Phase crossover in columnar tris-(1,3,4-oxadiazoles) with pendant quinoxalines

Four new series of bis-(1,3,4-oxadiazoles) 1a–c and tris-(1,3,4-oxadiazoles) 1d containing peripheral quinoxalines were prepared and their mesomorphic behavior was reported. Except for compound 1c (n=12), all other compounds 1a–d formed columnar phases which were confirmed by powder X-ray diffractometer (XRD). Compounds 1d have the highest clearing temperatures and the widest temperature range of columnar phases than those of other compounds 1a–b. Compounds 1d exhibited a phase crossover between the rectangular and hexagonal columnar phases. A value of N_cell=2.85 and 2.16 Å within a column slice of 9.0 Å thick was obtained for compound 1a (n=12) and 1d (n=12), indicating that two molecule was correlated within columns in columnar phases. All compounds 1a–d (n=12) showed good stabilities at temperature below T_dec=412.5–420.8 °C on thermogravimetric analysis. The PL spectra of all compounds 1a–d (n=12) showed one intense peak at λ_max=517–520 nm, and these photoluminescent emissions originated from peripheral quinoxaline moiety.