Supramolecular Modification of a Metal-Organic Framework Increases Flexibility and Switchability: Experimental Insights into Structural Deformation of ZIF-8

Abstract

Use of chemical modulators during the synthesis of a coordination framework is an important strategy to affect and tune properties of porous materials. Herein, we introduce an approach to understanding structural modulation and deformation of soft, porous crystals in the context of gas adsorption by using atomic force microscopy (AFM) nanoindentation and physisorption analysis. Modified nanocrystals of the prototypical zeolitic-imidazolate framework-8 (ZIF-8) were generated in triethylamine (Et3N) via an in-situ supramolecular modulation method. We demonstrate that using triethylamine as a modulator results in a conspicuous decrease in elastic modulus (0.7 ± 0.2 GPa) compared to the unmodified nanocrystals of ZIF-8 (2.1 ± 0.8 GPa). Argon gas adsorption analysis also reveals that the more flexible, amine-modified framework exhibited gradual and cooperative gate-opening structural transitions (P/P0 = 0.33 ± 0.02) compared to the steep and stepwise characteristics of unmodified samples of ZIF-8 (P/P0 = 0.45 ± 0.02). The differences in Argon gas pressure-induced structural deformations correspond to the decrease in elastic modulus of base-modified nanocrystals of ZIF-8, likely a consequence of the impact of Et3N on the relative number of point defects and hydrogen bonds in the framework. Our study demonstrates how a combination of AFM nanoindentation, gas uptake, and spectroscopic characterization can be applied to understand structural and energetic changes upon adsorption in flexible framework materials.