Supramolecular Photocatalysts for the Reduction of CO2

Photocatalytic reduction of CO2 into energy-rich compounds utilizing solar light as an energy source is expected to provide a solution to serious problems of the shortage of fossil resources and global warming. In this perspective, we summarize advances in supramolecular photocatalysts for the reduction of CO2, of which photosensitizer and catalyst units are connected via a bridging ligand. The first successful Ru(II)-Re(I) supramolecular photocatalysts reported in 2005 indicated molecular architecture for developing efficient supramolecular photocatalysts for CO2 reduction to CO with high selectivity and durability. On the basis of this architecture, both the bridging ligands and Re(I) catalyst unit were optimized to increase the photocatalytic activity. In addition, the compositional units of supramolecular photocatalytic systems were modified: (1) Ir(III) and Os(II) complexes, free- and metallo-porphyrins, and chlorophyll functioned as alternative or better photosensitizer units in comparison to the Ru(II) complexes, (2) Ru(II) carbonyl complexes reduced CO2 giving HCOOH selectively, and (3) dihydrobenzoimidazole derivatives were suitable sacrificial electron donors for evaluating the potential of supramolecular photocatalytic systems. These research studies have provided efficient photocatalytic systems for CO2 reduction with high selectivity, durability, and reaction rate under visible-light irradiation.