The Important Role of Carbonyl in Accelerating Reverse Intersystem Crossing for Selenium-Based Organoboron Narrowband...

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 (ΔE_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 ΔE_ST and ultrafast RISC rates of 7.5 × 10⁶ s^-1 for pSeXBNO, 1.5 × 10⁷ s^-1 for 1pSeXBN, and 4.8 × 10⁷ 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.