The Degradation Mechanism of Multi-Resonance Thermally Activated Delayed Fluorescence Materials

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