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 t₁ < 150 fs). The spectroscopic evidence points to a mechanism involving intersystem crossing from S₁ to the ³nπ* / ³ππ* T₂ state (t₂ = 400 ± 100 fs) and internal conversion to T₁ (with time constant for growth t₃ = 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.