial and biaxial smectic A phases in
compounds with highly polar groups such as a cyano group at
one end of the bent-core is mainly due to the partial bilayer
structure in which there is a dense region of biaxial aromatic
cores and relatively sparsely populated aliphatic regions, which
allow for considerable orientational freedom of the chains.
Fig. 11 Temperature variation of the layer birefringence in the biaxial
smectic A phase of compound 1i14. Stars are experimental data and the
continuous line is the theoretical fit referred to in the text.
J. Mater. Chem., 2002, 12, 943–950
949

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12 B. K. Sadashiva, V. A. Raghunathan and R. Pratibha, Ferro-
electrics, 2000, 243, 249.
Acknowledgement
13 I. Wirth, S. Diele, A. Eremin, G. Pelzl, S. Grande, L. Kovalenko,
N. Pancenko and W. Weissflog, J. Mater. Chem., 2001, 11, 1642.
14 B. K. Sadashiva, H. N. Shreenivasa Murthy and Surajit Dhara,
Liq. Cryst., 2001, 28, 483.
The authors thank Dr V.A. Raghunathan for help in the
X-ray experiments and Mr P.N. Ramachandran and Ms
K.N. Vasudha for technical support.
15 R. Pratibha, N. V. Madhusudana and B. K. Sadashiva, Science,
2000, 288, 2184.
16 R. Pratibha, N. V. Madhusudana and B. K. Sadashiva, Mol.
Cryst. Liq. Cryst., 2001, 365, 755.
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