Mixed p-π Resonance in Pyrene-Based Covalent Organic Frameworks: Shallow Trap Charge Dynamics for Efficient Photocatalysis in Reduction of U(VI).

Covalent organic frameworks (COFs) represent a versatile platform of photocatalysis for environmental remediation. Nevertheless, their efficiency is often constrained by the presence of deep trap states that lead to rapid charge carrier recombination. Here, we fine-tune the p-π resonance of pyrene-based COFs by functionalizing the 5,6 positions of benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzaldehyde (BT) with electron-withdrawing (-F), mixed (-F/-OCH3), and electron-donating (-OCH3) groups followed by condensation with 4,4',4″,4‴-(pyrene-1,3,6,8-tetrayl)tetraaniline to afford BTCOF-2F, BTCOF-Mix, and BTCOF-2OCH3, respectively. The mixed p-π resonance strengthens the internal electric field, reduces the exciton binding energy to 62.06 meV, and generates long-lived shallow trap states with a lifetime up to 1574.6 ps for BTCOF-Mix. As a result, BTCOF-Mix photocatalysis realizes a U(VI) uptake of 1600.1 mg g-1 and a rate constant of 0.084 min-1, outperforming those of BTCOF-2F (1090.2 mg g-1) and BTCOF-2OCH3 (1245.2 mg g-1). Notably, BTCOF-Mix achieves over 90% selectivity in the presence of competing ions and more than 99% reduction of U(VI) and ensuing recovery by visible-light photocatalysis from real mining wastewater. This work demonstrates that balancing p-π resonance constitutes a viable approach for modulation of the trap state energetics of COF photocatalysis toward solar-driven reduction and recovery of radioactive ions in aquatic environments.