Researchers light the way for organic glow-in-the-dark materials

Date:
November 29, 2021
Source:
Okinawa Institute of Science and Technology (OIST) Graduate
University
Summary:
Organic materials could soon be utilized for glow-in-the-dark
products, which has the potential to diversify and expand the
already US$400 million industry, thanks to new research.



FULL STORY ========================================================================== Glow-in-the-dark materials are utilized worldwide for emergency signs,
watches, and paint. This useful characteristic fuels a global market
worth approximately US$400 million. But the inorganic crystals that are currently needed to generate this ability to a high level of performance require rare-earth metals and fabrication temperatures of over 1000
degrees Celsius. Now, writing in Nature Materials, researchers from
the Okinawa Institute of Science and Technology Graduate University
(OIST) and Kyushu University, both in Japan, have developed a method
to generate a glow-in-the-dark light using the more- readily available
organic materials.


==========================================================================
"Not only are organic materials much more available and easier to work
with than inorganic materials but they are also soluble, which has the potential to diversify and expand the use of glow-in-the-dark objects,
as the characteristic could be added to inks, films, and textiles,"
stated Prof. Chihaya Adachi, director of the Center for Organic Photonics
and Electronics Research (OPERA), Kyushu University. "Another important application is their potential use in bioimaging, which could have a
myriad of benefits for health science." In 2017, researchers showed, for
the first time, that two organic materials could create a glow-in-the-dark effect. This was a great success and published in Nature. However,
the performance was almost 100 times weaker than with the inorganic
varieties. In fact, the researchers had to use an ultraviolet light to
generate the emissions, had to go into a dark room to see the light,
and could not expose the samples to oxygen. Now, the researchers have
produced a better outcome when they progressed from a method with two components to a method with three components and changed the molecules
they used. The result was emissions that lasted for over one hour at
room temperature, a tenfold improvement from the previous work.

"It's a four-stage process to create the glow-in-the-dark effect --
charge transfer, separation, recombination, and, finally, emissions,"
explained Prof.

Ryota Kabe, who leads OIST's Organic Optoelectronics Unit. "Within
the molecules, electrons are nestled in holes. An important part of
the process is separating the electrons from the holes. When the two
come back together, it generates the glow." In the previous research,
when the organic materials were energized by light, electrons would be transferred from a molecule dubbed the electron donor to a molecular
dubbed the electron acceptor. However, an issue was caused as the
electron acceptor could not store many electrons. When the electrons
returned to the donor, this recombination created the glow effect but
because the number of stored electrons was limited, the glow was not
strong and rapidly faded away.

However, in this new work, the researchers did several things differently.

Firstly, they used molecules that ensured the holes were the things that
moved rather than the electrons. This hole diffusion system reduced
the probability of the molecules reacting with the air so ensured the
samples would glow whilst exposed to oxygen. Secondly, the researchers
added in the third component -- a hole trapper, which kept the electron
and the hole separated for longer, allowing more holes to build up and increasing the resulting emissions period.

And finally, they used molecules that required less energy to move between
the different steps of the process, ensuring that the whole process took
less energy and allowing the emissions to be generated in visible light,
rather than just ultraviolet light.

"By tweaking the method, we've successfully improved the performance
of organic molecules by ten times the previous work," Prof. Kabe
concluded. "The organic molecules now work in air, though the
performance is still weak. We will continue to work to tune the emissions
until they are on par with those produced by the inorganic crystals." ========================================================================== Story Source: Materials provided by Okinawa_Institute_of_Science_and_Technology_(OIST)
Graduate_University. Original written by Lucy Dickie. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Kazuya Jinnai, Ryota Kabe, Zesen Lin, Chihaya Adachi. Organic long-
persistent luminescence stimulated by visible light in p-type
systems based on organic photoredox catalyst dopants. Nature
Materials, 2021; DOI: 10.1038/s41563-021-01150-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211129122805.htm

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