Multiple-Excitation Configurations Reduce Singlet–Triplet Energy Gaps in Multiple-Resonance Thermally Activated Delayed Fluorescence Emitters

• • The multiple-excitation configuration, involving HOMO→LUMO and HOMO→LUMO+1 singlet configurations, lowers the S1 energy, reducing ΔE_ST compared to the single-excitation picture where ΔE_ST is governed by 2K_HL. • • The empirical expression ΔE_ST ≈ f(2K_HL, ΔE_LUMO–LUMO+1) was validated against a test set of MR-type emitters, indicating that the energy gap between LUMO and LUMO+1 is a key parameter for tuning ΔE_ST. • • For IV-DABNA, STEOM-DLPNO-CCSD calculations quantified the contributions of excitation configurations to S1 and T1 states, providing a concrete example of how multiple-excitation effects manifest in a real MR-TADF emitter. • • This design principle offers a new strategy to achieve small ΔE_ST in MR-TADF emitters, which is essential for efficient reverse intersystem crossing (RISC) and high OLED efficiency, potentially overcoming the limitations of conventional single-excitation approaches.