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

ABSTRACT

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°C (heating process) and 50.0–64.0°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 π-conjugated molecule composed of benzene core, ethynyl linking group and benzimidazole terminated unit. The theory results of three-dimensional molecular conformations with dipole moment (µ), ratio of length to width (L/W), polarisability anisotropy (Δα) and average polarisability (Δα) calculated with density functional theory are also helpful in explaining the experimental results.

Graphical abstract

KEYWORDS:

• Benzimidazole liquid crystals
• difluro-substitution
• nematic mesophases
• birefringence
• density functional theory

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors thank for the financial support by National Science Foundation Committee of China (52273186, 51873100 and 62105194), Program for Science & Technology Innovation Team of Shaanxi Province, China (2018TD-030), Sanqin scholars innovation teams in Shaanxi Province, China, and International Science and Technology Cooperation Project of Shaanxi Province, China (2021KW-20).