Triplet state formation and quenching dynamics of 2-mercaptobenzothiazole in solution.

The photochemical dynamics of the thione 2-mercaptobenzothiazole (MBT) initiated by absorption of 330 nm ultraviolet light are investigated by ultrafast transient absorption spectroscopy. The lowest energy triplet state (T1) has mixed 3ππ*/3nπ* character and is populated with a quantum yield of 0.58 ± 0.01 from the photo-excited 1ππ* S2 state in methanol solution via rapid internal conversion to the 1nπ* S1 state (with time constant τ1 < 150 fs). The spectroscopic evidence points to a mechanism involving intersystem crossing from S1 to the 3nπ*/3ππ* T2 state (τ2 = 400 ± 100 fs) and internal conversion to T1 (with time constant for growth τ3 = 6.1 ± 0.4 ps). The remainder of the photoexcited molecules return to the ground state by S1 → S0 internal conversion. In methanol solution, the T1 state is long-lived but when the solvent is changed to styrene, triplet quenching is observed with a time constant of 107 ± 8 ps and assigned to the adduct-mediated energy transfer process MBT (T1) + styrene (S0) → 3[MBT-styrene] → MBT (S0) + Styrene (T1). Transient vibrational absorption spectroscopy observes the 3[MBT-styrene] biradical intermediate and determines its lifetime to be 700 ± 80 ps. Computational studies identify the mechanistic pathway for triplet quenching, which involves a curve crossing between two triplet states of the MBT-styrene adduct. The quenching process occurs with high efficiency, and no long-lived isomers of the initial adduct are observed.