18 November 2021

A counterion study of a series of [Cu(P^P)(N^N)][A] compounds with bis(phosphane) and 6-methyl and 6,6′-dimethyl-substituted 2,2′-bipyridine ligands for light-emitting electrochemical cells

The choice of counterion in heteroleptic copper(I) luminophores for LECs has a critical effect on the photoluminescence quantum yields of the solid materials, and on the device figures-of-merit.

The syntheses and characterisations of a series of heteroleptic copper(I) compounds [Cu(POP)(Mebpy)][A], [Cu(POP)(Me₂bpy)][A], [Cu(xantphos)(Mebpy)][A] and [Cu(xantphos)(Me₂bpy)][A] in which [A]⁻ is [BF₄]⁻, [PF₆]⁻, [BPh₄]⁻ and [BAr^F₄]⁻ (Mebpy = 6-methyl-2,2′-bipyridine, Me₂bpy = 6,6′-dimethyl-2,2′-bipyridine, POP = oxydi(2,1-phenylene)bis(diphenylphosphane), xantphos = (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane), [BAr^F₄]⁻ = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) are reported. Nine of the compounds have been characterised by single crystal X-ray crystallography, and the consequences of the different anions on the packing interactions in the solid state are discussed. The effects of the counterion on the photophysical properties of [Cu(POP)(N^N)][A] and [Cu(xantphos)(N^N)][A] (N^N = Mebpy and Me₂bpy) have been investigated. In the solid-state emission spectra, the highest energy emission maxima are for [Cu(xantphos)(Mebpy)][BPh₄] and [Cu(xantphos)(Me₂bpy)][BPh₄] (λ^em_max = 520 nm) whereas the lowest energy λ^em_max values occur for [Cu(POP)(Mebpy)][PF₆] and [Cu(POP)(Mebpy)][BPh₄] (565 nm and 563 nm, respectively). Photoluminescence quantum yields (PLQYs) are noticeably affected by the counterion; in the [Cu(xantphos)(Me₂bpy)][A] series, solid-state PLQY values decrease from 62% for [PF₆]⁻, to 44%, 35% and 27% for [BF₄]⁻, [BPh₄]⁻ and [BAr^F₄]⁻, respectively. This latter series of compounds was used as active electroluminescent materials on light-emitting electrochemical cells (LECs). The luminophores were mixed with ionic liquids (ILs) [EMIM][A] ([EMIM]⁺ = [1-ethyl-3-methylimidazolium]⁺) containing the same or different counterions than the copper(I) complex. LECs containing [Cu(xantphos)(Me₂bpy)][BPh₄] and [Cu(xantphos)(Me₂bpy)][BAr^F₄] failed to turn on under the LEC operating conditions, whereas those with the smaller [PF₆]⁻ or [BF₄]⁻ counterions had rapid turn-on times and exhibited maximum luminances of 173 and 137 cd m⁻² and current efficiencies of 3.5 and 2.6 cd A⁻¹, respectively, when the IL contained the same counterion as the luminophore. Mixing the counterions ([PF₆]⁻ and [BF₄]⁻) of the active complex and the IL led to a reduction in all the figures of merit of the LECs.

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