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The Interplay of Lateral and Tiered Interactions in Stratified Self-Organized Molecular Assemblies Robert S. Clegg, Scott M. Reed, Rachel K. Smith, Bridgette L. Barron, Jamieson A. Rear, and James E. Hutchison* Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403-1253 Received August 3, 1999 Hybrid self-assembled monolayers (SAMs) containing well-defined strata of different polarity enable insight into how fundamental interactions lead to higher order structure and may provide useful analogies for self-assembled multilayers, new hybrid materials, and functional biological interfaces. We report amidecontaining alkanethiol SAMs with internal polar sublayers that are two amide groups thick and nonpolar overlayers comprising either dodecyl or hexadecyl chains. The assemblies have been characterized by X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and external reflective infrared spectroscopy (FTIR-ERS). XPS demonstrates the SAMs are of monolayer thickness, chemisorbed to the gold substrate, and anisotropically oriented. Contact angle data show the methyl surface for n ) 16 is highly ordered, but the surface for n ) 12 is less well ordered. FTIR-ERS reveals that the alkyl chains for n ) 16 are close packed, but that those for n ) 12 are disordered. FTIR-ERS also shows that, although the two-amide sublayers are compositionally identical, they are well ordered and assume polyglycineII-like conformations for n ) 16, but they are poorly ordered for n ) 12. Comparison of these two SAMs to each other in the context of previously reported one- and three-amide SAMs leads to two conclusions. (1) The threshold n for alkyl chain length ordering in two-amide SAMs is 12 e n e 16. Thus, in SAMs with internal amide sublayers both one and two amide groups thick, the threshold number of methylenes required to form ordered alkyl regions is significantly increased compared to alkanethiol SAMs, demonstrating destructive interference of the amide region with the hydrocarbon ordering process. (2) In two-amide SAMs the formation of a well-ordered amide region depends on the ordering of an overlying hydrocarbon region. This behavior differs with that previously demonstrated for one- and three-amide SAMs, in which the amide groups assume characteristic conformations regardless of hydrocarbon region thickness and order. For two-amide SAMs, the apparent dependence of amide ordering on complementary ordering in the alkyl region provides evidence of an energetic interplay between the two sublayers, manifested as a “reverse ordering” effect. The previously unobserved elastic-elastic character of the buried interface in two-amide SAMs is contrasted with the rigid-elastic interface found in the one-amide SAMs.
Introduction A chemical understanding of how molecular level interactions lead to supramolecular structure is essential for the rational design of materials based on selfassembling units. New polypeptide materials based on first principles1 constitute but one area of intense interest in research aimed toward realizing desired bulk properties through the judicious choice of molecular building blocks.2 However, in the rational design of materials, serious obstacles remain due to the incomplete knowledge of how weak interactions can be used to build up larger, ordered assemblies.2e,3 Therefore, systems that probe the interplay of intermolecular interactions are desired for both fundamental research and materials science applications. (1) (a) Kelly, J. W. Structure 1997, 5, 595-600. (b) For a review, see: Urry, D. W. Biopolymers 1998, 47, 167-178. (c) Schneider, J. P.; Kelly, J. W. Chem. Rev. 1995, 95, 2169-2187. (2) (a) Harvey, P. D.; Fortin, D. Coord. Chem. Rev. 1998, 171, 351354. (b) Evans, C. C.; Bagieubeucher, M.; Masse, R.; Nicoud, J. F. Chem. Mater. 1998, 10, 847-854. (c) Deschenaux, R.; Donnio, B.; Rheinwald, G.; Stauffer, F.; Sussfink, G.; Velker, J. J. Chem. Soc., Dalton Trans. 1997, 4351-4355. (d) Shimizu, T.; Kogiso, M.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 6209-6210. (e) Whitesides, G. M.; Simanek, E. E.; Gorman, C. B. In Chemical Synthesis: Gnosis to Prognosis; Chatgilialoglu, C., Snieckus, V., Eds.; NATO ASI Series E, Volume 320; Kluwer Academic: Dordrecht, The Netherlands, 1996; pp 565-588. (f) Decurtins, S. Chimia 1998, 52, 539-546. (g) Schubert, U. S.; Lehn, J.-M.; Hassmann, J.; Hahn, C. Y.; Hallshmid, N.; Muller, P. In Functional Polymers; Patil, A. O., Schulz, D. N., Novak, B. M., Eds.; ACS Symposium Series 704; American Chemical Society: Washington, DC, 1998; pp 248-260. (h) Ranganathan, D. Pure Appl. Chem. 1996, 68, 671-674.
Self-assembled monolayers (SAMs) of simple and terminally functionalized n-alkanethiols on coinage metals are well-known.4 Several reports of the incorporation of amide groups5 and other internal functionalities6 deep within monolayers have appeared. We have developed a series of stratified, heteroatom-containing SAMs containing two types of material of contrasting polarity (Figure 1)san amide underlayer capable of cross-linking by hydrogen bonding5 and an overlayer of alkyl chains capable of close packing through van der Waals interactions.5g,7a,e Our interest in such “buried” amide groups is to develop (3) (a) Kortemme, T.; Ramierez-Alvarado, M.; Serrano, L. Science 1998, 281, 253-257. (b) Petka, W. A.; Harden, J. L.; McGrath, K. P.; Wirtz, D.; Tirrell, D. A. Science 1998, 281, 389-392. (c) Krejchi, M. T.; Atkins, E. D. T.; Waddon, A. J.; Fournier, M. J.; Mason, T. L.; Tirrell, D. A. Science 1994, 265, 1427-1432. (4) (a) Poirier, B. E.; Pylant, E. D. Science 1996, 272, 1145-1148. (b) Karpovich, D. S.; Schessler, H. M.; Blanchard, G. J. Thin Films 1998, 24, 44-81. (c) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (d) Liu, G.-y.; Xu, S.; Cruchon-Dupeyrat, S. Thin Films 1998, 24, 82-111. (e) Fenter, P. Thin Films 1998, 24, 112-149. (f) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (g) Finklea, H. O. In Electroanalytic Chemistry: A Series of Advances; Bard, A. J., Ed.; Dekker: New York, 1996; pp 110-336. (h) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719-729. (i) Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Chem. 1987, 59, 379A-385A. (j) Mrksich, M.; Whitesides, G. M. Annu. Rev. Biophys. Biomol. Struct. 1996, 25, 55-78. (k) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219-227. (l) Tien, J.; Younan, X.; Whitesides, G. M. Thin Films 1998, 24, 227-254. (m) Liu, J.; Kim, A. Y.; Wang, L. Q.; Palmer, B. J.; Chen, Y. L.; Bruinsma, P.; Bunker, B. C.; Exarhos, G. J.; Graff, G. L.; Rieke, P. C.; Fryxell, G. E.; Virden, J. W.; Tarasevich, B. J.; Chick, L. A. Adv. Colloid Interface Sci. 1996, 69, 131-180. (n) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550-575.
10.1021/la9910529 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/15/1999
Stratified Self-Organized Molecular Assemblies
Langmuir, Vol. 15, No. 26, 1999 8877
Figure 1. General arrangement of sublayers within these stratified, amide-alkyl SAMs. The amide underlayer is anchored to the gold substrate by a mercaptoethylene linker sublayer previously employed in well-ordered SAMs.5e-g
amide-containing SAMs amenable to electron-transfer studies,5e,h as well as to study the interaction of different stabilizing interactions, the effect of different sublayers on equilibrium assembly structures, and the nature of buried organic interfaces. A systematic study of the variation of the thickness of the alkyl overlayer recently demonstrated that a polar underlayer one amide group thick both destabilizes (relative to n-alkanethiol SAMs) and serves as a rigid, preattached “anchor layer” for a hydrocarbon overlayer.5g Further, highly ordered amide regions are formed in SAMs containing three glycine-based amide groups per adsorbate molecule underlying n-nonyl residues, and the amide chains assume 31 helical conformations stabilized by intermolecular hydrogen bonding as in bulk polyglycine II, as shown by both IR spectroscopy and thickness measurements.5f On the basis of the knowledge that, in ambient conditions, alkyl chains of sufficient length are required to form well-ordered hydrocarbon regions in one-amide5e,g and n-alkanethiol7a,e monolayers, an analogous threshold is reasonable for SAMs containing polar underlayers two amide groups thick (Cn-mAT/Au,8 m ) 2). The present study compares two examples of such SAMs in which the (5) (a) Lenk, T. J.; Hallmark, V. M.; Hoffman, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 46104617. (b) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. A.; Jeon, N.; Nuzzo, R. A. Langmuir 1995, 11, 4371-4382. (c) Chechik, V.; Scho¨nherr, H.; Vancso, G. J.; Stirling, C. J. M. Langmuir 1998, 14, 3003-3010. (d) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E., Jr.; Hussey, C. L. Langmuir 1998, 14, 124-136. (e) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239-5243. (f) Clegg, R. S.; Reed, S. M.; Hutchison, J. E. J. Am. Chem. Soc. 1998, 120, 24862487. (g) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 5319-5327. (h) Slowinski, K.; Fong, H. K. Y.; Majda, M. J. Am. Chem. Soc. 1999, 121, 7257-7261. (6) (a) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Ha¨ussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050-1053. (b) Cheng, J.; Sa`ghi-Szabo´, G.; Tossell, J. A.; Miller, C. J. J. Am. Chem. Soc. 1996, 118, 680-684. (c) Evans, S. D.; Goppert-Berarducci, K. E.; Urankar, E.; Gerenser, L. J.; Ulman, A. Langmuir 1991, 7, 2700-2709. (d) Miller, C.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 5225-5233. (e) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1993, 97, 6233-6239. (f) Tsao, M.-W.; Hoffman, C. L.; Rabolt, J. F.; Johnson, H. E.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1997, 13, 4317-4322. (7) (a) 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. (b) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1990, 112, 570579. (c) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (d) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365385. (e) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (f) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (g) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. (h) Stranick, S. J.; Parikh, A. N.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 11136-11142. (i) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368. (j) Camillone, N., III.; Leung, T. Y. B.; Scoles, G. Surf. Sci. 1997, 373, 333-349. (k) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927945.
Figure 2. Amide-containing alkanethiol adsorbate molecules for the self-organized assemblies considered in this study.
hydrocarbon overlayers are either dodecyl or hexadecyl based (C12-2AT/Au and C16-2AT/Au, respectively; see Figure 2). Here we report the characterization of these two-amide SAMs by contact angle goniometry, X-ray photoelectron spectroscopy, and external reflective IR spectroscopy, providing an understanding of a two-amide series of hybrid SAMs. The characterization data suggest that in the two-amide series the assembly structures depend not only on lateral stabilizing interactions within sublayers but also on a covalently mediated interplay between sublayers. As materials in which supramolecular structure is determined by interrelationships within and between component sublayers, these SAMs provide examples of how ensembles of weak interactions form the basis of extended structure. Although it is obvious that hydrogen bonding could dominate over the individually weaker hydrophobic interactions, this report demonstrates that an accumulation of weaker interactions can assist the ordering process of a collection of amide groups. By comparing the two new two-amide SAMs with previously studied one-5e,g and three-amide5f SAMs (Figure 2), the effect of varying the atomic composition of the precursors upon the conformations and packing in the resulting assemblies is observed, enabling an understanding of the dependence of the supramolecular structure upon the molecular constitution. The variation of amide-alkyl composition in the adsorbate molecules provides three useful lines of comparison. (1) In “same amide” analogues, the amount of alkyl material is varied while holding constant the atomic composition of the amide strata (C12-2AT/Au and C16-2AT/Au). (2) In “same alkyl” analogues, the amide sublayers are varied while holding constant the alkyl chain length (C12-1AT/Au and C12-2AT/ Au; C16-1AT/Au and C16-2AT/Au). (3) SAMs with different alkyl and amide components, but having similar spectroscopic features, are also compared (C16-2AT/Au and C9-3AT/Au). In this way, a moderate number of monolayers provides a well-defined context for the two examples of two-amide SAMs reported here, enabling elucidation of factors influencing assembly structure. Within this context, the two-amide SAMs strongly suggest that inter(8) In this nomenclature of the general form Cn-mAT/Au, Cn denotes an alkyl chain comprising n - 1 methylenes and a methyl terminus (covalently linked to the C-protecting amine which forms part of the first amide group). mA denotes m amide groups separated by single methylenes (forming m - 1 C- and N-substituted glycine residues). T/Au denotes the thiol-gold interface linked to the N-protecting carbonyl by an ethylene linker. Consistent with this nomenclature, n-alkanethiol SAMs on gold are referred to as CnS/Au; for example, ocatadecanethiol on gold is C18S/Au.
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Langmuir, Vol. 15, No. 26, 1999 Scheme 1a
molecular interactions in one stratum of a SAM can affect the order of an adjacent sublayer in specific and predictable ways. Unlike one- and three-amide SAMs, order in the amide region of two-amide SAMs is dependent upon order in the alkyl region, and close packing of the alkyl chains is destabilized by the presence of an amide underlayer. To explain this behavior, a “reverse ordering” effect is proposed for two-amide SAMs whereby an ordered, threeamide-like conformation is induced in the presence of a limited number of hydrophilic stabilizing interactions by increasing the number of hydrophobic interactions in the overlying alkyl region. Experimental Section Materials. Dichloromethane was distilled from calcium hydride immediately before use. All other solvents and reagents were used as received from Aldrich, Fluka, Bachem, or Lancaster as previously described.5e-g We5e and others7a,d-e have previously described the procedures for forming gold substrates and SAMs. Synthesis of C12-2AT and C16-2AT is analogous to the previously reported syntheses5f,g of 3-mercapto-N-n-hexadecylpropionamide (3a, C16-1AT), 3-mercapto-N-n-dodecylpropionamide (3b, C12-1AT), and 3-mercapto-N-(N′-(N′′-n-nonylacetamido)acetamido)propionamide (7c, C9-3AT). As a representative procedure, the synthesis of 3-mercapto-N-(N′-n-dodecylacetamido)propionamide (5b, C12-2AT) is briefly described here (see Scheme 1). The condensation of n-dodecylamine 2b and tert-butoxycarbonylglycine using 3-N′-dimethylaminopropyl-N-ethylcarbodiimide and catalytic 4-(dimethylamino)pyridine in dichloromethane at room temperature yields the corresponding amide.9a,b Removal of the tert-butoxycarbonyl functionality9c is accomplished using trimethylsilyl iodide (formed in situ from hexamethyldisilane (9) (a) Bodanszky, M. Principles of Peptide Synthesis; SpringerVerlag: New York, 1984; pp 9-58. (b) Sheehan, J. C.; Cruickshank, P. A.; Boshart, G. I. J. Org. Chem. 1961, 26, 2525-2528. (c) Schlu¨ssler, H.; Zahn, H. Chem. Ber. 1962, 95, 1076-1080. (d) Olah, G. A.; Narang, S. C.; Gupta, B. G. B.; Malhotra, R. Angew. Chem., Int. Ed. Engl. 1979, 18, 612-614. (e) Daeniker, H. U.; Druey, J. Helv. Chim. Acta 1957, 40, 2148-2156.
Clegg et al. and iodine9d) in dichloromethane at reflux. The resulting primary amine 4b and S-acetyl-3-mercaptopropionic acid (1)5e,9e are condensed as before.9a,b The thioester is hydrolyzed using 0.2 N methanolic sodium hydroxide followed by acidification.5b The product is washed with hexane and recrystallized from methanol/ chloroform to yield white microcrystalline solid C12-2AT 5b. Mp 143-145 °C. 1H NMR (300 MHz, 5% CD3OD in CDCl3): δ 0.89 (t, J ) 6.9 Hz, 3H); 1.20-1.35 (broad overlapping resonance, 18H); 1.46 (m, 2H); 1.61 (t, J ) 9.2 Hz, 1H); 2.46 (t, J ) 6.8 Hz, 2H); 2.81 (m, 2H); 3.26 (m, 2H); 3.90 (d, J ) 8.4 Hz, 2H); 6.28 (br, 1H); 6.59 (br, 1H). Synthesis of 3-mercapto-N-(N′-n-hexadecylacetamido)propionamide (5a, C16-2AT) is analogous to that of C12-2AT, using n-hexadecylamine 2a instead of n-dodecylamine 2b. Mp 162165 °C. 1H NMR (300 MHz, 5% CD3OD in CDCl3): δ 0.89 (t, J ) 6.9 Hz, 3H); 1.20-1.35 (broad overlapping resonance, 26H); 1.48 (m, 2H); 1.61 (t, J ) 9.3 Hz, 1H); 2.48 (t, J ) 6.8 Hz, 2H); 2.83 (m, 2H); 3.28 (m, 2H); 3.91 (d, J ) 8.5 Hz, 2H); 6.07 (br, 1H); 6.75 (br, 1H). Methods. Contact angles were measured using a captive drop technique7a on a goniometer constructed in our laboratory. Advancing and receding contact angles are defined as the maximum and minimum angles formed between the substratedroplet interface and the tangent to the probe droplet at its intersection with the film.10a External reflective infrared spectra were collected as 1024 signal-averaged interferograms on a Nicolet Magna 550 IR with 1 cm-1 data spacing using a SpectraTech 80 fixed reflectance accessory as previously described.5f X-ray photoelectron spectroscopy (XPS) was performed on a Kratos HSi spectrometer as previously described.5f Binding energies were referenced to Au (4f7/2) at 84.4 eV, and Shirley sensitivity factors were used. Peaks arising from significantly different chemical states of the same element were deconvoluted using the Kratos Vision software (i.e., C(1salkyl) from C(1scarbonyl), O(1scarbonyl) from O(1sadventitious)). SAM thicknesses were calculated from area integration of the Au (4f7/2) peak before and after sputtering for 30 s with an argon ion beam11c using takeoff angles of 45° and 30° and attenuation lengths determined by the method reported previously.5f Approximately 80% of the systematic error reported in the measured thicknesses is due to uncertainty in the attenuation length. At least three independent measurements on separate samples were obtained for all data sets. Molecular modeling was performed using Spartan 5.0 (Wavefunction, Inc.) as previously described.5f Modeling of SAM thicknesses reported in the text was performed using a parallel chain polyglycine II-like conformation of the amide region as described in ref 5f. (The modeled thickness for a parallel chain polyglycine I-like, rippled sheet conformation (putative structure from ref 13d) is 0.8 Å longer for each two-amide SAM.) The modeled thickness (10) (a) Dettre, R. H.; Johnson, R. E. J. Phys. Chem. 1965, 69, 15071515. (b) Laibinis, P. E.; Palmer, B. J.; Lee, S.-W.; Jennings, G. K. Thin Films 1998, 24, 1-41. (c) Using water as the probe liquid, we measure advancing contact angles of 118° on well-ordered methyl n-alkanethiol monolayers such as C12S/Au and C18S/Au, in agreement with values reported in the literature.7e (11) For reviews, see: (a) Briggs, D.; Riviere, J. C.; Hofmann, S.; Seah, M. P. In Practical Surface Analysis by Auger X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; Wiley & Sons: Chichester, 1983; pp 87-216. (b) Hochella, M. F., Jr. Rev. Mineral. 1988, 18, 573637. (c) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawadowski, T. A., Jr. Langmuir 1996, 12, 1172-1179. (d) Bomben, K. D.; Dev., S. D. Anal. Chem. 1988, 60, 1393-1397. (e) Clark, D. T.; Peeling, J.; Colling, L. Biochim. Biophys. Acta 1976, 453, 533-545. (f) Cadman, P.; Evans, S.; Scott, J. D.; Thomas, J. M. J. Chem. Soc., Faraday Trans. II 1975, 71, 1777-1784. (12) For reviews, see: (a) Porter, M. D. Anal. Chem. 1988, 60, 1143A1155A. (b) Crooks, R. M.; Xu, C.; Sun, L.; Hill, S. L.; Ricco, A. J. Spectroscopy 1993, 8, 29-39. (c) Greenleer, R. G. J. Chem. Phys. 1966, 44, 310-316. (13) (a) For a review, see: Krimm, S.; Bandekar, J. J. Adv. Protein Chem. 1986, 38, 181-364. (b) Cheam, T. C.; Krimm, S. J. Chem. Phys. 1985, 82, 1631-1641. (c) Dwivedi, A. M.; Krimm, S. Macromolecules 1982, 15, 177-185. (d) Bandekar, J.; Krimm, S. Biopolymers 1988, 27, 909-921. (e) Dwivedi, A. M.; Krimm, S. Biopolymers 1982, 21, 23772397. (f) Toniolo, C.; Bonora, G. M.; Pillai, V. N. R.; Mutter, M. Macromolecules 1980, 13, 772-774. (g) Drago, R. S. Physical Methods for Chemists: Second Edition; Saunders College Publishing: Ft. Worth, 1992; p 182.
Stratified Self-Organized Molecular Assemblies also takes into account the ∼25° alkyl chain tilt indicated by external reflective infrared spectroscopy (FTIR-ERS).
Langmuir, Vol. 15, No. 26, 1999 8879 Table 1. XPS Peak Assignments and Atomic Composition for Cn-2AT/Au n ) 12
Results Here the synthesis of alkanethiols with one or more amide bonds and the characterization of the two-amide SAMs is described. XPS enables determination of the thickness, atomic composition, and chemisorption of the SAMs. Contact angle goniometry (CAG) indicates the degree of order at the methyl surface of the films. FTIRERS provides the fullest information about the amide backbone conformations and the order in the amide and alkyl sublayers. A General Synthetic Method for Amide-Containing Alkanethiols. A general synthetic strategy for preparation of alkanethiols containing one to three amide bonds has been developed (Scheme 1). Starting with the n-alkylamine, the oligopeptide portion of the molecule is grown in the C f N direction using standard solution phase amide coupling9a,b and deprotection procedures.9c,d This coupling/deprotection sequence is reiterated until the desired number of amino acid residues is installed, followed by coupling of the thiol to the N-terminus in protected form as the thioacetate.9e Deprotection with base to yield the free thiol is facile and nearly quantitative.5b This synthetic strategy offers several advantages. The route is more straightforward and economical than solid phase peptide synthesis due to the use of readily available N-protected peptides and dipeptides, for only one peptide coupling would be afforded on a solid support. It is versatile, allowing access to a wide range of monolayer precursors and oligopeptides incorporating different amino acid residues and different proportions of alkyl and amide material. The route is also convenient, as the thioacetate linker (S-acetyl-3-mercaptopropionic acid)5e,9e is readily synthesized in multigram quantities. XPS Provides Confirmation of the Overall Film Orientation and Thickness.11a,b The XPS data show that the films are a single molecule thick, chemisorbed by the sulfur atoms to the gold, and anisotropic. The film thicknesses as measured by XPS agree with thicknesses predicted by molecular modeling (C12-2AT/Au, 25.0 ( 0.9 Å measured, 24.1 Å modeled; C16-2AT/Au, 29.1 ( 1.0 Å measured, 28.8 Å modeled). The S(2p3/2) peak at 162.0 ( 0.1 eV is as expected for sulfur bound to gold.7a-d,f The S(2p3/2:1/2) intensity ratio of 1.95 ( 0.07 indicates the absence of adventitiously adsorbed thiol.7a-d,f The remaining binding energies are as expected for amide and alkyl moieties.5a-b,e-f,11d,e Systematic variation of the relative intensities from those calculated from the molecular formulas (Table 1) demonstrates the anisotropy of the SAMs.5a,6c,7d,11f The accentuation of the C(1salkyl) peak indicates the presence of alkyl material primarily near the top of the films. Conversely, the N(1s), C(1scarbonyl), O(1scarbonyl), and S(2p) signals are attenuated by the overlying carbon. The attenuation is most pronounced for the S(2p) doublet, consistent with its location at the gold/ SAM interface. Contact Angle Measurements Indicate Structural Differences between the Assemblies. The hydrophobicity of a surface, as measured by the angle formed between a liquid droplet and the surface, depends on several factors including the order of the atoms in the top 5 Å of the surface.7b,d Low hysteresis between advancing and receding contact angles is also a sensitive indicator of order.10b The contact angle data show the methyl surface of the hexadecyl-tailed two-amide SAM is better ordered than that of the dodecyl-tailed analogue. The advancing contact
C(1salkyl) C(1scarbonyl) N(1s) O(1scarbonyl) S(2p)
n ) 16
BEa
obsb
predb
obsb
predb
285.1 287.5 399.9 531.2c 162.0d
71.6 8.8 8.9 8.7 2.1e
68.2 9.1 9.1 9.1 4.5e
77.3 7.2 6.9 7.1 1.6e
73.1 7.7 7.7 7.7 3.8e
a BE ) binding energy in eV. Peak positions are reproducible to (0.1 eV. b Atomic composition in atom %. Abbreviations: obs ) observed at 90° takeoff angle; pred ) predicted from the molecular formulas of the adsorbate molecules. Standard deviations for measured atom %: C(1salkyl), (0.5; C(1scarbonyl), (0.2; N(1s), (0.2; O(1scarbonyl), (0.4; S(2p), (0.1. c Deconvoluted from adventitious oxygen at 532.5 eV. d The binding energy shown is for S(2p3/2); S(2p1/2) is 1.18 ( 0.04 eV higher. e The atom % shown is for the entire S(2p) doublet.
Table 2. FTIR-ERS Spectral Mode Assignmentsa and Peak Positionsb for the Cn-2AT SAMsc assignment
n ) 12
n ) 16
Amide A: N-H(str)d Amide I: CdO(str)d Amide II: N-H(ipb)d Amide III: N-H(ipb), C-N(str)d CH3(as, ip) CH3(sym, FR) CH2(as) CH3(sym, FR) CH2(sym)
3420 (vw) ∼1640 1557 1287, 1251 2965 2937 (sh) 2920 2880 2852
∼1645 (vw) 1562 1283, 1249 2965 2938 2918 2878 2850
a Literature sources for band assignments: amide modes, ref 13; C-H(str) modes, refs 7c, e, g, and i. b In cm-1. c Nomenclature: str ) stretch; ipb ) in-plane bend; as ) asymmetric stretch; ip ) in-plane; sym ) symmetric stretch; FR ) Fermi resonance; vw ) very weak; sh ) shoulder. d For amide modes, the dominant vibrational contributors for parallel polyglycine II are listed as determined in ref 13.
angle with water for C16-2AT/Au is 118 ( 2°, as high as that of C18S/Au,7a,d,10c consistent with a high degree of order at the methyl surface. For C12-2AT/Au, the advancing contact angle is 113 ( 2°, consistent with decreased order among the alkyl chains.7d An even stronger indicator of a difference in order at these methyl surfaces comes from a comparison of contact angle hysteresis10b (27 ( 3° in C16-2AT/Au versus 46 ( 3° in C12-2AT/Au). Thus, contact angle goniometry suggests that structural differences in the hydrocarbon regions of these assemblies depend on alkyl chain length. This is confirmed by FTIR-ERS. Information About Molecular Orientation,7g,12 Conformation,13a and Order12a,b in Surface-Bound Assemblies Is Obtained by FTIR-ERS. First, the basis for extracting structural information from reflective IR spectra of amide-alkyl assemblies is presented, followed by the reflective IR data for the two-amide SAMs. The data (Table 2 and Figure 4) show that both the amide and alkyl regions are disordered in the dodecyl-tailed twoamide assemblies, whereas both regions are well ordered in the hexadecyl-tailed SAMs. In the discussion section, this behavior is compared to that of the one- and threeamide14 assemblies, in which the amide regions are well ordered independently of the order or disorder in the alkyl overlayers. The contrast between those SAMs and the two-amide SAMs reported here suggests that in two-amide assemblies alkyl ordering has a profound effect upon the adjacent amide regions. Amide backbone conformation is reliably indicated by the peak frequencies of the amide III bands.13a Four bulk analogues exist for the possible amide conformations in amide-containing SAMs. (1) The amide region may
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resemble the trans-amide groups of N-methylacetamide, in which amide III is a broad band at ∼1300 cm-1.13b,f (2) The amide region may comprise β-sheets as in antiparallel or parallel chain polyglycine I (PGI), in which amide III arises at 1410 and 1150 cm-1 (antiparallel PGI13c) and 1410 and 1162 cm-1 (parallel PGI, predicted13d). (3) The amide region may form a partial 31 helix with intermolecular hydrogen bonding to four neighboring chains, as in a segment of polyglycine II (PGII),19 in which amide III occurs at 1283 and 1249 cm-1.13e,f (4) The amide region may be disordered, resulting in a hyperchromic shift of amide III.7c,13g
In reflective IR, relative peak intensities scale with the perpendicularity of dipoles with respect to the substrate surface.7c,e-g,k,12c A number of methods have been published for deriving adsorbate orientation from the relative intensities of amide and C-H(str)16 modes.7c,g,i,17 Amide I (primarily CdO(str)16) oscillates perpendicular and amide II (mainly N-H(ipb)16) oscillates parallel to the molecular long axis in all three of the ordered amide conformations described above,13c-e,19 so tilting this axis from surface normal increases the amide I intensity and decreases amide II (Figure 3).5,20,21 The net orientations of amides A and III follow those of amides I and II, respectively,13c,d,21 so these peak intensities behave analogously.5a,b,f However, determining alkyl chain orientation in amide-containing SAMs is far less certain due to possible contributions of C-H(str)13a,f from glycyl methylenes and the ethylene linker adjacent to the sulfur.5e,f In this case, alkyl chain order and orientation may be assessed by evaluation of peak frequencies and widths5a-c,6c,f,7d and comparison to spectra of well understood assemblies.6c,7a,c-f,i,18 With progressive ordering in n-alkanethiol SAMs, CH2(as) undergoes a bathochromic shift7a,c-f,16,18 from 2928 to 2918 cm-1, and the full width at half-maximum (fwhm) narrows to 12 cm-1.6c,7e Both the Amide and Alkyl Regions Are Well Ordered in Two-Amide, Hexadecyl-Tailed SAMs. In the C16-2AT/Au spectrum (Figure 4C, Table 2), CH2(as) is red shifted (2918 cm-1), supporting a well-ordered alkyl overlayer. CH2(as) is also strong and quite narrow (fwhm ) ∼14 cm-1), again consistent with well-ordered chains.22 Except for the possible contribution of the glycyl methylene, the C-H(str) modes are practically identical to the analogous peaks in highly ordered C16-1AT/Au5g (Figure 4A) and C18S/Au,7a,e,g supporting similar chain tilt and packing for these types of assemblies. In the mid frequency region of the IR spectrum, amide III occurs as sharp peaks at 1283 and 1249 cm-1. This matches the amide III frequencies in C9-3AT/Au5f and bulk PGII,13e,f strongly supporting PGII-like amide backbone conformations5f,20 for C16-2AT/Au. The amide II peak is strong and reproducible from sample to sample, and amides I and A are absent, supporting PGII-like chains oriented with the helical axes perpendicular to the substrate plane.5f,13e,19-21 Comparison of C16-2AT/Au to C9-3AT/Au (Figure 4E) shows that the amide II intensity increases nearly linearly with the number of amide groups (Figure 5), further supporting similar backbone orientations.21 In Two-Amide, Dodecyl-Tailed Assemblies, Both the Amide and Alkyl Regions Appear To Be Disordered. In C12-2AT/Au (Figure 4D, Table 2), CH2(as) and CH2(sym) are slightly blue shifted (2920 and 2852 cm-1) and significantly broadened (fwhm ) 18 cm-1), consistent
(14) The alkyl chains in C9-3AT/Au were previously proposed5f to be perpendicular to the surface, based on the absence of CH2(as) and CH2(sym) in the reflective IR spectrum. We hypothesized that such a perpendicular orientation would be due to constraint by a decreased interchain spacing in a three-amide underlayer compared to a oneamide underlayer. Comparison to the recently reported analogue C91AT/Au,5g which has an alkyl overlayer compositionally identical to C9-3AT/Au,5g shows the C-H(str) IR peaks of C9-1AT/Au and C9-3AT/ Au to be indistinguishable. Other characterization techniques (contact angle goniometry and double layer capacitance5g) indicate that the alkyl region of C9-1AT/Au is disordered. It has been suggested that reflective IR is incapable of delivering orientational information for short alkyl chains (fewer than four to nine methylenes) due to an optoelectronic effect at the polarizable interface7e or due to breaking of the near degeneracy of the methylene bands.15 These suggestions are supported by the comparison of C9-1AT/Au and C9-3AT/Au and the complementary characterization data which are consistent with disordered chains. (15) Sinniah, K.; Cheng, J.; Terrettaz, S.; Reutt-Robey, J. E.; Miller, C. J. J. Phys. Chem. 1995, 99, 14500-14505. (16) For dipole moment nomenclature, see Table 2.
(17) (a) Cammarata, V.; Atanasoska, L.; Miller, L.; Kolaskie, C. J.; Stallman, B. J. Langmuir 1992, 8, 876-886. (b) Kewan, W. S. V.; Atanasoska, L.; Miller, L. Langmuir 1991, 7, 1419-1425. (c) Young, J. T.; Boerio, F. J.; Zhang, Z.; Beck, T. L. Langmuir 1996, 12, 1219-1226. (18) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150. (19) (a) Crick, F. H. C.; Rich, A. Nature (London) 1955, 176, 780781. (b) Ramachandran, G. N.; Sasisekharan, V. Biochim. Biophys. Acta 1965, 109, 314-316. (20) (a) Strong, A. E.; Moore, B. D. Chem. Commun. 1998, 473-474. (b) Yamada, N.; Koyama, E.; Imai, T.; Matsubara, K.; Ishida, S. Chem. Commun. 1996, 2297-2298. (c) Cha, X.; Ariga, K.; Kunitake, T. Bull. Chem. Soc. Jpn. 1996, 69, 163-168. (21) (a) Whitesell, J. K.; Chang, H. K.; Whitesell, C. S. Angew. Chem., Int. Ed. Engl. 1994, 33, 871-873. (b) Boncheva, M.; Vogel, H. Biophys. J. 1997, 73, 1056-1072. (c) Whitesell, J. K.; Chang, H. K. Science 1993, 261, 73-76. (22) This peak may be even narrower due to possible convolution with glycyl CH2(as)13f at 2932 cm-1.
Figure 3. Orientation of the net dipole transition moments for important amide IR vibrations.13a The dipole moments of amide II and III lie along a nearly coincident axis but are of different composition. According to the IR surface selection rule,7g,12c observed intensity increases in proportion to the magnitude of the dipole component perpendicular to the reflective substrate and decreases in proportion to the component parallel to the substrate. Thus, for orientation A the spectrum contains a strong amide II peak and a near-zero amide I peak, whereas for orientation B amide II is weaker and amide I is stronger.
Figure 4. FTIR-ERS spectra for alkanethiol SAMs containing internal amide groups: left, C-H(str) region; right, midfrequency region containing the amide I, II, and III peaks. Spectral mode assignments and comparison of peak positions to dissolved and drop-cast solid adsorbates are given in Table 2. A, C161AT/Au; B, C12-1AT/Au; C, C16-2AT/Au; D, C12-2AT/Au; E, C93AT/Au.
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Figure 5. Plot of amide II intensities versus number of amide groups for Cn-mAT/Au. The unfilled symbol denotes broadened amide III peaks consistent with poorly ordered amide strata (C12-2AT/Au). The dotted line indicates the linear increase of amide II intensity in C16-2AT/Au and C9-3AT/Au and is included as a guide to the eye. Triangles, C12-2AT/Au, C16-2AT/Au and C16-2AT/Au (polyglycine II-like amide III frequencies); circle, C12-2AT/Au and C16-2AT/Au (N-methylacetamide-like amide III peaks). Error bars denote the range of data.
with the presence of large numbers of gauche defects in the hydrocarbon overlayers.7a,c-f,18 This region of the spectrum closely resembles that of C12-1AT/Au5g (Figure 4B), which was previously shown to have a disordered alkyl overlayer. Amide III occurs as two slightly blue shifted bands at 1287 and 1251 cm-1 which are broader, weaker, and more poorly defined than those in C16-2AT/ Au, consistent with a tendency toward a PGII-like conformation but greater disorder in the amide sublayer. A weak to moderate absorbance occurs for amide I, and the intensity of amide II is reduced compared to C16-2AT/ Au (Figure 5), indicating a difference between the two SAM types in average orientation of the amide groups.21 In C12-2AT/Au, the intensity ratio of amide I to amide II varies from 1:3 to 1:6, corresponding to an average 10 ( 3° tilt23 of the N-H and CdO bonds from the surface plane.17 This tilt is consistent with increased disorder in the amide region but not with complete disorder.13,17,19 Amide A is present at ∼3420 cm-1 as an extremely broad, weak elevation in the baseline. In C12-2AT/Au, the amide III intensity and the amide I, II, and III frequencies are significantly more variable from sample to sample24 than those in C16-2AT/Au. This could arise from partial surface coverage or differences in adsorbate orientation,12c conformation,13 or order7g between the various samples. The presence of significantly less than a monolayer of adsorbate is ruled out by the XPS thickness measurements and the similarity of the C-H(str) peaks of C12-2AT/Au to those of C12-1AT/Au. The occurrence of weak PGII-like frequencies for amide III suggests that the amide backbones are disposed to conformations similar to those in C16-2AT/Au, but the broadening and slight red shift of these peaks support relatively less order in the amide sublayer. The difference in average amide group orientation also is consistent with (23) The uncertainty here corresponds to the range of data in the measurements. A systematic error of ∼(5% for ordered assemblies (see text) is generally assumed4f,7c,e,g,17 for the technique. Importantly, the value given here is the tilt of the dipole with respect to the surface plane, which may be due to both tilt of the molecular axis and twist of that axis out of the plane of tilt. (24) Variable IR spectra for C12-2AT/Au were found under identical conditions of substrate preparation, soaking solution concentration, solvent, and adsorption temperature.
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increased disorder in C12-2AT/Au. It is important to note that the surface selection rule7g,12c does not distinguish between positive and negative tilt,7c so the tilt value corresponds to the average absolute value of tilt for amide groups in the sample. This means that the dispersity of amide group orientations within a C12-2AT/Au sample may be quite large, but the amide groups in C16-2AT/Au are constrained to a very small range of orientations. The significance of the variability in the C12-2AT/Au samples remains unclear, but taken together with monolayer coverage and the increase in average amide group tilt, it is consistent with decreased order in the amide sublayer of these SAMs compared to C16-2AT/Au. The only compositional difference between these twoamide adsorbate molecules is the length of the alkyl tails; the amide regions comprise identical N- and C-substituted glycines. However, the spectral data show that both regions are ordered in C16-2AT/Au, whereas both regions are disordered in C12-2AT/Au. The contrast between the structural differences in the adsorbates and the assemblies may signal an interplay between the hydrocarbon and amide regions in the self-assembly process. This important point will be amplified in the next section, where the dependence of the amide conformation and order upon the length of the hydrocarbon tail is examined. Discussion The successive replacement of methylene groups with amide groups yields stratified monolayers with what may be thought of as buried organic-organic (or amide-alkyl) interfaces between the sublayers. Analogous buried interfaces commonly occur between regions of differing polarity in biological structures such as lipoproteins, lipidlinked proteins,25a,b and proteins in intimate contact with cell membranes25c,d and organelles,26 some of which contain covalent links across the buried interface as in the monolayer system considered here. In this system, as in many biological ones, differences in assembly structures can be traced to the numbers of hydrogen bonds and hydrophobic interactions as well as to interfacial interactions between the polar amide regions and the neighboring hydrocarbon regions. Here, in an analysis of the selfassembly behavior of the first two members of the twoamide series of SAMs, the data suggest an intriguing interplay between hydrogen bonding and hydrophobic interactions across the amide-alkyl interface. Energetic Trade-Offs between the Amide and Hydrocarbon Sublayers Are Involved in Forming Ordered Two-Amide SAMs. The IR data show that both the hydrocarbon and amide region are well ordered in C16-2AT/Au, and the amide IR peaks bear the signature of a PGII-like conformation13a,f as in three-amide SAMs (in which the alkyl overlayer is disordered14). However, with no change other than shortening of the alkyl chains, both regions of C12-2AT/Au are poorly ordered. A highly ordered PGII-like helical conformation does not predominate in dodecyl-tailed, two-amide SAMs, most likely because the number of hydrogen bonding interactions is insufficient to stabilize such a structure. On the basis of these two examples and in the context of the three-amide SAM, more than two sets of hydrogen bonds per precursor (25) (a) Voet, D.; Voet, J. G. Biochemistry; Wiley & Sons: New York, 1990; p 304. (b) For a review, see Sefton, B. M.; Buss, J. E. J. Cell. Biol. 1987, 104, 1449-1453. (c) Chapman, D.; Peel, W. E.; Quinn, P. J. Ann. N. Y. Acad. Sci. 1978, 308, 67-84. (d) Menger, F. M.; Wong, Y.-L. J. Org. Chem. 1996, 61, 7382-7390. (26) (a) Rothman, J. E.; Orci, L. Nature 1992, 355, 409-415. (b) Yeagle, P. L. The Structure of Biological Membranes; CRC: Boca Raton, FL, 1991; pp 539-602, 721-780.
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appear to be required to form ordered PGII-like amide sublayers in the presence of a disordered alkyl overlayer. These comparisons suggest that self-assembly of the hexadecyl chains in C16-2AT/Au, with a concomitant decrease in degrees of freedom of the covalently linked oligopeptide chains, provides a driving force for ordering of the amide region as well. Measurement of stability of films with various amide and alkyl components may enable determination of the thermodynamic contributions of the sublayers to overall monolayer stucture.27 This interplay of weak intermolecular interactions across the amide-alkyl interface in the two-amide SAMs, unobserved in the one-amide SAMs,5g suggests that different alkyl chain length ordering thresholds may occur in these two series. Assuming a sharp ordering threshold exists for two-amide SAMs (as in the n-alkanethiol and one-amide series), the examples reported here place that threshold between 11 and 15 methylenes. Compared to the n-alkanethiol series (where the threshold is ∼9 methylenes by FTIR-ERS7a,e), the data indicate that both types of amide underlayers interfere destructively with close packing of the alkyl chains. This suggests that a difference in mismatch between headgroup (amide cross section versus gold-sulfur spacing) and alkyl chain cross sections has an energetic effect on the assembly structures, resulting in a change in the stabilization provided per van der Waals contact.5g,7a Different Types of Buried Interfaces May Exist. In the one-amide SAM series, the amide regions form structurally invariant “anchor layers” (whether the alkyl tails contain 8 or 17 methylene groups) but hydrocarbon ordering depends on chain length. The buried amidealkyl interfaces in that case were described as having a rigid-elastic character.5g Recently, an analogous description of the n-alkanethiol SAM series was based on the apparent competition between alkyl chain length and gold-sulfur epitaxy.28 Such rigid-elastic interfaces differ fundamentally from rigid-rigid interfaces such as epitaxially grown metal monolayers on metal substrates. For amide-containing SAMs, a rigid-elastic interface is expected if the amide sublayer is structurally invariant. The alkyl tails, with smaller cross sections19,29 than the amide “headgroups,” would tilt to maximize van der Waals forces, forming ordered domains at sufficient chain lengths.7a This picture is consistent with the data for the previously reported one-amide series of SAMs.5g However, the amide-alkyl interfaces in the two-amide SAMs reported here do not have rigid-elastic character, as the amide underlayers adopt different structures for n ) 12 and n ) 16. Below the alkyl chain length ordering threshold, the amide region is also disordered (Figure 6A). Above that threshold, the driving force toward order in the hydrocarbon region appears to enforce order in the amide region as well (Figure 6B). Thus, the two-amide sublayers can be characterized as elastic, and we describe the amide-alkyl interface as having an elastic-elastic nature. For disordered elastic-elastic SAMs, vertical as well as lateral ordering may be decreased, so the interfacial demarcation may be decreased as well (Figure 6A). Above the chain length threshold for alkyl ordering, the ordered structure would depend on two sets of variable param(27) (a) Clegg, R. S.; Smith, R. K.; Hutchison, J. E. Manuscript submitted. (b) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 47314740. (c) Schessler, H. M.; Karpovich, D. S.; Blanchard, G. J. J. Am. Chem. Soc. 1996, 118, 9645-9651. (d) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528-12536. (28) In that study, the terminology “hard-soft” was used in place of “rigid-elastic” as applied here.7j (29) Sellers, J.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389-9401.
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Figure 6. Schematic views of Cn-2AT/Au (A, n ) 12; B, n ) 16) emphasizing order and disorder in the major strata and the relative degree of interfacial demarcation inferred by the structural comparisons in the text. Solid/parallel and dashed/ nonparallel striping depicts ordered and disordered sublayers, respectively, and does not convey information about the orientation or conformation of the adsorbate chains. A poorly demarcated interface, indicated by the dotted region, is shown for C12-2AT/Au. A well demarcated interface is shown by the boldface line for C16-2AT/Au.
eters: (a) an interfacial interchain spacing that is energetically accessible by each region and (b) an energetic compromise between competing interchain interactions within and across the interface. The proposed elasticelastic interface therefore involves a more complex interplay than the rigid-elastic interface. The number and strength of intermolecular interactions on each side of the interface would affect the interchain spacing and vertical demarcation at the interface, providing a mechanism for packing in one region to affect order in the other region. Such an amide-alkyl interplay may constitute a “reverse ordering” effect, whereby order in an underlayer may be induced via a structural change in an overlying region. Therefore, in amide-containing SAMs a rich higher order structural diversity appears to arise from variations in atomic constitution, analogous to the dependence of tertiary structure upon primary structure in proteins and of cell membrane function upon acyl chain length.26b Implications of the Elastic-Elastic Interface. Clearly, a process of rational design can be applied to the formation of well-ordered SAMs containing internal amide groups. Only a few stratified, alkane-based monolayer systems have been reported with more than one or two heteroatoms in each sublayer in which both the polar and nonpolar strata are well ordered.5f,6c In stratified systems, principles for making ordered alkyl regions are well accepted,30 but principles for designing ordered polar regions remain unclear. The present one-, two-, and threeamide alkanethiol system provides a well-defined, growing body of knowledge about the effect of systematically varying components upon the resulting higher order assembly structure, and design principles for such ordered polar regions may be possible. Several implications of the proposed “reverse ordering” effect at elastic-elastic interfaces, which contrasts fundamentally with epitaxial ordering, deserve note. (1) Order in the upper layers of self-assembled multilayers generally depends on order in the foundation layers,31a but multilayers have been reported in which disorder in anchor layers does not preclude order in overlying layers.31b By analogy to the two-amide SAMs reported here, in such (30) Sigal, G. B.; Bamdad, C.; Barberis, A.; Strominger, J.; Whitesides, G. M. Anal. Chem. 1996, 68, 490-497. (31) (a) Byrd, H.; Whipps, S.; Pike, J. K.; Ma, J.; Nagler, S. E.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 295-301. (b) O’Brien, J. T.; Zeppenfeld, A. C.; Richmond, G. L.; Page, C. J. Langmuir 1994, 10, 4657-4663.
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multilayers growth of an ordered upper layer might induce a latent ordering process in the anchor layer. (2) New ordered, cross-linked materials are challenging targets, as these often require successful crystallization to gain information about the viability of a proposed structurefunction relationship.4h,32 Heterogeneous SAMs may allow mixtures of order and disorder in addition to full order and relative disorder. Monolayer analogues of proposed bulk solids could facilitate a strategy of successive approximations in design of such materials. (3) New materials whose supramolecular properties arise from structured periodicity of hydrophobic and hydrophilic domains are targeted2d,3b,c as versatile analogues of polypeptides,33a polyacrylamide, and nylons.33b-d The hybrid monolayer system discussed here offers opportunities to improve the understanding of how interactions between polar and nonpolar domains affect supramolecular structure.2d (4) Finally, apt analogies to lipid-linked proteins25a,b are provided by these covalently linked alkylamide assemblies. For example, in Semliki forest virus, an acylated viral coat protein is anchored to a target cell membrane by a hexadecanoate chain, disrupting the lipid bilayer25c,d and facilitating insertion of genetic material.25b However, the functional requirement for a specific fatty acid residue is not understood.25a Further examination of the amide-alkyl interplay in monolayers could reveal principles applicable to biological analogues.25 Conclusions As a part of a systematic investigation of the fundamental effect of atomic composition upon supramolecular structure, two stratified self-assembled monolayers (SAMs) with amide and alkyl sublayers were formed from adsorbate molecules containing two amide bonds and either dodecyl or hexadecyl alkyl “tails”. Variation of the hydrocarbon overlayer thickness while maintaining a compositionally constant amide underlayer, along with comparison to previously reported SAMs in which alkyl and amide composition is varied, enables an understanding of the influence of the interplay between the weak intermolecular interactions in this system (hydrogen bonding and van der Waals contacts) upon supramolecular (32) (a) Nie, W. Adv. Mater. 1993, 5, 520-545. (b) Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Chem. 1987, 59, 379A-385A. (c) Murray, R. W. In Electroanalytical Chemistry: A Series of Advances, Vol. 13. Bard, A. J., Ed.; Marcel Dekker: New York, 1984; pp 191-368. (33) (a) Spek, E. J.; Wu, H.-C.; Kallenbach, N. R. J. Am. Chem. Soc. 1997, 119, 5053-5054. (b) Atkins, E. D. T.; Hill, M.; Hong, S. K.; Keller, A. Macromolecules 1992, 25, 917-924. (c) Bank, M. I.; Krimm, S. J. Polym. Sci., Polym. Phys. Ed. 1969, 7, 1785-1809. (d) Dasgupta, S.; Hammond, W. B.; Goddard, W. A., III J. Am. Chem. Soc. 1996, 118, 12291-12301.
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structure. FTIR-ERS, contact angle goniometry, and XPS provide an internally consistent structural characterization of the two-amide SAMs. Monolayer coverage is achieved for both types of two-amide SAMs, but for dodecyl-based hydrocarbon chains, the alkyl overlayer and the underlying amide stratum are both poorly ordered. However, when hexadecyl chains are used, both strata are well ordered. Importantly, the change in the order of the amide underlayer is induced with no change in its atomic composition and depends solely upon the alkyl overlayer thickness. Comparison of these two two-amide SAMs with previously reported one-5g and three-amide5f SAMs suggests the following conclusions. (1) The alkyl chain length ordering threshold for two-amide SAMs is between 11 and 15 methylenes, comparable to that for one-amide SAMs (where the threshold is 14 by FTIRERS5g) and several methylenes longer than for n-alkanethiol SAMs on gold. This implies a destructive interference of the amide underlayer upon the hydrocarbon ordering process in both one- and two-amide SAMs relative to n-alkanethiol SAMs, with subsequent structural implications which have been discussed. (2) The differences in the two-amide assembly structures appear to arise from differences in lateral interactions within sublayers as well as interfacial interactions between the sublayers. The buried interfaces within the two-amide SAMs can be characterized as elastic-elastic in nature, where a driving force for amide microcrystallization may be provided by structurally altering the hydrocarbon region. This energetic interplay between sublayers provides a mechanism for a proposed reverse ordering effect whereby order may be induced in a sublayer of the stratified assembly by making a structural change in an overlying region. Elastic-elastic interfaces and a reverse ordering effect may have important implications for self-assembled multilayers, control of bulk properties based on molecular contributions, hybrid materials development, and fundamental studies of lipid-linked protein structure and function. Acknowledgment. This research was supported by the National Science Foundation (NMR, CHE-9421182; XPS, CHE-9512185), the National Science Foundation CAREER Program (CHE-9702726), the Camille and Henry Dreyfus Foundation, the Oregon Medical Research Foundation, the Howard Hughes Medical Institute (Undergraduate Research Fellowship Program), and the University Club Foundation of Portland, Oregon. J.E.H. is an Alfred P. Sloan Fellow. LA9910529
