Organic material sets luminescent record

A new glow-in-the-dark organic material emits light for over an hour at room temperature when excited by ultraviolet or visible white light (Nature 2017, DOI: 10.1038/nature24010). The achievement is a new record, as other luminescent organic materials glow for at most a few minutes.

The new material’s long-lasting glow is an “extraordinary” finding, says photomaterials specialist Runfeng Chen of Nanjing University of Posts & Telecommunications, who was not involved in the work. Daniel Scherman of Paris Descartes University, who has developed nanophosphors for biological imaging applications, agrees and believes that the material could spark industrial interest. Potential applications include glow-in-the-dark paints for illuminating building corridors, roads, and street signs; glowing fabrics and windows; and biocompatible probes for biological imaging.

To make the new materials, Ryota Kabe and Chihaya Adachi of Kyushu University’s Center for Organic Photonics & Electronics Research melted together the electron donor N,N,N´,N´-tetramethylbenzidine (TMB) and the electron acceptor 2,8-bis(diphenylphosphoryl)dibenzo[b,d] thiophene (PPT). When PPT absorbs incident light, the energy creates an isolated electron and a corresponding positive charge, or hole. While the hole transfers to TMB, the electron jumps from one PPT molecule to another, getting passed around like a hot potato. The electron eventually recombines with the hole on a TMB molecule, causing light emission. Some electrons recombine quickly with a hole, but many remain isolated for a long time, causing the glow to persist for over an hour after the excitation light has been turned off. The material currently emits green light, but the researchers believe other colors will be readily accessible.

Luminescent inorganic materials that glow for long times—typically more than 10 hours but up to 360 hours in one extraordinary case—are widely used commercially on watches and signs. But they are not transparent, are rigid and insoluble, and require fabrication temperatures of over 1,000 °C.

The new material is transparent and potentially flexible, although flexibility was not achieved in this initial study. It is also soluble in organic solvents and does not require high-temperature processing.

Despite these advantages over inorganic luminescent materials, the organic material’s visible excitation is weak and could use improvement, says photonics materials expert Jianrong Qiu of South China University of Technology.

The new material is also sensitive to oxygen and water, Kabe and Adachi say. But they think transparent protective barriers could protect the material from exposure. In future work, they hope to create new versions of the material with improved properties.