Carbocation‐Based Multi‐Resonance Thermally Activated Delayed Fluorescent Emitters with Efficient Narrowband Electroluminescence

Multi‐resonance thermally activated delayed fluorescent (MR‐TADF) emitters hold great promise for ultrahigh‐definition displays, but are fundamentally restricted to wide‐energy‐gap heteropolycyclic systems typically with blue‐to‐green emissions, while their yellow‐to‐red emissions remain a major challenge. Here we propose a strategy for developing MR‐TADF emitters with narrow energy gaps (less than 2.20 eV) by doping positively‐charged carbenium ion (C+) into polycyclic skeletons to create strong short‐range charge transfer with electron‐rich nitrogen atoms, achieving a significant 160 nm emission redshift compared to the benchmark neutral boron‐based counterpart. Furthermore, steric isopropyl groups and bulky tetrakis(pentafluorophenyl)borate counter ions are synergistically integrated to suppress intermolecular aggregation, yielding high solid‐state photoluminescence quantum efficiencies up to 90%. Solution‐processed organic light‐emitting diodes based on the emitters exhibit promising external quantum efficiency of 29.4% with narrow full‐width at half‐maximum of 0.17 eV, opening the way for development of ion‐based MR‐TADF emitters toward efficient long‐wavelength narrowband electroluminescence.