Light-triggered chemistry in a single molecule

Controlling the chemical structure of matter at the atomic level with light seemed impossible until now. Now, scientists have developed a technique to control photochemical reactions at the level of individual molecules. An international team of researchers, including Tomáš Neuman from the Institute of Physics at CAS, has published a method for controlling molecular dynamics in Nature Nanotechnology. This breakthrough could open a new chapter in photochemistry research.

In the future, this could be used to perform photochemical reactions at the level of individual molecules, and thus perhaps combine molecules into nanostructures with new properties that will find applications in optoelectronics and nanotechnology.

Photochemical reactions are commonly present in nature and industry, and are behind, for example, the ability of the eye to register light or the reactions leading to the joining of molecules into chains (polymers), which find applications in 3D printing, among other things. In all these cases, however, a large number of molecules react with light at the same time, which complicates the use of photochemistry in the nanoscale world.

"From the complex experiments of alchemists and the empirical knowledge of later chemists, we have finally reached a direct experimental representation and control of the mechanisms behind many light-triggered chemical reactions that occur both in nature and in the chemical industry," comments Tomáš Neuman from the Institute of Physics of the CAS.

It was his theoretical calculations that helped interpret the results of a team of experimenters led by Guillaume Schull of the Centre national de la recherche scientifique (CNRS) in Strasbourg, France, with research supervised by Anna Roslawska, now at the Max Planck Institute in Stuttgart.

Interaction of light and matter: what happens between a photon and a single molecule

The international team used the ability of the tip of a scanning tunneling microscope (STM) to concentrate light to the atomic level to trigger a chemical reaction in an isolated molecule. These special tips work similarly to conventional TV antennas, except that instead of radio waves, they interact with light to guide the atomically sharp tip.

The tip can then be used as an atomic-sized flashlight, i.e., of dimensions approximately 100 times smaller than the wavelength of light. With this technique, scientists have succeeded in concentrating light into a volume comparable to the size of a single molecule, and in addition, by moving the tip over different parts of the molecule, they can influence its chemical structure.

At cryogenic temperatures, the international team was able to use this technique to induce the joint movement of two protons in the molecule - and they controlled the rate and the resulting state of the phenomenon, called tautomerisation, using changes in the wavelength of light and the movement of the tip. The tip of the microscope not only collects light, but also emits it, so that by rasterising each point, the scientists can obtain information about how much light is coming out of the molecule and what colours are represented in the emitted light.

Based on this knowledge, the scientists then evaluated the tautomerisation process. The method described provides a detailed view of the interaction of light with the phthalocyanine molecule, which absorbs light strongly and is used as a dye for plastics or textiles.

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