Multiple‐Resonance‐Induced Thermally Activated Delayed Fluorescence Materials Based on Indolo[3,2,1‐jk]carbazole with an Efficient Narrowband Pure‐Green Electroluminescence

Abstract Herein, we report two multiple‐resonance thermally activated delayed fluorescence emitters (VTCzBN and TCz‐VTCzBN) based on indolo[3,2,1‐ jk ]carbazole unit and boron‐nitrogen skeletons, whose emissions peaking at 496 and 521 nm with full width at half maximum of 34 and 29 nm, respectively. Meanwhile, fast rate constants of reverse intersystem crossing of above 10 6 s −1 are obtained due to small singlet‐triplet energy gaps and large spin‐orbital coupling values. Notably, planar molecular structures along the transition dipole moment direction endow them with high horizontal emitting dipole ratios of up to 94 %. Consequently, the corresponding organic light‐emitting diodes (OLEDs) show the maximum external quantum efficiencies of 31.7 % and 32.2 %, respectively. Particularly, OLED with TCz‐VTCzBN display ultra‐pure green emission with Commission Internationale de l′Eclairage coordinates of (0.22, 0.71), consistent with the green display standard of the National Television System Committee.