Non-conventional liquid crystals: synthesis and mesomorphism of non-symmetric trimers and tetramers derived from cholesterol

Abstract

Non-conventional liquid crystals such as oligomers formed by covalent tethering of two to several anisometric (mesogenic) cores through flexible spacer(s) represent a fascinating and emerging class of functional soft materials. Over the last two decades or so, a variety of such motifs have been accomplished to illustrate their significance. This manuscript reviews certain specific examples of nonlinear and linear oligomers with a special attention to linear trimers and tetramers. Following this, the preparation and phase transitional behaviour of linear non-symmetric trimers and tetramers have been presented. Linear trimers have been made by covalently binding three different functional anisometric segments viz., promesogenic-optically active-cholesterol, promesogenic-naphthalene and photochromic-azobenzene cores, through two flexible spacers of varying length and parity. Tetramers have been synthesised by linking cholesterol, naphthalene, biphenyl (a promesogenic) and tolane (a half-disc) mesogenic moieties via three flexible spacers. The occurrence of mesomorphism in these supermolecules has been evidenced by optical textural pattern, calorimetric and powder X-ray diffraction studies. The phase transitional behaviour of these compounds has been compared with closely related known trimers and tetramers, which have in fact, served as reference/model for the substances synthesised in this study.