Article pubs.acs.org/IC
Neutral Mononuclear Copper(I) Complexes: Synthesis, Crystal Structures, and Photophysical Properties Yinghui Sun,†,‡ Vincent Lemaur,§ Juan I. Beltrán,§ Jérôme Cornil,§ Jianwen Huang,† Juntong Zhu,† Yun Wang,† Roland Fröhlich,∥ Haibo Wang,† Lin Jiang,*,‡ and Guifu Zou*,† †
College of Physics, Optoelectronics and Energy, Institute of Chemical Power Sources, and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, 215006, China ‡ Institute of Functional Nano and Soft Materials, Soochow University, Suzhou, Jiangsu 215123, China § Service de Chimie des Matériaux Nouveaux, Université de Mons, Place du Parc 20, 7000 Mons, Belgium ∥ Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, 48149 Münster, Germany S Supporting Information *
ABSTRACT: Neutral green-emitting four-coordinate Cu(I) complexes with general formula POPCu(NN), where POP = bis[2-(diphenylphosphino)phenyl]ether and NN = substituted 2-pyridine-1,2,4-triazole ligands, were synthesized. The crystal structures of (POPCuMeCN)+(PF6)− (1), POPCuPhPtp (2a, PhPtp = 2-(5-phenyl-2H-[1,2,4]triazol-3-yl)-pyridine), and POPCu(3,5-2FPhPtp) (2d, 3,5-2FPhPtp = 2-(5-(3,5-difluorophenyl)-2H-1,2,4-triazol-3-yl)pyridine) were determined by single-crystal X-ray diffraction analysis. The electronic and photophysical properties of the complexes were examined by UV−vis, steady-state, and time-resolved spectroscopy. At room temperature, weak emission was observed in solution, while in the solid state, all complexes exhibit intense green emission with quantum yield up to 0.54. The electronic and photophysical properties were further supported by calculation performed at the (time-dependent) density functional theory level. One of the complexes was also tested as dopant in electroluminescent devices.
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INTRODUCTION In recent years, extensive research efforts have been dedicated to phosphorescent Cu(I) complexes based on diphosphine ligands due to their potential applications in solar energy conversion,1 luminescence-based sensors,2,3 organic lightemitting diodes (OLEDs),4−8 and probes of biological systems.9,10 Cu(I) complexes have several advantages versus the more conventional Ir(III), Os(II), and Pt(II) complexes extensively used as emitting materials in phosphorescent OLEDs,11−13 such as being abundant, cheap, and environmentally friendly. Although a number of cationic Cu(I) complexes chelated with bis[2-(diphenylphosphino)phenyl]ether (POP) and phenanthroline derivatives, and associated with negative counteranions such as BF4− and PF6−, have shown remarkable electroluminescent performance in devices, 4,5,14−23 especially in light-emitting electrochemical cells,24−27 the presence of the counteranion causes some problems for practical applications, such as the unsuitability for sublimation and vapor deposition processing.28 Consequently, charge-neutral phosphorescent Cu(I) complexes without counteranions are expected to enhance OLED device properties. Recently, some neutral and luminescent Cu(I) complexes have already been studied. Omary’s group reported a dinuclear and trimeric Cu(I) complexes exhibiting multicolor bright © XXXX American Chemical Society
phosphorescent emissions both in the solid state and in solution.29−31 Volger’s group also developed a type of CuI(tripod)X [tripod = 1,1,1-tris(diphenylphosphanylmethyl)ethane; X− = Br−, I−, PhS−, PhCC−] complexes which are phosphorescent in solution and in the solid state (λmax ≈ 465 nm).32 However, the reported quantum yields were rather low. Brase, Tsuboyama, and co-workers reported halogen-bridged dinuclear complexes [Cu(μ-X)dppb]2 (X = I, Br, Cl, and dppb = 1,2-bis[diphenylphosphino]benzene) with intense blue-green photoluminescence with lifetimes in the microsecond range and high quantum yield in the solid state (λmax = 492−533 nm; Φ = 0.6−0.8).21,33,34 Peter’s group also reported a neutral mononuclear Cu(I) complex supported by PNP ligand set that exhibited an unusually high quantum efficiency in the range of 0.16−0.70 and lifetimes as long as 150 ms in benzene solution;35 it was thermally stable and has been incorporated in OLEDs with high maximum external quantum efficiency of 16.1%.36 Interestingly, neutral three-coordinate Cu(I) complexes have been reported by Thompson’s and Osawa’s groups.37−40 Neutral mononuclear Cu(I) complexes containing phosphine mixed ligands were synthesized by treating their ionic counterparts by a base and used as dopant in OLEDs.41 Received: January 16, 2016
A
DOI: 10.1021/acs.inorgchem.6b00101 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Cu(I) complexes were also characterized by mass spectroscopy (HRMS, ESI). For 2a, the expected molecular ion peaks are observed at m/z 823.1811 and 823.2 ([M + H]+), calculated for C49H37CuN4OP2 822.1739, respectively. This further confirms the formation of neutral Cu(I) complexes. Similar results are observed for complexes 2b−2e, see the Experimental Section. The structures were solved by X-ray crystallographic analyses, and ORTEP drawings of the complexes are shown in Figure 1. Selected parameters of the molecular structures are given in Table 1, and all other data are available in the Supporting Information in CIF format.
Aiming at further widening the family of neutral Cu(I) complexes, herein we report an entire class of soluble Cu(I) complexes with intense green emission and high emission quantum yields in solid state at room temperature The complexes have been fully characterized using steady-state and time-resolved spectroscopy, and for some of them, crystals have been obtained allowing a full crystallographic analysis. The experimental spectroscopic data have been further rationalized by means of quantum-chemical calculations. We have also performed some preliminary studies by inserting them in polymeric matrixes for the fabrication of electroluminescent devices.
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Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) for Complexes 2a and 2d
RESULTS AND DISCUSSION The molecular structures of the Cu(I) complexes are depicted in Chart 1. The Cu(I) complexes were easily obtained in high
Cu(1A)−N(8A) Cu(1A)−N(1A) Cu(1A)−P(2A) Cu(1A)−P(1A) Cu−O N(8A)−Cu(1A)−N(1A) N(8A)−Cu(1A)−P(2A) N(1A)−Cu(1A)−P(2A) N(8A)−Cu(1A)−P(1A) N(1A)−Cu(1A)−P(1A) P(2A)−Cu(1A)−P(1A)
Chart 1. General Chemical Structures of POPCu(NN) Complexes 2a−2e
2a
2d
2.028(5) 2.085(5) 2.2361(18) 2.239(2) 3.163 81.3(2) 117.11(15) 114.29(14) 116.86(15) 109.71(15) 113.36(7)
2.038(2) 2.104(2) 2.2398(7) 2.2659(7) 3.192 80.14(8) 125.40(6) 114.84(6) 113.57(6) 109.55(6) 109.56(3)
The structure of precursor 1 points to a trigonal-planar coordination of Cu(I), similar to the ref 43, and not the commonly reported tetrahedral structure.45 P−Cu bonds lengths are 2.2476(10) and 2.2580(10) Å, while the P−Cu− P angle is 117.79(4)°, and the Cu−N bond length involving the CH3CN ligand is 1.930(4) Å (Table S2). The structure is similar to those of already reported monomeric threecoordinate Cu(I) compounds.38,46,47 The Cu−P and Cu−N distances of complexes of 2a and 2d range from 2.24 to 2.27 Å and 2.03−2.10 Å, respectively, which are comparable to those Cu(I) complexes containing diphosphine and diimine chelate ligands.16,48,49 The dihedral angles between the N−Cu−N and P−Cu−P planes for 2a and 2d are 87.99° and 88.41°, respectively. It reveals a distorted tetrahedral geometry around the Cu(I) ion for all structures.41,48,50,51 It can be noted that the POP ligand is bound to the metal only through its pair of phosphine donor atoms, the ether oxygen atom lying at a nonbonding distance from the
yields via a simple two-step reaction. Reaction of stoichiometric quantities of POP and [Cu(CH3CN)4PF6]42 gives ionic complexes (POPCuMeCN)+(PF6)− (1) first in fresh distilled THF solution under N2 at room temperature.43 Then, the solution of corresponding NN (substituted 2-pyridine-1,2,4triazole) ligands44 (presented in the Experimental Section) in THF was dropwise added and the mixture was stirred for 4 h. Signals in the 1H and 31P NMR spectra were characterized in solution by 1D NMR methods and display resolved signals of the POP and NN ligand protons, which are consistent with the formation of the Cu(I) complexes, see the Experimental Section and Figures S1−S6. The 31P NMR spectra of 2a−2f in CD2Cl2 at room temperature are shown in Figure S6; only one single resonance is in the range of −13.87 to −12.49 ppm, which suggests that the two P atoms of the POP ligand are equivalent for all of the complexes, and there is no evidence for the presence of PF6− from the precursor of 1. The synthesized
Figure 1. ORTEP drawing (50% probability ellipsoids) of 1, 2a, and 2d complexes where H atoms are omitted for clarity. B
DOI: 10.1021/acs.inorgchem.6b00101 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Cu(I) center (>3.2 Å). The geometry in the ground state of compounds 2a−2e has been also optimized at the density functional theory (DFT) level, and some important bond lengths and bond angles have been compared to those provided by the X-ray structures. Table S3 shows that the calculated bond lengths are systematically slightly overestimated (by maximum of 7%). The largest differences are found for the Cu−P distances, as already observed for other copper complexes.52 The dihedral angles between the N−Cu−N and P−Cu−P planes are estimated to be 86.49° and 86.62° for 2a and 2d, respectively, in good agreement with the distorted tetrahedral geometry found experimentally. Most of the geometric parameters involving the copper atom are not strongly modified when going from the optimized singlet state to the optimized triplet state. The major fluctuations are located on the Cu−N1 bond length that decreases by ∼0.08 Å and on the P1−Cu−P2 and N1−Cu−P2 (or N1−Cu−P1) angles that decreases or increases by ∼1.6°, respectively (see Table S4). On the basis of the optimized geometries, we analyzed the energy and nature of the frontier electronic levels in the ground state to shed light on the nature of the excited states in the absorption spectra, see Table 2 (and more details in Table S5). Table 2. DFT-Calculated Energy (in eV) of the HOMO and LUMO Levels in Complexes 2a−2e as Well as Their Energy Difference (ΔEH−L) HOMO LUMO ΔEH−L
2a
2b
2c
2d
2e
−4.81 −1.00 3.81
−4.84 −1.04 3.80
−5.04 −0.98 4.06
−5.07 −1.11 3.96
−5.26 −1.13 4.13
As qualitatively expected, the results show that the frontier electronic levels are stabilized upon addition of fluorine atoms due to their acceptor character. The impact is more pronounced for the highest occupied molecular orbital (HOMO) than for the lowest unoccupied molecular orbital (LUMO) (with a shift of 0.45 and 0.13 eV, respectively, going from 2a to 2e for the LUMO). The HOMO level is mainly localized on the Phptp ligand, especially on the triazole and the attached phenyl group (see Figure 2). The contributions on the copper atom and POP ligand are small and increase with the number of fluorine atoms in the HOMO. That the contribution of the copper atom to the HOMO level is growing when adding fluorine atoms can be easily understood by the fact that the highest occupied d orbitals of copper are located at lower energy than the HOMO level of the complexes. Since adding fluorine atoms stabilizes the frontier electronic levels, the HOMO and copper-centered electronic levels get closer in energy, leading to an increase in the mixture of their character. However, this increase in the localization on the copper atom is not linear with the number of fluorine atoms as a result of the change in the torsion angle between the triazole ring and the fluorophenyl ring. For example, 2c and 2e have a larger contribution on the copper atom because the fluorophenyl ring is not planar with respect to the triazole ring due to the presence of the fluorine atoms in positions 2 and 6. The twist angle reduces the delocalization over the fluorophenyl group, so that the HOMO level is more localized on the other part of the Phptp ligand, and in particular on the copper atom. In all cases, the HOMO − 1 to HOMO − 3 are copper-centered levels exhibiting also a large contribution on the POP ligand. Deeper
Figure 2. Representation of the optimized molecular geometry of complexes 2a−2e (center) and the localization of HOMO (left) and LUMO (right) orbitals in the ground state.
occupied levels are localized either on the POP, on Phptp ligand, or on both. This localization of the occupied orbitals, in particular the d orbitals of the copper, contrasts with previous studies on copper complexes.52 Here, the HOMO level is not described by a large amount of copper d orbitals. This points to significant changes in the electronic properties between neutral and charged copper complexes. The LUMO level is mainly localized on the Phptp ligand, in particular on the pyridine ring. This explains why adding fluorine atoms has a much smaller impact on the energy of the LUMO level compared to the HOMO level. For 2a, the contribution on the copper atom is only 6% and 25% on the POP ligand. While the weight on the copper atom remains constant for all complexes, the contribution on the POP ligand decreases with an increase in the number of fluorine atoms. Except for LUMO + 5 which is localized on the pyridine ring of Phptp, all the higher-lying unoccupied levels listed in Table S5 are mainly localized on the POP ligand. The photophysical data of all the investigated Cu(I) complexes in aerated and deaerated CH2Cl2 solutions and at 77 K in butyronitrile matrix, the solid-state powder [poly(methyl methacrylate) (PMMA) films], are summarized in C
DOI: 10.1021/acs.inorgchem.6b00101 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 3. Photophysical Data for the Studied Cu(I) Complexes aerated λabs (nm) (ε)a,b 2a 2b 2c 2d 2e
272 271 285 281 274
(2.39) (2.75) (2.92) (2.02) (1.98)
deaerated
77 K
solid state
λem (nm)a,c
τ (μs)
Φa,d
λem (nm)a
τ (μs)
Φa,d
λem (nm)b,e
τ (ms)
λem (nm)c
555 552 574 565 585
0.16 0.11 0.10 0.18 0.11
0.0047 0.0010 0.0046 0.0026 0.0010
555 548 550 562 570
4.9 17.1 2.0 8.7 7.7
0.0536 0.0216 0.0123 0.0454 0.01
510 529 526 527 532
0.23 0.21 0.19 0.18 0.19
531 524 513 526 512
2.7 1.3 1.5 2.6 4.6
τ (μs) (rel wt)
Φc,f
(15%), (20%), (21%), (17%), (25%),
0.17 0.24 0.23 0.54 0.24
10.0 (85%) 6.9 (80%) 7.1 (79%) 6.8 (83%) 12.6 (75%)
In CH2Cl2 solution. bMolar absorptivity (× 104 M−1 cm−1) given in parentheses. cλex = 366 nm. dQuantum yields are measured vs fac-Ir(ppy)3 (Φ = 97%) (ref 58) in deaerated CH2Cl2 solution. eIn butyronitrile solution. fQuantum yields in solid state are measured with an integrating sphere, Hamamatsu C9920. a
Figure 3. Absorption spectra of Cu(I) complexes 2a (), 2b (− − −), and 2e (···) in aerated CH2Cl2 solution at room temperature. ([C] ∼ 1 × 10−5 M.) Inset: emission spectra of 2a (), 2b (− − −), and 2e (···) in the solid state (λex = 366 nm).
Figure 4. TD-DFT calculated absorption spectra of 2a (), 2b (− − −), and 2e (···).
localized on the POP ligand upon fluorination. Despite the localization of both frontier orbitals on the same ligand, the intensity of the peak is weak, as observed experimentally, since the HOMO and LUMO levels are localized on different parts of Phptp (see Table S6). The calculated second absorption peaks range between 300 and 325 nm, in good agreement with the experimental results. Here, the main character of the band is MLCT which is consistent with the fact that contributions of d orbitals of copper are found in HOMO − 1, HOMO − 2, and HOMO − 3. The next two main important contributions of the bands are LLCT and LC in character in increasing order, except for complex 2e for which it is LC and LLCT. In all complexes, the major LC contribution involves the Phptp ligand while the LLCT contribution arises from POP to Phptp for complexes 2a and 2b and from Phptp to POP for complexes 2c−2e. The third absorption bands are calculated between 270 and 280 nm, once again in very good quantitative agreement with the experimental data. For complexes 2a and 2b, the bands involve LC transitions located on both ligand and a MLCT contribution. For complex 2c, an LLCT contribution (from Phptp to POP) is also involved and its percentage increases with the number of fluorine atoms. For complexes 2d and 2e, the LC contributions are not anymore dominant, explaining why the intensity of the third peak is weaker for these complexes. The emission spectra of 2a, 2b, and 2e in deaerated CH2Cl2 at 298 K and in butyronitrile matrixes at 77 K are depicted in Figure 5, while the inset in Figure 5 shows emission spectra in the solid state measured at 298 K. The spectra are broad without vibronic progressions, suggesting that the emissive excited states have a dominant charge-transfer character.16,52,53 In deaerated CH2Cl2, the λmax values of complexes 2a, 2b, and 2e are in the order 2a ≈ 2b < 2e which is consistent with the
Table 3. The electronic absorption spectra of the Cu(I) complexes in CH2Cl2 are depicted in Figure 3 and Figure S7. In analogy to previous works,34,48 the intense absorption bands located around 280 nm (ε = 19 800−29 200 M−1 cm−1) are assigned to the π−π* transitions involving the coordinated ligands. Broad shoulders (6800−20 000 M−1 cm−1) at ∼315 nm and weak bands (∼2000 M−1 cm−1) at ∼360 nm are tentatively attributed to metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT) transitions. It is known that the MLCT absorption bands of Cu(I) complexes containing a POP unit occur at higher energy compared to that of similar heteroleptic Cu(I) complexes containing 1,10phenanthroline and sterically crowded diphosphine ligands, due to the flattening distortion.41,48,52−55 In order to get a better insight into the optical properties of 2a−2e, we have calculated, at the time-dependent (TD) DFT level, the nature of the excited states contributing to the experimental absorption bands (see Figure 4, Figure S8 and Table S6). Our simulations reproduce nicely the position of the three measured absorption peaks. The low-energy absorption bands are predicted between 349 and 377 nm. For all the complexes, these bands involve HOMO to LUMO transitions mainly with a ligand-centered (LC) character. This contrasts with the typical assignment reported in literatures for Cu(I) complexes34,48,56,57 and originates from the fact that the HOMO and LUMO levels have a small weight on the copper atom according to our calculations. The time-dependent and LLCT character of these bands is quite small but is increasing for the former upon fluorination of the ligand since the contribution of the copper atom complexes to the HOMO level increases going from for the 2c, 2d, and 2e. In contrast, the LLCT character decreases since the LUMO level gets less D
DOI: 10.1021/acs.inorgchem.6b00101 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 5. Emission spectra of complexes 2a (), 2b (■), and 2e (○) in deaerated CH2Cl2 (λex = 366 nm) at room temperature. Inset: emission spectra of complexes 2a (), 2b (■), and 2e (○) in butyronitrile matrix at 77 K (λex = 366 nm).
Figure 6. TD-DFT representation of the character of the triplet emitting excited state of all copper complexes together with its wavelength (in nm). The red/blue (yellow/green) isosurfaces are generated by the Jmol program (http://www.jmol.org) combining for each atom the LCAO coefficients in all the occupied (unoccupied) molecular orbitals involved in the TD-DFT description of the triplet excited state and their CI contributions, as calculated in the geometry of the lowest triplet excited state (refs 59 and 60).
decrease in the number of electron-withdrawing fluorine groups. The complexes show luminescence in CH2Cl2 solutions at room temperature with low quantum yields (
