Unraveling Structure and Dynamics in Porous Frameworks via Advanced In Situ Characterization Techniques

Dynamic metal–organic frameworks (MOFs) represent a subgroup of frameworks featuring unique performance, as they are capable of adapting their pore size and/or the orientation of framework constituents in response to specific guest molecules such as gases or solutes and often outperform their rigid analogus in gas storage, sensing, or separation. In this review, the authors focus on recent methodical developments of advanced in situ diffraction and spectroscopic techniques for comprehensive characterization of porous frameworks. Examples for advanced instrumentation are highlighted for in situ nuclear magnetic resonance, electron paramagnetic resonance, and optical spectroscopies as well as X‐ray and neutron diffraction. Several examples of high‐resolution transmission electron microscopy (HRTEM) on MOFs are shown because HRTEM is an emerging technique for the characterization of time‐resolved structural dynamics in MOFs. These methods shed light on structural features and phase transitions of the host, its spin state, electronic structure, specific host–guest and guest–guest interactions, preferable adsorption sites, and bonding situation in the framework, focusing on the most prominent recent case studies. The synergistic development of novel in situ characterization methods and exploration of well‐defined model framework systems are crucial to advance the understanding of dynamic processes in porous materials in future. Structural switchability, accompanied by a large breathing amplitude of the unit cell, is a unique phenomenon in flexible metal–organic frameworks (MOFs). Recent advances in the development of in situ spectroscopic (optical, nuclear magnetic and electron paramagnetic resonance) and diffraction (X‐ray, neutron and electron) techniques for monitoring the dynamic behavior of switchable MOFs upon applying external stimuli are reviewed.