α-Fluoro esters incorporating a cyclohexane ring: some new chiral dopants for ferroelectric mixtures

Abstract

Several new homologous series of optically active α-fluoro esters incorporating a cyclohexane ring and a number of different cores have been synthesized. Several of these chiral dopants possess an enantiotropic chiral smectic C (S_c*) phase at elevated temperatures. They are characterised by a relatively high spontaneous polarisation, long pitch and low rotational viscosity. In addition they exhibit exceptional chemical, thermal and photo- and electro-chemical stability. It is shown that an ester group in a terminal position of the molecular core of these new chiral dopants leads to an increase in the spontaneous polarisation (compared with the values for the analogous ethers) with only a small increase in the rotational viscosity, as determined in a standard S_c base mixture. An ester group in a central position of the molecular core has the opposite effect on these parameters. It is shown that at least two aromatic rings are normally necessary for S_c formation. An optically active α-chloro ester incorporating a cyclohexane ring was also prepared and compared with the analogous α-fluoro ester. A higher spontaneous viscosity were determined for the α-fluoro ester. These new α-fluoro esters are excellent chiral dopants for chiral smectic C mixtures for use in the surface-stabilised ferroelectric liquid-crystal display.