PNNL

The Science

Proton-coupled electron transfer (PCET) reactions are fundamental to a wide range of chemical systems. These reactions are commonly governed by an intricate interplay between electron transfer, proton transfer, and a hydrogen-bond network that mediates proton motion. Researchers identified and characterized key intermediate states along a light-driven PCET reaction. They found that photoexcitation first redistributes electronic charge within a molecule, followed by the formation of a protonated intermediate associated with the reorganization of both the molecule’s electronic structure and the surrounding hydrogen-bond network. A unique combination of techniques, including high-resolution X-ray spectroscopies, reveals distinct molecular states before and after protonation, providing insight into how proton transfer is coupled to changes in the molecule and its environment.

The Impact

PCET reactions are central to many processes that support life and enable energy technologies. By revealing how electronic structure, proton transfer, and solvent organization are coupled at the molecular level, this research provides new principles for understanding and controlling chemical reactivity. These insights can guide the development of more efficient catalysts and energy-conversion systems, supporting future advances in energy and chemical technologies. Developing the ability to directly interrogate these complex processes also establishes new opportunities for studying and optimizing the molecular transformations critical to addressing global energy challenges.

Summary

PCET is foundational to catalysis, bioenergetics, and energy conversion, yet directly observing the interplay between electronic redistribution, protonation, and solvent reorganization remains challenging. Researchers combined femtosecond optical spectroscopy, ultrafast N K-edge X-ray absorption spectroscopy, and time-resolved X-ray solution scattering to capture the steps of a sequential PCET reaction in water with atomic-site specificity. Using a ruthenium polypyridyl model complex, they resolved the electron redistribution upon photoinduced metal-to-ligand charge transfer and subsequent (∼460 ps) protonation at a ligand nitrogen, as well as the concomitant rearrangement of the first solvation shell. Combined with advanced electronic structure and molecular dynamics simulations, the measurements revealed a marked localization of the excited-state electron density at the protonated N site, together with a switch from N···H–O to NH···O hydrogen-bonds. These results establish a multimodal X-ray framework for mechanistic insight into PCET and its control in catalysis, artificial photosynthesis, and biological energy flow.

Contact

Elisa Biasin, Pacific Northwest National Laboratory, elisa.biasin@pnnl.gov 

Funding

This work was supported by the U.S. Department of Energy (DOE); Office of Science; Basic Energy Sciences; Chemical Sciences, Geosciences, and Biosciences Division; Condensed Phase and Interfacial Molecular Science (CPIMS), FWP 16248 (E. B. and A. K.), and Atomic, Molecular, and Optical Sciences (AMOS) Programs, FWP 72684 (S. G. and N. G.), at Pacific Northwest National Laboratory (PNNL). E. S. R., N. P. R., and A. A. C. were supported by the DOE, Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, through SLAC National Accelerator Laboratory under Contract No. DE-AC02-76SF00515. B.I.P. acknowledges support by the NSF GRFP (No. DGE-1762114). C. B. L. gratefully acknowledges a SNSF Ambizione Grant (Grant Number 193436). Use of the Linac Coherent Light Source, SLAC National Accelerator Laboratory, is supported by the DOE, Office of Science, Basic Energy Sciences under Contract No. DE-AC02-76SF00515. A portion of this research was performed on project awards (60604, 61181) at the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science user facility at PNNL. This research also gratefully acknowledges computational resources provided by the PNNL Institutional Computing Program and National Energy Research Scientific Computing Center (NERSC). PNNL is operated by Battelle Memorial Institute for the United States DOE under DOE Contract No. DE-AC05-76RL1830. NERSC is a DOE Office of Science user facility operated under Contract No. DE-AC02-05CH11231.