Governing hybridized local and Charge-Transfer state in B,N-Embedded emitters for OLEDs

The primary factor limiting the luminous efficiency of multi-resonant thermally activated delayed fluorescence (MR-TADF) molecules is the slow reverse intersystem crossing (RISC) process. This work proposes an asymmetric weak electron donor modulation strategy which successfully introduces hybridized local-charge transfer (HLCT) states into the boron/nitrogen based MR-TADF materials with 1,3-bis(3,6-di-tert-butylcarbazol-9-yl)benzeneboron core (DtCzB). Benefiting from the enhancement of RISC process, the newly synthesized molecule DtCzB-SFN exhibits a fast fluorescence rate constant (k_F > 10⁸ s⁻¹) and high spin–orbit coupling parameter (SOC: <S₁|Ĥ_SOC|T₂ > is 0.9204 cm⁻¹), both notably higher than other MR-TADF materials based on the DtCzB core. Moreover, the maximum external quantum efficiency (EQE_max) of the sky blue organic light-emitting diode (OLED) utiziling DtCzB-SFN as emitter reaches 30.7 %. Additionally, due to the improved steric encapsulation effect in DtCzB-SFN, which can effectively suppress aggregation-caused quenching, the device maintains high efficiency even at high doping concentrations. Therefore, the strategy of goverining HLCT states in the MR-TADF luminescence system offers a promising approach for developing high-performance OLEDs.