Bright or Dark: Unexpected Photocatalytic Behavior of Closely Related Phenanthroline‐based Copper(I) Photosensitizers

In search of a better understanding of the structure‐property relationship of heteroleptic CuI photosensitizers, two structurally closely related complexes are investigated in detail. Although both compounds bear the same 4‐methoxyphenyl substituents at the phenanthroline moiety (i. e. at 5,6‐position in Cu1 and at 4,7‐position in Cu2), substitution at the 4,7‐position increases the attenuation coefficient in the 300–500 nm range by a factor of more than two. Even more drastic is the difference in emission lifetime (11 vs. 628 ns), which rises by a factor of almost 60 when positions 4 and 7 (i. e. Cu2) are chosen. A combination of solid‐state structural analysis, TDDFT calculations, transient absorption spectroscopy and quenching studies is then applied to elucidate the reasons for the superior photophysical properties of Cu2. Subsequently, a non‐emissive and long‐lived excited state (715 ns) has been revealed for Cu1. In a next step, both complexes were successfully used in the photocatalytic splitting of water and the reduction of CO2 to CO to test the influence of this behavior on the activity. Most importantly, Cu2 not only performs significantly better than Cu1, but also than the benchmark system and the noble‐metal‐based complex [Ir(dFppy)3]. Two closely related heteroleptic copper(I) photocatalysts are compared. Both complexes bear the same p‐methoxyphenyl substituents at the phenanthroline moiety, but at different positions. Surprisingly, significant differences in catalytic activity are found for the evolution of H2 and the reduction of CO2. This is mainly due to the presence of dark or bright long‐lived excited states.