Spin-Probe Method Reveals Chemistry Inside Framework Pores

A research group at Martin Luther University Halle-Wittenberg has built a new way of probing the chemical forces active inside the pores of molecular framework compounds, something that earlier approaches could only capture in a restricted way. The technique, described in the journal Small, may help scientists better grasp—and eventually steer—the properties of these materials.

Covalent organic frameworks, or COFs, are porous crystalline structures that many consider well suited to a wide range of uses, largely because of their uniform, nanoscale pores. Co-author Simona Bassoli explains the appeal: “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.” Putting that potential to use, however, depends on being able to fine-tune the chemical conditions within the pores, and tuning them first requires measuring them accurately. 

That measurement has proven difficult. Pore dimensions of only a few nanometers put optical techniques out of reach, and other direct measurement approaches have their own ceilings. Researchers have generally relied on tracking how much nitrogen gas the pores absorb at roughly minus 196 degrees Celsius. First author Sebastian Michler points out the drawback of that approach: “At these temperatures, all molecular motion freezes. You get a snapshot without any information about chemical interactions or important characterization of the pores’ properties.”

To test their new approach, Bassoli and Michler built two versions of a COF with deliberately opposite pore chemistries—one lined with water-attracting chains and the other with water-repelling ones—to see whether the method could pick up on the difference.

Instead of nitrogen, the team introduced a nanoscale magnetic probe that dissolves in water at room temperature. Exposed to microwaves inside a strong magnetic field, the probe emits a response signal that shifts depending on how it interacts with the pore walls, detected through electron spin resonance (EPR) spectroscopy. From these shifts, the researchers could tell how many probes attached inside the pores, how densely they gathered there, and what kind of bond held them to the pore wall—along with a clear readout of how water-attracting or water-repelling the pore surfaces actually were.

 Rather than replacing existing analytical methods, the approach adds a complementary layer of information about the same materials. The team’s next step is to test whether the probes can reveal other material characteristics and whether the technique extends to porous materials beyond COFs.