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

Abstract

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.

Intra-Cation versus Inter-Cation π-Contacts in [Cu(P^P)(N^N)][PF6] Complexes

“…The solid-state emission spectra are shown in Figure 10a. 6 ] are blue-shifted on going from solution to the powdered samples, and this follows the typical trend [9,12]. However, for the compounds containing ligand 2, a red-shift is observed from solution to solid (Figure 10b) and this is probably associated with the extensive intermolecular π-stacking between ligands 2 that is confirmed in the crystal structure of [Cu(xantphos)(2)][PF 6 ]·CH 2 Cl 2 .…”

“…[Cu(PˆP)(NˆN)] + complexes containing the wide bite-angle bisphosphanes 4,5-bis(diphenylphosphanyl)-9,9-dimethyl-9H-xanthene (xantphos, IUPAC PIN (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphane)) and bis (2-(diphenylphosphanyl)phenyl)ether (POP, IUPAC PIN oxydi(2,1-phenylene)]bis(diphenylphosphane)) (Scheme 1) have been widely investigated and show particular promise as materials exhibiting high photoluminescence quantum yields (PLQYs). alkyloxy [10], alkylthio [10], aryl [9,11], halo [12] or trifluoroalkyl [13] [9] and [Cu(xantphos)(6-PhSbpy)][PF6] (38%) (6-Mebpy = 6-methyl-2,2'-bipyridine, 6-Etbpy = 6-ethyl-2,2'-bipyridine, 6,6'-Me2bpy = 6,6'-methyl-2,2'-bipyridine, 6-PhSbpy = 6-phenylthio-2,2'-bipyridine) [10]. A recurring feature of the solid-state structures of [Cu(xantphos)(6-Xbpy)] + complexes is the accommodation of the X group within the bowl-shaped cavity of the xanthene unit.…”

Remote Modification of Bidentate Phosphane Ligands Controlling the Photonic Properties in Their Complexes: Enhanced Performance of [Cu(RN‐xantphos)(N^N)][PF6] in Light‐Emitting Electrochemical Cells

“…Therefore, on the basis of both TD‐DFT and UDFT calculations, the 3 LLCT (HOMO→LUMO) triplet state does not play any special role in the phosphorescent emission of HN‐xantphos‐ and BnN‐xantphos complexes. It is also worth mentioning that, as previously discussed for xantphos‐containing complexes, [ 29,31 ] the partial oxidation of the copper atom in the T 1 state leads to the geometry of the complex in T 1 deviating from the tetrahedral geometry obtained for S 0 and rather tends to adopt a square‐planar coordination geometry. This geometrical relaxation is partially restricted by the presence of Me substituents in the 6 and 6′‐positions of the bpy ligand, [ 29,31 ] and the emission from T 1 blue‐shifts upon increasing the number of Me groups.…”

“…This effect was already observed for similar series of Cu complexes. [ 29,30 ] Therefore, more negative reduction potentials are expected along both series of complexes. Regarding the highest‐occupied molecular orbital (HOMO), it is calculated at ‐6.00 eV for [Cu(xantphos)(bpy)] + and mainly resides over the metal atom with some extension to the neighboring phosphorus atoms (Figure 3).…”

“…This is consistent with the description of the first triplet state previously discussed for complexes bearing the xantphos ligand. [ 10,29–31 ]…”