The Chain Length Determines The Color Of The Molecule

Researchers at ETH Zurich have developed fluorescent polymers whose fluorescent color can be easily fine-tuned. Depending on their length, the polymers appear to be of different colors. Possible uses include biomedicine, security printing and solar energy.

Organic molecules, i.e. molecules containing carbon, which emit colored light when appropriately stimulated, are currently being intensively researched and developed around the world. The drivers of this research field are the display industry and the development of biomedical imaging processes. While with organic fluorescent dyes so far precise color gradations have mostly been achieved by mixing different molecules, ETH researchers have now developed an approach in which a palette of different colors can be generated in a molecule through chemical adaptations.

The scientists, led by Yinyin Bao, group leader in the group of ETH professor Jean-Christophe Leroux, used fluorescent organic polymers for this purpose. These polymers have to be thought of as flexible chains of different lengths. "The chains are built up symmetrically, and two components in them contribute to the fluorescence," explains Bao. "On the one hand there is a component in the middle of the chain, we call it the fluorophore, on the other hand a component that occurs once at each of the two chain ends". Between the fluorophore in the middle of the chain and the ends of the chain there are links, the number and structure of which the scientists can vary. When the polymer chain is so bent

The 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. “The interaction between the chain end and the fluorophore is responsible for the fluorescence of these polymers,” explains Bao. "The distance between the two components influences the interaction and thus the emitted color."

Using a method known as “living polymerization”, the researchers can determine the number of chain links. In doing so, they gradually add building blocks to the chain in a slow process. Once the desired length has been reached, the scientists can stop the process and add the chain end molecule. In this way, the researchers produced polymers with different colors: With fewer than 18 building blocks, the molecules fluoresce yellow, with 25 chain links green and with 44 or more links blue. “The specialty is that these differently luminous polymers are all composed of exactly the same components. The only difference is the chain length, ”says Bao.

OLED with a large color range
The research team, including scientists from ETH professors Chih-Jen Shih's group and from the Royal Melbourne Institute of Technology in Australia, published their work in the journal Science Advances. At the moment, the researchers can only produce fluorescent polymers in yellow, green and blue. However, the scientists are working on expanding the principle to include other colors, including red.

The new fluorescent polymers cannot be used directly as OLEDs (organic LEDs) in screens because their electrical conductivity is not high enough, explains Bao. However, it is conceivable to combine the polymers with semiconducting molecules in order to easily produce OLEDs with a large color gamut. In concentrated solar power, they could also collect sunlight more efficiently and thus increase the efficiency of the power plants. The ETH researcher sees the main areas of application in laboratory diagnostic processes in which fluorescence is used, for example in PCR, as well as in microscopy and in imaging processes in cell biology and medicine.