Formation of Ground-State Intermediate during Electronic Relaxation of Pyrimidine Nucleobases

Ultrafast electronic relaxation of nucleobases from the 1ππ* state to the ground electronic state (S0) is crucial for the photostability of DNA and RNA. However,
it has
been suggested that electronic relaxation of pyrimidine nucleobases,
nucleosides, and nucleotides in an aqueous environment generates an
electronically excited intermediate state with a lifetime of tens
to hundreds of picoseconds with a relatively high quantum yield (QY)
of 0.2–0.5. The generation of such a long-lived excited state
seems to be inconsistent with the photostability of these molecules.
A recent extreme ultraviolet time-resolved photoelectron spectroscopy
study by Miura et al. [J. Am. Chem. Soc. 2023, 145 (6), 3369] reinvestigated this problem and revealed that the
accurately determined QY for long-lived excited states is much too
low to allow an electronically excited reaction intermediate to exist.
In the present study, we investigated the nature of the reaction intermediate
using ultraviolet and infrared transient absorption spectroscopy,
along with quantum chemical calculations to show that the intermediate
is in the S0 state, and its infrared spectrum is compatible
with a metastable twisted CC species theoretically predicted
by Park et al. [J. Phys. Chem. Lett. 2022, 13 (30), 7072].