J. Phys. Chem. 1994,98, 270-273
270
Spectrofluorimetric Hydrodynamic Voltammetry: The Investigation of Electrode Reaction Mechanisms Richard G. Compton' and R. Geoffrey Wellington Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, U.K. Received: July 16, 1993; In Final Form: October 12, 19938
Spectrofluorimetric hydrodynamic voltammetry (SFV) is used to study the electroreduction of crystal violet in acetonitrile solution. The SFV technique is shown to have high sensitivity and to complement conventional electrochemical experiments through its identification of leucocrystal violet as an electrolysis product generated in low concentration in addition to the radical species formed as a one-electron adduct of the parent compound and previously thought to be produced exclusively. Mechanistic SFV studies show that the leucocrystal violet is formed via an EC process for which kinetic parameters are reported.
Introduction
We have recently introduced1 an experimental approach to the characterizationand estimationof intermediatesin electrolysis reactions by means of fluorescence spectroscopy. In particular the high sensitivity and selectivity of the latter technique may be exploited by locating a specially designed channel electrode flow cell inside a spectrofluorimeter. The practical implications and requirements of this were detailed previously,' and it was shown that in situ fluorescence measurements could be performed with high sensitivity and were able to reveal the existenceof otherwise unsuspected intermediates in electrochemical processes. The flow cell designed for spectrofluorimetric voltammetry employs a well-defined hydrodynamicregime. This can facilitate the quantitative study of fluorescent solution-phase electrogenerated species, since the spatial and temporal distribution of the species within the flow cell can be modeled for any suggested reaction mechanism and so the fluorescence intensity can be related to the electrode current. In particular, measurement of (a) the intensity and current under steady-state conditions as a function of the rate of mass transport (flow rate) within the cell and (b) the time evolution of the fluorescence signal after the electrode potential is stepped should permit the deduction of electrodereaction mechanisms and kinetics. The theoretical basis of such investigationswas described previously;' the purpose of this paper is the experimental realization of spectrofluorimetric hydrodynamicvoltammetry (SFV) in the elucidationof electrode processes with coupled homogeneous kinetics.
Experimental Seetion The SFV flow cell previously described' was used for all quantitative work. This comprises a demountable channel electrode constructed in optical quality synthetic silica (Suprasil 2) by Heraeus Silica and Metals Ltd., Weybridge, U.K. The electrode is a rectangular foil (ca. 4 mm X 4 mm), of platinum (Goodfellow Advanced Materials, purity 99.9596, thickness 0.025 mm), cemented onto a silica cover plate and polished flat. Electrical connection is made to the rear of the electrode via a hole through the cover plate and the cell assembled as previously described. The completecell is supported within a Perkin-Elmer front surface accessory (Perkin-Elmer Part 5212 3 130) and used in conjunction with a Perkin-Elmer Model LS50 luminescence spectrometer. The cell position inside the latter could be finely adjusted for optimal sensitivity with the front surface accessory so that the incidentexciting beam interrogated a length of solution of 9 mm X 1 mm immediately downstream of the electrode surface. *Abstract published in Aduunce ACS Absrru~u,December 1, 1993.
0022-3654/94/2098-0270$04.50/0
The width of this beam (1 mm) was significantly narrower than the electrode width (ca. 4 mm) and was positioned centrally with respect to the latter. Any resulting luminescence is detected in a direction perpendicular to the incident light. The flow cell was incorporated into a gravity-fedall-glass flow system capable of delivering flow rates in the range 10--10-1 om3 s-l. A platinum gauze counter electrode was placed downstreamof the working electrode,outsideof the spectrometer, and a reference electrode (saturated calomel or pseudo-silver) was located upstream. The potential of the silver wire pseudoreference electrode was found to be steady throughout the period of our experiments: consistent values for the oxidation potential of N,N,N',N'-tetramethyl- 1,4-~henylenediaminewere observed. Henceall potentials are reported relative to the saturatedcalomel electrode, +0.20 V being subtracted from the measured values to obtain the potentials quoted. Electrochemical measurements employed a potentiostat and scan generator (Oxford Electrodes, Oxford, U.K.) modified to boost the counter electrode voltage (up to 200 V).* The LS50 luminescence spectrometer was controlledby an Epson PC-AX2e personal computer, and the system was capable of measuring luminescence induced by excitation wavelengths between 200 and 800 nm. Complementary rotating disc measurements were conducted using Oxford Electrodes equipment. Dihydrofluorescein diacetate (DFDA; 97%), fluorescein diacetate (FDA, 98%), fluorescein (F;98%), crystal violet (CV+, 96%). and leucocrystal violet (LCV; 97%) were used as received from Aldrich. Typically, experimental solutions were made up to the desired concentration in dried3 acetonitrile (Fisons, dried, distilled) solution containing 0.1 mol dm-3 (recrystallized) tetrabutylammonium perchlorate (TBAP Fluka, purum) as supporting electrolyte. Solutions were purged of oxygen by outgassing with prepurified argon prior to electrolysis. Computer programs to model the fluorescence intensity/ electrode current/solution flow rate data were written in FORTRAN 77 and executed on the VAX/VMS cluster machines of Oxford University or on a Sun SPARC IPC workstation. Results and Discussion
We consider two chemical systems both based in acetonitrile solution containing 0.1 M TBAP supporting electrolyte which were studied using platinum electrodes. The first is concerned with the oxidation of dihydrofluoreqceindiacetate (DFDA),which is shown to be a simple process free from complications due to coupled homogeneous kinetics and which is used to verify the theoretical modeling' relating the fluorescence intensity to the solution flow rate and the electrode current. The second is concerned with the reduction of the dye crystal violet (CV+)0 1994 American Chemical Society
Electrode Reaction Mechanisms
The Journal of Physical Chemistry, Vol. 98, No. 1 , 1994 271
DFDA
which is found to proceed with the generation of leucocrystal
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Figure 1. Excitation and emission spectra of the electrochemical product of the oxidation of DFDA as determined by SFV experiments.
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species formed as a one-electron adduct of the parent compound previously thought to be produced exclusively. We consider first the DFDA system. Initial experiments utilized a platinum rotating disk electrode to oxidize the parent molecule. An oxidation wave was observed with a halfwave potential of +1.95 V (vs SCE). Analysis of the rotation speed dependence of the transport-limited current4 suggested that the proccas was overall a two-electron oxidationand that the diffusion coefficient of DFDA was 0.94 f 0.02 X It5cm2 s-l. The parent molecule was found to be nonfluorescent when excited at wavelengths between 200 and 700 nm. In contrast exploratory SFV experiments showed the product of the twoelectron oxidation to absorb with a maximum at 435 nm and to fluoresce at 469 nm. The excitation and emission spectra are shown in Figure 1. Consideration of the electrochemical and SFV data in parallel and in light of the known fluorescent properties of fluorescein and its derivatives536 suggested that the electrode process under examination involved the loss of two electrons and one proton as shown below. The process is thus of the simple A B type with no apparent kinetic complications.
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Figure2. Variation of the fluorescencesignal resulting from the oxidation of DFDA with flow rate for two representative conditions: (a, top) [DFDA] = 0.406 mM, electrode length = 0.413 cm, electrodewidth = 0.524 cm, cell depth = 0.036 cm; (b, bottom) [DFDA] = 0.26 mM, electrode length = 0.605 cm, electrode width = 0.482 cm, a l l depth = 0.038 cm. In each case the solid line drawn is that predicted theoretically as described in the text using a cell sensitivity parameter, S*, of 160.’
cm-1 and 4 = 0.01, and knowledge of these two constants is used in the modeling of the experimental results as described below. The value of the quantum yield was deduced from direct fluorimetric comparison of the species under investigation with the known fluorescencestandards 9,lO-diphenylanthraceneand 7-phenylbenzo[klfluoranthene;both of these have quantum yields of unity.7-8 Quantitative SFV experiments were next conducted using a platinum channel electrode, and the variation of fluorescence signal intensity with flow rate was measured with the electrode held at a potentialcorresponding to the transport-limitedoxidation of DFDA. Experiments were performed with a range of DFDA concentrationsbetweenO.l and0.7 mM. Threedifferent electrode sizes were also used with electrode lengths between 0.413 and 0.605 cm and widths ranging from 0.436 to 0.524 cm. The resultingdata was modeled using the theory presented previously1 for a simple oxidation process, taking into account the cell and electrode geometriesused and assuming that the product (B)had the same diffusioncoefficient as the precursor, DFDA (A). Good
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Figure 3. Transient signal showing the variation of the fluorescence intensity with time following a potential step at the working electrode between potentials corresponding to no current flow and to the transportlimited oxidation of DFDA. The experimental conditions were as follows: [DFDA]= 0.26 mM, electrode length = 0.605 cm, electrode width = 0.482 cm, cell depth = 0.038 cm, electrolyte flow rate = cm3s-I. The solid line drawn is that predicted theoretically as described in the text using a cell sensitivity parameter, S*, of 160.'
agreement was found under all conditions, and two examples of the variation of the fluorescence signal intensity with flow rate are shown in Figure 2. Excellent agreement between experiment and theory is apparent. As a further check on the flow cell and theoretical modeling, a measurement was made of the transient fluorescence signals at different flow rates induced by potential steps on the working electrode from a value corresponding to no current flow to a potential where the oxidation of DFDA is transport-limited. An example of such a transient response is shown in Figure 3 together with the theoretically predicted transient' for a simple redox event, and again the good agreement between experiment and theory is highlighted and is representative of all transients measured. The DFDA system demonstrates the validity of the experimental and theoretical approaches adopted in the SFV work. We turn next to the CV+ system, in which the electrode reaction is revealed to be coupled to previously unsuspected homogeneous kinetics. The reduction of CV+ in acetonitrile/O.l M TBAP was first investigated using a platinum rotating disk electrode. A oneelectron reversible reduction was observed at -0.59 V (vs SCE), in agreement with previous reports. A plot of the transportlimited current against the square root of rotation speed produced an excellent straight line and enabled the deduction of 1.3 X 10-5 cm2 s-I for the diffusion coefficient of CV+.4 ESR experiments have shown the product of the reduction is the radical CV.9 NMe2 I
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Cyclic voltammetry experiments showed that if the electrode potential was swept positive after severalcycles through theCV+/ CV' reduction peak, a tiny oxidative peak, otherwise absent, was observed at +0.91 V (vs SCE). This was attributed to the oxidation of leucocrystal violet (LCV). This suggestion was confirmed by direct voltammetry on solutions of authentic samples of LCV. It follows that small amounts of LCV are produced during the reduction of CV+. Next, preliminarySFV experimentswere undertaken to search for any luminescent species arising from the electrochemistry of CV+. A small signal was observed with an excitation maximum at 268 nm and an emission maximum at 360 nm. This is
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Figure4. Excitation and emission spectra of (a, top) the reduction products of CV+ and (b, bottom) an authentic sample of LCV.
attributable to LCV;'OJ1 Figure 4 shows the fluorescence signal seenvia SFV analysis of CV+ reduction and that from an authentic sample of LCV. The close similarity of the two spectra is apparent, and this also confirms that LCV is produced thermally from CV' and not photochemically;otherwise the electrochemicallygenerated spectrum would be increasingly distortedas the wavelength was increased toward 406 nm, where the crystal violet radical is known to absorb light reactively to give the leuco form.9 The electrode process can be summarized as shown in Scheme 1 and is an EC-type process with TBAP the likely source of protons for the C - ~ t e p .Measurements ~ (as above) made on the authentic sample of LCV gave an extinction coefficient of c = 50 900 dm3 mol-' cm-' (at 268 nm) and a fluorescence quentum yield of 4 = 0.012. Quantitative SFV experiments using a platinum channel electrode were next employed to find the variation of the fluorescence signal intensity due to LCV with the solution flow rate under conditionswhere the electrode was held at a potential corresponding to the transport-limited reduction of CV+. Experiments were performed using two electrode sizes (electrode length = 0.561 and 0.605cm; electrode width = 0.436 and 0.482 cm) with a range of CV+ concentrations between 0.09 and 0.50 mM. In addition to steady-state measurements, transient fluorescent responses were recorded when the working electrode potential was stepped between values correspondingto no current flow and to the transport-limited current for CV+ reduction. The data resulting from both sets of experimentswere modeled on the assumption of an EC process on the basis of previously developed theory'J2 using the measured cell and electrode geometries assuming that LCV and Cv'have diffusioncoefficients similar to that of CV+. Figure 5 shows the comparison between theory and the steady-state experiments for two different but representative experimental conditions when a rate constant of kl = (4.00f 0.03) X 10-4 s-l was assumed to compute the EC response. Good agreement is apparent, and this was typical of all the experimental data recorded under all geometries and for all substrate concentrationsexperimentallyinvestigatedmodeled using an EC process in the manner described with the rate constant specified. Figure 6shows a representativesignal intensityresponse as the potential is stepped to one which drives a transport-limited current for the system. The solid line shown represents that predicted theoretically1J2using exactly the same parameters as used to model the steady-state data. This level of agreement was
The Journal of Physical Chemistry, Vol. 98, No. I, 1994 213
Electrode Reaction Mechanisms
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Figure 6. Transient signal showing the variation of the fluorescence intensity with time following a potential step at the working electrode between potentialscorresponding to no current flow and to the transportlimited reduction of CVt.The experimental conditions were as follows: [CV+] = 0.30 mM, electrodelength = 0.561 cm, electrode width = 0.436 cm, cell depth = 0.032 cm, electrolyte flow rate = 0.0025 cm3s-'. The solid line drawn is that predicted theoretically as described in the text.
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Figures. Variation of the fluorescencesignalresulting from the reduction of CVt with flow rate for two representativeconditions: (a, top) [CV+] = 0.15mM, electrode length = 0.561 cm, electrode width = 0.436 cm, cell depth = 0.03 cm; (b, bottom) [CV+]= 0.208 mM, electrodelength = 0.605 cm, e lectrode width = 0.482 cm, cell depth = 0.036 cm. In each case the solid line drawn is that predicted theoretically as described in the text for an EC process using a cell sensitivity parameter, S*,of 70.1
typical of transients recorded a t all flow rates studied. The excellent independent agreement between theory and experiment vindicates both the mechanism and kinetics deduced for the electrode reaction. Conclusions We have demonstrated using quantitative SFV that the reduction of CV+ in acetonitrile solution proceeds via an EC mechanism. The high sensitivity of the SFV technique has been highlighted by this example. The leucocrystal violet electrogenerated via this route is a minor electrode reaction product in comparison with the directly formed radical Cv' product, as evidenced by both thevery small intensity of fluorescence observed (even though the values of c and 4 are quite high) and the low
first-order rate constant deduced for the conversion of CV' into LCV. Theoretical modeling showed that the mean concentration of the luminescent species throughout the sensitive part of the flow cell was less than ca. 1 pM yet could be easily detected and analyzed using the protocol presented to provide mechanistic and kinetic information. The technique of SFV should therefore complement conventional electrochemical methods for studying electrode reaction mechanisms in which electron transfer is coupled to homogeneous kinetic events.
References and Notes (1) Compton, R. G.; Fisher, A. C.; Wellington, R. G.; Winkler, J. J. Phys. Chem. 1992, 96, 8153. (2) Compton, R. G.; Waller, A. M . J . Electroanal. Chem. 1985, 195, 289.
( 3 ) Compton, R. G.; Barghout, R.; Eklund, J. C.; Fisher, A. C. Electroanalysis, in press. (4) Albery, W.J. Electrode Kinetics; Clarendon Press: Oxford, U.K., 1975; p 57. (5) King,A.T.;Lowe,K.C.;Mulligan,B. J.Biotech.Lett. 1989,10,873. ( 6 ) Chen, S.; Nakamura, H.; Tamura, Z . Chem. Pharm. Bull. 1979,27, 475. (7)
Birks,J. B. PhotophysicsofAromatic Molecules;Wiley-Intersciencc; Chichester, U.K., 1970. ( 8 ) Ginter, H.; Heinrich, G. J . Phorochem. 1982, 18, 9.
(9) Compton, R.G.; Coles, B. A.; Stearn, G. M.; Waller, A. M.J . Chem. Soc., Faraday Trans. 1 1988, 84, 2351. (10) Lueck, H.; McHale, J.; Edwards, W. J. Am. Chem. SOC.1992, 114, 2342.
(1 1) Kitamura, N.; Kawasaki, Y.; Tazuke, S . J. Photochem. Phorobiol, A: Chem. 1987, 40, 93. (12) Fisher, A. C.; Compton, R. G. J. Phys. Chem. 1991, 95, 7538.
