Near-infrared circularly polarized electroluminescence from axially chiral diplatinum(II) phosphors

Near-infrared (NIR) organic light-emitting diodes (OLEDs) that exhibit efficient circularly polarized electroluminescence (CPEL) are highly promising for applications in medical imaging/therapy, night-vision and wearable optoelectronics. However, their current performance is limited by two major challenges: poor device performance metrics, such as low external quantum efficiency (EQE) and maximum radiance (R_max), and a low CPEL dissymmetry factor (g_EL). These limitations are especially severe for devices operating beyond 750 nm, due to the lack of efficient chiral NIR emitters, the constraints of the energy gap law, and the inherent trade-off between EQE and g_EL. Herein, we report highly efficient NIR circularly polarized phosphorescent OLEDs (CP-PHOLEDs) by utilizing a pair of diplatinum(II) phosphors with an axially chiral bridging ligand. The introduction of the axial chirality leads to dramatic CPEL enhancement and emission redshift compared to another pair of model complexes with a centrally chiral bridging ligand. The solution-processed devices exhibit NIR CPEL at 780 nm with maximum EQE (EQE_max) of 8.46% and g_EL of +0.114/-0.111. Furthermore, by using a thermally activated delayed fluorescence-sensitized phosphorescence strategy, an improved EQE_max of 11.38%, a high R_max of 13943 mW sr^-1 m^-2, and comparable g_EL approaching 0.1 ( + 0.097/-0.083) at 760 nm have been realized. In addition, these complexes perform equally well on flexible substrates, and a NIR CP-PHOLED skin patch has been demonstrated. These results establish the most efficient NIR CP-OLEDs reported to date, achieving state-of-the-art overall device performance and chiroptical properties.