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Organic light-emitting diodes (OLEDs) have attracted much attention for their potential in applications as fullcolour, flexible displays1) and white lighting sources2) 3). In 1987, Tang and VanSlyke developed an OLED with a double-layered structure, in which a fluorescent compound, tris(8-hydroxyquinoline) (Alq3), was used as an emitter4). Charge recombination in an OLED generates both singlet and triplet excitons in a 1:3 ratio5) and thus the use of fluorescent emitters as dopants limits both the internal and external quantum efficiency (IQE and ηEQE, respectively) to 25% (Fig.1) and 5-7.5%, respectively. To increase quantum efficiency, phosphorescent dopants have been incorporated into OLEDs6). These dopants allow all the singlet and triplet excitons to be Abstract Thermally activated delayed fluorescence (TADF) emitters have recently attracted considerable attention as dopants for organic light-emitting diodes (OLEDs), and are considered promising alternatives to fluorescent and phosphorescent dopants. TADF emitters require a small singlet-triplet energy gap (∆EST). This study presents a molecular design strategy to achieve a small ∆EST. An electroluminescent dopant containing two electron-donating carbazolyl groups and an electron-accepting triphenyltriazine, m-bisCzTRZ, was prepared. The electron-donating moieties were introduced at the meta positions of the two phenyl rings in triphenyltriazine. Transient photoluminescence decay measurements confirmed that m-bisCzTRZ emitted blue TADF in a solid-state host layer and thus it showed promise as a blue dopant. The experimental ∆EST value for m-bisCzTRZ was small (0.09 eV), as determined from fluorescence and phosphorescence spectra of a mbisCzTRZ-doped host. These observations suggested that the use of meta-linked electron-donating and electronaccepting units could be a useful strategy to obtain TADF emitters.

[Paper] Meta-linking Strategy for Thermally Activated Delayed Fluorescence Emitters with a Small Singlet-Triplet Energy Gap