The chain length determines the molecular color

Researchers at ETH Zurich have developed fluorescent polymers whose color can be easily adjusted. Depending on their length, the polymers emit a different color. Possible applications are biomedicine, security printing and solar energy.

Extensive research and development work is currently being carried out around the world on carbon-containing or organic molecules which, when appropriately stimulated, emit colored light. This field of research is driven by the display industry and the development of biomedical imaging techniques. While precise color matching in organic fluorescent dyes was previously usually achieved by mixing different molecules, ETH researchers have now developed an approach with which a wide range of colors can be generated through chemical adjustments within the molecules themselves.

These polymers, visible here under UV light, consist of exactly the same components. The only difference is their chain length. Photo credit: Suiying Ye / ETH Zurich

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 viewed as moving chains of varying lengths. “The chains have a symmetrical structure and two components contribute to the fluorescence,” explains Bao. "One component, called a fluorophore, is in the middle of the chain, while the other component occurs once at each end of the chain." The fluorophore in the middle of the chain at each end of the chain connects links the number and structure of which scientists can adjust. If the polymer chain is bent so that one of its ends is near the fluorophore and the chain is exposed to UV light at the same time, it will fluoresce.

Distance affects the interaction

Scientists have now been able to show that the fluorescent color depends not only on the structure of the chain links and ends, but also on the number of chain links. "It is the interaction between the chain end and the fluorophore that is responsible for the fluorescence of these polymers," says Bao.

The distance from the end group (gray) to the fluorophore (yellow) influences the interaction of these two components and thus the emitted color (schematic representation). Photo credit: Ye et al. Scientific advances 2021

Using a method known as living polymerization, researchers can regulate the number of links in the chain. First, they gradually grow the chain through a slow process of adding building blocks to the fluorophore. Once the desired length is reached, the scientists can stop the process and create the chain end molecule at the same time. The researchers produced polymers with different colors: with fewer than 18 building blocks, the molecules fluoresce yellow; with 25 chain links, green; and blue with 44 or more links. “The special thing about it is that these differently luminescent polymers all consist of exactly the same components. The only difference is the chain length, ”says Bao.

Large color palette of OLEDs

The research team, which included scientists from ETH Professor Chih-Jen Shih's group and the Royal Melbourne Institute of Technology in Australia, published their work in the journal Advances in science. Currently, researchers can make 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) in displays because their electrical conductivity is not high enough, explains Bao. However, it should be possible to combine the polymers with semiconducting molecules in order to produce OLEDs with a large color range in a simple manner. In concentrated solar power plants, they could also collect sunlight more efficiently and thus increase the efficiency of the systems. Bao sees their main areas of application in laboratory diagnostic methods that use fluorescence, for example in PCR, as well as in microscopy and imaging methods in cell biology and medicine. Other possible uses would be security features on banknotes and certificates or in passports.

Source: ETH Zurich

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