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

Abstract 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.