Langmuir 1996, 12, 5365-5374
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Calix-4-resorcinarene Monolayers and Multilayers: Formation, Structure, and Differential Adsorption1 Frank Davis and Charles J. M. Stirling* Centre for Molecular Materials and Department of Chemistry, The University, Sheffield S3 7HF, U.K. Received June 3, 1996. In Final Form: July 26, 1996X A tetrapodal calix-4-resorcinarenethiol has been immobilized in a gold-thiol monolayer. Adsorption of a variety of polar substrates into the monolayer has been demonstrated by Fourier transform infrared spectroscopy. For many substrates, adsorption is reversible but not in the case of hydroxylactones for which adsorption appears to be followed by scission of the lactone ring and acylation of hydroxyl groups on the periphery of the bowl of the resorcarene. Evidence for this unusual behavior is presented. Some calix-4-resorcinarenes have been found to form multilayers under certain conditions. These multilayers have regular periodic rather than random structures and are able to adsorb substrates selectively. These multilayers disperse in polar solvents, but when pendant chains in a calix-4-resorcinarene multilayer are terminated by vinyl groups, UV irradiation apparently cross-links the chain with resulting stability of the multilayer and resistance to dispersal.
Introduction Calixarenes, cyclic (poly) phenol aldehyde oligomers, have attracted much attention in recent years because of their unusual properties, notably their conformational flexibility, their striking acid-base behavior, and their ability to form complexes with a variety of both polyfunctional adsorbates and metal ions.2,3 Alongside the widespread current work on resorcarenes, studies of self-assembled gold-thiol monolayers, notably in the hands of Whitesides,4 Crooks,5 Nuzzo,6 and Ulman7 have developed. These groups showed that closely packed monolayers of long, straight-chain organic thiols formed spontaneously when appropriately configured gold surfaces were immersed in dilute solutions of the thiols. Such monolayers can readily be interrogated by Fourier transform infrared (FTIR) spectroscopy,8 contact angle determination,9 and surface plasmon resonance.10 We now report on the first phase of our work on the restraining of appropriately constructed resorcarenes in gold-thiol monolayers. This report is divided into four sections: (i) formation of self-assembled monolayers of resorcarenes; (ii) selective adsorption/reaction with such monolayers; (iii) multilayering of resorcarenes; (iv) adsorption and reaction in multilayers. Throughout this paper which deals with calix-4-resorcinarenes, these compounds will be referred to as resorcarenes3 unless otherwise stated. Systematic naming of this class of compounds is byzantine; that of resorX
Abstract published in Advance ACS Abstracts, October 1, 1996.
(1) Preliminary accounts of parts of this work have been published: (a) Adams, H.; Davis, F.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1994, 2527. (b) Davis, F.; Stirling, C. J. M. J. Am. Chem. Soc. 1995, 117, 10385. (2) Gutsche, C. D. Calixarenes; Royal Society of Chemistry, Monographs in Supramolecular Chemistry, Cambridge, 1989. Aldrichem. Acta 1995, 28, 3. (3) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713. (4) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. R. J. Am. Chem. Soc. 1989, 111, 321. (5) Chailapakul, O.; Sun, L.; Xu, C. J.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459. (6) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (7) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (8) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (9) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (10) Jordan, C. E.; Frey, B. L.; Karnguth, S.; Corn, R. M. M. Langmuir 1994, 10, 3642.
S0743-7463(96)00543-4 CCC: $12.00
carene 1d is r-2,c-8,t-14,t-20-tetraundecylpentacyclo[19.3.1.13,7.19,13.115,19]octacosa-1(25),3,5,7(28),9,11,13(27),15,17,19(26),21,23-dodecaene-4,6,10,12,16,18,22,24octol. Results and Discussion We chose to work initially with resorcarenes because of their ease of assembly and potential for complexation of a variety of substrates. Because formation of a selfassembled gold-thiol monolayer was our initial objective, we prepared resorcarene (1b) as shown in Chart 1. Closely related compounds had been prepared11 earlier in Reinhoudt’s laboratories12 including the sulfide 1c. In our work, photoaddition of thiolacetic acid to 1a and subsequent hydrolysis to the thiol 1b proceeded smoothly. Self-Assembled Monolayers. Smooth gold surfaces on a primer layer of chromium on silicon wafers were prepared as described elsewhere.13 Immersion of these surfaces in 0.25 mM solutions of the resorcarenethiol 1b gave monolayers which showed consistent FTIR spectra and sessile drop contact angles of 28° indicating a hydrophilic surface.9 It appeared therefore that the structure of the monolayer was as shown in Chart 2. The IR C-H stretching frequencies indicated loose packing of the pendant chains in contrast to Reinhoudt’s work with sulfides.11 This work should be viewed against the background of earlier work on Langmuir-Blodgett monoand multilayers of resorcarenes.14 Gold-thiol monolayers possess the advantage of robustness enabling a wide range of chemical treatments without loss of the layer. In relation to the conformational behavior of resorcarenes, we believe that the tetrapodal anchoring selects the crown conformation with high preference versus others.15 We have determined the crystal structure1a of the related resorcarene 1d (Figure 1) which clearly shows the crown conformation in the solid state together with (11) van-Velzen, E. U. T.; Engbersen, J. F. J.; Delange, P. J.; Mahy, J. W. G.; Reinhoudt, D. N. J. Am. Chem. Soc. 1995, 117, 6853. (12) van-Velzen, E. U. T.; Engbersen, J. F. J.; Reinhoudt, D. N. Synthesis (Stuttgart) 1995, 8, 989. (13) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121. (14) Brake, M.; Bo¨hmer, V.; Kra¨mer, P.; Vogt, W.; Wortmann, R.; Supramol. Chem. 1993, 2, 65. Conner, M. D.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1993, 115, 1178. Conner, M. D.; Janout, V.; Kudelka, I.; Dedek, P.; Zhu, J.; Regen, S. L. Langmuir 1993, 9, 2389. (15) Hogberg, A. S. J. Am. Chem. Soc. 1980, 102, 6046.
© 1996 American Chemical Society
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Chart 1
Figure 1. X-ray structure of resorcarene 1d.
Chart 2
the novel situation of deep interdigitation of the pendant alkyl chains, a feature to which we return below. We believe this to be the first X-ray structure determined for a resorcarene with free hydroxyl groups and showing interdigitated pendant chains. X-ray structures of several resorcarenes have been determined, but resorcarenes have usually required conversion into, e.g., octaacetates to give material suitable for analysis.16 Adsorption in Resorcarene Monolayers. Considerable attention has been paid, notably by Aoyama and his co-workers17 to complexation of a variety of substrates by resorcarenes in bulk solutions. Thus ethylene glycol, glycerol, glucose, and riboflavin, for examples, all bind strongly to resorcarenes. Khare and Sugahara18 have likewise shown that resorcarene 1d selectively adsorbs and catalyzes the glycosidation of sugars such as ribose. As part of our program on selective interactions with highly oriented thin films,19-21 we initially examined (16) (a) (octa-acetate) Palmer, K. J.; Wong, R.-I.; Jurd, L.; Stevens, K.; Acta Crystallogr., B 1976, 32, 847. (b) (Mannich modification) Leigh, D. A.; Linnane, P.; Pritchard, R. G.; Jackson, G. J. Chem. Soc., Chem. Commun. 1994, 384. (17) Aoyama, Y.; Tanaka, Y.; Sugahara, S. J. Am. Chem. Soc. 1989, 111, 5397. (18) Khare, C.; Sugahara, S. J. Indian Chem. Soc. 1991, 68, 42. (19) Guo, B. Z.; Taylor, D. M.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1991, 479. (20) Neogi, P.; Neogi, S.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1993, 1134. (21) Davis, F.; Neogi, P.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1994, 1199.
adsorption of substrates in monolayers of resorcarenethiol, 1b. We chose substrates with the potential for adsorption by hydrogen bonding to the bowl circumference and which could readily be detected by FTIR. Results are in Table 1 and fall into three broad categories. First, those for which no adsorption can be detected. This category includes azide ion (entry 1), 3-acetoxybutyrolactone (entry 2), the vitamin C analogues (entries 3 and 4), and the squaric acid derivative (entry 5). The second category is an intermediate one, namely, that for which adsorption can be detected, but when the monolayer with attached adsorbate is immersed in pure solvent, adsorbate is lost from the monolayer and the spectrum of a fresh monolayer is regenerated. This applies to the glutaric acids 6, 7, and 8, sodium gluconate 9, poly(N-vinylpyrrolidone) 10, riboflavin 11, and butyrolactone 12. The third category comprises entries 13-18 in which adsorption onto the monolayer is clearly observable but for which washout into pure solvent does not occur even over considerable periods. In all of these cases, removal of the adsorbate occurs when the layer is immersed in dilute aqueous sodium hydroxide. The fourth category is that of N-acetylglucosamine. In this case, the adsorbate is not removed by washing in pure solvent or by immersion in either dilute acid or alkali. In almost all cases, adsorption is attended by shifts in the CdO stretching frequencies; values are given in Table 1, and in all of these cases, the issue of quantification arises. We have attempted to deal with this problem by examining the IR spectra of 1:1 bulk mixtures of resorcarene and three adsorbates. We recognize that band intensities are orientation sensitive for thin films and that accurate comparisons are not possible. For vitamin C, the intensity ratio CH2 (asymmetric stretching):carbonyl stretching is 0.9:1. With equimolecular mixtures (cast film from ethanol) the ratio is 1.1:1. Adsorption of IR radiation is proportional to cos2 of the angle between the vector of the IR radiation and the vibration concerned. This only has major effects when the system is highly ordered and the angles concerned are close to 0 or 90°. From the work of Reinhoudt11 it appears that only resorcarenes with spacefilling alkylthio groups beneath the bowl form well-ordered monolayers and our systems with only four “legs” are much more liquid-like in behavior. Therefore we think that the IR spectra indicate an approximate 1:1 combination of the species. The adsorption of vitamin C was quite fast, the carbonyl adsorption intensity after 10 min being about 75% of that after 30 min of exposure. Longer exposure times, up to 3 days did not affect the IR spectrum. This is comparable with the results of Aoyama17 on sugar adsorption in LB films of 1. Likewise, for glutaric acid, in both gold-thiol monolayers of 1b and multilayers (below), the bulk:thin film carbonyl intensity ratio was
Calix-4-resorcinarene Monolayers and Multilayers Table 1. Substrate Binding by a Monolayer of Calixresorcinarene 1b
a Exposure of monolayer to 1 mmol dm-3 aqueous solution, 16 h at 20 °C. b Exposure of monolayer + adsorbate to pure water, 16 h at 20 °C. c From ethanol. d Wash out in 10 mmol dm-3 aqueous NaOH 16 h at 20 °C. e Cast on CaF2 plate. f meso-/racemic mixture. g No wash out after exposure to 10 mmol dm-3 aqueous NaOH or 1 mol dm-3 HCl 16 h at 20 °C. h Insoluble in H2O, EtOH, and CH2Cl2.
Langmuir, Vol. 12, No. 22, 1996 5367
close to unity as it also was for N-acetylglucosamine in gold-thiol monolayers of 1b. We interpret these results as follows: For category 1 adsorbates, hydrogen bonding to the adsorbate either is absent, as for azide ion, or is weak as for the arylactones 3 and 4 and squaric acid 5. In relation to the results below for which strong adsorption of related compounds is evident, we think that the bulky phenyl group in 3 and 4 inhibits interaction with the upturned hydroxyl groups sufficiently to make solvation by the solvent, ethanol, competitive in the adsorption equilibrium. In the case of squaric acid, deposition from water was unsuccessful. The geometry of this substrate is not ideal for adsorption by the resorcarene monolayer and it is, in any case, highly solvated by water and very poorly soluble in ethanol. The final nonbinding substrate in this category is 3-acetoxybutyrolactone 2. Again, steric size, if not contributing to binding, appears to displace equilibrium toward the solvent sufficiently to make any adsorption undetectable. The second category of adsorption but with easy desorption is represented by butyrolactone, the glutaric acids, riboflavin, and poly(N-vinylpyrolidone). Infrared spectra show stoichiometric adsorption with the reservation made above. The lactone and double carboxyl functions fit appropriately, but pure solvent displaces the equilibrium in favor of solvent. For the third category, adsorption is readily observable and is not reversed in pure solvent. A particularly interesting series is that of butyrolactone, 3-hydroxybutyrolactone, and 3-acetoxybutyrolactone. Adsorption is strong and irreversible for the hydroxylactone and the IR carbonyl stretching frequency changes for the hydroxylactone from 1773 cm-1 in the liquid phase to 1745 cm-1 in the monolayer (Figure 2). There is, however, a corresponding change with butyrolactone. Likewise for vitamin C, the carbonyl stretching frequencies move from 1756 and 1690 cm-1 in bulk solid to 1735 and 1665 in the monolayer (Figure 3). Desorption did not occur even after long (9 day) immersion in water, and our observation that this absorption is reversed by treatment of the layer with aqueous sodium hydroxide tentatively suggests that acylation by the lactone of one of the rim hydroxyl groups occurs (Scheme 1). Such a process would demand significant nucleophilicity and ionization of a phenolic hydroxyl in these (neutral) conditions in water. This is appropriate given the low pKa of such multihydrogen bonded systems. We have determined approximate pKas for resorcarene 1d. It is readily soluble in buffers at pH 9.2 but not at pH 7 so that both protonated and deprotonated phenolic hydroxyl groups are present together. Possibly it is this juxtaposition that is accelerating what would otherwise be the very slow esterification of a phenol by a lactone. There is no reaction observable between resorcarene 1d and vitamin C in bulk conditions. Displacement of vitamin C did not occur when the layer incorporating vitamin C was treated with aqueous solutions of glucose, galactose, ethylene glycol, or glycerol, all known to bind strongly to resorcarenes in bulk.17 This observation offers the prospect of very precise selection between related substrates on the basis of reactivity and steric fit. For the final category, represented by N-acetylglucosamine, FTIR data suggest that acylation of the resorcarene hydroxyl groups is not occurring. The carbonyl stretching frequency is the same in bulk sugar as that in the adsorbate, and neither acid or base perturbs the equilibrium which is strongly on the adsorption side. Immobilization of substrate in the monolayer of the resorcarene is important. Attempts to prepare adducts under bulk solution and phase transfer conditions from
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Figure 2. IR spectra of (a) 2-hydroxybutyrolactone (neat liquid), (b) 3-hydroxybutyrolactone adsorbed on a gold-thiol monolayer of resorcarene 1b, and (c) attempted wash-off of adsorbate.
Figure 3. IR spectra of (a) vitamin C cast from aqueous solution onto a CaF2 plate. (b) SAM of resorcarene 1b. (c) Vitamin C adsorbed in a SAM of resorcarene 1b.
vitamin C and several other adsorbates were successful only with glutaric acid and riboflavin as previously reported.17 No interaction between bulk resorcarene 1d and aqueous vitamin C could be detected nor was vitamin C “solubilized” by resorcarene 1d in chloroform. The resorcarene is monomeric in ethanol and so hydrogenbonding between the bowls does not seem to be responsible. Possibly the greater flexibility of the free resorcarene solute is responsible. Multilayering in Resorcarenes. Multilayering in thin films by covalent-covalent,22 covalent-coordinate,23 covalent-ionic,24 and hydrogen bonding25 is well sum-
marized,26 and in the area of gold-thiol monolayers, Bard and his group27 have shown that self-assembled monolayers (SAMs) of octadecanethiol, on long exposure to solutions of the thiol, form multilayers some three to four layers thick, as determined by ellipsometry. Formation of the gold-thiol SAMs of resorcarene 1b, together with the propensity of the pendant chains to interdigitate known from the crystal structure of 1d (Figure 1), suggested the possibility that successive layers (22) Maoz, R.; Netzer, L.; Gun, J.; Sagiv, J. J. Chem. Phys. 1988, 85, 1059. Tillman, A.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101.
Calix-4-resorcinarene Monolayers and Multilayers
Langmuir, Vol. 12, No. 22, 1996 5369 Scheme 1
Table 2. Multilayer Deposition of Resorcarene 1d on Gold-Thiol Monolayers of 1b solvent
concn of 1d (mmol dm-3)
equiv no. of monolayersa,b
hexane hexane hexane dichloromethane propanone ethanol
1 3 10 10 10 10
21 42 44 7 c c
a Intensity of ν -1 stretching frequency in multilayer C-H 2921 cm vs monolayer. b After equilibration for 16 h at 20 °C. c No deposition.
Table 3. Multilayer Deposition of Resorcarene 1d on Different Substrates
Figure 4. IR spectra of (a) SAM of resorcarene 1b and (b) multilayer of resorcarene 1d.
of resorcarenes could be built up by a combination of interlayer hydrogen bonding between the bowl hydroxyl groups and van der Waals interactions between the interdigitated chains. When a gold-thiol SAM of resorcarene 1b was immersed in an ethanolic solution of resorcarene 1d, then removed and washed in pure ethanol, no changes in the reflectance-adsorbance FTIR spectrum (Figure 4) were discernible. Clearly, no multilayering had occurred, and the sessile water drop contact angle of the monolayer was unchanged. When, however, the gold-thiol SAM of 1b was immersed in a hexane solution of 1d, removal and (23) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. Lee, H.; Kepley, L. J.; Hong, H.-G.; Alehter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597. Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420. Akhter, S.; Lee, H.; Hong, H. G.; Mallouk, T. E.; White, J. M. J. Vac. Sci. Technol. A 1989, 7, 1608. Putvinski, T. M.; Schilling, M. L.; Katz, H. E.; Chidsey, C. E. D.; Mujsce, A. M.; Emerson, A. B. Langmuir 1990, 6, 1567. Katz, H. E.; Schilling, M. L.; Chidsey, C. E. D.; Putvinski, T. M.; Hutton, R. S. Chem. Mater. 1991, 3, 699. Evans, S. D.; Ulman, A.; Goppert-Berarducci, K. E.; Gerenser, L. J. J. Am. Chem. Soc. 1991, 113, 5866. Unemura, Y.; Tanaka, K.-I.; Yamagishi, A. J. Chem. Soc., Chem. Commun. 1992, 67. Yang, H. C.; Aoki, K.; Hong, H.-G.; Sackett, D. D.; Arendt, M. F.; Yau, S. L.; Bell, C. M.; Mallouk, T. E. J. Am. Chem. Soc. 1993, 115, 11855. Frey, B. L.; Hanken, D. G.; Corn, R. M.; Langmuir 1993, 9, 1815. Byrd, H.; Pike, J. K.; Talham, D. R. Chem. Mater. 1993, 5, 709. Byrd, H.; Whipps, S.; Pike, J. K.; Talham, D. R. Thin Solid Films 1994, 244, 768. Zeppenfeld, A. C.; Fiddler, S. L.; Ham, W. K.; Klopfenstein, B. J.; Page, C. J. J. Am. Chem. Soc. 1994, 116, 8817. (24) Bill, C. M.; Arendt, M. F.; Gomez, L.; Schmehl, R. H.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8374. Bell, C. M.; Keller, S. W.; Lynch, V. M.; Mallouk, T. E. Mater. Chem. Phys. 1993, 35, 225. (25) Sun, L.; Kepley, L. J.; Crooks, R. M. Langmuir 1992, 8, 2101. (26) Keller, S. W.; Kim, H.-N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817. (27) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. Langmuir 1994, 9, 1942.
substrate
adsorptiona
gold,c,d aluminumc,d monolayer of HO(CH2)10SHb,c monolayer of C18H37SHb,c monolayer of 1bb,c stainless steelc quartze NaCle,f silicon, GaAsf
yes yes no yes yes yes yes no
a Adsorption was calibrated against that of monolayers or LB multilayers of known layer number. b On gold. c Detected by grazing angle reflectance IR. d On glass. e Detected by transmission UV. f Detected by transmission IR.
washing in pure hexane left a film whose FTIR frequency spectrum was again identical with that of the monolayer but whose absorption intensity was some 30 to 40 times greater than that of the monolayer (Figure 4). The water contact angle of the layer increased from that of the gold thiol SAM at 28° to 92° in 2 h. The extent of multilayering was also calibrated using LangmuirBlodgett films of resorcarenes 1d of known (15 layers) multiplicity. This observation raised a number of other queries. First, was the multilayer formed ordered or random? In a series of check experiments it was established that sonication of the layer in pure hexane did not dislodge molecules from the surface as judged by the FTIR spectral intensities. X-ray photoelectron spectroscopy of the multilayers showed that the increase in νC-H stretching intensity in the infrared spectrum was not due to solvent (e.g., hexane) entrapment. The overall carbon to oxygen ratio and the ratio of carbon linked to oxygen to the total carbon were expected for the resorcarene itself. The increase in spectral intensities over that of the monolayer is dependent on the concentration of the resorcarene in free solution and on the solvent in which the monolayer is immersed (Table 2).
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Figure 6. Putative structure of a multilayer of 1d on SAM of 1b on Au.
Figure 5. (a) Neutron reflectance spectrum of an O-predeuterated multilayer of 1d on Al. (b) X-ray reflectance spectrum of a multilayer of 1d on Al.
The second question that presented itself was the generality of the phenomenon. We found that gold, aluminum, stainless steel, quartz, and sodium chloride would accept a deposit of the unfunctionalized resorcarene 1d from hexane solution. These deposits could be interrogated by a variety of techniques, notably IR and UV. Formation of multilayers was confirmed as before by comparison of spectral intensities with monolayers such as the gold-thiol SAM from 1b or from known multilayer dimensions derived from Langmuir-Blodgett films. We think that the overall structure of the multilayers on Au and Al is very similar except that the first adsorbed layer of 1b on Au is a bowl-up resorcarene anchored by four gold-thiol bonds whereas the first layer on Al of, for example, 1d is a bowl-down resorcarene anchored by multiple hydrogen bonds to the aluminum oxide overlayer. Deposition did not occur on every surface; silicon and gallium arsenide showed no tendency to receive resorcarene multilayers, presumably because in the former case an initial hydrophilic interaction between the surface and the resorcarene is needed. In this connection, neither did an SAM (hydrophobic surface) of octadecanethiol receive a resorcarene multilayer. The third issue was that of regularity or randomness in the monolayers. The C-H asymmetric stretching frequency in a typical multilayer was 2921 cm-1 as against that (2930 cm-1) of a self-assembled monolayer of 1b speaking of closer packing in the multilayer consistent both with interdigitation in the multilayer and its reluctance to incorporate other substrates (vide infra). These conclusions were reinforced in two ways: X-ray and neutron reflectometry data (Figure 5) give multilayer thicknesses of 420 ( 15 Å. X-ray reflectometry was measured using a conventional Siemens 0.20 diffractometer with Cu KR radiation. The sample was a selfassembled multilayer of 1d on Al/glass, of about 37 layers (by FTIR spectroscopy). Multilayer thicknesses were obtained from the frequency of the Kiessig fringes.28 Neutron reflectometry was measured on the D17 instrument at the Institut Laue-Langevin, Grenoble, France, on the same sample after it had been converted into the
per O-D derivative by soaking overnight in D2O. Infrared spectra showed removal of the O-H stretching band at 3250 cm-1 and its replacement by the O-D stretching band at 2540 cm-1. Coupled with the νC-H str intensification this corresponds to 18 double layers together with a (hydrophobic) capping layer for samples of 1d on aluminum used for the reflectometry experiments. The crystal structure dimensions of 1d (Figure 1) show a repeating distance of 23.3 Å for the interdigitated bilayer and a single layer distancescorresponding to the capping layersof 18.9 Å. For the 18 double layers plus one, this amounts to 420 Å, in good agreement with the reflectometry values. The neutron diffraction results also showed a low, broad Bragg peak, corresponding to a repeat bilayer spacing of about 23 Å. On this evidence, we think that multilayers have the structure depicted in Figure 6. Further evidence for close packing in these multilayers, albeit negative evidence, arose from attempted reptation into the multilayers. The interdigitation (reptation) of suitable, usually long straight chain, substrates into widely spaced surfaces has been described by Arduengo29 and his collaborators. They showed that when chains in SAMs were separated more widely than those in layers assembled, for example, from octadecanethiol, reptation of simple long chain substrates into the layer spaces occurred. We have carried out independent reptation studies based on the Arduengo systems confirming the general findings, but no reptation of long chain substrates bearing infrared active functional groups including ethyl laurate and resorcarene 1f could be observed in the resorcarene multilayers. Stability of Multilayers. The mechanical stability is such that, as mentioned above, sonication of the layers in hexane to the point at which the gold film peels from the gold-chromium-silicon base, does not disturb the film. When, however, the multilayer of 1d on a gold-thiol film of 1b is immersed in ethanol, two dramatic changes ensue: the contact angle of the film decreases from 92° to 28°sthat of the monolayer of 1bsand the infrared spectral intensity decreases some 40-fold to that of the monolayer (Figure 4). Clearly the multilayer has dispersed; solvation by ethanol of the individual resorcarene (28) For neutron reflectometry determinations, samples of calixarene 1d were pre-equilibrated with D2O. Infrared spectra showed removal of the O-H stretching band at 3250 cm-1 and its replacement by the O-D stretching band at 2540 cm-1. Full details will appear elsewhere: Davis, F.; Gerber, M.; Cowlam, N.; Stirling, C. J. M. Thin Solid Films, in press. (29) Arduengo, A. J.; Moran, J. R.; Rodriguez-Parada, J.; Ward, M. D. J. Am. Chem. Soc. 1990, 112, 6153.
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Figure 7. IR spectra of (a) a multilayer of 1d on 1b, (b) a multilayer of 1d on 1b with encapsulated glutaric acid, and (c) a monolayer of 1b with encapsulated glutaric acid (intensity scale ×25).
molecules is apparently more advantageous than interlayer hydrogen bonding and interdigitation. Aggregation of resorcarenes in bulk solution has been studied by Aoyama,30 and using vapor pressure osmometry, we have found average relative molar masses for 1d in hexane to be 7000 (heptamer or hexamer), in dichloromethane to be 3000 (trimer), but in ethanol to be 1100, corresponding to the monomer.31 It is not clear from our investigations as to what are the limiting factors in the accumulation of such multilayers, but two further observations are relevant to this phenomenon of multilayering. Resorcarene 1e with pendant chains of only five carbon atoms also forms multilayers. The compound is essentially insoluble in hexane, but deposition from toluene gives high water contact angles as before. Infrared spectroscopic intensities indicate somewhat fewer repeat units in the multilayer. Similar limitations have been observed in the extents of multilayering in Sagiv layers32 and in layers constructed by acid-base layering.33 Notably, such multilayering restrictions do not obtain when layers are assembled under pressure as in the Langmuir-Blodgett technique in which around 3000 layers can be assembled.34 We have interpreted the multilayering of resorcarenes such as 1d as a combination of hydrogen bonding with interdigitation. The failure of resorcarene 1f, incapable of hydrogen bonding, to form multilayers is consistent with this view. Additionally, by hydroboration of 1a we have made resorcarene 1g in which bulky substituents are attached to the ends of the tetrapodal chains. Unfortunately, we have not been yet able to obtain a crystal structure of this compound to reveal the dispositions of the chains. Tight interdigitation would not be expected and, consistently, the compound does not form multilayers. Adsorption and Reaction in Resorcarene Multilayers. The differential adsorption of a number of simple substrates by resorcarene monolayers has been discussed (30) Aoyama, Y.; Tanaka, Y.; Toi, Y.; Ogoshi, H. J. Am. Chem. Soc. 1988, 110, 634. (31) Vapor phase osmometry was performed with a Knower osmometer using benzil as standard. (32) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674. (33) Tredgold, R. H.; Winter, C. S.; El-Badawy, Z. I. Electron Lett. 1985, 21, 54. (34) Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937, 51, 964.
above. The formation of multilayers raises intriguing questions as to whether adsorption by such multilayers can occur and if so whether it is selective. Additionally, the question as to whether in such multilayers with their quasi-enforced juxtaposition of reactive functional groups, intralayer reactions can be observed. We have made initial studies of both of these possibilities. Treatment of the multilayer produced from the goldthiol monolayer of 1b and the unfunctionalized resorcarene 1d, with aqueous solutions of vitamin C showed no detectable (grazing angle FTIR) incorporation in the multilayer even after immersion for 200 times as long as needed for saturation in the monolayer. Evidently the molecule is not able to penetrate the tight multilayer structure. When, however, the gold-thiol monolayer of resorcarene 1b is treated successively with aqueous vitamin C and then with a hexane solution of resorcarene 1d, the multilayer forms as before (IR νC-H str intensity), and the vitamin C molecule is visible by its carbonyl stretching frequency, presumably buried at the level of the first layer. When this thin sandwich multilayer is immersed in ethanol, the multilayer disperses as before, but the vitamin C moiety remains as in the experiments with the monolayer. When the multilayer containing vitamin C is treated with dilute aqueous alkali as before, vitamin C is removed and the multilayer remains. Evidently small species such as hydroxyl ion can move freely within the multilayer structure. The question as to whether this freedom of movement is selective was dealt with in another series of experiments. The monolayer of resorcarene 1b was found to adsorb glutaric acid reversibly (above) and when the 1d on 1b multilayer was treated with aqueous glutaric acid, glutaric acid was adsorbed (Figure 7). Comparisons of the infrared intensities of the C-H antisymmetric bands with those ν(CdO) stretching bands of the encapsulated glutaric acid again showed a 1:1 correspondence. This suggested incorporation of one glutaric acid molecule for each resorcarene unit. The new structure is presumably a double sandwich (Figure 8) as it is difficult to envisage incorporation of glutaric acid among the hydrophobic chains of the resorcarene. Binding of glutaric acid is still weak; when the 1:1 glutaric acid/
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juxtaposition of the vinyl groups and the possibility of photopolymerization within such multilayers presented itself. The multilayers of 1a showed the accumulationdispersal phenomena as a function of solvent just as for 1d (Figure 10); however, when the multilayers from the vinylresorcarene were UV irradiated and subsequently washed with ethanol, only slight dispersion of the layers occurred as indicated by the IR spectra (Figure 10). Such behavior is consistent with cross-linking via the vinyl groups in the multilayer preventing dispersion. The multilayers of 1a adsorbed glutaric acid just as for multilayers of 1d; washout from the multilayers was unaffected by irradiation thus providing additional evidence for location of the acid between the resorcarene bowls. Conclusions Figure 8. Putative structure of glutaric acid encapsulated in a multilayer of 1d.
resorcarene multilayer is immersed in water, the acid escapes and the multilayer remains. This adsorption by the multilayer is selective; when 3,3- or 2,4-dimethylglutaric acids (7 and 8) are used, no incorporation into the multilayer can be detected. Aoyama has described36 the 1:1 complex of resorcarene 1d and glutaric acid, and we have found that multilayering with this complex also occurs. Layers are about 90 units thick, but the stoichiometry, as shown by relative νC-H and νCdO stretching intensities in the multilayer, shows less glutaric acid present than expected. Both the apparent greater thickness and lower inclusion of glutaric acid could however be artifacts of the orientation effect. If the complex forms multilayers with the side chains more tilted from the vertical, this would increase the νC-H signal and cause the effects seen. When the resorcarene complexes of the related acids, 7 and 8, are used, multilayering is less (7 and 23 layers, respectively) and little incorporation of either acid is detectable. Clearly these observations open up a wide range of possibilities for selective separation of closely related structures. Selective Multilayer Deposition. It was of interest to discover whether, in the formation of multilayers, there was any competition between related substrates for deposition. We checked this point by dipping SAMs of 1b in hexane solutions of equimolecular amounts of resorcarenes 1a and 1d. FTIR spectra of the resulting multilayers showed clearly (Figure 9) the preferential deposition of 1a as judged by the comparison of the multilayer spectra of each separately and the loss in the intensity of the sp3 C-H stretching frequencies of the pendant methyl groups of 1d. Resorcarene 1a is substantially less soluble in hexane than 1d and as deposition appears to be an equilibrium between multilayer and bulk solution as suggested earlier; this is a consistent result. Likewise, the shorter-legged resorcarene 1e on codeposition with a multilayer with 1d is also preferentially incorporated to judge by the relative intensities of the sp3 C-H adsorption at 2955 cm-1. In both examples of these competitive multilayerings, the resulting multilayer is thinner than that obtained with the pure resorcarene. Reasons for this behavior are not clear. Multilayer Polymerization. In resorcarene 1a, the pendant chains are terminated by vinyl groups. The resorcarene forms multilayers just as for resorcarene 1d. In such multilayers there is very probably enforced (35) Atis, L.; Dalcanale, E.; Du vosel, A.; Spera, S. J. Org. Chem. 1988, 53, 5475.
Resorcarenes immobilized in gold-thiol SAMs have been shown to interact differentially with a wide range of substrates. For strong binding, which appears to be covalent, substrates must possess a hydroxyl group to promote attachment and an acyl function capable of acyl transfer to the polyhydroxy rim of the resorcarene bowl. Resorcarenes with chains pendant from the bowl structure readily form multilayers by what appears to be a combination of hydrogen bonding and chain interdigitation, the latter suggestion being supported by X-ray data. Such multilayers are capable of discrimination in absorption of substrates such as between closely related glutaric acids. Multilayers formed from resorcarenes in which the pendant chains are terminated by vinyl groups become immune to dispersal in hydrogen-bonding solvents, presumably because of interchain polymerization. Experimental Section Resorcarene 1a (from EtOH-H2O) had mp >250 °C (lit.12 270 °C); 1d (from EtOH) had mp >250 °C (lit.17 270 °C); 1e (from MeOH) had mp >250 °C (lit.35 >315 °C); 1f (from light petroleum, bp 40-60 °C) had mp 131-133 °C (lit.17 132-132.5 °C). For neutron reflectometry measurements, the per-O-D derivative of resorcarene 1d was obtained by equilibration of the multilayer with D2O overnight. The IR O-H stretching band in 3250 1d was replaced by an O-D stretching band at 2540 cm-1. Resorcarenetetrathiol (1b). A solution of resorcarene 1a (1.1 g), thiolacetic acid (1.12 mL), and azoisobutyronitrile (0.133 g) in toluene was irradiated (λ ) 254 nm) in a Rayonet reactor for 1 h. Evaporation of the solvent, dissolution in ethanol, and precipitation with water gave crude thiol ester (65%) as a sandy colored solid showing νCdO 1675 cm-1. The thiolester (0.5 g), sodium hydroxide (1 g), and water (5 mL) were dissolved in methanol and stirred at 20 °C for 21 h under argon. Acidification (HCl) precipitated the crude resorcarene (82%), mp >250 °C from EtOH. 1H NMR (DMSO-d6) showed loss of signals at δ 5.0 and 5.7-5.8 ppm. Anal. Calcd for C68H104S4O8: C, 69.38; H, 8.84; S, 10.88. Found: C, 69.30; H, 8.63; S, 10.68. Treatment of resorcarene 1d in chloroform for up to a week with 2.5 M aqueous vitamin C and subsequent recovery showed no incorporation (FTIR) of vitamin C. There was no incorporation when the vitamin C solution was kept at pH values of 2, 7, and 13 nor when solid 1a was stirred with aqueous vitamin C or solid vitamin C stirred with a solution of the resorcarene. 1a (110 mg) and a 4-fold excess of vitamin C (70 mg) were dissolved in ethanol, and the solution was allowed to evaporate over several days. Crystals formed did not contain vitamin C (FTIR). Resorcarene (1g). Resorcarene 1a (690 mg) in dry tetrahydrofuran (10 mL) was treated with 0.5 M 9-BBN in tetrahydrofuran (10 mL) at 0 °C. After 20 h, solvent was removed and recrystallization of the residue from ethanol gave the resorcarene borane (0.5 g) whose FTIR spectrum (KBr disk) and 1H NMR (CDCl3) showed loss of the vinyl groups. The fast atom bombardment (FAB) mass spectrum showed a main peak at 1484 (calculated value 1528) and the 11B NMR spectrum gave a peak
Calix-4-resorcinarene Monolayers and Multilayers
Langmuir, Vol. 12, No. 22, 1996 5373
Figure 9. IR spectra of (a) a multilayer of resorcarene 1d, (b) a multilayer of resorcarene 1a, and (c) a codeposited multilayer of 1a and 1d.
Figure 10. IR spectra of (a) a monolayer of 1b, (b) a multilayer of 1d, (c) a multilayer of 1d after irradiation and wash-off, (d) a multilayer of 1a on 1d, and (e) multilayer (d) after irradiation and wash-off. (broad s at 18.3 ppm relative to BF3‚Et2O). Anal. Calcd for C100H156B4O8: C, 78.42; H, 9.29. Found: C, 78.15; H, 9.31. pKas of Resorcarene 1d. Ultraviolet spectra were determined on 0.1 mM solutions of the resorcarene in 50:50 (v/v) EtOH-H2O, at pHs 12.1, 9.2, 8.1, 7.0, and 3.1. Spectra at 7.0 and 3.1 were identical, with a single peak at 284 nm. Spectra were again identical at pHs 9.2 and 12.1 with a single peak at 300 nm. There was peak doubling at pH 8.1 suggesting 4-fold deprotonation between pH 7.0 and 9.2. 3-Acetoxybutyrolactone (2) was obtained by treatment of 3-hydroxybutyrolactone with acetic anhydride in pyridine.37 1H NMR: 4.1-4.7 (m, 3H), 2.1-2.8 (m, 2H), 2.0 (S, 3H). IR: νCdO str 1781, 1736. Self-Assembled Monolayers. n-Type silicon wafers polished on one side were cut to 50 mm × 20 mm and cleaned with piranha solution (care!). Chromium (5 nm) and then gold (50 nm) were deposited by thermal evaporation (Edwards E306A evaporator). The slides were coated by placing them in 0.25 mM ethanolic solutions of resorcarene 1b for 16 h under argon. The samples were then removed, rinsed repeatedly with ethanol, and dried in a stream of dry oxygen-free nitrogen.
Multilayers. These were assembled by placing substrates, e.g., monolayers of 1b on gold or aluminum-coated glass slides, in hexane solutions of resorcarenes 1d for 16 h (for concentrations see Table 2). For multilayers of 1a, 10 mM solutions were employed. In wash off experiments, the multilayers were immersed in pure solvent for up to 16 h. Adsorption by Mono- and Multilayers. Mono- and multilayers were immersed in aqueous or ethanolic solutions (Table 1) of adsorbate (0.01-10 mM) for up to 16 h and then washed quickly with pure solvent. For desorption experiments, the films containing adsorbates were immersed in pure solvent (H2O or EtOH) for 16 h. Multilayers of the adducts of glutaric acids (entries 6, 7, and 8 of Table 1) with resorcarene 1d, prepared as described previously,36 were assembled on aluminum-coated glass slides as for pure resorcarene samples. Ninety layers of the adduct with 6 were readily obtained and showed stoichiometric carbonyl (36) Tanaka, Y.; Kato, Y.; Aoyama, Y. J. Am. Chem. Soc. 1990, 112, 2807. (37) Jelinek, V. C.; Upson, F. W. J. Am. Chem. Soc. 1938, 60, 355.
5374 Langmuir, Vol. 12, No. 22, 1996 adsorption. Seven and 23 layers of the adducts of 7 and 8 were deposited but showed only weak carbonyl absorption for 7 and none for 8. Irradiation of multilayers of resorcarenes 1a and 1d was with a UVGL-58 mineralight (UVP Inc.)(λ ) 254 nm) for 1 h. Instrumentation. Infrared spectra were taken with a PerkinElmer 1725X instrument fitted with an MCT detector and a Harrick reflection accessory (Spectra-Tech) for the grazing incidence mode. The instrument was purged with argon. Contact angle measurements were made with a cathetometer/ graticule set-up. NMR spectra were determined with either Bruker AMX2-400 or AC-250 spectrometers. X-ray photoelectron spectra were measured on a VG Clam 2 spectrometer. X-ray Diffraction. For resorcarenes 1d and 1e threedimensional, low-temperature X-ray data were collected on an Enraf Nonius FAST area detector. The 9878 independent reflections (of 14 448 measured) for which |F||σ(|F|) > 4.0 were corrected for Lorentz and polarization effects, but not for absorption. The structure was solved by direct methods and refined by blocked cascade least squares on F2. Hydrogen atoms were included in calculated positions and refined in riding mode. Refinement converged at a final R ) 0.0776 (wR2 ) 0.1842 for all 14448 unique data, 829 parameters, mean and maximum δ/σ 0.000, 0.000), with allowance for the thermal anisotropy of all non-hydrogen atoms. Minimum and maximum final electron density -0.382 and 0.640 e Å-3. A weighting scheme w ) 1/[σ2(Fo2) + (0.1105P)2 + 0.00P] where P ) (Fo2 + 2*Fc2)/3 was used in the latter stages of refinement. Complex scattering factors
Davis and Stirling were taken from the program package SHELXL93 as implemented on the Viglen 486dx computers. Crystallization of samples of 1d and 1e from EtOH or MeOH gave crystals with loosely bound solvent. The crystals disintegrated on evaporation of solvent, and determinations were made at 140K. Full details will be published elsewhere.38
Acknowledgment. We thank Dr. N. Cowlam and Mr. M. Gerber for X-ray and neutron reflectometry measurements, Professor M. B. Hursthouse and Mr. H. Adams for X-ray crystallography, Dr. T. Richardson and Ms. V. C. Smith for Langmuir-Blodgett film preparation, Ms. S. Bradshaw for vapor phase osmometry determinations, Professor H. Dahn (Lausanne) for his interest and for samples of compounds at entries 3, 4, 5, 17, 18, and 20 of Table 1, Professor D. Reinhoudt (Twente) for helpful advice and information, Dr. J. C. Anderson for suggesting hydroboration of resorcarene 1a, Dr. B. F. Taylor for boron NMR determinations, Dr. Robert Short and Dr. L. O’Toole for X-ray photoelectron spectroscopy determinations, and the University of Sheffield, the Royal Society, and the Engineering and Physical Sciences Research Council for the support of this work. LA960543F (38) Adams, H.; Davis, F.; Hursthouse, M.; Maliki, A.; Stirling, C. J. M. To be submitted for publication in Acta Crystallogr.
