The influence of the benzimidazole-based nematic liquid crystals containing ethynyl and difluoro-substitution on mesomorphic and birefringence properties

A series of benzimidazole-based liquid crystal compounds containing laterally difluoro-substituted and ethynyl linking groups, namely, 2-(4-(2-(2,3-difluoro-4-(alkoxy) phenyl)ethynyl)phenyl)-1-methyl-1 H-benzimidazolem derivatives (nPF(2,3)EPMx), are synthesised and investigated for their mesomorphic properties. These compounds mainly display enantiotropic nematic mesophases in the ranges of 32.0-44.5 degrees C (heating process) and 50.0-64.0 degrees C (cooling process). 10PF(2,3)EPMx exhibits lower clearing points and broader nematic phase ranges than nonfluorinated reference compound 10PEPMx, because of their slightly increased dipole moment. nPF(2,3)EPMx displays much larger theoretical birefringence (0.43-0.55) and experimental birefringence (0.42-0.53) than the common tolane-based liquid crystals, which is ascribed to its large pi-conjugated molecule composed of benzene core, ethynyl linking group and benzimidazole terminated unit. The theory results of three-dimensional molecular conformations with dipole moment (mu), ratio of length to width (L/W), polarisability anisotropy (Delta alpha) and average polarisability (Delta alpha) calculated with density functional theory are also helpful in explaining the experimental results.