Multiple-Excitation Configurations Reduce Singlet–Triplet Energy Gaps in Multiple-Resonance Thermally Activated Delayed Fluorescence Emitters
The pursuit of efficient organic light-emitting diodes (OLEDs) has been significantly advanced by thermally activated delayed fluorescence (TADF) materials, particularly those employing multiple-resonance (MR) effects. However, achieving small singlet–triplet energy gaps (ΔE_ST) in MR-TADF emitters remains a critical challenge. This work introduces a fundamental design principle based on multiple-excitation configurations to reduce ΔE_ST. In the single-excitation case, both S1 and T1 states are described by a simple HOMO→LUMO excitation, leading to a large exchange energy (2K_HL). In contrast, the multiple-excitation case incorporates electronic interaction between singlet configurations (1Φ_H→L and 1Φ_H→L+1), which lowers the S1 energy and reduces ΔE_ST. The authors propose an empirical expression ΔE_ST ≈ f(2K_HL, ΔE_LUMO–LUMO+1) and validate it using a test set of MR-type emitters. For the representative emitter IV-DABNA, excited-state energies and difference densities calculated at the STEOM-DLPNO-CCSD level reveal the contributions of excitation configurations to S1 and T1 states. This work provides a new avenue for molecular engineering of MR-TADF emitters, potentially stimulating renewed interest in excited-state design principles for future OLED technologies.