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

We report PTZ-Dipp-SO2B and its analogue PTZ-Dipp-(Bu)SO2B – thermally activated delayed fluorescence (TADF) emitters built from phenothiazine donor, diisopropylphenyl spacer and 10H-dibenzo[b,e][1,4]thiaborinine 5,5-dioxide (SO2B), a strongly electron-accepting boracycle. PTZ-Dipp-(Bu)SO2B contains an additional n-butyl group attached to the SO2B core. In dilute low-polarity solutions, both molecules show a yellow-orange photoluminescence with λem = 561-588 nm, and red to near-infrared (NIR) PL in neat films, λem = 680-706 nm. The PL of PTZ-Dipp-(Bu)SO2B is systematically blue shifted in respect to PTZ-Dipp-(Bu)SO2B thanks to a ~0.1 eV difference in LUMO energy caused by 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 ΦPL in dilution, the PTZ-Dipp-(Bu)SO2B is a significantly more efficient luminophore in solid state than its counterpart. The OLED external quantum efficiency (EQE) at 7% load is 12.2% and in neat film is 1.0% for PTZ-Dipp-(Bu)SO2B, while the same values for PTZ-Dipp-SO2B are 5.4% and 0.4%, respectively. The difference in λem or λEL 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 formation of aggregates in solid state. Due to the similarity with behaviour observed earlier by some of the current authors and associated with solvation of the emitter with hexane molecules we decide to name this effect as internal solvation effect.