Ultrafast Triplet–Singlet Exciton Interconversion in Narrowband Blue Organoboron Emitters Doped with Heavy Chalcogens

Abstract Narrowband emissive organoboron emitters featuring the multi‐resonance (MR) effect have now become a critical material component for constructing high‐performance organic light‐emitting diodes (OLEDs) with pure emission colors. These MR organoboron emitters are capable of exhibiting high‐efficiency narrowband thermally activated delayed fluorescence (TADF) by allowing triplet‐to‐singlet reverse intersystem crossing (RISC). However, RISC involving spin‐flip exciton upconversion is generally the rate‐limiting step in the overall TADF; hence, a deeper understanding and precise control of the RISC dynamics are ongoing crucial challenges. Here, we introduce the first MR organoboron emitter ( CzBSe ) doped with a selenium atom, demonstrating a record‐high RISC rate exceeding 10 8 s −1 , which is even higher than its fluorescence radiation rate. Furthermore, the spin‐flip upconversion process in CzBSe can be accelerated by factors of ≈20000 and ≈800, compared to those of its oxygen‐ and sulfur‐doped homologs ( CzBO and CzBS ), respectively. Unlike CzBO and CzBS , the photophysical rate‐limiting step in CzBSe is no longer RISC, but the fluorescence radiation process; this behavior is completely different from the conventional time‐delaying TADF limited by the slow RISC. Benefitting from its ultrafast exciton spin conversion ability, OLEDs incorporating CzBSe achieved a maximum external electroluminescence quantum efficiency as high as 23.9 %, accompanied by MR‐induced blue narrowband emission and significantly alleviated efficiency roll‐off features.