On the nature of the long-lived "dark" state of isolated 1-methylthymine

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.