Volume 95 Issue 40 | p. 10 | News of The Week
Issue Date: October 9, 2017 | Web Date: October 5, 2017

Organic material sets luminescent record

Persistent glowing material could find applications in road paints and street signs
Department: Science & Technology
Keywords: photonics, luminescence
Films (1 cm2) of PPT, a TMB/PPT mixture, and TMB sit under weak ambient light (top). The mixture is also shown in the dark after getting excited by UV light. Only the TMB/PPT combination film glows in the dark.
Credit: Ryota Kabe
Photos of films of TMB and PPT constituents and a TMB+PPT mixture shows that the latter glows in the dark.
Films (1 cm2) of PPT, a TMB/PPT mixture, and TMB sit under weak ambient light (top). The mixture is also shown in the dark after getting excited by UV light. Only the TMB/PPT combination film glows in the dark.
Credit: Ryota Kabe

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.

Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Arthur Friedman (Wed Oct 11 21:18:38 EDT 2017)
I am curious about the use of tetramethylbenzidine in this interesting formulation. I believe that the unsubstituted benzidine, which was used to detect occult blood many years ago, is no longer used because it was causing bladder carcinomas in medical technicians who were using the reagent. Is it possible that this substituted benzidine may cause the same problems?
V, Jayathirtha Rao (Thu Oct 12 16:44:51 EDT 2017)
It is the "Cation radical" and Anion radical" combination leads to formation of excited state and further emission of light, It is not electron - hole combination as it is in inorganic chemiclas.
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