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Spectroelectrochemistry, Applications
Spectroelectrochemistry, Applications RJ Mortimer, Loughborough University, UK ã 2017 Elsevier Ltd. All rights reserved.
Symbols A b e E E0 Ep Epa Epc F
absorbance path length electron electrode potential reversible electrode potential peak potential anodic peak potential cathodic peak potential Faraday constant (96 485 C mol1)
Introduction Spectroelectrochemistry encompasses a group of techniques that allow simultaneous acquisition of spectroscopic and electrochemical information in situ in an electrochemical cell. Electrochemical reactions can be initiated by applying potentials to the working electrode, and the processes that occur are then monitored by both electrochemical and spectroscopic techniques. Electronic (UV-visible) transmission and reflectance spectroelectrochemistry has proved to be an effective approach for studying the redox chemistry of organic, inorganic and biological molecules, for investigating reaction kinetics and mechanisms, and for exploring electrode surface phenomena. In this article a selection of representative examples are presented, the emphasis being on the applications of transmission electronic (UV-visible) spectroelectrochemistry
k Keq n O R R t T « lmax
rate constant equilibrium constant number of electrons oxidized form reduced form gas constant (8.315 J K1 mol1) time temperature in kelvin molar extinction coefficient wavelength of maximum absorbance
strated in 1964 using o-tolidine, a colourless compound that reversibly undergoes a 2-electron oxidation in acidic solution to form an intensely yellow coloured species (eqn [1]). This system soon became a standard for testing spectroelectrochemical cells and new techniques. Figure 1 shows absorbance spectra, for a series of applied potentials, recorded in an electrochemical cell employing an optically transparent thin-layer electrode (OTTLE). Curve a was recorded after application of þ0.800 V vs saturated calomel electrode (SCE), which under thin-layer electrode conditions causes complete electrolytic oxidation of o-tolidine to the yellow form ([O]/[R]>1000, where O represents the oxidized form and R the reduced form). Curve g was recorded after application of þ0.400 V, causing complete electrolytic reduction ([O]/[R]<0.001), with the intermediate spectra corresponding to intermediate values of Eapplied. The absorbance at
½1
to the study of redox reactions and homogeneous chemical reactions initiated electrochemically within the boundaries of the diffusion layer at the electrode–electrolyte interface.
Organic Systems Many organic systems exhibit redox states with distinct electronic (UV-visible) absorption spectra and are therefore amenable to study with spectroelectrochemical techniques.
o-Tolidine The technique of transmission spectroelectrochemistry, using an optically transparent electrode (OTE), was first demonThis article is reproduced from the previous edition, Copyright 1999, Elsevier Ltd.
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438 nm reflects the amount of o-tolidine in the oxidized form, which can be calculated from the Beer–Lambert law. Determination of E0, the reversible electrode potential, and n, the number of electrons in the o-tolidine redox reaction, can be determined from the sequence of spectropotentiostatic measurements (Figure 1). For a reversible system, O þ ne Ð R
[2]
the [O]/[R] ratio at the electrode surface is controlled by the applied potential according to the Nernst equation: RT ½O ln [3] Eapplied ¼ E0 þ nF ½R surface In an OTTLE cell, on application of a new potential, the concentrations of O and R in solution are quickly adjusted to the same values as those existing at the electrode surface. Thus, at equilibrium:
Encyclopedia of Spectroscopy and Spectrometry, Third Edition
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Spectroelectrochemistry, Applications
Figure 1 Thin-layer spectra of 0.97 mM o-tolidine, 0.5 M ethanoic acid, 1.0 M HCIO4 for different values of Eapplied. Cell thickness 0.017 cm. Potential vs SCE: (a) 0.800 V, (b) 0.660 V, (c) 0.640 V, (d) 0.620 V, (e) 0.580 V, (f) 0.600 V, (g) 0.400 V. Reprinted with permission from DeAngelis TP and Heineman WR (1976) Journal of Chemical Education 53: 594–597. ã 1976 Division of Chemical Education, American Chemical Society.
½O ½R
¼ surface
½O ½R solution
[4]
The Nernst equation in a thin-layer cell can then be written as: RT ½O ln [5] Eapplied ¼ E0 þ nF ½R For the o-tolidine spectra, 438 nm is used as the monitoring wavelength and the ratio [O]/[R] is determined from the Beer–Lambert law: ½O ðA2 A1 Þ=Deb A2 A1 ¼ ¼ ½R ðA3 A2 Þ=Deb A3 A2
RT ðA2 A1 Þ ln nF ðA3 A2 Þ
Figure 2 Plot of Eapplied vs log([O]/[R]) from the spectra in Figure 1. Reprinted with permission from the DeAngelis TP and Heineman WR (1976) Journal of Chemical Education 53: 594–597. ã 1976 Division of Chemical Education, American Chemical Society.
semiquantitative information using a rapid scan spectrometer (RSS), the reduction of methyl viologen (the 1,10 -dimethyl4,40 -bipyridilium dication) under semi-infinite linear diffusion conditions is presented. Methyl viologen (MV2þ) undergoes two consecutive one-electron reductions to the radical cation (MV•þ) and neutral species (MV0) in an EE mechanism. In acetonitrile at an OTE coated with a tin oxide film, both waves appear reversible with peak potentials (Ep)1¼0.36 V and (Ep)2¼0.76 V vs Ag/AgCl (eqn [8]).
[6]
where A1 is the absorbance of the reduced form, A3 is the absorbance of the oxidized form, A2 is the absorbance obtained at an intermediate applied potential, De is the difference in molar absorptivity between O and R at 438 nm and b is the light path length in the thin-layer cell. Thus the Nernst equation can be expressed as Eapplied ¼ E0 þ
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½8
[7]
Figure 2 gives Eapplied vs log([O]/[R]) for the data from Figure 1. The plot is linear as predicted from eqn [7], the slope being 0.031 V, which corresponds to an n value of 1.92, with an intercept of 0.612 V vs SCE.
Methyl Viologen Mechanistic information is often available from spectroelectrochemical measurements. To illustrate the acquisition of
If the electrode is stepped some 0.200 V more negative than (Ep)1 during a chronoamperometric experiment, absorbance spectra taken by RSS show two absorbance bands at lmax equal to 390 and 602 nm. Interestingly, if the experiment is repeated
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with the potential stepped slightly beyond (Ep)2, the spectra taken are qualitatively identical to those obtained at (Ep)1. This can be interpreted as due to the equilibrium between the three methyl viologen redox species in the diffusion layer, which greatly favours the radical ion MV•þ, since Keq103 for the reaction: kf
MV 2þ þ MV 0 Ð2MV þ kb
[9]
Analysis of the spectroelectrochemical working curves for this mechanism shows that when the radical ion is being monitored spectrally, the slopes of the A vs t1/2 plots obtained by chronoamperometric reductions at potentials of the first and second waves, respectively, should be in a ratio of 1:1.20. This ratio assumes that the electrode reaction at both waves occurs at the diffusion-controlled rates, and that the three species are in thermodynamic equilibrium in the diffusion layer. The ratio for methyl viologen is 1.21 at lmax¼620 nm. The larger ratio of 1.79 at lmax¼390 nm is believed to be caused by band overlap from MV0, which absorbs near the 390 nm band of MV•þ. If the chronoamperometric electrolysis is continued beyond several seconds, the rate of growth of the absorbance at the shorter wavelength of 390 nm decreases considerably owing to the formation of a dimer that absorbs near the longer-wavelength band.
Pyrene Reduction Reduction of the polycyclic aromatic pyrene serves as another excellent example of an EE mechanism where follow-up chemical reactions complicate the overall mechanism (Figure 3). The one-electron reduction to the radical anion produces a ground doublet state with allowed transitions expected in the visible region of the spectrum. Spectra taken by RSS during chronoamperometric reduction at a potential 0.200 V more negative than Ep of the first wave Ep¼2.06 V vs SCE in acetonitrile–TEAP (tetraethylammonium perchlorate)) showed only a major band with a wavelength maximum at 492 nm in the visible region (Figures 3a and 3c). Reduction at Ep of the second wave produced spectra with wavelength maxima at 455 and 520–530 nm (Figure 3a, curve b). These maxima are similar to those for a spectrum obtained from the chemical reduction of pyrene and attributed to the dianion, except that the long-wavelength band at 602 nm reported earlier is absent. There is doubt, however, that this spectrum is the dianion because the second wave is irreversible; a new oxidation wave more positive in potential than the wave for the oxidation of the radical anion appears (Figure 3c); and no spectrum due to the free radical appears during chronoamperometric reduction at Ep of the second wave. In any EE mechanism where the waves are sufficiently separated that the equilibrium constant for the disproportionation reaction is large, the equilibrium
Figure 3 Spectra and cyclic voltammograms for the reduction of pyrene. (a) Curve a, spectrum of monoanion radical; lmax 492, 446, 385 nm; curve b, spectrum obtained from reduction at the second wave for pyrene (see (c)); lmax 455 and 520–530 nm. (b) Cyclic voltammogram for the reduction of pyrene to the monoanion radical in acetonitrile–TEAP at a tin oxide OTE. (c) Cyclic voltammogram for the reduction of pyrene. After reduction at the second wave, a new oxidation wave more positive than for the oxidation of the radical appears. Reprinted by courtesy of Marcel-Dekker, Inc. from Kuwana T and Winograd N (1974) Spectroelectrochemistry at optically transparent electrodes. I. Electrodes under semi-infinite diffusion conditions. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol 7, pp. 1–78. New York: Marcel-Dekker.
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between the three species (pyrene, radical anion and dianion) would favour the presence of the radical anion in the diffusion layer. The supposed absence of rapid electron exchange between pyrene and dianion to form the radical anion suggests an EEC mechanism in which the dianion undergoes a fast homogeneous chemical reaction to a species more stable than the radical. A likely candidate is the monoanion formed through protonation.
Inorganic Systems There is a wide range of inorganic systems amenable to study by the spectroelectrochemical approach. In particular, transition metal complexes, with their rich redox state-dependent electronic spectra, have been intensively studied.
Hexacyanoferrate(III/II) The hexacyanoferrate(III/II) (ferricyanide/ferrocyanide) system in aqueous solution is a well-known electrochemically reversible redox couple (eqn [10]). ½FeIII ðCNÞ6 3 þ e Ð ½FeII ðCNÞ6 4
[10]
Furthermore, as the hexacyanoferate(III) ion is brilliant yellow in colour and the hexacyanoferrate(II) ion is only very pale yellow, this redox couple is particularly suited as a model system for electronic (UV-visible) absorbance spectroelectrochemical studies. Figure 4 shows UV-visible absorption spectra recorded in a spectropotentiostatic experiment in an OTTLE cell on reduction of hexacyanoferrate(III) at a sequence of applied potentials. Curve a is at þ0.50 V vs SCE reference electrode, where the redox system is in the oxidized state ([FeIII(CN)6]3/ [FeIII(CN) 6]4>1000). Curve h is at þ0.00 V vs SCE, where the redox system is in the reduced state ([FeIII(CN)6]4/ [FeIII(CN)6]3>1000), while the intermediate spectra correspond to intermediate values of applied potentials. The inset plot in Figure 4 demonstrates the reversibility of this system in accordance with eqn [7].
[ TcIII(diars)2Cl2]+ The complex [TcIII(diars)2Cl2]þ (diars¼[1]) provides another example of a reversible redox couple for which the spectropotentiostatic method has been applied.
Figure 4 In situ UV-visible absorption spectra of 2.0 mM K3Fe(CN)6 in aqueous 1 M KCl at a sequence of applied potentials vs Ag/AgCl: (a) 0.50 V, (b) 0.28 V, (c) 0.26 V, (d) 0.24 V, (e) 0.22 V, (f) 0.20 V, (g) 0.17 V and (h) 0.00 V. Inset shows the plot of Eapplied vs log([O]/[R]): • at 312 nm and ▲ at 420 nm. Reprinted from Niu J and Dong S (1995) Electrochimica Acta 40: 823–828. ã 1995, with permission from Elsevier Science.
from the spectra in Figure 6 is shown in Figure 7 (E0¼0.091 V vs SSCE, n¼0.99).
Polypyridylruthenium(II) Complexes The prospect of developing new materials of relevance to the emerging field of molecular electronics, modelling electrontransfer processes in biological systems and producing new electroactive and photoactive catalysts has led in years to considerable interest in transition metal polypyridyl complexes. Two examples of the application of the OTTLE spectroelectrochemical technique to the study of these fascinating systems are described here.
Identification of mixed-valence states in polynuclear polypyridylruthenium(II) complexes Figure 5 shows a thin-layer cyclic voltammogram for this system and Figure 6 gives a series of spectra for a spectropotentiostatic experiment. Each spectrum was recorded 5 min after potential application so that ([O]/[R])solution is at equilibrium with the electrode potential. Spectrum h is the oxidized form, whereas spectrum a is the reduced form. A Nernst plot
Mixed-valence complexes provide an ideal way of studying electron transfer – the most fundamental process in chemistry – under controlled conditions. Polynuclear complexes containing polypyridylruthenium(II) moieties are of particular interest for the study of mixed valency because of their kinetic inertness in both the þII and þIII oxidation states, generally reversible
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Figure 5 Thin-layer cyclic voltammogram at 2 mV s1 of 0.87 mM [TcIII(diars)2Cl2]þ, 0.5 M TEAP in DMF. (SSCE¼Sodium chloride saturated calomel electrode.) Reprinted with permission from Hurst RW, Heineman WR, and Deutsch E (1981) Inorganic Chemistry 20: 3298–3303. ã 1981 American Chemical Society.
electrochemical behaviour, and good p-donor ability which allows interaction with bridging ligand orbitals. Spectroelectrochemical measurements can be used to probe electrogenerated mixed-valence states in such complexes. A good example (Table 1 and Figure 8) is the controlled-potential oxidation of the [2, 2] species of the complex [{Ru(bipy)2}2(m-OMe)2] [PF6]2 in an OTTLE cell. Oxidation of the [2, 2] species to the mixed-valence [2,3] state results in the collapse of the metal-toligand charge transfer (m.l.c.t.) bands at 589 and 364 nm and the generation of a new transition at 1800 nm (e¼5000 dm3 mol 1 cm1), which disappears on further oxidation to the RuIII state. The observations that this transition is not solvato-chromic and that the half-width of the peak is much narrower than the value predicted from Hush theory for vectorial intervalence charge-transfer bands both point to a class III (Robin and Day fully delocalized) mixed-valence state.
Electronic properties of hydroquinone-containing ruthenium polypyridyl complexes Ruthenium polypyridyl complexes bound to hydroquinone/ quinone moieties are expected to yield information on the behaviour of hydroquinone-type compounds in biological processes. Furthermore, ruthenium(II)-hydroquinone complexes involving O and N bonds are likely to absorb well into the visible region and therefore have potential as dyes in sensitized solar cells. A recent example in the application of
Figure 6 Spectra recorded during an OTTLE spectropotentiostatic experiment on 0.87 mM [TcIII(diars)2Cl2]þ, 0.5 M TEAP in DMF. Applied potentials vs SSCE: (a) 0.250 V; (b) 0.150 V; (c) 0.100 V; (d) 0.075 V; (e) 0.050 V; (f) 0.025 V; (g) 0.100 V; (h) 0.250 V. Reprinted with permission from Hurst RW, Heineman WR, and Deutsch E (1981) Inorganic Chemistry 20: 3298–3303. ã 1981 American Chemical Society.
spectroelectrochemistry to the study of hydroquinonecontaining ruthenium polypyridyl complexes is the oxidation of [Ru(bipy)2(HL0)]þ (H2L0¼1,4-dihydroxy-2,3-bis(pyrazol1-yl)benzene) (Figure 9). The spectral changes associated with the first two-electron oxidation step are reversible, and unstable long-lived intermediates are not present, as indicated by he clear isobestic points at 327, 398, 446 and 614 nm (Figure 9). After the first twoelectron oxidation the m.l.c.t. band at 490 nm blue shifts to approximately 416 nm, and a new feature appears at 700 nm for [Ru(bipy)2(HL0)]2þ. The presence of significant absorption features between 400 and 500 nm in the spectrum of the oxidized compound suggests that in the complex the metal centre is still in the ruthenium(II) state, consistent with interpretation from electrochemical data. The oxidized complex is therefore most likely the analogous ruthenium(II)–quinone species. After oxidation of the hydroquinone to quinone, the RuII ! bipy(p2*) m.l.c.t. shifts to the blue as a result of the stabilization of the t2g level when the s-donating ability of the ligand is decreased. Further oxidation results in the irreversible loss of the intense feature between 700 and 800 nm and of the band at 416 nm and the generation of a yellow complex likely to be a complex in which the pyrazole is bound to the ruthenium in a monodentate fashion.
Spectroelectrochemistry, Applications
Figure 7 Nernst plot for spectropotentiostatic experiment on 0.87 mM [TcIII(diars)2Cl2]þ, 0.5 M TEAP in DMF. Data at 403 nm from Figure 6 are used. Reprinted by courtesy of Marcel-Dekker, Inc. from Heineman WR, Hawkridge FM, and Blount HN (1984) Spectroelectrochemistry at optically transparent electrodes. II. Electrodes under thin-layer and semi-infinite diffusion conditions and indirect coulometric titrations. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol 13, pp. 1–113. New York: Marcel-Dekker.
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Figure 8 Successive electronic spectra of the dinuclear complex [{Ru (bipy)2} 2(m-OMe)2][PF6] 2 in propylene carbonate at 240 K recorded during electrochemical oxidation to the mixed-valence RuIIRuIII state, showing the disappearance of the RuII ! m.l.c.t. bands and the appearance of the near-IR band. Reprinted with permission from Bardwell DA, Horsburgh L, Jeffrey JC, et al. (1996) Journal of the Chemical Society, Dalton Transactions, 2527–2531.
Table 1 Electronic spectral data for the dinuclear complex [{Ru (bipy)2}2(m-OMe)2][PF6]2 in CH2Cl2 at 240 K Oxidation state
lmax (nm) (103 e(dm3 mol1 cm1))
[2,2] [2,3] [3,3]
572 (12), 420 (sh), 359 (15), 293 (79), 242 (58) 1800 (5), 480 (9), 340 (12), 292 (94), 242 (57) 580 (6), 380 (sh), 248 (64)
Reprinted with permission from Bardwell DA, Horsburgh L, Jeffrey JC, et al. (1996) Journal of the Chemical Society, Dalton Transactions, 2527.
Biological Systems Numerous biological redox systems have been studied by the spectroelectrochemical approach, including cytochromes, myoglobin, photosynthetic electron transport components, spinach ferrodoxin, blue copper proteins, retinal, and vitamin B12 and its analogues. Two classic examples are presented here.
Vitamin B12 Vitamin B12 (cyanocob(III)alamin) is an example of a quasireversible redox system that exhibits slow heterogeneous electron-transfer kinetics. Cyclic voltammetry alone suggests that the reduction of vitamin B12 is a single two-electron process at Epc¼0.93 V vs SCE to the Co(I) redox state
Figure 9 Spectroelectrochemical oxidation of [Ru(bipy)2 (HL0)] þ (H2L0¼1,4-dihydroxy-2,3-bis(pyrazol-1-yl)benzene) as a function of time between 0 and 20 min. Reprinted with permission from Keyes TE, Jayaweera PM, McGarvey JJ, and Vos JG (1997) Journal of the Chemical Society, Dalton Transactions, 1627–1632.
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Figure 11 Thin-layer spectra for reduction of vitamin B12 to B12r in a solution of 1 mM vitamin B12, Britton–Robinson buffer pH 6.86, 0.5 M Na2SO4. To obtain the spectra, the potential was stepped in 0.5 mV increments and maintained at each step for 3–5 min until spectral changes ceased. Applied potentials vs SCE: (a) 0.550 V; (b)0.630 V; (c) 0.660 V; (d) 0.690 V; (e) 0.720 V; (f) 0.770 V. Reprinted by courtesy of Marcel-Dekker, Inc. from Heineman WR, Hawkridge FM, and Blount HN (1984) Spectroelectrochemistry at optically transparent electrodes. II. Electrodes under thin-layer and semi-infinite diffusion conditions and indirect coulometric titrations. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol 13, pp. 1–113. New York: Marcel-Dekker.
Figure 10 (a) Thin-layer cyclic voltammogram of 1 mM vitamin B12, Britton–Robinson buffer pH 6.86, 0.5 M Na2SO4. (b) Plot of absorbance at 368 nm vs potential, recorded at effectively 0.003 mV s1, from spectra in Figures 11 and 12. Reprinted by courtesy of Marcel-Dekker, Inc. from Heineman WR, Hawkridge FM, and Blount HN (1984) Spectroelectrochemistry at optically transparent electrodes. II. Electrodes under thin-layer and semi-infinite diffusion conditions and indirect coulometric titrations. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol 13, pp. 1–113. New York: Marcel-Dekker.
(Figure 10a). However, thin-layer spectroelectrochemistry using a Hg–Au minigrid OTTLE in a spectropotentiostatic mode reveals that reduction takes place via two consecutive one-electron steps (Figures 11 and 12). Figure 11 shows thinlayer spectra for the reduction to B12r, which occurs in the potential range 0.580 to 0.750 V, and Figure 12 shows the spectral changes for the further reduction to B12s, which occurs in the range 0.770 to 0.950 V. Nernst plots for these two reduction processes (using eqn [7] above) give values of E1¼0.655 V, n¼1 and E2¼0.880 V, n¼1, respectively. The two one-electron reduction processes are clearly shown by the plot of absorbance at 363 nm vs potential in Figure 10b, the first one-electron reduction occurring in a region with no apparent cathodic current (Figure 10a).
Cytochrome c Often biological macromolecules will not undergo direct heterogeneous electron transfer with an electrode. Instead,
Figure 12 Thin-layer spectra for reduction of vitamin B12r to B12s in a solution initially of 1 mM vitamin B12, Britton–Robinson buffer pH 6.86, 0.5 M Na2SO4. To obtain the spectra, the potential was stepped in 0.5 mV increments and maintained at each step for 3–5 min until spectral changes ceased. Applied potentials vs SCE: (a) 0.770 V; (b) 0.820 V; (c) 0.860 V; (d) 0.880 V; (e) 0.900 V; (f) 0.920 V; (g) 1.000 V. Reprinted with permission from Rubinson KA, Itabashi E, and Mark Jr HB (1982) Inorganic Chemistry 21: 3771–3773. ã 1982 American Chemical Society.
mediator titrants are used that exchange electrons heterogeneously with the electrode and homogeneously with the macromolecules. Figure 13 gives spectra obtained for the reduction of a mixture of the haem proteins cytochrome c and cytochrome c oxidase, both initially in the fully oxidized state. Each spectrum was recorded after the coulometric addition of 5 109 equivalents of reductant, the methyl viologen
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Modified Electrodes Immobilization of chemical microstructures onto electrode surfaces has been a major growth area in electrochemistry in recent years. Compared to conventional electrodes, greater control of electrode characteristics and reactivity is achieved on surface modification. Potential applications of such systems include the development of electrocatalytic systems with high chemical selectivity and activity, coatings on semiconducting electrodes with photosensitizing and anticorrosive properties, electrochromic displays, microelectrochemical devices for the field of molecular electronics and electrochemical sensors with high selectivity and sensitivity. Spectroelectrochemical measurements, both ex situ and in situ, are frequently used in the characterization of modified electrodes. In the case of in situ spectroelectrochemical measurements, the modified electrode can be considered to be analogous to an OTTLE, the redox active layer being physically or chemically confined to the electrode surface. Electronic spectroelectrochemistry sees significant use in the study of electrodes modified with electrochromic surface films, for which some examples are given below.
Characterization of Electrochromic Materials
Figure 13 Spectrocoulometric titration of cytochrome c (17.5 mM) and cytochrome c oxidase (6.3 mM) by reduction with electrogenerated methyl viologen radical cation (MV%þ) at a SnO2 OTE. Each spectrum was recorded after 5 109 equivalents of charge (0.5 mC) were passed. Spectra correspond to titration from totally oxidized to totally reduced forms. The final two spectra around 605 nm were recorded after excess MV%þ was present. The inset shows titration curves at 550 and 605 nm. Reprinted with permission from Heineman WR, Kuwana T, and Hartzell CR (1973) Biochemical and Biophysical Research Communications 50: 892–900.
radical cation (MV%þ) electrogenerated at a SnO2 OTE. The reaction sequence is an EC catalytic regeneration mechanism:
½11 In solution, one MV%þ species can reduce a single haem site in cytochrome c or one of two in the oxidase. The absorbance increase (Figure 13) at 605 nm corresponds to the reduction of the two haem components of cytochrome c oxidase; the increase at 550 nm corresponds to the reduction of the haem in cytochrome c. Study of plots of absorbance change vs coulometric charge (see inset of Figure 13) indicates that MV%þ initially reduces one of the haem groups in cytochrome c oxidase, then the haem in cytochrome c, before it reduces the second haem of the oxidase.
Chemical species that can be electrochemically switched between different colours are said to be electrochromic. Electrochromism results from the generation of different visibleregion electronic absorption bands on switching between redox states. The colour change is commonly between a transparent (‘bleached’) state and a coloured state, or between two coloured states. In cases where more than two redox states are electrochemically available, the electrochromic material may exhibit several colours and be termed polyelectrochromic. Likely applications of electrochromic materials include their use in controllable light-reflective or light-transmissive devices for optical information and storage, antiglare car rear-view mirrors, sunglasses, protective eyewear for the military, controllable aircraft canopies, glare-reduction systems for offices, and ‘smart windows’ for use in cars and in buildings.
Prussian blue Prussian blue (PB; iron(III) hexacyanoferrate(II)) thin films can be switched to Prussian white (PW) on electrochemical reduction and to Prussian yellow (PX) on oxidation via the partially oxidized Prussian green (PG). For all these electrochromic redox reactions, there is concomitant ion ingress/ egress in the films for electroneutrality. The spectra of PX, PG, PB and PW are shown in Figure 14, together with two intermediate states between blue and green. The intense blue colour in the [FeIIIFeII(CN)6] chromophore of PB is due to an intervalence charge-transfer (CT) absorption band centred at 690 nm. The yellow absorption band in PX corresponds with that of [FeIIIFeIII(CN)6] in solution, both maxima (lmax¼425 nm) coinciding with the (weaker) [FeIII(CN)6] 3 absorption maximum. On increase from þ0.50 V vs SCE to more oxidizing potentials, the original PB peak shifts continuously to longer wavelengths with diminishing absorption, while the peak at 425 nm steadily increases, owing to the increasing
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Figure 14 Spectra of iron hexacyanoferrate films on ITO-coated glass at various potentials [(i) þ0.50 V (PB, blue); (i) 0.20 V (PW, transparent); (iii) þ0.80 V (PG, green); (iv) þ0.85 V (PG, green); (v) þ0.90 V (PG, green); (vi) þ1.20 V (PX, yellow)] vs SCE with 0.2 M KCl þ0.01 M HCl as supporting electrolyte. Reproduced with permission from Mortimer RJ and Rosseinsky DR (1984) Journal of the Chemical Society, Dalton Transactions, 2059–2061.
[FeIIIFeIII(CN)6] absorption. The reduction of PB to PW is by contrast abrupt, with transformation to all PW or all PB without pause, depending on the potential that is set. In the cyclic voltammogram of a PB-modified electrode, the broad peak for PBÐPX in contrast with the sharp PBÐPW transition emphasizes the range of compositions involved. This difference in behaviour, supported by ellipsometric measurements, indicates continuous mixed-valence compositions over the blueto-yellow range in contrast with the presumably immiscible PB and PW, which clearly transform one into the other without intermediacy of composition.
Conducting polymers Chemical or electrochemical oxidation of numerous resonance-stabilized aromatic molecules including pyrrole, thiophene, aniline, furan, carbazole, azulene and indole produces electronically conducting polymers. In their oxidized forms, such conducting polymers are ‘doped’ with counteranions (p-doping) and possess a delocalized p electron band structure, the energy gap between the highest occupied p electron band (valence band) and the lowest unoccupied band (the conduction band) determining the intrinsic optical properties of these materials. The doping process (oxidation) introduces polarons (in polypyrrole, for example, these are radical cations delocalized over ca. four monomer units), which are the major charge-carriers. Reduction of conducting polymers with concurrent counteranion exit removes the electronic conjugation, to give the ‘undoped’ (neutral) electrically insulating form.
Figure 15 Spectra of poly(m-toluidine) films on ITO in 1 M hydrochloric acid at (a) 0.20 V, (b) þ0.10 V, (c) þ0.20 V, (d) þ0.30 V vs SCE. Reproduced with permission from Mortimer RJ (1995) Journal of Materials Chemistry 5: 969–973.
All conducting polymers are potentially electrochromic in thin-film form, redox switching giving rise to new optical absorption bands in accompaniment with transfer of electrons/counteranions. Good examples are the polymers of aniline, o-toluidine and m-toluidine, which are easily prepared as thin films by electrochemical oxidation from aqueous acid solutions of the appropriate monomer. The electrical and electrochromic properties of such polyanilines depend not only on oxidation state but also on the protonation state, and polyanilines are in fact polyelectrochromic (transparent yellow to green to dark blue to black), the yellow–green transition being durable to repetitive colour switching. Spectra for a poly(m-toluidine)-modified electrode are illustrated in Figure 15. The two low-wavelength spectral bands observed are assigned to an aromatic p–p* transition (330 nm) related to the extent of conjugation between the adjacent rings in the polymer chain, and to radical cations formed in the polymer matrix (440 nm). With an increase in applied potential, the 330 nm band absorbance decreases and the 440 nm increases (Figure 15), the isobestic point indicating that the two species have the same chemical stoichiometry with differences only in electron(s). Beyond þ0.30 V, the conducting region is entered; the 440 nm band decreases as a broad free carrier electron band 800 nm is introduced. Response times for the yellow–green transition following a potential step can be determined using a diode array spectrophotometer (Figure 16).
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Figure 16 Spectra recorded at times indicated after potential switching of poly(m-toluidine) films on ITO in 1 M hydrochloric acid (a) Potential step 0.20 to þ0.40 V vs SCE. (b) Potential step þ0.40 to 0.20 V vs SCE. Reproduced with permission from Mortimer RJ (1995) Journal of Materials Chemistry 5: 969–973.
Viologens in Nafion In addition to being important mediator titrants, 1,10 -disubstituted-4,40 -bipyridiliums (viologens) are a major group of electrochromic materials. Electrochromism occurs in bipyridiliums because, in contrast to the bipyridilium dications, the radical cations formed on electroreduction have a delocalized positive charge, coloration arising from an intramolecular electronic transition. Suitable choice of nitrogen substituents to attain the appropriate molecular orbital energy levels can, in principle, allow colour choice of the radical cation. For short alkyl chain length, 1,10 -dialkyl-4,40 -bipyridiliums, both the dication and radical-cation states, are soluble in water and any electrochromic device (ECD) using such bipyridiliums would have the limitation of a low write–erase efficiency. One solution to this problem involves electrostatic binding of bipyridilium dications into anionic polyelectrolyte films.
When a Nafion-modified electrode is immersed in an aqueous solution of 1,10 -dialkyl-4,40 -bipyridilium (alkyl¼methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl), the 1,10 -dialkyl-4,40 bipyridilium accumulates in the anionic polyelectrolyte such that its concentration is considerably higher than that in the bulk solution. Figure 17 illustrates the uptake of 1,10 -dimethyl4,40 -bipyridilium into a Nafion film monitored by in situ spectral measurements. The coloured form in each case is purple, from the presence of monomeric (blue, lmax 600 nm) and dimeric (red, lmax 500 nm) viologen radical cations. For such viologen-incorporated Nafion films, the electrochromic response times are in excess of 60 s for both coloration and bleaching and independent of viologen size. Figure 18 shows absorbance spectra measured every 10 s in response to a potential step between the oxidized and reduced forms, for the case of the 1,10 -di-n-hexyl-4,40 -bipyridilium system. The longer
Figure 17 Spectra recorded at 0.90 V vs SCE during the 2nd, 4th, 6th, 8th and 10th cyclic voltammograms for an ITO/Nafion electrode in 0.1 mM 1,10 -dimethyl-4,40 -bipyridilium dichloride þ0.2 M KCI (pH 5.5). The vertical arrows indicate absorbance increase with scan number. For a comparable experiment in the absence of Nafion, the maximum absorbance was <0.01. Reproduced with permission from Mortimer RJ and Dillingham JL (1997) Journal of the Electrochemical Society 144: 1549–1553.
Figure 18 (a) Spectra recorded at t¼0, 10, 20, 30, 40 and 50 s in response to a potential step from þ1.00 to 0.90 V vs SCE for an ITO/Nafion/ 1,10 -di-n-hexyl-4,40 -bipyridilium electrode in 0.1 mM 1,10 -di-n-hexyl-4,40 -bipyridilium dibromide þ0.2 M KCl (pH 5.5). The vertical arrows indicate absorbance increase with time. (b) Spectra recorded at t¼0, 10, 20, 30, 40 and 50 s in response to a potential step from 0.90 to þ1.00 V vs SCE for an ITO/Nafion/1,10 -di-n-hexyl-4,40 -bipyridilium electrode in 0.1 mM 1,10 -di-n-hexyl-4,40 -bipyridilium dibromide þ0.2 M KCl (pH 5.5). The vertical arrows indicate absorbance decrease with time. Reproduced with permission from Mortimer RJ and Dillingham JL (1997) Journal of the Electrochemical Society 144: 1549–1553.
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poration of the methyl viologen system) to an inner layer of PB, is possible (Figure 19). The transparent/purple viologen dication/radical cation electrochromicity operates in the potential region where the PB is in its (reduced) transparent state; the bilayer electrode system thus exhibits yellow/green/ blue/transparent/purple colours.
See also: Ellipsometry; Spectroelectrochemistry, Methods and Instrumentation; UV–Visible Absorption Spectrometers; UV-Visible Absorption Spectroscopy, Biomacromolecular Applications; UV-Visible Absorption Spectroscopy, Dyes and Indicators Applications; UVVisible Absorption Spectroscopy, Organic Applications.
Further Reading
Figure 19 Spectra recorded at þ0.50 V (blue), 0.20 V (transparent) and 0.90 V (purple) vs SCE for an ITO/PB/Nafion/methyl viologen electrode in 0.1 mM 1,10 -dimethyl-4,40 -bipyridilium dichloride þ0.2 M KCl (pH 5.5). Reproduced with permission from Mortimer RJ and Dillingham JL (1997) Journal of the Electrochemical Society 144: 1549–1553.
response time for the oxidation (bleaching) reflects the slower diffusion of radical-cation dimers through the Nafion film compared to that of the monomeric radical cations. Five-colour polyelectrochromicity, by application of an outer Nafion layer (with subsequent electrostatic incor-
Heineman WR and Jensen WB (1989) Spectroelectrochemistry using transparent electrodes – an anecdotal history of the early years. ACS Symposium Series 390: 442–457. Heineman WR and Kissinger PT (1984) Large-amplitude controlled-potential techniques. In: Kissinger PT and Heineman WR (eds.) Laboratory Techniques in Electroanalytical Chemistry. New York: Marcel Dekker Chapter 3. Heineman WR, Hawkridge FM, and Blount HN (1984) Spectroelectrochemistry at optically transparent electrodes. II. Electrodes under thin-layer and semi-infinite diffusion conditions and indirect coulometric titrations. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol. 13, pp. 1–113. New York: Marcel-Dekker. Kuwana T and Heineman WR (1976) Study of electrogenerated reactants using optically transparent electrodes. Accounts of Chemical Research 7: 241–248. Kuwana T and Winograd N (1974) Spectroelectrochemistry at optically transparent electrodes. I. Electrodes under semi-infinite diffusion conditions. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol. 7, pp. 1–78. New York: Marcel-Dekker. Mortimer RJ (1994) Dynamic processes in polymer modified electrodes. In: Compton RG and Hancock G (eds.) Research in Chemical Kinetics, vol. 2, pp. 261–311. Amsterdam: Elsevier. Mortimer RJ (1997) Electrochromic materials. Chemical Society Reviews 26: 147–156. Murray RW (1984) Chemically modified electrodes. In: Bard AJ (ed.) Electroanalytical Chemistry. A series of Advances, vol. 13, p. 191. New York: Marcel-Dekker –3\68. Niu J and Dong S (1996) Transmission spectroelectrochemistry. Reviews in Analytical Chemistry 15: 1–171.
Symbols A b e E E0 Ep Epa Epc F
absorbance path length electron electrode potential reversible electrode potential peak potential anodic peak potential cathodic peak potential Faraday constant (96 485 C mol1)
Introduction Spectroelectrochemistry encompasses a group of techniques that allow simultaneous acquisition of spectroscopic and electrochemical information in situ in an electrochemical cell. Electrochemical reactions can be initiated by applying potentials to the working electrode, and the processes that occur are then monitored by both electrochemical and spectroscopic techniques. Electronic (UV-visible) transmission and reflectance spectroelectrochemistry has proved to be an effective approach for studying the redox chemistry of organic, inorganic and biological molecules, for investigating reaction kinetics and mechanisms, and for exploring electrode surface phenomena. In this article a selection of representative examples are presented, the emphasis being on the applications of transmission electronic (UV-visible) spectroelectrochemistry
k Keq n O R R t T « lmax
rate constant equilibrium constant number of electrons oxidized form reduced form gas constant (8.315 J K1 mol1) time temperature in kelvin molar extinction coefficient wavelength of maximum absorbance
strated in 1964 using o-tolidine, a colourless compound that reversibly undergoes a 2-electron oxidation in acidic solution to form an intensely yellow coloured species (eqn [1]). This system soon became a standard for testing spectroelectrochemical cells and new techniques. Figure 1 shows absorbance spectra, for a series of applied potentials, recorded in an electrochemical cell employing an optically transparent thin-layer electrode (OTTLE). Curve a was recorded after application of þ0.800 V vs saturated calomel electrode (SCE), which under thin-layer electrode conditions causes complete electrolytic oxidation of o-tolidine to the yellow form ([O]/[R]>1000, where O represents the oxidized form and R the reduced form). Curve g was recorded after application of þ0.400 V, causing complete electrolytic reduction ([O]/[R]<0.001), with the intermediate spectra corresponding to intermediate values of Eapplied. The absorbance at
½1
to the study of redox reactions and homogeneous chemical reactions initiated electrochemically within the boundaries of the diffusion layer at the electrode–electrolyte interface.
Organic Systems Many organic systems exhibit redox states with distinct electronic (UV-visible) absorption spectra and are therefore amenable to study with spectroelectrochemical techniques.
o-Tolidine The technique of transmission spectroelectrochemistry, using an optically transparent electrode (OTE), was first demonThis article is reproduced from the previous edition, Copyright 1999, Elsevier Ltd.
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438 nm reflects the amount of o-tolidine in the oxidized form, which can be calculated from the Beer–Lambert law. Determination of E0, the reversible electrode potential, and n, the number of electrons in the o-tolidine redox reaction, can be determined from the sequence of spectropotentiostatic measurements (Figure 1). For a reversible system, O þ ne Ð R
[2]
the [O]/[R] ratio at the electrode surface is controlled by the applied potential according to the Nernst equation: RT ½O ln [3] Eapplied ¼ E0 þ nF ½R surface In an OTTLE cell, on application of a new potential, the concentrations of O and R in solution are quickly adjusted to the same values as those existing at the electrode surface. Thus, at equilibrium:
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Spectroelectrochemistry, Applications
Figure 1 Thin-layer spectra of 0.97 mM o-tolidine, 0.5 M ethanoic acid, 1.0 M HCIO4 for different values of Eapplied. Cell thickness 0.017 cm. Potential vs SCE: (a) 0.800 V, (b) 0.660 V, (c) 0.640 V, (d) 0.620 V, (e) 0.580 V, (f) 0.600 V, (g) 0.400 V. Reprinted with permission from DeAngelis TP and Heineman WR (1976) Journal of Chemical Education 53: 594–597. ã 1976 Division of Chemical Education, American Chemical Society.
½O ½R
¼ surface
½O ½R solution
[4]
The Nernst equation in a thin-layer cell can then be written as: RT ½O ln [5] Eapplied ¼ E0 þ nF ½R For the o-tolidine spectra, 438 nm is used as the monitoring wavelength and the ratio [O]/[R] is determined from the Beer–Lambert law: ½O ðA2 A1 Þ=Deb A2 A1 ¼ ¼ ½R ðA3 A2 Þ=Deb A3 A2
RT ðA2 A1 Þ ln nF ðA3 A2 Þ
Figure 2 Plot of Eapplied vs log([O]/[R]) from the spectra in Figure 1. Reprinted with permission from the DeAngelis TP and Heineman WR (1976) Journal of Chemical Education 53: 594–597. ã 1976 Division of Chemical Education, American Chemical Society.
semiquantitative information using a rapid scan spectrometer (RSS), the reduction of methyl viologen (the 1,10 -dimethyl4,40 -bipyridilium dication) under semi-infinite linear diffusion conditions is presented. Methyl viologen (MV2þ) undergoes two consecutive one-electron reductions to the radical cation (MV•þ) and neutral species (MV0) in an EE mechanism. In acetonitrile at an OTE coated with a tin oxide film, both waves appear reversible with peak potentials (Ep)1¼0.36 V and (Ep)2¼0.76 V vs Ag/AgCl (eqn [8]).
[6]
where A1 is the absorbance of the reduced form, A3 is the absorbance of the oxidized form, A2 is the absorbance obtained at an intermediate applied potential, De is the difference in molar absorptivity between O and R at 438 nm and b is the light path length in the thin-layer cell. Thus the Nernst equation can be expressed as Eapplied ¼ E0 þ
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½8
[7]
Figure 2 gives Eapplied vs log([O]/[R]) for the data from Figure 1. The plot is linear as predicted from eqn [7], the slope being 0.031 V, which corresponds to an n value of 1.92, with an intercept of 0.612 V vs SCE.
Methyl Viologen Mechanistic information is often available from spectroelectrochemical measurements. To illustrate the acquisition of
If the electrode is stepped some 0.200 V more negative than (Ep)1 during a chronoamperometric experiment, absorbance spectra taken by RSS show two absorbance bands at lmax equal to 390 and 602 nm. Interestingly, if the experiment is repeated
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with the potential stepped slightly beyond (Ep)2, the spectra taken are qualitatively identical to those obtained at (Ep)1. This can be interpreted as due to the equilibrium between the three methyl viologen redox species in the diffusion layer, which greatly favours the radical ion MV•þ, since Keq103 for the reaction: kf
MV 2þ þ MV 0 Ð2MV þ kb
[9]
Analysis of the spectroelectrochemical working curves for this mechanism shows that when the radical ion is being monitored spectrally, the slopes of the A vs t1/2 plots obtained by chronoamperometric reductions at potentials of the first and second waves, respectively, should be in a ratio of 1:1.20. This ratio assumes that the electrode reaction at both waves occurs at the diffusion-controlled rates, and that the three species are in thermodynamic equilibrium in the diffusion layer. The ratio for methyl viologen is 1.21 at lmax¼620 nm. The larger ratio of 1.79 at lmax¼390 nm is believed to be caused by band overlap from MV0, which absorbs near the 390 nm band of MV•þ. If the chronoamperometric electrolysis is continued beyond several seconds, the rate of growth of the absorbance at the shorter wavelength of 390 nm decreases considerably owing to the formation of a dimer that absorbs near the longer-wavelength band.
Pyrene Reduction Reduction of the polycyclic aromatic pyrene serves as another excellent example of an EE mechanism where follow-up chemical reactions complicate the overall mechanism (Figure 3). The one-electron reduction to the radical anion produces a ground doublet state with allowed transitions expected in the visible region of the spectrum. Spectra taken by RSS during chronoamperometric reduction at a potential 0.200 V more negative than Ep of the first wave Ep¼2.06 V vs SCE in acetonitrile–TEAP (tetraethylammonium perchlorate)) showed only a major band with a wavelength maximum at 492 nm in the visible region (Figures 3a and 3c). Reduction at Ep of the second wave produced spectra with wavelength maxima at 455 and 520–530 nm (Figure 3a, curve b). These maxima are similar to those for a spectrum obtained from the chemical reduction of pyrene and attributed to the dianion, except that the long-wavelength band at 602 nm reported earlier is absent. There is doubt, however, that this spectrum is the dianion because the second wave is irreversible; a new oxidation wave more positive in potential than the wave for the oxidation of the radical anion appears (Figure 3c); and no spectrum due to the free radical appears during chronoamperometric reduction at Ep of the second wave. In any EE mechanism where the waves are sufficiently separated that the equilibrium constant for the disproportionation reaction is large, the equilibrium
Figure 3 Spectra and cyclic voltammograms for the reduction of pyrene. (a) Curve a, spectrum of monoanion radical; lmax 492, 446, 385 nm; curve b, spectrum obtained from reduction at the second wave for pyrene (see (c)); lmax 455 and 520–530 nm. (b) Cyclic voltammogram for the reduction of pyrene to the monoanion radical in acetonitrile–TEAP at a tin oxide OTE. (c) Cyclic voltammogram for the reduction of pyrene. After reduction at the second wave, a new oxidation wave more positive than for the oxidation of the radical appears. Reprinted by courtesy of Marcel-Dekker, Inc. from Kuwana T and Winograd N (1974) Spectroelectrochemistry at optically transparent electrodes. I. Electrodes under semi-infinite diffusion conditions. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol 7, pp. 1–78. New York: Marcel-Dekker.
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between the three species (pyrene, radical anion and dianion) would favour the presence of the radical anion in the diffusion layer. The supposed absence of rapid electron exchange between pyrene and dianion to form the radical anion suggests an EEC mechanism in which the dianion undergoes a fast homogeneous chemical reaction to a species more stable than the radical. A likely candidate is the monoanion formed through protonation.
Inorganic Systems There is a wide range of inorganic systems amenable to study by the spectroelectrochemical approach. In particular, transition metal complexes, with their rich redox state-dependent electronic spectra, have been intensively studied.
Hexacyanoferrate(III/II) The hexacyanoferrate(III/II) (ferricyanide/ferrocyanide) system in aqueous solution is a well-known electrochemically reversible redox couple (eqn [10]). ½FeIII ðCNÞ6 3 þ e Ð ½FeII ðCNÞ6 4
[10]
Furthermore, as the hexacyanoferate(III) ion is brilliant yellow in colour and the hexacyanoferrate(II) ion is only very pale yellow, this redox couple is particularly suited as a model system for electronic (UV-visible) absorbance spectroelectrochemical studies. Figure 4 shows UV-visible absorption spectra recorded in a spectropotentiostatic experiment in an OTTLE cell on reduction of hexacyanoferrate(III) at a sequence of applied potentials. Curve a is at þ0.50 V vs SCE reference electrode, where the redox system is in the oxidized state ([FeIII(CN)6]3/ [FeIII(CN) 6]4>1000). Curve h is at þ0.00 V vs SCE, where the redox system is in the reduced state ([FeIII(CN)6]4/ [FeIII(CN)6]3>1000), while the intermediate spectra correspond to intermediate values of applied potentials. The inset plot in Figure 4 demonstrates the reversibility of this system in accordance with eqn [7].
[ TcIII(diars)2Cl2]+ The complex [TcIII(diars)2Cl2]þ (diars¼[1]) provides another example of a reversible redox couple for which the spectropotentiostatic method has been applied.
Figure 4 In situ UV-visible absorption spectra of 2.0 mM K3Fe(CN)6 in aqueous 1 M KCl at a sequence of applied potentials vs Ag/AgCl: (a) 0.50 V, (b) 0.28 V, (c) 0.26 V, (d) 0.24 V, (e) 0.22 V, (f) 0.20 V, (g) 0.17 V and (h) 0.00 V. Inset shows the plot of Eapplied vs log([O]/[R]): • at 312 nm and ▲ at 420 nm. Reprinted from Niu J and Dong S (1995) Electrochimica Acta 40: 823–828. ã 1995, with permission from Elsevier Science.
from the spectra in Figure 6 is shown in Figure 7 (E0¼0.091 V vs SSCE, n¼0.99).
Polypyridylruthenium(II) Complexes The prospect of developing new materials of relevance to the emerging field of molecular electronics, modelling electrontransfer processes in biological systems and producing new electroactive and photoactive catalysts has led in years to considerable interest in transition metal polypyridyl complexes. Two examples of the application of the OTTLE spectroelectrochemical technique to the study of these fascinating systems are described here.
Identification of mixed-valence states in polynuclear polypyridylruthenium(II) complexes Figure 5 shows a thin-layer cyclic voltammogram for this system and Figure 6 gives a series of spectra for a spectropotentiostatic experiment. Each spectrum was recorded 5 min after potential application so that ([O]/[R])solution is at equilibrium with the electrode potential. Spectrum h is the oxidized form, whereas spectrum a is the reduced form. A Nernst plot
Mixed-valence complexes provide an ideal way of studying electron transfer – the most fundamental process in chemistry – under controlled conditions. Polynuclear complexes containing polypyridylruthenium(II) moieties are of particular interest for the study of mixed valency because of their kinetic inertness in both the þII and þIII oxidation states, generally reversible
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Figure 5 Thin-layer cyclic voltammogram at 2 mV s1 of 0.87 mM [TcIII(diars)2Cl2]þ, 0.5 M TEAP in DMF. (SSCE¼Sodium chloride saturated calomel electrode.) Reprinted with permission from Hurst RW, Heineman WR, and Deutsch E (1981) Inorganic Chemistry 20: 3298–3303. ã 1981 American Chemical Society.
electrochemical behaviour, and good p-donor ability which allows interaction with bridging ligand orbitals. Spectroelectrochemical measurements can be used to probe electrogenerated mixed-valence states in such complexes. A good example (Table 1 and Figure 8) is the controlled-potential oxidation of the [2, 2] species of the complex [{Ru(bipy)2}2(m-OMe)2] [PF6]2 in an OTTLE cell. Oxidation of the [2, 2] species to the mixed-valence [2,3] state results in the collapse of the metal-toligand charge transfer (m.l.c.t.) bands at 589 and 364 nm and the generation of a new transition at 1800 nm (e¼5000 dm3 mol 1 cm1), which disappears on further oxidation to the RuIII state. The observations that this transition is not solvato-chromic and that the half-width of the peak is much narrower than the value predicted from Hush theory for vectorial intervalence charge-transfer bands both point to a class III (Robin and Day fully delocalized) mixed-valence state.
Electronic properties of hydroquinone-containing ruthenium polypyridyl complexes Ruthenium polypyridyl complexes bound to hydroquinone/ quinone moieties are expected to yield information on the behaviour of hydroquinone-type compounds in biological processes. Furthermore, ruthenium(II)-hydroquinone complexes involving O and N bonds are likely to absorb well into the visible region and therefore have potential as dyes in sensitized solar cells. A recent example in the application of
Figure 6 Spectra recorded during an OTTLE spectropotentiostatic experiment on 0.87 mM [TcIII(diars)2Cl2]þ, 0.5 M TEAP in DMF. Applied potentials vs SSCE: (a) 0.250 V; (b) 0.150 V; (c) 0.100 V; (d) 0.075 V; (e) 0.050 V; (f) 0.025 V; (g) 0.100 V; (h) 0.250 V. Reprinted with permission from Hurst RW, Heineman WR, and Deutsch E (1981) Inorganic Chemistry 20: 3298–3303. ã 1981 American Chemical Society.
spectroelectrochemistry to the study of hydroquinonecontaining ruthenium polypyridyl complexes is the oxidation of [Ru(bipy)2(HL0)]þ (H2L0¼1,4-dihydroxy-2,3-bis(pyrazol1-yl)benzene) (Figure 9). The spectral changes associated with the first two-electron oxidation step are reversible, and unstable long-lived intermediates are not present, as indicated by he clear isobestic points at 327, 398, 446 and 614 nm (Figure 9). After the first twoelectron oxidation the m.l.c.t. band at 490 nm blue shifts to approximately 416 nm, and a new feature appears at 700 nm for [Ru(bipy)2(HL0)]2þ. The presence of significant absorption features between 400 and 500 nm in the spectrum of the oxidized compound suggests that in the complex the metal centre is still in the ruthenium(II) state, consistent with interpretation from electrochemical data. The oxidized complex is therefore most likely the analogous ruthenium(II)–quinone species. After oxidation of the hydroquinone to quinone, the RuII ! bipy(p2*) m.l.c.t. shifts to the blue as a result of the stabilization of the t2g level when the s-donating ability of the ligand is decreased. Further oxidation results in the irreversible loss of the intense feature between 700 and 800 nm and of the band at 416 nm and the generation of a yellow complex likely to be a complex in which the pyrazole is bound to the ruthenium in a monodentate fashion.
Spectroelectrochemistry, Applications
Figure 7 Nernst plot for spectropotentiostatic experiment on 0.87 mM [TcIII(diars)2Cl2]þ, 0.5 M TEAP in DMF. Data at 403 nm from Figure 6 are used. Reprinted by courtesy of Marcel-Dekker, Inc. from Heineman WR, Hawkridge FM, and Blount HN (1984) Spectroelectrochemistry at optically transparent electrodes. II. Electrodes under thin-layer and semi-infinite diffusion conditions and indirect coulometric titrations. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol 13, pp. 1–113. New York: Marcel-Dekker.
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Figure 8 Successive electronic spectra of the dinuclear complex [{Ru (bipy)2} 2(m-OMe)2][PF6] 2 in propylene carbonate at 240 K recorded during electrochemical oxidation to the mixed-valence RuIIRuIII state, showing the disappearance of the RuII ! m.l.c.t. bands and the appearance of the near-IR band. Reprinted with permission from Bardwell DA, Horsburgh L, Jeffrey JC, et al. (1996) Journal of the Chemical Society, Dalton Transactions, 2527–2531.
Table 1 Electronic spectral data for the dinuclear complex [{Ru (bipy)2}2(m-OMe)2][PF6]2 in CH2Cl2 at 240 K Oxidation state
lmax (nm) (103 e(dm3 mol1 cm1))
[2,2] [2,3] [3,3]
572 (12), 420 (sh), 359 (15), 293 (79), 242 (58) 1800 (5), 480 (9), 340 (12), 292 (94), 242 (57) 580 (6), 380 (sh), 248 (64)
Reprinted with permission from Bardwell DA, Horsburgh L, Jeffrey JC, et al. (1996) Journal of the Chemical Society, Dalton Transactions, 2527.
Biological Systems Numerous biological redox systems have been studied by the spectroelectrochemical approach, including cytochromes, myoglobin, photosynthetic electron transport components, spinach ferrodoxin, blue copper proteins, retinal, and vitamin B12 and its analogues. Two classic examples are presented here.
Vitamin B12 Vitamin B12 (cyanocob(III)alamin) is an example of a quasireversible redox system that exhibits slow heterogeneous electron-transfer kinetics. Cyclic voltammetry alone suggests that the reduction of vitamin B12 is a single two-electron process at Epc¼0.93 V vs SCE to the Co(I) redox state
Figure 9 Spectroelectrochemical oxidation of [Ru(bipy)2 (HL0)] þ (H2L0¼1,4-dihydroxy-2,3-bis(pyrazol-1-yl)benzene) as a function of time between 0 and 20 min. Reprinted with permission from Keyes TE, Jayaweera PM, McGarvey JJ, and Vos JG (1997) Journal of the Chemical Society, Dalton Transactions, 1627–1632.
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Figure 11 Thin-layer spectra for reduction of vitamin B12 to B12r in a solution of 1 mM vitamin B12, Britton–Robinson buffer pH 6.86, 0.5 M Na2SO4. To obtain the spectra, the potential was stepped in 0.5 mV increments and maintained at each step for 3–5 min until spectral changes ceased. Applied potentials vs SCE: (a) 0.550 V; (b)0.630 V; (c) 0.660 V; (d) 0.690 V; (e) 0.720 V; (f) 0.770 V. Reprinted by courtesy of Marcel-Dekker, Inc. from Heineman WR, Hawkridge FM, and Blount HN (1984) Spectroelectrochemistry at optically transparent electrodes. II. Electrodes under thin-layer and semi-infinite diffusion conditions and indirect coulometric titrations. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol 13, pp. 1–113. New York: Marcel-Dekker.
Figure 10 (a) Thin-layer cyclic voltammogram of 1 mM vitamin B12, Britton–Robinson buffer pH 6.86, 0.5 M Na2SO4. (b) Plot of absorbance at 368 nm vs potential, recorded at effectively 0.003 mV s1, from spectra in Figures 11 and 12. Reprinted by courtesy of Marcel-Dekker, Inc. from Heineman WR, Hawkridge FM, and Blount HN (1984) Spectroelectrochemistry at optically transparent electrodes. II. Electrodes under thin-layer and semi-infinite diffusion conditions and indirect coulometric titrations. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol 13, pp. 1–113. New York: Marcel-Dekker.
(Figure 10a). However, thin-layer spectroelectrochemistry using a Hg–Au minigrid OTTLE in a spectropotentiostatic mode reveals that reduction takes place via two consecutive one-electron steps (Figures 11 and 12). Figure 11 shows thinlayer spectra for the reduction to B12r, which occurs in the potential range 0.580 to 0.750 V, and Figure 12 shows the spectral changes for the further reduction to B12s, which occurs in the range 0.770 to 0.950 V. Nernst plots for these two reduction processes (using eqn [7] above) give values of E1¼0.655 V, n¼1 and E2¼0.880 V, n¼1, respectively. The two one-electron reduction processes are clearly shown by the plot of absorbance at 363 nm vs potential in Figure 10b, the first one-electron reduction occurring in a region with no apparent cathodic current (Figure 10a).
Cytochrome c Often biological macromolecules will not undergo direct heterogeneous electron transfer with an electrode. Instead,
Figure 12 Thin-layer spectra for reduction of vitamin B12r to B12s in a solution initially of 1 mM vitamin B12, Britton–Robinson buffer pH 6.86, 0.5 M Na2SO4. To obtain the spectra, the potential was stepped in 0.5 mV increments and maintained at each step for 3–5 min until spectral changes ceased. Applied potentials vs SCE: (a) 0.770 V; (b) 0.820 V; (c) 0.860 V; (d) 0.880 V; (e) 0.900 V; (f) 0.920 V; (g) 1.000 V. Reprinted with permission from Rubinson KA, Itabashi E, and Mark Jr HB (1982) Inorganic Chemistry 21: 3771–3773. ã 1982 American Chemical Society.
mediator titrants are used that exchange electrons heterogeneously with the electrode and homogeneously with the macromolecules. Figure 13 gives spectra obtained for the reduction of a mixture of the haem proteins cytochrome c and cytochrome c oxidase, both initially in the fully oxidized state. Each spectrum was recorded after the coulometric addition of 5 109 equivalents of reductant, the methyl viologen
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Modified Electrodes Immobilization of chemical microstructures onto electrode surfaces has been a major growth area in electrochemistry in recent years. Compared to conventional electrodes, greater control of electrode characteristics and reactivity is achieved on surface modification. Potential applications of such systems include the development of electrocatalytic systems with high chemical selectivity and activity, coatings on semiconducting electrodes with photosensitizing and anticorrosive properties, electrochromic displays, microelectrochemical devices for the field of molecular electronics and electrochemical sensors with high selectivity and sensitivity. Spectroelectrochemical measurements, both ex situ and in situ, are frequently used in the characterization of modified electrodes. In the case of in situ spectroelectrochemical measurements, the modified electrode can be considered to be analogous to an OTTLE, the redox active layer being physically or chemically confined to the electrode surface. Electronic spectroelectrochemistry sees significant use in the study of electrodes modified with electrochromic surface films, for which some examples are given below.
Characterization of Electrochromic Materials
Figure 13 Spectrocoulometric titration of cytochrome c (17.5 mM) and cytochrome c oxidase (6.3 mM) by reduction with electrogenerated methyl viologen radical cation (MV%þ) at a SnO2 OTE. Each spectrum was recorded after 5 109 equivalents of charge (0.5 mC) were passed. Spectra correspond to titration from totally oxidized to totally reduced forms. The final two spectra around 605 nm were recorded after excess MV%þ was present. The inset shows titration curves at 550 and 605 nm. Reprinted with permission from Heineman WR, Kuwana T, and Hartzell CR (1973) Biochemical and Biophysical Research Communications 50: 892–900.
radical cation (MV%þ) electrogenerated at a SnO2 OTE. The reaction sequence is an EC catalytic regeneration mechanism:
½11 In solution, one MV%þ species can reduce a single haem site in cytochrome c or one of two in the oxidase. The absorbance increase (Figure 13) at 605 nm corresponds to the reduction of the two haem components of cytochrome c oxidase; the increase at 550 nm corresponds to the reduction of the haem in cytochrome c. Study of plots of absorbance change vs coulometric charge (see inset of Figure 13) indicates that MV%þ initially reduces one of the haem groups in cytochrome c oxidase, then the haem in cytochrome c, before it reduces the second haem of the oxidase.
Chemical species that can be electrochemically switched between different colours are said to be electrochromic. Electrochromism results from the generation of different visibleregion electronic absorption bands on switching between redox states. The colour change is commonly between a transparent (‘bleached’) state and a coloured state, or between two coloured states. In cases where more than two redox states are electrochemically available, the electrochromic material may exhibit several colours and be termed polyelectrochromic. Likely applications of electrochromic materials include their use in controllable light-reflective or light-transmissive devices for optical information and storage, antiglare car rear-view mirrors, sunglasses, protective eyewear for the military, controllable aircraft canopies, glare-reduction systems for offices, and ‘smart windows’ for use in cars and in buildings.
Prussian blue Prussian blue (PB; iron(III) hexacyanoferrate(II)) thin films can be switched to Prussian white (PW) on electrochemical reduction and to Prussian yellow (PX) on oxidation via the partially oxidized Prussian green (PG). For all these electrochromic redox reactions, there is concomitant ion ingress/ egress in the films for electroneutrality. The spectra of PX, PG, PB and PW are shown in Figure 14, together with two intermediate states between blue and green. The intense blue colour in the [FeIIIFeII(CN)6] chromophore of PB is due to an intervalence charge-transfer (CT) absorption band centred at 690 nm. The yellow absorption band in PX corresponds with that of [FeIIIFeIII(CN)6] in solution, both maxima (lmax¼425 nm) coinciding with the (weaker) [FeIII(CN)6] 3 absorption maximum. On increase from þ0.50 V vs SCE to more oxidizing potentials, the original PB peak shifts continuously to longer wavelengths with diminishing absorption, while the peak at 425 nm steadily increases, owing to the increasing
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Figure 14 Spectra of iron hexacyanoferrate films on ITO-coated glass at various potentials [(i) þ0.50 V (PB, blue); (i) 0.20 V (PW, transparent); (iii) þ0.80 V (PG, green); (iv) þ0.85 V (PG, green); (v) þ0.90 V (PG, green); (vi) þ1.20 V (PX, yellow)] vs SCE with 0.2 M KCl þ0.01 M HCl as supporting electrolyte. Reproduced with permission from Mortimer RJ and Rosseinsky DR (1984) Journal of the Chemical Society, Dalton Transactions, 2059–2061.
[FeIIIFeIII(CN)6] absorption. The reduction of PB to PW is by contrast abrupt, with transformation to all PW or all PB without pause, depending on the potential that is set. In the cyclic voltammogram of a PB-modified electrode, the broad peak for PBÐPX in contrast with the sharp PBÐPW transition emphasizes the range of compositions involved. This difference in behaviour, supported by ellipsometric measurements, indicates continuous mixed-valence compositions over the blueto-yellow range in contrast with the presumably immiscible PB and PW, which clearly transform one into the other without intermediacy of composition.
Conducting polymers Chemical or electrochemical oxidation of numerous resonance-stabilized aromatic molecules including pyrrole, thiophene, aniline, furan, carbazole, azulene and indole produces electronically conducting polymers. In their oxidized forms, such conducting polymers are ‘doped’ with counteranions (p-doping) and possess a delocalized p electron band structure, the energy gap between the highest occupied p electron band (valence band) and the lowest unoccupied band (the conduction band) determining the intrinsic optical properties of these materials. The doping process (oxidation) introduces polarons (in polypyrrole, for example, these are radical cations delocalized over ca. four monomer units), which are the major charge-carriers. Reduction of conducting polymers with concurrent counteranion exit removes the electronic conjugation, to give the ‘undoped’ (neutral) electrically insulating form.
Figure 15 Spectra of poly(m-toluidine) films on ITO in 1 M hydrochloric acid at (a) 0.20 V, (b) þ0.10 V, (c) þ0.20 V, (d) þ0.30 V vs SCE. Reproduced with permission from Mortimer RJ (1995) Journal of Materials Chemistry 5: 969–973.
All conducting polymers are potentially electrochromic in thin-film form, redox switching giving rise to new optical absorption bands in accompaniment with transfer of electrons/counteranions. Good examples are the polymers of aniline, o-toluidine and m-toluidine, which are easily prepared as thin films by electrochemical oxidation from aqueous acid solutions of the appropriate monomer. The electrical and electrochromic properties of such polyanilines depend not only on oxidation state but also on the protonation state, and polyanilines are in fact polyelectrochromic (transparent yellow to green to dark blue to black), the yellow–green transition being durable to repetitive colour switching. Spectra for a poly(m-toluidine)-modified electrode are illustrated in Figure 15. The two low-wavelength spectral bands observed are assigned to an aromatic p–p* transition (330 nm) related to the extent of conjugation between the adjacent rings in the polymer chain, and to radical cations formed in the polymer matrix (440 nm). With an increase in applied potential, the 330 nm band absorbance decreases and the 440 nm increases (Figure 15), the isobestic point indicating that the two species have the same chemical stoichiometry with differences only in electron(s). Beyond þ0.30 V, the conducting region is entered; the 440 nm band decreases as a broad free carrier electron band 800 nm is introduced. Response times for the yellow–green transition following a potential step can be determined using a diode array spectrophotometer (Figure 16).
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Figure 16 Spectra recorded at times indicated after potential switching of poly(m-toluidine) films on ITO in 1 M hydrochloric acid (a) Potential step 0.20 to þ0.40 V vs SCE. (b) Potential step þ0.40 to 0.20 V vs SCE. Reproduced with permission from Mortimer RJ (1995) Journal of Materials Chemistry 5: 969–973.
Viologens in Nafion In addition to being important mediator titrants, 1,10 -disubstituted-4,40 -bipyridiliums (viologens) are a major group of electrochromic materials. Electrochromism occurs in bipyridiliums because, in contrast to the bipyridilium dications, the radical cations formed on electroreduction have a delocalized positive charge, coloration arising from an intramolecular electronic transition. Suitable choice of nitrogen substituents to attain the appropriate molecular orbital energy levels can, in principle, allow colour choice of the radical cation. For short alkyl chain length, 1,10 -dialkyl-4,40 -bipyridiliums, both the dication and radical-cation states, are soluble in water and any electrochromic device (ECD) using such bipyridiliums would have the limitation of a low write–erase efficiency. One solution to this problem involves electrostatic binding of bipyridilium dications into anionic polyelectrolyte films.
When a Nafion-modified electrode is immersed in an aqueous solution of 1,10 -dialkyl-4,40 -bipyridilium (alkyl¼methyl, ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl), the 1,10 -dialkyl-4,40 bipyridilium accumulates in the anionic polyelectrolyte such that its concentration is considerably higher than that in the bulk solution. Figure 17 illustrates the uptake of 1,10 -dimethyl4,40 -bipyridilium into a Nafion film monitored by in situ spectral measurements. The coloured form in each case is purple, from the presence of monomeric (blue, lmax 600 nm) and dimeric (red, lmax 500 nm) viologen radical cations. For such viologen-incorporated Nafion films, the electrochromic response times are in excess of 60 s for both coloration and bleaching and independent of viologen size. Figure 18 shows absorbance spectra measured every 10 s in response to a potential step between the oxidized and reduced forms, for the case of the 1,10 -di-n-hexyl-4,40 -bipyridilium system. The longer
Figure 17 Spectra recorded at 0.90 V vs SCE during the 2nd, 4th, 6th, 8th and 10th cyclic voltammograms for an ITO/Nafion electrode in 0.1 mM 1,10 -dimethyl-4,40 -bipyridilium dichloride þ0.2 M KCI (pH 5.5). The vertical arrows indicate absorbance increase with scan number. For a comparable experiment in the absence of Nafion, the maximum absorbance was <0.01. Reproduced with permission from Mortimer RJ and Dillingham JL (1997) Journal of the Electrochemical Society 144: 1549–1553.
Figure 18 (a) Spectra recorded at t¼0, 10, 20, 30, 40 and 50 s in response to a potential step from þ1.00 to 0.90 V vs SCE for an ITO/Nafion/ 1,10 -di-n-hexyl-4,40 -bipyridilium electrode in 0.1 mM 1,10 -di-n-hexyl-4,40 -bipyridilium dibromide þ0.2 M KCl (pH 5.5). The vertical arrows indicate absorbance increase with time. (b) Spectra recorded at t¼0, 10, 20, 30, 40 and 50 s in response to a potential step from 0.90 to þ1.00 V vs SCE for an ITO/Nafion/1,10 -di-n-hexyl-4,40 -bipyridilium electrode in 0.1 mM 1,10 -di-n-hexyl-4,40 -bipyridilium dibromide þ0.2 M KCl (pH 5.5). The vertical arrows indicate absorbance decrease with time. Reproduced with permission from Mortimer RJ and Dillingham JL (1997) Journal of the Electrochemical Society 144: 1549–1553.
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poration of the methyl viologen system) to an inner layer of PB, is possible (Figure 19). The transparent/purple viologen dication/radical cation electrochromicity operates in the potential region where the PB is in its (reduced) transparent state; the bilayer electrode system thus exhibits yellow/green/ blue/transparent/purple colours.
See also: Ellipsometry; Spectroelectrochemistry, Methods and Instrumentation; UV–Visible Absorption Spectrometers; UV-Visible Absorption Spectroscopy, Biomacromolecular Applications; UV-Visible Absorption Spectroscopy, Dyes and Indicators Applications; UVVisible Absorption Spectroscopy, Organic Applications.
Further Reading
Figure 19 Spectra recorded at þ0.50 V (blue), 0.20 V (transparent) and 0.90 V (purple) vs SCE for an ITO/PB/Nafion/methyl viologen electrode in 0.1 mM 1,10 -dimethyl-4,40 -bipyridilium dichloride þ0.2 M KCl (pH 5.5). Reproduced with permission from Mortimer RJ and Dillingham JL (1997) Journal of the Electrochemical Society 144: 1549–1553.
response time for the oxidation (bleaching) reflects the slower diffusion of radical-cation dimers through the Nafion film compared to that of the monomeric radical cations. Five-colour polyelectrochromicity, by application of an outer Nafion layer (with subsequent electrostatic incor-
Heineman WR and Jensen WB (1989) Spectroelectrochemistry using transparent electrodes – an anecdotal history of the early years. ACS Symposium Series 390: 442–457. Heineman WR and Kissinger PT (1984) Large-amplitude controlled-potential techniques. In: Kissinger PT and Heineman WR (eds.) Laboratory Techniques in Electroanalytical Chemistry. New York: Marcel Dekker Chapter 3. Heineman WR, Hawkridge FM, and Blount HN (1984) Spectroelectrochemistry at optically transparent electrodes. II. Electrodes under thin-layer and semi-infinite diffusion conditions and indirect coulometric titrations. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol. 13, pp. 1–113. New York: Marcel-Dekker. Kuwana T and Heineman WR (1976) Study of electrogenerated reactants using optically transparent electrodes. Accounts of Chemical Research 7: 241–248. Kuwana T and Winograd N (1974) Spectroelectrochemistry at optically transparent electrodes. I. Electrodes under semi-infinite diffusion conditions. In: Bard AJ (ed.) Electroanalytical Chemistry. A Series of Advances, vol. 7, pp. 1–78. New York: Marcel-Dekker. Mortimer RJ (1994) Dynamic processes in polymer modified electrodes. In: Compton RG and Hancock G (eds.) Research in Chemical Kinetics, vol. 2, pp. 261–311. Amsterdam: Elsevier. Mortimer RJ (1997) Electrochromic materials. Chemical Society Reviews 26: 147–156. Murray RW (1984) Chemically modified electrodes. In: Bard AJ (ed.) Electroanalytical Chemistry. A series of Advances, vol. 13, p. 191. New York: Marcel-Dekker –3\68. Niu J and Dong S (1996) Transmission spectroelectrochemistry. Reviews in Analytical Chemistry 15: 1–171.