High-efficiency long-wavelength NIR iridium complexes constructed by extending rigid coordination and optimizing peripheral donor position to break cocoon into butterfly

Abstract

Due to the non-radiative decay of excited states, it is extremely complicated for iridium (III) complexes to simultaneously harvest high-efficiency and long-wavelength emission in the near-infrared (NIR) region based on the energy-gap law. In order to address this issue, we designed and synthesized two butterfly-type iridium(III) complexes of (PyPz-TPA)2Ir(acac) and (PyQl-TPA)2Ir(acac), in which pyrene-pyrazine (PyPz) and pyrene-quinoxaline (PyQl) are respectively used as the rigid C^N bidentate coordinated cocoons, peripherally with two triphenylamine (TPA) units at 4, 5-positions. Their photophysical, electrochemical and electroluminescent properties are primarily investigated. It is found that the (PyQl-TPA)2Ir(acac) presents stronger spin-orbital coupling effect, a smaller ΔEST energy difference, and lower reorganization energy than (PyPz-TPA)2Ir(acac) by selecting rigid pyrene-quinoxaline coordination and optimizing peripheral TPA position. Furthermore, the (PyQl-TPA)2Ir(acac)-doped OLEDs exhibit a red-shifted NIR emission peaked at 784 nm with a record external quantum efficiency of 5.42% using CBP as host matrix by solution process, which is a rare representation to shrink the limitation of energy-gap law in NIR region for iridium(III) complexes. This study provides a feasible strategy to construct high-efficiency NIR-emitting iridium(III) complexes by an artfully motivated strategy of extending and breaking the C^N coordinate cocoon into butterfly.