Tailored Design of π‐Extended Multi‐Resonance Organoboron using Indolo[3,2‐b]Indole as a Multi‐Nitrogen Bridge

Abstract Extending the π‐skeletons of multi‐resonance (MR) organoboron emitters can feasibly modulate their optoelectronic properties. Here, we first adopt the indolo[3,2‐b]indole (32bID) segment as a multi‐nitrogen bridge and develop a high‐efficiency π‐extended narrowband green emitter. This moiety establishes not only a high‐yield one‐shot multiple Bora–Friedel–Crafts reaction towards a π‐extended MR skeleton, but a compact N‐ethylene‐N motif for a red‐shifted narrowband emission. An emission peak at 524 nm, a small full width at half maximum of 25 nm and a high photoluminescence quantum yield of 96 % are concurrently obtained in dilute toluene. The extended molecular plane also results in a large horizontal emitting dipole orientation ratio of 87 %. A maximum external quantum efficiency (EQE) of 36.6 % and a maximum power efficiency of 135.2 lm/W are thereafter recorded for the corresponding device, also allowing a low efficiency roll‐off with EQEs of 34.5 % and 28.1 % at luminance of 1,000 cd/m 2 and 10,000 cd/m 2 , respectively.