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

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journal contribution
posted on 2020-04-15, 03:24 authored by Mengting Zhang, Yiming Sun, Shenghua Du, Diao Yuan, Pei Chen, Guoqing Liu, Jiazhen Dang, Xinbing Chen, Zhongwei An

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.

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].

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