Probing Molecular Motions in Metal-Organic Frameworks by Three-Dimensional Electron Diffraction

Area of Science:

• Materials Science
• Crystallography
• Nanotechnology

Background:

• Flexible metal-organic frameworks (MOFs) exhibit diverse functionalities and tunable pore structures, with dynamic motions being key to guest diffusion.
• Characterizing these molecular motions is vital for understanding MOF properties and applications.
• Traditional methods like NMR and SCXRD have limitations regarding sample purity and crystal size.

Purpose of the Study:

• To investigate molecular motions in MOFs using three-dimensional electron diffraction (3D ED).
• To demonstrate 3D ED's capability in analyzing dynamics of nanocrystalline materials and phase mixtures.
• To compare linker motion in different structural environments within MOFs.

Main Methods:

• Utilized three-dimensional electron diffraction (3D ED) for structural analysis of MOFs.
• Performed *ab initio* structure determination of MIL-140C and UiO-67.
• Analyzed atomic anisotropic displacement parameters (ADPs) and thermal ellipsoid models at 298 K and 98 K.

Main Results:

• Successfully applied 3D ED to study molecular motions in nanocrystalline MOFs (MIL-140C and UiO-67) present in a mixture.
• Observed and quantified dynamic motions of linker molecules within the MOF frameworks.
• Found that linker motions in MIL-140C were significantly influenced by π-π stacking interactions.

Conclusions:

• 3D ED is a powerful technique for probing atomic-level dynamics in MOFs, especially for nanocrystalline samples and mixtures.
• The findings provide insights into structure-dynamics relationships in MOFs.
• 3D ED offers an alternative to SCXRD for studying molecular motions with comparable accuracy.