Harnessing the Heavy‐Atom Effect and Linkage Engineering in Isomorphic COFs for Enhanced H ₂ O ₂ Photosynthesis

ABSTRACT The performance of photocatalytic covalent organic frameworks (COFs) is jointly regulated by their microscopic electronic structure and mesoscopic morphology. Heteroatom incorporation endows COFs with distinct optoelectronic properties, showing great promise for H ₂ O ₂ production. In this work, ten COFs from two categories were synthesized by precisely tuning heteroatom type and position to optimize their electronic and band structures. The thiourea‐based COF‐127 achieved a high H ₂ O ₂ production rate of 6672 µmol g ^{− 1} h ^{− 1} and an apparent quantum yield (AQY) of 11.03%. Although this material exhibits a relatively high oxygen adsorption energy barrier, the characteristic heavy‐atom effect of sulfur significantly enhances spin–orbit coupling, promotes the intersystem crossing process, and thereby efficiently generates triplet excitons. Simultaneously, sulfur introduction induces pronounced hole localization and electron delocalization, substantially improving the separation and migration efficiency of photogenerated charges, effectively compensating for its thermodynamic disadvantage during the adsorption step. In summary, the delicate synergy between the heavy‐atom effect of sulfur, the heteroatom type (S > O), and the linker geometry (aromatic ring > aliphatic chain) collectively modulates the band structure, carrier dynamics, and surface reaction energetics. This work establishes a rational design paradigm of “heavy‐atom foundation coupled with molecular structure optimization” for advanced COF photocatalysts.