(Invited) Electronic Structures of Metal Centers in OER Catalyst Models and Electron/Energy Relays in the Excited State Supramolecular Dinuclear Transition Metal Complexes

Abstract

Electronic Structures of Metal Centers in an OER Catalyst Model. One of the challenges in solar fuels generation is generating multiple redox equivalents for the oxygen evolution reaction (OER) (i.e., 2H2O→O2+4H++4e-) that is a four-electron, four-proton coupled process. Earth-abundant first-row transition metal oxides of cobalt, nickel, and their mixed-metal forms can drive this half-reaction at relatively low overpotentials. In collaboration with Nocera Group, we used a combination of in situ and ex situ X-ray absorption and emission spectroscopies as well as quantum mechanical calculations on OER thin films and their molecular and heterogeneous inorganic analogs. These studies provided insights into the electronic structures of the high-valent states involved in the mechanisms of O-O bond formation and for the high-valent states to drive the bond-forming and bond-breaking steps of OER reactions. Using a Co4O4 cluster as a model for the Co-based OER anode, CoPi, which has a similar topology and can host a stable, but electronically delocalized, mixed-valent Co(IV) state, we employed complementary X-ray spectroscopies, X-ray absorption (XAS) and 1s3p resonant inelastic X-ray scattering (Kβ RIXS), to effectively extract Co(IV) contributions within a spectroscopically active background (Figure 1). Co K- and L-edge X-ray absorption directly probe the 3d-manifold of effectively localized Co(IV), providing a handle on the covalency of the dπ-based redox active molecular orbital (MO) of Co4O4. Kβ RIXS is shown to be highly sensitive to metal cluster oxidation state and oxo-mediated metal-metal coupling in high-valent, delocalized mixed-valent species. We also probed the doubly oxidized Co(III)2(IV)2 state of the Co4O4 cluster, which features a cofacial Co(IV)2 site and is thus a molecular model of the active site in Co-OEC. Electron and Energy Relays in the Excited State Supramolecular Dinuclear Transition Metal Complexes. The rational design of multinuclear transition metal complexes (TMCs) for photochemical catalysis of homogeneous and/or heterogeneous multi-electron reactions (e.g. for producing solar fuels) requires a detailed understanding of the often unique and convoluted excited state charge and energy transfer dynamics in this class of compounds. Using combined optical and X-ray transient spectroscopic measurements, excited state electron electron relays in tetrapyridophenazine-bridged heteroleptic dinuclear Cu(I) bis(phenanthroline) complexes have been investigated as an exemplary system.