Theoretical Study of the Substituent Effect on the Electronic Structure and Spectral Properties of Six Salicylaldehyde Schiff Bases

Abstract

In the present work, the ground- and excited-state geometries for a series of salicylaldehyde hydrazones (SHs) were optimized including SH-Ph, Naph-SH-Ph, 3-OMe-SH-Ph, 4-NEt2-SH-Ph, 3-F-SH-Ph, and 3-NO2-SH-Ph at the PBE1PBE/6-31G(d,p) level, and the results agree well with the corresponding experimental data. By means of the TD-DFT method, the absorption and emission spectra were calculated based on the optimized ground-state and the excited-state geometries, respectively. It is found that the absorption and emission transition character can be altered by adjusting the electron-withdrawing and electron-donating groups or expanding the aromatic conjugation on salicylaldehyde. These results indicate that both absorption and emission properties are governed by the HOMO–LUMO gap (∆H–L) which is usually considered as the basis of the experimental design. In addition, the charge transport quality has been estimated approximately by the calculated reorganization energy (λ). The calculated results also show that the species of the substitute groups and the expanded aromatic ring affect the charge transfer rate and balance. All calculations reveal that introduction of strong electron-withdrawing or electron-donating substituents on the base of the expanded aromatic rings is expected to be a useful for the luminescent material design.