Heavy atom effect on the luminescent properties of multi-resonance thermally activated delayed fluorescence emitters: Theoretical insights

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters are one of the most excellent materials for high performance organic light-emitting diodes (OLEDs) with high color purity benefiting from their narrow full width at half maximum (FWHM) and great device performance. However, small spin-orbit coupling (SOC) is one of drawbacks for MR-TADF emitters and introduction of heavy atoms may be one effective solution. In this work, four MR-TADF molecules with different atoms (O, S, Se) are carried out based on the first-principles calculation, and excited state dynamics in both toluene and solid phase is investigated. Our calculation results indicate that heavy atoms could induce smaller adiabatic energy gap and larger SOC between the first singlet excited state and the first triplet excited state, which is advantageous for the conversion of singlet and triplet excitons, and thus the generation of thermally activated delayed fluorescence (TADF). Though the reorganization energy and full width at half maximum (FWHM) of emission are increased, the influence is quite limited. Besides, the fluorescent rates are also little influenced. Our calculation results indicate that heavy atom introduction is an efficient strategy to enhance the SOC values of MR-TADF emitters without influencing other properties significantly.