- Email: [email protected]
Properties of chemically prepared polypyrrole with an aqueous solution containing Fe2(SO4)3, a sulfonic surfactant and a phenol derivative
SYflTH|TIIC I I|TRLS ELSEVIER
Synthetic Metals 95 (1998) 191-196
Properties of chemically prepared polypyrrole with an aqueous solution containing Fe2(SO4) 3, a sulfonic surfactant and a phenol derivative Yasuo Kudoh *, Kenji Akami, Yasue Matsuya Advanced Materials Research Laboratory, Matsushita Research Institute Tokyo, Inc., 3-10-1 Higashimita, Tama-ku, Kawasaki 214-8501, Japan
Received 19 November 1997; received in revised form 30 March 1998; accepted 31 March 1998
Abstract This paper describes chemically prepared polypyrrole (PPy) with an aqueous solution containing Fe2(SO4) 3 as an oxidant. PPy having enhanced conductivity and environmental stability was obtained under the coexistence of a sulfonic surfactant and a phenol derivative with an electron-withdrawing group. The effect of the sulfonic surfactant was ascribed to a large-sized sulfonic anion being selectively incorporated into PPy as dopant, because an increase in the yield of PPy depended on the concentration of the surfactant. For PPy synthesized with a solution containing p-nitrophenol (pNPh), a clear increase in the doping ratio was recognized from elemental analysis. Moreover, the conductivity of PPy increased with increasing strength of the electron-withdrawing force of the substituent. The electron-withdrawing substituent of the phenol derivative seems to prevent undesirable side reactions so as to improve the regularity of the PPy backbone. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Polypyrrole; Chemical polymerization; Sulfonic surfactant; Phenol derivatives; Conductivity; Environmental stability
1. Introduction Intrinsic electroconducting polymers have been attracting much attention in the field of electronics as advanced materials because of their multifunctionality. So far, the electroconducting polymers have been proposed for many applications including solid electrolytic capacitors [1,2], solar cells [3], rechargeable batteries [4], field-effect transistors [5], electrochromic displays [6], electroluminescent devices [7], nonlinear optical devices [8], under layers for the metallization of printed circuit boards [9], conducting resists for electron-beam lithography [ 10] and electroconducting textiles for electromagnetic interference shielding [ 11 ]. Actually, aluminum and tantalum solid electrolytic capacitors whose electrolytes are composed of PPy have already been commercialized [ 12,13 ]. In addition, applications of polyaniline and polyethylenedioxythiophene to solid electrolytic capacitors were also proposed [ 14-16]. These capacitors showed ideal impedance-frequency characteristics, because the conductivities of the electroconducting polymers are much higher than those of conventionally used liquid electrolytes or manganese dioxide. Although they also showed high durability under high temperatures and high humidities, it greatly depended on the sealing epoxy resin * Corresponding author. Tel.: + 81 44 911 6351; fax: + 81 44 922 9766.
that can hinder the outer gas permeating. To develop the electroconducting polymer capacitors into widely used ones, it is necessary to improve the environmental stability of the electroconducting polymer. Moreover, also in the other applications of the electroconducting polymers, the superior environmental stability is an indispensable factor for their commercialization. We previously reported that PPy prepared with an aqueous solution containing Fe2(SO4)3 and a sulfonic surfactant showed high conductivity and superior environmental stability [ 17]. In this paper, we show that the addition of a phenol derivative having a strong electron-withdrawing substituent enhances the conductivity and environmental stability of PPy doped with surfactant anion.
2. Experimental All reagents including a pyrrole monomer, the oxidant (Fe2(SOn) 3" nH20), the surfactants, the additives and ethanol were purchased in their refined grades and used without any further purification. The water content of Fe2(SO4)3.nH20 was determined from a weight loss after heating at 300°C for 30 h. The surfactants used in this study were 40% aqueous solution of sodium alkylnaphthalenesulfonate (NaANS, mean molecular weight 338), sodium dodecylbenzenesulfonate (NaDBS, hard type) and 40% aqueous
0379-6779/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S03 79-6779 ( 9 8 ) 00054-X
192
E Kudoh et al. / Synthetic Metals 95 (1998) 191-196
solution of sodium alkylsulfonate (NaAS, mean molecular weight 328). The additives are summarized later in Table 2. Polymerization was started by pouring a pyrrole monomer solution into a stirred oxidant solution. The surfactant and additive were mixed in the pyrrole monomer solution in advance when necessary. All polymerizations were carded out under the condition of a stoichiometric excess concentration of the pyrrole monomer to the oxidant at 25°C. After the prescribed polymerization time, PPy was filtered from the solution by a Kiriyama funnel and a Kiriyama No. 5C filter paper. The filtrated PPy was washed by deionized water until the filtrate showed neutrality and further rinsed by ethanol several times. Finally, PPy was dried in a vacuum for about 10 h at 40°C and preserved in nitrogen at room temperature. The conductivity of PPy was measured with a compressed disc of 13 mm diameter prepared under a pressure of 30 MPa in the atmosphere at room temperature. A Loresta SP resistivity meter produced by Mitsubishi Yuka was used for the measurement. To evaluate environmental stability, while aging the PPy discs in air at 125°C, in nitrogen at 150°C and at 85°C/85% RH, the conductivity was intermittently measured in the atmosphere at room temperature. The doping ratios of PPy were calculated from the results of elemental analysis. For the purpose, PPy thoroughly washed with a Soxhlet extractor and ethanol for 12 h was given.
40
,
30 ~
3.0 2.5
~Eo r~ >
'1o
o
~m 2.0 "a
20
O l
1.5 ~
NPh 1.0 without pNPh 0 , ' ' ' 0.5 0.00 0.01 0.02 0.03 0.04 NaANSConcentration / M Fig. 1. Conductivity and yield of Playpreparedwith 0.05 M of pNPh (©) and without pNPh (A) vs. concentrationof suffactant NaANS. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration, 0.1 M.
50
3.5
40 ~E o ~0 30
3.0 2.5 2.0 o~ "o
20
8 lO
1.5 ~/
~ J
3. Results and discussion Figs. 1-3 show changes in conductivities and yields as a function of concentration of various surfactants and with or withoutp-nitrophenol (pNPh). We already reported that PPy prepared with an aqueous solution containing Fe2(SOn)3 and a sulfonic surfactant showed not only improved conductivity and an increased yield but also superior environmental stability [ 17]. These favorable properties were due to the sulfonic anion, originated from the surfactant, being incorporated into the PPy backbone as a dopant prior to the sulfate anion, originated from the oxidant. It is worth noting that the maximal conductivities were obtained at around 0.02 M of the surfactants. As will be mentioned later, 0.02 M is nearly equal to the concentration of positive charges in the PPy formed that should be canceled out by the dopant. In lower concentration regions, after the surfactant anion is incorporated into PPy, the insufficient dopant is covered with the sulfate anion. This is why the yield of PPy increases approximately in proportion to the surfactant concentration in the lower region. In higher concentration regions than 0.02 M, it seemed that the surfactant was adsorbed onto the surface of hydrophobic PPy. Therefore, the saturation of the yield is observed at the high concentration region of the surfactant in comparison to that observed at the maximal conductivity. As can be seen, NaAS offered the highest conductivity, followed by NaDBS and NaANS in that order, i.e., the conductivity decreases with increasing bulk of the dopant anion. This is most likely
3.5
i
1.0
withoutpNPh
0.5
i
0.00
0.01 0.02 0.03 0.04 NaDBS Concentration / M Fig. 2. Conductivity and yield of Play prepared with 0.05 M of pNPh (©) and without pNPh (A) vs. concentration of surfactant NaDBS. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration, 0. l M.
80
3.5
roe60~
o
70
3.0
tn 50
.~
_~ 40
~
*~ 30 201 10
20_~
1.5
"o
0'
0.00
pNPh --~-- withoutpNPh ,
,
,
0.01 0.02 0.03 NaAS Concentration / M
1.0
0.5 0.04
Fig. 3. Conductivity and yield of PPy prepared with 0.05 M of pNPh (O) and without pNPh (A) vs. concentration of surfactant NaAS. Polymerization temperature, 25°C; polymerization time, 60 rain; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4) 3 concentration, 0.1 M.
because, when the bulky anion was doped, the contribution of interchain hopping conduction decreases due to the layered structure of the PPy backbone and the dopant [ 18 ]. Furthermore, it is notable that the conductivity is enhanced by the addition of pNPh independent of the kind of the sur-
193
Y. Kudoh et al. / Synthetic Metals 95 (1998) 191-196
factants. Fig. 4 displays the effect of the molar ratio of pNPh to the pyrrole monomer on the conductivity and the yield of the resultant PPy. The conductivity greatly increases with the molar ratio of pNPh, whereas the yield of PPy slightly decreases. Table 1 summarizes the effect of added pNPh on the compositions and doping ratios of PPy along with conductivity data. To eliminate the influence of the surfactant adsorbed onto the PPy, the compared samples were prepared without any surfactant. By the addition of pNPh, a clear increase in the doping ratio was observed as well as increase in the conductivity. On the other hand, a decrease in the molar ratio of H/C was not recognized in PPy prepared with pNPh. Therefore, the enhancement of the conductivity is not due to the inhibition of the incorporation of 2,5-bis-(2-pyrrolyl) pyrrolidine into the PPy backbone as mentioned by Rapi et al. [ 19]. With polythiophene, it was reported that electrochemically polymerized polythiophene had higher conductivity and doping ratio than the chemically polymerized one [ 20]. This seemed to result from polythiophene with a more regulated backbone structure being formed in the electropolymerization. In addition, it is known that PPy polymerized at low temperatures shows increased conductivity [21,22]. This was ascribed to the regularity of the PPy backbone being 30
0.9
25
0.8
"rE 2C co
0.7 >. 0.6
0
012
O.0
014
016
018
110
112
L4 1.
0.5
1.6
pNPh / Pyrrole monomer / Molar Ratio Fig. 4. Conductivity and yield of PPy vs. molar ratio of pNPh to pyrrole monomer. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.094 M; Fe2(SO4) 3 concentration, 0.025 M; NaANS concentration, 0.02 M. Table 1 Data for elemental analysis and conductivity of PPy prepared with and without pNPh Doping Conductivity ratio = ( S c m - t )
Elemental analysis (wt.%)
With pNPh b Without pNPh
C
H
N
S
Fe
52.8
3.70
15.2
5.0
0.005
0.29
53.1
3.67
15.1
4.1
0.11
0.24
14.0 1.75
a 2 × molar ratio of S/N. b Concentration, 0.05 M; pyrrole monomer concentration, 0.375 M; Fe2(SO4) a concentration, 0.1 M; solvent, water; polymerization condition, 25°C/60 min.
improved because side reactions were prevented. Furthermore, it is observed that electropolymerized PPy with pNPh had greater absorption at the bipolaronic level and smaller absorption at the polaronic level [ 23 ]. This phenomenon was due to the bonding between ot positions being increased to improve the regularity of the PPy backbone. Chemically prepared PPy under the existence of pNPh also seems to have improved regularity similar to the electropolymerized one. Table 2 shows the conductivities of PPy prepared with various additives. The phenol derivatives were arranged in order of the strength of substituted groups' electron-withdrawing effect estimated from the Hammett substituent constant [24]. The conductivity of PPy clearly increases with increasing electron-withdrawing effect of the substituent. The maximal effect was obtained by using pNPh. Besides these phenol derivatives, the improvement of the conductivity was also recognized by other aromatic compounds having a nitro group, such as p-nitrobenzoic acid andp-nitrobenzylalcohol. Therefore, the improvement of the conductivity was essentially ascribed not to the hydroxy group of the phenol derivatives but to the electron-withdrawing substituents. The polymerization reaction of pyrrole, which is a sort of electrophilic substitution, is liable to occur between ot positions, because the resonance structure of the intermediate carbocation yielded by attack at the ot position is more stable than that of the carbocation yielded by attack at the 13position [25]. However, it is known that the difference of those stabilities is not so large that irregular bondings, such as between ot-13 positions and 13-13positions, may also be formed. From the foregoing, the enhancement of conductivity by the addition of pNPh appears to be based on the following mechanism. When pNPh is coexistent with the pyrrole monomer, the electron-withdrawing nitro group partially withdraws electrons of the pyrrole ring. As a result, polymerization reaction being electrophilic substitution is properly suppressed so as to decrease the probability of unfavorable side reactions. Namely, the enhanced conductivity caused by pNPh may be attributed to the regularity of the PPy backbone being improved. This supposition is also supported by the decrease Table 2 Data for conductivity of PPy prepared with various additives a Additive
p-Nitrophenol m-Nitrophenol p-Cyanophenol p-Hydroxybenzoic acid 2-Nitroresorcinol p-Nitrobenzoic acid m-Nitrobenzoic acid p-Nitrobenzyl alcohol None
Conductivity
Yield
(Scm -I )
(g)
25.6 23.9 22.2 17. l 19.0 23.5 18.6 20.7 15.5
2.79 2.73 2.86 2.91 2.84 2.74 2.67 2.76 2.84
a Pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration, 0.1 M; NaANS concentration, 0.04 M; additive concentration, 0.0375 M; solvent, water 200 cm3; polymerization condition, 25°C/60 rain.
194
Y. Kudoh et al. / Synthetic Metals 95 (1998) 191-196
in yield of PPy as well as the increase in conductivity being observed with increasing relative concentration of pNPh to the pyrrole monomer, as shown in Fig. 4. Fig. 5 displays the effect of the surfactant concentration on the conductivity decay observed in air at 125°C. PPy samples used here were prepared under the coexistence of NaANS and pNPh. The conductivity decay decreases with increasing NaANS concentration until 0.032 M. In the higher regions, the decay comes to a minimal level independent of the NaANS concentration. This can be explained as follows: As
can be seen from Fig. 1, the alkylnaphthalenesulfonic anion (ANS) is incorporated into the PPy backbone as dopant by high selectivity. As a result, the proportion of ANS to the total dopants increases with the concentration of added NaANS. The doping ratio of PPy prepared with pNPh can be calculated to be 0.29 as shown in Table 1. Assuming the doping ratio does not change by the addition of the surfactant and the whole oxidant is consumed by the polymerization reaction, the concentration of a monovalent dopant anion is calculated at 0.025 M using the following equation: Doping ratio × Concentration of pyrrole converted to PPy
101
= (0.29 × 0.1 × 2 ) / ( 2 + 0 . 2 9 )
X without NaANS --O--NaANS 0.008 M ---[]--- N~,~NS 0.016 M .--~,-- NaANS 0.024 M ~NaANS 0.032 M
~, lO°~
"~. '~ 10-~(, "~ 10-2' "~ 10.3 .N "~10 "4 10"5 1060
200 '
400 '
66 0 800 ' Time / h
10'00
1200
Fig. 5. Normalized conductivity of PPy prepared under the various concentrations of NaANS vs. time aged in air at 125°C. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration, 0.1 M; pNPh concentration, 0.05 M.
101
l O .~'~"10-1 10.2
i
5
For PPy prepared without pNPh, the doping ratio is calculated at 0.021 M in a similar way. Although the selectivity of ANS is extremely high, we have to consider that the sulfate anion existing in great excess to ANS is also incorporated into the PPy backbone as the dopant. Therefore, the concentration at which the dopant is predominantly sheared by ANS should be a little higher than 0.025 M. This is the reason why the conductivity decay shows a minimal level in the NaANS concentration region of 0.032 M or more. Figs. 6 and 7 display the conductivity decay of various PPy samples held in air at 125°C and at 85°C/85%RH, respectively. The improvements of the thermal and moisture stabilities are observed in all PPy prepared with the surfactants. It is worth noting that the environmental stability increases with 101
,
(a)
~ lO-1
'
DBs
o
o 10-2
104 10~
~ 10"~ z
:~ 10"3
None
~7
"~one
10 ~ 1 0 -~ 10 ~
,
10tl . / (b) 10°1~
,
.
,
.
10-s 101
f
.
,
,
,
,
,
Co)
,
,
10_1(
lO°1
o 10~I
None
N~ s
240
4()0
DBS
z 104
-
one
10"7 l
10"~
600 Time / h
8(]0
1000
1200
Fig. 6. Normalized conductivity of PPy prepared under various surfactants vs. time aged at 125°C in air: (a) with 0.05 M of pNPh; (b) without pNPh. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4) 3 concentration, 0.1 M; None, without surfactant; AS, with 0.036 M of sodium alkylsulfonate; DBS, with 0.03 M of sodium dodecylbenzenesulfonate, ANS, with 0.032 M of sodium alkylnaphthalenesulfonate.
1 O'S 0
' 200
' 400
' ' 600 800 Time / h
10'o 0
1200
Fig. 7. Normalized conductivity of PPy prepared under various surfactants vs. time aged at 85°C and 85%RH: (a) with 0.05 M of pNPh; (b) without pNPh. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration, 0.1 M; None, without surfactant; AS, with 0.036 M of sodium alkylsulfonate; DBS, with 0.03 M of sodium dodecylbenzenesulfonate, ANS, with 0.032 M of sodium alkylnaphthalenesulfonate.
Y. Kudoh et aL / Synthetic Metals 95 (1998) 191-196
101
4. Conclusions
i
lO4 10: -2
.•10 N
.E
10 ~
~
~ NaANS (0.04 M) / pNPh (0.05 M) ---[3~ NaANS (0.04 M) •--~.-- pNPh (0.05 M) -'-O'-- None
7° 104 10 .5
195
I
I
|
I
I
1000
2000
3000
4000
5000
6000
Time / h
Fig. 8. Normalizedconductivityof PPy prepared under various conditions vs. time aged in nitrogen at 150°C. Polymerizationtime, 60 rain; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration,0.1 M. increasing bulk of the doped surfactant anion, although the as-grown conductivity decreases as mentioned above. This suggests that the dedoping of the dopant is one of the main factors for the conductivity decay at high temperatures and high humidities, i.e., the bulkier the dopant anion, the slower is its thermal diffusion. Moreover, the thermal and moisture stabilities are clearly improved by the addition of pNPh. This is attributed to the decrease in the structural defects of the PPy backbone that are liable to be oxidized. For PPy prepared without the surfactant, the conductivity decay hardly depends on the addition of pNPh. Here, the conductivity decay is probably dominated not by the oxidation reaction but by the dedoping of the dopant because the molecular size of the sulfate anion is very small. Fig. 8 shows the conductivity decay observed in nitrogen at 150°C. In comparison with Fig. 6, it is clear that the conductivity decay of PPy doped with ANS is exceedingly hindered in the inert gas. This proves that the oxidation reaction to the PPy backbone is another main factor leading to the conductivity decay at elevated temperatures. Furthermore, in the case of PPy doped with ANS, it can be seen that the thermal stability is improved by the addition of pNPh. This is most likely caused by the improved regularity of the PPy backbone induced by the coexistence of pNPh with the pyrrole monomer. Although the electroconducting polymers are very interesting materials, commercial base application has remained limited. This is mainly due to their relatively low environmental stability. In addition, the situation, which seemed necessary to use an organic solvent for the preparation of PPy having improved conductivity and environmental stability, has also been an obstacle to commercialization. We found a novel chemical polymerization method using aqueous solution whereby highly conducting and environmentally stable PPy is easily obtained. Because water is an ideal solvent for mass production, this polymerization method and resultant PPy are expected to be applied in various fields as well as in solid electrolytic capacitors.
We found that PPy prepared with an aqueous solution containing Fe2(504)3, a sulfonic suffactant and a phenol derivative had high conductivity and superior environmental stability. These favorable properties seem to be caused by the surfactant anion being selectively incorporated into the PPy backbone as the dopant, and the electron-withdrawing substituent of the phenol derivative interacting with the pyrrole monomer so as to improve the regularity of the PPy backbone. It was shown that the conductivity decay of PPy was caused by the dedoping of the dopant and/or the oxidation of the PPy backbone. The combination of NaANS giving the bulkiest dopant and pNPh having the strongest electron-withdrawing substituent offered the most environmentally stable PPy. It is certain that PPy prepared by this novel polymerization method will greatly contribute to progress in the field of electronics.
Acknowledgements The authors gratefully acknowledge Professor T. Yamamoto, Tokyo Institute of Technology, for his valuable suggestions and helpful discussion to prepare this paper.
References [1] Y. Kudoh, S. Tsuchiya, T. Kojima, M. Fukuyama, S. Yoshimura, Synth. Met. 4143 (1991) 1133. [2] M. Satoh, H. Ishikawa, K. Amano,E. Hasegawa, K. Yoshino, Synth. Met. 65 (1994) 39. [3] A.J. Frank, S. Glenis, A.J. Nelson, J. Phys. Chem. 93 (1989) 3818. [4] T. Nakajima, T. Kawagoe, Synth. Met. 27-29 (1989) C629. [5] A. Tsumura, H. Koezuka,T. Ando,Appl. Phys. Lett. 49 (1986) 1210. [6] M. Gazard, in T.A. Skoteim (ed.), Handbook of Conducting Polymers, Vol. 1, Marcel Dekker, New York, 1986, p. 673. [7] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [8] T. Kaino,T. Kurihara, S. Tomaru, Nonlinear Opt. 4 (1993) 319. [9] J. Hupe, W. Kronenberg,World Patent No. WO 89/08375 (1989). [ 10] M. Angelopoulos,N. Patel, J.M. Shaw, in M. Aldissi (ed.), Intrinsically ConductingPolymer: An Emerging Technology,Kluwer, Dordrecht, 1993, p. 147. [ 11] R.V. Gregory,W.C. Kimbrell, H.H. Kuhn, Synth.Met. 27-29 (1989) C823. [ 12] Y. Kudoh, M. Fukuyama, T. Kojima, N. Nanai, S. Yoshimura, in M. Aldissi (ed.), Intrinsically Conducting Polymer: An Emerging Technology,Kluwer, Dordrecht, 1993, p. 191. [ 13] T. Fukami, K. Sakata, S. Arai, A. Kobayashi,T. Nishiyama,M. Ooi, NEC Tech. J. 46 (1993) 49 (in Japanese). [ 14] M. Ezoe, M. Abe, A. Otani, H. Higuchi,Jpn. Patent OpenNo. 3-35516 (1991). [15] F. Jonas, G. Heywang,Electrochim.Acta 39 (1994) 1345. [ 16] K. Akami, Y. Matsuya, Y. Kudoh, T. Kojima, Abstr., AutumnMeet. of Electrochemical Societyof Japan, 1997, p. 134 (in Japanese). [ 17] Y. Kudoh, Synth. Met. 79 (1996) 17. [ 18] G.R. Mitchell, F.J. Davis, C.H. Legge, Synth. Met. 26 (1988) 247. [ 19] S. Rapi, V. Bocchi, G.P. Gardini, Synth. Met. 24 (1988) 217.
196
Y. Kudoh et al. / Synthetic Metals 95 (1998) 191-196
[20] G. Tourillon, in T.A. Skotheim (ed.), Handbook of Conducting Polymers, Vol. l, Marcel Dekker, New York, 1986, p. 293. [21] M. Ogasawara, K. Funahashi, T. Demura, T. Hagiwara, K. Iwata, Synth. Met. 14 (1986) 61. [22] Y. Nakazawa, T. Ebine, M. Kusunold, H. Nishizawa, J. Hanna, H. Kokado, Jpn. J. Appl. Phys. 27 (1988) 1304.
[23] M. Fukuyama, N. Nanai, T. Kojima, Y. Kudoh, S. Yshimura, Synth. Met. 58 (1993) 367. [24] Nippon Kagakukai (ed.), Kagaku Benran (HandbookofChemistry), Maruzen, Tokyo, 1984, p. 11-364 (in Japanese). [25] R.T. Morrison, R.N. Boyd, Organic Chemistry, Prentice-Hall, Englewood Cliffs, NJ, 6th edn., 1992, p. 1064.
Synthetic Metals 95 (1998) 191-196
Properties of chemically prepared polypyrrole with an aqueous solution containing Fe2(SO4) 3, a sulfonic surfactant and a phenol derivative Yasuo Kudoh *, Kenji Akami, Yasue Matsuya Advanced Materials Research Laboratory, Matsushita Research Institute Tokyo, Inc., 3-10-1 Higashimita, Tama-ku, Kawasaki 214-8501, Japan
Received 19 November 1997; received in revised form 30 March 1998; accepted 31 March 1998
Abstract This paper describes chemically prepared polypyrrole (PPy) with an aqueous solution containing Fe2(SO4) 3 as an oxidant. PPy having enhanced conductivity and environmental stability was obtained under the coexistence of a sulfonic surfactant and a phenol derivative with an electron-withdrawing group. The effect of the sulfonic surfactant was ascribed to a large-sized sulfonic anion being selectively incorporated into PPy as dopant, because an increase in the yield of PPy depended on the concentration of the surfactant. For PPy synthesized with a solution containing p-nitrophenol (pNPh), a clear increase in the doping ratio was recognized from elemental analysis. Moreover, the conductivity of PPy increased with increasing strength of the electron-withdrawing force of the substituent. The electron-withdrawing substituent of the phenol derivative seems to prevent undesirable side reactions so as to improve the regularity of the PPy backbone. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Polypyrrole; Chemical polymerization; Sulfonic surfactant; Phenol derivatives; Conductivity; Environmental stability
1. Introduction Intrinsic electroconducting polymers have been attracting much attention in the field of electronics as advanced materials because of their multifunctionality. So far, the electroconducting polymers have been proposed for many applications including solid electrolytic capacitors [1,2], solar cells [3], rechargeable batteries [4], field-effect transistors [5], electrochromic displays [6], electroluminescent devices [7], nonlinear optical devices [8], under layers for the metallization of printed circuit boards [9], conducting resists for electron-beam lithography [ 10] and electroconducting textiles for electromagnetic interference shielding [ 11 ]. Actually, aluminum and tantalum solid electrolytic capacitors whose electrolytes are composed of PPy have already been commercialized [ 12,13 ]. In addition, applications of polyaniline and polyethylenedioxythiophene to solid electrolytic capacitors were also proposed [ 14-16]. These capacitors showed ideal impedance-frequency characteristics, because the conductivities of the electroconducting polymers are much higher than those of conventionally used liquid electrolytes or manganese dioxide. Although they also showed high durability under high temperatures and high humidities, it greatly depended on the sealing epoxy resin * Corresponding author. Tel.: + 81 44 911 6351; fax: + 81 44 922 9766.
that can hinder the outer gas permeating. To develop the electroconducting polymer capacitors into widely used ones, it is necessary to improve the environmental stability of the electroconducting polymer. Moreover, also in the other applications of the electroconducting polymers, the superior environmental stability is an indispensable factor for their commercialization. We previously reported that PPy prepared with an aqueous solution containing Fe2(SO4)3 and a sulfonic surfactant showed high conductivity and superior environmental stability [ 17]. In this paper, we show that the addition of a phenol derivative having a strong electron-withdrawing substituent enhances the conductivity and environmental stability of PPy doped with surfactant anion.
2. Experimental All reagents including a pyrrole monomer, the oxidant (Fe2(SOn) 3" nH20), the surfactants, the additives and ethanol were purchased in their refined grades and used without any further purification. The water content of Fe2(SO4)3.nH20 was determined from a weight loss after heating at 300°C for 30 h. The surfactants used in this study were 40% aqueous solution of sodium alkylnaphthalenesulfonate (NaANS, mean molecular weight 338), sodium dodecylbenzenesulfonate (NaDBS, hard type) and 40% aqueous
0379-6779/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S03 79-6779 ( 9 8 ) 00054-X
192
E Kudoh et al. / Synthetic Metals 95 (1998) 191-196
solution of sodium alkylsulfonate (NaAS, mean molecular weight 328). The additives are summarized later in Table 2. Polymerization was started by pouring a pyrrole monomer solution into a stirred oxidant solution. The surfactant and additive were mixed in the pyrrole monomer solution in advance when necessary. All polymerizations were carded out under the condition of a stoichiometric excess concentration of the pyrrole monomer to the oxidant at 25°C. After the prescribed polymerization time, PPy was filtered from the solution by a Kiriyama funnel and a Kiriyama No. 5C filter paper. The filtrated PPy was washed by deionized water until the filtrate showed neutrality and further rinsed by ethanol several times. Finally, PPy was dried in a vacuum for about 10 h at 40°C and preserved in nitrogen at room temperature. The conductivity of PPy was measured with a compressed disc of 13 mm diameter prepared under a pressure of 30 MPa in the atmosphere at room temperature. A Loresta SP resistivity meter produced by Mitsubishi Yuka was used for the measurement. To evaluate environmental stability, while aging the PPy discs in air at 125°C, in nitrogen at 150°C and at 85°C/85% RH, the conductivity was intermittently measured in the atmosphere at room temperature. The doping ratios of PPy were calculated from the results of elemental analysis. For the purpose, PPy thoroughly washed with a Soxhlet extractor and ethanol for 12 h was given.
40
,
30 ~
3.0 2.5
~Eo r~ >
'1o
o
~m 2.0 "a
20
O l
1.5 ~
NPh 1.0 without pNPh 0 , ' ' ' 0.5 0.00 0.01 0.02 0.03 0.04 NaANSConcentration / M Fig. 1. Conductivity and yield of Playpreparedwith 0.05 M of pNPh (©) and without pNPh (A) vs. concentrationof suffactant NaANS. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration, 0.1 M.
50
3.5
40 ~E o ~0 30
3.0 2.5 2.0 o~ "o
20
8 lO
1.5 ~/
~ J
3. Results and discussion Figs. 1-3 show changes in conductivities and yields as a function of concentration of various surfactants and with or withoutp-nitrophenol (pNPh). We already reported that PPy prepared with an aqueous solution containing Fe2(SOn)3 and a sulfonic surfactant showed not only improved conductivity and an increased yield but also superior environmental stability [ 17]. These favorable properties were due to the sulfonic anion, originated from the surfactant, being incorporated into the PPy backbone as a dopant prior to the sulfate anion, originated from the oxidant. It is worth noting that the maximal conductivities were obtained at around 0.02 M of the surfactants. As will be mentioned later, 0.02 M is nearly equal to the concentration of positive charges in the PPy formed that should be canceled out by the dopant. In lower concentration regions, after the surfactant anion is incorporated into PPy, the insufficient dopant is covered with the sulfate anion. This is why the yield of PPy increases approximately in proportion to the surfactant concentration in the lower region. In higher concentration regions than 0.02 M, it seemed that the surfactant was adsorbed onto the surface of hydrophobic PPy. Therefore, the saturation of the yield is observed at the high concentration region of the surfactant in comparison to that observed at the maximal conductivity. As can be seen, NaAS offered the highest conductivity, followed by NaDBS and NaANS in that order, i.e., the conductivity decreases with increasing bulk of the dopant anion. This is most likely
3.5
i
1.0
withoutpNPh
0.5
i
0.00
0.01 0.02 0.03 0.04 NaDBS Concentration / M Fig. 2. Conductivity and yield of Play prepared with 0.05 M of pNPh (©) and without pNPh (A) vs. concentration of surfactant NaDBS. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration, 0. l M.
80
3.5
roe60~
o
70
3.0
tn 50
.~
_~ 40
~
*~ 30 201 10
20_~
1.5
"o
0'
0.00
pNPh --~-- withoutpNPh ,
,
,
0.01 0.02 0.03 NaAS Concentration / M
1.0
0.5 0.04
Fig. 3. Conductivity and yield of PPy prepared with 0.05 M of pNPh (O) and without pNPh (A) vs. concentration of surfactant NaAS. Polymerization temperature, 25°C; polymerization time, 60 rain; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4) 3 concentration, 0.1 M.
because, when the bulky anion was doped, the contribution of interchain hopping conduction decreases due to the layered structure of the PPy backbone and the dopant [ 18 ]. Furthermore, it is notable that the conductivity is enhanced by the addition of pNPh independent of the kind of the sur-
193
Y. Kudoh et al. / Synthetic Metals 95 (1998) 191-196
factants. Fig. 4 displays the effect of the molar ratio of pNPh to the pyrrole monomer on the conductivity and the yield of the resultant PPy. The conductivity greatly increases with the molar ratio of pNPh, whereas the yield of PPy slightly decreases. Table 1 summarizes the effect of added pNPh on the compositions and doping ratios of PPy along with conductivity data. To eliminate the influence of the surfactant adsorbed onto the PPy, the compared samples were prepared without any surfactant. By the addition of pNPh, a clear increase in the doping ratio was observed as well as increase in the conductivity. On the other hand, a decrease in the molar ratio of H/C was not recognized in PPy prepared with pNPh. Therefore, the enhancement of the conductivity is not due to the inhibition of the incorporation of 2,5-bis-(2-pyrrolyl) pyrrolidine into the PPy backbone as mentioned by Rapi et al. [ 19]. With polythiophene, it was reported that electrochemically polymerized polythiophene had higher conductivity and doping ratio than the chemically polymerized one [ 20]. This seemed to result from polythiophene with a more regulated backbone structure being formed in the electropolymerization. In addition, it is known that PPy polymerized at low temperatures shows increased conductivity [21,22]. This was ascribed to the regularity of the PPy backbone being 30
0.9
25
0.8
"rE 2C co
0.7 >. 0.6
0
012
O.0
014
016
018
110
112
L4 1.
0.5
1.6
pNPh / Pyrrole monomer / Molar Ratio Fig. 4. Conductivity and yield of PPy vs. molar ratio of pNPh to pyrrole monomer. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.094 M; Fe2(SO4) 3 concentration, 0.025 M; NaANS concentration, 0.02 M. Table 1 Data for elemental analysis and conductivity of PPy prepared with and without pNPh Doping Conductivity ratio = ( S c m - t )
Elemental analysis (wt.%)
With pNPh b Without pNPh
C
H
N
S
Fe
52.8
3.70
15.2
5.0
0.005
0.29
53.1
3.67
15.1
4.1
0.11
0.24
14.0 1.75
a 2 × molar ratio of S/N. b Concentration, 0.05 M; pyrrole monomer concentration, 0.375 M; Fe2(SO4) a concentration, 0.1 M; solvent, water; polymerization condition, 25°C/60 min.
improved because side reactions were prevented. Furthermore, it is observed that electropolymerized PPy with pNPh had greater absorption at the bipolaronic level and smaller absorption at the polaronic level [ 23 ]. This phenomenon was due to the bonding between ot positions being increased to improve the regularity of the PPy backbone. Chemically prepared PPy under the existence of pNPh also seems to have improved regularity similar to the electropolymerized one. Table 2 shows the conductivities of PPy prepared with various additives. The phenol derivatives were arranged in order of the strength of substituted groups' electron-withdrawing effect estimated from the Hammett substituent constant [24]. The conductivity of PPy clearly increases with increasing electron-withdrawing effect of the substituent. The maximal effect was obtained by using pNPh. Besides these phenol derivatives, the improvement of the conductivity was also recognized by other aromatic compounds having a nitro group, such as p-nitrobenzoic acid andp-nitrobenzylalcohol. Therefore, the improvement of the conductivity was essentially ascribed not to the hydroxy group of the phenol derivatives but to the electron-withdrawing substituents. The polymerization reaction of pyrrole, which is a sort of electrophilic substitution, is liable to occur between ot positions, because the resonance structure of the intermediate carbocation yielded by attack at the ot position is more stable than that of the carbocation yielded by attack at the 13position [25]. However, it is known that the difference of those stabilities is not so large that irregular bondings, such as between ot-13 positions and 13-13positions, may also be formed. From the foregoing, the enhancement of conductivity by the addition of pNPh appears to be based on the following mechanism. When pNPh is coexistent with the pyrrole monomer, the electron-withdrawing nitro group partially withdraws electrons of the pyrrole ring. As a result, polymerization reaction being electrophilic substitution is properly suppressed so as to decrease the probability of unfavorable side reactions. Namely, the enhanced conductivity caused by pNPh may be attributed to the regularity of the PPy backbone being improved. This supposition is also supported by the decrease Table 2 Data for conductivity of PPy prepared with various additives a Additive
p-Nitrophenol m-Nitrophenol p-Cyanophenol p-Hydroxybenzoic acid 2-Nitroresorcinol p-Nitrobenzoic acid m-Nitrobenzoic acid p-Nitrobenzyl alcohol None
Conductivity
Yield
(Scm -I )
(g)
25.6 23.9 22.2 17. l 19.0 23.5 18.6 20.7 15.5
2.79 2.73 2.86 2.91 2.84 2.74 2.67 2.76 2.84
a Pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration, 0.1 M; NaANS concentration, 0.04 M; additive concentration, 0.0375 M; solvent, water 200 cm3; polymerization condition, 25°C/60 rain.
194
Y. Kudoh et al. / Synthetic Metals 95 (1998) 191-196
in yield of PPy as well as the increase in conductivity being observed with increasing relative concentration of pNPh to the pyrrole monomer, as shown in Fig. 4. Fig. 5 displays the effect of the surfactant concentration on the conductivity decay observed in air at 125°C. PPy samples used here were prepared under the coexistence of NaANS and pNPh. The conductivity decay decreases with increasing NaANS concentration until 0.032 M. In the higher regions, the decay comes to a minimal level independent of the NaANS concentration. This can be explained as follows: As
can be seen from Fig. 1, the alkylnaphthalenesulfonic anion (ANS) is incorporated into the PPy backbone as dopant by high selectivity. As a result, the proportion of ANS to the total dopants increases with the concentration of added NaANS. The doping ratio of PPy prepared with pNPh can be calculated to be 0.29 as shown in Table 1. Assuming the doping ratio does not change by the addition of the surfactant and the whole oxidant is consumed by the polymerization reaction, the concentration of a monovalent dopant anion is calculated at 0.025 M using the following equation: Doping ratio × Concentration of pyrrole converted to PPy
101
= (0.29 × 0.1 × 2 ) / ( 2 + 0 . 2 9 )
X without NaANS --O--NaANS 0.008 M ---[]--- N~,~NS 0.016 M .--~,-- NaANS 0.024 M ~NaANS 0.032 M
~, lO°~
"~. '~ 10-~(, "~ 10-2' "~ 10.3 .N "~10 "4 10"5 1060
200 '
400 '
66 0 800 ' Time / h
10'00
1200
Fig. 5. Normalized conductivity of PPy prepared under the various concentrations of NaANS vs. time aged in air at 125°C. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration, 0.1 M; pNPh concentration, 0.05 M.
101
l O .~'~"10-1 10.2
i
5
For PPy prepared without pNPh, the doping ratio is calculated at 0.021 M in a similar way. Although the selectivity of ANS is extremely high, we have to consider that the sulfate anion existing in great excess to ANS is also incorporated into the PPy backbone as the dopant. Therefore, the concentration at which the dopant is predominantly sheared by ANS should be a little higher than 0.025 M. This is the reason why the conductivity decay shows a minimal level in the NaANS concentration region of 0.032 M or more. Figs. 6 and 7 display the conductivity decay of various PPy samples held in air at 125°C and at 85°C/85%RH, respectively. The improvements of the thermal and moisture stabilities are observed in all PPy prepared with the surfactants. It is worth noting that the environmental stability increases with 101
,
(a)
~ lO-1
'
DBs
o
o 10-2
104 10~
~ 10"~ z
:~ 10"3
None
~7
"~one
10 ~ 1 0 -~ 10 ~
,
10tl . / (b) 10°1~
,
.
,
.
10-s 101
f
.
,
,
,
,
,
Co)
,
,
10_1(
lO°1
o 10~I
None
N~ s
240
4()0
DBS
z 104
-
one
10"7 l
10"~
600 Time / h
8(]0
1000
1200
Fig. 6. Normalized conductivity of PPy prepared under various surfactants vs. time aged at 125°C in air: (a) with 0.05 M of pNPh; (b) without pNPh. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4) 3 concentration, 0.1 M; None, without surfactant; AS, with 0.036 M of sodium alkylsulfonate; DBS, with 0.03 M of sodium dodecylbenzenesulfonate, ANS, with 0.032 M of sodium alkylnaphthalenesulfonate.
1 O'S 0
' 200
' 400
' ' 600 800 Time / h
10'o 0
1200
Fig. 7. Normalized conductivity of PPy prepared under various surfactants vs. time aged at 85°C and 85%RH: (a) with 0.05 M of pNPh; (b) without pNPh. Polymerization temperature, 25°C; polymerization time, 60 min; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration, 0.1 M; None, without surfactant; AS, with 0.036 M of sodium alkylsulfonate; DBS, with 0.03 M of sodium dodecylbenzenesulfonate, ANS, with 0.032 M of sodium alkylnaphthalenesulfonate.
Y. Kudoh et aL / Synthetic Metals 95 (1998) 191-196
101
4. Conclusions
i
lO4 10: -2
.•10 N
.E
10 ~
~
~ NaANS (0.04 M) / pNPh (0.05 M) ---[3~ NaANS (0.04 M) •--~.-- pNPh (0.05 M) -'-O'-- None
7° 104 10 .5
195
I
I
|
I
I
1000
2000
3000
4000
5000
6000
Time / h
Fig. 8. Normalizedconductivityof PPy prepared under various conditions vs. time aged in nitrogen at 150°C. Polymerizationtime, 60 rain; solvent, deionized water 200 cm3; pyrrole monomer concentration, 0.375 M; Fe2(SO4)3 concentration,0.1 M. increasing bulk of the doped surfactant anion, although the as-grown conductivity decreases as mentioned above. This suggests that the dedoping of the dopant is one of the main factors for the conductivity decay at high temperatures and high humidities, i.e., the bulkier the dopant anion, the slower is its thermal diffusion. Moreover, the thermal and moisture stabilities are clearly improved by the addition of pNPh. This is attributed to the decrease in the structural defects of the PPy backbone that are liable to be oxidized. For PPy prepared without the surfactant, the conductivity decay hardly depends on the addition of pNPh. Here, the conductivity decay is probably dominated not by the oxidation reaction but by the dedoping of the dopant because the molecular size of the sulfate anion is very small. Fig. 8 shows the conductivity decay observed in nitrogen at 150°C. In comparison with Fig. 6, it is clear that the conductivity decay of PPy doped with ANS is exceedingly hindered in the inert gas. This proves that the oxidation reaction to the PPy backbone is another main factor leading to the conductivity decay at elevated temperatures. Furthermore, in the case of PPy doped with ANS, it can be seen that the thermal stability is improved by the addition of pNPh. This is most likely caused by the improved regularity of the PPy backbone induced by the coexistence of pNPh with the pyrrole monomer. Although the electroconducting polymers are very interesting materials, commercial base application has remained limited. This is mainly due to their relatively low environmental stability. In addition, the situation, which seemed necessary to use an organic solvent for the preparation of PPy having improved conductivity and environmental stability, has also been an obstacle to commercialization. We found a novel chemical polymerization method using aqueous solution whereby highly conducting and environmentally stable PPy is easily obtained. Because water is an ideal solvent for mass production, this polymerization method and resultant PPy are expected to be applied in various fields as well as in solid electrolytic capacitors.
We found that PPy prepared with an aqueous solution containing Fe2(504)3, a sulfonic suffactant and a phenol derivative had high conductivity and superior environmental stability. These favorable properties seem to be caused by the surfactant anion being selectively incorporated into the PPy backbone as the dopant, and the electron-withdrawing substituent of the phenol derivative interacting with the pyrrole monomer so as to improve the regularity of the PPy backbone. It was shown that the conductivity decay of PPy was caused by the dedoping of the dopant and/or the oxidation of the PPy backbone. The combination of NaANS giving the bulkiest dopant and pNPh having the strongest electron-withdrawing substituent offered the most environmentally stable PPy. It is certain that PPy prepared by this novel polymerization method will greatly contribute to progress in the field of electronics.
Acknowledgements The authors gratefully acknowledge Professor T. Yamamoto, Tokyo Institute of Technology, for his valuable suggestions and helpful discussion to prepare this paper.
References [1] Y. Kudoh, S. Tsuchiya, T. Kojima, M. Fukuyama, S. Yoshimura, Synth. Met. 4143 (1991) 1133. [2] M. Satoh, H. Ishikawa, K. Amano,E. Hasegawa, K. Yoshino, Synth. Met. 65 (1994) 39. [3] A.J. Frank, S. Glenis, A.J. Nelson, J. Phys. Chem. 93 (1989) 3818. [4] T. Nakajima, T. Kawagoe, Synth. Met. 27-29 (1989) C629. [5] A. Tsumura, H. Koezuka,T. Ando,Appl. Phys. Lett. 49 (1986) 1210. [6] M. Gazard, in T.A. Skoteim (ed.), Handbook of Conducting Polymers, Vol. 1, Marcel Dekker, New York, 1986, p. 673. [7] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 (1990) 539. [8] T. Kaino,T. Kurihara, S. Tomaru, Nonlinear Opt. 4 (1993) 319. [9] J. Hupe, W. Kronenberg,World Patent No. WO 89/08375 (1989). [ 10] M. Angelopoulos,N. Patel, J.M. Shaw, in M. Aldissi (ed.), Intrinsically ConductingPolymer: An Emerging Technology,Kluwer, Dordrecht, 1993, p. 147. [ 11] R.V. Gregory,W.C. Kimbrell, H.H. Kuhn, Synth.Met. 27-29 (1989) C823. [ 12] Y. Kudoh, M. Fukuyama, T. Kojima, N. Nanai, S. Yoshimura, in M. Aldissi (ed.), Intrinsically Conducting Polymer: An Emerging Technology,Kluwer, Dordrecht, 1993, p. 191. [ 13] T. Fukami, K. Sakata, S. Arai, A. Kobayashi,T. Nishiyama,M. Ooi, NEC Tech. J. 46 (1993) 49 (in Japanese). [ 14] M. Ezoe, M. Abe, A. Otani, H. Higuchi,Jpn. Patent OpenNo. 3-35516 (1991). [15] F. Jonas, G. Heywang,Electrochim.Acta 39 (1994) 1345. [ 16] K. Akami, Y. Matsuya, Y. Kudoh, T. Kojima, Abstr., AutumnMeet. of Electrochemical Societyof Japan, 1997, p. 134 (in Japanese). [ 17] Y. Kudoh, Synth. Met. 79 (1996) 17. [ 18] G.R. Mitchell, F.J. Davis, C.H. Legge, Synth. Met. 26 (1988) 247. [ 19] S. Rapi, V. Bocchi, G.P. Gardini, Synth. Met. 24 (1988) 217.
196
Y. Kudoh et al. / Synthetic Metals 95 (1998) 191-196
[20] G. Tourillon, in T.A. Skotheim (ed.), Handbook of Conducting Polymers, Vol. l, Marcel Dekker, New York, 1986, p. 293. [21] M. Ogasawara, K. Funahashi, T. Demura, T. Hagiwara, K. Iwata, Synth. Met. 14 (1986) 61. [22] Y. Nakazawa, T. Ebine, M. Kusunold, H. Nishizawa, J. Hanna, H. Kokado, Jpn. J. Appl. Phys. 27 (1988) 1304.
[23] M. Fukuyama, N. Nanai, T. Kojima, Y. Kudoh, S. Yshimura, Synth. Met. 58 (1993) 367. [24] Nippon Kagakukai (ed.), Kagaku Benran (HandbookofChemistry), Maruzen, Tokyo, 1984, p. 11-364 (in Japanese). [25] R.T. Morrison, R.N. Boyd, Organic Chemistry, Prentice-Hall, Englewood Cliffs, NJ, 6th edn., 1992, p. 1064.