New liquid crystals for display mixtures

This thesis is concerned with the design, synthesis and properties of thermotropic liquid crystals based on the fluorene and 9,10-dihydrophenanthrene core units. Derivatives of 9,10-dihydrophenathrene were found to be both expensive to prepare and difficult to purify, and attention was concentrated on fluorene derivatives. Simple 2,7-disubstituted fluorenes are either monotropic or exhibit enantiotropic smectic phases. Accordingly a third benzene ring was directly coupled to the fluorene core to give a 2-phenylfluorene. The homologous series of 2-(4-cyanophenyl)-7-alkylfluorenes from ethyl to nonyl was prepared via a five step convergent synthesis, and the compounds were isolated as white crystalline solids. All the compounds exhibited enantiotropic nematic phases, and the heptyl, octyl and nonyl homologues also exhibited a smectic A phase. The parent compound 2-(4 cyanophenyl)fluorene with no alkyl chain was prepared by a similar route, but did not exhibit an enantiotropic mesophase. Some physical properties of 2-(4-cyanophenyl)-7-pentylfluorene were measured, and many values {e.g. dielectric anisotropy and birefringence) were similar to those for the commercially successful 4-pentyl-4" -cyano-p-terphenyl. However 2-phenylfluorene material had a higher viscosity. These properties could be related to the crescent shape of the 2-phenylfluorene core, which is dictated by the bridging methylene group. Lateral substitution of the fluorene core was investigated in an attempt to prepare low melting point derivatives. Four C9 alkylated compounds were prepared, and these were found to be oils. However no mesophases were detected. Some 2-(4 alkoxyphenyl)-7-alkylfluorenes were prepared as low birefringence analogues, but these were found to exhibit smectic phases at high temperatures. The analogous alkoxy compound 2-pentoxy-7-(4 cyanophenyl)fluorene was prepared but could not be satisfactorily purified. Preliminary investigations into partial reduction of the fluorene core were carried out, but these did not progress beyond an early stage