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

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 systems constructed by simply anchoring a well-established homogeneous photosensitizer (PS) on a solid scaffold. While ³PS* has been widely exploited for their persistence, ¹PS* defines a fresh paradigm for artificial photosystems specifically for aerobic photoredox processes where 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 aerobic aza-Henry reaction of N-aryl-tetrahydroquinone compared to common ³PS* benchmarks-both solution-dissolved and heterogenized forms. Furthermore, endowed with large singlet energy and 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 dark step involving a slow H-atom transfer process entails a moderate apparent quantum yield of ~20%, a discernable rate difference was established to be defined by the driving force for the initial photo-driven electron transfer processes which, in turn, are regulated by the electronic properties of 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 effective ¹MOF* based heterogeneous photoredox catalysts.