Metal-organic frameworks with engineered flexibility for pressuredriven applications

Metal-organic frameworks (MOFs) are a relatively new type of hybrid microporous materials, consisting of inorganic nodes which are interconnected in an ordered way in space by organic linker molecules. Some MOFs feature the fascinating ability to undergo reversible structural transformations in response to e.g. adsorption of guest molecules or external pressure. Unfortunately, few strategies exist to deliberately synthesize such flexible, or ‘breathing’ materials. In this project, we propose a more generalized concept to introduce flexibility in MOFs. Rather than relying on flexibility at the node-linker connection, which is the current paradigm in breathing MOFs, we will replace the rigid, often aromatic linkers present in many MOFs by their flexible, aliphatic counterparts. Such MOFs can shrink or expand by conformationally ‘(un)folding’ their linkers. Our strategy not only enables breathing, but directly offers ways for its fine-tuning. For instance, the pressure needed to shrink the framework could be increased by mixing both rigid and flexible linkers in one material, or by adding substituents on the used flexible linkers to make folding up more difficult. Such precisely engineered materials could be put to work as shock absorbers, or as easily regenerated adsorbents by pressure-induced desorption. Even MOF particles with a core of rigid linkers and an outer shell of flexible linkers, with a potential for triggered release of molecules, are within reach.