Mesogenic properties of two 1,3,4-oxadiazole-based bent-core geometrical isomers

Abstract

Two new symmetrical oxadiazole-based bent-core mesogens bearing cis and trans geometrical stereoisomer tails, geranyloxy and neryloxy, have been synthesised and the effect of the cis–trans geometric configuration of the terminal chains on the thermal profile and mesomorphic behaviour were investigated using differential scanning calorimetry and polarised optical microscopy. The trans–trans stereoisomer compound 4g with terminal geranyloxy chains exhibited a nematic phase characterised by the schlieren texture, which contained both 2- and 4-brush defects and remarkably no smectic mesophases. The cis–cis stereoisomer neryloxy derivative 4n has a much lower melting temperature but did not exhibit mesomorphism. In an attempt to further lower the melting point of the nematic mesophase, a binary mixture study was carried out between the geranyloxy and neryloxy 2,5-bis(4-hydroxyphenyl)-1,3,4-oxadiazole derivatives. The mesomorphism is retained up to 50% ratio, with a cross-over reached at 70% of the geranyloxy trans–trans stereoisomer 4g, resulting in monotropic behaviour of the mixture. Once neryloxy cis–cis stereoisomer homologue 4n is in excess, the mesophase is completely suppressed.