Around the world, there is currently a huge amount of research and development work being done on carbon-containing molecules or organic molecules that emit colored light after appropriate excitation. This research field is driven by the screen industry and the development of biomedical imaging techniques. Although precise color adjustment in organic fluorescent dyes has so far usually been achieved by mixing different molecules, ETH researchers have now developed an approach that can generate a wide range of colors through chemical adjustments within the molecules themselves.
Yinyin Bao, group leader in the group of ETH Professor Jean-Christophe Leroux, and his team of scientists turned to fluorescent organic polymers for this work. These polymers can best be thought of as moving chains of different lengths. “Chains have a symmetrical structure, and the two components inside them contribute to fluorescence,” explains Bao. “One component, called the fluorophore, is in the middle of the chain, while the other component occurs once at each of the two ends of the chain.” The connection of the fluorophore in the middle of the chain with each end of the chain are links whose number and structure scientists can adjust. If the polymer chain is bent so that one of its ends lies near the fluorophore and the chain is simultaneously irradiated with UV light, it fluoresces.
Distance affects interaction
Scientists have now been able to show that the color of fluorescence depends not only on the structure of the links and the ends of the chain, but also on the number of links in the chain. “It is the interaction of the chain end and the fluorophore that is responsible for the fluorescence of these polymers,” says Bao: “The distance between these two components affects their interaction, and thus the color that is emitted.”
Using a method called mercury polymerization, researchers can regulate the number of link chains. First, slowly grow the chain by the slow process of attaching the building blocks to the fluorophore. Once the desired length is reached, scientists can interrupt the process and simultaneously generate a molecule at the end of the chain. Thus, the researchers produced polymers of different colors: with less than 18 building blocks, the molecules fluoresce yellow; with 25 chain links, green; and with 44 or more connections, blue. “What’s special about this is that all of these differently luminescent polymers are made up of exactly the same components. The only difference is in the length of the chain,” says Bao.
Wide range of OLED colors
The research team, including scientists from the group of professors ETH Chih-Jen Shih and from the Royal Institute of Technology in Melbourne, Australia, published their work in the journal Scientific progress. Currently, researchers can produce fluorescent polymers in yellow, green and blue, but are working to extend the principle to other colors, including red.
These new fluorescent polymers cannot be used directly as OLEDs (organic LEDs) on screens because their electrical conductivity is not high enough, Bao explains. However, it should be possible to combine polymers with semiconducting molecules to easily produce wide-spectrum OLEDs. Used in concentrated solar power plants, they could also collect sunlight more efficiently and thus increase plant efficiency. Bao sees their main areas of application in laboratory diagnostic procedures using fluorescence, for example in PCR, as well as in microscopy and imaging in cell biology and medicine. Other potential uses would be as security features on banknotes and certificates or in passports.
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Material provided ETH Zurich. Original written by Fabio Bergamin. Note: Content can be edited for style and length.