Ultrafast metal-to-ligand charge transfer driven by bond shortening revealed with dual-edge computational X-ray spectroscopy

Understanding electron flow during chemical reactions is fundamental to ultrafast chemistry, particularly in transition metal complexes where redox processes involve intricate coupling between electronic and nuclear dynamics. While time-resolved X-ray spectroscopy provides a window into these dynamics, interpreting spectral data to identify transient intermediates and charge transfer mechanisms remains challenging. Here, we introduce a dual-edge computational spectroscopy approach that simultaneously simulates O K-edge and Cu L-edge X-ray absorption spectra for the paradigmatic CuO ₂ ⁺ system. We show that symmetric Cu--O bond shortening drives the ultrafast conversion from Cu(I):O ₂ to Cu(II):O ₂ ^∙- through metal-to-ligand charge transfer. Our peak-by-peak analysis along the binding coordinate directly resolves concurrent dioxygen reduction and copper oxidation, leveraging the interpretable ligand-centered O K-edge to decode the complex metal-centered L-edge spectrum. This work establishes a general protocol for extracting atomic-level electron flow from ultrafast X-ray spectra, with implications for metalloenzyme function, catalysis, and energy conversion.