Chain length determines molecular color

Date:
April 7, 2021
Source:
ETH Zurich
Summary:
Researchers have developed fluorescent polymers whose color can
be easily tuned. Depending on their length, the polymers emit a
different color.

Potential applications include biomedicine, security printing and
solar energy.



FULL STORY ========================================================================== Around the world, a huge amount of research and development work is
currently being done on carbon-containing, or organic, molecules that
emit coloured light after appropriate excitation. This research field is
driven by the display industry and the development of biomedical imaging techniques. While precise colour tuning in organic fluorescent dyes has so
far usually been achieved by mixing different molecules, ETH researchers
have now developed an approach that can generate a broad palette of
colours by way of chemical adjustments within the molecules themselves.


========================================================================== Yinyin Bao, a 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 thought of as moving chains
of varying lengths. "The chains have a symmetrical structure, and two components within them contribute to the fluorescence," Bao 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." Joining
the fluorophore in the middle of the chain with each end of the chain are
links whose number and structure scientists can adjust. If the polymer
chain is bent so that one of its ends comes to lie near the fluorophore
and the chain is simultaneously irradiated with UV light, it fluoresces.

Distance affects the interaction The scientists have now been able to
show that the fluorescence colour depends not only on the structure of
the chain links and ends, but also on 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," Bao says: "The distance
between the two components affects how they interact and thus the
colour that's emitted." Using a method called living polymerisation,
the researchers can regulate the number of chain links. First, they
gradually grow the chain by a slow process of attaching building
blocks to the fluorophore. Once the desired length is reached, the
scientists can terminate the process and simultaneously generate the
chain end molecule. This is how the researchers produced polymers with different colours: with fewer than 18 building blocks, the molecules
fluoresce yellow; with 25 chain links, green; and with 44 or more links,
blue. "What's special about this is that these differently luminescent
polymers are all composed of the exact same components. The only
difference is the chain length," Bao says.

Wide colour range OLEDs The research team, including scientists from
the group of ETH Professor Chih- Jen Shih and from the Royal Melbourne Institute of Technology in Australia, published their work in the journal Science Advances. Currently, the researchers can produce fluorescent
polymers in yellow, green and blue, but they are working on extending
the principle to include other colours, including red.

These new fluorescent polymers can't be used directly as OLEDs
(organic LEDs) in displays because their electrical conductivity is
not sufficiently high, Bao explains. However, it ought to be possible
to combine the polymers with semiconducting molecules in order to
produce wide colour range OLEDs in a simple way. Used in concentrated
solar power plants, they could also collect sunlight more efficiently
and thus increase the plants' efficiency. Bao sees their main areas of application in laboratory diagnostic procedures that use fluorescence,
for example in PCR, as well as in microscopy and imaging procedures in
cell biology and medicine. Other potential uses would be as security
features on banknotes and certificates or in passports.

========================================================================== Story Source: Materials provided by ETH_Zurich. Original written by
Fabio Bergamin. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Suiying Ye, Tian Tian, Andrew J. Christofferson, Sofia Erikson,
Jakub
Jagielski, Zhi Luo, Sudhir Kumar, Chih-Jen Shih, Jean-Christophe
Leroux, Yinyin Bao. Continuous color tuning of single-fluorophore
emission via polymerization-mediated through-space charge
transfer. Science Advances, 2021; 7 (15): eabd1794 DOI:
10.1126/sciadv.abd1794 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/04/210407143817.htm

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