Solvent-Induced Incremental Pore Collapse in Two-Dimensional Covalent Organic Frameworks

Nanoporous materials such as covalent organic frameworks (COFs) are attractive due to their extremely high surface areas and porosities, but they are susceptible to pore collapse and loss of crystallinity. Prior studies reported complete pore collapse above a threshold surface tension for the activation solvents. In this study, we show that the degree of pore collapse can be precisely controlled by using a mixture of high and low surface tension solvents for activation. We further demonstrate that pore collapse is an irreversible process, in which partially collapsed COFs can collapse further using high surface tension solvents but cannot recover their structural integrity by activation with low surface tension solvents. Finally, we show that the solvent surface tension is the most important characteristic that governs pore collapse of COFs during solvent activation, and the activation temperature, pressure, washing time, and number of activation cycles has negligible impact on the final COF porosity and crystallinity. This work provides novel insight into pore collapse of COFs, a method to systematically tune surface area, and guidance on the development of effective activation methods to produce highly crystalline and porous COFs. These findings will be useful for the preparation of COFs and other nanoporous materials.