Enhancing Spin-Orbit Coupling by Introducing a Lone Pair Electron with p Orbital Character in a Thermally Activated Delayed Fluorescence Emitter: Photophysics and Devices

Reverse intersystem crossing (RISC) is the rate-limited step for the radiative process of thermally activated delayed fluorescence (TADF) materials, which is important to achieve ideal photoluminescence and electroluminescence efficiency. Herein, we propose a new strategy of introducing a lone pair (n) electron with p orbital character to enhance spin-orbit coupling (SOC) for promoting the RISC process. A proof-of-concept TADF molecule with p orbital lone pairs, namely, MoCz-PCN, was developed, and three counterparts without any p lone pairs, namely, DMAc-PCN, DPAc-PCN, and SpiroAc-PCN, were constructed for comparison. The experimental data revealed that MoCz-PCN exhibits a ca. 1.9 times higher RISC rate than the counterparts, which can be ascribed to enhanced SOC. Moreover, a significant increase in external quantum efficiency is observed in the MoCz-PCN-based OLED device. These findings provide a feasible strategy to develop highly efficient TADF emitters by introducing a lone pair (n) electron with p orbital character.