Switching on and off Interlayer Correlations and Porosity in 2D Covalent Organic Frameworks

Abstract

Two-dimensional covalent organic frameworks (2D COFs) attract great interest owing to their well-defined pore structure, thermal stability, high surface area, and permanent porosity. In combination with a tunable chemical pore environment, COFs are intriguing candidates for molecular sieving based on selective host–guest interactions. Herein, we report on 2D COF structures capable of reversibly switching between a highly correlated crystalline, porous and a poorly correlated, nonporous state by exposure to external stimuli. To identify COF structures with such dynamic response, we systematically studied the structural properties of a family of two-dimensional imine COFs comprising tris­(4-aminophenyl)­benzene (TAPB) and a variety of dialdehyde linear building blocks including terephthalaldehyde (TA) and dialdehydes of thienothiophene (TT), benzodithiophene (BDT), dimethoxybenzodithiophene (BDT-OMe), diethoxybenzodithiophene (BDT-OEt), dipropoxybenzodithiophene (BDT-OPr), and pyrene (Pyrene-2,7). TAPB-COFs consisting of linear building blocks with enlarged π-systems or alkoxy functionalities showed significant stability toward exposure to external stimuli such as solvents or solvent vapors. In contrast, TAPB-COFs containing unsubstituted linear building blocks instantly responded to exposure to these external stimuli by a drastic reduction in COF layer correlation, long-range order, and porosity. To reverse the process we developed an activation procedure in supercritical carbon dioxide (scCO2) as a highly efficient means to revert fragile nonporous and amorphous COF polymers into highly crystalline and open porous frameworks. Strikingly, the framework structure of TAPB-COFs responds dynamically to such chemical stimuli, demonstrating that their porosity and crystallinity can be reversibly controlled by alternating steps of solvent stimuli and scCO2 activation.

Acid Exfoliation of Imine‐linked Covalent Organic Frameworks Enables Solution Processing into Crystalline Thin Films

“…We predict that this repulsion enables their exfoliation. A similar disrupted stacking model was recently validated by Medina and co‐workers, who observed a reduction in the crystallinity and porosity of several imine‐linked COF powders upon exposure to certain solvents …”

Ultralow Surface Tension Solvents Enable Facile COF Activation with Reduced Pore Collapse

“…The process of removing these impurities and yielding dry COFs is generally termed activation, 22,23 and activation is arguably as important as synthesis 22,23 but has received far less attention. Proper activation can effectively remove unreacted small molecules or generated oligomers, adsorbed solvents, and other guest molecules while maintaining COF crystallinity and pore structure [22][23][24][25][26][27] . The simplest and most commonly used activation process for COFs is solvent washing followed by vacuum drying 1,22,23 .…”

Strong‐Base‐Assisted Synthesis of a Crystalline Covalent Triazine Framework with High Hydrophilicity via Benzylamine Monomer for Photocatalytic Water Splitting

“…To further enhance the crystallinity, the CTFs were treated with supercritical carbon dioxide, and then the XRD patterns become smoother and noisy background was obviously decreased (Figure d). The reason for this phenomena may be due to that the supercritical carbon dioxide could reorganize the layer structures as reported in the literature . The strong peak at 7.5° corresponds to the (100) plane in the crystalline structure.…”