Tracking the evolution of charge and spin densities in transition metal complexes following charge-transfer excitations is crucial to understand and control electron transfer reactions in various photochemical applications. X-ray spectroscopies are of particular value in this regard as they give access to the valence electronic structure locally at the photocatalytically active site. Here, we show how femtosecond resonant inelastic X-ray scattering (RIXS) at the nitrogen K-edge provides unique information on the evolving charge and spin distributions following the optical excitation of a transition metal complex. For the prototypical metal-to-ligand charge-transfer (MLCT) excitation in [Ru(2,2′-bipyridine)3]2+, we demonstrate the selectivity of the method to local charge distributions at specific molecular sites as well as to the spin state of the system. Specifically, ultrafast ligand K-edge RIXS provides site-selectivity, which allows to distinguish between the electron-accepting ligand in the charge-transfer process and the remaining ligands. Thereby, excitation spectra specific to the different ligands can be extracted and, by comparison to quantum chemical spectrum simulations, can be related to changes in the local electronic structure resulting from the oxidation of the metal and the reduction of a specific bipyridine group. Furthermore, a temporal upper limit of (250 ± 40) fs for the associated electron localization following the MLCT excitation can be determined. Lastly, the selection rules of ligand K-edge RIXS allow to derive a general scheme for probing the initial spin dynamics. Ultrafast spin crossover typically takes place within the MLCT manifold and is challenging to follow with conventional spectroscopic techniques. Here, we establish femtosecond K-edge RIXS as a unique probe of the total spin state of the system opening up new opportunities to study ultrafast spin crossover in transition metal complexes at upcoming high-repetition rate X-ray sources.

Femtosecond resonant inelastic X-ray scattering at the ligand K-edge as a probe of charge and spin dynamcis in transition metal complexes

ACS Spring 2022

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