Metal–Organic Framework-Based Efficient Singlet Heterogeneous Photoredox Catalyst for Aerobic C–H Functionalization

Abstract

Effective light harvesting is critical in bioinspired heterogeneous photocatalysts to transport the absorbed light energy, compensating for diffusion-limited surface-only activity that is commonly achieved in solid systems constructed by simply anchoring a well-established photosensitizer (PS). While ³PS* has been widely exploited for their persistence, ¹PS* defines a fresh paradigm for artificial photosystems specifically for aerobic photoredox processes where the ¹O₂*-mediated oxidative path must be avoided. Endowed with a large chemically accessible surface area hosting ultrafast anisotropic singlet exciton transportation, three mesoporous Zr-MOFs, PCN-222(H₂), NU-1000, and SIU-100, displayed superior catalytic activities (t_0.5 ∼3 h and a TOF of 106 h^–1 at t_0.5) toward the aerobic aza-Henry reaction of N-aryl-tetrahydroquinone compared to common ³PS* benchmarks. Furthermore, with higher excited state redox potentials, ¹MOF* can be flexible in the initial photoproduct [amine^•+ and O₂^•–] formation through an oxidative or a reductive quenching pathway expanding the scope of the amine substrates. While a slow H-atom transfer process in the dark step entails a moderate apparent quantum yield of ∼20%, a discernible rate difference was established to be defined by the driving force for the initial photoinduced electron transfer processes, which, in turn, are regulated by the electronic properties of the MOF relative to the amine substrates. Nevertheless, it is the electronic property of the amine that dictates the product identity and distribution. With detailed mechanistic studies and wider substrate scopes, this study underscores the advantageous platform for developing effective¹MOF*-based heterogeneous photoredox catalysts.