Impact of Surrounding Environment on Hot-Exciton Based Organic Emitters for TADF Applications

Understanding thermally activated delayed fluorescence (TADF) in solid-state environments is crucial for practical applications. However, limited research focuses on how the medium affects TADF properties of hot-exciton-based emitters. In our study, we calculated and compared reverse intersystem crossing, radiative, and non-radiative decay rates of TADF emitters in gas, solvent, and solid phases. The designed emitters have a donor-acceptor-donor (D-A-D) structure, with donors such as triphenylamine (TPA) and diphenylamine thiophene (ThPA), combined with acceptors such as benzothiadiazole (BT), pyridine thiadiazole (PT) and thiadiazolobenzopyridine (NPT). We model the solvent and solid phases with the polarizable continuum model (PCM) and quantum mechanical/molecular mechanics (QM/MM) methods, respectively. Using density functional theory (DFT) and time-dependent DFT, we analyze how TADF emitters ' geometrical, electronic, and excited-state properties vary in these phases. Our results show that the solid-state environment significantly influences the geometry and TADF properties of emitters. In the presence of solid medium, our study indicates that non-radiative decay rates tend to be slower. On the other hand, radiative emission rates were found to be less influenced by the properties of the surrounding medium. Overall, our study connects emitter chemical structure and the surrounding environment's impact on excited-state characteristics and photochemical properties. The study addressed the challenges in designing efficient hot-exciton-based TADF emitters, including navigating the hot-exciton mechanism with design criteria, overcoming the energy gap law for near-infrared emission, and understanding the role of the solid-state environment in TADF applications. We investigated how the surrounding medium (gas, solvent, and solid) affects the geometrical, electronic, excited-state, and photochemical properties of hot-exciton-based TADF emitters. image