Azobenzene is one of the best-known molecules that can be switched between two forms by light. However, the question of how the molecule moves in the first few picoseconds (trillionths of a second) after it absorbs light has remained unresolved for nearly 50 years. A Korean research team has now shown that, during the initial stage of the reaction, the molecule changes shape through a coordinated motion of the two nitrogen atoms at its center, while its two benzene rings stay nearly in place. The finding is expected to provide useful information for designing materials and molecular-scale machines that operate with light.
KAIST (President Choongsik Bae) announced on October 1 that a research team led by Hyotcherl Ihee, Professor in the Department of Chemistry at KAIST and Director of the Center for Advanced Reaction Dynamics (CARD) at the Institute for Basic Science (IBS, President Sukbok Chang), has identified the process by which the light-responsive molecule azobenzene changes its structure.
Azobenzene consists of two benzene rings connected by a central linkage of two nitrogen atoms. When it absorbs light, the molecule can change from a form in which the two rings are on opposite sides of the linkage (the trans form) to a form in which they are on the same side (the cis form). The molecule keeps the same atoms, and only their positions change. Researchers have used this property to control the activity of drugs with light and to develop light-responsive materials and molecular machines.
However, the structures of the molecule before and after the change do not show how it moves between the two forms. Researchers have proposed different explanations. Some suggested that the two rings rotate significantly, some that the central linkage straightens, and others that several parts of the molecule twist together. The question has been difficult to resolve because the structures that form during the reaction exist for too short a time to be observed directly.
The research team used the X-ray free-electron laser at the Pohang Accelerator Laboratory (PAL-XFEL) to observe these structures. The researchers first started the reaction by irradiating azobenzene dissolved in methanol with a laser. They then measured how its structure changed over time using ultrafast X-ray pulses.
The main challenge was to detect the weak signal from azobenzene, which was obscured by the much stronger signal from the surrounding solvent. The team separated the two signals using an analysis method that mathematically removes the contribution of the solvent. Based on the measurements taken at each point in time, the researchers then reconstructed the motion of the molecule as a movie.
The results showed that azobenzene does not change shape by rotating its two large benzene rings significantly at the same time. Instead, the reaction begins with torsion about the carbon–nitrogen bonds. During this initial motion, the central nitrogen linkage moves in a coordinated motion, similar to the way the two pedals of a bicycle move, and this changes the overall shape of the molecule.
This finding helps explain why the reaction rate does not change significantly even when the surrounding liquid becomes more viscous. If the molecule had to rotate its two large rings, it would need to displace a large volume of the surrounding liquid. Because the motion occurs mainly in the central part of the molecule, much less liquid needs to be displaced.
"This study shows the pathway by which azobenzene changes its shape after it absorbs light," said Professor Hyotcherl Ihee. He added that the team expects the work to help researchers understand how a wide range of light-responsive molecules work, because it improves methods for observing the motion of organic molecules that react rapidly.
The study did not directly improve the performance of drugs or materials that use azobenzene. Its significance is that it identifies the pathway the molecule actually follows as it moves. This provides basic data that researchers can refer to when they design light-responsive materials and molecular machines.
Dr. Jungmin Kim and Dr. Hosung Ki, who both received their degrees from KAIST and are now at IBS, are co-first authors of the study. The findings were published online in the international journal Nature on September 30 (UK time).
Paper title: X-ray liquidography decodes complex motions in azobenzene isomerization, DOI: 10.1038/s41586-026-11068-4
This research was supported by the Institute for Basic Science (IBS) Research Center Program of the Ministry of Science and ICT.