Theoretical study of the mechanism of the solvent dependency of ESIPT in HBT.

2-(2′-Hydroxyphenyl)-benzothiazole (HBT) has been widely studied for use as a system for excited-state intramolecular proton transfer. However, the mechanism underlying the solvent dependency of HBT fluorescence spectra remains unclear. In this study, the HBT photochemical process in the S1 state was analysed using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The excited-state intramolecular proton transfer in the enol form of HBT was found to depend on the hydrogen-bond acceptability of the solvent. The twisting of the keto form of HBT is determined by whether HBT acts as a hydrogen-bond acceptor or donor. A specific stacking structure of the enol form of HBT was found to decrease the S1 → S0 transition energy, which corresponds to the experimental fluorescence spectra in a DMSO/H2O solution mixture.