A Novel Locked‑Donor‐Based MR‐TADF Emitter for Highly Efficient and Pure‐Green OLEDs

Multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) materials provide an attractive platform for narrowband emission. However, achieving simultaneously high efficiency and color purity in green MR‐TADF emitters remains challenging. Herein, a rigid locked donor unit, 13h‐indolo[2′,3′:4,5]pyrrolo[3,2,1‐jk]carbazole (DPCz), is developed by replacing carbazole with a more conjugated indolo[3,2‐b]indole motif and incorporated into an MR‐TADF emitter, MRIN‐DPCz . Benefiting from synergistic multiple‐resonance effects arising from the DPCz locking unit and the B‐N core, MRIN‐DPCz exhibits enhanced molecular rigidity, suppressed excited‐state relaxation, and narrowband green emission with a peak at 517 nm, a full width at half maximum of 33 nm, and a photoluminescence quantum yield of 93%. Phosphorescent‐sensitized top‐emitting OLEDs based on MRIN‐DPCz deliver high color‑ pure green electroluminescence at 529 nm with an ultranarrow bandwidth of 22 nm, a CIEy value of 0.75 approaching the BT.2020 standard, and a maximum external quantum efficiency of 51.7%. This work highlights locked‐donor engineering as a versatile molecular design strategy for high‐efficiency, color‐pure MR‐TADF emitters.