High‐Efficiency Blue‐Emitting Multi‐Resonance TADF Organic Light‐Emitting Diodes Constructed on Spiro Quinolinoacridine/Fluorene‐diphenylphosphine Oxide Positional Isomers

Abstract Improving the performance of blue‐emitting devices is one of the most important challenges in Organic Light‐Emitting Diodes (OLEDs). In recent years, Multi‐Resonance Thermally Activated Delayed Fluorescence (MR‐TADF) with a phosphorescent sensitization technique has shown its potential to enhance efficiency and mitigate efficiency roll‐off. It is reported herein the investigations on the design, the synthesis, and the study of quinolinoacridine‐based positional isomers (SQA‐1‐FPOPh₂, SQA‐3‐FPOPh2, and SQA‐4‐FPOPh₂) as host for blue phosphorescence‐sensitized MR‐TADF OLEDs. The molecular design strategy involves the spatial separation of the electron‐rich (quinolinoacridine) and electron‐poor (fluorene/diphenylphosphine oxide) fragments. The impact of the π‐conjugation disruption between the fluorene and the diphenylphosphine oxide on the electronic properties of the materials is discussed. These bipolar materials are successfully incorporated as hosts in blue MR‐TADF OLEDs employing different emitters BNCz or v‐DABNA and different phosphorescent sensitizers, FIrpic, CN‐Ir, or fac‐Ir(tpz)₃. Remarkably, when used with the v‐DABNA: CN‐Ir combination (0.5:20 wt.% respectively), SQA‐4‐FPOPh₂ presents excellent electroluminescent performances. These devices possess a narrowband emission peak at 472 nm, a full width at half‐maximum of 21 nm, and a high maximum external quantum efficiency of 22.9%. The excellent molecular orientation, which maximizes the light outcoupling efficiency (η_out), is proposed to be the origin of this high performance.