Synthesis of metal containing liquid crystals

Abstract

The primary objective of the present study was to develop mesogenic metal complexes. Part of this work was concerned with the synthesis of structural analogues of existing mesogenic bis-1,2-dithiolatonickel(II) complexes to explore the impact of structural variations on the mesophase behaviour of these complexes and to develop complexes with novel mesophase behaviour. The study was successfully continued to develop new mesogenic systems.
The work has shown that replacement of the hydrogen atom at the 2-position of the metallodithiolato ring by a phenyl or alkyl group diminishes the tendency of mesophase formation of the system, although the melting temperatures of the 2-alkyl substituted derivatives are greatly reduced relative to the non-substituted analogues.
A more general and shorter synthetic route - over literature methods - for tetra-substituted dithiolatonickel(II) complexes has been developed although the synthesised complexes were proved to be nonmesogenic.
A novel synthetic route to bis[1-(2'-fluoro-4'-((S)-(+)-7-methylnonyl) phenyl)ethene-1,2-dithiolato]nickel(II) (4.42) has been developed and the compound was shown to posses ferroelectric behaviour, which unfortunately persist only over a very short temperature range (ca. 0.5°C).
Attempts to restore this interesting phase by variation of the alkyl side chain length proved to be unsuccessful; extension of the chain length stabilises the mesophase, but with exclusion of the ferroelectric behaviour, while the reduction of the alkyl side chain length destabilises the mesogenic behaviour of the complexes. However, mixed ligand complexes were found to posses relatively wider mesophase range with a low crystal to liquid crystal transition temperatures (at or near room temperature).
Insufficient amounts of these materials have been obtained to date for the ferroelectric properties to be examined. Attempts to synthesise complexes with 3'-fluoro substitutents were unsuccessful due to the exclusive formation of the "incorrect" isomer in the preceding synthetic step.
A number of bis[methylidenearoylhydrazinato]nickel(H) complexes have been shown to posses mesogenic properties. The short chain homologue of these complexes showed a nematic phase while the higher homologues gives smectic mesophases. The corresponding Cu(II) complexes were found to be prone to decomposition just above their transition temperatures. Alkyl or aryl substituents at the azomethine carbon of the nickel derivatives results in non-mesogenic systems.