Excited state intramolecular proton transfer mechanism of a benzothiazole derivative fluorescent probe: Spontaneous ESIPT process

Abstract We have investigated the fluorescent mechanism of 4-(benzo[d]thiazol-2-yl)-2-(1,3-dithian-2-yl) phenol (HBT-Hg) and the excited state intramolecular proton transfer processes (ESIPT) for hydroxyphenyl benzothiazole-based fluorescent probe (HBH) by DFT/TDDFT method. After optimizing the geometries of HBH-Enol and HBH-Keto in the ground (S0) and first excited (S1) states at B3LYP/TZVP/IEFPCM computation level, we demonstrated that the intramolecular hydrogen bond was strengthened in S1 state and would facilitate the ESIPT process of HBH, which were further verified by the electron spectroscopy and the frontier molecular orbital results. In addition, the spectral results also show that the isomerism form (HBH-Keto) plays the role in the fluorescence emission. The potential energy curve analyses according to variational O-H coordinate also support proton transfer process because of the ultra-low potential energy barriers for HBH in S1 state. Thus, the intramolecular proton transfer is more likely to occur in the excited state and it turns out to be a spontaneous process.