In Prof. Tiefeng Wang's research group at Tsinghua University, our focus lies in designing the high-performance catalysts with industrial application prospects. While working on a project of heterogeneous hydroformylation reactions, we serendipitously found that the electron microscopy can be used to characterize the linker flexibility in MOFs. Therefore, we work with Dr. Xiao Chen to investigate the rotation properties of benzene rings in UiO-66 and analyze the influence of different functional groups on local flexibility.

The dynamic properties of MOFs

Metal organic frameworks (MOFs) have been extensively studied and have potential applications in separation, catalysis and pharmacy fields. The modifiability of ligands in MOFs expands the types of these materials, enables more application scenarios, and results in different macroscopic performances. However, the relationship between ligand functionalization and different properties remains vague. Previous researches attributed the different performance mainly to the static influences of ligand functionalization, like electronic modification of either MOFs structure or supported metals. It remains insufficient regarding the understanding of dynamic influences, which are essential in molecular machines  and gas separations. Few studies reported whether the functionalization can change the macroscopic performance through dynamic effects, partially because of the lack of suitable characterization methods, especially for the microscopic rotation properties of organic linkers in MOFs. Only ²H-NMR can provide some statistical information of apparent activation energies, while direct information, such as real-space images, has not been obtained. 

Advancements in electron microscopy technology now allow researchers to directly visualize the atomic structure of MOFs. However, the imaging of their dynamic properties remains blank although MOFs are widely considered flexible. Previous studies focused on the atomic-level imaging of the static structure, such as the changes of Zr nodes after dehydroxylation and the missing defects of organic linkers and Zr nodes. The linker dynamics have not been imaged in the real space, although the breathing and other flexibility properties are highly correlated to the adsorption and catalysis performances. 

What's new in our study

In this work, we study the dynamic influence of functional groups on UiO-66 type MOFs. Using iDPC-STEM (integrated differential phase contrast scanning transmission electron microscopy) technology, we directly ‘see’ the rotation properties of benzene ring in the BDC-X (p-benzenedicarboxylic acid) linkers of UiO-66-X. 

• Key findings

    Using iDPC-STEM (integrated differential phase contrast scanning transmission electron microscopy), we directly image the BDC-X linkers in UiO-66-X. With the change of funtional groups, the rotation properties of benzene rings in BDC-X changes accordingly, resulting in significant differences in the intensity profiles along the long axis. The full width at half maximum (FWHM) of BDC-X linkers significantly broadens, which is attrbuted to the rotation of the benzene ring. Therefore, the rigidity of MOFs is highly related to the functional groups, which follows the sequence of -OH > -NH₂ > -H > -CH₃ ~ -F > -Cl > -Br. 

    The rotation energies of UiO-66-X with different functional groups are also calculates, and the results are consistent with the iDPC-STEM images that UiO-66-OH shows the most rigidity while UiO-66-Br shows the largest flexibility. The intramolecular hydrogen bonds between -OH and carboxyl O contributes to the enhanced regidity of UiO-66-OH. 

    The functionalization of linkers can affect the macroscopic performance of MOFs through dynamic effects by changing the local rigidity, which is a complement analysis of static electronic effects. Using the reported experimental data of CO2 capture, we found a positive relationship between rigidity of UiO-66-X and CO₂ uptake, which is possibly attributed to the larger number of identical porous units. 

Conclusion and outlook

In conclusion, we studied the dynamic influence of functional groups on UiO-66-X samples. Using iDPC-STEM technology, we are able to directly ‘see’ the rotation properties of benzene rings in BDC-X linkers, and the rigidity against π-flipping is highly related to the functional groups. To the best of our knowledge this is the first use of electron microscopy to image the rotation properties of organic linkers in MOFs, which is an important complement to the spectral method and provides an approach to understand the local flexibility of MOFs from a more direct perspective. Among the UiO-66-X samples, UiO-66-OH sample shows the highest local rigidity, which is attributed to the strong intramolecular hydrogen bond. The benzene rings in UiO-66-OH and UiO-66-NH₂ showed basically the same orientation at RT, which has not been reported. Moreover, the difference in the dynamic properties would be responsible for the different macroscopic properties, and we observe a positive relationship between CO₂ uptake and local rigidity of UiO-66-X. These results of various rotation properties of UiO-66-X pave the way for their potential applications in capturing of small molecules, separation of organic compounds and molecular machines.