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On the Adsorption of Extended Viologens at the Electrode|Electrolyte Interface Magdalena Hromadova,*,† Viliam Kolivoska,† Romana Sokolova,† Miroslav Gal,† Lubomı´ r Pospı´ sil,†,‡ and Michal Valasek‡ †
J. Heyrovsk y Institute of Physical Chemistry of ASCR, v.v.i., Dolejskova 3, 18223 Prague, Czech Republic, and Institute of Organic Chemistry and Biochemistry ASCR, v.v.i., Academy of Sciences of the Czech Republic, Flemingovo n. 2, 16610 Prague, Czech Republic
‡
Received August 27, 2010. Revised Manuscript Received September 22, 2010 Extended viologens represent a group of organic molecules intended to be used as molecular wires in molecular electronic devices. Adsorption properties of a novel series of extended viologen molecules were studied at the mercury electrode|electrolyte interface. These compounds form compact monolayers around the potential of zero charge with a constant differential capacitance value of 2.5 ( 0.2 μF cm-2 independent of temperature, length of the molecule, and its bulk concentration. At more negative potentials their reduction in the adsorbed state takes place. We showed that the adsorption process is diffusion controlled and time needed to fully cover the electrode surface is independent of the electrode potential. A modified Koryta equation was employed for the calculation of the surface concentration of the adsorbates leading to the value of 5.3 10-11 mol cm-2 for the shortest wire and to 1.6 10-11 mol cm-2 for the longest one. Based on the space filling model and the differential capacitance value in the compact film region, it was postulated that these molecules lay flat on the electrode surface.
Introduction Extended viologens belong to a group of rigid linear molecules, which are easily dopable with electrons. Such species are often designated as molecular wires, since one of their dimensions (length) greatly exceeds the diameter of the molecule. Structurally simpler dialkyl viologens represent today one of the most frequently studied models of the conducting organic molecular wires.1-7 The electronic properties of the extended viologens have been studied only recently and their potential application in the field of molecular electronics has been scrutinized.8-12 Equally important is the ability of such molecular wires to be anchored in a well-defined manner on the conducting surfaces. In this work, we investigated the adsorption properties of a series of molecular wires containing up to six extended viologen units on the mercury| electrolyte interface. Their molecular structures are shown in the Scheme 1. Molecules containing one repeating unit will be *To whom correspondence should be addressed. E-mail: hromadom@ jh-inst.cas.cz. (1) Haiss, W.; van Zalinge, H.; Higgins, S. J.; Bethell, D.; H€obenreich, H.; Schiffrin, D. J.; Nichols, R. J. J. Am. Chem. Soc. 2003, 125, 15294. (2) Li, Z.; Han, B.; Meszaros, G.; Pobelov, I.; Wandlowski, T.; Blaszczyk, A.; Mayor, M. Faraday Discuss. 2006, 131, 121. (3) Lee, N. S.; Shin, H. K.; Qian, D. J.; Kwon, Y. S. Thin Solid Films 2007, 515, 5163. (4) Pobelov, I.; Li, Z.; Wandlowski, T. J. Am. Chem. Soc. 2008, 130, 16045. (5) Leary, E.; Higgins, S. J.; van Zalinge, H.; Haiss, W.; Nichols, R. J.; Nygaard, S.; Jeppesen, J. O.; Ulstrup, J. J. Am. Chem. Soc. 2008, 130, 12204. (6) Bagrets, A.; Arnold, A.; Evers, F. J. Am. Chem. Soc. 2008, 130, 9013. (7) Wang, Ch.; Batsanov, A. S.; Bryce, M. R.; Martı´ n, S.; Nichols, R. J.; Higgins, S. J.; Garcı´ a-Suarez, V. M.; Lambert, C. J. J. Am. Chem. Soc. 2009, 131, 15647. (8) Valasek, M.; Pecka, J.; Jindrich, J.; Calleja, G.; Craig, P. R.; Michl, J. J. Org. Chem. 2005, 70, 405. (9) Pospı´ sil, L.; Fiedler, J.; Hromadova, M.; Gal, M.; Valasek, M.; Pecka, J.; Michl, J. J. Electrochem. Soc. 2006, 153, E179. (10) Pospı´ sil, L.; Hromadova, M.; Fiedler, J.; Gal, M.; Valasek, M.; Pecka, J.; Michl, J. ECS Trans. 2006, 2, 35. (11) Pospı´ sil, L.; Hromadova, M.; Gal, M.; Valasek, M.; Fanelli, N.; Kolivoska, V. Collect. Czech. Chem. Commun. 2009, 74, 1559. (12) Porter, W. W., III; Vaid, T. P.; Rheingold, A. L. J. Am. Chem. Soc. 2005, 127, 16559.
17232 DOI: 10.1021/la1034239
Scheme 1. Chemical Structure of the Extended Viologen Molecules Containing n = 1-6 Repeating Unitsa
a
Compounds are labeled 1 to 6.
designated as 1, containing two units as 2, containing three as 3, and so on. The adsorption properties of the extended viologens on the electrode/electrolyte interface have not been reported to the best of our knowledge. On the other hand, the adsorption properties of 4,40 -bipyridine have been studied.13-16 The adsorption prewave was observed in the acidic aqueous solutions of 4,40 -bipyridine and was explained by a two-dimensional (2D) condensed phase formation of the doubly protonated reduction product, i.e., of the cation radical. Similar interfacial behavior has been observed for the adsorption of dialkyl viologens on mercury,17-23 gold,24-26 (13) Heyrovsky, M.; Novotny, L. Collect. Czech. Chem. Commun. 1987, 52, 54. (14) Sanchez-Maestre, M.; Rodrı´ guez-Amaro, R.; Mu~noz, E.; Ruiz, J. J.; Camacho, L. J. Electroanal. Chem. 1995, 390, 21. (15) Gomez, L.; Ruiz, J. J.; Camacho, L.; Rodrı´ guez-Amaro, R. J. Electroanal. Chem. 2004, 564, 179. (16) Gomez, L.; Ruiz, J. J.; Camacho, L.; Rodrı´ guez-Amaro, R. Langmuir 2005, 21, 369. (17) Kobayashi, K.; Fujisaki, F.; Yoshimine, T.; Niki, K. Bull. Chem. Soc. Jpn. 1986, 59, 3715. (18) Pospı´ sil, L.; Kuta, J.; Volke, J. J. Electroanal. Chem. 1975, 58, 217. (19) Pospı´ sil, L.; Kuta, J. J. Electroanal. Chem. 1978, 90, 231. (20) Kitamura, F.; Ohsaka, T.; Tokuda, K. J. Electroanal. Chem. 1993, 347, 371.
Published on Web 10/11/2010
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silver,27 and highly oriented pyrolytic graphite28,29 electrodes in the aqueous medium, where the 2D adsorption of the dialkyl viologen cation radical was confirmed. Additionally, Kobayashi et al.17 showed that the methyl viologen (MV2þ) adsorbs on the mercury electrode with a flat orientation at more positive potentials than the potential of zero charge, whereas its reduced form MVþ• (cation radical) adsorbs with a vertical orientation at the potentials more negative than the formal potential of the MV2þ/MVþ• redox couple. The reorientation of adsorbed MVþ• from a flat to the vertical orientation was also reported by Pospı´ sil and Kuta.19 Recently, the adsorption properties of several thiolated alkyl viologen derivatives (V2þ) were studied in the aqueous solution on the gold substrate.2,30-34 Surface concentration of the adsorbate was mainly evaluated from the first reduction wave of the alkyl viologen moiety, i.e., from the cyclic voltammetry of the V2þ/Vþ• couple. Li et al.2 performed a comprehensive STM study of the adsorption of hexyl viologen thiols and dithiols on the gold(111) electrodes. The authors identified three different types of adlayers in the aqueous solution: a low coverage disordered one, an ordered striped phase of the molecules laying flat on the electrode surface and a high coverage monolayer composed of the tilted viologen moieties. Haiss et al.32,34 observed that the surface coverage of the viologen-based dithiols on Au(111) in the aqueous solution increases with increasing reaction time, resulting in a change of the film orientation from a flat to a more upright standing conformation. The aim of this work is to follow the adsorption of a new class of extended viologens as shown in Scheme 1. Their electrochemical properties will be the subject of our subsequent report.
Experimental Section All extended viologen compounds were synthesized by Valasek et al. according to the general procedure described elsewhere.35 Their chemical structures are schematically shown in the Scheme 1. R-[4-(Methylsulfanyl)phenyl]-ω-methylsulfanyl-mono[(2,6-diphenylpyridinium-4,1-diyl)-1,4-phenylene-(2,6-diphenylpyridinium1,4-diyl)-1,4-phenylene]-bis(trifluoromethanesulfonate) is labeled in the text as 1, R-[4-(methylsulfanyl)phenyl]-ω-methylsulfanyl-di[(2,6-diphenylpyridinium-4,1-diyl)-1,4-phenylene-(2,6-diphenylpyridinium-1,4-diyl)-1,4-phenylene]-tetrakis(trifluoromethanesulfonate) as 2, R-[4-(methylsulfanyl)phenyl]-ω-methylsulfanyltri[(2,6-diphenyl-pyridinium-4,1-diyl)-1,4-phenylene-(2,6-diphenylpyridinium-1,4-diyl)-1,4-phenylene]-hexakis(trifluoromethanesulfonate) as 3, R-[4-(methylsulfanyl)phenyl]-ω-methyl-sulfanyl(21) Salas, R.; Sanchez-Maestre, M.; Rodrı´ guez-Amaro, R.; Mu~noz, E.; Ruiz, J. J.; Camacho, L. Langmuir 1995, 11, 1791. (22) Millan, J. I.; Rodrı´ guez-Amaro, R.; Ruiz, J. J.; Camacho, L. J. Phys. Chem. B 1999, 103, 3669. (23) Millan, J. I.; Ruiz, J. J.; Camacho, L.; Rodrı´ guez-Amaro, R. Langmuir 2003, 19, 2338. (24) Widrig, C. A.; Majda, M. Langmuir 1989, 5, 689. (25) Shimazu, K.; Yanagida, M.; Uosaki, K. J. Electroanal. Chem. 1993, 350, 321. (26) Arihara, K.; Ohsaka, T.; Kitamura, F. Phys. Chem. Chem. Phys. 2002, 4, 1002. (27) Millan, J. I.; Ruiz, J. J.; Camacho, L.; Rodrı´ guez-Amaro, R. J. Electrochem. Soc. 2002, 149, E440. (28) Arihara, K.; Kitamura, F.; Nukanobu, K.; Ohsaka, T.; Tokuda, K. J. Electroanal. Chem. 1999, 473, 138. (29) Tanaka, Y.; Sagara, T. J. Electroanal. Chem. 2008, 619-620, 65. (30) De Long, H. C.; Buttry, D. A. Langmuir 1992, 8, 2491. (31) Yan, J.; Dong, S.; Li, J.; Chen, W. J. Electrochem. Soc. 1997, 144, 3858. (32) Tang, X.; Schneider, T. W.; Walker, J. W.; Buttry, D. A. Langmuir 1996, 12, 5921. (33) Haiss, W.; Nichols, R. J.; Higgins, S. J.; Bethell, D.; H€obenreich, H.; Schiffrin, D. J. Faraday Discuss. 2004, 125, 179. (34) Haiss, W.; van Zalinge, H.; H€obenreich, H.; Bethell, D.; Schiffrin, D.; Higgins, S. J.; Nichols, R. J. Langmuir 2004, 20, 7694. (35) Valasek, M.; Betı´ k, R.; Pecka, J.; Michl, J.: J. Org. Chem. submitted for publication.
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Figure 1. Differential capacitance of 0.1 M KF in 20% ethanol/ water mixture in the absence (O) and presence (b) of 6 μM wire 1. Perturbation frequency 160 Hz, amplitude 5 mV, and t = 25 C. tetra[(2,6-diphenyl-pyridinium-4,1-diyl)-1,4-phenylene-(2,6-diphenylpyridinium -1,4-diyl)-1,4-phenylene]-oktakis(trifluoromethanesulfonate) as 4, R-[4-(methylsulfanyl) phenyl]-ω-methylsulfanylpenta[(2,6-diphenylpyridinium-4,1-diyl)-1,4-phenylene-(2,6-diphenylpyridinium-1,4-diyl)-1,4-phenylene]-dekakis(trifluoromethanesulfonate) as 5, and R-[4-(methylsulfanyl)phenyl]-ω-methylsulfanylhexa[(2,6-diphenylpyridinium-4,1-diyl)-1,4-phenylene-(2,6-diphenylpyridinium-1,4-diyl)-1,4-phenylene]-dodekakis(trifluoromethanesulfonate) as 6, respectively. Deionized water with a maximum resistivity of 18 MΩ cm was obtained by means of a Milli-Q RG purification system (Millipore Co., U.S.A.) and was used throughout the studies. Ethanol, p.a. (Lach-Ner, Czech Republic) and potassium fluoride, p.a. (Merck, Germany) were used as received. The electrochemical measurements were made using a laboratory-built electrochemical system consisting of a fast rise-time potentiostat and a lock-in amplifier (Stanford Research, model SR830). The instruments were interfaced to a personal computer via an IEEE-interface card (PC-Lab, AdvanTech Model PCL-848) and a data acquisition card (PCL-818) using 12-bit precision. A three-electrode electrochemical cell was used. The reference electrode (Ag|AgCl|1 M LiCl) was separated from the test solution by a salt bridge with a double fritted junction. A valve-operated static mercury drop electrode SMDE2 (Laboratornı´ Prı´ stroje, Prague) of area 0.0155 cm2 was used as a working electrode. Platinum net was used as the auxiliary electrode. Oxygen was removed from the solution by a stream of argon. A protecting argon layer blanketed the solution surface during the entire experiment. The differential capacitance measurements were obtained as an out-of-phase component of the complex admittance Y00 of the system divided by the angular frequency ω = 2πf of the applied ac voltage and the electrode area. A more detailed description of the electrochemical capacitance measurements is described elsewhere.36 The time resolution of the capacitance-time transients was given by the output time constant of the averaging filter of the lock-in amplifier, which was set to 0.03s for the measuring frequency 160 Hz. This means that the output signal reached the true value approximately in three times 0.03 s. Hence the data were collected with the time step of 0.1 s.
Results and Discussion The adsorption properties of molecular wires 1 to 6 were studied in 0.1 M potassium fluoride in 20% ethanol/water mixture due to their limited solubility in pure water. The adsorption studies were performed mainly by a phase-sensitive ac voltammetry and time-resolved differential capacity measurements employing a hanging mercury drop electrode (HMDE) as a working electrode. Figure 1 shows the differential capacitance C data as a (36) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Fundamentals and Applications, 2nd ed.; J. Wiley: New York, 2001; Chapter 10.
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Figure 2. Capacitance-time transients for 3.3 μM 1 in 0.1 M KF and 20% ethanol/water mixture at (a) -0.1, (b) -0.3, and (c) -0.5 V. Perturbation frequency 160 Hz, amplitude 5 mV, and t = 25 C.
Figure 3. Capacitance-time transients for (a) 2.5, (b) 3.3, (c) 4.3,
and (d) 6.0 μM 1 in 0.1 M KF and 20% ethanol/water mixture obtained at -0.5 V. Other experimental conditions are the same as in Figure 2.
function of the applied electrode potential for the shortest molecule, i.e., the dication 1. Similar capacitance curves were observed for all other compounds 2-6. Data in Figure 1 were obtained after a waiting period of 3 min at -0.7 V followed by the potential scan in either a positive or negative direction at the scan rate 5 mV/s. The capacitance values are practically independent of the electrode potential within a wide potential range (here between -0.1 and -0.8 V) forming a capacitance pit, which is usually an indication of a 2D condensed film formation.37,38 The capacitance values in the pit region are independent of temperature (measured between 10 and 47 C) and the pit width decreases with temperature increase (data not shown). We did not attempt any quantitative analysis of these temperature dependent data due to the fact that the pit width is limited on the cathodic side by the onset of the electrochemical reduction of the wire. The electron transfer process is discernible from the presence of a small peak at -1.04 V in the in-phase admittance component of the ac voltammetric curves (data not shown), i.e., at the same potential, where the reorientation peak appears in the differential capacitance plot of Figure 1. A full description of the electrochemical studies of wires 1-6 will be given in our subsequent communication. The independence of the differential capacitance C on the electrode potential in the capacitance pit region was also confirmed by the measurements of the capacitance-time (C-t) transients. (37) De Levie, R. Chem. Rev. 1988, 88, 599. (38) Double layer and electrode kinetics,; Delahay, P., Ed.; Interscience Publishers: New York, 1965.
17234 DOI: 10.1021/la1034239
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Figure 4. Plot of the electrode surface coverage θ as a function of the square root of time for 6 μM 1 in 0.1 M KF and 20% ethanol/ water mixture obtained at -0.1 V. Parameter ζ is the characteristic time of the film formation.
Figure 5. Graph of the dependence of the film formation time ζ on the concentration 1/c2 for three molecular wires (A) 1, (B) 2, and (C) 4, respectively. Other experimental conditions are the same as in Figure 2.
Figure 2 shows such curves obtained at three different electrode potentials. They converge to the same capacitance value of 2.3 μF cm-2 as long as the potential is selected within the capacitance pit. Interestingly, the capacitance reaches this constant value at a unique time, which is the characteristic time of the film formation ζ. Furthermore, the differential capacitance value was found to be independent of the bulk concentration of the wires (Figure 3), which suggests that the formation of a monolayer takes place. The rate of the film formation and ζ depend on the bulk concentration of the wires. Assuming a parallel-plate capacitor model the experimentally obtained C-t curves can be used for calculation of the electrode surface coverage θ θðtÞ ¼
CðtÞ - C0 C¥ - C0
ð1Þ
where C(t) is the differential capacitance at time t, C¥ is the capacitance value corresponding to the fully covered electrode, and C0 is the capacitance of the electrode|electrolyte interface in the absence of the adsorbate. Since both free and occupied areas coexist on the electrode surface during the film formation (0 < t < ζ) the capacitance values fall between C¥ and C0 (C¥ < C(t) < C0). For times t g ζ the capacitance value stays constant and equal to C¥ leading to the surface coverage θ equal to one. Figure 4 shows a representative θ transient obtained for 6 μM solution of 1 in 0.1 M KF and 20% ethanol/water mixture at -0.1 V plotted against the square root of time. It is clear that the Langmuir 2010, 26(22), 17232–17236
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Article Table 1. Characteristic Adsorption Parameters for the Molecular Wires of Different Lengtha
n
-2
C¥ (μF cm )
l (nm)
S (s mol2 m-6)
D (m2 s-1)
ΓMAX (mol cm-2)
Ateor (nm2)
Aexp (nm2)
1.04 10-10 5.3 10-11 2.8 3.1 2.3 2.35 21.0 10-4 0.85 10-10 2.4 10-11 4.9 6.9 2.5 4.07 5.29 10-4 -4 -10 -11 0.75 10 1.7 10 7.0 10.0 2.3 5.80 2.87 10 0.69 10-10 1.7 10-11 9.0 9.6 2.5 7.52 3.44 10-4 0.64 10-10 1.4 10-11 11.1 12.0 2.6 9.26 2.39 10-4 -4 -10 -11 0.60 10 1.6 10 13.2 10.6 2.5 11.0 3.23 10 a n = number of repeating units in the molecule, C¥ = differential capacitance of the fully covered electrode, l = length of the molecule, S = slope of the modified Koryta equation, D = diffusion coefficient, ΓMAX is the surface concentration, and Ateor and Aexp theoretical and experimental area occupied by one molecule.
1 2 3 4 5 6
surface coverage increases linearly with t1/2 during the monolayer formation, which suggests that the adsorption process is diffusion-controlled. Koryta39,40 showed that for a diffusion-controlled process the characteristic time ζ depends on the concentration of the adsorbate and does not depend on the applied potential. Koryta derived his equation for a dropping mercury electrode (DME). A modified version of the Koryta equation for the diffusion-limited adsorption on HMDE leads to the following expression for the characteristic time ζ41,42 ζ ¼
π Γmax 2 4D c2
ð2Þ
where D is the diffusion coefficient, Γ is the surface concentration in mol cm-2 and Γ = ΓMAX for t g ζ. A linear dependence of ζ on 1/c2 is expected for the diffusion-controlled adsorption. Figure 5 shows the experimentally obtained values of ζ as a function of the concentration (1/c2 dependence) for three selected molecular wires 1, 2, and 4. The typical concentrations used in the adsorption studies fell into the range of 0.5-50 μM. The higher concentrations were of no use since the capacitance changes on a freshly created mercury drop were faster than the time resolution of our experimental setup. All molecular wires 1-6 exhibited the linear dependence of ζ on 1/c2 confirming the diffusion-controlled nature of the adsorption. Table 1 summarizes the adsorption properties of all six molecular wires. The differential capacitance C¥ of the electrode| electrolyte interface in the presence of adsorbed monolayers of 1-6 is listed in the second column and C¥ was found to be independent of the length of the molecule. The C¥ value equals 2.5 ( 0.2 μF cm-2 and suggests that the monolayer has the same thickness under the assumption of the constant electric permittivity of the film. This would indicate the parallel orientation of the individual molecules on the electrode surface as shown in a schematic sketch in Figure 6 (left panel). Table 1 also contains the estimate of the molecular length l based on the space filling model of the molecules 1-6 as obtained by the Spartan’08 software (Wave function, Inc., U.S.A.). The experimental values of the slopes S of the graphs in Figure 5 are listed in the fourth column of Table 1 and can be considered to be πΓMAX2/4D according to the modified Koryta equation (eq 2). If the diffusion coefficient of the molecule is known one can obtain the experimental values of ΓMAX., i.e. the surface concentration of the adsorbate at θ = 1. The molecular wires are often considered to be analogous to methyl viologen MV. Therefore, we decided to estimate the diffusion coefficient of the wires D with molar mass M by using the equation (39) Koryta, J. Collect. Czech. Chem. Commun. 1953, 18, 206. (40) Z aklady polarografie; Heyrovsky, J., Kuta, J., Eds.; Nakladatelství Ceskoslovensk e akademie ved: Prague, 1962. (41) Delahay, P.; Trachtenberg, I. J. Am. Chem. Soc. 1957, 79, 2355. (42) Imhoff, D. W.; Collat, J. W. J. Phys. Chem. 1967, 71, 3048.
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Figure 6. Graphical sketch of the parallel (left panel) and perpendicular (right panel) orientation of the molecular wire 1 on the electrode|electrolyte interface. The symbols refer to molecular dimensions: r, molecular radius; l, molecular length.
D = DMV(MMV/M)1/3 (which can be easily derived from the Stokes-Einstein relation). The values of DMV = 1.68 10-10 m2 s-1 and MMV =257 g mol-1 are the diffusion coefficient and molar mass of methyl viologen dichloride. As the electrolyte contains 20% v/v ethanol in water, the DMV value differs from that measured in water DMV(H2O) = 4.4 10-10 m2 s-1. The value DMV was calculated from DMV(H2O) value using the Walden rule. The calculated D values for all chain lengths are listed in Table 1. It should be emphasized that the Stokes-Einstein relation is fully applicable only for the spherical molecules. However, we did not opt for the use of the Perrin equation,43 which was derived for the ellipsoids, since we wanted to keep the number of parameters only to those available experimentally. Knowing the D values, one can get the experimental ΓMAX values (see Table 1) and calculate the experimental area Aexp = 1/ΓMAXNA (NA being the Avogadro constant) occupied by one extended viologen molecule. The last two columns of Table 1 compare the experimentally obtained values Aexp with those calculated theoretically using the space filling model (Atheor). The molecules can be approximated by a cylinder with a constant radius r = 0.6 nm and the length l that scales linearly with the number of repeating units n in the molecule. If the molecule lays parallel to the surface, then Atheor = 2rl. If the molecules lay perpendicular to the surface and a close packed upright cylindrical arrangement is assumed then Atheor = 2 31/2 r2 = 1.25 nm2 independent of the compound (Figure 6, right panel). In such a case the capacitance C¥ would not be the same for all the wires, but would scale with the length of the molecule. The Aexp values are all much larger than 1.25 nm2, they scale with increasing n and are well comparable to the values Atheor calculated from the model based on the parallel orientation of the molecules. One can therefore conclude that the molecular wires 1 to 6 are adsorbed on the mercury|electrolyte interface with the pyridinium rings lying flat on the electrode surface. They form (43) Edward, J. T. J. Chem. Educ. 1970, 47, 261.
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a compact monolayer, the formation of which is diffusion controlled.
Conclusions In this contribution the adsorption properties of a series of organic molecules consisting of the repeated extended viologen units were studied on the mercury electrode|electrolyte interface. Knowledge of these adsorption properties is important step toward their future use as the conducting molecular wires in the molecular electronics assemblies. In a water/ethanol mixture all compounds form a compact film on the mercury electrode in a wide potential range close to the potential of zero charge. At more negative potentials the molecules undergo the reduction in the adsorbed state accompanied by their reorientation. The timeresolved differential capacitance measurements provided a constant value of the differential capacity equal to 2.5 ( 0.2 μF cm-2 independent of the temperature, length and the bulk concentration
17236 DOI: 10.1021/la1034239
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of the molecular wire. Based on this value and the capacitancetime transients the extended viologen molecules must be preferentially oriented with pyridinium rings parallel to the electrode| electrolyte interface. The surface concentrations of individual members of the homologous series were calculated using the modified Koryta equation for the diffusion controlled adsorption process leading to 5.3 10-11 mol cm-2 for the shortest extended viologen molecule and to 1.6 10-11 mol cm-2 for the longest one. Acknowledgment. This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (IAA400400802), Grant Agency of the Czech Republic (GACR 203/08/1157 and GACR 203/09/0705), and Ministry of Education of the Czech Republic (COST D36 OC140). V.K. acknowledges the scholarship award from the Institute of Chemical Technology, Prague.
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