Directional-Polarity Engineering in Bithiophene-Based Covalent Organic Frameworks Enables Selective...

Selective photocatalytic generation of ¹O₂ through electron-transfer pathways represents an attractive alternative to conventional triplet-sensitized photooxygenation, yet remains limited by uncontrolled oxygen activation and poor reactive oxygen species (ROS) selectivity. Herein, we report a directional-polarity engineering strategy that programs charge kinetics and interfacial-O₂ activation in bithiophene-based covalent organic frameworks (COFs) for selective O₂→•O₂ ^-→¹O₂ conversion. Systematic modulation of donor-acceptor polarity identifies a highly polarized bithiophene-imine-triazine framework that lowers exciton binding energy, accelerates ultrafast charge separation, and establishes directional electron transfer from bithiophene to adsorbed O₂. Mechanistic studies reveal that this polarized framework thermodynamically promotes O₂ activation to •O₂ ^- via a bithiophene-superoxide-mediated pathway, followed by hole-driven oxidation of •O₂ ^- at the adjacent phenylene units to generate ¹O₂. ¹⁸O₂/H₂ ¹⁸O isotope-labeling experiments and theoretical calculations reveal that ¹O₂-mediated Achmatowicz rearrangements are dominated by non-exchange pathways, together with minor favorable water-mediated oxygen-exchange pathways responsible for the observed oxygen scrambling during coupling furfuryl-alcohol oxidation and H₂O₂ formation. The optimized COFs achieve ¹O₂ generation rates of 769.6 µmol L^-1 min^-1 and H₂O₂ productivity of 52.7 mmol g^-1 h^-1 while enabling near-quantitative furfuryl-alcohol upgrading under ambient conditions. This work establishes directional-polarity engineering as a molecular strategy for steering electron-transfer oxygen activation and selective ROS photocatalysis in crystalline organic semiconductors.