Unveiling the Optimal Conjugation Threshold in Covalent Organic Frameworks for Photocatalytic Hydrogen Peroxide Production.
Solar-driven photocatalytic synthesis of hydrogen peroxide (H2O2) offers a sustainable alternative to the energy-intensive anthraquinone process. Donor-acceptor (D-A) covalent organic frameworks (COFs) are highly promising photocatalysts for this transformation; however, the precise correlation between electronic conjugation, D-A pairing, and catalytic efficiency remains poorly understood. Here, we systematically investigate the threshold of conjugation enhancement on photocatalytic H2O2 production by designing D-A COFs with varying degrees of π-conjugation. Using benzotrithiophene (BTT) as the electron donor, we integrated acceptor units featuring alkyne and extended-ring motifs (yielding TATAB-BTT and TATAP-BTT). We reveal a distinct structure-activity relationship: moderate conjugation (TATAB-BTT) optimizes energy-level matching and maximizes charge separation, achieving an outstanding H2O2 production rate of 1610.8 µmol g-1 h-1, outperforming the less-conjugated literature example, TAPB-BTT (557.0 µmol g-1 h-1). Conversely, excessive conjugation (TATAP-BTT) induces a structural mismatch that shifts the electronic configuration from D-A to D-D, severely trapping excitons and diminishing performance (780.0 µmol g-1 h-1. These findings demonstrate that an optimal conjugation threshold is critical for sustaining D-A charge-transfer dynamics, providing a fundamental molecular design principle for next-generation polymeric photocatalysts.