Establishing a simple and versatile design strategy to finely modulate emission colors while retaining high luminescence efficiency and color purity remains an appealing yet challenging task for the development of multi‐resonance‐induced thermally activated delayed fluorescence (MR‐TADF) materials. Herein, we demonstrate that the strategic introduction of electron‐withdrawing imine and electron‐donating amine moieties into a versatile boron‐embedded 1,3‐bis(carbazol‐9‐yl)benzene skeleton enables systematic hypsochromic and bathochromic shifts of narrowband emissions, respectively. By this method, effective electroluminescence color tuning was accomplished over a wide visible range from deep‐blue to yellow (461–571 nm), using the same MR molecular system, without compromising very narrow spectral features. Deep‐blue to yellow organic light‐emitting diodes with maximum external quantum efficiencies as high as 19.0–29.2 % and superb color purity could be produced with this family of color‐tunable MR‐TADF emitters.

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.

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 À 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.

Developing organic luminophores with unique capability of strong narrowband emission is both crucial and challenging for the further advancement of organic lightemitting diodes (OLEDs). Herein, a nanographitic fusednonacyclic p-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 skyblue 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.