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Intrinsically charged covalent organic frameworks (COFs) afford specific ionic nanochannels for mass transport, and thus become promising platforms to design membranes with unique selectivity. However, internal electrostatic repulsion and large-pore frameworks cause significant barriers that greatly limit membrane performances. Herein, we report a novel strategy to synchronously crystallize and upgrade cationic COF membranes by ion-pairing design. Ion-paired guest molecules are involved in electric-driven interfacial crystallization to offset the charges of the ionic monomers and host frameworks. This host-guest neutralization promotes the crystallization of compact COF membranes. Lateral dimension and charge nature of guest molecules fundamentally affect the ion-pairing efficiency. Tight encapsulation of the large-sized electronegative molecules effectually narrows the molecular sieving channels, yielding a significant elevation in membrane selectivity. The membrane rejection of organic ions with a size larger than 1.2 nm thus can be improved from below 50 % to above 85 % with a water permeance of ∼10 L m−2 h−1 bar−1. Prominently, our membranes demonstrate efficient recovery and pH-dependent release of bioactive pharmaceuticals, with a release rate that is 12 times higher in an acidic solution compared to a neutral environment. This work provides an ion-pairing strategy to regulate COF membranes for pharmaceutical industries and beyond. [Display omitted] • Ion-pairing is designed to crystallize and upgrade COF membranes. • Charged additives can be encapsulated to finely tune the COF membranes. • Encapsulation of the large electronegative molecules promotes membrane selectivity. • Ion-paired COF membranes realize the controlled release of bioactive pharmaceuticals. [ABSTRACT FROM AUTHOR]

Tight ultrafiltration (TUF) membranes featuring specific pore sizes are increasingly developed to bridge the gap between nanofiltration and ultrafiltration. So far, a wealth of efforts has been devoted to tackle the limitation found in TUF, but there still lacks a facile accessibility to upgrade TUF membranes with simultaneously improved permeance and selectivity. Herein, we report a large-pore covalent organic framework (LP-COF) as the building material to prepare ultra-permeable TUF membranes for fast separations. The LP-COF layers with an exceptionally large aperture size of up to ~3.6 nm are synthesized on macroporous substrates through a unidirectional diffusion method. The resultant LP-COF layers show a moderate crystallinity and low thickness down to ~60 nm, and allow ultrafast water permeation. Surprisingly, the optimal LP-COF membrane exhibits an unprecedented water permeance of ~3147 L m−2 h−1 MPa−1 with a high Congo red rejection (~92.6%), which is basically unchanged after several cycles of filtration. Moreover, the large-pore channels enable an unimpeded pass of ions, thus affording the membrane an excellent dye/salt separation competence, and largely exceeding state-of-the-art membranes. Therefore, this work opens up a new opportunity in producing high-performance TUF membranes by large-pore COFs for rapid and precise separations. [Display omitted] • A large-pore covalent organic framework (LP-COF) is designed to prepare TUF membranes. • Local nucleation and interfacial polymerization collectively promote the growth of ultrathin LP-COF layers. • Ordered channels of LP-COF membranes promise fast separations with high selectivities. • The optimized LP-COF membrane shows a preeminent dye/salt separation performance. • The extraction of nanoparticles from aggressive solvents is realized by the LP-COF membrane. [ABSTRACT FROM AUTHOR]