Mechanistic studies on dinuclear complexes that can activate CO 2 are rare. Based on the investigations done for the mononuclear compound (bpy)Re(CO) 3 Cl (bpy-Re, with bpy = 2,2′-bipyridine), many reports favor a mononuclear catalytic cycle, while the possibility of a dinuclear catalytic species is discussed in the literature in only a few cases. Here, we report the synthesis and characterization of a homobimetallic rhenium(I) compound, in which two (bipyridine)Re(CO) 3 Cl fragments are brought into close vicinity by attaching them to a xanthene backbone. First, photocatalytic investigations show a significant increase of the catalytic performance compared to the mononuclear parent compound. Second, spectroelectrochemical experiments demonstrate the remarkable fast formation of an intermediate with a Re−Re bond that forms upon reduction of the starting compound, but which is not able to activate CO 2 . Third, spectroscopic investigations under (photo)catalytic conditions were performed to shed light on the crucial intermediates emerging in the reaction cycle. The assignment of these intermediates is assisted by extensive density functional theory calculations. As a result, the enhanced photocatalytic activity is reasoned by inhibition of deactivation channels and a cooperative reaction mechanism, in which one metal center functions as a photosensitizer to assist the second, catalytically active, metal.

A ruthenium­(II) polypyridine-type complex based on the dipyridophenazine ligand with a directly fused imidazole unit (L1, dipyrido­[3,2-a:2′,3′-c]­phenazine-10,11-imidazole) has been synthesized, and its electrochemical and photophysical properties have been studied. The cyclic voltammogram of [Ru­(tbbpy)2(L1)]2+ (C1) (tbbpy is 4,4′-tert-butyl-2,2′-bipyridine) shows a cathodic shift of the phenazine-based reduction process compared to similar molecules, while the first detected reduction wave (−1.34 V vs Fc/Fc+) is assigned to the imidazole unit within the molecule. On the basis of the TD-DFT calculations, the strong visible absorption band exhibited by C1 is assigned to a metal-to-ligand charge transfer (MLCT) transition with a concurrent ligand-centered (LC) transition. At room-temperature, C1 features emission (Φ = 0.04) from its lowest excited states with time constants of 1.2 and 18.3 μs. These lifetimes are assigned to emission processes from the 3MLCT and 3LC state, respectively. This is the first time that a long-lived dual emission has been observed for a ruthenium­(II) complex bearing a directly fused extended π-system. Furthermore, the emission of C1 is quenched upon water addition. In contrast to related compounds based on a dipyridophenazine ligand, the excited state energy is not shifted, and the lifetime is drastically decreased to 169 ns.

This study reveals the effect of an anthracene moiety on the resulting Cu(i) complexes and their photo- and electrochemical properties. As a result, unprecedented excited state lifetimes were found for such Cu(i) photosensitizers containing an extended π-system.