The Important Role of Carbonyl in Accelerating Reverse Intersystem Crossing for Selenium‐Based Organoboron Narrowband Blue Emitters

ABSTRACT Narrowband emissive polycyclic aromatic heterocycles (PAHs) featuring multiple resonance thermally‐activated delayed fluorescence (MR‐TADF) are capable of achieving high color purity with high exciton utilization efficiency via reverse intersystem crossing (RISC). However, thermally‐activated RISC remains the rate‐limiting step in MR‐TADF molecules due to the typically large singlet and triplet energy gap (Δ E ST ) and weak spin‐orbital coupling. To overcome this challenge, we introduce three carbonyl‐containing organoboron PAHs doped with selenium atoms (pSeXBNO, 1pSeXBN, and pSeXBN) for the first time. Introducing single or dual selenium‐embedded carbonyl heterocycles into the MR core enables significant orbital delocalization of carbonyls, resulting in small Δ E ST and ultrafast RISC rates of 7.5 × 10 6 s −1 for pSeXBNO, 1.5 × 10 7 s −1 for 1pSeXBN, and 4.8 × 10 7 s −1 for pSeXBN. The non‐sensitized OLED employing pSeXBN achieves an emission peak at 478 nm with a narrow bandwidth of 28 nm, along with a maximum external quantum efficiency (EQE) of 32.6% and retaining 28.4% at 1000 cd m −2 , representing state‐of‐the‐art performance for blue MR‐TADF materials. Moreover, bi‐color white OLED employing pSeXBN exhibits excellent performance with a maximum EQE of 30.9% and 23.2% retained at 1000 cd m −2 . These advances demonstrate the role of carbonyl here is of significant guidance in forwarding narrowband blue materials.