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Polymer Molecules Fluoresce After Chemical Adjustments

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ETH Zurich researchers have introduced a method that relies on chemical adjustments occurring in fluorescent organic polymer molecules to generate a broad palette of color. The approach is an alternative to mixing different molecules to achieve precise color tuning in organic fluorescent dyes.

The outcomes of both processes are necessary to biomedical imaging techniques and display technology, both of which require molecular fluorescence and the emission of color. The chemical method using polymer molecules rather than dye molecules, however, offers advantages in solar power concentration; the polymers would be able to collect sunlight more efficiently than current commonly used molecules.

According to ETH Zurich’s Yinyin Bao, the fluorescent polymers that the research group used can be visualized as moving chains of varying lengths. The chains feature a symmetrical structure. Two components within the chains contribute to fluorescence, Bao said. One component (the fluorophore) is positioned in the middle of the chain. The other component occurs once at each of the chain’s ends.

These polymers, seen here under UV light, are composed of the exact same components. The only difference is their chain length. Courtesy of Suiying Ye / ETH Zurich.
These polymers, seen here under UV light, are composed of the exact same components. The only difference is their chain length. Courtesy of Suiying Ye/ETH Zurich.
In addition to the presence of those components, the researchers were able to adjust the number and structure of individual chains’ links. Bending the polymer chain in such a way that one end comes to lie near the fluorophore, and simultaneously irradiating the chain with UV light, caused it to fluoresce.

The mechanism showed that the fluorescence color depended on the structure of the chain links and ends, as well as on the number of chain links. Using living polymerization to regulate the number of chain links, the team was able to produce polymers exhibiting different colors. That process involves gradually growing the chain by adding to the fluorophore before terminating it and simultaneously generating the chain-end molecule once the desired length is achieved.

It was specifically the interaction between the chain-end and the fluorophore that caused the fluorescence. “The distance between the two components affects how they interact and thus the color that’s emitted,” Bao said.

By varying the length of the chains and, as a result, the polymers themselves, the researchers’ molecules fluoresced yellow, green, and blue. The team of researchers from ETH Zurich and Australia’s Royal Melbourne Institute of Technology is now working to produce other colors.

In addition to solar power applications, the team said the polymers could combine with semiconducting molecules to produce wide color-range OLEDs in a simple manufacturing process. The polymers on their own are unable to be used directly as OLEDs due to their electrical conductivity not being sufficiently high.

Polymerase chain reaction (PCR), microscopy, and cell biology are among other applications Bao identified for the fluorescent polymers.

The research was published in Science Advances (www.doi.org/10.1126/sciadv.abd1794).

Photonics Handbook
GLOSSARY
polymerization
Process of synthesizing long molecular chain materials (polymers) by reaction of many small molecules (usually thousands) called monomers.
researchResearch & TechnologyeducationETH Zurichpolymerspolymerizationfluorescent dyefluorescent dye moleculesfluorescent dyessolarphotovoltaicsMicroscopybiosensingbioimagingcell biologylight sourcesmedicalmedicineTech Pulse

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