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Langmuir 1997, 13, 4892-4897
Langmuir and Langmuir-Blodgett Films of Amphiphilic and Nonamphiphilic TTF Derivatives and Their Mixtures Pilar Cea, Carlos Lafuente, Jose S. Urieta, Marı´a C. Lo´pez, and Fe´lix M. Royo* Departamento de Quı´mica Orga´ nica-Quı´mica Fı´sica, Facultad de Ciencias, Plaza de San Francisco, Ciudad Universitaria, 50009 Zaragoza, Spain Received April 28, 1997. In Final Form: July 1, 1997X We have studied the ability of two amphiphilic TTF derivatives to form both Langmuir films and Langmuir-Blodgett films under different experimental conditions. It has been found that these substances are capable of forming stable monolayers at the air-water interface and building up ordered LB films yielding Y-type LB films of high quality that were characterized by UV-vis and SEM having shown a reversibility behavior in the iodine doping process. Another nonamphiphilic TTF derivative, which does not form real monolayers at the air-water interface has been mixed with the amphiphilic TTF derivatives obtaining mixed stable monolayers and ordered LB films that show a conductive behavior after a doping process with iodine. Cyclic voltammetry studies of the three TTF derivatives and their mixtures have been performed.
Introduction Much interest has been centred on the application of the Langmuir-Blodgett (LB) technique to obtain conductive organic films due to the potential of this technique to build up mono- or multilayers in which the order at the molecular level can be controlled, offering exciting possibilities for applications such as the formation of ultrathin modified electrodes with high density of electroactive sites and regularity in molecular organization, as well as sensors, electro- and photoelectrochemical devices and photoelectrochemical storage devices. LB technique also has the advantage of creating films with large-scale order, providing new insights on electron transfer reactions at interfaces. Organic conductors can be formed from the combination of planar aromatic or heteroaromatic electron donors (e.g. tetratiafulvalene, tetrathiatetracenes, ...) and electron acceptors (tetracianoquinodimethane, metal-bis(dithiolate), ...). The synthesis of new organochalcogen donors has remained at the forefront of research with a few systems. In attempt to prepare organic conducting materials, properties, characterisation, and applications of the TTF and its derivatives have been studied in a systematic way.1-3 LB films of conjugated materials can exhibit increased electrical conductivity in their partially oxidized or reduced state. Only few as-deposited films are directly conductive. So, in most of cases, an ulterior treatment is necessary to obtain conductive films. Two methods can be used in order to obtain films in the conductive state:4 (i) chemical doping by oxidising reagents (e.g. iodine vapor). (ii) electrochemical methods. The first one limits the range of anions that can be inserted into the multilayer matrix and induces a disordered structure and ulterior reorganisations in the film structure. However, electrochemical methods can perform this conversion in a more controllable manner because the level of oxidation can be regulated by the potential and the charge. Good conductivity mixed films have been obtained with the ethylenedithiodioctadecylthio-tetrathiafulvalene, * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Lozac’h, N.; Stavaux, M. Adv. Heterocyl. Chem. 1980, 27, 151. (2) Hansen, T. K.; Becher, J. Adv. Mater. 1993, 5, 288. (3) Adam, M.; Mu¨llen, K. Adv. Mater. 1994, 6, 439. (4) Goldenberg, L. M. J. Electroanal. Chem. 1994, 379, 3.
S0743-7463(97)00431-9 CCC: $14.00
a
b
c Figure 1. (a) EDTTTF(SC18H37)2. (b) TTF(SC12H25)2(SC11H22COOH)2. (c) TTF(SC18H37)2(SC3H5O2)2.
EDTTTF(SC18H37)2, molecule5-7 (Figures 1a and 2a). This molecule has no hydrophilic group, giving nonstable monolayers at the air-water interface. As far as we know, this molecule has always been mixed with fatty acids in order to obtain stable Langmuir films and transfer them to a solid substrate. However steric hindrance (due both to the long hydrocarbon chains of the molecule and the (5) Vandevyver, M.; Rouillay, M.; Bourgoin, J. P.; Barraud, A.; Morand, J. P.; Noe¨l, O. J. Colloid Interface Sci. 1991, 141, 459. (6) Vandevyver, M.; Rouillay, M.; Bourgoin, J. P.; Barraud, A.; Gionis, V.; Kakoussis, V. C.; Mousdis, G. A.; Morand, J. P.; Noe¨l, O. J. Phys. Chem. 1991, 95, 246. (7) Dourthe, C.; Izumi, M.; Garrigou-Lagrange, C.; Buffeteau, T.; Desbat, B.; Delhae`s, P. J. Phys. Chem. 1992, 96, 2812.
© 1997 American Chemical Society
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Figure 3. Isotherms of (a) TTFA and (b) TTFE registered at 20 °C. Figure 2. (a) EDTTTF(SC18H37)2. (b) TTF(SC12H25)2(SC11H22COOH)2. (c) TTF(SC18H37)2(SC3H5O2)2 in 3-D.
presence of fatty acid) has been suggested to decrease the potential conducting properties of this compound.6 The following challenge, in the development of conducting LB films of TTF derivatives, should be the use of a stabilizant molecule that does not interfere with the doping process or, even more, supply it. In this paper, going on with our line of work in the field of electroactive films,8-10 we present for the first time the results of a detailed research into the electrochemical properties of the EDTTTF(SC18H37)2 films without using a fatty acid as the stabilizer molecule. The stabilizer molecules used here are other TTF derivatives with hydrophilic groups, the acid derivative, TTF(SC12H25)2(SC11H22COOH)2 (TTFA in an abridged form), and the ester derivative, TTF(SC18H37)2(SC3H5O2)2 (briefly TTFE), (Figures 1b,c, and 2b,c). These amphiphilic molecules are supposed to form real monolayers at the air water interface, and might be able to form mixed stable monolayers when are blended with the EDTTTF(SC18H37)2 molecule. The molecules chosen for this study have two different hydrophilic groups (acid and ester) and quite different hydrophobic chains to compare their effects on the stability of the mixed monolayers, the transference process, the structure of the films, the doping process, and the final conducting state. Experimental Section The synthesis of the TTF derivatives was carried out following the method described by Saito11 et al.12 Solutions of TTF derivatives were prepared in chloroform. This was HPLC grade (99.9%) purchased from Aldrich. The solutions were kept in dark bottles wrapped with aluminium foil in a refrigerator. In spite of this, the solutions were used as fresh as possible. This study has been performed with a teflon trough (460 × 210 mm2) designed by us and whose details have been reported before.13 The water used for the subphase was purified by deionization and then by passing it through a Millipore Milli-Q purification system. The resistivity of the water was 18.2 MΩ cm and its pH was 5.8. Some of the experiments required the use of KI3 in the subphase. The KI and I2 used in its preparation
were of the best grade commercially available. The surface pressure of the monolayers was measured with a Wilhelmy paper plate pressure sensor, and its behavior was checked with the π-A curve of the docosanoic acid monolayer. Blank experiments were performed by spreading pure chloroform on the water surface. The uncertainty in the area per molecule obtained from the isotherms is about (5%. With respect to the procedure, first, the solutions were spread on the aqueous surface of the trough allowing the solvent of each drop to evaporate before spreading the next one. In spite of this, after the spreading process we have waited 15 min more to allow the solvent that could remain to evaporate completely. Then the compression slowly began at a controlled sweeping speed of 0.5 Å2/molecule minute. Monolayer stability was investigated by monitoring the decrease in the area per molecule at constant surface pressure. The substrates (quartz and glass for the UV) spectroscopy and electron microscopy, respectively) were cleaned carefully as was previously described.10 The monolayers were transferred onto the substrates by the vertical dipping method. The hydrophilic substrates were initially immersed in the water subphase. The dipping speed was 0.2 cm/min for the first transference and 0.6 cm/min for the following ones. The transfer ratio of the monolayers was calculated from the ratio of the area of the substrate coated with the monolayer to the actual decrease in the surface area of the floating monolayer at the air-water interface. The decrease in molecular area due to the monolayer instability was also taken into account. The UV-vis spectra were acquired on an UVIKON 941 double beam spectrophotometer. The solution spectra were collected with a 1-cm path length quartz cell. The spectra of LB films were registered with a normal incident angle with respect to the film plane. Scanning electron microscopy, SEM, was performed by means of a JEOL JSM 6400 microscope. The doping iodination process was carried out dipping the films into an atmosphere saturated with iodine vapor during some minutes. Afterward, the films were withdrawn from the vessel containing the iodine. Then the films were allowed to evolve at the air or were heated at 40 °C (increasing the process speed) to achieve the conducting phase. Conductivity studies in the film plane were undertaken using a two-probe technique. The resistance between the two wires was measured with a Hewlett Packard 4329 A high resistance meter.
Results and Discussion (8) Morand, J. P.; Brzezinski, L.; Lo´pez, M. C. Thin Solid Films. 1992, 280, 210. (9) Cea, P.; Lafuente, C.; Urieta, J. S.; Lo´pez, M. C.; Royo, F. M. Langmuir 1996, 12, 5881. (10) Cea, P.; Morand, J. P.; Urieta, J. S.; Lo´pez, M. C.; Royo, F. M. Langmuir. 1996, 12, 1541. (11) Saito, G. Pure Appl. Chem. 1987, 59, 999. (12) Saito, G.; Imaeda, K.; Shi, Z.; Mori, T.; Enoki, T.; Inokuchi, A. Chem. Lett. 1986, 441. (13) Royo, F. M.; Lo´pez, M. C.; Ruiz, B.; Camacho, A.; Lozano, J. M.; Urieta, J. Rev. Acad. Cienc. Exactas, Fis., Quim. Nat. Zaragoza. 1993, 48, 177.
Langmuir Films. The surface pressure-area isotherms (Figure 3) of TTFA and TTFE were found to be independent upon both the concentration of the spreading solution (changing between 10-4 and 3 × 10-4 M) and the initial volume (between 0.2 and 1mL). The areas per molecule at 30 mN/m were 51 and 52 Å2/molecule for TTFA and TTFE respectively. These values are slightly greater than the area of two alkyl chains (in theory 20 Å2/alkyl chain; that is, 40 Å2/molecule). We can interpret this fact
4894 Langmuir, Vol. 13, No. 18, 1997
taking into account that these molecules are tilted with respect to the air-water interface because the TTF ring and the alkyl chains are not coplanar (Figure 2). Besides, in the TTFE there is a steric hindrance that comes from the methyl groups attached to the ester group.6 About the stability of these Langmuir films, we have studied the decrease with time in molecular area at the constant pressure of 30 mN/m. From these studies several conclusions have been obtained: (i) The stability of the monolayer (both in TTFA and TTFE) strongly depends on how the spreading process has been performed. A great improvement in the stability is achieved if we allow the solvent of each drop to evaporate before spreading the next one. The best results have been obtained for an spreading time of about 20-25 min (working with 0.7 mL of a solution 10-4 M). (ii) The stability of the monolayer increases with the decrease of the subphase temperature. For example, the time for a decrease of 10% in the area per molecule of a monolayer of TTFE at 30 mN/m is 3 min at 23 °C, 7 min at 20 °C, and 18 min at 14 °C. (iii) The stability of the TTFA monolayers is slightly greater than that of the TTFE monolayers in the same experimental conditions. This is a reasonable result due to the greater hydrophilic character of the acidic group. EDTTTF(SC18H37)2 is a nonamphiphilic molecule and its monolayers are not stable at the air-water interface. This phenomenon might be the reason of achieving nonreproducible isotherms, which are dependent on both the concentration and the volume of the expanded solution as has been reported before for this6 and other similar molecules.10 This has been explained arguing that aggregates and multilayers are formed at the air-water interface instead of true monolayers because of the interactions among the film forming molecules themselves are much more important than those between the EDTTTF(SC18H37)2 and the water molecules due to the non-hydrophilic features of EDTTTF(SC18H37)2. However, it has been verified experimentally that the addition of another molecule that forms stable Langmuir films leads to mixed stable monolayers. Mixed Langmuir films of EDTTTF(SC18H37)2 and fatty acids have been obtained at several ratios.7 But no references have been found about the mixture of EDTTTF(SC18H37)2 with other TTF derivatives. The surface pressure (π) vs area per molecule (A) isotherms for mixed EDTTTF(SC18H37)2 + TTFA and EDTTTF(SC18H37)2 + TTFE in several molar fractions have been registered under steady state conditions. It does not show any plateau which could be related to a 2D to 3D phase transition. Dourthe et al.7 reported the molecular area at 30 mN/m for the EDTTTF(SC18H37)2 as 40 Å2/molecule (this result was obtained from extrapolated values of the area per molecule of EDTTTF(SC18H37)2 + behenic acid, measured at several molar ratios). Our isotherms are in agreement with this result (Figure 4), indicating that a monolayer (and not multilayers) is formed at the air-water interface at least for Xm(EDTTTF(SC18H37)2) < 0.5. The compressed monolayers were quite stable even for molar fractions of EDTTTF(SC18H37)2 of 0.8 and 0.7 for the mixtures with TTFA and TTFE, respectively. Langmuir-Blodgett Films. Both monolayers, TTFA and TTFE, can be transferred onto a hydrophilic substrate, initially immersed in the water subphase, achieving an Y-type deposition with a transfer ratio of 1 and 0.9 respectively. The films had an homogeneous and uniform appearance to the naked eye. Mixtures of EDTTTF(SC18H37)2 with TTFA and TTFE were transferred onto
Cea et al.
Figure 4. Area per molecule in the mixed monolayers at π ) 30mN/m vs Xm: (a) TTFA + EDTTTF(SC18H37)2; (b) TTFE + EDTTTF(SC18H37)2 registered at 20 °C.
hydrophilic substrates in several molar ratios obtaining good-quality Y-type films. The films were yellowish. It has been observed that the quality of the films is also highly dependent on the quality of the monolayer, which depends on both how the molecules have been spread on the aqueous surface and the subphase temperature. The final stability of the compressed monolayers increases if during the spreading process the solvent of each drop is allowed to evaporate before adding the following one. The subphase temperature was lower than 18 °C (because higher temperatures decreases both the monolayer stability and the transfer ratio, obtaining films with a patching appearance). The quality of the films is much better if hydrophilic substrates are used, because hydrophobic substrates yields also to patched films with lower transference ratios. With this careful working method we have been able to obtain both stable monolayers at the air-water interface and high quality LB films with constant transfer ratio in successive layers (>50) using TTFA, TTFE, and their mixtures with EDTTTF(SC18H37)2. Those attainments are remarkable especially if we bear in mind that up till now and according to the previous reports6 a fatty acid was necessary to stabilize the Langmuir films of TTFE and to improve the transference process. In these mixtures, a mixed monolayer of TTFE + EDTTTF(SC18H37)2 with a molar fraction as high as 0.8
Langmuir and Langmuir-Blodgett Films
Figure 5. UV-vis spectra of a TTFA film: (a) before the doping process; (b) just after the doping process; (c-e) measurements were taken each 30 min.
Langmuir, Vol. 13, No. 18, 1997 4895
Figure 6. UV-vis spectra of a TTFA + EDTTTF(SC18H37)2 film: fine line 10 min and thick line 30 min in iodine atmosphere.
a
in EDTTTF(SC18H37)2 has been deposited with a transference ratio of 0.8. UV-vis Characterization. The films of TTFA, TTFE, and their mixtures with EDTTTF(SC18H37)2 have been characterized with UV-vis spectra. The initial films show bands that are red shifted compared with the solution spectra (about 5 nm). The bands at 268 and 322 nm can be assigned to the lowest π-π* intramolecular transitions of TTF unit, and the band at 408 can be assigned to the π-π* intramolecular electronic transition of chargetransfer absorption.14 These films have been doped with iodine vapor. Upon this treatment, remarkable changes in the UV-vis spectra have happened. The absorption intensity increases, and the new absorption bands, which are assigned to the isolated I3- species,15-17 appears at 298 and 380 nm (Figure 5). In the cases of pure TTFA and TTFE films, a reversible process takes place and the spectra return to the original ones. This behavior might be interesting in applications as sensor molecular devices in order to detect molecules capable of oxidizing the TTFA and TTFE in a reversible way. Mixed films of TTFA + EDTTTF(SC18H37)2 and TTFE + EDTTTF(SC18H37)2 were also studied before and after the iodination process. In these cases the process is not reversible, and quite similar results have been obtained in both mixed films. Before the iodination process the films were yellowish. After some minutes in iodine vapor the films turn into dark brown color and bands at 298 and 380 appear. It has been proved that the exposure time to iodine vapor is not a critical parameter. Figure 6 shows a TTFA + EDTTTF(SC18H37)2 LB film doped during 10 and 30 min. As can be seen, no great difference exists between them. This film was exposed to iodine vapor during 18 h obtaining the same spectra as that with only 30 min of exposure. If afterward the films are heated at 40 °C or they are allowed to evolve at air, they turn into a violet color. The 298 and 380 nm bands vanish, and finally a broad absorption peak with a maximum at 521 and a shoulder at 610 nm appears (Figure 7). These bands have been previously attributed to the ions I3- ordered in a linear chain.17,18
Figure 7. (a) UV-vis spectra of a mixed film of TTFA + EDTTTF(SC18H37)2 Xm ) 0.5. curves: (a) just after doping; (b, c) measurements taken each 30 min; (d-g) measurements taken each 60 min; (h) final conductive state after 36 h. (b) Enlargement of Part a in the 470-670 nm region in order to make the bands more visible.
(14) Torrance, J. B.; Scott, B. A.; Welber, B.; Kaufman, F. B.; Seidem, P. Phys. Rev. 1979, B19, 730. (15) Reddy, J. M.; Knox, K.; Robin, M. B. J. Chem. Phys. 1963, 40, 1082. (16) Robin, M. B. J. Chem. Phys. 1964, 40, 3369. (17) Marks, T. J.; Kalina, D. W. Extended Linear Chain Compounds; Miller, J. M., Ed.; Plenum Press: New York, 1979; Vol. 1. (18) Mizumo, M.; Tanaka, M.; Harada I. J. Phys. Chem. 1981, 85, 1789.
The UV-vis spectrum evolution is marked by a welldefined isobestic point at 500 nm. This proves that we are dealing with a simple chemical reaction. However, this isobestic point does not exist in the spectra registered during the first hour after the doping process, and it is evident that the decrease in intensity of 298 and 380 nm bands is much more important at the beginning of film
b
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exposure at the air than that observed after the first hour. This phenomenon can be interpreted considering the elimination of weakly bound iodine molecules during the first minutes after taking the film out of the doping vessel. We followed another doping method consisting in using a 10-3 M aqueous solution of KI3 in the subphase. The color of the films was violet, that is, the conductive phase was directly obtained. The two steps that take place during the iodination process may be explained by the following processes:5,6
3I2 + 2 (TTFderivative0) f 2I3- +2(TTFderivative•+) f 2I3- + (TTFderivative•+)2 (i) 2I3- + (TTFderivative•+)2 f (TTFderivative)2+I3- +3/2 I2 (ii) Conductivity Studies. The conductivity measurements of the LB films were performed using two electrical contacts to the film. The conductivity of the LB film was calculated following the equation:19
σ)
1 d RA
(1)
where σ is the conductivity, R is the resistance, d is the distance between the two contacts, and A is the area through which the electric current passes. By varying the distance between the electrodes, it was established that the contact resistance effect was negligible. It was assumed that the electroactive phase thickness is 10 Å per layer (chemical simulation). No anisotropy of conductivity was found in the film plane. The room-temperature films conductivity values have been found to be dependent upon the number of layers and the molar fraction of EDTTTF(SC18H37)2. The asdeposited LB films were insulators. The conductivity of the films after the iodination process in the violet phase was generally in the range 10-2-10-3 S cm-1. The films doped using KI3 in the subphase and those obtained doping the films after the transference with I2 showed a similar conductivity. Upon storage in air, the conductivity of the doped multilayer films suffered from a gradual drop, giving finally the insulating precursor film (1 week), although they can be doped again giving the same conductivity results. However if they are kept in a closed box, the conducting films remain quite stable for more than one month. The I3- ions, which exist in the first step of iodine treatment (as has been shown in the UV-vis study) do not give rise to the conduction process (insulating film). This is then, indicative that the TTF moiety is responsible for the conduction process. SEM. Figure 8 shows the electron microscopy photographs of TTFA, EDTTTF(SC18H37)2 + TTFA, and EDTTTF(SC18H37)2 + TTFA doped with iodine vapor. The photographs indicate that the films are formed by a matrix of a quite homogeneous surface (smooth background) and some aggregates (needle-shaped crystals in Figure 8a). The films doped with iodine show iodine crystallized on the surface, or perhaps clusters of TTFA + EDTTTF(SC18H37)2+ iodine. (19) Logsdon, P. B.; Prasade, J. P. Synth. Met. 1988, 26, 369.
Figure 8. SEM taken from films: (a) pure TTFA; (b) TTFA + EDTTTF(SC18H37)2, Xm ) 0.5; (c) same as part b but just after the doping process.
CV. TTFA monolayers have been transferred onto ITO (indium tin oxide electrode) substrates at several surface pressures obtaining films with different number of layers in each case. The oxidation and reduction processes yield potentials EpOx ) 0.56 V and EpRed ) 0.44 V (the reference electrode was Ag/AgCl, and the counter electrode was a platinum one) in a 0.1M solution in KCl, respectively (Figure 9). Then ∆Ep)0.120 V. However, in a reversible reaction at 25 °C, ∆Ep)0.059 V according to the bibliography.20 This
Langmuir and Langmuir-Blodgett Films
1.
2.
Figure 9. (1) CV of a four-layer TTFA film transferred at a pressure of 5 mN/m: (a) first cycle; (b) second cycle. (2) CV of a four-layer TTFA film transferred at a pressure of 15 mN/m: (a) first cycle; (b) second cycle.
discrepancy between the experimental and the theoretical value may be interpreted in two ways: (i) A strong irreversible process exists. (ii) The first and the second
Langmuir, Vol. 13, No. 18, 1997 4897
oxidation potential are joined in one, so the interchange is two electrons. This latest possibility has been previously observed in other films of TTF derivatives,21 and might be due to the increase of the interactions between the radical cation and the dication in the film.22 Nevertheless, in our case the first hypothesis is much more probable because during the second cycle both the oxidation and reduction peaks almost disappear as can be seen in Figure 9. This fact demonstrates that a strong irreversible process takes place. If the monolayers are transferred at pressures less of 15 mN/m both the oxidation and the reduction processes are observed, although the reduction process is more evident if the transference pressure is lesser (Figure 9a). At pressures greater than 15mN/m the reduction process can not be observed (Figure 9b). These experimental results suggest a quite irreversible process that avoid the observation of the reduction process. If the order in the layers (increasing pressure transference) increases, the irreversibility of the process also increases. This might be explained if we take into account that when KCl is used as the electrolyte, Cl- ions are expected to move into the LB film during oxidation, and during reduction, Clions move out of the surface layer and K+ ions move into the layer. It is supposed that the ions move relatively easily through small defects and regions of nonideal packing of the alkyl chains. In films obtained at low transference pressure the number of defects and holes are greater than those in monolayers transferred at high pressures. When more than eight monolayers are transferred the intensity both in the oxidation and reduction peaks is almost zero. A high number of layers avoid the oxidation of the TTFA probably due to the steric hindrance because of the long ordered hydrocarbon chains prevent the ions from arriving at the surface electrode. The cyclic voltammetry of TTFE, (TTFA+EDTTTF(SC18H37)2), and (TTFE+ EDTTTF(SC18H37)2) transferred onto ITO substrates has been performed. Similar results have also been obtained in these cases, with irreversible redox processes as explained before for multilayers of TTFA. Acknowledgment. TTF derivatives were kindly provided by Prof. J. P. Morand from the ENSCPB (France). We are grateful for financial assistance from Universidad de Zaragoza (Project No. UZ-96-CIENT-03) and financial assistance from D.G.I.C.Y.T. (Project No. PB 96-0723). P.C. gratefully acknowledges support from Gobierno de La Rioja and a travel grant to visit the ENSCPB laboratories given by La Comisio´n mixta CAI, Consejo Superior de Investigacio´n y Desarrollo (Gobierno de Aragon). LA9704314 (20) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. John Wiley & Sons. 1980. (21) Goldenberg, L. M.; Khodorkowsky, V. Y.; Becker, J. Y.; Lukes, P. J.; Bryce, M. R.; Petty, M. C.; Yarwood, J. Chem. Mater. 1994, 6, 1426. (22) Lee, C.; Bard, A. J. Electroanal. Chem. 1988, 239, 441.
