Stepwise Assembly of Acceptor−Sigma Spacer−Donor Monolayers

spacer through an amide bond to complete the acceptor-sigma spacer-donor assembly. ..... were carried out in a three-electrode cell using a CHI 660A...
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Langmuir 2006, 22, 3388-3395

Stepwise Assembly of Acceptor-Sigma Spacer-Donor Monolayers: Preparation and Electrochemical Characterization Smita Sarkar and S. Sampath* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India ReceiVed July 9, 2005. In Final Form: January 6, 2006 Self-assembled monolayers comprising benzoquinone-methylene spacer-ferrocene molecules have been prepared on gold surfaces using a stepwise assembly procedure. A base monolayer of cystamine is formed on a gold surface. Benzoquinone is then attached to the amine end of the cystamine monolayer by a Michael’s addition reaction. Subsequently, a diaminoalkane spacer of varying length is introduced. Finally, ferrocene is attached to the diamonoalkane spacer through an amide bond to complete the acceptor-sigma spacer-donor assembly. The distance between the two redox moieties has been varied systematically by altering the length of the alkyl chain spacer present between them. The quinone attachment to the cystamine monolayer leads to two different redox forms, a mono- and a diamino derivative. The pKa values have been evaluated for both of the derivatives. The monomolecular layer assembly has been characterized extensively using electrochemical techniques and the electrochemical kinetic parameters have been evaluated at different stages of modification.

Introduction Donor-acceptor interactions are essential for the sustenance and maintenance of several biological and synthetic processes. Toward this end, understanding electron-transfer (ET) reactions involving a donor and an acceptor has been at the forefront of research for a long time.1-3 For example, it is very essential to comprehend the ET kinetics between the donor and the acceptor moieties to develop systems mimicking photosynthesis.4,5 A large number of model systems based on donor-acceptor (D-A) assemblies including several dyads and triads have been synthesized,6-10 and various aspects such as the nature of the spacer and extrinsic factors such as temperature have been * Corresponding Author. E-mail: [email protected]. (1) (a) Bowler, B. E.; Raphael, A. L.; Gray, H. B. Prog. Inorg. Chem. 1990, 38, 259. (b) Taube, H. In Electron-Transfer Reactions in Solution; Achademic Press: New York, 1970. (c) Jortner, J. Biochim. Biophys. Acta 1980, 594, 193. (2) (a) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (b) Marcus, R. A. Annu. ReV. Phys. Chem. 1965, 16, 155. (c) Tunneling in Biological Systems; Chance, B., DeVault, D. C., Frauenfelder, H., Marcus, R. A., Schrieffer, J. R., Sutin, N., Eds.; Academic: New York, 1979. (3) (a) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vols. 1-5. (b) Page, C. C.; Moser, C. C.; Chen, X.; Dutton, P. L. Nature 1999, 402, 47. (c) Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996, 100, 13148. (4) (a) Anoxygenic Photosynthetic Bacteria; Blankenship, R. E., Madigan, M. T., Bauer, C. E., Eds.; Kluwer Academic Publishing: Dordrecht, The Netherlands, 1995. (b) Kochi, J. K. Acc. Chem. Res. 1992, 25, 39. (5) (a) Steinberg-Yfranch, G.; Liddell, P. A.; Hung, S.-C.; Moore, A. L.; Gust. D.; Moore, T. A. Nature 1997, 385, 239. (b) Warncke, K.; Dutton, P. L. Biochem. 1993, 32, 4769. (6) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (b) Imahori, H.; Ozawa, S., Ushida, K.; Takahashi, M.; Azuma, T.; Ajavakom, A.; Akiyama, T.; Hasegawa, M.; Taniguchi, S.; Okada, T.; Sakata, Y. Bull. Chem. Soc. Jpn. 1999, 72, 485. (c) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (7) (a) Perepichka, D. F.; Bryce, M. R.; Pearson, C.; Petty, M. C.; McInnes, E. J. L.; Zhao, J. P. Angew. Chem., Int. Ed. 2003, 42, 4636. (b) Simon, J. A.; Curry, S. L.; Schmehl, R. H.; Schatz, T. R.; Piotrowiak, P.; Jin, X.; Thummel, R. P. J. Am. Chem. Soc. 1997, 119, 11012. (8) (a) Zeng, Y.; Zinnet, M. B. J. Phys. Chem. 1992, 96, 8395. (b) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem. ReV. 1996, 96, 759. (c) Zaleski, J. M.; Chang, C. K.; Leroi, G. E.; Lukier, R. I.; Nocera, D. G. J. Am. Chem. Soc. 1992, 114, 3564. (9) (a) Padden-Row, M. N. Acc. Chem. Res. 1994, 27, 18. (b) Paulson, B. P.; Curtis, L. A.; Bal, B.; Class, G. L.; Miller, J. R. J. Am. Chem. Soc. 1996, 118, 378. (10) (a) Fukuzumi, S.; Okamoto, K.; Imahori, H. Angew. Chem., Int. Ed. 2002, 41, 620. (b) Okamoto, K.; Imahori, H.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 7014. (c) Fukuzumi, S.; Okamoto, K.; Yoshida, Y.; Imahori, H.; Araki, Y.; Ito, O. J. Am. Chem. Soc. 2003, 125, 1007.

analyzed to understand their effects on the interaction between the donor and the acceptor.6-9 Efforts have also been expended to promote ET reactions by adding electron-transfer catalysts.10 The D-A systems have also been used for developing a variety of novel applications such as electrogenerated chemiluminescence11 and molecular rectification.12 Most of the literature reports on D-A systems pertain to studies in the solution phase,13,14 and the efforts directed toward analyzing these systems after immobilization on a solid support matrix15-17 have not been substantial. Organic thin films based on selfassembly provide a convenient route to make such assemblies.18 Self-assembled monolayers (SAMs) based on alkanethiols on gold are among the most extensively investigated systems.19 Owing to their ease of preparation and high degree of ordering, they have been widely used for various applications in biomaterial fabrication, corrosion resistance, tribology, adhesion, wetting, molecular recognition, sensors, nanolithography, etc.20-22 (11) Lai, R. Y.; Kong, X.; Jenekhe, S. A.; Bard, A. J. J. Am. Chem. Soc. 2003, 125, 12631. (12) (a) Metzger, R. B. Acc. Chem. Res. 1999, 32, 950. (b) Martin, A. S.; Sambles, J. R.; Ashwell, G. J. Phy. ReV. Lett. 1993, 70, 218. (13) (a) Hackett, J. W., II; Turro, C. J. Phys. Chem. A 1998, 102, 5728. (b) Sumida, J. P.; Liddell, P. A.; Lin, S.; Macpherson, A. N.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. A 1998, 102, 5512. (c) Napper, A. M.; Reed, I.; Kaplan, R.; Zunmt, M. B.; Waldeck, D. H. J. Phys. Chem. A 2002, 106, 5288. (14) (a) Antonello, S.; Crisna, M.; Formaggio, F.; Moretto, A.; Taddei, F.; Toniolo, C.; Meran, F. J. Am. Chem. Soc. 2002, 124, 11503. (b) Paulson, B. P.; Curtiss, L. A.; Bal, B.; Class, G. L. Miller, J. R. J. Am. Chem. Soc. 1996, 118, 378. (c) Ryu, C. K.; Wang, R.; Schmehl, R. H.; Ferrere, S.; Luduikow, M.; Merkert, J. W.; Headford, C. E. L.; Elliot, C. M. J. Am. Chem. Soc. 1992, 114, 430. (15) (a) Lahav, M.; Katz, E.; Willner, I. Electroanalysis 1998, 10, 1159. (b) Pacsial, E. J.; Alexander, D.; Alvarado, R. J.; Tomasulo, M.; Raymo, F. M. J. Phys. Chem. B 2004, 108, 19307. (16) He, Z.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E.; Sabapathy, R. C.; Hussey, C. L. J. Electroanal. Chem. 1998, 458, 7. (17) Berchmans, S.; Ramalechume, C.; Lakshmi, V.; Yegnaraman, V. J. Mater. Chem. 2002, 12, 2538. (18) (a) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1998; Vol. 19, p 109. (b) Clegg, R. S.; Hutchison, J. E. In Electron Transfer in Chemisty; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol 4, p 541. (19) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (c) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733.

10.1021/la051857i CCC: $33.50 © 2006 American Chemical Society Published on Web 03/02/2006

Assembly of Acceptor-Sigma Spacer-Donor Monolayers

Willner and co-workers15 have reported the SAM formation with naphthoquinone and ferrocene but the monolayer design does not confer to the acceptor-σ spacer-donor architecture. The monolayer has a “Y” type structure where the top arms are attached to the redox units, whereas the bottom is attached to the gold surface. Hussey and co-workers16 have aimed at the synthesis of a naphthoquinone-ferrocene based monolayer, but upon reaction of the surface bound quinone with the ferrocene alkylamine, the quinonoid functionality is reported to be lost. Apart from this, Berchmanns and co-workers17 have reported on the synthesis of an anthraquinone-ferrocene based monolayer, but the structure of the monolayer is unclear. The present work reports on the development of a molecular assembly based on “quinone-sigma spacer-ferrocene” architecture on a gold surface. Ferrocene is a one electron donor having a well-defined and reversible electrochemical response both in the solution phase and in the monolayer form.23 Quinones constitute an interesting group of electroactive compounds24 which are prevalent in various systems involved in ET reactions such as photosynthetic reaction centers and mitochondria. Thus, a combination of benzoquinone and ferrocene would result in a good model system to understand the ET kinetics in a D-A assembly immobilized on a matrix. Additionally, this D-A assembly is amenable for possible sandwiching between two metal surfaces to obtain a molecular rectification device. In the present study, a benzoquinone-sigma spacer-ferrocene (BQσn-Fc) monolayer has been assembled on the surface of gold using a stepwise assembly procedure and characterized at each step of modification. The molecular rectification characteristics obtained with this configuration are communicated separately. Experimental Section Chemicals. Cystamine dihydrochloride (cyst), 1,4-benzoquinone (BQ), 1,3-diaminopropane (DA3), 1,5-diaminopentane (DA5), 1,8diaminooctane (DA8), 1,10-diaminodecane (DA10), ferroceneacetic acid (FAA), and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) were all products of Aldrich, U.S.A. and used without further purification. Potassium ferrocyanide trihydrate (K4Fe(CN)6‚3H2O), potassium ferricyanide (K3Fe(CN)6), potassium dihydrogen phosphate (KH2PO4), and di-potassium hydrogen phosphate (K2HPO4) were of analytical grade from SD Fine Chemicals, India. Analytical reagent grade acetonitrile (MeCN) and ethanol were used for the experiments. Acetonitrile was distilled over CaH2 under nitrogen, passed through activated alumina, and stored in a tightly capped bottle over 4 Å molecular sieves,25 and ethanol was distilled prior to use. Water was purified using a Milli-Q system (Millipore). All of the buffer solutions were made from reagent grade chemicals. Phosphate buffer (0.1 M, pH 7.2) was prepared from equimolar mixtures of KH2PO4 and K2HPO4. All of the quinone solutions were kept in the dark to prevent undesirable photochemical transformations.26 (20) (a) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (b) Sanghvi, R.; Kumar, A.; Lopez, G. P.; Stephanopoulas, G. N.; Wang, D. I. C.; Whitesides, G. M. Science 1994, 264, 696. (21) (a) Bardea, A.; Katz, E.; Buckmann, A. F.; Willner, I. J. Am. Chem. Soc. 1997, 119, 9114. (b) Laibinis, P. E.; Nuzzo, R. G.; Whitsides, G. M. J. Phys. Chem. 1992, 96, 6. 5097. (22) (a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (b) Chan, K. C.; Kim, T.; Schoer, J. K. Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875. (c) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (23) (a) Smalley, J. F.; Finklea, H. O.; Chidsey, C. E. D.; Linford, M. R.; Creager, S. E.; Ferraris, J. P.; Chalfant, K.; Zawodzinsk, T.; Feldberg, S. W.; Newton, M. D. J. Am. Chem. Soc. 2003, 125, 2004. (b) Carter, M. T.; Rowe, G. K.; Richardson, J. N.; Tender, L. M.; Terrill, R. G.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 2896. (24) Morton, R. A. In Biochemistry of Quinones; Academic: New York, 1955. (25) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. In Purification of Laboratory Chemicals; Pergamon Press: Elmsford, NY, 1996. (26) He, P.; Crooks, R. M.; Faulkner, L. R. J. Phys. Chem. 1990, 94, 1135.

Langmuir, Vol. 22, No. 7, 2006 3389 Substrate Preparation. The SAMs were prepared on polycrystalline Au disk electrode surfaces (4 mm dia, CH Instruments, USA). The gold electrodes were cleaned by immersing in two successive baths of freshly prepared piranha solution for a time period of 10 min each (Caution: piranha reacts violently when comes in contact with organic compounds). The extent of cleanliness of the electrode surface was monitored by recording the cyclic voltammogram in an equimolar (1 mM) solution of K4Fe(CN)6/K3Fe(CN)6 couple dissolved in 0.1 M KCl. A potential difference of 0.065 V (at a scan rate of 0.05 V/s) between the oxidation and reduction peaks indicated a clean gold surface. The cleaning procedure was repeated until reproducible cyclic voltammograms were obtained for the redox reaction of K4Fe(CN)6/K3Fe(CN)6. This process of cleaning results in an increase in the surface roughness factor (Rf) for the gold electrodes. The area under the oxide-stripping peak, obtained from the cyclic voltammogram of gold electrode recorded in 0.5 M H2SO4 solution by cycling between 0.5 and 1.4 V, was integrated to get the real surface area.27 An average roughness factor of 3 was obtained reproducibly. The coverage values given in the paper are corrected for the roughness factor. Monolayer Formation. Cystamine Monolayer. The formation of the base monolayer was carried out by immersing the cleaned gold electrode in a 5 mM aqueous solution of cystamine.2HCl for 12 h.28 The substrate was taken out and washed well with water to remove any physisorbed material. Benzoquinone Monolayer. The benzoquinone monolayer was formed by reacting the cystamine monolayer modified gold surface with a 0.5 M solution of benzoquinone in an equivolume ratio of MeCN and phosphate buffer solution.29 After modification with benzoquinone, the sample was washed well with EtOH to remove any trace of physisorbed quinone present on the surface. Diaminoalkane Modification. Four different diaminoalkanes were used to form the spacer between the quinone and the ferrocene. The dimainoalkanes used are 1,3-diaminopropane, 1,5-diaminopentane, 1,8-diaminooctane, and 1,10-diaminodecane. The conditions employed for the diaminoalkane reaction to the BQ-SAM depend on the length of the diminoalkane used. An ethanolic solution of amine in 30% v/v ratio was used for chain lengths n ) 3 and 5,28-30 and a 0.5 M ethanolic solution of diamines was used for high chain lengths, n ) 8 and 10. The surface modification reaction was carried out for 24 h, and subsequently, the substrate was washed well with EtOH. Attachment of Ferrocene Acetic Acid to the Monolayer. The diamine modified surface was immersed in a saturated solution of ferrocene acetic acid (FAA) in 0.1 M phosphate buffer solution (pH 7.2) for 48 h in the presence of a coupling agent, EDC. This resulted in the attachment of ferrocenyl groups with the formation of an amide bond between the surface amine groups and the carboxylic group of FAA. Upon modification, the gold surface was washed well with the buffer followed by EtOH to remove any unreacted ferrocene acetic acid molecules. The functionalized gold electrodes were stored in EtOH after each step of modification and the samples were dried well in a stream of argon and subsequently kept in a desiccator to remove the solvent before the experiments. Electrochemical Measurements. Electrochemical measurements were carried out in a three-electrode cell using a CHI 660A Electrochemical analyzer (CH Instruments, U.S.A.) or Autolab electrochemical system II PGSTAT30 (Ecochemie, Netherlands). The modified gold disk electrode served as the working electrode. The counter electrode was a platinum foil of area 4 cm2. A saturated calomel electrode (SCE) {Hg, Hg2Cl2/Cl- (saturated)} was used as the reference electrode in all of the experiments. The electrolyte (27) Angerstein- Kozlowska, H. In ComprehensiVe Treatise of Electrochemistry; Yeager, E., Bockris, J. O. M., Conway, B. E., Sarangapani, S., Eds.; Plenum: New York, 1994; Vol 9, p 24. (28) (a) Katz, E.; Solov’ev, A. A. J. Electroanal. Chem. 1990, 291, 171. (b) Katz, E.; Schmidt, H.-L. J. Electroanal. Chem. 1993, 360, 337. (29) Lukkari, J.; Kleemola, K.; Meretoja, M.; Kankare, J. Chem. Commun. 1997, 1099. (30) Budaveri, V.; Szucs, A.; Somlai, Cs.; Novak, M. Electrochim. Acta 2001, 47, 4351.

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Scheme 1

used was either 0.1 M HClO4 or 0.1 M phosphate buffer (pH 7.2). The electrolytes were deaerated with high purity N2 before each experiment, and an atmosphere of N2 was maintained in the cell during the measurements. Cyclic voltammetry was carried out at scan rates between 0.005 V/s and 20 V/s depending on the experiment. Impedance measurements were carried out using an ac signal of 0.005 V rms in two different frequency ranges, a range of 106 Hz to 0.01 Hz for the measurement of electrochemical rate constant and a range of 106 Hz to 0.1 Hz for monitoring the blocking characteristics of the monolayer. The ac impedance measurements to follow the blocking behavior of the monolayer were carried out at the formal potential of the redox probe, 0.1V vs SCE.

Figure 1. (a) Nyquist plots for gold electrode (1) after and (2) before modification with cystamine SAM, in the presence of 1mM Fe(CN)64-/3- in 0.1 M KCl. The impedance spectra have been obtained with 0.005 V rms as a signal at a dc bias of 0.1 V. The frequency range used is 0.1 Hz to 106 Hz. (b) Cyclic voltammograms of Au (i) before and (ii) after modification with cystamine at a scan rate of 0.05 V/s in 0.1 M phosphate buffer in the presence of dissolved oxygen. Scheme 2

Results and Discussion The acceptor-spacer-donor assembly has been constructed based on Schemes 1 and 2. The concentration of the coupling agent, EDC, is optimized to be 3 equiv with respect to ferrocene acetic acid. Cystamine Monolayer. Cystamine (2,2′-dithiobisethaneamine) has been widely used as a linker molecule to attach various functionalities onto metallic surfaces.28 It is known to form a defective monolayer on the surface of gold.31 The monolayer integrity has been probed using AC impedance spectroscopy32 and the apparent surface coverage (θ) is determined32 based on the following equation:

θ ) [1 - (Rct/R′ct)] × 100

(1)

where Rct indicates the charge-transfer resistance on an unmodified gold surface and R′ct is the corresponding charge-transfer resistance for the modified surface. The charge-transfer resistance is dependent on the coverage as well as the defects associated with the monolayer. Hence, the θ defined above is only an apparent coverage indicative of the blocking nature of the modified electrode. Figure 1a shows the impedance characteristics of the gold electrode before and after modification with cystamine in the presence of 1 mM Fe(CN)64-/3- in 0.1 M KCl. Based on the relative increase in the Rct values of the SAM modified surface as compared to the bare gold surface, it can be concluded that there is a blocking monolayer present on the surface. The hydrated Fe(CN)63- ion has a diameter of 6.0 Å32 and a diffusion coefficient (31) Shon, Y.-S.; Lee, S.; Colorado, R.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 7556. (32) Bandyopadhyay, K.; Vijaymohanan, K.; Shekhawat, G. S.; Gupta, R. P. J. Electroanal. Chem. 1998, 447, 11.

of 8.3 × 10-6 cm2/s. Another probe molecule, dissolved oxygen, has also been used to follow the blocking behavior. Figure 1b shows the cyclic voltammograms for the oxygen reduction on gold before and after adsorption of cystamine in 0.1 M phosphate buffer solution of pH 7.2. On a bare gold surface, the oxygen

Assembly of Acceptor-Sigma Spacer-Donor Monolayers

Figure 2. Cyclic voltammogram of the BQ-II SAM in 0.1 M phosphate buffer (pH 7.2) at a scan rate of 0.05 V/s. Arrows denote the peaks used for calculating the surface coverage.

reduction occurs at -0.2 V (Figure 1b; curve i). The electrochemical reduction of oxygen becomes less facile on a modified surface and shifts to a very negative potential (-0.8 V; figure not shown). Upon modification, there is a considerable decrease in the currents and no peak is observed at -0.2 V (Figure 1b; curve ii). This observation reveals that the monolayer modified electrode shows a perfect blocking behavior toward dissolved oxygen probe. The approximate sizes of the probe molecules, hydrated Fe(CN)63- and hydrated oxygen, are estimated to be 6 and 8.5 Å, respectively. Hence, the coverage based on accessibility should not be very different for the two probe molecules. However, the accessibility being higher for Fe(CN)64-/3- may be due to the fact that the protonated -NH2 groups on the monolayer probably concentrates the Fe(CN)64-/3species by electrostatically binding them leading to a good accessibility. The pKa of the -NH2 group of the cystamine monolayer has been determined to be 7.6 using an impedance based technique proposed by White and co-workers.33a Accordingly, the amines are in the protonated form in the Fe(CN)64-/3solution used for checking the blocking characteristics of the monolayer. Consequently, the Rct values have been observed to depend on the pH of the electrolyte, the values being large in the alkaline range (above a pH of 8), whereas the values decrease when the pH of the electrolyte is made acidic. This confirms that the electrostatic interactions play an important role in the chargetransfer resistance observed for the cystamine modified electrode toward Fe(CN)64-/3- couple. The apparent surface coverage (eq 1) of cystamine-modified gold has been studied as a function of adsorption time, based on the oxygen reduction current at -0.2 V (Supporting Information). Although a very high coverage is attained within 1 h, the adsorption time is kept at 12 h for further modification, to form a completely blocking monolayer. Benzoquinone Monolayer. The amino groups attached to the cystamine monolayer undergo Michael’s addition with the benzoquinone. The reaction of cystamine monolayer with 0.5 M solution of benzoquinone for 3 h results in two sets of redox peaks (Figure 2) with formal potentials at -0.2 and -0.5 V at a scan rate of 0.05 V/s. The plots of peak current vs scan rate for both of the peaks (Supporting Information) are linear showing that the redox activity is from adsorbed species.33b Voltammograms have been recorded for the intentionally physisorbed quinone, and it shows redox activity at a potential more positive (E0f ) 0.15 V) than the chemically attached species. So, the redox peaks observed after the Michael’s addition are due to immobilized quinones, but their origin could result from either of the two possibilities. (1) The same benzoquinone molecule undergoes two successive 1e- reduction processes to generate radical anion and dianion, respectively, or (2) the two redox (33) (a) Smith, C. P.; White, H. S. Langmuir 1993, 9, 1; Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385. (b) Southampton Electrochemistry Group; In Instrumental methods in electrochemistry; John Wiley: New York, 1985.

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peaks could indicate the presence of quinones in two different environments. Since an aqueous buffer electrolyte is used (0.1 M phosphate, pH 7.2) for the electrochemical studies, the stabilization of the radical anion and the dianion would not be possible. Additionally, it has been found that each of the redox peaks corresponds to a 2e- 2H+ process, based on the pH dependent studies as will be discussed later. Hence, the two redox peaks result from the possibility of the benzoquinone in two different environments. When an amine group is introduced, the electrochemical potential of quinone would shift to more negative values compared to that of the unsubstituted quinone due to the electron donating ability of the amines. Hence, the peak observed at -0.2 V is assigned to the monoamino derivative of quinone and that observed at -0.5 V to the diamino derivative as given in Scheme 1. Katz and co-workers28 and Budaveri and co-workers30 have reported similar shifts in the electrochemical potential when an amine is attached to the quinone. In the present study, it is observed that both mono- and diamino derivatives coexist in the monolayer. Lukkari and co-workers29 reported the formation of only the monoamino derivative of quinone when a base monolayer of 4-aminothiophenol is reacted with benzoquinone. This may be attributed to the difference in nucleophilicity of the amine group present in the aromatic amine as compared to the aliphatic amine, cystamine, used in the present study. Concentration Dependence. The concentration of BQ used in the reaction mixture is varied to understand the effect of availability of BQ on the formation of mono- and diamino derivatives. The voltammograms for the benzoquinone monolayer formed with three different BQ concentrations (time of reaction has been kept constant at 3 h) are shown in Figure 3. The surface concentrations of quinone are determined based on the areas under the redox peaks observed in the voltammograms and an electrode roughness factor of 3 (Table 1). A low quinone concentration of 1 mM in the deposition solution results in the formation of primarily the diamino derivative of quinone (BQ-I SAM), giving a redox peak at -0.5 V (Figure 3a). The redox activity of the monoamino derivative is observed to be very low. As the quinone concentration in the deposition medium is increased to 500 mM, a relative increase in the surface concentration of monoamino derivatives is observed (Figure 3b; BQ-II SAM). Further increase in the concentration of quinone to 1000 mM does not considerably increase the amount of monoamine derivative in SAM (Figure 3c). In the BQ-II SAM, the surface concentration of the monoamino derivative is 2.85 (( 0.1) × 10-10 mol/cm2 and that of the diamino derivative is 1.53 (( 0.1) × 10-10 mol/cm2. The surface concentration of quinone in the BQ-I SAM, which predominantly comprises the diamino derivatives of quinone, is calculated to be 3.00 (( 0.15) × 10-10 mol/cm2. It has been reported that benzoquinone occupies an area of 0.527 nm2 when lying flat and an area of 0.277 nm2 when oriented edge-wise34 on a platinum surface. This leads to a theoretical coverage of 3.15 × 10-10 and 5.99 × 10-10 mol/ cm2 for face-on and edge-wise orientations, respectively. Though the surface coverage values observed in the present studies do not yield any information on the orientations of the quinone species, vibrational spectroscopic evidence points to different orientations for the two derivatives as given in the subsequent paper. Time Dependence. The concentration of benzoquinone in the deposition medium is kept constant at 0.5 M while the reaction time is varied. At short time scales, the peak current of the diamino (34) Soriaga, M. P.; Wilson, P. H.; Hubbard, A. T. J. Electroanal. Chem. 1982, 142, 317.

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Figure 4. (a) Cyclic voltammograms of (i) bare Au and (ii) BQ-II SAM on gold in the presence of 1mM Fe(CN)64-/3- in 0.1 M KCl at a scan rate of 0.05 V/s. (b) Complex impedance plot of BQ SAM in the presence of 1mM Fe(CN)64-/3-.

Figure 3. Cyclic voltammograms of benzoquinone monolayers prepared under different conditions in 0.1 M phosphate buffer at a scan rate of 0.05 V/s. The concentrations of BQ used in the bulk solution for Michael’s addition reaction are (a) 1 mM; (b) 500 mM; and (c) 1000 mM. Table 1. Surface Concentrations and the Corresponding Coverage Ratios for the Redox Peaks Due to the Mono and Diamino Derivatives of Quinone for Three Different BQ SAMs Prepared from Different Concentrations of Quinone in the Reactant Solutiona peak I conc (mM)

Γ (mol/cm2)

peak II Epc (V)

Γ (mol/cm2)

coverage ratio Epc (V)

1 peak absent 3 × 10-10 -0.525 500 2.85 × 10-10 -0.250 1.53 × 10-10 -0.53 1000 2.87 × 10-10 -0.255 1.64 × 10-10 -0.53 a

peak I:peak II 1.86:1 1.75:1

The deposition time is kept constant at 3 h.

derivative is higher than that of the monoamino derivative (Supporting Information). The peak current ratio changes in favor of the monoamino derivative as the reaction time is increased. This value attains a maximum of unity at a reaction time of 3 h. Further increase in the reaction time has little effect on the peak current ratio. The charge under the redox peaks yields very similar results. The concentration of quinone in the reaction medium is optimized at 0.5 M with a reaction time of 3 h to obtain a high percentage of monoamino quinone. Structural Integrity of BQ-II SAM. Unlike the cystamine monolayer, the BQ-II SAM having both mono- and diamino derivatives of quinone show a good blocking behavior toward K4Fe(CN)6/K3Fe(CN)6 redox couple. Figure 4a shows the redox behavior of Fe(CN)64-/3- on bare gold as well as BQ-II SAM modified gold surfaces. The redox response is completely

suppressed with the BQ-II SAM modified surface (Figure 4a; curve ii). To quantitatively understand the blocking behavior, an electrochemical treatment using the Tafel approach based on a procedure reported by Krysinski and co-workers35 has been followed. The apparent rate constants, kapp, for the probe molecule have been calculated. The heterogeneous rate constant determined for the BQ-II SAM modified gold surface is 1.95 × 10-7 cm/s. This is five-orders of magnitude lower than the corresponding values obtained on a bare gold surface reflecting the excellent blocking characteristics of the BQ-II SAM. The blocking behavior has also been followed using AC impedance measurements in the presence of Fe(CN)64-/3-. A charge-transfer resistance of 48.8 kΩcm2 is determined based on the diameter of the semicircle obtained from the Nyquist plot (Figure 4b), and an apparent coverage of 95% is obtained using eq 1. The base monolayer of cystamine shows poor blocking ability due to the presence of a large number of defect sites in the SAM. However, upon covalent modification of cystamine with benzoquinone, a good blocking behavior is observed pointing to the reduction of defects in the monolayer. This is likely to be due to the size and orientation of the quinones that block the accessibility of the probe redox molecules. pH Dependent Study of BQ-II SAM. The redox reaction of a quinone species in the aqueous environment is represented by the eq 4 where, Q is quinone and QH2 is the corresponding hydroquinone

Q + 2H+ + 2e- ) QH2

(4)

The formal potential of a quinone depends on the pH of the medium according to 0 ) E0f - (2.3RTm/nF)pH EpH

(5)

where n is number of electrons involved in the redox process, m is number of protons associated with the reaction, and R, T, and F are the gas constant, temperature and Faraday constant, respectively. In aqueous solutions, the number of electrons and the number of protons involved in the redox process per quinone molecule is 2 provided the process proceeds below the pKa of (35) Krysinski, P.; Brzostowska-Smolska, M. J. Electroanal. Chem. 1997, 424, 61.

Assembly of Acceptor-Sigma Spacer-Donor Monolayers

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Figure 6. Cyclic voltammograms of BQ-II SAM (i) before and (ii) after reaction with 1,10-diaminodecane at a scan rate of 0.05 V/s in 0.1 M HClO4.

Figure 7. Cyclic voltammogram of the BQ-σ10-Fc SAM in 0.1 M HClO4 at a scan rate of 0.05 V/s.

Figure 5. (a) Cyclic voltammograms of BQ-II SAM as a function of pH of the solution at a scan rate of 0.05 V/s. Plot of formal potential (E0f ) of (b) redox peak I and (c) redox peak II as a function of pH.

the electrogenerated hydroquinone. In this pH regime, the slope of the plot of E0pH vs pH is expected to be -0.059 V/pH.15,36 Figure 5a shows the representative cyclic voltammograms for BQ-II SAM in buffer solutions of three different pH values. The formal potential, E0f , is plotted against the solution pH for both monoamino and diamino derivatives of the BQ-II SAM. There are two regions observed with different slopes, and the point at which the slope change occurs corresponds to the electrochemical pKa of the redox species being monitored. Figure 5, panels b and c, shows the plots of E0f vs pH, in the expanded pH range of 8.5-11, for the monoamino and diamino derivatives of BQ, respectively. A slope of -0.057 V/pH is obtained up to a pH of 9.5 (Figure 5b). The slope changes to -0.038 V/pH, and the break occurs at a pH of 9.6 for the monoamino derivative. In the case of diamino derivative (Figure 5c), the point of transition is at a pH of 10.5, and the slopes change from a value of -0.055 to -0.032 V/pH. The change in slope signifies a change in mechanism (2e-, 2H+ process to a 2e-, 1H+ process) beyond the pKa values. The difference in the pKa is ascribed to the electron donating effect of the second amine group in the diamino derivative of BQ. The pKa of the immobilized BQ species observed in the present studies is different from the bulk pKa of 9.9.36 Introduction of Diaminoalkane Spacer. The cyclic voltammograms of BQ-II SAM in 0.1 M HClO4, before and after the reaction with 1,10-diaminodecane, are shown in Figure 6. The voltammogram after the attachment of the diaminoalkane spacer (36) Bailey, S. I.; Ritchie, I. M. Electrochim. Acta, 1985, 30, 3.

does not show the presence of redox peaks at 0.17 V. Correspondingly, an increase in the peak currents for the redox couple present at -0.05 V is observed. The monoamino derivative that is present at 0.17 V (curve i) is converted to a diamino derivative at -0.05 V (curve ii) and the conversion is found to be quantitative. Prior to the diamine modification, the surface concentrations for both monoamino and diamino derivatives are determined to be 2.65 (( 0.1) × 10-10 and 1.61 (( 0.1) × 10-10 mol/cm2 respectively. The surface concentration obtained for the BQ-II SAM after diamine modification, 4.4 (( 0.1) × 10-10 mol/cm2. The quantitative conversion is observed for all the four spacer lengths used. Structural Integrity of BQ-DA10 SAM. The monolayer integrity after the diamine modification is followed using an external redox probe as described earlier. The heterogeneous rate constant of the Fe(CN)64-/3- in the presence of BQ-DA10 SAM is determined to be 4.35 × 10-8 cm/s based on the procedure reported by Krisynski and co-workers.35 This is smaller than the values obtained on a bare gold surface (1.50 × 10-2 cm/s) and on BQ-II SAM modified gold before the diamine attachment (1.95 × 10-7 cm/s). Thus, it is evident that the monolayer becomes better ordered after diamine attachment than that observed in the previous step. The relative increase in the order of the monolayer could be attributed to an increased interchain van der Waals interaction based on the long alkyl chains. The charge-transfer resistance calculated from the Nyquist plot (Supporting Information) shows a high value of 1.43 MΩ cm2 revealing that the monolayer is highly blocking in nature. Benzoquinone-σ10-Ferrocene Assembly. The diamino spacer attached to the BQ monolayer is further reacted with ferrocene acetic acid to obtain the final BQ-σ spacer-Fc assembly. The cyclic voltammetric response of the BQ-σ10-Fc monolayer in 0.1 M HClO4 shows two sets of redox peaks (Figure 7). The peaks centered at 0.25 V are due to ferrocene and the peaks having a formal potential of -0.05 V are due to benzoquinone. The plots of peak current, Ip, vs scan rate show a linear behavior for both ferrocene and benzoquinone (Supporting Information).

3394 Langmuir, Vol. 22, No. 7, 2006

The stability of the ferrocene monolayers are reported to be highly dependent on the nature of the electrolyte used.37 The ferrocene monolayer is found to be stable in perchloric acid and phthalate buffer but a constant loss in electroactivity of ferrocene is observed when the supporting electrolyte is changed to phosphate buffer, tris buffer, and HEPES buffer (pH 7.2). The loss in electroactivity results from the loss of ferricenyl moiety and not due to the desorption of the monolayer. Upon cycling in electrolytes such as phosphate buffer, tris buffer, and HEPES buffer, there is a constant reduction in peak currents corresponding to the redox peaks for ferrocene, but the redox peaks due to benzoquinone molecule are practically unchanged. A surface concentration of 1.27 (( 0.15) × 10-10 mol/cm2 has been calculated for ferrocene based on the voltammogram given in Figure 7. A theoretical surface coverage of 4.5 × 10-10 mol/ cm2 is possible if ferrocene is treated as a sphere with diameter of 0.66 nm.37 The coverage observed in the present studies is less than the theoretically expected value. It should be pointed out that only half of the quinone sites are available for further functionalization with ferrocene based on the previous step of modification. Blocking Behavior of A-σ Spacer-D SAM. The voltammogram for the assembly A-σ spacer-D in the presence of the freely diffusing redox probe, Fe(CN)64-/3-, shows small currents but not well-defined peaks (Supporting Information). The heterogeneous rate constant for the probe is determined35 to be 1.37 × 10-7 cm/s. This value is one order higher than the value obtained for the monolayer up to the diaminoalkane modification. The charge-transfer resistance (Rct ) 650 kΩ cm2) obtained from the complex impedance plots is two times lower than that obtained for the diamino derivative of the BQ SAM (BQ-DA10 SAM). This is likely to be due to the following reasons. (1) The defects on the monolayer have increased leading to reduced charge-transfer resistance or (2) the redox reaction of ferrocyanide/ ferricyanide couple is enhanced. In the present study, it is probable that the second reason is appropriate. This is in line with the observations on the voltammogram for the assembly A-σ spacer-D in the presence of Fe(CN)64-/3-. The currents associated with the voltammogram are higher than the currents observed in the previous step though the peaks are not sharp. This is rationalized as follows: Uosaki and co-workers38 have reported the electron exchange of ferrocenyl group with Fe(II) species in the solution and found that the ferrocene units in the monolayer act as a mediator for Fe(III) reduction. In the present study, the formal potential of ferrocene matches with the redox potential of the Fe(CN)64-/3- couple and hence the SAM probably acts as a mediator as observed by Uosaki and co-workers.38 Kinetic Parameter of the A-σ Spacer-D SAM. The heterogeneous rate constants associated with the redox species of the monolayer have been determined using impedance measurements. The potential separation between the anodic and cathodic peaks of ferrocene is small (0.029 V). The formal potential of ferrocene, E0f is not affected by the scan rate, for all spacer lengths. The invariance in the values of E0f indicates that electron-transfer coefficients Ra and Rc for anodic and cathodic processes are almost equal (Ra ) Rc ) 0.5). AC impedance spectroscopy is well suited to measure the electrochemical rate constants of adsorbed redox centers.41-43 In this technique, the system is marginally perturbed by a small (37) (a) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521 (b) Chidsey, C. E. D.; Bertocci, C. R.; Putvinski, T. M.; Mujsec, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (38) Sato, Y.; Itoigawa, H.; Uosaki, K. Bull. Chem. Soc. Jpn. 1993, 66, 1032. (39) Chidsey, C. E. D. Science 1991, 251, 919. (40) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173.

Sarkar and Sampath Table 2. Heterogeneous Rate Constants Determined for BQ and Fc in the BQ-σn-Fc Assembly by AC Impedance Spectroscopy, as a Function of Spacer Length rate constant from AC impedance measurements, s-1 spacer lengtha

-BQ

ferrocene

3 5 8 10

340 335 288 389

3809 1890 127.8 40

a The spacer length signifies the number of CH2 units in the diaminoalkane chain.

amplitude sinusoidal voltage and measurements are carried out at the formal potential of the redox species. The electrochemical rate constant, ks, is determined from the linear plot of cot φ vs frequency where φ is phase angle. The heterogeneous rate constants calculated for ferrocene and benzoquinone based on the initial linear region are listed in Table 2. It should be pointed out that the linearity predicted for the cot φ vs frequency plot for a tunneling process is not observed in the present studies (supporting Information). The exact reason is presently not clear. We believe that the order and the compactness of the assembly is one of the reasons for this observation. The compactness is not maintained in the present case at any stage. This is revealed in the symmetric and antisymmmetric methylene stretches being higher than the expected values of 2850 and 2920 cm-1, respectively (Supporting Information). This is true in all of the cases (right from cystamine stage) irrespective of the stage of modification. This might result in a variation in the chain structure within the same assembly. It may also lead to differences in the solvation around the redox centers. A distribution of redox centers will result in the distrubution of rate constants. This possibly may have an effect in the determination of rate constants by impedance measurements. However, the results are reproducible. Mukae and co-workers44 have reported a rate constant value of 9.9 s-1 for naphthoquinone attached to cystamine monolayer. Hong and Park45 have evaluated the kinetic parameters of hydroquinone terminated SAM to be 5 s-1 and 0.405 for the rate constant and transfer coefficient, respectively. Substantial work has been carried out toward evaluating the kinetic parameters of ferrocene monolayer.18,37,40,46,47 A range of values has been reported for the rate constant depending on the linker group, the distance of the redox moiety from the electrode surface, and the nature of the base monolayer. Uosaki and co-workers48 have reported a value of 1.2 × 104 s-1 for an undecylferrocenyl thiol monolayer. Creager and co-workers46 have reported rate constants of 1800 and 1300 s-1, respectively, for monolayers, where one of them is linked through a dodecanethiol spacer and the other linked through an amide bond with decanethiol spacer. Murray and co-workers47 have carried out extensive studies on the temperature dependence of rate constants for ferrocene monolayers with varying chain lengths. In the present studies, the heterogeneous rate constant of the ferrocenyl unit in the monolayer (41) Finklea, H. O.; Ravencroft, M. S.; Snider, D. A. Langmuir 1993, 9, 223. Brevnon, D. A.; Finklea, H. O.; Ryswyk, H. V. J. Electroanal. Chem. 2001, 500, 100. (42) Laviron, E. J. Electroanal. Chem. 1979, 7, 135. (43) Yan, J.; Dong, S.; Li, J. Chen, W. J. Electrochem. Soc. 1997, 144, 3858. (44) Mukae, F.; Takemura, H.; Takehara, K. Bull. Chem. Soc. Jpn. 1996, 69, 2461. (45) Hong, H.-G.; Park, W. Langmuir 2001, 17, 2485. (46) Sumner, J. J.; Weber, K. S.; Creager, S. E. J. Phys. Chem. B 2000, 104, 7449. (47) Richrdson, J. N.; Peck, S. R.; Curtin, L. S.; Tender, L. M.; Terrill, R. H.; Carter, M. T.; Murray, R. W.; Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1995, 99, 766. (48) Sato, Y.; Itoigawa, H.; Uosaki, K. Bull. Chem. Soc. Jpn. 1993, 66, 1032.

Assembly of Acceptor-Sigma Spacer-Donor Monolayers

decreases with increase in the spacer length (Table 2). The tunneling parameter as determined from the slope of the plot of ln ks vs number of CH2 units is 0.7( 0.03 per CH2 unit. The literature reports a wide range of values from 1.07 to 0.6 per CH2 unit.18a,23a,41,44,45

Conclusions A self-assembled monolayer based on the A-σ spacer-D configuration has been designed and synthesized successfully with a systematic variation of the alkyl chain spacer between the donor and acceptor moieties. The assembly has been characterized by electrochemical techniques at each step of monolayer formation. The reaction conditions, time of deposition and concentration of quinone in the deposition solution are found to play an important role in determining the surface concentration of the monoamino and diamino derivatives of BQ in the

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benzoquinone monolayer. The pKa values have been evaluated for the monoamino and diamino derivatives of benzoquinone. The heterogeneous rate constants for ferrocene show a distance dependence from the electrode surface while the BQ rate constant is almost invariant as a function of spacer length. A detailed spectroscopic and spectroelectrochemical characterization of this monolayer assembly is given in the subsequent paper. Acknowledgment. This work was supported by funds from DST and CSIR, New Delhi, India. The authors gratefully acknowledge the useful discussions with Prof. S. Bhattacharya. Supporting Information Available: Figures 1S-8S and Tables 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org. LA051857I