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 IrIII photosensitizers (IrPSs), [Ir(C^N)2 (N^NAryl )]+ , possessing ancillary ligands that 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 ]. 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-PhSi )-sensitized homogeneous systems (IrPS+ReI catalyst) gave a turnover number of 1340 (ΦCO =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.

Organic-inorganic hybrid photocatalyst for carbon dioxide reduction.

Efficient hybrid photocatalysts for carbon dioxide reduction were developed from dye-sensitized TiO2 nanoparticles and their catalytic performance was optimized by ternary organic/inorganic components. Thus, the hybrid system consists of (E)-2-cyano-3-(5'-(5''-(p-(diphenylamino)phenyl)thiophen-2''-yl)thiophen-2'-yl)-acrylic acid as a sensitizer and fac-[Re(4,4'-bis(diethoxyphosphorylmethyl)-2,2'-bipyridine)(CO)3Cl] as a reduction catalyst (ReP), both of which have been fixed onto TiO2 semiconductors (s-TiO2, h-TiO2, d-TiO2). Mott-Schottky analysis on flat-band potential (Efb) of TiO2 mesoporous films has verified that Efb can be finely modulated by volume variation of water (0 to 20 vol%). The increase of added water resulted in substantial positive shifts of Efb from -1.93 V at 0 vol% H2O, to -1.74 V (3 vol% H2O), to -1.56 V (10 vol% H2O), and to -1.47 V (20 vol% H2O). As a result, with addition of 3-10 vol% water in the photocatalytic reaction, conversion efficiency of CO2 to CO increased significantly reaching a TON value of ∼350 for 30 h. Catalytic activity enhancement is mainly attributed to (1) the optimum alignment of Efb by 3-10 vol% water with respect to the of the dye and Ered of ReP for smooth electron transfer from photo-excited dye to RePvia the TiO2 semiconductor and (2) the water-induced acceleration of chemical processes on the fixed ReP. In addition, the energy level was further tuned by variation of the dye and ReP amounts. We also found that the intrinsic properties of TiO2 sources (morphology, size, agglomeration) exert a great influence on the overall photocatalytic activity of this hybrid system. Implications of the present observations and reaction mechanisms are discussed in detail.

Photosensitization Behavior of Ir(III) Complexes in Selective Reduction of CO2 by Re(I)-Complex-Anchored TiO2 Hybrid Catalyst.

A series of cationic Ir(III) complexes ([Ir(btp)2(bpy-X2)]+ (Ir-X+: btp = (2-pyridyl)benzo[b]thiophen-3-yl; bpy-X2 = 4,4'-X2-2,2'-bipyridine (X = OMe, tBu, Me, H, and CN)) were applied as visible-light photosensitizer to the CO2 reduction to CO using a hybrid catalyst (TiO2/ReP) prepared by anchoring of Re(4,4'-Y2-bpy)(CO)3Cl (ReP; Y = CH2PO(OH)2) on TiO2 particles. Irradiation of a solution containing Ir-X+, TiO2/ReP particles, and an electron donor (1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole) in N,N-dimethylformamide at greater than 400 nm resulted in the reduction of CO2 to CO with efficiencies in the order X = OMe > tBu ≈ Me > H; Ir-CN+ has no photosensitization effect. A notable observation is that Ir-tBu+ and Ir-Me+ are less efficient than Ir-OMe+ at an early stage of the reaction but reveal persistent photosensitization behavior for a much longer period of time unlike the latter. Comparable experiments showed that (1) the Ir-X+ sensitizers are commonly superior compared to Ru(bpy)32+, a widely used transition-metal photosensitizer, and (2) the system comprising Ir-OMe+ and TiO2/ReP is much more efficient than a homogeneous-solution system using Ir-OMe+ and Re(4,4'-Y'2-bpy)(CO)3Cl (Y' = CH2PO(OEt)2). Implications of the present observations involving reaction mechanisms associated with the different behavior of the photosensitizers are discussed in detail.