The influence of the ring system with mono/diester and imine links, and terminal unsaturated carbon chains on mesomorphism and their quantum-mechanical study

Abstract

ABSTRACT Five homologous series of low molar mass liquid crystals containing mono/diester and imine-linked cores with terminal unsaturated carbon chains and/or a carboxylic group were synthesised. They were characterised with IR, UV and 1H-NMR spectroscopies. Liquid crystalline properties of all the derivatives were studied by polarised optical microscopy and their mesophase transition enthalpies and entropy changes measured by DSC. The 4-((4-n-alkoxybenzoyl) oxy) benzoic acids (n OPBA) exhibit enantiotropic smectic A and nematic mesophases, while all the 4-allyl-2-methoxyphenyl 4-n-alkoxybenzoates (n OBEg) and 4-allyl-2-methoxyphenyl (E)-3-(4-n-alkoxyphenyl) acrylates (n OCEg) do not show any liquid crystalline properties. Intermediate members of the 4-allyl-2-methoxyphenyl (E)-4-((4-n-alkoxybenzylidene) amino) benzoate series (n OPMABEg) and the 4-((4-allyl-2-methoxyphenoxy) carbonyl) phenyl 4-n-alkoxybenzoate series (n OB2Eg) exhibited only enantiotropic nematic mesophase behaviour. The optimised geometry of all the liquid crystalline compounds was calculated by density functional theory (DFT), and the effect of the twist angle ϴ of the ring system, the length of the core, HOMO-LUMO energies and electrostatic potential of the compounds on mesomorphism was examined. The effects of the linking groups, the ring systems, the length of the core and the terminal unsaturated carbon chain on mesophase behaviour were studied. The mesophase transitions of the five series were compared with each other and with structurally similar compounds to evaluate the effects of the ring systems on their mesophase behaviour.