Science1 publisher2 min readPublished
X-ray laser movies show azobenzene switches shape by pedalling its central nitrogen atoms
KAIST-led chemists filmed azobenzene's first picoseconds with ultrafast X-rays, finding a nitrogen-centred twist behind a switch debated for nearly 50 years. Designers of light-driven materials and molecular machines now have a measured mechanism to work from.
The Scientist · Science desk
What happened
- The reaction begins with torsion about the carbon-nitrogen bonds, then the two central nitrogen atoms move together like bicycle pedals while both benzene rings stay nearly in place.
- Earlier explanations had the two rings rotating a long way, the central nitrogen linkage straightening, or several parts of the molecule twisting at once.
- A laser pulse started the reaction in azobenzene dissolved in methanol, and X-ray pulses from the Pohang Accelerator Laboratory's free-electron laser tracked its structure over time.
- The study, published in Nature, was led by Hyotcherl Ihee of KAIST and the Institute for Basic Science, with Jungmin Kim and Hosung Ki as co-first authors.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- capability A switching motion confined to the molecule's core gives materials chemists a concrete prediction to test: azobenzene units should keep their speed in thick or crowded surroundings where ring rotation would be slowed.
- constraint The pathway was measured in a single solvent, so applying it to polymer films, crystals or the modified azobenzenes used to switch drugs needs experiments outside this report.
- precedent If the solvent-subtraction method carries over as Ihee expects, other light-responsive organic molecules that react too fast to observe become candidates for the same kind of X-ray movie.
A molecule's starting and finishing structures do not show the route between them, and the structures azobenzene passes through on the way exist too briefly to observe directly [4]. The switch keeps every atom and only rearranges them, carrying its two benzene rings from opposite sides of the nitrogen linkage to the same side [13]. Chemists have used that property to control drug activity with light and to build light-responsive materials and molecular machines [13].
The hard part of the experiment was the solvent. The weak signal from azobenzene sat under a much stronger one from the surrounding liquid [7]. Every structural claim in the paper depends on separating the two, so the subtraction is the step of the method to check first. The team removed the solvent's contribution mathematically, then assembled the measurements from each time point into a movie of the molecule's motion [7].
The strongest support comes from a separate observation. According to the report, the reaction rate does not change significantly when the surrounding liquid becomes more viscous [8]. Swinging two large rings would mean pushing a large volume of liquid aside, and a thicker liquid should slow that down. A motion concentrated in the middle of the molecule displaces much less [8]. I think this agreement is the best part of the case. The X-ray structures and the viscosity behaviour are different kinds of evidence, and the pedal pathway accounts for both.
The question of the first picoseconds had been open for nearly 50 years [1]. According to the team's results, azobenzene does not change shape by rotating both rings significantly at the same time, which removes one of the three proposed answers [5]. The phys.org account does not say how the result bears on the other two, the straightening linkage and the collective twist [3].
"This study shows the pathway by which azobenzene changes its shape after it absorbs light," said Ihee [11]. The report is plain that the study did not directly improve the performance of any drug or material that uses azobenzene [10].
What to watch
- An independent ultrafast measurement, by X-ray or electron scattering, that reproduces the pedal-like motion of the nitrogen atoms.
- Quantum-chemistry simulations of the same first picoseconds that either match the reconstructed movie or point to artefacts left by the solvent subtraction.