Langmuir 1998, 14, 3797-3807
3797
Host-Guest Complexation in Self-Assembled Monolayers: Inclusion of a Monolayer-Anchored Cationic Ferrocene-Based Guest by Cyclodextrin Hosts Rajaram C. Sabapathy, Sukanta Bhattacharyya, W. E. Cleland, Jr., and Charles L. Hussey* Department of Chemistry, University of Mississippi, University, Mississippi 38677 Received July 5, 1996. In Final Form: April 17, 1998 A novel ferrocenyl disulfide of the type ω,ω′-dithiobis[alkyl(dimethyl)(ferrocenylmethyl)ammonium bromide] was synthesized and coassembled with nonanethiol on vapor deposited Au(111) electrode surfaces. The coassembled ferrocene-based cationic guest monolayer assembly was found to bind with β- and γ-cyclodextrins to form stable host-guest complexes. The cyclodextrin hosts are believed to interact with the monolayer assembly in two ways: by inclusion of the ferrocene moiety within the cyclodextrin cavity and by interactions between the counterions (e.g., Br-, PF6-, or ClO4-) and the hydrophilic planes of the cyclodextrin molecules. Interactions between the monolayer and cyclodextrin are manifested primarily by a decrease in the voltammetric oxidation current for the surface-bound ferrocenyl redox centers after exposure to dilute solutions (e.g., 10 µM) of the cyclodextrin hosts. This decrease in electrochemical activity stems from the restricted access of charge-compensating anions to the ferrocenyl redox center. It is proposed that the unusual stability of the resulting cationic ferrocene-cyclodextrin complex is due to dual occupancy of the cyclodextrin hosts by the surface-bound cationic ferrocene-based guests and their counterions. Supporting evidence for the interaction of cyclodextrin hosts with the ferrocene-based cationic guest monolayer was obtained using infrared reflection-absorption spectroscopy (IRRAS).
Introduction The self-assembly of organosulfur compounds on gold and other surfaces, e.g., Ag and Pt, is a convenient route to customization of the molecular architecture of the electrode-solution interface. This has been achieved through the selective use of functionalized tail groups in the synthesis of the adsorbates.1 Through this method, the chemical and physical properties of the interface have been manipulated to generate a variety of surfaces with well-defined composition and structure.2 As a result of this surface engineering, the monolayer interface can be modified to exhibit quite unique recognitive properties for specific molecules. This behavior constitutes hostguest interactions (molecular recognition) at an interface; such interactions are achieved only by modifying the reactivity of the electrode so that it is selectively responsive to some species of interest. Investigations of host-guest interactions give insight into the interplay between molecular recognition phenomena and molecular organization.3 The potential applications of host-guest systems as molecular sensors require the immobilization of either the host molecule or the guest molecule on an inert surface. (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly, Academic Press: Boston, MA, 1991. (b) Tredgold, R. H. Order in Thin Organic Films; Cambridge University Press: New York, 1994; Chapter 6. (c) Ulman, A. Chem. Rev. 1996, 96, 1533 and references therein. (2) (a) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (c) Zhong, C.-J.; Porter, M. D. Anal. Chem. 1995, 67, 709A. (3) (a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (b) Sun, L.; Johnson, B.; Wade, T.; Crooks, R. M. J. Phys. Chem. 1990, 94, 8869. (c) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (d) Steinberg, S.; Tor, Y.; Sabatini, E.; Rubinstein, I. J. Am. Chem. Soc. 1991, 113, 5176. (e) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (f) Sun, L.; Kepley, L. J.; Crooks, R. M. Ibid. 1992, 8, 2101. (g) Sun, L.; Crooks, R. M.; Ricco, A. J. Ibid. 1993, 9, 1775. (h) Schierbaum, K. D.; Weiss, T.; Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N.; Gopel, W. Science 1994, 265, 1413.
Studies of the binding properties of host-based monolayers (receptors) to specific guests (substrates) are central to the understanding and further development of molecular sensors in self-assembled monolayers. To this end, hostguest interactions resulting in the formation of inclusion complexes have been studied extensively both in aqueous and in nonaqueous media.4 The immobilization of hostguest systems also affords a novel approach to the stepwise functionalization of surfaces; i.e., surface-confined molecules of either the host or guest can present a template upon which subsequent immobilization of the former or latter can take place.5 However, there are only a few reports concerning host-guest interactions between surface-confined monolayers and solution species.6-8 For example, Stirling and co-workers6 reported the immobilization of a tetrapodal calix-4-resorcinarenethiol on gold and its subsequent selectivity for vitamin C. On the other hand, Reinhoudt and co-workers7a,7b described the preparation and self-assembly of novel resorcin[4](4) (a) Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D., Eds.; Inclusion Compounds; Academic Press: New York, 1985; Volume 1-3. (b) Vogtle, F.; Weber, E., Eds.; Host-Guest Complex Chemistry; Springer-Verlag: Berlin, 1985. (c) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89. (d) Schneider, H.-J. Ibid. 1991, 30, 1417. (e) Seel, C.; Vogtle, F. Ibid. 1992, 31, 528. (f) Buckingham, A. D., Legon, A. C., Roberts, S. M., Eds. Principles of Molecular Recognition; Blackie Academic & Professional (Chapman & Hall): London, 1993. (5) Spinke, J.; Liley, M.; Guder, H.-J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821. (6) Adams, H.; Davis, F.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1994, 2527. (7) (a) Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Am. Chem. Soc. 1994, 116, 3597. (b) Thoden van Velzen, E. U.; Engbersen, J. F. J.; de Lange, P. J.; Mahy, J. W. G.; Reinhoudt, D. N. Ibid. 1995, 117, 6853. (c) Huisman, B.-H.; Thoden van Velzen, E. U.; van Veggel, F. C. J. M.; Engbersen, J. F. J.; Reinhoudt, D. N. Tetrahedron Lett. 1995, 36, 3273. (d) Huisman, B.-H.; Kooyman, R. P. H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Mater. 1996, 8, 561. (8) (a) Rojas, M. T.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 5883. (b) Zhang, L.; Godinez, L. A.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Angew. Chem., Int. Ed. Engl. 1995, 34, 235. (c) Rojas, M. T.; Koniger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336. (d) For an excellent review, see: Kaifer, A. E. Isr. J. Chem. 1996, 36, 389.
S0743-7463(96)00660-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/09/1998
3798 Langmuir, Vol. 14, No. 14, 1998
arene-based receptor molecules, and calix[4]arene derivatives7c on gold substrates, whereas Kaifer and coworkers8a reported on the host-guest interactions between surface-confined cyclobis(paraquat-p-phenylene) monolayers on gold and both catechol and indole in an aqueous medium. In a separate study, they also reported on the interaction between water soluble sulfonated calix[6]arene hosts and self-assembled ferrocenyl alkanethiol monolayers on gold.8b Very recently, Kaifer and co-workers8c reported the formation of mixed monolayers of β-cyclodextrin and pentanethiol (C5SH) on gold and the recognitive properties of such monolayers for micromolar quantities of ferrocene in solution. The cyclodextrins (CD) belong to a class of naturally occurring receptors that are cyclic molecules made up of 6-12 glucopyranose units linked through R-(1 f 4) glycosidic bonds. The most common of these macrocyclic oligosaccharides are R-, β-, and γ-CD, which contain six, seven, and eight glucose units, respectively.9 The most prominent structural feature of the cyclodextrin molecule is the rigid, well-constructed cylindrical cavity (toroidal
shape) capable of including a variety of guest molecules to form stable host-guest inclusion complexes, both in the solid and in aqueous solutions. Cyclodextrin is one of the most widely studied water soluble molecular receptors (hosts) and is capable of forming inclusion complexes with a variety of molecular substrates (guests) ranging in polarity from hydrophobic to ionic.9d,e,h This is attributed to the amphiphilic character of cyclodextrin with its two large hydrophobic surfaces located on the inside and outside of the torus and to the presence of two hydrophilic edges where primary and secondary hydroxyl groups are located. Lately, a growing interest in the behavior of cyclodextrins as ideal host molecular receptors has spawned numerous studies involving either the derivatization and self-assembly of CDs on gold surfaces8c,10 or the casting of thin polymer films of the CDs on electrode surfaces.11 One example of a class of nonpolar type guest molecules that has been studied quite extensively with regard to (9) (a) Harata, K.; Uedaira, H. Bull. Chem. Soc. Jpn. 1975, 48, 375. (b) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; SpringerVerlag: Berlin, 1978. (c) Saenger, W. Angew. Chem., Int. Ed. Engl. 1980, 19, 344. (d) Szejtli, J. Cyclodextrins and Their Inclusion Complexes; Akademiai Kiado: Budapest, 1982. (e) Li, S.; Purdy, W. C. Chem. Rev. 1992, 92, 1457. (f) Ueno, A.; Minato, S.; Osa, T. Anal. Chem. 1992, 64, 2562. (g) Odashima, K.; Kotato, M.; Sugawara, M.; Umezawa, Y. Ibid. 1993, 65, 927. (h) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 803. (i) Donze, C.; Coleman, A. W. J. Inclusion Phenom. Mol. Recognit. Chem. 1995, 23, 11. (j) Breslow, R.; Halfon, S.; Zhang, B. Tetrahedron 1995, 51, 377. (10) (a) Maeda, Y.; Kitano, H. J. Phys. Chem. 1995, 99, 487. (b) Moore, L. W.; Springer, K. N.; Shi, J.-X.; Yang, X.; Swanson, B. I.; Li, D. Adv. Mater. 1995, 7, 729. (c) Henke, C.; Steinem, C.; Janshoff, A.; Steffan, G.; Luftmann, H.; Sieber, M.; Galla, H.-J. Anal. Chem. 1996, 68, 3158. (d) Weisser, M.; Nelles, G.; Wohlfart, P.; Wenz, G.; Mittler-Neher, S. J. Phys. Chem. 1996, 100, 17893. (e) Nelles, G.; Weisser, M.; Back, R.; Wohlfart, P.; Wenz, G.; Mittler-Neher, S. Ibid. 1996, 118, 5039. (f) He, P.; Ye, J.; Fang, Y.; Suzuki, I.; Osa, T. Electroanalysis 1997, 9, 68. (11) (a) Kutner, W. Electrochim. Acta 1992, 37, 1109. (b) Kutner, W.; Doblhofer, K. J. Electroanal. Chem. 1992, 326, 139. (c) Kutner, W.; Storck, W.; Doblhofer, K. J. Inclusion Phenom. Mol. Recognit. Chem. 1992, 13, 257. (d) Lepretre, J.-C.; Saint-Aman, E.; Utille, J.-P. J. Electroanal. Chem. 1993, 347, 465. (e) D’Souza, F.; Hsieh, Y.-Y.; Wickman, H.; Kutner, W. J. Chem. Soc., Chem. Commun. 1997, 1191.
Sabapathy et al.
cyclodextrin hosts is the organometallic complexes,12 especially ferrocene.13 Although there are numerous reports describing the formation of stable inclusion complexes between ferrocene and cyclodextrins in solution, the article by Kaifer and co-workers8c is the only report describing the formation of a ferrocene-based inclusion complex by a cyclodextrin monolayer host anchored to a gold surface. It is conceivable that the ability of a ferrocene guest to form an inclusion complex with a cyclodextrin host in solution will not be the same when either the guest or host is immobilized on a surface. This is because the binding of a guest to a host under such conditions requires the close approach of the former to the latter before any kind of noncovalent interaction can occur. The driving force for inclusion complexation is believed to be the release of high-energy water molecules from the cyclodextrin cavities by guest molecules, the release of strain energy of the ring, and the opportunities for intermolecular interactions such as hydrogen bonding and London dispersion forces.4a Consequently, host(guest)-based monolayer-solution interfaces that do not participate in such interactions usually do not bind to the respective guest (host) molecules. In view of this limitation, making the monolayersolution interface either hydrophilic or hydrophobic with respect to the characteristics of the self-assembled host or guest may afford these modified surfaces with the means to reject or attract substrates, depending on the chemical nature of the latter.14 The amphiphilic nature of cyclodextrin confers on these immobilized molecular hosts the unique ability to interact with both polar and nonpolar guest molecules. It must be borne in mind that, with the unique geometry of the cyclodextrin molecule, any potential interaction with guests (either in solution or on the surface) can take place on two different levels, i.e., the inclusion of the species within the hydrophobic cavity or the inclusion within the hydrophilic plane defined by the hydroxyl groups of the CD. Generally, inorganic anions, e.g., Br-, ClO4-, and PF6-, have been shown to interact with the hydrophilic planes of the cyclodextrin molecule to form inclusion complexes that are similar in stability to complexes containing neutral species within the hydrophobic cavity.15 In this article, we report the synthesis and self-assembly of a novel amphiphilic ferrocene-based guest molecular substrate on vapor-deposited gold electrode surfaces. The (12) (a) Alston, D. R.; Slawin, A. M. Z.; Stoddart, J. F.; William, D. J. Angew. Chem., Int. Ed. Engl. 1985, 24, 786. (b) Colquhoun, H. M.; Stoddart, J. F.; William, D. J. Ibid. 1986, 25, 487. (13) (a) Harada, T.; Takahashi, S. J. Inclusion Phenom. 1984, 2, 791. (b) Maeda, Y.; Takashima, Y. Ibid. 1984, 2, 799. (c) Harada, A.; Takahashi, S. J. Chem. Soc., Chem. Commun. 1984, 645. (d) Ueno, A.; Moriwaki, F.; Osa, T.; Hamada, F.; Murai, K. Tetrahedron Lett. 1985, 26, 899. (e) Matsue, T.; Evans, D. H.; Osa, T.; Kobayashi, N. J. Am. Chem. Soc. 1985, 107, 3411. (f) Kobayashi, N.; Osa, T. Chem. Lett. 1986, 421. (g) Matsue, T.; Kato, T.; Akiba, U.; Osa, T. Ibid. 1986, 843. (h) Harada, A.; Hu, Y.; Yamamoto, S.; Takahashi, S. J. Chem. Soc., Dalton Trans. 1988, 729. (i) Strelets, V. V.; Mamedjarova, I. A.; Nefedova, M. N.; Pysnograeva, N. I.; Sokolov, V. I.; Pospisil, L.; Hanzlik, J. J. Electroanal. Chem. 1991, 310, 179. (j) Isnin, R.; Salam, C.; Kaifer, A. E. J. Org. Chem. 1991, 56, 35. (k) McCormack, S.; Russell, N. R.; Cassidy, J. F. Electrochim. Acta 1992, 37, 1939. (l) Kaifer, A. E. In Transition Metals in Supramolecular Chemistry; Fabrizzi, L., Poggi, A., Eds.; NATO ASI Series; Kluwer: Dordrecht, The Netherlands, 1994. (m) Yilmaz, V. T.; Karadag, A.; Icbudak, H. Thermochim. Acta 1995, 261, 107. (n) Luong, J. H. T.; Brown, R. S.; Schmidt, P. M. J. Mol. Recognit. 1995, 8, 132. (o) Godinez, L. A.; Sonal, P.; Criss, C. M.; Kaifer, A. E. J. Phys. Chem. 1995, 99, 17449. (p) Nielson, R. M.; Lyon, L. A.; Hupp, J. T. Inorg. Chem. 1996, 35, 970. (q) Castro, R.; Cuadrado, I.; Alonso, B.; Casado, C. M.; Moran, M.; Kaifer, A. E. J. Am. Chem. Soc. 1997, 119, 5760. (14) (a) Mrksich, M.; Grunwell, J. R.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 12009. (b) Anzai, J.; Guo, B.; Osa, T. Bioelectrochem. Bioenerg. 1996, 40, 35.
Host-Guest Complexation
Langmuir, Vol. 14, No. 14, 1998 3799 Scheme 1
cationic guest-based ferrocenylalkyl disulfide, 1, was
coassembled with nonanethiol (C9SH) and exposed to micromolar concentrations of β- and γ-CD. The resulting complexes were subsequently characterized with cyclic voltammetry (CV) and infrared-reflection absorption spectroscopy (IRRAS). Cationic/neutral ferrocene-based amphiphiles similar to 1 have been used in host-guest studies with cyclodextrin in solution13j,o,p and have also been used as potential redox-active surfactant micelles.16 Experimental Section Chemicals. Sulfuric acid (98%, H2SO4), perchloric acid (70%, HClO4), hydrogen peroxide (30%, H2O2), and 2-propanol (Optima Grade) were purchased from Fisher Scientific and were used as received. Sodium sulfate (99.99%, Na2SO4), potassium hexafluorophosphate (98%, KPF6), and nonanethiol (95%, C9SH) were obtained from Aldrich Chemical Co. and were also used as received. Absolute ethanol (McCormick Distilling Co. Inc.) and sodium perchlorate (99.99%, Acros Organics) were used without further purification. The cycloamyloses (β- and γ-CD) were supplied by Aldrich Chemical Co. and were used as received. Ultrapure water was obtained from a Milli-Q Plus water purification system (four cartridge Millipore system) and was used for the preparation of all aqueous solutions. Methylene chloride (CH2Cl2) and hexanes were dried by distillation from calcium hydride. Triethylamine (N(C2H5)3) was distilled from KOH before use. 8-Bromooctanoic acid, oxalyl chloride, cystamine dihydrochloride, N,N-dimethylaminomethylferrocene and anhydrous dimethylformamide (DMF) were obtained from Aldrich Chemical Co. and used without further purification. Synthetic Procedures. The 1H NMR spectra reported here were obtained on a Bruker AC-E 300 spectrometer using CDCl3 as the solvent and TMS (Me4Si) as the internal standard. Melting points were recorded using a Thomas-Hoover apparatus and are uncorrected. Flash column chromatography was performed on E. Merck silica gel 60 (230-400 mesh). For the removal of (15) (a) Lewis, E. A.; Hansen, L. D. J. Chem. Soc., Perkin Trans. 1973, 2, 2081. (b) Mochida, K.; Kagita, A.; Matsui, Y.; Date, Y. Bull. Chem. Soc. Jpn. 1973, 46, 3703. (c) Wojcik, J. F.; Rohrbach, R. P. J. Phys. Chem. 1975, 79, 2251. (d) Rohrbach, R. P.; Rodriguez, L. J.; Eyring, E. M.; Wojcik, J. F. Ibid. 1977, 81, 944. (e) Buvari, A.; Barcza, L. Inorg. Chim. Acta 1979, 33, L179. (f) Gelb, R. I.; Schwartz, L. M.; Radeos, M.; Laufer, D. A. J. Phys. Chem. 1983, 87, 3349. (g) Sanemasa, I.; Fujiki, M.; Deguchi, T. Bull. Chem. Soc. Jpn. 1988, 61, 2663. (h) Buvari, A.; Barcza, L. J. Inclusion Phenom. Mol. Recognit. Chem. 1989, 7, 379. (i) Taraszewska, J.; Wojcik, J. Supramol. Chem. 1993, 2, 337. (16) (a) Saji, T.; Hoshino, K.; Aoyagui, S. J. Chem. Soc., Chem. Commun. 1985, 865. (b) Facci, J. S. Langmuir 1987, 3, 525. (c) Donohue, J. J.; Buttry, D. A. Ibid. 1989, 5, 671. (d) Gomez, M. E.; Li, J.; Kaifer, A. E. Ibid. 1991, 7, 1571; 1797. (e) De Long, H. C.; Donohue, J. J.; Buttry, D. A. Ibid. 1991, 7, 2196. (f) Medina, J. C.; Gay, I.; Chen, Z.; Echegoyen, L.; Gokel, G. W. J. Am. Chem. Soc. 1991, 113, 365. (g) Zu, X.; Rusling, J. F. Ibid. 1997, 13, 3693.
solvents, “in vacuo” refers to the vacuum achieved by a water aspirator attached to a rotary evaporator. All elemental analyses were performed by Atlantic Microlab, Inc., Atlanta, GA. The synthesis of compound 1 was carried out according to the procedure outlined in Scheme 1. Synthesis of N,N ′-Bis(8-bromooctanoyl)cystamine (1a). A mixture of 8-bromooctanoic acid (2.23 g, 10 mmol), 10 mL of a 2.0 M oxalyl chloride solution in methylene chloride, 4 drops of triethylamine, and anhydrous methylene chloride (60 mL) was stirred for 8 h at room temperature. The solvent and excess oxalyl chloride were removed in vacuo to give crude 8-bromooctanoyl chloride as an oil, which was used in the next step of the synthesis. A saturated aqueous solution of cystamine dihydrochloride was made alkaline by the addition of solid NaOH, and the aqueous phase was extracted with CH2Cl2 several times. The organic extracts were dried (Na2CO3) and then concentrated in vacuo to give the free base cystamine as a colorless oil. The crude acid chloride was redissolved in 50 mL of methylene chloride, and 2.8 mL of triethylamine was added. The solution was cooled in an ice bath and cystamine (0.60 g, 4 mmol) in methylene chloride (25 mL) was added dropwise over a 15 min period with stirring. The resulting mixture was stirred at room temperature for 8 h, after which it was washed with 1 M HCl (2 × 20 mL), water (20 mL), 1 M NaOH (2 × 15 mL) and repeated with water (2 × 20 mL). The organic layer was dried with Na2SO4 and removal of the solvent in vacuo gave a white solid, which was then recrystallized from a methylene chloride-hexanes mixture to give pure 1a: 1.55 g (70%); mp 79 °C; 1H NMR (CDCl3) δ [ppm] 1.30-1.48 (m, 12 H), 1.61-1.69 (m, 4H), 1.85 (quin, J ) 7.0 Hz, 4H), 2.22 (t, J ) 7.5 Hz, 4H), 2.83 (t, J ) 7.5 Hz, 4H), 3.41 (t, J ) 7.0 Hz, 4H), 3.58 (q, J ) 6.8 Hz, 4H), 6.31 (t, J ) 6.8 Hz, 2H). Anal. Calcd for C20H38Br2S2N2O2: C, 42.71; H, 6.81; N, 4.98; S, 11.40. Found: C, 42.94; H, 6.72; N, 4.96; S, 11.51. Synthesis of N,N ′-Bis[8-dimethyl(ferrocenylmethyl)ammoniumoctanoyl]cystamine Dibromide (1). A mixture of 1a (0.56 g, 1 mmol) and N,N-dimethylaminoferrocene (0.54 g, 2.2 mmol) in anhydrous DMF (10 mL) was heated at 50-60 °C (bath temperature) for 7 h under a nitrogen atmosphere. The clear solution was cooled to room temperature and diluted with diethyl ether (200 mL), which resulted in the precipitation of the quaternary ammonium salt (1) as a thick yellow oil. The ether solution was decanted and the oily precipitate was triturated several times with diethyl ether until the washings were colorless. Drying of the residue under a vacuum (0.1 mmHg) produced pure 1 as a bright yellow oil: 0.53 g (50%). An analytically pure sample of 1 was obtained by dissolving the yellow oil in anhydrous methylene chloride and reprecipitating 1 by the addition of anhydrous diethyl ether, followed by the removal of the solvent in vacuo and then drying under a vacuum. 1H NMR (CDCl3) δ [ppm]: 1.36-1.45 (m, 12H), 1.65-1.82 (m, 8H), 2.35 (t, J ) 7.2 Hz, 4H), 2.95 (t, J ) 6.8 Hz, 4H), 3.16 (s, 12H), 3.52-3.62 (m, 8H), 4.29 (s, 10H), 4.36 (t, J ) 1.8 Hz, 4H), 4.49 (t, J ) 1.8 Hz, 4H), 4.71 (s, 4H), 8.05 (t, J ) 6.5 Hz, 2H). Anal. Calcd for C46H72S2N4O2Br2Fe2: C, 52.68; H, 6.92; N, 5.34; S, 6.11. Found: C, 52.38; H, 6.98; N, 5.38; S, 6.20. Preparation of Au Substrates. Glass microscope slides (Fisher Scientific) served as the substrate. They were cleaned by ultra sonication in successive baths of “piranha” solution (1:3 by volume 30% H2O2/concentrated H2SO4), deionized water (Millipore), and 2-propanol. [Caution! Care must be exercised with the handling of the “piranha” solutions as they are
3800 Langmuir, Vol. 14, No. 14, 1998
Sabapathy et al.
Scheme 2
Figure 1. Typical cyclic voltammogram of a SAM of 1 on a gold electrode recorded in a 1.0 M NaClO4 solution; ν ) 100 mV/s. potentially explosive and therefore should be disposed of appropriately after each use.17] The slides were then oven dried and coated with ∼50-60 Å of a chromium (Alfa Æ SAR, 99.99%) adhesion layer that was deposited at a rate of 0.2-0.3 Å/s by thermal evaporation. The substrates were then coated with 1500-1600 Å of gold (Alfa Æ SAR Premion, 99.9985%) deposited at a rate of 5-6 Å/s. The gold-coated slides prepared by this method were analyzed by X-ray diffraction techniques and were found to exhibit strong Au(111) characteristics.18 The deposition was carried out at a base pressure of 5 × 10-7 Torr using an Edwards Auto 306 vacuum coater equipped with a Sycon Model STM-100/MF film thickness monitor. After the coating procedure was complete, the vacuum chamber was backfilled with high purity nitrogen gas, and the freshly coated gold slides were used immediately for the preparation of the SAMs. Preparation of the SAMs. The gold electrodes were taken directly from the vacuum coater and immersed in an ethanolic solution containing compound 1 at a concentration of ca. 1 mM for about 24 h. There was no significant change in the surface coverage of 1 beyond this immersion time. Alternatively, SAMs composed of mixed monolayers of 1 and nonanethiol (C9SH) were also fabricated by immersing the Au/1 electrodes prepared as described above in solutions containing an equimolar concentrations of 1 and nonanethiol (C9SH). The total concentration of the adsorbates in the coating solution was maintained at ca. 1 mM. The gold electrodes (Au/1 + C9SH) were then rinsed with copious quantities of absolute ethanol, followed by Millipore water to remove any excess adsorbate. Prior to any electrochemical characterization, the modified electrodes were immersed in a 1.0 M KPF6 solution to exchange bromide (Br-) in the monolayer for the more inert hexafluorophosphate (PF6-) anion. Following electrochemical and/or the spectroscopic characterization, the gold electrodes modified with 1 and C9SH were exposed to dilute solutions of β- and γ-CD solutions. After a period of time, the modified electrodes were rinsed with copious amounts of Millipore water to remove excess cyclodextrin from the electrode surface, and the gold electrodes modified with cyclodextrin were characterized again. The entire self-assembly protocol and the three stages where electrochemical and/or spectroscopic characterization was carried out are outlined in Scheme 2. These steps where electrochemical and/or spectroscopic characterization was carried out are indicated with an asterisk. For comparison purposes, a 1.0 mM solution of 1 + 0.25 mM of β-cyclodextrin (precomplexed in solution) was prepared and subsequently self-assembled on the gold electrodes. (17) (a) Dobbs, D. A.; Bergman, R. G.; Theopold, K. H. Chem. Eng. News 1990, 68, (17), 2. (b) Wnuk, T. Ibid. 1990, 68 (26), 2. (c) Matlow, S. L. Ibid. 1990, 68 (30), 2. (18) (a) He, Z.; Bhattacharyya, S.; Cleland, W. E., Jr.; Hussey, C. L. J. Electroanal. Chem. 1995, 397, 305. (b) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. M. Jeon, N.; Nuzzo, R. A. Langmuir 1995, 11, 4371.
Electrochemical Measurements. All voltammetric measurements were carried out in a three-electrode cell using an EG&G Princeton Applied Research Corp. (PARC) Model 283 potentiostat/galvanostat employing PARC Model 270/250 Research Electrochemistry Analysis Software (v. 4.23), running on an IBM-compatible 486 computer. Gold-coated substrates (25 × 25 mm and 40 × 25 mm) served as the working electrodes. They were clamped against a PTFE O-ring in a joint on the side of the electrochemical cell. The O-ring provided a liquid tight seal and also defined the area of the working electrode, which was estimated to be about 1.54 cm2. Only half of the 40 × 25-mm electrode was modified with the ferrocene host, leaving the other half as a control. The reference electrode was a sodium-saturated calomel electrode (SSCE) isolated in a tube fitted with a Luggin capillary. All of the potentials reported in this paper were measured with respect to this reference. The counter electrode was a platinum wire spiral immersed in a solution of the supporting electrolyte. The supporting electrolytes used were HClO4 and NaClO4, and all experiments were carried out at room temperature (ca. 24 °C). The solution in the electrochemical cell was deaerated with high-purity N2 gas before any electrochemical measurements were initiated. Spectroscopic Characterization of the SAMs. Surface reflection spectra were obtained using a Bruker Model IFS66 FTIR spectrometer equipped with a narrow-band, liquid N2-cooled MCT (Hg-Cd-Te) detector. A Plexiglass glovebox covered the spectrometer sample compartment. The glovebox and the spectrometer were purged with high-purity N2 gas from the bleedoff of a liquid-N2 tank. All spectra were acquired at 2 cm-1 resolution using a zero filling factor of 2. The surface spectra of the monolayer films before and after immersion in the molecular host solutions (β- and γ-CD) were obtained with p-polarized light incident at a grazing angle of 86° from the surface normal by coadding 128 signal-averaged scans in the 4000-400 cm-1 region. All of the spectra presented herein have been minimally baseline corrected and smoothed using a routine computer program. The transmission spectra of the cyclodextrin hosts were obtained using a Perkin-Elmer Paragon 500 FTIR spectrometer. The spectra were obtained at 4.0 cm-1 resolution by coadding 32 signalaveraged scans of the β- and γ-CD dispersed in KBr in the 4400450 cm-1 region.
Results and Discussion Electrochemical Characterization of Gold Electrodes Modified with Compound 1. Self-assembled monolayers of 1 on vapor-deposited Au(111) surfaces were investigated using cyclic voltammetry. The voltammetric response of these monolayers in HClO4 and NaClO4 (1.0 M) was characterized by the one-electron reversible oxidation of the surface-confined ferrocenyl tail group,
Host-Guest Complexation
[Fe(cp)2 h Fe(cp)2+ + e-] (Figure 1). The formal oxidation potential of the Fe(cp)20/+ redox couple of 1 was found to be ca. +0.43 V vs SSCE. The separation between the anodic and cathodic peak potentials (∆Ep) was typically smaller than 20 mV at moderate scan rates (
