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.