NOVEL GLASS-FORMING LIQUID-CRYSTALS .3. HELICAL SENSE AND TWISTING POWER IN CHIRAL NEMATIC SYSTEMS

Based on nematic and chiral precursors comprising of acetylenic, pheny l, naphthyl and terminal cyano groups in conjugation and cyclohexane a s the central core, a series of novel nematic and chiral nematic glass -forming liquid crystals with a T-g greater than or equal to 60 degree s C were synthesized for an investigation of handedness and helical tw isting power, HTP, as functions of chemical structure. Three chiral bu ilding blocks with a single asymmetric carbon centre were employed: (S )-(-)-1-phenylethanol, (S)-(-)-alpha-methyl-2-naphthalenemethanol, and (S)-(-)-1-phenylethylamine, were all found to yield a left-handed cho lesteric mesophase. With reference to (S)-(-)-1-phenylethanol, the HTP value was found to be nearly doubled in the presence of a strong anch oring plane furnished by a naphthyl group surrounding the asymmetric c arbon centre, but reduced by one third in the presence of intermolecul ar hydrogen bonding through the amide group. All these observations we re 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 f ound to give a left-handed cholesteric mesophase and an HTP value a qu arter of that of (S)-(-)-1-phenylethanol under otherwise identical str uctural settings, both remaining unaccounted for from a molecular poin t of view. The cyclohexane ring to which chiral as well as nematic pre cursors are attached was found to improve miscibility with cyclohexane -based nematic hosts and to elevate the T-g of the blends with an impr oved morphological stability but a very modest increase in HTP, all in comparison to chiral precursors as the dopant. The optical elements p repared on a single glass substrate and between a pair of substrates u sing the presently reported materials showed broad selective wavelengt h reflection bands with a monodomain feature characterized by pronounc ed side band oscillations.