Covalent organic frameworks (COFs) represent a promising energy material with the ability to harness, convert, and store energy. Despite being relatively young, this material class holds immense potential for applications in battery technology and hydrogen manufacture.
However, the synthesis of COFs has remained a challenge for scientists over the past two decades, often relying on trial and error due to a lack of understanding of the synthesis process.
To address this, Prof. Emiliano Cortés, Professor of Experimental Physics and Energy Conversion at LMU, and Christoph Gruber, who is researching this topic in Cortés’s team, have collaborated with the research group of LMU chemist Prof. Dana Medina, who specializes in the synthesis of COFs, to investigate the formation mechanism of COFs at the nano level.
Using a special microscope, the team followed the formation mechanism of the COFs, leading to groundbreaking results published in the journal Nature. These were accompanied by a video showing the processes that occur during synthesis in real-time.
Synthesis of the molecular frameworks demands precise control of the reaction and self-assembly of the molecular building blocks present. “Only when you have this control is it probable to obtain a highly crystalline structure with an extensive order and, ultimately, the desired functionality,” says Medina. “However, our knowledge, particularly of the early stages of nucleation and growth, is full of gaps. And this has thwarted the development of effective synthesis protocols. We, therefore, were extremely intrigued to visualize the reaction as it unfolds and set the focus on the earliest stages when the mixed molecular components are starting to react.”
Gruber’s pioneering investigation into COF formation began with a seemingly unconventional approach: iSCAT (interferometric scattering) microscopy, which is typically used by biophysicists to study protein interactions.
“The measurement principle is based on the fact that even the tiniest of particles, made up of just a few molecules, scatter incident light. If these scattered light waves overlap, we get interference – just like water waves in a pool. That is to say, we get larger and smaller waves depending on how the waves overlap. We record these light patterns with a high-resolution camera and, with subsequent image processing, we obtain pictures that reveal, for example, nano-scale COF particles,” explains Gruber.
The iSCAT method is suitable for capturing dynamic processes in real-time, allowing researchers to observe the synthesis live. This precise control over molecular self-assembly is crucial for the synthesis of molecular frameworks.
Upon the initiation of the reaction, the researchers were astonished to witness the emergence of minuscule structures within the transparent reaction medium.
“The images showed us that nanometer-scale droplets can play an essential role in the synthesis. Although they are extremely small, they control the entire kinetics at the beginning of the reaction,” says Gruber. “Nothing was known about their existence before now, but for the formation of the COFs we studied, the nano-droplets turned out to be extremely important. If they are absent, the whole reaction happens too quickly, and the desired order is lost.”
Using the iSCAT method, the LMU team managed to record a film showing the formation of the molecular frameworks from the beginning – with a sensitivity of just a few nanometers. “Existing techniques couldn’t capture the start of the reaction, with these nano-scale and millisecond-long processes, in real-time,” says Cortés. “Through our research, we’ve now managed to close this gap in our knowledge. At the same time, we’re getting a holistic picture of the early stages of the reaction and the progressive formation of the COFs.”
In addition, the researchers utilized the video footage and subsequent analyses to develop an energy-efficient synthesis concept. “Building on our results, we discovered how to rationally design the reaction conditions,” explains Medina. “By adding normal table salt, for example, we were able to massively reduce the temperature, such that the molecular frameworks form at room temperature as opposed to 120 degrees Celsius.”
The researchers are confident that their findings will revolutionize the approach to synthesizing over 300 different COFs, potentially propelling advancements in industrial COF production. Furthermore, the implications of the results could extend to the synthesis of other materials and chemical reactions that have not previously been observed in real-time. The researchers at LMU are enthusiastic about capturing new molecular performances on film.
Journal reference:
Christoph G. Gruber, Laura Frey, Roman Guntermann, Dana D. Medina & Emiliano Cortés. Early stages of covalent organic framework formation imaged in operando. Nature, 2024; DOI: 10.1038/s41586-024-07483-0
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