T1 Population as the Driver of Excited-State Proton-Transfer in 2-Thiopyridone.

Excited-state proton transfer (ESPT) is a fundamental process in biomolecular photochemistry, but its underlying mediators often evade direct observation. We identify a distinct pathway for ESPT in aqueous 2-thiopyridone, by employing transient N 1s X-ray absorption spectroscopy and multi-configurational spectrum simulations. Photoexcitations to the singlet S2 and S4 states both relax promptly through intersystem crossing to the triplet T1 state. The T1 state, through its rapid population and near nanosecond lifetime, mediates nitrogen site deprotonation by ESPT in a secondary intersystem crossing to the S0 potential energy surface. This conclusively establishes a dominant ESPT pathway for the system in aqueous solution, which is also compatible with previous measurements in acetonitrile. Thereby, the hitherto open questions of the pathway for ESPT in the compound, including its possible dependence on excitation wavelength and choice of solvent, are resolved.