Enhancing Reverse Intersystem Crossing in TSCT-TADF Emitters: Heavy Atom Modulation of Multiresonance Acceptors

With the rapid development of thermally activated delayed fluorescence (TADF) materials, achieving efficient reverse intersystem crossing (RISC) to mitigate triplet-triplet annihilation has emerged as a prominent research focus. This study investigates five derivative molecules, featuring varied bridging atoms/groups (O, S, Se, -CH₂-), designed from the reported TADF molecule AC-BO with through-space charge transfer (TSCT) properties. Utilizing time-dependent density functional theory coupled with a PCM solution model, their excited state behaviors were simulated in a toluene environment. Interestingly, it was observed that RISC in AC-BO and one derivative, AC-BCO, occurs predominantly via the T₂ state rather than the typical T₁ state (³LE_B, where B denotes the fluorene bridge), distinguishing it from conventional TSCT-TADF compounds, where RISC typically involves transitions between the ³CT and ¹CT states. This distinctive mode is attributed to reduced spin-orbit coupling (SOC) between ¹CT and ³LE_B, with T₂ representing a significant contributor to the RISC process through its ³CT character. Introduction of heavy atoms enhances the electron-withdrawing ability of the acceptor unit, leading to the T₁ transitions exhibiting ³MRCT characteristics and increased SOC, thereby favoring RISC via ³MRCT to ¹CT transitions. This study not only deepens our understanding of transition mechanisms in TSCT-TADF compounds but also provides crucial insights into the molecular design and regulation of excited triplet states.