Synthesis of highly chiral multisubstituted binaphthyl compounds as potential new biaxial nematic and NLO materials

Abstract

A number of multi substituted binaphthalene compounds were synthesized in an attempt to obtain a highly chiral biaxial nematic (N_b) phase or a good NLO material. Introduction of two pairs of substituents into the binaphthalene molecule

with one of these being a nitrile group and R = C₁₀ was successful but neither this compound nor the above esters showed any mesophases. Introduction of additional pairs of substituents proved to be difficult. Friedel-Crafts acylation to add a third pair through a ketone gave complicated mixtures of products obtained by cleavage of the ether followed by acylation. Bromination, which could lead to either the necessary nitrile substituent or esters via the acid obtained from the nitrile generally gave mixtures of bromides as expected. Some of these could be separated by chromatography and successfully converted to the nitrile. However, attempts to hydrolyze these nitrile groups usually gave mixtures of various products of incomplete hydrolysis producing compounds that did not have the appropriate substituents for observing mesophases. No mesophases were observed in any of the binaphthalene compounds prepared. The (R)-(+) optical isomer of the above nitrile with R = C₁₀ was also prepared to study its potential use as a chiral dopant. This isomer had a melting temperature below that of the racemic material indicating that the latter is a racemic compound. A few mixture studies using primarily the racemic dopant with known liquid crystals indicated that low percentages (< 10 per cent) of this dopant gave good mixtures. The best mixtures were obtained using a cyanobiphenyl (8OCB) host which showed an enhanced nematic phase. Mixtures of a small amount of the chiral dopant in room temperature nematics gave highly coloured cholesteric (twisted nematic) phases.