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Simple Acridan‐Based Multi‐Resonance Structures Enable Highly Efficient Narrowband Green TADF Electroluminescence

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) offers an exceptional solution for narrowband organic light‐emitting diode devices in terms of color purity and luminescence efficiency, while the development of new MR skeleton remains an exigent task. It is hereby demonstrated that a simple modification of the B (boron)−N (nitrogen) framework by sp3‐carbon insertion will significantly bathochromic shift the short‐range charge‐transfer emission, boost the reverse intersystem crossing process, and improve the device performances. The bis(acridan)phenylene‐based skeleton developed in this contribution presents a non‐planar conformation with functional sites to facilely introduce isolating units, deriving two luminophores with quantum yields approaching 90% in film state and narrowband emission. Corresponding green‐emissive devices realize superior performances compared to the planar carbazolyl‐based MR‐TADF analogs, featuring a maximum external quantum efficiency (EQEmax) up to 28.2% and small efficiency roll‐off without the involvement of any sensitizing host.