Number and Position of Methoxy Substituents Govern Photophysics and Visible-Light Photocatalysis in Earth-Abundant Cu(I) Complexes.

Earth-abundant Cu(I) photosensitizers are promising systems for visible-light-driven photocatalysis, but further design guidelines are required to relate targeted ligand modification to catalytic performance. Herein, we present five new heteroleptic Cu(I) complexes with two or three methoxy substituents at the bathocuproine backbone and two single-crystal structures to assess how substitution number and position affect structure, photophysics, redox properties, and photocatalytic performance. While all complexes retain the expected distorted tetrahedral Cu(I) coordination motif, their properties depend strongly on the substitution pattern. In particular, ortho/para substitution is much more effective than meta substitution in prolonging the excited-state lifetimes (up to 1.46 µs) and emission quantum yields (up to 8.1%). Benchmark studies in energy-transfer and electron-transfer photocatalysis reveal that there is no single optimal substitution pattern for all reactions. Ortho/para substitution appears to be advantageous for excited-state performance and triplet sensitization, whereas meta substitution benefits from higher excited-state potentials, highlighting that the catalytic performance of these complexes depends strongly on the specific substrate and reaction mechanism. Overall, these results demonstrate that the position of the methoxy substituents is more decisive than their number and establish application-specific methoxy substitution as a useful strategy for tailoring noble-metal-free Cu(I) photosensitizers toward photocatalytic applications.