title: Triplet state formation and quenching dynamics of 2-mercaptobenzothiazole in solution.
abstract: 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 mixed3ππ*/3nπ* character and is populated with a quantum yield of 0.58 ± 0.01 from the photo-excited1ππ* S2state in methanol solution via rapid internal conversion to the1nπ* S1state (with time constant τ1< 150 fs). The spectroscopic evidence points to a mechanism involving intersystem crossing from S1to the3nπ*/3ππ* T2state (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→ S0internal conversion. In methanol solution, the T1state 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 the3[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.
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK. A.Orr-Ewing@bristol.ac.uk.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Koyama
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK. A.Orr-Ewing@bristol.ac.uk.
authors.forename: Andrew J
authors.identifier: http://orcid.org/0000-0001-5551-9609
authors.initials: AJ
authors.lastname: Orr-Ewing