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 phenyl)-1,3,4-oxadiazole ( 3; n = 1) was determined by means of X-ray structural analysis. It crystallizes in the monoclinic space group p 2 1 / 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.