Dendritic thermally activated delayed fluorescence sensitizers for efficient deep-blue narrowband organic light-emitting diodes

Abstract

Here, we report the design and synthesis of two dendritic donor–acceptor sensitizers (2D2-mMe-BN and 2D2-pMe-BN) by integrating two carbazole dendron donors into a methyl-modified benzonitrile acceptor for deep-blue narrowband hyperfluorescence organic light-emitting diodes (OLEDs). It is found that the position of the methyl substituent has a considerable effect on modulating the lowest unoccupied molecular orbital (LUMO) energy levels and elevating the excited-state energy levels of the dendrimers. 2D2-mMe-BN with a meta-methyl group demonstrates balanced photophysical properties with a high singlet-state (S₁) energy of 3.03 eV, while retaining a rapid reverse intersystem crossing rate (k_RISC) of 3.84 × 10⁶ s⁻¹, which enables efficient harnessing of triplet excitons and then Förster resonance energy transfer (FRET) to a deep-blue multi-resonance narrowband emitter. In contrast, 2D2-pMe-BN with para-methyl substitution exhibits enlarged singlet–triplet energy splitting and much lowered k_RISC that impairs triplet harvesting despite its similar high S₁ energy. Consequently, solution-processed hyperfluorescence OLEDs based on a 2D2-mMe-BN sensitizer reveal the best device performance with a state-of-the-art external quantum efficiency of 19.0% and narrowband deep-blue emission with Commission Internationale de l’Éclairage (CIE) coordinates of (0.142, 0.056) that are close to the BT.2020 blue standard, shedding light on the meta-methyl substituted dendrimers as a promising candidate toward high-efficiency narrowband deep-blue OLEDs.