Abstract
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 [BArF4]− (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), [BArF4]− = 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] (λemmax = 520 nm) whereas the lowest energy λemmax 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 [BArF4]−, 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)][BArF4] 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.
Highlights
Residual excess POP was removed in the purification steps
We have previously reported the performances of light-emitting electrochemical cells (LECs) containing [Cu(POP)(Mebpy)][PF6], [Cu(POP)(Me2bpy)][PF6], [Cu(xantphos)(Mebpy)][PF6] and [Cu(xantphos)(Me2bpy)][PF6] in their active layers, but under different device driving conditions.[21, 58]
It is important to notice that the added ionic liquids (ILs) was not always the same in all studies, and this can affect the performance of the device, as we show here
Summary
The design of LECs allows the production of large-area lighting devices on substrates such as glass, metals and flexible materials including polymers, paper[9] and textile fibres.[3, 4, 10,11,12] LECs utilise mobile ionic species either as the luminophore or blended with the semiconductor within the same active layer.[2, 3, 13,14,15] This layer functions as an ionic conductor.[5, 16] In its simplest implementation, the single-layer active composite is sandwiched between an air-stable cathode and anode.[2]. [Cu(xantphos)(N^N)][PF6] compounds, in which the N^N ligand is bpy or a methyl-substituted derivative, exhibit TADF.[20], Currently the operational model of a LEC is described both by the electrochemical doping model (ECDM) and the electrodynamic model (EDM) Both theoretical mechanisms rely on mobile ions in the active layer. The product was precipitated from CH2Cl2 with Et2O, followed by centrifugation and decantation of the supernatant In [Cu(POP)(Mebpy)][PF6]·0.5CH2Cl2·0.3Et2O, the region of the solvent contained disordered CH2Cl2 and Et2O molecules which were modelled over two sites with partial occupancies 0.5 and 0.3, respectively. The aromatic rings in the [BPh4]– or [BArF4]– anions are labelled E
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