The liquid crystalline and optical properties of chiral symmetrical dimers with two azo groups and 1,2-propanediol chiral moiety

Abstract

Six symmetric dimers were synthesised with 1,2-propanediol as the chiral center, azobenzene as the side arm, and substituents with different electron withdrawal ability groups ( C₄H₉O, C₂H₅O, CH₃O, H, F and CF₃) as the end groups. All dimers exhibit photoisomerism with E to Z isomerisation being much faster than Z to E. The end groups, which have different electron-donating and withdrawing abilities, significantly influence the liquid crystal and optical properties of the dimers. Specifically, E to Z photoisomerisation rates are faster for dimers with C₄H₉O, C₂H₅O, and CH₃O end groups than for those with H, F, and CF₃ end groups. The dimers with C₄H₉O, C₂H₅O and CH₃O end groups display a monotropic chiral nematic (N*) phase on cooling, while those with H, F, and CF₃ end groups do not exhibit liquid crystal properties. Notably, (CH₃O-Azoben-C₄)₂-PD shows obvious selective reflectivity in the visible light range on cooling. Using short alkyl chains (CH₃O) as terminal groups reduces the melting and crystallisation temperature and weakens the alternating arrangement of high and low polarisability, which is conducive to liquid crystal phase formation.