Theoretical and experimental study on intramolecular charge-transfer in symmetric bi-1,3,4-oxadiazole derivatives

Abstract

Abstract The photophysical properties of 2,2′-bis-(4-alkoxyphenyl)-bi-1,3,4-oxadiazole (BOXD- n ), have been studied by a combination of spectroscopic techniques and theoretical calculations. Interestingly, strong fluorescence red-shift in polar solvents was observed in this highly symmetric molecule, which indicates an efficient charge transfer (CT) occurred in the excited state. The change of molecular dipole moment between the ground state and CT excited state was calculated to be 9.9 D. Theoretical calculations at Density Functional Theory level revealed that the first singlet excited state of BOXD- n shows both π – π * and CT characters. Clear evidence for charge transfer from alkoxy benzene to the central bi-1,3,4-oxadiazole group can be observed by analysing the atomic charge and electron density change, though it is not significant. Since BOXD-1 retained a planar conformation and became more quinoid-like in the excited state, a planar intramolecular charge transfer is assigned for it. By further comparing these results with the half molecule terminated by CH 3 group (2-tetradecyloxyphenyl, 5-methyl-1,3,4-oxadiazole), it could be concluded that intramolecular charge transfer property was enhanced in this DAAD arrangement of BOXD- n .