Dynamics of Photoexcited 5-Bromouracil and 5-Bromo-2'-Deoxyuridine Studied by Extreme Ultraviolet Time-resolved Photoelectron Spectroscopy in Liquid Flat Jets

The UV-induced photo-relaxation dynamics of 5-bromouracil (BrU) and 5-bromo-2’-deoxyuridine (BrUrd) in aqueous solution were investigated using femtosecond time-resolved photoelectron spectroscopy with an extreme ultraviolet (XUV) probe in a flat liquid jet. Upon excitation to the 1ππ* state by 4.66 eV UV photons, both molecules exhibited rapid relaxation into lower-lying electronic states followed by decay to the S0 ground state. By employing a 21.7 eV XUV probe pulse, we were able to differentiate the evolution of the excited state population from the initially excited 1ππ* state to an intermediate electronic state, which eventually relaxed into the S0 ground state within a lifetime of ~200 fs. Computational results identify this intermediate as the 1πσ* excited state, accessed by a 1ππ*/1πσ* conical intersection. In contrast to thymine, formation of neither the 1nπ* state nor a long-lived triplet state was observed. Our findings suggest that although the 1πσ* state is largely repulsive, relatively little C-Br bond fission occurs for BrU and BrUrd in aqueous solution owing to solvent caging effects. Hence, their role as radiosensitizers upon direct UV photoabsorption may be limited.