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Deprotonation-Induced Efficient Delocalization of Polaron in SelfDoped Poly(anilinephosphonic acid) Toru Amaya,*,† Yasushi Abe,†,‡ and Toshikazu Hirao*,† †
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan Osaka R&D Laboratory, Daihachi Chemical Industry Co., Ltd., 3-5-7 Chodo, Higashiosaka, Osaka 577-0056, Japan
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polyaniline was demonstrated by Epstein et al. in 1990.5 The strong self-doping remains the doping state even in the presence of a base. Highly sulfonated polyaniline is reported to retain the conductivity under basic conditions (up to pH 12).6 Another advantage of self-doped polyaniline is high solubility in water. Conformational change of polyaniline sulfonic acid, poly(2-methoxyaniline-5-sulfonic acid) (PMAS) (Figure 1b),7 has been reported by Kane-Maguire, Wallace, and co-workers, where the conformational change from extended coil to compact one is induced as the pH of the PMAS solution increases.8 We also reported the redox-induced reversible conformational switching of PMAS.9 On the other hand, we have successfully synthesized a selfdoped conducting polyaniline with phosphonic acid and phosphonic acid monoethyl ester directly attached to the backbone, poly(2-methoxyaniline-5-phosphonic acid) (PMAP)10,11 and poly(2-methoxyaniline-5-phosphonic acid monoethyl ester) (PMAPE), respectively (Figure 2).12 These
INTRODUCTION Polyanilines are widely regarded as one of prominent conductive polymers because of their availability and practical applicability.1,2 Electrical conductivity of polyaniline is induced by doping with simple protonation of polyaniline emeraldine base, where a conductive polysemiquinone radical cationic (polaronic) state is considered to be formed via reorganization of the electronic structure (Figure 1a).1 The conductivity
Figure 2. Structures of PMAP and PMAPE and this work.
are the first examples of polyaniline where phosphonic acid or phosphonic acid monoester directly attaches to the backbone. Differing from sulfonic acid, two acidic protons are available in phosphonic acid. The second acid moiety not used for doping would provide the features such as tolerance to base and acid/ base complexation (or salt formation). The complexation (or salt formation) would effect to the conformation of the main chain. Conformation of polyanilines is reflected to the absorption spectra.3 The diagnostic peaks are polaron band
Figure 1. (a) Doping of polyaniline (emeraldine base). (b) Poly(2methoxyaniline-5-sulfonic acid) (PMAS).
strongly depends on the polymer main-chain conformation of the acid-doped emeraldine salt of polyaniline.3 The acid-doped emeraldine salt of polyaniline taking the extended coil conformation shows much larger conductivity compared to that taking the compact coil conformation.3 On the other hand, polyanilines are not able to exhibit the sufficient conductivity if acid doping is not enough. A covalently attached acid moiety on the polyaniline backbone can dope itself without an external dopant. As such self-doped conducting polyanilines,4 sulfonated © XXXX American Chemical Society
Received: August 6, 2014 Revised: October 28, 2014
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Figure 3. UV−vis−NIR spectra of PMAP/pyridine (1:1) (4.9 × 10−4 M aqueous solution) in the range (a) pH 5.95−8.98 and (b) pH 8.98−13.03, and PMAP (4.9 × 10−4 M aqueous solution) in the range (c) pH 7.12−10.02 and (d) pH 10.02−12.97. Concentration is calculated based on the aniline unit. tion spectra were recorded on a JASCO V-670 spectrometer. Acidity of solutions was recorded on a HORIBA D-51 pH meter. Variable pH UV−Vis−NIR Absorption Spectral Experiments. PMAP (9.96 mg, 0.049 mmol for the experiment in Figure 3, parts a and b; 10.10 mg, 0.049 mmol for the experiment in Figure 3, parts c and d), or PMAPE (10.02 mg, 0.044 mmol for the experiment in Figure 5a, 10.10 mg, 0.044 mmol for the experiment in Figure 5b), and 1 M aqueous pyridine solution (49 μL, 0.049 mmol for the experiments in Figure 3, parts a and b; 22 μL, 0.022 mmol for the experiment in Figure 5a) were diluted with water in measuring flask to 100 mL total. The solution was transferred into a beaker with a stirrer bar. A pH meter was immersed in the solution. To the stirred solution, 1 M aqueous NaOH solution (ca. 15 μL) was added until the acidity of the solution became around pH 6.0 for the experiments in Figures 3a,b and 5a, and around pH 7.0 for the experiments in Figures 3c,d and 5b. Approximately 3 mL of the solution was transferred to a cuvette (1 cm path length), and the absorption spectrum of the solution was measured at 25 °C. The solution in the cuvette was returned to the beaker. Then, 1 M aqueous NaOH solution was added, and the absorption spectrum of the solution was again measured at 25 °C. This sequence was repeated until around pH 13 in increments of about 0.5. After pH 12, amounts of the added 1 M aqueous NaOH solution were several milliliters; therefore, the effect of the dilution with 1 M aqueous NaOH solution is relatively large in a range of pH 12 to 13. Electrical Conductivity Measurements. The indium tin oxide (ITO) having a gap (width, 200 μm; height, 150 nm)/glass was ultrasonically washed with solvents in order of neutral detergent, water, acetone, ethanol, and desalted water twice for each process. It was stored in desalted water. Before usage, it was dried by an air
(around 440 nm) and further localized polaron band (around 780 nm), which corresponds to extended and compact coil conformations, respectively.3 Steadily increasing free carrier tail from 1000 nm arises from delocalization of electrons in the polaron band promoted by a straightening-out of the polymer chain (extended coil), which can be used as a diagnosis for metallic conductive materials.3,13 In this context, the present study was undertaken to investigate pH-dependency (from acidic to basic) of PMAP and PMAPE in view of their conformation and delocalization of electrons by following these peaks (Figure 2). Herein, we report the characteristic pHdependent behavior on the absorption spectra based on the difference between PMAP and PMAPE, where we encountered the unusual phenomena that the absorption for free carrier tail in PMAP increased with the increasing of the pH in the solution. Especially, it is worthy of remark that considerable degree of the free carrier tail is remained in PMAP under basic conditions up to pH around 11. To the best of our knowledge, such deprotonation-induced efficient delocalization of polaron has never been observed in polyanilines, not even in the related self-doped polyaniline with phosphonic acid attached on the backbone through methylene unit.14
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EXPERIMENTAL SECTION
General Data. Preparation of PMAP and PMAPE is reported in our previous papers.10−12 Milli-Q water was used in the experiments using UV−vis−NIR absorption spectroscopy. UV−vis−NIR absorpB
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duster. The aqueous PMAP/pyridine (1:1) solution (1 wt % based on PMAP) was dropped onto the ITO/glass substrate, which was dried with hot air from a dryer. The measurements of the electrical resistance for a two-probe method were conducted using a circuit tester (CUSTOM, CX-180N) under environmental conditions.
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RESULTS AND DISCUSSION A 1 M aqueous NaOH solution was added to the acidic solution of PMAP, and the spectral change was followed by UV−vis−NIR spectroscopy. Parts a and b of Figure 3 show the variable pH spectra for the aqueous solution of PMAP/pyridine (1:1 based on the aniline unit). Here, pyridine was added to improve the solubility because PMAP itself shows poor solubility in water. PMAP was present as an extended coil conformation of the emeraldine salt at pH 5.95 because typical polaron band and free carrier tail were observed at 430 nm and >1000 nm, respectively (Figure 3a), where the color of the solution was brown. In a range of pH 5.95 to 8.98, the peak for polaron band decreased, but the peak for free carrier tail increased (Figure 3a), where the color changed from brown to green. The broad peak at around 700 nm, which is likely to be further localized polaron band, appeared as the pH became basic (Figure 3a). Decreasing of the polaron band and appearance of the further localized polaron band may be accounted for by partial conformational change from extended coil to compact coil of the emeraldine salt, as reported in PMAS.8 Increasing of the free carrier tail with increasing of the pH value is unusual in the conducting polyanilines because dedoping usually occurs with increasing of pH value.1 In the case of PMAS, the free carrier tail decreases monotonically by the conformational change from extended coil to compact one of emeraldine salt as the pH value increases, although the dedoping does not occur due to self-doping.8 In contrast, the increasing of the free carrier tail of PMAP continued even though the pH reaches basic region (up to pH around 9.50). This unusual observation should be explained by conformational straightening of the main chain due to the repulsion between the negatively charged phosphonate anions which do not involve doping (Figure 4). In the study of the related self-
Figure 5. UV−vis-NIR spectra of PMAPE/pyridine (1:0.5) (4.4 × 10−4 M aqueous solution) in the range (a) pH 6.20−13.00 and PMAPE (4.4 × 10−4 M aqueous solution) in the range (b) pH 7.07− 12.94. Concentration is calculated based on the aniline unit.
around 700 nm. The former peak is considered to be assigned to be charge transfer (CT) transition from benzenoid to quinoid moieties in polyaniline main chain,6,15 showing dedoping. The color of the solution changed from green to purple. To investigate the effect of pyridine, the similar experiments were carried out without pyridine. Because of the low solubility of PMAP in acidic aqueous solution, these experiments were performed from pH 7.12 to 12.97. As a result, similar spectral change was observed (Figure 3, parts c and d). The free carrier tail continued to increase until pH 10.02, and kept a high intensity until pH around 11. Thus, the resistance to base in the absence of pyridine became slightly stronger than that in the presence of pyridine. The pH dependence of the electrical conductivity was investigated for the drop-casted films of PMAP/pyridine (1:1). The film formed with the pH 5.61 aqueous solution showed 9 S m−1. Increasing of the basicity (pH >7.32) caused the severe decreasing of the conductivity to be off the scale. Decreasing of the carriers (consistent with the decreasing the polaron band around 430 nm, Figure 1) and/or inefficient carrier hopping between the interchains in the bulk conditions might explain these results.
Figure 4. Proposal for the mechanism of efficient delocalization of polaron under basic conditions.
doped polyaniline with phosphonic acid attached to the backbone through methylene unit, absorption spectra of the sodium salts were reported by Chan et al.14 They described the remaining of slight polaronic character, but NIR region was not followed. Anyway, the behavior on increasing of free carrier tail by treatment with base was not described in the Chan’s report. The free carrier tail of PMAP started decreasing from pH around 9.50 (Figure 3b), but kept high intensity until pH around 10.5. Even at pH around 11, the free carrier tail was still present with almost 60% intensity of the maximum value at 1200 nm (Figure 3b). The polaron band around 430 nm also decreased in the range pH 8.98−13.03. In this range, the broad peak around 550 nm appeared with disappearance of the peak C
dx.doi.org/10.1021/ma5016209 | Macromolecules XXXX, XXX, XXX−XXX
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(7) Shimizu, S.; Saitoh, T.; Uzawa, M.; Yuasa, M.; Yano, K.; Maruyama, T.; Watanabe, K. Synth. Met. 1997, 85, 1337. (8) Strounina, E. V.; Shepherd, R.; Kane-Maguire, L. A. P.; Wallace, G. G. Synth. Met. 2003, 135−136, 289. (9) Amaya, T.; Saio, D.; Koga, S.; Hirao, T. Macromolecules 2010, 43, 1175. (10) Amaya, T.; Abe, Y.; Inada, Y.; Hirao, T. Tetrahedron Lett. 2014, 55, 3976. (11) Abe, Y.; Amaya, T.; Hirao, T. Bull. Chem. Soc. Jpn. 2014, 87, 1026. (12) Amaya, T.; Abe, Y.; Inada, Y.; Hirao, T. Synth. Met. 2014, 195, 137. (13) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1994, 65, 103. (14) Chan, H. S. O.; Ho, P. K. H.; Ng, S. C.; Tan, B. T. G.; Tan, K. L. J. Am. Chem. Soc. 1995, 117, 8517. (15) Premvardhan, L. L.; Wachsmann-Hogiu, S.; Peteanu, L. A.; Yaron, D. J.; Wang, P.-C.; Wang, W.; MacDiarmid, A. G. J. Chem. Phys. 2001, 115, 4359.
Figure 5a shows the variable pH UV−vis−NIR spectra (pH 6.20−13.00) for PMAPE/pyridine (1:0.5 based on the monomer unit). The observed spectral change was very simple, which is in sharp contrast to that for PMAP/pyridine. PMAPE was present as an extended coil conformation of the emeraldine salt at pH 6.20 because typical polaron band and free carrier tail were observed at around 450 nm and >1000 nm, respectively. Both peaks for polaron band and free carrier tail decreased as the pH became basic (Figure 5a). Concomitantly, the broad peak at 710 nm, which is likely to be assigned to further localized polaron band, appeared and slightly increased until pH 8.13 (Figure 5a). Then, the peak at around 600 nm assignable to CT band between quinoid and benzenoid forms, increased. The final spectrum (pH 13.00) is typical for emeraldine base of polyaniline.6,15 This behavior suggests the dedoping of PMAPE via compact coil conformation of emeraldine salt. The color changed from greenish brown (pH ∼6.0) to green (pH ∼9.0), and blue-purple (pH ∼13.0). Effect of pyridine was also investigated by the similar experiments without pyridine. Because of the low solubility of PMAPE in acidic aqueous solution, these experiments were performed from pH 7.07 to 12.94. As a result, almost the same spectral change was observed (Figure 5b). This behavior is little affected with pyridine under the conditions.
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CONCLUSION In conclusion, we revealed that a self-doped polyaniline PMAP exhibits deprotonation-induced efficient delocalization of polaron. These results were obtained from the investigation of the pH-dependent behavior of PMAP and PMAPE using UV− vis−NIR spectroscopy. The observed behavior of PMAP includes increasing of the free carrier tail with increasing of the pH value to the basic region (up to pH around 9.50). This is unusual in the conducting polyanilines because dedoping usually occurs with increasing of pH value. In contrast to PMAP, PMAPE shows the expected spectral change as a proton doped conducting polyaniline which includes the monotonic decrease of the polaron band and free carrier tail with increase of pH value. The contrasting results between PMAP and PMAPE would be derived from the difference of di- or monoacid in phosphonate moiety. Their anionic repulsion may induce the main chain conformation more extended even under the basic conditions. These results would provide a valuable guide of polymer design for conductive polyanilines.
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AUTHOR INFORMATION
Corresponding Authors
*(T.A.) E-mail:
[email protected]. *(T.H.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581. (2) Lee, K.; Cho, S.; Park, S. H.; Heeger, A. J.; Lee, C.-W.; Lee, S.-H. Nature 2006, 441, 65. (3) Xia, Y.; Wiesinger, J. M.; MacDiarmid, A. G. Chem. Mater. 1995, 7, 443. (4) Freund, M. S.; Deore, B. Self-Doped Conducting Polymers; John Wiley & Sons, Ltd.: New York, 2007. (5) Yue, J.; Epstein, A. J. J. Am. Chem. Soc. 1990, 112, 2800. (6) Wei, X.-L.; Wang, Y. Z.; Long, S. M.; Bobeczko, C.; Epstein, A. J. J. Am. Chem. Soc. 1996, 118, 2545. D
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