ABSTRACT Multi‐Resonance (MR) thermally activated delayed fluorescence (TADF) materials, featuring narrow band emission and high efficiency, are being utilized in blue organic light‐emitting diode (OLED) applications. However, compared to conventional donor–acceptor (D–A) type TADF emitters, these molecules exhibit relatively large singlet‐triplet energy gaps and slow reverse intersystem crossing (RISC) rates, indicating the need for further research to overcome these limitations. Herein, we designed deep blue MR‐TADF molecules by integrating intramolecular short‐range charge transfer and long‐range charge transfer to minimize the singlet‐triplet energy gap and accelerate the RISC rate through hybridized excited states maintaining a narrow emission bandwidth. Compared with conventional MR‐TADF molecules emitting only by short‐range charge transfer, the new MR‐TADF molecule is differentiated in that intramolecular long‐range charge transfer within the polyaromatic hydrocarbon framework additionally contributes to the emission process along with short‐range charge transfer for accelerated RISC. As a result, the bf DOB‐BN2 MR‐TADF emitter exhibited narrow blue emission at 447 nm with a narrow full width at half maximum (FWHM) of 20 nm, a small singlet‐triplet energy gap of 0.04 eV, and an ultrafast RISC rate of 2.1 × 10 6 s − 1 . In a blue TADF OLED device, the new TADF molecule showed a high external quantum efficiency of 37.5% with color coordinates (0.139 and 0.065).
Lee et al. (Tue,) studied this question.