Nanoscale Flexing Mechanism of a Metal–Organic Framework Determined by Atomic Force Microscopy

Flexible metal-organic frameworks (MOFs) are a unique set of compounds with applications in diverse areas. The nanoscale mechanism through which they flex is unproven. Herein, we use in situ atomic force microscopy to observe the crystal surface of Ga-MIL-53 MOF, [Ga(OH)(BDC)] (1) (BDC - 1, 4-benzenedicarboxylate) as it undergoes flexing transformations during the guest exchange between N,N-dimethylformamide (DMF) and ethanol (EtOH)-containing 1. 1·0.96DMF undergoes a flexing expansion transformation on guest exchange to form 1·xEtOH through the passage of wavefronts of cooperatively transforming, consecutive rows of unit cells parallel to the (011) plane, resulting in whole (011) layers of unit cells transforming by a layer-by-layer shear mechanism. The reverse process involves 1·xEtOH undergoing a flexing contraction transformation on guest exchange to form 1·0.96DMF through a layer-by-layer shear mechanism involving layers of unit cells parallel to the (011̅) plane transforming in a cooperative manner. This proves a nanoscale mechanism through which a MOF can flex and the coexistence of phases with different degrees of expansion within a crystal, thus providing a missing link in the multilength scale understanding of MOF flexing transformations, which will support future design and application of flexible MOFs and other extended crystalline solids.