Supramolecular Photocatalysts for the Reduction of CO₂

Abstract

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.

Molecular Porous Photosystems Tailored for Long‐Term Photocatalytic CO2 Reduction

“…The increases in catalytic activity observed when BIH is used rather than BNAH (TOFs ca. 1.2 h −1 ) is in line with the known capacity of BIH to be a stronger reductant resulting in an increased thermodynamic driving force as suggested by DFT calculations (Figure c) . Thus BIH was used in the following as sacrificial electron donor, while TEOA is still required as base, to overcome the thermodynamic limitation of CO 2 reduction and to deprotonate the oxidized BIH, which then acts as a two‐electron donor …”

“…1.2 h À1 )isinline with the known capacity of BIH to be as tronger reductant resulting in an increased thermodynamic driving force as suggested by DFT calculations ( Figure 2c). [1,39] Thus BIH was used in the following as sacrificial electron donor, while TEOAi ss till required as base,t oovercome the thermodynamic limitation of CO 2 reduction [40] and to deprotonate the oxidized BIH, which then acts as at wo-electron donor. [1] As the Cp*Rh-based catalytic center can be considered to be the same in both materials (see also DFT calculations above), the higher activity of the PerBpyCMP-based catalyst might be attributed to its increased capability to absorb light in the visible part (Figure 1e).…”

Re(bpy)(CO)3Cl Immobilized on Bipyridine‐Periodic Mesoporous Organosilica for Photocatalytic CO2 Reduction

“…In both cases, the BPy ligands of the complexes are connected by a short linker: (Si−O) n −Si, in which n =1 in P1 and n =4 in P2. These molecular structures are therefore analogous to those of binuclear Ru‐Re molecular photocatalysts that have been used for CO 2 reduction, in which the two complexes are bound together with non‐conjugated molecular linkers such as ‐CH 2 CH(OH)CH 2 ‐, ‐(CH 2 ) n ‐ ( n =2, 4, 6) or ‐CH 2 OCH 2 ‐ . However, the complexes in P1 and P2 are also immobilized by connection to the silica framework by Si−C covalent bonds and therefore cannot approach each other.…”

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

“…The latter systems were able to lead to higher turnover numbers (TON CO ) compared to intermolecular systems that consisted of a 1:1 mixture of the corresponding mononuclear units. Compound Ru‐Re (Figure ) showed a TON CO of 170 after 16 hours of irradiation with light at λ >500 nm using 1‐benzyl‐1,4‐dihydronicotinamide (BNAH) as the sacrificial electron donor . Porphyrins have also been used as the photosensitizers, and covalently linked to a Re unit, but only a few studies have dealt with their photochemical CO 2 reduction activity …”