Rational Charge‐Transfer Modulation for High‐Performance Narrowband Blue Thermally Activated Delayed Fluorophores

ABSTRACT Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters are attractive for ultrahigh‐definition organic light‐emitting diode (OLED) displays due to their narrowband emission and high color purity. However, the detrimental trade‐off between electroluminescence efficiency and broadened emission band remains a critical challenge for blue MR‐TADF materials. Herein, a rational charge‐transfer (CT) engineering strategy is proposed to design and synthesize four MR‐TADF molecules: SB‐DOP, DB‐DOP, DB‐DFN and DB‐DFNP, which feature the progressively extended linear conjugation. Benefiting from the precise regulation of the CT strength for rapid spin‐flip process, DB‐DFN achieves a high reverse intersystem crossing rate constant of 3.824 × 10 6 s −1 , along with a high photoluminescence quantum yield approaching 100% and a narrow full width at half maximum (FWHM) of 20 nm. In addition, the extended linear conjugation endows DB‐DFN with a 95% horizontal dipole orientation ratio. The optimized OLED employing DB‐DFN as a blue emitter exhibits a high external quantum efficiency of 42.12% with mitigated efficiency roll‐off at high luminance. The present study breaks through the “efficiency‐purity” trade‐off dilemma, providing new insights and practical examples for the design of high‐efficiency, low‐roll‐off narrowband blue OLED materials.