Time-dependent density functional theory study on the excited-state intramolecular proton transfer in salicylaldehyde

Time-dependent density functional theory method was performed to investigate the excited state intramolecular hydrogen bond dynamics of salicylaldehyde (SA). The geometric structures and IR spectra in the ground state S0 state and the excited state S1 state of SA are calculated using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) methods, respectively. In addition, the absorption and fluorescence peaks are also calculated using TDDFT methods. It is noted that the calculated large Stokes shift is in good agreement with the experimental results. Furthermore, our results have demonstrated that the excited state intramolecular proton transfer (ESIPT) process happens upon photoexcitation, which are distinct monitored by the formation and disappearance of the characteristic peaks of IR spectra involved in the formation of hydrogen bonds in different states and in the potential energy curves. We find that the hydrogen bonded quasi-aromatic chelating ring in the excited state becomes smaller which can facilitate the ESIPT process. The results presented here suggest that the ESIPT process of the SA molecule in the excited state can be attributed to the electronegativity change of O1 induced by excitation.