Tuning chiroptical properties and device hyperfluorescence efficiency via acceptor modification in an...

Multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters offer an attractive solution for the fabrication of colour-saturated organic light-emitting diodes (OLEDs). Still, their rigid frameworks often lead to slow reverse intersystem crossing and aggregation-caused quenching. To address these challenges and simultaneously introduce chiroptical properties in solution-processable materials, here we present the first examples of chiral intramolecular Förster resonance energy transfer (FRET) MR-TADF dendrimers, CzPBN-CzBN and CzPBA-CzBN. These emitters combine an MR-TADF core with chiral paracyclophane (CzP)-based donor dendrons linked to different acceptor units, benzonitrile for CzPBN-CzBN and benzoic acid for CzPBA-CzBN. While both dendrimers show narrowband TADF emission, the nature of the acceptor impacts their chiroptical properties. Specifically, CzPBA-CzBN exhibits circularly polarised luminescence (CPL), whereas CzPBN-CzBN does not due to a weaker circular dichroism signal. Conversely, in solution-processed hyperfluorescent (HF) OLEDs, the device with CzPBN-CzBN achieved a superior maximum external quantum efficiency (EQE_max) of 19.7% and lower efficiency roll-off (EQE of 18.9% at 1000 cd m^-2) compared to the device with CzPBA-CzBN (EQE_max/1000 = 17.2/16.5%). These findings reveal an intrinsic trade-off between CPL response and HF device performance, demonstrating how precise modulation of the excited-state manifold via acceptor engineering can be used to tailor the properties of intramolecular sensitised TADF emitters.