Light-controlled Switches Get Glow Up

Okinawa Institute of Science and Technology Graduate University

Whether it's neurons responding to electrical signals or plants stretching towards light, nature is filled with examples of biological materials that respond to their environment. Now, researchers want to develop new responsive materials for a wide range of applications, such as sensing, or low-energy technologies. However, achieving a balance of useful properties at the molecular level is challenging.

Published in Chemical Science, researchers at the Okinawa Institute of Science and Technology (OIST) have designed and synthesized new multi-functional compounds that change structure under UV light. Depending on the light conditions, the molecules reversibly switch between two stable forms through a ring-opening and ring-closing reaction. These molecular switches have a range of desirable properties, for example glowing with fluorescence after their structure changes, making it easy to see which state the molecule is in.

A chemical reaction is drawn on screen, with a molecule containing four ring structures shown on the left. On the right is the molecule after irradiation with 285 nm light. A carbon-oxygen bond breaks to result in a fluorescent compound, containing 3 rings within its structure.

The molecular scaffold 'opens' when under 285 nm wavelength light, with one carbon-oxygen bond breaking. In this 'open' state (B-OH), the molecule exhibits fluorescence, so it's easy to detect this structural change. The reaction reverses under 325 nm light. The molecule on the left side of the reaction (DHB) is intrinsically chiral, with the two enantiomers (mirror image forms of the molecule) showing opposite circular dichroism (CD) spectra, which measures the difference in absorption of left or right circularly polarized light.

Professor Christine Luscombe, head of OIST's Pi-Conjugated Polymers Unit and senior author of this study, comments, "Optical and smart technologies require materials that respond to light and other external stimuli. By designing new molecular switches and understanding the mechanisms by which they change their structure and properties, this research provides new insights into the development of multifunctional photoresponsive materials."

In the paper, the team also demonstrated directionally-selective control of the ring-opening reaction, incorporating different side groups into the starting molecule to selectively make different products.

Reaction schemes are shown, alongside the results of bond dissociation energy (BDE) calculations, with partial NMR spectra inset to the right to show where products were seen.

The results of different ring-opening reactions are shown. By changing the position of the side groups on one of the rings within the starting structure, the researchers could influence which bond was broken, and which product formed.

This study was supported by interdisciplinary collaboration between several OIST Units and Core Facilities members. First author Dr. Fathy Hassan comments, "By combining organic synthesis, computational chemistry, spectroscopy, crystallography, and advanced characterization techniques, we gained a deeper understanding of the structure, mechanism, and properties of the molecular switch."

The authors now plan to embed their molecular switches within larger polymers to create new materials. They hope to explore the potential applications of these in responsive and photonic systems.

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