Effect of nitrogen substitution and π-conjugation on photophysical properties and excited state intramolecular proton transfer reactions of methyl salicylate derivatives: Theoretical investigation

Effects of nitrogen substitution and pi-conjugation on the strength of intramolecular hydrogen bond (H-bond), photophysical properties, and excited-state intramolecular proton transfer (ESIPT) reactions of MS derivatives have been systematically investigated using DFT and TD-DFT methods at the B3LYP/TZVP level. The H-bond strength of all studied compounds becomes stronger in the S-1 than in the S-0. This result is confirmed by vibrational IR spectra and topological analysis. The H-bond strength of nitrogen substituted compounds is lower than those of compounds with the pi-extended conjugation. For photophysical properties, the HOMO-LUMO energy gaps in normal and tautomer forms of nitrogen substituted compounds are insignificantly increased, whereas those of compounds with the pi-extended conjugation are notably decreased. The slight widening of the energy gaps results in the slight blue-shifts of the absorption and emission peaks of nitrogen substituted compounds compared to those of MS, whereas the significant narrowing of the energy gaps makes the incredible red-shifts of both electronic spectra of compounds with the pi-extended conjugation. Thus, remarkably large Stokes shifts of about 1000 cm(-1) are observed for compounds with the pi-extended conjugation (M3HN, M2HQC, and M3HQC). In addition, potential energy curves along the PT reaction indicate that the ESIPT of all studied compounds should occurred due to low PT barriers in the exothermic reaction. Dynamic simulations reveal that the ESIPT reactions of studied compounds are observed in an ultrafast time scale (38-130 fs). Most of them have remarkably high PT probabilities (0.84-1.00) except M3HP (0.56), which are in good agreement with the results of H-bond strength. Therefore, the photophysical properties of studied compounds can be significantly tuned by controlling the effect of pi-conjugation.