Orbital-Level Signatures of Electron and Proton Motion Revealed by Ultrafast X-ray Absorption and Scattering

Proton-coupled electron transfer (PCET) involves coupled electronic, nuclear, and solvent motions that are notoriously difficult to disentangle. Here, we provide a multimodal, element- and site-specific view of PCET in solution by combining femtosecond optical spectroscopy, ultrafast soft X-ray absorption at the nitrogen K-edge, and time-resolved X-ray scattering, supported by time-dependent density functional theory (TDDFT) and molecular dynamics simulations. In a ruthenium polypyridyl model complex, we directly track photoinduced electron redistribution, ligand-site protonation on a ~100 ps timescale, and the accompanying solvent reorganization. Spectral signatures at the N K-edge reveal orbital-level changes associated with charge transfer and protonation, while X-ray scattering captures reorganization of the solute–solvent hydrogen-bonding network. This X-ray-anchored framework establishes a general approach to resolving elementary PCET steps and guiding control strategies in catalysis, artificial photosynthesis, and bioenergetics.