Charge-transfer transition regulation of thermally activated delayed fluorescence emitters by changing the valence of sulfur atoms

A sulfur (S) atom generally undergoes sp3 hybridization when linking two moieties, not only for the sulfane status but also its fully oxidized form, sulfone. In the research reported here an S atom was used as a bridge to construct two thermally activated delayed fluorescence (TADF) emitters, Trz-S-DMAc and Trz-SO2-DMAc. The only difference between these two emitters is the valence of the S atoms in the bridges; however, their electron distributions are greatly affected and the charge-transfer (CT) transitions can be well-modulated between intermolecular and intramolecular modes. In a Trz-S-DMAc molecule, the S atom breaks the conjugation between donor (D) and acceptor (A) moieties, and their CT transition can only then occur with other Trz-S-DMAc molecules. The OLEDs based on Trz-S-DMAc achieve the best external quantum efficiency (EQE) of 18.1% at a high concentration of 55 wt% and maintain a satisfactory EQE of 13.0% for a non-doped device. Whereas for Trz-SO2-DMAc, the sulfone group acts as a second A moiety rather than a simple bridge. The Trz-SO2-DMAc is thus a typical TADF emitter with an effective intramolecular CT transition. The optimized OLED based on Trz-SO2-DMAc achieves a maximum EQE of 18.3%, whereas a significant efficiency drop can be observed with increasing doping concentrations. It is expected that these results will help to increase the understanding of how to link donor and acceptor moieties via sp3 atoms to construct TADF emitters and the roles of the different CT transitions.