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.