This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. 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 working paper posted to the ChemRxiv preprint server, 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.
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