Novel glass-forming liquid crystals. III Helical sense and twisting power in chiral nematic systems

Abstract

Based on nematic and chiral precursors comprising of acetylenic, phenyl, naphthyl and terminal cyano groups in conjugation and cyclohexane as the central core, a series of novel nematic and chiral nematic glass-forming liquid crystals with a T _g ≧ = 60°C were synthesized for an investigation of handedness and helical twisting power, HTP, as functions of chemical structure. Three chiral building blocks with a single asymmetric carbon centre were employed: (S)-(−)-1-phenylethanol, (S)(−)-α-methyl-2-naphthalenemethanol, and (S)-(−)-1-phenylethylamine, were all found to yield a left-handed cholesteric mesophase. With reference to (S)-(−)-1-phenylethanol, the HTP value was found to be nearly doubled in the presence of a strong anchoring plane furnished by a naphthyl group surrounding the asymmetric carbon centre, but reduced by one third in the presence of intermolecular hydrogen bonding through the amide group. All these observations were properly accounted for by a molecular interaction model of a steric nature while allowing for hydrogen bonding. Consistent with previous observations on naturally occurring (−)-cholesterol, (+)-estrone was found to give a left-handed cholesteric mesophase and an HTP value a quarter of that of (S)-(−)-1-phenylethanol under otherwise identical structural settings, both remaining unaccounted for from a molecular point of view. The cyclohexane ring to which chiral as well as nematic precursors are attached was found to improve miscibility with cyclohexane-based nematic hosts and to elevate the T _g of the blends with an improved morphological stability but a very modest increase in HTP, all in comparison to chiral precursors as the dopant. The optical elements prepared on a single glass substrate and between a pair of substrates using the presently reported materials showed broad selective wavelength reflection bands with a monodomain feature characterized by pronounced side band oscillations.