Achieving High‐Performance Multi‐Resonance Thermally Activated Delayed Fluorescence (MR‐TADF) Blue Emitter Using Benzothiophene Heterocyclic Fusion Strategy

To meet the growing demands in the field of ultra-high-definition display, multi-resonance thermally activated delayed fluorescence (MR-TADF) materials have attracted extensive attention due to their narrowband emission characteristics. However, the development of these materials still faces many challenges, such as limited chemical modification methods and severe device efficiency roll-off. Herein, the benzothiophene (BZT) heterocyclic fusion strategy was adopted to chemically modify the classical single-boron and dual-boron MR-TADF emitters into two new derivatives (SS-B and SS-2B). By combining theory with experiments, the intrinsic mechanism of the improvements in emission properties was revealed due to the fusion incorporation of BZT units, resulting in the high-efficiency and high-color-purity doped blue organic light-emitting diode (OLED) with effective suppression of efficiency roll-off. The full width at half maximum (FWHM) of their electroluminescence spectra are only 26 and 29 nm for SS-B and SS-2B, respectively. Their maximum external quantum efficiencies (EQE_max) are as high as 34.34% and 37.23% respectively, and the EQE roll-off of SS-2B is only 4.9% at 100 cd·m^-2. This work not only provides a better understanding of the photophysical nature of boron/nitrogen (B/N) molecular systems, but also offers a new strategy for improving the electroluminescence performance of MR-TADF materials.