How a Single Atom Influences the Spatiotemporal Response of Flexible MOFs: Insights from Theory and Experiment

Flexible metal-organic frameworks (MOFs) are porous materials exhibiting spatiotemporal responses to environmental changes, which significantly affect their sorption properties and offer potential technological breakthroughs. In this study, we investigate two isostructural thiazolo[5,4-d]thiazolate MOFs, UAM-1S and UAM-1O, which differ by a single atom, sulfur or oxygen, in the angular dicarboxylate group. Despite this subtle modification, the materials trigger distinct structural adaptation mechanisms: a continuous in UAM-1S and a discrete in UAM-1O. Using a combination of experimental and theoretical approaches, including microcrystal electron diffraction and DFT analysis, we reveal the factors driving different transition mechanism. Appropriate treatment of UAM-1O, combined with single-crystal X-ray diffraction analysis, revealed the structure of the explosive metastable open phase, corroborating theoretical predictions. Furthermore, a time-resolved in situ powder X-ray diffraction data set was collected under varying CO₂ at pressures exceeding the cp-op structural transition pressure at 195 K, enabling the application of the Kolmogorov-Johnson-Mehl-Avrami equation to analyze the kinetics of adsorption. Holistically, our work enhances the understanding of the key factors responsible for the time-dependent response of flexible materials with implications for the design of dynamic materials.