From Mononuclear to Dinuclear Iridium(III) - ACS Publications

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From Mononuclear to Dinuclear Iridium(III) Complex: Effective Tuning of the Optoelectronic Characteristics for Organic LightEmitting Diodes Xiaolong Yang,† Xianbin Xu,† Jing-shuang Dang,† Guijiang Zhou,*,† Cheuk-Lam Ho,‡,§ and Wai-Yeung Wong*,‡,§ †

MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, and Institute of Chemistry for New Energy Materials, Department of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, P. R. China ‡ Institute of Molecular Functional Materials, Department of Chemistry, and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, P. R. China § HKBU Institute of Research and Continuing Education, Shenzhen Virtual University Park, Shenzhen 518057, P. R. China S Supporting Information *

ABSTRACT: Phosphorescent dinuclear iridium(III) complexes that can show high luminescent efficiencies and good electroluminescent abilities are very rare. In this paper, highly phosphorescent 2-phenylpyrimidine-based dinuclear iridium(III) complexes have been synthesized and fully characterized. Significant differences of the photophysical and electrochemical properties between the mono- and dinuclear complexes are observed. The theoretical calculation results show that the dinuclear complexes adopt a unique molecular orbital spatial distribution pattern, which plays the key role of determining their photophysical and electrochemical properties. More importantly, the solution-processed organic light-emitting diode (OLED) based on the new dinuclear iridium(III) complex achieves a peak external quantum efficiency (ηext) of 14.4%, which is the highest ηext for OLEDs using dinuclear iridium(III) complexes as emitters. Besides, the efficiencies of the OLED based on the dinuclear iridium(III) complex are much higher that those of the OLED based on the corresponding mononuclear iridium(III) complex.

1. INTRODUCTION Highly phosphorescent cyclometalated iridium(III) complexes have been widely employed to fabricate organic light-emitting diodes (OLEDs) because of their unique optoelectronic properties including high phosphorescent quantum yields (ΦP), relatively short triplet excited-state lifetimes (τP), and facile emission color tuning by manipulation of the ligand structure.1,2 Generally, the strong phosphorescence of the iridium(III) complex is induced by the triplet metal-to-ligand charge transfer (3MLCT), reflecting the character of both the metal center and chelated organic ligand. Therefore, the organic ligands exhibit great influences on the properties of these iridium(III) complexes, and various organic ligands have been developed to manipulate their emission color and chargeinjection/transporting properties.3−7 However, the role played by the metal center has often been overlooked because the iridium(III) complexes reported in most literature reports only have one metal center.3−7 In the course of searching for novel iridium(III) complexes, only a few dinuclear iridium(III) complexes have been obtained by either coupling two mononuclear iridium(III) complexes through active sites8−11 or reacting the iridium(III) chloro-bridged dimer with various prepared bridge ligands.12−20 Apart from the tedious and © XXXX American Chemical Society

complicated synthesis processes, these methods usually result in the weak tuning of the properties of dinuclear iridium(III) complexes. In addition, the resultant dinuclear iridium(III) complexes usually show weak or even no phosphorescence at room temperature,8−19 which makes them unsuitable for the development of high-performance OLEDs. In recent years, only a few examples of a highly efficient dinuclear iridium(III) complex have been developed. Kozhevnikov and co-workers showed that ditopic bis-terdentate cyclometallating ligands could be used to synthesize dinuclear iridium(III) complexes with high ΦP of up to 0.65.21 Bryce and co-workers also reported two highly efficient phosphorescent dinuclear iridium(III) complexes that can be used to fabricate green-emitting OLED with remarkably high peak external quantum (ηext), luminance (ηL), and power (ηP) efficiencies of 11%, 37 cd A−1, and 14 lm W−1, respectively.22 Although the dinuclear iridium(III) complexes reported in these two papers are much more efficient than before, the development of dinuclear iridium(III) complexes with the desired properties for highperformance OLEDs is still a great challenge. Received: November 13, 2015

A

DOI: 10.1021/acs.inorgchem.5b02625 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of the Dinuclear Iridium(III) Complexes D1 and D2 and Their Parent Complex M1

Figure 1. Perspective views of D1 and D2. The solvent and H atoms are omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.

efficiencies obtained by solution-processed orange-red-emitting OLEDs.27−31

Herein, we report a very simple and effective way to obtain dinuclear cyclometalated iridium(III) complexes by using a very simple bis-N^C-coordinating ligand, 2-(4-tert-butylphenyl)pyrimidine, as the bridge ligand in a quasi-one-pot reaction. The resulting iridium(III) complexes show high ΦP and an unexpectedly large red-shift effect in the phosphorescent wavelength compared to that of the corresponding mononuclear congener. Furthermore, a simple solution-processed orange-red-emitting OLED using the dinuclear iridium(III) complex as the emitter shows ηext, ηL, and ηP of 14.4%, 27.2 cd A−1, and 19.5 lm W−1, respectively, which are not only among the highest efficiencies achieved by dinuclear iridium(III) complex-based OLEDs11,21−26 but also among the highest

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization. As outlined in Scheme 1, the 2-(4-tert-butylphenyl)pyrimidine ligand was synthesized by reacting 2-chloropyrimidine with (4-tertbutylphenyl)boronic acid through a Suzuki cross-coupling reaction. Then the ligand was reacted with IrCl3 salt to give the cyclometalated chloro-bridged reaction intermediate complexes. In order to improve the solubility of the possible reaction intermediate complexes, the conventional reaction solvent, i.e., a mixture of 2-ethoxyethanol and H2O,3−7 was replaced by a mixture of tetrahydrofuran (THF) and water B


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Inorganic Chemistry

2.2. Optical Properties and Theoretical Calculations. In their UV−vis spectra (Figure 2), the dinuclear iridium(III)

(H2O) in this step. After the reaction of intermediate complexes with thallium(I) acetylacetonate [Tl(acac)] in CH2Cl2 at room temperature, the dinuclear iridium(III) complexes D1 and D2 were obtained, accompanied by the formation of mononuclear complex M1 simultaneously. These complexes were fully characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. According to their 1H NMR spectra [Figure S1 in the Supporting Information (SI)], these complexes can be easily recognized as one kind of mononuclear complex (M1) and two kinds of dinuclear complexes (D1 and D2). In the 1H NMR spectrum of M1, the singlet resonance signal with a chemical shift at ca. 1.07 ppm indicates 18 protons, which can be assigned to the two tert-butyl groups of the ligands. However, in the 1H NMR spectra of D1 and D2, besides the singlet resonance signals at around 1.15 ppm showing 18 protons for the two tert-butyl groups of the peripheral ligands, there are other singlet resonance signals at around 0.80 ppm exhibiting 9 protons for the tert-butyl group on the bridge ligand. When the 1H NMR spectra of D1 and D2 are compared, great differences can be found in the region from 7.05 to 6.80 ppm. For example, the proton at the 3 position of the bridge ligand in D1 shows a triplet resonance signal located on the right side of its adjacent signals, while the proton at the same position in D2 displays a triplet resonance signal at the center of its adjacent signals (see Figure S1 in the SI). The single crystals of D1 and D2 were successfully prepared by slow evaporation of their THF/hexane solutions. The structures of these complexes were investigated by single-crystal X-ray crystallography. Figure 1 depicts the crystal structures of the dinuclear iridium(III) complexes D1 and D2. The details of each structure are given in Table S1. In these crystals, the central bridge ligand can chelate two Ir atoms through its two N^C-coordinating sites, and therefore the central bridge ligand and two Ir atoms are almost coplanar. As a result, one dinuclear iridium(III) complex consists of two fused distorted octahedral coordination geometries. This kind of coordination pattern in the cyclometalated iridium(III) complex is very rare. The average bond lengths of Ir−C (ca. 2.01 Å) and Ir−N (ca. 2.03 Å) are similar to those of other mononuclear iridium(III) complexes (Table S2).3 The two Ir atoms in D1 and D2 show similar distances of ca. 5.49 Å, which is almost 2-fold longer than the lengths of typical Ir−Ir bonds (from ca. 2.6 to ca. 3.1 Å) in some multimetallic clusters,32,33 indicating the absence of metal−metal interactions.34 Besides these common characteristics, the difference between D1 and D2 is more obvious. The locations of the two acetylacetonate (acac) ligands as well as the cyclometalating ligands in D1 and D2 are vey different. As depicted in Figure 1, the two acac ligands are located on opposite sides of the central bridge ligand plane in D1 and on the same side of that in D2. Thus, D1 exhibits point-symmetry character to some extent and D2 possesses an internal mirror plane. Accordingly, D1 is a collection of racemic enantiomers and D2 represents the meso isomer (Scheme 1). The different molecular structure symmetries of D1 and D2 lead to different molecular dipole moments. During the purification process, D1 was obtained using a mixture of petroleum ether and ethyl acetate (7:3, v/v) as the eluent and then D2 using a mixture of petroleum ether, dichloromethane, and ethyl acetate (2:1:1, v/ v/v) as the eluent, indicating a smaller molecular dipole moment of D1 than that of D2, which is also confirmed by the theoretical calculation results (2.24 D for D1 vs 6.51 D for D2).

Figure 2. UV−vis absorption and photoluminescent spectra of M1, D1, and D2 at room temperature.

complexes D1 and D2 display two distinct absorption bands, which are similar to the absorption band for the mononuclear iridium(III) complex M1. The intense absorption bands of D1 and D2 in the UV region with profiles similar to that of M1 can be safely ascribed to the π−π* transitions related to the pyrimidine-based ligand. The weaker absorption parts extended to the visible region can be attributed to both singlet and triplet MLCT states.35 Differently, the MLCT absorptions of D1 and D2 are much stronger than those of M1 and significantly redshifted by ca. 60 nm compared to those of M1. The featureless emission profiles for both the di- and mononuclear iridium(III) complexes in solutions at room temperature reveal the dominant 3MLCT characteristics, while the structured emissions of these complexes at 77 K (Figure S2) indicate the admixture of 3MLCT/3LC (LC = ligand-centered) properties. At room temperature, the solution of M1 shows green phosphorescence with the peak at ca. 526 nm, while both D1 and D2 emit bright-red phosphorescence, peaking at ca. 606 nm (Figure 2 and Table 1). Such a dramatic emission red shift of ca. 80 nm has never been observed by conversion of a mononuclear iridium(III) complex to a dinuclear iridium(III) counterpart.9,19,21,36,37 The great emission red shifts from M1 to D1 and D2 have also been observed in the solid films (Figure S3). The ΦP values of D1 (0.61) and D2 (0.49) in solutions are much higher than those of most other dinuclear iridium(III) complexes,8−19 which makes D1 and D2 good phosphorescent emitters for fabricating highly efficient OLEDs. In addition, compared with the τP value of 1.02 μs for M1, the much shorter τP value of 0.18 μs for both D1 and D2 will relieve the undesired triplet−triplet annihilation effect in OLEDs. Hence, the simple dinuclearization strategy proposed in this paper can effectively tune the photophysical behavior of phosphorescent iridium(III) complexes to induce the preferred properties for OLEDs. To gain insight into the different photophysical featiures of the mono- and dinuclear iridium(III) complexes, time-dependent density functional theory (TD-DFT) calculations were performed. The results (Figure 3 and Table S3) reveal that the highest occupied molecular orbital (HOMO) of M1 is mainly located on the dπ orbital of the Ir center and the π orbitals of the two chelated phenyl rings, and the lowest unoccupied molecular orbital (LUMO) of M1 is mainly distributed at the π orbitals of the two pyrimidine rings. However, the HOMOs of C


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Inorganic Chemistry Table 1. Photophysical and Electrochemical Data for M1, D1, and D2 M1 D1 D2

λabs, nm (log ε, M−1 cm−1)a

λem,a nm

ΦPb

τP,c μs

269 (4.85), 340 (4.19), 379 (3.95), 411 (3.79), 460 (3.67) 267 (4.75), 338 (4.22), 411 (3.94), 476 (3.86), 518 (3.47) 266 (4.76), 338 (4.24), 407 (3.99), 476 (3.88), 518 (3.46)

526 606 607

0.99 0.61 0.49

1.02 0.18 0.18

E

ox

1/2 d

, V

0.65 0.45, 0.80 0.50, 0.88

Epc,d V

HOMO, eV

LUMO, eV

−2.30 −2.21 −2.16

−5.45 −5.25 −5.30

−2.51 −2.59 −2.64

Measured in CH2Cl2 at a concentration of 2 × 10−5 M. bMeasured in a degassed CH2Cl2 solution relative to fac-[Ir(ppy)3] (ΦP = 0.97). cRecorded in a degassed CH2Cl2 solution with a 369 nm light source. dRecorded in CH3CN versus Fc/Fc+. a

and thus ensure good conjugation among the dπ orbitals of the two Ir centers and the π orbitals of the chelated phenyl ring in the bridge ligand to elevate the HOMO levels of D1 and D2, as the TD-DFT results clearly indicate. In addition, the conjugation effect will increase the electron density on the phenyl ring in the bridge ligand and induce a stronger nuclear shielding effect. Hence, the chemical shifts for the aromatic protons together with the protons from the tert-butyl group on the phenyl ring of the bridge ligand would move to the high field (Figure S1). On the other hand, the two sp2-hybridized N atoms in the pyrimidine ring of the bridge ligand donate a lone electron pair to the empty orbital of the two IrIII centers to form Ir−N bonds. This bonding mode will lower the electron density on the N atoms, which will enhance the electronaccepting ability of the pyrimidine ring in the bridge ligand to some extent and stabilize the LUMOs of D1 and D2 accordingly. Therefore, D1 and D2 possess higher HOMO and lower LUMO levels than M1, resulting in much lower HOMO → LUMO transition energies. Besides, the calculation results also show that the excitation energy of S0 → T1 for M1 is significantly higher than those for D1 and D2 (Table S3). Hence, the remarkable red-shifted MLCT absorption bands and phosphorescent emissions are reasonable. Clearly, the effective tuning of the photophysical behaviors of the iridium(III) complexes by this dinuclearization strategy is fulfilled by altering the electronic characteristics of the bridge ligand via a high degree of metalation.

Figure 3. Computed energy levels and HOMO and LUMO patterns of M1, D1, and D2.

D1 and D2 are dominantly located on the two Ir centers (ca. 51%) and the π orbitals of the chelated phenyl ring in the bridge ligand (ca. 26%), and the LUMOs of D1 and D2 are dominantly located on the π orbitals of the pyrimidine ring in the bridge ligand (ca. 87%). The two peripheral ligands show a negligible influence on the HOMOs and LUMOs of D1 and D2. From the single-crystal X-ray structures of D1 and D2, the two Ir centers will show high coplanarity with the bridging unit

Table 2. EL Data for OLEDs Based on the Mono- and Dinuclear Iridium(III) Complexes device

dopant

Vturn‑on, V

luminance L, cd m−2

A1

D1 (3.0 wt %)

3.3

11837 (14.7)

A2

D1 (5.0 wt %)

3.1

18410 (14.9)

A3

D1 (7.0 wt %)

3.0

8971 (14.3)

B1

D2 (3.0 wt %)

4.0

7624 (13.9)

B2

D2 (5.0 wt %)

3.0

16486 (13.9)

B3

D2 (7.0 wt %)

3.7

9028 (13.9)

C

M1 (7.0 wt %)

3.2

47227 (13.1)

ηext, % a

8.9 (4.9) 8.6b 6.1c 14.4 (5.3)a 14.2b 12.1c 5.0 (4.9)a 4.8b 4.1c 3.4 (5.1)a 3.1b 2.4c 6.4 (4.7)a 6.3b 5.6c 2.7 (4.7)a 2.5b 2.1c 6.9 (7.5)a 3.9b 6.3c

ηL, cd A−1

ηp, lm W−1

λmax, nm (CIE)d

16.8 (4.9) 16.1 11.5 27.2 (5.3) 26.8 22.8 9.3 (4.7) 9.1 7.6 6.1 (5.1) 18.9 17.5 11.5 (4.7) 11.4 10.1 4.8 (4.5) 4.5 3.7 26.5 (7.5) 15.1 16.2

12.0 (4.3) 8.3 3.8 19.5 (4.1) 14.3 7.9 7.0 (3.9) 4.5 2.4 4.2 (4.3) 12.1 6.9 8.7 (3.9) 6.0 3.5 3.5 (4.1) 2.1 1.2 11.2 (7.1) 9.0 11.1

598 (0.56, 0.44) 598 (0.56, 0.44) 598 (0.56, 0.44) 598 (0.56, 0.44) 598 (0.56, 0.44) 598 (0.56, 0.44) 528 (0.34, 0.63)

Maximum values of the devices. Values in parentheses are the voltages at which they were obtained. bValues collected at 100 cd m−2. cValues collected at 1000 cd m−2. dValues were collected at 10 V, and CIE coordinates (x, y) are shown in parentheses.

a

D


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Inorganic Chemistry

Figure 4. EL spectra of (a) devices A2, B2, and C and (b) device A2 at different driving voltages.

Figure 5. J−V−L relationships of (a) devices A2 and (b) B2.

2.3. Electrochemical Properties. The cyclic voltammetry (CV) investigations (Figure S4) show that M1 exhibits a reversible oxidation potential at ca. 0.65 V, while D1 and D2 exhibit two discernibly reversible oxidation waves assigned to the successive redox processes for the two IrIII centers.18,19,24,25 The first oxidation potentials of D1 and D2 (0.45 and 0.50 V, respectively) are noticeably lower than that of M1, resulting in the higher HOMO levels of D1 and D2 (−5.25 and −5.30 eV, respectively) than that of M1 (−5.45 eV), which may facilitate the hole-injection process. The lower first oxidation potentials of D1 and D2 may be due to the effect of delocalization of the positive charge over the two IrIII cations. Furthermore, the reduction peaks for D1 and D2 (ca. −2.20 V) appear at less negative potential compared to that for M1 (ca. −2.30 V), leading to the lower LUMO levels of D1 and D2 (−2.59 and −2.64 eV, respectively) than that of M1 (−2.51 eV), which indicates the improved electron-injection ability of D1 and D2. These electrochemical results are in good agreement with the theoretical calculation results and also imply that D1 and D2 have some degree of bipolar character, which is a highly desired property for the development of high-performance OLEDs. 2.4. Electroluminescent (EL) Properties. To test their EL properties, solution-processed OLEDs based on D1 and D2 were fabricated with a very simple structure of indium−tin oxide (ITO)/PEDOT:PSS (45 nm)/(3.0, 5.0, and 7.0 wt %) D1 or D2:TCTA (30 nm)/TPBi (45 nm)/LiF:Al (1:100 nm) (Figure S5). In the devices, PEDOT:PSS [poly(3,4-ethylenedioxythiophene):polystyrenesulfonate] is the hole-transporting material, TCTA [4,4′,4″-tris(carbazol-9-yl)triphenylamine] acts as the host material, and TPBi [1,3,5tris(N-phenylbenzimidazole-2-yl)benzene] is the electrontransporting material. Table 2 and Figures 4 and 5 show the EL data, spectra, and current density (J)−voltage (V)−

luminance (L) curves. For comparison, solution-processed reference devices based on M1 were also fabricated with the same structure, and the best performance was obtained by the device C at a doping level of 7.0 wt % (Table 2 and Figure S6). All of the devices show EL spectra similar to the typical emissions of the corresponding phosphorescent complexes, indicating efficient energy transfer from the TCTA host to these emitters. It turns out that the 5.0 wt % doped device using the dinuclear iridium(III) complex D1 as the emitter displays the best performance (Figure 6). The device A2 based on D1 exhibits a low turn-on voltage of 3.1 V with the peak values ηext, ηL, and ηP of 14.4%, 27.2 cd A−1, and 19.5 lm W−1, respectively. These efficiencies can remain as high as 14.2%, 26.8 cd A−1, and 14.3 lm W−1 at a practical luminance of 100 cd m−2, indicating the very low efficiency roll-off. As for the device B2 based on D2, it shows the peak values ηext, ηL, and ηP of 6.4%, 11.5 cd

Figure 6. Efficiency versus luminance for devices A2 and B2. E


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Inorganic Chemistry A−1, and 8.7 lm W−1, respectively. Its inferior performance may be due to the larger permanent dipole moment of D2 (6.51 D as calculated) compared to that of D1 (2.24 D as calculated) because the larger permanent dipole moment usually tends to cause detrimental molecular aggregation and quenching effects and thereby impair the device efficiencies.38 Besides, the higher ΦP value of D1 than that of D2 may be another reason for the better EL performance of the device A2 than that of the device B2. Nevertheless, to the best of our knowledge, the peak efficiencies achieved by the device A2 are not only among the highest efficiencies achieved by dinuclear iridium(III) complexbased OLEDs11,20−25 but also among the highest efficiencies obtained by simple solution-processed orange-red-emitting iridium(III) complex-based OLEDs.27−31 The highest efficiencies recently afforded by a green-emitting dinuclear iridium(III) complex-based OLED are 11%, 37 cd A−1, and 14 lm W−1.22 The previously reported solution-processed orange-red-emitting OLED based on a mononuclear iridium(III) complex exhibited peak efficiencies of 11.2%, 13.4 cd A−1, and 5.9 lm W−1.30 Moreover, the efficiencies, especially the ηext and ηP values of the device A2 based on the dinuclear iridium(III) complex D1, are much higher than those of the reference device C based on the mononuclear iridium(III) complex M1 (6.9%, 26.5 cd A−1, and 11.2 lm W−1). The superior performance of the device A2 over that of the reference device C may result from the following two aspects: the improved and balanced charge-injection/transporting properties of D1 compared to those of M1; the smaller permanent dipole moment of D1 (2.24 D as calculated) compared to that of M1 (3.80 D as calculated). Therefore, these results vividly indicate the great potential of using 2-phenylpyrimidine-based dinuclear iridium(III) complexes to develop highly efficient solutionprocessed OLEDs.

4. EXPERIMENTAL SECTION 4.1. General Information. The starting materials were commercially available and were used directly without further purification. All reactions were performed under a nitrogen atmosphere. 1H and 13C NMR spectra were determined using a Bruker Avance 400 MHz spectrometer in CDCl3. Chemical shifts were recorded on the ppm scale and referenced to the solvent residual peak. Elemental analyses were performed on a Flash EA 1112 elemental analyzer. Fast atom bombardment mass spectrometry (FAB-MS) spectra were recorded on a Finnigan MAT SSQ710 system. UV−vis absorption spectra of these complexes were measured at room temperature on a Shimadzu UV-2250 spectrophotometer in CH2Cl2. Photoluminescent spectra and lifetimes of these complexes were tested on an Edinburgh Instruments Ltd. (FLSP920) fluorescence spectrophotometer. The phosphorescence quantum yields (ΦP) were determined in degassed CH2Cl2 solutions at room temperature against a fac-[Ir(ppy)3] standard (ΦP = 0.97).39 CV measurements were carried out on the Princeton Applied Research (PARSTAT 2273, Advanced Electrochemical System) equipment in CH3CN solutions containing nBu4NPF6 (0.1 M) as the supporting electrolyte with a scan rate of 100 mV s−1. The conventional three-electrode configuration was employed, which consists of a glassy carbon working electrode, a platinum plate counter electrode, and a platinum wire reference electrode calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference. The HOMO and LUMO energy levels were calculated using the oxidation (Eox1/2) and cathodic peak (Epc) potentials according to the equations EHOMO = −(Eox1/2 + 4.8) eV and ELUMO = −(Epc + 4.8) eV. 4.2. 2-(4-tert-Butylphenyl)pyrimidine. Under a nitrogen atmosphere, 2-chloropyrimidine (1.92 g, 16.8 mmol), (4-tert-butylphenyl)boronic acid (3.00 g, 16.8 mmol), and Pd(PPh3)4 (0.97 g, 0.80 mmol) were added to a mixture of 1,2-dimethoxyethane (25 mL), ethanol (5 mL), and a K2CO3 solution (2 M, 20 mL). The reaction mixture was heated to 90 °C and stirred for 24 h. After cooling to room temperature, the mixture was extracted with CH2Cl2 several times. The organic layers were combined, dried over anhydrous MgSO4, and concentrated. The residue was purified on a silica column using a mixture of petroleum ether and ethyl acetate (3:1, v/v) as the eluent to give 3.36 g of colorless solid in 94% yield. 1H NMR (400 MHz, CDCl3, δ): 8.79 (d, J = 4.8 Hz, 2H), 8.37 (dd, J = 8.4 and 1.6 Hz, 2H), 7.52 (d, J = 8.4 Hz, 2H), 7.16−7.14 (m, 1H), 1.38 (s, 9H). 13C NMR (100 MHz, CDCl3, δ): 164.74, 157.15, 154.13, 134.76, 127.90, 126.62, 125.57, 118.75, 34.84, 31.22. FAB-MS (m/z): 212 ([M]+). Anal. Calcd for C14H16N2: C, 79.21; H, 7.60; N, 13.20. Found: C, 78.98; H, 7.48; N, 13.11. M1, D1, and D2. Under a nitrogen atmosphere, 2-(4-tertbutylphenyl)pyrimidine (0.30 g, 1.4 mmol) and IrCl3·nH2O (0.30 g, 0.93 mmol) were added to a mixture of THF and H2O (3:1, v/v, 35 mL). The reaction mixture was heated to 110 °C for 42 h with stirring. After cooling to room temperature, the reaction mixture was poured into H2O (10 mL). The mixture was extracted with CH2Cl2 several times. The organic layers were combined and dried over anhydrous sodium sulfate. After removal of the solvent, the residue and thallium(I) acetylacetonate (0.28 g, 0.92 mmol) were dissolved in CH2Cl2 (20 mL) under a nitrogen atmosphere. After stirring for 24 h at room temperature, the solvent was removed. The residue was first purified on preparative TLC plates using a mixture of petroleum ether and ethyl acetate (7:3, v/v) as the eluent to give M1 (first color band) and D1 (second color band), followed by using a mixture of petroleum ether, dichloromethane, and ethyl acetate (2:1:1, v/v/v) as the eluent to obtain D2 (third color band). Caution! The reactant thallium(I) acetylacetonate is extremely toxic, and one should pay more attention to manage it. M1. Yellow solid (156 mg, 23% yield). 1H NMR (400 MHz, CDCl3, δ): 8.73 (dd, J = 4.4 and 2.0 Hz, 2H), 8.66 (dd, J = 5.6 and 2.4 Hz, 2H), 7.82 (d, J = 8.0 Hz, 2H), 7.09 (t, J = 5.6 Hz, 2H), 6.91 (dd, J = 8.0 and 1.6 Hz, 2H), 6.27 (s, 2H), 5.21 (s, 1H), 1.81 (s, 6H), 1.07 (s, 18H). 13C NMR (100 MHz, CDCl3, δ): 184.93, 177.32, 157.01, 154.82, 153.56, 146.70, 138.68, 128.74, 127.09, 118.96, 116.34, 100.56,

3. CONCLUSION In summary, two dinuclear iridium(III) complexes, D1 and D2, were obtained with dramatically tuned character compared to their reference mononuclear iridium(III) complex M1. With great contributions from the bridge ligand to the HOMOs and LUMOs of dinuclear iridium(III) complexes D1 and D2, the emission color can be switched from green (ca. 526 nm) for the reference mononuclear iridium(III) complex M1 to red (ca. 606 nm) for the dinuclear iridium(III) complexes D1 and D2. Because of the high ΦP (0.61), short τP (0.18 μs), and small permanent dipole moment of the dinuclear iridium(III) complex D1, the solution-processed device based on D1 shows outstanding performance with the peak values ηext, ηL, and ηP of 14.4%, 27.2 cd A−1, and 19.5 lm W−1, respectively, which are among the highest efficiencies achieved by dinuclear iridium(III) complex-based OLEDs. The efficiencies of the device based on D1 also outperform those of the device based on the mononuclear iridium(III) complex M1. These results have nicely demonstrated a new strategy to synthesize neutral dinuclear cyclometalated iridium(III) complexes, which can substantially tune the phosphorescent properties of the resulting dinuclear iridium(III) complexes and endow them with high ΦP and short τP values for the development of promising solution-processed OLEDs. It also provides the possibility of the design and synthesis of numerous highly phosphorescent dinuclear iridium(III) complexes for more potential applications. F


Inorganic Chemistry

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34.50, 31.08, 28.66. FAB-MS (m/z): 714 ([M]+). Anal. Calcd for C33H37IrN4O2: C, 55.52; H, 5.22; N, 7.85. Found: C, 55.37; H, 5.31; N, 7.71. D1. Orange-red solid (78 mg, 13% yield). 1H NMR (400 MHz, CDCl3, δ): 8.64−8.60 (m, 4H), 8.45 (d, J = 5.6 Hz, 2H), 7.80 (d, J = 8.0 Hz, 2H), 6.99 (t, J = 5.2 Hz, 2H), 6.95 (dd, J = 8.4 and 1.6 Hz, 2H), 6.91 (t, J = 5.6 Hz, 1H), 6.51 (d, J = 2.0 Hz, 2H), 5.49 (s, 2H), 5.23 (s, 2H), 1.83 (s, 6H), 1.82 (s, 6H), 1.15 (s, 18H), 0.78 (s, 9H). 13 C NMR (100 MHz, CDCl3, δ): 184.93, 177.75, 156.75, 154.93, 153.98, 153.41, 138.84, 129.89, 129.49, 126.88, 122.25, 118.62, 116.11, 115.96, 100.37, 34.54, 34.37, 31.30, 31.14, 28.60, 28.57. FAB-MS (m/ z): 1214 ([M]+). Anal. Calcd for C52H58Ir2N6O4: C, 51.38; H, 4.81; N, 6.91. Found: C, 51.22; H, 4.67; N, 6.73. D2. Orange-red solid (90 mg, 15% yield). 1H NMR (400 MHz, CDCl3, δ): 8.65−8.61 (m, 4H), 8.42 (d, J = 5.6 Hz, 2H), 7.84 (d, J = 8.4 Hz, 2H), 6.99−6.97 (m, 4H), 6.92 (t, J = 5.2 Hz, 1H), 6.86 (s, 2H), 5.47 (s, 2H), 5.17 (s, 2H), 1.82 (s, 6H), 1.81 (s, 6H), 1.20 (s, 18H), 0.80 (s, 9H). 13C NMR (100 MHz, CDCl3, δ): 185.51, 184.81, 177.64, 156.88, 155.00, 154.33, 153.59, 149.11, 142.81, 138.87, 129.70, 128.86, 127.17, 122.98, 119.21, 116.07, 112.18, 100.47, 34.71, 34.46, 31.44, 31.14, 29.70, 28.66. FAB-MS (m/z): 1214 ([M]+). Anal. Calcd for C52H58Ir2N6O4: C, 51.38; H, 4.81; N, 6.91. Found: C, 51.09; H, 4.92; N, 6.63. 4.3. X-ray Crystallography. Single crystals of D1 and D2 were obtained from a THF/hexane solution by slow evaporation of the solvents. The crystals were mounted on a glass fiber, and the data were collected on a Bruker SMART CCD diffractometer (Mo Kα radiation and λ = 0.71073 Å) in Φ and ω scan modes at ca. 100 K. Their structures were solved by direct methods, followed by difference Fourier syntheses, and then refined by full-matrix least-squares techniques against F2 using the SHELXL-97 program on a PC.40 4.4. Computational Details. DFT calculations by using B3LYP were performed for all of the iridium(III) complexes. The basis set used for C, H, N, and O atoms was 6-31G, while effective core potentials with a LanL2DZ basis set were employed for Ir atoms.41,42 The excitation behaviors of the complexes were computed by the TDDFT method based on optimized geometries at the ground states. All calculations were carried out by using the Gaussian 09 program.43 4.5. OLED Fabrication and Measurements. The precleaned ITO glass substrates were treated with ozone for 20 min before PEDOT:PSS was spin-coated on the surface of ITO glass substrates to form a 45-nm-thick hole-injection layer. Then, the ITO glass substrates with a PEDOT:PSS layer were cured at 120 °C for 30 min in air. The emission layer (30 nm) was obtained by spin coating a chloroform solution of M1, D1, or D2 (x wt %) in TCTA at various concentrations. The obtained ITO chip was dried in a vacuum oven at 60 °C for 10 min, and it was transferred to the deposition system for organic and metal deposition. TPBi (45 nm), LiF (1 nm), and Al cathode (100 nm) were successively evaporated at a base pressure of less than 10−6 Torr. The EL spectra and CIE coordinates were recorded with a PR650 spectra colorimeter. The L−V−J curves of the devices were measured by a Keithley 2400/2000 source meter, and the luminance was measured using a PR650 SpectraScan spectrometer. All of the experiments and measurements were carried out under ambient conditions.

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Article

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Tengfei Project from Xi’an Jiaotong University, the Fundamental Research Funds for the Central Universities, the Program for New Century Excellent Talents in University, the Ministry of Education of China (NECT-09-0651), the Key Creative Scientific Research Team in Shaanxi Province (2013KCT-05), the China Postdoctoral Science Foundation (Grant 20130201110034), and the National Natural Science Foundation of China (Grants 20902072 and 21572176). W.-Y.W. acknowledges financial support from the Areas of Excellence Scheme of the University Grants Committee (AoE/P-03/08), the National Basic Research Program of China (2013CB834702), the Science, Technology and Innovation Committee of Shenzhen Municipality (JCYJ20140419130507116), Hong Kong Research Grants Council (HKBU203313), and Hong Kong Baptist University (FRG2/13-14/083).

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02625. NMR and CV characterization, details of the computational studies, and the EL performance of a M1-based reference device (PDF) CIF file for D1 (CIF) CIF file for D2(CIF) G


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Inorganic Chemistry

Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision A.01; Gaussian, Inc.: Wallingford, CT, 2009.

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