Achieving High-Performance Solution-Processed Orange OLEDs with

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 10227−10235

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Achieving High-Performance Solution-Processed Orange OLEDs with the Phosphorescent Cyclometalated Trinuclear Pt(II) Complex Xiaolong Yang,† Bo Jiao,‡ Jing-Shuang Dang,† Yuanhui Sun,† Yong Wu,† Guijiang Zhou,*,† and Wai-Yeung Wong*,§

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†

MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Institute of Chemistry for New Energy Material, Department of Chemistry, School of Science, State Key Laboratory for Mechanical Behavior of Materials, and ‡Key Laboratory for Physical Electronics and Devices of the Ministry of Education, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China § Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P. R. China S Supporting Information *

ABSTRACT: Cyclometalated Pt(II) complexes can show intense phosphorescence at room temperature. Their emission properties are determined by both the organic ligand and the metal center. Whereas most of the related studies focus on tuning the properties by designing different types of organic ligands, only several reports investigate the key role played by the metal center. To address this issue, phosphorescent Pt(II) complexes with one, two, and three Pt(II) centers are designed and synthesized. With more Pt(II) centers, the cyclometalated multinuclear Pt(II) complexes display red-shifted emissions with increased photoluminescence quantum yields. Most importantly, solution-processed organic light-emitting diodes (OLEDs) with the conventional device structure using the multinuclear Pt(II) complexes as emitters show excellent performance. The controlled device based on the conventional mononuclear Pt(II) complex shows a peak external quantum efficiency, current efficiency, and power efficiency of 6.4%, 14.4 cd A−1, and 12.1 lm W−1, respectively. The efficiencies are dramatically improved to 10.5%, 21.4 cd A−1, and 12.9 lm W−1 for the OLED based on the dinuclear Pt(II) complex and to 17.0%, 35.4 cd A−1, and 27.2 lm W−1 for the OLED based on the trinuclear Pt(II) complex, respectively. To the best of our knowledge, these efficiencies are among the highest ever reported for the multinuclear Pt(II) complex-based OLEDs. KEYWORDS: organic light-emitting diodes, phosphorescence, trinuclear Pt(II) complex, triphenylamine, efficiency the properties of organic complexes.14,15 In comparison, little attention is paid to the question that what if one more metal center is incorporated into the organic complexes.16−18 At present, only a handful of studies investigated the properties of multinuclear Pt(II) complexes. Kozhevnikov and co-workers synthesized some cyclometalated dinuclear Pt(II) complexes (Scheme 1) and reported that the dinuclear Pt(II) complexes could show red-shifted phosphorescence emissions with enhanced emission efficiencies compared with their corresponding mononuclear Pt(II) complexes.19,20 Zhu and coworkers reported some cyclometalated dinuclear Pt(II) complexes and fabricated OLEDs using these dinuclear Pt(II) complexes as emitters.21−24 The solution-processed OLED based on Pt2L3 could show a maximum luminance of 205 cd m−2 and a maximum current efficiency (CE) of 0.1 cd A−1.22

1. INTRODUCTION Organic light-emitting diodes (OLEDs) have drawn increasing attention from both scientific and industrial communities because of their great potential application in next-generation displays and solid-state lightings.1,2 As the key active materials in OLEDs, emitters take the unique position of deciding the emission colors as well as the OLED performance. Since Pt(II) and Os(II) complexes were successfully used as emitters for OLEDs,3,4 phosphorescent organic complexes containing heavy-metal Ir(III) or Pt(II) centers become a research hot spot because they can fully utilize electrogenerated singlet (25%) and triplet (75%) excitons to reach 100% internal quantum yield.5−11 Generally, phosphorescence emissions of organic complexes originate from triplet metal-to-ligand chargetransfer states (3MLCTs) or a mixture of 3MLCTs with ligandbased triplet excited states.12,13 Therefore, the metal center has a significant effect on the emission properties of related organic complexes. However, to date, numerous efforts have been made to design and synthesize various organic ligands to manipulate © 2018 American Chemical Society

Received: December 2, 2017 Accepted: March 5, 2018 Published: March 5, 2018 10227

DOI: 10.1021/acsami.7b18330 ACS Appl. Mater. Interfaces 2018, 10, 10227−10235

Research Article

ACS Applied Materials & Interfaces

phosphorescent multinuclear Pt(II) complexes have great potential applications in fabricating high-performance OLEDs. However, the efficiencies of OLEDs based on multinuclear Pt(II) complexes are still very low. This may result from their low PLQYs, for example, 0.009 for Pt2L3, or the lack of the functional group to facilitate the charge injection/transport process. To solve these problems, in this study, three triphenylamine-based Pt(II) complexes have been designed and synthesized. The triphenylamine core is chosen for two reasons. (1) The electron-rich triphenylamine core has the advantage of improving the hole injection/transport ability of the resultant complexes.31 (2) The star-shaped triphenylamine core is very suitable for synthesizing multinuclear Pt(II) complexes with a relatively rigid molecular structure to improve their PLQYs.32 The phosphorescence emission peaks are redshifted from ca. 577 nm for the mononuclear Pt(II) complex to ca. 590 nm for the multinuclear Pt(II) complex. Furthermore, the PLQYs are increased from 0.24 for the mononuclear Pt(II) complex to 0.74 for the trinuclear Pt(II) complex. Most importantly, compared with the reference OLED based on the mononuclear Pt(II) complex, the solution-processed orange OLEDs based on the multinuclear Pt(II) complexes exhibit dramatically improved performance; that is, the electroluminescence (EL) efficiencies are greatly increased, representing the highest values for OLEDs based on multinuclear Pt(II) complexes so far. Compared with the studies that incorporate two metal centers into one molecule to enhance the emission efficiencies,16−20,33,34 this work further demonstrates that it is promising to improve the emission efficiencies by incorporating one more metal center to form the trinuclear complex emitter, which implies that Pt(II) complexes with suitable organic ligands containing four or five Pt(II) centers may display stronger emission. However, even though this study provides a new strategy to synthesize the multinuclear Pt(II) complexes with much higher yields compared with convectional ways, the yield significantly drops with increasing metal centers. Therefore, a new efficient synthetic method should be developed to obtain complexes with metal centers more than three. There is another point that is worth mentioning. Maybe, some simple and efficient synthetic routes will be discovered in the future, and then complexes bearing four or even more metal centers will be popular. However, even though the PLQY would increase up to 1.0 for complexes with four or even more metal centers, it is diseconomy to fabricate OLEDs based on these multinuclear complexes because of their high cost. Therefore, there exits a trade-off between the efficiency and the cost. Hence, it seems that dinuclear and trinuclear Pt(II) complexes may be a wise choice for this trade-off. Nevertheless, this study provides a new way to develop efficient phosphorescent emitters for high-performance OLEDs.

Scheme 1. Chemical Structures of Some Di- and Trinuclear Pt(II) Complexes19,20,22,23,25,30

The device based on Pt2L4 showed a peak external quantum efficiency (EQE) of 8.86% at a current density of 0.76 mA cm−2.23 Tronnier and Strassner synthesized a series of cyclometalated dinuclear N-heterocyclic biscarbene Pt(II) complexes.25 At a doping level of 30 wt %, the thermaldeposited OLED based on Pt2L5 could show the highest EQE, CE, and power efficiency (PE) of 8.5%, 24.9 cd A−1, and 10 lm W−1, respectively. Although trinuclear Pt(II) complexes were also reported,26−30 only two trinuclear Pt(II) complexes (Scheme 1) were used for fabricating OLEDs.30 The OLED based on Pt3L1 displayed the best performance with a peak luminance of 375 cd m−2 and a PE of 0.1 cd A−1. The abovementioned studies indicate that multinuclear Pt(II) complexes can show very different properties compared with conventional mononuclear Pt(II) complexes. With a rigid molecular structure, the photoluminescence quantum yields (PLQYs) of Pt(II) complexes can be increased by incorporating one more Pt(II) center (e.g., Pt2L1 and Pt2L2). Therefore,

2. RESULTS AND DISCUSSION 2.1. Synthesis. The synthetic route to the mono-, di-, and trinuclear Pt(II) complexes is illustrated in Scheme 2. The detailed synthetic procedures are described in the Supporting Information. At first, three formyl-substituted triphenylamine compounds were obtained through the Vilsmeier−Haack reaction.35 Then, the organic ligands (L1−L3) were prepared by treating 2-aminobenzenethiol with the corresponding formyl-substituted triphenylamine compounds in the presence of H2O2 and HCl. The mononuclear Pt(II) complex MPt was synthesized according to the reported procedure by reacting K2PtCl4 with ligand L1 at a molar ratio of ca. 1.0.36 However, if 10228


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ACS Applied Materials & Interfaces Scheme 2. Synthetic Route to the Mono-, Di-, and Trinuclear Pt(II) Complexes

Table 1. Photophysical and Thermal Data and Highest Occupied Molecular Orbital (HOMO)/Lowest Unoccupied Molecular Orbital (LUMO) Levels for the Pt(II) Complexesa λabs (nm) MPt DPt TPt

264 (4.42), 303 (4.25), 375 (4.21), 451 (4.38) 254 (4.75), 325 (4.30), 386 (4.39), 485 (4.73) 255 (5.16), 310 (4.92), 392 (4.92), 479 (5.12)

λem (nm) at rt 494 (3.2 ns), 577 (0.19 μs) 513 (1.5 ns), 591 (0.19 μs) 512 (2.3 ns), 588 (0.24 μs)

λem (nm) at 77 K/in film 570 (1.83 μs)/577 (1.86 μs) 583 (1.33 μs)/592 (1.33 μs) 579 (1.97 μs)/590 (1.0 μs)

PLQY (in THF/film)

Td (°C)

HOMO/LUMO (eV)

0.24/0.28

266

−5.24/−2.66

0.53/0.61

345

−5.27/−2.68

0.74/0.81

315

−5.28/−2.69

λabs was measured in THF at a concentration of 10−5 M; log ε values are shown in parentheses; λem and corresponding lifetime shown in parentheses were measured in THF solutions at rt and 77 K as well as in doped films; the PLQY was measured in degassed THF solutions relative to fac-[Ir(ppy)3] (PLQY = 0.96)37 at rt; the PLQY of the doped film was measured using an integrating sphere; Td was tested under a N2 atmosphere at a heating rate of 20 K min−1; and the HOMO and LUMO energy levels were calculated using the first anodic peak potential (Epa) and cathodic peak potential (Epc) according to the equations EHOMO = −(Epa + 4.8) eV and ELUMO = −(Epc + 4.8) eV, respectively. a

their well-defined chemical structures were confirmed by NMR spectroscopy (Figures S2−S4). 2.2. Physical Characterization. We studied the thermal properties of MPt, DPt, and TPt by thermogravimetric analysis (TGA). The TGA results showed that the decomposition temperatures (Td) of these complexes were higher than 260 °C (Table 1 and Figure S5), indicating their good thermal stability. UV−vis absorption spectra of MPt, DPt, and TPt in tetrahydrofuran (THF) at room temperature (rt) are shown in Figure 1a. Strong absorption bands peaking at ca. 260 nm observed for all of these complexes can be assigned to π−π* ligand-centered (LC) transitions. Because of their high molar extinction coefficient (ε > 24 000 M−1 cm−1) and broad profile, the arresting absorption band located in the range of 425−490 nm can be ascribed to the intraligand charge transfer (ILCT) (from the triphenylamine group to the benzothiazole unit). In detail, the more the electron-acceptor moiety (benzothiazolyl) exists in the Pt(II) complex, the stronger the absorption in the range of 425−490 nm observed (log ε = 4.38, 4.73, and 5.12 for MPt, DPt, and TPt, respectively). Besides, the onset electronic transition absorptions have been significantly red-shifted from 495 nm for MPt to 520 nm for both DPt and TPt (Figure 1a).

the feeding ratio between K2PtCl4 and organic ligand was 2.0 for DPt or 3.0 for TPt to achieve a high yield according to the conventional method, the coordination sites in the organic ligand should be fully utilized. In this case, nearly insoluble large chloride-bridged precursor Pt(II) complexes with the configuration of linear for DPt or bulky for TPt should be formed (see Figure S1a in the Supporting Information) to make the synthesis of DPt and TPt very difficult. Therefore, DPt and TPt were hard to prepare using the conventional method (the yield of TPt < 1%). To solve this problem, we adopted a new strategy to facilitate their synthesis by adding the terminating ligand of 2-phenylpyridine (Hppy). After adding Hppy (1/2 amount to K2PtCl4), the much smaller chloridebridged precursor Pt(II) complex (Figure S1b) would be formed more easily, which could be beneficial to improve the yield. Although the addition of Hppy was an economically ineffective way because of the formation of (2-phenylpyridinato-N,C 2 ′ )Pt(2,4-pentanedionato-O,O) [(ppy)Pt(acac)], DPt and TPt had been successfully synthesized in much higher yields (9.2 and 5.6%, respectively). At last, all desired Pt(II) complexes were obtained as orange solids, and 10229


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Figure 1. Absorption and photoluminescence of the Pt(II) complexes: (a) absorption spectra at rt, (b) PL spectra at rt, (c) PL spectra in doped polymethyl methacrylate (PMMA) films at ca. 10.0 wt %, (d) PL spectra at 77 K, and (e) PL spectra of MPt in toluene, dichloromethane (DCM), THF, and dimethylformamide (DMF) at rt (the numbers stand for the peak wavelengths).

more Pt(II) centers, which is the case for MPt and DPt. However, the emission maximum for TPt shows a slight hypochromic effect with respect to that of DPt. This interesting phenomenon has not been reported previously, but it is reasonable because the lowest-energy absorption band of TPt is also slightly blue-shifted compared with that of DPt. Another interesting observation is that with increasing the number of Pt(II) centers, the intensities of the fluorescence are decreased, whereas those of the phosphorescence are enhanced. Accordingly, the PLQYs are increased from 0.24 for MPt to 0.53 for DPt and 0.74 for TPt in solutions, which demonstrates the success of our design strategy. The solvatochromism behaviors of these Pt(II) complexes were investigated in different solvents (toluene, dichloromethane, THF, and DMF). As shown in Figures 1e and S6, the fluorescence emission peaks of these Pt(II) complexes are red-shifted with increasing the polarity of the solvent. For example, the fluorescence emission of MPt displays a vibronic structure with a peak located at 485 nm in toluene, whereas this emission becomes broad and structureless with the peak being red-shifted to 515 nm in DMF. The positive solvatochromism indicates the strong intramolecular charge transfer from the electron-donor (arylamine) to the electron-acceptor (benzothiazole) moieties, implying that the fluorescences of these Pt(II) complexes could be dominated by 1ILCT states.42 However, the phosphorescence emissions of these Pt(II) complexes show a very weak solvatochromic effect, implying that the phosphorescence emissions mainly result from the 3π−π* states.43 2.3. Theoretical Calculation. To provide deep understanding about the nature of the excited states of these complexes with different numbers of metal centers, theoretical calculations based on density functional theory (DFT) and time-dependent DFT were performed. We first investigated the

These results further imply the ILCT character of the intense absorptions in the range of 425−490 nm. It is interesting that the absorption peak of TPt (479 nm) is slightly blue-shifted compared with that of DPt (485 nm). In the longer wavelength region, the typical low-energy metal-to-ligand charge-transfer (MLCT) absorption bands cannot be detected clearly, which indicates the low participation of the Pt(II) center in frontier molecular orbitals of these Pt(II) complexes. Upon UV light excitation, all complexes display dual-band emission spectra in THF solutions (Figure 1b). As listed in Table 1, the short lifetimes (3.2 ns for MPt, 1.5 ns for DPt, and 2.3 ns for TPt) of these high-energy emissions peaking at around 500 nm imply their fluorescent character which origins from singlet states. The low-energy emissions in the range from 577 to 591 nm are phosphorescence from triplet excited states, indicated by their much longer lifetimes in the microsecond regime (0.19 μs for MPt, 0.19 μs for DPt, and 0.24 μs for TPt). When doped in PMMA films, these Pt(II) complexes show emissions in the low-energy region similar to those in solutions. However, the emission lifetimes are greatly increased to the range of 1.0−1.86 μs, indicating their triplet character. Besides, the intensities of low-energy emissions have been significantly increased because of the absence of oxygen in films and the effective restriction of intramolecular motion by the rigid PMMA matrix to promote radiative decay of the triplet states. This result further evidences the phosphorescent character of the low-energy emissions. As shown in Figure 1d, compared with the emissions recorded at rt, the intensities of the lowenergy emissions are also greatly enhanced, and the maximum emission wavelengths are blue-shifted at 77 K, which have been widely observed for other phosphorescent organometallic emitters.38−41 According to the previous studies,19,20 phosphorescence emissions should be notably red-shifted by introducing 10230


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band compared with DPt. This calculation result and the experimental absorption result are in good accord. To further understand the nature of the electronic transitions, natural transition orbitals (NTOs) for the S0 → T1 excitations were evaluated based on the geometry of T1 state for these Pt(II) complexes. The calculated results suggest that the phosphorescence emissions of these Pt(II) complexes mainly arise from the 3π−π* excited states, which has been experimentally supported by the solvatochromic behavior in solvents with different polarities. As shown in Figure 3, the

properties of HOMOs and lowest unoccupied molecular orbitals (LUMOs) of these Pt(II) complexes (Figure 2 and

Figure 2. HOMO and LUMO distributions for the Pt(II) complexes.

Table 2). Both HOMOs and LUMOs of these Pt(II) complexes are mainly located on the organic ligands, with minor contributions from the Pt(II) centers, and the HOMO → LUMO transitions make over 80% contribution to both T1 and S1 of these Pt(II) complexes, indicating that the HOMO → LUMO transitions show dominated LC characters. Therefore, it is reasonable that no obvious MLCT absorption bands could be detected for MPt, DPt, and TPt (Figure 1a). As indicated by the more dispersed patterns of the frontier molecular orbitals of DPt and TPt, the introduction of more Pt(II) centers as well as electron-acceptor benzo[d]thiazole groups to form the dinuclear and trinuclear Pt(II) complexes will effectively extend the conjugation length as well as enhance the donor−acceptor charge-transfer process. Besides, although the contribution from Pt(II) centers to the HOMOs is minor, it increases with the introduction of more Pt(II) centers into complexes, which may also help to enhance the charge-transfer process to some extent. Therefore, the effective extension of the conjugation length and the enhancement of the donor−acceptor chargetransfer process should be responsible for the red shift of absorption from the mononuclear complex to the multinuclear complex. Compared with DPt, TPt possesses one more electron-acceptor benzo[d]thiazole moiety and Pt(II) center. However, the third electron-acceptor moiety and Pt(II) center make more contribution to the HOMO than to the LUMO of TPt, meaning that the HOMO is more dispersive than the LUMO in TPt, which may stabilize the energy level of the HOMO more than that of the LUMO, leading to an enlarged energy gap. Hence, TPt shows a slightly blue-shifted absorption

Figure 3. NTOs for S0 → T1 excitation for the Pt(II) complexes at optimized T1 geometries.

weights of the hole-particle contribution to the excitations of these Pt(II) complexes are over 97%, and both hole and particle orbitals are obviously localized on the cyclometalating ligand (>90%, Table S1 in the Supporting Information), which reveals the predominated LC excitation character of T1 states. Although the contributions from Pt(II) centers to both hole and particle orbitals are very similar for these Pt(II) complexes, the distributions of both hole and particle orbitals for DPt and TPt are more dispersive on the cyclometalating ligand as well as the Pt(II) centers with increasing the number of Pt(II) centers, which will stabilize the excited states, and thereby lead to redshifted emissions. Because of the distribution of hole and particle orbitals of TPt being very similar to that of DPt, which indicates very similar excited states, TPt can show phosphorescence emission almost identical to DPt. This theoretical calculation result is consistent with the experimental observations. 2.4. Electrochemical Properties. The redox potentials of these Pt(II) complexes were measured by cyclic voltammetry (CV) in MeCN solutions containing n-Bu4NPF6 as the

Table 2. Theoretical Calculation Results for the Pt(II) Complexes Based on Their Optimized S0 Geometriesa

a

contribution from Pt to HOMO (%)

contribution from Pt to LUMO (%)

state

λcal (nm)

f

MPt

4.08

4.14

5.30

4.71

TPt

6.04

4.82

444 569 491 597 495 591

0.7606

DPt

S1 T1 S1 T1 S1 T1

1.0317 0.7873

percentage contribution of H → L to S1 or T1 H H H H H H

→ → → → → →

L L L L L L

(95.91%) (90.53%) (97.70%) (88.34%) (97.55%) (81.30%)

H → L: HOMO-to-LUMO transition; λcal: calculated wavelength; and f: oscillator strength. 10231


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Figure 4. Device configuration and molecular structures of the materials for the OLEDs.

Figure 5. EL characteristics of OLEDs based on MPt, DPt, and TPt: (a) EL spectra, (b) J−V−L properties, (c) curves of EQE vs luminance, and (d) curves of CE and PE vs luminance.

supporting electrolyte. The ferrocene/ferrocenium couple (Fc/ Fc+) was used as the reference (Figure S7). Two irreversible oxidation processes were observed for MPt, with the anodic peak potentials being located at 0.44 and 0.99 V, which are assigned to the oxidation process of the electron-rich triphenylamine core and the Pt(II) center, respectively.44,45 Complexes DPt and TPt also showed two irreversible oxidation processes similar to MPt. However, the first anodic peak potentials of DPt and TPt were 0.47 and 0.48 V, respectively.

Their slightly increased anodic peak potentials may be caused by the fact that more electron-acceptors in DPt and TPt will lower the electron density of the triphenylamine core. Therefore, the HOMO levels of DPt and TPt (−5.27 and −5.28 eV, respectively) are slightly deeper than that of MPt (−5.24 eV), but they are still higher than that of (ppy)Pt(acac) (−5.32 eV) without triphenylamine group,46 indicating the influence of the triphenylamine core on the HOMO levels. During the cathodic scan process, MPt, DPt, and TPt showed 10232


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ACS Applied Materials & Interfaces Table 3. EL Data for the Solution-Processed Devices

a

device

emitter

λmax EL (nm)

V (V)a

Lmax (cd m−2)

EQE (%)b

CE (cd A−1)b

PE (lm W−1)b

A1 A2 A3 B1 B2 B3 C1 C2 C3

MPt (7 wt %) MPt (9 wt %) MPt (11 wt %) DPt (7 wt %) DPt (9 wt %) DPt (11 wt %) TPt (7 wt %) TPt (9 wt %) TPt (11 wt %)

580 580 580 592 592 592 592 592 592

3.8/7.8/10.7 3.3/7.6/10.6 3.6/8.9/12.5 3.4/7.8/11.7 3.3/6.8/10.2 3.5/8.2/13.2 3.0/6.5/10.2 3.0/7.1/10.0 3.7/8.5/11.8

21 204 23 784 18 934 23 555 22 895 17 542 30 355 45 475 33 812

5.7/4.8 6.4/5.6 4.7/4.4 8.1/6.7 10.5/10.1 6.9/6.2 14.0/12.9 17.0/15.9 11.2/10.3

12.8/10.7 14.4/12.5 10.5/9.7 16.5/13.7 21.4/20.5 14.0/12.6 29.1/27.0 35.4/33.2 24.0/21.2

11.4/4.3 12.1/5.2 7.5/3.4 13.9/5.5 12.9/9.4 8.0/4.9 17.7/13.1 27.2/14.6 13.2/7.7

Driving voltages (V) in the order of at 1, 1000, and 10 000 cd m−2. bEQE, CE, and PE in the order of the maximum value and at 1000 cd m−2.

and better charge balance with more Pt(II) centers, demonstrating the great potential of developing highly efficient phosphorescent emitters with multi-metal centers.

similar reduction behaviors, with the cathodic peak potentials being located at −2.14, −2.12, and −2.11 V, respectively, which may result from the reduction of benzo[d]thiazole moieties. Anyway, with more benzo[d]thiazole moieties, trinuclear complex TPt possesses a deeper LUMO level of −2.69 eV, which may be beneficial to the charge injection process in OLEDs. 2.5. EL Characterization. To evaluate the EL properties of these Pt(II) complexes, solution-processed OLEDs were fabricated with the conventional device structure of indium tin oxide (ITO)/PEDOT:PSS (45 nm)/x wt % Pt emitter:TCTA (30 nm)/TPBI (45 nm)/LiF (1 nm)/Al (100 nm), in which the poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer was the hole injection/transport layer. The compound 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) was used as the host, and these Pt(II) complexes were doped at the levels of 7, 9, and 11 wt % (Figure 4). The compounds 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) and LiF were used as the electron transport and the electron injection materials, respectively. The EL characteristics of OLEDs based on MPt, DPt, and TPt are shown in Figures 5, S8, and S9. The key EL data are summarized in Table 3. Although the EL spectra show feeble emissions located around 510 nm, all of the OLEDs can emit intense orange light with EL emission peaks similar to the phosphorescence peaks in PL spectra obtained from the doped films (Figure 1c), indicating that the EL emissions predominately originate from the triplet excited states of these Pt(II) complexes. The turn-on voltages of these OLEDs are in the range of 3.0−4.0 V, which decrease slightly as the number of Pt(II) centers in the Pt(II) complexes increases. The devices based on the mononuclear Pt(II) complex MPt show common efficiencies with the peak EQEs, CEs, and PEs over 4.7%, 10.5 cd A−1, and 7.5 lm W−1, respectively. However, the devices based on the dinuclear Pt(II) complex DPt display much better performance. The peak efficiencies are significantly increased up to 10.5%, 21.4 cd A−1, and 12.9 lm W−1 and can still remain as high as 10.1%, 20.5 cd A−1, and 9.4 lm W−1, respectively, at a high luminescence of 1000 cd m−2. To the best of our knowledge, this is the best performance ever reported for OLEDs using dinuclear Pt(II) complexes as emitters.21−25,47−49 The most exciting result is that the device based on the trinuclear Pt(II) complex TPt achieves unexpected high peak efficiencies of 17.0%, 35.4 cd A−1, and 27.2 lm W−1 with very low efficiency roll-off at a high luminescence of 1000 cd m−2. This is a benchmark for OLEDs based on trinuclear Pt(II) complexes.30 The remarkably improved device performance can be attributed to the significantly increased PLQYs from the mononuclear to the dinuclear and the trinuclear Pt(II) complex

3. CONCLUSIONS Phosphorescent Pt(II) complexes with one, two, and three Pt(II) centers have been designed and synthesized to investigate the effects of the heavy-metal center on the properties of the resultant emitters. Compared with the mononuclear Pt(II) complexes, the multinuclear Pt(II) complexes exhibit red-shifted absorption and emission bands. Moreover, by incorporating more Pt(II) centers, the phosphorescence emission can be greatly enhanced, with the PLQYs increasing from 0.24 to 0.74 in solutions. Accordingly, the device performance of solution-processed orange OLEDs using these Pt(II) complexes is remarkably improved. The peak EQE, CE, and PE of the device based on the trinuclear Pt(II) complex are 17.0%, 35.4 cd A−1, and 27.2 lm W−1, respectively, which are over two-fold higher than those of the device based on the mononuclear Pt(II) complex. These results demonstrate that it is promising to increase the PLQYs as well as the related EL efficiencies by incorporating more heavy-metal centers into the organometallic emitters, providing a new way to develop high-performance phosphorescent materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18330. Detailed synthesis of the Pt(II) complexes, NMR, TGA, CV tests, and EL efficiencies of other devices (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.Z.). *E-mail: [email protected] (W.-Y.W.). ORCID

Yong Wu: 0000-0002-9460-3579 Guijiang Zhou: 0000-0002-5863-3551 Wai-Yeung Wong: 0000-0002-9949-7525 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support received from the National Natural Science Foundation of China (nos. 21602170 and 21572176), the Natural Science Foundation of 10233


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better than one: bright phosphorescence from non-stereogenic dinuclear iridium (III) complexes. Dalton Trans. 2016, 45, 6949−6962. (18) Lanoë, P.-H.; Tong, C. M.; Harrington, R. W.; Probert, M. R.; Clegg, W.; Williams, J. A. G.; Kozhevnikov, V. N. Ditopic bisterdentate cyclometallating ligands and their highly luminescent dinuclear iridium(III) complexes. Chem. Commun. 2014, 50, 6831− 6834. (19) Kozhevnikov, V. N.; Durrant, M. C.; Williams, J. A. G. Highly Luminescent Mixed-Metal Pt(II)/Ir(III) Complexes: Bis-Cyclometalation of 4,6-Diphenylpyrimidine As a Versatile Route to Rigid Multimetallic Assemblies. Inorg. Chem. 2011, 50, 6304−6313. (20) Culham, S.; Lanoë, P.-H.; Whittle, V. L.; Durrant, M. C.; Williams, J. A. G.; Kozhevnikov, V. N. Highly Luminescent Dinuclear Platinum(II) Complexes Incorporating Bis-Cyclometallating PyrazineBased Ligands: A Versatile Approach to Efficient Red Phosphors. Inorg. Chem. 2013, 52, 10992−11003. (21) Wang, Y.; Luo, J.; Liu, Y.; Zhang, Z.; Tan, H.; Yu, J.; Lei, G.; Zhu, M.; Zhu, W. Dinuclear cyclometalated platinum (II) complexes: Synthesis, photophysics, and monomolecular electroluminescence. Org. Electron. 2012, 13, 1646−1653. (22) Luo, J.; Liu, Y.; Chen, Q.; Shi, D.; Huang, Y.; Yu, J.; Wang, Y.; Zhang, Z.; Lei, G.; Zhu, W. Synthesis, optoelectronic properties of a dinuclear platinum (II) complex containing a binary cyclometalated ligand in the single-emissive-layer PLEDs. Dalton Trans. 2013, 42, 1231−1237. (23) Xiong, W.; Meng, F.; Tan, H.; Wang, Y.; Wang, P.; Zhang, Y.; Tao, Q.; Su, S.; Zhu, W. Dinuclear platinum complexes containing aryl-isoquinoline and oxadiazole-thiol with an efficiency of over 8.8%: in-depth investigation of the relationship between their molecular structure and near-infrared electroluminescent properties in PLEDs. J. Mater. Chem. C 2016, 4, 6007−6015. (24) Su, N.; Meng, F.; Chen, J.; Wang, Y.; Tan, H.; Su, S.; Zhu, W. Near-infrared emitting pyrazole-bridged binuclear platinum complexes: Synthesis, photophysical and electroluminescent properties in PLEDs. Dyes Pigm. 2016, 128, 68−74. (25) Tronnier, A.; Strassner, T. (C^C*) Cyclometalated binuclear Nheterocyclic biscarbene platinum(ii) complexes-highly emissive phosphorescent emitters. Dalton Trans. 2013, 42, 9847−9851. (26) Lu, W.; Chan, M. C. W.; Cheung, K.-K.; Che, C.-M. π-π Interactions in Organometallic Systems. Crystal Structures and Spectroscopic Properties of Luminescent Mono-, Bi-, and Trinuclear Trans-cyclometalated Platinum (II) Complexes Derived from 2, 6Diphenylpyridine. Organometallics 2001, 20, 2477−2486. (27) Liu, Q.-D.; Jia, W.-L.; Wu, G.; Wang, S. Binuclear and Starburst Organoplatinum (II) Complexes of 2, 2’-Dipyridylamino Derivative Ligands: Structures, Fluxionality, and Luminescence. Organometallics 2003, 22, 3781−3791. (28) Kui, S. C. F.; Sham, I. H. T.; Cheung, C. C. C.; Ma, C.-W.; Yan, B.; Zhu, N.; Che, C.-M.; Fu, W.-F. Platinum (II) Complexes with πConjugated, Naphthyl-Substituted, Cyclometalated Ligands (RC^N^N): Structures and Photo- and Electroluminescence. Chem.Eur. J. 2007, 13, 417−435. (29) Shao, P.; Sun, W. Trinuclear Platinum (II) 4, 6-Diphenyl-2, 2’bipyridyl Complex with Bis(diphenylphosphinomethyl) phenylphosphine Auxiliary Ligand: Synthesis, Structural Characterization, and Photophysics. Inorg. Chem. 2007, 46, 8603−8612. (30) Shi, D.; Wang, Y.; Liu, Y.; Zhang, Z.; Luo, J.; He, J.; Chen, Q.; Lei, G.; Zhu, W. Star-Shaped Trinuclear Cyclometalated Platinum (II) Complexes as Single-Component Emitters in White-Emitting PLEDs. Chem.Asian J. 2012, 7, 2096. (31) Kwon, O.; Barlow, S.; Odom, S. A.; Beverina, L.; Thompson, N. J.; Zojer, E.; Brédas, J.-L.; Marder, S. R. Aromatic amines: A comparison of electron-donor strengths. J. Phys. Chem. A 2005, 109, 9346−9352. (32) Rausch, A. F.; Murphy, L.; Williams, J. A. G.; Yersin, H. Improving the Performance of Pt(II) Complexes for Blue Light Emission by Enhancing the Molecular Rigidity. Inorg. Chem. 2012, 51, 312−319.

Shaanxi Province (no. 2016JQ2011), the China Postdoctoral Science Foundation (no. 2015M580831), the Fundamental Research Funds for the Central Universities (nos. cxtd2015003 and xjj2016061), and Creative Scientific Research Team in Yulin City.

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REFERENCES

(1) Schwartz, G.; Reineke, S.; Rosenow, T. C.; Walzer, K.; Leo, K. Triplet Harvesting in Hybrid White Organic Light-Emitting Diodes. Adv. Funct. Mater. 2009, 19, 1319−1333. (2) Fleetham, T.; Li, G.; Li, J. Phosphorescent Pt (II) and Pd (II) Complexes for Efficient, High-Color-Quality, and Stable OLEDs. Adv. Mater. 2017, 29, 1601861. (3) Ma, Y.; Zhang, H.; Shen, J.; Che, C. Electroluminescence from triplet metal-ligand charge-transfer excited state of transition metal complexes. Synth. Met. 1998, 94, 245−248. (4) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 1998, 395, 151−154. (5) Chou, P.-T.; Chi, Y. Phosphorescent dyes for organic lightemitting diodes. Chem.Eur. J. 2007, 13, 380−395. (6) Fan, C.; Yang, C. Yellow/orange emissive heavy-metal complexes as phosphors in monochromatic and white organic light-emitting devices. Chem. Soc. Rev. 2014, 43, 6439−6469. (7) Yang, X.; Zhou, G.; Wong, W.-Y. Functionalization of phosphorescent emitters and their host materials by main-group elements for phosphorescent organic light-emitting devices. Chem. Soc. Rev. 2015, 44, 8484−8575. (8) Tseng, C.-H.; Fox, M. A.; Liao, J.-L.; Ku, C.-H.; Sie, Z.-T.; Chang, C.-H.; Wang, J.-Y.; Chen, Z.-N.; Lee, G.-H.; Chi, Y. Luminescent Pt(II) complexes featuring imidazolylidene-pyridylidene and dianionic bipyrazolate: from fundamentals to OLED fabrications. J. Mater. Chem. C 2017, 5, 1420−1435. (9) Liao, J.-L.; Chi, Y.; Wang, J.-Y.; Chen, Z.-N.; Tsai, Z.-H.; Hung, W.-Y.; Tseng, M.-R.; Lee, G.-H. Pt(II) Phosphors Featuring Both Dicarbene and Functional Biazolate Chelates: Synthesis, Luminescent Properties, and Applications in Organic Light-Emitting Diodes. Inorg. Chem. 2016, 55, 6394−6404. (10) Hsu, C.-W.; Ly, K. T.; Lee, W.-K.; Wu, C.-C.; Wu, L.-C.; Lee, J.J.; Lin, T.-C.; Liu, S.-H.; Chou, P.-T.; Lee, G.-H.; Chi, Y. Triboluminescence and Metal Phosphor for Organic Light-Emitting Diodes: Functional Pt(II) Complexes with Both 2-Pyridylimidazol-2ylidene and Bipyrazolate Chelates. ACS Appl. Mater. Interfaces 2016, 8, 33888−33898. (11) Bullock, J. D.; Salehi, A.; Zeman, C. J.; Abboud, K. A.; So, F.; Schanze, K. S. In Search of Deeper Blues: Trans-N-Heterocyclic Carbene Platinum Phenylacetylide as a Dopant for Phosphorescent OLEDs. ACS Appl. Mater. Interfaces 2017, 9, 41111−41114. (12) Chou, P.-T.; Chi, Y.; Chung, M.-W.; Lin, C.-C. Harvesting luminescence via harnessing the photophysical properties of transition metal complexes. Coord. Chem. Rev. 2011, 255, 2653−2665. (13) Yersin, H.; Rausch, A. F.; Czerwieniec, R.; Hofbeck, T.; Fischer, T. The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs. Coord. Chem. Rev. 2011, 255, 2622−2652. (14) You, Y.; Park, S. Y. Phosphorescent iridium(III) complexes: toward high phosphorescence quantum efficiency through ligand control. Dalton Trans. 2009, 1267−1282. (15) Kalinowski, J.; Fattori, V.; Cocchi, M.; Williams, J. A. G. Lightemitting devices based on organometallic platinum complexes as emitters. Coord. Chem. Rev. 2011, 255, 2401−2425. (16) Turnbull, G.; Williams, J. A. G.; Kozhevnikov, V. N. Rigidly linking cyclometallated Ir (III) and Pt (II) centres: an efficient approach to strongly absorbing and highly phosphorescent red emitters. Chem. Commun. 2017, 53, 2729−2732. (17) Daniels, R. E.; Culham, S.; Hunter, M.; Durrant, M. C.; Probert, M. R.; Clegg, W.; Williams, J. A. G.; Kozhevnikov, V. N. When two are 10234


Research Article


physical and electroluminescent properties. Dyes Pigm. 2017, 138, 162−168.

(33) Yang, X.; Feng, Z.; Zhao, J.; Dang, J.-S.; Liu, B.; Zhang, K.; Zhou, G. Pyrimidine-Based Mononuclear and Dinuclear Iridium (III) Complexes for High Performance Organic Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2016, 8, 33874−33887. (34) Yang, X.; Xu, X.; Dang, J.-s.; Zhou, G.; Ho, C.-L.; Wong, W.-Y. From mononuclear to dinuclear iridium (III) complex: effective tuning of the optoelectronic characteristics for organic light-emitting diodes. Inorg. Chem. 2016, 55, 1720−1727. (35) Mongin, O.; Mallegol, T.; Gmouh, S.; Meziane, M. A.; Blanchard-Desce, M. Practical and efficient synthesis of tris (4formylphenyl) amine, a key building block in materials chemistry. Synthesis 2005, 1771−1774. (36) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.; Bau, R.; Thompson, M. E. Synthesis and characterization of phosphorescent cyclometalated platinum complexes. Inorg. Chem. 2002, 41, 3055−3066. (37) Hofbeck, T.; Yersin, H. The Triplet State of fac-Ir(ppy)3. Inorg. Chem. 2010, 49, 9290−9299. (38) Lu, K.-Y.; Chou, H.-H.; Hsieh, C.-H.; Yang, Y.-H. O.; Tsai, H.R.; Tsai, H.-Y.; Hsu, L.-C.; Chen, C.-Y.; Chen, I-C.; Cheng, C.-H. Wide-Range Color Tuning of Iridium Biscarbene Complexes from Blue to Red by Different N^N Ligands: an Alternative Route for Adjusting the Emission Colors. Adv. Mater. 2011, 23, 4933−4937. (39) Chan, S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung, K.-K.; Zhu, N. Organic Light-Emitting Materials Based on Bis(arylacetylide)platinum(II) Complexes Bearing Substituted Bipyridine and Phenanthroline Ligands: Photo- and Electroluminescence from 3 MLCT Excited States. Chem.Eur. J. 2001, 7, 4180−4190. (40) Kavitha, J.; Chang, S.-Y.; Chi, Y.; Yu, J.-K.; Hu, Y.-H.; Chou, P.T.; Peng, S.-M.; Lee, G.-H.; Tao, Y.-T.; Chien, C.-H.; Carty, A. J. In Search of High-Performance Platinum(II) Phosphorescent Materials for the Fabrication of Red Electroluminescent Devices. Adv. Funct. Mater. 2005, 15, 223−229. (41) Mao, H.-T.; Zang, C.-X.; Wen, L.-L.; Shan, G.-G.; Sun, H.-Z.; Xie, W.-F.; Su, Z.-M. Ir(III) Phosphors Modified with Fluorine Atoms in Pyridine-1,2,4-triazolyl Ligands for Efficient OLEDs Possessing Low-Efficiency Roll-off. Organometallics 2016, 35, 3870−3877. (42) Farley, S. J.; Rochester, D. L.; Thompson, A. L.; Howard, J. A. K.; Williams, J. A. G. Controlling emission energy, self-quenching, and excimer formation in highly luminescent N^C^N-coordinated platinum (II) complexes. Inorg. Chem. 2005, 44, 9690−9703. (43) Sun, W.; Zhang, B.; Li, Y.; Pritchett, T. M.; Li, Z.; Haley, J. E. Broadband nonlinear absorbing platinum 2, 2’-bipyridine complex bearing 2-(benzothiazol-2’-yl)-9, 9-diethyl-7-ethynylfluorene ligands. Chem. Mater. 2010, 22, 6384−6392. (44) Hu, Z.; Wang, Y.; Shi, D.; Tan, H.; Li, X.; Wang, L.; Zhu, W.; Cao, Y. Highly-efficiency red-emitting platinum (II) complexes containing 4’-diarylamino-1-phenylisoquinoline ligands in polymer light-emitting diodes: Synthesis, structure, photoelectron and electroluminescence. Dyes Pigm. 2010, 86, 166−173. (45) He, Z.; Wong, W.-Y.; Yu, X.; Kwok, H.-S.; Lin, Z. Phosphorescent platinum(II) complexes derived from multifunctional chromophores: Synthesis, structures, photophysics, and electroluminescence. Inorg. Chem. 2006, 45, 10922−10937. (46) Luo, J.; Liu, Y.; Shi, D.; Wang, Y.; Zhang, Z.; Yu, J.; Lei, G.; Chen, Q.; Li, J.; Deng, X.; Zhu, W. Synthesis and optoelectronic properties of a carbazole-modified platinum (II) complex in polymer light-emitting devices. Dalton Trans. 2012, 41, 1074−1081. (47) Ma, B.; Djurovich, P. I.; Garon, S.; Alleyne, B.; Thompson, M. E. Platinum Binuclear Complexes as Phosphorescent Dopants for Monochromatic and White Organic Light-Emitting Diodes. Adv. Funct. Mater. 2006, 16, 2438−2446. (48) Wu, X.; Liu, Y.; Wang, Y.; Wang, L.; Tan, H.; Zhu, M.; Zhu, W.; Cao, Y. Highly efficient near-infrared emission from binuclear cyclometalated platinum complexes bridged with 5-(4-octyloxyphenyl)1,3,4-oxadiazole-2-thiol in PLEDs. Org. Electron. 2012, 13, 932−937. (49) Su, N.; Meng, F.; Wang, P.; Liu, X.; Zhu, M.; Zhu, W.; Su, S.; Yu, J. Near-infrared emission from binuclear platinum (II) complexes containing pyrenylpyridine and pyridylthiolate units: Synthesis, photo10235


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