Unraveling the Electronic Structure of Photocatalytic Manganese Complexes by L-Edge X-ray Spectroscopy

X-ray absorption (XAS) and resonant inelastic scattering (RIXS) of a number of Mn^2+,3+,4+ complexes relevant for photo-electrooxidation of water is studied theoretically using the RASSCF/RASSI approach. This enables us to quantify spin-orbit coupling induced mixing of states with different multiplicities in the valence- and core-excited electronic states, evidencing the mostly spin-forbidden character of transitions in RIXS spectra. The notably different patterns of spectroscopic features in this series of substances not only provide insight into their electronic structure but open the possibility for tracing redox evolution by means of X-ray spectroscopy. Specific findings deduced from the analysis of the shape of the RIXS spectra concern the gap between the ground and first excited valence states relevant for catalytic activity. This gap is substantially lower for Mn³⁺ as compared to the even oxidation states.