Enhancing the reverse intersystem crossing (RISC) rates and efficiencies of MR-TADF emitters with a U-shaped molecular structure for solution-processed OLEDs

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters hold great potential for applications in organic light-emitting diodes (OLEDs). However, owing to their inherently rigid and planar molecular structures and the localized charge transfer (LCT) characteristics, these emitters typically exhibit poor solubility and low reverse intersystem crossing (RISC) rates, which are unfavorable for high-performance solution-processed OLEDs. Herein, we constructed three U-shaped MR-TADF emitters (BN-N-TTz, BN-N-PCz and BN-N-BN) by introducing triazine, phenylcarbazole and MR-TADF units at the 1- and 8-positions of a naphthalene ring. This U-shaped molecular architecture endows the emitters with excellent solubility. Moreover, this structure not only enhances spin-orbit coupling between the S₁ and T₁ states but also reduces the energy difference between the two states (ΔE_ST), thereby increasing RISC rates to as high as 3.17 × 10⁵ s⁻¹. Notably, the solution-processed OLED based on BN-N-BN achieved the highest EQE of 27.6% without sensitizers. This represents one of the best performances among solution-processed OLEDs based on MR-TADF emitters to date. This simple approach reveals the great potential for developing solution-processable emitters suitable for high-performance rigid and planar molecular structures.