Effect of Purification Solvent on Polymer Impurities and Device

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Cite This: ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Effect of Purification Solvent on Polymer Impurities and Device Performance Shunsuke Kodama,† Junpei Kuwabara,*,‡ Xin Jiang,‡ Iori Fukushima,§ and Takaki Kanbara*,‡

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†

Advanced Technology Research & Development Center (Shimodate), Hitachi Chemical Co., Ltd., 1919 Morisoejima, Chikusei City, Ibaraki 308-0861, Japan ‡ Tsukuba Research Center for Energy Materials Science (TREMS), Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba City, Ibaraki 305-8573, Japan § Advanced Technology Research & Development Center, Hitachi Chemical Co., Ltd., 48 Wadai, Tsukuba City, Ibaraki 300-4247, Japan S Supporting Information *

ABSTRACT: Poly(arylamine)s were synthesized by polycondensation of 4-n-octylaniline with 4,4′-dibromobiphenyl using the Buchwald−Hartwig aryl amination. Both the NH and the Br end groups were properly modified upon addition of an end-capping reagent in an appropriate ratio. The synthesized polymers contained many impurities, such as Pd, Br, and Cl, which decrease organic light-emitting diode performance. An investigation to reduce the impurities in the polymer showed that the purification solvent plays the key role in reducing the concentration of impurities in the polymer; purification with a nonchlorinated solvent, anisole, provided a highly pure poly(arylamine) even with a simple purification procedure. Moreover, the highly purified polymer material improved carrier mobility in hole-only devices. KEYWORDS: poly(arylamine), Buchwald−Hartwig aryl amination, impurity in polymer, purification solvent, hole injection material, hole-transporting material, organic light-emitting diode (OLED), hole-only device (HOD)

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reprecipitation, filtration, and washing with solvents. In addition, chelating agents and absorbent materials are effective for removing heavy metals derived from catalysts and ingredient residue such as Pd.11 Because any purification step cannot remove the impurities incorporated into the polymer through covalent bonds, synthetic procedures should avoid becoming contaminated with these impurities including unexpected bond connections via side reactions and undesired terminal structures. In this study, we focused on poly(triarylamine), which is expected to be applied as a hole injection and transporting material for solution coating processes.12−15 In general, Cu, Ni, and Pd catalyzed crosscoupling reactions have been used for preparation of poly(triarylamine).16−20 However, these cross-coupling reactions require organometallic reagents, such as organotin or boronic acid compounds. Because these organometallic reagents tend to be expensive and waste containing metals is generated after the polycondensation reactions, simple adaptation of these reactions is difficult in industrial

INTRODUCTION Organic light-emitting diodes (OLEDs) have become popular in displays and lighting applications.1−6 There are two major types of fabrication methods for OLEDs: the vapor-deposition process and the solution coating process, such as spin-coating and inkjet printing. The solution coating process is more suitable than the vapor-deposition for large-area fabrication process because solution processes are expected to decrease the takt time of manufacturing and the loss of expensive OLED materials. The efficiency and lifetime of OLEDs have been improved by separating the functions by layer. Appropriate polymer materials for each layer have been developed in recent years.7 In addition to the development of high-performance materials, the supply of a high-purity polymer is also an important topic for practical application because the purity of the materials used in the device affects OLED performance.8,9 In another example, Liu et al. reported that charge transport ability of a conjugated polymer in OFET devices decreases with increase in Pd residue in the polymer.10 The solution process has stricter requirements for purity in materials than the deposition process because it does not have any sublimation steps which are used in the deposition process. Common purification techniques of polymer materials are © XXXX American Chemical Society

Received: April 23, 2019 Accepted: June 28, 2019

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DOI: 10.1021/acsapm.9b00385 ACS Appl. Polym. Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Polymer Materials Scheme 1. Synthesis of P1

Synthesis of P1. A mixture of (Pd[P(t-Bu)3]2) (20.6 mg, 0.040 mmol), P(t-Bu)3·HBF4 (11.6 mg, 0.040 mmol), 4-n-octylaniline (218.8 μL, 1.00 mmol), 4,4′-dibromobiphenyl (312 mg, 1.00 mmol), and NaOt-Bu (288.4 mg, 3.00 mmol) were stirred in degassed toluene (4.0 mL) for 24 h at 100 °C under a nitrogen atmosphere. After cooling to room temperature, chloroform (15.0 mL) was added into the reaction mixture to dissolve the precipitate. The solution was stirred with an NaS2CN(C2H5)2 aqueous solution overnight and washed with water. After separation of the organic phase, the solvents were removed in vacuo. The precipitate was resolved in chloroform (3.5 mL), and the solution was filtered through Celite to remove the insoluble materials. A reprecipitation from chloroform/methanol produced P1 with 68% yield. Mn = 13400, Mw/Mn = 3.3. 1H NMR (400 MHz, CDCl3): δ = 7.44 (d, J = 8.4 Hz, 4H), 7.13 (d, J = 8.4 Hz, 4H), 7.08 (br, 4H), 2.57 (t, J = 6.6 Hz, 2H), 1.60 (br, 2H), 1.28 (br, 10H), 0.88 (t, J = 6.8 Hz, 3H). The spectral data are essentially the same as the reported data.26 Synthesis of P2. The polycondensation reaction of 4-n-octylaniline (223.2 μL, 1.02 mmol), 4,4′-dibromobiphenyl (312 mg, 1.00 mmol), 1-bromo-3,5-dimethylbenzene (5.5 μL, 0.040 mmol) was carried out according to the above-mentioned method giving P2 with 50% yield. Mn = 16200, Mw/Mn = 1.75. The 1H NMR spectrum shows signals of the terminal 3,5-dimethylbenzene units at 6.74, 6.66, and 2.22 ppm. Synthesis of P3 and P4. P3 and P4 were synthesized according to the above-mentioned method except that the purification solvents were o-dichrolobenzene and anisole, respectively. Additional Purification of P1−P3. Metal scavenger Si-thiol (200 mg) was added to a solution of the polymer (100 mg) in anisole (0.9 mL). The suspension was vigorously stirred for 2 h at 120 °C. After cooling to room temperature, the suspension was filtrated with a 0.2 μm filter, and reprecipitation from anisole/methanol gave the further purified polymer. Fabrication and Characterization of Hole-Only Devices (HODs). The HODs were fabricated in the following configuration: ITO glass/hole injection layer (HIL)/Al electrode. The patterned ITO glass was cleaned in an ultrasonic bath of detergent, ultrapure water, alcohol, and high volatility solvent and then treated in an ultraviolet-ozone chamber. Polymer ink was prepared from each polymer, dopant, and toluene. A thin layer (100 nm) of the HIL was spin-coated using the above ink onto the cleaned ITO under air conditions. The prepared thin layer was annealed at 210 °C for 30 min on a hot plate to remove the residual toluene. The thicknesses of the polymer layers were measured using a Kosaka Laboratory Surfcorder ET-200 automatic microfigure measuring instrument. After annealing, the substrates were transferred to an N2-filled glovebox. Al (100 nm) was then deposited onto the active layer by conventional thermal evaporation at a chamber pressure lower than 5 × 10−4 Pa. The devices were encapsulated with a desiccant using UVcuring glue. The UV-glue was dispensed onto the edge of a piece of glass in the air. The devices were covered with the above UV-glue coated glass and desiccant in the glovebox. The devices were then sealed by irradiation under ultraviolet light by attaching the glass on top of the devices. Finally, the current density−voltage (J−V) curve measurements were carried out on a System Engineers CO EAS-62C light-emitting characteristic evaluation apparatus using a Keithley Instruments Ltd. source meter SMU2400 as a DC power supply. The Supporting Information accompanying this article provides additional experimental details.

manufacturing. In contrast, the Buchwald−Hartwig aryl amination reaction has advantages in cost because it consists of coupling halogenated aryl compounds with arylamine derivatives, which are easily obtainable and inexpensive. Hartwig et al. have reported synthesis of poly(triarylamine)s from aromatic dihalides and aromatic secondary diamines.21,22 We have also reported synthesis of poly(triarylamine)s from aromatic dihalides and aromatic primary amines.23−26 These reactions afford polymers with halogen and NH terminal units when the terminal units are not treated at the end of the polymerization. These terminal structures decrease the device performance.27,28 For example, a halogenated terminal can be the hole carrier trap during device operation and inhibit the carrier flow.28 An NH terminal is susceptible to oxidation because the terminal structure is a diphenylamine structure. Diphenylamine derivatives are known as oxidation inhibitors.29 The diphenylamine structure can be easily oxidized to form a radical unit under a hole-rich environment. These radical sites cause degradation of the polymer and formation of an oxidative dimer.30 In this paper, we report the preparation of a terminally modified and high-purity poly(triarylamine) using the Buchwald−Hartwig aryl amination. We established a simple and effective end-capping method for poly(triarylamine). In addition, this research revealed that a proper choice of purification solvent succeeded in significantly reducing the impurities in the polymer. The polymer with low impurity concentration showed better performance in terms of carrier mobility.

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EXPERIMENTAL SECTION

Materials. 4,4′-Dibromobiphenyl, 4-n-octylaniline, 1-bromo-3,5dimethylbenzene, bis(tritert-butylphosphine)palladium(0) (Pd[P(tBu)3]2), tritert-butylphosphonium tetrafluoroborate (P(t-Bu)3· HBF4), sodium tert-butoxide (NaOt-Bu), sodium N,N-diethyldithiocarbamate trihydrate (NaS2CN(C2H5)2·3H2O), and other chemicals were received from commercial suppliers and used without further purification. Si-thiol (3-mercaptopropyl silica gel) was purchased from Biotage Japan. Anhydrous toluene was purchased from Kanto Chemical and used as a dry solvent. General Methods. NMR spectra were recorded on Bruker Avance-400 and Avance-600 NMR spectrometers. 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. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) spectra were recorded on an AB Sciex MALDI TOF/TOF 5800 using DCBT (trans-2-[3-(4-tertbutylphenyl)-2-methyl-2-propenylidene]malononitrile) as a matrix. Energy dispersive X-ray fluorescence (EDX) spectroscopy measurements were carried out on a Shimadzu EDX-7000 benchtop spectrometer to measure and estimate the impurities in the polymer. All manipulations for the reactions were carried out under nitrogen atmosphere using the standard Schlenk technique. Synthesis. A typical procedure for the polycondensation reaction is as follows: The samples (P2−P4) are different in terms of the solvent which was added after the reaction during the purification process. B


Article

ACS Applied Polymer Materials Scheme 2. Synthesis of P2−P4

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RESULTS AND DISCUSSION Poly(arylamine) Synthesis Using the Buchwald− Hartwig Aryl Amination Reaction. The reaction of 4,4′dibromobiphenyl and 4-n-octylaniline in a 1:1 molar ratio was conducted to synthesize poly(arylamine), P1, under previously optimized conditions (Scheme 1).23 P1 with a molecular weight of Mn = 13400 was obtained with 68% yield (Table S1). For treatment of the terminal Br and NH moieties in the polymer, 1-bromo-3,5-dimethylbenzene was added as an endcapping reagent at the initiation of polycondensation (Scheme 2). The molar ratio of monomers and end-capping reagent was determined to obtain a similar molecular weight to P1. As a result, the reaction in Scheme 2 afforded P2 with a molecular weight of Mn = 16200 with 50% yield. The 1H NMR spectrum of P2 exhibits a singlet signal at 2.2 ppm, which is assigned to the methyl proton originating from the end-capping group (Figure 1).

OH terminal structure is considered to be derived from decomposing tert-butoxy groups coming from the base.31 In contrast, the spectrum of P1, which was synthesized without an end-capping reagent, reveals the presence of Br terminals (Figure S2). From these results, the terminal structure of the polymer was properly modified under the conditions described in Scheme 2. Reducing Concentration of Impurities in the Polymer. Next, we investigated the impurity concentrations of Pd, Br, and Cl in the synthesized polymers. The impurity concentrations were determined by energy dispersive X-ray spectrometry (EDX). The measurements of P1 and P2 show the presence of several tens to several hundred ppm of Pd and Br (Table1). In addition, several hundreds to more than a thousand ppm of Cl were detected although Cl was not used in the polycondensation reaction. The spectra show the absence of Na, K, P, and S, which may have caused contamination during the synthesis and purification processes. To decrease Pd concentration, a heterogeneous metal scavenger, Si-thiol, was used as an additional purification procedure. As a result, the additional purification process decreased the Pd and Br concentration of P1 and P2. However, Cl concentration was scarcely reduced. These results suggest that Br may be bound to Pd but Cl is unrelated to Pd. Possible forms of Cl are a Cl anion, a low-molecular-weight compound, or a polymer-bound Cl. For examination of the above possibilities, P2 solution was washed with water and reprecipitated in methanol again to remove the free Cl anions. Even after washing, the Cl concentration did not decrease (Table S2). Then, the polymer was washed with hexane and ethyl acetate several times to remove the low-molecular-weight compounds. The Cl concentration of the polymer was not reduced. These results show that Cl exists as part of the polymer rather than as a Cl anion or as part of the low-molecular-weight compounds. Because any purification process cannot remove the covalently bonded Cl, an overall process was investigated to decrease the Cl concentration. When o-dichlorobenzen was used as a good solvent for reprecipitation instead of chloroform, Cl concentration was drastically reduced (Table 1, P3). Cl concentration was further decreased by applying anisole; EDX shows no Cl element in the polymer (Table 1, P4). These results indicate that chlorinated solvent is a Cl source, which forms the C−Cl bond in the polymer. In addition, the Pd and Br concentrations were also decreased to less than 30 ppm without an additional purification process using the heterogeneous metal scavenger. From these results, we found the advantages of the

Figure 1. 1H NMR spectrum of P2 (CDCl3, 400 MHz).

To verify whether the end-capping reagent was introduced precisely to the polymer terminal site or not, the terminal structure of P2 was further investigated by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MASS). The mass spectrum of P2 shows the absence of polymers with a Br terminal structure (Figure 2). Although the main peak corresponds to the cyclic product, the spectrum shows the peak corresponding to the polymer modified with the end-capping reagent. In addition, we observed debrominated end and OH end structures. The C


Article

ACS Applied Polymer Materials

Figure 2. MALDI-TOF-MASS spectrum of P2 and proposed structures.

Table 1. Properties of P1−P4 impurity concentrations before scavenger treatment (ppm)

impurity concentrations after scavenger treatment (ppm)

polymer

end-capping reagent

purification solvent

Pd

Br

Cl

Pd

Br

Cl

P1 P2 P3 P4

none Br-aryla Br-aryla Br-aryla

chroloform chroloform o-DCBb anisole

211 134 692 23

137 60 250 ≤1

761 1528 85 17

80 36 20

72 34 13

801 1263 99

a

1-Bromo-3,5-dimethylbenzene. bo-Dichlorobenzene.

nonhalogenated solvent, anisole, as a purification solvent to effectively decrease the impurities of Cl, Pd, and Br in the polymer via a simple purification process, which does not require Soxhlet extraction or high-performance liquid chromatography methods. In general, a reaction solvent has been investigated for optimization of reaction conditions in the synthesis of polymer materials. This finding provides unique information on the importance of purification solvents for obtaining high-purity materials in an effective way. Influence of Impurity Concentration in Polymers on HOD Performance. The device performances of P2−P4 were investigated to evaluate the influence of impurity concentration. Because poly(arylamine)s are expected to be used as hole injection and transporting materials, we fabricated a holeonly device with P2 and P3 after treatment with a metal scavenger and P4 without treatment and measured the J−V properties of each device (Figure 3). The J−V curve shifted to the low-voltage side in the order of P4, P3, and P2 (Figure 3). P4 showed slightly higher current density than P3 at the same voltage, although P4 was not purified by the metal scavenger. The high performance of P4 is more pronounced when compared with P2. Combined with the results of the impurity concentrations in Table 1, the device performances strongly correlate with the concentration of Cl in the polymers. The Cl moieties in the polymers probably act as hole-trapping sites, and higher voltage is needed to cause the carriers in the devices to flow. From these results, we found that anisole is a suitable purification solvent to avoid undesired incorporation of Cl and remove metal impurities without metal scavenger treatment.

Figure 3. J−V properties of HOD using each polymer.

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CONCLUSIONS In this report, we obtained the target poly(arylamine) via polycondensation of 4-n-octylaniline with 4,4′-dibromobiphenyl using the Buchwald−Hartwig aryl amination. The addition of an end-capping reagent properly modified both the NH and the Br ends. We were able to reduce the impurities in the polymer by choosing a nonhalogenated solvent, anisole, as the purification solvent. These findings provide beneficial insights into the purification procedure because purification solvent has not been regarded as an important factor for the purity of polymeric materials. Establishing the end-capping and purification procedures enabled us to obtain high-purity material without any special purification steps such as scavenger treatment. The simply obtained pure material shows improved carrier mobility in the HOD. This research provides basic and widely applicable information for D


Article

ACS Applied Polymer Materials

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preparation of high-quality polymeric materials for optoelectronic applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.9b00385. 1 H NMR spectra of P1 and P2, MALDI-TOF-MASS spectrum of P1, results of polymerization for P1−P4, and detailed data of impurity concentration in P1 after washing (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.K.). *E-mail: [email protected] (T.K.). ORCID

Junpei Kuwabara: 0000-0002-9032-5655 Takaki Kanbara: 0000-0002-6034-1582 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 spectra. The authors thank Dr. H. Sakuma for useful discussions.

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REFERENCES

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Effect of Purification Solvent on Polymer Impurities and Device

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