Chemists Shine a Light on Well-Known Chemical Reactions

Proton Transfer Forms Basis of Many Light-Driven Organic Reactions

In nature, a large number of organic and biological reactions are driven by light. In photosynthesis, plants use sunlight to convert carbon dioxide and water into glucose and oxygen. In the human body, vitamin D can be made in the skin from exposure to sunlight.

Judy Wu & Team Members

These examples are just a fraction of the biologically important reactions which depend on light.

In a recent paper, published in the journal Proceedings of National Academy of Sciences, chemists at the University of Houston unfold fundamental principles that underlie light-driven proton transfer in organic molecules.

“We recognized that when light strikes an organic compound, there is a pattern that determines whether or not a proton transfer reaction will likely happen,” said Judy Wu, assistant professor of chemistry in the College of Natural Sciences and Mathematics and corresponding author for the paper.

Photoexcitation: Transfer of Light Energy to Electrons

In a light-driven organic reaction, the first step is the excitement of an electron. When light hits an electron, it gives the electron a jolt of energy. This process is called photoexcitation, and the gained energy is useful for driving a number of reactions.

Wu, along with her research group, looked at the effects of aromaticity and antiaromaticity, a common molecular feature of cyclic organic compounds, on light-driven reactions. Compounds with aromatic character often are stable and unreactive. Compounds with antiaromatic character are unstable, reactive, and often have only fleeting existence.

When light shines on an aromatic molecule, it can gain antiaromatic character because of a redistribution of its electrons. This means the molecule transforms from a stable and unreactive state, to an unstable and highly reactive state. Once this happens, the key driving force is returning to a lower energy configuration. A proton transfer reaction is one way of doing so.

Proton Transfer Reactions: An Escape Route for Photoexcited Molecules to Get Rid of Antiaromaticity

“When light strikes a molecule and prompts a proton transfer reaction, it may seem that only a bare proton shifts its position,” Wu said, “but what really is happening is an enormous change in the electronic structure of the whole molecule. Our paper shows that that change is the result of aromatic molecules getting rid of ‘antiaromaticity’ in their photoexcited states.”

Co-authors of the paper include postdoctoral researcher Chia-Hua Wu, chemistry graduate student Lucas José Karas, and Uppsala University collaborator Professor Henrik Ottosson. This research was supported by funding from the National Science Foundation and National Institutes of Health.

- Rachel Fairbank, College of Natural Sciences and Mathematics