Allosteric Modulation toward Nonionic Covalent Organic Framework Nanosheets

Nonionic covalent organic framework nanosheets (nCOFNs) are new-generation building blocks for diverse applications. However, the solution-phase synthesis of highly crystalline, large-sized nCOFNs remains particularly challenging, as it requires the simultaneous regulation of chemical interactions for intralayer framework construction and physical interactions for interlayer stacking. Allostery, a ubiquitous natural process for regulating the structure and activity of biomacromolecules, awaits exploration in synthetic functional materials. Herein, we showcase an allosteric modulation strategy for the solution-phase synthesis of β-ketoenamine-linked nCOFNs. The secondary amine, as an allosteric modulator, facilitates enol-keto tautomerization reversibility, affording allosteric activation for in-plane assembly, while its steric hindrance affords allosteric inhibition for out-of-plane assembly. The modulator with a high N exposure factor and moderate-size side groups can achieve an optimal balance between two allosteric effects. Consequently, a series of high-quality nCOFNs with high crystallinity and aspect ratios exceeding 1000 can be mildly synthesized under ambient conditions with yields of up to 82%. The excellent solution processability of nCOFNs enables their facile assembly into membranes that exhibit ultrahigh methanol permeance of 127 L m-2 h-1 bar-1, superior durability in continuous filtration for over 1000 h, and high efficiency in purifying high-value pharmaceuticals. Furthermore, we demonstrate the practical application potential by assembling nCOFNs into large-area flat-sheet (∼430 cm2) and hollow fiber membranes.