New mesogens with cubic phases: hydrogen-bonded bipyridines and siloxane-containing benzoic acids - I. Preparation and phase behaviour

Several 4-(oligodimethylsiloxyl)alkoxybenzoic acids and their hydrogen-bond ed complexes with 4,4-dipyridyl or 1,2-bis(4-pyridyl)ethane were prepared a nd their phase behaviour studied by DSC and polarized optical microscopy. T he neat acids showed no liquid crystalline phases. The 4,4'-dipyridyl compl ex of 4-(n-heptamethyltrisiloxyl) hexyloxybenzoic acid (Si(3)C(6)BA) exhibi ts an optically isotropic, highly viscous liquid crystalline phase below a smectic A phase. The 1,2-bis(4-pyridyl) ethane complex of Si(3)C(6)BA also shows an optically isotropic liquid crystalline phase above its smectic C p hase. Its behaviour is similar to that of the well known cubic D phase foun d in 4'-n-alkoxy-3'-nitrobiphenyl-4-carboxylic acids. In the hydrogen-bonde d mesogens studied herein, the cubic phase appears to assemble spontaneousl y in order to take account of the chemical incompatibility between the silo xane moiety terminating the H-bonded complex and the stiff aromatic cores. The transition temperatures of the cubic phases in these materials is aroun d 100 degreesC, hence they are amenable to a variety of physical measuremen ts.