Excited‐State and Steric Hindrances Engineering Enable Fast Spin‐Flip Narrowband Thermally Activated Delayed Fluorescence Emitters with Enhanced Quenching Resistance

Abstract Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials have great potential for applications in ultrahigh‐definition (UHD) organic light‐emitting diode (OLED) displays, that benefit from their narrowband emission characteristic. However, key challenges such as aggregation‐caused quenching (ACQ) effect and slow triplet‐to‐singlet spin‐flip process, especially for blue MR‐TADF materials, continue to impede their development due to planar skeletons and relatively large Δ E ST s. Here, an effective strategy that incorporates multiple carbazole donors into the parent MR moieties is proposed, synergistically engineering their excited states and steric hindrances to enhance both the spin‐flip process and quenching resistance. As expected, the designed materials namely 5Cz‐BNO and 5Cz‐BN exhibit bright blue and green emissions with narrow full‐width at half‐maximums (FWHMs) around 23 nm, together with significantly improved reverse intersystem crossing (RISC) rates. The OLEDs based on 5Cz‐BNO and 5Cz‐BN with doping concentrations from 5 to 20 wt % achieve high maximum external quantum efficiency (EQE max ) values exceeding 30 % with suppressed efficiency roll‐offs and improved operational stability. This work offers an effective approach for designing doping‐insensitive blue and green MR‐TADF materials with fast spin‐flip processes by integrating the engineering of excited states and steric hindrances.