Alkanethiol Structure and Supporting Electrolyte Effects on the

S. Abraham John, Fusao Kitamura, Koichi Tokuda, and. Takeo Ohsaka*. Department of Electronic Chemistry, Interdisciplinary. Graduate School of Science ...
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Langmuir 2000, 16, 876-880

Alkanethiol Structure and Supporting Electrolyte Effects on the Electrochemical and in Situ Fourier Transform IR Spectral Properties of Asymmetric Alkyl Viologen on the Electrode Surface

The asymmetric viologen (bromide salt) N-ethyl-N′-octadecyl viologen (1) was synthesized and characterized by the reported procedure.4 n-Octanedithiol (n-OdiT) was purchased from Aldrich, and n-octanethiol (n-OT), tert-octanethiol (t-OT), and tertdodecanethiol (t-DDT) (99%) were purchased from Wako Pure Chemicals Co. and used as received. The alkanethiol monolayers

S. Abraham John, Fusao Kitamura, Koichi Tokuda, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Received June 16, 1999. In Final Form: September 21, 1999

Introduction Recently we have studied the electrochemical behavior of an asymmetric alkyl viologen (viologen with different alkyl substituents in the two N atoms), N-ethyl-N′octadecyl viologen (1), confined on the self-assembled monolayers (SAMs) of n-alkanethiols (CH3(CH2)nSH; n ) 3-17) coated on Au electrodes in the presence of different supporting electrolytes.1,2 The 1 confined on n-alkanethiolcoated electrodes (n ) 3-17) showed a broad redox wave in the presence of Cl-, NO3-, SO42-, and ClO4- ions. On the other hand, the electrochemical behavior of 1 in the presence of PF6- ion largely depended on the alkyl chain length of the thiols. When 1 was confined on the SAMs of alkanethiols of n > 9, it showed no redox response. On the other hand, it showed a redox response at the alkanethiols of n < 9 coated Au electrodes. The observed difference in the electrochemical behavior of 1 in the presence of PF6- ion has been interpreted from the difference in the structures of alkanethiols and the hydrophobicity of PF6- ion.1,2 In the present investigation we have used SAMs of tert-thiols and dithiol-coated Au electrodes to study the electrochemical behavior of 1 in the presence of PF6- ion with reference to the n-alkanethiol-coated electrodes. In addition, we report interesting electrolyte anion dependent in situ FT-IR spectral properties of 1 on tert-thiol-coated Au electrode. Experimental Section The polycrystalline Au electrode (2 mm diameter) surface was polished to a mirror finish with fine emery paper (Sankyo Rikagaku Co., Japan) and then aqueous slurries of successively finer alumina powder (1 and 0.06 µm) on a polishing microcloth, sonicated for 10 min in water. The Au electrode was then electropolished by potential cycling in 0.05 M H2SO4 in the potential range of -0.2 to +1.5 V at the potential scan rate of 100 mV s-1 for 20 min or until the cyclic voltammetric characteristic for a clean Au electrode was obtained. Such an electrode was considered as a bare Au electrode. The real surface area was calculated from the charge required to reduce the surface oxide layer using the previously established formula,3 0.43 mC cm-2. The geometric area of the Au electrode was calculated using the diameter of the employed Au electrode. The surface roughness was calculated as the ratio of the real surface area to the geometric area, and it was found to be 1.2. * Corresponding author. E-mail: [email protected]. Fax: +81-45-924-5489. Tel: +81-45-924-5404. (1) John, S. A.; Kitamura, F.; Nanbu, N.; Tokuda, K.; Ohsaka, T. Langmuir 1999, 15, 3816. (2) John, S. A.; Ohsaka, T. J. Electroanal. Chem., in press. (b) John, S. A.; Kitamura, F.; Tokuda, K.; Ohsaka, T. Manuscript in preparation. (3) Gileadi, E.; Eisner, K. E.; Penciner, J. Interfacial ElectrochemistrysSAn Experimental Approach; Addison-Wesley: Reading, MA, 1975.

were formed by immersing the Au electrode into a 1-10 mM ethanol solution of each alkanethiol for 1-2 h. The electrode was then washed with ethanol and water and dried in air. Then the electrode was immersed in an aqueous solution of 1 for 30-90 min and transferred to the supporting electrolyte for electrochemical measurements. The electrochemical instrumentation has already been described.1 The surface coverage (Γ/mol cm-2) was estimated by graphically integrating the cyclic voltammogram recorded at various scan rates and corrected with a surface roughness factor. The calculated Γ values of 1 are in the range of (2.2-2.8) × 10-10 mol cm-2. All solutions were thoroughly deoxygenated by purging with nitrogen gas, and during the electrochemical experiments a nitrogen atmosphere was maintained above the solution. For in situ Fourier transfer infrared reflection adsorption spectroscopy (FT-IRRAS) studies, the Au disk electrode (0.78 cm2 area) on which the 1-confined SAMs of the thiols were coated was used as the working electrode. A platinum wire and Ag/AgCl (NaCl sat.) were used as the counter and the reference electrodes, respectively. The FT-IRRAS measurements were carried out in a mode of subtractively normalized interfacial FT-IR spectroscopy (SNIFTIRS) with a FTS-7T Fourier transform infrared spectrometer (Bio-Rad Laboratories) equipped with a wide-band MCT detector. A total of 320 scans were collected and averaged for each spectrum. The angle of incidence at the air/CaF2 window was set to 60°. Light polarization was controlled by a wire-grid polarizer. All the measurements were performed at room temperature.

Results and Discussion Characterization of SAMs of Alkanethiols and 1-Confined SAMs of Alkanethiols on Au Electrodes by Cyclic Voltammetry. Cyclic voltammetric blocking experiments using ferrocyanide redox marker offered useful information about the electron-transfer barrier properties of SAMs of alkanethiol.5-7 Since the electron transfer between a solution species and the electrode must occur either by tunneling through the monolayer or by approaching the electrode via a pinhole or defect in the monolayer, the extent of surface passivation to electron transfer is useful to detect the defects in the monolayer. (4) Gomez, M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797. (5) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol.19, p 109. (6) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (7) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (b) Reese, S.; Fox, M. A. J. Phys. Chem. B 1998, 102, 9820.

10.1021/la990775p CCC: $19.00 © 2000 American Chemical Society Published on Web 11/16/1999

Notes

Figure 1. (A) CVs obtained for (a) bare and (b) n-OT-, (c) n-OdiT-, (d) t-OT-, and (e) t-DDT-coated Au electrodes. (B) CVs obtained for 1-confined SAMs of (f) n-OT, (g) n-OdiT, and (h) t-DDT coated on Au electrodes in 1 mM K4[Fe(CN)6] solution containing 0.5 M KCl. Scan rate ) 0.1 V s-1.

For a typical impermeable monolayer, the faradaic current should be completely attenuated, and the only current response to the applied voltage is due to capacitive charging of the electrode. Figure 1A shows the cyclic voltammograms (CVs) of bare Au electrode and the Au electrodes modified with SAMs of n-octanethiol (n-OT), n-octanedithiol (n-OdiT), tert-octanethiol (t-OT), and tertdodecanethiol (t-DDT) in an aqueous solution containing 1 mM K4[Fe(CN)6] and 0.5 M KCl. A well-defined CV, characteristic of a diffusion-limited redox process was observed at the bare Au electrode (Figure 1A(a)). The SAM of n-OT coated on a Au electrode efficiently blocked the electrode reaction of ferrocyanide (Figure 1A(b)), in agreement with previous studies and indicating that the monolayer scarcely contains defects.6 In the cases of the n-OdiT- and t-DDT-coated electrodes, an increase in the peak current was observed at potentials higher than 0.2 V (Figure 1A(c) and (e)). On the other hand, the CV recorded at the t-OT-coated Au electrode showed a welldefined redox reaction of ferrocyanide, indicating that the

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t-OT SAM contains more defects or vacant sites in the monolayer (Figure 1A(d)) when compared to the SAMs of n-OdiT- and t-DDT-coated electrodes. The barrier property of the t-OT-coated Au electrode is characteristic of shortchain alkanethiol monolayers and is indicative of low packing density and coverage.6 Recently, it has been reported that the SAMs of dithiols are lying with their axis parallel to the electrode surface and contain defects in the monolayer.8,9 The dominant form of defect is likely a vacancy, and thus the SAM of an ODiT-coated Au electrode is less resistant toward ferrocyanide.9 In the case of the t-DDT SAM, we believe that the presence of the long alkyl chain after the tertiary group may lead to form a densely packed monolayer, and thus it efficiently blocked the electron-transfer reaction of ferrocyanide when compared to the t-OT-coated Au electrode. On the basis of the CV data, t-OT forms the most permeable SAM, and the SAMs of n-OdiT, t-DDT, and n-OT become less permeable in that order. Figure 1B shows the CVs obtained for the SAMs of n-OT, n-OdiT, and t-DDT after being immersed in an aqueous solution of 1 (100 mM) for 45 min and then transferred to the 1 mM solution of K4[Fe(CN)6] containing 0.5 M KCl. When the CV was recorded for the 1-confined SAM of n-OT, a small increase in the faradaic current was observed (Figure 1B(f)) when compared to the pure SAM of n-OT coated on Au electrode. On the other hand, the ferrocyanide response was greatly increased at the 1-confined SAMs of n-OdiT and t-DDT (Figure 1B(g) and (h)) when compared to the pure SAMs of n-OdiT- and t-DDT coated on Au electrodes. The observed increase in the response of ferrocyanide clearly indicates that the 1-confined SAMs of n-OdiT and t-DDT are greatly disordered and contain defects in their monolayers. Thus the electron-transfer reaction of ferrocyanide occurred at the Au electrode coated with SAMs of n-OdiT and t-DDT. It is concluded from the observed CVs that the 1-confined SAMs of t-ODT and n-OdiT are highly disordered and less resistant toward ferrocyanide, while only a slight disorder is expected for the 1-confined SAM of n-OT and thus almost completely blocked the electron-transfer reaction of ferrocyanide. In a separate experiment, we have calculated the amount of 1 confined on the SAMs of n-OT, n-OdiT, and t-DDT coated on Au electrodes by recording the CVs in the presence of 0.1 M KCl. It was found that only a marginal change in the Γ values of 1 was observed, indicating that the disorder of the SAMs is not related to the amount of 1 confined on the SAM-coated Au electrodes. The cause for the disorder is mainly due to the difference in the structure and packing of the SAMs on Au electrodes. Electrochemical Behavior of 1 Confined on SAMs of n-OT, n-OdiT, t-OT, and t-DDT on Au Electrodes. Since the electrochemical behavior of 1 on n-alkanethiolcoated electrodes in the presence of PF6- ion is different from that of other anions such as Cl-, NO3-, ClO4-, and SO42-,1,2 we are interested in examining the electrochemical behavior of 1 confined on the SAMs of n-OdiT and t-OT with respect to PF6- ion. Several significant features were observed for the 1 confined on the SAMs of n-OdiT and t-OT when compared to 1 confined on the SAM of n-OT in 0.1 M NH4PF6. The 1 confined on the SAMs of n-OdiT and t-OT was reduced at less negative potential (-0.505 V) than that on the SAM of n-OT (-0.533 V) (not shown). In addition, the calculated Γox/Γred ratio of 1 (8) Kobayashi, K.; Horiuchi, T.; Yamada, H.; Matsushig, K. Thin Solid Films 1998, 331, 210. (9) Kohli, P.; Taylor, K. K.; Harris, J. J.; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962.

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Figure 2. CVs obtained for 1 confined on t-DDT-coated electrodes in 0.1 M of (a) KCl, (b) NaClO4, and (c) NH4PF6. Scan rate ) 0.2 V s-1.

confined on the SAMs of n-OdiT and t-OT (0.78) was greater than that of 1 on the SAM of n-OT (0.50). The observed less negative reduction potential and greater Γox/Γred ratio of 1 confined on the SAMs of n-OdiT and t-OT suggest that 1 confined on these SAMs are more permeable to water molecules than 1 confined on the SAM of n-OT. The observed result is consistent with the weak barrier property of the 1-confined n-OdiT and t-OT SAMs toward ferrocyanide, where the redox reaction of ferrocyanide was not completely blocked (vide supra). The CVs obtained for the assembly of 1 on the SAM of t-DDT in the presence of 0.1 M of KCl, NaClO4, and NH4PF6 are shown in Figure 2. In KCl and NaClO4, the assembly of 1 showed broad redox waves at -0.43 and -0.47 V, respectively, with equal Γox and Γred (Figure 2a,b). In t-DDT-coated Au electrode the redox moiety (bipm2+) of 1 is assembled close to the electrode surface and not to the exterior of the thiol monolayer. This was supported by our own exsitu FT-IR data of 1 on n-alkanethiol-coated Au electrodes2 and the earlier FT-IR data reported for the mixed SAM of hexadecanethiol and N-octadecyl-4,4′-bipyridinium dibromide.10 In the presence of PF6- ion, 1 confined on the t-DDT-coated electrode showed sharp reduction and oxidation peaks at -0.518 and -0.445 V, respectively, and Γox was smaller than that of Γred. On the other hand, the 1 confined on the n-DDT-coated electrode showed no redox response in the presence of PF6- ion.1,2 This indicates that the 1-confined n-DDT SAM is less permeable to water molecules than the 1-confined t-DDT SAM owing to the densely packed structure of n-DDT compared with t-DDT. The sharp and more negative reduction potential of 1 in the presence of PF6- ion indicates that PF6- strongly interacts with the bipm2+ moiety of 1 when compared to ClO4- and Cl- ions. When the hydrophilic bipm2+ moiety of 1 was reduced to hydrophobic bipm+• moiety of 1, water molecules are lost from the monolayer. Under this condition, the PF6- ions may form an insoluble salt with bipm+• moiety of 1 and became electroinactive.11 Thus, the Γox of 1 is less than that of Γred in the PF6- ion (Figure 2c). This is confirmed from the observed increase in Γox (10) Lee, K. A. B.; Mowry, R.; McLennan, G.; Finklea, H. O. J. Electroanal. Chem. 1988, 246, 217. (11) Mortimer, R. J.; Anson, F. C. J. Electroanal. Chem. 1982, 138, 325.

Notes

Figure 3. Potential difference SNIFTIR spectra obtained for 1 confined on t-DDT-coated Au electrode in 0.1 M NaClO4. Sampling potentials are indicated in the figure. Reference potential ) 0 V vs Ag/AgCl (NaCl sat.).

when the same electrode used in PF6- was transferred to Cl- or ClO4- ions (not shown). On the other hand, bipm+•Cl- and bipm+•ClO4- salts are highly soluble and therefore Γox is equal to Γred (Figure 2a,b). The observed different redox behavior of 1 in the presence of PF6- ion compared with Cl- and ClO4- ions was further studied by in situ FT-IR spectral studies (vide infra). SNIFTIR Spectral Studies of 1 Confined on t-DDTCoated Electrodes in the Presence of KCl, NaClO4, and NH4PF6. One of the characteristic features of the radical cation of viologen (bipm+•) is the tendency to dimerize in aqueous media.12 The tendency for the formation of a dimer could result from the hydrophobic nature of the radical cation compared with the parent dication. In situ UV-vis,12 electron spin resonance,13 Raman,14,15 and IR16-20 spectral techniques have been used to investigate the formation of dimer. In the FT-IR studies, the dimerization was strongly supported by observing the abnormally strong intensity of the fundamental, totally symmetric ring modes (which should be forbidden in the IR).14 Figure 3 shows the series of potential difference SNIFTIR spectra obtained for the 1 (surface coverage of 2.74 × 10-10 mol cm-2) confined on a t-DDT-coated electrode in the presence of 0.1 M NaClO4. At the applied potentials of -0.20 and -0.40 V, where 1 is in the oxidized state, no clearly defined peaks were obtained (data not shown). However, at an applied potential of -0.50 V, (12) Kosower, E. M.; Cotter, J. L. J. Am. Chem., Soc. 1964, 86, 5524. (b) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49. (c) Monk, P. M. S. The Viologens: Physiochemical Properties, Synthesis and Applications of the Salts of 4, 4′-bipyridine; Wiley: Chichester, 1998. (13) Evans, A. G.; Evans, J. C.; Baker, M. W. J. Chem. Soc., Perkin Trans. 2 1977, 1787. (14) Hester, R. E.; Suzuki, S. J. Phys. Chem. 1982, 86, 4626. (15) (a) Lu, T.; Cotton, T. M.; Hurst, J. K.; Thompson, D. H. J. Phys. Chem. 1988, 92, 6978. (b) Ghoshal, S.; Lu, T.; Feng, Q.; Cotton, T. M. Spectrochim. Acta, Part A 1988, 44, 651. (16) Ito, M.; Sasaki, H.; Takahashi, M. J. Phys. Chem. 1987, 91, 3932. (17) Christensen, P. A.; Hamnett, A. J. Electroanal. Chem. 1989, 263, 49. (18) Brienne, S. H. R.; Cooney, R. P.; Bowmaker, G. A. J. Chem. Soc., Faraday Trans. 1991, 87, 1355. (19) Osawa, M.; Yoshi, K. Appl. Spectrosc. 1997, 51, 512. (20) Bae, I. T.; Huang, H.; Yeager, E. B.; Scherson, D. A. Langmuir 1991, 7, 1558.

Notes

Figure 4. Potential difference SNIFTIR spectra obtained for 1 confined on t-DDT-coated Au electrode in 0.1 M NH4PF6. Sampling potentials are indicated in the figure. Reference potential ) 0 V vs Ag/AgCl (NaCl sat.).

several well-defined negative going bands were observed in the region of 1100-1700 cm-1, i.e., sharp intense bands at 1188, 1335, 1510, and 1598 cm-1 and a less intense band at 1166 cm-1. When the potential was switched to -0.60 V, the bands of 1598, 1188, and 1166 cm-1 were increased and a new sharp band was observed at 1627 cm-1. The bands observed at 1627 and 1188 cm-1 were assigned to B2u modes.15 The other bands were ascribed to Ag modes, dominated by C-N stretches.15,19 The bands observed at 1166, 1335, 1510, and 1598 cm-1 were assigned to the dimer of the radical cation of 1, while the bands at 1188 and 1627 cm-1 were assigned to the radical cation of 1.17-20 Similar spectra were also observed for methyl and heptyl viologens (MV2+ and HV2+). It has been shown that HV2+ on the Ag electrode shows bands at 1635, 1596, 1508, 1460, 1335, 1240, 1183, 1165, and 1025 cm-1 at about -0.50 V.19 The bands observed at 1635 and 1183 cm-1 bands have been attributed to the monomeric radical, whereas other bands have been assigned to the radical dimer with a plane-to-plane configuration linked by π bonding.19 Similar spectral features have also been reported for the MV2+ solution species on Pt electrode.17 The observed spectral features in Figure 3 at an applied potential of -0.60 V are comparable to those reported for the electrogenerated radical cations of MV2+ and HV2+, though there is some shift in the bands.17,19 Thus we assign the bands at 1627 and 1188 cm-1 to the monomeric radical cation, while the other bands are assigned to the radical cation dimer. The dimeric bands are assigned to totally symmetric (Ag) ring modes under an assumption of D2h molecular symmetry.19 Figure 4 shows similar potential difference SNIFTIR spectra obtained for the assembly of 1 on the t-DDT-coated electrode in 0.1 M NH4PF6. No spectral features were observed at applied potentials of -0.20 and -0.40 V. Surprisingly a single band at 1197 cm-1 was observed at -0.50 V in the presence of PF6- ion, in contrast to several bands observed in ClO4- and Cl- ions. When the potential was further switched to -0.60 V, the intensity of the 1197 cm-1 band was increased. In addition, an additional band appeared at 1653 cm-1. The observed bands of 1197 and 1653 cm-1 were assigned to the monomeric radical cation

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of 1. The absence of the bands characteristic of the dimer in the presence of PF6- ion strongly demonstrates that PF6- ion does not favor the dimerization of radical cation of 1 on the t-DDT-coated Au electrode. The observed different spectral features of radical cation of 1 in the presence of Cl-, ClO4-, and PF6- anions are explained by the difference in the hydrophobicity of these ions. Since PF6- ion is more hydrophobic than ClO4- and Cl- ions, the bipm+•PF6- salt may experience a more hydrophobic environment on the electrode surface, for example, similar to that of nonaqueous solution at room temperature.12 It has been already shown that the radical cation of bipm+• dimerizes in an aqueous solution and also in nonaqueous solvent (methanol) at decreased temperatures.12,13 In the nonaqueous solvents such as ethanol and acetonitrile the viologen radical cation exists as a monomer rather than dimer at room temperature.12 The electrogenerated radical cation of HV2+ on Pt electrode has been reported in acetonitrile and water using the potential difference FTIR spectroscopic technique.18 The electrogenerated radical cation of HV2+ in water shows the spectral features characteristic of the dimer at 1168, 1340, 1512, and 1602 cm-1 along with the bands of the monomer at 1189 and 1636 cm-1.18 On the other hand, in acetonitrile the bands ascribed to the dimer were absent and the spectral features corresponding to the monomer were only observed at 1180 and 1636 cm-1.18 Thus we propose that the bipm+• moiety of 1 in PF6- ion experiences a nonaqueous environment similar to acetonitrile and shows the spectral features characteristic of the monomer (Figure 4) as reported for the radical cation of HV2+ in acetonitrile, while in the presence of Cl- and ClO4- ions, it experiences a hydrophilic environment similar to water and shows the spectral features of both the dimer and the monomer (Figure 3) as reported for the radical cation of HV2+ in water.18 Recently, we have studied the potential difference SNIFTIR spectra for the SAM of 1 on the Au electrode in the presence of PF6- ion and observed monomer and dimer spectral features.1 Since the SAM structure of 1 contains defects and pinholes in the monolayer,2 we believe that the bipm+• moiety of 1 experiences a polar environment in the monolayer and showed monomer-dimer spectral features in the PF6- ion. On the other hand, the 1-confined t-DDT SAM contains less defects and pinholes (vide supra). Therefore, the bipm+• moiety of 1 experiences a less polar environment and showed purely monomer spectral features on the SAM of t-DDT in PF6- ion. Conclusions The present work demonstrates that the SAMs of alkanethiols in which 1 is confined and electrolyte ions strongly influence the electrochemical and SNIFTIR spectral properties of 1. The n-OT forms a more densely packed monolayer than n-ODiT-, t-OT, and t-DDT, which was verified from its efficient blocking property toward ferrocyanide. Further the electrochemical behavior of 1 was also studied in the presence of Cl-, ClO4-, and PF6ions. The 1-confined t-DDT SAM electrode showed a broad redox wave with equal Γox and Γred values in the presence of Cl- and ClO4- ions, while it showed a sharp redox wave with a difference in the Γox and Γred values in the presence of PF6- ion. In the potential difference SNIFTIR spectral studies, the electrogenerated radical cation of 1 confined on the t-DDT-coated electrode showed the spectral features of both the monomer and the dimer in the presence of Cland ClO4- ions. On the other hand, only monomer spectral features were observed in the presence of PF6- ion. The observed difference in the spectral features for the 1 in the presence of Cl-, ClO4-, and PF6- ions was explained

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by the difference in the hydrophobicities of these anions. In the presence of Cl- and ClO4- ions, the bipm+• moiety of 1 experienced a hydrophilic environment (similar to water) and showed the monomer and the dimer spectral features. On the other hand, the bipm+• moiety of 1 experienced a hydrophobic environment (similar to acetonitrile) in the presence of PF6- ion and showed the monomer spectral features. We have demonstrated electrolyte anion dependent FT-IR spectral properties of viologen on the electrode surface for the first time.

Notes

Acknowledgment. The present work was financially supported by Grants-in-Aid for Scientific Research in Priority Areas “New Polymers and Their Nano-Organized Systems” (No.277/10126219), “Exploratory Research” (No.09875207), and “Scientific Research (A)” (No.10305064) from the Ministry of Education, Science, Sports and Culture, Japan (Monbusho). S.A.J. thanks Monbusho for the fellowship. LA990775P