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Next-generation banknotes and passports could benefit from a novel approach to fine-tuning the color of fluorescent polymers, which could also advance microscopy and imaging techniques used in cell biology and medicine.
Although global research has focused on organic, carbon-containing molecules that emit colored light with the appropriate excitation, precise color tuning in organic fluorescent dyes has only been realized by mixing different molecules.
Researchers from ETH Zürich in Switzerland have developed a novel approach to perfecting the color of polymers by making chemical changes within the molecules themselves. The polymers can be tuned to emit a different color by varying its length and end group, allowing researchers to generate a broad spectrum of colors.
Dynamic Chains
The polymers can be thought of as moving chains of varying lengths, says Dr Yinyin Bao, from the Institute of Pharmaceutical Science at ETH Zürich. In the center of the chain is a fluorophore, which is joined to the chain ends via links or monomers whose number and structure can be altered.
“The chains have a symmetrical structure, and two components within them contribute to the fluorescence,” he explains. “One component, called the fluorophore, sits in the middle of the chain, while the other component occurs once at each of the chain’s two ends.”
If the polymer chain is bent so that one end is adjacent to the fluorophore and the chain is irradiated with UV light at the same time, it fluoresces. The researchers have demonstrated that the fluorescence color depends on the structure of the chain links and ends, and the number of chain links.
“It’s the interaction of the chain end and the fluorophore that’s responsible for the fluorescence of these polymers,” says Bao, a leader in the group of Professor Jean-Christophe Leroux. “The distance between the two components affects how they interact and thus the color that’s emitted.”
Controlling Chain Length for Color
Researchers controlled the number of chain links using a method called living polymerization. The chain was created through a slow process of attaching building blocks to the fluorophore, which was terminated once the chain reached the desired length, with the chain end molecule created at the same time.
This allowed researchers to produce polymers with different colors. If the chain had fewer than 18 building blocks attached to the fluorophore, it fluoresced yellow; if it had 25 chain links, it was green, and chains of more than 44 links fluoresced blue.
“What’s special about this is that these differently luminescent polymers are all composed of the exact same components,” enthuses Bao. “The only difference is the chain length.”
Expanding the Palette
At present, the researchers can only generate fluorescent polymers in blue, yellow, and green; they hope to expand the color palette to other colors, including red. Because the system is so versatile, the researchers expect that other fluorophores, monomers, and chain end groups can be exploited to generate polymers with emission ranges across the color spectrum.
In their paper published in Science Advances, the researchers note that the monomers they used are commercially available, which “further highlights the simplicity of our developed system since there is no need to prepare functionalized monomers via multistep syntheses to obtain tunable emission.”
Potential Applications
Fluorescent polymers often find use in displays such as LEDs, however, these new fluorescent polymers are unsuitable for direct use as organic LEDs as their electrical conductivity is not high enough. However, Bao believes it should be possible to join these polymers with semiconducting molecules to create broad color range OLEDs.
Boa envisages these polymers will be utilized in laboratory diagnostic procedures that require fluorescence, such as PCR (polymerase chain reaction), or in microscopy and imaging techniques used in cell biology and medicine.
Other potential applications include in concentrated solar power plants where they could collect sunlight more efficiently, and therefore increase the plant’s productivity, or as security features, printed on banknotes, passports, or certificates.
References and Further Reading
Bergamin, F. (2021) Chain length determines molecular colour, ETH Zürich: https://ethz.ch/en/news-and-events/eth-news/news/2021/04/chain-length-determines-molecular-colour.html. Accessed 20 April 2021.
Suiying, Y. et al (2021) Continuous color tuning of single-fluorophore emission via polymerization-mediated through-space charge transfer, Science Advances: DOI: 10.1126/sciadv.abd1794/ https://advances.sciencemag.org/content/7/15/eabd1794. Accessed 20 April 2021.
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