Secondary Donor Engineering for High-Efficiency Orange-Red Thermally Activated Delayed Fluorescence Emitters

Achieving efficient orange-red emission in thermally activated delayed fluorescence (TADF) emitters remains challenging due to the hard to achieve great balance between a small singlet-triplet energy gap and high radiative decay rates. In this study, a modification site engineering strategy for secondary donors to optimize excited-state properties for orange-red emitters is proposed. Two isomeric emitters, ND28DBT and ND37DBT, were synthesized by introducing dibenzothiophene (DBT) units at different positions of a naphthalimide-dimethylacridine (NAI-DMAc) core. The theoretical analysis indicates that ND37DBT possesses a more favorable excited-state configuration, which facilitates an efficient reverse intersystem crossing (RISC) process and accelerates the radiative decay rate. As a result, ND37DBT showed a high photoluminescence quantum yield (PLQY) (70%), a fast RISC rate (6.25 × 10⁵ s^-1), and an excellent external quantum efficiency (EQE) (22.1%) with minimal roll-off in 100 cd m^-2. This work demonstrates that precise control of secondary donor modification sites offers a powerful molecular design strategy for developing high-efficiency orange-red TADF materials.