Driving Forces and Spin-Gated Reactivity of N5-Hydropyrimidopteridinetetraone Radicals in Photocatalysis

Pyrimidopteridinetetraones (PPTs) are potent excited-state oxidants that also function as hydrogen atom transfer (HAT) catalysts. Here, we investigate the photobasicity and mechanistic role of N-hydropyrimidopteridinetetraone radicals (PPTH^•) in catalyst turnover. Optical and electron paramagnetic resonance (EPR) spectroscopy, quantum chemical calculations, and thermochemical analysis reveal that excited-state protonation of PPTs occurs at the N5 position of the heterocycle, generating PPTH^• as a key catalytic intermediate. Formation of the exocyclic N-H bond from the excited state is associated with a bond dissociation free energy (BDFE) of 131 kcal mol^-1 from the singlet and 106 kcal mol^-1 from the triplet excited state, while subsequent homolytic N-H cleavage (BDFE ≈ 57 kcal mol^-1) enables catalyst regeneration. Spin-resolved mechanistic analysis shows that singlet radical encounters proceed uniformly via a direct HAT (dHAT) pathway across aliphatic, benzylic, and redox-active substrates with low activation free energies. In contrast, triplet encounters access substantially higher-energy reaction manifolds and can give rise to substrate-dependent mechanistic trajectories, including dHAT, concerted proton-coupled electron transfer (cPCET), or stepwise electron and proton transfer sequences. These findings establish a spin-state-dependent framework for HAT in PPT photoredox catalysis, highlighting how electron affinity and (photo)basicity govern excited-state reactivity and positioning PPTs as a platform for proton-coupled electron transfer chemistry.