University of Osaka: flexible TADF molecules give orange-to-red OLEDs with up to 15% external quantum efficiency

An international team led by researchers at the University of Osaka (Japan) has shown that flexible molecules can serve as efficient metal-free emitters of orange-to-red light, according to a report from ResearchSEA asiaresearchnews.comResearchSEAasiaresearchnews.comOpen source ↗. The work was published in the Journal of the American Chemical Society, and OLEDs made with the new molecules reached an external quantum efficiency of up to 15%, the authors reported.

Many high-performance OLEDs use emitters containing expensive metals such as iridium or platinum. This has driven interest in metal-free materials based on thermally activated delayed fluorescence (TADF). Metal-free materials that efficiently emit orange-to-red light have been hard to make.

Through-space charge transfer without rigid locking

One promising type of TADF molecule works by through-space charge transfer (TSCT): charge moves across space between an electron-donating part and an electron-accepting part held face to face. Such molecules are usually made rigid to keep the two parts close together. How flexible molecules able to rearrange would behave in this role was unclear.

The team, which includes collaborators from Europe, designed large molecules with an "S" shape: two donor units and a central acceptor unit joined by rotatable connections. These connections let the molecules adopt various conformations, including ones with closely stacked donor and acceptor units that favor TSCT. Theoretical calculations showed that the molecules keep the electronic characteristics required for TSCT without permanent conformational locking asiaresearchnews.comResearchSEAasiaresearchnews.comOpen source ↗.

Spectroscopy and device performance

The researchers used time-resolved spectroscopic techniques together with quantum chemical calculations. According to lead author Nae Aota, light emission is governed both by the variety of shapes the molecules adopt before absorbing light and by the changes in shape after excitation.

OLEDs made with these molecules emitted orange-to-red light with an external quantum efficiency of up to 15%. Senior author Youhei Takeda said the best device stayed efficient even at high brightness, which he described as a relatively high level of performance for such materials. The collaboration involved Professor Przemyslaw Data under the Horizon Europe program and Professors Marc Etherington and Piotr de Silva through JST's ASPIRE program.

The authors state that the design concept, which uses flexibility instead of rigidifying molecules, gives new insight into the link between molecular structure, molecular rearrangements, and light emission. They expect further development to lead to TADF materials emitting at even longer wavelengths, with uses in OLED displays and biological imaging asiaresearchnews.comResearchSEAasiaresearchnews.comOpen source ↗.