Synthesis of self-doped conducting polyaniline bearing phosphonic acid

Article abstract of DOI:10.1016/j.tetlet.2014.04.115

As a self-doped conducting polyaniline bearing phosphonic acid, poly(2-methoxyaniline-5-phosphonic acid) (PMAP) was synthesized via oxidative polymerization of 2-methoxyaniline-5-phosphonic acid. The pyridinium salt of thus-obtained PMAP was water-soluble and its film exhibited conductivity.

Full text of DOI:10.1016/j.tetlet.2014.04.115

Accepted Manuscript
Synthesis of self-doped conducting polyaniline bearing phosphonic acid
Toru Amaya, Yasushi Abe, Yuhi Inada, Toshikazu Hirao
PII:
S0040-4039(14)00752-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.04.115
TETL 44577
Reference:
Tetrahedron Letters
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22 April 2014
30 April 2014
Please cite this article as: Amaya, T., Abe, Y., Inada, Y., Hirao, T., Synthesis of self-doped conducting polyaniline
bearing phosphonic acid, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.04.115
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Synthesis of self-doped conducting
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Toru Amaya, Yasushi Abe, Yuhi Inada, Toshikazu Hirao

1
Tetrahedron Letters
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Synthesis of self-doped conducting polyaniline bearing phosphonic acid
Toru Amayaa,*, Yasushi Abea,b, Yuhi Inadaa, Toshikazu Hiraoa,*
aDepartment of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan
bOsaka R&D Laboratory, Daihachi Chemical Industry Co.,LTD., 3-5-7 Chodo, Higashiosaka, Osaka 577-0056, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
As a self-doped conducting polyaniline bearing phosphonic acid, poly(2-methoxyaniline-5-
phosphonic acid) (PMAP) was synthesized via oxidative polymerization of 2-methoxyaniline-5-
phosphonic acid. The pyridinium salt of thus-obtained PMAP was water-soluble and its film
exhibited conductivity.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Conducting polymer
Polyaniline
Phosphonic acid
Self-doping
Water-soluble
1
2
3
4
5
6
7
8
9
Conducting polymers have attracted considerable interest due 33 possessing acid group. The latter should make the polymer
to their wide application for polymer-based electronics and 34 design more flexible.
biosensors.1 To exhibit their conductivity, the charge carriers
35
In the present study, we chose polyaniline phosphonic acid 1
must become mobile in the polymer chain. Therefore, the partial
oxidation or reduction of intrinsically conducting -conjugated
polymer is usually required to form the conductive state, and this
process is called doping.2 In contrast to such redox-mediating
doping, polyaniline (emeraldine base) can be doped by simple
protonation to lead to conductive polysemiquinone radical
36 as a synthetic target (Figure 1b). Differing from sulfonic acid,
37 two acidic protons are available in phosphonic acid. This feature
38 would provide a base resistance property or further acid/base
39 complexation. Alkylphosphonic acid is known to have an enough
40 acidity to dope the polyaniline (emeraldine base).9 So far, self-
10 cationic (polaronic) state via reorganization of the electronic
11 structure (Figure 1a).3
12
Organic conducting polymers that contain covalently bound,
13 charged, functional groups that impact the properties of the
14 polymer are referred to as self-doped conducting polymers.4 A
15 covalently attached acid group to the polyaniline backbone can
16 dope itself without an external dopant. As such self-doped
17 conducting
polyanilines,
sulfonated
polyaniline
was
18 demonstrated by Epstein et al. in 1990.5 On the other hand, there
19 is a great demand for highly water-soluble conducting polymers,
20 particularly in applications with processing using aqueous
21 solution or operated in biological environments.6 From this point
22 of view, self-doped polyaniline possessing a hydrophilic group is
23 also attractive. In our previous study, we have developed the
24 hybrid systems consisted of poly(2-methoxyaniline-5-sulfonic
25 acid) and transition metals,7 and reported their catalytic
26 application for the oxidation reaction in aqueous solution.7c-f
27 Despite such attractive features for self-doped polyaniline,
28 development of self-doped polyanilines is still limited, especially
29 most of them have a quite low electrical conductivity.8
30 Concerning the synthesis, there are mainly two strategies to
31 introduce an acid group. One is post-synthetic modifications of
32 polyaniline. The other is the polymerization of monomer
Figure 1. (a) Doping of polyaniline (emeraldine base). (b)
Polyaniline phosphonic acid 1.

2
Tetrahedron
41 doped polyaniline with phosphonic acid attached to the
42 backbone through methylene unit was synthesized.8g,h In our
43 designed polyaniline phosphonic acid, a directly attached acid
44 group to the backbone is expected to show more efficient self-
45 doping. Herein, we describe the synthesis of poly(2-
46 methoxyaniline-5-phosphonic acid) (PMAP, 1b) via oxidative
47 polymerization of 2-methoxyaniline-5-phosphonic acid. The
48 pyridinium salt of thus-obtained PMAP was water-soluble and its
49 film exhibited conductivity.
50
To synthesize 1a, oxidative polymerization of the monomer
51 2a10a or 2b10b was investigated using (NH4)2S2O8 as an oxidant
52 (Scheme 1a). However, the reaction conditions to afford the
53 conducting polymers were not found from both monomers
54 despite several attempts of the conditions. To improve the
55 reactivity for polymerization, monomer 6 possessing methoxy
56 group was designed. The synthetic scheme for 6 is shown in
57 Scheme 1b. Our developed Pd-catalysed coupling reaction was
58 employed to introduce phosphonate group.11 Commercially
59 available bromide 3 was coupled with HPO(OEt)2 in the presence
60 of Pd(OAc)2 (10mol%) to give diethylphosphonate 4 in 69%
61 yield. Hydrogenation of nitro group followed by acid-catalysed
62 hydrolysis gave the monomer 6 (2 steps 72% yield). The thus-
63 obtained monomer 6 was polymerized with (NH4)2S2O8 in 2.5 M
64 aqueous pyridine solution at -5 °C for 5 days (Scheme 1c). After
65 the treatment with 1 M aqueous HCl solution, the polymer 1b
66 was obtained in 53% yield as a black solid. The obtained polymer
67 1b has a weight average molecular weight (Mw) of 2000 and a
68 polydispersity index of 2.7, which were determined by GPC
69 analysis (see Figure S1 in Supplementary Material). Although
70 PMAP 1b did not exhibit a good solubility in an acidic to neutral
71 aqueous solution, it was soluble in a basic aqueous solution. The
72 pyridinium salt of PMAP 1b was also soluble in neutral water.
Scheme 1. (a) Polymerization of 2. (b) Synthesis of monomer 6.
(c) Synthesis of PMAP 1b.
73
Figure 2 shows UV-vis-NIR spectra for 1b. Aqueous solution
74 of the PMAP/pyridine complex (molar ratio = 1:1 based on the
75 monomer unit) exhibited pH 5.35, and its color was brown. This
76 complex has a maximum absorption at 281 and a shoulder peak
77 around 420 nm. The shoulder peak around 420 nm should be
78 attributed to a polaron band.12 Steadily increasing absorption
79 starting from around 1000 nm was also observed (free carrier
80 tail). Such free carrier tail is characteristic of doped conducting
81 polyanilines.12 On the other hand, the pH 8.98 aqueous solution
82 of the PMAP/pyridine complex (molar ratio = 1:1 based on the
83 monomer unit) has maximum absorptions at 283 and 683 nm,
84 and a shoulder peak was observed around 430 nm. The broad
85 absorption around 680 nm should be assigned as a further
86 localized polaron band.12 It is worth noting that the absorption
87 from around 1000 nm was also found in the weakly basic
88 solution.
Figure 2. UV-vis-NIR spectra for PMAP 1b at 298 K (4.9 x 10-4
M based on the monomer unit): PMAP/pyridine (1:1) in H2O (pH
5.35) (solid line) and in the pH 8.98 aqueous solution (dashed
line).
89
ESR spectrum of the aqueous solution of the PMAP/pyridine
90 complex (molar ratio = 1:2 based on the monomer unit) showed a
91 single resonance line centered on around g = 2.0036 without
92 hyperfine coupling in water (Figure 3). This is characteristic of
93 delocalized free radicals.13
94
The electrical resistance of the polymer film was measured by
95 the direct-current (DC) method (Table 1). The film was formed
96 by drop-casting the aqueous 1% (w/w) suspension or solution of
97 the pyridinium salt of PMAP 1b on a glass substrate, on which a
98 pair of indium tin oxide (ITO) electrodes with a gap (width: 200
99 m, height: 150 nm, see the figure in Table 1) were placed. An
100 aqueous suspension and solutions of PMAP 1b were prepared
101 with changing the molar ratio of pyridine to the PMAP 1b.
Figure 3. ESR spectrum for PMAP/pyridine (1:2) in H2O at 298 K
(4.9 x 10-2 M based on the monomer unit).
102 Addition of two molar equivalents of pyridine based on the 105 pyridinium salts of PMAP were ranged from 14 to k, which
103 monomer unit was required to prepare 1% (w/w) PMAP 1b 106 corresponds to 0.19 to 0.13 S/cm in terms of conductivity (Table
104 solution in water. The electrical resistances of the film for the 107 1, entries 1-3). The sheet resistance of the drop-cast film was

3
108 measured by a four-point probe method. The film was formed by 142 University for the conductivity measurements by the four-point
109 drop-casting the aqueous 2% (w/w) solution of the pyridinium 143 probe method, and Dr. H. Yamamoto and Prof. T. Kozawa at
110 salt of PMAP 1b (molar ratio = 1:2 based on the monomer unit) 144 Osaka University for the film thickness measurements. We also
111 on a glass substrate. The sheet resistance was 3.8 x 106 /□. The 145 thank Comprehensive Analysis Center of the Institute of
112 spin-coating film was also formed with the aqueous 2% (w/w) 146 Scientific and Industrial Research, Osaka University for the ICP-
113 solution of the pyridinium salt of PMAP 1b (molar ratio = 1:2 147 MS measurements and SEM analysis.
114 based on the monomer unit) on a glass substrate. The photo of
115 the film was shown in Figure 4. It was a transparent light brown
116 film. The thickness was 18 nm. The scanning electron
117 microscopy (SEM) images of the spin-coating film were shown
118 in Figure S3 in Supplementary Material. The uniformly formed
119 surface of the film was observed.
148 Supplementary Material
149
Supplementary data associated with this article can be found,
150 in the online version, at http://dx.doi.org/ .
151 References and notes
120 Table 1. Electrical resistance and conductivity for the drop-cast
121 films of the aqueous 1% (w/w) suspension or solution of the
122 pyridinium salts of PMAP 1b
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Electrical
resistance
(k)
Molar
equivalents of
pyridinea
Suspension or
solution
Conductivity
(S/cm)b
Entry
1
2
3
1
2
4
Suspension
Solution
Solution
14
20
20
0.19
0.13
0.13
123 aBased on the monomer unit. bConductivity was calculated on the assumption
124 that the gap (see the below figure) was filled with the polymer. Therefore, the
125 obtained value is the minimum value.
126
127
128
129
Figure 4. Photo of the spin-coating film formed with the aqueous
2% (w/w) solution of the pyridinium salt of PMAP 1b (molar ratio
= 1:2 based on the monomer unit) on a glass substrate (26 x 26 mm)
and its thickness.
130
9. Chan, H. S. O.; Ng, S. C.; Ho, P. K. H. Macromolecules 1994, 27,
2159-2164.
131
In conclusion, the self-doped conducting polyaniline PMAP
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2793. (b) Mucha, A.; Kunert, A.; Grembecka, J.; Pawełczak, M.;
Kafarski, P. Eur. J. Med. Chem. 2006, 41, 768-772.
11. (a) Hirao, T.; Matsunaga, T.; Ohshiro, Y.; Agawa, T. Synthesis
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132 1b was successfully synthesized via oxidative polymerization.
133 This is the first synthesis of the polyaniline bearing phosphonic
134 acid directly attached to the backbone. Self-doping of PMAP 1b
135 was clearly shown by UV-vis-NIR and ESR measurements. The
136 pyridinium salt of PMAP 1b was water-soluble and its film
137 exhibited conductivity. The electrical materials application of
138 PMAP is now underway.
139 Acknowledgments
140
We thank Dr. T. Suenobu and Prof. S. Fukuzumi at Osaka
141 University for the ESR measurements, Dr. R. Tsuji at Osaka