Photoexcitation Dynamics of Thymine in Acetonitrile and an Ionic Liquid Probed by Time‐resolved Infrared Spectroscopy

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Femtosecond transientIRabsorption spectroscopy was used to probe the decay mechanism of electronically excited thymine (a naturally occurring pyrimidine base inDNA) dissolved in an ionic liquid ([Bmim][BF4]) orCD3CNafter the absorption ofUVlight (267 nm). In both solvents, an absorption band grew on a picosecond timescale, along with decaying bleach and evolving red‐shifted absorption signals. A population analysis of the observed kinetic data suggested that most of the photoexcited thymine underwent a sub‐picosecond non‐radiative relaxation to the vibrationally hot ground electronic state. About 4% (16%) of the excited thymine in the ionic liquid (CD3CN) relaxed to an intermediate electronic state, which relaxed into a low‐lying triplet state by intersystem crossing (ISC) (ISCdid not relax to the ground electronic state within the experimental period (1 ns)). The lowISCyield for thymine in an ionic liquid was correlated with molecular properties of the solvent. This observation is significant because theISCto triplet state transition for excited thymine has been considered as a precursor to cyclobutane–pyrimidine dimer formation, which led to functional damage of the base afterUVabsorption. This finding may shed light on the photostability ofDNAin ionic liquids.