A peripheral dual-donor strategy for high-efficiency blue multi-resonance thermally activated delayed fluorescence emitters with concentration-dependent excited-state tuning

Abstract

Developing efficient, high-color-purity blue emitters remains a critical challenge for next-generation organic light-emitting diodes (OLEDs). Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials provide high efficiency and high color purity, but their practical application is limited by slow reverse intersystem crossing (RISC) and aggregation-induced quenching or spectral broadening in the solid state. Here, we present a peripheral dual-donor strategy for blue MR-TADF emitters by introducing bulky C₁–N-linked bis(tert-butylcarbazolyl) (BTC) units at the para position of nitrogen/boron/oxygen- (NBO) and nitrogen/boron/nitrogen- (NBN) doped MR cores. This approach enhances molecular rigidity and steric hindrance, suppressing nonradiative decay and aggregation, while enabling multi-pathway charge transfer through multiple-resonance, through-space, and through-bond mechanisms to improve RISC. We systematically investigate the effects of doping concentration on excited-state nature and photophysical properties, revealing that NBN-BTC maintains an MR-dominated emissive state across a wide concentration range, whereas NBO-BTC evolves from MR to long-range charge transfer character with increasing concentration. Both emitters exhibit photoluminescence quantum yields exceeding 95% and favorable horizontal transition dipole ratios. Blue OLEDs based on these materials achieve maximum external quantum efficiencies of 32.6% and 38.9%, with emission full widths at half-maximum of 30 and 26 nm. This work demonstrates that peripheral dual-donor and MPCT strategies enable high-performance blue OLEDs and provide insight into concentration-dependent excited-state tuning in MR-TADF systems.