Organic materials that glow in the dark

Researchers have succeeded in developing, for the first time ever, materials that glow in the dark and are based on organic molecules. The new materials eliminate the use of precious metals and the processing processes at high temperatures required for the production of existing similar materials. In addition to reducing the production cost of these materials compared to those existing today, the innovative organic materials should also allow improved flexibility, transparency, and biocompatibility, properties that will lead to the development of a variety of new applications.

[Translation by Dr. Nachmani Moshe]

Glow-in-the-dark dyes that are flexible and transparent and cheaper and simpler to manufacture are on the horizon, thanks to new research conducted at Kyushu University. In a groundbreaking study, the researchers were able to achieve stable light emission for more than one hour from organic materials, a development that could be an important advance in a variety of innovative applications, for example, in the field of biological imaging.

Based on a process known as 'persistent luminescence', materials that glow in the dark work by slowly releasing energy absorbed from the surrounding light. Such materials, used in wristwatches and emergency signs, are currently based on inorganic compounds and include rare metals such as europium and dysprosium. At the same time, these materials are expensive, require high temperature conditions for their production, and scatter the light - this is in contrast to the transparency feature - when they are ground to obtain a powder used in coloring materials.

Carbon-based organic materials - similar to those used in plastics and paints - may overcome many of these shortcomings. They can be excellent sources of light emission and are already common today in devices such as organic light emitting diodes (OLEDs). Despite this, achieving a stable and long-term emission is quite a challenge, and the longest emission duration from organic materials under indoor lighting conditions and at room temperature was only a few minutes until now. Researchers from the Center for Photonics and Electronics Research at Kyushu University were able to overcome this threshold using simple mixtures of two suitable molecules. Using layers created by fusing together electron-donating molecules in combination with electron-accepting molecules, the emission remains stable for more than an hour, without the need for high temperatures or strong lighting. "Many organic materials can harness energy absorbed from lighting in their environment while emitting light of a different color, however, this light emission is usually fast because the energy is stored directly within the molecules that produce the emission," says the lead author of the article in which these findings were published. "In contrast, our mixtures keep the energy within electric charges that are separated over a greater distance. This additional step allows us to significantly slow down the release of the energy in the form of light, and thus we achieve the effect of materials that glow in the dark."

Within the mixtures, the absorption of light by an electron-accepting molecule contributes to the molecule excess energy that can be used to remove an electron from an electron-donating molecule. The transfer of the electron is efficient in the same way that a positive charge is transferred in the opposite direction. The excess electron can next jump to another molecule and move further away from the positively charged molecule, thus increasing the distance between the charges. The separated charges reconnect at a gradual rate, some slowly and some quickly, when the phase in which they merge together results in the release of their excess energy in the form of light, for a period of more than an entire hour. The composition of the materials and the processes for their preparation are similar to those underlying organic solar cells and OLEDs. After the accumulation of separated charges similar to the stage that occurs in a solar cell, the charges are trapped and eventually fuse together while emitting light, just like in OLED devices. The main difference in the innovative mixtures lies in the fact that the charges in the content can exist in a separated state for relatively long periods of time. "We believe that our organic materials will have a great ability to reduce the cost of glow-in-the-dark materials, so the first area where we will see their impact will be in applications that require large areas, such as corridors where luminous signs are placed or roads with luminous markings aimed at increasing safety," says One of the researchers. "In the next step we can start thinking about utilizing the benefits of organic materials for the development of fibers, fabrics and windows that glow in the dark, or even biological detectors for medical imaging."

The main challenge further down the road to making the innovative materials practically effective is the sensitivity of the process to oxygen and water. Barriers that protect against oxygen and water are already used today in organic electronic components and in inorganic materials that glow in the dark, so the researchers are convinced that they can find a solution to this challenge. At the same time, the researchers will also look for new molecular structures that can increase the duration of the emission and its efficiency, and in addition, that will allow a change in the effect of the emitted light.

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