Ancillary Ligand Effects on Heteroleptic Ir(III) Dye in Dye-Sensitized Photocatalytic CO<SUB>2</SUB> Reduction: Aryl-Substitution on Bipyridine Ligand and its Control on Catalytic Performance.
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We report the synthesis and photochemical and -physical properties as well as the catalytic performance of a series of heteroleptic Ir(III) photosensitizers (IrPS), [Ir(C^N)2(N^NAryl)]+, possessing ancillary ligands which are varied with aryl-substituents on bipyridyl unit [C^N = (2-pyridyl)benzo[b]thiophen-3-yl (btp); N^NAryl = 4,4′-Y2-bpy (Y = -Ph or -PhSi(Ph)3]. Ancillary ligand effect was dominant in the light driven energy transfer process. Their photosensitizing abilities were tested in a homogeneous condition for CO2 reduction (IrPS + Re(I) catalyst) as well as a semi-heterogeneous photocatalytic system consisting of Ir(III) photosensitizer and Re(I) catalyst supported on TiO2 semiconductor (IrPS + TiO2/Re(I). We found that the π-extension of bipyridyl ligand by aryl-substitution puts bipyridyl ligand in use as an electron relay unit performing charge accumulation before delivering to the catalytic center, greatly improving the overall CO2-to-CO conversion activities.