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

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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 (Sc*) 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 Sc 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 Sc 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.