Radical TADF: Quartet‐Derived Luminescence with Dark TEMPO

Abstract High‐spin states in organic molecules offer promising tuneability for quantum technologies. Photogenerated quartet excitons are an extensively studied platform, but their applications are limited by the absence of optical read‐out via luminescence. Here, a new class of synthetically accessible molecules with quartet‐derived luminescence is demonstrated, formed by appending a non‐luminescent TEMPO radical to thermally activated delayed fluorescence (TADF) chromophores previously used in OLEDs. The low singlet‐triplet energy gap of the chromophore opens a luminescence channel from radical‐triplet coupled states. A set of design rules is established by tuning the energetics in a series of compounds based on a naphthalimide (NAI) core. Generation of quartet states is observed and the strength of radical‐triplet exchange is measured. In DMAC‐TEMPO, up to 72% of detected photons emerge after reverse intersystem crossing from the quartet state repopulates the state with singlet character. This design strategy does not rely on a luminescent radical to provide an emission pathway from the high‐spin state. The large library of TADF chromophores promises a greater pallet of achievable emission colours.