Shining rings: The effect of the rigid core and benzazole heterocycles on the properties of luminescent calamitic liquid crystals

Abstract

We report the molecular designing of non-symmetrical luminescent liquid crystals based on the 1,2,3-benzotriazole (BTZ), 2,1,3-benzoxadiazole (BOX) and 2,1,3-benzothiadiazole (BTD) heterocycles. The compounds showed stable nematic and smectic mesophases in a wide range of temperatures (between 70 and 170 °C). Translational order parameter investigations based on X-ray diffraction experiments demonstrated high ordered layers, closely dependent of the molecular structure and the smectic organization. The molecules exhibited intense photoluminescence under UV–vis light excitation in solutions. The emission spectrum for BTZ revealed a maximum in the blue region (447 nm), while BOX and BTD emit in the yellow one (around 550 nm). The fluorescence quantum yields of the BTZ-based derivatives solution are up to 64%, significantly higher than BOX and BTD. Moreover, in solid films, the efficiencies remained in the same range of solution. DFT calculations indicate dihedral angles of 24-30° between the benzazole-phenyl rings, which may suppress π-stacking and consequently reduce fluorescence quenching in the solid phase. This work contributes to general knowledge, where asymmetric cores improve mesophase stabilization and photoluminescence preservation in the solid state compared to their symmetric pairs.