Influence of acetylene-containing peripheral chains on the mesomorphic properties of triphenylene-based liquid crystals

Abstract

Three series of novel acetylene-containing triphenylene-based liquid crystals have been prepared. One class of material is symmetrical with the acetylenic unit located at the terminus of a peripheral alkynyloxy chain in a peripheral position at the terminus of an alkoxy chain. These symmetrical hexa-substituted materials possess high melting points which renders mesomorphism monotropic. A second class comprises materials where only one of the six peripheral units contains a terminal alkyne chain. The asymmetry confers relatively low melting points and an enantiotropic hexagonal columnar mesophase is exhibited for all materials, albeit to a lower temperature than the known parent systems. The third series of materials also comprises unsymmetrical structures, here containing one acetylenic unit conjugated with the triphenylene core and five alkoxy chains. This third series of materials show enhanced π–π interactions and enhanced space-filling effects of the acetylenic unit close to the core which enhance mesophase stability markedly over the hexaalkoxy analogues. This result shows that rigidity of peripheral chains, as opposed to molecular symmetry, is the significant factor for the reduction of mesophase stabilities.