In this study a detailed scrutiny of the electronic structure changes during the redox events of the oxidative and reductive quenching cycles of the representative homoleptic and heteroleptic octahedral iridium [Ir­(bpy) x (ppy)3–x ] x+ (x = 0, 1, 2, and 3) and ruthenium [Ru­(bpy) x (ppy)3–x ] x−1+ (x = 1, 2, and 3) photoredox catalysts is provided through the corresponding electron density difference Δρ­(r) distributions. The systematic analysis of the Δρ­(r) distributions provides intuitive insights into the details of the metal- and ligand-centered electron transfer processes that take place in the different excited- and ground-state redox steps of classical photoredox catalysis. In addition to the structural metrics, the measured ground-state reduction potentials were also reproduced with great accuracy, typically within 0.15 V, when using the TPSSh functional in combination with the Def2-TZVP basis set coupled to reparameterized implicit solvation model (SMD). We computed the excited-state reduction potentials of these ruthenium and iridium complexes without using TD-DFT, but by directly computing the solution-state Gibbs free energy of the triplet 3MLCT state, giving good agreement with respective experiments. The analyzed Δρ­(r) maps reveal the characteristic features of metal- and ligand-centered reductions and oxidations in both ground- and excited states and metal-to-ligand charge transfers (MLCT), sometimes perturbed by additional ligand-to-ligand charge transfer (LLCT) contributions. One of the most interesting features of ligand-centered redox processes is the localization of the accumulated electron density at one redox-active ligand in the case of heteroleptic systems [Ir­(bpy)­(ppy)2]+ and [Ru­(bpy)­(ppy)2]0, which is in contrast to the delocalized nature of the ligands-hosted charge in homoleptic photoredox catalysts, such as the classical [Ru­(bpy)3]2+ system.

In this study, a detailed look at the electronic structure changes induced by photon absorption and of the succeeding redox events of the oxidative and reductive quenching cycles of ruthenium–carbene and ruthenium–pyridine photoredox catalysts is provided through an arsenal of density functional theory-based techniques including electron density difference Δρ(r) maps, spin-density distributions, and the non-covalent interaction analysis. We introduced an efficient computational protocol to obtain accurate equilibrium structures and ground-state reduction potentials for these types of complexes, substantiated via a direct comparison to empirical X-ray structures and cyclic voltammetry measurements, respectively. Moreover, we demonstrated the utility of a hitherto unexplored approach to compute excited-state redox potentials based on the Gibbs free energy of the triplet metal-to-ligand charge transfer state (3MLCT). The analyzed Δρ(r) maps revealed the characteristic features of, for example, metal- and ligand-centered reductions and oxidations in both ground and excited states and MLCT processes, disclosing the active participation of carbene ligands in the redox events of homoleptic systems. Beyond analyzing ligand–ligand non-covalent interactions and redox-active behaviors of carbene and pyridine ligands side by side, the effect of such groups on the kinetics of 3MLCT to 3MC transition was scrutinized.

The electronic structures of the ground and excited electronic states involved in the oxidative and reductive quenching cycles of 12 relevant ruthenium, iridium, and copper photoredox catalysts (S 0 , T 1 , D ox , and D red ) are characterized using the recently developed effective oxidation state (EOS) analysis, allowing the monitoring of metal and ligand oxidation states (OSs) along the catalytic cycles. The formal oxidation state assignments derived from the EOS analysis are in agreement with those commonly assumed for these complexes in both ground and excited states. Rather clean and separate ligand-and metal-centered redox events along the different quenching cycles are observed in most of the studied molecular systems. The reliability index obtained for the OS assignations can be readily interpreted in terms of the ionic/covalent character of metal−ligand interactions and ligand non-innocent character. In addition, EOS analysis reveals the high-degree localization of the ligand-centered redox event to one or two redox-active ligand(s) in heteroleptic complexes. Ligand-and metal-condensed spin populations were also computed and analyzed for all the open-shell species involved in this study, showing promises for rapid oxidation state assignments in certain systems, especially Ru complexes, however, suffering from severe defects in other cases.