Currently, host-guest interactions in metal–organic frameworks (MOFs) are mainly understood via theoretical simulations, bulk characterization techniques, and x-ray diffraction, all of which can average out information related to defects, interfaces, and spatial heterogeneity among adsorption sites. Since these structural features can directly impact the functionality of MOFs we applied local probes to investigate these effects. Specifically, we applied both 3D electron diffraction and 4D scanning transmission electron microscopy to structurally characterize and spatially visualize the effect of gas-loading and diamine functionalization in MOF crystals. In addition, complementary infrared scattering scanning near-field optical microscopy measurements confirmed spatial variations of chemisorbed CO2 inside an individual crystal.
The results of our multimodal approach was the ability to map heterogeneity arising from host-guest interactions and material functionalization. Our 3D electron diffraction data has provided the only ab initio electrostatic potential map of any variant of Mg2(dobpdc), which has been an elusive structure. Our 4D-STEM images are, to our knowledge, the only dual-space (i.e., real and reciprocal space) visualizations of a MOF. Most importantly, although we have applied this methodology to a specific material of interest in gas storage and separation, we hope to introduce these microscopy techniques to the community as effective tools, complementary to the established bulk techniques, by which to evaluate and optimize synthetic and post-synthetic modification procedures of MOFs.