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

Abstract In search of a better understanding of the structure‐property relationship of heteroleptic Cu I 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 CO 2 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 ].