Nature-Inspired, Highly Durable CO₂ Reduction System Consisting of a Binuclear Ruthenium(II) Complex and an Organic Semiconductor Using Visible Light

A metal-free organic semiconductor of mesoporous graphitic carbon nitride (C₃N₄) coupled with a Ru(II) binuclear complex (RuRu′) containing photosensitizer and catalytic units selectively reduced CO₂ into HCOOH under visible light (λ > 400 nm) in the presence of a suitable electron donor with high durability, even in aqueous solution. Modification of C₃N₄ with Ag nanoparticles resulted in a RuRu′/Ag/C₃N₄ photocatalyst that exhibited a very high turnover number (>33000 with respect to the amount of RuRu′), while maintaining high selectivity for HCOOH production (87-99%). This turnover number was 30 times greater than that reported previously using C₃N₄ modified with a mononuclear Ru(II) complex, and by far the highest among the metal-complex/semiconductor hybrid systems reported to date. The results of photocatalytic reactions, emission decay measurements, and time-resolved infrared spectroscopy indicated that Ag nanoparticles on C₃N₄ collected electrons having lifetimes of several milliseconds from the conduction band of C₃N₄, which were transferred to the excited state of RuRu′, thereby promoting photocatalytic CO₂ reduction driven by two-step photoexcitation of C₃N₄ and RuRu′. This study also revealed that the RuRu′/Ag/C₃N₄ hybrid photocatalyst worked efficiently in water containing a proper electron donor, despite the intrinsic hydrophobic nature of C₃N₄ and low solubility of CO₂ in an aqueous environment.