Mesomorphic properties improved via lateral fluorine substituent on benzoxazole-terminated mesogenic compounds

ABSTRACT

Here, laterally monofluorinated heterocyclic mesogenic compounds, 2-[4-[2-[4-alkoxy-phenyl]-ethynyl]-3-fluorophenyl]-benzoxazole derivatives (nPEFPBx) with hydrogen, methyl and nitro terminal substituents (coded as nPEFPBH, nPEFPBM and nPEFPBN, respectively) at 5-position of benzoxazole unit, are synthesised and characterised. They display enantiotropic nematic mesophase with mesophase ranges of 22–30°C and 32–39°C on heating and cooling for nPEFPBH, 67–92°C and 87–115°C for nPEFPBM, 31–84°C and 42–51°C for nPEFPBN (n < 7), respectively, which is better than nonfluorinated and laterally difluorinated analogous . Improved mesophase property is ascribed to reduced dipole moment resulting from introduction of lateral fluorine atom in opposite direction of polar benzoxazole unit. This indicates that monofluorine substituent contributes to improve nematic mesophase for benzoxazole-terminated mesogenic compounds. In addition, the compounds nPEFPBx have high birefringence of 0.513–0.650 because of large π-conjugated mesogenic core composed of benzene, ethynyl and benzoxazole groups, which suggests a possible application in liquid crystal mixture to enhance birefringence. Meanwhile, except nPEFPBN, the compounds nPEFPBx show intense photoluminescence emission at 406–407 nm in methylene chloride solution with exciting them at absorption maxima.

Graphic Abstract

KEYWORDS:

• Laterally monofluorinated mesogenic compound
• liquid crystal
• birefringence
• ethynyl group
• nematic mesophase

Acknowledgments

The authors thank for the financial support by National Science Foundation Committee of China (51773112, 51873100 and 21673134), Program for Science & Technology Innovation Team of Shaanxi Province (2018TD-030) and the Fundamental Research Funds for the Central Universities (GK201801001 and GK201703030).

Disclosure Statement

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

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Funding

This work was supported by National Science Foundation Committee of China [51773112, 51873100, 21673134]; the Program for Science & Technology Innovation Team of Shaanxi Province [2018TD650030]; the Fundamental Research Funds for the Central Universities [GK201801001 and GK201703030].