Theoretical insights into site-specific heavy-atom effects on MR-TADF emitters: modulation of spin–orbit coupling and color purity

Introducing heavy atoms is a promising strategy to enhance triplet-to-singlet upconversion in multi-resonance thermally activated delayed fluorescence (MR-TADF) materials. By bolstering spin-orbit coupling (SOC), this approach suppresses triplet-triplet annihilation and consequently alleviates efficiency roll-off in devices. However, the embedded heavy atoms at different sites often exert a dual influence on both the SOC and the color purity. This study systematically investigates three types of heavy-atom-embedded MR-TADF emitters derived from a QAO core, with modifications at the bay position, on a spiro-fused periphery, and through a noncovalent face-on interaction. The high-level quantum chemical calculations reveal that the enhancement of SOC strongly depends on the heavy atom's position relative to the luminescent core. Direct embedding into the luminescent core significantly increases SOC but causes red-shifted and broadened spectra. In contrast, placing heavy atoms farther from the luminescent core diminishes their effect. Notably, non-covalent incorporation of heavy atoms into the luminescent core can balance increasing SOC while preserving a narrow full width at half maximum (FWHM). This investigation provides theoretical insights for designing efficient MR-TADF materials which simultaneously suppress efficiency roll-off and achieve high color purity.