Visible light-induced reduction of carbon dioxide sensitized by a porphyrin–rhenium dyad metal complex on p-type semiconducting NiO as the reduction terminal end of an artificial photosynthetic system

Visible light-induced, two-electron reduction of carbon dioxide to carbon monoxide, catalyzed by a newly synthesized metal complex dyad composed of a zinc porphyrin as a light-harvesting sensitizer and a rhenium bipyridyl complex as a catalytic moiety for the reduction, was attained for the first time on the p-type semiconductor NiO without any sacrificial electron donor. The fluorescence of the dyad adsorbed on the NiO nanoparticles (20–30 nm diameter) was efficiently quenched with a very short lifetime (29 ps) compared with the intrinsic value of 1.6 ns in DMF. The radical anion of the zinc porphyrin was actually formed upon excitation of 5,10,15,20-tetra(4-carboxy)phenylporphyrinatozinc (ZnTCPP) (5) on NiO by fast electron injection from NiO to the excited ZnTCPP with a lifetime of ∼180 ns, corresponding to the charge recombination with the hole on NiO. An electrochemical cell composed of the dyad-adsorbed NiO particle layers on an FTO cathode and platinum as a counter electrode in CO2-saturated DMF solution showed a constant cathodic photocurrent upon visible light irradiation (λ = 420 nm) accompanied by the formation of the two-electron-reduced product (CO). The photoreduction was further enhanced by a factor of 5.5 in a system prepared by co-adsorbing ZnTCPP (5) as a light- and electron-harvesting moiety and the dyad ZnDMCPP-Re(bpy)(NHAc) (1) as a catalytic moiety on NiO, as a promising example of the design of a CO2 photoreduction system as the reduction terminal end of an artificial photosynthetic system.