Fabian
Brunner
a,
Azin
Babaei
b,
Antonio
Pertegás
b,
José M.
Junquera-Hernández
b,
Alessandro
Prescimone
a,
Edwin C.
Constable
a,
Henk J.
Bolink
b,
Michele
Sessolo
*b,
Enrique
Ortí
*b and
Catherine E.
Housecroft
*a
aDepartment of Chemistry, University of Basel, Spitalstrasse 51, CH-4056 Basel, Switzerland. E-mail: catherine.housecroft@unibas.ch
bInstituto de Ciencia Molecular, Universidad de Valencia, 46980 Paterna, Valencia, Spain. E-mail: enrique.orti@uv.es; michele.sessolo@uv.es
First published on 8th November 2018
The synthesis and characterization of five [Cu(P^P)(N^N)][PF6] complexes in which P^P = 2,7-bis(tert-butyl)-4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (tBu2xantphos) or the chiral 4,5-bis(mesitylphenylphosphino)-9,9-dimethylxanthene (xantphosMes2) and N^N = 2,2′-bipyridine (bpy), 6-methyl-2,2′-bipyridine (6-Mebpy) or 6,6′-dimethyl-2,2′-bipyridine (6,6′-Me2bpy) are reported. Single crystal structures of four of the compounds confirm that the copper(I) centre is in a distorted tetrahedral environment. In [Cu(xantphosMes2)(6-Mebpy)][PF6], the 6-Mebpy unit is disordered over two equally populated orientations and this disorder parallels a combination of two dynamic processes which we propose for [Cu(xantphosMes2)(N^N)]+ cations in solution. Density functional theory (DFT) calculations reveal that the energy difference between the two conformers observed in the solid-state structure of [Cu(xantphosMes2)(6-Mebpy)][PF6] differ in energy by only 0.28 kcal mol−1. Upon excitation into the MLCT region (λexc = 365 nm), the [Cu(P^P)(N^N)][PF6] compounds are yellow to orange emitters. Increasing the number of Me groups in the bpy unit shifts the emission to higher energies, and moves the Cu+/Cu2+ oxidation to higher potentials. Photoluminescence quantum yields (PLQYs) of the compounds are low in solution, but in the solid state PLQYs of up to 59% (for [Cu(tBu2xantphos)(6,6′-Me2bpy)]+) are observed. Increased excited-state lifetimes at low temperature are consistent with the complexes exhibiting thermally activated delayed fluorescence (TADF). This is supported by the small energy difference calculated between the lowest-energy singlet and triplet excited states (0.17–0.25 eV). The compounds were tested in simple bilayer light-emitting electrochemical cells (LECs). The optoelectronic performances of complexes containing xantphosMes2 were generally lower with respect to those with tBu2xantphos, which led to bright and efficient devices. The best performing LECs were obtained for the complex [Cu(tBu2xantphos)(6,6′-Me2bpy)][PF6] due to the increased steric hindrance at the N^N ligand, resulting in higher PLQY.
[Cu(P^P)(N^N)]+ complexes in which P^P is bis(2-(diphenylphosphino)phenyl)ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) (Scheme 1) and N^N is a bpy or phen ligand have been studied systematically in an effort to enhance their photoluminescent properties and device performances. We have investigated the influence of introducing alkyl and phenyl substituents into the 6- and 6,6′-positions of bpy in [Cu(P^P)(bpy)]+ complexes, and demonstrated improved emission and device performance with LECs containing [Cu(xantphos)(Mebpy)][PF6], [Cu(xantphos)(Etbpy)][PF6] and [Cu(POP)(Etbpy)][PF6] having lifetimes longer than 15, 40 and 80 h, respectively.25,26 Weber et al. have studied the influence of substituents in the 4,4′-positions of bpy in [Cu(xantphos)(4,4′-R2bpy)][BF4] on their photo- and electroluminescent properties and found a direct correlation between the σ-Hammett parameters of the R groups in 4,4′-R2bpy and the device efficiency.27 Most attention has focused on the structural and electronic modification of the N^N ligand in [Cu(P^P)(N^N)]+ to tune the emission properties of the complexes and their LEC behaviour.9,24,28–35
For the P^P ligand in [Cu(P^P)(N^N)]+ complexes, the commercially available POP and xantphos are common choices, although there is some interest in complexes incorporating 1,2-bis(diphenylphosphino)benzene36 and 9,9-dimethyl-4,5-bis(di-tert-butylphosphino)xanthene.37 In the latter case, it has been observed that the photoluminescence (PL) of [Cu(P^P)(N^N)]+ species is significantly reduced when the phenyl groups in xantphos (Scheme 1) are replaced by tert-butyl substituents, attributed to vibrational quenching effects.37 Thus, for good photophysical properties it appears crucial to retain aryl substituents on the phosphorus atoms in ligands related to xantphos or POP.
Here, we report on the effects of modifying the xantphos ligand and investigate the influence on both the structural and electronic properties. It has previously been reported that in [Cu(POP)(N^N)]+ and [Cu(xantphos)(N^N)]+ complexes, the HOMO is mainly centred on copper with small contributions from phosphorus, while the LUMO is localized on the N^N ligand.24,26 Thus, structural modifications made to the P^P ligand will have little if any effect on the energy level of the HOMO and any structural tuning should lead primarily to changes in steric effects. We have introduced tert-butyl groups into the 2,7-positions of xantphos to give tBu2xantphos (Scheme 1); tBu2xantphos has previously been used in combination with palladium(II) in vinylarene hydroamination catalysis.38 The introduction of peripheral tert-butyl groups was expected to additionally result in a larger spatial separation of complex ions in the active layer in a LEC and therefore have an influence on the electroluminescent properties. The other P^P ligands investigated in this work were 4,5-bis(dimesitylphosphino)-9,9-dimethylxanthene (xantphosMes4) and 4,5-bis(mesitylphenylphosphino)-9,9-dimethylxanthene (xantphosMes2), in which the PPh2 groups in xantphos were replaced by either PMes2 or PPhMes units (Scheme 1), thereby tuning the Tolman cone angle39 of the phosphane. The N^N ligands chosen for the investigation were bpy, 6-Mebpy and 6,6′-Me2bpy (Scheme 1). The photophysical properties of [Cu(xantphos)(N^N)][PF6] (N^N = bpy, 6-Mebpy and 6,6′-Me2bpy) have been previously reported,24–26 giving a benchmark series for the present investigation.
Scheme 2 Syntheses of xantphosMes4 and xantphosMes2. Conditions: (i) nBuLi, dry heptane, TMEDA, reflux, 20 min; (ii) Mes2PCl, THF, 0 °C, 1 h; (iii) MesPhPCl, THF, 0 °C, 1 h. |
Fig. 1 Crystallographically determined structure of the (S,S)-enantiomer of xantphosMes2. See also Fig. S3.† |
The 31P NMR spectra of xantphosMes4 and xantphosMes2 exhibit resonances at δ −36.2 and −25.8 ppm, respectively, consistent with one phosphorus environment in each compound. 1H and 13C NMR spectra (see Experimental section in the ESI†) were assigned by 2D methods and were in accord with functionalization in the 4,5-positions of the xanthene unit (Scheme 2). The 1H NMR spectra are shown in Fig. S4 and S5.† The 1H NMR spectrum of xantphosMes2 (Fig. S5†) also shows the presence of a subspecies in solution, present in <10% based on integration. The chemical shifts of the low intensity signals and the presence of diagnostic NOESY peaks suggest the major and minor species are structurally related, and we assign them to the (rac)- and (meso)-forms, respectively. Based on the preference seen in the solid-state, we propose that the dominant species is the (rac)-form. Thus, the bisphosphane is preorganized to give particular diastereoisomers upon complexation with copper(I) and this indeed is the case as discussed later.
Heteroleptic [Cu(P^P)(N^N)]+ complexes with P^P = tBu2xantphos or xantphosMes2 were prepared using the established procedure24 by the addition of a mixture of the xantphos and bpy ligands to a solution of [Cu(MeCN)4][PF6] in CH2Cl2. This procedure avoids competitive formation of homoleptic [Cu(P^P)2]+ complexes.63 Homoleptic [Cu(N^N)2]+ complexes where N^N is a bpy derivative, and heteroleptic [Cu(xantphos)(N^N)]+ species are typically red and yellow respectively. In all cases, the reaction mixture remained orange after being stirred for 1–2 hours, suggesting incomplete conversion to the heteroleptic complex. To force full conversion, an additional 0.2 equivalents of the bisphosphane ligand were added, resulting in the solution turning yellow. After being stirred for a further 2.5 hours, the solvent was removed and the excess ligand was removed by washing with Et2O. This procedure yielded [Cu(tBu2xantphos)(bpy)][PF6], [Cu(tBu2xantphos)(6-Mebpy)][PF6], [Cu(tBu2xantphos)(6,6′-Me2bpy)][PF6], [Cu(xantphosMes2)(bpy)][PF6] and [Cu(xantphosMes2)(6-Mebpy)][PF6] as yellow powders. In the reaction between [Cu(MeCN)4][PF6], xantphosMes2 and 6,6′-Me2bpy, the yellow colour corresponding to a heteroleptic complex was not observed and only a red solid, identified by NMR spectroscopy and mass spectrometry as [Cu(6,6′-Me2bpy)2][PF6],64 could be isolated. We suggest that the steric demands of the substituents in 6,6′-Me2bpy combined with the two mesityl groups in xantphosMes2 militate against the formation of [Cu(xantphosMes2)(6,6′-Me2bpy)][PF6].
The five heteroleptic compounds were characterized by 1H, 13C and 31P NMR spectroscopies, elemental analysis, ESI mass spectrometry and IR spectroscopy (see Fig. S6–S10†), as well as representative single crystal structures. The ESI mass spectrum of each complex containing tBu2xantphos exhibited a base peak corresponding to the [M–PF6]+ ion. The ESI mass spectrum of [Cu(xantphosMes2)(bpy)][PF6] showed a base peak envelope at m/z 725.4 assigned to [Cu(xantphosMes2)]+ and a lower intensity peak envelope at m/z 881.5 arising from [M − PF6]+. For [Cu(xantphosMes2)(6-Mebpy)][PF6], the ESI mass spectrum exhibited only a peak envelope for the [Cu(xantphosMes2)]+ ion (m/z 725.4). Elemental analysis and the NMR spectra confirmed the formation of a heteroleptic complex. The solution NMR spectroscopic properties are discussed after the solid-state structures.
Complex cation | P–Cu–P chelating angle/° | N–Cu–N chelating angle/° | Angle between P–Cu–P and N–Cu–N planes/° | N–C–C–N torsion angle/° |
---|---|---|---|---|
a Values for the conformer with 6-Mebpy oriented with the 6-Me group away from the xanthene bowl as in Fig. 2b. b Values for the conformer with 6-Mebpy oriented with the 6-Me group lying over the xanthene bowl. | ||||
[Cu(tBu2xantphos)(bpy)]+ | 113.56(2) | 79.90(7) | 84.1 | −0.4(3) |
[Cu(tBu2xantphos)(6-Mebpy)]+ | 112.70(7) | 80.1(2) | 89.3 | 2.5(8) |
[Cu(xantphosMes2)(bpy)]+ | 115.96(2) | 79.08(6) | 86.1 | 15.0(2) |
[Cu(xantphosMes2)(6-Mebpy)]+ | 115.25(3) | 78.6(1)a | 89.3a | −23.3(6)a |
79.4(1)b | 89.0b | −22.5(6)b |
On going from [Cu(P^P)(bpy)]+ to [Cu(P^P)(6-Mebpy)]+, the introduction of the 6-methyl group in the bpy unit lowers the symmetry of the cation. The methyl substituent can, in principle, lie over the xanthene ‘bowl’ or be remote from it.26Fig. 2a shows that in [Cu(tBu2xantphos)(6-Mebpy)][PF6], the methyl group lies over the xanthene unit. In contrast, in [Cu(xantphosMes2)(6-Mebpy)][PF6], the 6-Mebpy ligand is orientationally disordered over two sites, each with 50% occupancy. Fig. 2b depicts the conformer in which the Me group is remote from the xanthene unit; the second conformer is structurally related to [Cu(tBu2xantphos)(6-Mebpy)]+ (Fig. 2a). The N–C–C–N torsion angles in Table 1 demonstrate that the bpy unit is significantly more twisted in the cations containing the xantphosMes2 ligand than those with tBu2xantphos. This appears to be associated with the fact that in both [Cu(xantphosMes2)(bpy)][PF6] (Fig. 3) and [Cu(xantphosMes2)(6-Mebpy)][PF6] one methyl group of one mesityl substituent is directed towards the middle of the bpy domain (Fig. 3b). This spatial proximity is characterized by CMe(Mes)⋯centroidpyridine distances of 3.98 and 4.37 Å (HMe(Mes)⋯centroidpyridine = 3.16 and 3.56 Å) in the [Cu(xantphosMes2)(bpy)]+ cation. Corresponding separations in [Cu(xantphosMes2)(6-Mebpy)]+ are 4.00 and 4.38 Å (3.24 and 3.57 Å) and 3.85 and 4.39 Å (3.27 and 3.48 Å) for the two partial occupancy 6-Mebpy sites.
Fig. 4 Aromatic region of the 1H NMR spectrum (500 MHz, acetone-d6) of [Cu(tBu2xantphos)(bpy)][PF6]. See Fig. S15† for the full spectrum. See Scheme 3 for atom labelling. |
On going from [Cu(tBu2xantphos)(bpy)][PF6] to [Cu(tBu2xantphos)(6-Mebpy)][PF6], the symmetry of the cation is lowered and phenyl rings D (see Scheme 3) split into two sets, those proximate to the methyl group of 6-Mebpy and those on the side of the unsubstituted pyridine ring (Fig. 2a). Fig. 6 shows the aromatic region of the solution 1H NMR spectrum of [Cu(tBu2xantphos)(6-Mebpy)][PF6], in which the sets of D rings are labelled D and D′. In the NOESY spectrum at 298 K, exchange (EXSY) peaks are observed between pairs of signals for protons D2/D2′ and D3/D3′; the D4/D4′ EXSY peaks appear too close to the diagonal in the NOESY spectrum to be cleary resolved. NOESY cross peaks (no EXSY) are observed between MeCq1 and MeCq1′ (Fig. S18†). These observations are consistent with inversion of the chelate ring (‘copper flip’ in Fig. 5) and no inversion of the xanthene bowl.
Scheme 3 Structures of [Cu(tBu2xantphos)(6-Mebpy)]+ and [Cu(xantphosMes2)(6-Mebpy)]+ with labelling for NMR spectroscopic assignments. |
Fig. 6 Aromatic region of the 1H NMR spectrum (500 MHz, acetone-d6) of [Cu(tBu2xantphos)(6-Mebpy)][PF6]. See Scheme 3 for atom labelling. |
The single crystal structures of [Cu(xantphosMes2)(bpy)][PF6] and [Cu(xantphosMes2)(6-Mebpy)][PF6] reveal that the two PPhMes groups of the xantphosMes2 ligand are mutually oriented as shown in Fig. 3a and 5. This desymmetrizes the xanthene unit (labelled rings C and C′). In addition, the equatorial and axial positions of the Ph and Mes substituents with respect to the chelate ring leads to chemical shift differences in the 1H NMR spectrum for pairs of phenyl rings (D and D′) and mesityl groups (E and E′). Fig. S19† shows the 1H NMR spectrum of [Cu(xantphosMes2)(6-Mebpy)][PF6], and Fig. 7 and S20† show exchange peaks observed in the NOESY spectrum. Exchange peaks between the signals for phenyl proton D2/D2′ and D4/D4′ is consistent with the ‘copper flip’ shown in Fig. 5. This leads to equivalence of the outer rings of the xanthene unit as confirmed by the EXSY peak between the signals for protons C3/C3′. The EXSY peak between signals for the xanthene methyls MeCq1 and MeCq1′ (Fig. S20†) confirms the inversion of the xanthene bowl (Fig. 5). This contrasts with [Cu(tBu2xantphos)(6-Mebpy)][PF6] where no exchange (only NOESY) peaks are observed (see above and Fig. S18†).
Fig. 7 Part of the NOESY spectrum (500 MHz, acetone-d6) of [Cu(xantphosMes2)(6-Mebpy)][PF6] showing exchange (EXSY) peaks between pairs of protons C3 and C3′, D2 and D2′, and D4 and D4′. See also Fig. S20.† |
Earlier, we noted an orientational disorder of the 6-Mebpy ligand in the solid-state structure of [Cu(xantphosMes2)(6-Mebpy)][PF6]. The disorder was modelled with a 50% occupancy of each orientation and Fig. 5a and b show the [Cu(xantphosMes2)(6-Mebpy)]+ with the two orientations of 6-Mebpy. The structure in Fig. 5a corresponds to the top diagram in the scheme in Fig. 5, while Fig. 5b corresponds to the bottom diagram in the scheme. The disorder therefore parallels a combination of the two dynamic processes which we propose the cation undergoes in solution.
Variable temperature (VT) NMR spectra were recorded for an acetone-d6 solution of [Cu(xantphosMes2)(6-Mebpy)][PF6]. The 31P NMR spectrum (Fig. S21†) shows only one signal over the range 298–180 K. Fig. 8 shows the effect of temperature on the alkyl region of the 1H NMR spectrum of [Cu(xantphosMes2)(6-Mebpy)][PF6]. The collapse of the signals for mesityl-methyl protons MeE2 and MeE2′ and the appearance of four signals for these methyls below 218 K are consistent with freezing out the rotation of the mesityl groups. A similar temperature dependence is observed for the mesityl E3 protons in the aromatic region of the spectrum (Fig. S22†). Significant shifting of the xanthene methyl protons MeCq1 and MeCq1′ (Fig. 8) and 6-Mebpy protons A6 and MeB6 (Fig. S22†) can be attributed to changes in their magnetic environments as the mesityl groups adopt a static configuration. Both 31P and 1H VT NMR spectra are consistent with the presence of only one conformer in solution.
Fig. 8 Alkyl regions of the variable temperature 1H NMR spectra (500 MHz, acetone-d6) of [Cu(xantphosMes2)(6-Mebpy)][PF6]. See Scheme 3 for atom labelling. |
Fig. 9 Cyclic voltamogramms of [Cu(P^P)(N^N)][PF6] compounds in CH2Cl2 at a scan rate of 100 mV s−1 referenced to internal Fc/Fc+ = 0 V. |
Complex cation | E ox1/2/V (Epc − Epa/mV) | E oxpc/V | E redpa/V |
---|---|---|---|
a Values taken from ref. 24. | |||
[Cu(tBu2xantphos)(bpy)]+ | +0.76 (90) | −2.20 | |
[Cu(tBu2xantphos)(6-Mebpy)]+ | +0.83 (90) | −2.22 | |
[Cu(tBu2xantphos)(6,6′-Me2bpy)]+ | +0.85 (100) | −2.28 | |
[Cu(xantphosMes2)(bpy)]+ | +0.80ir | ||
[Cu(xantphosMes2)(6-Mebpy)]+ | +0.84 (70) | ||
[Cu(xantphos)(bpy)]+a | +0.76 (110) | ||
[Cu(xantphos)(6-Mebpy)]+a | +0.85 (100) | ||
[Cu(xantphos)(6,6′-Me2bpy)]+a | +0.90 (150) |
Along the series [Cu(tBu2xantphos)(N^N)][PF6] with N^N = bpy to 6-Mebpy to 6,6′-Me2bpy, the copper oxidation shifts to higher potentials (as observed for the analogous [Cu(xantphos)(N^N)][PF6] series, Table 224) while the reduction moves towards more negative potentials. This demonstrates an increase in the HOMO–LUMO gap as the steric demand of the bpy ligand increases. This trend was also observed for a related series of compounds and has been rationalized using DFT calculations.24
Two geometry minima were found for [Cu(xantphosMes2)(6-Mebpy)]+, which show a different relative orientation of the 6-Mebpy ligand and correspond to the two conformations observed in the single-crystal structure determination (Fig. 5). They possess close energies, the conformation with the 6-methyl group lying over the xanthene bowl being more stable that with the Me group away from the xanthene unit by only 0.28 kcal mol−1. This is in good agreement with the occupancy of 50% experimentally found for each conformation as discussed above.
The geometry of the first triplet excited state (T1) was also optimized at the UB3LYP level for all the [Cu(P^P)(N^N)]+ cations, and the most significant geometry parameters are also included in Table S1.† The molecular geometries in the T1 state significantly differ from those in the ground state S0. As discussed below, the T1 state implies a charge transfer from a molecular orbital that mainly involves a d orbital of the Cu atom to a molecular orbital spreading over the bpy ligand. Consequently, the metal atom is partially oxidized and tends to adopt the square-planar coordination sphere expected for four-coordinate d9 Cu(II) complexes, instead of the tetrahedral conformation typical of d10 Cu(I) coordination complexes. This effect can be studied by following the changes in the angle formed by the N–Cu–N and P–Cu–P planes, which decreases in going from S0 to T1 as the molecule becomes more planar (Table S1†). The distortion degree from the tetrahedral structure in going from S0 to T1 is indeed limited by number of methyl groups in the 6,6′-positions of the bpy ligand, because substituents in these positions impede the movement of the ligands towards more planar dispositions.24 In this way, the [Cu(tBu2xantphos)(bpy)]+ complex, with no substituent in the 6,6′-positions, shows the largest reduction (25.6°) passing from 82.8° in S0 to 57.2° in T1. The [Cu(tBu2xantphos)(6-Mebpy)]+ complex, including one Me group in the 6-position, undergoes a smaller reduction of 23.3° (from 87.9 to 64.6°), and the [Cu(tBu2xantphos)(6,6′-Me2bpy)]+ complex, featuring Me substituents in both the 6- and 6′-positions, shows a reduction of only 15.4° (from 86.1 to 70.7°). Thus, the presence of Me groups in the 6- and 6′-positions strongly affects the degree of geometrical relaxation of the T1 excited state, and limits its stabilization. The energy position of the T1 state relative to S0, and thereby the emission properties of the complexes, therefore depend not only on the electron-donating or electron-withdrawing character of the substituent groups present in the ligands but also on the positions where the substituents are introduced and on the structural effects they induce.
Fig. 10 shows the evolution of the energy calculated for the highest-occupied (HOMO) and lowest-unoccupied molecular orbital (LUMO) of the five complexes. The atomic orbital composition of the molecular orbitals remains almost unchanged along the series and only the contour plots computed for [Cu(tBu2xantphos)(bpy)]+ are displayed as a representative example. As reported previously for similar copper(I) complexes,24,26,66 the HOMO appears mainly centred on the metal with a small contribution from the phosphorus atoms, and the LUMO spreads over the bpy ligand. The addition of tBu groups to the xantphos moiety has a negligible effect on the energy of the HOMO, and the [Cu(tBu2xantphos)(N^N)]+ (N^N = bpy, 6-Mebpy and 6,6′-Me2bpy) complexes have a slightly higher HOMO energy, around −5.97 eV (Fig. 10), than that calculated for the reference complex [Cu(xantphos)(bpy)]+ (−6.00 eV) at the same theoretical level.66 Replacement of PPh2 moieties by PPhMes groups in the xantphos ligand has a more significant effect moving up the HOMO of [Cu(xantphosMes2)(bpy)]+ and [Cu(xantphosMes2)(6-Mebpy)]+ to −5.81 and −5.89 eV, respectively. There are small changes in this energy within each series, as the structural changes introduced in the complexes are made in regions where the HOMO is not centered. The HOMO energies, even considering the small changes described, are quite close, in good agreement with the close Eox1/2 values reported for the complexes in the Electrochemistry section. The LUMO undergoes a small destabilization when a Me group (a weak electron donor) is added to the bpy ligand, a destabilization that becomes more pronounced when a second group is added. The HOMO–LUMO energy gap increases in the [Cu(tBu2xantphos)(N^N)]+ series with the number of Me substituents of the bpy ligand in agreement with the experimental CV data (Table 2). It is therefore expected that excited states described by HOMO → LUMO transitions appear at bluer wavelengths as more Me groups are attached to the N^N ligand.
CH2Cl2 solutiona,b | Powderb | Me-THF at 77 K | Thin film | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Complex cation | UV-Vis MLCT λmax/nm | λ maxem/nm | PLQY (non-degassed/degassed)% | τ 1/2(av)a,b (non-degassed/degassed)/μs | λ maxem/nm | PLQY/% | τ 1/2(av)a,b/μs | λ maxem/nm | PLQY/% | τ 1/2(av)a,b/μs | λ maxem/nm | PLQY/% |
a Solution concentration = 2.5 × 10−5 mol dm−3 unless stated otherwise. sh = shoulder. b λ exc = 365 nm. c Solution concentration = 5.0 × 10−5 mol dm−3. d Values taken from ref. 24. e Values taken from ref. 26. f Biexponential fit using the equation τ1/2(av) = ΣAiτi/ΣAi where Ai is the pre-exponential factor for the lifetime. | ||||||||||||
[Cu(tBu2xantphos)(bpy)]+ | 384 | 652 | 0.3/0.4 | — | 584 | 3.0 | 1.95f | 597 | 7 | 27.6 | — | < 1 |
[Cu(tBu2xantphos)(6-Mebpy)]+ | 382 | 605, 628sh | 0.4/0.9 | 0.205/0.581 | 552 | 16 | 6.32f | 578 | 25 | 56.3 | 569 | 6 |
[Cu(tBu2xantphos)(6,6′-Me2bpy)]+ | 375 | 566c | 0.6/2.4c | 0.222/1.05c | 522 | 59 | 13.8f | 555 | 46 | 92.1 | 550 | 23 |
[Cu(xantphosMes2)(bpy)]+ | 385 | 655c | 0.2/0.2c | — | 589 | 1.9 | 1.19f | 594 | 11 | 20.0 | 594 | < 1 |
[Cu(xantphosMes2)(6-Mebpy)]+ | 381 | 645, 623shc | 0.3/0.4c | 0.115/0.213 | 547 | 26 | 6.62f | 587 | 19 | 19.7 | 584 | 6 |
[Cu(xantphos)(bpy)]+d | 383 | 620, 650 | 0.5/0.5 | 0.075/0.104 | 587 | 1.7 | 1.3 | 613 | — | 11 | — | — |
[Cu(xantphos)(6-Mebpy)]+e | 379 | 635, 605 | 1.0/1.8 | 0.27/0.78 | 547 | 34 | 9.6 | — | — | — | 574 | 9.7.7 |
[Cu(xantphos)(6,6′-Me2bpy)]+e | 378 | 635, 606 | 1.6/10 | 0.45/3.4 | 539 | 37 | 11 | — | — | — | 555 | 21.8 |
To get a better understanding of the nature of the electronic excited states involved in the absorption spectra, the time-dependent DFT (TD-DFT) approach was used to study the lowest lying singlet (Sn) and triplet (Tn) excited states. Table 4 collects the energies and oscillator strengths (f) computed for the S1 and T1 states of all the complexes, together with those obtained for [Cu(xantphos)(bpy)]+ included as a reference. In all cases, both the S1 and the T1 state result from the HOMO → LUMO monoexcitation, with a contribution exceeding 95%. This excitation implies an electron transfer from the Cu(P^P) moiety of the complex to the bpy ligand, supporting the MLCT character of S1 and T1. The broad absorption band observed experimentally at around 380 nm therefore originates in the S0 → S1 transition, and the calculated values, although somewhat displaced to the red, reproduce the experimentally observed shift of the absorption maxima to bluer wavelengths as the number of Me substituents of the bpy ligand increases. This is also in good accord with the increase of the HOMO–LUMO gap along each series of complexes predicted above (Fig. 10).
Complex cation | S1 | T1 |
---|---|---|
E (eV nm−1) (f) | E (eV) | |
[Cu(xantphos)(bpy)]+ | 2.816/440 (0.08) | 2.569 |
[Cu(tBu2xantphos)(bpy)]+ | 2.803/442 (0.09) | 2.554 |
[Cu(tBu2xantphos)(6-Mebpy)]+ | 2.823/439 (0.10) | 2.577 |
[Cu(tBu2xantphos)(6,6′-Me2bpy)]+ | 2.923/424 (0.07) | 2.694 |
[Cu(xantphosMes2)(bpy)]+ | 2.664/465 (0.06) | 2.470 |
[Cu(xantphosMes2)(6-Mebpy)]+ | 2.750/451 (0.06) | 2.582 |
The powder and solution emission spectra are displayed in Fig. 12 and S24,† respectively, and data are given in Table 3. Upon excitation into the MLCT region (λexc = 365 nm), all the compounds show an emission in the orange to yellow region. On going from [Cu(tBu2xantphos)(bpy)][PF6] to [Cu(tBu2xantphos)(6-Mebpy)][PF6] to [Cu(tBu2xantphos)(6,6′-Me2bpy)][PF6], or from [Cu(xantphosMes2)(bpy)][PF6] to [Cu(xantphosMes2)(6-Mebpy)][PF6], the introduction of additional methyl groups in the bpy ligand shifts the emission to higher energies (Table 3). Additionally, the photoluminescent quantum yield (PLQY) increases for solution and especially for powder emission along the series (Table 3). [Cu(tBu2xantphos)(bpy)][PF6] and [Cu(xantphosMes2)(bpy)][PF6], just like [Cu(xantphos)(bpy)][PF6],24 exhibit low PLQY values, especially in solution. This is most probably due to the accessibility of the copper centre, which leads to solvent quenching of the excited state. Consequently, PLQY values in the powder samples are higher than in solution. The emission in the solid state is in all cases blue-shifted compared to the solution emission. This trend has also been observed for related complexes containing POP or xantphos.25,26
The theoretical results reproduce the trends observed in the experimental emission spectra. The emitting T1 state shifts to higher energies as more methyl groups are added to the bpy ligand (Table 4). The [Cu(tBu2xantphos)(6,6′-Me2bpy)]+ complex is predicted to have the higher energy T1, followed by [Cu(tBu2xantphos)(6-Mebpy)]+ and [Cu(xantphosMes2)(6-Mebpy)]+, and finally by the complexes with no methyl substituent. This is in good agreement with the emission wavelengths observed in experimental spectra (Table 3). The broad and mostly unstructured shape of the emission band (Fig. S24†) also agrees with the MLCT nature predicted for the emitting HOMO → LUMO T1 state.
As discussed above, the geometry relaxation of the emitting T1 state leads to the flattening of the tetrahedral coordination environment. This flattening is more hindered as the number of methyl substituents attached to positions 6 and 6′ of the bpy ligand is increased, and the relaxation of the T1 triplet is impeded thus leading to higher emission energies. Inspection of Table 3 shows that increasing the steric hindrance of the bpy ligand is beneficial for the emissive properties. Less flattening of the tetrahedral coordination environment of the copper centre gives rise to higher emission energies and, as expected, longer excited state lifetimes and higher PLQYs. The same is true for the series of [Cu(xantphos)(N^N)][PF6] (Table 3).24,26 However, from a synthetic point of view, the unsuccessful attempt to [Cu(xantphosMes2)(6,6′-Me2bpy)][PF6] demonstrates a limitation in the combined steric properties of the xantphosMes2 and 6,6′-Me2bpy ligands.
The photophysical properties of [Cu(tBu2xantphos)(bpy)][PF6] and [Cu(xantphosMes2)(bpy)][PF6] are similar both in solution and in the solid state (Table 3). In the case of the [Cu(P^P)(6-Mebpy)][PF6] complexes, a similar behaviour is observed in the solid state emission. Both complexes are blue shifted compared to their [Cu(P^P)(bpy)][PF6] analogues but the emission profile is still very similar and the difference in peak position is again low (Δλ = 5 nm). It is notable that the shift difference when going from [Cu(P^P)(bpy)][PF6] to [Cu(P^P)(6-Mebpy)][PF6] complexes is larger in the complexes containing xantphosMes2 (Δλ = 42 nm) compared to tBu2xantphos complexes (Δλ = 32 nm). On the other hand, the solution emission profiles differ significantly for the two [Cu(P^P)(6-Mebpy)][PF6] complexes (Fig. S24†). The emission maximum of [Cu(tBu2xantphos)(6-Mebpy)] is blue shifted compared to [Cu(tBu2xantphos)(bpy)] (Δλ = 47 nm) as expected, but in the case of [Cu(xantphosMes2)(6-Mebpy)][PF6] the emission maximum is blue-shifted very little compared to [Cu(xantphosMes2)(bpy)][PF6] (Δλ = 10 nm). Both [Cu(tBu2xantphos)(6-Mebpy)][PF6] and [Cu(xantphosMes2)(6-Mebpy)][PF6] also show a second unstructured emission feature at 628 nm and 623 nm respectively, which is not observed in the solid-state emission profile.
To investigate whether the compounds showed thermally activated delayed fluorescence (TADF) at room temperature, low temperature emission spectra and excited state lifetimes were recorded in frozen Me-THF at 77 K (Table 3 and Fig. 13). All complexes show a red shift in emission of 5–40 nm compared to the solid state emission and a greatly enlarged excited state lifetime. This indicates the possibility that all complexes are TADF emitters at room temperature. The energy difference between the lowest energy singlet and triplet excited states has been calculated to lie between 0.17 and 0.25 eV (Table 4), and is small enough to allow the occurrence of TADF processes.22
Fig. 13 Normalized emission spectra of [Cu(P^P)(N^N)][PF6] complexes in Me-THF at 77 K (λexc = 410 nm). |
Low temperature data further support the idea that the position of the emission bands of the [Cu(tBu2xantphos)(N^N)] complexes is strongly affected by the flattening of the tetrahedral environment in the T1 state. In solution, this flattening is not impeded and the difference in peak position between [Cu(tBu2xantphos)(bpy)][PF6] and [Cu(tBu2xantphos)(6-Mebpy)][PF6] is 86 nm (0.30 eV) (Table 3). In powder, the flattening is more restricted and the difference decreases to 62 nm (0.25 eV). Finally, at 77 K, where the relaxation is even more impeded for all the complexes, the emission maxima range between 597 and 555 nm, in a window of just 42 nm (0.15 eV).
[Cu(P^P)(N^N)]+ | t on (min) | Lummax (cd m−2) | t 1/2 (min) | Eff. (cd A−1) |
---|---|---|---|---|
[Cu(tBu2xantphos)(bpy)]+ | 1.1 | 20 | 5.1 | 0.2 |
[Cu(tBu2xantphos)(6-Mebpy)]+ | 4.5 | 230 | 53.8 | 2.3 |
[Cu(tBu2xantphos)(6,6′-Me2bpy)]+ | 1.0 | 370 | 4.9 | 3.7 |
[Cu(xantphosMes2)(6-Mebpy)]+ | 0.7 | 50 | 34.6 | 0.5 |
In general, the device lifetime (t1/2, time to decay to one-half of the peak luminance) for complexes containing tBu2xantphos was found to be low, approximately 5 minutes in the cases of bpy and 6,6′-Me2bpy and above 50 minutes for the LECs with [Cu(tBu2xantphos)(6-Mebpy)][PF6]. The low lifetime of the complexes with bpy and 6,6′-Me2bpy might be due to a reduced stability of the materials toward charge transport, as seen from the corresponding LECs voltage profile which drastically increases after only few minutes of operation (Fig. 14a).
The optoelectronic performance of complexes containing the xantphosMes2 ligand were in general lower as compared to those involving tBu2xantphos. We could not observe any electroluminescence from [Cu(xantphosMes2)(bpy)][PF6], perhaps due to its low PLQY both in solution and in the solid state. Moderate electroluminescence was measured for [Cu(xantphosMes2)(6-Mebpy)][PF6], with fast turn-on (<1 minute) and a maximum luminance of 50 cd m−2.
The spectral shape and position of the electroluminescence (EL, Fig. 14c) signals correlate with the PL maxima observed for the complexes in solution and in the solid state. For the tBu2xantphos-containing complexes, the EL maxima blue-shift from 584 nm to 575 and 557 nm when increasing the substitution at the bpy, i.e. going from bpy to 6-Mebpy and 6,6′-Me2bpy, respectively. As highlighted in the inset of Fig. 14c, this shift corresponds to a colour variation from the orange to the green region of the CIE 1931 colour space. The EL spectrum of [Cu(xantphosMes2)(6-Mebpy)][PF6] peaks at 582 nm, in agreement with the PL signal of the thin-film (Table 3).
The [Cu(P^P)(N^N)][PF6] compounds show a broad MLCT-absorption around 380 nm which shifts to higher energies on going from bpy to 6-Mebpy to 6,6′-Me2bpy. Upon excitation into the MLCT band, the [Cu(P^P)(N^N][PF6] complexes emit in the yellow to orange region; additional Me groups in the bpy ligand result in a blue-shift in the emission. The MLCT nature of the absorption and emission is supported by DFT calculations, which associate the lowest-energy S1 and T1 excited states to the HOMO → LUMO monoexcitation implying an electron transfer from the Cu(P^P) moiety to the bpy ligand. In solution, PLQY values are low, but in the solid state, PLQYs of 26 and 59% are observed for [Cu(xantphosMes2)(6-Mebpy)][PF6] and [Cu(tBu2xantphos)(6,6′-Me2bpy)]+, respectively, compared to benchmark values of 34 and 37% for [Cu(xantphos)(6-Mebpy)][PF6] and [Cu(xantphos)(6,6′-Me2bpy)][PF6]. Increased excited state lifetimes at low temperature are consistent with the complexes being TADF emitters and this is supported by a calculated energy difference between S1 and T1 of 0.17–0.25 eV.
The compounds were tested in simple bilayer LECs. The optoelectronic performance of complexes containing the xantphosMes2 ligand were generally lower than those with tBu2xantphos, which led to bright and efficient devices. The current efficiency of the LECs follows the trend observed for the PLQY, increasing with increasing substitution at the bpy ligand. In particular, luminances as high as 370 cd m−2 were obtained for the complex [Cu(tBu2xantphos)(6,6′-Me2bpy)][PF6], which correspond to an efficiency of 3.7 cd A−1. These encouraging results suggest that tBu2xantphos is a promising ligand to develop novel and efficient copper emitters for LECs and OLEDs.
Footnote |
† Electronic supplementary information (ESI) available: Synthetic experimental details. Fig. S1, S2 and S6–S10: IR spectra of ligands and complexes. Fig. S3 and S11–S14: ORTEP-style plots of crystal structures. Fig. S4, S5 and S15–S22: additional NMR figures. Fig. S23 and S24: Solution absorption and emission spectra. Table S1: Selected structural parameters calculated at the B3LYP-D3/(def2svp + def2tzvp) level. CCDC 1844060–1844063 and 1860879. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt03827a |
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