Control of gate adsorption characteristics of flexible metal-organic frameworks by crystal defect

Abstract

Abstract Flexible metal-organic frameworks (flexible MOFs) are known to exhibit a step-wise adsorption behavior induced by structural transition. This adsorption behavior is referred to as the “gate” adsorption behavior as it resembles the opening and closing of a gate. Gate adsorption behavior has been reported to originate from the balance between stabilization owing to the guest adsorption and destabilization that occurs due to structural transition. Although the peculiar adsorption of flexible MOFs is expected to have a wide range of applications, control of the gate adsorption behavior is essential. Here, we focus on CID-4, a member of flexible MOFs with an interdigitated structure. The study's approach to control the gate adsorption behavior of CID-4 is to create the crystal defect through coexistence with mono/bidentate organic ligands during synthesis. Depending on the extent of the crystal defect, the degree of destabilization owing to structural transition can be varied, resulting in different gate pressures. Essentially, by adding a mono organic ligand during the synthesis of CID-4 particles, the crystal defect due to the switch from bidentate to mono organic ligands was confirmed via X-ray diffraction (XRD) and thermal gravimetric analysis (TGA) measurements. Moreover, the gate adsorption behavior of CID-4 with crystal defects dramatically shifted to a higher relative pressure. The free energy analysis based on the crystal structure of interdigitated-type MOFs also supported our strategy to control the gate adsorption behavior by creating the crystal defect.