Strategy of Modulating\nNonradiative Decay for Approaching\nEfficient Thermally Activated Delayed Fluorescent Emitters

The photoluminescence quantum yield (PLQY) of an emitter\nreflects\nthe extent of the nonradiative process and significantly influences\nthe performance of the electroluminescent devices. This study presents\na new strategy for designing thermally activated delayed fluorescence\n(TADF) materials based on suppressing nonradiative decay. The substituents\nand their positions can modulate exciton behavior in a molecule. We\nmodified the diboraanthranene framework with various alkyl substituents\nas models to explore the strategy for designing donor–acceptor–donor\n(D–A–D)-type TADF emitters. Experimental results and\ntheoretical simulations indicate that suppressing nonradiative decay\ncan enhance PLQYs and the delayed component (Φ<sub>d</sub>)\nof PLQY. Based on the theory of nonradiative decay, our findings suggest\nthat the local C–H stretching modes originating from the alkyl\ngroups are the main contributors to the nonradiative decay rates of\nthe first excited singlet state (<i>k</i><sub>nr,S</sub>). Notably, our simulations demonstrate that the PLQYs of the five\nemitters are increased by alkyl deuteration and can be nearly optimized\nto 100% through perdeuteration. MECzDBA- and EECzDBA-based green organic\nlight-emitting diodes (OLEDs) achieve superior external quantum efficiencies\nof 29.4 and 28.6%, respectively, with reduced efficiency roll-off\nof only 5.1 and 5.2% at 1000 cd m<sup>–2</sup>. The outcomes\nfrom our study suggest a molecular design strategy that successfully\ncounteracts the nonradiative decay in TADF materials, providing a\nnew direction for creating high-efficiency organic emitters.