Banana-shaped mesogens: observation of a direct transition from the antiferroelectric B2 to nematic phase

Article abstract of DOI:10.1039/b106458b

The synthesis and characterisation of the first banana-shaped mesogens which exhibit a direct transition from the antiferroelectric B2 phase to the nematic phase are reported.

Full text of DOI:10.1039/b106458b

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Banana-shaped mesogens: observation of a direct transition from the
antiferroelectric B2 to nematic phase†
R. Amaranatha Reddy, B. K. Sadashiva* and Surajit Dhara
Raman Research Institute, C.V. Raman Avenue, Bangalore 560 080, India. E-mail: sadashiv@rri.res.in
Received (in Cambridge, UK) 19th July 2001, Accepted 22nd August 2001
First published as an Advance Article on the web 19th September 2001
The synthesis and characterisation of the first banana-
shaped mesogens which exhibit a direct transition from the
antiferroelectric B2 phase to the nematic phase are re-
ported.
texture. Fig. 1 shows a typical texture wherein the broken-fans
are appearing from the nematic phase and this phase transition
is enantiotropic. Compounds 5, n = 13 and n = 14, are
enantiotropic in both nematic and the lower temperature phase
while compound 5, n = 16, is also enantiotropic but without the
nematic phase. The N?I transition enthalpy is about 0.4 kJ
mol21. A DSC thermogram obtained for compound 5, n = 13,
is shown in Fig. 2 and all the transitions can be seen clearly.
In order to confirm the nature of the lower temperature phase
of these compounds, X-ray investigations were carried out. The
X-ray diffraction experiments were conducted using Cu-Ka
radiation from a rotating anode generator with a flat graphite
crystal monochromator. The diffraction patterns were collected
on an image plate (Marresearch). The unoriented sample was
contained in a Lindemann capillary and the temperature was
controlled to within 0.1 °C. The diffraction pattern of an
unoriented sample of the mesophase of compound 5, n = 14,
During the last couple of years there has been an upsurge in the
synthesis and phase transition studies of compounds composed
of banana-shaped or bent-core molecules because of the unusual
and very interesting mesophases they exhibit. The detection of
ferroelectricity in the mesophase of such an achiral compound
by Niori et al.,1 which was later determined to be antiferro-
electric in the ground state2–4 by current response measure-
ments, have stimulated further research in this field. Several
hundred compounds with a bent molecular shape have been
synthesised so far and at least seven different banana (B) phases
characterised.5,6 It has been observed that in many homologous
series of compounds only one banana phase exists in one
homologue. However, smectic A, smectic C or nematic phases
have been obtained either individually or in combination
particularly for homologues with short terminal chains. Al-
though the nematic phase has been observed in many series of
bent-core molecules,7–11 there is no example of a compound
exhibiting a direct transition from the B2 phase to nematic. Very
recently there was a report of a case in which the B2 phase and
a nematic phase are observed in the same substance and the two
are separated by smectic A and smectic C phases.12 It would be
very interesting and useful from an experimental point of view
if one could get a direct transition from the antiferroelectric B2
phase to the nematic phase. We have achieved this objective and
report our results in this communication.
The synthesis and characterisation of four compounds
composed of banana-shaped molecules, three of which repre-
sent the first examples which show a direct transition from the
antiferroelectric B2 to nematic (N) phase transition, is described
here. The general method of preparation of the compounds is
shown in Scheme 1. As reported10 by us earlier, 2,7-dihydroxy-
naphthalene 1 was reacted with 2-fluoro-4-benzyloxybenzoic
acid 2 to give the diester 3. The protective benzyloxy groups
were removed using 5% Pd–C as catalyst in an hydrogen
atmosphere to yield the diphenol 4. In the final step (iii)
compound 4 was reacted with two equivalents of an appropriate
(E)-4-n-alkoxy-a-methylcinnamic acid to furnish compound
5.‡ All the compounds were purified by column chromatog-
raphy (silica gel, 60–120 mesh) and were crystallised several
times using suitable solvents.
Scheme 1 Reagents and conditions: i DCC, cat. DMAP, dry CHCl3, room
temp., 12 h, 78%; ii cat. 5% Pd–C, H2, 1,4-dioxane, 55 °C, 90%; iii (E)-4-n-
alkoxy-a-methylcinnamic acid, DCC, cat. DMAP, dry CHCl3, room temp.,
15 h, 65–70%.
The liquid-crystalline properties of the compounds were
investigated by using optical polarising microscopy and
differential scanning calorimetry (DSC). The transition tem-
peratures together with the associated enthalpies are sum-
marised in Table 1. On examination of the samples under a
polarising microscope the following are observed. Compound
5, n = 12, melts into a nematic phase and on cooling the
isotropic (I) liquid shows typical schlieren texture. On further
cooling, small broken-fan like texture emanates from the
nematic phase and the field of view is completely filled with this
Table 1 Transition temperatures and the associated enthalpies (in italics) as
determined by DSC
Compound
n
Phase transition temp/°C; enthalpy/kJ mol21
5
5
5
5
12
13
14
16
Cr 119.5
75.56
Cr 117.5
59.28
Cr 107.0
58.6
Cr 108.0
52.36
[B2 116.0]
13.23
B2 120.0
13.92
B2 123.0
14.15
B2 128.0 I
16.84
N 127.0 I
0.49
N 125.0 I
0.43
N 124.0 I
0.34
† Electronic supplementary information (ESI) available: Fig. 1 in colour.
See http://www.rsc.org/suppdata/cc/b1/b106458b/
1972
Chem. Commun., 2001, 1972–1973
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106458b

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Fig. 3 Switching current response obtained in the B2 phase of compound 5,
n = 14 at 120 °C by applying a triangular voltage (cell thickness: 6 mm, 250
Vpp, 29 Hz). (Vpp = volt peak-to-peak.)
Fig. 1 Photo-micrograph of the B2 phase appearing on cooling from a
homogeneously aligned nematic phase, compound 5, n = 13 (magnification
3300).
The presence of this uniaxial nematic phase, as evidenced by
1
numerous s = 1 disclinations and fewer s = 2 defects above
the B2 phase helps to align the latter and may contribute to
making the measurement of other physical properties of this
interesting antiferroelectric phase rather easy.
The authors thank Dr V. A. Raghunathan for help with the X-
ray measurements and Mrs K. N. Vasudha for technical
support.
Notes and references
‡ The chemical structure of the compounds listed in Table 1 was determined
from their spectral data. Typical data obtained for compound 5, n = 13 is
given below. gmax (KBr) cm21 2910, 2850, 1740, 1720, 1605, 1508, 1415,
1250, 1190; dH (400 MHz, CDCl3) 8.22 (t, J 8.72 Hz, 2H, Ar-H), 7.94 (d,
J 8.92 Hz, 2H, Ar-H), 7.9 (s, 2H, Ar-CHNC), 7.72 (d, J 2.0 Hz, 2H, Ar-H),
7.47 (d, J 8.76 Hz, 4H, Ar-H), 7.39 (dd, J 2.08 Hz, 2H, Ar-H), 7.13–7.17 (m,
4H, Ar-H), 6.96 (d, J 8.68 Hz, 4H, Ar-H),4.01 (t, J 6.56 Hz, 4H, 2 3 Ar-
OCH2), 2.27 (d, J 0.88 Hz, 6H, 2 3 Ar-CNC(CH3)), 1.81 (quin., J 6.68 Hz,
4H, 2 3 Ar-OCH2-CH2), 1.26–1.48 (m, 12H, 6 3 CH2), 0.88 (t, J 6.64 Hz,
6H, 2 3 CH3); dC (400 MHz, CDCl3) 166.3, 162.1, 161.5, 159.9, 156.4,
149.0, 141.7, 134.3, 133.3, 131.8, 129.7, 129.3, 127.6, 124.2, 121.1, 118.6,
117.8, 115.1, 114.6, 111.4, 111.2, 68.2, 31.9, 29.6, 29.5, 29.3, 29.1, 25.9,
22.6, 14.2, 14.0.
Fig. 2 Differential scanning calorimetric scan of compound 5, n = 13
showing clearly the transition from the B2 to the nematic phase. The insert
shows the amplified N?I transition.
shows a diffuse peak in the wide angle region with d = 4.6 Å,
indicating liquid-like in-plane order. In addition, three orders of
lamellar reflections (in the ratio 1+2+3) in the small angle region
could be clearly seen. From this as well as the textural
characteristics we have identified this mesophase to be a B2
phase.
Finally, the characteristic feature of the B2 phase, namely the
switching current response, was carried out on the mesophase of
compound 5, n = 14. An electro-optical switching cell was
constructed using two ITO coated glass plates pre-treated with
polyimide which enables the sample to align homogeneously.
The cell thickness was adjusted to be 6 mm by using appropriate
spacers. The sample was filled in the isotropic phase and cooled
slowly. A very good alignment of the nematic phase was
obtained. On further slow cooling under a low frequency AC
triangular voltage, well formed domains of the B2 phase over
the entire field of view were seen. From the visual observations
under the polarising microscope, we see colourful circular
domains which switch above a threshold voltage. Simultane-
ously two very sharp current peaks could be seen on the
oscilloscope screen for the half period of the triangular voltage
(250 Vpp). A typical switching current response for this
compound is shown in Fig. 3. The current was measured across
a 10 kW resistance. The apparent ‘saturated polarisation’ Ps was
obtained by measuring the area under the peaks and is about 560
nC cm22. These observations clearly indicate the antiferro-
electric nature of this phase.
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