Tracing Covalent Organic Framework Assembly through Kinetic Deceleration

The formation mechanisms of covalent organic frameworks (COFs) have long been hypothesized but rarely substantiated by direct experimental evidence. Gaining mechanistic insights into this process is essential for the rational design and scalable synthesis of COFs. In this study, we employed the Schiff-base condensation, known for its high reactivity, to construct a series of β-ketoenamine COFs under mild, acid-free conditions. By intentionally slowing down the reaction kinetics, we successfully captured and structurally characterized millimeter-sized single crystals of an intermediate supramolecular assembly (ISA), which serves as a key intermediate in the COF assembly pathway. Systematic studies of structurally related systems further reveal that the intrinsic reactivity of the monomers plays a decisive role in guiding framework evolution. Notably, crystalline COFs can be synthesized without catalysts or modulators, establishing a green, sustainable synthetic route. Overall, this work provides fundamental insights into COF growth mechanisms, enabling greener, scalable routes to high-quality crystalline COFs.