Researchers use light to control high-speed chemical reactions in a new way

Many natural and synthetic chemical systems react and change their properties in the presence of certain types of light. These reactions can occur too quickly for ordinary instruments. For the first time, researchers adopted a new technique for observing high-speed reactions. A particular type of reaction observed with this method could lead to a new optical nanotechnology.

In chemistry, molecules can be manipulated in different ways to produce different things. Isomerization, for example, is a process that modifies the arrangement of a molecule while leaving the constituent atoms unchanged. This process is found in natural systems such as the retina of the eye and artificial systems such as certain types of chemical synthesis. In many cases, isomerization essentially makes a particular region of molecules more or less ordered.

Photoisomerization is a type of isomerization that is activated by light and takes place faster than blinking. Professor Takashi Kato of the Department of Chemistry and his colleagues subjected the liquid crystal molecules of the azobenzene chemical compound to specific frequencies of ultraviolet light. The photoisomerization of a single molecule of azobenzene usually occurs at a time scale of several hundred femtoseconds (a quarter of a billionth of a second). That's about a billionth to a trillion times the time it takes to blink! The researchers discovered that the molecule then triggers molecular interactions in liquid crystals at time scales of several hundreds of picoseconds (billions of seconds).

"We have shown how to change the shape of azobenzene molecules from a straight stem shape to a slightly curved shape in a process triggered by a photo-irradiation of UV light.This curvature could result in a mechanical or electronic function, "said Kato. "The reaction propagates through the neighboring molecules of the sample, which means that it is an extremely efficient process."

This reaction does not occur, however, in isolation; it occurs in a sample of soft matter whose function depends on the constituent molecules and their behavior. In this case, the soft material can mean an artificial muscle, flexible photographic sensors or even things that have not yet been imagined. The important fact is that the initial reaction, which usually takes only hundreds of femtoseconds, triggers a reaction of the surrounding soft matter in a hundred picoseconds in an efficient manner.

"This is the fastest intermolecular movement ever seen in soft matter, in fact, we wanted to observe so quickly that we had to use very specialized methods to acquire data and visualize what was spent during these tiny delays, "continued Kato. "This would not have been possible without some unique spectral instruments handcrafted by my colleague, Associate Professor Masaki Hada of the University of Tsukuba."

These methods are known as ultra-fast transient transmission spectroscopy, which is a precise means of recording the composition of a molecular sample, and time-resolved, ultra-fast electron diffraction analogous to radiography. and allowing to observe the images of the reaction. Note that both methods are called "ultra-fast", which simply shows that other methods would have been insufficient to capture data with the desired temporal resolution by the researchers.

"I have been working as a chemist for more than 35 years on ordered molecular assemblies, such as self-assembly systems, and since then I have been a graduate student.This research advances the fundamental chemistry of photosensitive molecules in soft matter. as well as their ultra-fast photomechanical applications "concluded Kato. "It's a real privilege for me and my colleagues to work on this type of project, which we hope will contribute to the design of molecular-based materials, such as soft-body mechanisms and photo-materials. functional. "