Liquid crystal dimers possessing chiral rod‐like anisometric segments: synthesis, characterization and electro‐optic behaviour

Abstract

Several new optically active liquid crystal dimers comprising pro‐mesogenic cholesterol and a chiral diphenylacetylene (tolane) segment, covalently linked in an end‐to‐end fashion through a flexible spacer, have been synthesized and investigated for their mesomorphic behaviour with the aid of optical, calorimetric and X‐ray diffraction studies. Five unsymmetrical dimers, designed on the basis of recent work, involve molecular structural variations of the tolane mesogenic entity with a view to stabilizing a wide thermal range smectic A (SmA) phase featuring the electroclinic effect. Three different chiral chains, namely, (S)‐1‐methylheptyloxy, (S)‐2‐methylbutyloxy, (3S)‐3,7‐dimethyloctyloxy, with or without polar (nitro or fluoro) lateral substituents, were incorporated, while keeping the length (C₆) of the spacer constant. As expected, all the dimers exhibited a SmA phase. A few also showed chiral nematic (N*) and/or twist grain boundary and/or chiral smectic C (SmC*) phases. Remarkably, some of these oligomesogens, upon melting, had a stable SmA phase over a wide thermal interval (100–150°C); this state seems to be stable for a long period of time. Electro‐optic studies, including optical tilt angle as well as temporal response as a function of temperature, were carried out in the SmA phase. The SmC* phase was also investigated for its electrical switching and optical tilt angle, as well as spontaneous polarization as a function of temperature. These studies showed that the mesophase response to an applied field is weak and is independent of variations in the dimer investigated.

Acknowledgements

We are grateful to the late Prof. S. Chandrasekhar for many helpful discussions. I. S. wishes to thank the Council of Scientific and Industrial Research (CSIR), for grant of a senior research fellowship. Financial support by CSIR, No. 01(1727)/02/EMR‐II, is gratefully acknowledged.