Carbazole Hybrid Bipolar Host Materials

Aug 22, 2012 - By conjugating carbazole moiety to the different positions of the rigid skeleton 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole, a series ...
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Simple Phenanthroimidazole/Carbazole Hybrid Bipolar Host Materials for Highly Efficient Green and Yellow Phosphorescent Organic Light-Emitting Diodes Hong Huang,† Yixin Wang,† Shaoqing Zhuang,† Xiao Yang,† Lei Wang,*,† and Chuluo Yang*,†,‡ †

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, P. R. China Department of Chemistry, Wuhan University, Wuhan 430072, P. R. China

‡

ABSTRACT: By conjugating carbazole moiety to the different positions of the rigid skeleton 1,2-diphenyl-1Hphenanthro[9,10-d]imidazole, a series of hybrid bipolar phosphorescent hosts was synthesized, and their photophysical properties were investigated. The introduction of a rigid phenanthroimidazole moiety greatly improves their morphological stability, with high decomposition temperatures (Td) and high glass transition temperatures (Tg) in the range of 394−417 and 113−243 °C, respectively. The highly efficient green and orange phosphorescent organic light-emitting diodes (PhOLEDs) have been achieved by employing these compounds as the phosphorescent hosts. For the device of ITO/MoO3 (10 nm)/NPB (80 nm)/TCTA (5 nm)/mPhBINCP:9 wt % Ir(ppy)3 (20 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (100 nm), a maximum luminous efficiency (ηc,max) of 77.6 cd/A, maximum power efficiency (ηp,max) of 80.3 lm/W, and maximum external quantum efficiency (ηEQE,max) of 21% were obtained. Furthermore, these hosts are also applicable for the orange phosphorescent emitter (fbi)2Ir(acac), a yellow PhOLEDs with pPhBICP as host, for which a performance of ηc,max of 57.2 cd/A, ηEQE,max of 19.3%, and ηp,max of 59.8 lm/W was achieved. These results demonstrated that the phenanthroimidazole unit is an excellent electron-transporting group for constructing the bipolar phosphorescent host.

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In addtion, Chang et al. reported a green device with ηEQE,max up to 20% (72.7 lm/W) by introducing triazole-containing electron transport/charge confining bipolar host material.6 But in most of reported bipolar phosphorescent hosts, electrondeficient groups mainly concentrate on oxadiazole,7 pyridine,8 phenylbenzimidazole,9 and diphenylphosphine oxide10 moieties. The development of facile synthesized and novel host materials containing new moieties is still interesting for practical applications. Phenanthroimidazole derivatives have been proved to be efficient for electron injection or hole blocking11 as well as blue emission.12 For example, 4,4′-bis(1-phenylphenanthro[9,10d]imidazol-2-yl)biphenyl (BPPI)12b was used as both electron transport layer and fluorescence host material, the doublelayered device based on BPPI exhibits a higher maximum luminance and a lower turn-on voltage, which could be attributed to its excellent electron mobility. However, until now, to the best of our knowledge, bipolar phosphorescent host materials incorporating phenanthroimidazole as the electron deficient moiety have not been exploited. Carbazole derivatives have been widely used in PhOLEDs because their good hole mobility and sufficiently large triplet energy levels13 satisfy the phosphorescent dopants. For instance, the ambipolar cabazole

INTRODUCTION Phosphorescent organic light-emitting diodes (PhOLEDs) can theoretically achieve 100% internal quantum efficiency,1 which has attracted considerable attention since the first report about applying the phosphorescent host−dopant system to reduce the concentration quenching and the triplet−triplet annihilation. In the PhOLEDs, the phosphorescent hosts play a vital role and serve as a recombination center for holes and electrons to generate the electronically excited states. Recently, bipolar hosts have aroused considerable interest because they can provide more balance in electron and hole fluxes, generate broad charge recombination zones, and simplify device structure.2 Among them, a lot of phosphorescent hosts doped with the Ir(ppy)3 [fac-tris(2-phenylpyridine)iridium(III)] to exhibit the highly efficient green phosphorescent emission have been reported. For example, Tao et al. reported a carbazole/ oxadiazole hybrid molecule o-CzOXD3 for green phosphorescent host, for which a maximum external quantum efficiency (ηEQE,max) of 20% (59.3 lm/W) was achieved. Then, they replaced the carbazole with triphenylamine to form the compound m-TPA-o-OXD,4 through optimizing the green device configuration, and a maximum ηEQE,max of 21% (86.4 lm/ W) was obtained. Chou and Cheng designed and synthesized a hybrid carbazole/phosphine oxide based bipolar host material (BCPO),5 and the green device with BCPO as host showed a maximum luminous efficiency (ηc,max) of 83.4 cd/A, ηEQE,max of 21.6%, and maximum power efficiency (ηp,max) of 87.5 lm/W. © 2012 American Chemical Society

Received: June 12, 2012 Revised: August 7, 2012 Published: August 22, 2012 19458

dx.doi.org/10.1021/jp305764b | J. Phys. Chem. C 2012, 116, 19458−19466

The Journal of Physical Chemistry C

Article

Computational Details. The geometrical and electronic properties were performed with the Amsterdam Density Functional (ADF) 2009.01 program package. The calculation was optimized by means of the B3LYP (Becke three parameters hybrid functional with Lee−Yang−Perdew correlation functionals)16 with the 6-31G(d) atomic basis set. Then the electronic structures were calculated at the τ-HCTHhyb/6311++G(d,p) level.17 Molecular orbitals were visualized using ADFview. Device Fabrication and Measurement. The holeinjection material MoO3, hole-transporting material 1,4-bis[(1-naphthylphenyl)amino]biphenyl (NPB), electron/excitonblocking material 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), and electron-transporting material 3,3′-(5′-(3-(pyridine-3-yl)phenyl)-[1,1′:3′,1″-terphenyl]-3,3″-diyl)dipyridine (TmPyPB) were commercially available. Commercial ITO (indium tin oxide) coated glass with sheet resistance of 20 Ω per square was used as the starting substrates. Before device fabrication, the ITO glass substrates were precleaned carefully and treated by oxygen plasma for 2 min. Then the sample was transferred to the deposition system. MoO3 (10 nm) was first deposited onto the ITO substrate, followed by NPB (80 nm), TCTA (5 nm), the emissive layer, and TmPyPB (45 nm). Finally, a cathode composed of lithium fluoride (1 nm) and aluminum (100 nm) was sequentially deposited onto the substrate in the vacuum of 10−6 Torr. The current density− voltage−brightness (J−V−L) curves of the devices were measured with a Keithley 2400 Source meter equipped with a calibrated silicon photodiode. The EL spectra were measured bya PR655 spectrometer. The ηEQE,max values were calculated according to previously reported methods.18 All measurements were carried out at room temperature under ambient conditions. Synthesis of 2-(4-Bromophenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (1). A mixture of phenanthrenequinone (2.1 g, 10.0 mmol), aniline (4.5 g, 50.0 mmol), 4-bromobenzaldehyde (1.9 g, 10.0 mmol), ammonium acetate (3.1 g, 40.0 mmol), and acetic acid (150 mL) was refluxed for 24 h under nitrogen.19 After that, the mixture was cooled to room temperature, and the solid was filtered. The solid product was washed with water and CH3OH. The product was dried under vacuum and used directly for the next step. White powder (yield: 88%). 1H NMR (CDCl3, 400 MHz): δ (ppm) 8.85−8.83 (d, J = 7.2 Hz, 1H), 8.78−8.76 (d, J = 8.4 Hz, 1H), 8.72−8.70 (d, J = 8.4 Hz, 1H), 7.76−7.73 (m, 1H), 7.68−7.61 (m, 4H), 7.51 (s, 3H), 7.43 (s, 4H), 7.26 (m, 1H), 7.18−7.16 (d, J = 8.4 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ (ppm) 149.78, 138.49, 137.35, 131.46, 130.86, 130.32, 130.03, 129.40, 129.36, 129.03, 128.35, 128.28, 127.41, 127.05, 126.35, 125.78, 125.09, 124.15, 123.44, 123.16, 122.91, 122.70, 120.86. MS (APCI): calcd for C27H17BrN2 448.1, found 449.2 (M + 1)+. Synthesis of 2-(3-Bromophenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (2). Compound 2 was prepared according to the same procedure as compound 1 but using 3-bromobenzaldehyde instead of 4-bromobenzaldehyde. White powder (yield: 85%). 1H NMR (CDCl3, 400 MHz): δ (ppm) 8.87−8.85 (d, J = 7.6 Hz, 1H), 8.79−8.77 (d, J = 7.6 Hz, 1H), 8.72−8.70 (d, J = 8.0 Hz, 1H), 7.82 (s, 1H), 7.77−7.74 (m, 1H), 7.69−7.63 (m, 4H), 7.53−7.51 (m, 3H), 7.46−7.41 (m, 2H), 7.26 (s, 1H), 7.18−7.12 (m, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 149.88, 138.36, 132.48, 131.86, 130.32, 130.09, 129.62, 129.47, 129.00, 128.37, 127.71, 127.44, 127.02, 126.37, 125.84, 125.17,

derivative 9,9-bis(9-methylcarbazol-3-yl)-4,5-diazafluorene (MCAF)14 and pyrimidine derivative (2,4,6-tris(3-(carbazol-9yl)phenyl)pyrimidine (TCPM)15 are successfully used as host for blue, green, and red triplet emitters. Here, the combinations of phenanthroimidazole moieties and carbazole are expected to provide a potential novel bipolar host for triplet emitters. By attaching a carbazole moiety to the different positions of the 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole skeleton, a series of phenanthroimidazole/carbazole hybrid were synthesized, and their photophysical properties and orbital energy levels were optimized. These novel bipolar phosphorescent hosts were found to possess sufficiently large triplet energy levels for the green phosphorescent emitter Ir(ppy)3 and the yellow phosphorescent emitter (fbi)2Ir(acac). Green device D using 1(3-(9H-carbazol-9-yl)phenyl)-2-phenyl-1H-phenanthro[9,10d]imidazole (mPhBINCP) as host exhibits high efficiencies of 77.6 cd/A and 80.3 lm/W (ηEQE,max = 21%) at the normal Ir(ppy)3 emission. Even the yellow device E doping the (fbi)2Ir(acac) into host 2-(4-(9H-carbazol-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (pPhBICP) exhibits ηc,max of 57.2 cd/A, ηEQE,max of 19.3%, and ηp,max of 59.8 lm/W, respectively. By the way, the performances of PhOLEDs based on these phosphorescent hosts are attractive compared to Ir(ppy)3- and (fbi)2Ir(acac)-based PhOLEDs with similar configurations reported recently.3

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EXPERIMENTAL SECTION Materials and Measurements. All the reagents and solvents used for the synthesis were purchased from Aldrich and were used without further purification. 1 H NMR and 13C NMR spectra were measured on a BrukerAF301 AT 400 MHz spectrometer. Elemental analyses of carbon, hydrogen, and nitrogen were performed on an Elementar (Vario Micro cube) analyzer. Mass spectra were carried out on an Agilent (1100 LC/MSD Trap) instrument using ACPI ionization. UV−vis absorption spectra were recorded on a Shimadzu UV−vis−NIR spectrophotometer (UV-3600). PL spectra were recorded on Edinburgh instruments (FLSP920 spectrometers). Differential scanning calorimetry (DSC) was performed on a PE Instruments DSC 2920 unit at a heating rate of 10 °C/min from 30 to 300 °C under nitrogen. The glass transition temperature (Tg) was determined from the second heating scan. Thermogravimetric analysis (TGA) was undertaken with a PerkinElmer Instruments unit (Pyris1 TGA). The thermal stability of the samples under a nitrogen atmosphere was determined by measuring their weight loss while heating at a rate of 10 °C/min from 30 to 700 °C. Cyclic voltammetry measurements were carried out in a conventional three-electrode cell using a Pt carbon working electrode of 2 mm in diameter, a platinum wire counter electrode, and an Ag/AgNO3 (0.1 M) reference electrode (4.6 V below vacuum) on a computer-controlled EG&G potentiostat/galvanostat (model 283) at room temperature. Reduction cyclic voltammograms (CVs) of all compounds were collected in dichloromethane containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the supporting electrolyte. The onset potential was determined from the intersection of two tangents drawn at the rising and background current of the CV. All solutions were purged with a nitrogen stream for 10 min before measurement. The procedure was performed at room temperature and a nitrogen atmosphere was maintained over the solution during measurements. 19459

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C39H25N3: C, 87.45; H, 4.70; N, 7.85. Found: C, 87.66; H, 4.92; N, 7.42. Synthesis of 1-(4-(9H-Carbazol-9-yl)phenyl)-2-phenyl-1Hphenanthro[9,10-d]imidazole (pPhBINCP). The compound pPhBINCP was prepared according to the same procedure as the compound pPhBICP but using compound 3 instead of compound 1. White powder (yield: 75%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.93−8.91 (d, J = 7.6 Hz, 1H), 8.82−8.80 (d, J = 8.0 Hz, 1H), 8.74−8.72 (d, J = 8.0 Hz, 1H), 8.20−8.18 (d, J = 8.0 Hz, 2H), 7.81−7.66 (m, 8H), 7.57−7.49 (m, 5H), 7.41− 7.34 (m, 7H). 13C NMR (100 MHz, CDCl3): δ (ppm) 151.08, 140.43, 139.00, 137.30, 130.75, 129.58, 129.44, 129.16, 128.38, 128.35, 128.07, 127.42, 127.14, 126.43, 126.31, 125.80, 125.09, 124.34, 123.82, 123.15, 122.92, 122.87, 120.73, 120.64, 120.62, 109.47. MS (APCI): calcd for C39H25N3 535.2, found 536.2 (M + 1)+. Anal. Calcd for C39H25N3: C, 87.45; H, 4.70; N, 7.85. Found: C, 87.34; H, 4.84; N, 7.82. Synthesis of 1-(3-(9H-Carbazol-9-yl)phenyl)-2-phenyl-1Hphenanthro[9,10-d]imidazole (mPhBINCP). The compound mPhBINCP was prepared according to the same procedure as the compound pPhBICP but using compound 4 instead of compound 1. White powder (yield: 70%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.89−8.87 (d, J = 8.0 Hz, 1H), 8.80−8.78 (d, J = 8.0 Hz, 1H), 8.72−8.70 (d, J = 8.0 Hz, 1H), 8.09−8.07 (m, 2H), 7.85−7.84 (m, 2H), 7.74−7.53 (m, 8H), 7.44−7.40 (m, 4H), 7.31−7.24 (m, 4H), 7.12 (s, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 151.11, 140.31, 140.10, 139.54, 131.26, 129.81, 129.43, 129.13, 128.56, 128.33, 127.93, 127.78, 127.73, 127.60, 127.39, 127.13, 126.41, 126.19, 125.79, 125.13, 124.33, 123.67, 123.12, 122.86, 120.80, 120.50, 120.45, 109.31. MS (APCI): calcd for C39H25N3 535.2, found 536.2 (M + 1)+. Anal. Calcd for C39H25N3: C, 87.45; H, 4.70; N, 7.85. Found: C, 87.54; H, 4.95; N, 7.51.

124.16, 123.16, 122.88, 122.76, 122.36, 120.92. MS (APCI): calcd for C27H17BrN2 448.1, found 449.1 (M + 1)+. Synthesis of 1-(4-Bromophenyl)-2-phenyl-1H-phenanthro[9,10-d]imidazole (3). Compound 3 was prepared according to the same procedure as compound 1 but using 4-bromoaniline instead of aniline. White powder (yield: 90%). 1H NMR: (DMSO-d6, 400 MHz): δ (ppm) 8.94−8.91 (d, J = 8.4 Hz, 1 H), 8.89−8.86 (d, J = 8.4 Hz, 1 H), 8.71−8.68 (m, J = 1.2, 8.0 Hz, 1 H), 7.80−7.67 (m, 7 H), 7.58−7.54 (m, 3 H), 7.51−7.49 (m, 2 H), 7.51−7.49 (t, J = 7.2 Hz, 1 H), 7.09−7.07 (d, J = 8.0 Hz, 1 H). 13C NMR (100 MHz, CDCl3): δ (ppm) 150.90, 139.33, 138.44, 137.49, 130.89, 130.18, 129.51, 129.33, 129.05, 128.36, 128.30, 127.81, 127.35, 127.07, 126.41, 125.76, 125.04, 124.21, 123.10, 122.82, 122.80, 120.69. MS (APCI): calcd for C27H17BrN2 448.1, found 449.3 (M + 1)+. Synthesis of 1-(3-Bromophenyl)-2-phenyl-1H-phenanthro[9,10-d]imidazole (4). Compound 4 was prepared according to the same procedure as compound 1 but using 3-bromoaniline instead of aniline. White powder (yield: 84%). 1H NMR (CDCl3, 400 MHz): δ (ppm) 8.89−8.87 (d, J = 7.6 Hz, 1H), 8.80−8.78 (d, J = 8.0 Hz, 1H), 8.72−8.70 (d, J = 8.4 Hz, 1H), 7.78−7.73 (m, 2H), 7.70−7.52 (m, 5H), 7.48−7.47 (m, 2H), 7.36−7.31 (m, 4H), 7.20−7.18 (m, 1H). 13C NMR (100 MHz, CDCl3): δ (ppm) 150.96, 140.05, 133.19, 132.30, 131.34, 129.60, 129.48, 129.27, 128.49, 128.44, 128.05, 127.50, 126.58, 125.93, 125.20, 124.35, 123.48, 123.22, 122.97, 122.83, 120.80. MS (APCI): calcd for C27H17BrN2 448.1, found 449.2 (M + 1)+. Synthesis of 2-(4-(9H-Carbazol-9-yl)phenyl)-1-phenyl-1Hphenanthro[9,10-d]imidazole (pPhBICP). A mixture of carbazole, compound 1 (0.45 g, 1.0 mmol), CuI (10.0 mg, 0.05 mmol), 18-crown-6 (13.2 mg, 0.05 mmol), and K2CO3 (0.83 g, 6.0 mmol) in 1,3-dimethyltetrahydropyrimidin-2(1H)one (DMPU) (2.0 mL) was heated at 170 °C for 48 h under nitrogen. After cooling to room temperature, dichloromethane was added and the mixture was filtered. The solvent was removed under reduced pressure, and the residue was purified by column chromatography on silica gel with dichloromethane as eluent. White powder (yield: 84%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.92 (s, 1H), 8.81−8.79 (d, J = 8.4 Hz, 1H), 8.75−8.73 (d, J = 8.0 Hz, 1H), 8.14−8.12 (d, J = 8.0 Hz, 2H), 7.85−7.76 (m, 3H), 7.70−7.62 (m, 6H), 7.56−7.54 (m, 3H), 7.42−7.41 (d, J = 4.0 Hz, 4H), 7.31−7.21 (m, 4H). 13C NMR (100 MHz, CDCl3): δ (ppm) 140.53, 130.80, 130.40, 129.15, 128.40, 127.43, 126.61, 126.39, 126.02, 124.19, 123.57, 123.17, 120.92, 120.35, 120.21, 109.78. MS (APCI): calcd for C39H25N3 535.2, found 536.2 (M + 1)+. Anal. Calcd for C39H25N3: C, 87.45; H, 4.70; N, 7.85. Found: C, 87.64; H, 4.84; N, 7.52. Synthesis of 2-(3-(9H-Carbazol-9-yl)phenyl)-1-phenyl-1Hphenanthro[9,10-d]imidazole (mPhBICP). The compound mPhBICP was prepared according to the same procedure as the compound pPhBICP but using compound 2 instead of compound 1. White powder (yield: 84%). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.88 (s, 1H), 8.77−8.75 (d, J = 8.4 Hz, 1H), 8.71−8.69 (d, J = 8.4 Hz, 1H), 8.14−8.12 (d, J = 7.6 Hz, 2H), 7.93−7.91 (m, J = 6.4 Hz, 1H), 7.76−7.49 (m, 10H), 7.37− 7.24 (m, 6H), 7.13−7.11 (d, J = 8.0 Hz, 1H), 7.04−7.02 (d, J = 8.4 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ (ppm) 140.67, 137.48, 130.55, 130.14, 129.48, 129.12, 129.01, 128.39, 128.05, 127.44, 126.38, 125.98, 125.16, 124.16, 123.35, 123.13, 122.89, 120.83, 120.20, 120.02, 109.71. MS (APCI): calcd for C39H25N3 535.2, found 536.2 (M + 1)+. Anal. Calcd for

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RESULTS AND DISCUSSION As shown in Scheme 1, the compounds pPhBICP, mPhBICP, pPhBINCP, and mPhBINCP were facilely synthesized by the copper(I)-catalyzed Ullmann cross-coupling of carbazole with the corresponding phenanthroimidazole bromide. The crude white powders were purified by chromatography and the yields of all target products were above 70%. All the 1,2-diphenyl-1Hphenanthro[9,10-d]imidazole bromide intermediates were synthesized in a one-pot reaction with similar procedures in high yield. Such a synthetic route provides a common and facile way to construct phenanthroimidazole derivatives, which is very important for its commercial application. Repeated temperature-gradient vacuum sublimations are required for further purification of these new hosts. The chemical structures of all the target products were characterized by 1H and 13C NMR spectroscopy, mass spectroscopy, and elemental analysis. The excellent thermal stability of these new phosphorescent hosts is indicated by their high decomposition temperatures (Td, corresponding to 5% weight loss), in the range of 395−417 °C (Table 1), determined through thermogravimetric analysis (TGA) (Figure 1). The high Td values indicate that these compounds would be capable of enduring the vacuum thermal sublimation process. In addition, all the compounds exhibit high glass transition temperatures (Tg) in the range of 113−243 °C by differential scanning calorimetry, which are distinctly higher than that of 1,3-di(9H-carbazol-9-yl)benzene (mCP) (60 °C),13f indicating that the introduction of the rigid phenanthroimidazole moiety improves their morphological stability greatly. Especially, the compound pPhBICP with a 19460

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solution. These red shifts can be explained by their relative strong π−π stacking interactions. All of the results indicated that the carbazole linking positions have important effects on the photophysical properties of these hosts. As shown in Figure 3, the triplet energies (ET) of these hosts determined by the highest-energy vibronic sub-band of the phosphorescence spectra at 77 K followed the sequence mPhBINCP (2.66) > mPhBICP (2.61) ≈ pPhBINCP (2.61) > pPhBICP (2.53), which is a reflection of the degree of πconjugation between the carbazole and phenanthroimidazole moieties; all the values are sufficient for confining green and orange phosphors. The electrochemical properties of pPhBICP, mPhBICP, pPhBINCP, and mPhBINCP were studied in solution through cyclic voltammetry (CV) measurements. Their cyclic voltammograms are shown in Figure 4 and the respective electrochemical data are summarized in Table 1. These phosphorescent hosts displayed reversible oxidation waves within the electrochemical window of CH2Cl2, and an additional peak at around 0.9 V was found for these compounds during the oxidation scan, which most likely resulting from the electrochemical property of the C3 and C6 of the carbazole.21 The oxidation and reduction potentials of pPhBICP, mPhBICP, pPhBINCP, and mPhBINCP are 1.15/−2.01, 1.20/−1.97, 1.21/−1.95, and 1.21/−1.96 V, respectively, which depend on the linkage modes of the carbazole and phenanthroimidazole moieties. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of these compounds were calculated from the onset potentials of oxidation and reduction. The HOMO levels of these hosts vary in a range from −5.75 to −5.81 eV, and LUMO levels vary in a range from −2.56 to −2.65 eV (see Table 1). To gain insight into the electronic structures of these compounds, a density functional theory (DFT) calculation was performed at the B3LYP/6-31G(d) level. As shown in Figure 5, the HOMO orbitals disperse in different moieties with the different linking modes of the carbazole; for example, when the carbazole is linked through the C2-phenyl, the HOMO orbitals mainly populate on the carbazole, and when the carbazole is linked through the N-phenyl, the HOMO levels mainly disperse on the phenanthroimidazole moieties. It should be pointed out that, in the compound pPhBICP, part of the HOMO is dispersed on the phenanthroimidazole moiety for the efficient conjugation in the para-position of C2-phenyl. For all the compounds, the LUMO orbitals mainly dispersed on the eletron-deficient moieties of phenanthroimidazole. The calculated HOMO/LUMO values are in range from −5.27 to −5.51/−2.29 to −2.40 eV, which is in good agreement with the experimental results (Table 1).

Scheme 1. Synthetic Routes of pPhBICP, mPhBICP, pPhBINCP, and mPhBINCPa

a

Reagents and condition: (a) CuI, K2CO3, 18-crown-6, 180 °C, 48 h.

para-substituent on the C2-phenyl of the phenanthroimidazole shows a high Tg of 243 °C, which can be rationalized by the smallest energetic disorder in the para-position of the C2phenyl of the phenanthroimidazole.20 The high Tg and Td of the hosts can improve the film morphology and reduce the possibility of phase separation upon heating, prolonging the lifetime of the devices. Figure 2 shows the absorption and photoluminescence of these new host materials in dilute CH2Cl2 (5.0 × 10−6 M) and in the film state. Key optical parameters are summarized in Table 1. It is apparent that all phosphorescent hosts except pPhBICP possess nearly similar absorption and PL peaks in solution or solid state. All the compounds show a maximum absorption peak at ca. 258 nm and a shoulder at around 290 nm, which may originate from the benzene rings and the carbazole-centered n−π* transition, respectively. In addition, for the compound pPhBICP, the absorption band between 300 and 380 nm could be assigned to the efficient intramolecular charge transfer owing to the π-conjugation between the carbazole and phenanthroimidazole moieties when the carbazole is linked in the para-position of the C2-phenyl of 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole for pPhBICP. All of the compounds emit deep-blue light with the emission peaks in the range of 421−431 nm in the film state, which are red-shifted by about 30−35 nm with respect to those in

Table 1. Photophysical, Electrochemistry, and Thermal Data of Phenanthroimidazole Derivatives λmax,abs (nm) a

compd

solution

pPhBICP mPhBICP pPhBINCP mPhBINCP

336/289/257 287/258 289/257 289/258

λmax,em (nm)

HOMO/LUMO (eV)

film

solution

film

Eg (eV)

obsdb

calcdc

ET (eV)d

Tg/Td (°C)

368/349 362/340 361/339 362/337

401/382 388/369 386/368 386/368

431/409 424/403 421/401 421/400

3.11 3.15 3.24 3.25

−5.75/−2.64 −5.80/−2.65 −5.81/−2.57 −5.81/-2.56

−5.27/−2.29 −5.29/−2.30 −5.43/−2.39 −5.51/-2.40

2.53 2.61 2.61 2.66

243/395 138/417 113/395 113/411

a

a

Measured in CH2Cl2 solvent at room temperature. bDetermined from the onset of the oxidation and reduction potentials. cValues from DFT calculation. dMeasured in 2-methyltetrahydrofuran at 77 K. 19461

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Figure 1. DSC (left) and TGA (right) curves of compounds pPhBICP, mPhBICP, pPhBINCP, and mPhBINCP at a heating rate of 10 °C/min.

Figure 2. UV absorption and PL spectra of pPhBICP, mPhBICP, pPhBINCP, and mPhBINCP in CH2Cl2 solution (left) and in the film state (right).

For a better understanding of the effect of the structural change on the energy levels and the carrier injection and transport ability, hole-only and the electron-only devices of these compounds were fabricated.5 The configuration of the hole-only device is ITO/NPB (10 nm)/host (30 nm)/NPB (10 nm)/Al (100 nm), while the electron-only device contains the following layers: ITO/BCP (10 nm)/host (30 nm)/BCP (10 nm)/LiF (1 nm)/Al (100 nm). NPB [N,N′-bis-(1-naphthyl)N,N′- diphenyl-1,10-biphenyl-4,4′-diamine] and BCP [2,9dimethyl-4,7-diphenyl-1,10- phenanthroline] layers were used to prevent electron and hole injection from the cathode and anode, respectively.22 As shown in Figure 6, for the compounds pPhBICP, pPhBINCP, and mPhBINCP, the hole-only devices give slightly lower current density than the electron-only devices. However, for the mPhBICP, the current density of the hole-only device is higher than that of the electron device. These results demonstrate that the electron injection from cathode into the host mPhBICP are more difficult in comparison with the compounds pPhBICP, pPhBINCP and mPhBINCP, which means that devices based on mPhBICP will exhibit poor performance. To evaluate the performance of the carbazole/phenanthroimidazole hybrids as hosts, phosphorescent organic lightemitting devices were fabricated by using a green emitter, Ir(ppy)3, as the guest with the simple configuration of ITO/ MoO3 (10 nm)/NPB (80 nm)/TCTA (5 nm)/host:9 wt % Ir(ppy)3 (20 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (host was pPhBICP for device A, mPhBICP for device B, pPhBINCP for device C, and mPhBINCP for device D). In these devices, ITO (indium tin oxide) and Al (aluminum) are the anode and cathode, respectively. NPB is the hole-transporting layer and TCTA [4,4′,4″-tris(N-carbazolyl)triphenylamine] is the exciton blocking layer. Ir(ppy)3-doped hosts were used as the emitting

Figure 3. The phosphorescence spectra of pPhBICP, mPhBICP, pPhBINCP, and mPhBINCP in a frozen 2-methyltetrahydrofuran in 77 K.

Figure 4. Cyclic voltammograms of pPhBICP, mPhBICP, pPhBINCP, and mPhBINCP.

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Figure 5. Spatial distributions of the frontier orbitals of pPhBICP, mPhBICP, pPhBINCP, and mPhBINCP.

Figure 6. Current density versus voltage characteristics of the hole-only and electron-only devices for (a) pPhBICP, (b) mPhBICP, (c) pPhBINCP, and (d) mPhBINCP.

wavelengths of the EL spectra of green devices A−D exhibit an emission similar to the phosphorescent emitter Ir(ppy)3. This suggests that the emission peaks are indeed derived from the dopants and the triplet energy transfer from the hosts to the dopant are complete. Additionally, the electroluminescence (EL) of all devices remained quite stable under different driving voltages. The J−V−L characteristics, efficiency-versus-current density curves, of devices A−D are shown in Figure 8b,c. All the phosphorescent devices display low turn-on voltages from 2.7 to 3.1 V. The devices A−D attain a maximum current efficiency of 69.5, 60.2, 76.6, and 77.6 cd/A and a maximum power efficiency of 74.7, 54.0, 79.5, and 80.3 lm/W, respectively. The maximum ηEQE,max reached 21.0% for devices C and D, which means that they are capable of potential practical application in OLEDs. For the green PhOLEDs, the current/power efficiencies of the devices hosted by these materials are in the order device C ≈ device D ≈ device A > device B. The poor performance of device B can be explained by the electron-only device of mPhBICP, which demonstrates that the electron injection from cathode into host mPhBICP is more difficult. The efficiencies

layer, and the best electroluminescence (EL) performance was achieved with 9 wt % Ir(ppy)3 for all the hosts. TmPyPB (3,3′(5′-(3-(pyridin-3-yl)phenyl)-[1,1′:3′,1″-terphenyl]-3,3″-diyl)dipyridine) is used as both the electron-transporting layer and hole-blocking layer. MoO3 and LiF (lithium fluoride) served as the hole- and electron-injecting layers, respectively. Energy level diagrams of the devices are shown in Figure 7. Key device performance data are summarized in Table 2. Figure 8a depicts normalized EL spectra of these green devices. Peak

Figure 7. Energy level diagram of the devices A−F based on the phenanthroimidazole derivatives. 19463

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Table 2. EL Performances of Devices Based on the Phenanthroimidazole Derivatives device

host

Vona(V)

Lmaxb (cd/m2)

voltage for Lmax(V)

ηc,maxc (cd/A)

ηp,maxd (lm/W)

ηEQE,max (%)

A B C D E F G H

pPhBICP mPhBICP pPhBINCP mPhBINCP pPhBICP mPhBICP pPhBINCP mPhBINCP

2.7 3.1 2.7 2.8 2.8 3.1 3.0 2.8

38 560 35 550 38 270 42 160 31 950 34 350 33 200 32 700

11.0 11.0 9.3 10.0 11.0 12.8 11.8 11.0

69.5 60.2 76.6 77.6 57.2 49.6 57.3 52.9

74.7 54.0 79.5 80.3 59.8 46.4 55.3 54.7

18.9 16.6 21.0 21.0 19.3 16.9 19.3 17.5

CIE(x, y)e (0.30, (0.30, (0.30, (0.30, (0.51, (0.51, (0.50, (0.49,

0.63) 0.63) 0.64) 0.63) 0.49) 0.48) 0.48) 0.49)

a

The turn-on voltage (L = 1 cd/m2). bMaximum luminance. cMaximum current efficiency. dMaximum power efficiency. eThe brightness is 100 cd/ m2.

Figure 8. (a) EL spectra of the devices A−D based on the phenanthroimidazole derivatives. (b) Current density−voltage−luminance characteristics (J−V−L) of devices A−D. (c) Current, power efficiency versus current density curves for devices A−D. (d) External quantum efficiency versus current density for devices A−D.

(100 nm)/TCTA (5 nm)/host:4 wt % (fbi)2Ir(acac) (20 nm)/ TmPyPB (45 nm)/LiF (1 nm)/Al (host is pPhBICP for device E, mPhBICP for device F, pPhBINCP for device G, and mPhBINCP for device H) (Figure 9). The best EL performance is achieved in device E using the pPhBICP as host with ηc,max of 57.2 cd/A, ηEQE,max of 19.3%, and ηp,max of 59.8 lm/W, respectively. It can be ascribed to the optimized energy gap between Et of pPhBICP (Et/2.53) and (fbi)2Ir(acac) (Et/ 2.22), which is a good compromise between energy level barriers and reverse energy transfer from gust back to host. The devices F−H with mPhBICP, pPhBINCP, and mPhBINCP as host attain ηc,max of 49.6, 57.3, and 52.9 cd/A and ηp,max of 46.4, 55.3, and 54.7 lm/W, respectively, which are comparable to those reported for yellow PhOLEDs.4,23

of the other three device are much better than those of the CBP-hosted devices.3 The high efficiency could be attributed to the following aspects. First, the bipolar properties of the hosts could balance the charge transfer and improve the carrier recombination ratio in the emissive layer. Second, the high triplet energies of these hosts could efficiently suppress adverse energy back-transfer from the guest to the host, consequently resulting in the good performances of these devices. Finally, the good thermal stability restrains the strong bimolecular interactions of the phosphorescent Ir(ppy)3 emitter to reduce the triplet−triplet annihilations. Anyway, these device performances, especially for the power efficiencies, are attractive for green phosphorescent OLEDs based on Ir(ppy)3 reported up to now. Similar trends are also observed in devices E−H by employing the widely used yellow triplet emitter bis(2-(9,9diethyl-9H-fluoren-2-yl)-1-phenyl-1H-benzoimidaz-ol-N,C3)iridium(acetylacetonate) [(fbi)2Ir(acac)] as the dopant under the following device structures: ITO/MoO3 (10 nm)/NPB

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CONCLUSIONS In summary, the electron-deficient moiety 1,2-diphenyl-1Hphenanthro[9,10-d]imidazole was used as a skeleton, and a series of novel simple bipolar phenanthroimidazole/carbazole 19464

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Figure 9. (a) Current density−voltage−luminance characteristics (J−V−L) of devices E−H. (b) Current, power efficiency versus current density curves for devices E−H. (c) External quantum efficiency versus current density for devices E−H. (d) EL spectra of the devices E−H. (2) (a) Liu, H.; Cheng, G.; Hu, D.; Shen, F.; Lv, Y.; Sun, G.; Yang, B.; Lu, P.; Ma, Y. Adv. Funct. Mater. 2012, 22, 2830−2836. (b) Hung, W.-Y.; Chi, L.-C.; Chen, W.-J.; Mondal, E.; Chou, S.-H.; Wong, K.-T.; Chi, Y. J. Mater. Chem. 2011, 21, 19249−19256. (c) Hung, W.-Y.; Tu, G.-M.; Chen, S.-W.; Chi, Y. J. Mater. Chem. 2012, 5410−5418. (d) Ding, J.; Wang, Q.; Zhao, L.; Ma, D.; Wang, L.; Jing, X.; Wang, F. J. Mater. Chem. 2010, 20, 8126−8133. (e) Tanaka, D.; S., H.; Li, Y. J.; Su, S. J.; Takeda, T.; Kido, J. Jpn. J. Appl. Phys. 2007, 46, L10−L12. (3) Tao, Y.; Wang, Q.; Yang, C.; Wang, Q.; Zhang, Z.; Zou, T.; Qin, J.; Ma, D. Angew. Chem., Int. Ed. 2008, 47, 8104−8107. (4) Tao, Y.; Wang, Q.; Yang, C.; Zhong, C.; Qin, J.; Ma, D. Adv. Funct. Mater. 2010, 20, 2923−2929. (5) Chou, H. H.; Cheng, C. H. Adv. Mater. 2010, 22, 2468−2471. (6) Chang, C.-H.; Kuo, M.-C.; Lin, W.-C.; Chen, Y.-T.; Wong, K.-T.; Chou, S.-H.; Mondal, E.; Kwong, R. C.; Xia, S.; Nakagawa, T.; Adachi, C. J. Mater. Chem. 2012, 22, 3832−3838. (7) Tao, Y.; Wang, Q.; Yang, C.; Zhong, C.; Zhang, K.; Qin, J.; Ma, D. Adv. Funct. Mater. 2010, 20, 304−311. (8) Su, S.-J.; Cai, C.; Kido, J. Chem. Mater. 2011, 23, 274−284. (9) Takizawa, S.-y.; Montes, V. A.; Anzenbacher, P. Chem. Mater. 2009, 21, 2452−2458. (10) Jeon, S. O.; Lee, J. Y. J. Mater. Chem. 2012, 22, 4233−4243. (11) Yuan, Y.; Li, D.; Zhang, X.; Zhao, X.; Liu, Y.; Zhang, J.; Wang, Y. New J. Chem. 2011, 35, 1534−1540. (12) (a) Zhang, Y.; Lai, S.-L.; Tong, Q.-X.; Lo, M.-F.; Ng, T.-W.; Chan, M.-Y.; Wen, Z.-C.; He, J.; Jeff, K.-S.; Tang, X.-L.; et al. Chem. Mater. 2011, 24, 61−70. (b) Wang, Z.; Lu, P.; Chen, S.; Gao, Z.; Shen, F.; Zhang, W.; Xu, Y.; Kwok, H. S.; Ma, Y. J. Mater. Chem. 2011, 21, 5451−5456. (c) Zhang, Y.; Lai, S.-L.; Tong, Q.-X.; Chan, M.-Y.; Ng, T.-W.; Wen, Z.-C.; Zhang, G.-Q.; Lee, S.-T.; Kwong, H.-L.; et al. J. Mater. Chem. 2011, 21, 8206−8214. (13) (a) Ho, C.-L.; Wang, Q.; Lam, C.-S.; Wong, W.-Y.; Ma, D.; Wang, L.; Gao, Z.-Q.; Chen, C.-H.; Cheah, K.-W.; Lin, Z. Chem. Asian J. 2009, 4, 89−103. (b) Ho, C.-L.; Wong, W.-Y.; Wang, Q.; Ma, D.; Wang, L.; Lin, Z. Adv. Funct. Mater. 2008, 18, 928−937. (c) Wong, W. Y.; Ho, C. L.; Gao, Z. Q.; Mi, B. X.; Chen, C. H.; Cheah, K. W.; Lin, Z. Angew. Chem. Int. Ed. 2006, 45, 7800−7803. (d) Ho, C.-L.; Lin, M.-F.; Wong, W.-Y.; Wong, W.-K.; Chen, C. H. Appl. Phys. Lett. 2008, 92,

hybrid host materials have been designed and synthesized. The photophysical and electrochemical properties of these hosts can be tuned through the different linkage between the electronic donor and acceptor components. Green device D hosted by mPhBINCP achieved ηc,max and ηp,max as high as 77.6 cd/A and 80.3 lm/W, respectively. Yellow device E hosted by pPhBICP exhibit ηc,max of 57.2 cd/A, ηEQE,max of 19.3%, and ηp,max of 59.8 lm/W, respectively. It is noteworthy that the high EL performances are realized by using simple device architecture. All of these results make these host materials attractive for practical application.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was supported by the central allocation grant from NSFC/China (21161160442) and Wuhan Science and Technology Bureau (NO: 01010621227).

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