Tracking Electron, Proton, and Solvent Motion in Proton-Coupled Electron Transfer with Ultrafast X-rays

TL;DR

This work tackles the challenge of disentangling electron, proton, and solvent motions during proton-coupled electron transfer in solution. It adopts a multimodal approach combining femtosecond optical spectroscopy, site-specific N K-edge X-ray absorption spectroscopy, and time-resolved X-ray solution scattering, coupled with TDDFT and MD simulations, on the Ru-based model [Ru(bpy)2(bpz)]2+. The study directly observes photoinduced electron redistribution, ligand-site protonation on a timescale of approximately 100 picoseconds, and concomitant solvent reorganization, providing an atomistic framework to separate electronic, nuclear, and solvation dynamics in PCET. This integrated methodology offers design principles for tuning solvent environments to optimize PCET in catalysis, artificial photosynthesis, and biological energy processes.

Abstract

Proton-coupled electron transfer (PCET) is foundational to catalysis, bioenergetics, and energy conversion, yet capturing and disentangling the coupled motions of electrons, protons, and solvent has remained a major experimental challenge. We combine femtosecond optical spectroscopy, site-specific ultrafast soft X-ray absorption spectroscopy, and time-resolved X-ray scattering with advanced calculations to disentangle the elementary steps of PCET in solution. Using a ruthenium polypyridyl model complex, we directly resolve photoinduced electron redistribution, ligand-site protonation within 100 ps, and the accompanying solvent reorganization. This unified multi-modal approach provides an orbital-level, atomistic picture of PCET, showing how electronic, nuclear, and solvation degrees of freedom can be separated experimentally. Our results establish a general X-ray framework for understanding and ultimately controlling PCET in catalysis, artificial photosynthesis, and biological energy flow.

Figures (3)

• Figure 1: (A) Overview of the excited-state dynamics of [Ru(bpy)$_2$(bpz)]$^{2+}$ in de-ionized water (pH 7) and 0.1 M HCl solution (pH 1). In acidic conditions, we probe the timescale of protonation with optical transient absorption (OTA), and capture the site-specific electronic dynamics and the coupled solvent reorganization with X-ray spectroscopies. (B) OTA spectra measured 1 ps after 400 nm excitation of [Ru(bpy)$_2$(bpz)]$^{2+}$ in neutral and acidic conditions. OTA spectra measured 100 and 300 ps after photoexcitation at pH 1 showing spectral evolution due to excite state protonation of the $^{3}$MLCT${_\text{bpz}}$ state. The gray-shaded area shows the inverted ground-state UV-Vis absorption spectrum. (C) Kinetic traces measured at the excited-state absorption (376 nm) and ground-state bleach (486) show different excited-state dynamics at pH 1 and pH 7.
• Figure 2: Ultrafast N K-edge response of [Ru(bpy)$_2$(bpz)]$^{2+}$ and assignment with TDDFT. (A) Schematic molecular orbital diagram illustrating the N K-edge XAS process. The red vertical arrow shows a representative transition from the N 1s to a $\pi^{*}$ orbital localized on the bpz ligand. (B) Transient N K-edge spectra recorded at 1, 100, and 300 ps after 400 nm excitation at pH 1. The gray shaded area represent the negative scaled ground-state absorption. (C) Kinetic traces at 399.5 eV (blue, left axis) and 398.9 eV (magenta, right axis) with 95% confidence intervals. Solid lines show the fits obtained from a sequential kinetic model, in which the time constants are fixed to the values obtained from the global analysis of the OTA data. For the GSB, the model includes a 2 ps component (VR) and a 3 ns component (decay of the excited-state). For the positive transient, an additional 97 ps component is included to account for excited-state protonation. (D) TDDFT calculations of the $^{1}$GS N K-edge spectra (gray fill, shown inverted) and transient spectra corresponding to the $^{3}$MLCT${_\text{bpz}}$ (blue) and $^{3}$MLCT${_\text{bpzH}}$ (orange) states. (E-G) Calculated N K-edge spectra (black, 0.25 eV Lorentzian-broadened) with underlying transitions for (E) $^{1}$GS, (F) $\;^{3}\mathrm{MLCT}_{\mathrm{bpz}}$, and (G) $\;^{3}\mathrm{MLCT}_{\mathrm{bpzH}}$. Insets: (E) molecular structure indicating the color-coded individual Ns: bpy-N (orange), coordinated bpz-N (green), peripheral bpz-N (purple; dark purple marks the protonated N); (F,G) spin-density isosurfaces of the corresponding excited-states, plotted at an isovalue of $\pm$0.005 a.u. The color of the transition reflects the N site involved; labels “L” (LUMO), “L+1/2/3” denote the most contributing unoccupied orbitals. (H) Most relevant ground-state unoccupied molecular orbitals shown as isosurfaces with isovalue of $\pm$0.03 a.u., which are used to label the dominant features in panels E-G.
• Figure 3: Ultrafast X-ray scattering reporting on solvent reorganization upon photoexcitation of [Ru(bpy)$_2$(bpz)]$^{2+}$. (A) Schematic illustration of the experimental setup. Following 530 nm excitation, elastic X-ray scattering was recorded on a forward detector and radially integrated as a function of the scattering vector Q. (B) Experimental difference scattering signals ($\Delta S$) at 1, 100, and 300 ps, showing a growing low-Q feature on the timescale of protonation. (C) Kinetic traces of the low-Q scattering signal (integrated over 0.25–0.35 Å$^{-1}$) at pH 1 (blue) and pH 7 (magenta) with 95% confidence intervals. Fits (black solid lines) use single- or double-exponential rise models and are a guide to the eye. (D) Structural evolution pathway of the local hydrogen bonding network for photoexcited [Ru(bpy)$_2$(bpz)]$^{2+}$. (E) Simulated difference scattering curves for the $^{3}$MLCT${_\text{bpz}}$ (blue) and $^{3}$MLCT${_\text{bpzH}}$ (orange), reproducing the low-Q increase upon protonation. (F) Radial distribution functions (RDFs) between N atoms and water hydrogens (N–H$_\mathrm{w}$) and oxygens (N–O$_\mathrm{w}$), calculated for the GS, MLCT${_\text{bpz}}$, and MLCT${_\text{bpzH}}$ states. The N–O$_\mathrm{w}$ curves are vertically offset by 0.3 for clarity.