Electrochemical and Spectroscopic Behavior of Iron (III) Porphyrazines

J. Phys. Chem. B 2008, 112, 11517–11528

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Electrochemical and Spectroscopic Behavior of Iron(III) Porphyrazines in Langmuir-Scha¨fer Films Gaetano Garramone,‡,† Daniela Pietrangeli,† Giampaolo Ricciardi,*,† Sabrina Conoci,⊥ Maria Rachele Guascito,*,§ Cosimino Malitesta,§ Daniela Cesari,§,| Serena Casilli,# Livia Giotta,3 Gabriele Giancane,# and Ludovico Valli*,# Dipartimento di Chimica, UniVersita` della Basilicata, Via N. Sauro, 85, I-85100 Potenza, Italia, Laboratorio di Chimica Analitica, Dipartimento di Scienza dei Materiali, UniVersita` del Salento, Via Arnesano 73100 Lecce, Italia, Istituto di Scienze dell’Atmosfera e del Clima, CNR, Str. ProV. Lecce-Monteroni Km 1.2, 73100 Lecce, Italia, LabonChip R&D, Microfluidic DiVision, CPG Group, STMicroelectronics, Stradale Primosole 50 - 95121 Catania, Italia, Dipartimento di Ingegneria dell’InnoVazione, UniVersita` del Salento, Via Monteroni 73100 Lecce, Italia, and Laboratorio di Chimica Fisica, Dipartimento di Scienza dei Materiali, UniVersita` del Salerno, Strada ProVinciale Lecce-Monteroni, 73100 Lecce, Italia ReceiVed: April 16, 2008; ReVised Manuscript ReceiVed: June 30, 2008

Thin films of a newly synthesized iron(III) porphyrazine, LFeOESPz (L ) ClEtO, OESPz ) ethylsulfanylporphyrazine), have been deposited by the Langmuir-Scha¨fer (LS) technique (horizontal lifting) on ITO or gold substrates. Before deposition, the floating films have been investigated at the air-water interface by pressure/area per molecule (π/A) experiments, Brewster angle microscopy (BAM) and UV-vis reflection spectroscopy (RefSpec). The complex reacts with water subphase (pH 6.2) forming the µ-oxo dimer, which becomes the predominant component of the LS films (LS-Fe) as indicated by optical, IR, XPS, and electrochemical data. LS-Fe multilayers exhibit, between open circuit potential (OCP) and +0.90 V (vs SCE), two independent peak pairs with formal potentials, Esurf (I) and Esurf(II) of +0.56 V and +0.78 V, respectively. According to dynamic voltammetric and coulometric experiments the peak pair at +0.56 V is attributed to one-electron process at the iron(III) centers on the monomer, while the peak pair at +0.78 V is associated to a four-electron process involving µ-oxo-dimer oligomers. LS-Fe films prove to be quite stable electrochemically between OCP and +0.90 V. The electrochemical stability decreases, however, when the potential range is extended both anodically and cathodically outside these limits, due to formation of new species. Upon incubation with TCA solutions, LS-Fe films show remarkable changes in the UV-vis spectra, which are consistent with a significant µ-oxo dimer f monomer conversion. Addition of TCA to the electrochemical cell using a LSFe film as working electrode, results in a linear increase of a cathodic current peak near -0.40 V as the TCA concentration varies in the 0.1-2.0 mM range. This behavior is interpreted in terms of TCA inducing a progressive change in the composition of the LS-Fe films in favor of the monomeric iron(III) porphyrazine, which is responsible for the observed increase in the cathodic current near -0.40 V. Introduction Porphyrazines (Pzs), sometimes called azaporphyrins, and their transition metal complexes are small-ring tetrapyrroles with high potential in a wide array of applications ranging from advanced materials1-17 to biomedicine.5,18-20 Very recently, the complex speciation and reactions of a novel water-soluble iron(III) porphyrazine with NO and H2O2, have been extensively investigated by van Eldik et al., which also opens interesting perspectives in the use of these complexes as effective catalysts.21 The Pz ligand is composed of four pyrrole rings bridged * Corresponding authors. E-mail: (G.R.) [email protected]. † Dipartimento di Chimica, Universita ` della Basilicata. ‡ Present address: Dipartimento di Scienze Chimiche e Ambientali, Universita` dell’Insubria, via Valleggio 11, 22100 Como, Italy. ⊥ LabonChip R&D, Microfluidic Division, CPG Group, STMicroelectronics. § Laboratorio di Chimica Analitica, Dipartimento di Scienza dei Materiali, Universita` del Salento. | Istituto di Scienze dell’Atmosfera e del Clima, CNR. # Dipartimento di Ingegneria dell’Innovazione, Universita ` del Salento. 3 Laboratorio di Chimica Fisica, Dipartimento di Scienza dei Materiali, Universita` del Salerno.

by nitrogen atoms, the most relevant electronic effect of which is to induce a remarkable stabilization of the lowest oxidation states of the coordinated transition metal ions, relative to the porphyrin counterparts.22-24As a matter of fact, in MPzs (M ) Mn, Fe), the formal MIII/MII redox potential is found to shift anodically by more than 400 mV (vs SCE) relative to the porphyrin analogs, which makes these systems also of potential interest in electrocatalysis.23,25 Moreover, due to the presence of the aza bridges, Pzs are strong σ donors, much stronger than porphyrins, to the effect that several physicochemical properties of their transition metal complexes are significantly modified/ enhanced relative to metalloporphyrins. For instance, in iron(III) porphyrazine µ-oxo dimers, [FePz]2Os, the bridging oxygen is remarkably more basic, i.e. more sensitive to protic environments, than in porphyrin µ-oxo dimers, [FeP]2Os.23 Just as found for porphyrins,26 the acid-base behavior of iron(III) porphyrazines is expected to change significantly upon incorporation in monolayer films or similar supramolecular assemblies, where packing interactions are known to cooperate with protic stimuli in controlling, both thermodinamically and kinetically, the monomer f dimer conversion. Indeed it has been shown that,

10.1021/jp803418b CCC: $40.75  2008 American Chemical Society Published on Web 08/22/2008

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SCHEME 1: Schematic Representation of the Molecular Structure of the LFeOESPz Complex

while stirring chloroform solutions of iron(III) porphyrins26 with aqueous solutions at pH 7 results in a nearly complete monomer f dimer conversion over a period of 3 days, spreading identical solutions on water surfaces to form ordered monolayers leads to a rapid monomerfdimer conversion. Furthermore, iron(III) porphyrazines and porphyrins are expected to exhibit different acid-base behavior in a supramolecular environment, as packing interactions strongly rely on the size and nature of the macrocycle framework. This is especially true when considering porphyrazines bearing peripheral heteroatoms, i.e., nitrogen or sulfur, which may enable a significant out-of-plane coordinating capability of the macrocycle.1,4,17,27 Conversion of monomeric iron(III) porphyrazines into their µ-oxo dimers is accompanied by striking changes in several physicochemical properties. For instance, the intense feature appearing in the visible region of their electronic absorption spectrum shifts to the red by more than 110 nm going from the monomer to its µ-oxo dimer. Moreover, while the µ-oxo dimer exhibits dual emission, i.e. it shows emission upon excitation into the Soret and the Q-band, the monomer is not luminescent.28 Finally, the electrochemical behavior of the redox active iron(III) center, is expected to vary significantly upon dimerization, both in solution and in a supramolecular environment. Therefore, the response of iron(III) porphyrazines to protic stimuli can be monitored through the changes in their optical and redox properties, which may give these tetrapyrrolic systems a potential as protic sensors. This prompted us to build up multilayered films, using as building block a newly synthesized iron(III) porphyrazine, the iron(III) 2-chloroethoxy-2,3,7,8,12,13,17,18-octakis(ethylthio)porphyrazine, ClEtOFeOESPz (hereafter denoted LFeOESPz), in conjunction with the Langmuir-Scha¨fer technique (see Scheme 1 for a schematic drawing of the LFeOESPz molecular structure). Water subphases of known pH were employed to verify the effect of the proton concentration on the chemical composition of the films. The structure and the qualitative chemical composition of the floating films at the air-water interface was investigated not only by the registration of Langmuir isotherms, but also through utilization of two innovative techniques, Brewster angle microscopy (BAM) and reflection spectroscopy (RefSpec) in the UV-vis wavelength range. The morphology of the LS films deposited on hydrophobised silicon surfaces was inspected by atomic force microscopy (AFM). The spectroscopic behavior of multilayers deposited onto indium tin oxide (ITO) substrates was studied by transmission UV-vis and ATR-FTIR spectroscopy. To verify the effects of the changes in chemical composition of the supramolecular assemblies on the redox behavior of the iron(III) centers, the redox response of LS films (LS-Fe), deposited both on ITO or Au substrates, as well as their stability under cyclic redox treatments, were studied in aqueous media by voltammetric techniques. In addition, the optical and electrochemical response of the LS films to proton stimuli was explored. LS films based on the

free base ethylsulfanylporphyrazine, H2OESPz, were also examined for the sake of comparison. For the description of the main optical and structural features of H2OESPz LS films (LS-H) we rely on a previous paper, where the fabrication and characterization of LS films based on a very similar porphyrazine, methylsulfanylporphyrazine (H2OMSPz), have been discussed in detail.29 Experimental Section Materials. All chemicals, including NaClO4, trichloracetic acid (TCA), and acetic acid (AA), and solvents (Aldrich Chemicals Ltd.) were reagent or analytical grade and used in the syntheses or electrochemical experiments as supplied. Solvents used in physical measurements were spectroscopic or HPLC grade. Silica gel used for chromatography was Merck Kieselgel 60 (70-230 mesh). Microanalyses were performed at the Butterworth Laboratories Ltd., Teddington, U.K. Atomic Force Microscopy (AFM) Measurements. The AFM images of LS-Fe films deposited on 1,1,1,3,3,3-hexamethyldisylazane- (EMDS-) hydrophobized silicon surfaces (1, 4, and 10 layers) were acquired in air by using a Digital 3100 instrument in tapping mode. Commercially available tapping etched silicon probes (Digital) with a pyramidal shape tip having a nominal curvature of 10 nm and a nominal internal angle of 35° were used. Fluorescence Confocal Microscopy Measurements. Fluorescence images of LS-Fe films deposited onto silicon hydrophobic substrate (1, 4 and 10 layers) were acquired under ambient conditions by Leica TCS SP2 AOBS confocal microscope. The instrument is equipped with 2 excitation lasers sources: Ar at 458 nm-514 nm and HeNe 543nm and 633 nm, respectively. The light emission is monitored by an acoustic optical beam splitter detector with the option of 2 channels for light recording: one channel (photomultiplier R 6357) that collects the light emission in the spectral range 185-900 nm and the other one (photomultiplier R 6358) that collects light in the spectral range 185-835 nm. Electrochemical Measurements. The electrochemical experiments were carried out through a CH-Instruments Model620A Electrochemical Analyzer, and a conventional three-electrode system. The 0.1 M NaClO4 electrolyte solution was preliminarily deoxygenated by bubbling nitrogen, and a nitrogen atmosphere was maintained in the cell throughout the measurements. ITO/ LS-Fe (30 monolayers), Au/LS-Fe (30 monolayers) and ITO/ LS-H (40 monolayers) were used as working electrodes without further preparation. Working electrodes were constructed by attaching a steel bar to the ITO glass or gold substrates with a screw. The ITO or gold substrates (1.5 cm × 1 cm) were vertically dipped into the electrolyte solution to expose the LSfilm coated surface solely to the electrolyte. A Pt wire was always employed as counter electrode and a saturated calomel electrode (SCE) as reference electrode. UV-Visible Spectroscopy. LS-Fe films deposited onto ITOglass slides were analyzed by transmission visible spectroscopy

Behavior of Iron(III) Porphyrazines in the 300 - 860 nm wavelength range using a Perkin-Elmer Lambda 850 UV-vis spectrometer. The spectral resolution was 1 nm. A bare ITO-glass slide was employed as a blank. XPS Spectroscopy. XPS spectra were performed using a Thermo VG Theta Probe instruments (from VG Scientific) equipped with an Al KR monochromatic source (hν ) 1486.6 eV). Survey and high-resolution spectra were acquired in CAE mode at pass energies of 200 and 150 eV respectively, spot size 400 µm. High-resolution spectra were referenced to the C1s main component binding energy of the adventiousus hydrocarbon at 285 eV. FTIR Measurements. Midinfrared spectra were acquired with a Perkin-Elmer Spectrum One FTIR spectrometer equipped with a DTGS detector. The spectral resolution used for all measurements was 4 cm-1. The remarkable reflectivity of indium tin oxide in the mid-infrared range allowed analyzing LS films deposited onto ITO-coated glass slides by FT-IR reflection spectroscopy. To this purpose a specular multireflection apparatus, designed specifically for thin film analyses (AmplifIR-SensIR technologies) and suitably accommodated into the sample area of the FTIR spectrometer, was employed. In this device, the IR beam is externally reflected between the sample surface and an adjustable gold-coated mirror. According to the specular reflection principles and the geometry of the apparatus the IR beam passes through the film, interacts with the sample and is then reflected by the ITO layer toward the Au-coated mirror. The distance of the mirror with respect to the sample was adjusted for achieving the maximum number of specular reflections of the beam before being conveyed to the detector. A total of 32 interferograms were recorded and averaged for each spectrum. Syntheses. H2OESPz. The free base porphyrazine was prepared according to the procedure reported in ref 29. LFeOESPz. H2OESPz (0.100 g, 0.126 mmol) and FeBr2 (35 mg, 0.163 mmol) were dissolved in 2-chloroethanol (12 mL). A catalytic amount of symmetric collidine was added and the stirred mixture was refluxed under purified N2. After 8 h the free base porphyrazine was no longer present as indicated by UV/vis spectroscopy and by TLC chromathography (TLC 5 × 10 cm plates, silica gel 60 F254; 9:1 v/v CH2Cl2/ClC2H4OH). Crystallization of the reaction mixture at -20 °C afforded a dark solid, that, after recrystallization from 2-chloro-ethanol at -20 °C, washing with small amounts of cold ethanol (3 mL × 3), and drying in Vacuo gave the desired compound (reddish platelets, yield >90%) (Found: C, 43.12; H, 4.34; N, 12.49; S, 27.85. Anal. Calcd for C34H44N8S8OClFe: C, 43.97; H, 4.77; N, 12.06; S, 27.62). 1H NMR (500 MHz, CDCl3, 297 K), δ/ppm: 1.371 and 1.426 (4H, O-CH2 and CH2-Cl); 3.182 (m, 24 H, -CH3); 11.293, 11.930 (m, 16H, S-CH2). [Fe(OESPz)]2O. The complex was prepared by quantitative conversion of pure LFeOESPz according to the procedure reported in ref 25.

J. Phys. Chem. B, Vol. 112, No. 37, 2008 11519 SCHEME 2: Schematic Representation of the Molecular Structure of the [FeOESPz]2O Complex

Figure 1. Evolution of the electronic absorption spectrum of a LFeOESPz chloroform solution ([LFeOESPz] ) 1.2 × 10-6 M) layered with an aqueous solution at pH 8.0 (a) and at pH 6.2 (b).

Results and Discussion Optical and Redox Behavior of LFeOESPz and [FeOESPz]2O in Solution. To fully understand the physicochemical properties of LFeOESPz and its µ-oxo dimer (see Scheme 2 for a schematic representation of the latter compound) in LS multilayer assemblies, it is useful to briefly analyze the essential aspects of their spectroscopic and electrochemical behavior in solution. The spectroscopic behavior is considered first. The UV-vis electronic absorption spectra of LFeOESPz and [FeOESPz]2O in chloroform are reported in Figure S1. The spectra were taken at a concentration sufficiently low (ca. 10-6

M) to prevent significant aggregation of the complexes. Therefore, the evident breadth of the absorption bands can be traced either to inhomogeneous broadening caused by multiple conformations accessible in solution or to the presence of additional excited states due to the peripheral substituents. Similar to ClFeOESPz,25 LFeOESPz shows a prominent feature in the visible, at ca. 592 nm, two distinct bands in the red region, at 670 and 800 nm, and a broad envelope in the near-UV terminating with an intense absorption at 350 nm, commonly denoted Soret band. The µ-oxo dimer exhibits a very different


Figure 2. Π vs A Langmuir curve of floating films of LFeOESPz spread over an aqueous subphase at pH 6.2.

spectral behavior. As seen in Figure S1, it shows a quite intense absorption at 702 nm (the Q-band), followed to the blue by a plateau, which coalesces within the Soret band peaking at ca. 355 nm. Layering chloroform solutions of LFeOESPz with aqueous solutions of known pH the monomer f µ-oxo dimer conversion takes place at the interface, and the process can be followed spectrophotometrically. As inferred from Figure 1, at pH 8 the conversion of LFeOESPz into the µ-oxo dimer is complete within 15 h, whereas, at pH 6.2, it is only partial over the same period of time. The spectral changes exhibit highly isosbestic behavior, suggesting that the monomer f µ-oxo dimer conversion does not involve formation of intermediates, or that they are not detectable spectrophotometrically. It is worth noting that layering chloroform solutions of the µ-oxo dimer with aqueous solutions, gave the same results at pH 8.0. At pH 6.2 the dimer did not show, instead, significant conversion to the monomer. The µ-oxo dimer f monomer conversion became fully reversible only at pH ∼2.8. This is at variance with porphyrinic systems, that are known to show a reversible behavior in the whole 10-0.8 pH range.26 A detailed cyclic voltammetric study of LFeOESPz and [FeOESPz]2O complexes in solution will be reported elsewhere. However, electrochemical data relevant to the interpretation of the redox behavior of LS films are anticipated here. In DMF the monomer exhibits between +0.90 V and -1.40 V (vs SCE) three quasi-reversible, diffusion-controlled one-electron processes, at +0.19, -0.36, and -0.95 V (vs SCE) (Figure S2). The anodic process occurs at nearly the same potential as in ClFeOESPz25 and ClFeOEPz,23 where it was assigned to the FeIII/FeII couple. Notably, the FeIII/FeII potential is anodically shifted by some 300-400 mV relative to iron(III) porphyrins, on account of the porphyrazine ligand stabilizing more effectively than the porphyrin ligand the Fe(III) and Fe(II) oxidation states.23,25 The first and second reductions are due to reduction of the porphyrazine π-system. The iron µ-oxo dimer is stable in DMF and undergoes, between +0.9 and -1.4 V (vs SCE), two oxidations and four reductions (Figure S3). Oxidations occur at E1/2 ) +0.37 and +0.62 V, while reductions are seen at E1/2 ) -0.26, -0.48, and -0.92 V and at Epc ) -1.30 V. Coulometric experiments showed that all waves involve one-electron processes (i.e., 0.5 e-/macrocycle), but the wave at -0.92 V, which proved to be a two-electron reduction. On the basis of the redox behavior of the monomer,

Garramone et al. which does not show any ligand oxidation in the investigated anodic region, the two anodic waves seen in the voltammogram of the µ-oxo dimer can be associated to formation of (stable) ferrous-ferric and ferrous-ferrous dimeric forms. Langmuir Experiments. In this section, we examine the salient features of floating films obtained by spreading the complex LFeOESPz on a water subphase at pH 6.2. The Π vs A Langmuir curve of floating films of LFeOESPz is illustrated in Figure 2. The Langmuir curve does not evidence a peculiar feature other than the smooth transition to the condensate state, as already observed by us in a previous study on another tetraazaporphyrin.29 Also earlier literature data manifested that most of simple porphyrin derivatives do not constitute uniform Langmuir monolayers30 and exhibit a generally low collapse pressure.31 The limiting area per molecule (obtained by extrapolation to zero surface pressure of the steepest and linear portion of the Langmuir curves) is 18 Å2, indicating, as suspected on the basis of solution experiments, that the porphyrazine molecules lie mostly in dimeric sites occupying about a quarter the area expected for closely packed monomers (ca. 80 Å2). Notably, the area/molecule we find is absolutely smaller than that expected even for an edge-on or tilted orientation of molecules on the water surface. Probably substantial interaction between porphyrazine rings predominates over the interaction with the polar water molecules in the subphase. It seems that this porphyrazine derivative is too hydrophobic to form stable, pure Langmuir monolayers at the air-water interface. Such a reduced value suggests that the floating film of LFeOESPz consists of more than a single monolayer in thickness. This behavior is also consistent with the observation that the so-called hydrophile-lipophile balance (HLB)32 remarkably leans toward the hydrophobic counterpart. A plausible rationale of such a small value of the limiting area is that, upon compression of the floating layer, a significant fraction of dimeric and monomeric molecules are forced out of the interface in different arrangements. It is also probable that a small percentage of molecules are effectively at the air-water interface, while the rest essentially extends far from the contact with the water subphase because the hydrophobic nature of the whole macrocycle forces the molecules out of the interface. Brewster angle microscopy (BAM) is a rather recent and powerful technique usually utilized to monitor directly the morphology of floating layers at the air-water interface.33 In actual interfaces, the intensity of the reflected radiation is mainly determined by the interfacial characteristics and evidence a minimum at the Brewster angle. Both the thickness of the floating layers and the roughness at the interface are critical parameters that determine the BAM images. Since the BAM investigation was carried out during the compression of the floating layer, this method provided essential information about the presence of 3D aggregates on the water surface. For example, when LFeOSPz was spread on the subphase, the existence of wide areas of uncovered subphase and, at the same time, of very large three-dimensional aggregates floating onto the clean water surface was observed, even immediately after the spreading solvent evaporation. This is illustrated in Figure 3, parts a and b, respectively, where images taken at a surface pressure of 0 mn/m are reported. According to the Langmuir curve, the floating film in Figure 3b is much more than a singular monolayer in thickness and, as apparent in the upper portion of the image, not really homogeneous. During compression such domains coalesce and broaden and consequently even at very

Behavior of Iron(III) Porphyrazines

J. Phys. Chem. B, Vol. 112, No. 37, 2008 11521

Figure 4. RefSpec spectra of LFeOESPz floating layers.

Figure 3. BAM images of LFeOESPz floating layers taken at a surface pressure of 0 mN/m (a, b) and of 2.5 mN/m (c).

low surface pressures (Π ) 2.5 mN/m) a thick and homogeneous condensed phase is originated. This is clearly evidenced in Figure 3c that shows a uniform layer whose morphology will not change during compression, according with the constancy of the slope of the Π vs A Langmuir curve. The novel and lighter gray color is in accordance with the increased thickness of the floating layer and will be further confirmed in the following also by reflection spectroscopy carried out directly at the air-water interface. Such a rigid floating film is not transferable by the usual vertical dipping method, but multilayers have been fabricated by the horizontal lifting (Langmuir-Scha¨fer) technique onto different substrates. The molecular organization of LFeOESPz floating layers at the air-water interface was investigated by spectroscopic measurements through reflection of light. We have studied reflection of light under normal incidence at the water surface covered with the tetraazaporphyrin derivative layer. The reflection method constitutes a powerful investigation technique on the chromophore behavior on the water surface and was first introduced by Kuhn and Mo¨bius.34 It is particularly tailored for this purpose, since only chromophores at the interface contribute to the enhanced reflection. The difference ∆R in reflectivity from the chromophore floating layer on the subphase and reflectivity from the bare subphase surface was monitored as a function of wavelength. The corresponding reflection spectra from LFeOESPz on the water surface at different fixed surface pressures after reaching equilibrium (i.e., no variation in surface pressure and enhanced reflectivity) are shown in Figure 4. Such spectra further confirm the potentialities and suitability of this analytical approach for investigations of floating films at the air-water interface.

In fact, the typical pattern of tetrapyrrolic macrocycles on the water surface is exhibited in such spectra with a Soret band whose maximum is at 366 nm and a broadband in the Q-region. Comparison of these spectra with the chloroform solution spectra of the monomeric LFeOESPz and the corresponding µ-oxo dimer provides useful elements for interpretation of reflection spectra at the air-water interface and rationalization of organization inside the floating layer. The position and shape of both the Soret and Q bands in the reflection spectra is consistent with a dominant contribution of the µ-oxo dimer on the water surface, although a minor contribution of the monomer cannot be excluded a priori. It has already been observed that the behavior of some macrocycles at the air-water interface is changed in comparison with the one in solution:26 this has been correlated with the remarkably enhanced concentration of the molecules on the water surface, thus allowing reactions such as dimerization.35 Unfortunately in our case, unlike the study carried out by Hopf et al.,26 the Langmuir curve cannot give further indication of µ-oxo dimer generation since it only suggests that in the floating film large 3D aggregates have been originated; this is consistent with the presence of both the monomer and/or µ-oxo dimer aggregates. From reflection spectra, it is apparent that, upon compression, the absolute reflection enhances because on the average the surface density grows, while its profile does not change. The continuous increase in ∆R, together with the constancy of the maximum wavelength, suggest that, after initial aggregation after solution spreading, the self-organized and associated clusters are dragged on the water surface and coalesce, while consequently the surface density of the floating layer enhances. As can be detected, the reflection spectra undergo continuous and monotonic enhancement during the compression process. This could be also rationalized considering that, after initial preaggregation, the 3D domains of aggregated molecules are constrained together but significant reorganization on the water surface does not take place. This spectral behavior is also consistent with the markedly hydrophobic character of the porphyrazine moieties (from the surface pressure vs area per repeat unit curve) and the BAM images. In the ∆R spectra on the water surface, the position of the two main peaks does not depend on the values of Π for all analyzed surface pressures, thus suggesting that the average arrangement of molecules is not varying while increasing the Π pressure. Morphology and General Properties of Deposited Films. The morphology of LS-Fe multilayers deposited on silicon substrates was inspected by AFM. Figure 5 reports both the


Garramone et al.

Figure 5. 2D top view AFM images and section analysis of LS-Fe monolayer (a), LS-Fe 4 layers (b), and LS-Fe 10 layers (c).

AFM images and the section analysis obtained for 1, 4 and 10 LS-Fe layers, respectively. In the case of LS-Fe monolayer in Figure 5a, the film morphology appears constituted by regions where the film uniformely covers the silicon surface, inteconnected by regions characterized by macrodomains whose general appereance is similar to what typically observed for well-ordered Langmuir-Blodgett monolayers.36 The section analysis of these macrodomains reveals that they are 50-100 nm in lateral size and 1.5-8 nm high. The roughness mean square (rms) calculated in the inspected area (1 × 1 µm) was about 3.4 nm. By increasing the number of LS-Fe layers to 4, the films appears continuous and uniform (Figure 5b). The section analysis shows the presence of structures that are placed on top of each other, very similar in appearance to microdomains observed in the monolayer. Actually, the lateral size of these structures was 50-100 nm, whereas the high point was 1.6-9 nm high. The roughness mean square (rms) calculated in the inspected area (1 × 1 µm) was about 3.0 nm. Finally, the film of 20 layers LS-Fe appears less uniform than the previous case (Figure 5c). In particular, it is characterized by the presence of dots which cover almost completely

Figure 6. Transmission electronic absorption spectra of LS-Fe (black), LS-Fe-M (red), and LS-Fe-D (blue) films (30 layers on ITO).

the substrate surface. The section analysis evidence that such dots have lateral size in the range 20-80 nm and are 1.7-5.8



SCHEME 3: Proposed Intermolecular Interaction Mode of µ-Oxo Dimers in the LS-Fe filma

a Water caps on iron centers belonging to terminal macrocycles are not indicated.

nm high, respectively. The rms calculated in the inspected area (1 × 1 µm) was about 3.5 nm. LS-Fe multilayers deposited on ITO were also characterized by transmission UV-vis, XPS, and FT-IR spectroscopies. The electronic absorption spectrum of the LS-Fe film (30 runs/layers) is displayed in Figure 6. It is clearly dominated by the µ-oxo dimer features, with possible intensity borrowing from monomerssmost likely present as mono- and/or bis-aqua complexes. Both the energy and shape of these features reproduce those of the µ-oxo dimer in solution, thus indicating that packing interactions have scarce impact, if any, on the π-system of the µ-oxo dimer. A plausible explanation is that, as indicated by previous X-ray structural studies on iron(III) porphyrazines, the µ-oxo dimers self-assemble in condensed phase through Fe · · · S, rather than π-π, intermolecular interactions, just as proposed in Scheme 3.25 Isolated or Fe · · · S linked µ-oxo dimers are likely to be capped by water molecule(s) at one or both terminal iron centers. A complementary explanation is that the residual monomers may act as “spacers” between µ-oxo dimers, thus contributing to preserve the electronic structure characteristics of the “isolated” dimer π-system. It should be noted, in passing, that formation of weakly π-interacting dimeric (or oligomeric) aggregates is not peculiar of the investigated iron(III) porphyrazine µ-oxo dimers. Indeed, tetramers generated by cofacial arrangement of two µ-oxo dimers have been clearly identified in the crystalline structure of mono-aqua-µ-oxo-bis(hemiporphyrazinato)iron(III) complexes.37 That the LS-Fe multilayer is mainly constituted by (aggregated) dimers, is somewhat at variance with the results of RefSpec experiments on floating films, suggesting that multilayer deposition on prepared substrate produces a striking alteration of both rate processes and equilibria in the key step. The kinetic acceleration occurring upon deposition can be readily understood in terms of the extremely high local concentrations in the dimeric sites, while the shift in the equilibrium may find a partial explanation in the difficulty of dissociation of the (protonated) dimer. Removal of LS-Fe multilayers from the ITO surface followed by dissolution in chloroform, afforded a solution whose electronic absorption spectrum exhibited only the spectral features of the dimer, indicating that even the residual iron(III)porphyrazines readily dimerize upon redissolution in chloroform. It is worthwhile to mention that multilayers constituted by sole LFeOESPz monomerssmost likely in the form of either mono- or bis-aqua-

Figure 7. FT-IR spectra taken from LFeOSPz and [FeOESPz]2O cast films (A and B), respectively, and from LS-Fe film (30 monolayers) (C).

monocations, [(H2O)FeOESPz]+ or [(H2O)2FeOESPz]+s obtained by spreading chloroform solutions of LFeOESPz on water surface at pH 2.0, show a totally different optical spectrum. As inferred from Figure 6, the transmission electronic absorption of these films, denoted as LS-Fe-M, are characterized by a single absorption in the visible, near 650 nm, with considerable broadening due to intermolecular interactions. In turn, transmission electronic absorption spectra of multilayers constituted by hydrated [FeOESPz]2O entities (denoted as LSFe-D) obtained by spreading chloroform solutions of the µ-oxo dimer on water surface at pH 6.2, exhibit a twin band in the visible, with maxima at 570 and 670 nm (Figure 6), generated by “exciton-like” interactions between the µ-oxo dimers. The important implication arising from the compared spectral behavior of LS-Fe, LS-Fe-M, and LS-Fe-D multilayers is that the constituent molecular building blocks adopt quite different self-assembling modes depending on the nature of the floating film and deposition procedure. XPS spectra of LS-Fe films are characterized in the Fe2p region by a narrow and intense peak at 708.8 eV and a weaker peak at 711.1 eV. These data point to the presence of two nonequivalent iron atoms, hence supporting the hypothesis that LS-Fe films are constituted by dimeric entities incorporating minority amounts of monomers. Inhomogeneous electronic effects related to packing interactions and/or axial coordination of water molecules do not allow for an unambiguous assignment of the observed peaks to monomeric and dimeric species. We note, however, that the measured BEs are well within the range reported for iron(III) tetrapyrrolic systems. The peak at 711.1 eV is consistent with the binding energy of Fe(III) reported for analogous systems composed by monomeric and dimeric iron(III) porphyrins.38,39 Only for Fe in (TPPFe)2N species a BE of 708.5 eV has been reported.40 This low BE value was attributed to a substantial increase in the electron density on the iron center due to the strong electron-donating ability of the nitride. Worth mentioning, the Fe2p peaks are not accompanied by shakeup satellites, which is consistent with the film being mostly composed by dimers where the constituent subunits are antiferromagnetically coupled and by low-spin bisaqua-monomers. That µ-oxo dimers coexist with minority amounts of monomers in the LS-Fe film, is further supported by FTIR spectra showing in the 1660-600 cm-1 spectral region


Garramone et al.

Figure 8. Fluorescence of LS-Fe monolayer (60 µm) collected in the range 640 -740 nm) through confocal microscopy. Excitation wavelengths are 633 (A) and 488 nm (B).

the typical spectral features of the dimer and weak, though detectable, signals due to the monomer (Figure 7). The presence of scarcely π interacting µ-oxo dimers in the LS-Fe film is also consistent with fluorescence surface images taken by confocal microscopy upon excitation at 488 and 633 nm. Representative images for LS-Fe monolayer reported in Figure 8 show dual, i.e. red and green luminescence (wavelength range ) 640-740 nm), in contrast to strongly π interacting µ-oxo dimers whose luminescence is expected to be substantially quenched by aggregation. Electrochemical Behavior of the LS-Fe Films. The electrochemical response of LS-Fe films coated on ITO and Au substrates was investigated by cyclic voltammetry using H2O/ NaClO4 0.1 M as electrolytic solution. ITO slides are electrochemically inactive between -1.00 and +1.40 V (vs SCE), i. e., in the potential range accessible to the H2O/NaClO4 0.1 M medium, so they can be used as conductive substrates for electrochemical measurements (see Figure 9 inset). The electrochemical results on the LS-Fe films are discussed taking as reference the redox behavior of LS-H films deposited on the same substrate. Figure 9 shows cyclic voltammograms obtained for ITO (inset) and ITO/LS-H electrodes, starting from 0.00 V, at a sweep rate of 10 mV s-1. The electrochemical behavior of LS-H films was characterized by a well-defined oxidation peak at Ep ) +1.50 V (Ep ) +1.20 V for H2OESPz in DMF solution) without a corresponding peak in reduction. The electroactivity was lost after a single scan, as expected for a totally irreversible process. No relevant electrochemical processes were recorded between 0.00 and +1.00 V even after successive scans. The charge density |Q| (C cm-2) associated to the anodic branch of the voltammogram (first scan, corrected for the contribution of the pristine ITO baseline) was calculated to be ≈ 7.0 mC cm-2, which corresponds to the oxidation of almost 7.3 × 10-8 mol cm-2 of porphyrazine units. This represents the total number of electroactive centers Γ* expressed as molxcm-2 calculated from eq 1 in the hypothesis n ) 1, which is five times larger than the value 1.2 × 10-8 mol cm-2 extrapolated from the limiting area of 52 Å2 (determined from the π-A Langmuir curve) assuming 100% efficiency.

Γ* ) |Q|/nF

(1)

This discrepancy can be traced to the difficulties in correcting the contributions of the baseline interferences at potentials larger than +1.00 V, where the system is likely to be affected by overoxidation and what is observed is probably a multielectron process (n > 1). As for the voltammetric behavior of LS-H film in the cathodic region, a poorly resolved pair of peaks at -0.58 V (IVc) and -0.27 V (IVa) is observed between 0.00 and -1.00 Vsa solvent discharge is seen at larger potential values on ITO.

Figure 10 shows voltammograms obtained in oxidation for ITO/LS-Fe films, starting from the measured OCP’s, at a sweep rate of 10 mV s-1. When the LS-Fe film was electrochemically oxidized and then reduced between OCP and +0.90 V, two well defined anodic waves, at +0.58 V (Ia) and +0.84 V(IIa), and two cathodic waves, at +0.54 V (Ic) and +0.73 V (IIc), were observed. The Ia-Ic, and IIa-IIc are coupled processes, as it was established by running voltammograms with different anodic end-scan, namely, +0.70 and +0.90 V (curves a and b in Figure 10). The formal potentials, Esurf(I) and Esurf(II), taken by averaging the anodic and cathodic peak potentials for each pair, were +0.56 and +0.78 V, respectively (cf. also Table 1). As shown in Figure 10 (curve a) the shape of the Ia/Ic pair is quasisymmetric with a peak separation, ∆E pI, of 40 mV and nearly equal height of reduction and oxidation current peaks. What is more, the anodic and cathodic peak half-width is of ≈300 mV, as expected for a solid-state redox process dominated by repulsive interactions.41,42 The relative charge estimated in oxidation was ≈200 µC cm-2. The second pair is characterized by a peak separation, ∆E pII, of ≈ 110 mV with a nearly equal height of reduction and oxidation peaks. Because of the not negligible overlap of the IIa/IIc and Ia/Ic pairs, the IIa/IIc peak half-widths could only approximately be estimated. The charge estimated in oxidation was almost half the charge related to the first pair ≈100 µC cm-2. The charge involved in the first anodic branch (almost 300 µC cm-2) represents (n ) 1) only 10% the total charge of 2.7 mC cm-2 theoretically predicted for 30 monolayers (assuming 100% efficiency), thus indicating that, at the used scan rate, only 2-3 monolayers on the substrate surface are involved in the redox process or that, alternatively, the electron transfer is due to aggregated dimeric entities. To determine the dynamic characteristics of the electrodic processes in the oxidation region, cyclic voltammograms were recorded at different scan rates between 1 and 200 mV s-1, in two different potential ranges: from OCP to +0.75 V and from OCP to +0.90 V. A first relevant datum is that the anodic peak Ia related to Ia/Ic redox process is not seen at a scan rate of 1 mV s-1. This could be due to the presence of an irreversible chemical reaction occurring during the electron transfer or to the instability of the reduced species.43 The total amount of charge involved between OCP and +0.90 V was estimated to be 1.6 mC cm-2 that corresponds to 60% the charge calculated for 30 monolayers, assuming 100% efficiency and n ) 1. This implies that at this scan rate even the monolayers not strictly in contact with the substrate are involved in the redox process. On increasing the scan rate (V g 10 mV s-1), the redox peak currents show a linear dependence on V, indicating a surfacecontrolled electrode process for both pairs (Figure S4). Both the anodic and cathodic peak potentials of the LS-Fe film showed a shift for both pairs upon increasing the scan rate. Figure S5 shows the variation of Epa and Epc as a function of log V for the Ia/Ic and IIa/IIc peak pairs. In the 10-200 mV scan range a linear dependence is evident for both peaks, as expected for a totally irreversible process for a surface reduction/ oxidation when the adsorption obeys to a Langmuir isotherm and the control by diffusion is absent, indicative of a diffusionless system.44 According to this model a graph of Ep ) f(log V) yields two straight lines with a slope equal to -2.3RT/RnF and intercept -(2.3RT/RnF) log(RnF/RTKs) + Esurf for cathodic peak (eq 2). For the anodic peak the slope is 2.3RT/(1 - R)nF



Figure 9. Cyclic voltammograms of ITO/LS-H film modified electrode in 0.1 M NaClO4. Scan rate: 10 mV s-1. Volts are vs SCE. Inset: Cyclic voltammograms of ITO bare electrode in 0.1 M NaClO4 from -1.00 to +1.60 V.

Figure 10. Cyclic voltammograms of ITO/ LS-Fe film modified electrode in 0.1 M NaClO4 obtained by using different anodic endscan, namely, +0.70 and +0.90 V (curves a and b). Scan rate: 10 mV s-1. Volts are vs SCE.

TABLE 1: Summary of the Formal Potentials Esurf (V) Obtained in CV Experiments for the Investigated LS Films in 0.1 M NaClO4 in the Potential Range As Reported in the Text (Scan Rate 10 mV s-1; Volts Ws SCE) LS films ITO/LS-H ITO/LS-D ITO/LS-M ITO/LS-Fe Au/LS-Fe ITO/LS-Fee Au/LS-Fee

Esurf(I)

Esurf(II)

Ep(III)

Esurf(IV)

+0.12c +0.070 n.a. +0.13c +0.09

-0.43 -0.22d -0.23 -0.28 (not resolved) n.a. -0.21 -0.20

+1.50a +0.97a +0.53 +0.56 +0.59 +0.48 +0.52

+0.78 +0.78 +0.69b +0.70b

a

Epa(II). b Epc(II). c Epa(III). d Epc(IV). e LS films cycled in potential ranges as reported in Figure S4, inset.

and the intercept is [2.3RT/(1 - R)nF) log((1 - R)nF/RTKs] + Esurf (eq 3),

Epc-Esurf ) (-2.3RT/RnF) log(V) (2.3RT/RnF) log(RnF/RTKs) (2) log(V) + [2.3RT/(1 -

Epa-Esurf ) [2.3RT/(1 - R)nF]

R)nF] log[(1 - R)nF/RTKs] (3) where Esurf(I) ) +0.56 V and Esurf(II)) +0.78 V are estimated at V ) 10 mV s-1. From the slopes of the curves (Figure S5) the values of R ≈ 1 and n ≈ 0.8, and R ≈ 0.5 and n ≈ 4 were obtained for the first and second pair, respectively. From the intercepts Ks ) (RnF/RT)Va, where Va is the scan rate when Epa ) Esurf, values of 0.08 s-1 and 0.02 s-1 could be obtained

Figure 11. Voltammograms of the ITO/LS-Fe-M and ITO/LS-Fe-D films modified electrodes in 0.1 M NaClO4. Scan rate: 10 mV s-1. Volts are vs SCE.

for the surface electrochemical rate constants, Ks(I) and Ks(II), respectively. Integration of oxidation and reduction peaks, at scan rates of 10, 20, 50, 100 and 200 mV s-1 gave nearly a constant charge |Q| of almost 300 µC cm-2. In particular, the charge associated to the first pair, both in oxidation and in reduction, was two times larger than the charge associated to the second pair, suggesting that if the first pair is a one-electron process (n ) 1) and the second pair a four-electron process (n ) 4) probably an oligomeric structure which involves a minimum 4 × 2 units. All these results suggest an irreversible, diffusionless, surface confined electrochemical behavior, in which all the electro-active centers in the film that are oxidized in the anodic scan are quasifully reduced during the cathodic scan. A relevant question to be answered, at this point, is which are the molecular sites where the electron transfers involving the first and second redox pairs, occur. Since, as discussed above, the CV of the LS-H multilayer does not exhibit any oxidation process between OCP and +0.90 V, the anodic waves appearing at +0.56 V and +0.78 V in the voltammogram of the LS-Fe film must originate from metal-centered processes. Inspection of the LS-Fe-M and LS-Fe-D film CVs (see Figure 11 and Table 1), reveals that between OCP and +0.90 V only the former shows a well defined redox pair, with Esurf(I) ) +0.52 V. This value is very close to the Esurf(I) measured for the LSFe film, hence suggesting that the first redox pair here originates

11526 J. Phys. Chem. B, Vol. 112, No. 37, 2008 from a FeIII/FeII process occurring on a “monomeric” iron(III) porphyrazine. The LS-Fe-D film exhibits, instead, only an irreversible multielectron oxidation at +0.97 V. It is plausible, therefore, that this wave shares a common origin with the second redox pair of the LS-Fe film, the observed discrepancies in shape and position of this process on passing from LS-Fe-D toLS-Fe films being reasonably traced to the different structure and composition of the pertinent multilayers. Extension of the Potential Range: Electrochemical Stability and Chemical Modifications of the LS-Fe Films. The charge (200 µC cm-2) associated to the first pair did not change significantly, both in oxidation and in reduction, after 10 cycles. After 60 cycles the current of the first redox pair decreased to some extent, and the |Qox| charge decreased by ∼20%, while Esurf(I) did not change appreciably (see Table 1). By contrast, on extending the potential range as indicated in Figure S6 (curve 1), a significant decrement of Ipa and Ipc and an increment of IIpa were observed for the first and second waves, respectively. A rapid decrement of Ipa and Ipc, not coupled to a significant Esurf(I) variation was observed after 10 cycles, indicating that the film electroactivity was only partially lost after some cycles of charge and discharge. The redox behavior of the LS-Fe films in oxidation was, instead, greatly affected by prereduction at potentials more negative than OCP, both on ITO and gold substrates. This is apparent from Figure S6, where CVs recorded for the LS-Fe film starting from film’s OCP to +0.90 V using different cathodic end scans, namely, OCV (curve 1), +200 mV (curve 2), -100 mV (curve 3), -200 mV (curve 4) and -400 mV (curve 5), are displayed. From the voltammograms on ITO, a decrement of the currents and a cathodic shift of the first pair is evident. In turn, the second pair becomes totally irreversible. The peak IIIa at +0.04 V, was only seen if the cathodic potential end scan was set to ca. -0.20 V. Finally, a well defined couple of peaks IVc/IVa at -0.30 V and -0.12 V (Esurf) -0.21 V) was observed when the cathodic end scan was -0.40 V. The same behavior was observed when gold was used as a support for LS-Fe film deposition, as inferred from the data in Table 1 and the inset of Figure S6 (it should be noted, in passing, that the peak IIIc at 0.00 V in Figure S6 originates from gold reduction). When the LS-Fe film was preoxidized and then reduced at a potential smaller than OCP, its behavior in oxidation changed in an interestingly way. Esurf(I) shifted toward more cathodic potentials, the peak pair at Esurf(II) disappeared, and a well resolved peak pair appeared in reduction at Esurf(IV) ) -0.21 V on ITO and at -0.22 V on Au. This behavior could be rationalized by supposing that electrochemical oxidation induces a significant conversion of the µ-oxo dimer oligomers into the “monomeric” iron(III) porphyrazines. Comparison of the voltammograms of Figure S6 and the electrochemical data gathered in Table 1, according to which the oxidized LS-Fe films show two pairs of peaks at the same Esurf(I) and Esurf(IV) values as the LS-M films, but not the characteristic IIa/IIc couple of the µ-oxo dimer response in the LS-Fe films, lends supports to this suggestion. Spectroscopic and Redox Response of LS-Fe Films to Acidic Environments. The spectroscopic and redox response of LS-Fe films to acidic environments, namely aqueous TCA solutions, was also explored. TCA was considered as a test case because it is a widely investigated water soluble organohalide pollutant.45,46

Garramone et al.

Figure 12. Evolution of the transmission UV-vis spectra of LS-Fe film upon repeated washings with distilled water.

Transmission electronic absorption spectra of LS-Fe films deposited on ITO/glass slides in equilibrium with a 0.01 M TCA solution were investigated first. Initially the LS-Fe film was put in equilibrium with the TCA solution, by dipping the coated slide for ca.15 min in the bathing medium. Subsequently the slide was rinsed with distilled water and dried. As seen in Figure 12, the spectrum of the LS-Fe film (black trace) underwent remarkable modifications upon incubation with the organic acid solution. Besides an appreciable blue shift of the Soret band, it showed isosbestic growth of a peak at 595 nm and intensity lowering of the shoulder at ca. 700 nm. These spectral changes indicate that a significant µ-oxo dimer f monomer conversion is operative when LS-Fe films are exposed to TCA. Exposure of LS-Fe multilayers to trichloroacetate ion in neutral solutions did not induce significant spectral changes, thus confirming that they only occur in response to protons. Washing the films with distilled water resulted in the apparent restoring of initial electronic absorptions, although an overnight incubation was necessary for completing the process. As for the redox response of LS-Fe films to acidic environments, addition of TCA to the electrochemical cell using a LSFe film as working electrode, resulted in an evident increase of the cathodic current peak (Figure 13) at a potential value near -0.40 V. Moreover the cathodic peak current increased almost linearly on increasing the TCA concentration in the 0.1-2.0 mM range. Information obtained from spectroscopic experiments may provide a plausible explanation for this electrochemical behavior. Since, as discussed above, TCA induces a progressive change in the composition of the LS-Fe film in favor of the monomeric iron(III)porphyrazine component, the observed increase in the cathodic current near -0.40 V has to be ascribed to reduction of the monomeric porphyrazine. That the LS-Fe-M film shows a redox couple with E(surf) ) -0.23 V, is in line with this suggestion. Interestingly, the LS-Fe film modified electrode proved to be substantially insensitive to strong inorganic acids such as HClO4. Indeed, LS-Fe modified electrodes did not show any detectable electrochemical signal in the potential range of interest when working in HClO4 solutions, regardless of the concentration used. This suggests that besides the proton and its concentration, also the nature of the counterion plays a role in determining the response of the film to protic environments. In particular, the trichloroacetate ion seems to be more suited


Figure 13. (a) Cyclic voltammograms of ITO/LS-Fe film modified electrode recorded in 0.1 M NaClO4: (black line) 0.0 mM TCA, (red line) 0.49 mM TCA, (yellow line) 0.74 mM TCA, (blue line) 0.99 mM TCA, and (magenta line) 1.48 mM TCA. Scan rate 10 mV s-1. Volts are vs SCE. (b) Plot of cathodic peak currents against the concentration of TCA in 0.1 M NaClO4.

than the perchlorate ion, in terms of structure and polarity, to penetrate the substantially hydrophobic matrix of the film and, hence, to assist in the mobility of the proton toward the inner layers. Multireflection FT-IR spectra acquired on LS-Fe film after incubation with TCA (not reported), showing the typical signature of the trichloroacetate ion, support this view. The relevant implication arising from our results is that LSFe film modified electrodes may afford a selective optical and redox response to acidic compounds in aqueous solutions. The selectivity and sensitivity as well as the stability of iron(III) porphyrazine modified electrodes as acidic sensors can be tuned by varying the polarity of the coating film in conjunction with the electronic properties of the metallomacrocycle. This will require further synthetic efforts to apply suitable chemical functions at the periphery of the ligand. Conclusions Thin films of a newly synthesized iron(III)porphyrazine, LFeOESPz, have been deposited by the Langmuir-Scha¨fer technique (horizontal lifting) on ITO or Au surfaces. Before deposition, the floating films have been investigated at the air-water interface by three different approaches: surface pressure/area per molecule (π/A) curves, Brewster angle microscopy and UV-vis reflection spectroscopy. The π/A curves are practically featureless and with a low limiting area per molecule (about 20 Å2), suggesting that 3D aggregates have been generated on the water surface and that such domains during compression have been simply dragged and coalesce. Such hypothesis has been further confirmed by BAM images and RefSpec analysis; in particular, the spectral analysis clearly shows a monotonous increase of ∆R, usually ascribable to an increase of density of the floating domains. The complex reacts with water subphase (pH 6.2) at the water/chloroform interface forming the µ-oxo dimer, which becomes the prevalent molecular building block of the LS films, as indicated by optical, IR,

J. Phys. Chem. B, Vol. 112, No. 37, 2008 11527 XPS, and electrochemical studies. In particular, when the LSFe film is electrochemically oxidized and then reduced between OCP and +0.90 V, two independent anodic waves, with formal potentials, Esurf(I) and Esurf(II) of +0.56 V and +0.78 V, respectively, are reproducibly observed. Dynamic voltammetric and coulometric studies indicate that the first wave corresponds to a one-electron process, while the second wave corresponds to a four-electron process, involving µ-oxo-dimer oligomers. Comparison of these electrochemical results with voltammetric results on LS-Fe-M and LS-Fe-D films indicates that the first and second waves appearing in the voltammogram of the LSFe film are associated to iron(III) centers localized on the monomer and the µ-oxo-dimer, respectively. The LS-Fe film shows a considerable stability when electrochemically oxidized and then reduced between OCP and +0.75 V. The stability is reduced significantly when the investigated potential range is extended both anodically and cathodically, due to formation of new species. In particular, it is found that electrochemical preoxidation induces a significant conversion of the µ-oxo dimer oligomers into the “monomeric” iron(III)-porphyrazines. The spectroscopic and redox response of the LS-Fe films to protic environments was also investigated. Incubation of LS-Fe films with TCA solutions induces remarkable UV-vis spectral changes, which are consistent with a significant µ-oxo dimer f monomer conversion. Addition of TCA to the electrochemical cell using a LS-Fe film as working electrode, resulted in an evident increase of the cathodic current peak at a potential value near -0.40 V. Moreover the cathodic peak current increased almost linearly on increasing the TCA concentration in the 0.1-2.0 mM range. This behavior is interpreted in terms of TCA inducing a progressive change in the composition of the LS-Fe film in favor of the monomeric iron(III) porphyrazine, which is responsible for the observed increase in the cathodic current near -0.40 V. Our results represent a favorable starting point for possible application of the target iron(III) porphyrazines in electrocatalysis and in the field of protic sensors. Acknowledgment. The authors gratefully acknowledge the Centro Interdipartimentale “Laboratorio di Ricerca per la Diagnostica dei Beni Culturali”, Universita` degli Studi di Bari (Italy), for XPS measurements. Supporting Information Available: Figures showing electronic absorption spectra of LFeOESPz and [FeOESPz]2O (Figure S1), cyclic voltammogramms of LFeOESPz and [FeOESPz]2O in DMF solution (Figures S2 and S3), plots of anodic and cathodic peak currents against V and log V (Figures S4 and S5), and CVs of ITO/LS-Fe and Au/LS-Fe films in extended potential ranges (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Sun, P.; Zong, H.; Salaita, K.; Ketter, J. B.; Barrett, A. G. M.; Hoffman, B. M.; Mirkin, C. A. J. Phys. Chem. B 2006, 110, 18151. (2) Guinchard, X.; Fuchter, M. J.; Ruggiero, A.; Duckworth, B. J.; Barrett, A. G. M.; Hoffman, B. M. Org. Lett. 2007, 9, 5291. (3) Fuchter, M. J.; Hoffman, B. M.; Barrett, A. G. M. J. Org. Chem. 2006, 71, 724. (4) Fuchter, M. J.; Hoffman, B. M.; Barrett, A. G. M. J. Org. Chem. 2005, 70, 5086. (5) Lee, S.; White, A. J. P.; Williams, D. J.; Barrett, A. G. M.; Hoffman, B. M. J. Org. Chem. 2001, 66. (6) C., Z.; Zhao, M.; Goslinski, T.; Stern, C.; Barrett, A. G. M.; Hoffman, B. M. Inorg. Chem. 2006, 45, 3983. (7) Zhao, H.; Zhong, C.; Stern, C.; Barrett, A. G. M.; Hoffman, B. M. J. Am. Chem. Soc. 2005, 127, 9769. (8) Donzello, M. P.; Ercolani, C.; Stuzhin, P. A.; Chiesi-Villa, A.; Rizzoli, C. Eur. J. Inorg. Chem. 1999, 2075.

11528 J. Phys. Chem. B, Vol. 112, No. 37, 2008 (9) Donzello, M. P.; Dini, D.; D’arcangelo, G.; Ercolani, C.; Zhan, R.; Ou, Z.; Stuzhin, P. A.; Kadish, K. M. J. Am. Chem. Soc. 2003, 125, 14190. (10) Donzello, M. P.; Ou, Z.; Monacelli, F.; Ricciardi, G.; Rizzoli, C.; Ercolani, C.; Kadish, K. M. Inorg. Chem. 2004, 43, 8626. (11) Bergami, C.; Donzello, M. P.; Ercolani, C.; Monacelli, F.; Kadish, K. M.; Rizzoli, C. Inorg. Chem. 2005, 44, 9862. (12) Bergami, C.; Donzello, M. P.; Monacelli, F.; Ercolani, C.; Kadish, K. M. Inorg. Chem. 2005, 44, 8532. (13) Fujimori, M.; Suzuki, Y.; Yoshikawa, H.; Awaga, K. Angew. Chem., Int. Ed. Engl. 2003, 115, 6043. (14) Suzuki, Y.; Fujimori, M.; Yoshikawa, H.; Awaga, K. Chem. Eur. J. 2004, 10, 5158. (15) Stuzhin, P. A.; Ercolani, C. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: Amsterdam, 2003; Vol. 15; p 263. (16) Donzello, M. P.; Ercolani, C.; Stuzhin, P. A. Coord. Chem. ReV. 2006, 250, 1530. (17) Belviso, S.; Ricciardi, G.; Lelj, F. J. Mater. Chem. 2000, 2, 297. (18) Ristori, S.; Salvati, A.; Martini, G.; Spalla, O.; Pietrangeli, D.; Rosa, A.; Ricciardi, G. J. Am. Chem. Soc. 2007, 129, 2728. (19) Salvati, A.; Ristori, S.; Pietrangeli, D.; Oberdisse, J.; Calamai, L.; Martini, G.; Ricciardi, G. Biophys. Chem. 2007, 131, 43. (20) Salvati, A.; Ristori, S.; Oberdisse, J.; Spalla, O.; Ricciardi, G.; Pietrangeli, D.; Giustini, M.; Martini, G. J. Phys. Chem. B 2007, 111, 10357. (21) Theodoridis, A.; Maigut, J.; Puchta, R.; Kudrik, E. V.; Van Eldik, R. Inorg. Chem. 2008, 47, 2994. (22) Yaping, N.; Fitzgerald, J. P.; Carroll, P.; Wayland, B. B. Inorg. Chem. 1994, 33, 2029. (23) Fitzgerald, J. P.; Aggerty, B. S.; Reingold, A. L.; Mai, L.; Brewer, G. A. Inorg. Chem. 1992, 31, 2006. (24) Fitzgerald, J. P.; Lebenson, J. R.; Wang, G.; Yee, G. T.; Noll, B. C.; Sommer, R. D. Inorg. Chem. 2008, 47, 4520. (25) Ricciardi, G.; Bavoso, A.; Bencini, A.; Rosa, A.; Lelj, F.; Bonosi, F. J. Chem. Soc., Dalton Trans. 1996, 13, 2799. (26) Hopf, F. R.; Mo¨bius, D.; Whitten, D. G. J. Am. Chem. Soc. 1976, 98, 1584.

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JP803418B

Electrochemical and Spectroscopic Behavior of Iron (III) Porphyrazines

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