The synthesis and mesomorphic properties of 4,4″-dialkyl-2,2′,3- and 2,2′,3′-trifluoro-1,1′∶4′,1″-terphenyls for high dielectric biaxiality ferroelectric liquid crystal mixtures

Abstract

Two series of liquid crystalline terphenyls with terminal alkyl chains have been synthesised with three lateral fluoro substituents in close proximity to each other. Two terminal alkyl chains were employed to minimise viscosity, maximise solubility and generate low melting points. The first series of materials employs fluoro substituents at an outer-edge position to maximise the smectic phase C phase stability, whereas the second series has the fluoro substituents in the centre of the molecule in an attempt to minimise melting points, but surprisingly the melting points of the former compounds are much lower. The synthetic routes to all of the materials involved low-temperature lithiations to generate arylboronic acids which were then involved in sequential, selective palladium-catalysed cross-coupling reactions. Many of the materials produced have extremely low melting points and some exhibit the smectic C phase at room temperature. In general, the 2,2′,3- and 2,2′,3′-patterns of lateral fluoro substitution in 1,1′∶4′,1′′-terphenyls generate materials of low viscosity and high lateral dipole. In particular, many materials from the first series have low melting points and exhibit the smectic C phase, hence they show great promise for the formulation of ferroelectric mixtures with a high dielectric biaxiality which is very important in τV minimum driving schemes; whereas the materials from the second series are nematogens and hence could be useful for vertically aligned nematic devices.