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 <i>in situ</i> atomic force microscopy to observe the crystal surface of Ga-MIL-53 MOF, [Ga(OH)(BDC)] (<b>1</b>) (BDC - 1, 4-benzenedicarboxylate) as it undergoes flexing transformations during the guest exchange between N,N-dimethylformamide (DMF) and ethanol (EtOH)-containing <b>1</b>. <b>1</b>·0.96DMF undergoes a flexing expansion transformation on guest exchange to form <b>1</b>·<i>x</i>EtOH 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 <b>1</b>·<i>x</i>EtOH undergoing a flexing contraction transformation on guest exchange to form <b>1</b>·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.