Fused‐Nonacyclic Multi‐Resonance Delayed Fluorescence Emitter Based on Ladder‐Thiaborin Exhibiting Narrowband Sky‐Blue Emission with Accelerated Reverse Intersystem Crossing

Abstract Developing organic luminophores with unique capability of strong narrowband emission is both crucial and challenging for the further advancement of organic light‐emitting diodes (OLEDs). Herein, a nanographitic fused‐nonacyclic π‐system ( BSBS‐N1 ), which was strategically embedded with multiple boron, nitrogen, and sulfur atoms, was developed as a new multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitter. Narrowband sky‐blue emission with a peak at 478 nm, full width at half maximum of 24 nm, and photoluminescence quantum yield of 89 % was obtained with BSBS‐N1 . Additionally, the spin‐orbit coupling was enhanced by incorporating two sulfur atoms, thereby facilitating the spin‐flipping process between the excited triplet and singlet states. OLEDs based on BSBS‐N1 as a sky‐blue MR‐TADF emitter achieved a high maximum external electroluminescence quantum efficiency of 21.0 %, with improved efficiency roll‐off.