Solvent effect on ESIPT process of N-(8-Quinolyl) salicylaldimine: A DFT/TD-DFT calculation

The investigations of the solvent effect on ESIPT process of N-(8-Quinolyl) salicylaldimine (QS) are carried out using DFT and TD-DFT methods. We choice TOL, THF, ACE and DMSO solvents to simulate the experimental solvent conditions. The normal and tautomeric structures of the QS in the S0 and S1 states are optimized by means of DFT and TD-DFT method in four different kinds of solvents. Through the analysis of the critical geometrical parameters related to hydrogen bond interactions, it could be found that QS molecule preform to be planar or nearly planar in the S1 state, and more importantly, the excited-state hydrogen bond is strengthened. By comparing IR vibrational spectra, RDG isosurfaces and scatter graphs in the four solvent conditions, we found that the hydrogen bond is strengthen after photoexcitation, and weakened with the increase of the dielectric constant of the solvents. The calculated absorption and fluorescence spectra are basically agreement with existing evidence from previous experiment. The energy barriers of ground and excited states of QS molecule under the different solvent conditions have been analyzed by established potential energy curves (PECs) in the S0 and S1 states, and the corresponding energy barriers are increased with the effect of change from TOL to DMSO solvents. Therefore, with the increasing dielectric constant of solvents, the ESIPT become favored.