<p></p><p> Damage to RNA from ultraviolet radiation
induce chemical modifications to the nucleobases. Unraveling the excited states
involved in these reactions is essential, but investigations aimed at
understanding the electronic-energy relaxation pathways of the RNA nucleotide
uridine 5’-monophosphate (UMP) have not received enough attention. In this
Letter, the excited-state dynamics of UMP is investigated in aqueous solution.
Excitation at 267 nm results in a trifurcation event that leads to the
simultaneous population of the vibrationally-excited ground state, a longlived <sup>1</sup>n<sub>O</sub>π*
state, and a receiver triplet state within 200 fs. The receiver state
internally convert to the long-lived <sup>3</sup>ππ* state in an ultrafast time
scale. The results elucidate the electronic relaxation pathways and clarify
earlier transient absorption experiments performed for uracil derivatives in
solution. This mechanistic information is important because long-lived nπ* and ππ*
excited states of both singlet and triplet multiplicities are thought to lead
to the formation of harmful photoproducts.</p><p></p>