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Reactivity in Self-Assembled Monolayers: Effect of the Distance from the Reaction Center to the Monolayer-Solution Interface Victor Chechik and Charles J. M. Stirling* Centre for Molecular Materials and Department of Chemistry, The University of Sheffield, Sheffield S3 7HF, U.K. Received March 12, 1997. In Final Form: August 4, 1997X Monolayers containing a reactive p-nitrophenyl ester group at different levels with respect to the monolayer interface have been self-assembled on a gold surface. Analysis of grazing angle IR spectra, surface plasmon resonance (SPR), and wettability measurements suggests disordered organization of the alkane chains in the monolayers. Kinetics of monolayer reactions with external reagents (alkylamines) have been studied and compared with those of the same process in bulk medium. Burying of a reaction center under the surface and other structural changes of monolayers were shown to have only a minor effect on the rates of reaction, implying that these monolayers could be easily penetrated by guest molecules. The higher reaction rates with monolayers than in bulk solution are possibly due to a weak binding of the external reagent to the monolayer prior to reaction.
Introduction Self-assembly of organic thiols1-3 or disulfides4,5 on a gold surface produces well-organized stable monolayers. Exceptionally high affinity of gold for thiolates allows incorporation of virtually any functional group in such monolayers.6 This fact, and the development of several analytical techniques sensitive enough to provide structural information on as little as one layer of molecules, has made it possible to design and monitor reactions proceeding at solid-liquid or solid-gas interfaces. It was shown in recent years that functional groups on the surface of self-assembled monolayers can undergo simple chemical transformations typical of bulk media.7 However, little is known about the effects which incorporation of reactive groups in monolayers has on their reactivity. Literature data are scarce and controversial. In some cases, reactions were reported to be significantly accelerated in monolayers,8 whereas other experiments suggest that reactivities in monolayers are of the same X Abstract published in Advance ACS Abstracts, December 1, 1997.
(1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (b) Bishop, A. R.; Nuzzo, R. G. Curr. Opin. Colloid Interface Sci. 1996, 1, 127-136. (2) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (3) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (4) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368. (5) (a) Offord, D. A.; John, C. M.; Griffin, J. H. Langmuir 1994, 10, 761-766. (b) He, Z.; Bhattacharyya, S.; Cleland, W. E.; Hussey, C. L. J. Electroanal. Chem. 1995, 397, 305-310. (6) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506-512. (7) (a) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 16211627. (b) Lee, Y. W.; Reed-Mundell, J.; Zull, J. E.; Sukenik, C. N. Langmuir 1993, 9, 3009-3014. (c) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (8) (a) Liu, M.; Nakahara, H.; Shibasaki, Y.; Fukuda, K. Chem. Lett. 1993, 967-970. (b) Arslanov, V. V.; Sheinina, L. S.; Bulgakova, R. A. 7th International Conference on Organised Molecular Films, Ancona, Italy, 1995; University of Ancona: Ancona, Italy, 1995; p 87. (c) Ahuja, R.; Caruso, P.-L.; Mobius, D.; Paulus, W.; Ringsdorf, H.; Wildburg, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 1033-1036.
order of magnitude or even lower than those in bulk.9-12 It is however important to identify factors which govern such surface reactions, not only because of their fundamental value but as they also could potentially be used in designing sensor devices and could mimic biological reactions.3,13 The objective of this work was to determine the effect of structural changes in the monolayer on the reactivity of a functional group embedded therein. For this purpose, reactive groups have been incorporated into bifunctional adsorbates (Figure 1) and a model reaction with external reagent has been used to test their reactivity. The second, “ballast” chain of the molecules was used to influence the order of monolayers and change the position of the reaction center with respect to the monolayer interface. We intended to address three distinctive questions: (1) Is the reactivity of a functional group in the monolayer different from that in the bulk solution? (2) How does the burying of a functional group deeply inside the monolayer affect its reactivity? (3) If the order of the monolayer is disrupted by accommodating a bulky group on the “ballast” chain of the adsorbate (Figure 1C), what effect will that have on the reactivity? Experimental Design Several methods allowing for incorporation of two different functional groups in gold-thiol monolayers have been reported. The most popular is to codeposit two different thiols, thus giving mixed monolayers (Figure 2A).14 It was reported however that these monolayers tend to phase segregate into domains comprising only (9) Ahmad, J.; Astin, K. B. Langmuir 1990, 6, 1797-1799 and references cited therein. (10) Neogi, P.; Neogi, S.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1993, 1134-1136. (11) Zhuang, X.; Lackritz, H. S.; Shen, Y. R. Chem. Phys. Lett. 1995, 246, 279-284. (12) Fryxell, G. E.; Rieke, P. C.; Wood, L. L.; Engelhard, M. H.; Williford, R. E.; Graff, G. L.; Campbell, A. A.; Wiacek, R. J.; Lee, L.; Halverson, A. Langmuir 1996, 12, 5064-5075. (13) Somorjai, G. A. Chem. Rev. 1996, 96, 1223-1235. (14) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164.
S0743-7463(97)00276-X CCC: $15.00 © 1998 American Chemical Society Published on Web 01/06/1998
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Chechik and Stirling Chart 1
Figure 1. Schematic illustration of different positions of the reactive functional group (shown as black circles) in a monolayer: protruding above the interface (A), buried inside the monolayer (B), and incorporated into a disordered monolayer (C). Approach of an external reagent (shown as open circles) to the functional group is shown with arrows.
Figure 2. Schematic formation of bifunctional monolayers using a mixture of different thiols (A), unsymmetrical disulfides (B), dialkyl sulfides (C), and cystine-based Y-type compounds (D).
molecules of the same type.15 Self-assembly of unsymmetrical disulfides (Figure 2B) may well suffer from the same problem, as these compounds are known to dissociate during deposition, producing independent thiolate moieties.16 Although recent observations suggest that in some cases phase segregation does not take place,17 the evidence accumulated so far does not allow one to exclude this possibility altogether. An alternative method using organic sulfides (Figure 2C) was found to give disordered, unstable monolayers.18 To avoid these complications, a new straightforward method to make reasonably-wellpacked bifunctional monolayers with uniform distribution of functional groups has been used. Cystine-based Y-type disulfides were readily available adsorbates satisfying these criteria. They were synthesized using traditional methods for peptides. The choice of the reactive functional group in the substrate was governed by the following principles: (1) It should be IR active, as IR spectroscopy is one of the few techniques providing direct structural information of the monolayers.19 (2) The model reaction between this group and external reagent should follow a straightforward mechanism, be fast, and be not accompanied by side processes. (3) The monolayer must be sufficiently stable and unreactive in the absence of external reagent. We have incorporated a (p-nitrophenoxy)carbonyl group in our substrates and used primary amines as external reagents. This system was found to fulfill all the above (15) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636-7646. (16) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 17661770. (17) Schonherr, H.; Ringsdorf, H.; Jaschke, M.; Butt, H. J.; Bamberg, E.; Allinson, H.; Evans, S. D. Langmuir 1996, 12, 3898-3904. (18) (a) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365-385. (b) Zhang, M.; Anderson, M. R. Langmuir 1994, 10, 2807-2813. (19) Ulman, A. ACS Symp. Ser. 1991, 447, 144-159.
requirements. Aminolysis of p-nitrophenol esters, one of the favorite reactions of peptide chemists, is a very selective process giving rise to high yields of the target compounds.20 Furthermore, the p-nitrophenol moiety could be used as a protecting group for carboxylic acids during peptide coupling (so-called backing-off procedure21), enabling reduction of the number of steps in the synthesis of adsorbates 1-7. The compounds used in this work are shown in Chart 1. Compounds 1-5 have a p-nitrophenyl ester group incorporated at different levels with respect to the monolayer interface; the substrates 6 and 7 have a bulky tert-butoxy group with the potential to disrupt the order of monolayers. Results and Discussion Preparation of Y-type Monolayers. All monolayers were prepared by dipping precleaned gold slides into a 1 mM solution of disulfides 1-7 in dichloromethane for 16 h. This solvent was chosen because of the relatively high reactivity of the p-nitrophenolate moiety, precluding use of nucleophilic solvents such as alcohols, commonly employed for the preparation of monolayers. Characterization of Monolayers (1-5). (A) IR Spectra. It was important from the outset to be clear about the nature of the monolayers within which the reactions to be studied occur. To get a better understanding of the organization of the monolayers, they were characterized by wettability measurements, surface plasmon resonance (SPR), and grazing angle IR spectroscopy. This type of IR spectroscopy involves selection rules for the observation of functional groups. The observed intensity is proportional to cos2 θ, where θ is the angle between the surface normal and the transitional dipole moment vector.22 Those vibrations whose transitional dipoles are oriented perpendicular to the monolayer surface therefore show maximum absorbances, while vibrations with dipoles parallel or nearly parallel to the surface are correspondingly weak in the spectra. IR spectra of monolayers 1-5 in the C-H stretching region are in Figure 3. Band assignments were according to literature precedent.23 (20) Bodanszky, M. In The Peptides: Analysis, Synthesis, Biology; Gross, E., Meienhofer, J., Eds.; Academic Press: New York, 1979; Vol. 1, pp 105-196. (21) (a) Goodman, M.; Stueben, K. C. J. Am. Chem. Soc. 1959, 81, 3980-3983. (b) Stewart, F. H. C. Aust. J. Chem. 1965, 18, 887-901. (22) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. (23) Hill, I. R.; Levin, I. W. J. Chem. Phys. 1979, 70, 842-851.
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Figure 4. Plot of νa(CH2) (O) and νs(CH2) (]) peak intensities versus number of CH2 units in monolayers 1-5.
Figure 3. IR spectra of monolayers 1 (a), 2 (b), 3 (c), 4 (d), and 5 (e) and bulk spectra of peptide 5 (arbitrary scale) in (f) solid state (KBr disk) and (g) solution in dichloromethane.
The νa(CH3)/νa(CH2) intensity ratio is an important indicator for the degree of order and tilt of alkane chains. If the chains are completely disordered, this ratio should be the same for monolayers and bulk medium.24 In the other extreme, if the chains are well-ordered and perpendicular to the surface, transitional dipole moments for CH2 stretching vibrations will be parallel to the surface, and these peaks are therefore invisible in the spectrum.25 Comparison of spectra e, f, and g in Figure 3 shows that the νa(CH3)/νa(CH2) intensity ratio is significantly higher for monolayer 5 as compared to both solution and solid state bulk spectra. Bulk spectra of compounds 1-4 (not shown for the sake of clarity) show similar behavior. This demonstrates that alkane chains in monolayers 1-5 are preferentially orientated toward the solution interface. The intensities of ν(CH3) peaks remain almost constant for monolayers 1-5. This indicates the similar degree of coverage for all monolayers.26 The intensities of ν(CH2) peaks, however, increase with the number of methylene units (Figure 4). It is interesting to note that the intensities of ν(CH2) peaks increase much more sharply in monolayers 3-5. Higher C-H stretching band intensities of methylene groups suggest that the CH2 units which are incremental in the “ballast” chains of monolayers 3-5 are less ordered (or more tilted) than the CH2 groups present in monolayers 1 and 2. On the assumption of fully stretched conformations for the alkane chains of the adsorbates, the conclusion is that the alkane chains below the level of the carbonyl function are better orientated toward the interface than the portion of the “ballast” chain protruding above. (24) Golden, W. G.; Snyder, C. D.; Smith, B. J. Phys. Chem. 1982, 86, 4675-4678. (25) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700-2704. (26) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.
Figure 5. Lower frequency IR spectra of peptide 4 in (a) cast film and (b) monolayer.
The CH2 stretching vibrations are sensitive to the nature of interactions between adjacent alkane chains.26 In wellpacked, crystalline samples these modes are usually observed at lower frequencies (the effect is more pronounced with νa(CH2)). It can be seen from Figure 3 that both CH2 peaks in monolayer 5 have shifted to higher frequency as compared to the crystalline bulk spectrum. The appearance of νa(CH2) at 2928 cm-1 and νs(CH2) at 2858 cm-1 in monolayers 1-5 is typical of disordered structures.27 Monolayers of disulfides 6 and 7 show νa(CH2) and νs(CH2) peaks at 2930 and 2960 cm-1, respectively, which also suggests the disordered state of the alkane chains in these monolayers. IR spectra of monolayers 1-5 in the lower frequency region are essentially the same, and a typical example, along with the corresponding bulk spectrum, is in Figure 5. The most intense peak in the bulk spectrum, the amide I peak, is lost in the spectrum of the monolayer, which (27) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150.
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Chechik and Stirling Table 1. Experimental and Maximum Theoretical Thicknesses of Monolayers 1, 4, and 5 monolayer
SPR-based thicknessa
max theor thicknessb
1 1c 4 5
19 21 19 20
21 21 21 23d
a Values (5%, units Å. b Calculated using CPK models, units Å. Measured in contact with ethyl acetate. d Calculated assuming that the portion of the “ballast” chain, protruding above the level of the nitro group, is tilted to provide maximum packing.
c
Table 2. Second-Order Rate Constantsa of Monolayer Reactions with n- and sec-BuNH2
Figure 6. Plot of sessile (O), advancing (]), and receding (3) contact angles of monolayers 1-5 versus length of the “ballast” chain.
indicates orientation of CdO bonds parallel to the surface.28 The amide II peak is overlapped by the νa(NO2) peak in the bulk spectrum and is almost invisible in the spectrum of the monolayer (the peak at 1553 cm-1 in the spectrum of the monolayer disappears after treatment with amines, which demonstrates that this peak belongs to the reactive nitrophenolate rather than to the unreactive amide group). (B) Wettability of Monolayers (1-5). It can be seen from Figure 6 that the monolayers with “ballast” chains terminating below the level of NO2 function (1-3) have the same contact angles, whereas monolayers 4 and 5 are more hydrophobic. This suggests that the nitrophenyl group dominates the surface of monolayers 1-3 and determines the value of the contact angles. In monolayers 4 and 5, however, the “ballast” chain protrudes above the interface and renders the surface more hydrophobic. The high contact angles of monolayer 5 are typical of a disordered alkane surface2 and may be compared with the value of 80° quoted for the mixed monolayer containing equimolar amounts of HS(CH2)11OH and HS(CH2)21CH3.29 These monolayers have a hydroxyl group buried 10 CH2 units below the interface and are well-ordered.30 (C) SPR Analysis. We employed the surface plasmon resonance technique to evaluate the degree of coverage and the thickness of the monolayers under study. This method is sensitive to both the refractive index and thickness of the thin film.31 We assumed the former parameter to be equal to 1.5, which is a reasonable approximation for most complete monolayers.32 The thickness calculated using this value is therefore reliable provided the monolayer is complete. For incomplete monolayers, however, the actual refractive index will be lower, ultimately resulting in a lower value of calculated thickness, dependent upon the degree of coverage of monolayers.33 The SPR data along with the maximum theoretical thicknesses of monolayers are in Table 1. The close correspondence between SPR data and maximum thicknesses suggests that the monolayers (28) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371-4382. (29) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164-7175. (30) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 38823893. (31) Knoll, W. MRS Bull. 1991, 16, 29-39. (32) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731-4740. (33) DeBono, R. F.; Loucks, G. D.; Manna, D. D.; Krull, U. J. Can. J. Chem. 1996, 74, 677-688.
b
monolayer
k2(n-BuNH2)
k2(sec-BuNH2)
k2(n-BuNH2)/ k2(sec-BuNH2)
1 1b 1c 1d 2 3 4 5 6 7
0.21 2.44 0.019 0.16 0.16 0.085 0.14 0.11 0.23 0.20
0.010 0.13 0.0018
21 19 11
0.0065 0.0050 0.0092 0.0062 0.014 0.012
25 17 15 18 16 17
a In ethyl acetate at 22 °C. Values (15%, units dm3 mol-1 s-1. In water. c In dichloromethane. d With n-C12H25NH2.
completely cover the gold surface and are not significantly tilted with respect to the monolayer interface. The fact that the thickness of monolayer 1 is almost the same in both the gas phase and the solution phase (Table 1) implies that there is no rearrangement of the surface in contact with solution. The measured thickness of monolayers is significantly greater than the maximum distance between the gold surface and the carbonyl function (14 Å, estimated from a CPK model), thus showing that this group is buried under the surface of monolayers. To summarize, it can be concluded from IR, SPR, and wettability data that the monolayers studied fully cover the gold surface. The alkane chains in these monolayers are disordered, and the carbonyl function is most likely to be buried under the monolayer interface. Reactivity of Functionalized Monolayers. Monolayers 1-7 were allowed to react with solutions of n- and sec-butylamines. The latter amine was used to discover whether the increased steric demand of the branched nucleophile would make penetration into the monolayer more difficult and as a consequence decrease reactivity. Reactions were monitored by grazing angle IR spectroscopy with disappearance of peaks at 1772, 1553, and 1349 cm-1 being used to derive rate constants. Reactions were found to be first order with respect to amine. Kinetic runs were carried out under pseudo-first-order conditions, the amine being in large excess with respect to the monolayer. Second-order rate constants were determined, and results are in Table 2. The data of Table 2 show that the rate constants decrease with an increase in the length of the “ballast” alkane chain. This reflects burying of the reactive carbonyl group deeper under the monolayer interface. The only exception of this trend is a slow reaction of monolayer 3. This might be due to a slightly different organization of this monolayer, in which the “ballast” chain terminates at the level of the phenyl ring, and hydrophobic interactions between these functions might stabilize the monolayer and hamper access of the external reagent. Interestingly, monolayers 6 and 7, which have bulky substituents, react faster than monolayers 2-5. Both
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Table 3. Second-Order Bulk Rate Constantsa for the Reaction of Ester 8 with Amines solvent
k2(n-BuNH2)
k2(sec-BuNH2)
k2(n-BuNH2) /k2(sec-BuNH2)
ethyl acetate ethyl acetateb dichloromethane
0.059 0.047 0.0048
0.0032
18.4
0.00022
21.8
a
At 22 °C. n-C12H25NH2.
Values (5%, units dm3 mol-1 s-1.
b
With
monolayers 6 and 7 have the same reactivity as ester 1, although, in the all-stretched conformation, the “ballast” branch of ester 7 should be longer than that of ester 3. Apparently, disorder caused by the bulky Boc group facilitates penetration of the monolayer by the external reagent. In order to compare the reaction at the monolayer surface with the same process in bulk medium, we have studied a model bulk reaction:
BocNH(CH2)5COOC6H4NO2-p + RNH2 f 8 BocNH(CH2)5CONHR + p-HOC6H4NO2 The choice of the model substrate 8 was determined by solubility considerations. The product from ester 6 with n-butylamine is not sufficiently soluble in ethyl acetate to allow accurate kinetic measurements, but the rate constant was estimated to be 0.078 dm3 mol-1 s-1, insignificantly different from that of the model compound. Kinetic runs were carried out under pseudo-first-order conditions, with the concentration of amine used in monolayer experiments. Results are in Table 3. It is surprising that the ratio of reaction rates in dichloromethane and ethyl acetate k(dichloromethane)/ k(ethyl acetate) is almost the same for the monolayers as in bulk solution. It is not possible to monitor bulk reaction in water because of solubility problems, but literature data on similar reactions suggest that the k(water)/ k(dichloromethane) ratio for the monolayer is also close to the bulk medium value.34 Moreover, it is clear from Table 2 that the rate constants for monolayer reactions are not very sensitive to the structure of the monolayer, changing by a factor of only 2 on going from monolayer 1 to monolayer 5. Even the k2(n-BuNH2)/k2(sec-BuNH2) ratio is constant within experimental error. This very close similarity of solvent and substituent effects on the reaction in monolayer and bulk solution suggests similar transition structures for both situations. Importantly, however, reaction with the monolayer is several times faster than in bulk. It is unlikely that local dielectric environment of the reaction center in monolayers is responsible for this effect. In this case, the reaction in monolayers would have been much less sensitive to solvent variations than the bulk reaction, as the strong effect of local environment on reactions in monolayers implies that the reaction center primarily interacts with the adjacent functionalities in the monolayer and not with the solvent molecules; sensitivity to solvent variations would therefore be reduced. Because the ratio of reaction rates in different solvents is almost the same in bulk and in monolayers, we expected the absolute values of reaction rates also to be similar. A tentative hypothesis to account for the faster reaction in monolayers is preassociation of the reagents, involving a weak equilibrium binding of the external (34) (a) Oleinik, N. M.; Litvinenko, L. M.; Kurchenko, L. P.; Terekhova, S. E.; Gel'bina, Zh. P. J. Org. Chem. USSR (Engl. Transl.) 1976, 12, 2304-2311. (b) Knowles, J. R.; Parsons, C. A. Chem. Commun. 1967, 755-757.
Figure 7. Schematic illustration of weak equilibrium binding of butylamines to monolayers 1-7.
reagent to the monolayer prior to reaction (Figure 7). Such an equilibrium process (which might be favored by dipoledipole or Van der Waals interactions) would have the effect of increasing the effective concentration of the amine in the vicinity of the monolayer, which in turn increases the rate constant. This association does not, however, significantly influence the structure of the transition and ground states of reaction, thus accounting for the similarity in behavior between monolayers and bulk solution. Whatever the reason for the rate acceleration of the reaction in monolayers, the most striking result of our studies is that the burying of a functional group below the surface of a monolayer has little effect on its reactivity. This implies that the permeability of these monolayers is not a limiting step in the overall reaction. The fast reaction rates obtained in this work are also in agreement with recently reported rapid aminolysis of trifluoroethyl esters incorporated in gold-thiol monolayers.12 It is however surprising that gold-thiol monolayers which are firmly attached to the gold surface could so easily change their conformation to accommodate the reagent molecules. This failure of disordered monolayers 1-7 to prevent penetration by the external species is in sharp contrast with some previously reported studies on well-packed ordered monolayers. The burying of a functional group inside a well-ordered monolayer was found to block its reactivity almost entirely,4,10 and electrochemical measurements showed that such monolayers cannot be penetrated by external molecules.35 This sharp difference in behavior between well-packed and disordered monolayers is reminiscent of the behavior of solid and liquid phases proposed for monolayers at the air-water interface.36 Recent molecular dynamic calculations for alkanethiol monolayers revealed the existence of several phases at different area per molecule values.37 They also described a sharp increase in the translational diffusion coefficient of molecules in less well-packed monolayers. Disordered monolayers organized in a liquid-like structure might therefore be much more flexible and permeable than the quasi crystalline well-packed monolayers. Increased disorder, as in monolayers 6 and 7, has only a minor effect on their flexibility and results in almost unchanged reaction rates (Table 2). (35) (a) Takehara, K.; Takemura, H.; Ide, Y. Electrochim. Acta 1994, 39, 817-822. (b) Campbell, D. J.; Herr, B. R.; Hulteen, J. C.; Van Duyne, R. P.; Mirkin, C. A. J. Am. Chem. Soc. 1996, 118, 10211-10219. (c) Terrettaz, S.; Becka, A. M.; Traub, M. J.; Fettinger, J. C.; Miller, C. J. J. Phys. Chem. 1995, 99, 11216-1224. (36) Gaines, G. L. Insoluble Monolayers: Liquid-Gas Interfaces; Interscience: New York, 1966. (37) Lee, S. H.; Kim, H. S. Bull. Korean Chem. Soc. 1996, 17, 700706.
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Langmuir, Vol. 14, No. 1, 1998
Conclusions We have shown that burying of a functional group deeply inside a disordered monolayer does not significantly affect its reactivity. Further disruption of the order by incorporation of a bulky group into the monolayer also has only a minor effect on reaction rates. Comparison with literature data on well-ordered monolayers4,10,35 known to block any penetration of guest molecules into the monolayer suggests that disordered monolayers exist in a liquid-like phase and are much more easily permeable than well-packed quasi crystalline monolayers. A model reaction with monolayers was found to proceed several times faster than the same process in bulk. Taking into account the surprisingly small difference in solvent/ substituent effects on the reaction in the monolayer and the bulk medium, we suggest that this acceleration is probably due to loose binding of a reagent to the monolayer prior to reaction, thus increasing its concentration in the vicinity of the reaction center. This process could be favored by dipole-dipole or Van der Waals interactions. While little is known about interactions of external molecules with monolayers in solution, gas phase experiments38 suggest the existence of similar equilibrium binding of guest species by the surfaces. Experimental Section Materials. HPLC grade dichloromethane and ethanol were purchased from Aldrich and used as such. Other solvents were dried according to published recommendations39 and distilled before use. Deionized water was used in all experiments. Preparation of Monolayers. Gold substrates were prepared by thermal evaporation of 5 nm of Cr followed by 100 nm of Au onto silicon wafers which were precleaned by heating in piranha solution (7:3 mixture of concentrated sulfuric acid and 30% hydrogen peroxide) at 90 °C for 1 h. Caution! Piranha solution reacts violently with almost any organic materials and should be handled with utmost care!2 Prior to deposition of monolayers, the gold substrates were treated with concentrated HNO3 for 10 min, washed exhaustively with water and ethanol, and dried. Monolayers were prepared by dipping the precleaned substrates into 1 mM dichloromethane solutions of disulfides (1-7) for 16 h at room temperature. The substrates were then rinsed with dichloromethane and ethanol and dried. Instrumentation. IR spectra of monolayers were determined with a Perkin-Elmer 1725 X instrument fitted with an MCT detector and a grazing angle accessory (Spectra-Tech Inc.). Freshly cleaned (concentrated HNO3, 10 min) bare gold slides were used as background. The sample compartment of the spectrometer was purged with nitrogen to prevent interference from water vapor. Bulk spectra were recorded as cast films on NaCl disks. SPR data were acquired using an instrument built in these laboratories giving a resonance angle resolution of 0.006°. The values of thickness were calculated from the shift of resonance angle using the standard Fresnel equation.40 Further details of SPR measurements will be published elsewhere. Contact angles were recorded with a CCD camera attached to a Power Macintosh computer. Water drops (1 µL) were generated with a micrometer syringe (Agla). Electronic images of sessile, advancing, and receding drops were stored in the computer and (38) (a) Huisman, B. H.; Kooyman, R. P. H.; Van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Mater. 1996, 8, 561. (b) Davis, F.; Neogi, P.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1994, 1199-1200. (c) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1990, 112, 570-579. (d) Thomas, R. C.; Houston, J. E.; Crooks, R. M.; Kim, T.; Michalske, T. A. J. Am. Chem. Soc. 1995, 117, 3830-3834. (e) Okahata, Y.; Matsuura, K.; Ito, K.; Ebara, Y. Langmuir 1996, 12, 10231026. (39) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon Press: Oxford, 1980. (40) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380-390.
Chechik and Stirling analyzed using ClarisDraw software. At least three drops were analyzed for each slide. Kinetic Measurements. Substrates covered with monolayers 1-5 were immersed in thermostated amine solutions (1% w/w) at 22.0 ( 0.5 °C. The amines were shown to be unreactive toward ethyl acetate on the time scale used. At intervals (corresponding to ca. 10-20% increase in conversion), slides were withdrawn, washed with ethanol, dried, and analyzed by IR spectroscopy. Reactions were monitored to 70-80% conversion. After the last measurement, the slides were dipped into a 5% solution of n-BuNH2 in ethyl acetate for 5 min to bring reaction to completion, washed, and analyzed by IR. This spectrum was smoothed and subtracted from all the preceding smoothed spectra prior to integration. This procedure was found to be crucial for achieving high accuracy of the measurements. The integral intensities of peaks at 1772, 1553, and 1349 cm-1 were shown to produce the same values of reaction rates within experimental error, showing that there is no change in orientation of nitrophenolate moieties during the reaction, and IR intensities therefore are proportional to the actual amount of material present in the monolayer. All kinetic data reported are the average of at least two runs. For kinetic runs in bulk media the appearance of p-nitrophenol was monitored at 310 nm. We used the same temperature and amine concentration as in monolayer experiments. The concentration of the ester was 50-100 times smaller than the concentration of the amine. The reaction mixture was homogeneous. In a separate experiment, reactions were found to be first order with respect to amine and ester concentrations. Deprotection of Boc-Protected Amino Components.41 A few drops of trifluoroacetic acid were added to the Boc-protected compound to give an oil. After 10 min, excess trifluoroacetic acid was evaporated off, and the residue was flushed with argon, triturated with ether, and dried over NaOH in a vacuum desiccator. Mixed Anhydride Couplings.42 The acid component in dry THF was cooled to -15 °C and a weighed amount of dry triethylamine in THF was added to the well-stirred mixture followed by isobutyl chloroformate in THF. After 5 min, a precooled solution of the trifluoroacetate salt of the amino component in THF was added followed by another portion of triethylamine in THF. Cooling was maintained for 0.5 h, and the mixture was allowed to warm to room temperature in 2-5 h. It was then filtered, evaporated, and separated by flash chromatography. Although this method is not usually accompanied by racemization,42 no special precautions were made to obtain enantiomerically pure compounds, and the optical purity of the products was not investigated. 6-[(N-Boc-cystinyl)amino]caproic Acid p-Nitrophenyl Ester (6). 6-(N-Boc-amino)caproic acid p-nitrophenyl ester 843 (3 g, 8.51 mmol) was deprotected, and the residue was dissolved in THF (5 mL) to make solution a. Di-Boc-L-cystine (1.875 g, 8.51 mmol), triethylamine (861 mg, 8.51 mmol), isobutyl chloroformate (1.162 g, 8.51 mmol), solution a, and triethylamine (1.722 g, 17.02 mmol) were coupled in THF (30 mL). Flash chromatography eluting with DCM/EtOAc ) 4:1 (Rf 0.27) gave peptide 6 (55%). Mp: 157-158 °C. 1H NMR (CDCl3) δ: 8.25 (d, 2H, CHArCNO2), 7.75 (br t, 1H, NHCH2), 7.26 (d, 2H, CHArCO), 5.54 (br d, 1H, NHOCO), 4.73 (m, 1H, CH), 3.27 (m, 2H, CH2N), 2.92 (m, 2H, CH2S), 1.76 (m, 2H, CH2CH2N), 1.61 (m, 2H, CH2CH2CO), 1.45 (s + m, 11H, CH3 + CH2CH2CH2CH2CH2). IR (cast film), cm-1: ν(NH) 3337, ν(C-Me) 1366, ν(CdOBoc) 1685, ν(CdOester) 1760, amide I 1658, amide II + νa(NO2) 1522, νs(NO2) 1347. Anal. Calcd for C40H56N6O14S2: C, 52.85; H, 6.21; N, 9.24. Found: C, 52.80; H, 6.02; N, 9.03. 6-[((6-N-Boc-amino)caproyl)cystinyl)amino]caproic Acid p-Nitrophenyl Ester (7). Peptide 6 (1 g, 2.2 mmol) was deprotected and the residue dissolved in THF (3 mL) to make solution a. 6-(Boc-amino)caproic acid (611 mg, 2.64 mmol), (41) Schnabel, E.; Klostermeyer, H.; Berndt, H. Liebigs Ann. Chem. 1971, 749, 90-108. (42) Meienhofer, J. In The Peptides: Analysis, Synthesis, Biology; Gross, E., Meienhofer, J., Eds.; Academic Press: New York, 1979; Vol. 1, pp 263-314. (43) Tesser, G. I.; Fisch, H.-U.; Schwyzer, R. Helv. Chim. Acta 1974, 57, 1718-1730.
Reactivity in Self-Assembled Monolayers triethylamine (267 mg, 2.64 mmol), isobutyl chloroformate (355 mg, 2.60 mmol), solution a, and triethylamine (445 mg, 4.4 mmol) were coupled in THF (10 mL). Flash chromatography eluting with DCM/EtOAc ) 1:3 (Rf 0.41) gave 0.7 g (56%) of peptide 7. Mp: 99.5-101.5 °C. 1H NMR (CDCl3) δ 8.25 (br d, 3H, CHArCNO2 + NHCOCH), 7.26 (d, 2H, CHArCO), 6.55 (br d, 1H, NHCH), 5.27 (m, 1H, CH), 4.54 (br s, 1H, NHOCO), 3.30 (m, 2H, CH2NHCOCH), 3.10 (m, 2H, CH2NHOCO), 2.92 (m, 2H, CH2S), 2.60 (t, 2H, CH2COO), 2.26 (t, 2H, CH2CON), 1.77 + 1.65 + 1.47 (m, 12H, CH2CH2CH2), 1.42 (s, 9H, CH3). IR (cast film), cm-1: ν(NH) 3295, ν(CsMe) 1365, ν(CdOBoc) 1683, ν(CdOester) 1762, amide I 1636, amide II + νa(NO2) 1524, νs(NO2) 1348. Anal. Calcd for C52H78N8O16S2: C, 55.01; H, 6.92; N, 9.87. Found: C, 55.08; H, 6.90; N, 9.59. Synthesis of Disulfides 1-5. 6-[(N-Boc-cystinyl)amino]caproic acid p-nitrophenyl ester 6 (100 mg, 0.22 mmol) was deprotected and dissolved in dry THF (2 mL) to make solution a. The acid chloride (0.264 mmol) in THF (5 mL) was cooled to 0 °C. Solution a was added followed by triethylamine (66.7 mg, 0.66 mmol) in THF (1 mL), and the mixture was stirred at 0 °C for 2 h, filtered, and evaporated. The crude product was purified by flash chromatography. 6-[(N-Caproylcystinyl)amino]caproic Acid p-Nitrophenyl Ester (1). Flash chromatography eluting with DCM/ petroleum ether/ethyl acetate ) 1:2:3 (Rf 0.4) gave 75 mg (75%) of peptide 1. Mp: 120.5-122.5 °C. 1H NMR (CDCl3) δ: 8.27 (m, 3H, NHCOCH + CHArCNO2), 7.25 (d, 2H, CHArCO), 6.58 (br d, 1H, NHCH), 5.29 (m, 1H, CH), 3.26 (m, 2H, CH2N), 2.90 (m, 2H, CH2S), 2.60 (t, 2H, CH2COO), 2.25 (t, 2H, CH2CO), 1.25-1.85 (m, 12H, CH3CH2CH2 + CH2CH2CH2), 0.87 (t, 3H, CH3). IR (cast film), cm-1: ν(NH) 3294, ν(CdO) 1763, amide I 1637, amide II + νa(NO2) 1525, νs(NO2) 1347, ν(CsO) 1209. Anal. Calcd for C42H60N6O12S2: C, 55.74; H, 6.68; N, 9.29. Found: C, 55.96; H, 6.84; N, 9.11. 6-[(N-Capryloylcystinyl)amino]caproic Acid p-Nitrophenyl Ester (2). Flash chromatography eluting with ethyl acetate/petroleum ether ) 4:3 (Rf 0.41) gave 100 mg (94%) of peptide 2. Mp: 131-133 °C. 1H NMR (CDCl3) δ: 8.27 (m, 3H, NHCOCH + CHArCNO2), 7.26 (d, 2H, CHArCO), 6.42 (br d, 1H, NHCH), 5.32 (m, 1H, CH), 3.30 (m, 2H, CH2N), 2.92 (m, 2H, CH2S), 2.60 (t, 2H, CH2COO), 2.26 (t, 2H, CH2CO), 1.25-1.85 (m, 16H, CH3CH2CH2 + CH2CH2CH2), 0.86 (t, 3H, CH3). IR (cast film), cm-1: ν(NH) 3294, ν(CdO) 1759, amide I 1637, amide II + νa(NO2) 1524, νs(NO2) 1347, ν(CsO) 1209. Anal. Calcd for C46H68N6O12S2: C, 57.48; H, 7.13; N, 8.74. Found: C, 57.15; H, 7.05; N, 8.47.
Langmuir, Vol. 14, No. 1, 1998 105 6-[(N-Decanoylcystinyl)amino]caproic Acid p-Nitrophenyl Ester (3). Flash chromatography eluting with DCM/ petroleum ether/ethyl ether ) 9:1:2 (Rf 0.34) gave 90 mg (80%) of peptide 3. Mp: 127.5 °C. 1H NMR (CDCl3) δ: 8.26 (m, 3H, NHCOCH + CHArCNO2), 7.25 (d, 2H, CHArCO), 6.49 (br d, 1H, NHCH), 5.31 (m, 1H, CH), 3.28 (m, 2H, CH2N), 2.90 (m, 2H, CH2S), 2.60 (t, 2H, CH2COO), 2.25 (t, 2H, CH2CO), 1.20-1.85 (m, 20H, CH3CH2CH2 + CH2CH2CH2), 0.84 (t, 3H, CH3). IR (cast film), cm-1: ν(NH) 3286, ν(CdO) 1761, amide I 1637, amide II + νa(NO2) 1524, νs(NO2) 1346, ν(CsO) 1209. Anal. Calcd for C50H76N6O12S2: C, 59.03; H, 7.53; N, 8.26. Found: C, 58.92; H, 7.57; N, 8.16. 6-[(N-Laurylcystinyl)amino]caproic Acid p-Nitrophenyl Ester (4). Flash chromatography eluting with DCM/petroleum ether/ethyl ether ) 3:2:2 (Rf 0.29) gave 60 mg (51%) of peptide 4. Mp 128 °C. 1H NMR (CDCl3) δ: 8.25 (m, 3H, NHCOCH + CHArCNO2), 7.25 (d, 2H, CHArCO), 6.42 (br d, 1H, NHCH), 5.30 (m, 1H, CH), 3.27 (m, 2H, CH2N), 2.90 (m, 2H, CH2S), 2.60 (t, 2H, CH2COO), 2.25 (t, 2H, CH2CO), 1.20-1.85 (m, 24H, CH3CH2CH2 + CH2CH2CH2), 0.85 (t, 3H, CH3). IR (cast film), cm-1: ν(NH) 3285, ν(CdO) 1762, amide I 1638, amide II + νa(NO2) 1524, νs(NO2) 1346, ν(CsO) 1210. Anal. Calcd for C54H84N6O12S2: C, 60.42; H, 7.89; N, 7.83. Found: C, 60.22; H, 7.84; N, 7.52. 6-[(N-Stearoylcystinyl)amino]caproic Acid p-Nitrophenyl Ester (5). Flash chromatography eluting with DCM/ethyl ether ) 6:1 (Rf 0.33) gave 93 mg (68%) of peptide 5. Mp: 126129 °C. 1H NMR (CDCl3) δ 8.26 (m, 3H, NHCOCH + CHArCNO2), 7.25 (d, 2H, CHArCO), 6.47 (br d, 1H, NHCH), 5.31 (m, 1H, CH), 3.27 (m, 2H, CH2N), 2.90 (m, 2H, CH2S), 2.59 (t, 2H, CH2COO), 2.25 (t, 2H, CH2CO), 1.20-1.85 (m, 36H, CH3CH2CH2 + CH2CH2CH2), 0.87 (t, 3H, CH3). IR (cast film), cm-1: ν(NH) 3290, ν(CdO) 1761, amide I 1637, amide II + νa(NO2) 1524, νs(NO2) 1346, ν(CsO) 1209. Anal. Calcd for C66H108N6O12S2: C, 63.84; H, 8.77; N, 6.77. Found: C, 63.74; H, 8.84; N, 6.73.
Acknowledgment. The authors would like to thank Mr. A. Sabot for help with SPR measurements. Support from EPSRC and Glaxo Wellcome is gratefully acknowledged. LA970276T
