Enhancing the Reverse Intersystem Crossing (RISC) Rates and Efficiencies of Multi-Resonance Thermally Activated Delayed Fluorescence (MR-TADF) Emitters with a U-shaped Molecular Structure for Solution-Processed OLEDs

Most B-N multiple resonance (MR) induced thermally activated delayed fluorescence (TADF) emitters, due to their inherently rigid and planar molecular structures and short-range charge-transfer characteristics, typically exhibit poor solubility and low reverse intersystem crossing (RISC) rates, which are highly undesirable for high-performance solution-processible OLEDs. Herein, we developed a simple yet feasible approach to fabricate three novel solution-processible small-molecule MR-TADF emitters by connecting either one MR-TADF unit and a triazine unit, one MR-TADF unit and a phenylcarbazole unit, or two MR-TADF units to a naphthalene bridge. The solution-processed OLEDs based on BN-N-TTz and BN-N-PCz achieved maximum external quantum efficiencies (EQEs) of 25.2% and 19.5%, respectively, with both having narrow emission bandwidths of only 29 nm full-width at half-maximum (FWHM). Notably, the solution-processed electroluminescent devices based on BN-N-BN achieved a maximum EQE of 27.6%, while BN-N-BN exhibited a very high RISC rate. This represents the excellent performance among solution-processible OLEDs based on MR-TADF emitters. This simple approach reveals the great potential for developing solution-processible emitters suitable for high-performance rigid and planar molecular structures.