Absorption, fluorescence, Raman spectroscopic and density functional theoretical studies on the singlet and triplet excited state decay of 3-amino-5-mercapto-1,2,4-triazole.

The excited state decay process of N-heterocyclic compounds is attracting increasing attention due to their fundamental applications in pharmaceutical and biological sciences. In this study, 3-amino-5-mercapto-1,2,4-triazole (AMT) was investigated in solid, protic, and aprotic solvents using vibrational and electronic spectroscopies combined with density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. The steady absorption and resonance Raman spectra indicated that the AMT structure was quite sensitive to the polarity and proton of the solvent, and the pH environments. The intermolecular hydrogen bonding may contribute significantly to the decay channels of the singlet excited S(ππ*) state process. Moreover, ns-transient absorption spectroscopy detected the short-time triplet species with ∼200 ns lifetime in solvents. The DFT and TDDFT calculations interpreted the photophysical and photochemical process from the excited S(ππ*) state, including the singlet and triplet decay mechanisms.