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

Hyperfluorescence (HF), a relatively new phenomenon utilizing the transfer of excitons between two luminophores, requires careful pairwise tuning of molecular energy levels and is proposed to be the crucial step towards the development of new, highly effective OLED systems. To date, barely 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 exciton transfer has not yet been proposed. Herein, we present a rational approach, which allows, 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 the rotation of the donor. This structural change prevents TADF, and guides bandgaps and excited state energies to simultaneously pursue FRET and TTS processes. New OLED devices utilizing the presented emitters show excellent external quantum efficiency (up to 27%) and a narrow full width at half maximum (40 nm), 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.

National Centre for Research and Development, Poland, Grant No. LIDER/21/0077/L-11/19/NCBR/2020 European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant agreement No. 847413 National Science Centre, Poland, Grant No. 2020/39/B/ST4/01952 Minister of Science and Higher Education entitled ``PMW′′ in the years 2020–2024; agreement no. 5005/H2020-MSCA-COFUND/2019/2 Wrocław Centre for Networking and Supercomputing Centre of Informatics of the Tricity Academic Supercomputer and Network Poznań Supercomputing and Networking Center affiliated to the Institute of Bioorganic Chemistry of the Polish Academy of Sciences Polish National Science Centre, grant no. 2017/25/B/ST5/02488 Rector of the Silesian University of Technology (04-040-SDU-10-22-04 and 04/040/RGJ21/0149) EU's Horizon 2020 ERA-Chair project ExCEED, grant agreement No 952008