Tracking photoinduced electron and proton transfer and the coupled solvent reorganization with femtosecond X-rays*

Light-induced electron transfer (ET) and proton-coupled electron-transfer (PCET) reactions are ubiquitous in chemistry, playing an important role in both natural and artificial solar energy conversion. To advance these applications, it is vital to achieve a molecular-level understanding of this important class of reactions. I will present investigations with time-resolved X-ray methods of light-triggered ET and PCET reactions in metal-based donor-acceptor complexes, with focus in considering explicitly the interaction of such molecular systems with the surrounding solvent.

As previously demonstrated, time-resolved X-ray solution scattering (XSS) can directly probe solvation dynamics and is uniquely sensitive to photoinduced changes in solute-solvent hydrogen bonding. Simultaneously, time-resolved X-ray spectroscopy can probe charge reorganization with element specificity as well as provide direct evidence of protonation. I have utilized these methods to capture ultrafast ET and coupled solvent reorganization in a tri-metallic mixed-valence complex in a series of solvent environments with varying hydrogen bonding ability. I will also show femtosecond XSS and N K-edge X-ray absorption data reporting on PCET in a Ru-polypyridyl complex in aqueous environment and coupled solvent reorganization. The experimental data will be shown together with theoretical models necessary to their interpretation, including molecular dynamics and time-dependent density functional theory calculations.