Columnar metallomesogens derived from 1,3,4-oxadiazoles and X-ray crystal structure of dichlorobis[2,5-bis(3,4,5-trimethoxyphenyl)-1,3,4-oxadiazole] palladium(II)

Three series of palladium(II) complexes 1-3 derived from 1,3,4-oxadiazoles exhibiting mesogenic properties are prepared and characterized. It was found that the formation of mesophases was sensitive to the numbers of alkoxy side chains. The compounds 3 appended with twelve side chains exhibited columnar (Col) phases, however, all other compounds 1-3 appended with four or eight side chains formed crystalline phases. Some derivatives (n = 8, 10, 12) of the series 3 were in fact mesogenic at room temperature. Powder XRD data indicated that the formation of columnar mesophase varied from rectangular (Col_r) to hexagonal arrangements (Col_h) was observed as carbon chains length increases. The crystal and molecular structure of palladium(II) complex of 2,5-bis(3,4,5-trimethoxy pheny 1)-1,3,4-oxadiazole (3; n = 1) was determined by means of X-ray structural analysis. It crystallizes in the monoclinic space group p2₁/n, with a = 8.7736 (3) Å, b = 19.1568 (6) Å, c = 15.7834 (5) Å, β= 100.106 (1)°, and Z = 2. The geometry at palladium center is perfectly square planar, and two trans-chlorine atoms are almost perpendicularly located to the molecular plane determined by four 1,3,4-oxadiazole rings. The fluorescent properties of these compounds were also examined. All λ_max peaks of the absorption and photoluminescence spectra of compounds 4 and 6 occurred at ca. 303-312 nm and 358-391 nm, respectively; whereas the quantum yields of some compounds were relatively low.