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

Abstract

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'-Y₂ -bpy (Y=-Ph or -PhSi(Ph)₃ ]. We found that the π-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 (^tBu IrP-Ph^Si )-sensitized homogeneous systems (IrPS+Re^I catalyst) gave a turnover number of 1340 (Φ_CO =24.2 %) at the early stage of photolysis (<5 h). This study demonstrates that the π-character modulation on the ancillary bipyridyl ligand is critical for forthcoming catalytic performance.

Keywords: intraligand electron delocalization; iridium; photochemical CO2 reduction; photochemistry; sensitizer; supported catalysts.