Langmuir 1996, 12, 591-593
591
Spectroelectrochemistry of Methyl Viologen at Mercury Film Electrodes R. O. Lezna* and S. A. Centeno INIFTA, Conicet, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Sucursal 4, Casilla de Correo 16, La Plata (1900), Argentina Received March 14, 1995. In Final Form: June 14, 1995
I. Introduction The interest in the class of compounds known as “viologens”, alkyl derivatives of 4,4′-bipyridyl, has not declined since first reported by Michaelis1 as redox indicators in biological investigations. Years later, they were the parent compounds for a successful type of herbicides, the “paraquats”. Herbicidal properties of viologens were ascribed to their strong reductant properties (particularly to the ability to reduce molecular oxygen).2 The redox potential is one of the most cathodic shown by organic systems that possess a fairly high degree of reversibility. A prominent change in color between the two oxidation states, along with a fast response, made the viologens suitable candidates for several attempts to build electrochemical display devices.3 It has been found that compounds that are not themselves electroactive, e.g., biological molecules, can be reduced in a cyclic process by electrochemically reducing the viologen, which, in turn, chemically reduces the electroinactive species. Because of this role, viologens are known as mediators or relays.4 Viologen-based polymers have also been widely reported in the electrochemical literature.5 This paper, the first part of a more extensive study with different anions, discusses some spectroelectrochemical properties of methyl viologen (MV2+) at a thin mercury film electrode/fluoride solution interface. At the same time the experimental approach, which may be of interest for other optical studies, is detailed. II. Experimental Section The working electrode was a thin Hg film (MFE), ca. 3 µm thick, deposited on a glass-encased Pt disk (area 0.28 cm2) according to the method reported by Bewick et al.6 This MFE was optically and electrochemically stable for several hours. Electromechanical oscillations, which may lead to artificial reflectance changes, were not observed. A platinum foil, separated from the main compartment by a sintered plug, was used as the counter electrode. Solutions were made with redistilled Millipore water and p.a. grade 0.1 M NaF. The dichloride salt of methyl viologen (1,1′-dimethyl-4,4′-bipyridynium; Sigma, 2.5 H2O/mol, kept under dry conditions) was added to the cell to obtain concentrations ranging from 5 × 10-5 to 2 × 10-3 M. Potentials were measured and are quoted against a saturated calomel electrode (SCE). Optical measurements were obtained with p-polarized light, at an incident angle of 59° using either a reflectance spectrometer specially designed for electro* To whom correspondence should be addressed. Fax: +54 21 53 01 89 or +54 21 25 50 04. E-mail: INIFTA5@ CESPIVM2.UNLP.EDU.AR. (1) Michaelis, L.; Hill, E. S. J. Gen. Physiol. 1933, 16, 859. (2) Pospisil, L.; Kuta, J.; Volke, J. J. Electroanal. Chem. 1975, 58, 217. (3) Van Dam, H. T.; Ponjee, J. J. J. Electrochem. Soc. 1974, 121, 1555. (4) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49 and references therein. Bewick, A.; Lowe, A. C.; Wederell, C. W. Electrochim. Acta 1983, 12, 1899. (5) Bookbinder, D. C.; Wrighton, M. J. Electrochem. Soc. 1983, 130, 1080. (6) Bewick, A.; Robinson, J. J. Electroanal. Chem. 1976, 71, 131.
0743-7463/96/2412-0591$12.00/0
Figure 1. (a) Differential capacitance of the MFE/NaF (0.01 M) interface (120 Hz, amplitude (rms) 2.8 mV, scan rate, ϑ ) 2.32 mV‚s-1). (b) Linear sweep voltammogram of the MFE in 0.1 M NaF (initial potential -0.4 V, ϑ ) 10 mV‚s-1).
Figure 2. (a) (s) Voltammogram of the MFE/0.1 M NaF/1.4 × 10-3 M MV2+ system (ϑ ) 100 mV‚s-1, initial potential -0.4 V, reversing potentials, -0.9 V (S1) and -1.3 V (S2)). (b) (- - -) Splitting of peak A2 after repeated pulsing of the MFE across A2/C2. chemical systems with the aid of potential modulation7 or a computerized optical multichannel analyzer fitted with a cooled Si diode array.8 This rapid-scan spectrometer, with a 14-bit resolution, was employed to obtain integral spectra after potential steps, monitoring the ensuing transient through successive spectra comprised of 50 exposures of 0.03 s each. Diffraction orders higher than 1 were sorted out by appropriate filters. Electrochemical experiments were carried out in a conventional way. All measurements were taken at room temperature, 25 °C, under rigorous deoxygenation.
III. Results and Discussion III.1. Characterization of the MFE. The differential capacitance of the MFE was determined as a function of potential in 0.01 M NaF, Figure 1a. The diagram shows that the MFE/solution interface behaves as an ideally polarizable electrode, its shape and minimum, -0.4 V, coinciding with those obtained with massive Hg electrodes. The linear sweep voltammetry of the working electrode in the presence of 0.1 M NaF is exhibited in Figure 1b. The charging region of the double layer was found to extend from -0.4 to -1.2 V without evidence of any faradaic processes. III.2. Electrochemical Measurements. Figure 2a corresponds to linear potential sweeps, run at 100 mV‚s-1, (7) Lezna, R. O.; Tacconi, N. R.; Arvia, A. J. An. Asoc. Quı´m. Argent. 1988, 76, 25. (8) Lezna, R. O.; Juanto, S.; Zagal, J. H. J. Electroanal. Chem. 1995, 389, 197.
© 1996 American Chemical Society
592
Langmuir, Vol. 12, No. 2, 1996
Notes
Figure 3. Alternating current voltammogram between -0.4 and -1.3 V of the MFE/0.1 M NaF/1.6 × 10-3 M MV2+ interface (area 0.28 cm2, frequency 11 Hz, amplitude (rms) 1.6 mV, ϑ ) 2.19 mV‚s-1 (single scan): (A) in-phase component, (B) quadrature component.
of 1.4 × 10-3 M MV2+. The scan marked S1 was reversed at -0.9 V.
MV2+ + e- S MV•+
(1)
Equation I represents the first step in the reduction of MV2+ identified as C1 in Figure 2a. Its counterpart, A1, can be seen on the back scan.9 Peak currents, IpC1, were found to obey a linear relationship, for several concentrations, with the square root of the sweep rate, ϑ1/2, the slope ratio of the straight lines, for different amounts of MV2+ in solution, being equal to the concentration ratio. Furthermore, the relation IpC1/IpA1 was determined to be close to unity while the peak potential separation, ∆Ep, was equal to 59 mV. Therefore, the first wave seems to conform to the guidelines expected for a reversible process, within the time scale of the experiment, controlled by the diffusion of MV2+ to the electrode. However, evidence for the presence of adsorption appeared in other studies, vide infra. When the negative limit was pushed to -1.3 V, scan S2, the cation radical, MV•+, undergoes a further reduction as follows,
MV•+ + e- w MV0
(II)
producing a solid deposited on the MFE, current peak C2, which is reoxidized to MV2+ in the process indicated as A2 on the positive-going scan. Peak A2 was observed to split into two current maxima after modulating the potential (or pulsing several times) across the wave C2/A2 as in the procedure required for optical measurements, Figure 2b. This result indicates the pulsing induces a reorganization in the solid, leading to distinct structures with defined oxidation energies. A2 was seen to decay in height with decreasing sweep rates whereas, at the same time, A1 presented a higher current than that detected on scan S1. This behavior arises as a consequence of the chemical reaction
MV2+ + MV0 S 2MV•+
(III)
Voltammograms with alternating current were run at 11 Hz, Figure 3. The in-phase component (A), rather structureless, just indicates the two waves described above. On the other hand, the quadrature signal (B) clearly deconvolutes reduction I into two processes, hinting at adsorption as playing a role in the response. (MV•+)ads might be the adsorbed species as no clear (optical) evidence (9) Steckhan, E.; Kuwana, T. Ber. Bunsen-Ges. Phys. Chem. 1974, 78, 253.
Figure 4. (a) Integral change of reflectance vs wavelength taken with a diode array setup, (MFE/0.1 M NaF/1.3 × 10-3 M MV2+, ∆R/R ) (R-0.85V - R-0.4V)/R-0.4V, first reduction wave (MV•+), spectra were averaged 50× in memory, exposure time 0.03 s, φ ) 59°, p-polarization). (b) ∆R/R ) R-1.2V - R-0.85V)/ R-0.4V, second reduction wave (MV0), MFE/0.1 M NaF/1.3 × 10-3 M MV2+, other conditions as in (a)).
Figure 5. Contribution of (MV•+)2 to the whole absorption (∆R/R ) (R-0.75V,back scan - R-1.0V)/R-0.4V).
of dimers was found at this stage.10 As the potential is made more negative, the formation of MV0 is discerned, attended by several reorganizations within the solid that arise, on the out-of-phase current, as sharp maxima. The latter result is consistent with the shape of the voltammogram, for the same process, after successive potential steps, vide supra. A similar profile in the equivalent optical signal was obtained by differential reflectance measurements at 600 nm (not shown). III.3. Optical Measurements. The system was further investigated by analyzing the reflected light intensity as a function of both wavelength and time for potential steps whose height (negative) was chosen to cover the different reactions detected in the voltammetries. Figure 4a is the optical (integral) signal obtained after the potential has been pulsed from -0.4 V, where only uncolored MV2+ is in the solution, to -0.85 V, where MV•+ is being generated at the interface under conditions of diffusion control. The ensuing spectrum coincides with that of MV•+ monomer in aqueous solution.11 As the potential is made more negative, to -1.2 V, to span the reaction II region, a band at 425 nm is detected superimposed on the MV•+ spectrum (MV•+ is still seen at negative potentials because of chemical process III. (10) Kobayashi, K.; Fujisaki, F.; Yoshimine, T.; Niki, K. Bull. Chem. Soc. Jpn. 1986, 59, 3715. (11) Watanabe, T.; Honda, K. J. Phys. Chem. 1982, 86, 2617.
Notes
Absorption of MV0 is reported to be at 395 nm in ethanol solutions.11 Therefore, a red shift to 425 nm is brought about by the interaction between the solid and the electrode. A deconvoluted band of MV0 is shown in Figure 4b as calculated from the difference between the normalized spectra taken at -1.2 and -0.85 V. A maximum can be observed at 425 nm, its intensity gradually falling off toward the red with practically no contribution at 600 nm. Spectra of the system were also collected each 100 mV while the potential was linearly scanned at 5 mV‚s-1 from -0.45 to -1.2 V and back. The intensity of the MV•+ band at 600 nm was monitored as a function of potential, the resulting curve going over an absolute maximum at ca. -0.75 V on the back scan. Moreover, when MV•+ reaches its highest concentration at the interface, a small band energies at 870 nm, attended by a blue shift in the absorption, the picture being suggestive of (MV•+)2 in the
Langmuir, Vol. 12, No. 2, 1996 593
optical path. To cut down on the dominant MV•+ contribution to the total intensity at -0.75 V, the spectrum measured on the negative-going scan, at -1.0 V, was subtracted from the signal at -0.75 V on the back sweep (at -1.0 V no contribution is detected from the 870 nm, dimer band). Figure 5 shows the outcome, in good agreement with data reported for the absorbance of (MV•+)2 species.12 Acknowledgment. We thank Professor Arvia for helpful comments and Miss E. Ferna´ndez for technical assistance. This work was supported by grants from Conicet, RA and CIC, Province of Buenos Aires. LA9502002 (12) Kosower, E. M.; Cotter, J. L. J. Am. Chem. Soc. 1964, 86, 5524.
