Electrochemical Characteristics of Self-Assembled Monolayers of a

K. Vengatajalabathy Gobi, Fusao Kitamura, Koichi Tokuda, and Takeo Ohsaka. The Journal of Physical Chemistry B 1999 103 (1), 83-88. Abstract | Full Te...
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Langmuir 1998, 14, 1108-1115

Electrochemical Characteristics of Self-Assembled Monolayers of a Novel Nickel(II) Pentaazamacrocyclic Complex on a Gold Electrode K. Vengatajalabathy Gobi, Takeyoshi Okajima, Koichi Tokuda, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received June 30, 1997. In Final Form: December 18, 1997 Ethyl disulfide possessing two nickel(II) pentaazamacrocyclic redox centers at its end carbon atoms, 1, adsorbs on gold electrodes from methanol solutions and yields stable, electroactive self-assembled monolayers (SAMs). Cyclic voltammetry of SAMs of 1 shows a stable redox wave at 0.55 V in aqueous 0.1 M Na2SO4 (pH 2), corresponding to the Ni2+/3+ redox reaction. For SAMs of 1 with the surface coverage (Γ) of nickel(II) redox centers of (1.41 ( 0.04) × 10-10 mol cm-2, the cyclic voltammograms (CV) show very small peakto-peak separations (∆Ep < 20 mV) with the ∆Efwhm (full-width at half-maximum of CV peaks) values ranging from 140 to 150 mV, indicating electrostatic repulsive interactions among the surface-immobilized nickel(II) redox centers. The constructed SAM of 1 was defined as a square close-packed monolayer with the polyazamacrocyclic ring of the nickel(II) redox center oriented perpendicular to the electrode surface. Coadsorption of 1 with electroinactive alkyl/aryl disulfides furnishes mixed monolayers, and the Γ of the electroactive nickel(II) redox centers can be varied by the mole ratio of 1 and coadsorbate in the adsorption solution. In mixed monolayers, the ∆Efwhm decreases as the Γ of 1 decreases from the limiting monolayer surface coverage, suggesting a homogeneous distribution of 1 and coadsorbate in the constructed mixed monolayers. The formal potentials (E°′) of the solution-dissolved 1 and the self-assembly of 1 with different Γ values, equivalent to 0.3-1 monolayer, are nearly equal, evidencing the same microenvironment around the nickel(II) redox centers in both the self-assembly and the solution phase. Exchange between the surface-immobilized gold-alkanethiolates with disulfides in solution has been observed and was found to accompany ordering of the monolayer. The apparent rate constant for the heterogeneous electrontransfer reaction, kapp, has been determined by fast-scan cyclic voltammetry to be 1.3 × 103 s-1 for the SAM of 1. An increase in the kapp with a decrease in the Γ of 1 in mixed monolayers was observed.

Introduction The chemistry of SAM and multilayer films has been a topic of great interest to electrochemists for decades. Much attention has been devoted to the preparation of highly organized monolayers or multilayers at the electrode-solution interface. Deposition of LangmuirBlodgett monolayers on electrode surfaces constitutes one of the most common methods to build interfacial amphiphilic structures.1-4 Another approach is based on the self-assembly of lipophilic molecules ending in hydrolytically unstable groups such as -SiCl3 or sulfur functional groups such as thiol or disulfide on Au surfaces.1,5,6 Among the many different chemical strategies that could be used to prepare such modified electrodes, the gold-alkanethiol self-assembly method has attracted many researchers to construct highly organized molecular thin films on gold electrodes,1,7 including structural derivatives of ferro* To whom correspondence should be addressed. Fax: +81-45921-1089. E-mail: [email protected]. (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films-From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (b) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109. (2) (a) Daifuku, H.; Aoki, K.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1985, 183, 1. (b) Zhang, X.; Bard, A. J. J. Phys. Chem. 1988, 92, 5566. (c) Gomez, M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797. (d) Lindholm-Sethson, B.; Orr, J. T.; Majda, M. Langmuir 1993, 9, 2161. (3) Zhang, X.; Bard, A. J. J. Am. Chem. Soc. 1989, 111, 8098. (4) Li, J.; Kaifer, A. E. Langmuir 1993, 9, 591. (5) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (6) (a) Evans, S. D.; Ulman, A. Chem. Phys. Lett. 1990, 462. (b) Krysinski, P.; Chamberlain, R. V.; Majda, M. Langmuir 1994, 10, 4286.

cene,8-12 viologen,13 and a few other systems.14,15 The results of electrochemical studies of these monolayers suggest a need to expand the range of redox species that can be immobilized by thiolate chemisorption. As applications of the SAMs in electrochemistry are explored further, it will become increasingly desirable to incorporate new redox active molecules by the goldalkanethiol self-assembly method. Tetraazamacrocyclic complexes of transition metals have proved to be effective electrocatalysts in the reduction of CO2,16 nitrate,17 and oxygen18 and oxidation of H2O2,19 cystine,20 glucose,21 etc. (7) Bubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (8) (a) Chidsey, C. E. D. Science 1991, 51, 919. (b) Redepenning, J.; Flood, J. M. Langmuir 1996, 12, 508. (9) Tender, L.; Carter, M. T.; Murray, R. W. Anal. Chem. 1994, 66, 3173. (10) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (11) Long, H. C. D.; Donohue, J. J.; Buttry, D. A. Langmuir 1991, 7, 2196. (12) Creager, S. E.; Rowe, G. K. J. Eleectroanal. Chem. 1994, 370, 203. (13) Long, H. C. D.; Buttry, D. A. Langmuir 1992, 8, 2491. (14) Redepenning, J.; Tunison, H. M.; Finklea, H. O. Langmuir 1993, 9, 1404. (15) (a) Zak, J.; Yuan, H.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772. (b) Forster, R. J. Inorg. Chem. 1996, 35, 3394. (16) Beley, M.; Collin, J.-P.; Ruppert, R.; Sauvage, J.-P. J. Am. Chem. Soc. 1986, 108, 7461. (17) (a) Beley, M.; Collin, J.-P.; Ruppert, R.; Sauvage, J.-P. J. Chem. Soc., Chem. Commun. 1984, 1315. (b) Taniguchi, I.; Nakashima, K.; Matsushita, K.; Yasukouchi, K. J. Electroanal. Chem. 1987, 224, 199. (18) (a) Bowers, M. L.; Anson, F. C. J. Electroanal. Chem. 1984, 171, 269. (b) Gobi, K. V.; Ramaraj, R. J. Chem. Soc., Chem. Commun. 1992, 1436. (19) Taraszewska, J.; Roslonek, G.; Darlewski, W. J. Electroanal. Chem. 1994, 371, 223.

S0743-7463(97)00694-X CCC: $15.00 © 1998 American Chemical Society Published on Web 02/05/1998

Ni(II) Pentaazamacrocyclic SAMs

Especially, the usually unstable oxidation states of transition metal ions (Ni(III), Ni(I), Cu(III), Cu(I), etc.) can be stabilized by complexing with polyazamacrocycles and the accession of those oxidation states can be modulated over a wide range.22 Covalent attachment of these polyazamacrocyclic complexes to the gold surface could provide an interesting comparison with previously studied,19,22,23 solution-dissolved and electrode-adsorbed polyazamacrocyclic complex-based catalysts as well. We were interested in constructing a monolayer of redox active nickel(II) complex on the electrode surface and synthesized the bis(pentaazamacrocyclic) dinickel(II) complex, 1 {dinickel(II) (2,2-bis(1,3,5,8,12-pentaazacyclotetradec-3-yl)ethyl disulfide) perchlorate}.

Herein we report the results of a study of the coadsorption of 1 with three different electroinactive disulfides on bulk Au electrodes. The three different coadsorbates, ethyl disulfide, 2-hydroxyethyl disulfide, and phenyl disulfide, were chosen so as to exhibit different interactions with the redox nickel(II) headgroup and among themselves. The presently designed highly positive-charged redox headgroup and its simple small alkyl chain may favor the construction of a highly ordered monolayer of 1 and the solvation of nickel(II) redox centers at the electrode-solution interface.24,25 The E°′ of the nickel(II) redox centers in the monolayer was determined at different Γ values of 1. Further, the interchange between the immobilized gold-alkanethiolates with the disulfides in the adsorption solution is observed by taking advantage of the electroactivity of 1. Cyclic voltammetry has been used to estimate the kapp. The subsequent findings are representative of the testing of more than five samples of each type of monolayer. Experimental Section Materials. Details of the preparation of 1 will be reported elsewhere.26 Complex 1 was characterized by elemental analysis, 13C NMR, and IR spectral measurements. Coadsorbates, ethyl (20) Halbert, M. K.; Baldwin, R. P. Anal. Chem. 1985, 57, 591. (21) Santos, L. M.; Baldwin, R. P. Anal. Chem. 1987, 59, 1766. (22) (a) Haines, R. I.; McAuley, A. Coord. Chem. Rev. 1981, 39, 77. (b) Lappin, A. G.; McAuley, A. Adv. Inorg. Chem. 1988, 32, 241. (23) Taniguchi, I.; Matsushita, K.; Okamoto, M.; Collin, J.-P.; Sauvage, J.-P. J. Electroanal. Chem. 1990, 280, 221. (24) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (25) Finklea, H. O.; Hanshew, D. D. J. Electroanal. Chem. 1993, 347, 327. (26) Gobi, K. V.; Okajima, T.; Tokuda, K.; Ohsaka, T. J. Chem. Soc., Dalton Trans., submitted for publication.

Langmuir, Vol. 14, No. 5, 1998 1109 disulfide (Analar grade, Kanto), 2-hydroxyethyl disulfide (Reagent grade, Aldrich), and phenyl disulfide (Analar grade, Kanto), were used as received. Deionized water, which was purified by passage through a Millipore Milli-Q filtration system, and spectral grade organic solvents were used to prepare the adsorption solution and for all measurements. Analar grade supporting electrolytes and inorganic acids were purchased from Kanto Chemical Co. Monolayer Fabrication Procedure. The Au electrodes were prepared by sealing an annealed 1.0 mm diameter Au rod in an insulating diaphragm (poly(chlorotrifluoroethylene)). The exposed gold surface was polished with aqueous slurries of successively finer alumina powder (down to 0.06 µm) on a polishing microcloth, sonicated for 10 min in water, and rinsed with water. Electrodes were etched for 3 min in a 1:3:4 (in volume) mixture of concentrated HNO3/concentrated HCl/water and sonicated in deionized water for 30 min.27 The Au electrode was then electropolished by potential cycling in 0.5 M H2SO4 in the potential range of -0.2 to +1.5 V at the potential scan rate of 100 mV/s for 20 min or until the CV characteristic for a clean Au electrode was obtained. Such an electrode was considered as a bare Au electrode. The real surface area was calculated from the charge required to reduce the surface oxide layer using the previously established formula,28 0.43 mC cm-2. The geometric area of the Au electrode was calculated using the diameter (1 mm) of the employed Au electrode. The surface roughness was calculated as the ratio of the real surface area to the geometric area. Once cleaned, the electrodes were immersed in 10 mL of methanol (Wako, UV spectral grade) containing different mole fractions of 1 and a coadsorbate at a total disulfide concentration of 0.5 mM, unless otherwise specified. Solutions containing the disulfides were passed through a 0.2 µm filter prior to use as the deposition solution. Electrodes were immersed in the coating solutions for different time periods, up to 48 h, and were rinsed sequentially with copious amounts of methanol and water prior to use in electrochemical experiments. The amount of surface-adsorbed 1 was determined as the charge of the anodic wave (Qpa) by the cut and weigh method. The glassy carbon electrode (GC-20 (diameter: 3 mm), Tokai Carbon Co., Ltd.), used for the cyclic voltammetry of solution-dissolved species, was polished with alumina powder, sonicated for 10 min in water, and rinsed with water. Measurements. Electrochemical measurements were performed in a conventional two-compartment three-electrode cell using an electrochemical measurements system (BAS 100 B/W) controlled by an IBM personal computer. The working electrode and the counter electrode (a platinum spiral wire) were separated by a porous glass. An NaCl-saturated Ag/AgCl electrode was used as the reference electrode. In cases of cyclic voltammetry using fast potential scans, INTLINITiR drop due to solution resistance was electronically compensated by an automatic compensator equipped to the BAS 100 B/W. Solutions were deaerated by bubbling the solvent-saturated N2 gas for at least 30 min prior to electrochemical measurements. During the measurements, the N2 gas was passed over the solution. Electronic absorption spectra were obtained with a Hitachi U-3300 spectrophotometer. Elemental analyses were performed at the Institute of Physical and Chemical Research, Japan. 13C NMR spectra were recorded on a JEOL JNM-GX-270 FT NMR spectrophotometer.

Results and Discussion Cyclic Voltammetry of 1 Self-Assembled on a Gold Surface. Typical CVs of self-assembly of 1 on the Au electrode, which was constructed by dipping in 0.5 mM 1 in methanol for 36 h, recorded in an aqueous 0.1 M Na2SO4 (pH 2) solution are shown in Figure 1. A symmetrical reversible redox wave was observed at 0.55 V and the ∆Ep values, although not zero, were typically very small, (27) Creager, S. E.; Hockett, L. A.; Rowe, G. K. Langmuir 1992, 8, 854. (28) Gileadi, E.; Eisner, K. E.; Penciner, J. Interfacial ElectrochemistrysAn Experimental Approach; Addison-Wesley: Reading, MA, 1975.

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Figure 2. Geometry of the postulated square close packing of nickel(II) redox centers on the gold surface. The shaded square (8.5 Å × 8.7 Å) represents the free space available in the SAM for counteranions. The length of the ethanethiolate chain connecting the nickel(II) redox center is calculated,1,5 ∼9.5 Å. The relative size of the hydrated ClO4- anion has been shown. Figure 1. CVs of the Au electrode modified by dipping in a methanol solution of 0.5 mM 1 for 36 h. The CVs were recorded in aqueous 0.1 M Na2SO4 (pH 2) at potential scan rates of 500, 400, 300, 200, 100, 50, and 20 mV s-1.

suggesting a fast electron-transfer kinetics. The E°′ (estimated as the average of the anodic and cathodic peak potentials) of the CVs recorded for the self-assembly of 1 corresponds to that of the metal-based Ni2+/3+ redox reaction of analogous nickel(II) polyazamacrocyclic complexes dissolved in solution.22 The CVs (Figure 1) are consistent in all respects with that anticipated for an electrochemically reversible reaction involving a surfaceconfined species. For example, the ∆Ep values were typically very small, the peak shapes are independent of potential scan rate, υ, the ratio of ipa to ipc at a given υ is unity, and in addition, the peak height varies linearly with υ in the range υ ) 0.02-2 V s-1, unlike the υ1/2 dependence expected for a freely diffusing species. Further, the observation of a single, sharp redox wave for the self-assembly of the disulfide molecule, 1, possessing two identical electroactive nickel(II) metal centers establishes that the formation of monolayers bound via sulfur to gold substrates proceeds from thiols or disulfides, whereby the produced films are essentially identical.29,30 Repeated scanning does not affect the CVs, demonstrating that the monolayer is stable upon potential cycling in acidic aqueous solution. To interpret the coverage of nickel(II) redox centers on gold surfaces in terms of the packing density and the orientation of the immobilized nickel(II) redox centers, we determined the Γ values. The faradaic charge Qpa associated with converting the self-assembly of 1 from the 2+ to the 3+ oxidation states has been estimated. Qpa was independent of υ, 130 ( 4 nC (the standard deviation was obtained from four independent experiments). On the basis of this value, we calculated the Γ value as the number of moles of electroactive nickel(II) headgroups per square centimeter: the surface charge density is 16.6 ( 0.5 µC cm-2 and Γ is (1.72 ( 0.05) × 10-10 mol cm-2. X-ray crystal data of an analogous nickel(II) macrocyclic complex 2, (3-methyl-1,3,5,8,12-pentaazacyclotetradecane)nickel(II) diperchlorate,31 were used to calculate the expected values of the molecular area and monolayer (29) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (30) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128. (31) Fabbrizzi, L.; Lanfredi, A. M. M.; Pallavicini, P.; Perotti, A.; Taglietti, A.; Ugozzoli, F. J. Chem. Soc., Dalton Trans. 1991, 3263.

surface coverage for the redox active nickel(II) headgroups in two different orientations: 216 Å2 and 7.7 × 10-11 mol cm-2 if the macrocycle of the nickel(II) complex is coplanar to the electrode surface in the self-assembly and 118 Å2 and 1.41 × 10-10 mol cm-2 for the perpendicular orientation. The experimentally observed Γ value is larger than those for both coplanar and perpendicular orientations. Surface coverages would be inflated by microscopic surface roughness of the Au electrode. The surface roughness of the employed bulk Au electrode was determined to be 1.22 ( 0.05; hence the experimental Γ value becomes (1.41 ( 0.04) × 10-10 mol cm-2 and corresponds to an area occupied per molecule of 118 Å2. Thus the constructed self-assembly of 1 is consistent with a square close-packed monolayer with the macrocyclic ring of the nickel(II) complex oriented perpendicular to the gold surface. The molecular area occupied by an amphiphilic cyclam derivative with the macrocyclic ring parallel to the air/water interface was found to be 185 Å2/molecule,32 which further supports our proposed orientation in the SAM. Acevedo and Abruna,33 Bretz and Abruna,34 and Finklea and Hanshew25,35 observed similar close-packed monolayers of charged osmium and ruthenium redox centers immobilized at electrode surfaces. The close packing of nickel(II) redox centers appears also to be reasonable in terms of the size of the perchlorate counteranions, which must be immobilized along with the nickel(II) redox centers in the SAM. The effective radius of an alkyl chain (2.49 Å) in Figure 2 was calculated from the experimental value of an area occupied by a molecule of docosanethiol in the hexagonal close-packing arrangement at the Au electrode surface (Au(111))36 of 21.4 Å2. Considering the relevant dimensions in this figure, the free space available between four adjacent hydrocarbon chains is calculated to exceed a square of 8.5 Å × 8.7 Å. The radius of a hydrated perchlorate anion37 is 2.24 Å, and thus the two perchlorate counteranions of the nickel(II) redox center can reside conveniently in the SAM itself; in addition, we envision that the relatively smaller perchlorate anions could also be immobilized on top of the nickel(II) redox centers. Here note that the coordination of axial ligands (32) Ducharme, D.; Salesse, C.; Leblanc, R. M.; Meller, P.; Mertesdorf, C.; Ringsdorf, H. Langmuir 1993, 9, 2145. (33) Acevedo, D.; Abruna, H. D. J. Phys. Chem. 1991, 95, 9590. (34) Bretz, R. L.; Abruna, H. D. J. Electroanal. Chem. 1996, 408, 199 (35) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (36) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (37) Mingos, D. M. P.; Rohl, A. L. Inorg. Chem. 1991, 30, 3769.

Ni(II) Pentaazamacrocyclic SAMs

accompanies only small changes in the molecular volume and dimensions38 and hence the Ni2+/3+ redox reaction, where the Ni3+ ion is of octahedral geometry with axially coordinated supporting electrolyte anions/solvent molecules,22 would not affect the compactness and order of the SAM. Monolayers of ferrocene derivatives exhibit ideal CVs only when the Γ of the ferrocene redox centers is relatively low.10,24,39 Monolayers of dicationic ruthenium complex-alkanethiol exhibited multiple redox peaks initially, and repeated immersions in the adsorption solution eliminate the multiple peaks and a single redox wave is observed finally.25,35 So the remarkable CVs observed for the SAM of 1 are attributed to the hydrophilicity of Ni2+ and Ni3+ redox centers and to the short alkyl chain length. Relative Adsorption Behavior of 1 in Mixed Monolayers. One of the aims of the present investigation is to evaluate the effect of the intermolecular interactions between the adsorbates on the relative tendencies of 1 and the coadsorbate to become immobilized by the selfassembly process. Mixed monolayers were constructed from adsorption solutions containing different mole fractions of 1 and coadsorbate. The CVs of Au electrodes modified by dipping into the adsorption solution for 36 h were recorded in aqueous 0.1 M Na2SO4 (pH 2). The affinity of ferrocene-alkanethiols for the gold surface was found to be a function of the polarity of ferrocene derivative, and the electrostatic effect plays a key role in determining the mixed-monolayer composition and the limiting Γ value.12 A positively charged ferrocene-alkanethiol was very reluctant to be immobilized on the electrode surface even at very low surface coverages (65% (Figure 3c). In a complementary (38) (a) Bosnich, B.; Mason, R.; Pauling, P. J.; Robertson, G. B.; Tobe, M. L. J. Chem. Soc., Chem. Commun. 1965, 97. (b) Barefield, E. K.; Bianchi, A.; Billo, E. J.; Connolly, P. J.; Paoletti, P.; Summers, J. S.; van Derveer, D. G. Inorg. Chem. 1986, 25, 4197. (c) Thom, V. J.; Fox, C. C.; Boeyens, J. C. A.; Hancock, R. D. J. Am. Chem. Soc. 1984, 106, 5947. (39) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500. (40) Doblhofer, K.; Figura, J.; Fuhrhop, J.-H. Langmuir 1992, 8, 1811

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Figure 3. CVs in aqueous 0.1 M Na2SO4 (pH 2) of the SAM of 1 (a) before and after immersion in a solution of 0.5 mM ethyl disulfide for (b) 60 h, (c) 200 h, and (d) 275 h. The potential scan rate was 200 mV s-1.

experiment, a monolayer of ethyl disulfide was fabricated41 by dipping a bare Au electrode in 0.5 mM ethyl disulfide for 36 h. Such a monolayer was then immersed in a solution of 0.5 mM 1 for 7 days and the CVs were recorded in 0.1 M Na2SO4 (pH 2). The observed CV (not shown) is identical with that of Figure 3a with almost equal Qp, ip and E°′. These observations clearly indicate that the selfassembling of disulfides onto the Au electrode surface is an equilibrium process. Similar results were observed for the self-assembly of nonelectroactive thiols/disulfides29 and ferrocene-terminated alkanethiols24 by X-ray photoelectron spectroscopy and cyclic voltammetry. Figure 4 presents Qpa values of 1 plotted against the mole fraction of 1, χ1, in the adsorption solution for the three sets of mixed monolayers examined: 1 with ethyl disulfide, 2-hydroxyethyl disulfide, and phenyl disulfide. Figure 4a,b shows a preferential adsorption of 1 over the coadsorbates, ethyl disulfide and 2-hydroxyethyl disulfide, and yielded the limiting Qpa value, 130 nC, with χ1 as low as 0.5. The favored (facile) attainment of surface coverages of 1 equivalent to those of close-packed monolayer (vide supra) of nickel(II) redox centers at χ1 ) 0.5 shows that the self-assembling of nickel(II) redox centers on a gold surface would not be hindered by the electrostatic repulsive interactions between the immobilizing, dicationic nickel(II) headgroups. The change of coadsorbate between ethyl disulfide and hydroxyethyl disulfide does not provide any specific change in the adsorption phenomenon of 1, although the presence of hydroxyl groups can be expected to result in intermolecular interactions among coadsorbates and the nickel redox center. The plot for 1 coadsorbed with phenyl disulfide (Figure 4c) shows a negative deviation from linearity at higher surface coverages. This could not be ascribed to intermolecular Coulombic repulsive interactions between nickel(II) redox centers, since no electrostatic repulsive interactions were observed for the other two systems (Figure 4a,b). The observed behavior can be thus explained by the possible hydrophobic interactions among the phenyl groups. At χ1 ) 0.5 or lower, on the other hand, a positive deviation from the linearity was observed (Figure 4c). The occur(41) (a) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (b) The surface coverage was found to be 99 µC cm-2, which corresponds to a close-packed monolayer of ethyl disulfide.

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Figure 4. Anodic peak charges of the Ni2+/3+ redox couple against the mole fraction of 1 with three different coadsorbates: (a) ethyl disulfide; (b) 2-hydroxyethyl disulfide; (c) phenyl disulfide. The total concentration of disulfides is 0.5 mM in methanol. The Au electrodes were dipped in each adsorption solution for 36 h.

rence of both positive and negative deviations implies that the process of surface immobilization might not be kinetically but thermodynamically controlled. Thus, the relative composition of each component in the mixed monolayers seems to be controlled by intermolecular interactions between the immobilized molecules and, importantly, the electrostatic repulsions between the immobilized nickel(II) redox centers did not exhibit characteristic features in governing the mixed monolayer composition. Adsorption Process and Formal Potential vs Surface Coverage. Both electroactive and nonelectroactive thiols were reported to adsorb on Au electrodes beyond the monolayer surface coverage, even to multilayers of n > 6; however, the reason is not clear.24,34,42 The electroactivity of 1 can be used to observe the adsorption behavior of the gold-alkanethiolate assembling process conveniently and successfully by recording the CVs of the modified electrode. Figure 5 shows the CVs of the Au electrodes modified by dipping in a 0.5 mM methanol solution of 1 for different time periods. The Ni2+/3+ redox couple was observed around 0.56 V even at the initial periods of immersion itself (Figure 5a-d). Figure 5h shows a plot of Qpa against the immersion time for adsorption of 1. The amount of adsorbed nickel(II) macrocyclic complexes increased sharply at the beginning and reached a limiting value within 1 h (Figure 5h). As the surface immobilization starts (Figure 5a-d), the double-layer charging current decreases, indicating that (42) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. Langmuir 1993, 9, 1941.

Gobi et al.

Figure 5. CVs of Au electrodes modified by dipping in 0.5 mM 1 for different time periods: (a) 7 s; (b) 30 s; (c) 2 min; (d) 15 min; (e) 4 h; (f) 8 h; (g) 36 h. All voltammograms were collected in aqueous 0.1 M Na2SO4 (pH 2) at a potential scan rate of 100 mV s-1. Current scale: S ) 0.3 µA for all the CVs. (h) Relation between the adsorption time and Qpa of nickel(II) redox centers. The dotted line is a guide to the eye at Qpa ) 130 nC.

the electrode-solution interface experiences a relatively low dielectric environment by the replacement of solvent molecules by gold-alkanethiolates. At longer immersion periods, 4-36 h, although the amount of adsorbed species does not increase (Figure 5h), the capacitive current decreased significantly and Ni2+/3+ redox peaks became smoother and sharper (Figure 5e-g). To check the time factor, a bare Au electrode modified by 4 h dipping (Figure 5e) in 0.5 mM 1 was transferred into pure methanol solution and the CVs were recorded in aqueous 0.1 M Na2SO4 (pH 2) after 12 and 36 h, in independent experiments. The recorded CV (not shown) was similar to that of Figure 5e itself. This implies that the adsorption of disulfide molecules occurs rapidly on the gold surface, leading to the formation of randomly placed redox active molecules at initial periods. As the exposed time passes, the adsorbate molecules present in the defective sites are replaced by fresh molecules present in the adsorption solution and hence the self-assembly becomes more ordered and compact. The observed behavior further supports the exchange of disulfides immobilized on the gold surface with thiols in solution, observed by Biebuyck and Whitesides.29 The effect of the concentration of 1 in the adsorption solution on the Γ value of nickel(II) redox centers was studied. The Au electrodes were modified by dipping in a methanol solution of 1 for 1 day and the charge of the anodic peak was determined from the CVs recorded in aqueous 0.1 M Na2SO4 (pH 2). The limiting Γ value, 1.45 × 10-10 mol cm-2, was acquired at a concentration of 1 as low as 0.02 mM. These results imply that the monolayer surface coverage can be acquired even at a low concentration of 1 and the results coincide with the data obtained

Ni(II) Pentaazamacrocyclic SAMs

Langmuir, Vol. 14, No. 5, 1998 1113

Figure 6. (a-c) CVs in 0.1 M Na2SO4 (pH 2) of mixed monolayers formed from methanol solutions containing different mole ratios of 1 and ethyl disulfide. The mole fraction of 1 (χ1) in the adsorption solution: (a) 1.0; (b) 0.5; (c) 0.2. (d) CV of solution-dissolved 2 × 10-4 M 1 in aqueous 0.1 M Na2SO4 (pH 2) at the GC electrode. The potential scan rate was 100 mV s-1. Current scales: S ) 40 nA (a-c) and 4 µA (d).

for octadecanethiol by contact angle and ellipsometry thickness measurements.5 Another aspect investigated was the relationship between the Γ value and the E°′ of the redox center in the monolayer. For self-assembly of electroactive molecules, the anodic and cathodic peak potentials are equal, hence E°′ ) Epa ) Epc, and depend on the Γ value according to the Brown and Anson equation,

E°′self-ass. ) E°′soln + (RT/nF)(rr - ro)Γt

(1)

where rr and ro are interaction parameters representing intermolecular interactions among reduced and oxidized forms, respectively.43 Accordingly, the E°′ of the selfassembly (E°′self-ass.) can vary with the Γ value when the difference between rr and ro is significant. For a redox reaction of an electroactive SAM involving charged species, Redepenning et al.14 studied the variation of E°′ of SAM (due to the potential difference across the monolayersolution interface, Φint) by changing the electrolyte concentration. Φint decreases with an increase in the concentration of supporting electrolyte,14,44 resulting in the E°′ of SAM being insensitive to the concentration of supporting electrolyte at high concentrations.34 Moreover, Smith and White44 recently concluded that the shift in the E°′ of a SAM involving charged species to a 10-fold increase in the concentration of z:z supporting electrolyte will not always be (59/z) mV and will decrease with an increase in the dielectric constant () of the electrolyte solution (water ) 78). In the present investigation, an optimum electrolyte concentration of aqueous 0.1 M Na2SO4 was used45 and the observed E°′ values of the Ni2+/3+ redox couple of the SAM and of the solution-dissolved 1 were compared. Figure 6 shows the CVs of mixed monolayers of 1 with different Γ values and of the solutiondissolved 1. The peak current of the observed CVs for solution-dissolved 1 at the GC electrode in aqueous 0.1 M (43) Brown, A. P.; Anson, F. C. Anal. Chem. 1977, 49, 1589. (44) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398. (45) The E°′ of the SAM of 1 is unchanged when the electrolyte concentration of Na2SO4 was increased to 0.5 and 1 M.

Na2SO4 (pH 2) varies linearly with υ1/2, indicating a masstransfer controlled electron-transfer redox reaction. A nearly identical CV, with the same E°′ value, was obtained (not shown) for solution-dissolved 1, when a bare Au electrode was used in place of a GC electrode. The E°′ of SAM of 1 and mixed monolayers of different Γ values are equal to that of the solution-dissolved 1 (E°′ ) 0.56 V, estimated as the average of the anodic and cathodic peak potentials) and the difference in the formal potentials (∆E°′ ) E°′self-ass. - E°′soln) is marginal (Figure 6). Thus it is expected that the environment around the redox centers in mixed monolayers at all Γ values is almost the same as that for a dissolved species in aqueous solution. The same E°′ for both solution-dissolved and surface-adsorbed species and the ∆Efwhm values significantly larger than the thermodynamically ideal value (90.6 mV) at different Γ values (Table 1) imply that although the nickel(II) redox centers in the self-assembly experience electrostatic repulsive interactions, the difference between rr and ro is not of considerable amount to furnish a change in E°′ with Γ. This conclusion can be further supported by the self-assembly of redox active species where the charge of the monolayer changes from neutral to +1/-1 charge.10,12,24,39 The E°′ of the ferrocene/ferrocenium couple of the self-assembly of ferrocene-alkanethiol varies largely with the Γ values and the ∆E°′ (E°′self-ass. - E°′soln) is significant (>100 mV) even at a Γ value of ferrocene as low as