Crystal dynamics in multi-stimuli-responsive entangled metal-organic frameworks

Abstract

An understanding of solid-state crystal dynamics or flexibility in metal–organic frameworks (MOFs) showing multiple structural changes is highly demanding for the design of materials with potential applications in sensing and recognition. However, entangled MOFs showing such flexible behavior pose a great challenge in terms of extracting information on their dynamics because of their poor single-crystallinity. In this article, detailed experimental studies on a twofold entangled MOF (f-MOF-1) are reported, which unveil its structural response toward external stimuli such as temperature, pressure, and guest molecules. The crystallographic study shows multiple structural changes in f-MOF-1, by which the 3 D net deforms and slides upon guest removal. Two distinct desolvated phases, that is, f-MOF-1 a and f-MOF-1 b, could be isolated; the former is a metastable one and transformable to the latter phase upon heating. The two phases show different gated CO₂ adsorption profiles. DFT-based calculations provide an insight into the selective and gated adsorption behavior with CO₂ of f-MOF-1 b. The gate-opening threshold pressure of CO₂ adsorption can be tuned strategically by changing the chemical functionality of the linker from ethanylene (−CH₂−CH₂−) in f-MOF-1 to an azo (−N=N−) functionality in an analogous MOF, f-MOF-2. The modulation of functionality has an indirect influence on the gate-opening pressure owing to the difference in inter-net interaction. The framework of f-MOF-1 is highly responsive toward CO₂ gas molecules, and these results are supported by DFT calculations.