Synthesis, mesomorphic and fluorescent properties of 1,3,4-oxadiazoles/thiadiazoles with a terminal 3-fluoro-4-cyanophenyl group

A series of new liquid crystalline 1,3,4-oxa/thiadiazoles with a terminal 3-fluoro-4-cyanophenyloxy group were synthesized, and their structures were characterized by nuclear magnetic resonance (NMR) and electrospray ionization mass spectrometry (ESI-MS). The liquid crystal properties were studied by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). All of the 1,3,4-oxadizoles exhibited only Nematic mesophase, while the 1,3,4-thiadiazole analogues displayed Smectic A (SmA) and Smectic C (SmC) over a wide temperature range. The fluorescent properties were investigated using ultraviolet–visible spectrophotometer (UV–Vis) and fluorescence spectrometer. The solutions of all final compounds in CH2Cl2 individually displayed one absorption band with λabs-max values at 319–340 nm, and exhibited an intense emission with λem-max values at 432–448 nm, and the photoluminescence quantum yields reached 0.52–0.66. The effects of the heterocyclic structural units, the terminal alkoxy chain length and the lateral fluorine atom on liquid crystalline and fluorescent properties were comparatively discussed.