Binuclear rhenium(I) complexes with 1,2-bis(4,4'-methyl-[2,2']bipyridyl)-ethane and 1,2-bis(4,4'-methyl-[2,2']bipyridyl)-dodecane as bridging ligands and their mononuclear analogues have been synthesized and characterized by their spectroscopic and electrochemical properties. First reduction potentials and luminescence properties as well as the reductive quenching of the emissive state with TEOA were not affected by the alkyl linker. By means of a detailed comparison of the photocatalytic CO(2) reductions of the monometallic and the bimetallic complexes a great beneficial effect on the activity depending on the proximity of the centres was found. In high dilution the overall kinetics in the CO(2) photoreduction of mononuclear complexes are clearly monometallic. If the proximity of the centres is adjusted according to the lifetime of the OER (one electron reduced species) the photocatalytic activity is greatly improved showing a clear bimetallic mechanism. In the binuclear rhenium complexes, both the facile generation of a free coordination site and binuclear interactions for effective two electron transfer can be realized.

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.

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.