Electron Density Difference Analysis on the Oxidative and Reductive Quenching Cycles of Classical Iridium and Ruthenium Photoredox Catalysts

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 Delta rho(r) distributions. The systematic analysis of the Delta rho(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 (MLCT)-M-3 state, giving good agreement with respective experiments. The analyzed Delta rho(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.