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Preparation and characterization of poly(4-alkyltriphenylamine) by chemical oxidative polymerization
Synthetic Metals 129 (2002) 123–128
Preparation and characterization of poly(4-alkyltriphenylamine) by chemical oxidative polymerization Chihiro Takahashi, Shinta Moriya, Nobutoshi Fugono, Hee Cheong Lee*, Hisaya Sato Department of Organic and Polymeric Materials Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan Received 3 September 2001; received in revised form 6 December 2001; accepted 27 December 2001
Abstract High molecular weight poly(4-alkyltriphenylamine) was synthesized by chemical oxidation polymerization. 13 C NMR spectra of the obtained polymers revealed that coupling reaction occurred exclusively at the para-position of unsubstituted N-phenyl rings. Poly(4-nbutyltriphenylamine) (P-nBTPA) showed better solubility and processability than polymers prepared from 4-methyltriphenylamine (MTPA) and 4-t-butyltriphenylamine (tBTPA) because of the high molecular weight and the flexible long n-butyl group in the side chain. Thus, PnBTPA allows us to prepare the film from its solution in organic solvent with ease. The rigid structure of polymeric backbone affords high glass transition temperature of 182 8C, indicating high thermal stability. Extended conjugation over polymer chain results in red shift of UV absorption up to 375 nm and low ionization potential compared to monomer. The results of the ionization potential measurement and the redox behavior suggest that the polymer has great potential as a hole-transporting material (HTM). The drift mobility of P-nBTPA measured by a standard time-of-flight (TOF) method was found to be of the order of 105 cm2/(V s). The results of the photoconductivity measurement revealed that the photoconductivity of P-nBTPA showed much higher than that of poly(N-vinylcarbazole). # 2002 Published by Elsevier Science B.V. Keywords: Chemical oxidative polymerization; Poly(4-alkyltriphenylamine); Hole-transporting compound; Drift mobility; Photoconductivity
1. Introduction Triarylamine derivatives have been most widely used as hole-transporting materials (HTMs) in organic photoconductors and electroluminescent devices [1]. Among the triphenylamine (TPA) derivatives, N,N,N0 ,N0 -tetraphenylbenzidine (TPD) is one of the most widely used as HTM [1]. Their hole-transporting abilities are based on the fact that they are easily oxidized to form stable radical cation [2]. However, the low molecular weight compounds from TPA derivatives has some problems such as the lack of the thermal stability and the mechanical strength [1]. It has been known that the crystalline structure from amorphous state or the aggregation of the materials is developed during the device driving, resulting in the decrease of the device performance. In order to solve these problems, it is proposed to prepare oligomers or polymers for HTM [3,4]. Some potential advantages of polymers over small molecules include the possibility of extended p-conjugation, generally
*
Corresponding author.
0379-6779/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 0 1 0 - 3
high glass transition temperature (Tg), excellent film-forming property and stable film morphology. Several attempts have been made to prepare polymers having TPD units. New palladium-catalyzed method has been applied to synthesize high molecular weight triarylamine dendrimers [5]. In these reactions, aryl bromides are coupled with arylamines or diarylamines [6,7]. Nickel(0)-catalyzed condensation polymerization of aryl dihalides has also been applied to synthesize poly(triphenylamine) [8]. Recently we reported that 4-methyltriphenylamine polymer (P-MTPA) was simply prepared by the chemical oxidative polymerization using ferric chloride as catalyst [9]. However, the molecular weight and the solubility of resulting polymer were not good enough to make a film. Generally, it has been known that the introduction of bulky substituent in the side chain improves the solubility of polymer and the film formation property. In this work, high molecular weight poly(4-n-butyltriphenylamine) (P-nBTPA) having long n-butyl group in the side chain was obtained by the oxidative polymerization of nBTPA, and its properties were investigated and compared with P-MTPA and P-tBTPA having different alkyl group in the side chain.
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2. Experimental
2.2. Polymerization procedure
2.1. Preparation of 4-alkyltriphenylamine
P-MTPA, P-tBTPA and P-nBTPA were synthesized as shown in Scheme 2, where ‘R’ is the alkyl group in the side chain, such as methyl, n-butyl or t-butyl. P-nBTPA was synthesized as follows. To a two-necked 50 ml flask equipped with a magnetic stirrer were added nBTPA and CHCl3 under nitrogen atmosphere. A quarter portion of FeCl3 was added to the reaction mixture at the interval of 1 h. After the polymerization, the reaction mixture was poured into methanol to recover the product followed by washing with methanol several times. Collected powder was dissolved in CHCl3 to remove the insoluble part by filtration. The filtrate was concentrated and reprecipitated with acetone containing small amount of aqueous ammonia. The product was filtered and dried in vacuo at 50 8C for 12 h. P-MTPA and P-tBTPA were also prepared by the same experimental procedure with P-nBTPA.
All the chemicals were obtained commercially and used without further purification except otherwise noted. Xylene was dried over sodium. Chloroform was distilled over CaH2. Tri-t-butylphosphine was diluted with dry xylene. 4-Methyltriphenylamine (MTPA), 4-n-butyltriphenylamine (nBTPA) and 4-t-butyltriphenylamine (tBTPA) were synthesized as shown in Scheme 1, where ‘R’ is the alkyl group in the side chain such as methyl, n-butyl or t-butyl. To a three-necked 300 ml flask equipped with a magnetic stirrer were added diphenylamine (18.9 g, 0.0886 mol), 1-bromo4-n-butylbenzene (18.9 g, 0.0886 mol), palladium(II) acetate (19.9 mg, 0.0886 mmol), t-BuONa (10.2 g, 0.106 mol) and 0.1 M P(t-Bu)3 xylene solution (3.54 ml, 0.354 mmol). Xylene (80 ml) was added via a syringe and the mixture was stirred at 120 8C for 3 h under nitrogen atmosphere. The reaction mixture was cooled down to 80 8C and water was poured into the reaction mixture. The organic layer was washed with water and concentrated by evaporation followed by purification using column chromatography to remove by-product(s) and unreacted materials. The white solid of nBTPA was obtained after evaporation of eluent and dried in vacuo at 50 8C for 12 h. The yield was 91.5%. 1 H NMR (CDCl3) d from TMS: 0.95, 1.38, 1.62 and 2.58 ppm [3H; –CH3, 2H; –CH2, 2H; –CH2, 2H; –CH2] and 6.90–7.25 ppm [m, 14H; aromatic protons]. 13 C NMR (CDCl3) d from TMS: 13.9, 22.4, 33.6 and 35.1 ppm [–CH3, –CH2, –CH2, –CH2] and 122.3–148.2 ppm [eight signals; aromatic carbons]. MTPA and tBTPA were also prepared by the same experimental procedure with nBTPA. MTPA; 1 H NMR (CDCl3) d from TMS: 2.30 ppm [s, 3H; –CH3] and 6.90–7.24 ppm [m, 14H; aromatic protons]. tBTPA; 1 H NMR (CDCl3) d from TMS: 1.34 ppm [s, 9H; –CH3] and 6.94–7.28 ppm [m, 14H; aromatic protons].
2.3. Measurement Molecular weight was estimated by gel permeation chromatography (GPC) equipped with a JASCO 880-pump and a JASCO UV-970 detector. The packing material was synthesized from divinylbenzene and styrene and packed into the stainless steel column in our laboratory. The polymer was eluted with chloroform and its molecular weight was calibrated against polystyrene standards. 1 H and 13 C NMR spectra were recorded on chloroform-d solution at 50 8C with tetramethylsiline as an internal standard using a JEOL a-500 spectrometer (1 H: 500 MHz; 13 C: 125 MHz). Glass transition temperature (Tg) was measured by Rigaku Thermo DSC 8230 at the heating rate of 10 8C/min. UV absorption spectra were recorded on JASCO V-570 spectrophotometer in chloroform (10 mg/ml) or UV–VIS array detector (JASCO MD-1510) connected with GPC columns (Shodex K-803, K-802) to investigate the dependence of UV absorption on molecular weight. Cyclic voltammetry
Scheme 1. Synthetic route of 4-alkyltriphenylamine.
Scheme 2. Polymerization of 4-alkyltriphenylamine.
C. Takahashi et al. / Synthetic Metals 129 (2002) 123–128
was conducted for the cast film on platinum working electrode immersed in dry acetonitrile containing tetra-n-butylammonium perchlorate (0.1 M) as an electrolyte under nitrogen atmosphere using a one-compartment cell with a polarization unit (TOHO PS-06). Platinum spiral was used as a counter electrode and Ag/AgCl as a reference electrode. Hole mobility was determined by a time-of-flight (TOF) method with the device consisting of Al/Ti-phthalocyanine/ polymer/semitransparent gold and xenon lamp. Ionization potential was measured for cast film (film thickness: 2 mm) on slide glass by RIKENKEIKI AC-1. Photoconductivity was measured with the device consisting of ITO/polymer including C60 (0.2 wt.%)/semitransparent gold using He–Ne laser (NEC GLS-5410) and digitizing oscilloscope (GOULD, DSO 630). Film thickness was determined by a profilometer (DEKTAK II, Solan).
3. Results and discussion 3.1. Polymerization results In a previous communication describing the synthesis of P-MTPA, the catalyst, ferric chloride, was added to the reaction medium at the beginning of the reaction in a lump resulting in inhomogeneous dispersion and consequently heterogeneous polymerization reaction to produce oligomer with low yield or high content of gel [9]. We found that the periodic addition of FeCl3 is efficient to provide high molecular weight polymer in this work. The feeding method of
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oxidant, FeCl3, applied in this work was as follows. A quarter portion of FeCl3 was added to the reaction medium at the intervals of 1 h in all synthetic experiments except for no. 15 as shown in Table 1. When FeCl3 was added in a lump, we could not obtain the reproducible results on molecular weight, molecular weight distribution and yield probably because of the heterogeneity of the reaction medium. The polymerization results by one spot addition of FeCl3 are shown in Table 1. It can be seen that molecular weight, molecular weight distribution and yield are different from reactor to reactor. Using the periodic feeding method of FeCl3, we investigated the dependence of the yield and the molecular weight by the oxidative polymerization of nBTPA on experimental conditions such as reaction temperature, reaction time and mole ratio of FeCl3 to monomer. The effect of the mole ratio was examined at the reaction temperature of 50 8C and the reaction time of 6 h. When the mole ratio was 2, the polymeric product was not obtained. We can see from Table 1 that the molecular weight and the yield increase with the mole ratio of oxidant from 2 to 5. However, when the mole ratio was 5, the reaction was rigorous to form much amount of gel up to 80.2% due probably to cross-linking. The effects of the reaction temperature and the reaction time were also investigated in the range of 30–60 8C and 4–8 h at the constant mole ratio of oxidants, 4. As shown in Table 1, the reaction at 30 and 40 8C of the reaction temperature could not provide high molecular weight polymers even though long reaction time, which increased undesired gel portion only. The best result, high molecular weight and high yield of soluble part in chloroform, was obtained at the
Table 1 Polymerization conditions and resultsa,b Temperature (8C)
Time (h)
Yield (%)
Mn (103)
Mw (103)
Monomer: 4-n-butyltriphenylamine ac 4 bc 4 cc 4 1 2 2 3 3 4 4 5 5 4 6 4 7 4 8 4 9 4 10 4 11 4
60 60 60 50 50 50 50 30 40 40 50 50 50 60
6 6 6 6 6 6 6 8 6 8 5.5 6 7 4
89.2 95.4 70.0 0 70.9 98.9 98.5 81.2 80.9 98.7 88.1 98.9 98.7 96.1
(0.02) (42.9) (80.2) (0.01) (0.01) (73.9) (8.50) (42.9) (95.5) (24.8)
3.6 5.3 2.0 – 2.8 6.8 5.8 3.3 4.3 3.0 5.1 6.8 3.1 6.8
19 27 15 – 19 26 15 14 21 20 20 26 14 27
Monomer: 4-methyltriphenylamine 12 4 13 4 14 4
50 50 60
4 5 4
95.4 (4.80) 91.3 (23.8) 86.4 (54.9)
4.8 4.9 3.2
24 25 11
Monomer: 4-t-butyltriphenylamine 4 15c
50
8
83.0 (13.0)
7.0
29
No.
FeCl3/monomer
a
Values in parentheses denote insoluble part in chloroform. Monomer concentration: 0.25 mol/l. c FeCl3 was added in a lump. b
(46.5) (71.2) (35.4)
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reaction condition of 60 8C and 4 h while longer reaction time at 50 8C results in serious gel formation up to 95.5% and low molecular weight 3.1 k of soluble part. For comparison purpose of reactivity, MTPA was also polymerized at the same experimental conditions with nBTPA of sample no. 11 which produce high molecular weight P-nBTPA as shown in Table 1 (no. 14). However, the gel content of the product was higher than that of P-nBTPA implying higher reactivity of MTPA. From Table 1, we can see that the polymerization condition of no. 12 is desirable to produce high molecular weight P-MTPA in high yield of CHCl3 soluble part. 3.2. Polymer structure 13
C NMR spectrum of the P-nBTPA (no. 11) is shown in Fig. 1. Signals were assigned by considering the NMR spectra of P-MTPA reported previously [9]. Signals at 13.9, 22.4, 33.6 and 35.1 ppm are assigned to –CH3, –CH2–, –CH2–, –CH2– of n-butyl group in the side chain, respectively. The linear structure as shown in the inset of Fig. 1 can be confirmed considering the number of peaks in the aromatic region of 13 C NMR spectrum. Eight signals in aromatic region indicate exclusive coupling reaction at the para-position of the unsubstituted N-phenyl rings and the linear structure. Eight aromatic peaks were also observed in P-MTPA and P-tBTPA polymers, indicating the linear structure for these polymers again. 3.3. Polymer characterizations The UV absorption spectra were monitored by the multichannel detector coupled with GPC. Fig. 2 shows the dependence of UV absorption maxima of P-MTPA, P-nBTPA and
Fig. 2. UV absorption maximum of polymers vs. retention time (i.e. molecular weight). In this experiment, an array UV detector was used equipped with small pore size columns (Shodex KF 802, KF 802.5). Red shift of UV absorption can be interpreted as p–p transition energy lowered by the extended conjugation over the biphenylene linkage formed by polymerization.
P-tBTPA on retention time. The retention time in Fig. 2 can easily be transferred to molecular weight of standard polymer. UV absorption maximum of nBTPA dimer was dramatically red shifted against monomer from 305 to 354 nm, and gradually shifted to 375 nm when molecular weight became higher than ca. 7000 which corresponds to the retention time of 13 min. The red shift can be attributed to the extended conjugation length by the polymerization. The peak maximum is almost identical with those of P-MTPA and P-tBTPA implying that the alkyl substituent does not make significant effect on the electronic structure of the aromatic ring.
Fig. 1. 13 C NMR spectrum of P-nBTPA. Eight aromatic signals prove the linear structure of polymer and the coupling reaction at the para-position of unsubstituted phenyl ring. Similar results were also observed in P-MTPA and P-tBTPA.
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Table 2 Physical properties of polymer prepared and related monomers Polymer and monomers
R
Mn (103)
lmax (nm)
IP (eV)
Epa (V)
Tg (8C)
P-MTPA P-tBTPA P-nBTPA TPD MTPA
Methyl t-Butyl n-Butyl Methyl Methyl
6.1 7.0 6.8 0.52 0.26
374 376 375 354 305
5.30 5.29 5.30 5.50 5.64
1.05 1.01 1.02 0.83, 0.94 0.92
207 177 182 52
It is considered that charge transport phenomenon in organic materials is governed by mainly the redox reaction of charge transporting molecules [2]. Cyclic voltammetry (CV) is a preliminary characterization method to determine the redox properties of the organic and polymeric materials. Chloroform solution of polymer was cast on a platinum electrode, and the cyclic voltammogram was obtained in acetonitrile containing tetrabutylammonium perchlorate (0.1 M) at a scanning rate of 100 mV/s. Oxidation potentials of polymers are shown in Table 2 together with those of TPA and TPD monomers. The three types of polymers (P-MTPA, P-tBTPA and P-nBTPA) showed approximately the same oxidation and ionization potentials. Thus, the alkyl substituents seem not to make a large effect on the electrochemical properties of the polymers, which is consistent with the results by UV measurement. As shown in Fig. 3, the cyclic voltammogram of P-nBTPA (no. 11 in Table 1) shows an oxidation wave of which peak top is at 1.02 V (versus Ag/ AgCl) and a reduction wave is at 0.83 V with shoulder at ca. 0.7 V. According to the report for polydiphenylamine [10] or poly(4-aminobiphenyl) [11], only one oxidation peak was observed although the polymer should also be oxidized reversibly in two separate steps. The redox peaks observed here are much broader than those observed in acrylate polymer containing TPD in the side chain showing a couple of well defined redox peaks [12]. This observation indicates that in partially oxidized unit on polymer backbone, the oxidized triarylamine unit has an electrochemical effect on
the adjacent units through delocalization over biphenylene linkage and nitrogen atoms. In Table 2, the ionization potentials of three polymers synthesized show lower values than those of MTPA and TPD. In the same manner with the red shift of UV absorption, this can be interpreted as extended p-conjugation through the biphenylene unit and nitrogen atom, stabilizing the oxidized state of polymer, consequently lowering the ionization potential. Tg of the polymers obtained by the oxidative polymerization is also shown in Table 2. Tg of TPD is 52 8C [13], of which low Tg limits the operating temperature of the devices because of recrystallization, poor thermal and mechanical stability [14,15]. As expected, Tg of the polymers is much higher than that of TPD. Tg of P-tBTPA and P-nBTPA was slightly lower than that of P-MTPA. This is due to the fact that the bulky and flexible group in the side chain provides the space for rigid polymer backbone to move with ease. P-MTPA was insoluble in common organic solvents such as THF, toluene and chlorobenzene but not in chloroform. On the other hand, P-tBTPA and P-nBTPA showed better solubility in common organic solvents, such as THF, toluene, chloroform and chlorobenzene. Excellent transparent orange films on the order of 10–30 mm thickness could be obtained from the toluene solution of P-nBTPA (no. 11) by a wire-bar-coating method while the film obtained from P-tBTPA was brittle. The film from P-nBTPA was conducted to the measurement of hole drift mobility by the standard TOF method. The mobility, m, is calculated according to the following equation: m¼
Fig. 3. Cyclic voltammogram of P-nBTPA.
L2 tT V
where L is the sample thickness, tT the transit time, and V is the applied voltage. Transit time, tT, was obtained from the onset as shown in the inset of Fig. 4. The transit time was determined to be 4.78 ms at the applied voltage of 60 V (19.7 kV/cm), which leads to the mobility of 3:25 105 cm2/(V s). The logarithm of the mobility of P-nBTPA was plotted against the square root of applied field E (V/cm) as shown in Fig. 4. The drift mobility of P-nBTPA was on the order of 105 cm2/(V s) regardless of applied voltage. The dependence of the drift mobility on applied voltage is still in debate where positive and zero slopes were observed in some papers while negative slope was also observed in other papers [1,16].
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Monomers were coupled exclusively at the para-position. P-nBTPA polymer showed the excellent properties of film formation, drift mobility and photoconductivity required in HTM. We found that the biphenylene linkage of polymer rather than substituents at the para-position makes a significant effect on the electronic structure of TPA unit through the extended conjugation as observed in red shift of UV absorption with molecular weight and low ionization potential compared to TPA monomer. The results of the ionization potential measurement and the redox behavior suggest that P-nBTPA has a great potential as HTMs. Acknowledgements This work is supported in part by grants from JSPS. We thank Prof. Kengi Ogino for his helpful comments. References Fig. 4. Drift mobility of P-nBTPA is constant on the order 105 cm2/(V s) regardless of the voltage applied.
Fig. 5. Photoconductivity of various polymers. P-nBTPA shows the highest photoconductivity value in three polymers.
The photoconductivity was measured for the device made from P-nBTPA (no. 11). Fig. 5 shows the results of the photoconductivity measurement together with those of poly(N-vinylcarbazole) (PVK) and N,N0 -diphenyl-N,N0 bis(4-methylphenyl)-1,4-phenylenediamine (TPD)–benzaldehyde (BzA) which was polymerized from TPD and BzA [17]. P-nBTPA showed much higher photoconductivity than TPD–BzA and PVK [17]. In conclusion, we were able to polymerize 4-alkyltriphenylamine to high molecular weight polymers, affording high Tg and thermal stability by the simple oxidative polymerization.
[1] S. Miyata, H.S. Nalwa, Organic Electroluminescence Materials and Devices, Gordon and Breach, Amsterdam, 1999. [2] T. Tsutsui, MRS Bull. (1997) 39; C.P. Lin, T. Tsutsui, S. Saito, New Polym. Mater. (1995) 4277; D.U. Kim, E. Aminaka, T. Tsutsui, S. Saito, Jpn. Appl. Phys. 34 (1995) 6255; E.S. Kolb, R.A. Gaudiana, P.G. Mehta, Macromolecules 29 (1996) 2359; A. Kraft, P.L. Burn, A.B. Holmes, D.D.C. Bradley, R.H. Friend, J.H.F. Martens, Synth. Met. 55 (1993) 4163; E. Bellmann, S.E. Shaheen, S. Thayumanavan, S. Barlow, R.H. Grubbs, S.R. Marder, B. Kippelen, N. Peyghambarian, Chem. Mater. 10 (1998) 1668. [3] Y. Shirota, Y. Kuwabara, H. Inaba, T. Wakimoto, H. Nakada, Y. Yonemoto, S. Kawami, K. Imai, Appl. Phys. Lett. 65 (1994) 807. [4] S. Tokito, H. Tanaka, K. Noda, A. Okada, Y. Taga, IEEE Trans. Electron. Devices 44 (1997) 1239. [5] J. Louie, J.F. Hartwig, A.J. Fry, J. Am. Chem. Soc. 119 (1997) 11695. [6] J. Louie, M.S. Driver, B.C. Hamann, J.F. Hartwig, J. Org. Chem. 62 (1997) 1268. [7] J.F. Marcoux, S. Wagaw, S.L. Buchwald, J. Org. Chem. 62 (1997) 1568. [8] J.P. Chen, G. Klaerner, J.I. Lee, D. Markiuwics, V.Y. Lee, R.D. Miller, J.C. Scott, Synth. Met. 107 (1999) 129. [9] K. Ogino, A. Kanegae, R. Yamaguchi, H. Sato, J. Kurjata, Macromol. Rapid Commun. 20 (1999) 103. [10] N. Comisso, D. Daolio, G. Mengoli, R. Salmaso, S. Zecchin, G. Zotti, J. Electroanal. Chem. 255 (1988) 97. [11] J. Guay, M. Leclerc, L.H. Dao, J. Electroanal. Chem. 251 (1988) 31. [12] K. Ogino, T. Nomura, T. Shichi, S.-H. Park, H. Sato, T. Aoyama, T. Wada, Chem. Mater. 9 (1997) 2768. [13] H.J. Bolink, C. Arts, V.V. Krasnikov, G.G. Malliaras, G. Hadziioannou, Chem. Mater. 9 (1997) 1407. [14] E. Han, L. Do, Y. Niidome, M. Fujihara, Chem. Lett. 969 (1994). [15] S. Tokito, Y. Taga, Appl. Phys. Lett. 66 (1995) 673. [16] H. Kageyama, K. Ohnishi, S. Nomura, Y. Shirota, Chem. Phys. Lett. 277 (1997) 173. [17] J.M. Son, T. Mori, K. Ogino, H. Sato, Y. Ito, Macromolecules 32 (1999) 4849.
Preparation and characterization of poly(4-alkyltriphenylamine) by chemical oxidative polymerization Chihiro Takahashi, Shinta Moriya, Nobutoshi Fugono, Hee Cheong Lee*, Hisaya Sato Department of Organic and Polymeric Materials Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan Received 3 September 2001; received in revised form 6 December 2001; accepted 27 December 2001
Abstract High molecular weight poly(4-alkyltriphenylamine) was synthesized by chemical oxidation polymerization. 13 C NMR spectra of the obtained polymers revealed that coupling reaction occurred exclusively at the para-position of unsubstituted N-phenyl rings. Poly(4-nbutyltriphenylamine) (P-nBTPA) showed better solubility and processability than polymers prepared from 4-methyltriphenylamine (MTPA) and 4-t-butyltriphenylamine (tBTPA) because of the high molecular weight and the flexible long n-butyl group in the side chain. Thus, PnBTPA allows us to prepare the film from its solution in organic solvent with ease. The rigid structure of polymeric backbone affords high glass transition temperature of 182 8C, indicating high thermal stability. Extended conjugation over polymer chain results in red shift of UV absorption up to 375 nm and low ionization potential compared to monomer. The results of the ionization potential measurement and the redox behavior suggest that the polymer has great potential as a hole-transporting material (HTM). The drift mobility of P-nBTPA measured by a standard time-of-flight (TOF) method was found to be of the order of 105 cm2/(V s). The results of the photoconductivity measurement revealed that the photoconductivity of P-nBTPA showed much higher than that of poly(N-vinylcarbazole). # 2002 Published by Elsevier Science B.V. Keywords: Chemical oxidative polymerization; Poly(4-alkyltriphenylamine); Hole-transporting compound; Drift mobility; Photoconductivity
1. Introduction Triarylamine derivatives have been most widely used as hole-transporting materials (HTMs) in organic photoconductors and electroluminescent devices [1]. Among the triphenylamine (TPA) derivatives, N,N,N0 ,N0 -tetraphenylbenzidine (TPD) is one of the most widely used as HTM [1]. Their hole-transporting abilities are based on the fact that they are easily oxidized to form stable radical cation [2]. However, the low molecular weight compounds from TPA derivatives has some problems such as the lack of the thermal stability and the mechanical strength [1]. It has been known that the crystalline structure from amorphous state or the aggregation of the materials is developed during the device driving, resulting in the decrease of the device performance. In order to solve these problems, it is proposed to prepare oligomers or polymers for HTM [3,4]. Some potential advantages of polymers over small molecules include the possibility of extended p-conjugation, generally
*
Corresponding author.
0379-6779/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 0 1 0 - 3
high glass transition temperature (Tg), excellent film-forming property and stable film morphology. Several attempts have been made to prepare polymers having TPD units. New palladium-catalyzed method has been applied to synthesize high molecular weight triarylamine dendrimers [5]. In these reactions, aryl bromides are coupled with arylamines or diarylamines [6,7]. Nickel(0)-catalyzed condensation polymerization of aryl dihalides has also been applied to synthesize poly(triphenylamine) [8]. Recently we reported that 4-methyltriphenylamine polymer (P-MTPA) was simply prepared by the chemical oxidative polymerization using ferric chloride as catalyst [9]. However, the molecular weight and the solubility of resulting polymer were not good enough to make a film. Generally, it has been known that the introduction of bulky substituent in the side chain improves the solubility of polymer and the film formation property. In this work, high molecular weight poly(4-n-butyltriphenylamine) (P-nBTPA) having long n-butyl group in the side chain was obtained by the oxidative polymerization of nBTPA, and its properties were investigated and compared with P-MTPA and P-tBTPA having different alkyl group in the side chain.
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2. Experimental
2.2. Polymerization procedure
2.1. Preparation of 4-alkyltriphenylamine
P-MTPA, P-tBTPA and P-nBTPA were synthesized as shown in Scheme 2, where ‘R’ is the alkyl group in the side chain, such as methyl, n-butyl or t-butyl. P-nBTPA was synthesized as follows. To a two-necked 50 ml flask equipped with a magnetic stirrer were added nBTPA and CHCl3 under nitrogen atmosphere. A quarter portion of FeCl3 was added to the reaction mixture at the interval of 1 h. After the polymerization, the reaction mixture was poured into methanol to recover the product followed by washing with methanol several times. Collected powder was dissolved in CHCl3 to remove the insoluble part by filtration. The filtrate was concentrated and reprecipitated with acetone containing small amount of aqueous ammonia. The product was filtered and dried in vacuo at 50 8C for 12 h. P-MTPA and P-tBTPA were also prepared by the same experimental procedure with P-nBTPA.
All the chemicals were obtained commercially and used without further purification except otherwise noted. Xylene was dried over sodium. Chloroform was distilled over CaH2. Tri-t-butylphosphine was diluted with dry xylene. 4-Methyltriphenylamine (MTPA), 4-n-butyltriphenylamine (nBTPA) and 4-t-butyltriphenylamine (tBTPA) were synthesized as shown in Scheme 1, where ‘R’ is the alkyl group in the side chain such as methyl, n-butyl or t-butyl. To a three-necked 300 ml flask equipped with a magnetic stirrer were added diphenylamine (18.9 g, 0.0886 mol), 1-bromo4-n-butylbenzene (18.9 g, 0.0886 mol), palladium(II) acetate (19.9 mg, 0.0886 mmol), t-BuONa (10.2 g, 0.106 mol) and 0.1 M P(t-Bu)3 xylene solution (3.54 ml, 0.354 mmol). Xylene (80 ml) was added via a syringe and the mixture was stirred at 120 8C for 3 h under nitrogen atmosphere. The reaction mixture was cooled down to 80 8C and water was poured into the reaction mixture. The organic layer was washed with water and concentrated by evaporation followed by purification using column chromatography to remove by-product(s) and unreacted materials. The white solid of nBTPA was obtained after evaporation of eluent and dried in vacuo at 50 8C for 12 h. The yield was 91.5%. 1 H NMR (CDCl3) d from TMS: 0.95, 1.38, 1.62 and 2.58 ppm [3H; –CH3, 2H; –CH2, 2H; –CH2, 2H; –CH2] and 6.90–7.25 ppm [m, 14H; aromatic protons]. 13 C NMR (CDCl3) d from TMS: 13.9, 22.4, 33.6 and 35.1 ppm [–CH3, –CH2, –CH2, –CH2] and 122.3–148.2 ppm [eight signals; aromatic carbons]. MTPA and tBTPA were also prepared by the same experimental procedure with nBTPA. MTPA; 1 H NMR (CDCl3) d from TMS: 2.30 ppm [s, 3H; –CH3] and 6.90–7.24 ppm [m, 14H; aromatic protons]. tBTPA; 1 H NMR (CDCl3) d from TMS: 1.34 ppm [s, 9H; –CH3] and 6.94–7.28 ppm [m, 14H; aromatic protons].
2.3. Measurement Molecular weight was estimated by gel permeation chromatography (GPC) equipped with a JASCO 880-pump and a JASCO UV-970 detector. The packing material was synthesized from divinylbenzene and styrene and packed into the stainless steel column in our laboratory. The polymer was eluted with chloroform and its molecular weight was calibrated against polystyrene standards. 1 H and 13 C NMR spectra were recorded on chloroform-d solution at 50 8C with tetramethylsiline as an internal standard using a JEOL a-500 spectrometer (1 H: 500 MHz; 13 C: 125 MHz). Glass transition temperature (Tg) was measured by Rigaku Thermo DSC 8230 at the heating rate of 10 8C/min. UV absorption spectra were recorded on JASCO V-570 spectrophotometer in chloroform (10 mg/ml) or UV–VIS array detector (JASCO MD-1510) connected with GPC columns (Shodex K-803, K-802) to investigate the dependence of UV absorption on molecular weight. Cyclic voltammetry
Scheme 1. Synthetic route of 4-alkyltriphenylamine.
Scheme 2. Polymerization of 4-alkyltriphenylamine.
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was conducted for the cast film on platinum working electrode immersed in dry acetonitrile containing tetra-n-butylammonium perchlorate (0.1 M) as an electrolyte under nitrogen atmosphere using a one-compartment cell with a polarization unit (TOHO PS-06). Platinum spiral was used as a counter electrode and Ag/AgCl as a reference electrode. Hole mobility was determined by a time-of-flight (TOF) method with the device consisting of Al/Ti-phthalocyanine/ polymer/semitransparent gold and xenon lamp. Ionization potential was measured for cast film (film thickness: 2 mm) on slide glass by RIKENKEIKI AC-1. Photoconductivity was measured with the device consisting of ITO/polymer including C60 (0.2 wt.%)/semitransparent gold using He–Ne laser (NEC GLS-5410) and digitizing oscilloscope (GOULD, DSO 630). Film thickness was determined by a profilometer (DEKTAK II, Solan).
3. Results and discussion 3.1. Polymerization results In a previous communication describing the synthesis of P-MTPA, the catalyst, ferric chloride, was added to the reaction medium at the beginning of the reaction in a lump resulting in inhomogeneous dispersion and consequently heterogeneous polymerization reaction to produce oligomer with low yield or high content of gel [9]. We found that the periodic addition of FeCl3 is efficient to provide high molecular weight polymer in this work. The feeding method of
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oxidant, FeCl3, applied in this work was as follows. A quarter portion of FeCl3 was added to the reaction medium at the intervals of 1 h in all synthetic experiments except for no. 15 as shown in Table 1. When FeCl3 was added in a lump, we could not obtain the reproducible results on molecular weight, molecular weight distribution and yield probably because of the heterogeneity of the reaction medium. The polymerization results by one spot addition of FeCl3 are shown in Table 1. It can be seen that molecular weight, molecular weight distribution and yield are different from reactor to reactor. Using the periodic feeding method of FeCl3, we investigated the dependence of the yield and the molecular weight by the oxidative polymerization of nBTPA on experimental conditions such as reaction temperature, reaction time and mole ratio of FeCl3 to monomer. The effect of the mole ratio was examined at the reaction temperature of 50 8C and the reaction time of 6 h. When the mole ratio was 2, the polymeric product was not obtained. We can see from Table 1 that the molecular weight and the yield increase with the mole ratio of oxidant from 2 to 5. However, when the mole ratio was 5, the reaction was rigorous to form much amount of gel up to 80.2% due probably to cross-linking. The effects of the reaction temperature and the reaction time were also investigated in the range of 30–60 8C and 4–8 h at the constant mole ratio of oxidants, 4. As shown in Table 1, the reaction at 30 and 40 8C of the reaction temperature could not provide high molecular weight polymers even though long reaction time, which increased undesired gel portion only. The best result, high molecular weight and high yield of soluble part in chloroform, was obtained at the
Table 1 Polymerization conditions and resultsa,b Temperature (8C)
Time (h)
Yield (%)
Mn (103)
Mw (103)
Monomer: 4-n-butyltriphenylamine ac 4 bc 4 cc 4 1 2 2 3 3 4 4 5 5 4 6 4 7 4 8 4 9 4 10 4 11 4
60 60 60 50 50 50 50 30 40 40 50 50 50 60
6 6 6 6 6 6 6 8 6 8 5.5 6 7 4
89.2 95.4 70.0 0 70.9 98.9 98.5 81.2 80.9 98.7 88.1 98.9 98.7 96.1
(0.02) (42.9) (80.2) (0.01) (0.01) (73.9) (8.50) (42.9) (95.5) (24.8)
3.6 5.3 2.0 – 2.8 6.8 5.8 3.3 4.3 3.0 5.1 6.8 3.1 6.8
19 27 15 – 19 26 15 14 21 20 20 26 14 27
Monomer: 4-methyltriphenylamine 12 4 13 4 14 4
50 50 60
4 5 4
95.4 (4.80) 91.3 (23.8) 86.4 (54.9)
4.8 4.9 3.2
24 25 11
Monomer: 4-t-butyltriphenylamine 4 15c
50
8
83.0 (13.0)
7.0
29
No.
FeCl3/monomer
a
Values in parentheses denote insoluble part in chloroform. Monomer concentration: 0.25 mol/l. c FeCl3 was added in a lump. b
(46.5) (71.2) (35.4)
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reaction condition of 60 8C and 4 h while longer reaction time at 50 8C results in serious gel formation up to 95.5% and low molecular weight 3.1 k of soluble part. For comparison purpose of reactivity, MTPA was also polymerized at the same experimental conditions with nBTPA of sample no. 11 which produce high molecular weight P-nBTPA as shown in Table 1 (no. 14). However, the gel content of the product was higher than that of P-nBTPA implying higher reactivity of MTPA. From Table 1, we can see that the polymerization condition of no. 12 is desirable to produce high molecular weight P-MTPA in high yield of CHCl3 soluble part. 3.2. Polymer structure 13
C NMR spectrum of the P-nBTPA (no. 11) is shown in Fig. 1. Signals were assigned by considering the NMR spectra of P-MTPA reported previously [9]. Signals at 13.9, 22.4, 33.6 and 35.1 ppm are assigned to –CH3, –CH2–, –CH2–, –CH2– of n-butyl group in the side chain, respectively. The linear structure as shown in the inset of Fig. 1 can be confirmed considering the number of peaks in the aromatic region of 13 C NMR spectrum. Eight signals in aromatic region indicate exclusive coupling reaction at the para-position of the unsubstituted N-phenyl rings and the linear structure. Eight aromatic peaks were also observed in P-MTPA and P-tBTPA polymers, indicating the linear structure for these polymers again. 3.3. Polymer characterizations The UV absorption spectra were monitored by the multichannel detector coupled with GPC. Fig. 2 shows the dependence of UV absorption maxima of P-MTPA, P-nBTPA and
Fig. 2. UV absorption maximum of polymers vs. retention time (i.e. molecular weight). In this experiment, an array UV detector was used equipped with small pore size columns (Shodex KF 802, KF 802.5). Red shift of UV absorption can be interpreted as p–p transition energy lowered by the extended conjugation over the biphenylene linkage formed by polymerization.
P-tBTPA on retention time. The retention time in Fig. 2 can easily be transferred to molecular weight of standard polymer. UV absorption maximum of nBTPA dimer was dramatically red shifted against monomer from 305 to 354 nm, and gradually shifted to 375 nm when molecular weight became higher than ca. 7000 which corresponds to the retention time of 13 min. The red shift can be attributed to the extended conjugation length by the polymerization. The peak maximum is almost identical with those of P-MTPA and P-tBTPA implying that the alkyl substituent does not make significant effect on the electronic structure of the aromatic ring.
Fig. 1. 13 C NMR spectrum of P-nBTPA. Eight aromatic signals prove the linear structure of polymer and the coupling reaction at the para-position of unsubstituted phenyl ring. Similar results were also observed in P-MTPA and P-tBTPA.
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Table 2 Physical properties of polymer prepared and related monomers Polymer and monomers
R
Mn (103)
lmax (nm)
IP (eV)
Epa (V)
Tg (8C)
P-MTPA P-tBTPA P-nBTPA TPD MTPA
Methyl t-Butyl n-Butyl Methyl Methyl
6.1 7.0 6.8 0.52 0.26
374 376 375 354 305
5.30 5.29 5.30 5.50 5.64
1.05 1.01 1.02 0.83, 0.94 0.92
207 177 182 52
It is considered that charge transport phenomenon in organic materials is governed by mainly the redox reaction of charge transporting molecules [2]. Cyclic voltammetry (CV) is a preliminary characterization method to determine the redox properties of the organic and polymeric materials. Chloroform solution of polymer was cast on a platinum electrode, and the cyclic voltammogram was obtained in acetonitrile containing tetrabutylammonium perchlorate (0.1 M) at a scanning rate of 100 mV/s. Oxidation potentials of polymers are shown in Table 2 together with those of TPA and TPD monomers. The three types of polymers (P-MTPA, P-tBTPA and P-nBTPA) showed approximately the same oxidation and ionization potentials. Thus, the alkyl substituents seem not to make a large effect on the electrochemical properties of the polymers, which is consistent with the results by UV measurement. As shown in Fig. 3, the cyclic voltammogram of P-nBTPA (no. 11 in Table 1) shows an oxidation wave of which peak top is at 1.02 V (versus Ag/ AgCl) and a reduction wave is at 0.83 V with shoulder at ca. 0.7 V. According to the report for polydiphenylamine [10] or poly(4-aminobiphenyl) [11], only one oxidation peak was observed although the polymer should also be oxidized reversibly in two separate steps. The redox peaks observed here are much broader than those observed in acrylate polymer containing TPD in the side chain showing a couple of well defined redox peaks [12]. This observation indicates that in partially oxidized unit on polymer backbone, the oxidized triarylamine unit has an electrochemical effect on
the adjacent units through delocalization over biphenylene linkage and nitrogen atoms. In Table 2, the ionization potentials of three polymers synthesized show lower values than those of MTPA and TPD. In the same manner with the red shift of UV absorption, this can be interpreted as extended p-conjugation through the biphenylene unit and nitrogen atom, stabilizing the oxidized state of polymer, consequently lowering the ionization potential. Tg of the polymers obtained by the oxidative polymerization is also shown in Table 2. Tg of TPD is 52 8C [13], of which low Tg limits the operating temperature of the devices because of recrystallization, poor thermal and mechanical stability [14,15]. As expected, Tg of the polymers is much higher than that of TPD. Tg of P-tBTPA and P-nBTPA was slightly lower than that of P-MTPA. This is due to the fact that the bulky and flexible group in the side chain provides the space for rigid polymer backbone to move with ease. P-MTPA was insoluble in common organic solvents such as THF, toluene and chlorobenzene but not in chloroform. On the other hand, P-tBTPA and P-nBTPA showed better solubility in common organic solvents, such as THF, toluene, chloroform and chlorobenzene. Excellent transparent orange films on the order of 10–30 mm thickness could be obtained from the toluene solution of P-nBTPA (no. 11) by a wire-bar-coating method while the film obtained from P-tBTPA was brittle. The film from P-nBTPA was conducted to the measurement of hole drift mobility by the standard TOF method. The mobility, m, is calculated according to the following equation: m¼
Fig. 3. Cyclic voltammogram of P-nBTPA.
L2 tT V
where L is the sample thickness, tT the transit time, and V is the applied voltage. Transit time, tT, was obtained from the onset as shown in the inset of Fig. 4. The transit time was determined to be 4.78 ms at the applied voltage of 60 V (19.7 kV/cm), which leads to the mobility of 3:25 105 cm2/(V s). The logarithm of the mobility of P-nBTPA was plotted against the square root of applied field E (V/cm) as shown in Fig. 4. The drift mobility of P-nBTPA was on the order of 105 cm2/(V s) regardless of applied voltage. The dependence of the drift mobility on applied voltage is still in debate where positive and zero slopes were observed in some papers while negative slope was also observed in other papers [1,16].
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Monomers were coupled exclusively at the para-position. P-nBTPA polymer showed the excellent properties of film formation, drift mobility and photoconductivity required in HTM. We found that the biphenylene linkage of polymer rather than substituents at the para-position makes a significant effect on the electronic structure of TPA unit through the extended conjugation as observed in red shift of UV absorption with molecular weight and low ionization potential compared to TPA monomer. The results of the ionization potential measurement and the redox behavior suggest that P-nBTPA has a great potential as HTMs. Acknowledgements This work is supported in part by grants from JSPS. We thank Prof. Kengi Ogino for his helpful comments. References Fig. 4. Drift mobility of P-nBTPA is constant on the order 105 cm2/(V s) regardless of the voltage applied.
Fig. 5. Photoconductivity of various polymers. P-nBTPA shows the highest photoconductivity value in three polymers.
The photoconductivity was measured for the device made from P-nBTPA (no. 11). Fig. 5 shows the results of the photoconductivity measurement together with those of poly(N-vinylcarbazole) (PVK) and N,N0 -diphenyl-N,N0 bis(4-methylphenyl)-1,4-phenylenediamine (TPD)–benzaldehyde (BzA) which was polymerized from TPD and BzA [17]. P-nBTPA showed much higher photoconductivity than TPD–BzA and PVK [17]. In conclusion, we were able to polymerize 4-alkyltriphenylamine to high molecular weight polymers, affording high Tg and thermal stability by the simple oxidative polymerization.
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