1650
J. Phys. Chem. C 2008, 112, 1650-1657
Behavior of the Heterogeneous Electron-Transfer Rate Constants of Arenes and Substituted Anthracenes in Room-Temperature Ionic Liquids Stephen R. Belding,† Neil V. Rees,† Leigh Aldous,‡ Christopher Hardacre,‡ and Richard G. Compton*,† Physical & Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford, OX1 3QZ, United Kingdom, and School of Chemistry and Chemical Engineering/QUILL, Queen’s UniVersity Belfast, Belfast, Northern Ireland BT9 5AG, United Kingdom ReceiVed: October 26, 2007; In Final Form: NoVember 9, 2007
We report the anodic oxidation of several arenes and anthracenes within room-temperature ionic liquids (RTILs). In particular, the heterogeneous electron-transfer rates (k0) for substituted anthracenes and arenes are also investigated in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) and found not to obey the outer-sphere Marcus-type behavior of these compounds in contrast to the behavior in traditional organic solvents, in particular the predictions for k0 with molecular size and solvent static dielectric constant. To obtain the electron-transfer rate for 9-phenylanthracene, the dimerization and heterogeneous electrontransfer kinetics of its electrogenerated radical cations is studied in [C2mim][NTf2] and eight other RTILs and are both found to be largely independent of the solution viscosity.
Introduction The use of room-temperature ionic liquids (RTILs) in electrochemical studies over the past decade has often focused on investigating their unusual physical and voltammetric properties.1-3 Now that they are rapidly becoming established as electrochemical solvents, an increasing amount of work is being performed to understand the differences in electrochemical kinetics in these solvents.4-9 Some theoretical work has investigated the effect of RTILs on the double-layer structure and heterogeneous electron-transfer rates compared to traditional organic solvents.10,11 It has been shown that, for example, the solvent reorganization energies associated with an electron transfer are similar in RTILs than for acetonitrile, and the charge-transfer process is Marcus-like.11 While the behavior simulated for the RTILs could be interpreted using the continuum model, it was recognized that the mechanisms by which an RTIL responds to an applied field are quite different from a traditional, polar solvent. In the latter, solvent molecules reorient, whereas in an RTIL the molecules must move relative to each other.11 These differences must impact upon the overall observed rate of electron transfer, even if the activation free energies are similar. That is, while inner-sphere electron-transfer rates might be expected to be very similar in RTILs compared to polar organic solvents, outer-sphere electron transfers will be very different due to the screening response mechanisms. The solvent reorientation in polar solvents is characterized by the longitudinal dielectric relaxation time, τL, which appears in the pre-exponential factor in the Sumi-Marcus model (see eq 1):12 k0 ) Q
(ψr )
1/2
[(
exp - Br +
ψ r
)]
(1)
* To whom correspondence should be addressed. Email: richard.
[email protected]. Phone: +44 (0) 1865 275413. Fax: +44 (0) 1865 275410. † Oxford University. ‡ Queen’s University Belfast.
where Q ) Kpκel0 exp[-B(δ - σ)]/2τLxπ and ψ ) NAe2/ 32π0RT(1/op - 1/s). In this expression, Kp is the equilibrium constant for the formation of the precursor complex, κel0 exp[-B(δ - σ)] is the electronic transmission probability comprised of a maximum probability κel0, with B a constant, σ the distance of closest approach, δ an arbitrary distance, op and s are the high and low-frequency limits of the dielectric constant for the solvent, and NA, e, 0, R, and T have their usual meanings. However, at present there is no accepted analogous relaxation time for RTILs, due to the complexity of the relaxation processes occurring (rotation, translation, and other as yet unidentified concerted molecular motions13,14). Full theoretical results for outer-sphere electron transfer have yet to be published, with only experimental reports of non-Marcus behavior in RTILs.6 There is an increasing body of literature on homogeneous chemistry in RTILs,4,7,15-19 but there is still much to learn of the effects that these solvents confer onto apparently well-known reaction pathways. In this paper we present results from a kinetic study into the electrochemical oxidation of arenes and substituted anthracenes. The aims were to investigate the heterogeneous electron-transfer kinetics of the anthracenes and arenes in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]). Mono- and disubstituted anthracenes have previously been shown to follow an outer-sphere electron-transfer model consistent with Marcus theory,12 and comparison is made to these results. Since electron transfers have been found to be slower in RTILs than in organic solvents, conventional cyclic voltammetry at a microdisk has been chosen as the method of investigation as it is more applicable than hydrodynamic methods such as the high-speed channel electrode, which would normally be preferred.20,21 Some of these compounds exhibit coupled homogeneous kinetics, and so in order to extract the electron-transfer rates for these compounds it was necessary to model these kinetics according to previously reported schemes. In particular, the dimerization reaction of 9-phenylanthracene
10.1021/jp7103598 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/10/2008
Arenes and Substituted Anthracenes in RTILs
Figure 1. Structures of RTIL cations and anions.
Figure 2. (a) Cyclic voltammogram of 9-PA in [C2mim][NTf2] recorded at a voltage scan rate of 100 mV s-1. (b) A double-potential step transient fitted for DA and DB.
(9-PA) was examined in a range of RTILs to investigate the effect of viscosity on homogeneous and heterogeneous kinetics in RTILs. Experimental Reagents. 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C2mim][NTf2]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C4mim][NTf2]), 1-butyl-3-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([C4mpyrr][NTf2]), and their bromide salt precursors were prepared and purified by standard literature procedures.22,23 1-Octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C8mim][NTf2]) was also prepared according to literature.22 1-Butyl-3-methylimidazolium tetrafluroborate ([C4mim][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), and 1-butyl-3-methylimidazolium triflate ([C4mim][OTf]) were kindly donated by Merck KGaA and were used as received. 1-Butyl-3-methylimidazolium nitrate ([C4mim][NO3]) was prepared as reported previously.24 Tris(ethyl)hexylammonium bis(trifluoromethylsulfonyl)amide ([N6,2,2,2][NTf2]) was prepared by metathesis of n-hexyl-triethylammonium bromide (Aldrich, 99%) and tris(n-hexyltetradecylphosphonium chloride), both used as received, with lithium bis(trifluoromethyl)sulfonylamide and subsequently purified according to the literature methods.22,23 The aromatic reagents 9-PA (Lancaster, 98%), 9-chloroanthracene (9-CA, Aldrich, 97%), 9-aminoanthracene (9-AA, Aldrich, 97%), 9,10-diphenylanthracene (DPA, Aldrich, 97%), 9,10-dichloroanthracene (DCA, Aldrich, 97%), pyrene (Aldrich, 97%), perylene (Aldrich, 97%), chrysene (Aldrich, 97%), 1,2benzanthracene (1,2-BA, Aldrich, 97%), and 2,3-benzanthracene
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1651 (2,3-BA, Aldrich, 97%) were all used as received without further purification. Solutions were made up initially in a carrier solvent (dichloromethane) and added to a small quantity of RTIL to a known concentration (typically ca. 10 mM). The solution was then placed under vacuum for several hours to remove moisture, carrier solvent, and oxygen. Solutions were made and used at 295 ( 1 K. Apparatus. A µAutolab Type II potentiostat (Eco Chemie, Utrecht, Netherlands) was used in conjunction with GPES v.4.9 control software for the electrochemical measurements. Platinum microdisk electrodes with diameters of 10 and 15 µm were used as working electrodes with a silver wire as combined reference and counter electrode (a two-electrode arrangement was justified by sub-nanoamp currents).25 Chronoamperometry. Single- and double-potential step transients were also measured in order to confirm the solution concentration and determine the diffusion coefficients for the 9-PA neutral (DA) and radical cation (DB) species (as these are typically different in RTILs1). This was achieved by fitting the single potential-step transient with the Shoup-Szabo expression26 using Origin software (Microcal Inc., version 7) to extract both the concentration and DA. A fitting program written inhouse27 was used with these values to then fit the full doublepotential step transient to find DB. Waveshape Analysis. Experimental waveshapes were modeled using DigiSim v3.03b simulation software (Bioanalytical Systems Inc., West Lafayette, Indiana) using a hemispherical regime and an electrode radius reduced by the appropriate 2/π factor.28 Although the use of a 1-dimensional model for simulation is grossly inaccurate for waveshape modeling of microelectrode voltammetry at these voltage scan rates in traditional organic/aqueous solvents, the slow mass transport in RTILs (diffusion coefficients are typically 2-3 orders of magnitude smaller) renders it acceptable.28,29 Results and Discussion 1. The Oxidative Dimerization of 9-PA. The substituted anthracenes were selected for study because they are well-known to show outer-sphere heterogeneous electron-transfer kinetics. Of these compounds, the monosubstituted anthracenes are known to be reactive, and in particular 9-PA has been shown convincingly to undergo oxidative self-dimerization.30 To derive values for the heterogeneous electron-transfer kinetics therefore, it was necessary to consider the associated homogeneous dimerization kinetics. The radical cations derived from monosubstituted anthracenes have been known to be reactive, for example, undergoing nucleophilic attack from solvent (or other reactant) molecules31-34 or self-dimerization.30 In particular, 9-PA predominantly displays self-dimerization in acetonitrile with relatively low reactivity,30,35 making it an ideal test system for kinetic measurement in RTILs before the other monosubstituted anthracenes were studied (vide infra). First, the oxidation of 9-PA was studied in [C2mim][NTf2]. A solution of 9-PA was prepared as above, and cyclic voltammograms (CVs) recorded at a range of voltage scan rates spanning 3 orders of magnitude from 5 mV s-1 to 5 V s-1. Figure 2 shows a typical CV and transient. Once the concentration and diffusion coefficients were found, the waveshapes were analyzed as described above using the kinetic scheme for the EC2EE mechanism shown in Figure 3a.32,33 The mechanistic details of the dimerization of aromatic radical cations have been the subject of much debate over the last three decades. Figure 3a shows the direct radical-radical coupling (RRC) mechanism with separate steps for dimerization
1652 J. Phys. Chem. C, Vol. 112, No. 5, 2008
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Figure 3. EC2EE mechanisms for oxidative dimerization of 9-substituted anthracenes.
and deprotonation (which may or may not be experimentally observable),4 while 3b shows the radical-substrate coupling (RSC) model. Parker has reported that a mixed RRC-RSC mechanism can be used to explain the literature results, with the RRC mechanism becoming dominant at higher concentrations,33 but in the case of 9-PA has reported that the RRC pathway is most likely.35 Recent work by Hapiot supports this route for the electrodimerization of some aromatic molecules in RTILs.4 Save´ant has shown for substituted biphenyls that the RSC mechanism can explain the observed kinetic behavior.36 Figure 3c shows the concerted RRC mechanism due to Heinze.37-39 This procedure was followed for the oxidation of 9-PA in the remaining 8 RTILs, and the results given in Table 1. When initially modeling the experimental data, it was not possible to discriminate the dimerization and deprotonation steps as characterized by the rate constants kdim and kH in Figure 3a, and so for simplicity the simulation software was programmed with a single combined dimerization and deprotonoation step characterized by the rate k2. In all cases, the dimerization was found to be irreversible and a value of Keq ) 1010 was assumed. Likewise the transfer coefficient R was assumed to be 0.5 for all electron transfers. Other results derived from the modeling are given in Table 2. These include constants for the B-D and C-E processes, which are relatively insensitive due to the irreversibility of the coupling reaction. In particular the formal potentials stated for the B-D and C-E oxidations are derived purely for the purposes of modeling and do not reflect any experimental measurement. Figures 4 and 5 show a selection of typical results from the waveshape modeling, demonstrating very close agreement with
the experimental voltammetry. Modeling was undertaken separately using the alternative RSC dimerization mechanism shown in Figure 3b. However, it was not possible to fit the relative sizes and positions of the forward and reverse peaks to the experimental results for any combination of the simulation parameters. Figure 3c shows the concerted mechanism due to Heinze, which assumes a diffusion-limited RRC dimerization reaction where the slow kinetics are due to Coulombic repulsions between the like-charged reactants rather than an activation barrier.37-39 We therefore conclude that the mechanism shown in Figure 3a above is the dominant process, in agreement with reported results for the dimerization of dithiafulvalene in [C2mim][NTf2].4 Figure 6 shows that the dimerization rate constant, k2, is approximately constant within an order of magnitude, with a very weak dependence on solvent viscosity, η, at low values of η, suggesting that the dimerization is activation rather than diffusion-controlled. The figure also shows for comparison the calculated limits of diffusion control given by the standard Smoluchowski equation40 kdiff ≈
8000RT 3η
(2)
as well as the Smoluchowski equation adapted for Coulombic screening41 kdiff ≈
(
8000RT δ 3η eδ - 1
)
(3)
with δ ) ZAZBe2/4π0r RABRT where Zi is the charge on ion i, RAB is the sum of the radii of the two species A and B (taken
k0(A-B)/cm s-1 k2/dm3 mol-1 s-1 Ef0(A-B)/V DA/cm2 s-1 DB/cm2 s-1 η/mPa‚s at 298 K r45 a
[C2mim] [NTf2]
[C4mim] [NTf2]
[C8mim] [NTf2]
[C4mpyrr] [NTf2]
[N6,2,2,2] [NTf2]
[C4mim] [BF4]
[C4mim] [PF6]
[C4mim] [NO3]
[C4mim] [OTf]
(1.1 ( 0.1) × 1 0-2 (6.5 ( 1.0) × 1 03 1.20 ( 0.01 3.7 × 10-7 2.8 × 10-7 281 12.3
5.0 × 10-3 (1.2 ( 0.3) × 103 1.29 ( 0.01 1.3 × 10-7 1.0 × 10-7 521 11.6
5.0 × 10-3 (2.6 ( 1.1) × 10 3 1.17 ( 0.01 9.0 × 10-8 7.0 × 10-8 741a
1.0 × 10-2 (2.3 ( 0.9) × 10 3 1.19 ( 0.01 9.1 × 10-8 7.0 × 10-8 801 11.9
(6.5 ( 2.0) × 10-2 (5.2 ( 0.8) × 10 3 1.47 ( 0.01 7.5 × 10-8 5.8 × 10-8 1671 10.0b
(1.6 ( 0.1) × 10-2 (4.4 ( 1.8) × 103 1.23 ( 0.01 1.1 × 10-7 9.0 × 10-8 11043 11.7
(3.1 ( 0.3) × 10-3 (6.0 ( 1.5) × 10 3 1.14 ( 0.01 6.4 × 0-8 5.5 × 10-8 2751 11.4
(4.0 ( 2.0) × 10-3 (8.0 ( 1.9) × 10 3 1.32 ( 0.01 6.1 × 10-8 5.0 × 10-8 22824
(9.5 ( 1.1) × 10-2 (2.8 ( 0.9) × 10 3 1.27 ( 0.01 8.4 × 10-8 7.0 × 10-8 9044 13.2
At 295 K. b This value is for [N5,2,2,2][NTf2].
k0(B-D)/cm s-1 k0(C-E)/cm s-1 Ef0(B-D)/V Ef0(C-E)/V DD/cm2 s-1 DC ) DE/cm2 s-1
[C2mim] [NTf2]
[C4mim] [NTf2]
[C8mim] [NTf2]
[C4mpyrr] [NTf2]
[N6,2,2,2] [NTf2]
[C4mim] [BF4]
[C4mim] [PF6]
[C4mim] [NO3]
[C4mim] [OTf]
2.0 × 10-3 (7.8 ( 2.5) × 10-3 1.45 ( 0.06 1.18 ( 0.01 2.8 × 10-7 1.9 × 10-7
8.0 × 10-4 5.0 × 10-3 1.42 ( 0.04 1.25 ( 0.12 1.0 × 10-7 7.0 × 10-8
1.0 × 10-4 5.0 × 10-3 1.28 ( 0.01 1.16 ( 0.03 7.0 × 10-8 5.5 × 10-8
8.0 × 10-4 7.5 × 10-3 1.36 ( 0.01 1.17 ( 0.01 7.0 × 10-8 5.5 × 10-8
(7.3 ( 2.0) × 10-4 (5.0 ( 1.0) × 10-3 1.56 ( 0.02 1.46 ( 0.01 5.8 × 10-8 4.0 × 10-8
(8.2 ( 1.0) × 10-4 (1.4 ( 0.2) × 10-2 1.45 ( 0.02 1.18 ( 0.05 9.0 × 10-8 7.0 × 10-8
(2.0 ( 0.1) × 10-3 (2.5 ( 0.1) × 10-3 1.35 ( 0.01 1.09 ( 0.01 5.5 × 10-8 3.5 × 10-8
1.0 × 10-3 (1.5 ( 0.1) × 10-3 1.43 ( 0.03 1.30 ( 0.01 5.0 × 10-8 3.5 × 10-8
1.0 × 10-3 (7.0 ( 2.3) × 10-3 1.39 ( 0.01 1.25 ( 0.02 7.0 × 10-8 5.0 × 10-8
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TABLE 2: Secondary Results from Modeling the Oxidation of 9-PA
Arenes and Substituted Anthracenes in RTILs
TABLE 1: Key Results for the Oxidation of 9-PA in Various RTILs
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Figure 4. Experiment (solid lines) vs simulation (circles) for 9-PA in [C2mim][NTf2] at the following voltage scan rates (all in V s-1): (a) 0.01, (b) 0.1, (c) 1.0, (d) 5.0.
Figure 5. Experiment (solid lines) vs simulation (circles) for the following RTILs and voltage scan rates. (a) [C2mim][NTf2], 1.0 V s-1; (b) [C4mim][NTf2], 10 V s-1; (c) [C8mim][NTf2], 1.5 V s-1; (d) [C4mpyrr][NTf2], 3 V s-1; (e) [N6,2,2,2][NTf2], 0.5 V s-1; (f) ([C4mim][BF4]), 0.5 V s-1; (g) [C4mim][PF6], 0.1 V s-1; (h) [C4mim][NO3], 5 mV s-1; (i) [C4mim][OTf], 10 mV s-1.
here to be 8 Å42), and the other terms have their usual physical meanings. The observation that the calculated diffusioncontrolled limits due to eq 3 are significantly greater than the derived values for k2, support the earlier supposition that mechanism 3c was not likely to be dominant in this case. Figure 7 shows the lack of variation in the ratio of diffusion coefficients (DA/DB) with viscosity. While this ratio has been shown to be at least 0.96 for several large, charge-delocalized molecules in acetonitrile, where the polar solvent molecules
appear to solvate neutral and charged species approximately equally,46 it has also been well documented that this is not so for RTILs where the solvation by ions is markedly different for neutral and charged species,47 and in this case the ratio is approximately 1.25. Differences in the degree of solvation of charged and neutral species are thus expected to be reflected in differences in the diffusion coefficients of the species. The ratio appears to have a weak dependence on the RTIL anion, as reported in the literature,48 although all lie close to the average ratio (shown by a solid line).
Arenes and Substituted Anthracenes in RTILs
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1655
Figure 9. Mechanistic scheme for oxidation of arenes.
Figure 6. Variation of the dimerization rate constant, k2, with RTIL viscosity. Data points show RTILs with (9) the same anion and (O) the same cation. The calculated diffusion-controlled rate constants are also shown for: (2) eq 2 for uncharged species, (b) eq 3 with ZA ) ZB ) +1, and (∆) eq 3 with ZA ) +1, ZB ) -1 (see text).
Figure 10. General shape of eq 1.
Figure 7. Variation of the ratio of diffusion coefficients with RTIL viscosity. Solid line shows the average value and does not represent a trend line.
Figure 11. Experimental plot of k0 vs solvodynamic radius for anthracenes (squares) and arenes (circles).
TABLE 3: Simulation Results for the Oxidation of 9-CA and 9-AA in [C2mim][NTf2] 9-CA -1
Figure 8. The observed variation of k0 for the oxidation of 9-PA with (a) RTIL viscosity and (b) RTIL static dielectric constant.
Finally, Figure 8 shows the variations in the heterogeneous electron-transfer rates (k0) derived from the waveshape modeling of the first 9-PA oxidation across the nine RTILs. The viscosity of the RTILs varies by a factor of almost an order of magnitude, while their static dielectric constants are all very similar (these values compare with s ) 36 for acetonitrile, s ) 78 for water, and s ) 8.5 for dichloromethane). There is no clear systematic dependence of k0 on either viscosity or static dielectric constant.
k0(A-B)/cm s k0(B-D)/cm s-1 k0(C-E)/cm s-1 k2/dm3 mol-1 s-1 Ef0(A-B)/V Ef0(B-D)/V Ef0(C-E)/V DA/cm2 s-1 DB ) DD/cm2 s-1 DC ) DE/cm2 s-1
9-AA
(6.5 ( 1.3) × 2.010-3 (7.8 ( 2.5) × 10-3 (3.0 ( 1.7) × 106 1.39 ( 0.03 1.57 ( 0.03 1.37 ( 0.02 2.9 × 10-7 2.0 × 10-7 1.7 × 10-7 10-3
(4.0 ( 1.1) × 10-3 (1.5 ( 0.5) × 10-4 1.0 × 10-3 (5.7 ( 1.0) × 106 0.62 ( 0.01 0.82 ( 0.01 0.57 ( 0.02 1.1 × 10-7 1.0 × 10-7 7.0 × 10-8
2. The Heterogeneous Electron-Transfer Rates for the Oxidation of Substituted Anthracenes and Arenes in [C2mim][NTf2]. An analogous procedure to that described above for 9-PA was carried out for the electrochemical oxidation of 9-chloroanthracene (9-CA) and 9-aminoanthracene (9-AA). Single- and double-potential step transients were used to ascertain the diffusion coefficients of neutral species and the
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TABLE 4: Simulation Results for the Oxidation of Arenes in [C2mim][NTf2] DPA s-1
k0(A-B)/cm k0(B-D)/cm s-1 k0(C-E)/cm s-1 k2/dm3 mol-1 s-1 Keq Ef0(A-B)/V Ef0(B-D)/V Ef0(C-E)/V R (for all ET) DA/cm2 s-1 DB ) DD/cm2 s-1 DC ) DE/cm2 s-1
(3.1 ( 1.2) × N/A N/A N/A N/A 1.24 ( 0.02 N/A N/A 0.5 1.4 × 10-7 1.3 × 10-7 N/A
DCA 10-3
perylene
(7.6 ( 2.3) × N/A N/A N/A N/A 1.43 ( 0.01 N/A N/A 0.5 2.3 × 10-7 1.9 × 10-7 N/A
10-3
(1.3 ( 0.5) × N/A N/A N/A N/A 0.99 ( 0.01 N/A N/A 0.5 2.0 × 10-7 1.58 × 10-7 N/A
pyrene 10-2
radical cations as well as confirming the concentration of the solution. Cyclic voltammetry was carried out over 3 orders of magnitude in voltage scan rate, and the resulting waveshapes fitted using DigiSim according to the same RRC mechanistic scheme as set out in Figure 3a (chosen based on literature reports for 9-CA35), with results given in Table 3. As before, the coupling reaction was found to be irreversible and was modeled with Keq ) 1010, and the transfer coefficient (R) was taken to be 0.5 for all electron transfers. Results for the B-D and C-E processes are relatively insensitive due to the irreversibility of the coupling step but are included here for completeness. As before, the formal potentials stated for the B-D and C-E oxidations are derived purely for the purposes of modeling and do not reflect any experimental measurement. Identical experiments were then carried out on the 9,10disubstituted anthracenes, DPA and DCA. These are known to follow simple E reactions, with no homogeneous reactions.12 Potential step experiments were conducted to determine DA and DB, and waveshapes were modeled as before. The results are listed in Table 4. Similar experiments and analysis were then conducted on the arenes: pyrene, perylene, chrysene, 1,2-benzanthracene, and 2,3benzanthracene. However, modeling was carried out according to the mechanistic scheme as shown in Figure 9 due to the reported formation of Wegner salts.48 Uniquely, perylene was excellently modeled by a simple E reaction (see Table 3 for results). These anthracenes have previously been shown to follow an outer-sphere electron transfer in acetonitrile12 and obey the Marcus expression relating the standard electrochemical rate constant (k0) to the molecular solvodynamic radius (r) derived from the diffusion coefficient k0 ) Q
(ψr )
1/2
[(
exp - Br +
ψ r
)]
(1)
where Q ) Kpκel0 exp[-B(δ - σ)]/2τLxπ and ψ ) (NAe2/ 32π0RT)(1/op - 1/s), which has the general shape given in Figure 10. Figure 11 shows the experimentally observed variation of k0 with r, and the apparently poor agreement between eq 1 and the experimental data appears to support previous reports that this Marcus relation appears to break down in ILs.6 Conclusion The dimerization kinetics of 9-PA in a range of RTILs have been measured in order to determine their heterogeneous electron-transfer rates and found to be activation controlled and follow a RRC-type mechanism involving the direct coupling of the radical cations.29,30 There is no clear dependence of the second-order rate constant, k2, on viscosity within experimental error.
chrysene
(2.3 ( 0.6) × (1.3 ( 0.1) × 10-3 0.01 1.0 × 104 (2.2 ( 0.4) × 103 1.31 ( 0.01 1.40 ( 0.06 1.20 ( 0.01 0.5 3.2 × 10-7 2.4 × 10-7 1.0 × 10-7 10-2
1,2-benzanthracene 2,3-benzanthracene
(1.1 ( 0.3) × (3.1 ( 0.5) × 10-3 (5.3 ( 0.5) × 10-3 (7.3 ( 0.5) × 103 1.0 × 1010 1.64 ( 0.02 1.75 ( 0.04 1.57 ( 0.01 0.5 1.1 × 10-7 1.0 × 10-7 7.0 × 10-8 10-2
(9.7 ( 1.5) × 10-3 1.0 × 10-3 5.0 × 10-3 (4.7 ( 0.6) × 103 1.0 × 1010 0.28 ( 0.01 0.43 ( 0.01 0.28 ( 0.01 0.5 1.3 × 10-7 1.0 × 10-7 8.0 × 10-8
(1.1 ( 0.3) × 10-2 1.0 × 10-3 5.0 × 10-3 (8.0 ( 2.0) × 103 1.0 × 1010 0.61 ( 0.02 0.75 ( 0.03 0.58 ( 0.01 0.5 1.0 × 10-7 8.3 × 10-8 6.3 × 10-8
The heterogeneous rate constants for the oxidation of 9-PA have also been derived in the same range of RTILs and also found not to show any clear variation with either viscosity or static dielectric constant of the medium. A more definitive study currently in progress will examine these aspects of electron transfer in RTILs in greater detail. The heterogeneous rate constants of several arenes and substituted anthracenes have been found to be 2-3 orders of magnitude slower than in acetonitrile and do not follow the Marcus model for the dependence of k0 on solvodynamic radius that is expected for outer-sphere electron transfers.12 Acknowledgment. S.R.B. and N.V.R. thank St John’s College and EPSRC, respectively, for funding. References and Notes (1) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. Chem. Phys. Chem. 2004, 5, 1106. (2) Silvester, D. S.; Compton, R. G. Z. Phys. Chem. 2006, 220, 1247. (3) Endres, F. Chem. Phys. Chem. 2002, 3, 144. (4) Lagrost, C.; Carrie´, D.; Vaultier, M.; Hapiot, P. J. Phys. Chem. A 2003, 107, 745. (5) Lagrost, C.; Preda, L.; Volanschi, E.; Hapiot, P. J. Electroanal. Chem. 2005, 585, 1. (6) Fietkau, N.; Clegg, A. D.; Evans, R. G.; Villagran, C.; Hardacre, C.; Compton, R. G. Chem. Phys. Chem. 2006, 7, 1041. (7) Evans, R. G.; Compton, R. G. Chem. Phys. Chem. 2006, 7, 488. (8) Silvester, D. S.; Wain, A. J.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Electroanal. Chem. 2006, 596, 131. (9) Skrzypczak, A.; Neta, P. J. Phys. Chem. A 2003, 107, 7800. (10) Reed, S. K.; Lanning, O. J.; Madden, P. A. J. Chem. Phys. 2007, 126, 084704. (11) Lynden-Bell, R. M. Electrochem. Commun. 2007, 9, 1857. (12) (a) Clegg, A. D.; Rees, N. V.; Klymenko, O. V.; Coles, B. A.; Compton, R. G. J. Am. Chem. Soc. 2004, 126, 6185, and references therein. (b) Sumi, H.; Marcus, R. A. J. Chem. Phys. 1986, 84, 4894. (13) Asaki, M. L. T.; Redondo, A.; Zawodzinski, T. A.; Taylor, A. J. J. Chem. Phys. 2002, 116, 10377. (14) Paul, A.; Samanta, A. J. Phys. Chem. B 2007, 111, 4724. (15) Lagrost, C.; Hapiot, P.; Vaultier, M. Green Chem. 2005, 7, 468. (16) Grodkowski, J.; Neta, P. J. Phys. Chem. A 2002, 106, 9030. (17) Lee, Y.-M.; Kwon, O.-Y.; Yoo, I.-K.; Ryu, K.-G. J. Microbiol. Biotechnol. 2007, 17, 600. (18) D’Anna, F.; Frenna, V.; Noto, R.; Pace, V.; Spinelli, D. J. Org. Chem. 2006, 71, 9637. (19) Li, D.; Zhang, Y.; Wang, H.; Tang, J.; Wang, B. J. Appl. Polym. Sci. 2006, 102, 4254. (20) Rees, N. V.; Dryfe, R. A. W.; Cooper, J. A.; Coles, B. A.; Compton, R. G.; Davies, S. G.; McCarthy, T. D. J. Phys. Chem. 1995, 99, 7096. (21) Rees, N. V.; Alden, J. A.; Dryfe, R. A. W.; Coles, B. A.; Compton, R. G. J. Phys. Chem. 1995, 99, 14813. (22) Bonhote, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundram, K.; Gratzel, M. Inorg. Chem. 1996, 32, 1168. (23) MacFarlane, D. R.; Meakin, P.; Sun, J.; Amini, N.; Forsyth, M. J. Phys. Chem. B 1999, 103, 4164. (24) Broder, T. L.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Crossley, A.; Compton, R. G. New J. Chem. 2007, 31, 966. (25) Compton, R. G.; Banks, C. E. Understanding Voltammetry; World Scientific: London, 2007. (26) Shoup, D.; Szabo, A. J. Electroanal. Chem. 1982, 140, 237. (27) Evans, R. G.; Klymenko, O. V.; Saddoughi, S. A.; Hardacre, C.; Compton, R. G. J. Phys. Chem. B 2004, 108, 7878.
Arenes and Substituted Anthracenes in RTILs (28) Alden, J. A.; Hutchinson, F.; Compton, R. G. J. Phys. Chem. B 2007, 101, 949. (29) Silvester, D. S.; Wain, A. J.; Aldous, L.; Hardacre, C.; Compton, R. G. J. Phys. Chem. B 2007, 111, 5000. (30) Parker, V. D. Acta Chem. Scand. 1970, 24, 3171. (31) Peover, M. E. in Electroanalytical ChemistrysA Series of AdVances; Bard, A. J., Ed.; Marcel Dekker: New York, 1967; Vol. 2, p 1. (32) Parker, V. D. Acc. Chem. Res. 1984, 17, 243. (33) Hammerich, O.; Parker, V. D. AdV. Phys. Org. Chem. 1984, 20, 55. (34) Matsui, J.; Park, H.; Otsuka, K.; Oyama, M. J. Electroanal. Chem. 2003, 558, 49. (35) Hammerich, O.; Parker, V. D. Acta Chem. Scand. B 1982, 36, 519. (36) Amatore, C.; Save´ant, J. M. J. Electroanal. Chem. 1983, 144, 59. (37) Heinze, J. In Microelectrodes: Theory and Applications, NATO ASI Series; Montenegro, M. I., Queiros, M. A., Daschbach, J. L., Eds.; Kluwer Academic Publishers: Dordrecht, 1991; p 283. (38) Hu¨bler, P.; Heinze, J. Ber. Bunsen. Phys. Chem. 1998, 102, 1506.
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1657 (39) El-Desoky, H.; Heinze, J.; Ghoneim, M. M. Electrochem. Commun. 2001, 3, 697. (40) Cox, B. G. Modern Liquid Phase Kinetics; Oxford University Press: Oxford, 1994; p 59. (41) Strehlow, H. Rapid Reactions in Solution; VCH Verlagsgesellschaft: Weinheim, 1992; Chapter 2. (42) Weinga¨rtner, H.; Sasisanker, P.; Daguenet, C.; Dyson, P. J.; Krossing, I.; Slattery, J. M.; Schubert, T. J. Phys. Chem. B 2007, 111, 4775. (43) Wang, J.; Tian, Y.; Zhao, Y.; Zhuo, K. Green Chem. 2003, 5, 618. (44) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156. (45) Weinga¨rtner, H. Z. Phys. Chem. 2006, 220, 1395. (46) Wang, R. L.; Tam, K. Y.; Compton, R. G. J. Electroanal. Chem. 1997, 434, 105. (47) Evans, R. G.; Klymenko, O. V.; Price, P. D.; Davies, S. G.; Hardacre, C.; Compton, R. G. Chem. Phys. Chem. 2005, 6, 526. (48) Kro¨hnke, C.; Enkelmann, V.; Wegner, G. Angew Chem., Int. Ed. 1980, 19, 912.
