The role of the bridging ligand in photocatalytic supramolecular assemblies for the reduction of protons and carbon dioxide

► Review of catalysts for photocatalytic proton reduction. ► Review of catalysts for photocatalytic CO2 reduction. ► Comparison of the bridging ligands in supramolecular assemblies. Molecular photocatalysts, combining a light absorber and a catalytic active center via a bridging ligand, have received much attention during the last decade. Therefore, many new types of photocatalysts containing novel bridging ligands are reported. The bridging ligand is expected to play an important role and does not only represent the chemical connection between a photosensitizing unit and a catalytically active center, but can also greatly affect the overall catalytic activity and stability of the photocatalysts. Moreover, a well-chosen combination of the three parts photosensitizer-bridge-catalyst is expected to yield intramolecular photocatalysts superior over intermolecular systems. This review will discuss the coordination, photophysical, electrochemical and photocatalytic properties of multinuclear metal complexes based on a selection of heterocyclic bridging ligands that show significant photocatalytic activity for hydrogen generation or CO2 reduction. Of particular interest in the discussion will be the effect of the bridging ligand on the photocatalytic properties of the compounds. Based on the results obtained, possible design principles for the optimization of the photocatalytic properties of this type of compounds will be suggested.