Hybridized Charge-Transfer Window within a Fully Conjugated Multi-Resonance Thermally Activated Delayed Fluorescence Framework for Ultrafast Reverse Intersystem Crossing and High-Efficiency in Deep-Blue Organic Light-Emitting Diodes.

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 bfDOB-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 × 10⁶ 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).