A series of heteroleptic copper(I) photosensitizers of the type [(P^P)Cu(N^N)] with an extended π-system in the backbone of the diimine ligand has been prepared. The structures of all complexes are completely characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. These novel photosensitizers were assessed with respect to the photocatalytic reduction of protons in the presence of triethylamine and [Fe (CO) ]. Although the solid-state structures and computational results show no significant impact of the π-extension on the structural properties, decreased activities were observed. To explain this drop, a combination of electrochemical and photophysical measurements including time-resolved emission as well as transient absorption spectroscopy in the femto- to nanosecond time regime was used. Consequently, shortened excited state lifetimes caused by the rapid depopulation of the excited states located at the diimine ligand are identified as a major reason for the low photocatalytic performance.

Four different heteroleptic [Cu(N^N)(P^P)]PF6 complexes, which combine classical bidentate diimine ligands and sterically demanding diphosphine ligands, are studied by a combination of ultrafast time-resolved spectroscopy and quantum chemical calculations. The light-induced excited state processes, accompanied by a structural change, are discussed with respect to the application of these complexes as a new class of noble-metal-free photosensitizers in proton reducing systems. In particular, the influence of different substituents in the ligand backbone on the photophysical properties is highlighted.

This study provides detailed mechanistic insights into light-driven hydrogen production using an abundant copper−iron system. It focuses on the role of the heteroleptic copper photosensitizer [Cu(P ∧ P)(N ∧ N)] + , which can be oxidized or reduced after photoexcitation. By means of IR, EPR, and UV/vis spectroscopy as well as computational studies and spectroelectrochemistry, the possibility of both mechanisms was confirmed. UV/ vis spectroscopy revealed the reorganization of the original heteroleptic photosensitizer during catalysis toward a homoleptic [Cu(N ∧ N) 2 ] + species. Operando FTIR spectroscopy showed the formation of a catalytic diiron intermediate, which resembles well-known hydrogenase active site models.