The future of organic glow-in-the-dark materials is shining brightly now thanks to a study reporting a way to make the easily-processed, low-cost materials stable in air (Nat. Mater. 2021, DOI: 10.1038/s41563-021-01150-9).
Commercial materials that exhibit the glowing effect known as long-persistent luminescence (LPL) typically include metal oxides and rare-earth metals. These materials, which put the glow in glow-in-the-dark watch faces, tend to require high-temperature processing and must be ground into fine particles and dispersed in solvents or polymers for use in applications.
Organic LPL materials are metal-free, readily soluble, and easily processed in liquid solvents. Those properties make the materials easy to put in paints, polymers, and fabrics for use in emergency signs, safety apparel, and other applications. Compared with their inorganic counterparts, however, organic LPL materials have tended to glow weakly and become dim quickly, especially upon exposure to air.
Now, a team including Chihaya Adachi of Kyushu University and Ryota Kabe of the Okinawa Institute of Science and Technology reports organic LPL materials that glow for more than 1 h in air.
The new materials consist of neutral semiconducting electron donors (carbazols and benzimidazols) and cationic electron acceptors (pyrylium compounds). Unlike earlier organic LPL systems in which light-emission is mediated by electron-based charge transfer, the new materials do the job by shuttling positively charged species, which resist oxidation. The materials also include a third component—nitrogen and sulfur-based positive-charge trapping materials—that helps the materials glow even longer.