Chain length determines molecular color

There is a tremendous amount of research and development currently underway around the world on carbon-containing or organic molecules that emit colored light after appropriate excitation. The driving force behind this area of ​​research is the manufacture of displays and the development of biomedical imaging techniques. While fine-tuning the color in organic fluorescent dyes until now has usually been achieved by mixing different molecules, ETH researchers have now developed an approach that can generate a wide gamut of colors by chemically adjusting the molecules themselves.

Yingin Bao, Team Leader of ETH Professor Jean-Christophe Leroux, and his team of scientists turned to fluorescent organic polymers for this work. These polymers are best thought of as moving chains of varying lengths. “The chains are symmetrical and the two components within them promote fluorescence,” explains Bao. “One component, called a fluorophore, is in the middle of the chain, and the other occurs once at each of the two ends of the chain.” To the fluorophore in the middle of the chain, links are attached at each end of the chain, 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 simultaneously irradiated with UV light, it fluoresces.

Distance affects interaction

Now scientists have been able to show that the color of fluorescence depends not only on the structure of the links and ends of the chain, but also on the number of links in the chain. “The fluorescence of these polymers is due to the interaction between the end of the chain and the fluorophore,” says Bao. “The distance between the two components affects their interaction and therefore the emitted color.”

Using a technique called living polymerization, researchers can adjust the number of chain links. First, they gradually build up the chain, slowly attaching building blocks to the fluorophore. Once the desired length is reached, scientists can stop the process and simultaneously generate a molecule at the end of the chain. Here’s how the researchers made different colored polymers: from less than 18 building blocks, molecules fluoresce in yellow; 25-link chain, green; and with 44 or more links, blue. “What’s special about this is that all of these differently luminescent polymers are made up of the same components. The only difference is the length of the chain, ”says Bao.

OLED LEDs with a wide color range

The research team, which included scientists from the team of ETH Professor Chih-Jen Shi and from the Royal Melbourne Institute of Technology in Australia, published their work in the journal. Science achievements… Researchers can currently produce fluorescent polymers in yellow, green and blue, but they are working to expand this principle to include other colors, including red.

These new fluorescent polymers cannot be used directly as OLEDs (organic light emitting diodes) in displays because their electrical conductivity is not high enough, Bao explains. However, it should be possible to combine polymers with semiconducting molecules to easily produce OLEDs with a wide color gamut. Used in concentrated solar power plants, they can also collect sunlight more efficiently and thus increase the efficiency of power plants. Bao sees the main areas of their application in laboratory diagnostic procedures using fluorescence, such as PCR, as well as microscopy and imaging procedures in cell biology and medicine. Other possible uses are security features for banknotes and certificates or passports.

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reference

Ye S, Tian T, Christofferson AJ, Erickson S, Jagelski J, Luo Z., Kumar S, Shi KJ, Leroux JK, Bao Y .: Continuous adjustment of the emission color of one fluorophore through the transfer of charge through space, mediated by polymerization. Science achievements, April 7, 2021, DOI: 10.1126 / sciadv.abd1794