Synthesis of π-Conjugated Polymers Containing Fluorinated Arylene

Article pubs.acs.org/Macromolecules

Synthesis of π-Conjugated Polymers Containing Fluorinated Arylene Units via Direct Arylation: Efficient Synthetic Method of Materials for OLEDs Wei Lu,† Junpei Kuwabara,*,† Takayuki Iijima,‡ Hideyuki Higashimura,‡ Hideki Hayashi,§ and Takaki Kanbara*,† †

Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan ‡ Advanced Materials Research Laboratory, Sumitomo Chemical Co., Ltd., 6 Kitahara, Tsukuba 300-3294, Japan § Nagoya Municipal Industrial Research Institute, 3-4-41, Rokuban, Atsuta-ku, Nagoya 456-0058, Japan S Supporting Information *

ABSTRACT: Polycondensation via direct arylation of tetrafluorobenzene or octafluorobiphenyl was investigated for the synthesis of π-conjugated polymers consisting of fluorinated arylene units. The optimization of reaction conditions revealed that a combination of Pd(OAc)2 and PtBu2Me-HBF4 is the most efficient catalytic system for the polycondensation reactions. The polycondensation reactions produced four types of π-conjugated polymers having low highest occupied molecular orbital (HOMO) levels due to the strong electron-withdrawing nature of the fluorine substituents. Owing to the low HOMO levels, the synthesized polymers served as an efficient hole-blocking layer (HBL) in OLEDs.

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dibromo-9,9-dioctylfluorene.8 The polycondensation reaction gave the corresponding polymer with a high molecular weight without the use of organometallic monomers. Because this simple procedure gives high-molecular-weight polymers possessing a high film-forming ability, the synthetic method is available for syntheses of materials for organic electronic devices. In addition to the high reactivity of tetrafluorobenzene for direct arylation, the selection of tetrafluorobenzene as a monomer plays an important role in providing unique optoelectronic properties to the obtained polymer because of the strong electron-withdrawing nature of the fluorine substituents. In general, π-conjugated polymers with strong electron-withdrawing substituents possess a low HOMO level,12 which is an essential element of the hole-blocking materials in OLEDs.2,13 In addition, π-conjugated polymers containing tetrafluorobenzene units showed better performance as luminescent materials in OLEDs than analogous polymers containing phenylene units because tetrafluorobenzene units lead to a facile electron injection from the electrode interface owing to local polarity on the polymers.14 Therefore, developing a method that uses direct arylation of tetrafluorobenzene is expected to produce valuable optoelectronic materials. To prove the usability of the previously developed polycondensation reaction for producing optoelectronic

INTRODUCTION π-Conjugated polymers have attracted considerable attention as important materials for organic electronic devices such as organic light emitting diodes1,2 (OLEDs) and polymer solar cells.3 The development of π-conjugated polymers has been achieved based on reliable synthetic methods such as Yamamoto polycondensation and polycondensation using transition-metal-catalyzed cross-coupling reactions including Suzuki−Miyaura coupling and Migita−Kosugi−Stille coupling.4 Because the importance of π-conjugated polymers as optoelectronic materials continues to grow in academia and industry, reconsideration of synthetic methodology is required for developing efficient and environmentally friendly processes. The conventional synthetic methods that use cross-coupling reactions generally require preparation of organometallic reagents such as organoboron or organotin compounds as monomers. In some cases, the polycondensation produces a stoichiometric amount of toxic byproduct such as trialkyltin bromide. To overcome these issues, polycondensation using dehydrohalogenative cross-coupling reactions, so-called direct arylation, was investigated.5−10 This synthetic strategy enables us to avoid prior preparation of organometallic monomers and the treatment of toxic byproduct. Ozawa and co-workers successfully synthesized poly(3-alkylthiophene)s with a high molecular weight and high regioregularity via direct arylation.6 Because the C−H bonds in fluorobenzenes possess high reactivity for direct arylation,11 we demonstrated the polycondensation reaction of 1,2,4,5-tetrafluorobenzene with 2,7© 2012 American Chemical Society

Received: March 10, 2012 Revised: April 27, 2012 Published: May 8, 2012 4128

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materials, we expanded the synthetic method using direct arylation of tetrafluorobenzene, and evaluated the obtained polymers as hole-blocking materials for OLEDs.

Table 1. Results of the Polycondensation Reactions and Thermophysicalproperties of the Polymersa

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entry

RESULTS AND DISCUSSION Reexamination of Optimal Reaction Conditions. We previously reported that the polycondensation reaction of 2,7dibromo-9,9-dioctylfluorene and 1,2,4,5-tetrafluorobenzene in the presence of Pd(OAc)2 (5 mol %), PtBu2Me-HBF4 (10 mol %), and K2CO3 (2 equiv) in dimethylacetamide (DMAc) for 6 h produced poly[(9,9-dioctylfluorene-2,7-diyl)-(2,3,5,6-tetrafluoro-1,4-phenylene)] (PDOF-TP) with a molecular weight of 26700 and an 83% yield (eq 1).8 The procedure yielded a

1 2 3 4

polymer PDOFTP 3,6POCTP PDOPTP PDOFOD

Mnb

Mw/Mnb

yield (%)c

Tgd (°C)

31 500

3.45

81

−

8300

1.75

66

12 500

1.65

43 200

2.34

Tme (°C)

Td5f (°C) 412

56

190, 231, 247 −

88

52

−

366

95

−

218, 249

429

433

Reactions were carried out for 24 h at 100 °C using Pd(OAc)2 (5 mol %), PtBu2Me-HBF4 (10 mol %), and K2CO3 (2 equiv) in DMAc. b Estimated by GPC calibrated on polystyrene standards. cThe products were obtained by reprecipitation from CHCl3/MeOH. d Glass-transition temperature. eMelting point. fThe 5% weight-loss temperatures under inert atmosphere. a

Scheme 1. Polycondensation of 1,2,4,5-Tetrafluorobenzene with Various Dibromoaromatic Compounds

higher molecular weight polymer in a shorter reaction time than conventional methods using a cross-coupling reaction.14,15 Alternatively, we also found that in the presence of pivalic acid, the polycondensation of thiophene derivatives with dibromoarylene analogues proceeded via direct arylation without the use of a phosphine ligand.9 In general, added carboxylic acids work as carboxylate ligands that assist the deprotonation step in direct arylation reactions, which is called a concerted metalation−deprotonation pathway.16 To apply the results of the thiophene derivatives to the polycondensation of tetrafluorobenzene, the reaction upon addition of pivalic acid and without the use of PtBu2Me-HBF4 was examined (see Supporting Information, Table S1). The reaction with pivalic acid in the absence of the phosphine ligand did not yield any polymeric product (Table S1, entry 2). In addition, the reaction with pivalic acid and PtBu2Me-HBF4 gave PDOF-TP with a molecular weight of 19300 and 70% yield (Table S1, entry 3). These results show that PtBu2Me is necessary for the reaction and that the addition of pivalic acid has a slightly negative effect on the polycondensation. On the basis of these results, reaction conditions without the addition of carboxylic acids were determined to be the optimal conditions for this polycondensation reaction. Syntheses of Various Polymers. For a prolonged reaction time of 24 h, PDOF-TP with a high molecular weight of 31500 was obtained with 81% yield (Table 1, entry 1). Then, the polycondensation reactions of 1,2,4,5-tetrafluorobenzene with various dibromoarylene analogues for 24 h were carried out under the optimized conditions (Scheme 1 and Table 1). Under the same conditions as those for the synthesis of PDOFTP, the polycondensation reaction of 1,2,4,5-tetrafluorobenzene with 2,7-dibromo-N-octadecylcarbazole produced an insoluble solid in common organic solvents such as CHCl3, DMF, and DMSO. Owing to its insolubility, further characterization of the polymer was not performed. Insolubility of the polymer is presumably due to the cross-linking structure of the polymer.9 Owing to the high reactivity of the C−H bonds at the 3- and 6-positions in the carbazole derivatives,17 direct arylation likely occurred at these C−H bonds as well as at the

C−H bonds of 1,2,4,5-tetrafluorobenzene, which resulted in the formation of the cross-linking structures. In contrast, the polycondensation reaction of 3,6-dibromo-N-octadecylcarbazole gave poly[(N-octadecylcarbazole-3,6-diyl)-(2,3,5,6-tetrafluoro-1,4-phenylene)] (3,6-POC-TP), which was soluble in tetrachloroethane (Table 1, entry 2).8 The improved solubility of 3,6-POC-TP indicated a relatively low cross-linking structure. Because 3,6-dibromo-N-octadecylcarbazole does not have active C−H bonds at the 3- and 6-positions, the side reaction in the carbazole moiety would be suppressed. In a similar manner, the polycondensation reaction of 2,8-dibromo10,10-dioctyl-N-methylphenazasiline provided a soluble polymer, poly[(10,10-dioctyl-N-methylphenazasiline-2,8-diyl)(2,3,5,6-tetrafluoro-1,4-phenylene)] (PDOP-TP) with 88% yield (Table 1, entry 3). Because the 19F NMR spectrum of PDOP-TP exhibited a single resonance at −141.4 ppm, terminal or cross-linking structures were minor (see Supporting Information). The polycondensation reaction of 2,7-dibromo-9,9-dioctylfluorene with 2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl instead of 1,2,4,5-tetrafluorobenzene was carried out under the optimized conditions (eq 2, Table 1, entry 4). The reaction gave poly[(9,9-dioctylfluorene-2,7-diyl)-(2,2′,3,3′,5,5′,6,6′-octafluoro4,4′-diphenylene)] (PDOF-OD) with a molecular weight of 43200 and 95% yield. The high molecular weight and yield of PDOF-OD indicate high reactivity for 2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl in direct arylation in comparison with 1,2,4,5tetrafluorobenzene. The high reactivity can probably be attributed to the extended π-conjugated structure of octafluorobiphenyl, because a C−H bond in an extended πconjugated molecule tends to possess higher reactivity than that in a small π-conjugated molecule.18 For example, a bithiophene 4129

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derivative showed a higher reactivity for direct arylation than monothiophene derivatives.9 The 1H NMR spectrum of PDOF-OD exhibits signals of the repeating dioctylfluorene unit (Figure S-4, Supporting Information). However, the 19F NMR spectrum exhibited minor signals at −134.1 and −134.5 ppm as well as the signals of the repeating unit at −135.0 and −139.0 ppm (Figure 1a). The integral ratio between the minor

Figure 2. UV/vis absorption spectra of PDOF-TP, 3,6-POC-TP, PDOP-TP, and PDOF-OD in thin film state.

in the wide band gap of PDOF-OD and absorption in a shortwavelength region. PDOP-TP exhibited the longest wavelength absorption among the polymers probably due to the flat structure of the phenazasiline unit possessing a wide π conjugation.21 The absorption spectra in the solution state were quite similar to those in the thin film state (Table 2). The small difference in the absorption spectra between the solution state and the film state suggests weak interchain interactions such as π−π stacking in the film state. Table 2 summarizes the detailed optical properties of the polymers including absorption and fluorescent maxima, fluorescent quantum yield, and optical band gap. The optical band gaps (Eg) were obtained from the absorption edges (λedge) in the film state. The HOMO levels of the polymers were obtained from ionization potentials. PDOF-TP possessed the deepest HOMO level (−5.96 eV) among the synthesized polymers. Because the HOMO level of the analogous polymer without any fluorine atoms, poly[(9,9-dihexylfluorene-2,7-diyl)(1,4-phenylene)], is −5.36 eV,22 four fluorine atoms on the phenylene unit in PDOF-TP strongly affect the HOMO level, resulting in HOMO levels that are lower by 0.6 eV. The correlation between HOMO levels and performance as holeblocking layers in OLEDs will be discussed later. Powder X-ray diffraction (XRD) patterns of the drop-cast films of PDOF-TP exhibited a peak at 7.9°. On the basis of the XRD patterns of poly(9,9-dioctylfluorene-2,7-diyl),23 the peak was assigned to a lamellar morphology with 1.1 nm in layer spacing (Figure S-7, Supporting Information). However, poly[(9,9-dihexylfluorene-2,7-diyl)-(1,4-phenylene)] was reported to exhibit no apparent diffraction peak in the XRD measurement, which indicated an amorphous structure.24 These results suggest that fluorine atoms on the phenylene unit provide relatively high crystallinity in PDOF-TP.25 The XRD patterns of PDOF-OD indicated a lamellar morphology with 1.5-nm layer spacing. However, no apparent peak was observed in the films of 3,6-POC-TP and PDOP-TP, indicating amorphous nature of the polymers. Differential scanning calorimeter (DSC) measurements of 3,6-POC-TP and PDOP-TP also supported an amorphous nature based on the existence of a glass-transition temperature Tg and the absence of a melting point (Table 1). Evaluation of Polymers as OLED Materials. PDOF-TP was expected to serve as a hole-blocking material on an OLED cathode because of the low HOMO level of PDOF-TP. To investigate the performance of the polymers as hole-blocking materials, devices with the configuration of ITO/PEDOT− PSS/emitting polymer-1/emitting polymer-2/HBL/LiF/Al

Figure 1. 19F NMR spectra of (a) PDOF-OD and (b) PDOF-TP (377 MHz, CDCl3).

unit and the repeating unit was 1:25. This value is inconsistent with the estimated value (1:127) based on the degree of polymerization (n = 63) under the assumption that the minor signals were assigned to a terminal octafluorobiphenyl unit. These results suggest the branching structures were formed by a Pd-catalyzed oxidative C−H/C−H cross-coupling reaction between C−H bonds in the fluorine unit and octafluorobiphenyl.19 The result of the MALDI−TOF−MASS of PDOFOD also supports the formation of branching structures (Figure S-6, Supporting Information). The high reactivity of the octafluorobiphenyl monomer might cause the side reaction, which is the oxidative coupling reaction, to form the branching structures. It is noteworthy that the 19F NMR spectrum of PDOF-TP exhibited the sole signal of the repeating unit without minor signals (Figure 1b). Physical Properties. Figure 2 shows absorption spectra of the polymers in the thin film state. The absorption of 3,6-POCTP appeared in the short-wavelength region in contrast to the other polymers. The short-wavelength absorption of 3,6-POCTP can be attributed to the limited π-conjugation on the main chain of the polymer due to the linkage at the 3- and 6positions on the carbazole unit.20 The absorption spectrum of PDOF-OD had a similar trace to that of PDOF-TP but appeared in a relatively short-wavelength region. The octafluoro-4,4′-diphenylene unit of PDOF-OD should be twisted owing to the steric hindrance of the fluorine atoms. The effect of the twisted diphenylene structure is likely to result 4130

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Table 2. Optical Properties and Energy Level of Polymersa thin film

solution

a

polymer

λmax (nm)

λem (nm)

ΦF

λmax (nm)

λem (nm)

Eg (eV)c

EHOMOd

ELUMOe

PDOF-TP 3,6-POC-TPb PDOP-TP PDOF-OD

344 302 (340) 361 335

385 409 406 373

0.66 0.04 0.15 0.36

344 304 (341) 364 338

388 410 412 376

3.14 3.10 3.02 3.18

−5.96 −5.74 −5.68 −5.82

−2.82 −2.64 −2.66 −2.78

Solution data are given in CHCl3, and value in parentheses are prominent shoulders. bTetrachloroethane was used as solvent. cEg = optical gap. EHOMO was obtained by from ionization potential. eELUMO = EHOMO + Eg.

d

the device with PDOF-OD was observed among the fabricated devices. Although PDOF-OD possesses a low HOMO level (−5.82 eV), the PDOF-OD layer provided a negative effect presumably due to its high resistance, which was confirmed by the voltage−current characteristics (Figure S-9, Supporting Information). To summarize, the device with PDOF-TP demonstrated the best performance as a hole-blocking material compared with the other polymers owing to its low HOMO level and relatively low resistance. Polymers bearing polar groups have been found to serve as materials for an electron injection layer on an OLED cathode.2 Therefore, PDOF-TP has the potential to serve as an electron injection material because the strong electron-withdrawing nature of the fluorine substituents causes local polarity on the polymers,14a which might also contribute to the improvement of current efficiency.

were fabricated with hole-blocking layers (HBL) made from PDOF-TP, PDOP-TP, and PDOF-OD. The details of OLED fabrication and the structure of emitting polymers are described in the Experimental Section. Evaluations of 2,7-POC-TP and 3,6-POC-TP were not performed because of their low filmforming ability. For comparison, the device without a HBL was also fabricated. Figure 3 shows luminance versus current

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CONCLUSION Polycondensation via the direct arylation of tetrafluorobenzene was successively extended to provide new π-conjugated polymers. In addition to the high reactivity of tetrafluorobenzene in polycondensation, the introduction of a tetrafluorobenzene unit has significance in lowering the HOMO level of the polymers. Owing to the low HOMO level, the obtained polymers, especially PDOF-TP, served as an effective hole-blocking material in OLEDs. The present research provides an efficient methodology for producing materials for optoelectronic devices.

Figure 3. Luminance vs current density plots for the devices (ITO/ PEDOT−PSS/emitting polymer-1/emitting polymer-2/hole-blocking layer/LiF/Al).

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density plots of these devices. The luminance of the device with PDOF-TP was higher than that of the device without a HBL over the entire current density region. In terms of current efficiency, the device with PDOF-TP exhibited a higher value (1.52 CdA−1) than that of the device without a HBL (1.02 CdA−1) at 6 V, which indicated PDOF-TP served as an efficient hole-blocking material in the device. The positive effect of PDOP-TP in the HBL was observed only in the low current density region. The low performance of PDOP-TP as a holeblocking material is attributed to its higher HOMO level (−5.68 eV) compared with that of PDOF-TP (−5.96 eV). The three above-mentioned OLED devices exhibited luminescence with a maximum at around 455 nm (Figure S-8a (Supporting Information), the coordinates of the CIE chromaticity at 6 V: x = 0.17, y = 0.19). Because the luminance spectra are quite similar to the photoluminescence spectrum of the device without a HBL (Figure S-8b, Supporting Information), the electroluminescence of the devices occurred at the emitting layer and emission from the HBL was negligible. In contrast, the device with PDOF-OD exhibited broad luminescence, which was significantly different from those of the other devices. This difference in luminescence suggests that a smooth recombination in the emitting layer is prevented by the presence of PDOF-OD. In addition, the lowest luminance of

EXPERIMENTAL SECTION

1,2,4,5-tetrafluorobenzene, 2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl, 2,7-dibromo-9,9-dioctylfluorene, Pd(OAc)2, PtBu2Me-HBF4, K2CO3, and other chemicals were received from commercial suppliers and used without further purification. Anhydrous dimethylacetamide (DMAc) were purchased from Kanto Chemical and used as a dry solvent. Poly(ethylene-1,2-dioxy-thiophene)/poly(stylene surfonic acid) (PEDOT:PSS) (CLEVIOS P VP AI 4083) was purchased from H.C.Starck Inc. The epoxy resin (PX681C/NC) was purchased from Robnor resins Ltd. Tripentaerythritol octaacrylate (TPEA) was purchased from Koei Chemical Co., Ltd. 3,6-Dibromo-N-octadecylcarbazole,8 2,7-dibromo-N-octadecylcarbazole,9 and 2,8-dibromo10,10-dioctyl-N-methylphenazasiline21 were prepared according to the literature methods. Syntheses of Emitting polymer-1 and 2 were described in Supporting Information. All manipulations for the reactions were carried out under nitrogen atmosphere using a standard Schlenk technique. NMR spectra were recorded on a Bruker AVANCE-400 NMR spectrometer and a JEOL JNM-ECS-400 NMR spectrometer. 1 H and 13C{ 1H} spectra were measured with tetramethylsilane (TMS) as internal standard. 19F NMR spectra were measured with hexafluorobenzene as external standard (−162.9 ppm). Gel permeation chromatography (GPC) measurements were carried out on a SHIMADZU prominence GPC system equipped with polystyrene gel columns, using CHCl3 as an eluent after calibration with polystyrene standards. A high-temperature gel permeation chromatography (HT-GPC) measurement for 3,6-POC-TP was 4131

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carried out on a TOSHO HLC-8121GPC/HT DF-8020 at 140 °C, using 1,2-dichlorobenzene as an eluent after calibration with polystyrene standards. MALDI-TOF-MS spectra were recorded on AB SCIEX MALDI TOF/TOF 5800 using dithranol as a matrix. UV/ vis absorption spectra were recorded on a JASCO V-630iRM spectrophotometer. Fluorescence spectra were recorded on a JASCO FP-6200 spectrophotometer in a concentration of 1 × 10−5 M at room temperature. Fluorescence quantum yields were obtained by a Hamamatsu Photonics absolute PL quantum yield measurement system C9920−02. Column chromatography was carried out with silica gel 60 (Kanto, 40−100 μm, neutral). Purifications by High Performance Liquid Chromatography (HPLC) were carried out on a JAI LC-9201 using CHCl3 as an eluent. X-ray diffraction patterns were recorded on a Philip X’Pert MRD with CuKα radiation (λ = 1.542 Å). The thermal properties were measured on SII EXSTAR6200 DSC instrument and an EXSTAR TG/DTA6300 instrument. Polycondensation of 1,2,4,5-Tetrafluorobenzene and 2,8Dibromo-10,10-dioctyl-N-methylphenazasiline. A mixture of Pd(OAc)2 (5.6 mg, 0.025 mmol), PtBu2Me-HBF4 (12.4 mg, 0.05 mmol), 2,8-dibromo-10,10-dioctyl-N-methylphenazasiline (297 mg, 0.5 mmol), 1,2,4,5-tetrafluorobenzene (55.8 μL, 0.5 mmol) and K2CO3 (138 mg, 1.0 mmol) was stirred in dimethylacetamide (1.0 mL) for 24 h at 100 °C under nitrogen atmosphere. After cooling to room temperature, the mixture was poured into aqueous solution of ethylenediaminetetraaceticacid disodium salt (pH = 8). The suspension was stirred for 1 h at room temperature. The precipitates were separated by filtration and washed with 1 M HCl solution, distilled water, MeOH, and hexane. The gray solid was dissolved in CHCl3 and the solution was filtered through a plug of Celite to remove insoluble materials. A reprecipitation from CHCl3/MeOH gave gray solid of poly[(10,10-dioctyl-N-methylphenazasiline-2,8-diyl)(2,3,5,6-tetrafluoro-1,4-phenylene)] (PDOP-TP) in 88% yield. 1H NMR (400 MHz, CDCl3, 293 K): δ 7.67 (2H, br), 7.60 (2H, d, J = 9.2 Hz), 7.24 (2H, d, J = 9.2 Hz), 3.66 (3H, br), 1.26 (24H, br), 0.99 (4H, br), 0.83 (6H, t, J = 6.8 Hz). 13C{1H} NMR (100 MHz, CDCl3, 293 K): δ 151.2, 144.4 (d, JC−F = 241.3 Hz), 135.8, 131.9, 121.3, 119.5, 118.8, 115.5 (m), 38.5, 33.5, 31.9, 29.2, 23.6, 22.7, 14.1, 13.3. 19F NMR (377 MHz, CDCl3, 293 K): δ −141.38 (s). Mn = 12500, Mw/Mn = 1.65. Polycondensation of 2,2′,3,3′,5,5′,6,6′-Octafluorobiphenyl and 2,7-dibromo-9,9-dioctylfluorene. A mixture of Pd(OAc)2 (11.2 mg, 0.050 mmol), PtBu2Me-HBF4 (24.8 mg, 0.1 mmol), 2,7dibromo-9,9-dioctylfluorene (548 mg, 1.0 mmol), 2,2′,3,3′,5,5′,6,6′octafluorobiphenyl (298 mg, 1.0 mmol), and K2CO3 (276 mg, 2.0 mmol) were stirred in anhydrous dimethylacetamide (2.0 mL) for 24 h at 100 °C under nitrogen atmosphere. After cooling to room temperature, the mixture was poured into aqueous solution of ethylenediaminetetraaceticacid disodium salt (pH = 8). The suspension was stirred for 1 h at room temperature. The precipitates were separated by filtration and washed with 1 M HCl solution, distilled water, MeOH, and hexane. The gray solid was dissolved in CHCl3 and the solution was filtered through a plug of Celite to remove insoluble materials. A reprecipitation from CHCl3/MeOH gave gray solid of poly[(9,9-dioctylfluorene-2,7-diyl)-(2,2′,3,3′,5,5′,6,6′octafluoro-4,4′-diphenylene)] (PDOF-OD) in 95% yield. 1H NMR (400 MHz, CDCl3, 293 K): δ 7.94 (2H, d, J = 8.4 Hz), 7.59 (4H, br), 2.06 (4H, br), 1.15 (20H, br), 0.83 (6H, t, J = 7.2 Hz), 0.77 (4H, br). 13 C{1H} NMR (100 MHz, CDCl3, 293 K): δ 151.6, 145.6 (dd, JC−F = 14.5, 48.5 Hz), 143.2 (dd, JC−F = 14.5, 48.5 Hz), 141.5, 129.2, 126.1, 125.0, 123.1 (m), 120.4, 105.9 (m), 55.7, 40.1, 31.8, 29.9, 29.2, 23.8, 22.6, 14.0. 19F NMR (377 MHz, CDCl3, 293 K): δ −135.03 (s), −138.95 (s). Mn = 43200, Mw/Mn = 2.34. Measurement of HOMO and LUMO Level. A highest occupied molecular orbital (HOMO) level of a polymer was obtained from an ionization potential of the polymer, and a lowest unoccupied molecular orbital (LUMO) level was obtained from the ionization potential and the energy difference between the HOMO and the LUMO. For the measurement of the ionization potential, a photoelectron spectrometer (manufactured by Riken Keiki Co., Ltd.; AC-2) was used. The energy difference between the HOMO and the

LUMO was obtained by measuring an absorption spectrum of the polymer film using an UV/vis spectrophotometer and from the absorption edge thereof. Device Fabrication and Evaluation of OLEDs. The devices with PDOF-TP, PDOP-TP, and PDOF-OD as a hole-blocking layer were fabricated as follow. A 70 nm-thick PEDOT−PSS film was spin-coated on the ITO substrate and dried at 200 °C for 10 min. The emitting polymer-1 film was spin-coated from a xylene solution to form a uniform 20 nm-thick film on top of the PEDOT−PSS layer and annealed at 180 °C for 60 min. The emitting polymer-2 film was spincoated from a xylene solution with TPEA (emitting polymer-2:TPEA = 8:2) to form a uniform 105 nm-thick film on top of the PEDOT layer and dried at 200 °C for 20 min. The hole-blocking polymer films (PDOF-TP, PDOP-TP, and PDOF-OD) were spin-coated from a chloroform solution to form a uniform 10 nm-thick film on top of EL layer. Then, in order to form a cathode layer, lithium fluoride of 0.5 nm and aluminum of 100 nm were successively deposited under vacuum. The device was encapsulated by glass plate with epoxy resin. The structures of emitting polymer-1 and -2 are as follow. The electroluminescence spectra, current efficiency and current density− voltage curves of OLEDs were obtained by OLED TEST SYSTEM (manufactured by Tokyo System Kaihatsu). The emitting light was collected with a silicon photodetector placed in front of the device.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and spectroscopic data and current density vs voltage plots. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (J.K.) [email protected]; (T.K.) kanbara@ ims.tsukuba.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Chemical Analysis Center of University of Tsukuba for the measurements of NMR and MALDI-TOFMS. This work was partly supported by the Collaborative Research Program of Institute for Chemical Research, Kyoto University and Industrial Technology Research Grant Program in 2011 from New Energy and Industrial Technology Development Organization (NEDO) of Japan. 4132

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dx.doi.org/10.1021/ma3004899 | Macromolecules 2012, 45, 4128−4133