Glow-in-the-dark organic substances are under investigation by a team of researchers hoping to enhance their naturally occurring properties by improving techniques that are already at the cutting edge of materials science. This would help the $400 million global glow-in-the-dark materials industry to grow and expand into new markets sustainably, and replace toxic substances used to make artificial glow-in-dark materials today.
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Glow-in-the-dark materials and coatings currently meet a small but consistent global demand each year. This demand is driven by manufacturers of emergency signage, paint, toys and games, watches, safety and rescue equipment, and a host of other products.
Glow-in-the-dark materials in production today are made from inorganic crystals, often using rare-earth metals and typically consuming large amounts of energy for the high fabrication temperatures required for crystal growth (over 1,000 °C).
Materials scientists have been investigating organic materials with glow-in-the-dark properties as an alternative to synthesizing organic materials with the desired traits in a resource and energy-intensive manufacturing process.
Previously, organic glow-in-the-dark materials had been successfully synthesized by scientists, showing that a sustainable method for manufacturing materials with these properties was possible.
But these were far less luminous than their inorganic counterparts. New research has tweaked the previous method for synthesizing organic glow-in-the-dark materials to produce organic materials that perform as well as conventional inorganic materials.
This breakthrough may open the path for wider use of glow-in-the-dark materials, provided it can scale to manufacturing capacities as the researchers expect. Future applications for glow-in-the-dark materials that perform better and cost less (financially and environmentally) will be novel: glow-in-the-dark roads, walls, factories, and parks are just some of the potential uses for glow-in-the-dark materials that have not yet been fully explored.
Benefits of Organic Glow-in-the-Dark Materials
The scientists behind the new technique, from the Okinawa Institute of Science and Technology (OIST) and Kyushu University, Japan, wanted to develop more readily available materials for glow-in-the-dark products that were easier to work with and were soluble.
Their study, published in Nature Materials in 2021, makes the case that organic materials meet these requirements and could open up markets for new glow-in-the-dark products.
Researchers say that inks, films, and textiles can all be made to glow in the dark. Glow-in-the-dark products also have a potentially important role to play in bioimaging, which could lead to new developments in medical science.
The new study builds on very recent research into organic glow-in-the-dark materials. A 2017 paper published in Nature demonstrated for the first time how two organic materials could be combined to create an organic compound that glows in the dark.
The glow-in-the-dark performance of these new compounds, however, was close to 100 times weaker than typical inorganic materials used for glow-in-the-dark products. Samples would only glow if emissions were generated with ultraviolet (UV) light, the room was darkened, and samples were not exposed to any oxygen.
To improve on this, scientists added another organic component to the compounds, creating three instead of two-part compounds. They also tweaked the type and dispersion of molecules in the compound.
The result was a glow-in-the-dark substance whose light emissions lasted for over an hour at room temperature. This was ten times better than the previous demonstration, and the researchers behind it are confident that more improvements can be achieved in the next few years.
This would help to replace inorganic materials typically used for glow-in-the-dark products with organic compounds that cost much less in terms of energy, resources, and finances to manufacture.
Tweaking the Manufacturing Method for Better Organic Glow-in-the-Dark Materials
The method developed in 2017 uses four stages to create the glow-in-the-dark effect. These stages are charge transfer, separation, recombination, and emissions.
In charge transfer, electrons nestle in holes in the molecules. These must be separated from each other so that the electrons can return to recombine with the molecules in these holes. That recombination causes an emission of photons from the molecule, generating glow.
The original method used light to energize the organic materials, causing electrons to transfer from a so-called electron donor molecule to an electron acceptor molecule. The different molecules were the two organic components referred to in the two-components method.
However, electron acceptor molecules can only store so many electrons, resulting in only a few returning to recombine with the donor molecule. This in turn results in relatively weak glowing performance.
Improving the method, the authors of the latest study tweaked several factors.
First, they found organic molecules whose surface topology meant that holes would move to the electrons, rather than electrons moving to the holes. This system, referred to as hole diffusion, meant that molecules were less likely to react with air. As a result of this, glow-in-the-dark properties were achieved that could withstand exposure to oxygen.
A third component was also added. A hole trapper separated the electron and the hole for longer, allowing for more holes to build up and increase the length of time the material would emit light for.
Finally, the team found molecules that could move between the steps of the process with less energy required. As the whole process could now work with less energy, the materials could now emit visible light rather than only UV light that prior research had achieved.
All of this improved the new method for organic glow-in-the-dark materials by 10 times.
The Future of Organic Glow-in-the-Dark Materials
The researchers of the 2021 study are confident that more improvements to their technique can be made. These improvements would be needed to bring the materials in line with typical inorganic materials that are still 10 times stronger than the best organic varieties.
References and Further Reading
Jinnai, K. et al. (2021). Organic long-persistent luminescence stimulated by visible light in p-type systems based on organic photoredox catalyst dopants. Nature Materials. https://doi.org/10.1038/s41563-021-01150-9.
Kabe, R., Adachi, C. (2017) Organic long persistent luminescence. Nature 550, 384–387. https://doi.org/10.1038/nature24010
Phys.org. (2021) Researchers light the way for organic glow-in-the-dark materials. Available at: https://phys.org/news/2021-11-glow-in-the-dark-materials.html
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