"After Half a Century, the 'Secret Door' Opens... Principle of Light-Responsive 'Molecular Switch' Unveiled"

"What is the principle behind azobenzene's shape-shifting ability?"

The secret that countless researchers could not unlock for half a century has finally been revealed. By capturing the structures of two short-lived intermediates at the atomic level and linking them, the research team was able to reconstruct the reaction process as a "molecular video", which became the key to opening the door.

'Azobenzene' is a representative "molecular switch" that responds to light. This latest discovery is expected to provide new clues for designing materials that move in response to light, as well as ultra-small molecular machines.

AI-generated image. KAIST

On October 1, KAIST announced that the research team led by Professor Lee Hyocheol from the Department of Chemistry, in collaboration with the Center for Advanced Reaction Dynamics at the Institute for Basic Science (IBS), had identified the structural transformation process of "azobenzene".

Azobenzene consists of two benzene rings connected by two nitrogen atoms. When exposed to light, the two rings, which initially face opposite directions, can change to face the same direction. The atoms themselves remain the same, but their positions change.

So far, this property has been utilized in studies that control drug activity with light or create light-responsive materials and molecular machines.

However, simply knowing the molecular shapes before and after the transformation was not enough to understand the entire process (the motion). The reason is that the structure changes too rapidly during the transition, making it difficult to observe the underlying principle. For the same reason, there have been ongoing debates among researchers about how much the two rings move, whether the central connecting part straightens, and whether multiple sections twist together.

To resolve these questions, the joint research team tracked the molecular transformation process using the X-ray free-electron laser at the Pohang Accelerator Laboratory. They irradiated azobenzene dissolved in methanol with a laser to initiate a reaction and measured the structural changes over time with an ultrafast X-ray beam.

During these measurements, the team separated the molecular signals—as one would pick out a quiet voice in a noisy room—and used the resulting data at each moment to reconstruct a video of the molecule's movement.

As a result, it was observed that azobenzene does not change its structure by simultaneous rotation of both benzene rings. Instead, the portion joining carbon and nitrogen atoms twists first, while the two sides of the central nitrogen linkage interlock and move together like bicycle pedals, leading to a complete transformation of the overall structure.

(Front row) Professor Hyochul Lee, (back row from left) Postdoctoral researcher Jungmin Kim, Senior researcher Alekos Segalina, Research fellow Hoseong Ki. KAIST

The team expects that this discovery will help explain why the reaction speed does not change significantly even if the viscosity of the surrounding liquid increases. If the large rings rotated simultaneously, they would need to push aside a lot of surrounding liquid—much like trying to turn large furniture in a narrow space. However, when primarily the central region is moving, there is less need to displace the liquid around it.

Professor Lee stated, "This study is meaningful in that it clarifies the actual pathway of molecular movement, providing fundamental data for designing light-responsive materials and molecular machines. In the future, the joint research team will further develop methods to observe the motion of rapidly reacting organic molecules, thereby contributing to understanding the operating principles of diverse photo-responsive molecules."

This research was co-first-authored by IBS researchers Kim Joongmin and Ki Hoseong (both KAIST graduates). The results were published online in the international journal "Nature" on September 30 (local time in the UK).

This content was produced with the assistance of AI translation services.