Nitrogen/carbonyl (N/C═O) based multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters are attractive due to their narrowband delayed emission. Here, four MR-TADF emitters were developed by sterically wrapping a tert-butylated DiKTa core. Through modulation of short-range and through-space charge transfer, triplet harvesting efficiency was boosted from 1% in tDiKTa to 52% in tDiKTa-GCz (kRISC = 1.37 × 106 s-1) in degassed toluene. Surprisingly, however, these improvements were accompanied by rapid photodegradation, while that same degradation was not observed in toluene under air. Our studies revealed that triplet excited states trigger hydrogen-atom transfer (HAT) from the solvent to the carbonyl group of the emitter, leading to fast, irreversible chemical change. Moreover, this phenomenon was found to be general for several previously published MR-TADF compounds. Using natural population analysis and bond dissociation energies, we show that the electrophilicity of the donor hydrogen atom governs the observed reactivity. This work uncovers a previously unrecognized photochemical vulnerability in N/C═O MR-TADF systems, and suggests that researchers studying them must be aware of their sensitivity toward HAT-induced photodegradation. We also call for a careful re-examination of published data on these compounds, as even brief exposure to light was sufficient to cause observable degradation in certain solvents.
Si et al. (Mon,) studied this question.