DOI: https://doi.org/10.29363/nanoge.dynamic.2023.005
Publication date: 15th February 2023
Switchable metal-organic frameworks change their structure in time and selectively open their pores, adsorbing guest molecules, leading to highly selective separation, pressure amplification, sensing, and actuation applications. While numerous studies have addressed the function of switchable MOFs under quasi-equilibrium conditions, only a few studies have addressed their dynamic response and spatiotemporal evolution experimentally. The energetic landscape and transition paths characterising the switching process of dynamic MOFs, are virtually unexplored. The three-dimensional engineering of metal-organic frameworks has reached a high level of maturity, but spatiotemporal evolution opens a new perspective towards engineering materials in the fourth dimension (time) by t-axis design, in essence exploiting the deliberate tuning of activation barriers.
This work demonstrates the first example in which an explicit temporal engineering of a switchable, gate pressure MOF (DUT-8, [M1M2(2,6-ndc)2dabco]n, 2,6-ndc = 2,6,-naphthalenedicarboxylate, dabco = 1,4 diazabicyclo[2.2.2]octane, M1= Ni, M2 = Co) is presented. Spatiotemporal response of DUT-8 is governed by an activation barrier reflected in the gate opening pressure and the activity of the pore opening stimulating guest molecule. It can be deliberately tuned by adjusting the stiffness of the metal node. Finally, we used the variation in metal and metal content to tune the gate opening rates.
Various in situ time-resolved techniques, ranging from ensemble adsorption and advanced synchrotron X-ray diffraction experiments to individual crystal analysis, were applied to analyse the switching kinetic stimulated by vapour adsorption.
Individual crystal transformation rates are orders of magnitude faster than rates estimated based on ensemble methods. However, slow-transforming ensembles show a delayed induction of individual crystal transformations. Differences in the spatiotemporal response of crystal ensembles originate from induction times that vary statistically. In DUT-8(Co/Ni) it widens characteristically with increasing cobalt content, reflecting increasing activation barriers.
The support by DFG (FOR2433) is gratefully acknowledged.
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