Polar effect in columnar unsymmetric 1,3,4-oxa(thia)diazoles

Three new series of heterocyclic 1,3,4-oxa(thia)diazoles 1a–c were prepared and their mesomorphic behavior studied. All compounds 1a exhibited monotropic columnar phases, which were confirmed by powder XRD diffractometer. Compounds 1b were not mesogenic. The formation of columnar phases was sensitive to the substituent groups attached on the phenyl moiety; those with electron-donating groups (R=H, OCH₃) were not liquid crystals, in contrast, other derivatives with electron-withdrawing groups (R=F, NO₂) formed enantiotropic columnar phases. A N_cell and R_ar value equal to 4.73 and 20.20 Å² of compound 1a (n=16) within a slice of 9.0 Å thick were obtained, indicating that two molecules was correlated to give a supplemental more disc-like molecule within columns. This is the first example, showing that the formation of columnar phases could be controlled by polar groups. The PL spectra of two compounds 1a–c showed one intense peak at λ_max=505–511 nm, and these photoluminescent emissions originated from quinoxaline moiety.