Nickel-Mediated Synthesis of Fused Heterocycle-Linked Covalent Organic Frameworks for Self-Sensitized Photocatalysis.

Conjugated and chemically robust covalent organic frameworks (COFs) incorporating metal ions hold great promise in diverse applications due to their finely tuned optoelectronic properties. However, their synthesis typically relies on post-synthetic modifications to stabilize linkages and introduce chelation sites, and metal incorporation is almost exclusively achieved through post-synthetic chelation of pre-formed COFs. Such approaches often result in compromised crystallinity, incomplete conversion and limited control over modification sites. Here, we report a direct, one-pot nickel-mediated Friedländer annulation strategy to construct Ni-chelated fused heterocycle-linked COFs (NNiN, NNiO, and NNiS). The fused heterocycle linkages act as an additional structural regulation factor, enabling precise tuning of framework packing and metal-chelation environment, thereby enhancing crystallinity and optoelectronic properties simultaneously. The resulting COFs exhibit extended π-conjugation, efficient Ni chelation and strong charge-transfer absorption in the visible-light region. Combining conjugated robust linkages, accessible single-site Ni centers, and self-photosensitizing capability, these frameworks provide a powerful platform for photocatalysis. They achieved up to 98% conversion in carbon-chalcogen coupling reactions, while retaining crystallinity and catalytic activity over multiple cycles. This work thus offers a general strategy for designing stable, π-conjugated, and metal-functionalized COFs, broadening opportunities for heterogeneous catalysis and beyond.