Article pubs.acs.org/ac
Determination of Heterogeneous Electron Transfer and Homogeneous Comproportionation Rate Constants of Tetracyanoquinodimethane Using Scanning Electrochemical Microscopy Chandima B. Ekanayake, Manjula B. Wijesinghe, and Cynthia G. Zoski* Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003, United States S Supporting Information *
ABSTRACT: We report the use of scanning electrochemical microscopy (SECM) in determining the heterogeneous electron transfer and homogeneous comproportionation kinetics of tetracyanoquinodimethane (TCNQ) in acetonitrile at Pt tip UMEs (radius 12.5−1 μm). TCNQ undergoes two consecutive one-electron reductions with comproportionation occurring between TCNQ2−, the product of the second reduction, and bulk TCNQ to produce TCNQ−. A standard rate constant, k01 = 2.9 ± 0.5 cm/s, for the first reduction was determined by tip voltammetry with total positive feedback and a large Pt substrate. A comparatively smaller rate constant, k02 = 0.44 ± 0.05 cm/s, for the second reduction was determined in the absence of comproportionation by tip voltammetry with the tip shielded from the bulk TCNQ solution by the TCNQ− diffusion layer of the large Pt substrate. A comproportionation rate constant, kcomp = 1 × 106 M−1 s−1, was determined by tip pulse chronoamperometry at coaxially aligned tip and substrate UMEs of the same radius. Diffusion coefficients of the TCNQ species and standard potentials for the reductions were also determined. Experimental results were compared with 2D transient axisymmetric simulations and reported analytical equations.
T
where kcomp and kdisp represent the forward and backward homogeneous rate constants for homogeneous reaction 4. The Gibbs free energy associated with comproportionation reaction 4 is negative and hence the reaction is spontaneous, and kcomp is very large.10,11 Electrochemical investigations have focused on measurements of the diffusion coefficients of the TCNQ species, heterogeneous kinetic parameters associated with consecutive reductions 1 and 2, and the comproportionation rate constant of reaction 4. Results from some of these studies are summarized in Table 1, in addition to our results shown in the fifth column. Heterogeneous kinetic measurements have been made with either ultramicroelectrode and nanometer size ultramicroelectrodes (UMEs) in steady-state voltammetry (SSV) or scanning electrochemical microscopy (SECM) investigations or with macro-sized electrodes in cyclic voltammetry (CV), normal pulse voltammetry (NPV), or Fourier transform large amplitude ac voltammetry (FT ACV) investigations. As shown in Table 1, reported heterogeneous rate constants for the first reduction range from 0.005 to 7 cm/s, while those for the second reduction range from 0.006 to 0.3 cm/s, indicating that the second reduction either occurs at the same rate as the first reduction or at
etracyanoquinodimethane (TCNQ) and its derivatives are a well-known class of conducting organic compounds.1,2 The conductivity of TCNQ is due to its high electron acceptor ability which is attributed to the presence of four cyano groups and a large conjugation system. These attributes permit the formation of stable, solid complexes with a wide variety of electron-donating compounds3−5 and have been important in the recent use of TCNQ and its derivatives in the development of biosensors,6 organic light emitting diodes,7,8 and organic field effect transistors.9 With these recent applications, there is continued interest in the fundamental electrochemical behavior of TCNQ which involves two consecutive reductions coupled with a comproportionation and disproportionation homogeneous chemical reaction:10−24 TCNQ + e− → TCNQ−
first reduction
(1)
TCNQ− + e− → TCNQ2 −
second reduction
(2)
TCNQ + 2e− → TCNQ2 −
overall reduction
(3)
kcomp
TCNQ2 − + TCNQ HooooI 2TCNQ−
Received: December 24, 2012 Accepted: March 1, 2013 Published: March 26, 2013
kdisp
comproportionation/disproportionation © 2013 American Chemical Society
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Table 1. Comparison of Reported Diffusion and Mass Transfer Coefficients and Kinetic Data According to Electrochemical Technique, Technique Parameter, and Electrode Size with the Results of This Work (Column 5)a studies technique DTCNQ/10−5 cm2 s−1 DTCNQ−/10−5 cm2 s−1 DTCNQ2−/10−5 cm2 s−1 technique parameter a, μm m, cm s−1 k01, cm s−1 k02, cm s−1 kcomp, M−1 s−1
Zoski et al.18
Sun and Mirkin22
SECM 1.44
SECM 1.98
1.36
Nioradze et al.19
this work
Rongfeng and Evans11
Lehmann and Evans13
Bano et al.21
SECM 2.0
SECM 1.45
CV 1.44
NPV 1.44
ACV 1.66
1.60
1.35
1.35
1.35
1.47
0.92
0.91
0.91
1.05
tp/s = 0.050
f/s−1 = 9.0
155 0.0096 reversible 6.5 × 10−3 1 × 108
500, 750 0.012 0.3 0.3
−1
d/μm = 2.6
d/μm = 0.03−0.15
d/μm = 0.10−0.16
d/μm = 0.18−0.26
v/V s
12.5 0.01
0.076 −0.386 1.32−6.68 1.1
0.532 1.26−2.10 7
1.0−12.5 0.13−0.54 2.9 0.44 1 × 106
1000 0.0075 5 × 10−3 6 × 10−3 1 × 106
= 0.1
a d refers to tip-substrate separation in SECM, a to electrode radius, m to mass-transfer coefficient (mSECM α D/d, mCV α (D/(RT/Fv))1/2, mNP α (D/ (πtp))1/2, mAC α (Df)1/2), k0 to heterogeneous rate constant, f to frequency, v to scan rate, and tp to pulse duration. ACV, alternating current voltammetry; CV, cyclic voltammetry; NPV, normal pulse voltammetry.
reduction, k01, was determined using the SECM steady-state approach involving total positive feedback at a substrate that is large in comparison to the UME tip and held at a potential well positive of the standard potentials for TCNQ reduction. This approach is in contrast to the quasi-steady-state partial feedback/ partial shielding approach that was recently reported where the substrate was held at the standard potential of the first reduction.19 The heterogeneous rate constant for the second reduction, k02, was determined in the absence of comproportionation using a quasi-steady-state, total shielding approach where the potential of the large substrate is held at a value in the potential range of the limiting current plateau of the first reduction wave.18 This creates a quasi-steady-state diffusion layer of TCNQ− that totally shields the tip from the bulk TCNQ solution and which serves as the bulk solution. Tip generated TCNQ2− is then oxidized at the substrate in a total positivefeedback mode. This approach has not been used in previous SECM heterogeneous kinetic investigations and is an alternative approach to other electrochemical methods where solutions of anions of the neutral species are prepared using bulk electrolysis.11−13,21 The diffusion coefficient of TCNQ2−, standard potential E02 of the second reduction, and comproportionation rate constant kcomp, were determined using a SECM configuration with coaxially aligned UMEs of the same metallic radius for both tip and substrate. SECM experiments involving coaxially aligned UMEs of similar dimension have been used to evaluate the chemical reactions from two electrogenerated species in picoliter volumes29 and oxidative etching kinetics of metals.30 Using this configuration under steady-state conditions, we show that the tip and substrate currents are not affected by comproportionation, thus allowing DTCNQ2− and E02 to be determined. In contrast, we also demonstrate that short-time transient experiments in a SECM coaxial configuration and involving pulse chronoamperometry allows one to probe the comproportionation reaction and determine kcomp. This approach is in contrast to short-time transient experiments in SECM using fast scan cyclic voltammetry which have been reported for other applications.31−34
a rate that is much slower. Because small currents are measured in SSV and SECM with UME tips, kinetic measurements can be made without interference from electrode capacitive effects and uncompensated resistance, which can be especially important in nonaqueous solvents and at macro-sized electrodes.25 Additionally, in SECM, a UME is approached to a substrate electrode held at a potential where the product of the half-reaction at the UME tip regenerates the starting material. The rate of heterogeneous electron transfer can be measured by observing the feedback current, detected as a function of the distance, d, between the tip and substrate, and noting deviations from the behavior for a diffusion-controlled reaction. An advantage in measuring large heterogeneous rate constants with SECM is that the mass transfer rate, which is proportional to D/d, where D is the diffusion coefficient of the molecule under study, can be varied by changing the tip−substrate distance so that it is small compared to the tip radius, a, thus leading to a mass transfer rate that is large compared to k0.26,27 In contrast, the mass transfer rates for CV, NPV, and FT ACV are proportional to (D/(RT/Fv))1/2, (D/ πtp)1/2, (Df)1/2, respectively, where v, tp, and f refer to sweep rate, pulse duration, and frequency. As shown in Table 1, larger mass transfer rates are possible with SECM. Comproportionation rate constants ranging from 106 to 108 M−1 s−1 have resulted from studies involving SSV over a wide range of electrolyte concentrations at a UME10 or with CV and NPV at macroelectrodes. There is closer agreement among investigations involving diffusion coefficient determinations.12,13,18,19,21 We report here the heterogeneous kinetics of the second reduction and the homogeneous kinetics associated with the comproportionation reaction using SECM. We also determine the heterogeneous kinetics for the first reduction in order to compare the value with previous SECM measurements at nanometer-size UMEs. SECM has been used extensively in heterogeneous kinetic determinations as well as in studies involving homogeneous chemical reactions.27,28 We also determined diffusion coefficients DTCNQ, DTCNQ−, and DTCNQ2−, in addition to standard potentials, E01 and E02, and transfer coefficients, α1 and α2. The heterogeneous rate constants, k01 and k02, for the first and second reductions, and the comproportionation rate constant, kcomp, were determined using SECM in either a tip voltammetry or a pulsed chronoamperometric mode, respectively. The heterogeneous rate constant for the first
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EXPERIMENTAL SECTION Chemicals. 7,7,8,8-Tetracyanoquinodimethane (TCNQ; 98%, Aldrich) and tetra-n-butylammoniumperchlorate (TBAP; 4023
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RESULTS AND DISCUSSION Linear Sweep Voltammetry (LSV) at Inlaid Disk UMEs. Steady-state LSVs normalized by the limiting current of the first reduction, iT∞1, and recorded at UMEs of 1, 2.5, and 12.5 μm radius are shown in Figure 1A. The steady-state LSVs have the
99% Fluka) were used as received. Ferrocene (98% Fluka) was repurified by recrystallization from hexane. Acetonitrile (MeCN; Fisher, HPLC grade) was stored over neutral activated alumina (MP Biomedicals Super 1) to remove trace water. Ultra-high purity argon (Airgas) was used in all experiments. Electrodes and Electrochemical Cell. Platinum (Pt) (99.99%: 25, 10, and 5 μm diameter, hard; 2 μm diameter, Wollaston; Goodfellow) wire was used in fabricating UMEs, and SECM tips and substrates as described elsewhere.35,36 SECM tips were shaped with RG = 3 and 5; the corresponding UME substrate of identical metallic radius was fabricated with RG ≥ 10. UMEs used in recording SSVs were fabricated with RG ≥ 10. A 1 mm radius Pt disk electrode insulated with KEL-F (CH Instruments, Texas) was also used as a substrate. All electrodes were polished with 0.05 μm alumina on microcloth pads (Buehler, Lake Bluff, IL) prior to use. Pt wires (0.5 mm diameter, 99.99%, as drawn, Goodfellow) were used as both counter and quasi-reference electrodes. The Pt quasi-reference electrode was frequently calibrated with respect to the ferrocenium/ferrocene couple in MeCN. All potentials are reported vs Ag/AgCl (sat). All solutions were presaturated with argon prior to each experiment, transferred under argon pressure to a Teflon cell that was capped with a Teflon top and wrapped with parafilm, and kept under a MeCN presaturated argon blanket.37 Instrumentation. A CHI 900 SECM (CH Instruments, Texas) coupled with a home-built Faraday cage on a vibration table (VH3030W-OPT, Newport) was used in all SECM experiments. SSV experiments at UMEs were performed on a CHI 760 D potentiostat (CH Instruments, Texas). SECM Experiments. The tip was positioned using previously reported procedures.29,38 In short, the tip was positioned in the vicinity of the substrate using positive feedback approach curves where the tip and the substrate potentials were set at 0.000 and 0.500 V vs Ag/AgCl, respectively (i.e., corresponding to the first reduction). The distance, d, was determined by comparing experimental positive feedback approach curves with those simulated with Comsol Multiphysics (3.5a) or by comparing experimental limiting currents of tip LSVs recorded under total positive feedback (tpf) (i.e., ES = 0.500 V and ET scanned from 0.500 to 0.000 V at 50 mV/s) with simulated LSV limiting currents.18 Prolonged exposure of TCNQ to light results in a gradual color change of the solution from yellow to green, which indicates photoreduction of TCNQ to TCNQ−.39,40 Therefore, experiments were conducted with minimal exposure to light and within reasonably short times. Trace amounts of H2O led to irreproducibility of the second reduction; storage of MeCN over neutral activated alumina eliminated this problem.
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Figure 1. Steady-state voltammetry for TCNQ reduction. (A) Steadystate LSVs for the reduction of TCNQ to TCNQ2−. Expansion of the steady-state LSVs for the (B) first reduction and (C) second reduction. Solution: 1.0 mM TCNQ/0.1 M TBAP in MeCN under argon. UME: 1, 2.5, and 12.5 μm radius Pt disk. Initial potential: 0.5 V vs Ag/AgCl. Sweep rate: 50 mV s−1. Quiet time: 10 s.
characteristic sigmoidal shape showing two, well-separated reductions corresponding to the reduction of TCNQ to TCNQ2− through TCNQ−. The limiting currents, iT∞1 and iT∞2, halfwave potentials, E1/2,1 and E1/2,2, and wave shape parameter, E1/4 − E3/4, for each reduction are tabulated in Table 2 for each UME. The average limiting current ratio of the two reductions leads to an apparent number of electrons, napp (i.e., iT∞2/iT∞1), of 1.96 ± 0.01, in good agreement with a value of 1.98 ± 0.02 predicted from chronoamperomeric simulations.11 In the first reduction, E1/2,1 shifts negative from 260 to 255 mV and the E1/4 − E3/4 potential difference increases from 56 to 60 mV as the UME radius decreases from 12.5 to 1 μm. These E1/4 − E3/4 values can be compared to a value of 56.4/n for a completely reversible wave.25 The increasingly negative shift in E1/2 and increase in E1/4 − E3/4 with decreasing UME radius is more apparent in Figure 1B and is indicative of heterogeneous kinetic behavior. In contrast, Figure 1C shows that E1/2,2 for the second reduction is fixed around −302 ± 2 mV, with E1/4 − E3/4 increasing from 57 to 64 mV as the UME radius decreases from 12.5 to 1 μm. This behavior can be attributed to comproportionation reaction 4 between TCNQ and TCNQ2−; it can be detected experimentally when the second electron transfer is slower than the first and the diffusion coefficients of the TCNQ species are unequal.13 The use of a range of UMEs allows comproportionation to be more easily detected and its rate constant, kcomp, to be measured, in theory, by placing the
DIGITAL SIMULATIONS
SECM experiments were simulated with COMSOL Multiphysics (3.5a, COMSOL, Inc., Burlington, MA) with the Chemical Engineering Module in 2D axial symmetry under transient conditions. Two configurations were used as shown in Figure S1 in the Supporting Information: an SECM tip of radius a and RG = 3 or 5 positioned above either (a) an infinitely larger substrate or (b) a UME substrate of the same radius a and RG ≈ ∞ . Diffusion equations, kinetic parameters, and boundary and potential program conditions are given in the Supporting Information (S1). 4024
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Table 2. Tabulation of Experimental and Calculated Limiting Currents, iT∞1 and iT∞2 and Experimental E1/2 Potentials and E1/4 − E3/4 Potential Differences for the Reduction of TCNQ for UME radii of 1.0, 2.5, and 12a tip radius 1 μm 2.5 μm 12.5 μm
iT∞1 iT∞2 iT∞1 iT∞2 iT∞1 iT∞2
(iT,∞)exp, nA
(iT,∞)theo, nA
napp
E1/2, mV
E0′, mV
E1/4 − E3/4, mV
0.504 ± 0.003 0.983 ± 0.005 1.528 ± 0.002 2.985 ± 0.002 7.245 ± 0.001 14.181 ± 0.001
0.504 0.974 1.528 2.951 7.247 13.994
1.95 ± 0.03
254 ± 2 −303 ± 2 258 ± 1 −302 ± 1 260 ± 1 −303 ± 1
256 ± 2 −293 ± 2 260 ± 1 −292 ± 1 262 ± 1 −293 ± 1
60 ± 2 64 ± 2 58 ± 1 60 ± 1 56 ± 1 58 ± 1
1.95 ± 0.01 1.96 ± 0.01
a The ± corresponds to the standard deviation of three measurements. Limiting current values were calculated using iT∞1 = 4nFDTCNQCa and iT∞2 = iT∞1 + 4nFDTCNQ−Ca (DTCNQ = 1.45 × 10−5 cm2 s−1, DTCNQ− = 1.35 × 10−5 cm2 s−1). All potentials are reported vs Ag/AgCl (satd). E0 was calculated from E0 = E1/2 + (RT/nF) ln(DO/DR).
−1.62091.41 Only results satisfying the λ′ 10) and simulated i/iT∞1 vs. t responses over a range of kcomp. When ET is pulsed to −0.5 V, iT/iT∞1 initially decreases rapidly. In the absence of comproportionation, tip generated TCNQ2− diffuses to the substrate where it is oxidized to TCNQ and iT/iT∞1 increases toward a steady value with time due to positive feedback. In the presence of comproportionation (i.e., kcomp > 104 M−1 s−1), the increase toward a steady value occurs more quickly due to the production of TCNQ− in the center of the gap (i.e., Figure 4C) which then diffuses in part to the tip, where it reduces to TCNQ2− and in part to the substrate, where it oxidizes to TCNQ. With increasing time, tip and substrate currents reach approximately the same steady current value. The tip approaches this steady current value more quickly than the substrate and is more sensitive to kcomp largely due to the positive feedback that occurs at the tip. The best fit between experimental and simulated tip current responses was achieved with kcomp = 1 × 106 M−1 s−1. Figure S12 in the Supporting Information shows a similar result at d = 1.35 μm (d/a = 0.51) for the same concentration as does Figure S13 in the Supporting Information for a solution of 0.5 mM TCNQ/0.1 M TBAP at d = 0.54 μm (d/a = 0.20). This value is well within the working range of kcomp values from 1 × 105 M−1 s−1 to 1 × 108 M−1 s−1 that can be measured in our SECM experiments. Our SECM value of 1 × 106 M−1 s−1 is also in good agreement with earlier investigations which predicted a comproportionation rate constant ≥1 × 106 M−1 s−1 at a 12.5 μm radius UME using low supporting electrolyte voltammetry10 and values of 1 × 106 M−1 s−1 and 1 × 108 M−1 s−1 in two different studies involving curve fitting of cyclic and normal pulse voltammograms recorded at macro-electrodes.11,13
Figure 3. Determination of diffusion coefficients for TCNQ− (A) and TCNQ2− (B) and E0 (C) using SECM tip voltammetry. ET was scanned at 50 mV s−1 from 0.0 V to −0.50 V after a 5 s quiet time, with ES = (A) 0.000 V, (B) −0.288 V, and (C) −0.500 V. Experimental tip LSVs and corresponding substrate LSVs were compared with simulations generated with a range of diffusion coefficients (DTCNQ− and DTCNQ2−) from 0.68 × 10−5 cm2/s to 2.74 × 10−5 cm2/s (A) and (B), and with E0 ranging from −0.268 V to −0.298 V (C). Tip, 12.5 μm radius Pt disk UME (RG = 3). Substrate, 12.5 μm radius Pt disk UME (RG = ∞). d = 6.35 μm (L = 0.51). Solution, 1 mM TCNQ/0.10 M TBAP in MeCN presaturated with Ar. Other simulation parameters: E0TCNQ/TCNQ− = 0.260 V, DTCNQ = 1.45 × 10−5 cm2 s−1, DTCNQ− = 1.35 × 10−5 cm2 s−1, k01 = 2.9 cm s−1, k02 = 0.44 cm s−1. Simulated currents were normalized by iT∞1 = 6.99 × 10−9 A.
the gap, tSECM = d2/2D, is in competition with the time scale of the comproportionation reaction, tCOMP = 1/kcompC*.23,26,28 Figure 4C shows concentration profiles for TCNQ, TCNQ−, and TCNQ2− at the middle of a 0.1 s pulse period (i.e., at 0.05 s), in the absence (kcomp = 0 M−1 s−1) and presence (kcomp = 1 × 106 M−1 s−1) of comproportionation, where formation of a TCNQ− layer between the tip and substrate only occurs when comproportionation is considered. Both TCNQ and TCNQ2− concentrations decrease in the gap region around the UMEs concomitant with the increase in TCNQ− concentration when comproportionation occurs. Simulated transient tip and substrate current responses (Figure 4D) were sampled at 0.02 and 0.05 s during the 0.1 s pulse period and over a range of tip−substrate distances in order to select conditions that would maximize measurement of kcomp. Figure 5 shows simulated results for (A) tip and (B) substrate which are graphed in terms of normalized current difference, Δi/ iT∞1, versus normalized tip−substrate distance, d/a, to generate an approach curve. The normalized currents reflect the difference between current simulated in the presence and absence of comproportionation and sampled at 0.02 s, as shown schematically in Figure S10 in the Supporting Information. Figure S11 in the Supporting Information shows the approach curve at a 0.05 s sampling time, midway through the pulse period. This approach
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CONCLUSIONS SECM was used to determine diffusion and kinetic parameters for the consecutive 2e TCNQ reduction in which comproportionation occurs. Two SECM configurations were used. In the first, a UME tip was positioned over a much larger substrate and steady-state or quasi-steady state tip voltammetry was used in determining diffusion coefficients, standard potential, and heterogeneous rate constant for the first reduction and the heterogeneous rate constant for the second reduction. For the first reduction, total positive feedback was used to determine k01 = 2.9 ± 0.5 cm s−1. In the second reduction, k02 = 0.44 ± 0.05 cm s−1 was determined by totally shielding tip-generated TCNQ2− from the bulk TCNQ solution to prevent comproportionation from occurring. Partial feedback-partial shielding and total shielding were used in determining DTCNQ and DTCNQ−, and E10, respectively, for the first reduction. 4027
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Figure 4. Comproportionation investigation under SECM transient conditions. (A) Schematic of electrodes and reactions. (B) Tip potential program. ET is pulsed from 0.500 V to −0.500 V after a 2 s quiet time and 0.1 pulse width. ES = 0.500 V. (C) Concentration profiles of TCNQ, TCNQ−, and TCNQ2− in the tip−substrate gap sampled in the middle of the pulse (at 0.05 s of the potential program); concentration scale, millimolar. (D) Tip current (solid red line) and corresponding substrate current (dotted line). Tip (RG = 3) and substrate (RG = ∞), 2.5 μm radius disk. d = 5 μm. Concentration: 1 mM TCNQ. Additional simulation parameters: E0TCNQ/TCNQ− = 0.260 V, E0TCNQ−/TCNQ2− = −0.288 V, DTCNQ = 1.45 × 10−5 cm2 s−1, DTCNQ− = 1.35 × 10−5 cm2 s−1, and DTCNQ2− = 0.92 × 10−5 cm2 s−1, k01 = 2.9 cm s−1, k02 = 0.44 cm s−1. All potentials are vs Ag/AgCl.
Figure 6. Comproportionation rate constant determination. Experimental chronoamperometric tip (A) and substrate (B) responses (red dots) were compared with simulated data (solid lines) for comproportionation rates ranging from 0 M−1 s−1 to 1 × 108 M−1 s−1. Tip: 2.65 μm radius Pt disk UME (RG = 3). Substrate: 2.65 μm radius Pt disk UME (RG = ∞). d = 0.56 μm (d/a = 0.21). ET pulse: 0.5 V to −0.5 V. ET pulse width: 0.1 s. ES = 0.5 V. Quiet time: 2 s, where ET = ES = 0.5 V. Solution: 1 mM TCNQ/0.10 M TBAP in MeCN presaturated with Ar. iT∞1 = 1.6 × 10−9 A. Additional simulation parameters: E0TCNQ/TCNQ− = 0.260 V, E0TCNQ−/TCNQ2− = −0.288 V, DTCNQ = 1.45 × 10−5 cm2 s−1, DTCNQ− = 1.35 × 10−5 cm2 s−1, and DTCNQ2− = 0.92 × 10−5 cm2 s−1. All potentials are vs Ag/AgCl.
Figure 5. Normalized current difference for (A) tip (2.5 μm radius, RG = 3) and (B) substrate (2.5 μm radius, RG = ∞), 0.02 s after initiating a pulse as a function of normalized distance d/a over a range of comproportionation rate constants. Simulations and experiments used ET pulsed from 0.500 V to −0.500 V after a 2 s quiet time, with ES = 0.5 V. Concentration: 1 mM TCNQ.
The diffusion coefficient for TCNQ2−, standard potential, and comproportionation rate constant for the second reduction were determined using a coaxially aligned UME tip and substrate of the same metallic radius. In this tip-on-tip configuration, comproportionation was confined to the tip−substrate gap 4028
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Analytical Chemistry
Article
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rather than occurring in the bulk solution. In the steady-state, DTCNQ2− and E02 could be determined without interference from comproportionation. In order to determine a comproportionation rate constant, a potential pulse was used because the tip current was found to approach a steady value within 0.1 s. Comparison of experimental and simulated tip and substrate chronoamperograms led to kcomp = 1 × 106 M−1 s−1 . The SECM techniques reported here can be further extended to more complex compounds like hydroxyquinones, which also undergo a two step reduction with comproportionation during the second reduction.42,43 However, in addition to the comproportionation reaction, hydroxyquinone radicals also undergo an association reaction with neutral hydroxyquinone.
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ASSOCIATED CONTENT
S Supporting Information *
(section S1) Comsol multiphysics simulations; (section S2) diffusion coefficient and E0 determinations for TCNQ and its reduced species under steady and quasi-steady state SECM conditions for the first reduction; (section S3) results of multiphysics simulations for SECM determination of diffusion coefficients and formal potentials at a large substrate for the second TCNQ reduction; (section S4) results of multiphysics simulations for SECM determination of diffusion coefficients and formal potentials at a UME substrate for the second TCNQ reduction; (section S5) figures related to determination of kcomp. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: 575-646-2649. Phone: 575-646-5292. E-mail: czoski@ nmsu.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support of the National Science Foundation (Grant CHE-0809966) is gratefully acknowledged. REFERENCES
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