Efficient TADF-OLEDs with ultra-soluble copper(I) halide complexes containing non-symmetrically substituted bidentate phosphine and PPh₃ ligands

Abstract

High efficiency copper(I) halide complexes containing rigid diphosphine ligand have attracted much attention. However, rigid bidentate phosphine ligands were rarely reported so far and limited to structural symmetric diphosphines. Here, a series of four-coordinate mononuclear copper(I) halide complexes containing non-symmetrically substituted bidentate phosphine and PPh₃ Ligands, CuX(dpts)(PPh₃) [dpts = 2-trimethylsilyl-3,4-bis(diphenylphosphine)thiophene, X = I (1), Br (2), Cl (3)] and in comparison to CuX(dppt)(PPh₃) [dppt = 3,4-bis(diphenylphosphino)thiophene, X = I (4), Br (5), Cl (6)], were synthesized, and their molecular structures and photophysical properties were investigated. The introduction of trimethylsilyl group into diphosphine ligand, not only greatly improves the solubility of the complexes, but also quantum efficiency, and fine-tunes the emission color as well. These complexes exhibit intense bluegreen to yellowish green emissions in powder state at room temperature and have peak wavelengths at 485–535 nm with microsecond lifetimes (τ = 4.8–48.9 μs, Φ = 0.03–0.52). The emission of the complexes 1–6 mainly originates from MLCT (metal to ligand charge transfer), XLCT (halide to ligand charge transfer) and intraligand transitions. Solution-processed, nondoped and doped devices of complex 2 exhibit yellow green emission with CIE(x,y) of (0.43, 0.51). The nondoped device gives a maximum external quantum efficiency (EQE) of 7.74% and a maximum luminance of 234 cd/m2. These promising results open the door to solution-processed, efficient TADF (thermally activated delayed fluorescence) OLED (organic light-emitting diode) devices with ultra-soluble and abundant copper emitters.