Toward Controlled Partial Desolvation of Guest-Responsive Metal–Organic Frameworks for Precise Porosity Control

Abstract

Desolvation of guest-responsive metal–organic frameworks (MOFs) under dynamic vacuum often leads to the collapse of the pores, whereas supercritical CO2 (SC-CO2) drying is found to be the best alternative way to overcome the challenge of MOF desolvation. Nevertheless, some of the MOFs collapse during desolvation by the SC-CO2 drying method. SC-CO2 drying always leads to complete desolvation of the MOFs except the metal-coordinated solvent molecules. However, controlled and partial desolvation of the MOFs can be a possible way to restrict the pore collapse of the frameworks. The importance of nuanced desolvation is demonstrated for two isostructural tetracarboxylic acid-based Cu-MOFs (DUT-202, DUT-203). These MOFs switch to a contracted pore phase upon complete desolvation either by vacuum treatment or the SC-CO2 drying method. Therefore, a controlled desolvation technique has been used to desolvate DUT-202 and 203, while the strongly confined N,N-dimethylformamide (DMF) remains coordinated, which is essential to retain the microporous nature of the framework. Acetone-exchanged Cu-MOFs were treated under an argon flow to remove the weakly connected solvent molecules inside the pores but not the strongly trapped guest molecules, and the desolvated phases were found to have an open pore phase with microporous nature. Interestingly, the contracted pore phase can be reopened by heating in DMF for several hours.