Smectic A-hexagonal columnar-B7 phase transition of acute-angle bent-core molecules

Abstract

Acute-angle bent-shaped molecules comprising a 1,7-naphthalene central core, alkylthio terminal chains and long side wings comprising three phenyl rings connected by imine and ester linkages were synthesised. Their distinct mesomorphic properties and phase structures were revealed. Despite their acute shape, these molecules exhibited a polar switchable hexagonal columnar phase (Colh), banana B7 phases and calamitic smectic A (SmA) phase, enabling the first observation of SmA-Colh-B7 and Colh-B7 phase transition behaviours. The formation of a hexagonal columnar phase between the layered smectic A and B7 phases supports the proposed Colh phase model in which a tube-like assembly incorporating smectic layers generates hexagonal columns. Molecular packing in the optically uniaxial SmA phase was also discussed in terms of birefringence. Overall, the structural transformation of one-dimensional SmA layers into deformed hexagonal columnar and B7 phases was found to depend on the combined effects of molecular asymmetry, alkylthio terminal tails and the balance between flexible chain lengths and rigid mesogenic units.