Exceeding flexpectations: a combined experimental and computational investigation of structural flexibility in...

Designing sorbents for the separation of molecules with sub-angstrom differences in size requires precise control over pore size and environment, which can be challenging to establish in the presence of structural flexibility. However, metal-organic frameworks (MOFs) that incorporate 3-dimensional (3D) linkers-ditopic ligands with 3-dimensional, sterically bulky cores-are well-suited to address this challenge, as 3D linkers enable sub-angstrom level control over pore size by mitigating the effects of structural flexibility. In this study, we used a combined computational and experimental approach to quantify flexibility in two systems of MOFs with increasing linker bulkiness, leveraging these systems to distinguish between two classes of flexibility: global and local. Specifically, we used density functional theory (DFT) calculations to understand the electronic energy landscapes of MIL-53(Al), MIL-47(V) and their corresponding 3D linker analogues of increasing bulkiness. We further characterized the mechanical properties of these materials with DFT calculations of elastic tensors and in practical compression conditions using in situ variable pressure X-ray diffraction experiments. Finally, we illustrated the importance of establishing sub-angstrom level pore control by demonstrating the effects of each type of flexibility on the adsorption properties of MOFs using grand canonical Monte Carlo simulations.