Supramolecular dimeric liquid crystals. The liquid crystalline behaviour of mixtures ofα-(4-pyridyloxy)-ω -[4-(4-butylphenylazo)phenoxy]alkanes and 4-octyloxybenzoic acid

The thermal behaviour of binary mixtures of 1-(4-pyridyloxy)-5-[4-(4-butylphenylazo)phenoxy]pentane (Bu-azo-5-Pyr) and 1-(4-pyridyloxy)-6-[4-(butylphenylazo)phenoxy]hexane (Bu-azo-6-Pyr) with 4-octyloxybenzoic acid is reported. Both systems are miscible over the complete composition range; this miscibility is attributed to the formation of a hydrogen bond between the unlike species, a view confirmed by IR spectroscopy. Bu-azo-5-Pyr and Bu-azo-6-Pyr do not exhibit liquid crystallinity but enhanced liquid crystal behaviour is observed for the mixtures with the acid. In particular, smectic A behaviour is injected into the phase diagrams of both systems. The clearing temperatures of the mixtures containing Bu-azo-6-Pyr are considerably higher than those for the analogous mixture containing Bu-azo-5-Pyr. In addition, the entropy change associated with the clearing transition for the equimolar mixture containing Bu-azo-6-Pyr is significantly higher than that for the analogous mixture containing Bu-azo-5-Pyr. These observations strongly suggest the formation of a supramolecular dimeric liquid crystal. For the Bu-azo-6-Pyr-based system the specific molecular interaction between the pyridyl and acid fragments has not only strongly enhanced the liquid crystalline behaviour but also increased the degree of molecular ordering.