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 (3LEB, 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 3LEB, 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.