Sensitized Photochemical CO2 Reduction by Hetero‐Pacman Compounds Linking a ReI Tricarbonyl with a Porphyrin Unit

The hetero‐Pacman architecture places two different metal coordination sites in close proximity, which can support efficient energy and/or electron transfer and allow for cooperative activation of small molecules. Here, the synthesis of dyads consisting of a porphyrin unit as photosensitizer and a rhenium unit as catalytically active site, which are held together by the rigid xanthene backbone, is presented. Mononuclear [(NN)Re(CO)3(Cl)] complexes for CO2 reduction in which NN represents a bidentate diimine ligand (e.g., bipyridine or phenanthroline) lack light absorption in the visible region, resulting in poor photocatalysis upon illumination with visible light. To improve their visible‐light absorption, we have focused on the incorporation of a strongly absorbing free base or zinc porphyrin unit. Resulting photocatalytic experiments showed a strong dependence of the catalytic performance on both the type of photosensitizer and the excitation wavelengths. Most notably, the intramolecular hetero‐Pacman system containing a zinc porphyrin unit showed much better catalytic activity in the visible region (excitation wavelengths >450 nm) than the free base porphyrin version or the corresponding mononuclear rhenium compound or an intermolecular system comprised of a 1:1 mixture of the mononuclear analogues. The hetero‐Pacman architecture places a porphyrin and a rhenium terpyridine unit in close distance to each other. This molecular design allows for wavelength‐independent light‐driven activation of CO2 when a zinc porphyrin unit is used as photosensitizer. Time‐resolved spectroscopic measurements confirmed the electronic communication between the two functional units and suggested the involvement of a charge‐separated state.