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

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 (ΔEST). This study presents a molecular design strategy to achieve a small ΔEST. 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 ΔEST value for m-bisCzTRZ was small (0.09 eV), as determined from fluorescence and phosphorescence spectra of a m-bisCzTRZ-doped host. These observations suggested that the use of meta-linked electron-donating and electron-accepting units could be a useful strategy to obtain TADF emitters.