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New organic material glows longer and stronger without depending on rare metals

Science is now one step closer to bringing the glow-in-the-dark effect often used in signs and watches to a wider variety of applications without the need for rare metals.

Applying a new strategy for combining carbon-based organic molecules, researchers at Kyushu University and the Okinawa Institute of Science and Technology (OIST), both in Japan, have dramatically improved the length and strength of the glow produced by the versatile materials. As reported in a paper in Nature Materials, these novel organic materials have the potential to be more easily formed into paints and fibers than their rare-metal-containing counterparts.

The new work builds off the same research group’s discovery in 2017 of the world’s first organic system for producing the glow-in-the-dark effect at room temperature by melting together two metal-free molecules. Formally known as persistent luminescence, the glow-in-the-dark phenomenon is also often referred to as phosphorescence, though this term is also applied to another emission mechanism commonly found in organic materials.

While commercial glow-in-the-dark materials based on inorganic compounds containing rare-earth metals already achieve excellent performance, their inorganic nature often limits how they can be processed.

“Organic materials are more readily available than rare-metal-containing inorganic materials, and their solubility makes them easier to process,” explains Chihaya Adachi, professor and leader of the research at Kyushu University. “In addition to new applications for light-storing materials such as inks, films, and fibers, we expect organics to also enable bio-imaging applications in the future.”

However, the length and strength of the emission from the organic material they developed in 2017 were only about one-hundredth that of inorganic materials, and the glow was quickly extinguished in the presence of oxygen.

“By changing our design strategy, we have now succeeded in improving the performance of organic persistent luminescence by about 10 times over our previous report,” says Ryota Kabe, assistant professor and leader of the research at OIST.

At the heart of the emission, the mechanism is the excitation of a negatively charged electron into a state of higher energy by the absorption of light. Transfer of a lower-energy electron from a nearby donor molecule to fill the 'hole' left behind by the excited electron leads to one molecule having one electron more than normal and the other one less – a situation known as a charge-transfer state.

Emission occurs when the excited electrons return to molecules missing an electron and give off their extra energy as light. So the key to achieving a long-lasting glow is to get the charges to separate by hopping between molecules, which slows down their eventual return.

In this study, the researchers chose a material combination in which it is effectively the holes — the voids left by excited electrons –that hop between molecules, rather than electrons. Holes are generally more stable and less reactive with oxygen, so the light emission from the material was much longer in the air than was the case with their previous materials, where the excited electrons were mobile.

By employing an absorbing material that could be excited with lower-energy light, the materials could not only be energized with ultraviolet light but also with green and even orange light. Additionally, the researchers were able to further stabilize the energy storage state by adding a third organic material that essentially traps the holes, delaying their return and extending the emission duration.

“We have now succeeded in achieving a longer duration and emission under atmospheric conditions,” comments Kabe. “While performance is still below that of inorganic materials, we hope to achieve performance that exceeds that of inorganics with further research.”

The researchers also hope that the organic materials developed in this research will help to expand and diversify sustainable industries without the need for rare metals.

“Time and time again, we are finding that precise control of organic charge-transfer materials enables the expression of a variety of emission properties, not only for glow-in-the-dark applications but also organic LEDs and lasers. I look forward to the new possibilities that a further deepening of the science will bring in the future,” says Adachi.

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