Peripheral Para ‐Chalcogen‐π‐Oxygen Fusion Strategy for High‐Performance Deep‐Blue Narrowband MR‐TADF OLEDs

Abstract

ABSTRACT Narrowband deep‐blue emitters featuring a targeted emission peak (465 ± 3 nm) and high reverse intersystem crossing (RISC) rate are highly desirable for ultra‐high‐definition displays. Herein, guided by the para ‐chalcogen‐π‐oxygen alignment strategy, two novel derivatives ( para ‐O‐BN and para ‐S‐BN ) are constructed by peripherally fusing rigid five‐membered chalcogen‐containing (O, S) heterocycles into a classical double‐boron multi‐resonance thermally activated delayed fluorescence (MR‐TADF) skeleton. The extended π‐conjugation and enhanced molecular rigidity yield nearly identical ultra‐narrow deep‐blue emission band at ∼465 nm, with a full width at half maximum below 20 nm in solution. Also, the para ‐arranged chalcogens localize electron density on the central benzene ring inserted between heteroatoms, thereby weakening central boron‐oxygen charge transfer and subtly tuning the HOMO. Importantly, the small singlet‐triplet energy gap (Δ E ST ) delivers a fast RISC rate ( k RISC ∼10 5 s − 1 ) and high photoluminescence quantum yield exceeding 90%. Substituting oxygen with sulfur further accelerates RISC process in para ‑S‑BN with negligible spectral shift. OLEDs incorporating para ‑O‑BN / para ‑S‑BN demonstrate maximum external quantum efficiencies ( EQE max ) of 34.1% and 40.2%, respectively. Notably, sensitizing devices with exciplex host and phosphorescent sensitizer achieve EQE max of 34.5/35.2% and retain 23.7%/23.8% even at 10 000 cd m −2 , with lifetimes of 121.5/244.1 h to 50% of 1000 cd m −2 .