Chapter 12: Studies of Metal–Organic Frameworks: Xenon for Probing Framework Porosity, Breathing and Gating Behavior

During the past decade, metal–organic frameworks (MOFs) – novel micro- and mesoporous materials of extraordinarily high porosity – have attracted increasing research interest. In addition to the established solid-state NMR spectroscopic techniques for the characterization of their frameworks, ¹²⁹Xe NMR provides a valuable tool for characterizing the porosity of these materials. Moreover, host–guest interactions and guest mobility can also be studied using xenon. Apart from rigid MOFs, some of these compounds exhibit pronounced structural changes under the influence of adsorbed species induced by the resulting host–guest interactions (“breathing effect”). In extreme cases, this breathing effect even results in “structure switching” between an entirely “closed” state which cannot be penetrated by guest molecules and a highly porous state capable of adsorbing guest molecules. This switching (“gate pressure effect”) is dependent on the type of the adsorbed species, the pressure, and the temperature. ¹²⁹Xe NMR spectroscopy is a useful method to investigate these amazing processes.