Hannover 2020 – scientific programme
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MO: Fachverband Molekülphysik
MO 9: Femtosecond Spectroscopy I
MO 9.5: Talk
Wednesday, March 11, 2020, 12:00–12:15, f102
Population of long-lived excited states in 7- and 9-methylpurine probed by femtosecond transient vibrational absorption spectroscopy — •Rebecca Holtmann, Amke Nimmrich, Hendrik Boehnke, and Friedrich Temps — Institute of Physical Chemistry, Christian-Albrechts-University Kiel, Germany
We investigated the photo-induced dynamics of 7- and 9-methylpurine (7-MePur, 9-MePur) as model systems for purine-based DNA building blocks. The ensuing processes upon photoexcitation at λ = 267 nm were probed by structure-sensitive time-resolved vibrational absorption spectroscopy. After excitation to the S2(ππ*) state, the excited-state population is transferred to the S1(nπ*) state by ultrafast internal conversion within τ1 ≈ 300 fs. Aided by ab initio calculations, our results reveal two parallel deactivation pathways from the 1nπ* state for both isomers: Recovery of the ground state associated with vibrational cooling within τ2 ≈ 10 ps and intersystem crossing to a 3ππ* state with τ3 = 250 ps for 7-MePur and τ3 = 480 ps for 9-MePur, respectively. Exclusively for 9-MePur, an additional deactivation pathway on the nanosecond time scale from the initially populated S2(ππ*) state has been identified. In summary, canonical nucleobases like adenine and guanine typically feature excited-state lifetimes of < 1 ps. Consequently, the reported long-lived excited states involved in the electronic deactivation after photoexcitation in 7-MePur and 9-MePur highlight the crucial effects of structural modifications on the dynamics of the purine related nucleobases.