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Synthesis and properties of benzoxazole-terminated mesogenic compounds containing tolane with high birefringence and large dielectric anisotropy

ABSTRACT New benzoxazole-terminated mesogenic compounds containing tolane moiety with high birefringence and large dielectric anisotropy, namely, 2-[4-[2-[4-alkoxyphenyl]ethynyl]-2,6-difluoro-phenyl]-benzoxazole derivatives (nPEDFPBx), were synthesised and investigated. Among them, the ones bearing methyl and nitro substituents at 5-position of benzoxazole moiety exhibit stable mesophase ranges of 30–71°C and 55–81°C during heating and cooling processes, respectively, while the non-substituted compounds exhibit no mesophase except the ones with long terminal alkoxy chain (n = 10, 12). Birefringence of nPEDFPBx is obtained from DFT theoretical calculation (0.446–0.583) and experimental measurement (0.395–0.499), respectively, where high birefringence is ascribed to their large π-conjugated molecule composed of benzene ring, acetylene linkage and benzoxazole moiety. Meanwhile, it is found that nPEDFPBx displays large dielectric anisotropy because of their high dipole moment. As an example, the compound 5PEDFPBM is doped into the liquid crystalline mixture as an additive, the properties of the resulting mixture, such as birefringence, dielectric anisotropy, clearing point and response time, are improved obviously, indicating high potential application for liquid crystalline mixture. GRAPHICAL ABSTRACT