Mechanoresponsive Porosity in Metal-Organic Frameworks

Abstract

The application of mechanical stress to porous MOFs provides the possibility to reversibly modify their structures and control their properties. Structural design involving the incorporation of certain geometric motifs allows to achieve MOF-based mechanical metamaterials exhibiting negative linear compressibility or negative Poisson’s ratio. The stabilization of metastable states in flexible MOFs is a key aspect to induce a shape-memory effect, which can be achieved by subtle changes in the characteristics of MOFs such as particles size or presence of defects. The use of mechanical stress has potential to control the transition between the metastable state and the more thermodynamically stable state. By combining flexible MOFs with other materials in a spatially controlled manner and tuning their interaction, mechanical forces can be efficiently propagated across different length scales to exploit and modify MOF structural transformations. Flexible metal-organic frameworks (MOFs) constitute a very promising class of materials because of the wide application prospects derived from their dynamic behaviors. Since their discovery, most of these crystalline porous solids have demonstrated the capacity to adapt their structures in response to guest molecule accommodation; however, researchers are moving beyond this typical host–guest process using more widely applicable and controllable physical stimuli such as mechanical stress, which provides a simple but powerful tool for fine-tuning flexible MOF structures and their porous properties in a controlled manner. This opinion summarizes the progress in the development of mechanoresponsive MOFs, and presents novel opportunities and future challenges associated with the combination of mechanical forces with flexible MOF systems. Flexible metal-organic frameworks (MOFs) constitute a very promising class of materials because of the wide application prospects derived from their dynamic behaviors. Since their discovery, most of these crystalline porous solids have demonstrated the capacity to adapt their structures in response to guest molecule accommodation; however, researchers are moving beyond this typical host–guest process using more widely applicable and controllable physical stimuli such as mechanical stress, which provides a simple but powerful tool for fine-tuning flexible MOF structures and their porous properties in a controlled manner. This opinion summarizes the progress in the development of mechanoresponsive MOFs, and presents novel opportunities and future challenges associated with the combination of mechanical forces with flexible MOF systems. 2D or 3D crystalline porous structures in which organic building blocks, entirely composed of light elements (boron, carbon, nitrogen, oxygen, and silicon), are linked by strong covalent bonds. a dynamic tessellation (periodic arrangement in a plane of regular polygons sharing edges with no overlaps and no gaps) where the polygons are hinged at their vertices and can freely and reversibly rotate to create different periodic arrangements. the relative volume change of an object when exposed to mechanical stress. class of materials with unconventional properties due to the topological arrangement of their structural building blocks into periodic lattice. When external stimuli such as pressure is applied, their mechanical response, via the bending, buckling, or hinge tessellation of structural motifs, can lead to nonlinear phenomena such as negative linear compressibility or negative Poisson’s ratio. according to IUPAC definition, MOFs are a class of coordination polymers with an open framework containing potential voids. MOF is obtained by the assembly between metal ions and organic ligands, through the coordination bond formation, to give crystalline 2D or 3D networks. a counterintuitive mechanical response of an object which expands in one direction upon isotropic (uniform) compression. also called auxetic behavior, it is the counterintuitive mechanical response of an object which expands or contracts in the direction perpendicular to the direction of stretching or compression, respectively. tendency of an object to expand or contract in the direction perpendicular to the direction of compression or extension, respectively. tendency of an object to resist transverse deformation when opposing forces are applied on parallel faces of an object. tendency of an object to deform along an axis when opposing forces (tension or compression) are applied along that axis. For an isotropic material (same deformation in response to force regardless of orientation), the moduli are connected via the equation E = 2G(1 + ν).