Kinetic-Directed Thermodynamic Repair Enables the Synthesis of High-Strain 2D Sub-Stoichiometric COFs

Two-dimensional covalent organic frameworks (2D COFs) are promising photocatalysts due to their tunable electronic structures and ordered π-stacking. Sub-stoichiometric COFs, featuring periodically unreacted functional groups, offer enhanced property tunability while presenting significant synthetic challenges, especially for strained frameworks like [4+3]-type COFs. Although previous studies have shown that kinetic conditions strongly influence COF structure, strategies to systematically tune these pathways for precise optimization of crystallinity are still lacking. This work establishes a general kinetic-controlled strategy wherein modulation of reaction duration or solvent polarity governs the degree of polymerization of kinetic intermediates, directing their subsequent thermodynamic repair into highly crystalline COFs. Using COF-221 and COF-222 as models, we demonstrate that varying polymerization time yields amorphous kinetic intermediates with different degrees of polymerization. These intermediates exhibit markedly different crystallization behaviors during thermal repair; only intermediates with an optimal degree of polymerization convert into highly crystalline COFs. Alternatively, employing a high-polarity solvent (e.g., nitrobenzene) improves monomer solubility and directly produces intermediates with a suitable polymerization degree, effectively merging nucleation and self-repair into a single step and simplifying what would otherwise require separate kinetic polymerization and thermodynamic repair stages. The resulting COF-222, achieves efficient charge separation and a 98.6% removal rate in visible-light-driven uranium reduction.