Chalcogen‐Bridged Double‐Boron Multiresonance Thermally Activated Delayed Fluorescence Emitters: Controlling Excited‐State Energetics, Vibronic Coupling, and Spin‐Conversion Dynamics for High‐Performance Organic Light‐Emitting Diodes

Multiresonance thermally activated delayed fluorescence (MR‐TADF) emitters are promising for narrowband organic light‐emitting diodes (OLEDs), yet simultaneous control of emission energy, vibronic coupling, and triplet harvesting within a common framework remains challenging. Here, local chalcogen‐bridging effects are systematically investigated within a double‐boron MR‐TADF framework using unbridged DBN‐H and its O‐ and S‐bridged analogs, DBN‐O and DBN‐S . This matched series clarifies how bridge identity governs molecular geometry, vibronic coupling, and spin‐conversion dynamics. Combined photophysical measurements and computational analyses reveal that oxygen bridging rigidifies the MR framework, suppressing excited‐state structural reorganization and vibronic coupling. Consequently, DBN‐O affords narrowband deep‐blue electroluminescence at 453 nm with a full width at half maximum of 19 nm, a maximum external quantum efficiency (EQE _max ) of 25.7%, and Commission Internationale de l’Éclairage (CIE) chromaticity coordinates of (0.14, 0.06), close to the Rec.2020 blue primary. In contrast, DBN‐S forms a lower‐energy, more vibronically relaxed emissive state while exhibiting accelerated reverse intersystem crossing. Together with its near‐unity photoluminescence quantum yield and horizontal transition‐dipole orientation, these characteristics enable DBN‐S ‐based OLEDs to achieve an EQE _max of 31.4% with suppressed efficiency roll‐off. These results demonstrate how local O‐ and S‐bridging motifs can differentially balance spectral purity and triplet harvesting within a common double‐boron MR‐TADF framework.