Ambipolar Self‐Host Functionalization Accelerates Blue Multi‐Resonance Thermally Activated Delayed Fluorescence with Internal Quantum Efficiency of 100%

Abstract Emerging multi‐resonance (MR) thermally activated delayed fluorescence (TADF) emitters can combine 100% exciton harvesting and high color purity for their organic light‐emitting diodes (OLED). However, the highly planar configurations of MR molecules lead to intermolecular‐interaction‐induced quenching. A feasible way is integrating host segments into MR molecules, namely a “self‐host” strategy, but without involving additional charge transfer and/or vibrational components to excited states. Herein, an ambipolar self‐host featured MR emitter, tCBNDADPO, is demonstrated, whose ambipolar host segment (DADPO) significantly and comprehensively improves the TADF properties, especially greatly accelerated singlet radiative rate constant of 2.11 × 10 8 s −1 and exponentially reduced nonradiative rate constants. Consequently, at the same time as preserving narrowband blue emission with an FWHM of ≈28 nm at a high doping concentration of 30%, tCBNDADPO reveals state‐of‐the‐art photoluminescence and electroluminescence quantum efficiencies of 99% and 30%, respectively. The corresponding 100% internal quantum efficiency of tCBNDADPO supported by an ultrasimple trilayer and heavily doped device demonstrates the feasibility of the ambipolar self‐host strategy for constructing practically applicable MR materials.