Luminescent copper(i) complexes with bisphosphane and halogen-substituted 2,2′-bipyridine ligands

Heteroleptic [Cu(P∧P)(N∧N)][PF₆] complexes, where N∧N is a halo-substituted 2,2′-bipyridine (bpy) and P∧P is either bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) have been synthesized and investigated. To stabilize the tetrahedral geometry of the copper(i) complexes, the steric demands of the bpy ligands have been increased by introducing 6- or 6,6′-halo-substituents in 6,6′-dichloro-2,2′-bipyridine (6,6′-Cl₂bpy), 6-bromo-2,2′-bipyridine (6-Brbpy) and 6,6′-dibromo-2,2′-bipyridine (6,6′-Br₂bpy). The solid-state structures of [Cu(POP)(6,6′-Cl₂bpy)][PF₆], [Cu(xantphos)(6,6′-Cl₂bpy)][PF₆]·CH₂Cl₂, [Cu(POP)(6-Brbpy)][PF₆] and [Cu(xantphos)(6-Brbpy)][PF₆]·0.7Et₂O obtained from single crystal X-ray diffraction are described including the pressure dependence of the structure of [Cu(POP)(6-Brbpy)][PF₆]. The copper(i) complexes with either POP or xantphos and 6,6′-Cl₂bpy, 6-Brbpy and 6,6′-Br₂bpy are orange-to-red emitters in solution and yellow-to-orange emitters in the solid state, and their electrochemical and photophysical properties have been evaluated with the help of density functional theory (DFT) calculations. The emission properties are strongly influenced by the substitution pattern that largely affects the geometry of the emitting triplet state. [Cu(POP)(6,6′-Cl₂bpy)][PF₆] and [Cu(xantphos)(6,6′-Cl₂bpy)][PF₆] show photoluminescence quantum yields of 15 and 17%, respectively, in the solid state, and these compounds were tested as luminophores in light-emitting electrochemical cells (LECs). The devices exhibit orange electroluminescence and very short turn-on times (<5 to 12 s). Maximum luminance values of 121 and 259 cd m^-2 for [Cu(POP)(6,6′-Cl₂bpy)][PF₆] and [Cu(xantphos)(6,6′-Cl₂bpy)][PF₆], respectively, were achieved at an average current density of 100 A m^-2. External quantum efficiencies of 1.2% were recorded for both complexes.