A Fourier Transform Infrared Reflection−Absorption Spectroscopy

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A Fourier Transform Infrared Reflection-Absorption Spectroscopy Study of Redox Polyelectrolyte Films Cecilia Bonazzola and Ernesto J. Calvo* INQUIMAE, Departamento de Quı´mica Inorga´ nica, Analı´tica y Quı´mica Fı´sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabello´ n II, Ciudad Universitaria, AR-1428 Buenos Aires, Argentina

Fransisco C. Nart Instituto de Quı´mica de Sa˜ o Carlos, Universidade de Sa˜ o Paulo, C.P. 780, 13560-970 Sa˜ o Carlos, SP, Brazil Received September 9, 2002. In Final Form: November 17, 2002 Self-assembled polyelectrolyte multilayer films comprised of poly(allylamine) derivatized with an Os(bpy)2ClPyCH- complex (PAH-Os), and poly(vinylsulfonate), PVS, or poly(styrensulfonate), PSS, have been studied by Fourier transform infrared reflection-absorption spectroscopy. The infrared absorbances of the characteristic SO3-, CH2, NH3+, and aromatic bipyridine and pyridine groups have been characterized, and their intensity increases with the number of self-assembled layers and redox charge. The characteristic infrared signatures are the 1040 cm-1 band assigned to the aromatic ligands in the osmium complex (ν(Py)), PAH-Os, and the 1040 cm-1 (νs(SO3-)) and 1213 cm-1 (νa(SO3-)) bands for SO3 groups in PVS. The νs(SO3-) vibrational mode of PVS senses the local NH3+ environment of the cationic PAH-Os resulting in a band shift of 22 cm-1 for the first polyallylamine layer. Subtractively normalized Fourier transform infrared spectroscopy during the oxidation of the Os centers in the (PAH-Os)n(PVS)m multilayer reveals that different vibrational modes of bipyridine ligands in the osmium redox center of PAH-Os and the sulfonate groups of PVS are affected by charge-ligand electrostatic interaction and dipole reorganization in the multilayers.

Introduction Layer-by-layer (LBL) polyelectrolyte multilayers (PEMs) can be deposited by sequential and alternate immersion of a charged surface in solutions containing an oppositely charged polyanion and a polycation.1-9 In these electrostatically assembled multilayers, charge overcompensation occurs either by intrinsic charge balance between the polyions or by extrinsic charge compensation with salt counterions derived from the external electrolyte.10 The excess charged polymer profile into the multilayer has been shown to follow an exponential decay11,12 with the Donnan potential set up by the most external charged layers in contact with the electrolyte. Therefore strong electrostatic effects are expected between the charged groups in the polyions and the salt ions within these films. * To whom correspondence should be addressed. Telephone: 5411-45763378. Fax: 5411-45763341. E-mail: [email protected]. (1) Decher, G. Science 1997, 277, 1232. (2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (3) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (4) Lvov, Y.; Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396. (5) Lvov, Y.; Decher, G.; Mohwald, H. Langmuir 1993, 9, 481. (6) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid. Commun. 2000, 21, 319. (7) Arys, X.; Jonas, A. M.; Laschewsky, A.; Legras, R. Supramolecular Polymers; Ciferi, A., Ed.; Marcel Dekker: New York, 2000; pp 505563. (8) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (9) Ladam, G.; Gergeli, C.; Seger, B.; Decher, G.; Voegel, J. C.; Schaaf, P.; Cuisinier, F. J. G. Biomacromolecules 2000, 1, 674. (10) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (11) Klitzing, R. v; Mo¨hwald, H. Langmuir 1995, 11, 3354. (12) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mo¨hwald, H. Macromolecules 1999, 32, 2317.

Also some interpenetration of the polyions in the PEMs has been observed,13 which has been confirmed by neutron reflectivity in similar structures.1 In electroactive multilayers the charge of the constituent polyelectrolytes can be altered by oxidation-reduction of redox centers after film build up; this may result in changes in ion or salt population within the multilayer (extrinsic charge compensation).10 During oxidation and reduction of redox polymer films, exchange of ions with the electrolyte occurs to mantain the electroneutrality in the film. Schlenoff et al. studied self-assembled multilayers of poly(viologen), PBV, and poly(styrenesulfonate), PSS, polyanion multilayers, where the redox centers in the multilayer structure are electrochemically addressable via electron hopping between neighboring sites;13-15 thus directional electron transfer and rectifying effects can be achieved. Kim et al. studied electrostatically selfassembled multilayers comprised of functionalized cationic poly(amidoamine) dendrimers with ferrocene covalently bound and negatively charged enzyme glucose oxidase on gold.16 One of the most extensively studied LBL self-assembled systems has been poly(allylamine), PAH, and poly(styrenesulfonate), PSS, or poly(vinylsulfonate), PVS.17 Our previous studies with ferrocene and osmium complexs (13) Laurent, D.; Schlenoff, J. B. Langmuir 1997 13, 1552. (14) Stepp, J.; Schlenoff, J. B. J. Electrochem. Soc. 1997, 144, L155. (15) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (16) (a) Yoon, H. C.; Hong, M. Y.; Kim, H. S. Anal. Chem. 2000, 72, 4420. (b) Yoon, H. C.; Kim, H. S. Anal. Chem. 2000, 72, 922. (17) (a) Schmitt, J.; Gru¨newald, T.; Decher, G.; Pershan, P. S.; Kjaer, K.; Lo¨sche, M. Macromolecules 1993, 26, 7058. (b) Lvov, Y.; Haas, H.; Decher, G.; Mo¨hwald, H. J. Phys. Chem. 1993, 97, 12835. (c) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107.

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covalently attached to PAH, PAH-Fc18 and PAH-Os,19 respectively, have shown that in layer-by-layer nanostructures the enzyme glucose oxidase (GOx) can be electrically connected (“wired”) to an electrode surface with molecular recognition of glucose and generation of an electrical signal.20 The study of structure and dynamics of similar (PAH-Os)n(GOx)n films by ellipsometry21,22 and quartz crystal microbalance23,24 has shown film swelling upon oxidation due to the ingress of anions and solvent to compensate charge. Furthermore, the electrochemical oxidation and reduction of the films and enzyme redox catalysis result in electron hopping between adjacent redox sites in a redox concentration gradient in the organized multilayer.25 A systematic shift of the redox potential with the nature of the charge excess in the outermost layer in solutions of low ionic strength has been attributed to the Donnan equilibrium.21 Probe beam deflection (PBD) studies of ion exchange between (PAH-Os)n(PSS)n and the aqueous electrolyte have demonstrated that the film exchanges anions and protons with the electrolyte solution upon redox switching to equilibrate the chemical potential.27 Vibrational spectroscopy of organized LBL self-assembled polyelectrolyte multilayer films can complement information about the local environment of chemical groups exhibiting active infrared modes. Harris and Bruening employed Fourier transform infrared external reflection spectroscopy (FTIR-ERS) to follow the stepwise growth of PAH/PSS films.28 The authors characterized the absorbances due to sulfonate in the 1005-1220 cm-1 region, bands due to symmetric and antisymmetric -NH3+ deformations, and aromatic ring modes in the 1400-1625 cm-1 region. Bruening et al.29 also employed FTIR spectroscopy to follow the cross-linking of ultrathin PEM films formed by PAH and poly(acrylic acid), PAA, after heat-induced amide formation. Schlenoff and co-workers30 reported studies of the water content in poly(diallydimethylamonium) chloride, PDDA, and PSS multilayers by IR spectroscopy and thermal gravimetric analysis. Voegel and Shaaf reported the study of stabilizing effects of various polyelectrolyte multilayer films on the structure of adsorbed or embedded fibrinogen molecules by attenuated total reflection Fourier transform infrared spectroscopy.31 Caruso and co-workers32 studied protein-containing multilayer films of PSS and PAH assembled by sequential (18) Hodak, J.; Etchenique, R.; Calvo, E. J.; Singhal, K.; Bartlett, P. N. Langmuir 1997, 13, 2716. (19) Calvo, E. J.; Etchenique, R.; Pietrasanta, L.; Wolosiuk, A.; Danilowicz, C. Anal. Chem. 2001, 73, 1161. (20) Calvo, E. J.; Battaglini, F.; Danilowicz, C.; Wolosiuk, A.; Otero, M. Faraday Discuss. 2000, 116, 47. (21) Calvo, E. J.; Wolosiuk, A. J. Am. Chem. Soc. 2002, 124, 8490. (22) Forzani, Erica S.; Otero, Marcelo; Perez, Manuel A.; Teijelo, Manuel Lo´pez; Calvo, Ernesto J. Langmuir 2002, 18, 4020. (23) Forzani, E. S.; Pe´rez, M. A.; Teijelo, M. Lo´pez; Calvo, E. J. Langmuir 2002, 18, 9867-9873. (24) Calvo, Ernesto J.; Forzani, Erica S.; Otero, Marcelo Anal. Chem. 2002, 74, 3281. (25) Calvo, Ernesto J.; Forzani, Erica S.; Otero, Marcelo J. Electroanal. Chem., in press. (26) Calvo, Ernesto J.; Danilowicz, Claudia; Wolosiuk, Alejandro J. Am. Chem. Soc. 2002, 124, 2452. (27) Grumelli, D. E.; Wolosiuk, A.; Planes, G. A.; Barbero, C.; Calvo, E. J. J. Am. Chem. Soc., submitted. (28) (a) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006. (b) Harris, J. J.; Stair, J. L.; Bruening, M. L. Chem. Mater. 2000, 12, 1941. (29) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978. (30) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621. (31) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893. (32) Caruso, F.; Furlong, D. N.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1998, 14, 4559.

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adsorption of polyelectrolytes and anti-immunoglobulin G on solid substrates. Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS) studies confirmed the presence of the protein in the multilayer with the amide I and amide II bands and found evidence that the immuno-globulin in the film was not denatured. The present paper describes FTIR studies of LBL films comprised of cationic poly(allylamine) derivatized with Os(bpy)2PyCH2- complex, PAH-Os, and anionic PSS or PVS polyelectrolyte films. We focus on the infrared spectral changes that arise as a consequence of oxidationreduction of the Os(II)/Os(III) redox system within the polyelectrolyte multilayers. For this purpose we first compare the infrared absorbances of films comprised of unmodified PAH with PSS and PVS and osmium-derivatized PAH with PSS and PVS. We then employ subtractively normalized Fourier transform infrared spectroscopy (SNIFTIR) during the oxidation of the Os centers in the (PAH-Os)n(PVS)m to characterize changes in the electrostatic and dipolar environment of the osmium ligands and sulfonate groups in PVS of the assembled polyelectrolytes during electrochemical oxidation and reduction. Experimental Section Chemicals and Solutions. 3-Mercapto-1-propanesulfonic acid sodium salt, MPS (Aldrich), poly(vinyl sulfonic acid, sodium salt), PVS (Aldrich), poly(sodium 4-styrensulfonate), PSS (Aldrich), and (3-mercaptopropyl)trimethoxysilane (Sigma) were used as supplied. Poly(allylamine hydrochloride), PAH (Aldrich), was dialyzed against water for several days using a 12000 molecular weight cutoff membrane (Sigma D9652) and freezedried. Osmium redox polymer, PAH-Os was synthesized from Os(bpy)2Cl-(pyCHO)Cl complex and freeze-dried PAH as described elsewhere.33 Aqueous solutions of PAH, PVS, and PSS, 1 mM (in monomer), were employed for the film preparation by electrostatic adsorption. The osmium redox polymer aqueous solution was dialyzed against water for 3 days before use. A 20 mM MPS solution in 16 mM sulfuric acid (Merck) was prepared just before each experiment in order to avoid oxidation in air. In the fabrication of vapor-deposited gold on glass slides, (3-mercaptopropyl)trimethoxysilane, employed as received, was used as molecular adhesive. All the solutions were prepared with Milli-Q (Millipore) deionized water. Other reagents were analytical grade, and they were used without further purification. Film Preparation. Gold films (150 nm) evaporated on glass slides modified with 3-mercaptopropylsilane34 were employed as substrates. Gold vapor deposition was carried out in a vacuum coating system, Edward Auto 306, at 5 × 10-5 mbar. In the first step, the fresh gold film substrate was primed with sulfonate groups by immersion in a MPS solution for 2 h followed by rinsing with deionized water. After thiol adsorption, the first polycation layer was formed on thiol-modified Au substrate by immersion in PAH or PAH-Os solutions for 10 min. The next and subsequent layers were deposited onto the modified surface by alternate immersion in a solution of the respective polyanions (PVS or PSS) for 10 min and polycation solution, followed by thoroughly rinsing with distilled water and drying at the end of each adsorption step. The adsorption times were determined according to previous QCM kinetic results. In this manner, we built up LBL supramolecular structures of PVS/PAH, PVS/PAHOs, PSS/PAH, and PSS/PAH-Os by reverting the surface charge of the topmost layer. Fourier Transform Infrared Spectroscopy Measurements. FTIR spectroscopy was used in transmission (TIR) and reflection absorption spectroscopy (IRRAS) modes. TIR spectra were recorded using a Nicolet 510P spectrometer provided with a DTGS detector and a Balston 75-45 purge gas generator. IRRAS spectra were recorded with a Nicolet Nexus 670 spec(33) Danilowicz, C.; Corto´n, E.; Battaglini, F. J. Electroanal. Chem. 1998, 445, 89. (34) Goss, C. A.; Charych, D. H.; Mayda, M. Anal. Chem. 1991, 63, 65.

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Figure 1. Schematic representation of molecular structures uses in LBL self-assembled redox polyelectrolytes. trometer and a Whatman 75-52 purge gas generator, equipped with a liquid N2 cooled MCTA detector and Ca2F windows. TIR spectra of the polyanion and polycation solutions evaporated on an AgCl window or included in KBr pellets were performed to determine frequency positions and relative intensities of the characteristic bands. Reflection-absorption spectra were obtained for gold films modified with LBL polyelectrolyte using a purpose built reflectance set up with at 80° angle of incidence and a bare gold surface as background (reference). All spectra were collected at 4 cm-1 spectral resolution using 100 scans and are presented without smoothing correction. Electrochemical “in situ” experiments were carried out with a purpose built three-electrode PTFE cell with the Au electrode aligned against the CaF2 window leaving a small volume of electrolyte in the optical path35 to offset the water infrared absorption. Measurements in SNIFTIRS mode were carried out with the electrode potential at either 0.5 or 1.0 V corresponding to the fully reduced and fully oxidized osmium film, respectively. A Nicolet Magna 560 Fourier transform infrared (FTIR) spectrometer equipped with cryogenic MCTA detector was used in the experiments with a zinc selenide (Nicolet, Spectra-Tech) polarizer. The electrode potential was controlled with a potentiostat designed in the laboratory. The spectroelectrochemical cell was fitted with a Pd(H)/H+ reference electrode and a Pt auxiliary electrode. For electrolyte, a 0.5 M KF solution of pH 4.5 was employed. The spectra (300 interferogram scans each) were obtained first at reference potential (0.5 V) and then at 1.0 V with a resolution of 4 cm-1. The results are presented in the absorbance form, i.e., sample at 1.0 V (R) and reference (R0).

Results and Discussion The structures of the molecules employed in the construction of the LBL self-assembled polyelectrolyte films are shown in Figure 1. The reference infrared spectra of thin films for each polyelectrolyte employed in this study, namely, PAH, PAH-Os, PVS, and PSS, has been included as Supporting Information together with the characteristic modes for the different molecules (Table 1). The multilayer films were built by modification of a gold surface, and the spectra for the resulting films were recorded in the reflection-absorption mode. The strategy consists of building LBL and recording the spectra after each deposition step. (35) Iwasita, T.; Nart, F. C. In-situ Fourier Transform Infrared Spectroscopy: A tool to Characterize the Metal-Electrolyte Interface at a Molecular Level. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH: Weinheim, 1995; Vol. 4.

Typical results for an assembled polyelectrolyte film in the spectral region 1300-1000 cm-1 after the first adsorption cycles are shown in Figure 2 for PSS/PAH and PVS/PAH, respectively, and these results are similar to all the studied systems. A first MPS layer is useful to prime the surface with a negative charge that will hold electrostatically the subsequent positively charged polyelectrolyte layer. The spectrum of the mercaptopropanesulfonate-modified gold surface (Au/MPS) shows as dominant feature a peak at 1074 cm-1 assigned to the symmetric S-O3- stretching shifted to higher frequency with respect to the reference band at 1061 cm-1 for the MPS evaporated film. The strong and broad asymmetric S-O3 stretching band at 1200 cm-1 (Figure 1 Supporting Information) is not observed for the adsorbate, which suggests that the sulfonate group should be perpendicular to the Au surface, since the surface selection rule does not allow a mode with the dynamic dipole moment parallel to the surface. The very weak bands expected in the methylenestretching region (3000-2800 cm-1) could not be observed for the MPS monolayer since short alkanethiols cannot form a well-ordered self-assembled monolayer on Au.36 After adsorption of the first polycation layer (PAH or PAH-Os), the sulfonate band at 1074 cm-1 is shifted to 1052 cm-1 due to the interaction between the SO3- groups and the protonated amines of the polycationic polymer which replace small ions compensating the surface charge. This is consistent with the finding in sulfonated cationexchange resins that the sulfonate Raman bands are very susceptible to the countercation.37 After the adsorption of the subsequent first polyanion layer (PSS or PVS), a new broad band near 1200 cm-1 due to asymmetric stretching of the sulfonate group is apparent. This band arises from the sulfonic groups in the polyelectrolyte farther away from the surface and in random orientation so that the asymmetric S-O3 mode may have a component of the dynamic dipole moment perpendicular to the surface. Note that while for PVS there is a single peak at 1062 cm-1 in the reference spectrum, due to the sulfonate symmetric stretching, two bands are present for PSS at 1052 and 1035 cm-1. It may be possible (36) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (37) Wang, H.; Mann, C. K.; Vickers, T. J. Appl. Spectrosc. 1995, 49, 127.

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Figure 2. FT-IRRAS spectra of PSS/PAH (a) and PVS/PAH (b).

that the sulfonate interacts with a different environment in the self-assembled film being this interaction more significant for the PSS system. For the evaporated film, PSS presents the symmetric stretching at ca. 1044 cm-1 and another band at 1011 cm-1 related to the benzene ring is also observed for PSS. Figures 3 and 4 depict the Fourier transform infrared reflection absorption spectra (FT-IRRAS) of LBL multilayers comprised of (PAH-Os)n(PVS)n, (PAH)n(PVS)n and (PAH-Os)n(PSS)n, (PAH)n(PSS)n respectively. The characteristic features of the assembled polyelectrolyte infrared absorbances become more evident with increasing the number of bilayers. Broad signals can be observed in the regions of 1700-1400 and 3600-2700 cm-1. Comparing the systems with and without the osmium complex covalently attached to the cationic polyelectrolyte, we observe for the PVS/ PAH-Os a distinct band at 1420 cm-1 assigned to the aromatic ligands in the osmium complex, which we take as signature for the osmium complex in the LBL self-assembled films (Figure 3a). Note that PAH and PVS polyelectrolytes have no aromatic groups in their structure. In PAH-Os/PSS multilayers the band of the osmium complex aromatic rings at 1420 cm-1 is masked by the aromatic band of the poly(styrensulfonate). For both PVS/PAH and PVS/PAH-Os in the 20001300 cm-1 spectral region (Figure 3), we observe three main broad bands (1630, 1530, 1460 cm-1) which have different relative intensities in both cases. The ligand aromatic structures of the osmium complex covalently attached to PAH in the assembled multilayer PVS/ PAH-

Os may be responsible for the differences. Although the mode δa(NH3+) of the PAH polymer and the water trapped in the film contribute to the broad signal at 1630 cm-1 in both cases, the δs(NH3+) mode and one ν(Py) mode contribute mainly to the signal at 1530 cm-1 only for the PAH/Os film. The broad band observed at 1460 cm-1 has contributions not only of another ν(Py) ring vibration related to the ligands for PAH/Os but also of the δ(CH2-) bending modes present in both type of films. In the 13001000 cm -1 region, for both types of assembled multilayer polyelectrolyte films, the bands due to the sulfonate stretching vibrations are observed. The band of the asymmetric mode at 1215 cm-1 is different in shape from that observed for PVS evaporated film. It is apparent that the νs(SO3-) band around 1040 cm-1 changes not only intensity but also the shape when the number of bilayer increases. For PVS/PAH, a splitting of this band becomes evident at n ) 11, which could arise from the coupling between vibrational modes or to local anisotropic effects of the SO3- groups in the LBL polyelectrolyte multilayer. A similar trend can be observed in the PSS/PAH system (Figure 4a). The resulting signals come from the overlapping absorption bands of the polyelectrolyte components in the film. The characteristic bands due to SO3- asymmetric (1122-1181 cm-1) and symmetric (close to 1040 cm-1) stretchings have a similar behavior described for the PVS systems when the number of bilayers increases. Figure 5 depicts the increase in infrared absorbance at 1213 cm-1 (νa(SO3-) signature of the PSS) and 1420 cm-1 (ν(Py) signature of the PAH-Os) with the number of

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Figure 3. Layer-by-layer self-assembled polyelectrolyte FT-IRRAS spectra for (a) (PVS)n(PAH-Os)n and (b) (PVS)n(PAH)n.

bilayers of (PAH-Os)n(PVS)n and the osmium oxidation charge that also increases with the number of PAH-Os layers. A monotonic increase of both infrared absorbance peaks with the number of adsorbed layers similar to the trend found with the mass and thickness29,30 in similar LBL systems is observed. So far we have described the FTIR spectra for reduced Os(II) polyelectrolyte films. The electrochemical oxidation of the osmium polymer to Os(III) results in the simultaneous ingress of anions and cations in the film to compensate charge as shown by probe beam deflection studies27 and exchange of ions and water as shown by

the electrochemical quartz crystal microbalance31 and results in swelling driven by the redox transformation as demonstrated with ellipsometry.23 Proof for the exchange of anions with the electrolyte has also been found from the slope of peak potential vs the logarithm of the 1:1 electrolyte concentration under Donnan permselective conditions.21 It is interesting to obtain molecular level information with FTIR on the main functional groups in the multilayer during the oxidation of the polymer since this process introduces a change in the film charge and therefore dipole reorientation in the polyelectrolyte multilayers can take place.

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Figure 4. Layer-by-layer self-assembled polyelectrolyte FT-IRRAS spectra for (a) (PSS)n(PAH-Os)n and (b) (PSS)n(PAH)n.

Figure 6 depicts a typical surface cyclic voltammetry of the Os(II)/Os(III) redox couple in the (PAH-Os)11(PVS)11 multilayer in contact with 0.5 M KF-HF electrolyte of pH 4.5. At 0.5 V the film is fully reduced in the Os(II) form

while at 1.0 V all osmium sites are oxidized, and on this basis we have chosen 0.5 V as the reference and 1.0 V as the sample potentials for the “in situ” FTIR study . The electrolyte, 0.5 M KF was used to avoid the infrared

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Figure 5. Plot of absorbance at 1213 and 1420 cm-1 versus the number of self-assembled bilayer in (PVS)n(PAH-Os)n multilayer.

Figure 7. Differential FTIR (SNIFTIR) for (PVS)11(PAH-Os)11 in 0.5 M KF + HF of pH 4.5 at 1.0 V (B) and reference potential 0.5 V (A) and (C).

Figure 6. Cyclic voltammogram for Au electrode modified with MPS and (PVS)11(PAH-Os)11 in 0.5 M KF + HF of pH 4.5, v ) 0.1 V s-1.

absorption of oxoanions such as nitrate or perchlorate in the wavenumber window where the sulfonate of PVS group absorbs. In a typical in situ FTIR experiment, it is necessary to measure a reference spectrum at a potential, where the electrochemical process does not take place, and a sample spectrum, where the desired process takes place. A ratio of the two spectra is then obtained. This type of experiment was originally called subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS).35 In SNIFTIRS spectra usually there are bands in both up and down direction with respect to the baseline. Taking R0 as the reflectance in the reference spectrum and R the sample spectrum reflectance, positive bands (R0 > R) correspond to consumption of species and negative bands (R0 < R) correspond to appearance of new species. The SNIFTIRS spectrum is depicted in Figure 7b for the oxidized Os(III) polymer at 1.0 V (Pd-H2) with respect to the reduced Os(II) polymer at 0.50 V (Pd-H2) reference state shown in Figure 7a. Since the data in Figure 7 are presented in the absorbance scale, an increase (decrease) in absorbance with respect to the reference corresponds to a decrease (increase) in reflectance; hence positive bands in Figure 7b correspond to appearance and negative bands to the dissappearance of surface species, respectively.

The characteristic features in Figure 7b are positive bands at 1608 (aromatic), 1318 (aromatic), and 1031 cm-1 (sulfonate), a negative band at 1260 cm-1 (aromatic or νa(SO3-)), and bipolar bands at 1478-1470, 1461-1450 cm-1 (bipyridines), 1420-1432 cm-1 (related to ν(Py) at 1420 cm-1), and 1057-1048 cm-1 (related to the νs(SO3-) band 1040 cm-1).38 These bands disappear in the signal noise when the PAH-Os film is reduced back at 0.5 V (Figure 7c), showing that the process is reversible. The broad band around 1650 cm-1 is due to water in the film, which changes with time. Note that, unlike the absolute spectra described for the “ex situ” experiments, the difference between spectra for fully oxidized and fully reduced film is shown in Figure 7b. The bipolar nature of the bands at 1478-1470, 14611450, and 1420-1432 cm-1, assigned to the pyridine and bipyridine moieties, clearly indicate that there is a shift in the position of the bands with the change in the oxidation state of the osmium center. In particular, the more oxidized metallic center (Os(II) f Os(III)) produces a blue shift of the 1420 cm-1 band (signature of the osmium complex in PAH-Os) to 1432 cm-1. A blue shift can be assigned to a decrease in the back-bonding from the Os to the pyridine rings, giving rise to the bipolar band. A rather curious effect can be observed for the sulfonate moieties, since the SO3 group is not attached directly to the Os center and only a dipolar interaction can be (38) (a) Roeges, N. P. G. A Guide to the Complete Interpretation of Infrared Spectra of Organic Structures; John Wiley & Sons: New York, 1994. (b) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: New York, 1991.

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expected. In that case, the effect on the SO3 group could be more of electrostatic nature, but this would cause an opposite effect. Probably the change in the osmium charge density caused a general rearrangement of the dipoles and fields inside the film (intermingling polyelectrolyte layers), leading to the observed blue shift for the SO3 symmetric stretching. Connected with this result is the 1260 cm-1 negative band, which is likely due to the asymmetric stretching or the sulfonate group. This result points out the complexity of the interaction inside the film. The positive bands that appear at 1608, 1318, and 1031 cm-1 likely indicate a change in intensity, which is caused by an increase in the dynamic dipole moment. Since in the (PAH-Os)11(PVS)11 the only aromatic moieties are the pyridine rings, the decrease in the back-bonding is likely, causing the increase in the dipole moment. The FT-IRRAS reference spectrum of PAH-Os (Supporting Information) has as main absorption bands 1460, 1422, 1315, 1257, and 1017 cm-1, all related to the aromatic pyridine and bipyridine ligands of the osmium redox center. Therefore, except for the 1057-1048 cm-1 (νs(SO3-)) band, both sulfonate and aromatic pyridines and bipyridine groups have absorptions in the spectral region where the oxidation of the redox centers result in infrared spectral changes (Supporting Information). Then, the spectral changes that arise from the oxidation of the Os(II) complex in the polyelectrolyte multilayer should be related to changes in the charge-dipole interaction in the ligands around the Os center, in particular the pyridine and bipyridine ligands. Also the electrostatic environment of the positive Os(II) or Os(III) centers may change the dipole distribution in the multilayers affecting vibrational modes of the negatively charged sulfonate groups. Our preliminary resonant Raman spectroscopy studies of the same self-assembled redox polyelectrolyte system are in good agreement with the present evidence since the oxidation of the PAH-Os strongly affects the pyridine and bipyridine vibrational modes around the osmium redox center and the sulfonate groups of PVS.41 (39) Chenon, B.; Sandorfy, C. Can. J. Chem. 1958, 36, 1181. Brissete, E.; Sandorfy, C. Can. J. Chem. 1960, 38, 34. (40) Stroeve, P.; Vasquez, V.; Coelho, M. A. N.; Rabolt, J. F. Thin Solid Films 1996, 284/285, 708.

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Conclusions Two main conclusions can be drawn from the study of the layer-by-layer self-assembled redox polyelectrolytes by FT-IRRAS and SNIFTIR during oxidation-reduction of the osmium redox centers in the multilayers: The local NH3+ environment of the poly(allylamine) on the sulfonate groups results in ca. 22 cm-1 band shift for the νs(SO3-) absorbance similarly to the results of Raman shift of sulfonate features in the 1000-1200 cm-1 region in sulfonated cation exchange membranes which are sensitive to the associated cation.37 The oxidation of the osmium redox center in the polymer results in important changes in the infrared absorbances due to the aromatic ligands of the osmium when the charge is increased by +1. It is worth noting that the different vibrational modes of bipyridine ligands of the osmium redox center in PAH-Os and the sulfonate groups of PVS are affected differently as a result of osmium oxidation. We then conclude that introducing a positive charge in the Os center affects in a complex manner the different vibrational modes in the LBL electrostatically selfassembled multilayer due to dipole rearrangement in the multilayers. Acknowledgment. The authors acknowledge financial support from CAPES-SEPCyT Brazil-Argentina Cooperation Project 0017/02. Financial support came from the University of Buenos Aires (UBA) and Argentine Science Research Council (CONICET), and E.J.C. is a Research Fellow of CONICET (Argentina) and acknowledges a Guggenheim Fellowship 2000/2001. F. C. Nart acknowledges FAPESP and CNPq from Brazil for financial support. Supporting Information Available: The reference infrared spectra of thin films for each polyelectrolyte employed in this study, namely, PAH, PAH-Os, PVS, and PSS, characteristic modes for the different molecules (Table 1), FTIR spectra of MPS, PVS, PSS, PHA, and PHA-Os (Figure 1), and FTIR spectrum of bipyridine (Figure 2). This material is available free of charge via the Internet at http://pubs.acs.org. LA026528U (41) Fainstein, A.; Bonazzola, C.; Calvo, E. J. In preparation.