Chihaya Adachi (left) and Ryota Kabe (right) of Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) have developed the world's first glow-in-the-dark materials based on organic molecules. Image: Center for Organic Photonics and Electronics Research.Glow-in-the-dark paints with improved flexibility and transparency that are also cheaper and easier to manufacture are on the horizon courtesy of researchers from Kyushu University in Japan. In a ground-breaking demonstration, they were able to achieve light emission from organic materials that lasted for over an hour. These organic materials could find use in new applications such as bio-imaging.
Based on a process called persistent luminescence, glow-in-the-dark materials work by slowly releasing energy absorbed from ambient light. Used in watches and emergency signs, commercial glow-in-the-dark materials are based on inorganic materials that include rare metals such as europium and dysprosium. But these materials are expensive, require high temperatures to manufacture and scatter light – rather than being transparent – when ground into powders for paints.
Carbon-based organic materials – similar to those used in plastics and pigments – can overcome many of these disadvantages. They can be excellent light emitters and are already widely used in organic light-emitting diodes (OLEDs). But achieving long-lived emission has proved difficult: the longest emission from organics under indoor lighting at room temperature was, until now, only a few minutes.
Researchers at Kyushu University's Center for Organic Photonics and Electronics Research (OPERA) have now broken through this limit using simple mixtures of two molecules. In films formed by melting together molecules that donate electrons and molecules that accept electrons, the researchers demonstrated light emission from organic materials that lasted for over an hour, without the need for intense light sources or low temperatures.
"Many organic materials can use energy absorbed from light to emit light of a different color, but this emission is generally fast because the energy is stored directly on the molecule that produces the emission," explains Ryota Kabe, lead author of a paper in Nature on this research. "By contrast, our mixtures store the energy in electrical charges separated over a longer distance. This additional step allows us to greatly slow down the release of the energy as light, thereby achieving the glow-in-the-dark effect."
In these mixtures, absorption of light by an electron-accepting molecule, or acceptor, gives the molecule extra energy that it can use to remove an electron from an electron-donating molecule, or donor. This transfer of a negatively-charged electron from the donor to the acceptor is effectively the same as transferring a positive charge from the acceptor to the donor.
The extra electron on the acceptor can then hop to other acceptors and move away from the positively charged donor, resulting in the separation of positive and negative charges. These separated charges gradually combine again – some slowly and some more quickly – to release their energy as light over the span of almost an hour.
The mixtures and processes are similar to those found in organic solar cells and OLEDs. The separated charges build up like in a solar cell, but have nowhere to escape to and so eventually recombine to emit light like an OLED. The key difference in the newly developed mixtures is that the charges can exist in a separated state for very long periods of times.
"With organics, we have a great opportunity to reduce the cost of glow-in-the-dark materials, so the first place we expect to see an impact is large-area applications, such as glowing corridors or roadways for added safety," says Chihaya Adachi, director of OPERA. "After that, we can start thinking about exploiting the versatility of organic materials to develop glow-in-the-dark fabrics and windows, or even bio-compatible probes for medical imaging."
The first challenge to overcome on the road to practical applications is the sensitivity of the process to oxygen and water. Protective barriers are already used in organic electronics and inorganic glow-in-the-dark materials, so the researchers are confident that a solution can be found. At the same time, they are also looking into new molecular structures to increase the emission duration and efficiency, as well as to change the color of the emitted light.
This story is adapted from material from Kyushu University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.