Shine bright or live long: Substituent effects in [Cu(N^N)(P^P)]⁺-based light-emitting electrochemical cells where N^N is a 6-substituted 2,2′-bipyridine

We report [Cu(P^P)(N^N)][PF₆] complexes with P^P = bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) and N^N = 6-methyl-2,2′-bipyridine (Mebpy), 6-ethyl-2,2′-bipyridine (Etbpy), 6,6′-dimethyl-2,2′-bipyridine (Me₂bpy) or 6-phenyl-2,2′-bipyridine (Phbpy). The crystal structures of [Cu(POP)(Phbpy)][PF₆]·Et₂O, [Cu(POP)(Etbpy)][PF₆]·Et₂O, [Cu(xantphos)(Me₂bpy)][PF₆], [Cu(xantphos)(Mebpy)][PF₆]·CH₂Cl₂·0.4Et₂O, [Cu(xantphos)(Etbpy)][PF₆]·CH₂Cl₂·1.5H₂O and [Cu(xantphos)(Phbpy)][PF₆] are described; each copper(i) centre is distorted tetrahedral. In the crystallographically determined structures, the N^N domain in [Cu(xantphos)(Phbpy)]⁺ and [Cu(POP)(Phbpy)]⁺ is rotated ∼180° with respect to its orientation in [Cu(xantphos)(Mebpy)]⁺, [Cu(POP)(Etbpy)]⁺ and [Cu(xantphos)(Etbpy)]⁺; in each complex containing xantphos, the xanthene 'bowl' retains the same conformation in the solid-state structures. The two conformers resulting from the 180° rotation of the N^N ligand were optimized at the B3LYP-D3/(6-31G∗∗+LANL2DZ) level and are close in energy for each complex. Variable temperature NMR spectroscopy evidences the presence of two conformers of [Cu(xantphos)(Phbpy)]⁺ in solution which are related by inversion of the xanthene unit. The complexes exhibit MLCT absorption bands in the range 378 to 388 nm, and excitation into each MLCT band leads to yellow emissions. Photoluminescence quantum yields (PLQYs) increase from solution to thin-film and powder; the highest PLQYs are observed for powdered [Cu(xantphos)(Mebpy)][PF₆] (34%), [Cu(xantphos)(Etbpy)][PF₆] (37%) and [Cu(xantphos)(Me₂bpy)][PF₆] (37%) with lifetimes of 9.6-11 μs. Density functional theory calculations predict that the emitting triplet (T₁) involves an electron transfer from the Cu-P^P environment to the N^N ligand and therefore shows a ³MLCT character. T₁ is calculated to be ∼0.20 eV lower in energy than the first singlet excited state (S₁). The [Cu(P^P)(N^N)][PF₆] ionic transition-metal (iTMC) complexes were tested in light-emitting electrochemical cells (LECs). Turn-on times are fast, and the LEC with [Cu(xantphos)(Me₂bpy)][PF₆] achieves a maximum efficacy of 3.0 cd A^-1 (luminance = 145 cd m^-2) with a lifetime of 1 h; on going to the [Cu(xantphos)(Mebpy)][PF₆]-based LEC, the lifetime exceeds 15 h but at the expense of the efficacy (1.9 cd A^-1). The lifetimes of LECs containing [Cu(xantphos)(Etbpy)][PF₆] and [Cu(POP)(Etbpy)][PF₆] exceed 40 and 80 h respectively.