Understanding\nSpin–Orbit-Coupling-Induced Reverse\nIntersystem Crossing in DMAC-TRZ-Doped Organic Light-Emitting Diodes\nvia Magnetic-Field-Effect Measurement

An efficient reverse intersystem crossing (RISC) process\nin thermally\nactivated delayed fluorescence (TADF) material is a common way to\nobtain high-performance organic light-emitting diodes (OLEDs), but\nthe physical mechanism for the spin flipping of the RISC remains vague.\nHere, using magneto-electroluminescence (MEL) as an effective tool,\nwe found that the RISC (CT<sub>3</sub> → CT<sub>1</sub>) from\na triplet charge transfer (CT<sub>3</sub>) to the singlet charge transfer\n(CT<sub>1</sub>) state is decided by spin–orbit coupling (SOC)\nin metal-free OLEDs based on a typical TADF emitter DMAC-TRZ. By fitting\nand analyzing the current and concentration-dependent MEL data, it\nis found that the characteristic magnetic field of the SOC-induced\nRISC process is approximately 65–85 mT, which is obviously\nlarger than that (several mT) of the hyperfine-interaction-induced\nRISC process. Simultaneously, the dissociation effect of the electric\nfield on the CT<sub>3</sub> state causes the SOC-induced RISC process\nto decrease with increasing bias current. The different formation\nmethods of excited states lead to the nonmonotonic change of SOC-induced\nRISC process with the increase of dopant concentration. Furthermore,\nconsidering the orbital polarization of dipoles, the SOC mechanism\nis further verified by the measurement of magneto-photoluminescence\nto be the responsible for achieving the spin flipping in TADF molecules.\nTherefore, this work clarifies the underlying dynamic mechanism of\nthe RISC process in TADF-OLEDs.