Indolo[3,2,1‐jk]carbazole Embedded Multiple‐Resonance Fluorophors for Narrowband Deep‐blue Electroluminescence with EQE≈34.7 % and CIEy≈0.085

Abstract Multiple‐resonance (MR) organic emitters bearing small full‐width at half‐maximum (FWHMs) are of general interest in organic light‐emitting diodes. Indolo[3,2,1‐ jk ]carbazole (ICz) embedded MR‐fluorophors have demonstrated extremely small FWHMs, yet in the violet region with low electroluminescence efficiency. Herein, a strategic implementation of ICz subunits into MR fluorophors is proposed by taking advantage of the synergetic effect of para ‐positioned nitrogen atoms to enhance electronic coupling to decrease emitting energy gap. Deep blue emitters peaking at 441 and 447 nm with FWHMs of only 18 and 21 nm are thereof obtained, respectively, accompanied by ≈90 % photo‐luminance quantum yields. With the assistance of a thermally activated delayed fluorescence sensitizer to recycle excitons, the corresponding narrowband electroluminescent devices show unprecedent high maximum external quantum efficiencies of 32.0 % and 34.7 % with CIE y of 0.10 and 0.085, respectively.