Imaging the dynamic influence of functional groups on metal-organic frameworks

Abstract

Metal-organic frameworks (MOFs) with different functional groups have wide applications, while the understanding of functionalization influences remains insufficient. Previous researches focused on the static changes in electronic structure or chemical environment, while it is unclear in the aspect of dynamic influence, especially in the direct imaging of dynamic changes after functionalization. Here we use integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) to directly 'see' the rotation properties of benzene rings in the linkers of UiO-66, and observe the high correlation between local rigidity and the functional groups on the organic linkers. The rigidity is then correlated to the macroscopic properties of CO₂ uptake, indicating that functionalization can change the capability through not only static electronic effects, but also dynamic rotation properties. To the best of our knowledge this is the first example of a technique to directly image the rotation properties of linkers in MOFs, which provides an approach to study the local flexibility and paves the way for potential applications in capturing, separation and molecular machine.

Fig. 1. Crystal structure of UiO-66-X.

a Schematic model of UiO-66. The views from [110] and [001] projections are shown below. The long and short axes from the [110] projection are marked by blue and green arrows, respectively. The grey, red and white atoms represent C, O and H elements, respectively. b XRD patterns of UiO-66-X and simulated results of UiO-66-H. Source data are provided as a Source Data file.

Fig. 2. Characterizations of UiO-66-NH₂ and UiO-66-Br.

HAADF-STEM images and corresponding EDS mapping of UiO-66-NH₂ (a) and UiO-66-Br (d). iDPC-STEM images of UiO-66-NH₂ (b) and UiO-66-Br (e) from [110] orientation. The FFT pattern is shown as inset. Magnified iDPC-STEM images and simulated results of UiO-66-NH₂ (c) and UiO-66-Br (f). The intensity profile along the long axis and the model of BDC-NH₂ rotation are shown below. The dark lines in the intensity profile are experimental results, while the light lines are the simulated results. The scale bar is 1 nm. The grey, red, white, blue and brown atoms represent C, O, H, N and Br elements, respectively. Source data are provided as a Source Data file.

Fig. 3. Rotation properties of benzene ring in UiO-66-X.

iDPC-STEM images, simulated results and intensity profile along the long axis of UiO-66-H (a), UiO-66-CH₃ (b), UiO-66-OH (c), UiO-66-F (d) and UiO-66-Cl (e). The dark lines in the intensity profile are experimental results, while the light lines are the simulated results. The scale bar is 1 nm. f Summarized experimental and simulated FWHM of benzene rings along the long axis in UiO-66-X. The experimental FWHM is the averaged value obtained from more than 20 cells, and the error bar is the standard deviation. Source data are provided as a Source Data file.

Fig. 4. Investigating the rotation of benzene ring in UiO-66-X via DFT calculations.

a Schematic model of benzene rotation in UiO-66-OH. b Schematic model of two types of UiO-66-OH, with different hydrogen direction in the hydroxyl group of BDC-OH linkers. c The calculated rotation energy of UiO-66-X against the rotation degree of benzene ring. The grey, red and white atoms represent C, O and H elements, respectively. Source data are provided as a Source Data file.

Fig. 5. Influence of rotation energies on the experimental FWHM and CO₂ adsorption properties.

a Fitted FWHM against rotation energies. The fitted data are measured with UiO-66-X (X ≠ H), and the correlation is $\mathrm{FWHM}/\mathrm{\AA }=1.89\times \sin \left(\frac{0.18\mathrm{eV}}{E_{\mathrm{Rotate}}}\times \frac{\pi }{2}\right)+1.82$. The corresponding R² is 0.96. The values of FWHM and $E_{\mathrm{Rotate}}$ are obtained from iDPC-STEM images and DFT calculations, while other parameters of d, $E_0$ and $d_{\mathrm{atom}}$ are fitted. b Schematic model of the imaging of BDC-X linkers. c Correlation between CO₂ uptake and the rigidity of UiO-66-X. The values of CO₂ uptake are from Ref. and the adsorption conditions are 1 bar and 0 ^oC. Source data are provided as a Source Data file.