Manipulating Exciton Dynamics toward Simultaneous High-Efficiency Narrowband Electroluminescence and Photon Upconversion by a Selenium-Incorporated Multiresonance Delayed Fluorescence Emitter

Multiresonance thermal activated delayed fluorescence (MR-TADF) materials with an efficient spin–flip transition between singlet and triplet excited states remain demanding. Herein, we report an MR-TADF compound (BN–Se) simultaneously possessing efficient (reverse) intersystem crossing (ISC/RISC), fast radiative decay, close-to-unity quantum yield, and narrowband emission by embedding a single selenium atom into a common 4,4′-diazaborin framework. Benefitting from the high RISC efficiency accelerated by the heavy-atom effect, organic light-emitting diodes (OLEDs) based on BN–Se manifest excellent performance with an external quantum efficiency of up to 32.6% and an ultralow efficiency roll-off of 1.3% at 1000 cd m–2. Furthermore, the high ISC efficiency and small inherent energy loss also render BN–Se a superior photosensitizer to realize the first example of visible (λex > 450 nm)-to-UV (λem < 350 nm) triplet–triplet annihilation upconversion, with a high efficiency (21.4%) and an extremely low threshold intensity (1.3 mW cm–2). This work not only aids in designing advanced pure organic molecules with fast exciton dynamics but also highlights the value of MR-TADF compounds beyond OLED applications.