Synthesis, Structure, and Photophysical Properties of Two Four

Article pubs.acs.org/IC

Synthesis, Structure, and Photophysical Properties of Two FourCoordinate CuI−NHC Complexes with Efficient Delayed Fluorescence Zhiqiang Wang,*,† Caijun Zheng,‡ Weizhou Wang,† Chen Xu,*,† Baoming Ji,† and Xiaohong Zhang*,‡,§ †

College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang, Henan 471022, P. R. China Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China § Functional Nano & Soft Materials Laboratory (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R. China ‡

S Supporting Information *

ABSTRACT: Two luminescent cationic heteroleptic four-coordinate CuI complexes supported by N-heterocyclic carbene ligand and diphosphine ligand were successfully prepared and characterized. These complexes adopt typical distorted tetrahedral configuration and have high stability in solid state. Quantum chemical calculations show carbene units have contributions to both highest occupied molecular orbitals and lowest unoccupied molecular orbitals of these CuI−NHC complexes, the lowest-lying singlet and triplet excitations (S0 → S1 and S0 → T1) of [Cu(Pyim)(POP)](PF6) are dominated by metal-to-ligand charge transfer (MLCT) transition, while the S0 → S1 and S0 → T1 excitations of [Cu(Qbim)(POP)](PF6) are mainly MLCT and ligand-centered transitions, respectively. These CuI−NHC complexes show efficient long-lifetime emissions (λem = 520 nm, τ = 79.8 μs, Φ = 0.56 for [Cu(Pyim)(POP)](PF6), λem = 570 nm, τ = 31.97 μs (78.99%) and 252.2 μs (21.01%), Φ = 0.35 for [Cu(Qbim)(POP)](PF6)) in solid state at room temperature, which are confirmed as delayed fluorescence by investigating the emissions at 77 K.

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phosphorescent OLEDs. In this regard, CuI complexes with efficient TADF should be promising alternative emitters for high-efficiency OLEDs. Ever since McMillin group reported the strongly luminescent complex [Cu(dbp)(POP)]+ (dbp = 2,9-di-n-butyl-1,10-phenanthroline, POP = bis[2-(diphenylphosphino)phenyl]ether),17 most studies on luminescent CuI complexes focused on the homo- and heteroleptic complexes consisting of diphosphine and diimine ligands.4,11,18−22 However, the overall performances of OLEDs based on these CuI complexes are still far inferior to those of OLEDs based on IrIII and PtII complexes until now.10,12,23−28 Undoubtedly, ligands play a key role in improving the properties of luminescent CuI complexes. Thus, it is necessary to develop luminescent CuI complexes based on new type of ligands.

INTRODUCTION Phosphorescent complexes of transition metal, such as IrIII and PtII, have attracted much attention for their application in organic light emitting diodes (OLEDs), as they can make use of both singlet and triplet excitons resulting in high electroluminescent efficiency.1−3 However, the metals in these complexes are very scarce and highly expensive. Considering the abundant resource and low cost of copper metal, the luminescent CuI complexes have been considered as emitters for OLEDs in recent years.4−7 Although the smaller spin−orbit coupling constant of copper is not enough to promote efficient phosphorescence with short lifetime, many CuI complexes have smaller band gaps between the lowest singlet state (S1) and the lowest triplet state (T1), which can lead to efficient and shortlived thermally activated delayed fluorescence (TADF) at ambient temperature.8−16 It is well-known, OLEDs based on delayed fluorescence (DF) materials can harvest both singlet and triplet excitons in electroluminescence process as well as © XXXX American Chemical Society

Received: November 4, 2015

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DOI: 10.1021/acs.inorgchem.5b02546 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Chart 1. Molecular Structures of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6)

N-heterocyclic carbenes (NHCs) have strong σ-donating ability and weak π-accepting ability, which make them widely used as ligands in transition metal complexes.29 Recently, Thompson and Gaillard groups reported a series of cationic and neutral luminescent three-coordinate CuI−NHC complexes, and these complexes exhibited moderate-to-high emission efficiency.30−34 These findings inspire us to investigate the photophysical properties of four-coordinate CuI−NHC complexes. In this paper, we reported two cationic heteroleptic four-coordinate CuI−NHC complexes [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) (Chart 1), which consist of the chelating NHC ligands and the diphosphine ligand POP, with a counterion of PF6−. Both of these complexes exhibit high-efficiency TADF in solid state at ambient temperature, and emission wavelengths show desirable tunability through modification of NHC ligand. As far as we know, this is the first report on luminescent four-coordinate Cu I−NHC complexes.

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(1H, dd, J = 3.0, 2.5 Hz), 8.63−8.65 (1H, m), 10.28 (1H, s); electrospray ionization mass spectrometry (ESI-MS, m/z) = 236.1 (MPF6−). Synthesis of 3. Compound 2 (0.76g, 2 mmol) and Ag2O (0.23g, 1 mmol) reacted in CH3CN (10 mL) at 50 °C overnight; after the solvent was evaporated with a rotary evaporator, the dinuclear AgI− NHC complex 3 was obtained as white powder (0.78 g, yield 80%). The single crystal suitable for X-ray diffraction analysis was obtained by slowly evaporating the CH3CN solution of 3. 1H NMR (500 MHz, CD3CN) δ (ppm): 8.29−8.28 (2H, d, J = 4.5 Hz), 7.95−7.91 (2H, m), 7.85 (2H, s), 7.81 (1H, s), 7.79 (1H, s), 7.44−7.40 (4H, m), 7.36− 7.32 (6H, m), 7.29−7.26 (4H, m), 5.38 (4H, s); 13C NMR (125 MHz, CD3CN) δ (ppm): 181.01, 150.68, 148.89, 139.95, 136.41, 128.95, 128.39, 127.66, 124.08, 123.07, 120.56, 115.68, 55.77; ESI-MS (m/z) = 829.5 (M-PF6−); Anal. Calcd for C30H26Ag2F12N6P2: C, 36.91; H, 2.68; N, 8.61. Found: C, 36.81; H, 2.66; N, 8.65%. Synthesis of [Cu(Pyim)(POP)](PF6). Compound 3 (0.49g, 0.5 mmol) reacted with copper powder (0.13g, 2 mmol) for 3 h in degassed anhydrous CH3CN under N2 atmosphere at room temperature, and then the ligand POP (0.27 g, 0.5 mmol) was added and reacted for 1 h. After the precipitate was filtrated, the solvent was evaporated with a rotary evaporator. The residue was dissolved in dichloromethane/ethanol solution, and product was obtained as a pale green crystal by slowly evaporating the solvent (0.65 g, yield 60%). 1H NMR (400 MHz, CD3CN) δ (ppm): 7.89−7.95 (3H, m), 7.73 (1H, s), 6.98−7.34 (31H, m), 6.71 (2H, s), 6.62 (2H, s), 5.06 (2H, s); 13C NMR (125 MHz, CDCl3) δ (ppm): 158.21, 149.91, 148.24, 140.53, 135.04, 134.41, 133.35, 132.82, 131.87, 131.66, 131.58, 130.13, 129.96, 128.97, 128.76, 128.73, 128.69, 128.23, 127.29, 125.07, 124.18, 122.80, 122.56, 120.32, 117.50, 112.33, 55.19; 31P NMR (200 MHz, CDCl3) δ (ppm): −9.00 (s), −144.2 (quint). ESIMS (m/z) = 836.8 (M-PF6−); Anal. Calcd for C51H41CuN3F6OP3: C, 62.36; H, 4.21; N, 4.28. Found: C, 62.23; H, 4.24; N, 4.31%. Synthesis of 5. A mixture of benzyl chloride (2.52 g, 20 mmol) and 2-(1H-benzo[d]imidazol-1-yl)quinoline 4 (2.45 g, 10 mmol) was refluxed at 100 °C for 2 h. The excess benzyl chloride was removed under reduced pressure, and the residue was dissolved in acetone (50 mL); then KPF6 (2.76 g, 15 mmol) was added and reacted for 3 h. After the precipitate was filtrated, the organic solvent was evaporated with a rotary evaporator; a white powder was obtained (3.3 g, yield 68%). 1H NMR (500 MHz, DMSO-d6) δ (ppm): 5.95 (2H, s), 7.45− 7.47 (3H, m), 7.68−7.83 (5H, m), 7.95−8.04 (2H, m), 8.21−8.25 (3H, m), 8.89−8.91 (2H, d, J = 10 Hz), 10.95 (1H, s); ESI-MS (m/z) = 336.1 (M-PF6−). Synthesis of [Cu(Qbim)(POP)](PF6). Compound 5 (0.96g, 2 mmol) and Ag2O (0.23g, 1 mmol) reacted in CH3CN (10 mL) at 50 °C overnight; after the solvent was evaporated with a rotary evaporator, a AgI−NHC complex was obtained as white powder (0.91g, yield 76%). At room temperature, the AgI−NHC (0.59 g, 0.5 mmol) reacted with copper powder (0.13g, 2 mmol) for 3 h in degassed anhydrous CH3CN under N2 atmosphere, then the ligand POP (0.27 g, 0.5 mmol) was added and reacted for 1 h. After the precipitate was filtrated, the solvent was evaporated with a rotary evaporator. The residue was dissolved in dichloromethane/ethanol solution, and

EXPERIMENTAL SECTION

General. All starting materials, bis[2-(diphenylphosphino)phenyl]ether, 2-bromopyridine, imidazole, 2-bromoquinoline, benzimidazole, benzyl chloride, cuprous iodide (CuI), potassium tert-butylate (KOtBu), potassium hexafluorophosphate (KPF6), silver oxide (Ag2O), and copper powder were purchased from commercial suppliers and used as received. Solvents are of analytical grade and used without further purification, except for acetonitrile, which was distilled over CaH2 and degassed by N2. 1H NMR, 13C NMR, and 31P NMR spectra were recorded on the Bruker Avance 400 spectrometer or Bruker Avance 500 spectrometer. Mass spectra were performed on an LCQ Deca Xp Max instrument or a Bruker APEX II FT-ICR instrument. Elemental analysis was performed on a Vario III elemental analyzer. Thermogravimetric analysis (TGA) measurement was performed on a TG/DTA 6300 with a heating rate of 10 °C/min under N2 atmosphere. UV−visible absorption spectra were recorded on a Hitachi U-3010 UV−vis spectrophotometer. Steady-state emission spectra, emission lifetimes, and absolute emission quantum yields were recorded on an Edinburgh FLS980 spectrometer. Cyclic voltammetry was performed on a CHI620C electrochemical analyzer; the electrolytic cell was a conventional three-electrode cell consisting of a Pt working electrode, a Pt wire counter electrode, and a Ag/AgCl reference electrode. Synthesis of 2. A mixture of benzyl chloride (2.52 g, 20 mmol) and 2-(1H-imidazol-1-yl)pyridine 1 (1.45 g, 10 mmol) was refluxed at 100 °C for 2 h. The excess benzyl chloride was removed under reduced pressure, and the residue was dissolved in acetone (50 mL); then KPF6 (2.76 g, 15 mmol) was added and reacted for 3 h. After the precipitate was filtrated, the organic solvent was evaporated with a rotary evaporator; a white powder was obtained (2.7 g, yield 71%). 1H NMR (400 MHz, deuterated dimethyl sulfoxide (DMSO-d6)) δ (ppm): 5.54 (2H, s), 7.38−7.45 (3H, m), 7.49−7.52 (2H, m), 7.63 (1H, dd, J = 6.5, 5.6 Hz), 8.05 (2H, d, J = 8.2 Hz), 8.20 (1H, dd, J = 8.5, 8.1 Hz), 8.53 B


Article

Inorganic Chemistry Scheme 1. Synthesis of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6)

Ag2O and the NHC ligand precursor 2 at 50 °C in commercially available acetonitrile solvent, and the structure of 3 was confirmed by X-ray crystallography. Then the AgI− NHC complex 3 reacted with copper powder and the ligand POP in sequence at room temperature in degassed anhydrous acetonitrile solvent to give [Cu(Pyim)(POP)](PF6). Using the NHC ligand precursor 5 as substrate, [Cu(Qbim)(POP)](PF6) can be obtained by the same method. The intermediate Ag complex was not characterized in the synthesis of [Cu(Qbim)(POP)](PF6). Although these CuI−NHC complexes could be prepared by CuI as well as copper powder using a similar method,48 we used copper powder as the source of Cu(I) ion considering its advantage of nontoxicity. The structures of CuI−NHC complexes were characterized by 1H NMR, 13C NMR, and 31 P NMR and were confirmed by X-ray crystallography. Both of these CuI complexes are indefinitely stable to air and moisture in solid state at room temperature. TGA shows that they have high thermal stability, the decomposition temperatures (Td) are ∼343 °C for [Cu(Pyim)(POP)](PF6) and 340 °C for [Cu(Qbim)(POP)](PF6), respectively (Figure 1).

product was obtained as a yellow crystal by slowly evaporating the solvent (0.59g, yield 59%). 1H NMR (400 MHz, CD3CN) δ (ppm): 8.64−8.66 (1H, d, J = 8 Hz), 8.30−8.32 (1H, d, J = 8 Hz), 8.19−8.20 (1H, d, J = 4 Hz), 7.86−7.90 (2H, m), 7.39 (1H, s), 7.32 (1H, s), 6.99−7.30 (28H, m), 6.76−6.87 (8H, m), 5.38 (2H, s); 13C NMR (125 MHz, CDCl3) δ (ppm): 158.23, 158.18, 158.13, 150.19, 145.69, 141.85, 135.40, 134.37, 134.16, 133.39, 133.33, 133.27, 132.67, 132.61, 132.55, 132.14, 131.77, 131.49, 131.30, 131.17, 131.04, 130.51, 130.27, 129.85, 129.07, 128.79, 128.75, 128.72, 128.51, 128.47, 128.44, 128.11, 128.00, 127.95, 127.26, 126.91, 125.20, 125.14, 124.88, 124.41, 124.30, 124.18, 120.02, 113.16, 112.98, 112.35, 52.38; 31P NMR (200 MHz, CDCl3) δ (ppm): −8.79 (s), −144.2 (quint). ESI-MS (m/z) = 936.9 (M-PF6−); Anal. Calcd for C59H45CuN3F6OP3: C, 65.46; H, 4.19; N, 3.88. Found: C, 65.31; H, 4.22; N, 3.90%. X-ray Crystallography. Crystal data of [Cu(Pyim)(POP)](PF6), [Cu(Qbim)(POP)](PF6), and AgI−NHC complex 3 were collected on a Bruker SMART APEX-II CCD diffractometer with Mo Kα radiation (λ = 0.071 073 Å). The data were corrected for Lorentzpolarization factors as well as for absorption. Structures were solved by direct methods and refined by full-matrix least-squares methods on F2 with the SHELXL-97 program.35 All nonhydrogen atoms were refined anisotropically, while hydrogen atoms were placed in geometrically calculated positions. CCDC reference numbers for [Cu(Pyim)(POP)](PF6), [Cu(Qbim)(POP)](PF6), and AgI−NHC complex 3 are 981195, 1402538, 981193, respectively. Density Functional Calculations. Density functional theory (DFT) calculations were performed with Gaussian 0936 software package employing the B3LYP functional37,38 using the LanL2DZ basis set39−41 for Cu and 6-31G* basis set42 for C, N, H, O, and P. Geometric parameters obtained from X-ray analyses were used as a starting point for geometry optimization in the ground state; geometry optimization were carried out in the gas phase, Cartesian coordinates of the optimized geometries are given in Tables S1 and S2, and frequency calculations were performed to confirm the optimized structures to be true minima on the potential energy surfaces. The optimized geometries were used for time-dependent density functional (TD-DFT) calculations. Hole and electron distributions were analyzed by using the multiwfn 3.5 program.43

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RESULTS AND DISCUSSION Synthesis and Characterization. 2-(1H-Imidazol-1-yl)pyridine 1, [1-(2-pyridyl)-3-benzyl-1H-imidazolium]+PF6− 2, 2(1H-benzo[d]imidazol-1-yl)quinoline 4, and [1-(2-quinolyl)-3benzyl-1H-benzo[d]imidazolium]+PF6− 5 were synthesized according to the reported methods.44−46 CuI−NHC complexes were prepared using a modified literature procedure.47 Taking [Cu(Pyim)(POP)](PF6) as an example (Scheme 1), first, a dinuclear AgI−NHC complex 3 was obtained by the reaction of

Figure 1. TGA curves of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6).

X-ray Structure Analysis. The structure of intermediate 3 was studied by single-crystal X-ray diffraction; the important bond lengths and angles are listed in Table 1. As shown in Figure 2, 3 is a dinuclear silver complex and has a symmetric structure. Two AgI ions adopt different coordination modes; Ag1 is coordinated by two carbene C atoms in two ligands, C


Article

Inorganic Chemistry Table 1. Selected Bond Lengths (Å) and Angles (deg) of 3, [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) 3 Ag1−Ag2 Ag1−C6 Ag2−N1 C6−Ag1−C6A N1−Ag2−N1A N1−Ag2−Ag1 C6−Ag1−Ag2

[Cu(Pyim)(POP)](PF6) 2.780(2) 2.092(4) 2.202(4) 165.2(2) 168.4(2) 84.18(10) 82.62(12)

Cu1−C1 Cu1−N3 Cu1−P1 Cu1−P2 C1−Cu1−N3 P1−Cu1−P2 C1−Cu1−P1 C1−Cu1−P2 P1−Cu1−N3 P2−Cu1−N3

1.966(6) 2.180(5) 2.2467(17) 2.2609(17) 80.0(2) 110.18(6) 121.75(17) 113.29(19) 109.55(14) 119.55(13)

[Cu(Qbim)(POP)](PF6) Cu1−C10 Cu1−N1 Cu1−P1 Cu1−P2 C10−Cu1−N1 P1−Cu1−P2 C10−Cu1−P1 C10−Cu1−P2 P1−Cu1−N1 P2−Cu1−N1

1.957(4) 2.289(3) 2.3133(16) 2.2818(16) 77.54(14) 109.02(4) 115.24(11) 129.28(12) 101.06(10) 117.27(9)

Figure 2. Crystal structure of 3 (30% probability ellipsoids; H atoms and the PF6− ion are omitted).

while Ag2 is coordinated by pyridine N atoms in two ligands. The two AgI ions are held together by a strong ligandsupported Ag···Ag interaction; the distance of Ag1−Ag2 was determined to be 2.780(2) Å. This kind of short Ag···Ag contact has been often observed in Ag(I)-NHC complexes due to argentophilicity.49,50 The molecular structures of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) determined by single-crystal X-ray diffraction are shown in Figures 3 and 4, the important bond lengths and angles are listed in Table 1. These two CuI−NHC complexes adopt typical distorted tetrahedral configuration. The bond lengths of Cu−N and Cu−P are similar to those of reported four-coordinate CuI complexes.11,12,18−27 Nevertheless, the bond lengths of Cu−Ccarbene of these complexes are

Figure 4. Crystal structure of [Cu(Qbim)(POP)](PF6) (30% probability ellipsoids; H atoms and the PF6− ion are omitted).

obviously longer than those of luminescent three-coordinate CuI−NHC complexes (1.862 Å−1.921 Å) reported by Thompson and Gaillard groups,30−34 which may be attributable to the increasing steric hindrance of four-coordinate CuI complexes. In addition, the imidazolylidene ring and pyridine ring of carbene ligand in [Cu(Pyim)(POP)](PF6) are nearly coplanar with a small dihedral angle of 7.02°. In contrast, the benzo[d]imidazolylidene unit and quinoline unit of carbene ligand in [Cu(Qbim)(POP)](PF6) have a wider dihedral angle of 23.57° due to their larger steric bluk. Density Functional Theory and Time-Dependent Density Functional Theory Calculations. To gain insight into the electronic structures of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6), DFT calculations were performed for these two complexes using the geometric parameters obtained from X-ray analyses as starting structure. The optimized geometries around Cu(I) ions remain similar to those determined by X-ray crystallography. For example, the calculated bond lengths of Cu−Ccarbene in [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) are 2.0282 and 2.0255 Å, respectively, which are very close to the experimental values (Table 1). The calculated molecular orbital (MO) surfaces are shown in Figure 5; these complexes have similar frontier MO compositions. The highest occupied molecular orbitals (HOMO) of these complexes have significant participation of central CuI ions, as well as contributions from the carbene units and the ligand POP. The lowest unoccupied molecular orbitals (LUMO) are mostly located on the pyridine unit for [Cu(Pyim)(POP)](PF6), on the quinoline unit for [Cu(Qbim)(POP)](PF6), and partly located on the carbene

Figure 3. Crystal structure of [Cu(Pyim)(POP)](PF6) (30% probability ellipsoids; H atoms and the PF6− ion are omitted). D


Article

Inorganic Chemistry

Figure 5. Calculated molecular orbitals of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) in the gas phase (isovalue = 0.02).

units; the contributions from central CuI ions are minimal. It can be found that the carbene units in these four-coordinate CuI−NHC complexes have significant contributions to both HOMO and LUMO, which indicates the carbene units have multiple influences to the photophysical properties. TD-DFT calculations were used to determine the lowest energy electronic transitions in [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6), and the results are summarized in Table 2. The S1 and T1 excitations of [Cu(Pyim)(POP)](PF6)

notable that the T1 excitation of [Cu(Qbim)(POP)](PF6) is mainly LC transition, while the contributions from MLCT transition are minimal. Photophysical Properties. The UV−visible absorption spectra of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) were recorded in dichloromethane solution at room temperature as shown in Figure 6. These spectra display intense

Table 2. Lowest Energy Transitions for [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) Determined from TD-DFT Calculations complex [Cu(Pyim)(POP)] (PF6)

[Cu(Qbim)(POP)] (PF6)

states

λ (nm)

f

S1

360.8

0.0486

T2

380.9

0.0000

T1

389.0

0.0000

S1

393.8

0.0379

T2

409.2

0.0000

T1

458.3

0.0000

major contribution HOMO → LUMO (76%) HOMO−1 → LUMO (20%) HOMO → LUMO (34%) HOMO−1 → LUMO (49%) HOMO → LUMO (56%) HOMO−1 → LUMO (28%) HOMO → LUMO (99%) HOMO → LUMO (86%) HOMO−4 → LUMO (14%) HOMO−3 → LUMO (16%) HOMO−2 → LUMO (21%)

Figure 6. Absorption spectra of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) in CH2Cl2 solution at room temperature and corrected emission spectra of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) in frozen 2-MeTHF solution at 77 K.

high-energy absorption bands assigned to LC π−π* transitions in near-UV region. Compared to the absorption spectra of NHC ligand precursors and the ligand POP (Figure S2), the absorption bands between 335 nm−400 nm (ε < 0.71 × 104 M−1 cm−1) of [Cu(Pyim)(POP)](PF6) should be assigned to MLCT transitions, which mixed with LLCT and LC transitions according to the TD-DFT calculation results, while the absorption bands between 350 nm−450 nm (ε < 0.73 × 104 M−1 cm−1) of [Cu(Qbim)(POP)](PF6) can be ascribed to the mixed MLCT and LC transitions. Meanwhile, upon expansion of conjugation system of NHC ligand, the charge transfer (CT) transition absorption band of [Cu(Qbim)(POP)](PF6) is more distinct and shows obvious red-shift compared to that of [Cu(Pyim)(POP)](PF6). These complexes do not show detectable emission in organic solutions at room temperature, which should be caused by (i) Jahn−Teller distortion that occurs in the excited state (ii) solvent-induced exciplex formation; this luminescence quenching mechanism of fourcoordinate CuI complexes has been confirmed by several groups.4,51,52 As shown in Figure 6, in frozen 2-MeTHF (THF = tetrahydrofuran) solution at 77 K, [Cu(Pyim)(POP)](PF6)

are mainly HOMO → LUMO and HOMO−1 → LUMO transitions. In the case of [Cu(Qbim)(POP)](PF6), the S1 excitation is dominated by HOMO → LUMO transition, while HOMO−4 → LUMO, HOMO−3 → LUMO, and HOMO−2 → LUMO transitions have significant contributions to the T1 excitation. On the basis of the TD-DFT calculations, we analyzed the hole and electron distributions of these complexes at S1 and T1 states (Figure S1). Analysis results show that the S1 and T1 excitations of [Cu(Pyim)(POP)](PF6) can be ascribed to metal-to-ligand charge transfer (MLCT) transition admixed with a small amount of ligand-to-ligand charge transfer (LLCT) transition and ligand-centered (LC) transition. Similar to [Cu(Pyim)(POP)](PF6), the S1 excitation of [Cu(Qbim)(POP)](PF6) is principally MLCT transition. However, it is E


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Inorganic Chemistry exhibits a structureless emission with λem = 535 nm, [Cu(Qbim)(POP)](PF6) shows a structured emission with λem = 481, 516, and 555 nm, and these emissions have triplet character with microsecond-scale lifetimes. Both of these complexes show intense emission in solid state at room temperature (Figure 7), and the emission data are

Because of the small band gaps between S1 and T1, many CuI complexes exhibit efficient TADF at room temperature. In general, for emitters with TADF, as the level of S1 state is higher than that of T1 state and the S1 → S0 transition is significantly more allowed than the spin-forbidden T1 → S0 transition, a blue-shift of emission and an effective reduction of the emission decay time will be observed with increasing temperature. Low-temperature measurements were performed for [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) in solid state to gain a deeper understanding of the emissive states. At a low temperature of 77 K, emission of [Cu(Pyim)(POP)](PF6) appears at 553 nm, and emission of [Cu(Qbim)(POP)](PF6) appears at 612 nm (Figure 7). It can be found that emissions of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) in solid state at 77 K red-shifted 33 and 42 nm compared to those at room temperature, respectively. As shown in Figure 8, emission decay times of these complexes in solid state at 77

Figure 7. Corrected emission spectra of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) in solid state at 298 K and at 77 K.

Table 3. Emission Data of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) in Solid State emission at 298 K complex

λem (nm)

τa (μs)

emission at 77 K ΦPLb

λem (nm)

[Cu(Pyim)(POP)] (PF6)

520

79.84

0.56

553

[Cu(Qbim)(POP)] (PF6)

570

31.97 (78.99%) 252.2 (21.01%)

0.35

612

τa (μs)

Figure 8. Emission decay behaviors of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) in solid state at 298 K and at 77 K.

160.7 (74.82%) 258.8 (25.18%) 69.24 (24.29%) 156.6 (55.48%) 342.1 (20.23%)

K are obviously longer than those at 298 K. Emission of [Cu(Pyim)(POP)](PF6) is biexponential decay with an average lifetime of 177.7 μs, while the emission decay behavior of [Cu(Qbim)(POP)](PF6) is fit to a three-exponential function with an average lifetime of 130.8 μs; the individual lifetimes are given in Table 3. When it is cooled from 298 to 77 K, the emission decay times of these complexes get longer by a factor of 2−3. Above experimental results indicate that emissions of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) in solid state at room temperature match with a TADF mechanism. However, the increases in lifetimes of these complexes appear smaller than those of most CuI complexes, which should be caused by the phosphorescence components in emissions at room temperature according to the recent reports of Yersin group.9,32 Thus, it is expected that emissions of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) in solid state at room temperature stem mainly from singlet state (TADF) and partly from triplet state (phosphorescence). Electrochemistry. The electrochemical properties of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) were investigated by cyclic voltammetry (CV) at room temperature in anhydrous CH3CN solutions. No reduction peaks were found within the solvent window. [Cu(Pyim)(POP)](PF6) displays two irreversible oxidation peaks (Figure 9); the first oxidation peak at 1.02 V can be attributed to the CuI/CuII oxidation process, while the oxidation peak at higher voltage (1.32 V)

a

Lifetime, weighting is given in parentheses. bAbsolute quantum yield, which is calculated by corrected emission spectrum obtained from an Edinburgh FLS980 spectrometer equipped with a barium sulfatecoated integrating sphere; relative error = ±10%.

summarized in Table 3. [Cu(Pyim)(POP)](PF6) displays a green emission with a structureless peak at 520 nm. The emission peak of [Cu(Qbim)(POP)](PF6) locates at 570 nm (yellow emission), red-shifts ∼50 nm compared to that of [Cu(Pyim)(POP)](PF6); it is caused by the larger conjugation system of NHC ligand in [Cu(Qbim)(POP)](PF6). The emission quantum yields of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) were determined to be 0.56 and 0.35, respectively, which are outstanding results for cationic CuI complexes.16,18,19,25−27,53 Emissions of these complexes have long lifetimes. Emission of [Cu(Pyim)(POP)](PF 6) is monoexponential decay with a lifetime of 79.8 μs, while emission decay behavior of [Cu(Qbim)(POP)](PF6) is fit to a biexponential function with an average lifetime of 39.2 μs; two individual lifetimes are 31.97 and 252.2 μs, respectively. F


Article

Inorganic Chemistry

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absorption spectra of NHC ligand precursors and ligand (POP). (PDF) X-ray crystallographic data in CIF format. (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (No. U1404209, 21372112, 21173113, 51373190) and the Natural Science Foundation of Henan Education Department, China (No. 13A150804).

Figure 9. CV of [Cu(Pyim)(POP)](PF6) and [Cu(Qbim)(POP)](PF6) in anhydrous CH3CN solutions.

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should be an LC oxidation process. [Cu(Qbim)(POP)](PF6) displays two quasi-reversible oxidation peaks at 1.11 and 1.52 V (Figure 9); similar to [Cu(Pyim)(POP)](PF6), these two oxidation peaks correspond to the metal-centered and LC oxidation processes, respectively. The oxidation potentials of [Cu(Qbim)(POP)](PF6) are obviously higher than those of [Cu(Pyim)(POP)](PF6), which can be attributed to the greater steric hindrance of NHC ligand in [Cu(Qbim)(POP)](PF6). As CuII ions prefer the square planar coordination geometry, oxidation on the center CuI ions will lead to significant geometrical rearrangement. The greater steric hindrance can provide a larger barrier in forming the flattened geometry resulting in a higher oxidation potential.

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CONCLUSIONS In summary, we successfully designed and synthesized two luminescent four-coordinate CuI−NHC complexes for the first time. Both of these CuI−NHC complexes adopt a typical distorted tetrahedral configuration and show high stability in solid state. Interestingly, these CuI−NHC complexes exhibited different lowest-energy electronic transitions. Both S1 and T1 excitations of [Cu(Pyim)(POP)](PF6) are principally MLCT transition, while S1 and T1 excitations of [Cu(Qbim)(POP)](PF6) are mainly MLCT and LC transitions, respectively. These CuI−NHC complexes have high-efficiency emission in solid state at room temperature, and their emission wavelength can be tuned effectively by modifying the structure of NHC ligand. In the solid state, when cooled from 298 to 77 K, emission spectra of these complexes red-shifted obviously, and the emission lifetimes increased by a factor of 2−3, which is consistent with a TADF mechanism. The moderate increases in emission lifetimes can be rationalized by the phosphorescence components in emissions of these complexes. These research results can provide a valuable insight into the photophysical behavior of four-coordinate CuI−NHC complexes.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02546. Cartesian coordinates of the optimized geometries, hole and electron distributions at S1 and T1 states, the G


Article

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