Mesophase Behaviour of 1,2,3-Triazole-based Nematic Liquid Crystals Influenced by Varying Alkyl Chains and Halogen Atom Substitution

To develop novel nematic-phase liquid-crystal (LC) materials with practical potential in optical and photovoltaic devices, the alkyl chain and halogen atom substitution in 1,2,3-triazole-based compounds were varied, and the effects on the mesophase behaviour were investigated. The 4-(4-propylcyclohexyl)phenyl 4-(1-alkyl-5-hydro/halo-1,2,3-triazol-4-yl)benzoate series 1 was successfully synthesised through tandem aerobic oxidative halogenation and click reaction via in situ activation. Except for 5-thiophene-1,2,3-triazole compound 1c-Th displaying no mesomorphism, the other compounds 1 form enantiotropic nematic mesophases, among which compounds 5-hydro-1,2,3-triazole 1c-H (C10) and 1d-H (C12) (long straight chain compounds) form two mesophases (smectic C phase and nematic phase). For 5-halo-1,2,3-triazole compounds 1c-X (x = halide), the mesomorphic temperature range was wider than for the corresponding compound 1c-H because of the effect of the halogen atom substituent on the triazole. Density functional theory (DFT) analysis showed that the structure-property relation for series 1c-X(H) could be defined by using the calculated dihedral angles, dipole moment, polarisability, and molecular electrostatic potential as parameters. Notably, introducing halogen atoms at the 5-position of 1,2,3-triazole can greatly broaden the mesogenic temperature range, ascribed to the weak intermolecular forces and large dipole moment in this class of compounds.