Heterogeneous Artificial Photosystem I: Photoinduced Proton-Coupled Electron Transfer in Zr–Metal–Organic Frameworks

Photosystem I bridges the disparity between ultrafast energy transduction and much slower chemical bond formation by generating NADH as the primary photoproduct via proton-coupled electron transfer (PCET); NADH subsequently serves as a redox shuttle for downstream multielectron reactions. Replicating such an elegant blueprint, or its function and mechanism within artificial systems, remains challenging. This study demonstrates a photoinduced PCET process in a pyrene-based zirconium-oxo metal-organic framework (MOF) NU-1000, operating in the singlet manifold, directly producing an NADH model analogue (HNH-H) along with a 2[pyrene•+]─[Zr-oxo node-O-] pair, without any cocatalysts. Spectroscopic analyses─including steady-state, time-resolved, and transient methods─reveal that in a stepwise two 1e- reduction process, the proton transfer from node-bound hydroxyl and aqua ligands in polar dimethylformamide solvent is not directly involved in the rate-defining step, evidenced by a kinetic isotope effect (KIE) of 1 at a very low [HNH+]. However, with increasing difficulty in the successive deprotonation from the anionic node, an inverse KIE (0.75) was observed at higher [HNH+]. Density functional theory-based computation supports a pre-equilibrium proton transfer to the first 1e- reduced radical intermediate HNH•, followed by the second electron transfer, consistent with an overall ET-PTET mechanism for this endoergic process. In nonpolar solvents that do not support proton transfer, the HNH• intermediate instead undergoes irreversible dimerization. The reduced HNH-H shuttle was exploited in the photocatalytic multielectron PCET-based reduction of maleate to succinate, achieving ∼30% consumption of maleate and ∼12% conversion to succinate in aprotic media without any external hole scavengers and proton source, relying solely on node-bound protons in NU-1000. Under these conditions, the benchmark photocatalyst Ru(bpy)32+ was ineffective. Introduction of ethanol as a regenerator establishes a fully catalytic, recyclable system with NU-1000. These results highlight the potential of Zr-oxo MOFs as platforms for entirely artificial, bioinspired photosystem I analogues.