Discovery of chromophores that emit light in the ultraviolet region when excited with visible light
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Fluorescence usually entails the conversion of light at shorter
wavelengths to light at longer wavelengths. Scientists have now
discovered a chromophore system that goes the other way around. When
excited by visible light, the fluorescent dyes emit light in the
ultraviolet region. According to the study published in the journal Angewandte Chemie,
such light upconversion systems could boost the light-dependent
reactions for which efficiency is important, such as solar-powered water
splitting.
Contact: Nobuhiro Yanai, Kyushu University (Japan)
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Discovery of Key TIPS-Naphthalene for Efficient Visible-to-UV Photon Upconversion under Sunlight and Room Light
Fluorescent dyes absorb light at shorter wavelengths (high energy,
e.g. blue light) and emit light at longer wavelengths (low energy, e.g.
red light). Upconversion of light is much more difficult to achieve.
Upconversion means that a fluorescent dye is excited with radiation in
the visible range but emits in the ultraviolet. Such dyes could be used
to run high-energy catalytic reactions such as solar-powered water
splitting just using normal daylight as an energy source. Such dyes
would expand the range of available excitation energy.
Nobuhiro Yanai and colleagues at Kyushu University, Japan, are
exploring multi-chromophore systems for their ability to upconvert
fluorescence light. Yanai explains how upconversion works: “Fluorescence
upconversion occurs when two chromophore molecules, which have been
excited in the triplet state by a sensitizer, collide. This collision
annihilates the sensitized energy and lifts the chromophores to a higher
energy level. From there, they emit the energy as radiation.”
In practice, however, it is difficult to achieve effective
upconverting chromophore designs—existing systems need high-intensity
radiation and still do not achieve more than ten percent efficiency.
“The main reason for the low efficiency is that the sensitizer
chromophore molecules also absorb much of the upconverted light, which
is then lost,” Yanai says.
In contrast, the donor–acceptor chromophore pair developed by Yanai
and colleagues exhibits energy levels that are so finely adjusted that
it achieved a record-high 20 percent upconversion efficiency. Almost no
back-absorption and low nonradiative loss occurred. The novel
chromophore pair consisted of an iridium-based donor, which was an
established sensitizer, and a naphthalene-derived acceptor, which was a
novel compound.
Low back-absorption and few radiative losses mean that the intensity
of the exciting radiation can be low. The researchers reported that
solar irradiance was sufficient to achieve high upconversion efficiency.
Even indoor applications were possible using artificial light. The
authors held an LED lamp above an ampoule filled with the chromophore
solution and measured the intensity of the emitted UV light.
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About the Author
Dr. Nobuhiro Yanai is an associate professor
at the Department of Chemistry and Biochemistry, Kyushu University,
Fukuoka, Japan. His group investigates photon upconversion and its
applications in various processes from solar energy conversion to
biotechnology.
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