Hyperporphyrinization-Enhanced Electron Transfer in Covalent Organic Frameworks for Metal-Free Photocatalytic C–H Functionalization

The development of efficient covalent organic framework (COF)-based photocatalysts has been hindered by topological constraints on photosensitizer incorporation and inefficient charge transfer resulting from electron localization across the linkages. To overcome these limitations, we report a hyperporphyrinization strategy in a novel COF (HP-COF), constructed by linking porphyrinic photosensitizers to bipyridine N-oxide moieties via imine bonds. The unprecedented hyperporphyrin effect in HP-COF originates from linker-to-linker charge transfer (LLCT) across the imine linkage, as demonstrated by comparison with an amide-linked analogue (NP-COF), where the nonconjugated linkage disrupts this delocalization pathway. Upon photoexcitation, the porphyrinic unit oxidizes the bipyridine N-oxide to generate a pyridine N-oxyl radical, which acts as a hydrogen atom transfer catalyst for C–H bond activation. In HP-COF, the LLCT process enhances photooxidative potential, electron delocalization, and charge transport, resulting in superior photocatalytic performance in metal-free C–H functionalization compared to both NP-COF and homogeneous analogues. HP-COF is readily recyclable and retains its photocatalytic activity over five consecutive cycles.