Deciphering TADF Mechanisms in Metal-Free Organic Emitters BrA-HBI: Utilizing Optimally Tuned Range-Separated Functionals for Insight

Abstract Metal-free organic emitters, characterized by their thermally activated delayed fluorescence (TADF) properties, offer considerable promise for the creation of highly efficient organic light-emitting diodes (OLEDs). Recently, Shao et al. presented a novel excited state intramolecular proton transfer (ESIPT) system BrA-HBI, demonstrating an emission quantum yield of up to 50% [ Adv. Funct. Mater. 32 , 2201256 (2022)]. However, many open issues cannot be answered solely by experimental means only and require detailed theoretical investigations. For instance, what causes the activation of TADF from the Keto * tautomer and leads to fluorescence quenching in the Enol * form? Herein, we provide a theoretical investigation on the TADF mechanism of the BrA-HBI molecule by optimally tuned range-separated functionals. Our findings reveal that ESIPT occurs in the BrA-HBI molecule. Moreover, we have disclosed the reason for the fluorescence quenching of the Enol * form and determined that the T 2 state plays a dominant role in the TADF phenomenon. In addition, double hybrid density functionals method was utilized to verify the reliability of optimally tuned range separation functionals on the calculation of the TADF mechanism in BrA-HBI. These findings not only provide a theoretical reference for development of highly efficient organic light-emitting diodes, but also demonstrate the effectiveness of the optimally tuned range-separated functionals in predicting the luminescence properties of TADF molecules.