Linking Thermal Expansion to Transient Porosity in a Pair of Highly Flexible Activated 3D Zn(II)-Based MOFs

Two Zn(II)-based mixed-ligand metal−organic frameworks (MOFs) were synthesised using solvothermal methods and fully characterized (SCXRD, PXRD, TGA, DSC, FTIR, HSM). The MOFs are formulated as [Zn2(NDC)2(AzPy)2]n˙n(DMF)3 (ZNA150-DMF) and [Zn(NDC)(AzPy)0.5]n˙n(DMF)0.5 (ZNA90-DMF), where NDC2− = 2,6-naphthalene dicarboxylate, AzPy = 4,4′-azopyridine and DMF = N,N′-dimethylformamide, and are synthesized at 150 °C and 90 °C, respectively. Single crystal X-ray diffraction analysis confirmed that ZNA150-DMF and ZNA90-DMF are non-interpenetrated and triply interpenetrated, respectively. Both MOFs are highly flexible, displaying large changes in their unit cell parameters and void space features upon desolvation with the void space of both desolvated forms (ZNA150-act and ZNA90-act) consisting of isolated voids, thus leading to transient porosity, where these isolated pockets temporarily connect into channels to facilitate sorption. ZNA90-act carbon dioxide sorption isotherms have inflection points indicating a pressure-sustained structural change, as proven by variable-pressure single crystal and powder X-ray diffraction studies on ZNA90-act immersed in a CO2 gas environment. Variable-temperature single crystal X-ray diffraction studies on both activated MOFs, in the absence of gas, show that these are highly flexible, displaying colossal thermal expansion properties with changes in void space features. These changes tend towards what was observed for ZNA90-act under CO2 pressure, thus possibly linking flexible MOF behavior under the two sets of conditions.