Substituent Effect on Intramolecular Charge Transfer of Symmetric Methoxy-Substituted Bi-1,3,4-oxadiazole Derivatives

Intramolecular charge-transfer characteristics of a series symmetric methoxy -substituted bi-1,3,4-oxadiazole derivatives with various substituted positions and quantities have been studied with a combination of experimental techniques and theoretical calculations to investigate the substituent effect. Different degrees of fluorescence red shift in polar solvents are observed in these compounds. The meta-substituted molecule (BOXD-m-OCH₃) exhibits a larger red shift (82 nm) than the other two monosubstituted molecules, BOXD-o-OCH₃ (40 nm) and BOXD-p-OCH₃ (37 nm); the polysubstituted molecules BOXD-D1 and BOXD-T1 show 80 and 104 nm red shifts, respectively, which are obviously larger than the monosubstituted molecules. The changes of molecular dipole moment between the ground state and charge transfer (CT) excited state are calculated to be on the same order with the degree of red shift (7.56 D in BOXD-o-OCH₃, 12.07 D in BOXD-m-OCH₃, 7.38 D in BOXD-p-OCH₃, 14.79 D in BOXD-D1, and 16.80 D in BOXD-T1). Theoretical calculations at the density functional theory level reveal that the first singlet excited state of all of these compounds shows both π-π* and CT characteristics and the charge has been proven to transfer from the terminal methoxy phenyl group to the central bioxadiazole group. The analysis of charge transfer based on electron density shows that the greater the amount substituent, the more charge would be involved in the intramolecular charge transfer. In addition, the negative barycenter has a tendency to locate close to the methoxy substituent, which would cause the difference in the charge-transferred distance. The transferred charge and CT distance work jointly and finally lead to differences in dipole moment variation. These findings could provide very good guidance for the design of molecules with intramolecular charge-transfer characteristics.