Studies of Some Heterocyclic Mesogens

Abstract

New mesogens have been prepared and studied by differential thermal analysis and optical microscopy to elucidate the effects on mesomorphic properties of replacing a benzene ring by differnt hiterocyclic rings.

Earlier workers have assessed the effects on nematic properties arising from such changes in the terminal groups of molecules of the type 4-MeO.C₆H₄.C₆H₄.N:CH.R₄ (where R is phenyl, pyridyl or certain five-membered heterocylic rings). By studying, the corresponding n-octyl ethers, the relative effects of the different terminal rings on smectic properties have now been established. The compounds exhibit smectic polymorphism and the types of phase have been examined. Compounds of the type 4-R.CH:N.C₆H₄-C₆H₄-N:CH-R-4′ showed some differences in trends compared with those for compounds with only one terminal group, R. Other compounds, including two series of the type 4-X'C₆H₄-CO-O-C₆H₄-CH:CH.R-4′ (where X is methoxy or n-octyloxy and R is phenyl, pyridyl or diazinyl), have been examined and show, in addition to the trends observed above, the effect of the heterocyclic system on π-bond interactions within the molecule.

To supplement earlier work and to complement the above studies of changing the terminal groups of molecules, compounds with pyridyl rings in non-terminal positions have also been studied.

The overall results relating to the mesophases formed and the trends in the crystal-mesophase or -amorphous liquid and mesophase-mesophase or -amorphous liquid transition temperatures are discussed in terms of changes in axial molecular polarizability produced by the hetro-atom either directly or indiectly.