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 (Colr) to hexagonal arrangements (Colh) 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 λmax 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.
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Wang, H. (Contributor), Lee, G. (Contributor), Sheu, H. (Contributor), Lai, C. K. (Contributor), Wang, Y. (Contributor) & Wen, C. (Contributor), The Cambridge Structural Database, 1 Jan 2006