Sulfur-Embedded Pure Green Multiple Resonance TADF Emitters: Optimizing Photophysical and Electroluminescent Properties.

Two pure green multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters are reported here by utilizing benzothienocarbazole groups with sulfur atoms connecting with the 3-position carbon atoms of carbazole units, which are named Th-Cz-BN3 and Th-Cz-BN6. Although their molecular structures are similar, significant differences in energy splitting and spin-orbit coupling between the singlet and triplet excited states are exhibited, resulting from the differences in steric configurations and consequent electronic structures. The Th-Cz-BN3 and Th-Cz-BN6-based devices display pure green emission with Commission Internationale de L'Eclairage (CIE) y values exceeding 0.7 and maximum external quantum efficiencies of over 34.0%. Moreover, the Th-Cz-BN3-based non-sensitized device exhibits remarkable efficiency roll-offs of 0.3 and 16.8% at 100 and 1000 cd m-2, respectively, which can be ascribed to the rapid rates of radiative transition and reverse intersystem crossing processes. The results not only achieve sulfur-embedded pure green MR-TADF emitters but also pave the way for designing highly efficient MR-TADF emitters with suppressed efficiency roll-offs.