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

Abstract

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, Dox, and Dred) 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), Dox (d9L0), and Dred (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 Dred (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)]+.