The goal of our research is to directly visualize and quantify how d electronic correlations and local solute-solvent interactions control photoinduced charge transfer and delocalization between two metal sites in a solvated molecule. This understanding is crucial for controlling photochemical energy conversion processes in liquid environments, and for testing current theories of electron transfer and solvation dynamics. We have recently utilized a combination of femtosecond x-ray scattering and spectroscopy methods at the Linac Coherent Light Source to study the metal-to-metal charge transfer (MMCT) transition of a hydrogen-bonding cyanide-bridged Fe-Ru mixed-valence complex in aqueous solution. Time-resolved x-ray emission spectroscopy reveals that the MMCT lifetime is 62 fs, followed by back-electron transfer (BET). Simultaneously, x-ray solution scattering was utilized to monitor the rearrangements of the solute-solvent distances. By comparing the data with molecular dynamics simulations, we identify coherent translational motions of the first solvation shell water molecules with amplitude of tenths of Å, 120-200 cm-1 frequency, and 100 fs relaxation time arising from solute-solvent hydrogen bonding changes upon MMCT and subsequent BET. Finally, Fe K-edge x-ray absorption spectroscopy was utilized to measure the hole charge density at the Fe atom in the MMCT state. In combination with state-of-the-art TDDFT calculations of the solvated complex, we measured an electron localization of 75% at the Ru atom in the MMCT state of the Fe-Ru complex. This study combines state-of-the-art experimental and theoretical methods to provide an atomistic view of coupled solvent reorganization and electron transfer and delocalization in a bimetallic mixed-valence complex in solutions. Preliminary results from follow up studies may also be discussed.

Visualizing how intra- and inter-molecular interactions drive electron transfer and delocalization with femtosecond x-rays

ACS Spring 2022

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