Time resolved photoelectron spectroscopy of iodide-4-thiouracil cluster: the pi/pi star state as a doorway for electron attachment

The photophysics of thiobases — nucleobases in which one or more oxygen atoms are replaced with sulfur atoms — have been shown to vary greatly depending on the location of sulfonation. Not only are direct dynamics of the neutral thiobase impacted, but also the dynamics of excess electron accommodation. In this work, time resolved photoelectron spectroscopy (TRPES) is used to measure binary anionic clusters of iodide and 4-thiouracil, Iˉ4TU. We investigate charge transfer dynamics driven by excitation at 3.88 eV, corresponding to the lowest ππ* transition of the thiouracil, and 4.16 eV, near the cluster vertical detachment energy. The photoexcited state dynamics are probed by photodetachment with 1.55 eV and 3.146 eV pulses. Excitation at 3.88 eV leads to signal from the valence-bound ion only, indicating a charge accommodation mechanism that does not utilize the dipole-bound anion as an intermediate. Excitation at 4.16 eV rapidly gives rise to dipole-bound and valence-bound ion signals, with a second rise in valence-bound signal corresponding to the decay of the dipole-bound signal. The dynamics associated with the low energy ππ* excitation of 4-thiouracil provide clear experimental proof for the importance of localized excitation and electron backfilling in halide-nucleobase clusters.