Synthesis and Liquid Crystal Properties of Chiral Alkyl-Cyano-Biphenyls (and -p-Terphenyls) and of Some Related Chiral Compounds Derived from Biphenyl

Abstract

Using only (-)-2-methylbutan-1-ol as an optically active starting material, other chiral aliphatic materials have been derived by synthetic procedures which preserve the optical activity, and a number of chiral mesogens incorporating the biphenyl (or p-terphenyl) ring system have been prepared. The new cholesterogens give a wide range of physical properties-melting points, cholesteric-isotropic liquid transition temperatures and pitch values. The twisting powers of the individual materials, i.e., the cholesteric pitch values, are related to the proximity of the chiral centre of the alkyl group to the ring system in which it is a substituent. Many of the compounds exhibit only cholesteric mesophases but some give additional smectic phases. The physical properties of the mesogens are assessed with particular reference to their applications in electro-optical displays or in surface thermography. In the latter case, the compounds provide a range of cholesterogens of good chemical/photochemical stabilities from which mixtures with thermochromic properties can be made. Examples are given of mixtures: with such materials, the sensitivity of colour play for a given mixture composition can be manipulated in a way which is not practical with sterol-type cholesterogens. With regard to the smectic properties exhibited by some of the compounds, of particular interest are those chiral smectic C phases which, as thin films in the planar texture, show iridescence colours through selective, temperature dependent reflection of light in the visible wavelength range.