Excited‐state proton transfer via higher excited state in 2‐mercaptobenzothiazole: Absorption, fluorescence, Raman spectroscopic study, and theoretical calculation

Excited‐state proton transfer (ESPT) photochemical reactions are attracting increasing attention because of their many applications in materials science and biology. 2‐Mercaptobenzothiazole (MBT) is investigated in solid, protic, and aprotic solvents using vibrational and electronic spectroscopies combined with density functional theory (DFT). The presence of the stable dimer conformer in the solid and solvents was confirmed by vibrational spectroscopy. Steady‐state absorption and fluorescence emission spectra of MBT in different pH solvent environments indicate that the intermolecular hydrogen bonding may reveal important insights into the ESPT mechanisms involving single and double proton transfers. With the aid of DFT and time‐dependent density functional theory calculations, we assigned the observed Raman spectra to the dimer in water and methanol solvents and carried out preliminary investigations into the effect of hydrogen bonding with the solvent on the excited‐state proton transfer process. The higher energy of thiol should attribute to the dual fluorescence and the blue‐shifted bands for thiol and red‐shifted band for thione isomer. Several decay reaction pathways are found that the higher excited S2 decays to either S2→S2/T2→T2 or S2/T2→T2/T1→T1 via triplet states. There are very close energy and structures for the minimum S2 and conical intersection (S2/S1), which can decay via two pathways: S2→S2/S1→S1 or S1/T1→T1.