Guest-modulation of the mechanical properties of flexible porous metal–organic frameworks†
The highly flexible hybrid nanoporous MOF MIL-53(Cr) was evoked as a potential medium to store mechanical energy via a structural switching from an open to a close pore form under moderate applied external pressures. Herein, we show that the inclusion of a low concentration of either polar or apolar molecules into the pores can finely tune the structural and energetic behaviour of this solid under compression–decompression. This allows a modulation of the material's storage abilities in the form of not only nano-springs/dampers but also shock adsorbers by confining n-alkane and water–alcohol, respectively. Predicting and further understanding the impact of each guest on the mechanical properties of this MOF are achieved by molecular dynamics simulations based on a refined version of a flexible force field able to accurately capture the breathing of the framework. A careful analysis of the host–guest interactions and the preferential conformations of the confined molecules validated by in situ X-ray diffraction and microcalorimetry data shed light on the microscopic mechanisms at the origin of the singular mechanical behaviour of each guest loaded material.