Experiments Show Light-Induced Molecular Strain

When molecules are exposed to ultraviolet (UV) light, they can quickly change their structure, creating strain—this refers to stress within a molecule's chemical bonds caused by an increase in its internal energy. These changes happen incredibly fast, typically within just tens of picoseconds (one millionth of a millionth of a second). Thanks to advanced technology at X-ray free electron laser (XFEL) facilities, scientists can now capture images of these rapid structural changes.

In a study published in The Journal of Physical Chemistry A, researchers discovered structural evidence of a strained bicyclic molecule (a molecule with two interconnected rings) that forms during a chemical reaction when cyclopentadiene absorbs UV light. Cyclopentadiene is often used in research because it can undergo a variety of reactions, making it a valuable model compound with broad implications for chemistry.

Highly strained molecules are important in fields like solar energy and pharmaceuticals, but strain doesn’t usually occur naturally in molecules—it requires external energy to be introduced. Understanding the processes that lead to the creation of strained ring structures is a major challenge in physical chemistry.

The research confirmed a prediction that controlled interactions between light and cyclopentadiene molecules can produce strained structures. The team also managed to distinguish between different types of ring strain, which could help improve future methods of molecule synthesis.

Using X-ray pulses from the Linac Coherent Light Source, the researchers studied how the structure of cyclopentadiene molecules changes when exposed to UV light, observing the transition to a highly strained state after the molecules were photoexcited.

The molecules were excited with UV light pulses at 243 nanometers, and the team compared the resulting X-ray scattering data to theoretical models of different strained molecules formed during the reaction. The results were consistent with the formation of bicyclo[2.1.0]pentene, a structure that was predicted but never directly observed before.

However, the experiments did not detect another potential product, tricyclo[2.1.0.0]pentane, which had also been hypothesized. These findings bring researchers closer to understanding how molecular strain influences the behavior of photoexcited hydrocarbons.