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 (T-1) has mixed (3)pi pi*/(3)n pi* character and is populated with a quantum yield of 0.58 +/- 0.01 from the photo-excited (1)pi pi* S-2 state in methanol solution via rapid internal conversion to the (1)n pi* S-1 state (with time constant tau(1) < 150 fs). The spectroscopic evidence points to a mechanism involving intersystem crossing from S-1 to the (3)n pi*/(3)pi pi* T-2 state (tau(2) = 400 +/- 100 fs) and internal conversion to T-1 (with time constant for growth tau(3) = 6.1 +/- 0.4 ps). The remainder of the photoexcited molecules return to the ground state by S-1 -> S-0 internal conversion. In methanol solution, the T-1 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 (T-1) + styrene (S-0) -> (3)[MBT-styrene] -> MBT (S-0) + Styrene (T-1). 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.