Enhancing the Operational Lifetime of OLEDs Through the Modulation of Rigidity and Excited‐State Properties

Blue-emitting multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters with high photoluminescence quantum yield (PLQY), high robustness with short-lived emission lifetime is particularly desired for the development of organic light-emitting diodes (OLEDs). In this study, a series of MR-TADF molecules featuring fused boron/nitrogen (B/N) and C ═ O/N frameworks is reported. These emitters namely BNO, BNDO, and BNTO are systematically designed and synthesized to investigate the impact of molecular rigidity or planarity toward their excited-state dynamics through stepwise intramolecular electrophilic acylation reactions. Computational studies and single-crystal X-ray diffraction data revealed the enhanced planarity of BNDO and BNTO. Upon photoexcitation, these compounds exhibited blue emission with PLQYs of approaching unity and short-lived delayed lifetimes (<2 µs). BNTO-based OLEDs achieved sky-blue emission peaking at 489 nm, a moderate device operational lifetime of over 500 h at 70% of the initial brightness (LT₇₀) at 1000 cd m^-2, which shows longer device stability when compared with BNO and greener emission when compared to BNDO. This study highlights that manipulation of the rigidity of compounds and emissive states of MR-TADF compounds is essential in achieving blue emission and improving OLED operational stability.