Solvation effect on the excited-state intramolecular proton transfer mechanism of 1-morpholinylmethyl-2-naphthol

Abstract

In the present work, the solvent dependent excited-state intramolecular proton transfer (ESIPT) dynamics of 1-morpholinylmethyl-2-naphthol (MN) in n-hexane and acetonitrile have been investigated, based on the time-dependent density functional theory, as well as IEFPCM model. The theoretical results obtained, including the primary bond lengths, bond angles, and IR vibrational spectra, predict the possibility of ESIPT in the S1 state for MN chromophore in n-hexane and acetonitrile. In the polar solvent, all the calculated results are consistent with the experimentally observed phenomena and the dual fluorescence emission mechanism is well explained. Furthermore, our theoretical study provides the understanding for the undetected fluorescence peak at about 389nm of the MN chromophore in n-hexane, as the fluorescence emission from keto-MN product of post-ESIPT process. Upon optical excitation, the normal enol form is excited to the first excited state, following which the proton may be transferred along the route (O17–H⋯N) with a relative large energy barrier (1.93kcal/mol) and generate the keto tautomer. However, the low energy barrier (0.26kcal/mol) for proton back-transfer facilitates the conformation transition from the keto to enol form.