Adsorption Deformation and Structural Transitions in Metal−Organic Frameworks: From the Unit Cell to the Crystal

Much attention has recently been focused on soft porous crystals, a fascinating subclass of metal−organic frameworks that behave in a remarkable stimuliresponsive fashion, presenting structural changes of large amplitude in response to guest adsorption, mechanical pressure, or variations in temperature. In this Perspective, we summarize the recently developed thermodynamic and mechanical theoretical models for the understanding of these materials, based on the concepts of adsorption stress and osmotic thermodynamic ensemble. We show how these models provide a coherent picture of adsorption-induced deformation and structural transitions in flexible metal− organic frameworks, all the way from the length scale of the unit cell to that of the full crystal. In particular, we highlight the new perspectives opened by these models, as well as some of the important open questions in the field. M attention has recently been focused on metal− organic frameworks (MOFs), a wide class of designer microporous materials that garner a lot of interest for their potential applications in the fields of separation, catalysis, strategic gas capture and storage, and drug delivery. Soft porous crystals (SPCs) represent a specific subclass of MOFs with remarkable flexibility. These materials feature dynamic crystalline frameworks with reversible structural transformations of large amplitude in response to variation of external physical stimuli such as gas adsorption, temperature, or mechanical pressure. SPCs display a large gamut of flexibility mechanisms depicted in Figure 1, including local framework dynamics, such as sorption-induced linker reorientation in ZIF8; negative thermal expansion, as in the IRMOF family of materials; gradual swelling upon uptake of solvent molecules, as demonstrated in the MIL-88 family; gate opening, which involves an abrupt structural transition between closedand open-pore structures that is induced by gas adsorption (e.g., in the ELM family); and the breathing phenomenon, which consists of two successive adsorption-induced crystal-to-crystal transformations, for example, in the case of the MIL-53 family of materials, from a large pore (lp) state to a narrow pore (np) state and back again to the lp state. A growing number of SPC structures were synthesized and reported in the literature, and although none of them have yet been directly used at the industrial scale, they have been proposed for a large range of potential practical applications. In addition to the general application of MOFs, flexible frameworks are expected to present an intrinsic interest due to their large-scale stimuliresponsive transformations, which could leverage novel nanobiotechnologies, such as sensing for detecting traces of organic molecules, slow release of drugs for long-release single-injection therapies, and specific gas separations. The adsorption-induced deformation and adsorption-induced transitions in flexible MOFs were first addressed in the literature with respect to their structures and energetics, both experimentally and using molecular simulation methods. From the theoretical point of view, there are two key fundamental concepts for understanding the behaviors reported in the literature. First, the adsorption-induced deformation can be viewed as a response of the porous material to an adsorptioninduced stress, exerted by the adsorbed fluid on the host solid. Second, a rigorous thermodynamic description of the interplay between adsorption and deformation including structural transitions is provided by the use of the osmotic ensemble, a semiopen statistico-mechanical ensemble in which the quantity of solid, temperature, and mechanical stress are fixed, while the quantity of guest molecules and the unit cell shape and volume may change. The osmotic ensemble has been used to build theoretical models of the influence of adsorption on the equilibrium between metastable host structures. Such Received: July 4, 2013 Accepted: September 12, 2013 The adsorption-induced deformation can be viewed as a response of the porous material to an adsorption-induced stress. Perspective