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We unexpectedly found that pore size threshold is distinctive to each pore shape in creating similar pore environment. We further dive in to investigate the impact of pore size and shape to pinpoint the size threshold and shape for favourable water molecules confinement. In this work, we constitute hydrophobic microenvironment entwined with “pseudo-hydrophilicity” strips to promote water confinement at low pressure. These distinct features can be combined in COFs and are highly desired for supermicropores but hardly accessible to other porous analogues. Particularly, simplistic design can generate permanent supermicroporous voids that are implausible to collapse.
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Upon judicious selection of building blocks, aromatic components can connect via covalent bonds to grant extended crystalline structure and hydrolytic stability in COFs. COFs as a crystalline porous framework material, can be constructed via bottom-up polymerization through topological design. We anticipate hydrophobic supermicroporous covalent organic frameworks (COFs) with compact pore void is indispensable for water cluster occupation. Nevertheless, the design of hydrophobic small-pore materials is precluded by a preconception that hydrophobic small pores would eventually repulse water molecules and cannot enable water confinement and uptake. In recent years, a variety of porous polymers bearing intrinsic porosity has been studied extensively for water confinement and uptake 4, 5, 6. However, it is difficult to prepare stable and well-defined channels from phase-separated polymers. To create porous structures, researchers have relied on bottom-up strategy via self-assembly of block polymers 9, 10, 11. However, the lack of mechanistic strength and immobile water state have precluded their real implementation in water uptake and transport. To develop water confinement systems, extensive studies have been conducted over the past 60 years on a variety of hydrogel polymers with various hydrophilic backbones and different functionalities 1, 2, 3. Water confinement is ubiquitous and it plays vital roles in geological, biological and artificial systems with many applications 1, 2, 3, 4, 5, 6, 7, 8. Our results reveal a platform based on microporous hydrophobic covalent organic frameworks for water confinement. Water confinement experiments with large-pore frameworks pinpoint thresholds of pore size where confinement becomes dominated by high uptake pressure and large exchange hysteresis.
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The hydrophobic microporous frameworks achieve full occupation of pores by water via synergistic nucleation and capillary condensation and deliver quick water exchange at low pressures. The frameworks are designed to constitute dense, aligned and one-dimensional polygonal channels that are open and accessible to water molecules. Here, we demonstrate water confinement across hydrophobic microporous channels in crystalline covalent organic frameworks. However, most studies are based on a preconception that small hydrophobic pores eventually repulse water molecules, which precludes the exploration of hydrophobic microporous materials for water confinement. Progress over the past decades in water confinement has generated a variety of polymers and porous materials.