Novel Bipolar Indole-Based Solution-Processed Host Material for

Mar 6, 2017 - A new bipolar host material 2-(2′-(5-(dibenzo[b,d]furan-4-yl)-1H-indol-1-yl)-1,1′-biphenyl]-4-yl)-1-phenyl-1H-benzo[d]imidazole (IND...
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Novel Bipolar Indole-Based Solution-Processed Host Material for Efficient Green and Red Phosphorescent OLEDs Yi Chen,†,⊥ Xiang Wei,‡,⊥ Jin Cao,‡ Jinhai Huang,§ Lei Gao,∥ Jianhua Zhang,*,‡ Jianhua Su,*,† and He Tian† †

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ‡ Key Laboratory of Advanced Display and System Applications, Ministry of Education, Shanghai University, 149 Yanchang Road, Shanghai 200072, China § Shanghai Taoe Chemical Technology Company, Ltd., Shanghai 200030, China ∥ School of Chemistry and Chemical Engineering, Queen’s University, Belfast BT7 1NN, Northern Ireland, United Kingdom S Supporting Information *

ABSTRACT: A new bipolar host material 2-(2′-(5-(dibenzo[b,d]furan-4-yl)-1H-indol-1-yl)-1,1′-biphenyl]-4-yl)-1-phenyl-1H-benzo[d]imidazole (INDY) has been designed and synthesized for solution-processed PHOLEDs. A hole-transporting indole conjugated dibenzofuran scaffold and an electron-transporting benzimidazole unit were orthogonally connected with incorporation of a 2,4′-biphenyl bridge, which endows a balanced bipolar charge transfer distribution and appropriate triplet energy level (>2.6 eV). The twisted conformation of INDY also ensures good thermal/ morphological stability with high Tg (148 °C). The solutionprocessed green PHOLEDs achieved great device efficiencies with 27.33 cd/A and 12.26 lm/W at 7 V. Meanwhile, the red solution-processed PHOLEDs gained excellent performance with the maximum efficiencies of 17.20 cd/A, 9.82 lm/W, and 12.61%. In addition, all of the red devices presented excellent color stability with the same CIE coordinate as (0.65, 0.35) under varying dopant contents and operating voltages. KEYWORDS: indole derivatives, bipolar host material, highly efficient, phosphorescent OLEDs, high thermal stability

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INTRODUCTION Phosphorescence OLEDs (PHOLEDs) have indeed attracted extensive attention from academic research and commercial industry for flat panel display and light devices. Since the previous work of host−dopant structure from Forrest’s group,1 considerable efforts have been focused on the design of host and phosphor materials to peruse an ideal PHOLEDs performance. As a mature technique in the display industry, vacuum-deposited PHOLEDs have already achieved excellent device efficiencies with external quantum efficiency (EQE) over 30%.2,3 Nevertheless, to realize the mass production of organic electronics with high resolution, low cost, and large area, more attention has been paid to solution-based processes instead of the vacuum-deposited process.4,5 To meet these stringent requirements from the market, it is also necessary to design solution-processable host materials with integrated properties as in thermal vacuum-deposited PHOLEDs. In principle, polymer host materials, such as poly(pphenylenevinylene) (PPV) and poly(9-vinylcarbazole) (PVK),6−9 can access the large-scale solution-processed PHOLEDs with high film density and uniform film morphology. However, the intrinsic deficiencies of these materials’ uncertain molecular structure, harsh purification © 2017 American Chemical Society

method, and low triplet energy state limit their application in commercial production. Compared to the polymers, small molecule host materials with merits of certain molecular structure, high purity, and stable thermal properties are believed to be an alternative choice for solution-processed PHOLEDs. However, the investigations on soluble small molecular host materials were limited by the poor solubility, which could easily form self-aggregation and crystallization with the result of uncontrolled film thickness and nonuniform surface morphology.10 To solve this issue, decoration of bulk alky/alkoxy chains on host molecules has been utilized to improve their solubility in common organic solvents.11,12 However, the introduction of these bulk substituents often reduces electron-transporting ability and then deteriorates device efficiency and stability.13 Therefore, it is still worth exploring the study on the design and optimum of novel small molecular structure for solutionprocessed PHOLEDs. Apart from the good solubility, other basic requirements, such as reasonable HOMO and LUMO energy levels to Received: November 30, 2016 Accepted: March 6, 2017 Published: March 6, 2017 14112

DOI: 10.1021/acsami.6b15358 ACS Appl. Mater. Interfaces 2017, 9, 14112−14119

Research Article

ACS Applied Materials & Interfaces Scheme 1. Structure and Synthetic Routes of Compound INDY

Figure 1. (a) Absorption and fluorescence spectra in toluene (r.t.) and phosphorescent spectra (77 K) in 2-methyltetrahydrofuran of INDY and (b) emission spectra in various solvents.

efficiencies of red solution-processed PHOLEDs by using a bipolar small molecule as the single host in the previous work. In addition, red devices showed color stability with CIE (0.65, 0.35) independent of dopant content and operating voltage.

maintain available carrier injection, high triplet energy level to suppress reverse energy transfer from guest to host, and balanced charge migration to broaden exciton recombination area, should also be satisfied for the novel soluble bipolar host material. Enlightened from the previous work, indole derivatives with high hole-transporting ability and satisfied solubility in organic solvents could be used as a donor.14,15 Meanwhile, the dibenzofuran moiety with high triplet energy level and good thermal stability is a good candidate as an electron acceptor.16,17 However, relatively weak electrontransportability of the dibenzofuran group limited its use as an electron-transporting unit in the bipolar host molecule. Hence, another electron deficient moiety benzimidazole with good electron-transporting ability18,19 could be imported to moderate molecular hole-transporting−electron-transporting property and to facilitate charge balance in the emitting layer.20 Therefore, we developed a 2,4′-biphenyl bridge to connect the hole-transporting indole conjugated dibenzofuran scaffold and the electron-transporting benzimidazole unit, named as 2-(2′-(5-(dibenzo[b,d]furan-4-yl)-1H-indol-1-yl)[1,1′-biphenyl]-4-yl)-1-phenyl-1H-benzo[d]imidazole (INDY). The steric hindrance with the help of biphenyl linker could interrupt the π-conjugation between the donor−acceptor system and maintain the triplet energy level.21−23 The distorted molecular structure could also improve the thermostability defect of indole derivatives and remedy the poor solubility issue. INDY owns good solubility in several organic solvents, which is appreciated in solution-process PHOLEDs. Hence, both green and red PHOLEDs based on host INDY were fabricated, and excellent device performance was obtained. After device optimizing, INDY-based green PHOLEDs achieved 27.33 cd/A for the maximum current efficiency and 12.26 lm/W for the maximum power efficiency at 7 V. Furthermore, the red device with conventional device structure achieved the maximum efficiencies of 17.20 cd/A, 9.82 lm/W, and 12.61%, which can be comparable to the best device

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RESULTS AND DISCUSSION A novel indole molecule denoted as INDY was synthesized as host material for green and red solution-process PHOLEDs. The molecule structure and its synthetic route were described in Scheme 1. The intermediates 5-(dibenzo[b,d]furan-4-yl)-1Hindole (1) and 1-(2-bromophenyl)-5-(dibenzo[b,d]furan-4-yl)1H- indole (2) were synthesized from raw materials according to the methods reported in the previous literature.24 The desired product INDY can be obtained by the Suzuki crosscoupling reaction of the (4-(1-phenyl-1H-benzo[d]imidazol-2yl)phenyl)boronic acid with the bromide intermediates (2) in 73−76% yields. The intermediates and end product were characterized by 1H and 13C NMR spectral and mass spectrometry. Further refinement of the final compounds was readily accomplished by sublimation in vacuum to reach a high purity to meet the requirement of OLEDs application. The absorption and photoluminescence spectra of INDY in toluene were carried out to investigate its photophysical properties. As depicted in Figure 1a, a broad absorption band around 335 nm was observed, which could be assigned to the π−π* transitions of INDY. According to the absorption edge, its corresponding optical band gap (approximate to the S1 level) could be calculated as 3.44 eV, which was higher than the commonly used green and red phosphor emitters. To study the effect of solvent polarity on the optical performance, the electronic absorption and fluorescence spectra of compound INDY in various solvents are measured (Figure 1b and Supporting Information Figure S1). All absorption spectra exhibit a similar emission peak in various solvents. However, the fluorescence emissions manifest solvatochromic effect with the increase of solvent polarity, which corresponds to the weak 14113

DOI: 10.1021/acsami.6b15358 ACS Appl. Mater. Interfaces 2017, 9, 14112−14119

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ACS Applied Materials & Interfaces

the surface morphology of the film made by spin-coating of host solution on the hole-transport layer and annealing under different annealing temperatures (50, 90, and 145 °C) for 30 min in nitrogen atmosphere, as depicted in Figure 2b. We found that the surface morphology of INDY films turned into quite smooth ones with the increasing of annealing temperatures with root-mean-square roughness (RMS) values of about 5.1 Å in 50 °C, 3.8 Å in 90 °C, and 3.5 Å in 145 °C. All these results indicated that compound INDY possesses uniform and excellent film morphological properties, beneficial for employing in the PHOLEDs. The electrochemical property of INDY was investigated by cyclic voltammetry (Figure. S2). It can be seen from the Figure S2 that the onset oxidation and reduction potential of compound INDY are 1.25 V and −1.89 V. The highest occupied molecular orbital energy level was calculated as −5.69 eV for INDY by comparison to ferrocene (4.8 eV versus vacuum), which was lower than those of other indole-based derivatives with unsubstituted indole rings (EHOMO ∼ 5.9 eV).27 And the lowest unoccupied molecular orbital energy level of −2.40 eV was obtained from onset reduction potential, which was consistent with the ELUMO (−2.25 eV) determined by the EHOMO and optical band gap. It is evident that the introduction of benzimidazole significantly affects the molecular orbitals (MOs) distribution. To further examine MOs and electronic structures of INDY, density functional theory (DFT) was performed by the restricted exchange-correlation method B3LYP using the Gaussian software package in organic molecular models.28 According to the calculation of DFT (Figure. 3), the HOMO

CT-state character from the indole moiety and benzimidazole group.25,26 Additionally, the triple energy level calculated from the phosphorescent spectra at 77 K was 2.63 eV for compound INDY, which qualifies it as a host for green and red phosphorescence electroluminescence. The thermal stabilization of compound INDY could be investigated by measuring the value of Td and Tg through thermal gravimetric analyses (TGA) and differential scanning calorimetry (DSC) (Figure 2 and Table 1). INDY exhibits great

Figure 2. (a) TGA and DSC curve of compound INDY. (b) AFM images of solution-processed film of host material at different annealing temperatures.

Figure 3. Spatial distributions of the frontier molecular orbitals of INDY.

thermal and morphological stabilities, with Td (corresponding to 5% weight loss) of up to 418 °C and Tg of up to 148 °C, which was higher than the other indole-substituted compounds owing to its twisted molecular geometry.18 To further investigate the morphological stability of INDY, the atomic force microscopy (AFM) could be used to analyze

orbitals of INDY are mainly distributed on the indole and dibenzofuran units, and a small contribution from the N-phenyl group, while the LUMO orbital energy level is distributed on the benzimidazole unit and extended to the N-phenyl moiety, providing a channel for hole-transporting−electron-transporting and improving bipolar-transporting ability. Since an ortho-

Table 1. Device Characteristics of Solution-Processed Green PHOLEDs device

Von (V)a

Lmax (cd/m2)

ηc (cd/A)b

ηp (lm/W)c

ηext (%)d

G1a G1b G1c G2a G2b G2c G3a G3b G3c G3d

5.0 4.2 4.0 8.6 5.3 4.2 6.1 5.3 5.1 5.5

6607 5577 5020 2558 6887 5577 1530 2060 2934 4301

15.98 18.60 12.16 14.94 20.96 18.60 22.63 23.26 27.33 24.14

7.01 9.13 6.82 4.91 10.31 9.14 9.10 10.30 12.26 10.83

4.19 4.80 3.16 3.72 5.22 4.81 5.67 5.82 7.10 6.16

CIE(x,y)e 0.35, 0.34, 0.35, 0.35, 0.33, 0.34, 0.33, 0.34, 0.33, 0.33,

0.61 0.62 0.61 0.61 0.63 0.62 0.63 0.62 0.63 0.63

a

At 1 cd/m2. bMaximum current efficiency. cMaximum power efficiency. dMaximum external quantum efficiency. eBy inverting the EL spectra into the CIE 1931 diagram. 14114

DOI: 10.1021/acsami.6b15358 ACS Appl. Mater. Interfaces 2017, 9, 14112−14119

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ACS Applied Materials & Interfaces linking strategy could increase molecular distortion, a more delocalized LUMO spatial distribution is also observed due to the molecular twisted degree as verified from the different dihedral angles between each adjacent aromatic rings (49.3° between N-phenyl and benzimidazole and 61.4° between indole and N-phenyl). This moderate structural tuning not only can adjust the HOMO−LUMO energy level to match the frontier orbital of HTL and ETL in PHOLEDs but also can contribute to charge hopping as a result of the intermolecular interaction beingenhanced. Electroluminescent Devices. To have a keen insight into the bipolar charge-transporting characteristics of this host material, devices with the configuration of ITO/PEDOT:PSS (35 nm)/host (100 nm)/MoO3 (5 nm)/Al (120 nm) for a hole-only device and ITO/host (100 nm)/LiF (1 nm)/Al (120 nm) for an electron-only device were fabricated. As depicted in Figure 4, hole and electron current densities both exhibited a

Figure 5. Device structure and energy levels of materials used in this article.

device gained the maximum device efficiencies of 18.74 cd/A and 9.13 lm/W. Since device G1c obtained great J−V−L properties, the reduction in the efficiency may be due to triplet exciton quenching as the result of the available high exciton density formed in the emissive layers (EMLs). Furthermore, to explore the effect of annealing temperatures made on the device performance, three green phosphorescence OLEDs with the dopant concentration of 12% under different annealing temperatures were designed and characterized. The J−V−L and efficiencies curves of these devices were depicted in the Figure 6 and all the related data summarized in the Table 1. From the current−voltage curves, the current densities of devices G2a (50 °C), G2b (90 °C), and G2c (145 °C) increased in sequence, which is consistent with the observation in the luminance−voltage curves. The turn-on voltage at the luminance of 1 cd/m2 also shows a regular trend. These features could be well-understood if the effect of annealing temperature on thin film morphologies aforementioned is taken into consideration. As we know, the morphologies of thin film could make a dramatic impact on the charge carrier transporting/recombination and then affect the device performance. Both device G2b (90 °C) and device G2c (145 °C) with high annealing temperature pretreatments presented better device efficiencies than device G2a (50 °C), indicating that the increase of annealing temperatures could improve the film morphology with better packing density and leading to a better device performance. Meanwhile, the best performance among the three green devices was obtained from device G2b (90 °C), which exhibited the maximum CE of and PE value up to 20.96 cd/A and 10.31 lm/W. In order to further optimize device structure with a balanced hole/electron mobility, four green devices G3a (20 nm), G3b (25 nm), G3c (30 nm), and G3d (35 nm) based on different HTL thicknesses with the dopant content of 12% and annealing temperature of 90 °C were formed and measured (as displayed in the Figure 6). Generally, the thicknesses of the holetransport layer (HTL) could impact the balance of holetransporting−electron-transporting and injection into the EMLs, giving rise to effective exciton recombination and good device efficiencies. Therefore, as the thickness of the holetransporting layer intensified from 20 to 35 nm, these devices exhibited regularly increased current densities and luminance as shown by J−V−L curves. When the thickness reach 30 nm, charge transporting balanced and then the exciton recombination area expanded, leading to higher efficiencies among the

Figure 4. Current density−voltage curves of hole-only device and electron-only device for INDY.

rising trend with the increase of voltage, which implied that INDY as host material revealed a bipolar carrier-transporting property. It was worth noting that the electron current density was found to be slightly higher than the hole current density at the same voltage leading to a balanced carriers transporting ascribed to the introduction of an electron deficient moiety and ortho-linkage strategy. To access the feasibility of INDY as a host material in the solution-processed PHOLEDs, three types of green PHOLEDs (G1, G2, and G3) with a conventional device configuration of ITO/PEDOT:PSS (35 nm)/TFB/INDY:G (35 nm)/TmPyPb (40 nm)/LiF (1 nm)/Al (120 nm) were fabricated. The energy levels and molecular structure of those materials used in this work were presented in Figure 5. To study the impact of dopant content on the electroluminescence properties, devices with three different dopant concentrations (10%, 12%, and 14%) were designed and characterized. From the current density−voltage−luminance curves in Figure 6, both luminance and current density of the devices increase in the order of G1a (10%) < G1b (12%) < G1c (14%) at the same driving voltage. With dopant content enrichment, the intermolecular distance is narrowed, making it easy for carrier hopping and transporting. Furthermore, the turn-on voltages of devices decrease from 5.0 V for G1a (10%) to 4.0 V for G1c (14%), which could be ascribed to direct charge carriers injection and recombination in the emitting material. With a moderate dopant concentration of 12%, the 14115

DOI: 10.1021/acsami.6b15358 ACS Appl. Mater. Interfaces 2017, 9, 14112−14119

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Figure 6. Type G1 (G1a (10%), G1b (12%), G1c (14%)): (a) J−V−L characteristic and (b) current and power efficiencies of Ir(ppy)2acac-doped PHOLEDs with the different dopant concentrations. Types G2 (G2a (50 °C), G2b (90 °C) and G2c (145 °C)): (a) J−V−L characteristic and (b) current and power efficiencies of PHOLEDs with the EML under different annealing temperatures. Types G3 (G3a (20 nm), G3b (25 nm), G3c (30 nm), and G3d (35 nm)): (a) J−V−L characteristic and (b) current and power efficiencies of PHOLEDs with the HTL in different thickness.

energy transfer as the ET of the host (ET > 2.6 eV) is higher than that of Ir(ppy)2acac (ET = 2.45 eV).29 The comparatively great performance of solution-processed INDY-host green phosphorescence OLEDs motivated us to further explore its application in the solution-processed red phosphorescence OLEDs. Therefore, a red device with the structure of ITO/PEDOT:PSS (35 nm)/TFB (30 nm)/ INDY:R (x%, 35 nm, 90 °C)/TmPyPb (40 nm)/LiF (1 nm)/Al (120 nm) was formed. The soluble red emissive Ir(mphmq)2acac dopant into the compound INDY as EML at different contents formed devices R1 (4%)−R4 (12%). The EL spectra in different dopant concentrations, J−V−L characteristics and efficiencies curves of the red devices were depicted in Figure 7 and all the related data collected in Table 2.

four devices with the CE of 27.33 cd/A and PE of 12.26 lm/W at 7 V. Taken together, the effects of dopant concentration, annealing temperature, and hole transporting layer thickness on electroluminescence performance of green device have been taken to better evaluate the practical utility of INDY as host material in PHOLEDs. The green solution-processed PHOLEDs achieved CE of 27.33 cd/A and PE of 12.26 lm/W with a device structure of ITO/PEDOT:PSS (35 nm)/TFB (30 nm)/ INDY:G (12%, 35 nm, 90 °C)/TmPyPb (40 nm)/LiF (1 nm)/ Al (120 nm). In addition, only one green emission peak around 527 nm was observed in the EL spectra and almost no emission change happened even under different operating voltages (as displayed in Figure S3), which could be ascribed to the effective 14116

DOI: 10.1021/acsami.6b15358 ACS Appl. Mater. Interfaces 2017, 9, 14112−14119

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Figure 7. (a) J-V-L characteristic and (b) External quantum efficiencies versus current density of Ir(mphmq)2acac-doped PHOLEDs with the different dopant concentration. (c) Current efficiencies versus luminance of Ir(mphmq)2acac-doped PHOLEDs with the different dopant concentration. (d) The EL spectra of red devices (R1-R4) at 8 V.

Table 2. Device Characteristics of Solution-Processed Red PHOLEDs device

Von (V)a

Lmax (cd/m2)

ηc (cd/A)b

ηp (lm/W)c

ηext (%)d

CIE(x,y)e

R1 R2 R3 R4

5.6 5.4 5.3 4.8

1284 1533 1713 1754

13.86 17.20 12.77 6.94

7.91 9.82 6.18 4.36

10.23 12.61 9.21 5.41

0.65, 0.35 0.65 0.35 0.65 0.35 0.65 0.35

a

At 1 cd/m2. bMaximum current efficiency. cMaximum power efficiency. dMaximum external quantum efficiency. eBy inverting EL spectra into the CIE 1931 diagram.

voltage, reaping the benefit from the suppression of backward energy transfer from dopant to host as INDY possessed higher triplet energy level ET (>2.6 eV) than the red emissive Ir(mphmq)2acac.14

With the dopant concentration increased, the current densities of devices R1, R2, R3, and R4 increase in sequence and the same regular increase trend is observed in the luminance−voltage curves, which could be ascribed to the melioration of charge-transporting ability. Since the J−V−L characteristics of devices R1−R4 improved with dopant content enrichment, the efficiencies of devices R3 and R4 dramatically decline, induced by triplet−triplet annihilation. Impressively, device R2 with dopant content of 6% achieves the maximum efficiencies of 17.20 cd/A, 9.82 lm/W, and 12.61%, which are better than the best device efficiencies of solution-processed red phosphorescence OLEDs using bipolar small molecule as the single host in previous work.15,30 For instance, Yang et al. reported solution-processed red PhOLEDs with a maximum current efficiency of 13.3 cd/A using a small molecule as host. Recently, solution-processed red PHOLEDs based on bipolar host material with maximum current and power efficiencies of 12.1 cd/A and 8.1 lm/W were reported. A possible reason for device R2 having higher efficiencies was that host INDY with its bipolar nature could harmonize carriers transporting and broaden the exciton recombination area resulting in available exciton radiation. Moreover, the excellent thermal stability with quite high Tg and uniform film morphology could reduce the carriers trap caused by the occurrence of film crystallization and aggregation during device operation and then integrated device capability could be ameliorated. From Figure S4, all of these devices presented a pure red peak around 618 nm with the CIE (0.65, 0.35) independent of dopant content and operating

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CONCLUSIONS In summary, a novel bipolar host material INDY was characterized and successfully applied in the green and red solution-processed PHOLEDs. The connection of indole group and benzimidazole unit by ortho-linking strategy constructed a distorted molecular conformation leading to great thermal stability with uniform film morphology with low RMS from AFM and high Tg up to 148 °C. The ortho-substituted compound INDY-based green solution-processed PHOLEDs achieved maximum current efficiency of 27.33 cd/A and maximum power efficiency of 12.26 lm/W at 7 V with the device condition of ITO/PEDOT:PSS (35 nm)/TFB (30 nm)/INDY:G (12%, 35 nm, 90 °C)/TmPyPb (40 nm)/LiF (1 nm)/Al (120 nm). Furthermore, its red solution-processed PHOLEDs gained great performance with the maximum CE of 17.20 cd/A, PE of 9.82 lm/W, and EQE of 12.61% at dopant concentration of 6%, which was relative to the best performances among the other solution-processed red PHOLEDs using small molecule as single host. All these results demonstrated that this ortho-substituted host material INDY could finely tune the balance between charge-transporting ability and film morphology due to appropriate distorted molecular structure, thus achieving great device performance. 14117

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J = 7.6, 1.2 Hz, 1H), 7.73 (dd, J = 6.8, 6.0 Hz, 1H), 7.69−7.64 (m, 1H), 7.59 (dd, J = 7.6, 1.2 Hz, 1H), 7.53 (d, J = 8.0 Hz, 1H), 7.43− 7.39 (m, 2H), 7.37 (t, J = 6.0 Hz, 2H), 7.31−7.25 (m, 2H), 7.24 (d, J = 3.2 Hz, 1H), 7.21−7.18 (m, 1H), 6.74 (d, J = 3.2 Hz, 1H). Synthesis of Product INDY. Intermediate 2 (0.88 g, 2.00 mmol), (4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl) boronic acid (0.70 g, 2.20 mmol) and K 2 CO 3 (2 M, 5 mL) were dissolved in tetrahydrofuran (20 mL) bubbled with argon and added into a three-neck boiling flask. After stirring for 15 min, tetrakis(triphenylphosphine)palladium (0.046 g, 0.04 mmol) was added to the mixture. The reaction mixture was refluxed for 4 h under argon atmosphere. After the reaction finished, 10 mL of H2O was poured into and extracted with dichloromethane for three times (10 mL once). The anhydrous sodium sulfate was added into the organic layer as desiccant. The crude product was gained by removing solvent in vacuum and further purified by SiO2 column chromatography to give a white solid (0.92 g, 1.46 mmol, 73.02%). 1H NMR (400 MHz, CDCl3): δ 8.15 (d, J = 1.2 Hz, 1H), 7.99 (d, J = 7.2 Hz, 1H), 7.91 (dd, J = 7.6, 1.2 Hz, 1H), 7.86 (d, J = 8.0 Hz, 1H), 7.69−7.51 (m, 7H), 7.47−7.29 (m, 10H), 7.25−7.18 (m, 4H), 7.03 (d, J = 8.4 Hz, 2H), 6.84 (d, J = 3.2 Hz, 1H), 6.55 (d, J = 3.2 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 151.96, 142.99, 139.74, 139.03, 138.12, 137.13, 136.80, 136.39, 132.68, 131.34, 129.85, 129.52, 128.82, 128.48, 128.12, 127.20, 126.62, 125.08, 124.24, 123.38, 123.03, 122.62, 121.70, 120.54, 119.79, 110.76, 110.46, 103.59. HRMS (ESI, m/z), [M + K]+: calcd for: C45H29KN3O, 666.1942; found, 666.1947.

EXPERIMENTAL SECTION

Materials and Measurements. Dibenzo[b,d]furan-4-ylboronic acid and 5-bromo-1H- indole used in the process were purchased from Energy Chemical and HWRK Chem without purification. And the benzimidazole boronic acid intermediate compound was purchased from Shanghai Tao e Chemical Technology Co., Ltd. The 1H and 13C NMR spectra, mass spectra, ultraviolet−visible absorption spectra, fluorescence spectra, cyclic voltammetry, TGA, and DSC analysis of the host material were measured according to the methods reported in previous work.31,32 And the 1H and 13C NMR spectra and mass spectra were shown in Figure S5. Device Fabrication and Performance Measurements. All of the layers were fabricated on a glass substrate with a patterned ITO, which is 150 nm thick and ∼10 Ω/sq. The ITO glasses were cleaned for 15 min in sequential ultrasonic baths of detergent, water, acetone, and isopropanol. After cleaning, the ITO slice was treated in a UV− ozone chamber for 15 min. The hole injection layer (HIL), holetransporting layer (HTL), and emissive layer (EML) were spin-coated on the substrates in a glovebox filled with N2 orderly. First, the HIL of PEDOT:PSS (Al 4083) was spin-coated onto the ITO substrate at a speed of 5000 rpm for 1 min and annealed at 150 °C for 30 min. The hole-transporting layer of TFB with chlorobenzene solution was subsequently spin-coated over the PEDOT:PSS layer at a speed of 1500 rpm for 1 min and annealed at 150 °C for 30 min. On top of the HTL layer, an EML layer of INDY:Ir(ppy)2acac dissolved in toluene with a concentration of 8 mg/mL was spin-coated at a speed of 800 rpm and annealed for 30 min. After the spin-coating processing, the ETL of TmPyPB, an electron injection layer (EIL) of lithium fluoride (LiF), and the cathode of aluminum (Al) were deposited sequentially in a high vacuum (∼10 −4 Pa) evaporation device. An ∼40 nm amount of TmPyPB was deposited on top of EMLs at a rate of 1 Å/s. Upon the EML, ∼1 nm of LiF was deposited and then followed by ∼120 nm of Al cathode. The voltage−current density, luminance, and EL spectra were measured using a programmable source meter (Keithley 2400), luminance meter (LS110, Konica Minolta), and a spectrophotometer (Spectrascan PR670, Photo Research).

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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.6b15358. Absorption spectra in various solvents, cyclic voltammograms, summary of EL spectra of green and red PHOLEDs, and 1H NMR, 13C NMR, and HRMS spectra (PDF)

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SYNTHESIS AND CHARACTERIZATION OF COMPOUNDS

AUTHOR INFORMATION

Corresponding Authors

Synthesis of Compound 1. 5-Bromo-1H-indole (3.92 g, 20 mmol), dibenzo[b,d]furan-4-ylboronic acid (5.02 g, 22 mmol) and K2CO3 (2 M, 20 mL) were mixed in 60 mL of tetrahydrofuran and added into a 250 mL three-neck boiling flask. After stirring for 30 min, tetrakis(triphenylphosphine)palladium (0.46 g, 0.40 mmol) was added to the reaction and the reaction mixture was refluxed for 5 h under argon atmosphere. After cooling the reaction mixture to room temperature, 20 mL of H2O was poured into the mixture and extracted with dichloromethane for three times (15 mL once). The mixture solution was layered, and the organic layer was separated. Anhydrous sodium sulfate was added into the combined organic layer as desiccant. After filtering, approximately 100 mL of organic solvent was obtained. The crude product was gained by removing the solvent in vacuum and then purified by SiO2 column chromatography to give a white solid (4.22 g, 14.90 mmol, 74.56%). 1H NMR (400 MHz, CDCl3): δ 8.26 (s, 1H), 8.18 (s, 1H), 8.00 (d, J = 7.6 Hz, 1H), 7.92 (dd, J = 7.6, 1.2 Hz, 1H), 7.76 (dd, J = 8.4, 1.6 Hz, 1H), 7.66 (dd, J = 7.6, 1.2 Hz, 1H), 7.61 (d, J = 8.4 Hz, 1H), 7.56 (d, J = 8.4 Hz, 1H), 7.49−7.40 (m, 2H), 7.36 (t, J = 7.6 Hz, 1H), 7.28 (dd, J = 8.0, 5.2 Hz, 1H), 6.68 (d, J = 2.0 Hz, 1H). Synthesis of Compound 2. A mixture of 5-(dibenzo[b,d]furan-4yl)-1H-indole (2.83 g, 10 mmol) and 1-bromo-2-iodobenzene (3.11 g, 11 mmol) in 5 g of trichlorobenzene and potassium carbonate (2.76 g, 20 mmol) was added into a round-bottom flask. Copper(I) iodide (0.14 g, 0.75 mmol) was added into the mixture, and then the reaction mixture was refluxed for 8 h under N2 atmosphere. After the mixture cooled to room temperature, the reaction solvent was removed in vacuum and further purified by SiO2 column chromatography, affording a white solid (2.20 g, 5.00 mmol, 50%). 1H NMR (400 z, CDCl3): δ 8.16 (d, J = 1.2 Hz, 1H), 7.92 (d, J = 7.6 Hz, 1H), 7.84 (dd,

*(J.S.) Fax: (+86) 21-64252288. E-mail: [email protected]. *(J.Z.) E-mail: jhzhang@staff.shu.edu.cn. ORCID

Lei Gao: 0000-0002-7215-7870 Jianhua Su: 0000-0002-4746-6022 He Tian: 0000-0003-3547-7485 Author Contributions ⊥

Y.C. and X.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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REFERENCES

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