Photocatalytic reduction of CO2 with rhenium(I) bipyridine complexes has been studied for several decades. Nonetheless, important parameters affecting the catalytic performance remain elusive to date. By using the standard catalyst [Re(dmb)(CO)3Cl] (dmb=4,4′‐dimethyl‐2,2′‐bipyridine), the effect of catalyst concentration and irradiation intensity is studied in detail and important correlations are revealed. The decomposition of the catalyst is investigated, and two main deactivation pathways are proposed, both of which involve the one‐electron‐reduced species and are likely to be valid for other homogeneous photocatalysts as well. The rate of deactivation is linked to the relative concentration of 1) the catalyst in its electronic ground state, 2) the catalyst in its excited state, 3) the one‐electron‐reduced species, and 4) quencher radicals. Adequate tuning of catalyst concentration and irradiation intensity leads to the highest quantum yield (Φ=0.53) reported to date for a single‐molecule system.

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Mononuclear iridium(III) complexes [Ir(mppy)(tpy)X] (mppy = 4-methyl-2-phenylpyridine, X = Cl, I) and binuclear analogues with various bis(2-phenylpyridin-4-yl) bridging ligands were synthesized and characterized by their spectroscopic and electrochemical properties. Kinetic measurements concerning the photocatalytic two electron reduction of CO2 to CO were investigated in order to determine the influence of intermolecular interactions between two active centers. A detailed comparison between the monometallic and the bimetallic complexes indicates an enhanced lifetime (TON) of the covalently linked complexes, causing an increased overall conversion of CO2. Additionally the deactivation pathways of the catalysts are examined.

A trinuclear complex consisting of one [Ru(dmb)3]2+ (dmb=4,4′‐dimethyl‐2,2′‐bipyridine) (Ru) and two [Re(dmb)(CO)3Cl] (Re) building blocks, [Re(CO)3Cl(dmb−dmb)Ru(dmb)(dmb−dmb)Re(CO)3Cl](PF6)2 (Re−Ru−Re), is presented. Photophysical properties of Re−Ru−Re and the individual components with different or no covalent linkages are thoroughly investigated and compared. To elucidate the role of the single covalent bonds, photocatalytic reduction of CO2 is performed with the trinuclear complex and a series of model systems featuring systematic absence of linkages between the metal centers. Photoluminescence spectra and quantum yields reveal efficient energy transfer from the excited state of Re to Ru if these fragments are covalently linked. Moreover, intramolecular electron transfer from the one‐electron reduced species of Ru to Re occurs if there is covalent bonding, leading to a higher photostability and thus the highest turnover number in photocatalytic CO2 reduction of 199 for the trinuclear complex Re−Ru−Re within the systems under investigation. Optimized experimental conditions reveal the highest turnover number (315) reported to date for ReI/RuII‐based homogeneous catalysts in photocatalytic CO2 reduction.