title: On the nature of the long-lived "dark" state of isolated 1-methylthymine.
abstract: The photoinduced excited-state relaxation dynamics of gaseous thymine and 1-methylthymine are studied by both femtosecond and nanosecond pump-probe ionization spectroscopy on the sub-picosecond to microsecond timescale. A threefold exponential decay is observed with time constants of 80±40 fs, 4.8±2 ps, and 280±30 ns for thymine and 70±40 fs, 3.4±1.1 ps, and 310±30 ns for 1-methylthymine using a 267 nm excitation and subsequent 800 nm multiphoton ionization. In addition, a vibrational spectrum in the NH stretch region of the long-lived "dark" electronic state of isolated 1-methylthymine is reported for the first time. This spectrum, in combination with the dependence of the dark-state ionization rate on the laser intensity, allows assignment of the dark state of 1-methylthymine to the lowest triplet state of the keto tautomer, thus excluding enol tautomers as well as the nπ* excited state and a hot electronic ground state from the consideration. Very similar excited-state relaxation dynamics of thymine and 1-methylthymine justify the conclusion that the long-lived dark state of isolated thymine is also of triplet nature.
authors.affiliation: Institut für Physikalische und Theoretische Chemie, Goethe-Universität Frankfurt am Main, Max-von-Laue-Strasse 7, 60438 Frankfurt am Main, Germany.
authors.forename: Maksim
authors.initials: M
authors.lastname: Kunitski
authors.forename: Yevgeniy
authors.initials: Y
authors.lastname: Nosenko
authors.forename: Bernhard
authors.initials: B
authors.lastname: Brutschy
mesh_terms: D013055:Spectrophotometry, Infrared; D013941:Thymine / Q000031:analogs & derivatives* / Q000737:chemistry; D013997:Time Factors
chemical_list: C030630:1-methylthymine; D013941:Thymine