Charge Transfer Excited State Promoted Multiple Resonance Delayed Fluorescence Emitter for High-Performance Narrowband Electroluminescence

摘要

Multipleresonance thermally activated delayed fluorescence(MR-TADF)emitters are promising candidates for narrowband organic light-emittingdiodes, but their electroluminescent performance is typically hinderedby the slow reverse intersystem crossing rate (k (RISC)). Herein, we present an effective strategy to introducea multichannel reverse intersystem crossing (RISC) pathway with largespin-orbit coupling by orthogonally linking an electron-donatingunit to the MR framework. Through delicate manipulation of the excited-stateenergy levels, an additional intersegmental charge transfer tripletstate could be "silently" induced without perturbingthe MR character of the lowest excited singlet state. The proof-of-conceptemitter CzBN3 not only affords 23-fold increase of k (RISC) compared with its prototypical MR skeletonbut also realizes close-to-unity photoluminescence quantum yield,large radiative rate constant, and very narrow emission spectrum.These merits enable high maximum external quantum efficiency (EQE(max)) of up to 37.1 and alleviated efficiency roll-off in thesensitizer-free device (EQE(1000) = 30.4), and a furtherboost of efficiency (EQE(max/1000) = 42.3/34.1) is realizedin the hyperfluorescent device. The state-of-the-art electroluminescentperformance validates the superiority of our molecular design strategy.