Institut für Physikalische und Theoretische Chemie, Goethe-Universität Frankfurt am Main, Max-von-Laue-Strasse 7, 60438 Frankfurt am Main, Germany.
Chemphyschem. 2011 Jul 11;12(10):2024-30. doi: 10.1002/cphc.201000985. Epub 2011 Apr 5.
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.
气态胸腺嘧啶和 1-甲基胸腺嘧啶的光致激发态弛豫动力学通过飞秒和纳秒泵浦探测电离光谱在亚皮秒到微秒时间尺度上进行了研究。使用 267nm 激发和随后的 800nm 多光子电离，观察到胸腺嘧啶的三倍指数衰减，时间常数为 80±40fs、4.8±2ps 和 280±30ns，而 1-甲基胸腺嘧啶的时间常数为 70±40fs、3.4±1.1ps 和 310±30ns。此外，首次报道了孤立的 1-甲基胸腺嘧啶的长寿命“暗”电子态中 NH 伸缩区域的振动光谱。该光谱与暗态电离率对激光强度的依赖性相结合，允许将 1-甲基胸腺嘧啶的暗态分配给酮式互变异构体的最低三重态，从而排除烯醇互变异构体以及 nπ*激发态和热电子基态。胸腺嘧啶和 1-甲基胸腺嘧啶非常相似的激发态弛豫动力学证明了孤立胸腺嘧啶的长寿命暗态也是三重态的结论。
