Layer-by-Layer Assembly and Characterization of Multilayers of a

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Layer-by-Layer Assembly and Characterization of Multilayers of a Manganese Porphyrin Linked Poly(4-vinylpyridinium) Derivative and Poly(styrenesulfonic acid-o-maleic) Acid Hong-Lei Wang,† Qing Sun,† Meng Chen,† Jun Miyake,‡ and Dong-Jin Qian*,† † ‡

Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, China Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, University of Osaka, 1-3 Machikane-Yama, Toyonaka, Osaka 560-8531, Japan

bS Supporting Information ABSTRACT: Multilayers of manganese(III) porphyrin-linked poly(vinylpyridinium) (MnTMPyP-PVPMe) polyelectrolyte and poly(styrenesulfonic acid-omaleic) acid (PSS) have been assembled on gold, quartz, and indium tin oxide surfaces by a layer-by-layer (LBL) technique. The assembly process was monitored by measuring their absorption spectra and frequency change after each assembly, both of which confirmed the formation of three-dimensional MnTMPyP-PVPMe/PSS multilayers. The Soret absorption band of porphyrin red shifted about 8 nm in the multilayer compared with that in the dilute aqueous solution. The average mass changes for each assembly of MnTMPyP-PVPMe and PSS were estimated to be about 2.9 and 0.25 μg/cm2, respectively. X-ray photoelectron spectra revealed that the as-prepared multilayers were composed of S 2p , C 1s , N 1s , O 1s, and Mn 2p , corresponding to polymers of MnTMPyP-PVPMe and PSS. A rough surface was observed after the assembly of MnTMPyP-PVPMe on the gold surface, but it became smoother when the PSS layer was adsorbed. The significant difference in the mass change and film morphology after the assembly of MnTMPyP-PVPMe compared to those after the assembly of PSS was ascribed to the reason that the MnTMPyP-PVPMe polyelectrolyte contained large metalloporphyrin macrocycles, which were axially coordinated to the pyridyl substituents of the PVP polymeric backbones. The cyclic voltammograms revealed two couples of redox waves in the phosphate electrolyte solution at pH 11, which corresponded to the electron-transfer processes of Mn(II)/Mn(III) and Mn(III)/Mn(IV) of polymeric manganese porphyrin MnTMPyP-PVPMe. The charge-transfer process was also investigated. Finally, the present MnTMPyP-PVPMe/PSS multilayers were used as a heterogeneous catalyst for the decoloration of an azo dye.

’ INTRODUCTION Metalloporphyrins and their derivatives have attracted significant attention because of their possible applications as photosensitizers, as catalysts in chemical and photochemical reactions, in analytical chemistry, and as models for many important biological processes.1,2 Examples of these applications include artificial photosynthesis, solar energy cells, gas sensors, nonlinear optics, molecular electronic devices, photoinduced hydrogen production and so on.3
5 To achieve such purposes, porphyrin derivatives are sometimes dissolved in suitable solutions, but in most cases, they are assembled on solid surfaces including commonly used glass, quartz, and various electrode surfaces as well as on the surfaces of porous and nanostructural materials6,7 because ordered layers of porphyrins on solid surfaces are advantageous to their practical uses in optoelectrical molecular devices, recognition systems, and heterogeneous catalytic reactions.8
10 Many methods have been developed for the assembly of ordered layers of porphyrins in the past decades on the basis of either covalent or noncovalent molecular interactions.11,12 The former covalent assembly method is generally carried out via r 2011 American Chemical Society

chemical binding, resulting in the strong immobilization of porphyrins on solid surfaces. An early example by Li and coworkers revealed that tetrapyridylporphyrin (TPyP) could be attached to a substrate surface via a coupling layer of (p-(chloromethyl)phenyl)trichlorosilane,13 which has been developed as a support layer for preparing various hybrid multilayers.14 However, in the latter, the covalent interaction (called supramolecular interaction) is usually via an intermolecular interaction, such as electrostatic interaction, a coordination bond, a hydrogen bond, and so on. For instance, on the basis of the immobilized TPyP porphyrin or bipyridyl (BPy) monolayer, many multiporphyrin arrays or building blocks have been prepared by using the layer-by-layer (LBL) technique via an interlayer coordinative bond or an electrostatic interaction.15,16 Both optical and electrochemical features confirmed that well-defined 3D films could be constructed directly on substrate surfaces, which showed very

Received: March 3, 2011 Revised: July 13, 2011 Published: July 18, 2011 9880

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Figure 1. Preparation of polymeric manganese porphyrins of MnTMPyP-PVP and MnTMPyP-PVPMe.

strong stability and potential applications for the formation of host
guest materials and heterogeneous catalysts. A recent review of organized porphyrinic materials by Drain and co-workers has provided many examples of porphyrin multilayers by the LBL technique, most of which were based on a charged substrate and polyelectrolytes as well as on coordinative binding.17 These LBL porphyrin multilayers have some possible applications, such as sensors for the modification of electrodes and for photovoltaics and nonlinear optical materials. Besides intermolecular electrostatic interaction and coordination binding, metalloporphyrins could also form organized multilayers via their central metal ions (axial coordination), as reviewed by Beletskaya and co-workers.18 These metalloporphyrin supramolecules and multilayers can be used as models for biological processes and as catalysts for many organic reactions. We are currently interested in the design and assembly of 3Dorganized ultrathin films of various porphyrins at interfaces as well as their optical, electrochemical, and catalytic properties.19 These films were constructed by using the Langmuir
Blodgett (LB), self-assembly, and LBL techniques. In the present work, a charged manganese(III) porphyrin of 5,10,15,20-tetra-(4pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) (MnTMPyP) was linked on a poly(4-vinylpyridine) (PVP) that was first based on the axial coordinative bond between central Mn(III) ions and the pyridyl substituents of PVP, the product of which was then reacted with iodomethane, resulting in the formation of its poly(4-vinylpyridinium) salt. This positively charged polyelectrolyte was then alternatively assembled on substrate surfaces with a negatively charged polymer of poly(styrenesulfonic acid-o-maleic) acid (PSS) based on an interlayer electrostatic interaction by the LBL method. The assembly process and the as-prepared multilayers were characterized by using UV
vis absorption spectroscopy, quartz crystal microbalance (QCM), X-ray photoelectron spectroscopy, and an electrochemical analyzer. Morphologies of the LBL multilayers were

characterized by using scan electron microscopy (SEM), fieldemission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). The present study provided an alternative route to preparing polymeric films of metalloporphyrins, which could act as a light-harvesting unit, optoelectronic materials, and heterogeneous catalysts for some organic reactions.

’ EXPERIMENTAL SECTION Materials. Manganese(III) 5,10,15,20-tetra-(4-pyridyl)-21H,23Hporphine chloride tetrakis(methochloride), iodomethane, 4-mercaptobenzoic acid, imidazole, (p-(chloromethyl)phenyl)trichlorosilane, poly(styrenesulfonic acid-o-maleic) acid, poly(4-vinylpyridine), and 4-(2-hydroxy-1-naphthylazo)benzene sulfonic acid sodium salt were purchased from Aldrich Chemical Co. N-Trimethoxysilylpropyl-N,N, N-trimethylammonium chloride was from Fluorochem. Chloroform, N, N0 -dimethylformate (DMF), and methanol were from Fisher Chemical Co. All chemicals were used as received without further purification. Ultrapure water (18.2 ΩM cm) was prepared with a Rephile filtration unit (China). Synthesis of Manganese Porphyrin-Linked Poly(vinylpyridine) and Poly(vinylpyridinium) Salt. Polymeric manganese porphyrin (MnTMPyP-PVP) was obtained via the axial coordination of the central Mn(III) ion of the porphyrin (MnTMPyP) with the pyridyl substituent of the PVP polymer according to Campestrini and Meunier’s method.20 Figure 1 shows a schematic drawing of the preparation of the porphyrin-linked polymer of MnTMPyP-PVP and its methylated product of the MnTMPyP-PVPMe polyelectrolyte. Briefly, the manganese porphyrin-linked polymer of MnTMPyP-PVP was obtained by stirring a mixture of MnTMPyP and PVP (the molar ratio of porphyrin to pyridyl substituents of PVP was about 1: 30) in a methanol solution for 24 h, which was then evaporated on a rotary evaporator, well washed with plenty of methanol and water to remove unreacted MnTMPyP, and finally dried under vacuum at room temperature. The molar fraction of porphyrin substituents of MnTMPyP in the MnTMPyP-PVP polymer was estimated from the absorption spectra. 9881

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Figure 2. Schematic drawing of the assembly of MnTMPyP-PVPMe/ PSS multilayers on the gold substrate surface. The methylated product of polyelectrolyte MnTMPyP-PVPMe was obtained by stirring a mixture of MnTMPyP-PVP and iodomethane (the molar ratio of pyridyl substituents in the MnTMPyP-PVP polymer to CH3I was about 1:1.5) in a chloroform solution for 12 h. The precipitate was filtered, well washed with plenty of chloroform, and then dried under vacuum at room temperature. The molar fraction of MnTMPyP in the MnTMPyP-PVPMe polyelectrolyte was estimated by the absorption spectra in a dilute methanol solution.

Layer-by-Layer Assembly of Three-Dimensional MnTMPyPPVPMe/PSS Multilayers. The LBL multilayers of MnTMPyPPVPMe/PSS were assembled on gold, quartz, and indium tin oxide (ITO) substrate surfaces for the characterization of spectral and electrochemical properties as well as the film morphology. The gold electrode (geometrical area of 3.1 mm2, CH Instruments, Inc.) was first polished with 0.5 and 0.03 μm alumina paste and then subjected to oxidation
reduction cycles at a scan rate of 100 mV/s (versus Ag/ AgCl) between 1600 and
100 mV in a 0.01 mol/L HClO4 electrolyte solution until a cyclic voltammogram (CV) of a clean polycrystalline gold electrode was obtained.21 The freshly cleaned gold electrode was immersed in a 20 mmol/L ethanol solution of 4-mercaptobenzoic acid overnight. Finally, the electrode was rinsed with a copious amount of ethanol and water. In this way, a negatively charged surface (covered with
SC6H4COOH, Figure 2) was achieved, which could be used to adsorb positively charged polyelectrolytes.22 For the multilayers assembled on a quartz or ITO electrode surface, a different modification method was used. That is, the cleaned hydrophilic substrate was first attached by one layer of positively charged alkylammonium salt (Figure S1), which was done by immersing the hydrophilic quartz or ITO substrate in a 5 mg/mL DMF solution of N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride overnight and then washing with DMF and methanol, resulting in a positively charged substrate surface (covered with
O3SiC3H6N(CH3)4). These kinds of positively charged substrates could be used to adsorb negatively charged polyelectrolytes.23 After the modification of the substrate surfaces, multilayers of MnTMPyP-PVPMe/PSS were assembled by the LBL method. Figure 2 shows a schematic drawing of the assembly processes on the gold substrate surface. Briefly, the negatively charged gold electrode (covered with one layer of 4-mercaptobenzoic acid) was immersed in a 2 mg/mL aqueous solution of MnTMPyP-PVPMe and then a 2 mg/mL PSS aqueous solution. After each assembly, the substrate was well washed with water and ethanol. The assembly time for each layer was about 30 min. For the LBL assembly on the quartz or ITO electrode surface, the first layer was negatively charged PSS and then the polyelectrolyte of MnTMPyP-PVPMe (Figure S1). Other experimental conditions were the same as those for the multilayer assembly on the gold surface. Quartz Crystal Microbalance Measurements. QCM measurements were carried out by using AT-cut gold-coated quartz crystals with a resonance frequency of 9 MHz (5 mm diameter, Seiko EG&G,

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Seiko Instruments Inc.). The frequency of the QCM was measured with a Seiko EG&G model 917 quartz crystal analyzer. The crystal was mounted in a cell by means of O-ring seals, with only one face in contact with the solution. The gold-coated quartz crystal plate was first immobilized by one layer of 4-mercaptobenzoic acid using a method similar to that for the gold electrode resulting in the formation of a negatively charged surface (covered with
SC6H4COOH), which was well washed with plenty of methanol and then alternately immersed into aqueous solutions of MnTMPyP-PVPMe and PSS for the assembly of LBL multilayers. After each assembly, the plate was well washed with water to remove unadsorbed polyelectrolytes. Characterization. UV
vis spectra for the metalloporphyrin and its polymeric derivatives in the dilute methanol solutions and LBL multilayers were acquired with the use of a Shimadzu UV-2550 UV
vis spectrophotometer. For the spectral measurement of the LBL multilayers, both sides of the modified quartz substrate were detected. Thus, the absorption intensity was divided by 2 when the surface density of MnTMPyP was estimated. XPS spectra were recorded using a VGESCALAB MKII multifunction spectrometer with nonmonochromatized Mg KR X-rays as the excitation source. The system was carefully calibrated by the Fermi edge of nickel and Au 4f2/7 and Cu 2p2/3 binding energies. A pass energy of 70 eV and a step size of 1 eV were chosen when taking spectra. In the analysis chamber, pressures of (1
2)  10
7 Pa were routinely maintained. The binding energies obtained in the XPS analysis were corrected by referencing the C 1s peak to 284.60 eV. Scan electron spectroscopy measurements were performed on a Shimadzu SSX-550 electron microscope. Field-emission scanning electron microscopy images were obtained on a Hitachi S-4800 microscope. The samples were assembled on the gold substrate surface in different layers. AFM images were also observed on the gold substrate surface by using an SPM-9500J3 scanning probe microscope (Shimadzu). Tapping mode was used with a tip fabricated from silicon (130 μm in length with a ca. 40 kHz resonance frequency) in air. Electrochemical Measurements. Electrochemical measurements for the LBL multilayers of MnTMPyP-PVPMe/PSS were performed on either gold or an ITO electrode by using an electrochemical analyzer (CHI 601b) in a 50 mM phosphate electrolyte solution (pH 11).24 A Pt wire and Ag/AgCl electrode were used as the auxiliary and reference electrodes, respectively. A gold or ITO electrode covered with layers of MnTMPyP-PVPMe was used as the working electrode. For cyclic voltammogram measurements of MnTMPyP in solution, freshly prepared glass carbon (GC) or gold was used as the working electrode. An initial potential of 0.6 V was applied for 2 s, and subsequent cyclic scans to a final potential of
0.4 or
0.8 V were carried out for 10 cycles. The CV curves and data of potentials and redox current intensities reported in the present work were for the 10th cycle. Chronocoulomograms were measured by setting a fixed initial potential Ei and several final potentials E. The charge Q(t) following each potential jump Ei f E was recorded versus the time t elapsed from the instant of the jump for 250 ms, after which the potential was stepped back to Ei. The Ei was set equal to 0 V, and E was in the range from
0.1 to
0.4 (in the solution) or
0.7 V (in the LBL multilayer) for measurements of Q(t)
t curves in a 50 mM phosphate electrolyte solution. All electrochemical measurements were made in an Ar atmosphere at room temperature. Catalytic Decoloration of Azo Dye. The peroxide oxidation of the azo dye catalyzed by multilayers of MnTMPyP-PVPMe/PSS was done by determining the decoloration curve (UV
vis absorption) of the dye in aqueous solution at pH 7.0 at room temperature. Hydrogen peroxide was used as an oxidant. The measurement was started after immersing the multilayer-modified quartz plate in the dye solution, which was composed of 1  10
5 mol/L azo dye, 4  10
4 mol/L H2O2, and 1  10
4 mol/L imidazole. 9882

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’ RESULTS AND DISCUSSION Synthesis of the Polymeric Metalloporphyrins. The watersoluble manganese(III) porphyrin of MnTMPyP was linked to PVP via axial coordination of Mn(III) ions to the pyridyl substituents of PVP. The produced MnTMPyP-PVP polymer has been used as a catalyst for olefin epoxidation and alkane hydroxylation.20 Although it was indeed a positively charged polymer (because of the linked MnTMPyP), during experiments, we found that MnTMPyP-PVP hardly acted as an alternative layer to form LBL multilayers with PSS by the conventional LBL method,23 which was probably because its charge distribution was not matched with that of PSS and the strong steric hindrance of the two polymeric backbones. Hence, we prepared its methylated MnTMPyP-PVPMe product by a simple reaction of MnTMPyP-PVP with iodomethane in chloroform solution. The produced MnTMPyP-PVPMe polyelectrolyte could be well dissolved in water and suitable for the assembly of LBL multilayers with PSS as discussed below. Absorbance of Manganese Porphyrin Derivatives in Solutions. Figure S2 shows the absorption spectra of the manganese porphyrin of MnTMPyP and its polymeric derivatives of MnTMPyP-PVP and MnTMPyP-PVPMe in dilute methanol solutions. All curves revealed several absorption bands between 300 and 700 nm corresponding to the Soret and Q-band absorption of the porphyrin macrocycles. The Soret absorption band appeared at 461 nm and the main Q band appeared at about 560 nm for all of these compounds, indicating that there was almost no influence on the π
π* electron transition after MnTMPyP was coordinatively bound to the pyridyl substituents of PVP. Besides the absorption of porphyrin macrocycles, another band appeared at about 250 nm that could be ascribed to the electron transition of pyridyl units of both MnTMPyP and PVP. The relative intensity of this absorption band became stronger for the polymers of MnTMPyP-PVP and MnTMPyPPVPMe than that for MnTMPyP, which was because there were many more pyridyl or pyridinium substituents in the polymers. On the basis of the absorption spectra of the porphyrin derivatives in the dilute solutions, the molar fractions of MnTMPyP relative to the pyridine substituents of PVP in the polymer of MnTMPyP-PVP and its methylated product of MnTMPyPPVPMe were estimated to be about 1:30. This value was very close to the molar ratio of MnTMPyP to PVP during the synthesis of MnTMPyP-PVP, where 100 mg of MnTMPyP and 400 mg of PVP were used as starting reactants. This means that almost all manganese porphyrins of MnTMPyP were anchored to the PVP resin through the axial coordination bond. Layer-by-Layer Assembly of Three-Dimensional MnTMPyPPVPMe/PSS Multilayers. Multilayers of MnTMPyP-PVPMe/ PSS were assembled on either silane-modified quartz and ITO substrate surfaces or the 4-mercaptobenzoic acid-modified gold electrode surface, the process of which was characterized by measuring their absorption spectra, the frequency changes of the quartz plate, or cyclic voltammograms after each assembly. Because the adsorption of the PSS layer did not lead to an obvious increase in the absorption of the Soret band of porphyrins, Figure S3 shows several absorption spectral curves for the multilayers after each assembly of the porphyrin polyelectrolyte of MnTMPyP-PVPMe. These curves revealed two groups of absorption bands: one appeared at about 469 nm corresponding to the Soret band of porphyrins together with weak Q bands between 500 and 650 nm; another one appeared in the wavelength range

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from 250 to 260 nm corresponding to the absorption of the pyridinium units of the adsorbed MnTMPyP-PVPMe. From Figure S2, it can be found that the Soret absorption band of porphyrin in the dilute methanol solutions (either MnTMPyP or its polymeric derivatives) appeared at about 461 nm and this band shifted to about 469 nm in the LBL multilayers of MnTMPyP-PVPMe/PSS. This red shift has often been observed in the organized ultrathin films of porphyrins or macrocyclic molecules and has been ascribed to a closely packed arrangement of porphyrin macrocycles in molecular assemblies.25 Previous work has revealed a larger red shift (sometimes about 20
30 nm) in the LB films of porphyrins because of the formation of J aggregates.26,27 If metal-mediated multiporphyrin arrays were formed, then this red shift could be reduced to about 6 nm because the coexisting metal ions weakened the interaction between porphyrin macrocycles.27 Here, a smaller red shift (8 nm) was observed, which could be attributed to the reason that the metalloporphyrin molecules of MnTMPyP were axially bound to the pyridyl substituents of PVP (Figure 1), thus their macrocycles could not be so closely packed to form J aggregates as those in the LB films. A plot of the Soret absorption intensity to the layer number was shown in Figure S4, which indicated that this absorption intensity was linearly increased with the layer number. This means that a similar number of polymeric porphyrins were adsorbed for each assembly. According to the Beer
Lambert law A = εcl, where A is the absorbance and ε, l, and c are the extinction coefficient, the thickness of the film, and the concentration of porphyrins within the film, respectively, we can calculate the surface density of MnTMPyP in the multilayers, dsurf = Aε
1. On the basis of the extinction coefficient of MnTMPyP in the dilute solutions, the estimated dsurf was about 2.7  10
10 mol/cm2. Thus, the average occupied molecular area of the MnTMPyP macrocycles in the LBL multilayers was about 0.61 nm2. Previous studies of the Langmuir monolayers of porphyrins have revealed that the average molecular area of the tetraarylporphyrin macrocycle was about 0.9 nm2 if the porphyrin macrocycles lie vertical to the aqueous surface and about 2.25 nm2 if the porphyrin macrocycles lie parallel to the aqueous surface.26,27 Furthermore, it has been found that porphyrin macrocycles usually overlapped with a tilt orientation in the monolayers and LB films, resulting in a smaller occupied average molecular area. For instance, we have found that the molecular area of the tetrapyridylporphyrin was about 0.6
0.65 nm2 on the water surface and LB films. Here, the average molecular area was about 0.61 nm2, which is close to that in the monolayer and LB films with an overlapped tilt orientation (J aggregates). However, if J aggregates were formed, then the Soret absorption should have a rather larger red shift (over 20 nm), but here the red shift was only 8 nm. Taking into account that MnTMPyP was axially coordinated to the polymer of PVP and not the simple “free” porphyrin molecules, we suggest that the porphyrin macrocycles were randomly arranged in the LBL multilayers with a weaker intermolecular π
π* interaction, the features of which will be discussed again together with the morphologies of the multilayers as shown in the SEM and AFM photographs below. QCM Response. The LBL assembly process was further characterized by measuring the frequency change on the gold surface in both solutions and in air during or after each assembly. As an example, Figure 3A shows the frequency change (ΔF) as a function of time (t) for the QCM gold resonator in the 2 mg/mL 9883

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Figure 3. (A) Frequency change during the adsorption of the MnTMPyP-PVPMe layer. (B) Plot of the frequency change against layer number after the adsorption of porphyrins.

aqueous solution of MnTMPyP-PVPMe, which indicated that the frequency decreased quickly at first and then the decrease became smaller and smaller. After about 30 min, the frequency did not show an obvious decrease. This frequency change suggested that the assembly was quick in the initial 20 min and then became slower to reach an equilibrium state between adsorption and desorption. The whole assembly of each layer could be completed within half an hour. The value of ΔF measured in air after each assembly of MnTMPyP-PVPMe and PSS was calculated. Figure 3B shows a plot of the total frequency change after the assembly of the metalloporphyrin polyelectrolyte against the layer number, which revealed an almost linear frequency change in the layer number (except for the initial two layers) and suggested that similar amounts of polyelectrolyte could be deposited for each assembly. The deviation of the initial layers with respect to the linear increase may be attributed to the fact that the first assembly was performed on the
SC6H4COOH-modified gold surface, the surface morphology of which was more uniform than that of the polymeric porphyrin layer, as will be shown in their SEM and AFM images below. This smooth surface had a smaller surface area, resulting in a weak adsorption of positively charged polyelectrolytes. The average frequency changes for the layers of MnTMPyP-PVP and PSS were about 2750 and 200 Hz, respectively. On the basis of ΔF =
2F02Δm/(AFq1/2μq1/2), where ΔF(Hz) is the frequency change caused by the adsorbed mass, F0 is the fundamental resonance frequency of 9 Hz, Δm(g) is the mass change, A is the electrode area (0.196 cm2), Fq is the density of the quartz (2.65 g/cm3), and μq is the shear module (2.95  1011 dyn/cm2),28 the mass changes for the layers of MnTMPyPPVP and PSS were estimated to be about 2.9 and 0.25 μg/cm2, respectively. Our previous studies have revealed mass changes of about 64 ng/cm2 for the SAMs of viologen thiol and about 0.38 μg/cm2 for an LBL multilayer of a poly(octylviologen) derivative and carbon nanotubes.29,30 That is, the mass change or surface coverage of viologens in the LBL multilayer of polymer and carbon nanotubes was much larger than that of viologen thiol (simple organic molecules). The possible explanation was that the large polymeric backbone could result in an increased roughness of the films, which led to stronger adsorption. Here, the mass change for the layers of MnTMPyP-PVP and PSS was even larger than that for the LBL multilayers of poly(octylviologen) derivatives and carbon nanotubes. These results

Figure 4. XPS spectra for the LBL multilayers of MnTMPyP-PVPMe/ PSS assembled on the quartz substrate surface.

suggested that much more metalloporphyrin polyelectrolyte (weight) was adsorbed during each assembly, which was attributed to the irregular polymeric backbone structure of the MnTMPyP-PVPMe polyelectrolyte and its random distribution in the LBL multilayers. Further evidence and discussion will be presented together with the SEM and AFM images after each assembly of the polymers of MnTMPyP-PSSMe and PSS below. X-ray Photoelectron Spectra. Element compositions for the LBL multilayers of MnTMPyP-PVPMe/PSS on the quartz substrate surfaces were detected by using the XPS spectra, which showed five peaks in the binding energy range from 100 to 800 eV except for Si from the substrate. As shown in Figure 4, the binding energies for these five peaks are 169.4, 287.2, 401.4, 532.5, and 643.8 eV, which could be assigned to S 2p, C 1s, N 1s, O 1s, and Mn 2p, respectively. Elements S, C (partially), and O (partially) were obtained from the negatively charged polymer of PSS, and those of Cl, C (partially), N, and Mn were obtained from the polyelectrolyte of MnTMPyP-PVPMe. These data confirmed that the LBL multilayers were composed of MnTMPyP-PVPMe and PSS. Very low contamination was detected for the halogens (Cl and I), which suggested that the small anionic counterions were replaced by the larger anionic polymer of PSS. This was in agreement with results obtained in previous work31 and was complied with the entropy-increasing process for the formation of the LBL multilayers. Morphologies of the LBL Multilayers. The morphologies of the LBL multilayers of MnTMPyP-PVPMe/PSS were investigated by using SEM, FESEM, and AFM techniques. Figure 5 shows several SEM images for the gold substrate surface covered 9884

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Figure 5. SEM images of gold substrate surfaces covered with (A) the carboxylate layer of SC6H4COOH, (B) the first layer of MnTMPyPPVPMe, (C) a double layer of MnTMPyP-PVPMe/PSS, and (D) three layers of MnTMPyP-PVPMe/PSS films. The inset photographs were obtained by using FESEM.

with an initial carboxylate layer of
SC6H4COOH, the first layer of MnTMPyP-PVPMe, a double layer of MnTMPyP-PVPMe/ PSS, and three layers of MnTMPyP-PVPMe/PSS hybrids. The inserted images are obtained by using FESEM with higher resolution. These photographs revealed the following features. First, a smooth surface was observed for the gold surface covered with an initial layer of
SC6H4COOH (Figure 5A), which indicated uniform coverage for the self-assembled monolayer (SAM) of
SC6H4COOH on the gold surface. The negatively charged surface (after immersion in an aqueous solution) was used to adsorb the positively charged polyelectrolyte of MnTMPyPPVPMe. Second, MnTMPyP-PVPMe formed a rough layer with some domains or defects on the SAM (Figure 5B). Some defects reached several hundred nanometers, the size of which could be decreased by increasing the assembly time, but could not be completely eliminated. However, after the negatively charged layer of PSS was adsorbed on the MnTMPyP-PVPMe surface, most defects could be covered or reduced to form a rather smooth surface with some domains or aggregates (Figure 5C). Finally, when more and more layers of polyelectrolytes of MnTMPyP-PVPMe and PSS were adsorbed to form LBL multilayers, the film became thicker and thicker, again resulting in a rough surface (Figure 5D), which could be attributed to the reason that more polyelectrolyte molecules were adsorbed after each assembly. These SEM and FESEM images suggested that the first layer of MnTMPyP-PVPMe polyelectrolyte randomly adsorbed and separated on the SC6H4COOH-modified gold surface, resulting in a positively charged polymer layer. Then, the negatively charged PSS layer was adsorbed. These PSS molecules mainly surrounded the polymer backbones or aggregates of MnTMPyPPVPMe and formed some “domains”. After several cycles, more and more molecules “accumulated” on top of these domains or aggregates; as a consequence, a rough surface was formed for the LBL multilayers.

Figure 6. AFM topographic images of the gold substrate surfaces covered by (A) the carboxylate layer of SC6H4COOH, (B) the first layer of MnTMPyP-PVPMe, (C) a double layer of MnTMPyP-PVPMe/ PSS, and (D) three layers of MnTMPyP-PVPMe/PSS films. (E) High profiles corresponding to the lines in the AFM images (A, 3 3 3 ; B, - 3 - 3 - 3 ; C, ---; and D, —).

To give more structural information about the multilayers of MnTMPyP-PVPMe and PSS on the substrate surface, we used AFM to observe the same multilayers as those for the SEM and FESEM measurement with a characterization of surface morphology and height distribution of the films or aggregates. Figure 6A
D shows the AFM images for the gold substrate surfaces covered by the initial carboxylate layer of
SC6H4COOH, the first layer of MnTMPyP-PVPMe, a double layer of MnTMPyPPVPMe/PSS, and three layers of MnTMPyP-PVPMe/PSS hybrids. Similar to the SEM images, these AFM photographs revealed a rather smooth surface of the gold surface covered by a SAM layer of
SC6H4COOH (Figure 6A). Then, some domains formed and closely packed when the first layer of the polyelectrolyte of MnTMPyP-PVPMe was adsorbed. The size for most domains was within 20 nm. With more and more layers of polyelectrolytes of MnTMPyP-PVPMe and PSS adsorbed, the domains became larger and larger with the height of the films increased (Figure 6C,D). 9885

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Figure 7. Cyclic voltammograms for the polyelectrolyte of MnTMPyP-PVPMe and its LBL multilayer with PSS in the 0.05 M phosphate electrolyte solution at pH 11.0. (A) A naked gold electrode in the MnTMPyP-PVPMe solution, (B) a naked glass carbon electrode in the MnTMPyP-PVPMe solution, (C) a gold electrode covered by one layer of MnTMPyP-PVPMe/PSS, and (D) a gold electrode covered by three layers of the MnTMPyPPVPMe/PSS film.

Figure 6E shows the height profiles for the monolayer and multilayers, which corresponding to the lines of AFM images in Figure 6A
D. These curves indicated the following two features. First, a smooth monolayer was formed for the SAM monolayer of
SC6H4COOH, which was then becoming a rough surface during multilayer formation. Some domains could reach about 150 nm after three layers of MnTMPyP-PVPMe were adsorbed. Second, the thickness of the films increased with the layer number. Both features were in agreement with those observed from the UV
vis absorption and QCM measurements and confirmed the formation of 3D LBL multilayers. Cyclic Voltammograms of the MnTMPyP-PVPMe-PSS LBL Multilayers. Figure 7 shows the CV curves for the gold electrode covered by one and three layers of MnTMPyP-PVPMe/PSS films together with those of the polyelectrolyte of MnTMPyPPVPMe in the phosphate buffer at pH 11. Freshly cleaned gold and GC electrodes were used for the CV measurements of MnTMPyP-PVPMe in the buffer solutions, which revealed two couples of waves in the potential range from
0.4 to 0.6 V vs Ag/ AgCl (Figure 7A,B). By taking the gold electrode as an example, the cathodic peaks appeared at about
0.12 and 0.50 V, with the corresponding anionic peaks at about
0.30 and 0.40 V, respectively. On the basis of the electrochemical behaviors of MnTMPyP in the electrolyte solutions,24,32 we can conclude that these two couples of redox waves were ascribed to the electron-transfer processes of Mn(II)TMPyP-PVPMe T Mn(III)TMPyP-PVPMe and Mn(III)TMPyP-PVPMe T Mn(IV)TMPyP-PVPMe, respectively. For the gold or ITO electrode covered by LBL multilayers of MnTMPyP-PVPMe/PSS, both couples of waves could be detected but the current intensity was weak. This may be because there was a silane and/or a PSS layer between MnTMPyPPVPMe and the electrode surface, which could increase the film resistance and hinder electron transfer between the electrode surface and the metalloporphyrins, which resulted in a significant decrease in the current intensity. Increasing the layers of porphyrin polyelectrolyte in the multilayers led to an increase in the

current intensity because more MnTMPyP-PVPMe molecules were adsorbed, but we failed to obtain a linear increase in the current intensity with respect to the layer number. This may also be because of the increase in the film resistance after the anionic polymer of PSS was adsorbed. Chronocoulometric Properties of the MnTMPyP-PVPMePSS LBL Multilayers. To obtain further information on the electrochemical properties of metalloporphyrins, we investigated the charge-transfer behaviors in the LBL multilayers by the potential step chronocoulometry method. This method has often been used to determine the product of D or D1/2C in the filmmodified electrodes, where D is the charge-transfer diffusion coefficient for the diffusion-like propagation of charge through the films and C is the concentration of the redox centers in the films.33 Figure 8A shows a series of Q(t)
t curves for the freshly cleaned GC electrode in the 50 mM phosphate electrolyte solution. The potential jump was from an initial potential of 0 V to several final potential Es. Es ranged from
0.1 to
0.4 V. The curves in Figure 8A indicated the following two features. (i) A very small charge increase was recorded for the potential jump from 0 to
0.1 V because the MnTMPyP molecules were still electroinactive. (ii) For the potential jumps from
0 to
0.2 to ∼
0.4 V, the charge increased quickly in the initial 50 ms and then gradually increased with time t. It has been pointed out that the initial quick increase was due to the flow of the capacitive current that was required to charge the interface.34 The gradual increase was due to the electroreduction of the metalloporphrins adsorbed on the GC electrode. Figure 8C shows a series of Q(t)
t curves for the gold electrode covered with one layer of MnTMPyP-PVPMe/PSS in the 50 mM phosphate electrolyte solution. The potential jump was from an initial potential of 0 V to several final potentials Es. Es ranged from
0.1 to
0.7 V. In this case, the curve for the potential jump from 0 to
0.1 V was similar to that observed in Figure 8A, suggesting that the MnTMPyP in the LBL multilayer was electroinactive. However, a very quick charge increase for the 9886

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Figure 8. Q(t) versus t curves for the electroreduction of metalloporphyrin (A, in solution; C, in the LBL multilayer) in the 0.05 M phosphate electrolyte solution. Contrell plot for the electroreduction of metalloporphyrin (B, in solution; D, in the LBL multilayer) in the 0.05 M phosphate electrolyte solution.

potential jump from 0 to
0.5 and
0.7 V in the initial 10 ms may be attributed to the fact that the MnTMPyP-PVPMe layer was closely adsorbed on the electrode surface resulting in a quick reduction reaction for the metalloporphyrin molecules. After such a quick increase, the charge increased gradually, which may be due to the fact that most metalloporphyrin molecules have been reduced. On the basis of the chronocoulometric results in Figure 8A,C, we can calculate the value of D or D1/2C according to the Cottrell equation Q(t) = 2nFAC(Dt)1/2/π 1/2, where n is the number of electrons, F is the Faraday constant, A is the electrode area, C is the concentration of electroactive species, and D is the diffusion coefficient. Figure 8B,D shows Cottrell plots of the quantity of electricity Q(t) with respect to the root of time (t1/2) for the metalloporphyrins in solution and LBL multilayers. According to the Cottrell equation, the slope of the curve equals 2nFACD1/2/π1/2. Thus, the charge-transfer diffusion coefficient D was about 4.8  10
8 cm2/s in the solution (based on the jump from 0 to
0.4 V). Theoretically, the diffusion coefficient in the LBL multilayers could also be calculated on the basis of the Cottrell equation. However, because the concentration of metalloporphyrins in the LBL multilayers was unknown, it was difficult to obtain the D value in the LBL multilayers according the slope of the curve of Q(t)
t1/2. Thus, we provide only the curves of Q(t)
t1/2 for MnTMPyP-PVPMe/PSS (Figure 8D). Potential Application as a Heterogeneous Catalyst. It is well known that metalloporphyrins in solution and modified solid surfaces can catalyze several kinds of organic reactions, such as olefin epoxidation and alkane hydroxylation,35 or catalyze the electrochemical reduction of CO2.36 Besides, they could combine with proteins to mimic some biological catalytic processes.37 Here, as an example, we investigated the decoloration of an azo dye catalyzed by using three layers of MnTMPyP-PVPMe/PSS LBL multilayers immobilized on the quartz substrate surface in an aqueous solution of the dye at pH 7.0 at room temperature. Figure 9 shows several typical absorption curves of the azo dye oxidized by hydrogen peroxide and catalyzed by three layers of

Figure 9. Absorption spectra for the azo dye oxidized by hydrogen peroxide and catalyzed by three layers of MnTMPyP-PVPMe/PSS multilayers immobilized on the quartz substrate surface in an aqueous solution containing 1  10
5 mol/L azo dye, 4  10
4 mol/L H2O2, and 1  10
4 mol/L imidazole at different reaction times (top to bottom: 0, 2, 15, 30, and 60 min).

MnTMPyP-PVPMe/PSS multilayers in aqueous solution at reaction times of 0, 2, 15, 30, and 60 min. These curves revealed that the main absorption of dye at about 478 nm gradually decreased with the reaction time, which was in agreement with the results reported by Nango and co-workers.38 A control experiment without the addition of the LBL multilayers did not show a significant decrease in dye absorption. Thus, we concluded that the as-prepared MnTMPyP-PVPMe/PSS multilayers could act as a heterogeneous catalyst for the oxidation of azo dyes. More research on the catalytic applications is currently underway in our laboratory and will be reported elsewhere.

’ CONCLUSIONS We have demonstrated the preparation of LBL multilayers of the water-soluble manganese porphyrin-linked poly(vinylpyridinium) 9887

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Langmuir polyelectrolyte of MnTMPyP-PVPMe with anionic polymer PSS on gold, quartz, and ITO substrate surfaces by the layer-by-layer method. The average mass change for each layer of MnTMPyPPVPMe and PSS could reach about 2.9 and 0.25 μg/cm2, respectively. However, because the porphyrin of MnTMPyP was axially coordinated with the pyridyl substituents of poly(vinylpyridine), its macrocycles were loosely packed in the multilayers, resulting in a small red shift of the Soret absorption band of porphyrins. Also because of the axially coordinated porphyrin macrocycles that altered the polymeric backbone of the polyelectrolyte, a rough surface for the LBL multilayers was formed and characterized by the SEM and AFM images. In addition, the multilayer-modified electrodes showed couples of redox waves corresponding to the immobilized manganese porphyrins though the current intensity was not very strong because of the film resistance caused by the coexisting polymeric backbones. It is expected that the present metalloporphyrin multilayers could act as a kind of heterogeneous catalyst for organic reactions.

’ ASSOCIATED CONTENT

bS

Supporting Information. Schematic drawing of the LBL multilayers on the quartz or ITO substrate surface. Absorption spectra for MnTMPyP, MnTMPyP-PVP, and MnTMPyPPVPMe as well as those for the MnTMPyP-PVPMe/PSS multilayers after the assembly of the porphyrins. Plots of absorption intensity against layer number. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86-21-65643666.

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