Electrochemical and Spectroscopic Characterization of Self

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Electrochemical and Spectroscopic Characterization of Self-Assembled Monolayers of Ferrocenylalkyl Compounds with Amide Linkages Rajaram C. Sabapathy, Sukanta Bhattacharyya, Montray C. Leavy, Walter E. Cleland, Jr., and Charles L. Hussey* Department of Chemistry, University of Mississippi, University, Mississippi 38677 Received September 15, 1997X Short chain ferrocenylalkyl disulfides, where n < 9, with the amide functionality (-OdCsN) at variable positions were synthesized and self-assembled on vapor-deposited gold. The effects of interchain hydrogen bonding on the stability, packing density, electron transfer rate and the formal potential of these selfassembled monolayers were investigated using cyclic voltammetry and infrared reflection-absorption spectroscopy. Interchain hydrogen bonding appeared to enhance the stability of these monolayers, although the packing densities were lower than that expected for a full monolayer. The electrochemical behavior of coadsorbed monolayers of ferrocenylalkyl disulfides and nonelectroactive unsubstituted alkanethiols was also examined. In these coadsorbed systems, an unexpected negative shift in the formal potential of the ferrocenyl compounds with the amide linkages was observed. This was attributed to disorder arising from the disruption of the hydrogen bonds within the monolayer caused by the alkanethiol “spacers”, thus making it slightly easier for the ferrocene subunits to undergo oxidation. Interchain hydrogen bonding within each surface-confined layer was also probed with infrared reflection-absorption spectroscopy. A general broadening of the amide stretch appearing around 3300 cm-1 in the surface spectra relative to the same amide stretch in the calculated surface spectra provided compelling evidence for interchain hydrogen bonding.

Introduction During the past decade, numerous studies have shown that the spontaneous adsorption of long chain alkanethiols, e.g., CH3(CH2)nSH, where n ) 9-21, on gold leads to the formation of well defined self-assembled monolayers (SAMs) whose composition and chemical and physical structure may be readily manipulated.1-13 The driving force for the spontaneous formation of such 2D assemblies involves chemical bonding of the molecules to the gold surface and intermolecular interactions within the monolayer. These intermolecular interactions usually involve the hydrocarbon tails and are derived mainly from the weak van der Waals (vdW) interactions that cause the adsorbed molecules to arrange themselves in an extended all-trans configuration on the gold surface. This fact has favored the use of organosulfur compounds with long chain alkanethiols, where n > 9, over compounds with shorter X Abstract published in Advance ACS Abstracts, December 15, 1997.

(1) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (2) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (3) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 101, 201. (4) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (5) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (6) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (7) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (8) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (9) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (10) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (b) Camillone, N.; Chidsey, C. E. D.; Liu, G.-Y.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94, 8493. (11) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (12) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991 (see also the references cited therein). (13) Ulman, A. Chem. Rev. 1996, 96, 1533.

chains because short chain alkanethiols tend to form less stable and more disordered monolayers compared to their longer chain counterparts.7,10,14,15 This general disorder and instability has been attributed to a greater concentration of gauche defects and to an overall diminution of the interchain vdW attraction energy within the monolayer.10,12,13,16,17 Lenk et al.18 reported the synthesis and self-assembly of a new semifluorinated thiol with an amide group incorporated into the backbone. They believed that the presence of the amide group in the chain provided orientational stability through intermolecular hydrogen bonding within the monolayer, thus enhancing the mechanical integrity of the assembly as a whole. Similarly, Whitesides and co-workers19 reported the preparation and self-assembly of a series of fluorinated alkanethiols with an amide (OdCsNH) moiety substituted in a position β to the thiol (SH) group. They investigated the susceptibility of several short chain alkanethiols self-assembled on gold toward exchange with longer chain alkanethiols. They concluded that one of the amide-containing alkanethiol monolayers exhibited significantly enhanced stability against thermal desorption (in vacuo) or exchange with hexadecanethiol (C16SH) in ethanol. Recently, Clegg et al.20 prepared an amide-containing alkanethiol selfassembled monolayer on gold and obtained a well-ordered and densely packed monolayer with evidence of hydrogen bonding between the neighboring amide moieties. (14) Ohtake, T.; Mino, N.; Ogawa, K. Langmuir 1992, 8, 2081. (15) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (16) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 1147. (17) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (18) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610. (19) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. A. Langmuir 1995, 11, 4371. (20) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239.

S0743-7463(97)01042-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/06/1998

SAMs of Ferrocenylalkyl Compounds

Langmuir, Vol. 14, No. 1, 1998 125

Monolayers have been functionalized with the amide moiety in the past for reasons other than stabilization; e.g., Rowe et al.21 incorporated an amide moiety into one of their ferrocenylalkanethiol derivatives to alter the polarity of the surface environment of the coadsorbed monolayer, but their investigation focused on issues like the competitive assembly process and the double-layer and interfacial solvation effects on redox reactions taking place in the SAMs. In our investigation, we have prepared several ferrocenylalkyl compounds (1-5) with the amide

moiety located strategically at different positions relative to the electroactive ferrocene tail group along the alkyl chain backbone. These amide-containing ferrocenylalkyl compounds have the potential of forming intermolecular hydrogen bonds at specific points within the monolayer (Figure 1). On the basis of previous results, the introduction of an amide moiety into a short chain alkanethiol is expected to stabilize the monolayer more than would be expected on the basis of the number of methylene groups present. With the exception of the work carried out by the Whitesides group,19 where a relatively short chain fluorinated alkanethiol was used, Lenk et al.18 and Clegg et al.20 both used relatively long alkyl chains in their studies, where n ) 12 and 19, respectively. Therefore, it would be difficult to assess the contribution of this additional intermolecular interaction (i.e., hydrogen bonding) within such a monolayer assembly because of the relatively robust vdW interactions already present, i.e., any contributions due to hydrogen bonding might be masked by the vdW interactions. Thus, by using a short chain ferrocenylalkyl compound containing an amide functionality, it should be possible to determine if hydrogen bonding helps to confer order and stability to an otherwise disordered and unstable monolayer through these interchain interactions. Compounds 1-3 are the derivatives of a disulfide, whereas compound 4 is the derivative of a thiol. The amide groups are located R, β, and γ to the ferrocene subunit for compounds 1-3, respectively. We chose to work mainly with the disulfide derivatives rather than the thiols because the latter exhibits both a higher vapor pressure and greater chemical instability in the presence of atmospheric oxygen; i.e., thiols are more prone to oxidation compared to disulfides.22 Furthermore, it was recently reported that the chemisorption of di-n-alkyl disulfides (21) (a) Rowe, G. K.; Creager, S. E. J. Electroanal. Chem. 1994, 370, 203. (b) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500. (22) Patai, S., Ed. The Chemistry of the Thiol Group; John Wiley & Sons: New York, 1974; Vols. 1 & 2.

Figure 1. Idealized models of compounds 1-5 self-assembled on a gold surface showing the proposed intermolecular hydrogen bonds within the monolayer assembly. Note the strategic positioning of the amide linkages in the different SAMs.

on gold produces monolayers that are indistinguishable from those obtained by the chemisorption of alkanethiols.23,24 Nevertheless, compound 4 was synthesized to serve as a comparison for the disulfide analogue, compound 3. Compound 5 is a cyclic disulfide of a thioctic acid derivative. The 1,2-dithiolane ring is expected to provide a wider “footprint” or base for attachment of the sulfur head group to the gold surface. Compounds 6 and 7 were prepared as control analogues to 5. They were synthesized either without the proper handle for hydrogen bonding, as in 6, or through the substitution of a non-hydrogen bonding ester (-OdCsO) group, as in 7. Similarly, compound 8 was synthesized to resemble 1, but without the amide functionality. These analogues served as controls to facilitate understanding of the effects of intermolecular hydrogen bonding within the monolayer.

(23) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (24) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825.

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The amide-containing short chain ferrocenylalkyl compounds were self-assembled on vapor-deposited Au(111) surfaces and probed electrochemically as well as spectroscopically. Varying the position of the amide group along the alkyl chain served not only to introduce interchain hydrogen bonding at different points within the monolayer, but also to anchor the bulky ferrocene subunit at a fixed position relative to the gold electrode surface, minimizing the possibility that the ferrocene subunit could be “buried” partially or wholly within the monolayer and thus helping to ensure that this electroactive group will be positioned at the monolayer-solution interface. The introduction of interchain hydrogen bonding within the monolayers of 1-5 had a pronounced effect on the stability, packing density, electron transfer rate, and the formal potential of the SAMs. This was determined from the improved voltammetric characteristics obtained from these self-assembled monolayers. Generally, we observed an overall enhancement of the stability of these short chain ferrocenylalkyl compounds on gold, even though the packing densities of the monolayers were found to be slightly lower than that expected for a full monolayer. This could be attributed to the ferrocenylalkyl chains being oriented and optimally spaced as a result of the directionality of the hydrogen bonds between neighboring amide groups. In the course of characterizing the surface-bound ferrocenylalkyl compounds, we discovered a means of observing the presence or absence of intermolecular hydrogen bonding and its effect within the monolayer. An unexpected negative shift in the formal potential was observed when a surface-confined ferrocenylalkyl compound (1-5) was coadsorbed with an unsubstituted alkanethiol. We believe that this negative shift in the formal potential is a reflection of the transition from a higher state of order to a lower state of order within the monolayer. The alkanethiol spacers are believed to disrupt the formation of interchain hydrogen bonds between neighboring ferrocenylalkyl chains. This apparent state of disorder results in an overall destabilization of the neutral ferrocene species relative to the oxidized ferrocenium species, thus making it slightly easier for the ferrocene to undergo oxidation within the coadsorbed monolayer assembly. However, the most compelling evidence for intermolecular hydrogen bonding within these monolayers was obtained when the monolayers were probed using IRRAS. It was also possible to calculate the surface reflection spectrum (derived from the bulk KBr spectrum) of most of the ferrocenylalkyl compounds examined in this study and to compare the calculated spectrum to the reflection spectrum. A general broadening of the amide stretch (NsH) appearing around 3300 cm-1 in the IRRAS spectrum of the monolayer relative to the same amide stretch in the calculated spectrum of the compound where hydrogen bonding is absent was taken to be indicative of the presence of intermolecular hydrogen bonding within the SAMs. Results and Discussion Synthesis of Compounds. The preparation of compounds 1-8 involves a general synthetic approach for the formation of ω-ferrocenylalkane disulfides and ω-ferrocenylalkanethiols via the coupling reaction of an appropriate acid chloride with an appropriate primary/ secondary amine or alcohol. Reactions of bis(5aminopentyl) disulfide with ferrocenylcarbonyl chloride

Sabapathy et al. Scheme 1

Scheme 2

Scheme 3

Scheme 4

and ferrocenylacetyl chloride yielded compounds 1 and 2, respectively (Scheme 1). Similarly, disulfide 3 was obtained by coupling β-ferrocenylethylamine with 3,3′dithiodipropionyl chloride (Scheme 2). For the preparation of thiol 4, β-ferrocenylethylamine was reacted with 3-bromopropionyl chloride to give an ω-bromopropionamide derivative, which was subsequently converted into 4 (Scheme 3). The coupling of thioctic acid chloride with β-ferrocenylethylamine, N-methyl-β-ferrocenylethylamine, and β-ferrocenylethanol gave compounds 5-7, respectively (Scheme 4). The synthesis of N-methyl-β-ferrocenylethylamine, which was required for the preparation of 6, was obtained from β-ferrocenylethylamine according to Scheme 5. Disulfide 8 was prepared in three steps starting from 6-bromo-1-hexanol, the key step being the coupling of the ferrocenylcarbonyl chloride with the bis(6-hydroxyhexyl) disulfide (Scheme 6).

SAMs of Ferrocenylalkyl Compounds Scheme 5

Scheme 6

Langmuir, Vol. 14, No. 1, 1998 127 Table 1. Electrochemical Data for Ferrocenyl Disulfides 1-5 (with the Amido Linkage) monolayer pure/ mixed χFc ) 0.67

E0′ (V)

∆Efwhma (mV)

∆Epb (mV)

1010Γc (mol cm-2)

1 1 + C8SH 2 2 + C9SH 3 3 + C7SH 4 4 + C7SH 5d 5 + C9SH

0.433 ( 0.002 0.424 ( 0.002 0.254 ( 0.004 0.240 ( 0.002 0.219 ( 0.001 0.211 ( 0.003 0.221 ( 0.002 0.213 ( 0.002 0.254 ( 0.004 0.245 ( 0.001

145 110 130 100 150 110 135 115 130 115

25 15 20 5 20 10 30 10 28 11

3.7 ( 0.2 1.08 ( 0.06 4.0 ( 0.1 0.73 ( 0.07 3.7 ( 0.2 0.34 ( 0.04 4.4 ( 0.2 0.99 ( 0.10 2.6 ( 0.1 0.85 ( 0.10

a Ideal ) 90.3 mV. b Ideal ) 0 mV. c Typical Γ ) 4.5 × 10-10 mol cm-2. d DMF was used instead of CH3CN as the solvent. Note: statistical analysis based on 95% C.I., and a sample population of N ) 30.

Table 2. Electrochemical Data for Ferrocenyl Disulfides 6-8 (without the Amido Linkage)

Characterization of the SAMs. The following sections describe the results of the electrochemical and spectroscopic characterization of the ferrocenylalkyl compounds (1-5) self-assembled onto vapor-deposited Au(111) surfaces. The primary goal of these investigations is to study the characteristics of SAMs that are derived from relatively short chain alkyl disulfides but modified to contain interchain hydrogen bonds within the monolayer film. The characterization proceeded as follows. First, electrochemical studies were carried out to evaluate both the inherent reactivity and the long-term stability of the monolayers. The electrochemical evaluation of the SAMs using cyclic voltammetry was greatly simplified by using electroactive ferrocene subunits as the tail group. The behaviors of nearly “isolated” amide moieties within a monolayer were also investigated when the ferrocenylalkyl compounds (1-5) were coadsorbed with unsubstituted alkanethiol spacers. Second, IRRAS was carried out in order to gain information about the structural details of the adsorbates.25-30 Electrochemical Studies. Parts a-e of Figure 2 show representative cyclic voltammograms in aqueous 1.0 M HClO4 of the SAMs prepared from compounds 1-5. In each voltammogram, the electrode potential was cycled between 0 and 0.8 V. As is characteristic for an adsorbed redox center, the voltammetric waves were symmetric about the potential axis. In addition, the anodic peak current (ipa) was found to increase linearly with the scan rate (ν). The SAMs resulting from the adsorption of 1-5 and 6-8 were characterized with respect to the following electrochemical parameters: surface coverage (ΓFc), formal potential (E0′), full-width at half maximum (∆EFWHM), and electron-transfer rate (∆Ep). A similar characterization was carried out for the coassembled monolayers of (1/C8SH), (2/C9SH), (3/C7SH), (4/C7SH), (5/C9SH), (6/C9SH), (7/C9SH), and (8/C8SH), and the results are tabulated in Tables 1 and 2. (a) Surface Coverage. Gold electrodes modified with 1-5 were subjected to at least 50 repetitive potential cycles (25) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (26) Porter, M. D. Anal. Chem. 1988, 60, 1143A. (27) Mielczarski, J. A. J. Phys. Chem. 1993, 97, 2649. (28) Smith, E. L.; Porter, M. D. J. Phys. Chem. 1993, 97, 8032. (29) Crooks, R. M.; Xu, C.; Sun, L.; Hill, S. L.; Ricco, A. J. Spectroscopy 1993, 8, 28. (30) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636.

monolayer pure/ mixed χFc ) 0.67

E0′ (V)

∆Efwhma (mV)

∆Epb (mV)

1010Γc (mol cm-2)

6 6 + C9SH 7 7 + C9SH 8 8 + C8SH

0.249 ( 0.001 0.262 ( 0.007 0.269 ( 0.001 0.278 ( 0.003 0.506 ( 0.003 0.515 ( 0.005

115 103 63 100 120 115

30 21 19 15 28 11

3.6 ( 0.1 0.80 ( 0.09 3.3 ( 0.1 0.80 ( 0.10 4.3 ( 0.2 0.33 ( 0.04

a Ideal ) 90.3 mV. b Ideal ) 0 mV. c Typical Γ ) 4.5 × 10-10 mol cm-2. Note: statistical analysis based on 95% C.I., and a sample population of N ) 30.

in 1.0 M HClO4. After 50 cycles, there was no further change in the appearances of the voltammograms or in the surface coverages for the different monolayers. The surface coverage of each thiolate was estimated from the charge passed for the oxidation of the ferrocene redox center during the anodic sweep.31 In addition to subjecting the monolayers of 1-5 to repetitive potential cycling, the modified electrodes were also sonicated for several minutes in the perchloric acid solution. This harsh treatment of the modified gold electrodes did not result in any significant decrease in coverage, illustrating the inherent stability of these monolayers. It must be borne in mind that the adsorbates used to form the monolayers in this study are based on molecules with relatively short ferrocenylalkyl chains, i.e., 6 < n < 8. The use of such short ferrocenylalkyl chains is not normally expected to result in adsorbed thiolates that are either stable or well packed.7,10,14,15 However, contrary to this, the monolayers formed using 1-5 have been found to be quite stable, considering their ability to resist desorption from the gold surface even after being subjected to the different treatments, e.g., electrochemical cycling32 and sonication in aqueous HClO4. Nevertheless, the surface coverages obtained for the adsorbates 1-4 are slightly lower than the theoretical value of 4.5 × 10-10 mol cm-2 that has been reported for a full monolayer based on an alkyl chain (i.e., n ) 8 or 11) with a ferrocene tail group.33-36 Generally, those elec(31) The surface coverages were estimated by using ΓFc ) QFc/nFA, where QFc is the charge passed for the electrolysis of the ferrocene sites, n is the number of electrons involved in the electron transfer process (n ) 1 for the ferrocene/ferrocenium redox couple), F is the Faraday constant, and A is the experimental surface area of the electrode. Values of QFc were obtained by integration of the area under the anodic wave and were corrected for any charging-current contributions. (32) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (33) Seiler, P.; Dunitz, J. D. Acta Crystallogr. 1979, B35, 1068.

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Figure 2. Representative cyclic voltammograms of SAMs prepared from compounds 1-5 recorded on gold electrodes: (a) 1; (b) 2; (c) 3; (d) 4; (e) 5. The experiments were carried out in 1.0 M HClO4 at ν ) 100 mV/s. Voltammograms shown represent the last scan in a repetitive cycle.

trodes modified so as to have the possibility for interchain hydrogen bonding gave surface coverages ranging from 2.6 × 10-10 to 4.2 × 10-10 mol cm-2. Although 3 and 4 are analogues of one another, the surface coverage for 4 was found to be slightly higher than that for 3. Even though it has been reported that monolayers derived from thiols and disulfides are essentially indistinguishable once chemisorbed on the Au surface,23,24 nevertheless, in our hands, the packing density resulting from a thiol precursor is always slightly better than that resulting from a disulfide-based precursor. A recent report suggests that both the surface coverage and the adsorption rate constant for disulfides are smaller than those of the corresponding thiol derivative.37 All of (34) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (35) Hickman, J. J.; Laibinis, P. E.; Auerbach, D. I.; Zou, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357. (36) Kondo, T.; Yanagisawa, M.; Fujihira, M. Electrochim. Acta 1991, 36, 1793. (37) Kondo, T.; Takechi, M.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 381, 203.

the coverages reported in Table 1 for the monolayers of 1-5, which are the average values of 30 different electrodes, were corrected for surface roughness, assuming a roughness factor of ca. 1.2-1.3.38 The averages are the average values of 30 different electrodes. The coverage of monolayers formed from compounds 1-4 was between 80 and 90% of a full monolayer, based on a theoretical value of 4.5 × 10-10 mol cm-2 calculated for a ferrocenylalkanethiolate monolayer. When a SAM of 8 was prepared and analyzed, a surface coverage close to the theoretical value expected for a monolayer was obtained. This implies that the interactions within a monolayer are directional in nature, and if sufficiently strong, hydrogen bonding between the amide groups can serve as natural spacers within the monolayer. In other words, the ferrocenylalkyl chains of 1-5 have a closest approach limit on the gold surface imposed by the (38) The surface roughness of the gold electrode was approximated from the electrochemistry of adsorbed iodine as proposed by: Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987, 233, 283.


distance between any two amide groups as a result of the formation of intermolecular hydrogen bonds within the monolayer. It is further believed that this minimum distance between two neighboring chains prevents the formation of a tightly packed monolayer, as was the case for monolayers of 1-4. A similar trend was observed in the surface coverage when a monolayer of 5 was compared to the monolayers of 6 and 7. The cyclic disulfides (5-7) used in this study have a wider base for attachment through the sulfur atoms in the 1,2-dithiolane ring. Thus, these molecules are expected to occupy a larger surface area compared to the self-assembly of a thiol or disulfide. This larger “footprint” probably accounts for the lower overall coverage obtained for monolayers prepared with 5-7. Thus far, we have only compared adsorbates with amide groups to those without, with respect to the packing behavior within the monolayer. A significant difference between the monolayers of 1, 2 or 3 lies in the position of the amide functionality along the alkyl chain. We propose that the strategic positioning of the amide group along the alkyl chain should result in the formation of intermolecular hydrogen bonds at different points within the respective monolayers. Furthermore, the specific points at which hydrogen bonds are formed may be important for achieving a higher degree of order within the monolayer. This higher level of order may be attained by anchoring the bulky ferrocene group at a fixed position away from the electrode surface. This would prevent the ferrocene subunit from being “buried” within the hydrophobic alkane environment. The amide linkages in 1-3 are located at different positions along the alkyl backbone. Varying the position of the amide group had only a secondary effect on the stability of the monolayers described here; thus, it appears that the actual position of the amide group is far less important compared to the presence of the amide moiety itself. In other words, the presence of the amide group had a stabilizing effect on these short chain monolayers, but it would be difficult to ascribe the relative contribution of either effect (i.e., presence vs position) to the stability and order in the monolayer. This is evident from the coverages obtained for the monolayers prepared from 1-3. (b) Formal Potential, E0′. Although the position of the amide group played only a secondary role in stabilizing the monolayers of 1-5, the location of the electronwithdrawing carbonyl and the electron-donating amino groups had a pronounced effect on the formal potential of the modified electrodes. The formal potentials39 of the individual SAMs of 1-5 ranged between 0.20 and 0.44 V, depending on the position of the amide group. Monolayers of 1 displayed the most positive formal potential of 0.433 V, whereas monolayers of 3 exhibited the least positive potential of 0.219 V. This is to be expected because the electroactive ferrocenyl moiety is located adjacent to the strong electron-withdrawing carbonyl functionality in 1, thus making it slightly harder for the ferrocene to undergo oxidation. Generally, the trend observed for the changes in the formal potentials of electrodes modified with 1-5 agreed well with the predicted potential values, based on the position of the amide moiety relative to the ferrocene tail group. Although the amide position is the same for 3 and 5, the formal potentials obtained for these monolayers are different. This difference may be attributed to the (39) The formal potential for the surface-bound ferrocenyl species was estimated from the average of the anodic (Epa) and cathodic (Epc) peak potentials, E0′ ) 1/2(Epa + Epc).

Langmuir, Vol. 14, No. 1, 1998 129

influence of the different solvents used for making the adsorbate solutions and to their effects on the structural behavior of the monolayer.40 Based on the surface coverage data alone, a monolayer formed from 3 would be less susceptible to solvation because of the higher degree of order and the packing density expected for such a monolayer compared to a monolayer formed from 5. Monolayers that exhibit poor packing densities are more prone to trapping solvent molecules within the monolayer, which could also contribute to the difference in the ferrocene formal potentials. This might be the case for 5, where in the presence of the perchlorate counterion the trapped DMF has a higher specific resistance compared to acetonitrile.41 The cyclic voltammogram in Figure 2e shows a double wave for the monolayer resulting from the adsorption of 5, whereas monolayers prepared from 1-4 did not exhibit any such behavior. The double waves are discernable even after numerous potential cycles, and the difference in the oxidation potentials of the resulting waves was about 35 mV. The double waves for 5 are believed to be the result of inhomogeneities within the monolayer that have led to the formation of more than one stable environment. Because of the dithiolane ring, the packing behavior of 5 is expected to be different from monolayers prepared from disulfides or thiols. Thus, it is conceivable that differences in the surface-attachment groups, differences in the solvent used to make the adsorbate solutions, or simply the presence of the amide group in this particular compound may have contributed to the development of inhomogeneities within the monolayer during the self-assembly stage. (c) Intermolecular Interactions. The organizational behavior of the monolayers formed from 1-5 was assessed from the full-width at half-maximum, ∆Efwhm, of the anodic voltammetric wave. This parameter provides a qualitative measure of the relative interaction taking place between the adsorbates within a monolayer. In an ideal case, where the interaction between the electroactive tail groups of the adsorbates is minimal, ∆Efwhm ) 3.53RT/nF (90.3/n mV at 24 °C) can be expected.42 However, experimental values of ∆Efwhm have often been found to be larger than 100 mV or significantly smaller than 90 mV (where n ) 1). Experimental values of ∆Efwhm exceeding the theoretical values have been attributed to the existence of multiple formal potentials, which in turn implies the presence of an ensemble of redox centers in different environments with a negligible rate of conversion between environments within the monolayer, e.g., a monolayer with redox centers both on the external surface and inside a hydrocarbon-like domain.43 Also, when the surface concentrations of the electroactive centers is high, stronger interactions between these centers might be expected to develop. It has also been speculated that wave broadening may be the result of variations in the surface charge that take place during faradaic reactions involving the attached electroactive centers, resulting in fluctuations of the surface potential.44,45 However, this may not be the case for monolayers prepared from 1-5 because of the unique directional nature of the hydrogen bonds within the (40) Anderson, M. R.; Mark, N. E.; Zhang, M. Langmuir 1996, 12, 2327. (41) Kadish, K. M.; Ding, J. Q.; Malinski, T. Anal. Chem. 1984, 56, 1741. (42) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 1980; p 522. (43) Finklea, H. O. Electroanalytical Chemistry; Bard, A. J.; Rubinstein, I., Eds.; Marcel Dekker, Inc.: New York, 1996; Vol. 19, p 109. (44) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398. (45) Fawcett, W. R. J. Electroanal. Chem. 1994, 378, 117.

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monolayers, which might preclude such a scenario. The ∆Efwhm values obtained for the SAMs of 1-5 were greater than the theoretical value of 90.3 mV, ranging from 130 to 155 mV and are typical of those found for pure SAMs terminated with ferrocene redox centers. Conversely, SAMs prepared from the control analogues 6-8 exhibited voltammetric waves with lower ∆Efwhm values than those observed for 1-5, yet still falling short of the ideal value of 90.3 mV. The experimental ∆Efwhm values are closer to the ideal value, and this implies that there is minimal lateral interaction between the redox centers. However, the value ∆Efwhm obtained for 7 is lower than the expected theoretical value. The development of phase-like behavior within the monolayers prepared with 7 may be responsible for the formation of current spikes in the voltammogram of 7 (not shown), an explanation put forth by other workers who have encountered similar behavior.46,47 (d) Electron Transfer Rate, ∆Ep. The incorporation of different redox-active centers into SAMs, e.g., ferrocene,34,46,48 viologen,49 quinone,48b,50 and [Ru(bpy)32+],51 has facilitated the investigation of a number of phenomena, including electron transfer rates and mechanisms. The electron transfer rate associated with these redox-active centers can be estimated from the peak separation ∆Ep;52 under ideal conditions a surface-confined species participating in a reversible electron transfer process would be expected to exhibit a value of ∆Ep ) 0 mV. The experimental ∆Ep values obtained from the voltammograms of 1-5 were found to be relatively small, ranging from 20 to 30 mV. These ∆Ep values were also found to be independent of the scan rate, which implies that the electron transfer is relatively fast. Furthermore, the small separation between the oxidation and reduction peak potentials of the Fc/Fc+ couple (Fc ) ferrocene) verifies that the redox centers are surface-confined.53 The control analogues 6-8 also gave similar small but nonzero ∆Ep values, indicating that the presence of the amide group had no impact on the electron transfer rate. Chidsey et al.34 showed that the apparent electrochemical reversibility of ferrocenylalkanethiolate monolayers has been tentatively attributed to the rapid lateral transport of the electrons from one ferrocene unit to an adjacent ferrocene unit, following electron shuttling from the conducting gold surface to the ferrocene plane. (e) Mixed Monolayer Systems. In studies involving the mixed monolayer systems, the ferrocenylalkyl disulfides (1-8) were coassembled with unsubstituted nalkanethiol spacers (Tables 1 and 2). The choice of the alkanethiol spacer was dependent on the length of the individual disulfides (1-8); an alkanethiol comparable in length to the individual ferrocenylalkyl disulfide was (46) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (47) Willman, K. W.; Rocklin, R. D.; Nowak, R.; Kuo, K.-N.; Schultz, F. A.; Murray, R. W. J. Am. Chem. Soc. 1980, 102, 7629. (48) (a) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (b) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. (c) Uosaki, K.; Sato, Y.; Kita, H. Electrochim. Acta 1991, 36, 1799. (d) Shimazu, K.; Sato, Y.; Yagi, I.; Uosaki, K. Bull. Chem. Soc. Jpn. 1994, 67, 863. (e) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385. (f) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 372, 117. (g) Sato, H.; Itoigawa, H.; Uosaki, K. Bull. Chem. Soc. Jpn. 1993, 66, 1032. (h) Sato, Y.; Frey, B. L.; Corn, R. M.; Uosaki, K. Ibid. 1994, 67, 21. (i) Shogen, S.; Kawasaki, M.; Kondo, T.; Sato, Y.; Uosaki, K. Appl. Organomet. Chem. 1992, 6, 533. (49) (a) Lee, K. A. B. Langmuir 1990, 6, 709. (b) Katz, E.; Itzhak, N.; Willner, I. Ibid. 1993, 9, 1392. (50) Sasaki, T.; Bae, I. T.; Scherson, D. A. Langmuir 1990, 6, 1234. (51) (a) Obeng, Y. S.; Bard, A. J. Langmuir 1991, 7, 195. (b) Sato, Y.; Uosaki, K. Dengki Kagaku 1993, 61, 816. (52) The peak separation was determined by using ∆Ep ) |Epa - Epc|. (53) Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. 1980, 114, 89.

Sabapathy et al.

Figure 3. Schematic representation of a mixed monolayer system involving 1. Top: ideal distribution of adsorbate 1 and the alkanethiol (C8SH) spacers. Bottom: Phase segregation of adsorbate 1 and the C8SH spacers, depending on the mole ratio and immersion time.

usually chosen. The engineering of such a mixed assembly should result in the confinement of the ferrocene moieties to a more noninteracting configuration. Figure 3 depicts the possible optimum arrangement for the mixed assembly of 1-5 under ideal conditions and provide a realistic depiction of the expected mixed assembly. The coassembly of the ferrocenylalkyl disulfides with alkanethiol spacers produces a more alkane-like (less polar) environment around the ferrocene moiety.54 Rowe and Creager55 have reported the electrochemical behavior of a nonfunctionalized monolayer of 4, i.e., without the amide functionality, in the presence of alkanethiols with different chain lengths. They showed that there was a direct correlation between the positive shift in the redox potential and the increase in the alkanethiol chain length in the coassembled SAMs. This increase in the nonpolar character around the ferrocene subunit was attributed to destabilization of the ferrocenium species (Fc+) relative to the neutral ferrocene species. Furthermore, they postulated that when a localized nonpolar environment is created around an electroactive species, a positive shift in the formal potential can be expected; i.e., the potential for a redox center such as ferrocene should shift to more positive values. However, no such positive shift was observed in the coassembled systems based on 1-5. Instead, there was a small negative potential shift. We propose that this small negative shift is the net result of two opposing effects: a “destabilization” effect and the normal effect of the hydrophobic environment on the ferrocene redox centers. The destabilization effect arises from the introduction of the alkanethiols into the monolayer assembly. The alkanethiols that were coassembled (54) The surface coverage (ΓFc) of ferrocene in the mixed monolayer depends on the immersion time and mole fraction of the alkanethiol. For this investigation, the immersion time was limited to 6 h and the mole fraction (χ) of the alkanethiols in the mixed solutions was 0.33. (55) (a) Creager, S. E.; Rowe, G. K. Anal. Chim. Acta 1991, 246, 233. (b) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (c) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1997, 420, 291.


Langmuir, Vol. 14, No. 1, 1998 131

Table 3. FT-IRRAS Data for Compounds 1 and 5a assignments

in KBr (cm-1)

amide A amide B CH2(asymm) CH2(fermi) CH2(sym) amide I amide II

3288, 3289 3092 (sh), 3098 2926, 2924 a, 2890 (sh) 2854, 2855 1633, 1637 1541, 1554

calculated (cm-1)

SAM on Au (cm-1)

3287, 3287 3338 (br), 3293 (br) 3086, 3086 3109 (w), 3104 2924, 2924 2916, 2923 a, 2892 (sh) 2852, 2852 2856, 2854 (sh) 1626, 1631 1638, 1644 1542, 1553 1546, 1549

a a ) peak not observed; asymm ) asymmetric stretch; sym ) symmteric stretch; sh ) shoulder; br ) broad resonance.

with 1-5 acted not only as spacers but served to disrupt the formation of interchain hydrogen bonds. This could result in the formation of a monolayer that is less ordered compared to a monolayer without the alkanethiol spacers. As a result of the imposed state of disorder within the monolayer, the ferrocene groups may be made more “accessible” than before, thereby making it easier for the ferrocene to undergo oxidation. This is equivalent to the stabilization of the ferrocenium (Fc+) species over neutral ferrocene (Fc). Even though the destabilization effect appears to be less obvious than the more apparent effect of the alkane-like environment on the monolayer, it must have a sufficiently strong influence on the redox potential to offset the expected positive shift in the formal potential. In order to test our supposition about the possible influence of the disruption of the interchain hydrogen bonding resulting from the coassembly of 1-5, the control analogues (6-8) were also coassembled with the alkanethiol spacers. The resulting data (Table 2) show that the formal potentials of the control analogues 6-8 all shifted in the positive direction, as expected. Another consequence of the coassembly of alkanethiols with 1-5 would be the isolation of the ferrocene redox centers from one another, thus minimizing or eliminating any possible lateral interactions that might be taking place within the pure monolayer systems. This is clearly reflected in the experimental values obtained for the mixed monolayer assemblies that appear to approach the more ideal behavior, i.e., ∆Efwhm ) 90.3/n mV (Tables 1 and 2). Overall, there was a general reduction of about 20-40 mV in the ∆Efwhm values for coassembled 1-8, which were all within 20 mV of the ideal value. Bulk and Surface Infrared Spectra of Compounds 1 and 5. For the purposes of this study, the spectral analysis will be confined to the 3500-2700 and the 17001300 cm-1 regions. One unifying structural feature of 1-5 is the presence of the amide (-CON-) functionality in the alkyl backbone. The characteristic vibrational excitations of the amide functionality are encompassed within these two spectral regions. Of particular interest are the presence and the location of the different amide bands (e.g., amides A, B, I, II, and III), which may provide evidence for the presence of the interchain interactions afforded by the amide groups. Table 3 summarizes the various assignments made for 1 and 5 in the bulk (KBr), as calculated, and on Au. (a) Peak Assignments for the Bulk (KBr) Spectra.56,57 The bulk spectra of 1 and 5 shown in Figures 4a and 5a show strong amide A bands appearing around 3300 (56) (a) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: Boston, 1991. (b) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 4th ed.; John Wiley & Sons, Inc.: New York, 1991. (57) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: Boston, 1990.

Figure 4. FTIR spectra of disulfide 1 in the 4000-500 cm-1 region: (a) transmission (bulk) spectrum of 1 dispersed in a KBr matrix; (b) surface reflection spectrum of 1 on gold.

Figure 5. FTIR spectra of cyclic disulfide 5 in the 4000-500 cm-1 region: (a) transmission (bulk) spectrum of 5 dispersed in a KBr matrix; (b) surface reflection spectrum of 5 on gold.

cm-1 that are attributable to the NsH stretching vibrations (1/3288 cm-1 and 5/3289 cm-1). There is also a much weaker (amide B) band seen around 3100 cm-1 (1/3092 cm-1 and 5/3098 cm-1) that is caused by the Fermi resonance-enhanced overtone of the NsH in-plane bending seen around the 1500-1600 cm-1 region called the amide II band (1/1541 cm-1 and 5/1554 cm-1). It has been shown that the spectra of molecules with the amide functionality generally show the two bands (i.e., the amide A and amide B) in this region.58 On the other hand, the strong amide II band involves the CsN stretch and also the CsNsH in-plane bend in the stretch-bend mode, both of which are very characteristic of noncyclic monosubstituted amides. Lenk et al.18 have calculated the positions of these vibrations in the absence of any Fermi resonance, and the reported values for the unperturbed amide A and amide B bands are 3314 and 3113 cm-1, respectively. Secondly, the carbonyl (CdO) functionality usually exists in a trans configuration with the NsH group, with the CdO stretching band (amide I) appearing very strongly in the 1680-1630 cm-1 region (1/1633 cm-1 and 5/1637 cm-1). The different torsional motions involved in the amide II band for such a trans configuration are also believed to be responsible for the amide III band, seen around 1310-1250 cm-1, although it is usually much (58) Miyazawa, T. J. Mol. Spectrosc. 1960, 4, 168.

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weaker compared to the amide I band (1/1253 cm-1 and 5/1260 cm-1).59 Lenk et al.18 suggested in their work that the hydrogen bonded amide groups may adopt either an R-helical structure, a parallel pleated sheet structure, or even an antiparallel arrangement of the amide groups from the adjacent chains, as proposed for their semifluorinated amido thiol in the bulk form. The peak assignments for the νsym and the νasym CsH stretching modes of the methylene (CH2) groups found in solid 1 and 5 were compared to the peak positions reported in the work by Allara et al.60 and Porter et al.10a The experimental νasym(CH2) stretch was observed at around 2926 and 2924 cm-1, while the νsym(CH2) stretch was seen around 2854 and 2855 cm-1 for 1 and 5, respectively. Both of these methylene stretches are in close agreement with the reported values for a more liquid-like state in the bulk KBr, rather than the expected ideal crystalline-like state. The peaks observed at the following locations, 1/1106 cm-1, 5/1103 cm-1; 1/1020 cm-1, 5/1030 cm-1; 1/1821 cm-1; 5/813 cm-1, are the characteristic peak assignments that can be attributed to the different types of CsH stretches found in the ferrocene moiety, e.g., the cyclopentadienyl ring CsC stretch (1112-1090 cm-1), the out-of-plane CsH bend (1000-1040 cm-1), and the in-plane CsH stretch (800-830 cm-1).57,61 (b) Peak Assignments for 1/Au and 5/Au Monolayers.12,56,57 The NsH stretching vibrations within the SAMs prepared from 1-5 provided the most compelling evidence to indicate the presence of intermolecular hydrogen bonding within the monolayers (Figures 4b and 5b). The calculated surface reflection spectra (Figure 6a,b) for 1 and 5 gave a theoretical peak frequency of 3287 cm-1 for the NsH stretch (amide A). Usually, the unassociated NsH stretching bands are relatively sharp, but they do tend to become much broader when associated with hydrogen bonding.57 This being the case, the spectra in Figures 4b and 5b show broad amide A peaks of medium intensity centered around 3338 and 3293 cm-1 for the monolayers prepared with 1 and 5, respectively. Therefore, the relatively broad NsH stretch in the monolayers, compared to the sharper peaks found in the bulk and in the calculated spectra of 1 and 5, strongly supports the presence of hydrogen bonding within monolayers of 1 and 5. The detection of the amide A band in the SAMs prepared from 1 and 5 further suggests a more or less perpendicular orientation for the NsH bond with respect to the surface. A similar conclusion regarding the orientation of the -CdO bond within the monolayer can also be made from the assignment of the amide I band, which appears around 1638 and 1644 cm-1 for 1 and 5, respectively, compared to the amide I band values seen in the calculated spectra of 1 and 5, i.e., 1626 and 1631 cm-1, respectively. The amide II peak can be seen around 1546 for 1 and 1549 cm-1 for 5. The peak frequency shift observed in the amide (A, I, or II) bands to higher values suggests that the interchain interactions, e.g., hydrogen bonding and the vdW interactions within the bulk KBr and the monolayer on gold, are different. Furthermore, Clegg et al.20 have suggested that any progression toward higher frequencies for the amide II peaks indicates an increasing restriction imposed on the NsH bending motion within the monolayer, which according to the authors was consistent with (59) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman and Hall: New York, 1975; Vol. 1. (60) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (61) (a) Rausch, M.; Vogel, M.; Rosenberg, H. J. Chem. Educ. 1957, 34, 268. (b) Rosenblum, M. Chemistry of the Iron Group Metallocenes; John Wiley & Sons: New York, 1965; Part 1.

Sabapathy et al.

Figure 6. Calculated surface reflection spectra derived from the bulk KBr spectra of (a) disulfide 1 and (b) cyclic disulfide 5.

the nature of hydrogen bonding within their monolayer system. The amide II band region, which was reported by Tam-Chang et al.19 to indicate the presence of hydrogen bonding, correlates well with the experimental value obtained in this study for 1 (1546 cm-1) and for 5 (1549 cm-1). It was also found that the ferrocene tail groups served as a useful tag to further characterize and understand the orientation of groups within the assembled monolayer. It was possible to assign absorption frequencies for the ferrocene subunit from the surface spectra of 1 and 5 that seemed to correlate well with the characteristic peaks observed in the bulk spectra of 1 and 5. From the spectra in Figures 4b and 5b, the ring CsC stretch, the (CsH)op bend, and the (CsH)ip stretch were located around 1105, 1030, and 815 cm-1 for self-assembled 1 and 5. The possibility of making these unique peak assignments implies that the ferrocene moiety is not lying parallel to the gold substrate surface. However, the exact orientation of the ferrocene group may be dependent on the configuration of the amide group within the monolayer that results from the directional nature of the hydrogen bonds between the adjacent groups. In addition to studying the behavior of the selfassembled amide-containing compounds, the surface spectra of coassembled mixtures of 1 + C8SH and 5 + C9SH were also recorded and analyzed (Figure 7a,b). These spectra support our assumption that there is negligible (if any) interchain hydrogen bonding within the coassembled monolayers. It is apparent from the surface spectra of the coassembled 1 + C8SH and 5 + C9SH that


Figure 7. Surface reflection spectra of the coassembled monolayers of (a) 1 + C8SH and (b) 5 + C9SH in the 3700-2400 cm-1 region. Note the absence of the NsH stretch around 3300 cm-1.

there is no broad NsH stretching mode in either of the spectra or for that matter any assignments for the NsH stretching mode. The absence of an observable NsH stretch may also be due to the diminutive absorption coefficient for this process within the coassembled monolayer. Both Lenk et al.18 and Clegg et al.20 have reported the absence of both the amide A and the amide I bands in their respective monolayer systems and thus arrived at a similar conclusion about the possible parallel orientation of both the NsH and CdO bonds within the monolayers studied. However, it must be borne in mind that the chain lengths of the monolayer systems studied by the two aforementioned groups are indeed very much longer compared to those used in this study. Furthermore, the positions of the amide functionality in the reported cases were close to the gold surface and there were long alkyl chains projecting beyond the amide groups. Hence, it should be expected that the orientation of the amide groups in 1 and 5 on the gold surface will be different to a lesser or greater extent as a result of the overall configuration of the monolayers. This large difference in the alkyl chain lengths might also account for discrepancies that arise when comparing the experimental peak frequencies with those reported for the other monolayer systems. It is well-established that SAMs with long alkyl chains are hexagonally packed in a commensurate (x3 × x3)R30° lattice on Au(111) surfaces with an approximate lateral spacing of 5 Å and an average tilt angle of ∼30°.12,13 However, Tam-Chang et al.19 found the organization of one of their self-assembled amide-containing hydrocarbon compounds, which was 15 carbon atoms long, to be quite different from that found in an unsubstituted n-alkanethiolate monolayer. This implied that the overall arrangement of these amide-functionalized alkyl chains could be different on the gold substrates for reasons involving changes in the spacing and tilt angles within the monolayer. Consequently, the difference between an unsubstituted and an amide-substituted alkanethiolate could possibly have some kind of influence on the relative stability and packing order of the monolayer assembly as a whole. The question that follows the premise made about the effect of the differences in the chain lengths is whether the relative order within these short chain monolayer systems has improved or worsened under the given conditions, i.e., with the incorporation of the amide groups

Langmuir, Vol. 14, No. 1, 1998 133

for the added intermolecular interaction within the monolayers. Generally, the νasym(CH3) and the νsym(CH2) stretching modes are used to make structural interpretations of the monolayers because of the minimal overlap that exists with those of the other modes; but in this case, there is no need to consider the former stretching mode. It was discovered that the bulk and surface values for the νsym(CH2) mode were in close agreement with the results reported by Porter et al.10a This might indicate that the structural integrity of the amide-containing alkyl chains within the bulk KBr was not significantly altered as a result of monolayer formation. In fact, there was only a small increase in the νsym(CH2) mode for 1 (2 cm-1) and a negligible decrease for 5 (1 cm-1). The relative order within the SAM might not be much worse than the order within the bulk KBr. It should be noted here that the assembly of 1 and 5 is expected to be different, considering the different head groups present for attachment to the Au substrate in either case, i.e., SsS versus the 1,2dithiolane ring. On the other hand, the positions of the peak frequencies for the νasym(CH2) modes provide better insight into the intermolecular environment of the alkyl chains within the molecular assemblies. Snyder et al.62 showed that the location of the νasym(CH2) peaks is a sensitive indicator of the extent of the lateral interactions between long n-alkyl chains. Comparing the νasym(CH2) modes in the bulk and in the monolayer, it appears that there is a general decrease in the peak frequencies, albeit the shift in 1 appears to be more significant than in 5. From this, it may be concluded that the relative order within the monolayer is no better than in bulk KBr. The experimental data do not indicate conclusively that this is the case or that the relative order within monolayers has improved with interchain hydrogen bonding. There may be regions within the monolayer where the relative order is better or worse as compared to that in the bulk form; or alternatively, the organization within these amidecontaining monolayers may be made up of varying degrees of a simple arrangement and a totally different, but more complex organizational arrangement. Conclusions We have demonstrated the role that intermolecular hydrogen bonding plays in stabilizing monolayers based on short alkyl chains. However, the organizational behavior of these short chain monolayers may be more complex and varied compared to the rather simplistic model proposed for SAMs derived from alkanethiols with longer chains. Nevertheless, we have shown here that the amide-containing short ferrocenylalkyl disulfides are able to self-assemble and form a fairly ordered and stable monolayer and that the relative order seen for these monolayers correlates with the presence of the amide groups within the monolayers. It is expected that any instability and disorder inherent within the monolayers of short and nonfunctionalized alkyl chains may be overcome to an extent through the incorporation of additional interchain forces, e.g., the formation of hydrogen bonds or electrostatic interactions. The use of ferrocene redox centers as tail groups has further simplified the electrochemical characterization of the resulting SAMs of these amide-containing alkyl chains. We are currently studying the behaviors of other short chain amide-containing ferrocenylalkyl disulfides, e.g., (62) (a) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (b) Snyder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. Ibid. 1986, 90, 5623.

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C4 and C5, and also the effects of altering the position of the amide functionality within the short and long alkyl chains of the ferrocenyl disulfides (i.e., presence versus position). The overall stability and orientation of a ferrocenylalkyl system with more than one amide functionality is also being investigated. Through these studies, it is hoped that a better understanding of intermolecular hydrogen bonding within the monolayer will be gained. This knowledge may aid in the engineering and design of other monolayer systems with greater stability and organization. For example, it would prove fruitful to be able to design monolayers that mimic the unique organizational and interactional behaviors found in naturally occurring systems such as polypeptides and proteins. 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. Heptanethiol (C7SH), octanethiol (C8SH), and nonanethiol (C9SH) were purchased from Aldrich Chemical Co. and were used without further purification. Acetonitrile (CH3CN) and dimethylformamide (DMF) were purchased from Spectrum Chemical, dried by refluxing over CaH2 for at least 48 h and then freshly distilled prior to use. Dry methylene chloride (CH2Cl2) and hexanes were also obtained by distillation from CaH2. Triethylamine was distilled from KOH before use. Anhydrous tetrahydrofuran (THF) was obtained by distillation from lithium aluminum hydride (LAH). β-ferrocenylethylamine, β-ferrocenylethanol, ferrocenylacetic acid, and bis(5-aminopentyl) disulfide were prepared according to the reported procedures.63,64 Ferrocenecarboxylic acid, oxalyl chloride, 3,3′-dithiodipropionic acid, 3-bromopropionic acid, thioctic acid (6,8-dithiooctanoic acid), ditert-butyl dicarbonate, and 6-bromo-1-hexanol were purchased from Aldrich Chemical Co. and used without further purification. Synthesis of Compounds Used To Prepare SAMs of 1-8. All IR spectra reported in the synthesis were obtained on a PerkinElmer Model 1600 FT-IR spectrometer. 1H NMR spectra were obtained on a Bruker AC-E 300 spectrometer using CDCl3 as 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). The progress of the reactions was monitored by TLC on Whatman precoated TLC plates (PE SIL G/UV). The TLC spots were visualized either by exposure to iodine (I2) vapors or by irradiation with UV light. For the removal of 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, Georgia. Bis[5-((ferrocenylcarbonyl)amino)pentyl] Disulfide (1). A mixture of ferrocenecarboxylic acid (1.38 g, 6 mmol), 10 mL of a 2.0 M oxalyl chloride solution in methylene chloride, 4 drops of pyridine, and anhydrous methylene chloride (50 mL) were stirred in the dark for 10 h at room temperature. The solvent and excess oxalyl chloride were removed in vacuo, and the residue was extracted with hot hexanes (4 × 25 mL). The combined hexanes extracts were concentrated in vacuo to give ferrocenylcarbonyl chloride (1.12 g, 75%) as a red solid, which was used in the next step of the synthesis. Triethylamine (1.1 mL, 8 mmol) was added with stirring to an ice-cooled solution of ferrocenylcarbonyl chloride (1.0 g, 4.0 mmol) in dry methylene chloride (15 mL). Then, bis(5-aminopentyl) disulfide (0.36 g, 1.5 mmol) in dry methylene chloride (10 mL) was added dropwise to the cold solution over a period of 15 min. The resulting mixture was stirred in the ice bath for 30 min and then at room temperature for 12 h. The reaction mixture was diluted with methylene chloride (30 mL) and transferred to a separatory funnel. The organic solution was washed successively (63) Lednicer, D.; Lindsay, J. K.; Hauser, C. R. J. Org. Chem. 1958, 23, 653. (64) Dirscherl, W.; Weingarten, F. W. Leibigs Ann. Chem. 1951, 574, 131.

Sabapathy et al. with 1 M NaOH (2 × 20 mL), brine (20 mL), 1 M HCl (2 × 20 mL), and repeated with brine (20 mL). The organic extract was dried (Na2SO4) and the solvent removed in vacuo to yield a yellow solid, which was then recrystallized from a mixture of hexanes and methylene chloride to give 1 as a bright yellow solid: 0.66 g (65%); mp 86-88 °C; IR (KBr) 3291, 3086, 2930, 2852, 1626, 1534, 1293, 1102, 818 cm-1; 1H NMR (CDCl3) δ [ppm] 1.45-1.80 (m, 12H), 2.70 (t, J ) 7.2 Hz, 4H), 3.38 (q, J ) 6.3 Hz, 4H), 4.20 (s, 10H), 4.33 (s, 4H), 4.69 (s, 4H), 5.99 (br s, 2H). Anal. Calcd for C32H40S2N2O2Fe2: C, 58.19; H, 6.10; N, 4.24; S, 9.71. Found: C, 58.24; H, 6.15; N, 4.31; S, 9.79. Bis[5-((ferrocenylacetyl)amino)pentyl] Disulfide (2). Ferrocenylacetic acid (1.95 g, 8 mmol) was dissolved in anhydrous methylene chloride (50 mL), and 4 drops of freshly distilled triethylamine was added. The solution was cooled in an ice bath and 20 mL of a 2.0 M oxalyl chloride solution in anhydrous methylene chloride was added slowly to the stirred solution over 10 min. The resulting solution was stirred at room temperature for 8 h. The solvent and excess oxalyl chloride were removed in vacuo to yield the crude ferrocenylacetyl chloride, which was used in the next step of the synthesis. Ferrocenylacetyl chloride was redissolved in 50 mL of anhydrous methylene chloride and 1.4 mL (10 mmol) of freshly distilled triethylamine was added. The solution was cooled in an ice bath and a solution of bis(5-aminopentyl) disulfide (0.72 g, 3 mmol) in anhydrous methylene chloride (20 mL) was added slowly to a stirred solution over 15 min. The resulting mixture was stirred at room temperature for 12 h, after which it was washed with 1 M HCl (2 × 25 mL), water (20 mL), and 1 M NaOH (2 × 10 mL) and with water again (2 × 20 mL). The organic solution was dried (Na2SO4) and the solvent removed in vacuo to yield a brown solid, which was recrystallized twice from a hexanesdiethyl ether mixture to yield pure 2 as a yellow solid: 1.26 g (60%); mp 92-93 °C; IR (KBr) 3306, 3086, 2930, 2859, 1647, 1534, 1109, 818 cm-1; 1H NMR (CDCl3) δ [ppm] 1.30-1.50 (m, 8H), 1.62-1.70 (m, 4H), 2.62 (t, J ) 7.2 Hz, 4H), 3.16 (q, J ) 6.5 Hz, 4H), 3.32 (s, 4H), 4.10-4.22 (m, 18H), 5.67 (br s, 2H). Anal. Calcd for C34H44S2N2O2Fe2: C, 59.31; H, 6.44; N, 4.07; S, 9.31. Found: C, 59.15; H, 6.60; N, 4.15; S, 9.58. 3,3′-Dithiobis[N-(β-ferrocenylethyl)]propionamide (3). A mixture of 3,3′-dithiodipropionic acid (0.84 g, 4 mmol) and 50 mL of anhydrous methylene chloride were cooled in an ice bath, and 6 mL of a 2.0 M oxalyl chloride solution in methylene chloride was added with stirring. To this ice-cooled mixture was slowly added freshly distilled triethylamine (1.1 mL, 8 mmol) in methylene chloride over 30 min with stirring; the ice bath was then removed, and the mixture was stirred for another 2 h at room temperature. Removal of the solvent and excess oxalyl chloride in vacuo yielded the crude acid chloride, which was used in the next step without any further purification. The crude 3,3′-dithiodipropionyl chloride was redissolved in anhydrous methylene chloride (35 mL), and 1.4 mL (10 mmol) of freshly distilled triethylamine was added. To this mixture, a solution of β-ferrocenylethylamine (2.30 g, 10 mmol) in anhydrous methylene chloride (10 mL) was added with stirring. The resulting mixture was stirred at room temperature for 10 h. Product 3 was isolated according to the same procedure described for compound 1. Recrystallization of the crude product was carried out using a hexanes-methylene chloride mixture to give 3 as a yellow solid: 1.77 g (70%); mp 104-105 °C; IR (KBr) 3281, 3085, 2933, 2860, 1641, 1554, 1249, 1104, 814 cm-1; 1H NMR (CDCl3) δ [ppm] 2.56 (t, J ) 6.6 Hz, 8H), 2.97 (t, J ) 6.6 Hz, 4H), 3.39 (q, J ) 6.6 Hz, 4H), 4.09 (s, 8H), 4.12 (s, 10H), 5.99 (br s, 2H). Anal. Calcd for C30H36S2N2O2Fe2: C, 56.97; H, 5.74; N, 4.43; S, 10.14. Found: C, 56.71; H, 5.73; N, 4.35; S, 10.05. N-(β-Ferrocenylethyl)-3-mercaptopropionamide (4). 3-Bromopropionic acid (1.52 g, 10 mmol) was dissolved in anhydrous methylene chloride (20 mL) and the solution stirred. This was followed by the dropwise addition of 5 mL of a 2.0 M oxalyl chloride solution in methylene chloride over 15 min. The resulting solution was stirred at room temperature for 4 h following the addition of 2 drops of triethylamine. The solvent and excess oxalyl chloride were removed in vacuo to give 3-bromopropionyl chloride as a pale yellow liquid. The acid chloride was redissolved in anhydrous methylene chloride, triethylamine (1.6 mL, 12 mmol) was added, and the

SAMs of Ferrocenylalkyl Compounds solution was cooled in an ice bath. A solution of β-ferrocenylethylamine (1.83 g, 8 mmol) in anhydrous methylene chloride was added, and the resulting solution was stirred in the ice bath for 1 h and another 2 h at room temperature. The reaction was then quenched by adding 15 mL of 1 M NaOH. The organic layer was washed with water (20 mL) and 1 M HCl (2 × 5 mL), and with water again (20 mL). The organic layer was concentrated in vacuo after drying (Na2SO4) to give N-(β-ferrocenylethyl)-3bromopropionamide as a yellow solid: 2.18 g (75%); mp 96-97 °C (hexanes-diethyl ether); 1H NMR (CDCl3) δ [ppm] 2.57 (t, J ) 6.9 Hz, 2H), 2.72 (t, J ) 6.7 Hz, 2H), 3.40 (q, J ) 6.8 Hz, 2H), 3.65 (t, J ) 6.8 Hz, 2H), 4.09, 4.12 (2s, 9H), 5.76 (br s, 1H). N-(β-Ferrocenylethyl)-3-bromopropionamide (1.45 g, 4 mmol) was redissolved in 15 mL of DMSO. Thiourea (0.38 g, 5 mmol) was added, and the mixture was stirred at room temperature for 10 h. Then, 25 mL of aqueous NaOH (10%, w/v) was added and the stirring was continued at room temperature for 30 min under a nitrogen atmosphere. The mixture was then acidified with 1 M HCl to pH ∼2-3, and the organic part was extracted with diethyl ether (3 × 25 mL). The combined organic extracts were washed with water (4 × 15 mL), dried (Na2SO4), and concentrated to give product 4 as a yellow liquid. After purification of the crude product by chromatography on silica gel (hexanes-ethyl acetate, v/v ) 50:50), 4 was obtained as a thick yellow oil: 0.88 g (70%); IR (KBr) 3284, 3086, 2930, 2852, 1640, 1548, 1435, 1257, 1109, 995, 804 cm-1; 1H NMR (CDCl3) δ [ppm] 1.70 (br s, 1H), 2.45 (t, J ) 6.6 Hz, 2H), 2.53 (t, J ) 6.5 Hz, 2H), 2.81 (t, J ) 6.6 Hz, 2H), 3.30-3.43 (m, 2H), 4.10-4.25 (m, 9H), 5.88 (br s, 1H). Anal. Calcd for C15H19SNOFe: C, 56.73; H, 5.95; N, 4.42; S, 10.11. Found: C, 56.34; H, 5.56; N, 4.42; S, 9.89. N-(β-Ferrocenylethyl)-6-thioctic Amide (5). A mixture of thioctic acid (0.83 g, 4 mmol), 3 mL of a 2.0 M oxalyl chloride solution in methylene chloride, 4 drops of freshly distilled triethylamine, and 20 mL of anhydrous methylene chloride were stirred in the dark for 8 h at room temperature. Excess oxalyl chloride and solvent were removed in vacuo to give the crude thioctic acid chloride, which was used in the next reaction. To a stirred and ice-cooled solution of the thioctic acid chloride were added freshly distilled triethylamine (0.70 mL, 5 mmol) in anhydrous methylene chloride (10 mL) and a solution of β-ferrocenylethylamine (1.15 g, 5 mmol) in anhydrous methylene chloride (15 mL). The resulting mixture was stirred in the dark at room temperature for 10 h, and amide 5 was isolated following the same procedure described for compound 1. The crude form of 5 was purified by recrystallization from a hexanes-diethyl ether mixture to yield analytically pure 5 as a yellow solid: 1.10 g (65%); mp 77-79 °C; IR (KBr) 3291, 3086, 3022, 2923, 2852, 1640, 1555, 1449, 1265, 1102, 811 cm-1; 1H NMR (CDCl3) δ [ppm] 1.42-1.55 (m, 2H), 1.63-1.76 (m, 4H), 1.91 (sextet, J ) 6.8 Hz, 1H), 2.15 (t, J ) 7.2 Hz, 2H), 2.46 (sextet, J ) 6.8 Hz, 1H), 2.55 (t, J ) 6.9 Hz, 2H), 3.06-3.23 (m, 2H), 3.37 (t, J ) 6.9 Hz, 2H), 3.57 (quin, J ) 6.8 Hz, 1H), 4.08-4.15 (m, 9H), 5.53 (br s, 1H). Anal. Calcd for C20H27S2NOFe: C, 57.55; H, 6.52; N, 3.36; S, 15.36. Found: C, 57.75; H, 6.31; N, 3.72; S, 15.22. N-(β-Ferrocenylethyl)-N-methyl-6-thioctic Amide (6). Compound 6 was prepared by coupling N-methyl-β-ferrocenylethylamine with thioctic acid. N-Methyl-β-ferrocenylethylamine was prepared according to the following sequence of reactions. β-Ferrocenylethylamine (1.83 g, 8 mmol) and freshly distilled triethylamine (1.6 mL, 12 mmol) were dissolved in methylene chloride (25 mL). Di-tert-butyl dicarbonate (1.74 g, 8 mmol) was added, and the mixture was stirred at room temperature for 10 h. The solution was then transferred to a separatory funnel and washed with 1 M HCl (10 mL) and water (2 × 20 mL). The organic solution was dried (Na2SO4) and concentrated as a yellow liquid to give the N-(tert-butyloxycarbonyl) [N-BOC] derivative of β-ferrocenylethylamine: 2.10 g (80%); 1H NMR (CDCl3) δ [ppm] 1.45 (s, 9H), 2.51 (t, J ) 6.9 Hz, 2H), 3.24 (q, J ) 6.9 Hz, 2H), 4.09, 4.11 (2s, 9H), 4.74 (br s, 1H). A mixture of the N-BOC derivative (1.64 g, 5 mmol) and LAH (0.38 g, 10 mmol) in anhydrous THF (40 mL) was heated under reflux for 12 h. The reaction mixture was cooled in an ice bath and then diluted with methylene chloride (50 mL), followed by the dropwise addition of a saturated aqueous solution of Na2SO4 until no further effervescence was observed. The mixture was stirred for 10 min, after which the clear solution was decanted

Langmuir, Vol. 14, No. 1, 1998 135 from the white precipitate. The precipitate was washed with methylene chloride (3 × 30 mL) and the combined organic solution was concentrated in vacuo to give N-methyl-β-ferrocenylethylamine as an oil: 0.97 g (80%); 1H NMR (CDCl3) δ [ppm] 1.68 (br s, 1H), 2.24-2.29, 2.46-2.48 (2m, 2H), 2.27, 2.42 (2s, 3H), 2.53, 2.69 (2t, J ) 6.8 Hz, 2H), 4.04-4.13 (m, 9H). The coupling of N-methyl-β-ferrocenylethylamine (0.73 g, 3 mmol) with thioctic acid chloride was carried out according to the procedure described in the preparation of amide 5. The crude product after purification by chromatography on silica gel (hexanes-ethyl acetate, v/v ) 1:1) gave pure 6 as a yellow oil: 0.77 g (60%); 1H NMR (CDCl3) δ [ppm] 1.45-1.58 (m, 2H), 1.621.77 (m, 4H), 1.86-1.95 (m, 1H), 2.10 (t, J ) 6.8 Hz, 1H), 2.31 (t, J ) 6.8 Hz, 1H), 2.42-2.61 (m, 3H), 2.90, 2.91 (2s, 3H), 3.083.25 (m, 2H), 3.36 (t, J ) 6.8 Hz, 1H), 3.45 (t, J ) 6.8 Hz, 1H), 3.58 (quin, J ) 6.6 Hz, 1H), 4.04-4.15 (m, 9H). Anal. Calcd for C21H29S2NOFe:0.5H2O: C, 57.27; H, 6.86; N, 3.18; S, 14.56. Found: C, 57.33; H, 7.04; N, 3.21; S, 14.70. Thioctic Acid β-Ferrocenylethyl Ester (7). Thioctic acid (1.03 g, 5 mmol) was converted to the corresponding acid chloride according to the procedure previously described for the preparation of amide 5. The crude acid chloride was dissolved in 20 mL of anhydrous methylene chloride, and the solution was cooled in an ice bath. A solution of β-ferrocenylethanol (0.92 g, 4 mmol) in 15 mL of anhydrous methylene chloride was added slowly with stirring. The ice bath was removed, and the solution was stirred at room temperature for 5 h. The solution was then transferred to a separatory funnel and washed with cold 1 M NaOH (2 × 10 mL), followed by water (2 × 20 mL). Evaporation of the solvent after drying (Na2SO4) yielded an oil that was purified by column chromatography on silica gel (hexanesmethylene chloride, v/v ) 40:60) to give 7 as a yellow oil: 1.25 g (75%); 1H NMR (CDCl3) δ [ppm] 1.42-1.55 (m, 2H), 1.62-1.75 (m, 4H), 1.92 (sextet, J ) 6.9 Hz, 1H), 2.34 (t, J ) 7 Hz, 2H), 2.47 (sextet, J ) 6.9 Hz, 1H), 2.68 (t, J ) 7 Hz, 2H), 3.09-3.25 (m, 2H), 3.58 (quin, J ) 6.5 Hz, 1H), 4.06-4.18 (m, 9H), 4.2 (t, J ) 7 Hz, 2H). Anal. Calcd for C20H26S2O2Fe: C, 57.42; H, 6.26; S, 15.33. Found: C, 57.61; H, 6.41; S, 15.33. Bis[6-((ferrocenylcarbonyl)oxy)hexyl] Disulfide (8). 6-Bromo-1-hexanol (1.8 g, 10 mmol) was dissolved in 30 mL of absolute ethanol; thiourea (0.91 g, 12 mmol) was added, and the resulting mixture was heated under reflux for 8 h. The reaction mixture was cooled to room temperature and 60 mL of aqueous NaOH (10%, w/v) was added. The mixture was stirred at room temperature for 1 h and then titrated with a saturated solution of iodine in ethanol until the brown color of iodine persisted. The solution was extracted using diethyl ether (3 × 25 mL). The combined ethereal extracts were washed with brine (2 × 20 mL), dried (Na2SO4), and concentrated to give bis(6-hydroxyhexyl) disulfide as a pale yellow oil: 1.25 g (85%); 1H NMR (CDCl3) δ [ppm] 1.33-1.50 (m, 8H), 1.53-1.63 (m, 4H), 1.65-1.78 (m, 4H), 2.70 (t, J ) 6.8 Hz, 4H), 3.63 (t, J ) 6.8 Hz, 4H). Ferrocenecarboxylic acid was converted into the corresponding acid chloride according to the procedure described in the preparation of amide 1. Bis(6-hydroxyhexyl) disulfide (0.40 g, 1.5 mmol) was dissolved in 30 mL of anhydrous methylene chloride, and triethylamine (0.6 mL, 4 mmol) was added; the solution was then cooled in an ice bath. Stirring was initiated, and a solution of ferrocenylcarbonyl chloride (1.0 g, 4 mmol) in 20 mL of anhydrous methylene chloride was added dropwise over a 15 min period. The ice bath was removed, and the mixture was stirred at room temperature for 5 h. The solution was then transferred into a separatory funnel and washed with 1 M NaOH (2 × 10 mL) and water (2 × 20 mL). The organic extract was dried (Na2SO4) and concentrated in vacuo. The residue on purification by column chromatography on silica gel (hexanesmethylene chloride, v/v ) 40:60) yielded pure 8 as an orange liquid: 0.73 g (70%); 1H NMR (CDCl3) δ [ppm] 1.46-1.54 (m, 8H), 1.66-1.80 (m, 8H), 2.70 (t, J ) 7 Hz, 4H), 4.19 (s, 10H), 4.20 (t, J ) 6.8 Hz, 4H), 4.38 (s, 4H), 4.80 (s, 4H). Anal. Calcd for C34H42S2O4Fe2: C, 59.14; H, 6.13; S, 9.29. Found: C, 59.17; H, 6.24; S, 9.34. Preparation of Au Substrates. Glass microscope slides (25 mm × 25 mm; Fisher Scientific) served as the substrate. They were cleaned by ultrasonication in successive baths of “piranha” solution (1:3 by volume 30% H2O2/concentrated H2SO4), deionized

136


water, and 2-propanol. [Caution: Care must be exercised with the handling of the “piranha” solutions as they are potentially explosive and therefore should be disposed of appropriately after each use.65] The slides were oven dried and coated with ∼50-60 Å of a chromium (Johnson Matthey Electronics) adhesion layer that was deposited at a rate of 0.1-0.2 Å/s by thermal evaporation. The substrates were then coated with ∼1500-1600 Å of gold (Alfa-ÆSAR; 99.9985%) deposited at a rate of 4-5 Å/s. The goldcoated slides prepared by this method were analyzed by X-ray diffraction techniques and found to exhibit strong Au(111) characteristics.66 Both depositions were carried out at a base pressure of ∼(7-9) × 10-7 Torr using an Edwards Auto 306 vacuum coater equipped with a Sycon Model STM-100/MF film thickness/rate monitor. After the coating procedure was completed, the vacuum chamber was back-filled with high-purity N2 gas, and the freshly coated gold slides were used immediately for the preparation of the different SAMs. Preparation of the SAMs. The gold electrodes were taken directly from the vacuum coater and immersed in the disulfide or thiol coating solutions as described below. SAMs of compounds 1-4 were prepared by immersing the gold electrodes in CH3CN solutions containing the compounds of interest at a concentration of ca. 1 mM for at least 24 h at room temperature. Due to the poor solubility of compound 5 in CH3CN, DMF was used as the solvent. The alkanethiols used in the coadsorption studies were as follows: (1/C8SH), (2/C9SH), (3/C7SH), (4/C7SH), and (5/C9SH). The total concentration of the adsorbates in the coating solution was maintained at ca. 1 mM. Likewise, monolayers and coassembled monolayers of compounds 6-8 were prepared in a similar manner. After self-assembly, the gold electrodes were rinsed with copious quantities of CH3CN or DMF followed by deionized water to remove any excess adsorbate prior to electrochemical and spectroscopic characterization. Electrochemical Characterization of the SAMs. Cyclic staircase voltammetry experiments (with a 1 mV step size) were carried out in a three-electrode cell using an EG&G Princeton Applied Research Corp. (PARC) Model 273 potentiostat/galvanostat employing PARC Model 273 Electrochemistry Analysis Software running on an IBM compatible 386 computer. The gold-coated substrates, which served as the working electrode, were clamped against the O-ring of an O-ring joint on the side of the electrochemical cell. The O-ring provided a liquid tight seal and defined the experimental area of the working electrode, which was estimated to be 1.54 cm2. The reference electrode was a saturated calomel electrode (SCE) brought to the working (65) (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. (66) (a) He, Z.; Bhattacharyya, S.; Cleland, W. E., Jr.; Hussey, C. L. J. Electroanal. Chem. 1995, 397, 305. (b) The polycrystalline gold substrates used by Tam-Chang et al.19 in their study were also reported to exhibit a preferred Au(111) orientation.

Sabapathy et al. electrode surface by a Luggin capillary. All of the potentials reported in this paper were measured with respect to this reference. The counter electrode was a Pt wire spiral immersed in the supporting electrolyte solution. The supporting electrolyte solution used was 1.0 M aqueous HClO4, and all experiments were carried out at room temperature (ca. 24 °C). The electrolyte solution in the electrochemical cell was deaerated with highpurity N2 gas before any electrochemical measurements were initiated. Spectroscopic Characterization of the SAMs. Infrared spectra were obtained using a Bruker Model IFS66 FTIR spectrometer equipped with a narrow-band, liquid N2-cooled MCT (Hg-Cd-Te) detector. A Plexiglas glovebox covered the spectrometer sample compartment. The glovebox and spectrometer were purged with high-purity N2 gas from the bleed-off of a liquidN2 tank. All spectra were acquired at 2 cm-1 resolution using a zero-filling factor of 2. Transmission spectra we obtained by coadding 32 signal-averaged scans of the solid materials dispersed in KBr in the 4000-400 cm-1 spectral region. The surface reflection spectra of the monolayer films (i.e., the ex situ spectra) were obtained with p-polarized light incident at a grazing angle of 86° from the surface normal by coadding 256 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. All spectroscopic characterization was carried out using gold substrates with dimensions (40 mm × 25 mm) slightly different from those used for the electrochemical studies. The simulated or calculated refectionabsorption spectra were calculated from the transmission spectra of compounds 1-5 dispersed in KBr according to the method described by Ihs et al.67 The details of the computer program used to carry out this calculation will be reported separately. By calculating the surface reflection spectrum for a given monolayer from the bulk transmission spectrum and then comparing the results to the experimental reflection spectrum, it is possible to account for any peak distortions in the former that could be due to an optical effect known as anomalous dispersion.68 This optical effect arises from the rapid change in the refractive index of a material on a surface and generally shifts IR bands to higher wavenumbers in the reflection spectrum, which in turn depends on the intensity and width of the band, and the wavenumber region where the band occurs.69

Acknowledgment. This research was supported by the Mississippi NSF EPSCoR Project (Grant No. RII-8902064). LA971042C (67) Ihs, A.; Uvdal, K.; Liedberg, B. Langmuir 1993, 9, 733. (68) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (69) (a) Allara, D. L.; Baca, A.; Pryde, C. A. Macromolecules 1978, 11, 1215. (b) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45.

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