Ultrafast X-rays reveal how the light-responsive molecular switch azobenzene changes shape
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.
An ultrafast X-ray study has revealed how the light-responsive molecule azobenzene alters its structure following absorption of light. Azobenzene, renowned for its ability to switch between two forms triggered by light, had its dynamic changes in the first few picoseconds (trillionths of a second) following light absorption remained unclear for nearly 50 years.
Led by Hyotcherl Ihee from KAIST's Department of Chemistry and IBS Center for Advanced Reaction Dynamics, the research team has shed light on this mystery. Their findings, published in Nature, show that in the initial stage of the reaction, azobenzene undergoes a coordinated motion where the two nitrogen atoms at its core move together, while its two benzene rings stay largely in place.
This coordinated movement of the nitrogen atoms, akin to the pedaling of a bicycle, represents the first step in the transformation from the trans form (with rings on opposite sides of the nitrogen linkage) to the cis form (with rings on the same side). The central nitrogen linkage's motion, rather than significant rotation of the large benzene rings, drives the overall shape change.
This discovery is crucial as it clarifies the pathway azobenzene follows when moving from one form to another, a detail that had previously eluded researchers due to the fleeting nature of these structures. The ultrafast X-ray measurements, conducted at the Pohang Accelerator Laboratory's X-ray free-electron laser, allowed the team to observe these ultrafast molecular motions.
The study's implications extend beyond just understanding azobenzene's behavior; it lays the groundwork for designing light-responsive materials and molecular-scale machines. By identifying the precise molecular choreography during the light-induced transformation, researchers can better predict and manipulate the properties of such materials.
The research highlights that the reaction rate remains relatively stable even in more viscous liquids, as the localized motion around the central nitrogen atoms requires less displacement of the surrounding solvent compared to a full ring rotation.
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