Hinged and Wide: A New P^P Ligand for Emissive [Cu(P^P)(N^N)][PF₆] Complexes

Abstract

Heteroleptic [Cu(BIPHEP)(N^N)][PF₆] complexes (BIPHEP = 1,1′-biphenyl-2,2′-diylbis(diphenylphosphane)), in which N^N is 2,2′-bipyridine (bpy), 6-methyl-2,2′-bipyridine (6-Mebpy), 6-ethyl-2,2′-bipyridine (6-Etbpy), or 5,5′-dimethyl-2,2′-bipyridine (5,5′-Me₂bpy), have been synthesized and characterized using multinuclear NMR spectroscopies and electrospray ionization mass spectrometry. The single crystal structures of [Cu(BIPHEP)(bpy)][PF₆]∙CH₂Cl₂, [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆]∙CH₂Cl₂, [Cu(BIPHEP)(6-Mebpy)][PF₆]∙Et₂O∙0.5H₂O and [Cu(BIPHEP)(6-Etbpy)][PF₆] confirm distorted tetrahedral {Cu(P^P)(N^N)} coordination environments. Each compound shows a quasi-reversible Cu⁺/Cu²⁺ process. In deaerated solution, the compounds are weak emitters. Powdered samples are yellow emitters (λ_em^max in the range 558–583 nm) and [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆] exhibits the highest photoluminescence quantum yield (PLQY = 14%). On cooling to 77 K (frozen 2-methyloxolane), the emission maxima are red-shifted and the excited state lifetimes increase from τ_1/2 < 8 μs, to τ_1/2 values of up to 53 μs, consistent with the compounds with N^N = 6-Mebpy, 6-Etbpy and 5,5′-Me₂bpy exhibiting thermally activated delayed fluorescence (TADF).

1. Introduction

Organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs) contribute to modern solid-state lighting technologies, and significant advances in the performances of these devices have been made in recent years [1,2,3,4,5,6]. The discovery of the photophysical phenomenon of thermally activated delayed fluorescence (TADF), which allows both singlet and triplet excitons to be harvested for light emission, has injected huge excitement into the field [7,8,9]. In the TADF process, after excitation of a compound into the lowest-lying singlet excited state, fast intersystem crossing to the excited triplet state, at a slightly lower energy (<0.38 eV [9]), takes place. As the triplet state is long lived, and emission as phosphorescence is rather slow, the thermal energy at room temperature allows repopulation of the singlet excited state from which fluorescence occurs. The emission at room temperature is, therefore, a combination of phosphorescence from the triplet excited state and fluorescence from the singlet excited state, allowing, in theory, a 100% internal quantum efficiency. Although organic compounds have been widely used for TADF applications [10,11] metal complexes are particularly effective and can be easily tuned through ligand modification to possess desired excited state energies [9]. The development of LECs which contain ionic copper(I) complexes in the emissive layer is especially appealing, since it capitalizes on the use of an Earth-abundant metal and [Cu(P^P)(N^N)]⁺ complexes (N^N = diimine, P^P = bisphosphane) which exhibit TADF [9]. Application of these copper(I) coordination compounds follows early results from McMillin and co-workers, who were the first to report that copper(I) complexes comprising PPh₃ or P^P ligands and 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen) exhibited low-lying metal-to-ligand charge transfer (MLCT) excited states [12,13].

To date, the P^P ligands most thoroughly explored for incorporation into emissive [Cu(P^P)(N^N)]⁺ complexes have been 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). Both are classed as wide bite–angle bisphosphanes [14], and the P–Cu–P bite angles in the solid-state structures of [Cu(POP)(bpy)][PF₆]^.CHCl₃, [Cu(POP)(bpy)][PF₆]^.Et₂O and [Cu(xantphos)(bpy)][PF₆] are 115.00(3)° [15], 111.97(3)° [16] and 113.816(14) [17], respectively. A wide variety of bpy-based ligands have been combined with POP and xantphos in systematic investigations of [Cu(P^P)(N^N)]⁺ emitters [3,17,18,19,20,21,22,23] and references cited therein]. In contrast, variations of the P^P ligand (other than POP and xantphos) have received less attention. Replacing the PPh₂ units in xantphos with PMes₂ units (Mes = mesityl) leads to a P–Cu–P angle in [Cu(xantphosMes₂)(bpy)][PF₆] (xantphosMes₂ = 9,9-dimethyl-9H-xanthene-4,5-diyl)bis[di(2,4,6-trimethylphenyl)phosphane]) of 115.96(2)° [24]. We have observed that introducing P^tBu₂ in place of PPh₂ groups in the xantphos derivative 9,9-dimethyl-9H-xanthene-4,5-diyl)bis[di(tert-butyl)phosphane] is detrimental, and gives rise to very weakly emitting [Cu(P^P)(N^N)][PF₆] compounds, likely due to the vibrational quenching effects of the tert-butyl substituents [25]. Retaining aromatic substituents is therefore critical for efficient energy to light conversion. Two sterically demanding ligands that fulfil this requirement are 1,1′-biphenyl-2,2′-diylbis(diphenylphosphane) (BIPHEP) and 1,1′-binaphthalene-2,2′-diylbis(diphenylphosphane) (BINAP) (Scheme 1). A search of the Cambridge Structural Database (v. 5.40 with February 2019 updates [26]) for {Cu(BINAP)(N^N)}-containing compounds, in which N^N is constrained to a chelating ligand, reveals only six entries [27,28,29,30] and for these, the range of P–Cu–P bond angles is 99.25–105.13°. Both BINAP and BIPHEP are expected to exhibit atropisomerism, but, at room temperature, interconversion of (R)- and (S)-BINAP is slow, whereas the atropisomers (R)- and (S)-BIPHEP (Scheme 1) interconvert rapidly in solution [31]. Coordination of BIPHEP to a metal centre can lead to chiral metal coordination compounds. The preparation and photophysical properties of [Cu(BINAP)(N^N)]⁺ complexes have been reported [28], but, even though BIPHEP is commercially available, no [Cu(BIPHEP)(N^N)]⁺ complexes appear to have been described. We were, therefore, inspired to investigate the structural and solution properties of such complexes. In this work, we describe the preparation and characterization of [Cu(BIPHEP)(bpy)][PF₆], along with three derivatives, in which the bpy metal-binding domain bears alkyl substituents in the 5- or 6-positions.

2. Results and Discussion

2.1. Synthesis and Characterization of [Cu(BIPHEP)(N^N][PF₆] Complexes

The [Cu(BIPHEP)(N^N)][PF₆] complexes, in which N^N is bpy, 6-Mebpy, 6-Etbpy or 5,5′-Me₂bpy, were prepared by adding CH₂Cl₂ solutions containing both BIPHEP and the respective 2,2′-bipyridine ligand to CH₂Cl₂ solutions of [Cu(MeCN)₄][PF₆]. This led to an immediate colour change from colourless to yellow, indicating the formation of the heteroleptic complexes without ligand redistribution; formation of a homoleptic [Cu(N^N)₂][PF₆] complex would have been accompanied by the development of a distinctive red colour. Attempts to prepare the analogous complex [Cu(BIPHEP)(6,6′-Me₂bpy)][PF₆] resulted in a mixture of homoleptic complexes as well as the desired heteroleptic compound, and since equilibration between hetero- and homoleptic complexes persisted in the solution, this reaction was not investigated further. The compounds [Cu(BIPHEP)(bpy)][PF₆], [Cu(BIPHEP)(6-Mebpy)][PF₆], [Cu(BIPHEP)(6-Etbpy)][PF₆] and [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆] were characterized by elemental analysis, mass spectrometry, and multinuclear NMR spectroscopies.

In the ESI-MS spectrum of each compound, the base peak was assigned to the [M–PF₆]⁺ ion (Figures S1–S4, see Supporting Information). The room temperature ¹H and ¹³C{¹H} NMR spectra were recorded in acetone-d₆ solution and signals were assigned (see Materials and Methods section) using 2D-methods. Aromatic ring and atom labelling are shown in Scheme 2. COSY, HMQC and HMBC spectra are displayed in Figures S5–S16 (see Supporting Information). Figure 1 shows that the signals arising from the coordinated bpy, 6-Mebpy, 6-Etbpy and 5,5′-Me₂bpy ligands are well resolved and sharp. The non-equivalent pyridine rings in [Cu(BIPHEP)(6-Mebpy)][PF₆] and [Cu(BIPHEP)(6-Etbpy)][PF₆] (Figure 1b,c) were readily distinguished, starting with the characteristic signal for proton H^A6 in the COSY spectrum and using the NOESY signal between the 6-alkyl group and proton H^B5. In [Cu(BIPHEP)(bpy)][PF₆] and [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆], the ¹H and ¹³C{¹H} NMR spectra at room temperature reveal that the rings (rings C in Scheme 1) in the biphenyl unit of BIPHEP are equivalent. Consistent with this is the observation of a single signal in the ³¹P{¹H} NMR spectrum for the cation, at δ −1.2 ppm and δ −0.6 ppm, respectively, for [Cu(BIPHEP)(bpy)][PF₆] and [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆]. In [Cu(BIPHEP)(6-Mebpy)][PF₆] and [Cu(BIPHEP)(6-Etbpy)][PF₆], the rings of the biphenyl unit are non-equivalent, and this is illustrated by comparing Figure 1a,d with Figure 1b,c, with a splitting of the signal for the H^C3 protons. This is consistent with the modelled structure [32] shown in Figure 2; the biphenyl rings remain non-equivalent, even with partial rotation about the trans-annular C–C bond of the biphenyl unit. At 298 K, the solution ³¹P{¹H} NMR spectrum of [Cu(BIPHEP)(6-Mebpy)][PF₆] (Figure 3) and [Cu(BIPHEP)(6-Etbpy)][PF₆] (Figure S17 in the Supporting Information) exhibits two signals of similar chemical shifts, in addition to the signal for the [PF₆]⁻ ion.

We have previously discussed in detail the dynamic processes observed for complexes of the types [Cu(POP)(6-Rbpy)]⁺ and [Cu(xantphos)(6-Rbpy)]⁺, where 6-Rbpy is a 6-substituted 2,2′-bipyridine [19,33]. In the ¹H-NMR spectra in Figure 1, it is noticeable that the regions associated with the phenyl rings of the PPh₂ units (rings labelled D) are broader for compounds containing asymmetrical 6-Mebpy and 6-Etbpy than for those with bpy or 5,5′-Me₂bpy. The ¹³C{¹H} NMR spectra in Figure 4 are instructive. In [Cu(BIPHEP)(bpy)][PF₆], all signals are well-resolved and have been assigned with the aid of HMQC and HMBC spectra. The two sets of C^D1/D1’, C^D2/D2’, C^D3/D3’ and C^D4/D4’ signals arise from pairs facing towards or away from the bpy alkyl substituent (Figure 2), and the spectra are consistent with unhindered rotation of the PPh₂ phenyl rings. Introducing the 6-alkyl substituent results in a broadening of all ¹³C{¹H} NMR resonances arising from rings C and D (Figure 4b), consistent with increased barriers to dynamic behaviour. An inspection of the changes in line shapes of the signals for C^D2 and C^D2’, and for C^D3 and C^D3 (highlighted in Figure 4 by the red hashed lines) suggests that the energy barrier to the rotation of one set of phenyls (presumably the rings closest to the alkyl substituent) is higher than that for the second set.

2.2. Single Crystal X-ray Diffraction

X-ray quality crystals of [Cu(BIPHEP)(bpy)][PF₆]∙CH₂Cl₂, [Cu(BIPHEP)(6-Mebpy)][PF₆]∙Et₂O∙0.5H₂O, [Cu(BIPHEP)(6-Etbpy)][PF₆] and [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆]∙CH₂Cl₂, were obtained by diffusion of Et₂O into CH₂Cl₂ solutions of the respective compound. The latter two compounds crystallize in the monoclinic space groups P2₁/c and C2/c, respectively, while [Cu(BIPHEP)(bpy)][PF₆]∙CH₂Cl₂ and [Cu(BIPHEP)(6-Mebpy)][PF₆]∙Et₂O∙0.5H₂O crystallize in the triclinic space group P−1. The solid-state structures of the [Cu(BIPHEP)(N^N)]⁺ cations are shown in Figure 5 and Figure 6, and selected structural parameters are summarized in

Table 1. Selected structural parameters for the [Cu(BIPHEP)(N^N][PF₆] compounds.

Cation in [Cu(BIPHEP)(N^N)][PF6]	Cu–P Distance/Å	Cu–N distance/Å	P–Cu–P Angle/Deg	N–Cu–N Angle/Deg	N–C–C–N Torsion Angle/Deg
[Cu(BIPHEP)(bpy)]+	Cu1–P1 = 2.2415(10); Cu1–P2 = 2.2292(9)	Cu1–N1 = 2.071(3); Cu1–N2 = 2.040(3)	P2–Cu1–P1 = 104.70(4)	N2–Cu1–N1 = 80.15(11)	−11.7(5)
[Cu(BIPHEP)(6-Mebpy)]+	Cu1–P1 = 2.2570(7); Cu1–P2 = 2.2549(7)	Cu1–N1 = 2.086(2); Cu1–N2 = 2.049(2)	P2–Cu1–P1 = 105.01(2)	N1–Cu1–N2 = 80.65(10)	−12.1(4)
[Cu(BIPHEP)(6-Etbpy)]+	Cu1–P1 = 2.2410(11); Cu1–P2 = 2.2322(10)	Cu1–N1 = 2.072(4); Cu1–N2 = 2.063(4)	P2–Cu1–P1 = 101.56(4)	N2–Cu1–N1 = 79.75(14)	6.4(5)
[Cu(BIPHEP)(5,5′-Me2bpy)]+	Cu1–P16 = 2.2248(6); Cu1–P41 = 2.2515(6)	Cu1–N2 = 2.0321(18); Cu1–N15 = 2.0751(18)	P16–Cu1–P41 = 106.24(2)	N2–Cu1–N15 = 80.51(7)	−10.3(3)

. The twist in the backbone of the BIPHEP ligand renders each [Cu(BIPHEP)(N^N)]⁺ cation chiral (Figure 7), and both enantiomers are present in the unit cell.

The copper atoms in the complexes exhibit distorted tetrahedral geometries, with τ₄ parameters [34] of 0.81 for [Cu(BIPHEP)(bpy)]⁺, 0.85 for [Cu(BIPHEP)(6-Mebpy)]⁺, 0.78 for [Cu(BIPHEP)(6-Etbpy)]⁺, and 0.80 for [Cu(BIPHEP)(5,5′-Me₂bpy)]⁺. For T_d symmetry, τ₄ = 1.00, and the values observed here indicate a distortion towards a trigonal pyramidal geometry. Figure 7 displays the four [Cu(BIPHEP)(N^N)]⁺ cations, each viewed along the P...P vector; the same enantiomer is shown for each complex. The Cu–P distances are typical (2.2248(6) to 2.2570(7) Å) and the same is true for the Cu–N distances (range = 2.0321(18) to 2.086(2) Å, Table 1). The P–Cu–P angles lie in the range 101.56(4)°–106.24(2)°. For the analogous POP and xantphos complexes with eight-membered rings, the bite angle of the coordinated phosphanes is around 113° for the complexes with 6-Mebpy or 6-Etbpy [18,19], and in the range 111.97(3)°–115.00(3)° for [Cu(POP)(bpy)][PF₆] and [Cu(xantphos)(bpy)][PF₆] [15,16,17]. As expected, the chelating 2,2′-bipyridine ligands exhibit similar N–Cu–N angles, in the range 79.75(14)°–80.65(10)°. The two rings of the bpy ligand are twisted in all the complex cations, with N–C–C–N torsion angles between 6.4(5)° in [Cu(BIPHEP)(6-Etbpy)]⁺, and −12.1(4)° in [Cu(BIPHEP)(6-Mebpy)]⁺.

There are no trends in the structural parameters as a function of the number or steric demands of the alkyl substituents attached to the bpy unit, and it appears that packing effects are dominant. In the complexes containing [Cu(BIPHEP)(bpy)][PF₆]⁺, [Cu(BIPHEP)(6-Mebpy)]⁺ and [Cu(BIPHEP)(5,5′-Me₂bpy)]⁺, intracation π-stacking interactions occur between a phenyl ring of a PPh₂ group and one ring of the BIPHEP backbone (Figure 5 and Figure 6b, and Figure S18 in the Supporting Information) although the angles between the ring planes of 25.7°, 15.1° and 18.0° mean that the interactions are not optimal. Short C–H...F cation...anion contacts feature in all of the structures, and the accommodation of solvent molecules in [Cu(BIPHEP)(bpy)][PF₆]∙CH₂Cl₂, [Cu(BIPHEP)(6-Mebpy)][PF₆]∙Et₂O∙0.5H₂O and [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆]∙CH₂Cl₂ also contributes to packing effects.

2.3. Electrochemical Behaviour

The electrochemical behaviour of the [Cu(BIPHEP)(N^N)][PF₆] compounds was investigated using cyclic voltammetry (CV), and cyclic voltammograms for CH₂Cl₂ solutions are shown in Figures S19–S22 (see Supporting Information). Each compound exhibits a reversible or quasi-reversible oxidation process (

Table 2. Cyclic voltammetric data for [Cu(BIPHEP)(N^N)][PF₆] complexes referenced to internal Fc/Fc⁺ = 0.0 V; CH₂Cl₂ (dried) solutions, with [ⁿBu₄N][PF₆] as the supporting electrolyte and a scan rate of 0.1 V s⁻¹. Processes are quasi-reversible.

Cation in [Cu(BIPHEP)(N^N][PF6]	${\mathit{E}}_{1/2}^{\mathit{o}\mathit{x}}$/V	Epc – Epa/mV
[Cu(BIPHEP)(bpy)]+	+0.59	100
[Cu(BIPHEP)(6-Mebpy)]+	+0.72	129
[Cu(BIPHEP)(6-Etbpy)]+	+0.73	133
[Cu(BIPHEP)(5,5′-Me2bpy)]+	+0.57	197
[Cu(POP)(bpy)]+	+0.72 a	110 a
[Cu(POP)(6-Mebpy)]+	+0.69 b	-
[Cu(xantphos)(bpy)]+	+0.76 a	110 a
[Cu(xantphos)(6-Mebpy)]+	+0.90 a	100 a

^a Data taken from reference [17]. ^b Data taken from reference [18].

) assigned to a Cu⁺/Cu²⁺ oxidation, but no ligand-based reduction processes were observed within the solvent-accessible window. For comparison, Table 2 also reports the oxidation potentials of [Cu(P^P)(bpy)][PF₆] and [Cu(P^P)(6-Mebpy)][PF₆] (P^P = POP and xantphos) [17,18], measured under the same conditions as the BIPHEP-containing compounds. For the BIPHEP complexes, the oxidation potentials increase on going from the complex with the unsubstituted bpy (+0.59 V), to 6-Mebpy (+0.72 V) and 6-Etbpy (+0.73 V). This follows the trend in the corresponding xantphos-containing compounds (Table 2), although, with the more flexible POP ligand, the introduction of the 6-methyl substituent has little effect. The trends for the BIPHEP-containing compounds are consistent with the introduction of the 6-alkyl substituent, hindering the flattening of the tetrahedral copper(I) towards square planar copper(II), thereby moving the oxidation to a higher potential. The Cu⁺/Cu²⁺ oxidation occurs at similar potentials in [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆] and [Cu(BIPHEP)(bpy)][PF₆] (Table 2), indicating that the presence of methyl groups in these positions has little stabilizing effect on the copper(I) oxidation state. The data in Table 2 reveal that metal oxidation in [Cu(BIPHEP)(bpy)][PF₆] occurs at significantly lower potential than in [Cu(POP)(bpy)][PF₆] and [Cu(xantphos)(bpy)][PF₆], suggesting that the flattening of the copper coordination sphere that accompanies oxidation from Cu⁺ to Cu²⁺ occurs more easily. This should also be reflected in the excited state lifetimes, which are discussed in the next section. We note that there is no clear correlation between the Cu⁺/Cu²⁺ oxidation potential in solution and the τ₄ value (see Section 2.2), which gives a measure of the distortion away from a tetrahedral geometry (for which τ₄ = 1.00) in the solid state.

2.4. Photophysical Properties

The solution absorption spectra of the [Cu(BIPHEP)(N^N)][PF₆] compounds in CH₂Cl₂ are displayed in Figure 8 and absorption maxima are given in

Table 3. Solution absorption maxima for the [Cu(BIPHEP)(N^N)][PF₆] complexes (CH₂Cl₂ with solution concentrations in the range 3.5–5.3 × 10⁻⁵ mol dm⁻³).

Cation in [Cu(BIPHEP)(N^N)][PF6]	λmax/nm (ε/dm3 mol−1 cm−1)
	Ligand-Based π*←π	MLCT
[Cu(BIPHEP)(bpy)]+	244 sh (32,000), 291 (19,400), 309 sh (12,400)	407 (3500)
[Cu(BIPHEP)(6-Mebpy)]+	292 (19,400), 301 (18,500), 316 sh (14,500)	400 (3400)
[Cu(BIPHEP)(6-Etbpy)]+	292 (22,500), 301 (22,100), 315 sh (15,800)	400 (4000)
[Cu(BIPHEP)(5,5′-Me2bpy)]+	248 sh (36,200), 301 (21,300), 319 sh (15,200)	390 (4400)

. The spectra for all four compounds are similar, with intense, high-energy (230–330 nm) absorptions arising from spin-allowed, ligand-based π*←π transitions. The broad absorption, with λ_max close to 400 nm, is assigned to the MLCT band, and values of λ_max and ε_max vary little across the series of compounds.

When excited into the MLCT band at 390 nm, deaerated solutions of [Cu(BIPHEP)(bpy)][PF₆] did not show a detectable emission, while [Cu(BIPHEP)(6-Mebpy)][PF₆], [Cu(BIPHEP)(6-Etbpy)][PF₆] and [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆] exhibited weak emissions, with values of λ_em^max = 645, 636 and 641 nm, respectively (Figure S23 in Supporting Information). Since these emissions were so weak, we focused on solid-state and frozen-glass emission behaviour. Emission maxima, photoluminescence quantum yields (PLQY) and excited state lifetimes (τ_1/2) are presented in

Table 4. Emission maxima, photoluminescence quantum yields (PLQY) and lifetimes (τ_1/2) for [Cu(BIPHEP)(N^N)][PF₆] complexes.

Cation in [Cu(BIPHEP)(N^N)][PF6]	Powder a			Frozen Glass (Me-THF, 77 K) b
	λemmax/nm c	PLQY/%	τ1/2/μs	λemmax/nm d	τ1/2/μs
[Cu(BIPHEP)(bpy)]+	566	4	3	615	7
[Cu(BIPHEP)(6-Mebpy)]+	568	7	3	595	45
[Cu(BIPHEP)(6-Etbpy)]+	583	3	1	590	53
[Cu(BIPHEP)(5,5′-Me2bpy)]+	558	14	8	600	49

^aλ_exc = 365 nm; ^b λ_exc = 410 nm. ^c λ_em^max estimated error ± 2 nm. ^d λ_em^max estimated error ± 5 nm.

. Powdered samples are yellow emitters with values of λ_em^max, in the range 558–583 nm (Figure 9a), and are similar to those observed for [Cu(P^P)(bpy)][PF₆], [Cu(P^P)(6-Mebpy)][PF₆] and [Cu(P^P)(6-Etbpy)][PF₆], in which P^P is POP or xantphos [17,18,19]. A blue shift in the emission maximum, upon going from liquid solution to a solid state, is consistent with previous observations of these families of compounds [19,22]. The highest PLQY for the solid-state compounds (Table 4) is 14% for [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆] and this also shows the longest excited state lifetime (8 μs). This may indicate better protection of the copper centre against quenchers (e.g., O₂ and H₂O) [22], although substitution in the 5,5′-positions, in the case of [Cu(POP)(5,5′-(CF₃)₂bpy)][PF₆] and [Cu(xantphos)(5,5′-(CF₃)₂bpy)][PF₆], proved less beneficial than a 6,6′-substitution pattern [17]. As noted earlier, we were not able to isolate [Cu(BIPHEP)(6,6′-Me₂bpy)][PF₆] and are not, therefore, able to compare the effects of going from 5,5′-Me₂bpy to 6,6′-Me₂bpy in the case of the BIPHEP co-ligand.

In order to investigate whether the [Cu(BIPHEP)(N^N)][PF₆] compounds were TADF emitters, their emission and excited state lifetimes were recorded at low temperature and compared to their values at room temperature. Since the repopulation of the singlet excited state by reverse intersystem crossing is suppressed at low temperatures (ΔE > kT), the excited state lifetime should be considerably longer, since phosphorescence from the triplet excited state should be the dominant process. The normalized emission spectra of the [Cu(BIPHEP)(N^N)][PF₆] complexes in frozen Me-THF at 77 K are depicted in Figure 9b, and the emission maxima and excited state lifetimes are summarized in Table 4. A comparison of the values of λ_em^max in the powders at room temperature with those at 77 K reveals that the emission maxima are red-shifted for all the complexes, consistent with the triplet excited state being lower in energy than the singlet excited state. A significant extension of the excited state lifetimes is observed (Table 4), and, for the compounds in which the bpy bears alkyl substituents, values of τ_1/2 are ca. 50 μs. These data are consistent with [Cu(BIPHEP)(N^N)][PF₆] (N^N = 6-Mebpy, 6-Etbpy and 5,5′-Me₂bpy) being TADF emitters. The increased excited state lifetime on going from 298 to 77 K is particularly noticeable for [Cu(BIPHEP)(6-Etbpy)][PF₆], where τ_1/2 increases from 1 μs at room temperature to 53 μs at 77 K. At the same time, the temperature-induced red-shift in this complex is the smallest in the series, and at 77 K, it has an emission maximum at the shortest wavelength compared to others in the series. This raises the question as to whether other effects such as non-radiative decay processes occur in [Cu(BIPHEP)(6-Etbpy)][PF₆].

Since [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆] exhibited the highest PLQY and longest excited state lifetime in the solid state, we decided to conduct preliminary tests in LECs. Simple bilayer devices were fabricated by spin-coating a solution of [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆] on indium tin oxide-coated glass substrates, covered with a film of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. Devices were finished with the thermal evaporation of an aluminium cathode. LECs exhibited yellow electroluminescence with λ_em^max = 586 nm, red-shifted with respect to the solid-state photoluminescence. However, even at a current density of 100 A m⁻², the maximum luminance was as low as 12 cd m⁻², with a maximum external quantum efficiency (EQE) of 0.03%. The LEC also showed only a short lifetime, with t_1/2 = 0.1 hours. In comparison, LECs containing [Cu(xantphos)(6-Mebpy)][PF₆], [Cu(xantphos)(6-Etbpy)][PF₆] and [Cu(POP)(6-Etbpy)][PF₆] in the emissive layers exhibit values of t_1/2 greater than 15, 40 and 80 h, respectively [18,19].

3. Materials and Methods

3.1. General

¹H and ¹³C{¹H} and ³¹P-NMR spectra were recorded on a Bruker Avance III-500 spectrometer (Bruker BioSpin AG, Fällanden, Switzerland) at 298 K. ¹H and ¹³C-NMR chemical shifts were referenced, with respect to residual solvent peaks (δ TMS = 0), and ³¹P-NMR chemical shifts, with respect to δ(85% aqueous H₃PO₄) = 0 ppm. Solution absorption and emission spectra were measured using a Shimadzu UV-2600 spectrophotometer and a Shimadzu RF-5301PC spectrofluorometer, respectively (Shimadzu Schweiz GmbH, 4153 Aesch, Switzerland). A Shimadzu LCMS-2020 instrument (Shimadzu Schweiz GmbH, 4153 Aesch, Switzerland) was used to record electrospray ionization (ESI) mass spectra; samples were introduced as MeCN solutions with added formic acid.

Quantum yields (CH₂Cl₂ solutions or powder samples) were measured with a Hamamatsu absolute photoluminescence quantum yield spectrometer C11347 Quantaurus-QY (Hamamatsu Photonics, 4500 Solothurn, Switzerland). Emission lifetimes and powder emission spectra were recorded using a Hamamatsu Compact Fluorescence lifetime Spectrometer C11367 Quantaurus-Tau (Hamamatsu Photonics), with an LED light source with λ_exc = 365 nm. Low temperature emission and lifetime measurements were made using an Edinburgh Instruments LP920-KS instrument (Edinburgh Instruments, Livingston, UK), in frozen 2-methyloxolane (Me-THF); a 410 nm excitation was obtained from pulsed third-harmonic radiation from a Quantel Brilliant b Nd:YAG laser (Quantal Laser, Lumibird, 22300 Lannion, France), equipped with a Rainbow optical parameter oscillator. The laser pulse duration and pulse frequency were ~10 ns and 10 Hz, respectively, with a typical pulse energy of 7 mJ. An iCCD camera (Andor, Oxford Instruments GmbH, Wiesbaden, Germany) was used for detection of the spectra. Single-wavelength kinetics were recorded using a photomultiplier tube.

6-Methyl-2,2′-bipyridine (6-Mebpy) and 6-ethyl-2,2′-bipyridine (6-Etbpy) [35] and [Cu(MeCN)₄][PF₆] [36] were prepared following the literature procedures. BIPHEP was bought from Alfa Aesar (Alfa Aesar GmbH, 76185 Karlsruhe, Germany), and 2,2′-bipyridine (bpy), 5,5′-dimethyl-2,2′-bipyridine (5,5′-Me₂bpy) and 6,6′-dimethyl-2,2′-bipyridine (6,6′-Me₂bpy) from Sigma-Aldrich (Merck, 9470 Buchs, Switzerland). All chemicals were used as received.

3.2. [Cu(BIPHEP)(bpy)][PF₆]

A colourless solution of bpy (39 mg, 0.25 mmol) and BIPHEP (131 mg, 0.25 mmol) in CH₂Cl₂ (20 mL) was added to a colourless solution of [Cu(MeCN)₄][PF₆] (93 mg, 0.25 mmol) in CH₂Cl₂ (20 mL), turning the solution yellow. After stirring at room temperature for 2 h, the solvent was removed in vacuo. The crude product was redissolved in CH₂Cl₂ and [Cu(BIPHEP)(bpy)][PF₆] (63 mg, 0.071 mmol, 28%), precipitated as a yellow powder by layering with Et₂O. ¹H-NMR (500 MHz, acetone-d₆, 298 K): δ/ppm 8.76 (dt, J = 8.2, 1.0 Hz, 2H, H^A3), 8.42 (dd, J = 5.2, 1.4 Hz, 2H, H^A6), 8.29 (td, J = 7.9, 1.6 Hz, 2H, H^A4), 7.67 (m, 2H, H^A5), 7.42–7.37 (m, 2H, H^D4/D4’), 7.34–7.26 (overlapping m, 16H, H^{D2+D2’+D3/D3’+D4/D4’+C5}), 7.26–7.17 (overlapping m, 6H, H^D3/D3’+C4), 7.10 (m, 2H, H^C6), 6.81 (m, 2H, H^C3). ¹³C{¹H} NMR (126 MHz, acetone-d₆, 298 K): δ/ppm 152.9 (t, J_PC = 2 Hz, C^A2), 151.4 (C^A6), 146.5 (t, J_PC = 9 Hz, C^C1), 140.2 (C^A4), 135.7 (t, J_PC = 3.5 Hz, C^C6), 135.0 (t, J_PC = 8 Hz, C^D2/D2’), 134.5 (t, J_PC = 8 Hz, C^D2/D2’), 132.9 (t, J_PC = 18 Hz, C^D1/D1’), 132.2 (t, J_PC = 14 Hz, C^C2), 131.6 (t, J_PC = 18 Hz, C^D1/D1’), 131.6 (C^C3), 131.5 (t, J_PC = 1 Hz, C^D4/D4’), 131.2 (t, J_PC = 1 Hz, C^D4/D4’), 130.6 (C^C5), 130.0 (t, J_PC = 5 Hz, C^D3/D3’), 129.4 (t, J_PC = 5 Hz, C^D3/D3’), 128.6 (t, J_PC = 3 Hz, C^C4), 127.2 (C^A5), 123.8 (C^A3). ³¹P{¹H} NMR (202 MHz, acetone-d₆, 298 K): δ/ppm −1.2 (broad, FWHM = 270 Hz, BIPHEP), −142.5 (septet, J_PF = 700 Hz, [PF₆]⁻). ESI MS: m/z 741.10 [M–PF₆]⁺ (base peak, calc. 741.16). Found C 61.53, H 4.37, N 3.22; C₄₆H₃₆CuF₆N₂P₃∙H₂O requires C 61.03, H 4.23, N 3.09.

3.3. [Cu(BIPHEP)(6-Mebpy)][PF₆]

A colourless solution of 6-Mebpy (43 mg, 0.25 mmol) and BIPHEP (131 mg, 0.25 mmol) in CH₂Cl₂ (20 mL) was added to a colourless solution of [Cu(MeCN)₄][PF₆] (93 mg, 0.25 mmol) in CH₂Cl₂ (20 mL), turning the solution yellow. After stirring at room temperature for 2 h, the solvent was removed under reduced pressure. The crude product was redissolved in CH₂Cl₂ and layered with Et₂O, to precipitate [Cu(BIPHEP)(6-Mebpy)][PF₆] (202 mg, 0.23 mmol, 92%) as a yellow powder. ¹H-NMR (500 MHz, acetone-d₆, 298 K): δ/ppm 8.72 (dt, J = 8.3, 1.0 Hz, 1H, H^A3), 8.62 (d, J = 7.9 Hz, 1H, H^B3), 8.29–8.21 (overlapping m, 2H, H^B4+A4), 8.04 (dt, J = 5.2, 0.8 Hz, 1H, H^A6), 7.73 (dd, J = 7.8, 0.9 Hz, 1H, H^B5), 7.54 (m, 1H, H^A5), 7.46–7.10 (overlapping m, 26H, H^{D2+D2’+D3+D3’+D4+D4’+C4+C4’+C5+C5’+C6+C6’}), 6.99 (m, 1H, H^C3/C3’), 6.88 (m, 1H, H^C3/C3’), 2.58 (s, 3H, H^Me). ¹³C{¹H} NMR (126 MHz, acetone-d₆, 298 K): δ/ppm 160.5 (C^B6), 153.5 (C^A2/B2), 152.7 (C^A2/B2), 151.0 (C^A6), 145.7 (br, C^C1+C1’), 140.6 (C^A4/B4), 140.2 (C^A4/B4), 136.2 (br, C^C6/C6’), 135.2 (br, overlapping, C^D2/D2’), 134.2 (C^D2/D2’), 134.1 (C^D2/D2’), 129.45 (C^D3/D3’), 129.4 (C^D3/D3’), 128.8 (br, C^C4+C4’), 127.6 (C^B5), 127.0 (C^A5), 123.9 (C^A3), 121.4 (C^B3); signals for C^C3, C^C5, C^C1, C^C2, C^D1 were poorly resolved. ³¹P{¹H} NMR (202 MHz, acetone-d₆, 298 K): δ/ppm −1.3 (broad, FWHM = 428 Hz, BIPHEP) −3.7 (broad, FWHM = 401 Hz, BIPHEP), −144.2 (septet, J_PF = 700 Hz, [PF₆]⁻). ESI MS: m/z 755.16 [M–PF₆]⁺ (base peak, calc. 755.18). Found C 61.89, H 4.83, N 3.38; C₄₇H₃₈CuF₆N₂P₃∙H₂O requires C 61.41, H 4.39, N 3.05.

3.4. [Cu(BIPHEP)(6-Etbpy)][PF₆]

A solution of 6-Etbpy (46 mg, 0.25 mmol) and BIPHEP (131 mg, 0.25 mmol) in CH₂Cl₂ (20 mL) was added to a solution of [Cu(MeCN)₄][PF₆] (93 mg, 0.25 mmol) in CH₂Cl₂ (20 mL), causing an immediate change from colourless to yellow. After stirring at room temperature for 2 h, the solvent was removed in vacuo. The crude material was redissolved in CH₂Cl₂. Layering with Et₂O precipitated [Cu(BIPHEP)(6-Etbpy)][PF₆] (208 mg, 0.23 mmol, 92%) as a yellow powder. ¹H-NMR (500 MHz, acetone-d₆, 298 K): δ/ppm 8.72 (d, J = 8.2 Hz, 1H, H^A3), 8.62 (d, J = 7.8 Hz, 1H, H^B3), 8.31 (t, J = 7.9 Hz, 1H, H^B4), 8.24 (td, J = 7.6, 1.6 Hz, 1H, H^A4), 7.91 (m, 1H, H^A6), 7.79 (d, J = 7.7 Hz, 1H, H^B5), 7.51 (m, 1H, H^A5), 7.42–7.12 (overlapping m, 25H, H^{D4+D4’+C5+C5’+D2+D2’+D3+D3’+C4+C4’+C6/C6’}), 7.08 (m, 1H, C^C6/C6’), 7.00 (m, 1H, H^C3), 6.88 (m, 1H, H^C3), 3.03 (q, J = 7.8 Hz, 2H, H^Et), 1.08 (t, J = 7.6 Hz, 3H, H^Et). ¹³C{¹H} (126 MHz, acetone-d₆, 298 K): 164.8 (C^B6), 153.6 (C^A2/B2), 152.4 (C^A2/B2), 151.0 (C^A6), 145.8 (C^C1/C1’), 145.6 (C^C1/C1’), 140.8 (C^B4), 140.2 (C^A4), 136.1 (C^C6+C6’), 135.3 (overlapping m, C^D2/D2’), 134.2 (C^D2/D2’), 134.0 (C^D2/D2’), 132.7–131.6 (overlapping m, C^{D1+D1’+C2+C2’}), 131.5 (br, C^C3+C3’), 131.0 (C^D4/D4’), 130.9 (C^D4/D4’), 130.8 (C^D4/D4’), 130.5 (C^D4/D4’), 130.0 and 129.8 (br, overlapping m, C^{D3/D3’+C5+C5’}), 129.4 (br, overlapping m, C^D3/D3’), 128.7 (br, C^C4/C4’), 128.5 (br, C^C4/C4’), 126.9 (C^A5), 125.4 (C^B5), 123.9 (C^A3), 121.6 (C^B3), 35.5 (C^Et), 13.2 (C^Et); see text discussion. ³¹P{¹H} NMR (162 MHz, acetone-d₆, 300 K) δ/ppm −1.3 (broad, FWHM = 320 Hz, BIPHEP), −4.0 (broad, FWHM = 320 Hz, BIPHEP), −144.2 (septet, J_PF = 708 Hz, [PF₆]⁻); see text discussion. ESI MS: m/z 769.28 [M–PF₆]⁺ (base peak, calc. 769.20). Found C 62.40, H 4.98, N 3.22; C₄₈H₄₀CuF₆N₂P₃∙0.5H₂O requires C 62.37, H 4.47, N 3.03.

3.5. [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆]

A solution of 5,5′-Me₂bpy (46 mg, 0.25 mmol) and BIPHEP (131 mg, 0.25 mmol) in CH₂Cl₂ (20 mL) was added to a solution of [Cu(MeCN)₄][PF₆] (93 mg, 0.25 mmol) in CH₂Cl₂ (20 mL). The yellow reaction mixture was stirred at room temperature for 2 h, and then solvent was removed under reduced pressure. The crude product was redissolved in CH₂Cl₂ and layering with Et₂O led to the precipitation of yellow [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆] (215 mg, 0.23 mmol, 92%). ¹H-NMR (500 MHz, acetone-d₆, 298 K): δ/ppm ¹H-NMR (500 MHz, acetone-d₆, 298 K) δ /ppm 8.55 (d, J = 8.2 Hz, 2H, H^B3), 8.04 (m, 2H, H^B4), 7.96 (m, 2H, H^B6), 7.44–7.39 (m, 2H, H^D4/D4’), 7.38–7.29 (overlapping m, 12H, H^{D2/D2’+D3/D3’+D4/D4’+C5}), 7.27–7.19 (m, 10H, H^{D2/D2’+D3/D3’+C4}), 7.15 (m, 2H, H^C6), 6.76 (m, 2H, H^C3), 2.30 (s, 6H, H^Me). ¹³C{¹H} NMR (126 MHz, acetone-d₆, 298 K) δ /ppm 151.5 (C^B6), 150.5 (t, J = 2 Hz, C^B2), 145.7 (t, J = 10 Hz, C^C1), 140.2 (C^B4), 137.3 (C^B5), 135.8 (t, J = 4 Hz, C^C6), 135.2 (t, J = 9 Hz, C^D2/D2’), 134.6 (t, J = 8 Hz, C^D2/D2’), 132.8 (t, J = 18 Hz, C^D1/D1’), 132.2 (t, J = 14 Hz, C^C2), 131.7 (t, J = 3 Hz, C^C3), 131.6 (C^D4/D4’), 131.3 (t, J = 18 Hz, C^D1/D1’), 131.1 (C^D4/D4’), 130.6 (C^C5), 130.0 (t, J = 5 Hz, C^D3/D3’), 129.3 (t, J = 5 Hz, C^D3/D3’), 128.6 (t, J = 3 Hz, C^C4), 122.8 (C^B3), 18.1 (C^Me). ³¹P{¹H} NMR (162 MHz, acetone-d₆, 300 K) δ/ppm −0.6 (broad, FWHM = 350 Hz, BIPHEP), −144.2 (septet, J_PF = 708 Hz, [PF₆]⁻). ESI MS: m/z 769.28 [M–PF₆]⁺ (base peak, calc. 769.20). Found C 58.24, H 4.30, N 3.02; C₄₈H₄₀CuF₆N₂P₃∙CH₂Cl₂ requires C 58.84, H 4.23, N 2.80.

3.6. Crystallography

Single crystal X-ray diffraction data were collected on a Bruker Kappa Apex2 diffractometer with data reduction, solution and refinement, using the programs APEX [37] and CRYSTALS [38]. Structural analysis used the program Mercury v. 4.1.2 [39,40].

3.7. [Cu(BIPHEP)(bpy)][PF₆]∙CH₂Cl₂

C₄₇H₃₈Cl₂CuF₆N₂P₃, M = 972.19, yellow plate, monoclinic, space group P2₁/c, a = 14.3304(6), b = 15.4136(6), c = 20.6025(7) Å, β = 105.844(2)°, U = 4377.8(3) ų, Z = 4, D_c = 1.475 Mg m⁻³, μ(Cu-Kα) = 3.397 mm⁻¹, T = 123 K. Total 46,656 reflections, 8189 unique, R_int = 0.071. Refinement of 6109 reflections (550 parameters) with I >2σ(I) converged at final R₁ = 0.0505 (R₁ all data = 0.0733), wR₂ = 0.1151 (wR₂ all data = 0.1308), gof = 0.9710. CCDC 1958102.

3.8. [Cu(BIPHEP)(6-Mebpy)][PF₆]∙Et₂O∙0.5H₂O

C₅₁H₄₉CuF₆N₂O_1.5P₃, M = 984.42, yellow block, monoclinic, space group C2/c, a = 29.5271(19), b = 16.7361(11), c = 20.5516(14) Å, β = 111.249(2)°, U = 9465.5(11) ų, Z = 8, D_c = 1.381 Mg m⁻³, μ(Cu-Kα) = 2.157 mm⁻¹, T = 123 K. Total 31,108 reflections, 8537 unique, R_int = 0.023. Refinement of 8430 reflections (584 parameters) with I >2σ(I) converged at final R₁ = 0.0540 (R₁ all data = 0.0544), wR₂ = 0.1197 (wR₂ all data = 0.1197), gof = 1.0146. CCDC 1958100.

3.9. [Cu(BIPHEP)(6-Etbpy)][PF₆]

C₄₈H₄₀CuF₆N₂P₃, M = 915.31, yellow block, triclinic, space group P−1, a = 10.5819(11), b = 13.4147(14), c = 15.0134(16) Å, α = 93.766(3) β = 100.826(3), γ = 92.405(3)°, U = 2085.5(4) ų, Z = 2, D_c = 1.457 Mg m⁻³, μ(Cu-Kα) = 2.377 mm⁻¹, T = 123 K. Total 29,365 reflections, 7594 unique, R_int = 0.032. Refinement of 6965 reflections (541 parameters) with I >2σ(I) converged at final R₁ = 0.0412 (R₁ all data = 0.0502), wR₂ = 0.0822 (wR₂ all data = 0.1385), gof = 1.3893. CCDC 1958101.

3.10. [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆]∙CH₂Cl₂

C₄₉H₄₂Cl₂CuF₆N₂P₃, M = 1000.25, yellow block, triclinic, space group P−1, a = 10.9543(6), b = 10.9953(6), c = 18.8408(11) Å, α = 94.9723(17), β = 92.9633(18), γ = 96.7453(17)°, U = 2240.7(2) ų, Z = 2, D_c = 1.482 Mg m⁻³, μ(Cu-Kα) = 3.334 mm⁻¹, T = 123 K. Total 29,278 reflections, 7850 unique, R_int = 0.030. Refinement of 7808 reflections (568 parameters) with I >2σ(I) converged at final R₁ = 0.0453 (R₁ all data = 0.0453), wR₂ = 0.0461 (wR₂ all data = 0.0461), gof = 1.1280. CCDC 1958099.

4. Conclusions

We have prepared and characterized the compounds [Cu(BIPHEP)(bpy)][PF₆], [Cu(BIPHEP)(6-Mebpy)][PF₆], [Cu(BIPHEP)(6-Etbpy)][PF₆] and [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆]. ¹H, ¹³C{¹H} and ³¹P{¹H} NMR spectra, including the use of 2D methods, reveal dynamic processes involving the BIPHEP ligand. The single crystal structures of [Cu(BIPHEP)(bpy)][PF₆]∙CH₂Cl₂, [Cu(BIPHEP)(5,5′-Me₂bpy)][PF₆]∙CH₂Cl₂, [Cu(BIPHEP)(6-Mebpy)][PF₆]∙Et₂O∙0.5H₂O and [Cu(BIPHEP)(6-Etbpy)][PF₆] were determined and confirmed distorted tetrahedral {Cu(P^P)(N^N)} coordination environments. Each compound shows a quasi-reversible Cu⁺/Cu²⁺ process, which moves to a higher potential upon the introduction of 6-methyl or 6-ethyl substituent into the bpy unit. The [Cu(BIPHEP)(N^N)][PF₆] compounds are weak emitters in the deaerated solution, whereas powdered samples are yellow emitters at room temperature, with values of λ_em^max between 558 and 583 nm, and PLQY values up to 14%. Evidence that the compounds with N^N = 6-Mebpy, 6-Etbpy and 5,5′-Me₂bpy exhibit TADF comes from the fact that their excited state lifetimes increase from τ_1/2 < 8 μs to τ_1/2 values of up to 53 μs on cooling from room temperature to 77 K, and that the emission maxima are red-shifted.