Unlocking Long‐Range Exciton Diffusion via Hydrogen‐Bonded‐to‐Covalent Organic Framework Transformation for Photocatalytic H 2 O 2 Synthesis

Controlling long‐range exciton diffusion remains a central challenge in organic photocatalysts because of the inherent trade‐off between extended π‐conjugation and long‐range structural order. Here, we report a topochemical polymerization strategy that converts a single‐crystalline hydrogen‐bonded organic framework (HOF) into a fully π‐conjugated covalent organic framework (COF) while largely preserving crystalline order. The preorganized hydrogen‐bonded precursor enables the formation of interlayer covalent linkages, resulting in a narrowed bandgap and enhanced electrical conductivity. Using power‐dependent femtosecond transient absorption spectroscopy, we quantitatively reveal exciton diffusion along the π‐conjugated channels with an average diffusion length of ∼12 nm. To illustrate the chemical consequences of exciton diffusion, the π‐conjugated COF is employed as a metal‐free photocatalyst for hydrogen peroxide (H 2 O 2 ) photosynthesis from O 2 and H 2 O without sacrificial agents. These results establish HOFs as versatile crystalline precursors for constructing previously inaccessible fully π‐conjugated COFs and highlight exciton diffusion as a key parameter linking framework structure to photocatalytic performance.