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

Femtosecond transient IR absorption spectroscopy was used to probe the decay mechanism of electronically excited thymine (a naturally occurring pyrimidine base in DNA) dissolved in an ionic liquid ([Bmim][ BF4 ]) or CD3CN after the absorption of UV light (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) (ISC did not relax to the ground electronic state within the experimental period (1 ns)). The low ISC yield for thymine in an ionic liquid was correlated with molecular properties of the solvent. This observation is significant because the ISC to 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 after UV absorption. This finding may shed light on the photostability of DNA in ionic liquids.