Coordination-Isomerism-Driven Threshold Pressure Control of CO ₂ and C ₂ H ₂ in Mixed-Ligand Switching Metal−Organic Frameworks

Flexible metal-organic frameworks (MOFs) promise access to superior control and enhanced performance for gas separation, storage, and release. For the development of smart materials with "on-demand" responsiveness, the <i>ad hoc</i> design of switching mechanisms is crucial to drive multiple structural transformations. In this study, doubly interpenetrated pillared Zinc(II)-MOFs were customized to fine-tune CO₂ and C₂H₂ sorption/desorption pressures by modulating their dynamic responses. The incorporation of a specifically designed asymmetric bipyridyl-acrylonitrile pillar, together with benzenedicarboxylate and/or amino-benzenedicarboxylate ligands, leverages reversible metal-coordination isomerism to control gas-stimuli responsiveness. The sophisticated mechanism and temporal response involve framework expansion, node rearrangement, and ligand displacement. Furthermore, evolution of the global dynamics of the framework under cyclical CO₂ stimuli was revealed, demonstrating a progression from switching behavior to a permanently open structure. The pore-opening threshold pressure for CO₂ and acetylene (C₂H₂)─from discrete pockets to one-dimensional and two-dimensional interconnected channels─was rationally governed by increasing the amine group content, which modulates the density of hydrogen bonds between the two interpenetrated frameworks. This intricate mechanism was elucidated through <i>in situ</i> PXRD under CO₂ atmosphere, calorimetry-coupled gas adsorption, and density functional theory calculations. Notably, C₂H₂-induced gate opening expands the feasibility of C₂H₂-responsive systems, offering safer and more efficient sorbents.