Incorporating B–S Bonds Into Multi‐Resonance Frameworks for Narrowband Emitters With Fast Reverse Intersystem Crossing and Facile Synthesis

ABSTRACT Boron/nitrogen‑embedded multi‑resonance thermally activated delayed fluorescence (MR‑TADF) emitters are highly attractive for developing wide‑color‑gamut and low‑power‑consumption organic light‑emitting diode (OLED) displays. However, their development and application face challenges of slow reverse intersystem crossing (RISC) and significant synthetic difficulties that arise from the borylation reaction. In this study, we introduce the incorporation of B–S bonds into the MR framework as a strategic design concept, achieved by designing molecules (O2S2‐B3N and MesO2S2‐B3N) that contain both B–S and B–O bonds within the same core. The introduction of B–S bonds promotes enhanced spin–orbit coupling relative to the control compound with only B–O bonds, enabling substantially faster RISC with rate constants exceeding 2.6 × 10 5 s −1 , while also facilitating streamlined borylation with yields reaching 85%. The B–S incorporation preserves narrowband emission, yielding full widths at half maximum (FWHMs) of 22–23 nm. In a non‑sensitized electroluminescent device, MesO2S2‐B3N retains a narrow emission profile (FWHM: 26 nm) and achieves a high external quantum efficiency of 30.2% with substantially improved efficiency roll‑off characteristics compared to the control compound. This B–S incorporation strategy provides a unified solution to both the slow RISC kinetics and the challenging synthesis in MR‑TADF materials, establishing a new route to high‑performance, readily accessible emitters.