Tuning Photocatalytic Activity in Porphyrin–Based Metal–Organic Frameworks by Controlling Charge Recombination Pathways

Porphyrin-based metal-organic frameworks (MOFs) are promising photocatalysts, but their efficiency is often limited by rapid charge recombination. Herein, we enhance photocatalytic performance by incorporating an electron acceptor, phenyl-C₆₁-butyric acid (PCBA), into the porphyrinic framework PCN-222 and its metalated analogues (M = Co, Ni, Cu) via a solvent-assisted ligand incorporation (SALI) method. The free-base composite PCBA@PCN-222(H₂) exhibited outstanding photocatalytic activity for aerobic thioanisole oxidation (>99% conversion), far outperforming PCBA@PCN-222(Cu) (23%), PCBA@PCN-222(Ni) (3%), and PCBA@PCN-222(Co) (2%). Electrochemical and picosecond time-resolved photoluminescence (TRPL) studies revealed that this superior activity originates from a remarkably long-lived charge-transfer (CT) state (τ_CR = 1.72 ns) in the free-base system. In contrast, the metalated MOFs exhibited intrinsic metal-associated quenching of the porphyrin units that competes with exciton migration within the MOF, and the CT states formed from a fraction of Q-state populations underwent ultrafast charge recombination (τ_CR = 70 ∼ 100 ps), which severely limited their efficiency. These findings provide a clear correlation between the lifetime of the photo-induced CT state and catalytic performance, highlighting the importance of the porphyrin's core electronic nature in designing efficient donor-acceptor photocatalysts.