Characterization of a Perylenediimide Self-Assembled Monolayer on

Article pubs.acs.org/JPCC

Characterization of a Perylenediimide Self-Assembled Monolayer on Indium Tin Oxide Electrodes Using Electrochemical Impedance Spectroscopy Barbara P. G. Silva, Daniel Z. de Florio, and Sergio Brochsztain* Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas, Universidade Federal do ABC, Avenida dos Estados, 5001, 09210-580 Santo André, São Paulo, Brazil S Supporting Information *

ABSTRACT: Self-assembled monolayers (SAMs) of N,N′bis(2-phosphonoethyl)-3,4,9,10-perylenediimide (PPDI), a perylene dye substituted with phosphonic acid groups, were deposited on indium tin oxide (ITO) substrates. Dye deposition was confirmed by UV−visible absorption spectroscopy and by electrochemical methods. Electrochemical characterization of the SAM was performed using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Two reversible redox waves were observed by CV for the PPDI monolayer, corresponding to E1/2 = −0.49 V (radical anion formation) and E1/2 = −0.90 V (dianion formation). The effect of applied bias on the EIS response was studied, comparing a region where PPDI was not reduced (applied bias = 0 V) with a region within the redox window of the imide (applied bias = −0.6 V). The EIS results were analyzed using either impedance (Nyquist and Bode) or capacitance (Cole−Cole) diagrams. Capacitance plots were shown to be by far more sensitive to study the faradaic activity of the SAM, allowing the determination of both the pseudocapacitance (Cpc) for charging the monolayer and the heterogeneous electron transfer rate constant (ket) from the electrode to the SAM. A molecular coverage of 7 × 10−11 mol/cm2 was calculated for the SAM from the pseudocapacitance. A value of ket = 41 s−1 was obtained, consistent with literature data for similar systems. We have previously reported the construction of thin films of a PDI derivative on quartz substrates.26,35 Quartz substrates, however, are not conductive and therefore not suitable for electrochemical measurements. In the present work, we described the construction of a monolayer of a PDI derivative self-assembled on conductive indium tin oxide (ITO) electrodes, allowing the characterization by electrochemical methods. For this purpose, we employed N,N′-bis(2-phosphonoethyl)3,4,9,10-perylenediimide (PPDI), a PDI derivative substituted with phosphonic acid groups (Scheme 1A). Phosphonic acid functionalities have been shown recently to be among the best ligands for ITO modification.44−47 We studied the PPDI monolayer employing both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), which is a very powerful AC technique, since it can resolve phenomena occurring at different time scales. The studies include the effect of applied bias on the EIS spectra of the PPDI monolayer. To our knowledge, there have been no other reports in the literature using EIS for the characterization of PDI selfassembled films. In addition, the studies were complemented

1. INTRODUCTION 3,4,9,10-Perylenediimides (PDI) are a class of red dyes wellknown for their great thermal and photostability and high fluorescence quantum yields in the monomeric state.1,2 They can be reduced chemically3,4 or electrochemically,5−7 giving stable radical anions and dianions. Thanks to this electron acceptor character, the PDI are among the best known n-type semiconductors.8−10 The versatile synthesis of the PDI allows the incorporation of virtually any substituent group to the imide nitrogen, allowing the modulation of solubility and aggregation properties.11 For this reason, the PDI are very suitable as building blocks for the construction of supramolecular assemblies with a variety of architectures,12 including nanofibers and nanobelts,11,13−17 molecular π-stacks,18 liquid-crystalline mesophases,19−22 and light harvesting assemblies.23,24 Our group has been interested in self-assembled thin films containing the PDI. Thin films containing PDI dyes have been grown on various substrates using several different techniques,25,26 including vapor deposition,27−29 Langmuir−Blodgett (LB),30−32 layer-by-layer electrostatic attraction,25,33 covalent grafting,34 and metal phosphonate chemistry.26,35 A number of applications have been devised for these films, such as molecular transistors,36 fluorescent sensors,37,38 light-emitting diodes,7,39 nanowires,40 and dye-sensitized solar cells.41−43 © 2014 American Chemical Society

Received: September 20, 2013 Revised: December 23, 2013 Published: January 27, 2014 4103

dx.doi.org/10.1021/jp409416b | J. Phys. Chem. C 2014, 118, 4103−4112

The Journal of Physical Chemistry C

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Scheme 1. (A) Schematic Structure of the Self-Assembled PPDI Monolayer; (B) Charge Distribution with Applied Bias = 0 V (Outside the Redox Window of PPDI); and (C) Charge Distribution with Applied Bias = −0.6 V (within the Redox Window of PPDI)a

a

The positive charges correspond to K+ cations.

immobilized redox active groups (mainly ferrocene) have also been studied by EIS.78,79 Most EIS studies are presented in the form of Nyquist and Bode impedance plots. Some authors, however, have treated the EIS data using capacitance (or Cole−Cole) plots, which are a rather convenient way to represent EIS data for surface confined redox species,80−84 since they allow a clear segregation of the faradaic response of the monolayer from the capacitive background. This can be accomplished by comparing EIS measurements registered with the electrode biased within and without the redox window of the confined redox pair.85−87 In the present work, we compare the use of Nyquist and Bode impedance plots with the corresponding capacitance plots for the PPDI monolayer.

with UV−visible absorption spectroscopy for a better characterization of the monolayer. The electrochemical behavior of surface confined redox species has been extensively studied and reviewed in the last few decades.48−50 Nevertheless, the great majority of the studies employed DC electrochemical techniques, such as CV, which has been the method of choice by most researchers in the field. EIS is a very powerful AC technique that has been an important tool in the evaluation of organic coatings and in corrosion studies.51−55 However, only recently was EIS applied to the characterization of thin film nanodevices, such as solar cells,56,57 fuel cells,58,59 sensors and biosensors.60−62 EIS has also been applied for the characterization of self-assembled monolayers (SAMs). Early studies employed nonelectroactive SAMs of thiols on gold, either in the presence or in the absence of redox pairs in solution.63−69 More recently the studies were extended to polyelectrolyte films70,71 and to SAMs deposited on different substrates, such as copper72 and indium tin oxide (ITO).73−77 SAMs and polyelectrolyte films containing

2. EXPERIMENTAL SECTION 2.1. Materials. PPDI was synthesized as described elsewhere.88,89 KCl was obtained from Synth. Concentrated NH4OH was purchased from Merck. Hydrogen peroxide 30% was supplied by Carlo Erba. HPLC grade chloroform and 4104



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ethanol were obtained from Mallinckrodt. All aqueous solutions were prepared with deionized water (Barnstead easypure RF system, Dubuque, IA, USA). 2.2. Deposition of a PPDI Self-Assembled Monolayer. ITO coated glass substrates with dimensions 1 × 2.5 cm2 were cleaned in an ultrasonic bath with chloroform (30 min, 40 °C), followed by ethanol and finally water (10 min, 25 °C each). After cleaning, the substrates were activated by immersion in a mixture of water, concentrated NH4OH and hydrogen peroxide 30% (5:1:1) during 30 min (80 °C), and finally rinsed with water and dried under argon. The activated ITO substrates were then immersed in a 1 mM PPDI aqueous solution for 24 h and then rinsed with water, resulting in the deposition of a monolayer of PPDI. 2.3. Instruments. UV/vis absorption spectra were registered with a Varian Cary 50 spectrophotometer. Cyclic voltammetry (CV) was performed using a Solartron 1287 Potentiostat/Galvanostat. The measurements were carried out at room temperature (25 ± 1 °C) in a three-compartment electrochemical cell with a platinum wire auxiliary electrode and a saturated Ag/AgCl reference electrode in 3 M KCl solution. The working electrode was either a bare ITO substrate or ITO covered with a PPDI self-assembled monolayer. The voltammograms were registered with the substrates immersed in 0.1 M KCl as the support electrolyte. The area of the ITO substrate exposed to the solution was 1 cm2 in all experiments. The solutions were purged with N2 for 10 min before the measurements, and a flow of N2 was kept above the solution during the measurements. Electrochemical impedance spectroscopy (EIS) was performed using the same equipment used for the CV (Solartron 1287, coupled with FRA Solartron 1260), with AC amplitude of 10 mV and frequency scanning from 106 to 0.05 Hz, with the collection of 60 points equally spaced. The studies were performed at different applied bias (namely 0, −0.2, −0.6, and −0.8 V). The electrochemical cell and the solutions used for EIS measurements were the same as in the CV experiments described above. EIS data were fitted using the Zview software (Scribner Associates) provided with the equipment.

Figure 1. Absorption spectrum of a PPDI monolayer self-assembled on ITO. The black line is the spectrum of bare ITO, showing that the substrate is transparent in this wavelength range. Inset: Absorption spectrum of PPDI in solution (1:1 EtOH/H2O).

PPDI, indicating a great extent of π-stacking in the films.11,12,35 The molecular surface coverage of the SAM can be estimated from the absorbance data using Γ = (Aλελ−1NA) × 10−3, where Aλ is the absorbance at a given wavelength, ελ is the molar absorptivity coefficient of PPDI in solution at the same wavelength, and NA is Avogadro’s number.25 At 498 nm, which is the absorption maximum of the SAM (Figure 1), A = 0.0128 and ε = 38 700 M−1 cm−1, yielding Γ = 3.3 × 10−10 mol/cm2. This value, however, might be overestimated (see below), since the absorbance of the monolayer was quite low (outside the range where Beer law holds) and therefore subject to large experimental errors. 3.2. Cyclic Voltammetry of the PPDI Monolayer. Figure 2 shows CV of the PPDI monolayer registered at different scan rates. The typical two step redox process usually observed with PDI derivatives was observed,5−7 giving first an anion radical (PDI−•, E1/2 = −0.49 V) which was further reduced to a dianion species (PDI2‑, E1/2 = −0.90 V). The reduction and oxidation areas under the CV are nearly the same, showing that

3. RESULTS AND DISCUSSION 3.1. Monolayer Deposition. We have previously reported the deposition of monolayers, as well as multilayered thin films of PPDI on quartz substrates.35,88 In that case, however, a tedious three step procedure was required for the priming of the surface, including silylation with 3-aminopropyltriethoxysilane, followed by phosphorylation with POCl3 and zirconation with ZrOCl2. PPDI was then adsorbed on the top of the zirconium layer. With ITO substrates, on the other hand, there was no need for any priming step, except routinely employed activation, since organic phosphonic acid groups are well-known to form quite stable self-assembled monolayers on ITO surfaces.90−93 Construction of self-assembled PPDI monolayers was easily accomplished by exposition of activated ITO to 1 mM PPDI aqueous solution for 24 h. The absorption spectrum of a monolayer of PPDI selfassembled on ITO is shown in Figure 1. The absorption spectrum of PPDI in an alcoholic aqueous solution, where the compound is in the monomeric state, is shown in the inset for comparison. It can be noticed that the sharp bands peaking at 492 and 527 nm observed in solution were replaced by two new broad bands at 498 and 553 nm in the monolayer. These results imply strong excitonic coupling between the aromatic rings of

Figure 2. CV of a PPDI monolayer registered at 50 (blue), 100 (red), and 200 mV/s (green). The inset shows the linear dependence of the peak currents with scan rate for the second reduction. 4105



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Figure 3. EIS spectra of a PPDI monolayer on ITO at different applied bias. (A) Nyquist plots. (B) Magnitude Bode plots. (C) Bode phase plots. The solid lines in parts B and C represent the fit according to the equivalent circuits in Figure 4 (see text).

exactly at the same position, giving ΔEp = 0, regardless of scan rate. In real situations, however, this is rarely observed, due to kinetic limitations.48−50 It can be seen that peak separations in the PPDI monolayer are approximately constant and in general less than the value expected for a one-electron diffusional process (ΔEpdiff = 59 mV), in agreement with the behavior expected for surface immobilized species. It should be noted, however, that the midpoint between the anodic and cathodic peaks, which gives the formal reduction potential (E1/2) of the dye in the films, remained constant for any peak separation. 3.3. EIS of the PPDI Monolayer. Figure 3 shows EIS spectra of the PPDI monolayer on ITO, recorded in 0.1 M KCl solutions at different applied bias. The impedance spectra at low frequencies are similar to those obtained with bare ITO (Figure S4), showing typical capacitor-like behavior,51−55 as expected for systems without added redox pair in solution. The Nyquist plots (Figure 3A) approach vertical lines parallel to the y axis, while the Bode magnitude plots (Figure 3B) consisted of straight lines with a slope close to −1. Accordingly, the phase angles (Figure 3C) approached 90° at the low frequency region, as expected for capacitor-like systems. The capacitor-like behavior arises from the fact that surface confined species, in the absence of redox pairs in solution, cannot support a steady-state direct current (at low frequencies the alternated current approaches the conditions found in a DC experiment), since there is a limited amount of the electroactive component.80−84 In the case of bare ITO (Figure S4), the system can be described by a simple equivalent circuit, consisting of a resistor in series with a capacitor (Figure 4A). In this model, Rs is the resistance of the electrolyte solution between the reference (Re) and working (We) electrodes, and Cdl is the electrical double-layer capacitance. At high frequencies, the capacitor behaves as a short-circuit element, allowing all the AC current to pass. In this case, the capacitor

the system was chemically reversible (all the reduced dye was reoxidized in the reverse scan). This result indicates that the PPDI dyes were quite robust and strongly attached to the substrate and did not decompose neither desorb, even when submitted to potentials down to −1 V, where the PDI units are doubly negatively charged. Furthermore, the absorption spectra of the PPDI monolayer remained nearly unchanged after the series of electrochemical experiments (Figure S1). In contrast to this behavior, Chen et al.93 reported that phenylphosphonic acid self-assembled on ITO was easily desorbed from the surface when the electrode was poised at potentials more negative than −200 mV. These facts suggest that strong πstacking interactions between neighbor perylene rings contribute to the stability of the PPDI monolayer. The areas under the voltammograms can be used to estimate the surface coverage of the films. Considering that the first and second reduction waves were merged in the CVs (Figure 2), it was more accurate to consider the whole area of the CV and treat it as a two-electron wave, what gives a value of Γ = 2.5 × 10−10 mol/cm2 in the monolayer. This value is lower than the surface coverage obtained from the absorption data. However, surface coverages obtained by CV are known to be overestimated, since it is not possible to remove completely the capacitive contributions, which increase the area under the CV (Figure S2). We show below that capacitive EIS spectroscopy is a very valuable tool to obtain the surface coverage of immobilized species free from parasitic (nonfaradaic) terms. As seen in the inset of Figure 2 and Figure S3, the cathodic and anodic peak currents increased linearly with scan rate, which is the behavior expected for surface confined redox species. In the case of diffusing redox species the peak currents are proportional to the square root of scan rate. The cathodic to anodic peak separations (ΔEp) are also reported in Figure 2. For an ideal film, the cathodic and anodic peaks should be 4106



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impedances at low frequencies decreased, reflecting the onset of faradaic activity, since at these potentials PPDI was reduced to the radical anion form (Scheme 1C). The general behavior of the EIS spectra in the presence of the faradaic process was still capacitive, because surface confined redox species cannot support a steady-state direct current, even when the monolayer is reduced (a transient current will appear during the process of charging the monolayer, falling to zero after a short time).81 The lower impedance resulted in shorter straight lines in the Nyquist diagrams (Figure 3A) as going to more negative bias. The effect of applied bias, however, can be better visualized in the Bode plots (Figure 3B,C). A pronounced decrease in |Z| of 1 order of magnitude, from 2 × 105 to 2 × 104 ohm, was observed at low frequencies, as the applied bias was moved from 0 to −0.8 V (Figure 3B). Furthermore, a decrease in phase angle at more negative bias (Figure 3C) could be observed in the intermediate frequency range (100−103 Hz). The effect of bias observed with the PPDI self-assembled monolayer was not observed with bare ITO, in which case only slight bias-related changes attributable to electrode polarization were observed (Figure S4). With the electrode poised within the redox window of PPDI, a more suitable equivalent circuit is the one shown in Figure 4C, which has been used by several authors to represent the EIS of surface confined redox species.80−87 This circuit takes in account, in addition to the elements in Figure 4B, the faradaic components R et , representing the resistance for electron transfer from the ITO surface to the PPDI molecules and the pseudocapacitance (Cpc), which is the faradaic capacity for charging the monolayer. As seen in Figure 3, the data at bias = −0.6 V were nicely fitted using this equivalent circuit (as discussed below in more detail). For this fit, the values of Rs = 188 ohm and CPC = 14 μF obtained for the neutral monolayer (with the circuit in Figure 4B) were maintained, giving Ret = 3 ohm and Cpc = 53 μF. A saturation effect can be noticed as the applied bias was moved to values more negative than the first redox potential of PPDI (E1/2 = −0.49 V). The EIS spectra recorded at bias = −0.8 V were almost identical to those recorded at bias = −0.6 V (Figure 3). These results suggest that all PPDI molecules were already reduced to the radical anion form at these negative potentials, so that a further decrease in the applied bias makes no difference. In this regard, it would be quite interesting to probe the behavior at potentials even more negative, approaching the second redox potential of PPDI (E1/2 = −0.90 V). Unfortunately, however, long time exposition to more negative potentials, as required for EIS measurements at low frequencies, caused damage to the ITO surface (metallic spots appeared and the films lost the electrochemical activity in an irreversible way), and hence we had to restrict our measurements to bias ≥ −0.8 V. Irreversible damage to the ITO surface at very negative applied potentials has also been reported by other authors.95 3.4. Capacitance Spectra of the PPDI Monolayer. Nyquist and Bode plots such as the ones shown in Figure 3 are the usual way to represent EIS data.51−55 In cases where a DC current can be established (for instance, when a redox pair is added to the solution as a charge carrier), Nyquist diagrams display one or more semicircles. However, in the case of surface confined species exposed to an inert electrolyte, a DC current cannot be established, and the system shows a capacitor like behavior. Nyquist plots, for instance, look like straight lines parallel to the y axis (that would be the case for an ideal

Figure 4. Equivalent circuits used to fit the EIS data in Figures 3 and 5. (A) Bare ITO. (B) PPDI monolayer on ITO at zero bias (no faradaic activity, Scheme 1B). (C) PPDI monolayer on ITO at applied bias within the redox window of PPDI (faradaic activity present, Scheme 1C).

impedance becomes null and the total impedance of the system can be then represented by the solution resistance (Rs), resulting in the horizontal lines observed in the Bode magnitude plots at high frequencies and in the zero phase angle, typical of resistors (Figure S4). After deposition of a PPDI monolayer, when the EIS spectra were recorded at bias = 0 V (where there is no redox activity, according to the CV in Figure 2), the system can still be represented by a series combination of a resistor with a capacitor (Figure 4B). Nevertheless, the capacitance in this case is the film capacitance (Cf), which is due to the presence of the monolayer of dielectric organic material (Scheme 1B), in contrast to Cdl observed with bare ITO, where the dielectric material consisted of a thin layer of water molecules separating the ITO surface from a layer of positive K+ cations.51−55 An excellent fit to the model in Figure 4B for applied bias =0 V was obtained using a constant phase element (CPC) instead of a pure capacitor, with the values of Rs = 188 ohm and CPC = 14 μF (α = 0.97). If a parallel plate capacitor (Helmholtz) model is assumed, the film thickness or the dielectric constant can be calculated with eq 1

C = εε0A /t

(1)

which relates the capacitance C to the thickness (t) and to the dielectric constant (ε) of the monolayer (ε0 is the vacuum permittivity constant and A is the area of the film). According with eq 1, if one knows the dielectric constant of the film, the thickness can be obtained. Conversely, if the thickness is known, the dielectric constant can be calculated. In a previous study,88 we measured the thickness of a multilayered PPDI film on silicon using ellipsometry, and found a thickness of 1.6 nm per layer. Using this value in eq 1 affords ε = 25 for the PPDI monolayer. This value of dielectric constant is quite high for an organic coating (typical values: 4−8) and can be reasoned that the aqueous solution occupied all the void space in the monolayer, increasing the dielectric constant well above a typical hydrophobic SAM. It is very likely that the PPDI molecules formed islands, leaving uncovered areas on the substrate. Therefore, eq 1 will afford effective thicknesses and dielectric constants, since they are the average between the different regions of the SAM. It is worth noticing that CattaniScholz et al.94 used eq 1 to calculate a value of ε = 15 for a bilayer of 2,6-diphosphonoanthracene, whose thickness was measured by an independent method. When the potential was moved to more negative bias, toward the region of the redox window of PPDI (Figure 2), the 4107



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circuit in Figure 4C, which contains the faradaic terms Ret and Cpc, in addition to Rs and Cf, and could also be fit with the same parameters as in Figure 3. Alternatively, the data can be analyzed by means of capacitance Bode plots, where the imaginary part of the capacitance is plotted against the frequency,85−87 as seen in Figure 5B for the PPDI monolayer with different applied bias. In contrast, the effect of applied bias on the Cole−Cole and Bode capacitance spectra of bare ITO was much smaller than in the presence of the electroactive monolayer. As seen in Figure S5, only slight differences were observed in the spectra for the different bias, showing that the effects observed in Figure 5 were indeed due to the faradaic activity of the PPDI monolayer. It has been proposed80−87 that subtraction of the blank response (nonfaradaic contributions, registered with the electrode poised outside the redox window, Scheme 1B) from the total response (registered with the electrode poised within the redox window, Scheme 1C) will give information exclusively on the redox process, free of parasitic contributions. The subtraction can be made either in the Cole−Cole plot or in the Bode capacitance plot.85−87 As seen in Figure 5 for the PPDI monolayer, the time scale of the two processes (charge transfer for bias at −0.6 V and neutral monolayer dipole relaxation for bias at 0 V) were quite distinct, and the difference spectra gave virtually the same result as the total response (Figure S6). Capacitance plots have been used to withdraw important information about electroactive thin films, such as molecular surface coverages and electron transfer rates.80−87 In the Cole− Cole diagram, the diameter of the semicircle (after subtraction of the blank response) gives the pseudocapacitance (Cpc), which corresponds to the total charge transferred to reduce the monolayer (Scheme 1C).80−87 The surface coverage (Γ) can then be obtained directly from Cpc by means of eq 4.

capacitor) and bring little information, as seen in Figure 3A for the PPDI monolayer. Cole−Cole plots represent a more instructive way to display EIS data on surface confined redox species.80−87 The treatment consists in plotting the real (C′) versus the imaginary (C″) part of the capacitance, obtained from the real and imaginary impedances through eqs 2 and 3. Cre = −Re[(jωZ)−1] −1

C im = −Im[(jωZ) ]

(2) (3)

A vertical straight line in a Nyquist plot is converted into a semicircle in a Cole−Cole plot. Figure 5A shows the data in

Γ = 4RTCpc/F 2A

(4)

Furthermore, the rate constant for electron transfer (ket) can be obtained from the frequency at the top of the faradaic semicircle (either in the Cole−Cole or in the Bode capacitance plot) through eq 5. ket = πf0

Figure 5. (A) Cole−Cole plot and (B) capacitance Bode plot of PPDI monolayer. The solid lines represent the fit using the same parameters as for the impedance plots in Figure 3

(5)

From Figure 5A, the pseudocapacitance of the PPDI monolayer was determined as Cpc = 65 μF, giving a value of Γ = 7 × 10−11 mol/cm2 through eq 4. This value is about onethird of the surface coverage calculated from the absorption and CV data. As discussed by Bueno et al.,85−87 the difference could arise from the capacitive contributions that cannot be discounted properly in the voltammetry experiment, while they are completely removed in the capacitive spectroscopy experiment. Therefore, this value of surface coverage is more reliable than those obtained from the absorption spectra or from the CV, showing that capacitance EIS is a very powerful technique to determine the surface coverage of immobilized redox species. This surface coverage is consistent with each PPDI molecule occupying in average an area of 240 Å2 on the surface. In a previous report,88 we measured the thickness of a densely packed (2 × 10−10 mol/cm2) multilayered PPDI film on silicon using ellipsometry and found a value of 1.6 nm per layer, with the molecules tilted by 29° from the substrate normal. Those results are consistent with a molecular area of 80 Å2 per molecule in densely packed PPDI films. Therefore, the

Figure 3 replotted in the form of Cole−Cole plots for the different applied bias. As can be observed, the Cole−Cole plots allow a better distinction between processes occurring with different time constants. For the analysis of the EIS data using this kind of plot, it is convenient to compare two different applied bias, one outside the redox window of the redox pair (no faradaic activity) and the other well within the redox window (faradaic activity present). For this purpose, we compared the data recorded at bias = 0 V with the data at bias = −0.6 V. The smaller semicircle in Figure 5A, corresponding to the data recorded at 0 V (circuit in Figure 4B), reflects the nonfaradaic terms Rs and Cf. The data were nicely fitted using the same parameters as for the impedance plots in Figure 3. The value of Cf = 14 μF obtained from the fit corresponds to the width of the semicircle, as expected. The larger semicircle recorded at −0.6 V, on the other hand, corresponds to the 4108



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decompose neither desorve from the surface, even when biased to quite negative potentials (down to −1 V). The use of impedance spectroscopy, especially in the form of Cole−Cole diagrams, was shown to be a very powerful method to study these monolayers, allowing a clear segregation between the background signal from the faradaic activity. A submonolayer coverage was found for the PPDI monolayer using EIS, suggesting that the SAM formed islands, leaving part of the ITO substrate uncovered.

surface coverage obtained here from Cole−Cole plots was about one-third of a full monolayer, suggesting that the PPDI molecules formed islands on the surface, leaving some regions of the substrate uncovered. These findings are in agreement with the high effective dielectric constant obtained with eq 1. They are also in agreement with the absorption data, which indicated that the PPDI molecules were π-stacked in the monolayer. It is worth comparing our results to other films of organic phosphonic acids from the literature. Hanson et al.91 found a surface coverage of 9 × 10−10 mol/cm2 (molecular area of 18.5 Å2) for well packed monolayers of octadecyl phosphonic acid self-assembled on silicon substrates, measured by quartz crystal microbalance. Aromatic phosphonic acids, however, occupy a larger area than the aliphatic alkyl chain analogues, giving lower surface coverages. For instance, the reported surface coverage90 for a densely packed SAM of hexylferrocene phosphonic acid on ITO was 3 × 10−10 mol/cm2, whereas for 9-phosphonoanthracene on silicon96 a surface coverage of 3.3 × 10−10 mol/cm2 was found. It is also worth comparing our results to other films containing similar aromatic imides. Kwan et al constructed thin films containing oligomers of 1,4,5,8-naphthalenediimides (which are closely related to the PDI), and reported surface coverages of 1 × 10−10 mol/cm2 for thiol derivatives on gold97 and 2 × 10−10 mol/cm2 for Langmuir−Blodgett films.98 Tang et al.25 found a molecular density of 6 × 10−11 mol/cm2 per layer for multilayered films of PDI derivatives grown by electrostatic attraction. Thus, the surface coverage of the PPDI monolayer studied here was comparable to other films of aromatic imides. The rate constant for electron transfer (ket) can be obtained from the frequency ( f 0) at the highest ordinate point in Figure 5A (or B) using eq 5,80−87 giving ket = 41 s−1 for the PPDI monolayer on ITO at −0.6 V bias. This value is of similar magnitude as other reported in the literature using the Cole− Cole analysis. Katz et al.,81 for instance, found ket = 77 s−1 for a surface confined viologen based rotaxane on gold electrodes. Nahir and Bowden80 studied electron transfer from thiolmodified gold electrodes to confined cytochrome c and found ket = 1.3 s−1 for a long thiol spacer (HS(CH2)13COOH), increasing to ket = 503 s−1 with a shorter spacer (HS(CH2)5COOH). Bueno et al.85 found ket = 13 s−1 for a ferrocene monolayer on gold with a undecanethiol spacer. They also found ket values ranging from 30 to 740 s−1 for the redox protein azurin bound to self-assembled monolayers of alkylthiols with different lengths on gold. Guo et al.82 have also studied the azurin-alkylthiol-gold system and found ket ranging from 0.08 to 54 s−1, depending on chain length. Moreover, the ket value found for the PPDI monolayer is comparable to the ones reported by Kwan et al.98 for monolayers of oligomeric 1,4,5,8-naphthalenediimides on gold. They found ket varying between 92 and 197 s−1, depending of the length of the oligomer. They reasoned that the slow electron transfer kinetics is controlled by the rate at which cations from the electrolyte can penetrate the films, since electron and ion transport must be coupled to maintain the electroneutrality of the films. This interpretation is in full agreement with the model presented in Scheme 1C for the PPDI monolayer.

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ASSOCIATED CONTENT

S Supporting Information *

Absorption spectra of the SAM before and after the electrochemical measurements. Blank signals (bare ITO) for the CV of the SAM taken with different scan rates. Plots of current peaks versus scan rate for the CV in Figure 2. Effect of applied bias on the impedance spectra of bare ITO. Difference spectra obtained by subtraction of the blank response from the total response for the PPDI monolayer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +55-11-49968260. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from Brazilian agencies FAPESP (Grant No. 2012/16358-9) and CNPq (Grant No. 480189/2011-0). D.Z.F. thanks CNPq for a PQ scholarship. B.P.G.S. is grateful to FAPESP for a doctoral fellowship.

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