Synergetic Modulation of Steric Hindrance and Excited State for Anti‐Quenching and Fast Spin‐Flip Multi‐Resonance Thermally Activated Delayed Fluorophore

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials hold great promise for advanced high‐resolution organic light‐emitting diode displays. However, persistent challenges, such as severe aggregation‐caused quenching (ACQ) and slow spin‐flip, hinder their optimal performance. We propose a synergetic steric‐hindrance and excited‐state modulation strategy for MR‐TADF emitters, which is demonstrated by two blue MR‐TADF emitters, IDAD‐BNCz and TIDAD‐BNCz, bearing steric‐demanding 8,8‐diphenyl‐8H‐indolo[3,2,1‐de]acridine (IDAD) and 3,6‐di‐tert‐butyl‐8,8‐diphenyl‐8H‐indolo[3,2,1‐de]acridine (TIDAD), respectively. These rigid and bulky IDAD/TIDAD moieties, with appropriate electron‐donating capabilities, not only effectively mitigate ACQ, ensuring efficient luminescence across a broad range of dopant concentrations, but also induce high‐lying charge transfer excited states that facilitate triplet‐to‐singlet spin‐flip without causing undesired emission redshift or spectral broadening. Consequently, implementation of a high doping level of IDAD‐BNCz resulted in highly efficient narrowband electroluminescence, featuring a remarkable full‐width at half‐maximum of 34 nm and record‐setting external quantum efficiencies of 34.3% and 31.9% at maximum and 100 cd m‐2, respectively. The combined steric and electronic effects arising from the steric‐hindered donor introduction offer a compelling molecular design strategy to overcome critical challenges in MR‐TADF emitters.