Observing Transient Breathing States of MIL‐53 Homologues Using In Situ Single Crystal 3D Electron Diffraction

The MIL-53 series of metal-organic frameworks (MOFs) is considered archetypal for flexible MOFs, with desolvated materials rapidly transitioning between open and closed phases in a stepwise breathing process in response to changes in temperature under ambient pressure conditions. Herein, the differing structures of MIL-53(Cr) and MIL-53(Ga) during breathing - hydrated and anhydrous, closed and open pore-are characterized by in situ single crystal 3D electron diffractionthrough varying sample conditions within the electron diffractometer. In doing so, the crystal structures of nine intermediate phases are uncovered that together represent the continuous breathing of MIL-53 under vacuum, in stark contrast to ambient pressure stepwise breathing. In addition, these structures offer insight into particle-to-particle structural heterogeneity that are averaged out by conventional powder X-ray diffraction measurements, and may begin to explain metal-dependent adsorption phenomena observed across the MIL-53 homologues. In situ 3D electron diffraction is therefore expected to become a powerful tool for in-depth structural investigations of flexible porous materials.