For years, research in the field of metal-organic frameworks primarily involved synthesising new materials from unprecedented combinations of ligands and metals, and seeing how well they could adsorb gases. Things got more exciting, at least in my opinion, when some of these materials were discovered to be dynamic — flexible enough that their lattices undergo reversible structural transitions in response to external stimuli.
MOF chemists took to exploring how far this phenomenon could stretch, looking at introducing flexibility by design, and testing framework responses to a range of physical and chemical stimuli. The most studied dynamic MOFs remain those whose pores breathe upon guest adsorption/desorption, taking on open pore (or large pore) versus closed pore (or narrow pore) configurations. Phase transitions triggered by mechanical stimuli, on the other hand, are much less common, and only the MIL-53/MIL-47 family (developed and studied by the late, great Gérard Férey), whose lozenge-shaped channels are well-placed to contract anisotropically under mechanical pressure, have boasted such transformations.
Now, Sebastian Henke at TU Dortmund and colleagues discover that mechano-switching behaviour extends to some members of the zeolitic imidazolate framework family. The team took to the Diamond Light Source synchrotron in the UK and investigated the responses of ZIF-4(Zn) and ZIF-4(Co) to mechanical pressure inside a diamond anvil cell, using synchrotron powder X-ray diffraction and mercury intrusion measurements. Interestingly, this isn’t the first time that such experiments have been performed on ZIF-4(Zn), but fully evacuating guest molecules from the material now leads to a previously-unobserved phase transition. This time, Henke and team find that the framework undergoes an almost isotropic unit cell contraction at 28 MPa, decreasing in volume by ~21%. This transition sees the framework transform from an open pore phase with continuous porosity to a closed pore phase with inaccessible porosity.
Surprisingly, ZIF-4(Zn) and ZIF-4(Co) demonstrate major differences in their responses. The ZIF-4(Co) transition requires a pressure of 50 MPa to be initiated, and fully returns to the open phase after decompression. The closed pore phase of ZIF-4(Zn), on the other hand, requires heating at 130 °C to revert back to the open framework. These behavioural differences are ascribed to the nature of the ligand-to-metal bonding, which in turn differs as a result of the respective electronegativities and electron configurations of the Zn2+ and Co2+ cations.
As well as these mechanically-induced structural changes being fundamentally interesting to study, the team estimated a gravimetric entropy change of 300 J K-1 kg-1 for the ZIF-4(Zn) phase transition, which could make these MOFs competitive for applications in mechanocalorics, where current benchmark materials typically exhibit gravimetric entropy changes of ∼100–150 J K-1 kg-1.