Enhancing the Red and Near Infrared OLED Efficiency of a TADF Emitter through an Internal Solvation Effect

We report thermally activated delayed fluorescence (TADF) emitters, namely, PTZ-Dipp-SO2B, and PTZ-Dipp-(Bu)SO2B, featuring an additional n-butyl group on the 10H-dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B) core enhancing its solid-state luminescence through an internal solvation effect. In dilute low-polarity solutions, both molecules show a yellow-orange photoluminescence with λ_em = 561-588 nm and red to near-infrared (NIR) PL in neat films, λ_em = 680-706 nm. The PL of PTZ-Dipp-(Bu)SO2B is systematically blue-shifted with respect to PTZ-Dipp-SO2B thanks to a ∼0.1 eV difference in LUMO energy caused by the σ-donor properties of the n-butyl group. These molecules were successfully applied as luminescent dopants to organic light-emitting diodes (OLEDs). Although both compounds have similar electronic structures and exhibit comparable Φ_PL in solution, the PTZ-Dipp-(Bu)SO2B is a significantly more efficient luminophore in the solid state than its counterpart. The OLED external quantum efficiency (EQE) at 7% load is 12.2% and in the neat film is 1.0% for PTZ-Dipp-(Bu)SO2B, while those for PTZ-Dipp-SO2B are 5.4% and 0.4%, respectively. The difference in λ_em or λ_EL between the two emitters is too small to explain the differences in EQE exclusively with the effects of the energy gap law. Instead, we believe that the n-butyl group disrupts the environment around the molecule through locally lowering the polarity and disturbing the formation of aggregates in the solid state. From previous observations of solvation of the emitter with hexane molecules, we term this phenomenon as the internal solvation effect.