Liquid crystalline conjugated oligomers: synthesis and mesomorphic properties of laterally and terminally alkyl-substituted oligo(1,4-phenyleneethynylene)s

Abstract

A new series of liquid crystalline conjugated oligomers, namely laterally and terminally alkyl-substituted derivatives of oligo(1,4-phenyleneethynylene)s, were synthesized, and the relationships between the molecular structures and the mesomorphic properties were investigated. The mesomorphic properties of the oligomer derivatives were examined by differential scanning calorimetry and polarizing microscopy. In the alkyl-substituted oligomers with varying core lengths, C₈H₁₇C₆H₄CC[C₆H₂(C₂H₅)₂CC]_nC₆H₄C₈H₁₇ where n = 2–5, the trimer (n = 2) exhibited a monotropic nematic phase, and the others showed enantiotropic nematic phases. In the alkyl-substituted tetramers, R¹C₆H₄CC[C₆H₂(R²)₂CC][C₆H₂(R³)₂CC][C₆H₂(R²)₂CC]C₆H₄R¹ where R¹, R², and R³ = alkyl chains of varying lengths and H, most of the tetramer 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 are strongly affected by the structural features of the oligomer molecules: (i) the length of the oligomer core, (ii) the length, number, and position of the lateral chains, and (iii) the length of the terminal chains.