Unlocking Long-Range Exciton Diffusion via Hydrogen-Bonded-to-Covalent Organic Framework Transformation for Photocatalytic H2O2 Synthesis.

Abstract

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 (H2O2) photosynthesis from O2 and H2O 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.