Tracking molecules on their hiking tour in the pore maze

In order to be able to observe the molecules in detail on their journey through the labyrinth of pores of the Covalent Organic Frameworks (COFs), the chemist Dr. Lars Grunenberg had to use a certain number of tricks.

As mini-reactors, molecular sponges, and high-tech materials, highly porous covalent organic frameworks (COFs) have been making a name for themselves for several years. The details of what goes on in the nanostructured pore channels are of great importance for molecular design. Now, a team of researchers from the Max Planck Institute for Solid State Research, in cooperation with the University of Stuttgart, has watched molecules migrate inside their pore space, providing essential insights to further establish COFs for their versatile range of applications in catalysis and modern energy technologies.

Only a few substances are as full of air as Covalent Organic Frameworks, or COFs for short. But it is what encloses the air that is interesting: a stable, crystalline network of organic molecules. The highly porous framework structures are interspersed with pore channels on the nanometer scale. Similar to the cavities of a sponge, but in a regular, tailor-made shape, the COFs offer an enormous surface area in a tiny space. This makes them promising materials for catalytic processes, gas storage, and even battery materials. “But for COFs to develop their full potential, we need to understand exactly how molecules migrate through the tiny one-dimensional pore channels and what influences them,” says chemist Dr. Lars Grunenberg, who did his PhD thesis with Prof. Bettina Lotsch at the Max Planck Institute for Solid State Research in Stuttgart and whose work is funded by the Cluster of Excellence e-conversion. “That’s why we developed a model system with which we can investigate the mobility of small molecules in the COFs,” he explains. The team collaborated with a research group led by Prof. Nils Hansen at the University of Stuttgart, who simulated the model system using theoretical calculations. The results were recently published in the scientific journal ACS Nano.

What slows down molecules

However, the researchers had to use a few tricks to watch the molecules on their journey through the labyrinth of pores in the COFs, explains Grunenberg: “There is one main reason why we are the first to look at these diffusion processes: it is not only preparative but above all analytically challenging. We use a special variant of NMR spectroscopy, known as pulsed field gradient NMR, to track the molecules, with which we can detect the spatial displacement of certain atomic nuclei.” For his investigations, the chemist synthesized two different COF structures – one variant with a higher porosity and one with reduced – and analyzed the diffusion of solvent molecules, as a probe, at various temperatures. Using the in situ NMR method, he was able to show that they move much more restrictedly in the case of the less porous, disordered COF network. “This also confirms our expectations. Just as in the case of the ordered COF network, where molecules encounter less obstacles, giving way to observing anisotropic diffusion, i.e., a directional movement of the molecules along the one-dimensional pore channels,” he says.

Dr. Lars Grunenberg, who did his PhD thesis with Prof. Bettina Lotsch at the Max Planck Institute for Solid State Research in Stuttgart, uses a special variant of NMR spectroscopy, known as pulsed field gradient NMR (on the right), to track molecules in the pores of Covalent Organic Frameworks (COFs).

Dynamics provide insights into the structure

In retrospect, this sounds plausible and uncomplicated, but the scientist had to push the NMR device to its limits for the measurements. “The aim was to build up strong, but short, magnetic gradient field pulses to measure, as the signal decays very fast, if the molecules are confined in these channels. This is technically demanding, but in return, it provides us with the desired information about the dynamics of the molecules and thus, the internal structure of the COFs,” reveals the researcher, who was also delighted that his successful experiments were supported by theoretical simulation models developed by the Stuttgart research team. The investigations show what influence so-called real structure effects, i.e., deviations from the idealized structure of a material due to defects or irregularities, can have on the physicochemical properties of COFs.

Scaffold structures with potential

With their publication, the scientists have created a robust basis for further investigations to study mass transport in COFs. The concept can be transferred to other molecules and thus offers the opportunity to study reactions in the pore channels of the COFs and reveal the effect of mass transport limitations on the reactivity. “This, in turn, allows us to draw conclusions about the design of the scaffold structures and how the molecules that build them must be composed,” explains Grunenberg. “Our investigations are a further building block of knowledge for expanding the field of application of COFs and further improving these versatile materials – be it for the adsorption of gases, or complex catalytic reactions such as electrochemical CO₂ reduction,” says the researcher, giving an outlook on the potential of these special organic framework materials.

Publication

Probing Self-Diffusion of Guest Molecules in a Covalent Organic Framework: Simulation and Experiment
L. Grunenberg, C. Keßler, T. Wei Teh, R. Schuldt, F. Heck, J. Kästner, J. Groß, N. Hansen, B. V. Lotsch
ACS Nano, Vol 18, Issue 25, June 2024
https://pubs.acs.org/doi/full/10.1021/acsnano.3c12167

Contact

Dr. Lars Grunenberg
Nanochemistry Department
Max Planck Institute of Solid State Research
Email: l.grunenberg@fkf.mpg.de

Prof. Bettina Lotsch
Nanochemistry Department
Max Planck Institute of Solid State Research
Email: b.lotsch@fkf.mpg.de

Website: https://www.fkf.mpg.de/lotsch