Electronic structure analysis of copper photoredox catalysts using the quasi-restricted orbital approach

In this computational study, the electronic structure changes along the oxidative and reductive quenching cycles of a homoleptic and a heteroleptic prototype Cu(I) photoredox catalyst, namely, [Cu(dmp)₂]⁺ (dmp = 2,9-dimethyl-1,10-phenanthroline) and [Cu(phen)(POP)]⁺ (POP = bis [2-(diphenylphosphino)phenyl]ether), are scrutinized and characterized using quasi-restricted orbitals (QROs), electron density differences, and spin densities. After validating our density functional theory-based computational protocol, the equilibrium geometries and wavefunctions (using QROs and atom/fragment compositions) of the four states involved in photoredox cycle (S₀, T₁, D^ox, and D^red) are systematically and thoroughly described. The formal ground and excited state ligand- and metal-centered redox events are substantiated by the QRO description of the open-shell triplet metal-to-ligand charge-transfer (³MLCT) (d⁹L^-1), D^ox (d⁹L⁰), and D^red (d¹⁰L^-1) species and the corresponding structural changes, e.g., flattening distortion, shortening/elongation of Cu-N/Cu-P bonds, are rationalized in terms of the underlying electronic structure transformations. Among others, we reveal the molecular-scale delocalization of the ligand-centered radical in the ³MLCT (d⁹L^-1) and D^red (d⁹L^-1) states of homoleptic [Cu(dmp)₂]⁺ and its localization to the redox-active phenanthroline ligand in the case of heteroleptic [Cu(phen)(POP)]⁺.