Synthesis of High-Triplet-Energy Host Polymer for Blue and White

Oct 23, 2014 - ... iridium dendrimer for solution-processed non-doped phosphorescent organic light-emitting diodes with flat efficiency roll-off and l...
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Synthesis of High-Triplet-Energy Host Polymer for Blue and White Electrophosphorescent Light-Emitting Diodes Fei Xu,†,§ Ji-Hoon Kim,† Hee Un Kim,† Jae-Ho Jang,† Kyoung Soo Yook,‡ Jun Yeob Lee,*,‡ and Do-Hoon Hwang*,† †

Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Korea Department of Polymer Science & Engineering and Center for Photofunctional Energy Materials, Dankook University, Yongin, Gyeonggi 448-701, Korea § Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: A high-triplet-energy host polymer consisting of 9(4-(bis(9-(2-ethylhexyl)-9H-carbazol-3-yl)methyl)phenyl)-9H-carbazole and tetraphenylsilane units was designed and synthesized. The triplet energy (2.67 eV) is one of the highest values reported for conjugated polymer hosts. Suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of −5.61 and −2.24 eV, respectively, were also observed. Blue phosphorescent polymers were obtained by introducing bis[2-(4,6-difluorophenyl)pyridinato-N,C2′]iridium(III) picolinate (FIrpic) into the host polymer while white phosphorescent polymers were synthesized by introducing red emissive bis[2-phenylquinoline-N,C 2 ′]iridium(III) picolinate ((Phq)2Irpic) into the blue phosphorescent one. Polymer lightemitting devices with the configuration ITO/PEDOT:PSS/PVK/ EML/TSPO1/LiF/Al [ITO, indium tin oxide; PEDOT, poly(3,4ethylenedioxythiophene); PSS, poly(styrenesulfonic acid); PVK, poly(N-vinylcarbazole); EML, the emitting layer was composed of polymer or polymer and 1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (OXD-7) in a doping ratio of 2:1); TSPO1, diphenylphosphine oxide-4-(triphenylsilyl)pheny] were subsequently fabricated. Efficient energy transfer from the host polymer to the blue and red iridium(III) complexes was observed owing to the high triplet energy of the host. One of the fabricated blue phosphorescent devices had a maximum luminous efficiency of 3.57 cd/A.

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INTRODUCTION

green, and red) are indispensable. To date, a number of red and green phosphorescent polymers have been reported, with maximum quantum efficiencies (EQEs) reaching 18.0% and 15.3%, respectively.6−9 However, fewer blue phosphorescent polymers have been reported thus far, which crucially limits the commercial application of single phosphorescent polymers.10,11 The most difficult issue for blue phosphorescent polymers is the lack of suitable host polymers with a high triplet energy level (ET)12 because most conjugated host polymers have ETs lower than that of a blue dopant.13−15 The ET should be higher because, otherwise, triplet excitons trapped on the blue dopant will transfer to the triplet state of the host polymer after electrical excitation. As a result, all triplet excitons cannot be utilized for light emission. On the other hand, if the ET of the

Solution-processable organic light-emitting diodes (OLEDs) have recently attracted significant attention for application to next-generation displays and solid-state lighting1,2 because the typical vapor deposition method for fabricating OLEDs has critical drawbacks such as low utilization of expensive OLED materials, high manufacturing cost, and limitation to its scalability. Over the past decades, numerous efforts have focused on developing light-emitting materials for solutionprocessable OLEDs. Among these, single phosphorescent polymers, in which heavy metal complexes are incorporated into the polymer via covalent bonds, have recently become popular because with these materials, 100% internal quantum efficiency can be theoretically achieved owing to the strong spin−orbit coupling effect, and phase segregation and triplet− triplet aggregation can be effectively inhibited.3−5 For application in display and solid state lighting, single phosphorescent polymers with three primary colors (blue, © 2014 American Chemical Society

Received: August 6, 2014 Revised: October 12, 2014 Published: October 23, 2014 7397

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Scheme 1. Synthetic Routes for Host and Iridium Complex Monomers

structure. As anticipated, an ET of 2.67 eV was observed for the synthesized host polymer PCztPSi, which is higher than that of the well-known blue iridium(III) complex, bis[2-(4,6difluorophenyl)pyridinato-N,C 2 ′]iridium(III) picolinate (FIrpic, 2.60 eV). This value is also higher than most of the ETs reported for conjugated host polymers such as a poly(mphenylene) derivative tethering carbazole unit (ET = 2.64 eV),26 poly(3,6-carbazole) (ET = 2.60 eV),27 and poly(3,6fluorene) (ET = 2.58 eV).28 In addition, blue and allphosphorescent white-emitting polymers are obtained by introducing FIrpic and red emissive bis[2-phenylquinolineN,C2′]iridium(III) picolinate ((Phq)2Irpic) into the synthesized host polymer as pendent groups. Efficient energy transfer from the host backbone to the blue and red phosphors is observed owing to the high ET of the host polymer. The synthesis, thermal stability, and optical and electrical properties of the synthesized polymers were systematically investigated. The synthetic routes to the iridium complex and host monomers are shown in Scheme 1, and that for the polymers are shown in Scheme 2.

host polymer is higher than that of the blue dopant, triplet excitons trapped on the host polymer can transfer to the triplet state of the blue dopant. Therefore, triplet excitons trapped not only on the blue dopant but also on the host polymer can induce light emission, and high electroluminescence (EL) efficiency can be achieved. Additionally, the synthesis of allphosphorescent white-emitting polymers remains a challenge owing to the lack of a suitable host polymer for a blue dopant.16,17 Thus, the development of high-ET host polymers for blue and white phosphorescent polymers is very urgent and important. Recently, two bipolar blue host polymers with the highest reported ET (2.96 eV) were synthesized by Wang and co-workers.18−20 Breakthroughs in EL performance for both blue and all-phosphorescent white-emitting polymers were obtained with EQEs of 9.0% and 7.0%, respectively. In this work, we designed and synthesized a novel high-ET host polymer through the typical Suzuki polymerization method21−24 by utilizing a small carbazole-based molecular host and tetraphenylsilane. 9-(4-(Bis(9-(2-ethylhexyl)-9Hcarbazol-3-yl)methyl)phenyl)-9H-carbazole 9-(2-ethylhexyl)-3yl)methyl)phenyl)-9H-carbazole was introduced into the polymer backbone as a high-ET host unit through modification of the 3,6-position of carbazole. Brunner et al.25 reported that the ET of conjugated polymers was mainly determined by the longest poly(p-phenylene) chains. Thus, tetraphenylsilane units (i.e., linkage group) were alternately polymerized with carbazole to block the extended π-conjugation of the backbone and limit the longest poly(p-phenylene) chain to a biphenyl

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes and chemical structures of the monomers and polymers are outlined in Schemes 1 and 2, respectively. The host monomer M1 was synthesized with a moderate yield from 3,6-dibromo-9(4-(bis(9-(2-ethylhexyl)-9H-carbazol-3-yl)methyl)phenyl)-9Hcarbazole and bis(pinacolato)diboron using PdCl2(dppf) as 7398

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Scheme 2. Synthetic Routes for Polymers (R = 2-Ethylhexyl)

Table 1. Structural and Thermal Properties of PCztPSi-Based Polymers M1:M2 or M1:M2:M3 [mol %]c copolymer PCztPSi PCztPSiB2.5 PCztPSiB5 PCztPSiB5R0.6 PCztPSiB7.5R0.7

Mwa

3

(×10 )

17.2 12.3 13.4 12.7 12.4

b

PDI

Td [°C]

yield [%]

feed ratio [%]

actual ratio [%]

2.25 1.87 1.91 1.77 1.74

469 456 453 458 446

65 52 58 45 46

50/0 50/2.5 50/5 50/5/0.6 50/7.5/0.7

50/0 50/2.1 50/4.5 50/4.5/− 50/6.0/−

a

Molecular weights were determined by GPC using polystyrene standards. bObtained from TGA measurements. Td values were recorded at 5% weight loss. cThe iridium contents in polymers were estimated by 1H NMR.

catalyst. The iridium complex monomers M2 and M3 were synthesized via the method reported by Nonoyama.29 Iridium trichloride hydrate was initially reacted with 2-(2,4difluorophenyl)pyridine or 2-phenylquinoline to give the blue or red (μ-chloro)-bridged iridium(III) dimers, respectively. Compounds M2 and M3 were subsequently synthesized by treatment of the dimers with compound 5 in 2-ethoxyethanol with 53% and 76% yield, respectively. The formation of M1− M3 was confirmed by 1H (compounds 1−5 and M1−M4; see Figure S1 in the Supporting Information) and 13C NMR spectroscopy and FAB-MS spectrometry. The PCztPSi-based polymers were prepared via a palladiumcatalyzed Suzuki coupling reaction between dibromoaryl and diborolanylaryl monomers. The host polymer is designated as PCztPSi. The blue phosphorescent polymers are designated as PCztPSiB2.5 and PCztPSiB5, where the corresponding feed ratios of the blue iridium monomer M2 were 2.5 and 5 mol %, respectively. All-phosphorescent white-emitting polymers are designated as PCztPSiB5R0.6 and PCztPSiB7.5R0.7, where the corresponding feed ratios of M2 were 5 and 7.5 mol % while those of

the red iridium monomer M3 were 0.6 and 0.7 mol %, respectively. As shown in the inset of Figure S2, the intensities of the Ha, Hb, Hb′, and Hd proton peaks of M2 increased with increasing feed ratio. The actual FIrpic content of the polymer was determined by comparing the integration of the 1H NMR signal at 5.69 ppm from the Hb proton of the main ligand of the iridium complex (Figure S2) with that at 6.10 ppm from the Hc proton of carbazole units (Figures S3 and S4). In contrast, the actual (Phq)2Irpic content of white-emitting polymers cannot be determined from the NMR spectra owing to its very low feed ratio. The existence of the (Phq)2Irpic unit was ensured by observing its characteristic photoluminescence (PL) and EL emission bands at the emission spectra of the (Phq)2Irpiccontaining white-light-emitting polymers. Feed ratios and actual monomer contents are listed in Table 1. As in other reported iridium-containing polymers, the actual iridium complex content of the polymers was slightly lower than the monomer feed ratio, possibly because of the low reactivity of the monomer toward polycondensation. All polymers were found to be soluble in common organic solvents, such as tetrahydrofuran (THF), chloroform, and toluene, with no 7399

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evidence of gel formation. The weight-average molecular weights (Mw) of the polymers, as determined by gel permeation chromatography (GPC) using a polystyrene standard, ranged from 12 300 to 17 200 with polydispersity indices (PDIs) of 1.74−2.25 after the purification processes. The yields from polymerization were 45%−65%. The synthesized polymers were characterized by 1H NMR (Figures S3 and S4), 13C NMR, and elemental analysis, and results were in good accordance with the polymer structures. The thermal transitions of the PCztPSi host polymer were studied by differential scanning calorimetry (DSC) under a nitrogen atmosphere as shown in Figure S5. No discernible exothermic and endothermic peaks were observed between 0 and 300 °C, suggesting good stability in this temperature range. The thermal properties of PCztPSi-based polymers were determined using thermal gravimetric analysis (TGA) as shown in Figure S6 and summarized in Table 1. All polymers exhibited good thermal stability, losing less than 5% of their weight upon heating to ∼440 °C in TGA under a nitrogen atmosphere. The thermal properties of the polymers are summarized in Table 1. The high thermal stability and amorphous natures of polymers are highly desirable for OLEDs, particularly for fabrication by solution process. Optical and Electrochemical Properties. To study the triplet energy level of the synthesized host polymer PCztPSi, the PL spectra in toluene were measured at room temperature and 77 K as shown in Figure 1. At 77 K, the emission peak at Figure 2. (a) Absorption and (b) PL spectra of host (PCztPSi) and blue-emitting (PCztPSiB2.5 and PCztPSiB5) polymers in film state.

from the host backbone to FIrpic within the polymer chain was inefficient in solution state. In contrast, as shown in Figure 2b, there were two distinct PL peaks at 401 and 474 nm in the film state, which were assigned to the emissions from the host polymer backbone and pendent FIrpic unit, respectively. The blue emissions observed from polymers PCztPSiB2.5 and PCztPSiB5 coincided with that of the pristine FIrpic. Furthermore, the relative intensity of the emission from the host polymer to the FIrpic unit noticeably decreased with increasing iridium content. This demonstrates that the energy transfer from the host polymer backbone to the FIrpic unit efficiently occurred in the rigid thin film and the triplet energy back transfer from FIrpic to the polymer was inhibited owing to the high ET of the latter. The UV−vis absorption and PL spectra of all-phosphorescent white-emitting polymers (PCztPSiB5R0.6 and PCztPSiB7.5R0.7) in dichloromethane solution and film states are shown in Figure S8 and Figure 3, respectively. As in the blue phosphorescent polymers, the white ones exhibited identical strong π−π* absorption peaks at 303 nm in both solution and film states. MLCT transitions from the Ir complexes were too weak to be discernible because of the low iridium complex content. In the solution state, the emission from the host backbone at 394 nm was dominant as shown in Figure S8b. Blue emission from FIrpic can be observed at around 470 nm while red emission from (Phq)2Irpic cannot be evidently observed. This indicates that the energy transfer from the host polymer to the (Phq)2Irpic unit was less effective than that to FIrpic. In contrast, three emission peaks at around 398, 473, and 577 nm were found in the PL spectra of the film state of PCztPSiB5R0.6 and PCztPSiB7.5R0.7, which can be ascribed

Figure 1. PL spectra of host polymer (PCztPSi) in toluene at room and low temperatures (77 K).

465 nm was used to determine the ET of PCztPSi, which was 2.67 eV. This value was higher than those obtained for FIrpic (ET = 2.60 eV) and (Phq)2Irpic (ET = 2.11 eV), which indicates that the triplet energy back transfer from FIrpic and (Phq)2Irpic to PCztPSi would be efficiently prevented. The ultraviolet−visible absorption (UV) and PL spectra of PCztPSi and the blue phosphorescent polymers (PCztPSiB2.5 and PCztPSiB5) in dichloromethane solution and film state are shown in Figure S7 and Figure 2, respectively. In both dichloromethane solution and film states, all polymers displayed strong π−π* absorption peaks at 302−303 nm while the low-energy absorption bands for the metal-to-ligand charge transfer (MLCT) transition in FIrpic30 were too weak to be discernible owing to the low Ir content of the polymers. In the copolymer solution in dichloromethane, the major emission was observed at around 394 nm from the host polymer while no clear emission was observed at around 470 nm from the incorporated FIrpic unit as shown in Figure S7b. These observations indicate that the intramolecular energy transfer 7400

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phosphorescent polymer PCztPSi5R0.6. However, when the ratio of the FIrpic unit further increased to 7.5%, the Φpl decreased again. It could be rationalized by the greater tendency of triplet−triplet annihilation at higher Ir(III) concentration. The electrochemical properties of the polymers were studied by cyclic voltammetry (CV), and the oxidation waves of the polymers are shown in Figure S10. The onset of the oxidation waves was between 0.88 and 0.89 V. According to the literature, the HOMO level can be estimated from the onset oxidation potential using the following equation: HOMO = −(4.8 + Eox,onset − EFc), where Eox,onset and EFc are the potentials of the polymer and ferrocene, respectively.34 The calculated HOMO levels were −5.60 to −5.61 eV, while the LUMO levels were −2.23 to −2.30 eV. The onset oxidation potentials and energy levels of the polymers are listed in Table 2. EL Device Properties. To evaluate the electroluminescence performance, the synthesized polymers were used as the emitting layer (EML) in phosphorescent polymer light-emitting diodes (PhPLEDs) fabricated with the configuration (device I) ITO/PEDOT:PSS/PVK/polymer/TSPO1/LiF/Al [ITO, indium tin oxide; PEDOT, poly(3,4-ethylenedioxythiophene); PSS, poly(styrenesulfonic acid); PVK, poly(N-vinylcarbazole); TSPO1, 4-(triphenylsilyl)phenyldiphenylphosphine oxide] as shown in Figure 4. PVK and TSPO1 were used as the hole-

Figure 3. (a) Absorption and (b) PL spectra of white-emitting (PCztPSiB5R0.6 and PCztPSiB7.5R0.7) polymers in film state.

to emissions from the host polymer, M2, and M3, respectively (Figure S9). Moreover, the emissions originating from FIrpic and (Phq)2Irpic became dominant in the film state, with intensities increasing gradually with increasing Ir complex content. According to the difference between the PL spectra of the solution and film states, we speculate that the reduction in the distance between FIrpic and (Phq)2Irpic as the state shifted from solution to solid would favor Dexter energy transfer from FIrpic to (Phq)2Irpic. Furthermore, the PL quantum yields (Φpl) of the polymer films were measured at 310 nm excitation wavelength. The Φpls of the polymers are in the range of 6.7−14.2%. Similar with the previous reported iridium-containing polymers,31−33 the Φpl of PCztPSiB2.5 decreased to 6.7% as introducing 2.5% of FIrpic unit into the host polymer PCztPSi (Φpl = 9.5%), possibly due to the energy lost during the exciton transfer from singlet to triplet state. As the ratio of the iridium content in the polymer increased more than 2.5%, the Φpl of the polymer increased. The highest Φpl of 14.2% was observed from the white

Figure 4. Configuration of PhPLED and molecular structures of holetransporting (PVK) and triplet-exciton-blocking (TSPO1) materials.

transporting and exciton-blocking layers, respectively. Figure 5 illustrates the possible energy transfer paths of triplet excitons inside the PCztPSi-based polymers as well as exciton confinement by PVK and TSPO1. The triplet energies of PVK (ET = 3.00 eV) and TSPO1 (ET = 3.36 eV) were higher than that of the host polymer PCztPSi (ET = 2.67 eV), leading to effective confinement of triplet excitons within the EML.

Table 2. Optical and Electrochemical Properties of PCztPSi-Based Polymers copolymer

λmax,absa [nm]

λPLa [nm]

Φplb [%]

Eox,onsetc [V]

HOMOc [eV]

LUMOd [eV]

Eg [eV]

ETe [eV]

PCztPSi PCztPSiB2.5 PCztPSiB5 PCztPSiB5R0.6 PCztPSiB7.5R0.7

303 303 303 303 303

401 400, 474 401, 475 398, 473, 577 401, 474, 573

9.5 6.7 8.4 14.2 10.4

0.89 0.88 0.89 0.89 0.89

−5.61 −5.60 −5.61 −5.61 −5.61

−2.24 −2.23 −2.25 −2.27 −2.30

3.37 3.37 3.36 3.34 3.31

2.67

a Measured in film. bΦpl of the thin films measured under the excitation of a 150 W xenon light source (310 nm). cHOMO levels were calculated from CV potentials using ferrocene as a standard [HOMO = −(4.8 + Eox,onset − EFc) eV]. dLUMO levels were derived via the equation Eg = HOMO − LUMO, where Eg obtained from the absorption spectra. eMeasured in toluene at low temperature (77 K).

7401

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the polymer backbone were observed. In the case of the white phosphorescent polymers PCztPSiB5R0.6 and PCztPSiB7.5R0.7, the host emission also disappeared completely as shown in Figure 6b. The triplet excitons, formed by both electrical injection and energy transfer, were trapped in the iridium complex because of the difference in the ET (Figure 5) and HOMO/LUMO energy levels of the host and Ir complexes. White emissions originating from FIrpic and (Phq)2Irpic were observed with two emission peaks at 471 and 578 nm, respectively. In particular, the intensity ratios for FIrpic and (Phq)2Irpic did not change significantly from PL to EL as in other reported all-fluorescent and fluorescent/ phosphorescent hybrid white-emitting polymers.3,4 The current density−voltage−luminance characteristics of the PCztPSi series of polymers are shown in Figure 7. Low

Figure 5. Triplet energy levels of materials included in white PhPLEDs. Possible energy flow in the device is also shown.

Additionally, inefficient back-transfer of triplet excitons from FIrpic (2.60 eV) and (Phq)2Irpic (2.11 eV) to the host polymer can be expected, owing to the high triplet energy level of the host backbone. The EL spectra of the fabricated devices are shown in Figure 6. In contrast to the PL spectra of the

Figure 7. Current density−voltage−luminance characteristics of (a) host (PCztPSi), blue-emitting (PCztPSiB2.5, PCztPSiB5, and PCztPSiB5:OXD-7) and (b) white-emitting (PCztPSiB5R0.6, PCztPSiB5R0.6:OXD-7, PCztPSiB7.5R0.7, and PCztPSiB7.5R0.7:OXD-7) polymers.

Figure 6. EL spectra of (a) host (PCztPSi), blue-emitting (PCztPSiB2.5, PCztPSiB5, and PCztPSiB5:OXD-7) and (b) whiteemitting (PCztPSiB5R0.6, PCztPSiB5R0.6:OXD-7, PCztPSiB7.5R0.7, and PCztPSiB7.5R0.7:OXD-7) polymers.

PhPLED turn-on voltages in the range of 5.5−7.5 V were observed, which can be attributed to the good charge injection of the device. The highest luminance from the blue and white PhPLEDs was observed with PCztPSiB5 (368 cd/m2) and PCztPSiB7.5R0.7 (315 cd/m2). The luminous efficiency (LE) and EQE as functions of luminance are shown in Figure 8 and Figure S11, respectively, and device performances are summarized in Table 3. Among PCztPSi, PCztPSiB2.5, and PCztPSiB5, the latter had the best device performance, with a maximum EQE of 1.73%, maximum LE of 3.57 cd/A, maximum power efficiency (PE) of 1.32 lm/W, and Commission Internationale de L’Eclairage (CIE) coordinates of (0.18, 0.36). As the FIrpic content increased, the EQE and LE of the PhPLEDs gradually increased. Between PCztPSiB5R0.6 and PCztPSiB7.5R0.7, the latter exhibited the best device performance, with a maximum EQE of 1.07%,

polymers, efficient energy transfer from the host backbone to the iridium complex was observed with both blue and white phosphorescent polymers. The substantial difference in the PL and EL spectra indicates that the predominant mechanism of the EL devices was charge trapping rather than energy transfer. In the case of the copolymer PCztPSi, two emission peaks were observed at 396 nm from the host backbone and at 599 nm due to exciplex emission. With the introduction of the iridium complex monomer into PCztPSi, the emission from the host backbone expectedly decreased. In the case of the blue phosphorescent polymers PCztPSiB2.5 and PCztPSiB5, the emission peak from FIrpic was observed at 472 nm. At an iridium complex content of 5%, no additional emissions from 7402

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the EL spectra of the blended system were similar to those of the pure polymers, and no emissions from OXD-7 were observed, which indicates complete energy transfer from the OXD-7 to the polymers. After introducing the electrontransport material OXD-7, the turn-on voltages were decreased about 0.5−1.5 V as shown in Figure 7 and Table 3, which could be attributed to the improved charge injection of the device. The highest luminance from the PhPLEDs (device II) was observed with PCztPSiB5R0.6:OXD-7 (317 cd/m2). The luminous efficiency and EQE as functions of luminance are also shown in Figure 8 and Figure S11, respectively. In the case of blue-emitting polymer PCztPSiB5, the device performance was slightly improved by introducing OXD-7 up to the maximum EQE of 1.75%. For the white-emitting polymers, the device performances were much improved about 2 times. In the case of PCztPSiB5R0.6, the EQE was increased from 0.69% to 1.72%. The device fabricated using PCztPSiB5R0.6:OXD-7 exhibited the best device performances with a maximum EQE of 1.72%, maximum LE of 4.06 cd/A, maximum PE of 2.04 lm/ W, and CIE coordinates of (0.44, 0.41), which is close to the standard CIE coordinates for warm white-light emission (0.44, 0.40). To investigate the relationship between film morphology and device performance, tapping-mode atomic force microscopy (AFM) measurements were applied on the pure polymers and polymers doped with OXD-7 as shown in Figure S12. The root-mean-square (RMS) surface roughness values were observed in the range of 0.53−1.30 nm. After blending with OXD-7, the film morphology turned more smooth, and the lowest surface roughness value was observed from PCztPSiB5R0.6:OXD-7 blend film.

Figure 8. Luminous efficiency−luminance curves of (a) host (PCztPSi), blue-emitting (PCztPSiB2.5, PCztPSiB5, and PCztPSiB5:OXD-7) and (b) white-emitting (PCztPSiB5R0.6, PCztPSiB5R0.6:OXD-7, PCztPSiB7.5R0.7, and PCztPSiB7.5R0.7:OXD-7) polymers.

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CONCLUSION The host polymer PCztPSi, having a high triplet energy level of 2.67 eV, was designed and synthesized. Using this as a host, blue and white phosphorescent polymers were synthesized by introducing the blue iridium complex FIrpic and red iridium complex (Phq)2Irpic into the side chain of the backbone. PhPLEDs were fabricated using the PCztPSi-based polymers. Efficient energy transfer from the host backbone to the blue and red iridium complexes was observed, which can be attributed to the higher triplet energy level of the host backbone. Among the blue PhPLEDs, the best performance was observed using PCztPSiB5, with a maximum EQE of 1.73%, maximum LE of 3.57 cd/A, and CIE coordinates of (0.18, 0.36). On the other hand, among the white PhPLEDs, the best performance was observed using PCztPSiB5R0.6 doped with OXD-7. Our results provide a novel avenue for the

maximum LE of 2.26 cd/A, maximum PE of 0.96 lm/W, and CIE coordinates of (0.33, 0.37), which is close to the standard CIE coordinates for white light emission (0.33, 0.33). In order to further optimize the device structure, we fabricated another series of PhPLEDs with the configuration (device II) of ITO/PEDOT:PSS/PVK/polymer:OXD-7(2:1)/ TSPO1/LiF/Al [OXD-7,1,3-bis[5-(4-tert-butylphenyl)-1,3,4oxadiazol-2-yl]benzene]. Polymers PCztPSiB5, PCztPSiB5R0.6, and PCztPSiB7.5R0.7, which showed good EL performances at device I were studied. OXD-7 was introduced to improve the electron-transporting capability, and the doping ratio of the polymer to OXD-7 was 2:1. The performances of the fabricated EL devices are summarized in Table 3. As shown in Figure 6, Table 3. EL Device Performance of PCztPSi-Based Polymers copolymer a

PCztPSi PCztPSiB2.5a PCztPSiB5a PCztPSiB5R0.6a PCztPSiB7.5R0.7a PCztPSiB5:OXD-7b PCztPSiB5R0.6:OXD-7b PCztPSiB7.5R0.7:OXD-7b

Vturn‑onc [V] 5.5 5.5 6.5 7.5 6.5 5.5 6.0 6.0

λmax,EL [nm] 396, 472 473 471, 471, 473 472, 473,

599

578 578 578 574

CIEd [x, y] (0.45, (0.19, (0.18, (0.43, (0.33, (0.23, (0.44, (0.32,

0.31) 0.34) 0.36) 0.40) 0.37) 0.33) 0.41) 0.37)

Lmax [cd/m2]

EQEmax [%]

LEmax [cd/A]

PEmax [lm/W]

68 217 368 309 315 218 317 300

0.30 0.87 1.73 0.69 1.07 1.75 1.72 1.54

0.41 1.66 3.57 1.55 2.26 3.45 4.06 3.30

0.20 0.75 1.32 0.51 0.96 1.78 2.04 1.55

a

Device I: ITO/PEDOT:PSS/PVK/polymer/TSPO1/LiF/Al. bDevice II: ITO/PEDOT:PSS/PVK/polymer:OXD-7(2:1)/TSPO1/LiF/Al. cThe voltage at a luminance of 1 cd/m2. dAt the maximum luminous efficiency. 7403

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Macromolecules

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112.9, 111.7, 109.0, 108.9, 56.5, 47.5, 39.5, 31.0, 28.8, 24.4, 23.1, 14.1, 10.9. Synthesis of 3,6-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan2-yl)-9-(4-(bis(9-(2-ethylhexyl)-9H-carbazol-3-yl)methyl)phenyl)-9H-carbazole (M1). A mixture of compound 3 (3.0 g, 3.0 mmol), bis(pinacolato)diboron (2.0 g, 7.5 mmol), PdCl2(dppf) (0.075 g, 0.09 mmol), and potassium acetate (1.8 g, 18.6 mmol) in DMF (70 mL) was stirred for 3 days at 80 °C. After the reaction mixture has cooled, it was poured into water and extracted with diethyl ether. The organic layer was washed with brine three times and dried over magnesium sulfate. After the solvent was removed under pressure, the crude product was purified through column chromatography using hexane/ethyl acetate (9:1, v/v) as eluent and recrystallized in dichloromethane and methanol to afford the product as a white solid. Yield: 1.38 g (42%). 1H NMR (CDCl3, 300 MHz): δ 8.70 (s, 2H), 8.02 (d, 2H), 7.99 (s, 2H), 7.85 (d, 2H), 7.55−7.36 (m, 14H), 7.17 (t, 2H), 6.09 (s, 1H), 4.17 (d, 4H), 2.07 (m, 2H), 1.41−1.25 (m, 40H), 0.95−0.83 (m, 12H). 13C NMR (CDCl3, 75 MHz): δ 145.1, 143.0, 141.3, 139.7, 135.0, 134.8, 132.1, 131.0, 128.0, 127.4, 126.6, 125.5, 123.1, 122.8, 122.6, 121.0, 120.3, 118.6, 109.3, 108.9, 83.6, 56.6, 47.5, 39.5, 31.0, 28.8, 24.9, 24.4, 23.1, 14.1, 10.9. FAB-MS: m/z = 1064.89 (M+). Synthesis of Methyl 6-(8-(3,6-Dibromo-9H-carbazol-9-yl)octyloxy)picolinate (4). 3,6-Dibromo-9-(8-bromooctyl)carbazole (1.0 g, 1.8 mmol), methyl 6-hydroxypicolinate (0.34 g, 2.2 mmol), K2CO3 (3.1 g, 22.4 mmol), and KI (0.28 g) were mixed in acetone (35 mL) and refluxed for 24 h. After cooling, the mixture was poured into water and extracted with dichloromethane. The organic layer was washed with brine three times and then dried over magnesium sulfate. After removal of the solvent, the crude product was purified through column chromatography using hexane/ethyl acetate (10:1, v/v) as eluent to give compound 4. Yield: 0.66 g (61%). 1H NMR (CDCl3, 300 MHz): δ 8.12 (s, 2H), 7.70−7.63 (m, 2H), 7.55 (d, 2H), 7.25 (d, 2H), 6.89 (d, 1H), 4.33 (t, 2H), 4.22 (t, 2H), 3.94 (s, 3H), 1.81−1.68 (m, 4H), 1.50−1.24 (m, 8H). 13C NMR (CDCl3, 75 MHz): δ 165.7, 163.7, 145.3, 139.2, 139.0, 128.9, 123.3, 123.2, 118.5, 115.2, 111.8, 110.3, 66.2, 52.7, 43.2, 29.2, 29.1, 28.8, 27.1, 25.9. Synthesis of 6-(8-(3,6-Dibromo-9H-carbazol-9-yl)octyloxy)picolinic Acid (5). To a suspension of compound 4 (0.6 g, 1.0 mmol) in methanol (10 mL) was added a solution of NaOH (0.14 g, 3.5 mmol) in water (10 mL). The mixture was heated at reflux for 5 h and gradually became clear. After completion of the reaction, the organic phase was evaporated under reduced pressure. The aqueous phase was acidified with concentrated HCl. The mixture was poured into water and extracted with ethyl acetate, and the combined organic layer was dried over magnesium sulfate. The solvent was removed by rotary evaporation to gain compound 5. Yield: 0.53 g (90%). 1H NMR (CDCl3, 300 MHz): δ 10.5 (br, 1H), 8.12 (s, 2H), 7.82−7.78 (m, 2H), 7.55 (d, 2H), 7.27 (d, 2H), 6.99 (d, 1H), 4.25 (m, 4H), 1.82−1.72 (m, 4H), 1.50−1.24 (m, 8H). 13C NMR (CDCl3, 75 MHz): δ 168.3, 163.7, 145.3, 139.2, 139.0, 128.9, 123.3, 123.2, 118.5, 115.2, 111.8, 110.3, 66.2, 52.7, 29.2, 29.1, 28.8, 27.1, 25.9. Synthesis of Bis[2-(4,6-difluorophenyl)pyridinato-N,C2′]iridium(III)(6-(8-(3,6-dibromo-9H-carbazol-9-yl)octyloxy)picolinate) (M2). Compound 5 (0.54 g, 0.9 mmol), sodium carbonate (0.45 g, 4.2 mmol), and blue iridium dimer (0.51 g, 0.42 mmol) were dissolved in 2-ethoxyethanol and refluxed under a nitrogen atmosphere for 12 h. After cooling to room temperature, the crude solution was poured into water, extracted with dichloromethane/brine, dried over magnesium sulfate, and concentrated in vacuo. The residue was purified through column chromatography using dichloromethane as eluent and recrystallized in dichloromethane and hexane to afford the product as a yellow solid. Yield: 0.57 g (53%). 1H NMR (CDCl3, 300 MHz): δ 8.66 (d, 1H), 8.27 (q, 2H), 8.15 (s, 2H), 8.01 (d, 1H), 7.90 (t, 1H), 7.73 (m, 2H), 7.64 (d, 1H), 7.56 (d, 2H), 7.28 (d, 2H), 7.11 (t, 1H), 6.93 (t, 1H), 6.82 (d, 1H), 6.33 (m, 2H), 5.70 (d, 1H), 5.39 (d, 1H), 4.25 (t, 2H), 3.60 (m, 2H), 1.82 (m, 2H), 1.25−0.97 (m, 10H). 13C NMR (CDCl3, 75 MHz): δ 173.3, 165.1, 164.0, 161.2, 154.4, 154.3, 151.0, 150.5, 150.5, 148.9, 148.2, 141.6, 139.2, 138.0, 137.8, 129.0, 123.4, 123.2, 123.1, 122.4, 122.2, 121.7,

design of polymer host material with high ET for efficient blue and white electrophosphorescent light-emitting diodes.

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EXPERIMENTAL SECTION

Materials. Iridium(III) chloride and carbazole were purchased from Alfa Aesar. Bis(pinacolato)diboron was obtained from TCI Chemicals. Tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4] was purchased from Strem Chemicals. 4-Bromobenzaldehyde, 1,1′bis(diphenylphosphino)ferrocene]dichloropalladium(II) [PdCl2(dppf)], 2-(2,4-difluorophenyl)pyridine, and tetraethylammonium hydroxide solution (20 wt % Et4NOH in H2O) were obtained from Aldrich. All reagents were used without further purification. Acetic acid (AcOH) was purchased from Daejung, and 2ethoxyethanol and nitrobenzene were obtained from Junsei Chemical. Anhydrous toluene was purchased from Aldrich, and N,Ndimethylformamide (DMF) was obtained from Alfa Aesar. 2Ethoxyethanol was used after purging with dry nitrogen for 10 min. 3,6-Dibromo-9-(8-bromooctyl)carbazole, 35 bis(4-bromophenyl)diphenylsilane (M4),36 methyl 6-hydroxypicolinate,37 2-phenylquinoline,38 and iridium dimers29 were prepared according to the literature. All manipulations involving IrCl3·3H2O and other Ir(III) species were performed in an atmosphere of dry nitrogen. Synthesis of 4-(9H-Carbazol-9-yl)benzaldehyde (1). Carbazole (4.0 g, 23.9 mmol), 4-bromobenzaldehyde (5.4 g, 28.7 mmol), K2CO3 (19.8 g, 143.5 mmol), and copper powder (1.5 g, 23.9 mmol) were dissolved in nitrobenzene (100 mL) under a nitrogen atmosphere and refluxed for 24 h. The mixture was then cooled to room temperature, and the solvent was removed by distillation. Ammonia solution was subsequently added to the crude product, and the mixture was stirred for another 2 h. The crude solution was poured into water, extracted with dichloromethane/brine, dried over magnesium sulfate, and concentrated in vacuo. The residue was purified through column chromatography using hexane/dichloromethane (3:1, v/v) as eluent and then recrystallized in dichloromethane and hexane to afford the product as a yellow solid. Yield: 4.48 g (69%). 1H NMR (CDCl3, 300 MHz): δ 10.12 (s, 1H), 8.17−8.13 (m, 4H), 7.78 (d, 2H), 7.54−7.30 (m, 6H). 13C NMR (CDCl3, 75 MHz): δ 191.1, 143.5, 140.1, 134.7, 131.5, 126.9, 126.3, 124.1, 121.1, 120.9, 109.9. Synthesis of 4-(3,6-Dibromo-9H-carbazol-9-yl)benzaldehyde (2). Compound 1 (3.5 g, 12.9 mmol) was dissolved in dichloromethane (100 mL), after which Br2 (1.0 mL, 38.7 mmol) in dichloromethane (5 mL) was added dropwise to the flask at 0 °C. The mixture was continuously stirred at room temperature for another 6 h. The crude solution was quenched with saturated NaOH solution and poured into water, extracted with dichloromethane/brine, dried over magnesium sulfate, and concentrated in vacuo. The residue was recrystallized in dichloromethane and methanol to afford the product. Yield: 4.76 g (86%). 1H NMR (CDCl3, 300 MHz): δ 10.12 (s, 1H), 8.24 (s, 2H), 8.18 (d, 2H), 7.71 (d, 2H), 7.51 (d, 2H), 7.35 (d, 2H). 13 C NMR (CDCl3, 75 MHz): δ 193.0, 145.7, 141.3, 134.5, 131.2, 127.3, 125.6, 123.8, 123.1, 120.3, 112.9. Synthesis of 3,6-Dibromo-9-(4-(bis(9-(2-ethylhexyl)-9H-carbazol-3-yl)methyl)phenyl)-9H-carbazole (3). To a mixture of compound 2 (2.0 g, 4.7 mmol) and 9-(2-ethylhexyl)carbazole (3.3 g, 11.8 mmol) in AcOH (100 mL), HCl (25 mL, 35.0−37.0%) was added slowly. The reaction mixture was refluxed for 48 h and then poured in water and filtered. The deposit was dissolved in dichloromethane and extracted with dichloromethane/brine three times. The organic layer was dried over magnesium sulfate. After removing the solvent by evaporation, the residue was purified through column chromatography using hexane/dichloromethane (3:1, v/v) as eluent and recrystallized in dichloromethane and methanol to afford the product as a white solid. Yield: 3.44 g (76%). 1H NMR (CDCl3, 300 MHz): δ 8.18 (s, 2H), 8.01 (d, 2H), 7.95 (s, 2H), 7.50−7.38 (m, 14H), 7.33 (d, 2H), 7.17 (t, 2H), 6.08 (s, 1H), 4.15 (d, 4H), 2.07 (m, 2H), 1.41−1.25 (m, 16H), 0.95−0.83 (m, 12H). 13C NMR (CDCl3, 75 MHz): δ 145.7, 141.3, 139.8, 139.7, 134.6, 134.5, 131.2, 129.3, 127.3, 126.5, 125.6, 123.8, 123.1, 122.8, 122.6, 121.0, 120.3, 118.6, 7404

dx.doi.org/10.1021/ma5015929 | Macromolecules 2014, 47, 7397−7406

Macromolecules

Article

121.1, 114.1, 113.9, 113.7, 113.5, 111.9, 110.3, 109.6, 98.0, 95.9, 69.5, 43.2, 29.0, 28.8, 28.7, 27.3, 26.9, 25.2. FAB-MS: m/z = 1147.25 (M+). Synthesis of Bis[2-phenylquinoline-N,C2′]iridium(III)(6-(8(3,6-dibromo-9H-carbazol-9-yl)octyloxy)picolinate) (M3). Compound 5 (0.60 g, 1.0 mmol), sodium carbonate (0.50 g, 4.7 mmol), and red iridium dimer (0.60 g, 0.47 mmol) were dissolved in 2ethoxyethanol and refluxed under a nitrogen atmosphere for 12 h. After cooling to room temperature, the crude solution was poured into water, extracted with dichloromethane/brine, dried over magnesium sulfate, and concentrated in vacuo. The residue was purified through column chromatography using dichloromethane as eluent and recrystallized in dichloromethane and hexane to afford the product as a reddish orange solid. Yield: 0.84 g (76%). 1H NMR (CDCl3, 300 MHz): δ 8.87 (d, 1H), 8.15 (s, 2H), 8.12−7.98 (m, 5H), 7.91 (d, 1H), 7.76 (t, 2H), 7.67 (t, 2H), 7.54 (d, 2H), 7.45 (m, 3H), 7.36 (t, 1H), 7.22 (d, 2H), 6.88 (m, 3H), 6.76 (d, 1H), 6.59 (m, 3H), 6.16 (d, 1H), 4.21 (t, 2H), 3.60 (m, 2H), 1.82 (m, 2H), 1.25−0.97 (m, 10H). 13C NMR (CDCl3, 75 MHz): δ 172.9, 171.3, 170.4, 163.9, 152.0, 151.7, 149.3, 147.2, 146.4, 146.1, 140.6, 139.2, 138.4, 138.3, 134.9, 133.1, 131.5, 129.5, 129.4, 129.1, 129.0, 128.6, 127.7, 127.5, 127.2, 126.9, 126.3, 126.1, 125.9, 125.6, 125.3, 123.4, 123.2, 121.8, 119.8, 119.7, 117.1, 115.7, 111.9, 110.3, 108.2, 69.1, 43.2, 29.1, 28.9, 28.8, 27.6, 27.0, 25.6. FAB-MS: m/z = 1175.31 (M+). General Procedure for Suzuki Polymerization with PCztPSiB2.5 as Example. In a 50 mL two-necked flask, M1 (0.40 g, 0.38 mmol), M4 (0.18 g, 0.36 mmol), M2 (21.5 mg, 0.019 mmol), Pd(PPh3)4 (13 mg), and tetrabutylammonium bromide (TBAB) (12.1 mg, 0.038 mmol) were dissolved in toluene (5 mL), after which tetraethylammonium hydroxide solution (5 mL, 20 wt % Et4NOH in H2O) was added. The mixture was heated to 110 °C and stirred for 36 h under an argon atmosphere. The polymer was then capped by adding phenylboronic acid and stirring the reaction mixture continuously for 6 h. Bromobenzene was subsequently added, and the mixture was continuously stirred for another 6 h. The whole mixture was poured into methanol. The precipitated polymer was recovered by filtration and purification by silica column chromatography with chloroform. The polymer was then purified further by Soxhlet extraction with acetone as solvent to remove oligomers. The reprecipitation procedure with chloroform/methanol was repeated several times. The resulting polymer was soluble in common organic solvents such as chloroform and toluene. The polymer yield was 52%. 1 H NMR (300 MHz, CDCl3): δ 8.41 (br, 2H), 8.16 (br, 0.09H), 7.96 (br, 4H), 7.80−7.60 (br, 12H), 7.50 (br, 6H), 7.37 (br, 14H), 7.15 (br, 4H), 6.31 (br, 0.09H), 6.07 (br, 1H), 5.69 (br, 0.04H), 5.39 (br, 0.04H), 4.36 (br, 0.09H), 4.13 (br, 4H), 2.03 (br, 2H), 1.43−1.20 (m, 16H), 0.88 (br, 12H). 13C NMR (CDCl3, 75 MHz): δ 145.1, 142.9, 141.3, 140.9, 139.7, 137.0, 136.4, 135.2, 134.8, 134.4, 133.1, 132.0, 131.0, 129.6, 128.8, 127.9, 127.4, 126.7, 126.5, 125.5, 123.9, 122.8, 122.6, 121.0, 120.3, 118.9, 118.6, 110.4, 108.9, 56.6, 47.5, 39.5, 31.0, 28.8, 24.4, 23.1, 14.1, 10.9. Element Anal. Found: C, 86.41; H, 7.29; N, 4.15. PCztPSi. M1 (0.40 g, 0.38 mmol) and M4 (0.19 g, 0.38 mmol) were used for polymerization. Yield: 65%. 1H NMR (300 MHz, CDCl3): δ 8.41 (br, 2H), 7.96 (br, 4H), 7.80−7.60 (br, 12H), 7.50 (br, 6H), 7.37 (br, 14H), 7.15 (br, 4H), 6.07 (br, 1H), 4.13 (br, 4H), 2.03 (br, 2H), 1.43−1.20 (m, 16H), 0.88 (br, 12H). 13C NMR (CDCl3, 75 MHz): δ 145.1, 142.9, 141.3, 140.9, 139.7, 137.0, 136.5, 135.3, 134.8, 134.4, 133.2, 132.0, 131.0, 129.6, 128.8, 127.9, 127.4, 126.8, 126.5, 125.6, 124.0, 122.8, 122.6, 121.0, 120.4, 118.9, 118.6, 110.4, 108.9, 56.6, 47.5, 39.5, 31.0, 28.8, 24.4, 23.1, 14.1, 10.9. Element Anal. Found: C, 86.63; H, 7.27; N, 4.09. PCztPSiB5. M1 (0.40 g, 0.38 mmol), M4 (0.17 g, 0.34 mmol), and M2 (43.1 mg, 0.038 mmol) were used for polymerization. Yield: 58%. 1 H NMR (300 MHz, CDCl3): δ 8.41 (br, 2H), 8.16 (br, 0.18H), 7.96 (br, 4H), 7.80−7.60 (br, 12H), 7.50 (br, 6H), 7.37 (br, 14H), 7.15 (br, 4H), 6.31 (br, 0.18H), 6.07 (br, 1H), 5.69 (br, 0.09H), 5.39 (br, 0.09H), 4.36 (br, 0.18H), 4.13 (br, 4H), 2.03 (br, 2H), 1.43−1.20 (m, 16H), 0.88 (br, 12H). 13C NMR (CDCl3, 75 MHz): δ 142.9, 141.3, 140.9, 139.7, 136.9, 136.4, 135.2, 134.8, 134.4, 133.1, 131.9, 131.0, 129.6, 128.8, 127.9, 127.4, 126.7, 126.5, 125.5, 123.9, 122.8, 122.6,

121.0, 120.3, 118.9, 118.6, 110.4, 108.9, 106.5, 56.5, 47.5, 39.5, 31.0, 28.8, 24.4, 23.1, 14.1, 10.9. Element Anal. Found: C, 85.69; H, 7.26; N, 4.27. PCztPSiB5R0.6. M1 (0.40 g, 0.38 mmol), M4 (0.165 g, 0.33 mmol), M2 (43.1 mg, 0.038 mmol), and M3 (5.3 mg, 0.0045 mmol) were used for polymerization. Yield: 45%. 1H NMR (300 MHz, CDCl3): δ 8.43 (br, 2H), 8.17 (br, 0.18H), 7.98 (br, 4H), 7.80−7.60 (br, 12H), 7.53 (br, 6H), 7.39 (br, 14H), 7.19 (br, 4H), 6.31 (br, 0.18H), 6.09 (br, 1H), 5.67 (br, 0.09H), 5.39 (br, 0.09H), 4.37 (br, 0.18H), 4.15 (br, 4H), 2.06 (br, 2H), 1.43−1.20 (m, 16H), 0.87 (br, 12H). 13C NMR (CDCl3, 75 MHz): δ 145.1, 142.9, 141.3, 140.9, 139.7, 136.9, 136.4, 135.3, 134.8, 134.4, 133.1, 132.0, 131.0, 129.5, 128.7, 127.9, 127.4, 126.8, 126.5, 125.5, 124.0, 122.8, 122.6, 121.1, 120.5, 118.9, 118.7, 110.4, 108.9, 56.6, 47.5, 39.6, 31.0, 28.8, 24.4, 23.0, 14.0, 11.0. Element Anal. Found: C, 85.02; H, 7.45; N, 3.36. PCztPSiB7.5R0.7. M1 (0.40 g, 0.38 mmol), M4 (0.155 g, 0.34 mmol), M2 (64.6 mg, 0.056 mmol), and M3 (6.2 mg, 0.0052 mmol) were used for polymerization. Yield: 46%. 1H NMR (300 MHz, CDCl3): δ 8.43 (br, 2H), 8.17 (br, 0.24H), 7.98 (br, 4H), 7.80−7.60 (br, 12H), 7.51 (br, 6H), 7.41 (br, 14H), 7.16 (br, 4H), 6.31 (br, 0.24H), 6.10 (br, 1H), 5.69 (br, 0.12H), 5.40 (br, 0.12H), 4.37 (br, 0.24H), 4.16 (br, 4H), 2.06 (br, 2H), 1.43−1.20 (m, 16H), 0.88 (br, 12H). 13C NMR (CDCl3, 75 MHz): δ 145.1, 142.9, 141.3, 140.9, 139.7, 136.9, 136.4, 135.3, 134.8, 134.4, 133.1, 132.0, 131.0, 129.6, 128.7, 127.9, 127.3, 126.8, 126.5, 125.5, 124.0, 122.8, 122.6, 121.1, 120.5, 118.9, 118.7, 110.4, 108.8, 56.7, 47.5, 39.6, 31.0, 28.8, 24.4, 23.0, 14.1, 11.0. Element Anal. Found: C, 84.96; H, 7.67; N, 3.30. Device Fabrication. Devices with the ITO/PEDOT:PSS/PVK/ EML/TSPO1/LiF/Al configuration were fabricated using the synthesized polymers (Figure 4) as follows. ITO glass substrates were consecutively washed with acetone, detergent, distilled water, and 2-propanol. After ultraviolet/ozone treatment for 10 min, a 60 nm thick layer of PEDOT doped with PSS (PEDOT:PSS, CH8000) was spin-coated onto the ITO substrates. The spin-coated film was baked at 140 °C for 10 min. A 10 nm thick layer of PVK in chlorobenzene was spin-coated onto the PEDOT:PSS layer and baked at 120 °C for 10 min. Two kinds of emitting layers were prepared: one is pure polymers, and another is polymers dopped with OXD-7. Polymers or polymers with the OXD-7 in a dopping ratio of 2:1 were dissolved in toluene, filtered through a 0.50 μm polytetrafluoroethylene (hydrophobic) syringe filter, spin-coated onto the PVK layer, and baked at 120 °C for 10 min. The active layers were around 40 nm thick, as determined using an Alpha-Step IQ surface profiler (KLA Tencor). TSPO1, which acts as a high-triplet-energy exciton-blocking layer (HBL) with electron transport properties, was subsequently deposited on the emissive layer.39 Finally, lithium fluoride (LiF) was deposited as an electron-injecting layer (EIL) at an evaporation rate of 1 Å/s, and aluminum was deposited by vacuum evaporation on top of the film through a mask at a deposition rate of 5 Å/s. Measurements. 1H and 13C NMR spectra were recorded using a Varian Mercury Plus 300 MHz spectrometer. The chemical shifts were expressed as parts per million (ppm), with chloroform as an internal standard (δ 7.26 ppm). The fast atom bombardment mass spectra were measured using a ZMS-DX303 mass spectrometer (FAB-MS; JEOL LTD). Elemental analysis was performed using a Vario Micro Cube at the Korea Basic Science Institute (Busan, Korea.) The number- (Mn) and weight-average molecular weights and polydispersity indices of the polymers relative to a polystyrene standard were determined via gel permeation chromatography using a Waters highpressure GPC assembly (Model M590). Thermal analyses were performed using a Mettler Toledo TGA/SDTA 851e under a N2 atmosphere, with heating and cooling rates of 10 °C/min. Ultraviolet− visible absorption was recorded using a Shimadzu UV−vis spectrophotometer (UV-1800). Photoluminescence emission spectra were recorded using a Hitachi F-7000 fluorescence spectrophotometer. The PL quantum yields were measured using a Quantaurus-QY C11347−11 (Hamamatsu Photonics) at 310 nm excitation wavelength from 150 W xenon light source. Cyclic voltammetry using a CH Instruments electrochemical analyzer was carried out in an acetonitrile (CH3CN) solution containing 0.10 M tetrabutylammonium tetra7405

dx.doi.org/10.1021/ma5015929 | Macromolecules 2014, 47, 7397−7406

Macromolecules

Article

fluoroborate (TBABF4) as supporting electrolyte. A Pt plate, Pt wire, and Ag/AgNO3 electrode were used as the working, counter, and reference electrodes, respectively. Atomic force microscopy was performed using SPM L-Trace II operating in the tapping mode in air. The current density−voltage−luminance (I−V−L) characteristics and EL spectra of the PhPLEDs were measured using a Keithley 2400 source measurement unit and CS 1000 spectrophotometer.

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ASSOCIATED CONTENT

* Supporting Information S

1

H NMR spectra of compounds (1-5), monomers (M1-M4), and polymers, DSC and TGA curves, absorption and PL spectra in dichloromethane solution, cyclic voltammograms, AFM images, and efficiency−luminance curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.-H.H.). *E-mail: [email protected] (J.Y.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Industrial Strategic Technology Development Program (No. 10039141, Development of core technologies for organic materials applicable to OLED lighting with high color rendering index) funded by the Ministry of Knowledge Economy (MKE, Korea), the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CASE-2014M3A6A5060936), and a National Research Foundation (NRF) grant funded by the Korean government (NRF-2014R1A2A2A01007318).

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dx.doi.org/10.1021/ma5015929 | Macromolecules 2014, 47, 7397−7406