Insights into Adsorption of Uncharged Macrocyclic Complexes into a

Li-Hua Bi , Kevin Foster , Timothy McCormac , Eithne Dempsey. Journal of Electroanalytical Chemistry 2007 605 (1), 24-30. Article Options. PDF (99 KB)...
0 downloads 0 Views 99KB Size
Download PDF
J. Phys. Chem. B 2006, 110, 14673-14677

14673

Insights into Adsorption of Uncharged Macrocyclic Complexes into a Nafion Film: Adsorption Characteristics and Analysis of Tetraphenylporphyrine Zinc(II) Takayuki Kuwabara and Masayuki Yagi* Faculty of Education and Human Sciences and Center for Transdisciplinary Research, Niigata UniVersity, 8050 Ikarashi-2, Niigata 950-2181, Japan ReceiVed: February 25, 2006; In Final Form: June 14, 2006

Tetraphenylporphine zinc(II) (ZnTPP) was found to be adsorbed from its CH2Cl2 solution into a Nafion (Nf) film. The characteristics of the adsorption of ZnTPP into the Nf film were studied using a visible absorption spectroscopic technique. The initial rate (V0, mol cm-2 s-1) for uptake of ZnTPP was saturated with increasing ZnTPP concentration (c0, M) in the solution. This kinetic profile was analyzed in terms of a MichaelisMenten model considering preequilibrium of ZnTPP adsorption between the solution and the outer layer of the Nf film, followed by diffusion to an inner bulk region, giving a maximum diffusion reflux of Vmax ) (2.2 ( 0.2) × 10-13 mol cm-2 s-1. This is different from the kinetics for the Nf/phthalocyanine zinc(II) (ZnPc) film, which gives a linear plot of V0 vs c0. This can be explained by the relatively slow diffusion of ZnTPP in the film compared to that of ZnPc because of steric factors: ZnTPP contains bulky tetraphenyl moieties attached perpendicular to a porphyrin ring, whereas ZnPc has higher planarity. The isotherm for the adsorption of ZnTPP into the Nf film was analyzed using a Langmuir isotherm equation, yielding an equilibrium constant of (3.6 ( 1.1) × 106 M-1 and a saturated amount of adsorbed ZnTPP of (1.8 ( 0.1) × 10-9 mol cm-2, suggesting monolayer adsorption of ZnTPP on the hydrophobic polymer network interfacial with hydrophilic transport channels without significant intermolecular overlap. This is in contrast to the multilayer adsorption mode suggested for the ZnPc adsorption. The tetraphenyl moieties could prevent the stacking of ZnTPP for multilayer adsorption.

Introduction Nafion (Nf) is attracting much attention as a promising polymer film to be applied to electronic devices for electrocatalysis1,2 and energy conversion.3,4 Nf consists of a poly(tetrafluoroethylene) backbone with perfluorinated pendant chains terminated by sulfonate groups in either the acidic (H+) or neutral (Na+, K+, etc.) form. The microstructure, morphology, and properties of Nf films have been studied extensively,5,6 and it is generally known that Nf films have two fundamentally distinct structural regions: (1) a hydrophobic region formed by the perfluorinated polymer network and (2) a hydrophilic ionic cluster region consisting of the sulfonate groups, countercations, and water molecules. The neighboring clusters are interconnected through channels that enable the charge and mass transport of the ions and solvent. Hybridization of Nf films with functional molecules could expand their application to a large variety of nanoscale devices and also provide fundamental information on the physical and chemical properties of the film. Because Nf films can easily adsorb cationic molecules and ions from solution by cation exchange of the sulfonate groups, many hybrid films of Nf with cationic molecules have been fabricated to provide much knowledge and evidence about their adsorptions and molecular characteristics in the film, which are useful for designing functional devices.7-10 In contrast, hybrid films of Nf with noncationic molecules have been relatively scarce owing to the difficulty of adsorbing noncationic molecules by cation exchange and restricted conditions for the preparation of such films from * To whom correspondence should be addressed. E-mail: [email protected]. Fax and Tel.: +81-25-262-7151.

mixed solutions with Nf. Therefore, the characteristics of noncationic molecules in Nf films have not been sufficiently resolved. Ferrocene, as a noncationic molecule, has been reported to be extracted from its aqueous solution by adsorption into an Nf film, as evidenced by the growth of the ferrocene/ferrocenium redox waves upon repetitive cycling of the potential at an Nfcoated electrode dipped in a ferrocene-saturated aqueous solution.11 However, the cyclic voltammogram (CV) data were not shown, and the adsorption of ferrocene was not discussed quantitatively. Recently, noncationic ZnPc was reported to be adsorbed well into an Nf film,12 and the adsorption was analyzed to reveal the molecular characteristics of ZnPc in the film. ZnPc diffuses very rapidly in the film, with a diffusion coefficient of D ) 1.9 × 10-6 cm2 s-1, and ZnPc is adsorbed in a BrunauerEmmett-Teller (BET) adsorption mode to be considerably aggregated in the film. However, the adsorption characteristics of uncharged molecules in the film could not be understood sufficiently because of the existence of only one example of an investigation of ZnPc adsorption into Nf films. Metal porphyrin and its derivatives are functional molecules with unique photochemical and electrochemical properties,13,14 and they are expected to be applied to a large variety of nanoscale devices in solar cells,15-17 electrocatalysts,18 sensors,19,20 gas separation apparatuses,21 and displays.22,23 It was reported that the cationic porphyrin derivatives meso-tetrakis(N-methyl-4-priridyl)porphyrin and meso-tetrakis(N,N,N-trimethyl4-aminophenyl)porphyrin with Pt(II), Pd(II), and Rh(III) ions are adsorbed into Nf films by cation exchange.24,25 However, adsorption of noncationic porphyrin derivatives into Nf films

10.1021/jp061211e CCC: $33.50 © 2006 American Chemical Society Published on Web 07/13/2006

14674 J. Phys. Chem. B, Vol. 110, No. 30, 2006

Kuwabara and Yagi

has not yet been reported. Herein, we report the first observation that ZnTPP is adsorbed into an Nf film and analyze the ZnTPP adsorption to gain insight into the adsorption of uncharged macrocyclic complexes. Experimental Section Materials. Nafion 117 solution (5 wt % alcoholic solution) was purchased from Aldrich Chemical Co. Inc. and diluted with CH3OH before use. ZnTPP was purchased from Aldrich Chemical Co. Inc. and used as received. CH2Cl2 was purchased from Wako Pure Chemical Industries, Ltd., and purified as described in the literature26 before use. Preparation of an Nf Film and Adsorption of ZnTPP into the Film. An Nf film was prepared by casting 10 µL of a 2.5 wt % Nf solution onto a glass substrate (1.0 cm2). The film thickness (L) was calculated as 1.0 µm by eq 1

L ) fVdsol/(AdNf)

(1)

where f is the weight fraction (0.025) of Nf in the casting solution, V is the volume (1.0 × 10-2 cm3) of the cast Nf solution, dsol is the density (0.83 g cm-3) of the Nf solution, dNf (2.0 g cm-3) is the density of the Nf film, and A (1.0 cm2) is the substrate area. The Nf film coated freshly on the glass substrate was dipped in a ZnTPP solution (0.45-12 µM, 2.5 mL) in CH2Cl2 to obtain ZnTPP adsorption into the Nf film [(Nf/ZnTPP)ads film]. The amount of ZnTPP adsorbed into the film was calculated from the change in the visible absorption spectrum of the ZnTPP solution before and after ZnTPP adsorption, using the molar absorption coefficient  ) 421 000 M-1 cm-1 at λmax ) 418 nm or  ) 15 900 M-1 cm-1 at λmax ) 547 nm for the solution. The ZnTPP concentration (cNf, M) in the film was determined from the amount of ZnTPP adsorbed in the film and the film volume. For comparison, another Nf film [(Nf/ZnTPP)mix film] was prepared from a mixed solution containing ZnTPP and Nf as follows: A 50 µM ZnTPP solution in DMF was prepared, and then a portion of the solution was mixed with a 5 wt % Nf solution in a weight ratio of 9:1 (ZnTPP solution/Nf solution) to obtain a mixture solution containing the known amount of ZnTPP and 0.5 wt % Nf. The mixture solution (42.5 µL) was cast onto a glass substrate (1.0 cm2) and then vacuum-dried at room temperature for 30 min to form an (Nf/ZnTPP)mix film. The film thickness was estimated to be 1.0 µm using V ) 42.5 µL, dsol ) 0.94 g cm-3, dNf ) 2.0 g cm-3, and A ) 1.0 cm2 according to eq 1. The ZnTPP concentration in the present (Nf/ ZnTPP)mix film was calculated to be 20 mM from its amount in the film and the film volume. Measurements. Absorption spectra of the ZnTPP solution were measured in a quartz cell with a path length of 1 mm or 1 cm using a photodiode array spectrophotometer (Shimadzu, Multispec-1500). The absorption spectra of the ZnTPP-containing Nf film were measured using the same apparatus. Photoluminescence spectra of the ZnTPP solution were measured in a quartz cell (1 cm × 1 cm) using a fluorescence spectrophotometer (Hitachi F-4010). Photoluminescence spectra of the ZnTPP-containing Nf film in water were recorded using the same apparatus from the rear side of the glass substrate at an angle of 45° to minimize a light-scattering effects. All photoluminescence measurements were carried out at 25 °C under an argon atmosphere. Results and Discussion The UV-vis absorption spectrum of the ZnTPP solution in CH2Cl2 exhibits a well-defined intense Soret band at 418 nm

Figure 1. Absorption spectral change of a 0.45 µM ZnTPP solution (2.5 mL) in CH2Cl2 when an Nf film is dipped into the solution. The dipping times are indicated in the figure. The light path length was 1 cm.

Figure 2. Absorption spectral change of an Nf film dipped into a 0.45 µM ZnTPP solution (2.5 mL) in CH2Cl2. The dipping times are indicated in the figure.

and weak Q-bands at 547 and 585 nm.27 When the Nf film was dipped in ZnTPP solution, the absorbance at 418 nm (A418) decreased with the dipping time, as shown in Figure 1. The absorbance of the Nf film at 433 nm (A433) increased simultaneously with the decrease in A418 of the solution, as shown by the absorption spectral changes of the Nf film in Figure 2. A418 for the solution and A433 for the Nf film are plotted as a function of dipping time in Figure 3. The increase in A433 corresponds to the decrease in A418, showing that ZnTPP is adsorbed into the Nf film from the solution. As is the case for Nf/ZnPc in the earlier report,12 ZnTPP is assumed to be incorporated into the film from a mass-transport channel for the ions and solvent molecules by a hydrophobic interaction with the perfluorinated polymer chains, diffusing through an interfacial region between the hydrophilic transport channel and the hydrophobic polymer network to the inner bulk of the film. When the ZnTPPcontaining film was immersed in CH2Cl2, no ZnTPP desorbed from the film into the CH2Cl2 was detected spectroscopically, showing that ZnTPP is adsorbed stably in the film. The (Nf/ZnTPP)ads film was characterized using UV-visible spectroscopy. The UV-visible absorption spectrum of the (Nf/ ZnTPP)ads film is shown in Figure 4, along with those of a (Nf/ ZnTPP)mix film and a ZnTPP solution in DMF, and the spectroscopic features are summarized in Table 1. The absorption spectrum of ZnTPP in DMF exhibits an intense absorption band at 425 nm ( ) 384 000 M-1 cm-1) as a Soret band and

Adsorption of ZnTPP into a Nafion Film

J. Phys. Chem. B, Vol. 110, No. 30, 2006 14675

TABLE 1: Summary of Visible Absorption and Photoluminescence Spectroscopic Data absorption film or solution (Nf/ZnTPP)ads

(Nf/ZnTPP)mix

DMF solution

CH2Cl2 solution

emission

λmax (nm)

 (104 M-1 cm-1)

assignment

433 598 653 718 432 557 597 650 425 519 558 598 418 547 585

15.6 0.94 3.22 0.90 17.5 1.17 1.33 2.67 38.4 0.50 1.37 0.73 42.1 1.59 0.26

Soret Q Q Q Soret Q Q Q Soret Q Q Q Soret Q Q

λexa (nm)

λmaxb (nm)

Φrelc

433

667

54.8

433

604 665

2.9 48.5

431

577 606 656

5.3 100 17.3

a λex is the excitation wavelength. b λmax is the maximum wavelength of the emission spectrum. c Φrel was calculated from Φrel ) Iem/(1 - 10-Abs), where Iem and Abs are the relative emission intensity at λmax and the absorbance at λex, respectively. Values were normalized by Φrel ) 100 at λmax ) 606 nm for the DMF solution.

Figure 3. Plots of absorbances at 418 nm of the ZnTPP solution (A418, O) and at 433 nm of the Nf film (A433, b) versus dipping time. The conditions are indicated in Figures 1 and 2.

Figure 4. Absorption spectra of (a) 1.5 µM ZnTPP solution in DMF, (b) the (Nf/ZnTPP)ads film (cNf ) 19 mM), and (c) the (Nf/ZnTPP)mix film (cNf ) 19 mM). The inset is magnified spectra for Q-bands.

three relatively weak bands at 519 ( ) 5000 M-1 cm-1), 558 ( ) 13 700 M-1 cm-1), and 598 nm ( ) 7300 M-1 cm-1) as Q-bands split by vibrational levels (Figure 4a). The (Nf/ ZnTPP)ads film provided a Soret band at 433 nm and three weak Q-bands at 598, 653, and 718 nm. The Soret band is broader and shifts by 8 nm to longer wavelength compared to the spectrum in DMF, and the main Q-band at 653 nm shifts by 95 nm to longer wavelength compared to 558 nm for the DMF solution (Figure 4b). Zhao et al. reported that the zinc(II) ion

Figure 5. Plots of absorbance at 433 nm (A433) of the (Nf/ZnTPP)ads film versus the concentration (cNf) in the film.

in ZnTPP is replaced by two protons to form free base porphyrin H2TPP, which is further protonated to [H4TPP]2+ in either Nf butanolic solution or Nf film because of the acidity of the sulfonic acid of Nf.28 The absorption peaks of [H4TPP]2+ in the Nf film are at 435 nm (a Soret band) and 604 and 650 nm (Q-bands). These are consistent with the bands of the present (Nf/ZnTPP)ads film (433 nm for the Soret band and 598 and 653 nm for Q-bands). These red shifts of the absorption peaks can be explained by the replacement of a zinc(II) ion by protons and subsequent protonation. A plot of the absorbance at 433 nm versus cNf in the film gives a linear relationship for the (Nf/ ZnTPP)ads film (Figure 5). The  value (at 433 nm) of [H4TPP]2+ in the Nf film was calculated to be 156 000 M-1 s-1 from the slope of the linear fit. The (Nf/ZnTPP)mix film gave a UV-visible absorption spectrum (Figure 4c) with a Soret band at λmax ) 432 and a main Q-band at 650 nm, which are close to those of the (Nf/ZnTPP)ads film (Soret band at 433 nm and main Q-band at 653 nm). The similarities in the absorption spectra of the two films contrast with the previous Nf/ZnPc system, in which the absorption spectrum of the (Nf/ZnPc)ads film (prepared by adsorbing ZnPc into a preformed Nf film) is quite different from that of a (Nf/ZnPc)mix film (prepared from a DMF solution containing Nf and ZnPc by solvent evaporation) because of the formation of a ZnPc aggregate in the (Nf/ZnPc)ads film.12 Photoluminescence spectra of the (Nf/ZnTPP)ads and (Nf/ZnTPP)mix films, as well as the ZnTPP solution in DMF,

14676 J. Phys. Chem. B, Vol. 110, No. 30, 2006

Figure 6. Photoluminescence spectra of (a) ZnTPP solution in DMF (5.0 µM, ×1/3), (b) (Nf/ZnTPP)ads film (cNf ) 10 mM), and (c) (Nf/ ZnTPP)mix film (cNf ) 10 mM). The excitation wavelengths (λex) were (a) 431 and (b,c) 433 nm.

Kuwabara and Yagi

Figure 8. Plots of the initial rate of ZnTPP uptake (V0, mol cm-2 s-1) versus the ZnTPP concentration in solution (c0, M). The solid line is a simulated curve based on a Michaelis-Menten kinetic model (eq 2). The inset shows Lineweaver-Burk plots and a simulated straight line based on eq 3.

(2.1 M) . V0 was estimated from the tangent of the initial change of w. It increased with c0 at low c0 and saturated above c0 ) 1.5 µM, as shown in Figure 8. The saturation of V0 can be interpreted as a preequilibrium of ZnTPP adsorption between the solution and the outer layer of the Nf film, followed by diffusion into the inner bulk region. The plot of V0 vs c0 was analyzed by eq 2 according to a Michaelis-Menten kinetic model considering the preequilibrium of ZnTPP adsorption

V0 )

Figure 7. Time courses of the amount of ZnTPP (w, mol cm-2) adsorbed into the Nf film from the solution at various ZnTPP concentrations. The ZnTPP concentrations in the solution were (9) 0.45, (b) 0.9, (0) 1.5, and (O) 6.0 µM.

are shown in Figure 6, and the maximum wavelengths (λmax) and relative emission yields (Φrel) are included in Table 1. ZnTPP in DMF emits an intense fluorescence with Φrel ) 100 at 606 nm and weak ones with Φrel ) 5.3 and 17.3 at λmax ) 577 and 656 nm, respectively (Figure 6a). The fluorescence spectrum of the (Nf/ZnTPP)ads film gives a distinct peak at λmax ) 667 nm with Φrel ) 54.8 (Figure 6b), which is close to the main fluorescence peak at λmax ) 665 with Φrel ) 48.5 for the (Nf/ZnTPP)mix film. The λmax values at the main fluorescences for both films shift considerably to longer wavelength compared to that for the DMF solution (λmax ) 606 nm), which is consistent with the formation of [H4TPP]2+ in the Nf film as suggested in the UV-visible absorption spectral measurements. (The emission peak for [H4TPP]2+ in the Nf film is at 670 nm.28) The Φrel values of the main fluorescence for both films are lower than that (Φrel ) 100) for the DMF solution. This might be due to a self-quenching of the excited state of ZnTPP molecules close to each other in the films at a concentration that is 2000 times higher (10 mM) than in the solution (5.0 µM). Figure 7 shows time courses of the amount (w, mol cm-2) of ZnTPP adsorbed into the Nf film from the ZnTPP solution with its various concentrations. For each concentration, w increased with time and saturated in more than 5 h. The saturated value of w increased from 7.0 × 10-10 to 1.9 × 10-9 mol cm-2 as c0 increased from 0.45 to 6.0 µM. For the adsorption of 1.9 × 10-9 mol cm-2 of ZnTPP (film volume, 1.0 × 10-4 cm3), cNf is 19 mM, which is 2 orders of magnitude lower than the maximum value of cNf reported earlier for the (Nf/ZnPc)ads film

Vmaxc0 c0 + Km

(2)

where Vmax (mol cm-2 s-1) and Km (M) are the maximum diffusion reflux from the outer layer of the film to the inner bulk region and a constant related to the effective adsorption of ZnTPP (onto the outer layer of the film) for its diffusion into the inner bulk region. Lineweaver-Burk plots according to eq 3

Km 1 1 ) + V0 Vmaxc0 Vmax

(3)

give a straight line, as shown in the inset of Figure 8, suggesting the adequacy of the kinetic analysis with the assumed preequilibrium of ZnTPP adsorption. The y and x intercepts of the straight line give values of Vmax ) (2.2 ( 0.2) × 10-13 mol cm-2 s-1 and Km ) (5.6 ( 1.3) × 10-7 M. The kinetic profile shown in Figure 8 is different from that for the earlier (Nf/ZnPc)ads film in which the linear plot of V0 vs c0 was given up to c0 ) 120 µM, which is 10 times higher than the present c0 conditions for the (Nf/ZnTPP)ads film. The difference can be basically explained by the relatively slow diffusion of ZnTPP into the film compared to that of ZnPc, which is responsible for the preequilibrium of ZnTPP adsorption. The slow diffusion can be attributed to steric factors in that ZnTPP contains bulky tetraphenyl moieties attached perpendicular to a porphyrin ring whereas ZnPc has a higher planarity. An adsorption isotherm was examined to determine the adsorption characteristics of ZnTPP into the Nf film. The plot of the amount of ZnTPP adsorbed at equilibrium (weq, mol cm-2) versus its concentration (ceq, M) in the solution provides the saturation, as shown in Figure 9. It was analyzed using the Langmuir adsorption isotherm of eq 4

weq )

awsceq 1 + aceq

(4)

Adsorption of ZnTPP into a Nafion Film

J. Phys. Chem. B, Vol. 110, No. 30, 2006 14677 explained as arising from the steric hindrance of the tetraphenyl moieties of ZnTPP. The present report undoubtedly provides confirmed evidence for ZnTPP adsorption and insight into the adsorption of uncharged macrocyclic complexes into Nf films. However, the adsorption characteristics of uncharged macrocyclic complexes are not yet sufficiently understood, and this could rather show that the adsorption characteristics of ZnPc reported earlier are unique. Additional examples are required to reveal comprehensively the adsorption of uncharged molecules into Nf films. This could also provide important fundamental information for obtaining a wide range of functional Nf films that are applicable to many kinds of electronic and photoelectronic nanoscale devices.

Figure 9. Adsorption isotherm of ZnTPP from its CH2Cl2 solution into the Nf film. weq and ceq are the amount of ZnTPP adsorbed at equilibium and the concentration in solution, respectively. The solid line is a simulated curve based on a Langmuir adsorption isotherm (eq 4). The inset shows a plot of 1/weq versus 1/ceq and a simulated straight line based on eq 5.

where ws (mol cm-2) and a (M-1) are the saturated amount of ZnTPP adsorbed and the adsorption equilibrium constant, respectively. Equation 4 can be transformed into eq 5

1 1 1 1 ) + weq ws aws ceq

(5)

A plot of 1/weq versus 1/ceq according to eq 5 gives a straight line, as shown in the inset of Figure 9. This shows that ZnTPP is adsorbed into the Nf film in a monolayer mode, which means that ZnTPP is adsorbed on the hydrophobic polymer network interface with the hydrophilic transport channel without significant intermolecular overlap. The slope and intercept give the values a ) (3.6 ( 1.1) × 106 M-1 and ws ) (1.8 ( 0.1) × 10-9 mol cm-2. The high a value is consistent with the fact that the adsorbed ZnTPP is hardly desorbed in CH2Cl2. The effective area for ZnTPP adsorption per unit area (1.0 cm2) of the Nf film can be estimated as 22 cm2 from the value of ws (1.8 × 10-9 mol cm-2) and the approximate area per ZnTPP molecule (2.01 nm2) assumed as a circle with a diameter equal to the molecular size (1.6 nm).27 This means that the effective area for ZnTPP adsorption is a factor of 22 larger than the Nf film area. ZnPc adsorption into an Nf film was analyzed earlier according to a BET adsorption isotherm based on a multilayer adsorption mode.12 The multilayer adsorption is responsible for the very high concentration (2.1 M) of ZnPc in the film. In contrast, ZnTPP was adsorbed in a monolayer mode. ZnTPP could be prevented from stacking in the film by the steric hindrance of its tetraphenyl moieties, resulting in the monolayer adsorption of ZnTPP, which could be responsible for the lower cNf value of ZnTPP compared to that of ZnPc. Conclusion The present article reports the first observation that noncationic tetraphenylporphine zinc(II) (ZnTPP) is adsorbed from its CH2Cl2 solution into Nafion (Nf) film. The kinetic analysis of the ZnTPP uptake suggests a preequilibrium of ZnTPP adsorption between the solution and the outer layer of the Nf film, followed by diffusion into the inner bulk region. ZnTPP was adsorbed into the Nf film in a monolayer mode, in contrast to the multilayer adsorption of ZnPc. These results can be

Acknowledgment. This research was partially supported by a Grant for Promotion of Niigata University Research Projects. A fellowship grant was provided by The Niigata Engineering Promotion, Inc. (T.K.). References and Notes (1) Yagi, M.; Kaneko, M. AdV. Polym. Sci. 2006, 199, 143. (2) Yagi, M.; Kaneko, M. Chem. ReV. 2001, 101, 21. (3) Doyle, M.; Rajendran, G. Polymer electrolyte membrane fuel cell systems (PEMFC). In Handbook of Fuel Cells: Fundamentals, Technology and Applications; Vielstich, W., Lamm, A., Gasteiger, H., Eds.; John Wiley & Sons: Chichester, 2003; Vol. 3, Part 3, p 351. (4) Deng, W.-Q.; Molinero, V.; Goddard, W. A. J. Am. Chem. Soc. 2004, 126, 15644. (5) Heitner-Wirguin, C. J. Membr. Sci. 1996, 120, 1. (6) Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Chem. ReV. 2004, 104, 4587. (7) Yagi, M.; Kinoshita, K.; Kaneko, M. J. Phys. Chem. 1996, 100, 11098. (8) Yagi, M.; Kinoshita, K.; Kaneko, M. J. Phys. Chem. B 1997, 101, 3957. (9) Yagi, M.; Sato, T. J. Phys. Chem. B 2003, 107, 4975. (10) Yagi, M.; Takahashi, M.; Teraguchi, M.; Kaneko, T.; Aoki, T. J. Phys. Chem. B 2003, 107, 12662. (11) Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 5007. (12) Kuwabara, T.; Teraguchi, M.; Kaneko, T.; Aoki, T.; Yagi, M. J. Phys. Chem. B 2005, 109, 21202. (13) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2003; Vols. 15-17. (14) Knor, G. Coord. Chem. ReV. 1998, 171, 61. (15) Huisman, C. L.; Goossens, A.; Schoonman, J. J. Phys. Chem. B 2002, 106, 10578. (16) He, J.; Benko, G.; Korodi, F.; Polivka, T.; Lomoth, R.; Akermark, B.; Sun, L.; Hagfeldt, A.; Sundstrom, V. J. Am. Chem. Soc. 2002, 124, 4922. (17) Yoshida, T.; Tochimoto, M.; Schlettwein, D.; Woehrle, D.; Sugiura, T.; Minoura, H. Chem. Mater. 1999, 11, 2657. (18) Zhao, F.; Zhang, J.; Woehrle, D.; Kaneko, M. J. Porphyrins Phthalocyanines 2000, 4, 31. (19) Spadavecchia, J.; Ciccarella, G. C. C.; Stomeo, T.; Rella, R.; Capone, S.; Siciliano, P. Chem. Mater. 2004, 16, 2083. (20) Sauer, T.; Caseri, W.; Wegner, G.; Vogel, A.; Hoffmann, B. J. Phys. D: Appl. Phys. 1990, 23, 79. (21) Nagase, K.; Hasegawa, U.; Kohori, F.; Sakai, K.; Nishide, H. J. Membr. Sci. 2005, 249, 235. (22) Locklin, J.; Shinbo, K.; Onishi, K.; Kaneko, F.; Bao, Z.; Advincula, R. C. Chem. Mater. 2003, 15, 1404. (23) Lelievre, D.; Bosio, L.; Simon, J.; Andre, J. J.; Bensebaa, F. J. Am. Chem. Soc. 1992, 114, 4475. (24) Maldotti, A.; Andreotti, L.; Molinari, A.; Borisov, S.; Vasil’ev, V. Chem. Eur. J. 2001, 7, 3564. (25) Vasil’ev, V. V.; Borisov, S. M. Sens. Actuators B: Chem. 2002, 82, 272. (26) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory Chemicals, 4th ed.; Reed Education and Professional Publishing Ltd: Oxford, U.K., 1996. (27) Ohgushi, T.; Zi-Chen Li; Fu-Mian Li; Komatsu, T.; Takeoka, S.; Tsuchida, E. J. Porphyrins Phthalocyanines 1999, 3, 53. (28) Zhao, F.; Zhang, J.; Kaneko, M. J. Photochem. Photobiol. A: Chem. 1998, 119, 53.