Utility of Squaraine Dyes for Dye-Sensitized Photocatalysis on Water or Carbon Dioxide Reduction

Red light-sensitized squaraine (SQ) dyes were developed and incorporated into dye-sensitized catalysts (DSCs) with the formula of SQ/TiO2/Cat, and their efficacies were evaluated in terms of performance on either water or carbon dioxide reduction. Pt nanoparticles or fac-[Re­(4,4′-bis-(diethoxyphosphorylmethyl)-2,2′-bipyridine)­(CO)3Cl] were used as each catalytic center within the DSC frame of SQ/TiO2/Pt (Type I) or SQ/TiO2/Re­(I) (Type II). In order to convey the potential utility of SQ in low energy sensitization, the following catalytic reductions were carried out under selective lower energy irradiation (>500 nm). Type I and II showed different catalytic performances, primarily due to the choice of solvent for each catalytic condition: hydrogenation was carried out in H2O, but CO2 reduction in dimethylformamide (DMF), and SQ was more stable in aqueous acid conditions for hydrogen generation than CO2 reduction in DMF. A suspension of Type I in 3 mL water containing 0.1 M ascorbic acid (pH = 2.66) resulted in efficient photocatalytic hydrogen evolution, producing 37 μmol of H2 for 4 h. However, in photocatalysis of Type II (SQ/TiO2/Re­(I)) in 3 mL DMF containing 0.1 M 1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole, the TiO2-bound SQ dyes were not capable of working as a low energy sensitizer because SQ was susceptible to dye decomposition in nucleophilic DMF conditions, resulting in DSC deactivation for the CO2 reduction. Even with the limitation of solvent, the DSC conditions for the utility of SQ have been established: the anchoring group effect of SQ with either phosphonic acid or carboxylic acid onto the TiO2 surface; energy alignment of SQ with the flat band potentials (Efb) of TiO2 semiconductors and the reduction power of electron donors; and the wavelength range of the light source used, particularly when >500 nm

Highly Selective and Durable Photochemical CO₂ Reduction by Molecular Mn(I) Catalyst Fixed on a Particular Dye-Sensitized TiO₂ Platform

A Mn­(I)-based hybrid system (OrgD-|TiO2|-MnP) for photocatalytic CO2 reduction is designed to be a coassembly of Mn­(4,4′-Y2-bpy)­(CO)3Br (MnP; Y = CH2PO­(OH)2) and (E)-3-[5-(4-(diphenylamino)­phenyl)-2,2′-bithiophen-2′-yl]-2-cyanoacrylic acid (OrgD) on TiO2 semiconductor particles. The OrgD-|TiO2|-MnP hybrid reveals persistent photocatalytic behavior, giving high turnover numbers and good product selectivity (HCOO– versus CO). As a typical run, visible-light irradiation of the hybrid catalyst in the presence of 0.1 M electron donor (ED) and 0.001 M LiClO4 persistently produced HCOO– with a >99% selectivity accompanied by a trace amount of CO; the turnover number (TONformate) reached ∼250 after 23 h of irradiation. The product selectivity (HCOO–/CO) was found to be controlled by changing the loading amount of MnP on the TiO2 surface. In situ FTIR analysis of the hybrid during photocatalysis revealed that, at low Mn concentration, the Mn–H monomeric mechanism associated with HCOO– formation is dominant, whereas at high Mn concentration, CO is formed via a Mn–Mn dimer mechanism