A Multi-Resonant Thermally Activated Delayed Fluorescence Emitter with a Twisted Second-Generation Carbazole Dendron Showing Suppressed Concentration Quenching and Its Use in Solution-Processed Organic Light-Emitting Diodes

Multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters have drawn significant interest for use in organic lighting-emitting diodes (OLEDs) as they typically have bright and narrowband emission. However, their rigid, planar structures result in poor solubility in organic solvents and a tendency to aggregate. This usually results in severe aggregation-caused quenching (ACQ), which hinders in particular, their application in solution-processed OLEDs. Here, a solution-processable MR-TADF emitter 2,7-tBuCzNB has been designed, synthesized and studied. The presence of eight tert-butyl groups and the use of second-generation donor dendrons help to enhance its solubility and to suppress the ACQ. 2,7-tBuCzNB exhibits narrowband green emission at 493 nm, with full-width at half maximum of 32 nm, and a high photoluminescence quantum yield (PL) of 93% in toluene. The PL values in 1–10 wt% doped films in mCP are slightly lower yet are as high as 80%. The solution-processed OLEDs using this emitter showed maximum external quantum efficiencies (EQEmax) of 11.4 and 10.6% at 5 and 10 wt% doping concentration, respectively. This work demonstrates a strategy to synthesize MR-TADF emitters that are solution processable for use in solution-processed OLEDs.