CHEMCOMM

Raman Research Institute, C.V. Raman Avenue, Bangalore 560 080, India. E-mail: sadashiv@rri.res.inCommunication

R. Amaranatha Reddy, B. K. Sadashiva* and Surajit Dhara

www.rsc.org/chemcomm

Banana-shaped mesogens: observation of a direct transition from theantiferroelectric B2 to nematic phase†

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 bananashaped mesogens which exhibit a direct transition from theantiferroelectric B2 phase to the nematic phase are reported.
During the last couple of years there has been an upsurge in thesynthesis and phase transition studies of compounds composedof banana-shaped or bent-core molecules because of the unusualand very interesting mesophases they exhibit. The detection offerroelectricity in the mesophase of such an achiral compoundby Niori et al.,1 which was later determined to be antiferroelectric in the ground state2–4 by current response measurements, have stimulated further research in this field. Several
hundred compounds with a bent molecular shape have beensynthesised so far and at least seven different banana (B) phasescharacterised.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. Although the nematic phase has been observed in many series ofbent-core molecules,7–11 there is no example of a compound
exhibiting a direct transition from the B2 phase to nematic. Veryrecently there was a report of a case in which the B2 phase anda nematic phase are observed in the same substance and the twoare separated by smectic A and smectic C phases.12 It would bevery 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 andreport our results in this communication.
The synthesis and characterisation of four compounds
composed of banana-shaped molecules, three of which represent the first examples which show a direct transition from theantiferroelectric B2 to nematic (N) phase transition, is described
here. The general method of preparation of the compounds isshown in Scheme 1. As reported10 by us earlier, 2,7-dihydroxynaphthalene 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 compound5.‡ All the compounds were purified by column chromatography (silica gel, 60–120 mesh) and were crystallised several
times using suitable solvents.
The liquid-crystalline properties of the compounds wereinvestigated by using optical polarising microscopy anddifferential scanning calorimetry (DSC). The transition temperatures together with the associated enthalpies are summarised in Table 1. On examination of the samples under apolarising microscope the following are observed. Compound
5, n = 12, melts into a nematic phase and on cooling theisotropic (I) liquid shows typical schlieren texture. On further
cooling, small broken-fan like texture emanates from thenematic phase and the field of view is completely filled with this† Electronic supplementary information (ESI) available: Fig. 1 in colour.
See http://www.rsc.org/suppdata/cc/b1/b106458b/

1972

texture. Fig. 1 shows a typical texture wherein the broken-fans
are appearing from the nematic phase and this phase transitionis enantiotropic. Compounds 5, n = 13 and n = 14, areenantiotropic in both nematic and the lower temperature phase
while compound 5, n = 16, is also enantiotropic but without thenematic 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 phaseof 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 collectedon an image plate (Marresearch). The unoriented sample wascontained in a Lindemann capillary and the temperature wascontrolled to within ±0.1 °C. The diffraction pattern of anunoriented sample of the mesophase of compound 5, n = 14,Scheme 1 Reagents and conditions: i DCC, cat. DMAP, dry CHCl3, roomtemp., 12 h, 78%; ii cat. 5% Pd–C, H2, 1,4-dioxane, 55 °C, 90%; iii (E)-4-nalkoxy-a-methylcinnamic acid, DCC, cat. DMAP, dry CHCl3, room temp.,
15 h, 65–70%.
Table 1 Transition temperatures and the associated enthalpies (in italics) asdetermined by DSC
CompoundnPhase transition temp/°C; enthalpy/kJ mol215125135

14516Cr 119.5
75.56
Cr 117.5
59.28
Cr 107.0
58.6
Cr 108.0
52.36

Chem. Commun., 2001, 1972–1973
This journal is © The Royal Society of Chemistry 2001

[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

DOI: 10.1039/b106458b

Fig. 1 Photo-micrograph of the B2 phase appearing on cooling from ahomogeneously aligned nematic phase, compound 5, n = 13 (magnification3300).

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.)The presence of this uniaxial nematic phase, as evidenced bynumerous s = ±1 disclinations and fewer s = ±12 defects above
the B2 phase helps to align the latter and may contribute tomaking the measurement of other physical properties of thisinteresting antiferroelectric phase rather easy.
The authors thank Dr V. A. Raghunathan for help with the Xray measurements and Mrs K. N. Vasudha for technicalsupport.

Notes and references

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 oflamellar reflections (in the ratio 1+2+3) in the small angle region
could be clearly seen. From this as well as the texturalcharacteristics we have identified this mesophase to be a B2phase.
Finally, the characteristic feature of the B2 phase, namely theswitching current response, was carried out on the mesophase ofcompound 5, n = 14. An electro-optical switching cell wasconstructed using two ITO coated glass plates pre-treated withpolyimide 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 cooledslowly. A very good alignment of the nematic phase wasobtained. On further slow cooling under a low frequency AC
triangular voltage, well formed domains of the B2 phase overthe entire field of view were seen. From the visual observations
under the polarising microscope, we see colourful circulardomains which switch above a threshold voltage. Simultaneously two very sharp current peaks could be seen on theoscilloscope screen for the half period of the triangular voltage(250 Vpp). A typical switching current response for thiscompound is shown in Fig. 3. The current was measured acrossa 10 kW resistance. The apparent ‘saturated polarisation’ Ps wasobtained by measuring the area under the peaks and is about 560
nC cm22. These observations clearly indicate the antiferroelectric nature of this phase.

‡ The chemical structure of the compounds listed in Table 1 was determinedfrom their spectral data. Typical data obtained for compound 5, n = 13 isgiven 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 ArOCH2), 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.
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