High-speed phosphorescence without using rare metals — World's highest levels of efficiency achieved by Osaka University and Kyushu University

A research group led by Assistant Professor Yosuke Tani of the Graduate School of Science at Osaka University, in collaboration with a group headed by Associate Professor Kiyoshi Miyata of the Graduate School of Science at Kyushu University, has achieved a phosphorescence efficiency of organic molecules that breaks the world record by a wide margin. The research group elucidated the key mechanism of high-speed phosphorescence. The research results were published in Chemical Science.

Phosphorescence is a phenomenon in which molecules in a high-energy state emit light while changing the direction of their electron spins. This is being developed with applications for organic electroluminescence (EL) and cancer diagnosis. It was previously thought that the use of rare metals such as iridium and platinum was vital in order to obtain high-efficiency phosphorescence. However, rare metals present challenges in terms of stable supply, and a mechanism for achieving high-efficiency phosphorescence with organic molecules without the use of rare metals had not been elucidated.

The research group performed a detailed investigation of the luminescence of thienyl diketone, a proprietary rare metal-free organic molecule, using time-resolved spectroscopy and other methods. As a result, they elucidated that the phosphorescence efficiency of this molecule is up to 38% in solution, more than two-fold the previous world record. Thin films prepared by adding small amounts of this molecule to polymers demonstrated phosphorescence efficiency of greater than 50% under atmospheric conditions. Furthermore, the source of this high-efficiency phosphorescence is revealed to be the high speed of phosphorescence. The estimated speed was approximately 5000 per second, which is more than an order of magnitude faster than conventional organic molecules and approaching that of phosphorescence using the rare metal platinum (about 8300 per second). The mechanism behind this high-speed phosphorescence was clarified using quantum chemical calculations. Phosphorescence arises from a high-energy state called the triplet state. However, light is not typically emitted in the pure triplet state. However, the triplet state of thienyl diketone was found to be mixed, approximately 1% mixing, with a separate state that had strong luminescent properties. Such mixing enabled high-speed phosphorescence. By elucidating this mechanism, the research group has obtained guidelines for molecular design for achieving high-speed phosphorescence without the use of rare metals.

Tani said, "This molecule was discovered by chance; at first we did not understand why it demonstrated such excellent performance. However, as the research progressed, our understanding deepened, and it was as if the pieces of the puzzle were fitting together. We now feel that there has never been a molecular material with such a clearly defined mechanism. On the other hand, I also feel there is still much more we can explore, and I look forward to its future applications."

Journal Information
Publication: Chemical Science
Title: Fast, efficient, narrowband room-temperature phosphorescence from metal-free 1,2-diketones: rational design and the mechanism
DOI: 10.1039/D4SC02841D