Spotlight on copper: Photoredox catalysis relies on visible-light excitation to accelerate a burgeoning number of chemical reactions. Initially, the technique relied primarily on complexes of precious metals, such as ruthenium or iridium, to absorb the light. Hossain et al. review recent progress in the use of copper complexes as an alternative. In addition to its Earth abundance, copper opens up a variety of distinct mechanisms involving electron transfer within the coordination sphere. Science , this issue p. eaav9713

The mainstream applications of visible‐light photoredox catalysis predominately involve outer‐sphere single‐electron transfer (SET) or energy transfer (EnT) processes of precious metal Ru^II or Ir^III complexes or of organic dyes with low photostability. Earth‐abundant metal‐based MⁿL_n‐type (M=metal, L_n=polydentate ligands) complexes are rapidly evolving as alternative photocatalysts as they offer not only economic and ecological advantages but also access to the complementary inner‐sphere mechanistic modes, thereby transcending their inherent limitations of ultrashort excited‐state lifetimes for use as effective photocatalysts. The generic process, termed visible‐light‐induced homolysis (VLIH), entails the formation of suitable light‐absorbing ligated metal–substrate complexes (MⁿL_n‐Z; Z=substrate) that can undergo homolytic cleavage to generate Mⁿ⁻¹L_n and Z^. for further transformations.