Tiny magnetic probes can be used to investigate the chemical forces at work inside the pores of molecular framework compounds. Until now, this had only been possible to a limited degree. Developed by a team at Martin Luther University Halle-Wittenberg (MLU), the method could help to better understand the properties of these materials and control these properties in the future. The study was published in the journal Small.
Covalent organic frameworks (COFs) are regarded as promising materials for a range of applications. Essentially, they are porous, crystalline compounds. Their regular, tiny pores are what make them special. "These pores make COFs suitable for a range of applications. They could be used to filter pollutants out of water or as catalysts for chemical reactions," says Simona Bassoli from the Institute of Chemistry at MLU. A prerequisite for their application is that the chemical environment inside the pores can be specifically fine-tuned. And to do that, one must first be able to measure it precisely.
However, doing exact measurements is a problem: Because the pores are only a few nanometres in size, optical methods fall short. And there is a limit to what other direct measurement methods can achieve. The standard method currently being used measures how many nitrogen molecules become absorbed in the pores at around minus 196 degrees Celsius. "At these temperatures, all molecular motion freezes. You get a snapshot without any information about chemical interactions or important characterisation of the pores' properties," adds Sebastian Michler from the Institute of Chemistry at MLU.
The idea for the new research project emerged from the incubator programme of the "BEAM" Research Training Group, where Bassoli and Michler are pursuing their PhDs. As part of this programme, PhD students learn how to apply for, plan and carry out their own research projects independently. The team produced two variants of COFs with contrasting pore properties: one with water-attracting and one with water-repelling molecular chains. This difference was intended to demonstrate whether the method could detect chemical differences.
Instead of nitrogen, the researchers added a special type of molecular-scale magnetic probe that dissolves in water at room temperature. These probes can be stimulated using microwaves within a strong magnetic field and thus transmit a response signal. The researchers used a technique known as electron spin resonance (EPR) spectroscopy. The basic idea was that when the probes interact with the walls of the pores, their response signal would change. The signals revealed how many probes were bound within the pores, the concentration at which they accumulated there, and the type of bond by which they adhered to the wall. The probes also clearly indicated whether, and to what extent, the molecular chains of the pores were hydrophilic or hydrophobic.
The method currently supplements established analytical techniques, as it provides different information about the same material. Next, the researchers would like to determine whether the probes can be used to identify other material properties, and whether the findings can be applied to other porous materials.
The research was funded by the German Research Foundation (DFG) as part of the Graduate Training Group 2670 "BEAM – Beyond Amphiphilicity".
Study: Michler S., Bassoli S. et al. Characterization of the Pore Environments in Covalent Organic Frameworks by Dynamic Spin Probe Exchange. Small (2026). doi: https://doi.org/10.1002/smll.75070
