Multi‐Resonance Building‐Block‐Based Electroluminescent Material: Lengthening Emission Maximum and Shortening Delayed Fluorescence Lifetime

Abstract Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials are considered a class of organic materials with exceptional electronic and optical properties, which make them promising for the applications in organic light‐emitting diodes (OLEDs). In this study, we improved, synthesized, and characterized a multiple‐resonance type emitter based on the assembly of MR‐building blocks (MR‐BBs). By optimizing the geometric arrangement of MR‐BBs, we were able to generate narrowband emission in the longer wavelength region and shorten the delayed excited‐state lifetime, resulting in improved emission efficiency compared to the parent molecule. Our proof‐of‐concept molecule, m ‐DBCz, exhibited narrowband yellowish‐green TADF emission with a full width at half‐maximum of 32 nm and a small singlet‐triplet energy gap of 0.04 eV. The OLED developed using m ‐DBCz as the emitter demonstrated electroluminescence at 548 nm and achieved a high external quantum efficiency (EQE) of 34.9 %. Further optimization of the device resulted in a high external quantum efficiency of 36.3 % and extremely low efficiency roll‐off, with EQE values of 30.1 % and 27.7 % obtained even at high luminance levels of 50 000 and 100 000 cd m −2 . These results demonstrate the full potential of MR‐TADF materials for applications on ultrahigh‐luminance OLEDs.