Synthesis, crystal structures, mesomorphic and photo-luminescent properties of 1,3,4-thia(oxa)diazole-based compounds with a terminal methoxy or methylthio group

A series of aromatically 2,5-disubstituted 1,3,4-thia(oxa)diazoles with a terminal methoxy or methylthio group were synthesized and characterized by means of 1H NMR, 13C NMR, MS and elemental analysis. The X-ray crystal structures of compounds 1a and 3b revealed that both of them adopted layered arrangement without short intermolecular interactions. The liquid crystal properties have been investigated by means of polarizing optical microscopy (POM), differential scanning calorimetry (DSC), variable temperature X-ray diffraction (VXRD) and thermogravimetric analysis (TGA). All compounds (except 2a and 2b) exhibited various mesophases (nematic, smectic C and/or smectic A) with wide temperature ranges and good thermal stability. In CH2Cl2 solution, all these compounds displayed a room temperature emission with λmax at 371–425 nm and quantum yields of 0.25–0.83. The effect of the sulfur and oxygen atoms on mesomorphic and photo-luminescent properties was discussed according to the positions in the central heterocyclic rings and the terminal groups.