Liquid crystalline alkyl-substituted oligo(p-phenyleneethynylene)s: synthesis and structure–property relationships

Abstract

A new series of liquid crystalline oligo(p-phenyleneethynylene)s, namely laterally and terminally alkyl-substituted derivatives of phenyl-terminated phenyleneethynylene hexamer (H(C6H4CC)6C6H5), were synthesised, and the relationships between molecular structures and mesomorphic properties were investigated. The mesomorphic properties of the hexamer derivatives were examined by differential scanning calorimetry and polarising microscopy. In the alkyl-substituted hexamer derivatives, R1C6H4CC[C6H2(R2)2CC][C6H2(R3)2CC][C6H2(R4)2CC][C6H2(R3)2CC][C6H2(R2)2CC]C6H4R1, where R1, R2, R3 and R4 = alkyl chains of various lengths or H, most of the hexamer derivatives exhibited nematic phases, whereas the others did not show any mesophases. It was found that the formation of the mesophase and the crystalline–nematic and nematic–isotropic transition temperatures were strongly affected by the length, number and position of the lateral chains, and by the length of the terminal chains.