Impact of Host Structure on Guest−Host Recognition at Self

Publication Date (Web): July 25, 2002. Copyright ... Guest−host recognition at self-assembled surfaces of two different synthetic calix[4]resorcinar...
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Impact of Host Structure on Guest-Host Recognition at Self-Assembled Surfaces of Tetrathiol and Tetrasulfide Derivatives of Calix[4]resorcinarene John D. Faull and Vinay K. Gupta* Department of Chemical and Biomolecular Engineering, University of Illinois, Urbana-Champaign, 600 South Matthews Avenue, Urbana, Illinois 61801 Received March 14, 2002 Guest-host recognition at self-assembled surfaces of two different synthetic calix[4]resorcinarene receptors, a tetrasulfide (R4SC7) and a tetrathiol (R4SH) host, was performed. The host structure was found to impact the molecular organization within the monolayers as well as the interfacial properties. Comparison of structural characterization of the SAMs formed from R4SC7 and R4SH on gold substrates revealed a larger area per molecule for the tetrasulfide host, which indicated that the dialkyl sulfide chains do not pack commensurately below the calix[4]resorcinarene headgroup and can lead to interstitial cavities. As a consequence, the self-assembled surfaces formed from R4SC7 are slightly more hydrophobic and thinner than the SAMs formed from R4SH. Guest-host recognition of a small neutral molecule, R-hydroxyγ-butyrolactone (HBL), occurred from aqueous solutions on both surfaces on a time scale of several hundred seconds. Adsorption isotherms for the two surfaces revealed that the difference in the free energy change due to adsorption is comparable to thermal energy. Whereas HBL bound to the SAMs formed from R4SH was found to almost completely desorb into an organic solvent, the binding to the R4SC7 surface was largely irreversible. Despite R4SH and R4SC7 possessing macrocyclic cavities of geometrically similar size, the significant difference in the desorption behavior indicates the role of incommensurate packing of the dialkyl sulfide chains below the calix[4]resorcinarene headgroup and the impact of monolayer structure on guest-host recognition at self-assembled surfaces.

Introduction In recent years, monomolecular surfaces such as selfassembled monolayers (SAMs) and Langmuir-Blodgett (LB) films have become a popular vehicle for investigation of interfacial interactions. In particular, the phenomenon of molecular recognition and association has been actively studied due to its relevance in immunoassays, sensing, and other applications based on selective affinity of molecules to surfaces as well as its significance to fundamental studies of supramolecular interactions in well-defined geometries.1-3 A significant number of investigations of molecular recognition have relied on SAMs of organosulfides on gold due to the ease of preparation and the flexibility in design of the chemical interface.4,5 It has been demonstrated that SAMs formed from al* To whom correspondence should be addressed. E-mail: [email protected]. Fax: 217-333-5052. Tel: 217-244-2247. (1) Tidwell, C. D.; Ertel, S. I.; Ratner, B. D.; Tarasevich, B.; Atre, S.; Allara, D. L. Langmuir 1997, 13, 3404-3413. Dawson, S. L.; Elman, J.; Margevich, D. E.; McKenna, W.; Tirrell, D. A.; Ulman, A. In Hydrogels and biodegradable polymers for bioapplications; Ottenbrite, R. M., Huang, S. J., Park, K., Eds.; ACS Symposium Series Vol. 627; American Chemical Society: Washington, DC, 1996; pp 187-196. Kooyman, R. P. H.; van den Heuvel, D. J.; Drijfhout, J. W.; Welling, G. W. Thin Solid Films 1994, 244, 913-916. Spinke, J.; Liley, M.; Schmitt, F. J.; Guder, H. J.; Angermaier, L.; Knoll, W. J. Chem. Phys. 1993, 99, 7012-7019. Prime, K. L.; Chu, Y. H.; Schmid, W.; Seto, C. T.; Chen, J. K.; Spaltenstein, A.; Zerkowski, J. A.; Whitesides, G. M. In Macromolecular assemblies in polymeric systems; Stroeve, P., Balazs, A. C., Eds.; ACS Symposium Series Vol. 493; American Chemical Society: Washington, DC, 1992; pp 227-239. Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426-429. Allara, D. L. Biosens. Bioelectron. 1995, 10, 771-783. Weiss, T.; Schierbaum, K. D.; Thoden van Velzen, U.; Reinhoudt, D. N.; Goepel, W. Sens. Actuators, B 1995, B26, 203-207. Spinke, J.; Liley, M.; Guder, H. J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821-1825. (2) Chailapakul, O.; Crooks, R. M. Langmuir 1993, 9, 884-888. (3) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329-1340. (4) 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.

kanethiols and thiolated ligands can bind metal ions, surfactants, and volatile organic compounds.3,6 More recently, attention has focused on SAMs formed from complex synthetic “host” receptors such as cyclodextrins7 and calix[n]resorcinarenes8-12 that contain a molecular cavity for targeted recognition of “guest” molecules. The challenge here has been to understand the role of molecular organization, structure, and forces in the recognition process and thereby to improve selectivity in binding of metal ions, sugars, organic vapors, steroids, and apolar molecules. Calix[4]resorcinarenes are macrocyclic host molecules that possess a bowl-shaped molecular cavity formed by four resorcinol units (Figure 1). These cyclic tetramers are prepared by acid-catalyzed condensation of resorcinol and aldehydes.13 In pioneering work, Aoyama and co(5) Ulman, A. Ultrathin organic films; Academic Press: Boston, 1991. Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (6) Sun, L.; Kepley, L. J.; Crooks, R. M. Langmuir 1992, 8, 21012103. Sigal, G. B.; Mrksich, M.; Whitesides, G. M. Langmuir 1997, 13, 2749-2755. (7) Rojas, M. T.; Koeniger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336-343. Maeda, Y.; Fukuda, T.; Yamamoto, H.; Kitano, H. Langmuir 1997, 13, 4187-4189. Weisser, M.; Nelles, G.; Wenz, G.; Mittler-Neher, S. Sens. Actuators, B 1997, B38, 58-67. (8) Adams, H.; Davis, F.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1994, 2527-2529. (9) Davis, F.; Stirling, C. J. M. Langmuir 1996, 12, 5365-5374. (10) Friggeri, A.; Van Veggel, F. C. J. M.; Reinhoudt, D. N.; Kooyman, R. P. H. Langmuir 1998, 14, 5457-5463. (11) Huisman, B. H.; Kooyman, R. P. H.; Van Veggel, F. C. J. M.; Reinhoudt, D. N. Adv. Mater. 1996, 8, 561-564. Cygan, M. T.; Collins, G. E.; Dunbar, T. D.; Allara, D. L.; Gibbs, C. G.; Gutsche, C. D. Anal. Chem. 1999, 71, 142-148. (12) Thoden van Velzen, E. U.; Engbersen, J. F. J.; de Lange, P. J.; Mahy, J. W. G.; Reinhoudt, D. N. J. Am. Chem. Soc. 1995, 117, 68536862. (13) Gutsche, C. D. Calixarenes Revisited; Monographs in Supramolecular Chemistry, No. 6; Royal Society of Chemistry: Cambridge, 1998. Gutsche, C. D. Calixarenes; Monographs in Supramolecular Chemistry, No. 1; Royal Society of Chemistry: Cambridge, 1989.

10.1021/la020256d CCC: $22.00 © 2002 American Chemical Society Published on Web 07/25/2002

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Figure 1. Chemical structures of calix[4]resorcinarene host molecules: (a) tetrasulfide host where the aromatic rings are covalently bound by oxygen bridges and the dialkyl sulfide chains are similar in length; (b) tetrathiol host (R4SH) where the macrocyclic cavity is not oxygen bridged; (c) tetrasulfide host (R4SC7) where the calix[4]resorcinarene headgroup is similar to that in (b) and the dialkyl sulfide chains are of unequal lengths.

workers demonstrated that calix[4]resorcinarenes could trap various guest molecules such as sugars in aqueous solutions within the bowl-shaped cavity.14 The trapping was driven by either π-acid/base interactions or by cooperative hydrogen bonding interactions between the guest and host molecules. Reinhoudt and co-workers15 extended the strategy to self-assembled surfaces. Calix[4]resorcinarenes were modified with dialkyl sulfides for physisorption16 on gold surfaces. The tetrasulfide calix[4]resorcinarene host had a cavity that was made rigid by bridging the oxygen atoms of neighboring aromatic rings (Figure 1a). One of the first studies reported selective recognition of tetrachloroethylene from the gas phase. However, similar selectivity of organic vapors by amorphous polymer films demonstrated that general dispersion interactions rather than specific interactions heavily influence recognition from the vapor phase.17 In contrast, more recent studies have shown that specific recognition of steroids or other neutral molecules occurs readily from aqueous solutions on SAMs of oxygenbridged tetrasulfide calix[4]resorcinarene (Figure 1a).10,18 We and others have also reported recognition by selfassembled surfaces wherein calix[4]resorcinarenes were modified with alkylthiols (Figure 1b) that strongly chemisorb onto gold.8,9,19 In a previous study,19 we used quantitative methods such as surface plasmon resonance (SPR) and polarization modulation IR reflection-absorption spectroscopy (PM-IRRAS) to demonstrate that sur(14) Aoyama, Y.; Tanaka, Y.; Toi, H.; Ogoshi, H. J. Am. Chem. Soc. 1988, 110, 634-635. Tanaka, Y.; Kato, Y.; Aoyama, Y. J. Am. Chem. Soc. 1990, 112, 2807-2808. Yanagihara, R.; Aoyama, Y. Tetrahedron Lett. 1994, 35, 9725-9728. Fujimoto, T.; Yanagihara, R.; Kobayashi, K.; Aoyama, Y. Bull. Chem. Soc. Jpn. 1995, 68, 2113-2124. Pietraszkiewicz, O.; Kozbial, M.; Pietraszkiewicz, M. Adv. Mater. Opt. Electron. 1998, 8, 277-284. Kikuchi, Y.; Kobayashi, K.; Aoyama, Y. J. Am. Chem. Soc. 1992, 114, 1351-1358. Tanaka, Y.; Ubukata, Y.; Aoyama, Y. Chem. Lett. 1989, 1905-1908. Aoyama, Y.; Tanaka, Y.; Sugahara, S. J. Am. Chem. Soc. 1989, 111, 5397-5404. (15) Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N. Synthesis 1995, 989-997. (16) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles, G. J. Phys. Chem. B 1998, 102, 3456-3465. Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365-385. (17) Grate, J. W.; Patrash, S. J.; Abraham, M. H.; Du, C. M. Anal. Chem. 1996, 68, 913-917. (18) Schierbaum, K. D.; Weiss, T.; Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N.; Goepel, W. Science 1994, 265, 1413-1415. Friggeri, A.; Van Veggel, F. C. J. M.; Reinhoudt, D. N. Chem.sEur. J. 1999, 5, 3595-3602. (19) Faull, J. D.; Gupta, V. K. Langmuir 2001, 17, 1470-1476.

faces of tetrathiol calix[4]resorcinarene could discriminate between guest molecules with small differences in the chemical structure and between two structural isomers. Recognition of guest molecules was driven by a combination of factors such as hydrogen bonding, hydrophobic interactions, and steric match. The kinetics of recognition on surfaces formed from the tetrathiol19 host was found to be significantly slower than the kinetics for the oxygenbridged tetrasulfide10 host, which hinted at interesting effects of the host structure on the association process. Here we focus on the structure of the host molecule forming the self-assembled monolayer and study its impact on the association of neutral molecules in aqueous solutions. We present results on molecular association on self-assembled monolayers formed from the tetrathiol (Figure 1b) and the tetrasulfide (Figure 1c) calix[4]resorcinarene host. The tetrasulfide host molecule that we use (Figure 1c) differs from those used in previous studies in two ways. First, the neighboring aromatic rings forming the macrocyclic cavity are not covalently bridged and instead the bowl-shaped structure is similar in flexibility to the tetrathiol calix[4]resorcinarene. Second, as the chains in the dialkyl sulfide tethers are of unequal lengths, the monolayers are expected to possess an organization that is distinct from that of the O-bridged host molecules in Figure 1a. The experimental results reported here demonstrate that the SAMs formed from calix[4]resorcinarene hosts provide a unique environment to study the relationships between molecular organization of the monolayer, the forces driving molecular recognition, and the structure of the guest and host molecules. Experimental Procedures Chemicals. The ethanol used in synthesis and monolayer preparation was absolute, 200 proof (Aaper Alcohol and Chemical Co., KY). The other solvents, hexadecane, methanol, toluene, acetonitrile (Fisher Scientific, PA), and tetrahydrofuran (Aldrich, WI), were reagent grade. 11-Mercaptoundecanol (Aldrich) was used as received for formation of self-assembled monolayers. Hexadecanethiol, octanethiol, and heptanethiol (Aldrich) were passed through a column of neutral alumina before use. For the synthesis of the host molecules, undecylinic aldehyde, resorcinol, thiolacetic acid, 9-borabicyclo[3.3.1]nonane (BBN), and AIBN were from Aldrich and used as received. The guest adsorbate used in the study was R-hydroxy-γ-butyrolactone (Acros, PA). Purified water was from an EasyPure UV system (Barnstead, IA).

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Synthesis of Tetrathiol (R4SH) and Tetrasulfide (R4SC7) Calix[4]resorcinarene. The synthesis of R4SH has been reported elsewhere.19 Briefly, precursor material was obtained by condensation of equimolar amounts of resorcinol and undecylinic aldehyde in ethanol under acidic conditions at 75 °C.20 This precursor material was purified, and literature procedures21 were followed for radical addition of thiolacetic acid under ultraviolet conditions. The resulting thiolester was subsequently hydrolyzed in basic methanolic solution and acidified to give the thiolated tetramer (R4SH). Synthesis of R4SC7 started with the same alkene precursor material as was prepared in the synthesis of R4SH. Using a 4× excess, heptanethiol (CH3(CH2)6SH) was added across the double bond in a solution of tetrahydrofuran (THF) under argon. 9-BBN was added at 0 °C to initiate the reaction, and the solution was allowed to warm slowly to room temperature. After 24 h, the solution was concentrated at reduced pressure and the product R4SC7 was recrystallized from ethanol three times. Ellman’s test was used to verify removal of excess heptanethiol. The product was characterized by IR, 1H NMR, and CHN analysis (see Supporting Information). Preparation of Self-Assembled Monolayers. Gold substrates for SPR were prepared on microscope glass slides (Fisher’s Finest Premium) by thermal evaporation of 500 Å of gold (99.999% purity) onto a 20 Å adhesion layer of chromium (Kurt Lesker, PA). Both layers were deposited at a rate of 0.3 Å/s. Reflective substrates suitable for ellipsometry and IR characterization were prepared by evaporating 1000 Å of gold onto a 50 Å thick adhesion layer of chromium. Substrates were stored under nitrogen and were rinsed with ethanol before use. Monolayers of R4SH and R4SC7 were prepared by immersing the gold substrates in ethanolic solutions of 0.05 mM concentration for at least 4 h at room temperature. The SAMs were rinsed extensively with ethanol. All glassware used in monolayer preparation was cleaned in piranha solution (a mixture of concentrated sulfuric acid and 30% hydrogen peroxide in a 7:3 ratio)4 and rinsed with copious amounts of water before use. Warning: piranha solution should be handled with caution as it can detonate unexpectedly.22 Surface Plasmon Resonance. A home-built surface plasmon instrument in the Kretschmann configuration was used.23 Details of the apparatus have been published elsewhere.19 For adsorption experiments, a Teflon flow cell was mounted onto the gold substrate using a Viton O-ring. Water, ethanol, and aqueous solutions of the adsorbates were pumped through the flow cell with a peristaltic pump (E-07553-70 and P-77200-50, Cole Parmer, IL) using Viton and Teflon tubing. Before measurements, the tubes and the cell were cleaned with piranha solution and rinsed thoroughly with high-purity water. Typically, a liquid flow rate of ∼2.4 mL/min was established within the cell. After a stable value for the angle corresponding to minimum reflectivity was achieved, ethanolic or aqueous adsorbate solutions were pumped through using a system of valves. After adsorption, pure solvents were used to rinse the surface (see Results and Discussion). For multiple experiments with the same monolayer, the surface was rinsed with ethanol for about 10 min to remove the guest adsorbate. SPR experiments were carried out at least twice on different surfaces to ensure reproducibility. Infrared Spectroscopy. IR characterization of the SAMs was performed using polarization modulation IRRAS with realtime sampling electronics. We implemented PM-IRRAS using a Magna 860 IR spectrophotometer (Nicolet, WI) with dual-channel capabilities to achieve high sensitivity and selectivity to surface species adsorbed on the gold substrates. The IR beam was polarized with a wire grid polarizer (Optometrics, MA). By use of a ZnSe PEM-90 modulator (Hinds Instruments, OR), the polarization of the IR beam was rapidly modulated between (20) Hogberg, A. G. S. J. Am. Chem. Soc. 1980, 102, 6046-6050. (21) Stacey, F. W.; Harris, J. F., Jr. J. Am. Chem. Soc. 1963, 85, 963. Oswald, A. A.; Griesbaum, K.; Hudson, B. E., Jr.; Bregman, J. M. J. Am. Chem. Soc. 1964, 86, 2877. Griesbaum, K.; Oswald, A. A.; Hudson, B. E., Jr. J. Am. Chem. Soc. 1963, 85, 1969. (22) Dobbs, D. A.; Bergman, R. G.; Theopold, K. H. Chem. Eng. News 1990, 68 (17), 2. (23) Kretschmann, E.; Raether, H. Z. Naturforsch., A 1968, 23, 21352136.

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Figure 2. PM-IRRAS spectra of monolayers formed from (a) R4SC7, (b) R4SH, (c) CH3(CH2)15SH, and (d) CH3(CH2)7SH on gold in the spectral region 3100-2700 cm-1. p-polarization and s-polarization states. After reflection from the gold surface, the IR beam was focused by a BaF2 lens on a liquid nitrogen cooled MCT detector. The detected signal was demodulated by a Synchronous Sampling Demodulator (GWC Instruments, WI)24 to measure the differential intensity, (Ip Is), due to the two different polarizations. A normalized differential reflectance spectrum of intensities, (Ip - Is)/(Ip + Is), was recorded. In the spectra shown here, 1024 scans were collected and averaged at a resolution of 4 cm-1. Contact Angles. Contact angles were measured using a Rame´-Hart goniometer (model 100-00). Two pairs of advancing and receding angles were measured for each drop. At least three separate drops were used for each surface; the reported angles are an average of all (12 or more) measurements for that sample. Contact angles for monolayers formed from R4SH, R4SC7, and mercaptoundecanol were measured using water as the probe liquid. Monolayers formed from hexadecanethiol (CH3(CH2)15SH) and octanethiol (CH3(CH2)7SH) were characterized with hexadecane.

Results and Discussion Impact of Host Structure on Molecular Organization in the Monolayers. The molecular organization of a monolayer is a sensitive function of the chemical and physical structure of the molecules that form the monolayers. Because no previous report has directly studied SAMs formed from thiol-derivatized and dialkyl sulfide derivatized calix[4]resorcinarene molecules with similar headgroups, we first compare the structure of monolayers of R4SC7 to that of SAMs formed from R4SH to assess the impact of the host structure. We perform the comparison within the context of the extensively characterized SAMs formed from n-alkanethiols (CH3(CH2)nSH) to highlight the differences in structure. Figure 2 shows PM-IRRAS spectra of self-assembled monolayers formed from R4SH and R4SC7. For comparison, SAMs formed from two n-alkanethiols are also shown. Because surface species on a metallic substrate preferentially absorb the p-polarized light in comparison to the s-polarized light and because isotropic atmospheric species absorb both polarizations equally,25 the difference spectrum of Ip - Is in PM-IRRAS is highly sensitive to the monolayer and the species associated with the solid surface.24 The peaks corresponding to the stretching modes of methylene (-CH2) groups can be clearly discerned in (24) Green, M. J.; Barner, B. J.; Corn, R. M. Rev. Sci. Instrum. 1991, 62, 1426-1430. (25) Greenler, R. G. J. Chem. Phys. 1969, 50, 1963-1968. Blanke, J. F.; Vincent, S. E.; Overend, J. Spectrochim. Acta, Part A 1976, 32A, 163-173. Golden, W. G.; Dunn, D. S.; Overend, J. J. Catal. 1981, 71, 395-404.

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the spectral region from 3000 to 2800 cm-1 (Figure 2). For the monolayer formed from R4SH, as reported previously, the asymmetric -CH2 stretching is observed at νas ) 29292930 cm-1 and the symmetric -CH2 stretching is observed at νs ) 2856 cm-1. No peaks are observed at frequencies corresponding to the methyl stretch (near 2936, 2964, and 2877 cm-1) because the R4SH does not contain methyl groups (Figure 1b). In comparison, the spectrum of R4SC7 shows the asymmetric and symmetric methylene absorption peaks at 2929 and 2956 cm-1, respectively, along with a broad shoulder at νas ) 2962 cm-1, which corresponds to the asymmetric stretch of the methyl (-CH3) group. This is consistent with the presence of the methyl on the dialkyl sulfide tether (Figure 1c). Reference spectra of monolayers formed from hexadecanethiol (CH3(CH2)15SH) and octanethiol (CH3(CH2)7SH) are also shown in Figure 2, and agree with those of previous studies.26,27 Both spectra show distinct peaks at νas ) 2964 cm-1 and νs ) 2877 cm-1 due to the asymmetric and symmetric stretch of -CH3 groups, respectively. The Fermi resonance peak is also observed near 2937 cm-1. Because νas and νs peaks for -CH3 occur at comparable frequencies for both SAMs, the spectra indicate similar disorder in the terminal methyl group.4,27-29 In contrast, the asymmetric stretch of the -CH2 groups in the SAMs formed from the CH3(CH2)7SH is observed at a higher wavenumber than that in a monolayer of CH3(CH2)15SH (2922 versus 2918 cm-1). Because the symmetric -CH2 peak in CH3(CH2)7SH is significantly weaker in intensity, a shift in its position is less clear. The shift in the -CH2 stretch toward a higher wavenumber indicates that alkyl chains in the SAMs formed from CH3(CH2)7SH contain a higher number of gauche conformations in contrast to the wellordered, all-trans configuration of alkyl chains in SAMs formed from CH3(CH2)15SH, where intermolecular interactions between the neighboring long chains are stronger.27 The decrease in intensity of absorption of the -CH2 stretch in the monolayer formed from CH3(CH2)7SH compared to that formed from CH3(CH2)15SH reflects the combined effect of the increase in disorder of the shorter chains and the smaller number of -CH2 groups per chain. The above information provides a useful framework to analyze the infrared spectra of the monolayers formed from R4SH and R4SC7. In these two SAMs, the asymmetric stretch of the -CH2 groups is observed at a higher wavenumber than in a monolayer of CH3(CH2)15SH (292930 versus 2918 cm-1). A similar shift toward higher wavenumbers is also observed in the symmetric -CH2 peak (2856 versus 2850 cm-1). These shifts indicate that the alkyl chains in the SAMs formed from both macrocyclic hosts possess more gauche conformations and less order. Previous reports by Reinhoudt and co-workers on Obridged dialkyl sulfide SAMs of calix[4]resorcinarene and our earlier study on R4SH have also shown that SAMs formed from these bulky macrocyclic hosts lack the wellordered nature of simple, long-chain alkanethiols. Figure 2 shows two interesting results in addition to the presence of the methyl peak in R4SC7 and the increase in disorder within SAMs of the macrocyclic hosts compared to those of the n-alkanethiols. First, the peak positions (26) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52-66. Allara, D. L.; Swallen, J. J. Phys. Chem. 1982, 86, 4675-4678. Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swallen, J. D. J. Chem. Phys. 1983, 78, 946-952. Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395-406. (27) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (28) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097-5105. (29) Stole, S. M.; Porter, M. D. Langmuir 1990, 6, 1199-1202.

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Figure 3. Space-filling representation of the optimized structure of R4SH.

for -CH2 absorption occur at comparable frequencies for R4SH and R4SC7, which indicates that the degree of conformational ordering within these two types of monolayers is similar. The result is surprising because in previous studies Thoden van Velzen et al.12 reported an asymmetric -CH2 peak absorption near 2925-27 cm-1 for monolayers formed from O-bridged calix[4]resorcinarene molecules with dialkyl sulfide tethers (Figure 1a). Van Velzen et al. estimated the cross-sectional area of a calix[4]resorcinarene headgroup as ∼140 Å2 and speculated that since four alkylthiol chains with an area of ∼4 × 20 Å2 do not fill the void space beneath the macrocyclic cavity, a host with eight dialkyl sulfides is necessary to form monolayers with higher order. In later studies,30 Scho¨nherr and co-workers used a computational model to correct the headgroup cross-sectional area of the host shown in Figure 1a to ∼117 Å2. Results in Figure 2 indicate that increasing the number of alkyl tethers from R4SH to R4SC7 does not necessarily enhance order. We used molecular modeling31 to analyze the structure of R4SH. The cross-sectional area of the calix[4]resorcinarene headgroup in R4SH and R4SC7 (Figure 1b,c) was estimated to be ∼85-95 Å2. Figure 3 shows a space-filling representation of R4SH,32 which suggests that the four alkylthiol chains instead of the dialkyl sulfide chains provide a better fit to the headgroup size. These simple geometric arguments alone indicate that R4SC7 should occupy a larger area per molecule and will consequently have a lower density than R4SH.33 (30) Scho¨nherr, H.; Vancso, G. J.; Huisman, B.-H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 1997, 13, 1567-1570. (31) The Quanta software package from Accelrys (Burlington, MA) was used. An estimate of the headgroup size of the O-bridged calix[4]resorcinarene in Figure 1a gave values close to those reported by Scho¨nherr and co-workers. (32) The CPK or Corey-Pauling-Koltun model is a space-filling representation of a molecule in which each atom is represented by a sphere. The radius of each sphere is proportional to the van der Waals radius of that atom (see: Koltun, W. L Biopolymers 1965, 3, 665).

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Figure 4. PM-IRRAS spectra of monolayers formed from R4SH (solid line) and R4SC7 (broken line) on gold in the spectral region 1900-1200 cm-1.

The second effect observed in Figure 2 supports the impact of the host structure on the organization of the monolayers. The IR spectra show comparable intensities for the asymmetric -CH2 (2929-30 cm-1) peak in both R4SH and R4SC7. Because the absorption due to -CH2 groups remains the same despite the increase in the number of methylene moieties in R4SC7, the spectra indicate that the density of R4SC7 within the monolayers is smaller than that of R4SH. Previous studies15 have suggested that formation of monolayers of dialkyl sulfide derivatized calix[4]resorcinarene molecules at elevated temperatures and in mixed solvents (e.g., ethanolchloroform) can lead to more well-ordered monolayers. To test this hypothesis, monolayers of R4SC7 were also formed at 60 °C in a mixed ethanol-chloroform (1:1) solvent. Increasing the temperature of formation showed no changes in the methylene or methyl absorption peaks for R4SC7, indicating that no significant changes in ordering occur. On the basis of these results, we conclude that the differences in the headgroup between molecules in Figure 1a and Figure 1b,c are sufficient to affect the formation and organization of monolayers. In the spectral region between 1200 and 1900 cm-1 (Figure 4), the IR absorption is much weaker. The spectra of both R4SH and R4SC7 are similar. Among the major peaks in the PM-IRRAS spectrum, several vibrational bands due to the resorcinol unit can be identified. The CdC stretching of the aromatic benzene rings is observed at ∼1610, ∼1500, and 1440 cm-1.34 The peak at 1440 cm-1 overlaps with the scissoring bend of -CH2 groups near 1460 cm-1. Tentatively, the peak at 1300 cm-1 is due to the benzene ring and the peak at ∼1260 cm-1 is due to the C-O stretching. However, definitive assignment of peaks in the lower wavenumber region is difficult owing to very weak absorption and significant distortion of the baseline due to the photoelastic modulation.35 Since the p-polarized light is sensitive to transition moments perpendicular to the surface, the absorption spectra in Figure 4 point to a vertical orientation of the bowl-shaped cavity in both monolayers. (33) Complete coverage was verified by exposing fully formed monolayers of R4SH and R4SC7 to a solution of mercaptoundecanol (MUD) and monitoring the shift in surface plasmon angle. No change in the surface plasmon angle was observed, indicating that neither insertion of the MUD nor displacement of the calix host occurred over time scales of 1 h. (34) Silverstein, R. M.; Bassler, G. C.; Morill, T. C. Spectrometric identification of organic compounds; 4th ed.; John Wiley & Sons: New York, 1981.

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Past studies have shown that wettability can be a sensitive function of the structure of self-assembled monolayers.4,28,36 Therefore, the wetting characteristics of both surfaces were also compared. Advancing contact angles of water were measured to be typically 68°-72° on all samples of R4SH and 74°-78° for R4SC7. Adams et al. have reported contact angle of 28° for sessile drops on monolayers of R4SH;8 higher contact angles have been reported when multilayers formed in solvents other than ethanol.9,37 We believe that no multilayering occurred in our samples for two reasons: SAMs were formed from ethanol and the PM-IRRAS, ellipsometric, and SPR measurements were not indicative of multilayering. The discrepancy in the contact angles can be, plausibly, due to the difference in the method of measurement, that is, sessile drop versus captive drop, and the disorder in the hydroxyl groups at the interface of the SAMs.38 Past studies on wettability of mixed SAMs have observed that contact angle hysteresis and, in particular, the receding angle are sensitive to the monolayer structure.4,28,36 Pinning39 of the receding edge of the water drop was observed on monolayers formed from R4SH, while a receding angle of 32°-36° was measured on monolayers formed from R4SC7. Investigations of mixed monolayers formed from mixtures of long and short n-alkanethiols, that is, SAMs wherein chain length asymmetry exists, have shown changes in contact angle hysteresis and hydrophobic nature of the surface when structural disorder of the chains exposes methylene and methyl groups to water. Bain and Whitesides4 have studied mixed monolayers of HO(CH2)11SH and HO(CH2)19SH and shown that the contact angle of water increases with monolayer disorder and shows a maximum value at an intermediate composition. Similar conclusions are also reported in other studies of mixed monolayers formed from CH3-terminated and OH-terminated thiols.28 The wetting properties on SAMs formed from R4SH and R4SC7 support the structural differences suggested by the analysis of the IR absorption data. Because the geometric differences and the IR spectra indicate that the dialkyl sulfide chains in R4SC7 cannot pack neatly beneath the calix[4]resorcinarene headgroup, the SAMs formed from R4SC7 show a slightly greater hydrophobic nature as evident by the larger advancing contact angles and the higher receding angle. This difference in hydrophobic nature with only a slight perturbation in the host structure makes these surfaces an attractive system for studying guest-host (35) The photoelastic modulator used in PM-IRRAS introduces a sinusoidal modulation in the phase retardation of the IR radiation. Consequently, the normalized differential reflectance spectrum shows a sinusoidal background. This periodic background is removed using baseline correction. In our instrument, the periodic baseline in combination with the cutoff wavenumber for the MCT detector (650 cm-1) gave rise to a large distortion in the baseline near low wavenumbers. (36) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. (37) Reinhoudt and co-workers have reported that SAMs formed from tetrasulfide-calix molecules with eight hydroxyl groups show high advancing contact angles (∼90°) and a low receding angle (∼37°). From the unusually strong absorption intensities in the IR spectra and a thickness measurement of ∼29 Å from the surface plasmon resonance, the authors concluded that a disordered bilayer structure caused the relatively high contact angles. (38) It has been hypothesized that surfaces containing high concentrations of OH groups can reorganize and exhibit high (>30°) contact angles (see for instance: Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156-161). (39) Microscopic pinning of the contact line during wettability studies is a well-known phenomenon. Many studies have reported that this behavior and the associated hysteresis in contact angle are believed to originate from the imperfections, heterogeneities, chemical disorder, and microscopic structure of surfaces (see: Nadkarni, G. D.; Garoff, S. Europhys. Lett. 1992, 20, 523-528 and references therein).

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Figure 5. Detector response from reflection due to (a) bare gold (circles) and a monolayer formed from R4SH (squares) and (b) bare gold (circles) and a monolayer formed from R4SC7 (squares). The surfaces are under pure ethanol. Symbols are the experimental data, and solid lines show the fit using a four-layer optical model. Sparse markers have been used for clarity (every 10th point is shown).

recognition and understanding the impact of molecular forces on the recognition. A third route to characterizing structural differences between self-assembled monolayers of the R4SH and R4SC7 is measurement of monolayer thickness using SPR. Figure 5 shows the shift in the SPR response between bare gold surfaces and fully formed self-assembled monolayers of R4SH and R4SC7. It is evident that the magnitude of the shift in the minimum in reflectivity or the surface plasmon resonance angle (θp) is smaller during the formation of a self-assembled monolayer of R4SC7. The intensity of reflected light was fit using a model based on Fresnel equations,40 and the thickness of the monolayer of R4SH was found to be 21 Å ((1 Å).41 For reference, a similar experiment for the chemisorption of CH3(CH2)15SH and CH3(CH2)7SH was performed. We determined a thickness of 25 Å ((1 Å) for the monolayer of CH3(CH2)15SH and 10 Å ((1 Å) for the monolayer of CH3(CH2)7SH. However, self-assembled monolayers formed from R4SC7 were determined to be smaller in thickness than those of either R4SH or CH3(CH2)15SH. The lower thickness of 13 Å ((1 Å) by SPR measurements for R4SC7 indicates that the dialkyl sulfide chains in R4SC7 do not fold neatly beneath the calix[4]resorcinarene headgroup. Ellipsometric measurements of thickness were consistent with the SPR results. A rough approximation of the density of macrocyclic hosts in the monolayer can be obtained by linear interpolation of the thickness, which indicates that the SAMs formed from R4SC7 have ∼30% of the calix[4]resorcinarene molecules relative to SAMs formed from R4SH. Based on the combined measurements of IR, contact angles, and SPR measurements, a schematic illustration of the monolayers is shown in Figure 6. Guest-Host Interactions at Self-Assembled Surfaces of R4SH and R4SC7. To investigate the role of molecular structure and the forces in guest-host association properties of SAMs formed from R4SH and R4SC7, we selected R-hydroxybutyrolactone (HBL) as the guest molecule (inset, Figure 7a). Past studies have demon(40) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380-390. (41) In fitting the thickness of the monolayer, we assumed a constant refractive index of 1.5. No measurements of the refractive index of a monolayer of the calix[4]resorcinarene have been reported in the literature. In past studies, Shirshov et al. (Supramol. Sci. 1997, 4, 491494) have reported that thermally evaporated, 25-100 nm thick films of calix[4]arene show an average refractive index of 1.56. Reinhoudt and co-workers used a refractive index of 1.45 for fitting SPR reflectivity curves for monolayers of the O-bridged tetrasulfide host. Molar refractivity calculations yield a refractive index of 1.576 for R4SH, 1.526 for R4SC7, and 1.46 for CH3(CH2)15SH.

Figure 6. Schematic illustration of monolayers formed from (a) R4SC7 and (b) R4SH. See text for discussion. (Not drawn to scale.)

strated19,42 that HBL associates selectively with the calix[4]resorcinarene host. Surface plasmon resonance and PMIRRAS were used to investigate the recognition of guest molecules from aqueous solutions by the SAMs. The reflectivity curve was measured as a function of time while the solution of the guest molecule was pumped over the self-assembled monolayer. Association at the surface was monitored by measuring the change in the SPR angle (∆θp(t)). A typical temporal evolution of the SPR angle (∆θp(t)) when SAMs formed from R4SH and R4SC7 hosts were placed in contact with an aqueous solution of HBL is shown in Figure 7a. Both surfaces show a rapid rise in ∆θp, indicating fast association of HBL with the self-assembled surfaces of R4SH and R4SC7. The magnitude of the maximum shift (∆θmax) is ∼0.1-0.12°, which indicates the molecular dimension of the adsorbed layer (∼3-5 Å). A subsequent water rinse causes a slight decrease in ∆θmax but does not wash out the HBL. In the previous study, we used a series of guest molecules to demonstrate that steric (42) Davis and Stirling have used IR spectroscopy and reported that R-hydroxybutyrolactone associates with SAMs formed from calix[4]resorcinarene derivatives.

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Figure 7. (a) Shift in SPR angle (∆θp) caused by adsorption from a 1 mM aqueous solution of HBL on monolayers formed from R4SC7 (dotted curve), R4SH (solid curve), and mercaptoundecanol (dotted-dashed curve). The arrow indicates the time at which the surface was rinsed with pure water. (b) Schematic of the complex formed between HBL and the macrocyclic cavity in the R4SH or R4SC7 molecule (see text for discussion). For clarity, only the macrocyclic cavity is shown. Dashed lines indicate hydrogen bonds.

forces, hydrogen bonding, and hydrophobic forces drive selective recognition on the self-assembled surfaces of R4SH. The role of hydrophobic forces is apparent as interaction of HBL with a hydrophilic surface formed from HO(CH2)11SH produces no measurable shift in the SPR angle (Figure 7a).43 Interestingly, the interaction of HBL with a SAM formed from R4SC7 shows that the time scale of the rise in ∆θp and the magnitude of the angular shift are comparable to those of R4SH. We have also compared the orientation of the HBL associated with the R4SH and R4SC7 surfaces by using PM-IRRAS to probe each surface before and after association. As in our previous study,19 after the surfaces bind HBL the IR spectra show a large positive peak at ∼1744 cm-1 and weaker peaks at 1260 and 1125 cm-1. The peak at ∼1744 cm-1 is due to the carbonyl, CdO, bond in HBL, while the peaks at 1260 and 1125 cm-1 can be attributed to the C-C(dO)-O- bond stretch in HBL.34 Because the frequencies corresponding to both the CdO bond and the C-C(dO)-O bond are discerned in the IR spectra, it can be inferred that the HBL molecule is oriented such that the transition moments of these two bonds have components along the normal to the surface. Figure 7b shows a plausible scheme of the associated complex that conforms with the IR results and shows the (43) The shift in the SPR angle, ∆θp, is due to interaction of HBL with the R4SH and not the gold substrate. The evanescent wave in SPR penetrates the bulk solution to distance of ∼160-200 nm, and ∆θp can, therefore, show a shift due to bulk changes in refractive index of the aqueous solution. However, we have measured the dn/dc for aqueous solutions of HBL to be 0.000 009 4 mM-1, which indicates that the change in refractive index is negligibly small for 1 mM aqueous solutions of HBL. This is also supported by the near-zero ∆θp when the aqueous solution of HBL is in contact with a SAM formed from mercaptoundecanol.

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Figure 8. (a) Magnitude of the shift in SPR angle (∆θp) for adsorption of HBL from aqueous solutions of different bulk concentrations (C) onto self-assembled surfaces of R4SC7 (squares) and R4SH (circles). Dashed lines are drawn as a guide to the eye. (b) Plot of experimental data from (a) on transformed coordinate axes. Solid lines are fits to the Langmuir isotherm. See text for discussion.

potential hydrogen bond between the OH group of HBL and the macrocyclic cavity. Differences in the structure and monolayer organization between R4SH and R4SC7 do not show significant effects in SPR and IR measurements on the molecular association with HBL. Because the thermodynamic affinity of HBL to the surface is a sensitive function of the forces driving the interaction and because the SAMs formed from R4SH and R4SC7 differ in their hydrophobic nature and molecular organization, we also measured the association of HBL as a function of its bulk concentration in solution. Figure 8a shows a plot of the shift in ∆θp observed for HBL concentration varying between 0.01 and 10 mM. Both surfaces show a Langmuir type behavior with saturation of the response at high concentration.44 This behavior is in stark contrast to the measurements reported by Friggeri and co-workers10 for SAMs formed from the O-bridged calix[4]resrocinarene host (Figure 1a). Friggeri et al. reported that ∆θp for a guest molecule such as benzoic acid increased linearly with bulk concentration, which was interpreted to result from physisorption and formation of multilayers of the guest. The experimental data in Figure 8a were fit to an equation derived from a Langmuir isotherm:

C C 1 ) + ∆θp exp(-∆G°ads/RT) ∆θsat ∆θsat The plot C/∆θp versus C in Figure 8b shows that the observed data follow the linear fit. Regression analysis gives ∆θsat ) 0.166 ( 0.006 and ∆G°ads ) -4.6 ( 0.5 kcal/ mol for R4SC7 and ∆θsat ) 0.1 ( 0.002 and ∆G°ads ) -5.2 ( 0.6 kcal/mol for R4SH. Comparison of the complexation energy with measurements in bulk solution will be important to understand how preorganization of host (44) Because the thickness of the adsorbed HBL layer is extremely small (