Plate Nº 74 · recorded October 2, 2026
Chemistry & MaterialsReported finding
Ultrafast X-Rays Solve 50-Year Mystery of How Azobenzene Switches Shape
Scientists at KAIST have captured the first picoseconds of azobenzene's light-driven shape change, revealing a bicycle-pedal motion of two nitrogen atoms that settles a 50-year debate.
By Marcus Bennett3 min read687 words
In brief
- A KAIST-led team observed azobenzene's shape change in its first picoseconds using the PAL-XFEL X-ray free-electron laser, resolving a question open for nearly 50 years.
- The molecule switches via coordinated 'bicycle-pedal' motion of its two central nitrogen atoms, while its two benzene rings stay nearly in place.
- The findings, published in Nature (2026), provide reference data for designing light-responsive materials and molecular machines.
For nearly half a century, chemists could see what azobenzene looks like before and after it absorbs light — but not how it gets from one form to the other. A team led by KAIST has now filled in that missing journey, and the answer is more compact than anyone's textbook sketch might suggest.
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), led the research. The findings appear in the journal Nature.
A molecule that flips with light
Azobenzene consists of two benzene rings — flat, hexagonal carbon frameworks — joined by a central linkage of two nitrogen atoms. When the molecule absorbs light, it can switch from a "trans" form, in which the two rings sit on opposite sides of the linkage, to a "cis" form, in which they sit on the same side.
No atoms are added or removed. Only their positions change. That simplicity has made azobenzene a workhorse for chemists, who use it to control drug activity with light and to build light-responsive materials and molecular-scale machines.
The problem is that the starting and ending structures say nothing about the route between them. Researchers have floated competing explanations for decades. Some proposed that the two rings rotate dramatically around the central linkage. Others argued the linkage itself straightens out, or that several parts of the molecule twist together simultaneously.
Settling the debate was hard for a simple reason: the intermediate structures exist for only a few picoseconds — trillionths of a second — far too briefly to observe directly with conventional tools.
Filming a molecule with X-ray flashes
To catch the motion, the team used the X-ray free-electron laser at the Pohang Accelerator Laboratory (PAL-XFEL) in South Korea. The researchers kicked off the reaction with an optical laser aimed at azobenzene dissolved in methanol, then probed the molecule's changing structure with ultrafast X-ray pulses at successive moments in time.
The signal from azobenzene itself was weak and largely buried under the much stronger signal from the surrounding solvent. To dig it out, the team applied an analysis method that mathematically subtracts the solvent's contribution. From the time-resolved measurements, they reconstructed the molecular motion as a kind of movie.
Pedals, not propellers
The movie revealed something the competing models had missed. Azobenzene does not switch shape by swinging its two bulky benzene rings through the surrounding liquid. Instead, the reaction begins with torsion — a twisting — around the carbon–nitrogen bonds that connect the rings to the central linkage.
During this initial motion, the two central nitrogen atoms move together in a coordinated fashion, much like the two pedals of a bicycle. The rings stay nearly in place while this pedal-like motion in the molecule's core reshapes the whole structure.
This mechanism neatly explains an old puzzle: the reaction rate barely changes when the surrounding liquid becomes more viscous. If the molecule had to rotate its two large rings, it would have to shove aside a large volume of liquid, and a thicker liquid would slow it down. Because the motion is concentrated in the central part of the molecule, far less liquid needs to be pushed out of the way.
Why it matters
"This study shows the pathway by which azobenzene changes its shape after it absorbs light," Ihee said. He added that the team expects the work to help researchers understand how a wide range of light-responsive molecules behave, because it improves the methods used to watch organic molecules that react rapidly.
The study does not directly improve the performance of azobenzene-based drugs or materials. Its value is more foundational: it identifies the pathway the molecule actually follows, giving researchers concrete reference data for designing light-responsive materials and molecular machines.
Dr. Jungmin Kim and Dr. Hosung Ki, both KAIST graduates now at IBS, are co-first authors of the study.
Publication: Jungmin Kim et al., "X-ray liquidography decodes complex motions in azobenzene isomerization," Nature (2026). DOI: 10.1038/s41586-026-11068-4
via Phys.org Chemistry (Source)
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