Abstract
1,4-Azaborine-based arenes are promising electroluminescent emitters with thermally activated delayed fluorescence (TADF), offering narrow emission spectra and high quantum yields due to a multi-resonance (MR) effect. However, their practical application is constrained by their limited operational stability. This study investigates the degradation mechanism of MR-TADF molecules. Electroluminescent devices incorporating these compounds display varied operational lifetimes, uncorrelated with excitonic stability or external quantum efficiency roll-off. Bulk electrolysis reveals significant instability in the radical cationic forms of MR-TADF compounds, with device lifetime linked to the Faradaic yield of oxidation. Comprehensive chemical analyses corroborate that the degradation byproducts originated from intramolecular cyclization of radical cation, followed by hydrogen atom transfer. The mechanism is further supported by enhanced stability observed in a deuterated MR-TADF emitter, attributed to a secondary kinetic isotope effect. These findings provide insights into the stabilizing effects of deuteration and mechanism-driven strategies for designing MR-TADF compounds with improved operational longevity.