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Abstract
UV-photo-excited natural and modified DNA and RNA bases are highly important for their spectacular photophysical and photochemical properties and their applications in medicine. Thiobases are sulfur-substituted bases with a minor change to the natural base but with very different photophysical properties. The excited state dynamics of this class of molecules are the focus of this study, emphasizing the topography of their potential energy surfaces and the non-negligible wavelength dependence changes of the internal conversion and intersystem crossing lifetimes. Uracil, which is a natural base of RNA, with its three modifications, including 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil, are investigated with experimental time-resolved photoelectron spectroscopy. The underlying mechanisms of the excited state dynamics are explained with the help of high-level quantum chemistry calculations. The experiments are performed in the picosecond and also nanosecond pump-probe delay ranges. A general model of sequential exponential decay explains the relaxation dynamics of all molecules with a difference in that i) the excited state dynamics of uracil are dominated by internal conversion through singlet excited states and only show a minor population of the triplet state. ii) three modified bases show a high quantum yield of the triplet state, and depending on the degree and position of thionation, they show either efficient population and depopulation of the triplet state or slightly longer intersystem crossing lifetime for triplet state formation followed by longer than nanoseconds trapping in the lowest triplet state. A comparison between picosecond and nanosecond studies revealed that, while the deactivation model is unchanged for uracil, 4-thiouracils, and 2,4-dithiouracil at extended times and is described by the triple exponential decay model, 2-thiouracil shows an extra time constant assigned to the triplet state and requires four decay channels for describing nanosecond dynamics. This extra channel is associated with the double-minimum characteristics of the lowest triplet state in 2-thiouracil. INDEX WORDS: Excited States, Photoelectron Spectroscopy, Internal Conversion, Intersystem Crossing, Singlet, and Triplet Excited States, Uracil, 2-Thiouracil, 4-Thiouracil, 2,4-Dithiouracil.