Tandem rigidification and π-extension as a key tool for the development of a narrow linewidth yellow hyperfluorescent OLED system.

Hyperfluorescence (HF), a relatively new phenomenon utilizing excitons transfer between two luminophores, requires careful pairwise tuning of molecular energy levels and is proposed to be the crucial step toward the development of new, highly effective OLED systems. To date, barely a few HF yellow emitters with desired narrowband emission but moderate external quantum efficiency (EQE <20%) have been reported. This is because a systematic strategy embracing both Förster Resonance Energy Transfer (FRET) and triplet to singlet (TTS) transition as complementary mechanisms for effective excitons transfer has not yet been proposed. Herein, we present a rational approach, which allows to through subtle structural modification, a pair of compounds built from the same donor and acceptor subunits, but with varied communication between these ambipolar fragments, to be obtained. The TADF-active dopant is based on a naphthalimide scaffold linked to the nitrogen of a carbazole moiety, which through the introduction of an additional bond leads not only to π-cloud enlargement but also rigidifies and inhibits rotation of the donor. This structural change prevents TADF, and allows to guide bandgaps and excited states energies to simultaneously pursue FRET and TTS process. New OLED devices utilizing the presented emitters show excellent external quantum efficiency (up to 27%) and narrow full width at half maximum (40nm), which is a consequence of very good alignment of energy levels. The presented design principles prove that only a minor structural modification is needed to obtain commercially applicable dyes for HF OLED devices.