Abstract: Covalent organic framework (COF) is a highly promising photocatalyst for hydrogen peroxide (H₂O₂) photosynthesis, but its photocatalytic performance is limited by slow exciton transfer dynamics and poor reaction selectivity. Herein, we report a p-π conjugation engineering strategy by introducing chlorodiphenylphosphine units into COF framework (CTDDD-P), thereby enabling precise modulation of excited-state electronic structures and the H₂O₂ photosynthesis pathway. Phosphorus functionalization significantly enhances the photocatalytic H₂O₂ production performance and reaction stability of COF in air. Theoretical calculations reveal that p-π conjugation induces significant electronic delocalization, strengthens spin-orbit coupling, and reduces the singlet-triplet energy gap, thereby promoting intersystem crossing process. More importantly, femtosecond transient absorption spectroscopy directly resolves the ultrafast processes of exciton evolution and demonstrates the accelerated transition from singlet to triplet excitons and the prolonged triplet lifetime. In situ electron paramagnetic resonance spectroscopy and radical scavenging experiments demonstrate that the H₂O₂ photosynthesis mechanism is primarily driven by a triplet exciton energy transfer pathway, rather than the traditional oxygen reduction pathway involving superoxide radicals. The triplet exciton can effectively sensitize ground-state triplet oxygen to generate singlet oxygen, thereby increasing the selectivity of H₂O₂ production. This work has successfully modulated the electronic structure through p-π conjugation and achieved highly efficient triplet exciton generation, providing new insights into the design of high-performance COF-based photocatalysts.

Key words: p-π conjugation, Triplet excitons, Singlet oxygen, Photocatalytic H₂O₂ production, Femtosecond transient absorption spectroscopy, Spin-orbit coupling