Realizing Molecular Excited-State Regulation through Donor Molecular Engineering: The Intrinsic Relationship between TADF and HLCT Mechanisms

For developing high-performance pure organic light-emitting diode (OLED) emitters, thermally activated delayed fluorescence (TADF) and hybrid local and charge-transfer (HLCT) mechanisms are two pivotal strategies by harvesting triplet excitons via reverse intersystem crossing. The intrinsic relationship between the two in molecular design is revealed in this study: the switching between TADF and HLCT mechanisms can be achieved by adjusting the strength of the donor groups. In this work, five D-π-A-π-D type blue and green OLED emitters with acridine/phenoxazine/phenothiazine groups as donors were designed and synthesized. Compared to reported HLCT molecules adopting carbazole/diphenylamine groups as donors, the excited states of the molecules in this work were successfully regulated to TADF. The solution-processed blue/green OLED device based on FIP-DPAC/FIP-PXZ achieved the maximum external quantum efficiency (EQEmax) of 10.01%/17.45%. A strategy for regulating HLCT and TADF mechanisms of OLED emitters through donor molecular engineering was elucidated in this study.