Samsung Advanced Institute of Technology

1,4-Azaborine-based arenes gained prominence as electroluminescent emitters 24 that exhibit thermally activated delayed fluorescence (TADF). These materials 25 display exceptionally narrow emission spectra and high photoluminescence 26 quantum yields, benefits arising from the multi-resonance (MR) effect. The 27 practical application of MR-TADF emitters is often constrained by their limited 28 operational stability. In this study, we explore the mechanism responsible for the 29 degradation of a series of MR-TADF molecules. Electroluminescent devices 30 employing these compounds show varied operational lifetimes, which do not align 31 with either the excitonic stability of the emitter molecules or the degree of roll-off 32 in external quantum efficiency. Our bulk electrolysis study reveals a considerable 33 instability of the radical cationic forms of the MR-TADF compounds. A direct 34 correlation is observed between device lifetime and the Faradaic yield for 35 oxidative degradation of the emitter molecules. Comprehensive chemical analyses 36 suggest that the degradation byproducts originate from intramolecular cyclization 37 in the radical cation, preceded by intermolecular hydrogen atom transfer. 38 Quantum chemical calculations indicate that this intramolecular cyclization 39 accelerates the overall reaction, implying that cyclization reactivity is crucial for 40 the intrinsic stability of the MR-TADF compound upon hole trapping. Our study 41 offers an explanation for the beneficial effects of deuteration on the intrinsic 42 stability and lays the groundwork for developing mechanism-based strategies to 43 design MR-TADF compounds with greater operational longevity.