Unravelling the impact of sulfur atom oxidation and donor–acceptor effects on the performance of blue TADF emitters: a detailed computational study†

Blue TADF materials demonstrate significant potential for OLED and photovoltaic applications. Nevertheless, systematic studies are essential to explore the relationship between molecular structures and luminescence properties to develop blue-TADF emitters. In this study, a series of new 24 donor–acceptor–donor (D–A–D) type molecules with different electron donors and acceptors are designed theoretically, and their photophysical properties are analyzed by using DFT and TD-DFT methods. We examined the combined impact of sulfur oxidation and the symmetric incorporation of a nitrogen heteroatom, with positional modifications (2-dipyridyl and 3-dipyridyl), within the phenyl ring of the acceptor group. The findings suggest that enhancing both the donating and accepting strength of the molecules results in an orthogonal geometry and a small ΔE_ST, accompanied by an enhanced charge-transfer (CT) character. Upon sulfur oxidation, the magnitude of SOC decreases, resulting in a reduction of ΔE_ST attributed to screening and lone pair effects. Through quantum chemical calculations, we have theoretically identified 12 promising blue TADF molecules, featuring small ΔE_ST, increased SOC magnitude, and higher RISC (∼10⁺⁰⁷ s⁻¹) rates. Overall, our current study provides a robust molecular design approach and reliable computational method for designing a blue TADF emitter.