Smectic-phase-induced interdigitated orientation of polythiophenes bearing phenylnaphthalene mesogen

Abstract

Polythiophenes bearing mesogenic phenylnaphthalene side chains at 3-position, poly(3-{10-[6-(4-octylphenyl)naphthalene-2-yloxy]decyl}thiophene-2,5-diyl) and poly(3-{10-[4-(6-octyl-2-naphthyl)phenyloxy]decyl}thiophene-2,5-diyl), in which each mesogenic core linked reversely, and their model liquid crystals corresponding to each phenylnaphthalene side chains were synthesised and compared each other to investigate liquid crystalline property and molecular arrangement. Yamamoto polymerisation of corresponding 2,5-dibromothiophene monomers afforded soluble polythiophenes. They had similar number average molecular weights about 10 kDa and exhibited similar photophysical properties, i.e, UV–vis absorption λmax due to polythiophene backbones around 390 nm and fluorescence λmax around 540 nm in solution (560 nm in film state) with energy transfer from an excited phenylnaphthalene side group to the backbone. The respective model liquid crystals showed different behaviours of phase transition temperatures, enthalpies, and reversibility, whilst both polymers had similar phase transition behaviours. A well-developed fan-shaped texture in the temperature range between isotropic liquid and glass phases was observed for the polymers by polarising optical microscopy. It is suggested that both polymers had an almost identical smectic A1 phase, which led to glass state by retaining the interdigitated molecular arrangement through a crystal E phase on cooling.