Exploring the excited state proton transfer mechanisms of flavonoid probe in three representative solvents

Abstract

The role of intermolecular hydrogen bond is often ignored in the study of the solvent effect on the excited state proton transfer (ESPT) processes. The ESPT mechanisms of 2-(4-(dimethylamino)phenyl)-3‑hydroxy-6,7-dimethoxy-4H-chromen-4-one (HOF) in three representative solvents, which were toluene, dimethyl sulfoxide (DMSO) and ethanol (EtOH), were investigated through time-dependent density functional theory (TD-DFT) method, and the effect of intermolecular hydrogen bond on the ESPT was fully considered in this paper. The results show that the enhancement of intra- and inter-molecular hydrogen bonds is the key factor of proton transfer process. The change of electron density distribution is also more conducive to the ESPT processes. Due to the existence of intermolecular hydrogen bond in HOF-DMSO complex, the energy barrier of ESPT in DMSO is higher than that in toluene. In EtOH solvent, the excited state double proton transfer (ESDPT) reaction can occur between HOF and EtOH along two different reaction channels with the assistance of dual intermolecular hydrogen bonds. Although the ESPT modes in DMSO and EtOH solvents are different, the energy barriers are basically the same. The energy barriers of ESPT processes in two polar solvents increase obviously due to intermolecular hydrogen bond. Moreover, in polar solvents, a relatively large red-shift was observed in the fluorescence spectra.