This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 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), has identified the process by which the light-responsive molecule azobenzene changes its structure. The team has 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.
The findings are published in the journal Nature. 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 structure of the molecule before and after the change does not show how it moves between the two forms.
Researchers have proposed different explanations. Some suggest 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.
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