FOR 2433: Switchable Metal-Organic Frameworks (MOF-Switches)

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Porous materials play a key role in gas and liquid phase separations, energy storage, as catalysts and for optical and chemical sensing. Metal-Organic Frameworks (MOFs) stand out among other porous materials due to their extremely high porosity and modular tunability. While the majority of porous solids (and MOFs) is rigid, a novel and unique class of switchable MOFs was discovered in recent years. These materials only open their pores dynamically, as a response to the presence of gases or liquids at a characteristic concentration associated with unprecedented, step-wise unit cell volume changes (more than 240 %) during gas uptake. Such switchable MOFs are able to specifically respond or even recognize certain types of molecular species by opening their pores, resulting in a step-wise change of physical (i.e. magnetism, optical density, bulk density, etc.) and chemical characteristics (catalytic activity, reactivity). Moreover, they reversibly close their pores in the absence of the respective species. A principle understanding of the dynamic phenomena in such materials would represent a unique technological basis for the design of switchable catalysts, filters, threshold sensors, or stimulus induced drug delivery by receptor systems with integrated key-lock functionality. However, so far the discovery of switchable MOFs (also named gating, or breathing MOFs) was essentially accidental. Today only a limited number of such compounds are known, and it is impossible to rationally predict new switchable structures, because the underlying microstructural principles, responsible for such a high degree of flexibility, are not understood. For the technological development of switchable MOFs in separation, catalysis, or sensing, a fundamental understanding of the underlying structural principles and gas-solid interaction mechanisms is needed. The new research unit primarily addresses the fundamentals of porosity switching phenomena in the solid state and the underlying principles. Targeting idealized model materials, the role of network constituents on the degree of flexibility will be studied in a collaborative and closely coordinated experimental and theoretical approach in order to derive a predictive model for framework flexibility. Parallelized physical characterization tools will be established enabling the application of in situ global scattering techniques (XRD) and in situ local probe spectroscopies (NMR, EPR, EXAFS) in order to analyze the microscopic structural transformations and dynamics induced by host/guest interactions during adsorption/desorption. Only an interdisciplinary effort in a focused research unit can provide the structure required to develop a predictive framework for switchable MOFs fostering an intense cooperation of theoreticians, synthetic chemists, and physicists.