Variations of Diffusion Coefficients of Redox Active Molecules in Room

Oct 28, 2008 - Variations of Diffusion Coefficients of Redox Active Molecules in Room Temperature Ionic Liquids upon Electron Transfer. Dodzi Zigah ...
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14952

J. Phys. Chem. B 2008, 112, 14952–14958

Variations of Diffusion Coefficients of Redox Active Molecules in Room Temperature Ionic Liquids upon Electron Transfer Dodzi Zigah,†,‡ Jalal Ghilane,§ Corinne Lagrost,†,‡ and Philippe Hapiot*,†,‡ UniVersite´ de Rennes 1, Sciences Chimiques de Rennes (Equipe MaCSE), CNRS, UMR 6226 Campus de Beaulieu. Bat 10C. 35042 Rennes Cedex, France, UniVersite´ Denis Diderot - Itodys, and CNRS, UMR 7086. 1 rue Guy de la Brosse, 75005 Paris, France ReceiVed: June 24, 2008; ReVised Manuscript ReceiVed: August 11, 2008

In ionic liquids, the diffusion coefficients of a redox couple vary considerably between the neutral and radical ion forms of the molecule. For a reduction, the inequality of the diffusion coefficients is characterized by the ratio γ ) Dred/Dox, where Dred and Dox are the diffusion coefficients of the electrogenerated radical anion and of the corresponding neutral molecule, respectively. In this work, measurements of γ have been performed by scanning electrochemical microscopy (SECM) in transient feedback mode, in three different room temperature ionic liquids (RTILs) sharing the same anion and with a series of nitro-derivative compounds taken as a test family. The smallest γ ratios were determined in an imidazolium-based RTIL and with the charge of the radical anion localized on the nitro group. Conversely, γ tends to unity when the radical anion is fully delocalized or when the nitro group is sterically protected by bulky substituents. The γ ratios, standard potentials of the redox couple measured in RTILs, and those observed in a classical organic solvent were compared for the investigated family of compounds. The stabilization energies approximately follow the γ ratios in a given RTIL but change considerably between ionic liquids with the nature of the cation. Introduction Room-temperature ionic liquids (RTILs) are salts that exist in the liquid state at temperatures around 298 K.1 They generally consist of a combination of weakly coordinating inorganic anions and bulky unsymmetrical organic cations (imidazolium, quaternary onium). Numerous investigations have highlighted the beneficial use of ionic liquids by contrasting the results with those obtained in conventional solvents. The special characteristics of RTILs (high thermal stability, good conductivity, wide potential windows, etc.) make these solvents attractive media for electrochemical investigations2 and have been proposed for numerous applications in the general field of chemistry.3 However, fundamental descriptions of RTILs, including the properties of interfaces, solvations of molecules in a purely ionic environment, transport properties, and so forth, are still open questions. Electrochemical techniques allow detailed analyses of electron transfer and mass transport processes.4 As examples, diffusion coefficients have been measured for several organic molecules such as ferrocene,5-9 tetrathiafulvalene,5 N,N,N′,N′-tetraalkylphenylenediamine,10 and some other aromatic compounds.6 In commonly used RTILs, classical values for diffusion coefficients are on the order of 1 × 10-7 cm2 s-1.11 These values are about 2 orders of magnitude lower than those measured in a conventional electrolyte for the same dissolved molecules. The slow diffusion observed in the RTILs has been explained by the higher viscosity of RTILs as compared to those of classical organic solvents.2,4 The diffusion coefficients of N,N,N′,N′tetramethyl-para-phenylenediamine (TMPD) and N,N,N′,N′* To whom correspondence should be addressed. E-mail: philippe.hapiot@ univ-rennes1.fr. † Universite ´ de Rennes 1. ‡ CNRS, UMR 6226. Campus de Beaulieu. § Universite ´ Denis Diderot and CNRS, UMR 7086 1 rue Guy de la Brosse.

SCHEME 1: Investigated Redox Couples: Nitrotoluene (1), Nitromesitylene (2), 2,4,6-Tri-tert-butylnitrobenzene (3), 2-Methyl-2-nitropropane (4), Anthracene (5)

tetrabutyl-para-phenylenediamine (TBPD) were determined in a series of RTILs of increasing viscosity.10 For both compounds, the diffusion coefficient, D, was found to be inversely proportional to the viscosity in agreement with the classical StokesEinstein equation: D ) kBT/pπηr where kB is the Boltzmann constant, T the absolute temperature, η the viscosity, r the hydrodynamic radius, p is a constant equal to 4 or 6, depending if slip or stick conditions are applied.10,12 Similar conclusions were obtained for the variations of diffusion coefficients in different mixtures of ionic liquids with organic solvents8 and in recent studies concerning the ferrocene oxidation.9 This conformity to the Stokes-Einstein relation qualitatively suggests that RTILs behave just as conventional solvents. However, further experimental observations indicate other specific behaviors of RTILs that were first evidenced by a strong stabilization of electrogenerated radical anion by the cation of the RTIL7 and have several consequences on the kinetics of electron transfer itself13 and on the associated chemical reactions.14,15 Concerning the mass transport, the diffusion coefficients of the oxidized and reduced states of a redox couple, characterized by the ratio γ ) Dred/Dox, could be very different in RTILs.2,4 In contrast, this phenomenon is generally negligible in organic electrolyte because of the weak variation of the diffusion coefficient with the mass of the molecule.16 Spectacular dissimilarities in the diffusion coefficients between reactant and

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Variations of Diffusion Coefficients

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SCHEME 2: Anion TFSI and Cations Used To Prepare the Ionic Liquids: [BMIM], [Et3BuN], [Pyr]

electrogenerated species were revealed for the one-electron reduction of dioxygen to superoxide in commonly used RTILs (γ around 0.02).17,18 This observation suggests that the solvated O2.- travels with a large number of associated cations from the RTIL, resulting in a much larger molecule. Regarding the size of the reactant and its electrogenerated species, the dioxygen system could be considered as a rather extreme case. Nevertheless, such dissimilarities have also been reported for the oxidation of TMPD to its radical cation in four RTILs sharing the same [TFSI] anion.19 The ratio of the diffusion coefficients of the radical cation to the neutral molecule was determined to be around 0.5 and to decrease down to 0.33 between the dication and the neutral molecule. In this connection, the ratio γ that monitors the evolution of the diffusion coefficients upon the electron transfer appears as a convenient way for evaluating this specific effect. In a first approximation, one may consider that the influence of the viscosity on the diffusion of the ionic liquid will not considerably vary between the neutral and charged species. Besides the interest in understanding the properties of ionic liquids, we are also reminded that the consequences of an inequality of diffusion coefficients on electrochemical measurements are complex and often difficult to detect.20,21 As a test family, we chose to investigate the reduction of a series of aromatic and aliphatic nitro compounds (see Scheme 1) in three different ionic liquids and to measure the variations of their diffusion coefficients with the redox state. The reduction of nitro compounds to form a stable radical anion has been widely documented in conventional electrolytes, providing accurate electrochemical data related to their reduction in organic solvents.22 For most of them, the reduction in RTILs is monoelectronic and leads to the formation of the corresponding stable radical anion.23 As another interest for this family, it is relatively easy to modulate the charge localization in the radical anion, or the protection of the NO2 environment, where most of the charge is present, by introducing different substituent groups. Their properties were compared with the behavior of anthracene, for which the produced radical anion could be considered as a standard of charge-delocalized radical anion with little internal reorganization during the charge transfer.24 As previously mentioned, the determination of the diffusion coefficients ratio γ is not straightforward and often relies on a change in a diffusion regime. In this purpose, we have recently proposed the use of the scanning electrochemical microscopy (SECM) in transient feedback mode to make such measurements in ionic liquids.18 This method is classically dedicated to the investigations of interface,25 but could advantageously be used to determine γ ratios in ionic liquids.18,26 The discrepancies with the simple behavior (when diffusion coefficients have been taken as equal) could be transformed in a convenient method for characterizing the transport properties of species dissolved in RTIL,18,27 and the possibilities of the method were emphasized with the O2/O2.- couple.18 Another interesting aspect of this method is the possibility of a variation of the diffusion pathways through the adjustment of the substrate-microelectrode distances. Because of the small values of the diffusion coefficients in RTILs, other types of transport may interfere with the diffusion like those due to the “natural convection” effects28 or “superconcentration” dependence.29

Concerning the choice of the RTIL, we have considered three typical RTILs that are based on the cations depicted in Scheme 2 (imidazolium, pyrrolidinium, and quaternary ammonium) in combination with the same TFSI anion. By keeping the same anion, we can expect to panel the strength of possible interactions between the ionic liquids cation and the nitro radical anion produced upon the electrochemical reduction. Experimental Section Chemicals. The RTILs, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [BMIM][TFSI], butyltriethylammonium bis(trifluoromethylsulfonyl)amide) [Et3BuN][TFSI], and N-methyl-N-butylpyrrolidinium bis{(trifluoromethyl)sulfonyl}amide [Pyr][TFSI], were prepared from aqueous LiN(CF3SO2)2, [Li][TFSI] (Solvionic, France), and the corresponding bromide or chloride salts according to standard procedures.30 The samples were purified by repeated washing with H2O and filtered over neutral alumina and silica. Prior to each experiment, vacuum pumping carefully dried RTILs overnight, and the amount of residual water was measured with Karl Fischer titration (Karl Fischer 652 Metrohm). The amount of water measured in our samples ranged from 100 to 200 ppm. Electrodes and Cyclic Voltammetry Measurements. Disk microelectrodes (5 µm radius) were made by sealing gold or platinum wires (Goodfellow) in a soft glass tube that was subsequently ground at one end, according to previously published procedures.24 The glass sheaths were conically shaped with an outer diameter of approximately 50-100 µm (typical Rg ≈ 10). Prior to use, the microelectrodes were polished using decreasing sizes of diamond and alumina pastes. The microelectrodes were characterized by cyclic voltammetry and by typical approach curves recorded on conducting and insulating surfaces. The reference electrode was a quasireference electrode made with platinum covered by semioxidized polypyrrole and prepared according to the published procedure.31 Its potential was checked versus the ferrocene/ferrocenium couple used as an internal probe. A platinum wire (0.5 mm diameter) was used as the auxiliary electrode. For γ-E° correlations, E° values were determined as the halfsum between the forward and return peak potentials of a cyclic voltammogram recorded under linear diffusion conditions. In RTILs, E° values were corrected from the diffusion coefficient ratio, γ, according to the following expression: E1/2 ) E° + RT/2F log γ.21 All measurements were referenced against the ferrocene/ferrocenium cation that was proposed as a standard redox couple in the RTILs.2,4 SECM Experiments. All SECM measurements were performed using the CHI900B instrument from CH-Instruments equipped with an adjustable stage for the tilt angle correction. The electrochemical cell was that purchased with the SECM and used in a typical 4-electrode configuration. The electrode potentials were controlled with the bipotentiostat of the CHI 900B. The working microelectrode was polarized at a potential chosen at the level of the current diffusion plateau of the considered redox couple. Similarly, the conducting substrate (either platinum or carbon) was polarized at a potential much more positive that the E° of the investigated redox couple in a

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Figure 1. Cyclic voltammetry in RTILs on a 4-5 µm radius microlectrode: (a) 4-nitrotoluene (2 × 10-2 mol.L-1) in [Et3BuN][TFSI], (b) 2-nitromesitylene (10-2 mol L-1) in [Et3BuN][TFSI], (c) 2,4,6-tri-tert-butylnitrobenzene (saturated solution) in [Et3BuN][TFSI], (d) anthracene in [Et3BuN][TFSI], (e) 2,4,6-tri-tert-butylnitrobenzene (saturated solution) in [BMIM][TFSI], (f) nitrotoluene (2 × 10-2 mol L-1) in [Pyr][TFSI]. Scan rates V ) 0.1 V s-1 for curves a and c-f and V ) 0.05 V s-1 for b.

way to limit the influences of the ohmic drop or both electron transfer kinetics. In such conditions, electron transfers could be considered as infinitely fast both at the microelectrode and substrate surfaces. Because of the influence of the difference of the diffusion coefficients in RTIL, approach curves in RTILs cannot be directly used to align the sample.18 Thus, geometric alignments of the sample, cell, and tip electrode were done in an organic solvent. Before filling the cell with the RTIL solution, the first experiment consists of recording the approach curves in DMF using the ferrocene/ferrocenium couple as a mediator. Approach curves were recorded at three different points of the substrate in order to decrease the tilt angle and to maximize the minimum approach distance between the substrate and the tip that we can reach. This minimum approach distance was generally below 1 µm. Then, the SECM cell is rigorously cleaned with acetone and dried under an argon flux for 1 h. Just before the experiments begin, the RTIL solution containing the investigated redox couple at a typical concentration of 10-2 mol L-1 is placed under argon for 1 h in order to remove all oxygen traces that could modify the SECM response. Calculations. Numerical simulations of the transient responses were obtained using the commercially available program, Comsol Multiphysics 3.4, which allows the resolution of the diffusion differential equations based on finite elements.32 We used the same general procedure as depicted in the literature and a similar geometry representation.33 The size of the box was chosen to be at least 2-5 times larger than the size of the electrode with a total number of points around 100000. The

stability of the calculations was tested by varying the mesh sizes, especially for the case where γ differs considerably from the unity. Results and Discussion Preliminary Cyclic Voltammetry Investigations. As explained before, the effect of a difference in the diffusion coefficient, γ, can be evidenced by cyclic voltammetry in conditions where a change of the diffusion regime occurs. First, we used cyclic voltammetry on a microelectrode to check that the consequences of a difference of the diffusion coefficients are visible during the reduction of the NO2 compounds in RTILs (see Figure 1). The reductions of the aromatic nitro derivative compounds were investigated in the three ionic liquids [Et3BuN][TFSI], [BMIM][TFSI], and [Pyr][TFSI]. Studies of the reductions of the aliphatic nitro compound and of anthracene were not possible in [BMIM][TFSI] because of their reduction potentials being too negative. These two redox couples were only investigated in [Et3BuN][TFSI], which offers a sufficiently large electrochemical window.34 The curves in Figure 1 show the one-electron reversible reduction of the investigated compounds to form the corresponding radical anions with no considerable interference due to chemical complications, i.e., good stability of the electrogenerated radical anion in agreement with previous reports.23 The voltammograms correspond to an intermediate regime of diffusion between steady-state radial diffusion, which is characterized by S-shape voltammograms, and semi-infinite linear diffusion that is characterized by peak-shaped voltammograms.

Variations of Diffusion Coefficients It is noticeable that the “peak-shaped” feature is more pronounced during the reverse scan than that recorded during the forward scan. This suggests a more pronounced linear diffusion character for the reoxidation of the radical anion than for the reduction of the neutral molecule. The competition between the linear diffusion, for which peak-shaped voltammograms are obtained, to the spherical diffusion, for which S-shaped voltammograms are obtained, depends on a single parameter p ) (RT/FV)(D/r2) (where R is the gas constant, D the diffusion coefficient, F the Faraday constant, r the radius of the microelectrode, and V the scan rate).21 Keeping constant the electrode size and scan rate, the observed asymmetries can be ascribed to a change in the diffusion regime due to a decrease of the diffusion coefficient of the radical anion versus that of the neutral molecule.17 As another remarkable point, it is clear that the asymmetry depends on the considered molecules. For example, the asymmetry is more visible on the nitrotoluene reduction (Figure 1a) than in the case of the 2,4,6-tri-tert-butylnitrobenzene (Figure 1c) or of the anthracene reduction (Figure 1d), indicating that γ is closer to unity in the two last situations. Determination of Diffusion Coefficient Ratios by SECM. From the previous qualitative analyses, we observed that γ depends both on the molecule structure and on the RTIL nature. To get better insights, measurements of γ ) Dred/Dox are required. SECM in transient feedback mode proved to be a convenient method and was used in this purpose.18,27 Indeed, γ has no effect on the steady state current (typical conditions used for recording the SECM approach curves), but it affects how this steady state is reached.27 Thus, the principle of this method consists of measuring the current at the tip electrode as a function of the time when the microelectrode is maintained at a fixed chosen distance from a conducting surface (a platinum or carbon substrate, see Experimental Section). The applied potentials to the substrate and microelectrode are chosen in such a way that the electron transfers are fast at both interfaces. Practically, the following procedure was used. First, the value of the steady-state current is measured and considered for adjusting the value of the dimensionless distance L ) d/a, where d is the substratemicroelectrode distance and a the radius of the microelectrode. As L is known, the ratio Imin/IST provides a unique determination of γ ) Dred/Dox.18,27 Then, if required, the knowledge of the time at which the minimum current Imin is obtained allows the absolute measurements of Dred and Dox considering that the radius of the electrode is known.18 Notice that the procedure does not require the knowledge of the initial concentration of the dissolved species. This is an advantage when working with molecules like 2,4,6-tri-tertbutyl-nitrobenzene that are not very soluble in the RTIL and for which the concentrations may be difficult to determine. Typical transient SECM curves are represented on Figure 2a-d for the different compounds dissolved in the ionic liquid under the plot of the normalized current I d IT/Iinf where IT is the current at the microelectrode and Iinf the current recorded when the microelectrode is far from the substrate. On each figure, the curves display the variations of the current with time for different microelectrode-substrate distances. When the microelectrode is far from the substrate, the steadystate current is rapidly obtained and the normalized current I is equal to 1. For smaller separate distances, the curves show a different pattern. During the first few seconds of the measurement, a decrease of the current occurs until a minimum value of the

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Figure 2. Transient SECM experiments on a 4-5 µm radius microelectrode: (a) nitrotoluene in [Et3BuN][TFSI] L ) 0.50 (red), 1.08 (blue), ∞ (black); (b) nitrotoluene in [BMIM][TFSI] L ) 0.41 (red), 0.66 (blue), ∞ (black); (c) nitrotoluene in [Pyr][TFSI] L ) 0.46 (red), 0.78 (blue), ∞ (black); (d) anthracene in [Et3BuN][TFSI] L ) 1.00 (red), 1.45 (blue), ∞ (black). Concentrations ∼ 2 × 10-2 mol L-1.

current, Imin, is reached. Then, an enhancement of the normalized current is observed until the steady state IST current is reached (see Scheme 3). It is visible in Figure 2 that the time required to reach steady-state current decreases with the separate distances, corresponding to shorter diffusion lengths.27 Measured Variations of γ and Correlations with the Redox Potentials. The obtained values of the diffusion coefficients for the neutral species, Dox, are in the range of those measured by other direct electrochemical techniques for similar organic molecules,2,4 hence validating our measurements (see Table 1). Both Dox and Dred in the RTILs are 2-3 orders of magnitude lower than those classically measured in a solvent like acetonitrile. This effect is in agreement with the high viscosities of RTILs.2,4,7 Concerning the γ ratios, values range between 0.5 and 0.2. They depend both on the RTIL cation and on the molecule structure. As a general tendency, the lower values of γ, i.e., when the diffusion coefficients display the highest change upon the electron transfer, are found in [BMIM][TFSI]. The γ values in [Et3BuN][TFSI] and [Pyr][TFSI], both RTILs prepared with quaternary ammonium cations, are in the same range and higher than those determined in [BMIM][TFSI]. In a given RTIL among the aromatic nitro-compounds, the lowest γ values are obtained for the nitrotoluene/nitrotoluene radical anion couple (see Table 1). On the contrary, for the bulky 2,4,6-tri-tert-butyl-nitrobenzene, γ is much closer to the unity and the 2-nitromesithylene appears as an intermediate case. All these variations fall in line with what it is expected for ion associations between the electrogenerated radical anion and one or several cations of the RTIL. Indeed in radical anions of nitroderivatives, the highest charge density is found on the NO2 moiety,22 and ion associations are expected to be directly related to the accessibility of the NO2 group in the radical anion state to the RTIL cations. As expected, the unprotected nitrotoluene radical anion displays the lowest γ value and is the most associated among the aromatic compounds. The introduction of two tert-butyl groups increases the value of γ. The protection of the NO2 limits its accessibility to the RTIL cation, which results in a smaller difference of the diffusion coefficients between the neutral molecules and its radical anion. In this last

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SCHEME 3: Different Phases of the Current Evolution in Transient SECM in Feedback Modea

a Area (1) corresponds to the first seconds of the experiment where the current decreases because of the absence of the reverse reaction at the conducting substrate. In the area (2), the current starts to enhance but the steady state is not accomplished. In the last area (3), the current has reached a steady state value that corresponds to a situation where the difference between the diffusion coefficient of the electro-generated radical anion and the compound does not affect the feedback loop.

TABLE 1: Values Extracted from the SECM Measurements [BMIM][TFSI] 1 2 3 4 5 a

[Et3BuN][TFSI]

[Pyr][TFSI]

mesitylene

E°a

γ

107Doxb 107Dredb

E°a

γ

107Doxb 107Dredb

E°a

γ

107Doxb 107Dredb

4-Nitrotoluene 2-Nitro-mesitylene 2,4,6-tri-tert-butyl-nitrobenzene 2-Methyl- 2-nitropropane Anthracene

-1.37 -1.55 -1.65

0.21 0.32 0.50

3.6 0.77 5.4 1.7 2.03 1.0

-1.53 -1.76 -1.83 -2.02 -2.35

0.37 0.35 0.56 0.19 0.63

2.4 0.92 1.9 0.65 0.73 0.41 1.9 0.35 1.2 0.74

-1.46 -1.68 -1.74

0.34 0.31 0.57

3.9 1.3 1.9 0.6 2.0 1.2

In V vs Fc+/Fc. Measured from the E1/2 in cyclic voltammetry experiments and corrected from the γ values. b In cm2.s-1.

case, a value around γ ) 0.5 is measured that could be compared with γ ) 0.6 obtained for the anthracene couple for which the radical anion presents a very high delocalization of the negative charge over the whole molecule. We may also notice that γ considerably differs from unity even for such highly delocalized radical anion. Among all the investigated molecules, the aliphatic 2-methyl-2-nitropropane displays the highest variation of diffusion coefficients with γ around 0.2. Because the delocalization is not possible within the corresponding radical anion, the charge localization is expected to be the highest. However, even if a γ ratio in the range of 0.2 is already considerable and is not encountered in organic solvent, this variation of diffusion coefficients remains relatively modest as compared with that reported for the O2/O2.- couple for which values of γ that are 10 times lower have been determined. In that sense, the case of the reduction of O2 appears as an extreme situation. For the 2-methyl-2-nitropropane radical anion, we have to take into account a protective effect due to the methyl groups that also limits the accessibility of the NO2 to the RTIL cations. γ ratios closer to unity are also noticed for the measurements made in the two quaternary ammonium based RTILs suggesting lower ion associations. Similarly, this could be ascribed to a protective effect due to the large steric hindrance around the NO2 that limits the ion interactions with the bulky Et3BuN+ cation. To verify these assumptions, it is interesting to compare the γ ratios with the corresponding standard potentials, E°, of the same couple. Indeed, ion associations should affect both the standard potential by a stabilization of the radical anion and the γ ratio because of the increase of the effective radius of the radical anion solvated by the RTIL cations. The number of interacting cations with the radical anion is still

an open question. For purposes of simplicity, we will discuss the data by considering simple ion-pair associations as the same qualitative tendency for γ and E° are expected in the case of one or more interacting cations with the electrogenerated radical anion. However, it is noticeable that computational studies of RTIL mixtures have suggested that each anion interacts with more than one simple cation.35 Thus, in the framework of a simple model considering the ion pair formation, the standard potential for a concerted reductive formation of an ion pair in the RTIL, E°RTIL, is related to the standard potential of the ox./red. couple and to the standard free energy of the ion pair, ∆G°ion-pair, according to ° ° the following equation: ERTIL ) Eox/red + RT/F log Kion-pair 36 ° ° ) Eox/red - ∆Gion-pair. E°RTIL variations between RTILs or redox couples are indicative of modification of the ion pair energy that, in a simple electrostatic model, depends on the distance from the cation to the radical anion, the charge density on the radical anion, and the polarity of the media. ° (∆Gion-pair ∝q/dε where dRTIL-NO2 is the distance between the NO2 group and the cation of the RTIL, q is the charge on the NO2 group in the radical anion, and  is the apparent dielectric constant of the RTIL). Simultaneously, in the framework of the classical Stokes-Einstein equation, the diffusion coefficient D is proportional to 1/r, where r is the radius of the effective sphere for the ion-paired radical anion. In this simple model, the modifications of γ characterize the relative variations of size of the radical anion providing a rough estimation of the number of interacting RTIL cations. Thus, one could expect that these two values γ and E° be correlated. For easier evaluations, it seems better to compare the E° and γ in RTILs with those measured in acetonitrile (+ 0.1 mol L-1 NBu4PF6) because the ion-pair associations are negligible in such an organic electrolyte and γ ratios are close

Variations of Diffusion Coefficients

Figure 3. Variation of 1/γ ) Dox/Dred as function of the stabilization energy ∆E °RTIL-ACN for the aromatic NO2-compounds of Table 11: (1) nitrotoluene; (2) nitromesitylene; (3) 2,4,6-tri-tert-butylnitrobenzene; (4) 2-methyl-2-nitropropane; (5) anthracene.

to unity.19,37 Figure 3 shows the variations of 1/γ ) Dox/Dred that reflect the size increase of the nitro compound upon reduction, as a function of ∆E°RTIL-ACN ) E°RTIL - E°ACN for the different nitro-compounds. ∆E°RTIL-ACN characterizes the variations of standard potentials when passing from the acetonitrile to the RTIL.38 In this study, all potential measurements both in RTILs and in acetonitrile were standardized to the ferrocene/ferrocinium couple in the same media for which the change of its E° have been reported to be small when passing from an RTIL to another one.4 In agreement with our previous measurements, we found all E° values are positively shifted when passing from the acetonitrile to the RTILs.23 The ∆E°RTIL-ACN variations depend on the nature of the RTIL cation. They are larger for [BMIM][TFSI] than for [Et3BuN][TFSI] or [Pyr][TFSI] indicating an higher stabilization of the electrogenerated radical anion by the [BMIM] cation. As explained before, this stronger interaction could be explained by a lower distance from the charge center of the radical anion to the RTIL cation due to the planar geometry of the imidazolium ring and by its ability to form hydrogen bonds.23 Previous calculations performed with the DFT method (calculations made in gas phase) with similar cations and radical anions of organic molecules have indeed suggested that the stabilization energy is higher with the imidazolium cation.14 In line with this observation, the lowest γ ratios are also found in [BMIM][TFSI]. More generally, for a given RTIL, the highest variations of diffusion coefficients correspond approximately to the largest ∆E°RTIL-ACN values. However, when changing the nature of cation of the ionic liquid, all these values do not seem to correlate in a simple way. As seen in Figure 3, a given stabilization energy ∆E°RTIL-ACN does not lead to the same γ when the cation of the ionic liquid is different. In a model of simple ionic interactions and Stokes-Einstein relation, this observation suggests that the size of the ion-pair, and thus the number of interacting cations, does not lead to the same stabilization energy when changing the RTIL. If we assume that the variations of standard potential of the ferrocene/ferrocenium couple, which is used as internal probe, are negligible from one RTIL to another, we could conclude that this effect illustrates a modification in the solvent environment equivalent to a modification of the apparent dielectric constant of the RTIL. Conclusion In room temperature ionic liquids, organic molecules display some important variations of their diffusion coefficients when

J. Phys. Chem. B, Vol. 112, No. 47, 2008 14957 reduce to their corresponding radical anions. The difference that is characterized by the ratio γ)Dred/Dox is found to increase when the charge is localized in radical anion and conversely to decrease when the radical anion is highly delocalized. However, even in the case of the anthracene/anthracene radical anion, γ is found to be largely different from the unity. γ tends to the unity when bulky substituents protect the NO2 group or when the RTIL cation is bigger. All these observations fall in line with what it is expected for ion-pairing associations between the electrogenerated radical anion and the RTIL cation. In a simple representation as for the Stokes-Einstein law, the γ ratio provides an estimation of the size of the radical anion. In a given RTIL, γ approximately correlates with the stabilization energy brought by the ion pairing estimated from the relative variations of the standard potentials measured in the RTIL and in acetonitrile. However, there is no correlation between γ and the stabilization energy when passing from one RTIL to another, which could be seen as the consequence of different effective dielectric environments in a simple electrostatic description of the ion-pair. Acknowledgment. The authors thank the Agence National de la Recherche, Contract ANR-06-BLAN-0296-02 “R.E.EL” for financial support and the “Re´gion Bretagne” for studentship grant (D.Z.). Supporting Information Available: SECM experiments in transient feedback mode and simulations of the experimental curves. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Welton, T. Chem. ReV. 1999, 99, 2071. (b) Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, 2003. (c) Ionic Liquids: Industrial Applications for Green Chemistry; Rogers, R.,.; Seddon, K. R.; ACS Symposium Series 818; American Chemical Society: Washington, DC; 2002. (2) (a) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. ChemPhysChem. 2004, 5, 1106. (b) Silvester, D. S.; Compton, R. G. Z. Phys. Chem. 2006, 220, 1247. (3) (a) Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2003. (b) Green Industrial Applications of Ionic Liquids; Roger, R. D., Seddon, K. R., Volkov, S., Eds.; NATO Sciences Series; Kluwer: Dordrecht, The Netherlands, 2002; Vol 92. (c) Chiappe, C.; Pieracinni, D. J. Phys. Org. Chem. 2005, 18, 275. (4) Hapiot, P., Lagrost, C. Chem. ReV. 2008, 108, 275. (5) Fuller, J.; Carlin, R. T.; Osteryoung, R. A. J. Electrochem. Soc. 1997, 144, 3881. (6) Kosmulski, M.; Osteryoung, R. A.; Ciszkowska, M. J. Electrochem. Soc. 2000, 147, 1454. (7) Lagrost, C.; Carrie´, D.; Vaultier, M.; Hapiot, P. J. Phys. Chem. A 2003, 107, 745. (8) Comminges, C.; Barhdadi, R.; Laurent, M.; Troupel, M. J. Chem. Eng. Data 2006, 51, 680. (9) Rogers, E. I.; Silvester, D. S.; Poole, D. L.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. C 2008, 112, 2729. (10) Evans, R. G.; Klymenko, O. V.; Hardacre, C.; Seddon, K. R.; Compton, R. G. J. Electroanal. Chem. 2003, 556, 179. (11) Zistler, M.; Wachter, P.; Wasserscheid, P.; Gerhard, D.; Hinsch, A.; Sastrawan, R.; Gores, H. J. Electrochim. Acta 2006, 52, 161. (12) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 6103. (13) Brooks, C. A.; Doherty, A. P. J. Phys. Chem. B 2005, 109, 6276. (14) Lagrost, C.; Gmouh, S.; Vaultier, M.; Hapiot, P. J. Phys. Chem A. 2004, 108, 6175. (15) Lagrost, C.; Hapiot, P.; Vaultier, M. Green Chem. 2005, 7, 468. (16) Stokes-Einstein law predicts that D vary as the inverse of the radius of the equivalent sphere and thus as the third root of molecular mass D ∼ -1/3 M . (17) Buzzeo, M. C.; Klymenko, O. V.; Wadhawan, J. D.; Hardacre, C.; Seddon, K. R.; Compton, R. G. J. Phys. Chem. A 2003, 107, 8872.

14958 J. Phys. Chem. B, Vol. 112, No. 47, 2008 (18) Ghilane, J.; Lagrost, C.; Hapiot, P. Anal. Chem. 2007, 79, 7383. (19) Evans, R. G.; Klymenko, O. V.; Price, P. D.; Davies, S. G.; Hardacre, C.; Compton, R. G. ChemPhysChem 2005, 6, 526. (20) See for example:(a) Andrieux, C. P.; Hapiot, P.; Save´ant, J.-M. J. Electroanal. Chem. 1984, 172, 49. (b) Andrieux, C. P.; Hapiot, P.; Save´ant, J.-M. J. Electroanal. Chem. 1985, 186, 237. (21) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001. (22) (a) Chauhan, B. G.; Fawcett, W. R.; Lasia, A. J. Phys. Chem. 1977, 81, 1476. (b) Fawcett, W. R.; Lasia, A. J. Phys. Chem. 1978, 82, 1114. (c) Peover, M. E.; Powell, J. S. J. Electroanal. Chem. 1969, 20, 427. (d) Petersen, R. A.; Evans, D. H. J. Electroanal. Chem. 1987, 222, 129. (e) Evans, D. H.; Gilicinski, A. G. J. Am. Chem. Soc. 1992, 96, 2528. (f) Save´ant, J. M.; Tessier, D. Faraday Discuss. Chem. Soc. 1982, 74, 57. (g) Kraiya, C.; Singh, P.; Evans, D. H. J. Electroanal. Chem. 2004, 563, 203. (23) Lagrost, C.; Preda, L.; Volanschi, E.; Hapiot, P. J. Electroanal. Chem. 2005, 585, 1. (24) Andrieux, C. P.; Garreau, D.; Hapiot, P.; Save´ant, J.-M. J. Electroanal. Chem. 1988, 243, 321. (25) Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001. (26) Carano, M.; Bond, A. M. Aust. J. Chem. 2007, 60, 29. (27) Martin, R. D.; Unwin, P. R. J. Electroanal. Chem. 1997, 439, 123. (28) For more quantitative descriptions of diffusion concentration profiles at an ultramicroelectrode corrupted by natural convection, see the following: Amatore, C.; Knobloch, K.; Thouin, L. J. Electroanal. Chem. 2007, 106, 17. (29) (a) Variations of the diffusion coefficients with the concentration of the dissolved molecule were reported for the oxidation of concentrated solutions of ferrocene in RTILs.29b,c(b) Brooks, C. A.; Doherty, A. P. Electrochem. Commun. 2004, 6, 867. (c) Eisele, S.; Schwarz, M.; Speiser, B.; Tittel, C. Electrochim. Acta 2006, 51, 5304.

Zigah et al. (30) (a) Bonhoˆte, P.; Diaz, A. P.; Papageorgiou, N.; Kalyasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168. (b) Sun, J.; Forsyth, G. R.; MacFarlane, D. R. J. Phys. Chem. B 1998, 102, 8858. (31) Ghilane, J.; Hapiot, P.; Bard, A. J. Anal. Chem. 2006, 78, 6868. (32) Comsol Multiphysics 3.4; http://www.comsol.com. (33) Lefrou, C. J. Electroanal. Chem. 2006, 592, 103. (34) Despite imidazolium cation based RTILs presenting large cathodic windows, redox couples with too negative E° values display irreversible behaviors at low scan rates. It is likely that they are able to catalyze the reduction of the [BMIM] cation. [BMIM] cation is also known to react with electrogenerated bases. See for example:(a) O’Toole, S.; Pentlavalli, S.; Doherty, A. P. J. Phys. Chem. B 2007, 111, 9281. (35) Del Popolo, M. G.; Mullan, C. L.; Holbrey, J. D.; Hardacre, C.; Ballone, P. J. Am. Chem. Soc. 2008, 130, 7032. (36) (a) Save´ant, J.-M. J. Phys. Chem. B 2001, 105, 8995. (b) Save´ant, J.-M. J. Am. Chem. Soc. 2008, 130, 4732. (37) Standard potentials in acetonitrile + NBu4PF6 versus the ferrocene couple were determined as E°ACN )-1.62,-1.84,-1.89,-2.10 V versus Fc/Fc+, respectively, for the 4-nitrotoluene, 2-nitromesitylene, 2,4,6-tritert-butyl-nitrobenzene, and 2-methyl-2-nitropropane, respectively. Ion pairing could be considered negligible in this organic solvent containing alkyl ammonium supporting salt. (38) (a) As suggested in the literature,38b all potential measurements both in RTILs and in acetonitrile were standardized against the ferrocene/ ferrocenium couple used as an internal probe.4 For this redox couple, changes of its E° were found to be small when passing from an RTIL to another one in comparison with highly delocalized aromatic systems where ionic pairing is expected to be weak.7 This falls in line with the low variation of γ for the ferrocene/ferrocenium couple in RTILs based on [TFSI] as anion.9. (b) Hultgren, V. M.; Mariotti, A. W. A.; Bond, A. M.; Wedd, A. G. Anal. Chem. 2002, 74, 3151.

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