Synthesis and optical properties of novel pyrido[1,2-a]benzimidazole-containing 1,3,4-oxadiazole derivatives

A series of novel substituted 1,3,4-oxadiazole derivatives containing pyrido1,2-a]benzimidazole moiety were synthesized and characterized using FTIR, ¹H NMR, ¹³C NMR, and HRMS. An efficient tandem reaction was employed as a key step in constructing the pyrido1,2-a]benzimidazole moiety under very mild condition. The structure of compound 4a was established by X-ray crystallography. The UV-vis absorption and fluorescence spectral characteristics of these compounds were investigated in several solvents. Compounds 4a-i display similar absorptions, with absorption peaks ranging from 330 to 339 nm in acetonitrile, while the absorption maxima of compound 4j bearing a diphenylamino group on the benzene ring is red-shifted distinctly to 377 nm due to the strong electron-donating property of its substituent and extended π-conjugated system. All these target heterocyclic compounds present blue-green emissions (461-487 nm) in dilute solutions and show high quantum yields of fluorescence (Ф_PL=0.65−0.99) in dichloromethane.