An international team led by UOsaka researchers used specially designed molecules with movable segments to develop a new approach to designing light-emitting materials, with potential applications in advanced technologies such as smartphone displays

Osaka, Japan – Most of us spend much of our day staring at organic light-emitting diodes, or OLEDs, which convert electricity into light to produce the colorful displays on smartphones, computer monitors, and televisions. Although metal-free light-emitting materials that produce all colors are in demand, it has been difficult to make ones that efficiently emit orange-to-red light. Now, researchers led by the University of Osaka have shown that such materials do not need to be held in a rigid shape to work well: flexible molecules that can change shape can also emit orange-to-red light efficiently.

Many high-performance OLEDs rely on light-emitting molecules containing expensive metals such as iridium or platinum. This has driven interest in metal-free materials that function on a principle called thermally activated delayed fluorescence (TADF). One promising type of TADF molecule works via through-space charge transfer, in which charge moves across space between electron-donating and electron-accepting parts held face to face. These molecules are usually designed to be rigid to keep these two parts close together. However, how flexible molecules capable of rearranging would behave in this role has remained unclear.

In a study published in the Journal of the American Chemical Society, the team, which includes collaborators from Europe, designed large, complex molecules with movable segments and found that they efficiently emit orange-to-red light through this mechanism. “We used time-resolved spectroscopic techniques together with quantum chemical calculations to understand how these materials emit light,” says lead author Nae Aota. “We discovered that their light emission is governed both by the variety of shapes the molecules already adopt before absorbing light and by the changes in shape that occur after they are excited. Working with international collaborators gave me a valuable opportunity to learn different approaches to research and deepen my understanding of the complex photophysical processes involved.”

The new molecules have an “S” shape, with two donor units and a central acceptor unit joined by rotatable connections. Unlike previously reported rigid architectures, the rotatable connections allow these molecules to adopt various conformations, including those with closely stacked donor and acceptor units favorable for through-space charge transfer. Theoretical calculations revealed that the molecules can retain the electronic characteristics required for TSCT even without permanent conformational locking.

“OLEDs made with these molecules produced orange-to-red light with an external quantum efficiency of up to 15%, and the best device stayed efficient even at high brightness, representing a relatively high level of performance for such materials,” explains senior author Youhei Takeda. “This achievement was made possible through international collaborations with Professor Przemyslaw Data under the Horizon Europe program and with Professors Marc Etherington and Piotr de Silva through JST’s ASPIRE program. By combining our complementary expertise, we were able to gain deeper insights into the relationship between molecular flexibility and light emission.”

This new molecular design concept, which makes use of flexibility instead of rigidifying molecules to hold them in a light-emitting shape, has provided new insight into the connection between molecular structure, molecular rearrangements, and light emission. The team expects that further development will lead to highly efficient TADF materials that emit at even longer wavelengths, with applications not only in OLED displays but also in biological imaging.
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The article, “Efficient Long-Wavelength Through-Space Charge Transfer Thermally Activated Delayed Fluorescence in Conformationally Adaptive Emitters without Conformational Locking,” will be published in the Journal of the American Chemical Society DOI: https://doi.org/10.1021/jacs.6c12459

About The University of Osaka
The University of Osaka was founded in 1931 as one of the seven imperial universities of Japan and is now one of Japan's leading comprehensive universities with a broad disciplinary spectrum. This strength is coupled with a singular drive for innovation that extends throughout the scientific process, from fundamental research to the creation of applied technology with positive economic impacts. Its commitment to innovation has been recognized in Japan and around the world. Now, The University of Osaka is leveraging its role as a Designated National University Corporation selected by the Ministry of Education, Culture, Sports, Science and Technology to contribute to innovation for human welfare, sustainable development of society, and social transformation.
Website: https://resou.osaka-u.ac.jp/en