Turning conventional non-TADF units into high-lying reverse intersystem crossing TADF emitters: different symmetric D–A–D-type modified donor units

Abstract

In a variety of skeletal structures of delayed fluorescence molecular materials, the donor–acceptor–donor (D–A–D) type has been widely considered for improving the efficiency of the reverse intersystem crossing (RISC) process. Herein, three new D–A–D molecules (PTZ-MPS, TPA-MPS and PCz-MPS) bearing 9,9-dimethylthioxanthene-S,S-sulfur dioxide (MPS) as the electron acceptor group are designed and investigated using theoretical calculations. PTZ-MPS shows the feature of the high-lying reverse intersystem crossing process, which is conducive to improving the exciton utilization of organic light-emitting diodes (OLEDs). PTZ-MPS has a much smaller singlet–triplet energy splitting (ΔE_S1T3 = 0.03 eV) than TPA-MPS (ΔE_S1T3 = 0.32 eV) and PCz-MPS (ΔE_S1T3 = 0.59 eV). However, it has a much larger spin–orbital coupling (SOC) strength (〈S₁|Ĥ_SOC|T₃〉 = 1.013 cm⁻¹) than TPA-MPS (〈S₁|Ĥ_SOC|T₃〉 = 0.311 cm⁻¹) and PCz-MPS (〈S₁|Ĥ_SOC|T₃〉 = 0.354 cm⁻¹), which makes it easy to induce a sufficient RISC from the T_n state to the S₁ state. The ΔE_ST and SOC are the two most important factors in determining TADF molecules. Therefore, PTZ-MPS is expected to be a potential high-lying excited state delayed fluorescence material candidate, and our work demonstrates that high-performance TADF materials can also be obtained successfully by designing rational molecules.