An Ir(III) complex with 1-phenylisoquinoline (piq) ligands [Ir(piq)(2)(dmb)](+)(Ir, dmb = 4,4'-dimethyl-2,2'-bipyridine) exhibited strong absorption in the visible region, and the lifetime of its excited state was very long (tau = 2.8 mu s). Photochemical reduction of Ir efficiently proceeded with 1-benzyl-1,4-dihydronicotinamide (BNAH) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as reductants, giving the one-electron reduced species (OERS), which was stable in solution at ambient temperature. The OERS of the Ir complex possessed strong reductive power, sufficient to supply an electron to fac-Re(dmb)(CO)(3)Br (Re). The photocatalytic reduction of CO2 proceeded efficiently using a mixed system constructed with Ir as a redox photosensitizer and Re as a catalyst, selectively giving CO (Phi(CO) = 0.16 using BNAH at lambda(ex) = 480 nm). Ir was a more suitable photosensitizer for evaluating the activity of the Re catalyst in the photocatalytic reaction compared to [Ru(dmb)(3)](2+) (Ru) because the Ir complex was more stable in the photocatalytic reaction, and its decomposition products did not function as catalysts for CO2 reduction while the decomposition products of the Ru complex functioned as catalysts for the reduction of CO2 to HCOOH, inducing a drastic perturbation of the product distribution. A supramolecular photocatalyst (Ir-Re), in which the Ir(III) photosensitizer and the Re(I) catalyst were connected by a bridging ligand, was newly synthesized. When using BNAH, Ir-Re possessed a greater photocatalytic ability (Phi(CO) = 0.21, TONCO = 130) than the corresponding mixed system of the Ir and Re mononuclear complexes. Using BIH as the reductant, both Ir-Re and the mixed system showed very high photocatalytic activity (Phi(CO) = 0.40-0.41, TONCO = 1700).