Photochemical CO₂ Reduction Using a Ir(III)-Rh(III) Supramolecular Photocatalyst

There is immense scope and interest in the development of photocatalytic CO₂ conversion. We have successfully developed a new supramolecular photocatalyst (Ir-Rh) in which the Ir(III) complex [Ir(piq)₂(BL)]⁺ (piq = 1-phenylisoquinoline, BL = 1,2-bis(4'-methyl[2,2'-bipyridin]-4-yl)ethane) as a photosensitizer and the Rh(III) complex [Rh(BL)(Cp*)Cl]⁺ (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) as the catalyst are connected by a bridging ligand BL. The photophysical properties and the redox behavior of all complexes were investigated. Ir-Rh efficiently photocatalyzed CO₂ reduction in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH) as the sacrificial electron donor, selectively yielding formic acid as the major product (S_HCOOH > 90%) with minor amounts of CO and H₂. However, the combined system of the corresponding mononuclear complexes produced a smaller amount of formic acid. The mechanism of the photocatalytic reaction was explored through in situ UV-vis absorption studies and electrochemical measurements.