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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2021-12, Vol.5 (48), p.1792-17934
• Main Authors: Meyer, Marco, Mardegan, Lorenzo, Tordera, Daniel, Prescimone, Alessandro, Sessolo, Michele, Bolink, Henk J, Constable, Edwin C, Housecroft, Catherine E
• Format: Article
• Language: English
• Subjects: Chemistry; Coordination compounds; Copper; Crystallography; Electrochemical cells; Electroluminescence; Emission analysis; Emission spectra; Ionic liquids; Light emission; Photoluminescence; Single crystals; Solid state
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d1dt03239a

The syntheses and characterisations of a series of heteroleptic copper( i ) compounds [Cu(POP)(Mebpy)][A], [Cu(POP)(Me 2 bpy)][A], [Cu(xantphos)(Mebpy)][A] and [Cu(xantphos)(Me 2 bpy)][A] in which [A] − is [BF 4 ] − , [PF 6 ] − , [BPh 4 ] − and [BAr F 4 ] − (Mebpy = 6-methyl-2,2′-bipyridine, Me 2 bpy = 6,6′-dimethyl-2,2′-bipyridine, POP = oxydi(2,1-phenylene)bis(diphenylphosphane), xantphos = (9,9-dimethyl-9 H -xanthene-4,5-diyl)bis(diphenylphosphane), [BAr F 4 ] − = 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 Me 2 bpy) have been investigated. In the solid-state emission spectra, the highest energy emission maxima are for [Cu(xantphos)(Mebpy)][BPh 4 ] and [Cu(xantphos)(Me 2 bpy)][BPh 4 ] ( λ em max = 520 nm) whereas the lowest energy λ em max values occur for [Cu(POP)(Mebpy)][PF 6 ] and [Cu(POP)(Mebpy)][BPh 4 ] (565 nm and 563 nm, respectively). Photoluminescence quantum yields (PLQYs) are noticeably affected by the counterion; in the [Cu(xantphos)(Me 2 bpy)][A] series, solid-state PLQY values decrease from 62% for [PF 6 ] − , to 44%, 35% and 27% for [BF 4 ] − , [BPh 4 ] − and [BAr F 4 ] − , 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)(Me 2 bpy)][BPh 4 ] and [Cu(xantphos)(Me 2 bpy)][BAr F 4 ] failed to turn on under the LEC operating conditions, whereas those with the smaller [PF 6 ] − or [BF 4 ] − 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 ([PF 6 ] − and [BF 4 ] − ) of the active complex and the IL led to a reduction in all the figures of merit of the LECs. 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 t