Ancillary Ligand Effects on Heteroleptic IrIII Dye in Dye-Sensitized Photocatalytic CO2 Reduction: Photoaccumulation of Charges on Arylated Bipyridine Ligand and Its Control on Catalytic Performance

Herein, we report the synthesis, and photochemical and -physical properties, as well as the catalytic performance, of a series of heteroleptic Ir-III photosensitizers (IrPSs), [Ir(C<^>N)(2)(N<^>N-Aryl)](+), possessing ancillary ligands that are varied with aryl-substituents on bipyridyl unit [C<^>N=(2-pyridyl)benzo[b]thiophen-3-yl (btp); N<^>N-Aryl=4,4 '-Y-2-bpy (Y=-Ph or -PhSi(Ph)(3)]. We found that the pi-extension of bipyridyl ligand by aryl-substitution put bipyridyl ligand in use as an electron relay unit that performed charge accumulation before delivering to the catalytic center, greatly improving the overall CO2-to-CO conversion activities. In a typical run, the aryl-substituted IrPS ((IrP)-Ir-tBu-Ph-Si)-sensitized homogeneous systems (IrPS+Re-I catalyst) gave a turnover number of 1340 (phi(CO)=24.2 %) at the early stage of photolysis (<5 h). This study demonstrates that the pi-character modulation on the ancillary bipyridyl ligand is critical for forthcoming catalytic performance.