Simultaneous Emission Spectral Narrowing and Spin‐Flip Acceleration in Multi‐Resonance Organoboron Emitters via Sequential Indolo‐Fusion π‐Extension

Abstract Advanced cutting‐edge organic light‐emitting diode (OLED) technologies require narrowband electroluminescence (EL) with high quantum efficiency to growing performance demand. Here, sequential indolo‐fusion π‐extension as a rational design strategy to further narrow the emission bandwidth of multi‐resonance thermally activated delayed fluorescence (MR‐TADF) is introduced. By fusing one or two additional indole moieties to the periphery of Cz‐B, a typical MR‐TADF motif, gradual spectra narrowing, and accelerated triplet–singlet exciton spin conversion are concurrently achieved. Suppressing vibronic couplings induced by high‐frequency vibrational modes is vital for achieving extreme spectral narrowing. OLEDs based on a doubly indolo‐fused MR‐TADF emitter, IDCz‐B‐II, exhibited ultra‐narrowband sky‐blue EL with a full width at half maximum of 21 nm (0.11 eV), maximum external EL quantum efficiency of 25.9%, and reduced efficiency roll‐off, significantly outperforming the Cz‐B‐based device. The present indolo‐fusion strategy provides a simple yet promising approach for developing isomeric MR‐TADF materials with different fusing positions and orientations.