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Langmuir 1998, 14, 4180-4185
Order and Structure in Langmuir-Blodgett Mono- and Multilayers of Resorcarenes Frank Davis, Andrew J. Lucke, Katharine A. Smith, and Charles J. M. Stirling* Department of Chemistry, and Centre for Molecular Materials, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K. Received January 20, 1998. In Final Form: April 28, 1998 Langmuir-Blodgett (LB) films of 19 resorcarenes and pyrogallenes have been assembled. It has been shown that a wide variety of structure, in both the bowl rim of these molecules and in the pendant legs, can be tolerated in the deposition process, except when these lead to very rigid monolayer structures. The behavior of these resorcarenes and pyrogallenes under pressure on the water surface has been studied through their Π-A curves, which are presented and interpreted. Functionalization of the bowl-rim hydroxyl groups notably reduces collapse pressures and increases the surface area. This is consistent with destabilization of the cone conformer of these molecules. LB multilayers have been examined by low-angle X-ray diffractometry, and the results reveal the significance of interdigitation of pendant legs, the disruption of multilayer packing by bulky substituents, and the requirement of long straight hydrophobic chains for efficient multilayering.
Introduction The nature and structure of ultrathin films of organic compounds is a very important contemporary aspect of nanochemistry. Studies of such thin films can offer insights into subtle inter- and intramolecular interactions, and these thin films have widespread applications including as sensing elements,1 as pyroelectrically active surfaces,2 in corrosion prevention,3 in selective ionbinding,4 and in modification of surface hydro- and oleophobicity.5 Calixarenesscyclic aldehyde/phenol oligomersshave been extensively investigated in the past decade.6-8 The interest in these compounds resides in their great structural versatility coupled with their interactions with metal ions6 and with both neutral and charged organic species.6 The archetypal simple calixarene (structure A, Chart 1) is modified (structure B) in the resorcarenes and pyrogallenes, giving a bowl-like structure surrounded by 8 (resorcarenes) or 12 (pyrogallenes) hydroxyl groups at positions 1, 2, and 6. Additionally, the benzylic carbons attached at positions 3 and 5 can bear chains which control the conformations of the bowl-like structures. Ultrathin films of organic compounds have been prepared by essentially two procedures. Langmuir-Blodgett (LB) films are assembled by using amphiphilic molecules mobile on a suitable surface (usually water).9 The surface pressures of such films can be controlled over wide ranges that can be accurately measured. The relationship between the surface area per molecule and pressure (the (1) Schierbaum, K. D.; Weiss, T.; VanVelzen, E. U. T.; Engbersen, J. F. J.; Reinhoudt, D. N.; Gopel, W. Science 1994, 265, 1413. (2) Richardson, T.; Greenwood, M. B.; Davis, F.; Stirling, C. J. M. Langmuir 1995, 11, 4623. (3) Ramachandran, S.; Tsai, B.-L.; Blanco, M.; Chen, H.; Tang, Y.; Goddard, W. A. Langmuir 1996, 12, 6419. (4) Davis, F.; O’Toole, L.; Short, R.; Stirling, C. J. M. Langmuir 1996, 12, 1892. (5) Davis, F.; Stirling, C. J. M. U.K. Pat. PCT/GB97/01036, 1997. (6) Gutsche, C. D. Calixarenes; Monographs in Supramolecular Chemistry; Royal Society of Chemistry: 1989. (7) Calixarenes, a Versatile Class of Macrocyclic Compounds; Vicens, J., Bo¨hmer, V., Eds.; Kluwer: Dordrecht, 1991. (8) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713. (9) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991.
Chart 1. Generic Structures of Calixarenes and Resorcarenes
Π-A curve) can reveal interactions between the amphiphilic molecules in the monolayer. Multilayers of known thickness and molecular orientation can be assembled. The other general technique based on reaction between the substrate and the organic material leads to self-assembled monolayers (SAMs).9 These are often gold-thiol monolayers, but other interactions such as oxygen-silicon can be employed.9 In this paper we report on the formation of LB films of a wide variety of resorcarenes and pyrogallenes and we compare the results with earlier studies from our laboratories of the SAMs of related compounds.10 Previous relevant work has been carried out on monolayers of resorcarene 1 by Kurihara11 and co-workers, who studied their interactions with subphase constituents, notably sugars, with comparisons being drawn with bulk-phase interactions. In addition, selective binding of metal ions by resorcarene monolayers on water has been demonstrated.12 Results and Discussion Langmuir isotherms from monolayers of the series of resorcarenes (Charts 2 and 3) are shown in Figures 1-4, 6, and 7. Resorcarene 4 was too insoluble for study. (10) Adams, H.; Davis, F.; Stirling, C. J. M. J. Chem. Soc., Chem. Commun. 1994, 2527. (11) Kurihara, K.; Ohto, K.; Tanaka, Y.; Aoyama, Y.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 444. (12) Moreira, W. C.; Dutton, P. J.; Aroca, R. Langmuir 1995, 11, 3137.
S0743-7463(98)00078-X CCC: $15.00 © 1998 American Chemical Society Published on Web 06/30/1998
LB Mono- and Multilayers of Resorcarenes Chart 2. Structures of Resorcarenes 1-16
Resorcarenes 1 and 2 have similar areas per molecule, approaching 1.4 nm2 at high pressures (Figure 1). CoreyPauling-Koltun (CPK) models confirm that this area corresponds with that of the bowl-like headgroup. When the alkyl chains are extended to 17 atoms (resorcarene 3), the area per molecule drops well below 1.4 nm2 to about 0.7 nm2 (Figure 1), indicating possible formation of a bilayer. This may be the result of energetically favored interdigitation of the long alkyl side chains, as previously reported.13 This phenomenon has been seen in selfassembled multilayers (SAMs) of resorcarenes with C11 pendant chains such as 1 and in their crystal structures. However this would lead, if carried through to the LB film, to a multilayer with a surface covered with phenolic hydroxyl groups. The water contact angles for this film are in excess of 90°, which indicates that if this bilayer is formed on the water surface and transferred, there must be some rearrangement process occurring. Resorcarene 5 contains a 21-atom chain with a sulfur atom at position 11. The monolayer behavior of this resorcarene is closely analogous to that of simple resorcarenes 1 and 2 (Figure 1), with none of the apparent doubling shown by the C17 chain resorcarene 3. When position 2 of the resorcarene is substituted, modification of the monolayer behavior occurs. Alkenyl (13) Davies, F.; Stirling, C. J. M. Langmuir 1996, 12, 5365.
Langmuir, Vol. 14, No. 15, 1998 4181 Chart 3. Structures of Bridged Resorcarenes 17-19
pyrogallene 6 showed analogous behavior to that for resorcarene 2 (Figure 2), but it is not clear why the area per molecule is 20% larger. However for resorcarene 7, the 2-methylated version of 2, the behavior is closely similar. Monolayers of resorcarenes 8 and 9, which contain hydrophilic headgroups, display steep isotherms (Figure 3), indicating good packing of the headgroups of the resorcarenes which now have larger bowls. For 8, 9, and 10, the collapse pressures are high, but for resorcarene 11 with an aryl azo substituent at position 2, a larger area is accompanied by a lower collapse pressure. The bowlrim structure is less hydrophilic and apparently more readily lifts from the water surface, causing collapse. The hydrophilic prolyl derivative 9 and the bulky mercuric acetate derivative 10 show surface areas per molecule consistent with the structures. When the ends of the tetrapodal chains are made hydrophilic, as in the chain-terminal hydroxyl derivative 12 (Figure 4), the picture changes. The collapse pressure is much lower, and the film, after the initial compression to a surface area of 2 nm2, is much more compressible. We attribute this behavior to the inclusion, in the surface, of the pendant hydroxyl groups (Figure 5). At pressures above the plateau, the surface areas are consistent with formation of a bilayer. Apparently, either the pendant hydroxyl groups or the bowl-like headgroups or both are removed from the water surface (Figure 5). The bowllike headgroup (1.4 nm2) is larger in cross-section than the four pendant hydroxyl chains (about 0.8 nm2), and the area per molecule above the plateau is intermediate between these two values. This is best explained by the monolayer consisting of a 50:50 mix of the two perhaps alternating orientations (Figure 5b). Figure 4 shows the effect of compression and expansion of a monolayer of resorcarene 12. There is considerable hysteresis in the plateau section of the Π-A curve, but subsequent expansion and compression follow the same Π-A curve. This shows that the transition between the two phases of the monolayer of 12 is reversible upon reexpansion of the monolayer and reproducible from one compression to the next. Compression above 30 mN m-1 caused the appearance of visible aggregates in the
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Figure 1. Π-A curves of resorcarenes 1-3 and 5.
Davis et al.
Figure 4. Π-A curves for resorcarene 12: first compression (0); first expansion (]); second compression (O). Each cycle of compression and expansion takes 5 min.
arrangements in the monolayer, perhaps with resorcarenes lying on their sides, as found for some calix-8arenes.4 In the case of 13, with bulky and very hydrophobic silyloxy groups, both collapse pressure and area per molecule are consistent with the reduced amphiphilicity. Resorcarenes 17-19 (Figure 7) have rigid cavitand structures and display steep isotherms, indicating good headgroup packing and surface areas per molecule are consistent with formation of a monolayer. Resorcarene 19 has a significantly larger surface area than the others, possibly adopting a more splayed structure to minimize unfavorable dipole-dipole interactions. Figure 2. Π-A curves of resorcarenes 6 and 7.
Figure 3. Π-A curves of resorcarenes 8-11.
monolayer and led to the Π-A curve becoming irreproducible, indicating irreversible collapse to form particulate aggregates. When the bowl-rim hydroxyl groups are modified, by silylation (resorcarene 13) or acetylation (resorcarenes 14, 15, 16), severe changes in the isotherms (Figure 6) occur. The surface areas per molecule increase sharply, and the collapse pressures decrease. The increase in surface area is consistent with destabilization of the cone conformer in favor of the flattened cone,14 and the lower collapse pressures are consistent with the less hydrophilic headgroups. The acetylated phenyl resorcarene 15 showed normal monolayer behavior although the surface area per molecule at collapse is lower than that for the alkenyllegged acetate 14. This is possibly due to different packing (14) Palmer, K. J.; Wong, R. Y.; Jurd, L.; Steven, K. Acta Crystallogr. 1976, 32B, 847.
Structures of Resorcarene and Pyrogallene LB Multilayers Low-Angle X-ray Studies. We have prepared LB films of the resorcarenes, containing 51 layers and investigated the structures of these films by low-angle X-ray diffractometry. The results are shown in Table 1. These measurements reveal important new information about such multilayers. The most striking was the very large improvement in order, as shown by the number of Bragg peaks, obtained when the hydroxyl groups were acetylated. 1H NMR studies15 have shown that resorcarene acetates are much more flexible in solution than the parent hydroxyl compounds, and this flexibility appears to enable them to form more ordered multilayers. In all cases, the repeat spacing is too small for there to be two fully extended separate resorcarenes in each bilayer. Either tilting or interdigitation of the legs is occurring. The repeat spacing of an interdigitated bilayer of 1 is known to be 2.32 nm from X-ray crystallographic studies,10 much less than that measured for its LB film (2.92 nm). Resorcarene 3 gave a poorly resolved Bragg peak, indicating the structure is disordered and providing more evidence for there being rearrangement during or after transfer of the monolayer to the substrate. The multilayers of the pyrogallenes 6 and 8 had structures similar to those of the resorcarenes. Insertion at the 2-position of a methyl group in 7 produced noticeable lowering of the multilayer repeat spacing. This could be a function of the larger surface area per molecule, allowing more splaying of the legs or efficient interdigitation. Resorcarene 10 gave a very low broad Bragg peak, indicating that the presence of the bulky HgOAc group disturbed multilayer packing and increased spacing. Resorcarene 11 showed a similar effect. No Bragg peaks were observed for multilayers of 15, indicating that the (15) Ho¨gberg, A. G. S. J. Am. Chem. Soc. 1980, 102, 6046.
LB Mono- and Multilayers of Resorcarenes
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Figure 5. Possible low- and high-pressure (a-c) arrangements of resorcarene 12 on the water surface.
Figure 6. Π-A curves of hydroxy-modified resorcarenes 1316.
presence of long hydrophobic chains is necessary to form ordered multilayers. Resorcarene 17, with interlinked bowls, also gave no Bragg peaks, perhaps due to the rigidity of its monolayer, causing formation of poorly ordered multilayers. Resorcarene 12 showed novel behavior in that it could be deposited at two separate pressures, above and below the plateau to give quite different multilayers. At 15 mN m-1, the resorcarene deposited to form a Z-type multilayer which gave a single broad Bragg peak, indicating a repeat spacing of 3.09 nm. However at 25 mN m-1, the resorcarene deposited a Y-type film to give a multilayer which gave a single sharp Bragg peak showing a repeat spacing of 1.50 nm. Rearrangement of Z-type LB films to form Y-type films is well-known and, from this repeat spacing, appears to be occurring to give a bilayer of fully extended resorcarenes for the loosely packed films deposited at 15 mN m-1. The films deposited at 25 mN m-1 give a more
Figure 7. Π-A curves of bridged resorcarenes 17-19.
ordered structure with a smaller spacing. In normal Y-type films the repeat unit is a bilayer where molecules in adjoining layers are of opposite orientation. However in a film deposited from a monolayer in which the molecules are already in a 50:50 mixture of orientations (Figure 5b), adjoining layers are identical. Therefore the repeat unit is a monolayer rather than a bilayer, and a smaller spacing, approximately that of a fully extended single molecule, was measured. XPS and Infrared Studies of the Multilayers. IR and XPS determinations were carried out on selected films. LB films of compound 11 have already been well characterized.16 Since the mercury acetate derivative 10 had such a poorly resolved X-ray structure, IR and XPS studies were made to check whether the compound had reacted on the water surface. IR studies showed that the HgOAc (16) Omar, O.; Ray, A. K.; Davis, F.; Hassan, A. Supramol. Chem. 1997, 4, 417.
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Table 1. Low-Angle X-ray Diffractometry of Resorcarene LB Multilayersa resorcarene 1 2 3 5 6 7 8 9 10 11 12 13 14 15 16 17 a
deposition pressure (mN m-1)
no. of Bragg peaks
40 1 40 4 40 1 40 2 40 1 40 1 40 2 15-45 no deposition 35 1 25 1 15 1 25 1 unstable monolayer 25 7 15 0 25 10 15 0
bilayer spacingb (nm) 2.92 2.60 3.84 3.68 2.94 2.18 2.90 3.04 3.18 3.09 1.50 2.50 3.79
51 layers. b Derived from Bragg peaks.
Figure 9. IR spectra of resorcarene 16 as a KBr disk (upper spectrum) and 9 LB layers in transmission on CaF2 (solid line) and in reflection on Au/glass (dashed line).
Figure 8. XPS spectrum of an LB film of resorcarene 10.
group was still present (similar peaks for LB and bulk samples at 1575 cm-1), and XPS studies (Figure 8) showed the presence of Hg in the multilayer in approximately the expected concentrations. IR studies on the two most ordered multilayers, those of 14 and 16, were performed. Figure 9 shows spectra for an LB film of 16 in reflection and transmission and as a KBr disk in transmission. From the spectra, it was calculated17 that the side chains are inclined at an average angle of 55° to the surface and the carbonyl groups are inclined at an angle of 70° to the surface. This result combined with the bilayer spacing indicates that the side chains are tilted but not interdigitated. Similar values of 60° for the side chains and 65° for the carbonyl groups were found for 14. High values for the methylene stretching frequency (2925 cm-1) show there are some gauche conformers present and indicate that the side chains are liquidlike in behavior. An LB film of resorcarene 12 with hydroxyl-terminated legs showed even higher side chain disorder (2928 cm-1), as predicted. Contact Angle Measurements. Sessile contact angles of water were measured on all the LB films deposited. They all gave contact angles in excess of 90°, indicating a reasonably well-ordered hydrocarbon surface, as expected. There were two exceptions. Resorcarene 12 gave a contact angle of 90° when deposited at 15 mN m-1 but when deposited at 25 mN m-1 gave a contact angle of 62°. This indicates a hydrocarbon surface for the LB film (17) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700.
formed at the lower pressure and the possibility of hydrophilic hydroxyl groups in the surface when a higher deposition pressure is used. The contact angle is similar to the stable value obtained for a gold-thiol monolayer terminated with hydroxyl groups.18 Resorcarene 15 had a contact angle of 70°, which would be consistent with a structure in which the resorcarene molecules are lying on their sides, giving a surface presenting both phenyl and acetate groups. Conclusions A wide range of resorcarene structures could be deposited as LB multilayers, providing the monolayers were not too rigid. Numerous substituents can be incorporated without preventing deposition. Resorcarenes in which hydroxyl groups have been acetylated give far superior LB films. This may be due to increased structural flexibility. From the X-ray diffraction and IR studies of resorcarenes 14 and 16, it appears that these form extremely well-ordered LB multilayers with the side chains tilted and not interdigitated. The other compounds have similar structures although the very low bilayer spacing of 7 may be a result of interdigitation. Thin films of these compounds have a potential sensor application due to the wide range of structures available, their known thermal stability, and their ability to form complexes with various guests. These are under investigation in our laboratories, and the work will be reported later. Experimental Section Resorcarenes 1, 2,6,15 3,19 4,6,15 5,20 6, 7, 8,6,15 9,2111,16 14,20 15,6,15 16, 17,20 and 1822 (Charts 1-3) were synthesized as previously described. (18) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156. (19) Tanaka, Y.; Kobuke, Y.; Sokabe, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 693.
LB Mono- and Multilayers of Resorcarenes Tetramercuriacetoresorcarene (10). Resorcarene 1 (1.95 g; 1.8 mmol), mercuric acetate (2.52 g; 7.9 mmol), and chloroform (90 mL) were combined and stirred overnight. The mixture turned pale red, and the product was isolated by precipitation with ethanol, filtration, and recrystallization from chloroform to yield resorcarene 10 (3.30 g; 88%) as a pink powder. IR (KBr) 1575 cm-1 (acetate). 1H NMR (CDCl3, 250 MHz) δ 9.1-9.0 (br s, 2H, ArOH), 7.2 (s, 1H, ArH), 4.3 (t, 1H, RCHAr2), 2.2 and 1.3 (br s, 20H, CH2), 2.1 (s, 3H, OAc), 0.9 (t, 3H, CH3). Anal. Calcd for C80H120O12Hg4: C, 46.3; H, 5.9. Found: C, 46.6; H, 5.7. Hydroxyl-Terminated resorcarene (12). Concentrated hydrochloric acid (3.2 mL) was added slowly to a stirred solution of resorcinol (1.97 g; 17.9 mmol) and 1,1-dimethoxy-11-undecanol23 (4.15 g; 17.9 mmol) in ethanol (30 mL) at 0 °C. The reaction mixture was kept at 55 °C for 18 h under argon. After cooling, the yellow solution was poured into water (250 mL) to yield a yellow precipitate. This was collected by filtration, washed with warm water (6 × 100 mL), and dried (4.58 g; 92%). The crude material was recrystallized from methanol/chloroform to yield resorcarene 12 (3.87 g; 78%), mp 239 °C (dec). IR (Nujol mull) 3217, 1620, 1504, 1296, 1223, 1168, 1084, 903, 844 cm-1. 1H NMR (D6-acetone, 250 MHz) δ 8.51 (br s, 2H, ArOH (exchanges with D2O)), 7.57 (s, 1H, ArH), 6.25 (s, 1H, ArH), 4.32 (t, J ) 7.9 Hz, 1H, RCHAr2), 3.57 (m, 2H, RCH2OH), 2.91 (s, 1H, OH (exchanges with D2O)), 2.32 (m, 2H, RCH2CHAr2), 1.51 (m, 2H, RCH2CH2OH), 1.27 (s, 14H, CH2). FAB-MS (m/z) 1114 (8%, M+ + 1). Anal. Calcd for C68H104O12‚H2O: C, 72.2; H, 9.4. Found: C, 71.9; H, 9.6. O-Octatrimethylsilylresorcarene (13). To a flame-dried flask under argon containing 2 (3.05 g; 2.8 mmol) and THF (300 mL) was added triethylamine (11.65 g; 115.0 mmol). The mixture turned bright red, and chlorotrimethylsilane (8.68 g; 56.0 mmol) was added dropwise over 2 h. The mixture was stirred at 50 °C for 16 h and then cooled and quenched with water. The mixture was extracted with ether (500 mL), and the ether extract was washed with brine (4 × 250 mL), dried, and evaporated under reduced pressure. The product was recrystallized from ethanol and dried at 1 mmHg for 24 h at 20 °C to yield 13 (2.40 g; 57%) as fine white needles, mp 77-78 °C. IR (KBr) 2926, 2853, 1638, 1607, 1249, 1129, 1103, 917, 844, 751 cm-1. 1H NMR (CDCl3, 250 MHz) δ 7.15 (s, 2H), 6.28 (s, 2H), 6.18 (s, 2H), 6.02 (s, 2H), 5.85 (m, 4H), 4.96 (m, 8H), 4.40 (t, 4H, J ) 6.3 Hz), 2.04 (m, 8H), 1.75 (m, 8H), 1.25 (m, 48H), 0.35 (s, 12H), 0.04 (s, 12H). FABMS (m/z) 1618. Anal. Calcd for C92H160O8Si8: C, 68.3; H, 10.0. Found: C, 68.4; H, 10.1. Quinone-Bridged Resorcarene Cavitand (19). A flamedried flask was charged with resorcarene 2 (4.03 g; 3.8 mmol), 2,3-dichloronaphthoquinone (4.81 g; 14.8 mmol), and dry DMSO (250 mL) under argon. The reaction solution was stirred until it was homogeneous. Cesium carbonate (5.63 g; 17.3 mmol) was added, and the reaction was stirred at 20 °C for 72 h. The product was precipitated from the mixture by addition of water and the resulting yellow solid collected and washed with water (1.5 L) at 80 °C. The product was dried at 1 mmHg for 24 h at 20 °C to yield 19 (4.53 g; 71%) as a yellow powder, mp 180-182 °C (dec). IR (KBr) 4442 (bound H2O), 2925, 2853, 1679, 1607, 1578, 1489, 1320, 1262, 965, 906 cm-1. 1H NMR (CDCl3, 250 MHz) δ 8.16 (m, 8H), 7.71 (m, 8H), 7.65 (s, 2H), 7.15 (s, 2H), 7.10 (s, 2H), 6.23 (s, 2H), 5.78 (m, 4H), 4.95 (m, 8H), 4.23 (t, 4H, J ) 6.4 Hz), 2.1-1.9 (m, 16H), 1.5 (bs, 4H, H2O), 1.3-1.1 (m, 48H). Anal. Calcd for C108H104O16‚2H2O: C, 76.6; H, 6.4. Found: C, 76.8; H, 6.3. Deposition of Resorcarenes. LB depositions were made on a trough previously described.24 Solutions of the resorcarenes (1 mg/mL) were prepared in either chloroform or 10% ethanol/ chloroform, depending on solubility. All monolayer and multilayer experiments were carried out on a pure water (Millipore) subphase, pH ) 5.0-5.5 at 20 °C with no added salts. The (20) Van Velzen, E. U. T.; Engbersen, J. F. J.; Reinhoudt, D. N. Synthesis 1995, 8, 989. (21) Matsushita, Y.; Matsui, T. Tetrahedron Lett. 1993, 34, 7433. (22) Van Velzen, E. U. T. Ph.D. Thesis, University of Twente, Netherlands, 1994. (23) Trost, B. M.; Kulawiec, R. J. J. Am. Chem. Soc. 1993, 115, 2027. (24) Tredgold, R. H. Rep. Prog. Phys. 1987, 50, 1609.
Langmuir, Vol. 14, No. 15, 1998 4185 substrates for LB film deposition were glass microscope slides coated with 100 nm of freshly evaporated aluminum, CaF2 plates, or n-type silicon wafers. They were cleaned prior to use with aqueous NaOH/H2O2 and were then exposed to hexamethyldisilazane vapor, giving a hydrophobic surface.24 Resorcarenes were deposited as multilayers on silicon wafers, CaF2, or aluminum-coated glass microscope slides. Thirty minutes was allowed to elapse after spreading the monolayers. They were then compressed to the required pressure (Table 1) for 10 min before deposition, as this was found to improve deposition behavior. The rate of deposition was 5 mm min-1, and samples were dried for 20 min after the first deposition and 5 min after subsequent depositions. Shorter drying times lead to return of the resorcarene to the water surface. For the hydroxyl resorcarenes 1-3, 6, 7, and 8, a deposition pressure of 40 mN m-1 was required, as lower pressures lead to incomplete and irreproducible transfer. Apart from poor deposition on the first downstroke, deposition was Y-type, and up to 51 layers could easily be deposited. Resorcarene 10 proved much more difficult to transfer. At 30 mN m-1 the first layer peeled off on the second immersion of the substrate. At 40 mN m-1 the film became extremely rigid, as shown by movement of the barriers, causing movement of the Wilhelmy plate, and no transfer could be observed. At an intermediate pressure (35 mN m-1) poor Z-type deposition was observed. Fifty layers were deposited to give a film which was visually patchy. Resorcarene 11 was easily deposited at 25 mN m-1 to give good quality multilayers, as previously reported.16 Resorcarenes 14 and 16 were also deposited easily, albeit at a lower pressure, to form good-quality multilayers. Z-type deposition was observed for 9, but even at high pressures, return to the water surface on the downstroke was observed and multilayers could not be assembled. Monolayers of 13 were too unstable at the air-water interface to permit formation of multilayers. Resorcarenes 12 and 15 could be deposited as Z-type films at 15 mN m-1. Higher pressures caused collapse of the monolayer for 15, but 12 formed a stable monolayer at 25 mN m-1 and gave good Y-type deposition at this pressure. Resorcarene 17 formed a rigid monolayer on water but could be deposited as a Y-type multilayer at 15 mN m-1, although transfer ratios could be irregular. The other bridged resorcarenes 18 and 19 formed extremely rigid monolayers on the water surface, and no evidence of deposition could be found for a variety of pressures, dipping speeds, and drying times. X-ray Diffraction and Infrared and Contact Angle Measurements. These were carried out using a Philips PW1380 horizontal diffractometer fitted with a graphite crystal monochromator and using the Cu KR line (wavelength 0.1542 nm). IR spectra were measured in reflection on a Perkin-Elmer 1725X spectrometer with a Harrick reflection accessory (angle of incidence 80°) and a MCT detector. Spectra shown are the average of 100 scans. X-ray photoelectron spectra were acquired on a VG Clam 2 spectrometer, using Mg KR X-rays (1253.6 eV). The X-ray source was operated at 100 W, with the pass energy of the analyzer set to 100 eV. This configuration sacrifices energy resolution for the sake of sensitivity and was found necessary due to the low relative abundance of some of the elements present in the sample. The takeoff angle was either 30° or 70° relative to the sample surface; this gives a sampling depth of approximately 3 or 5 nm. Peak areas were converted into atom number ratios using sensitivity factors obtained from XPS analysis of standard compounds of known composition. Contact angles were determined by placing five 1 µL drops of water on the surface of the LB film and measurement of CCD camera images manipulated with ClarisDraw software.
Acknowledgment. We thank the University of Sheffield and EPSRC for funding. Dr. Z. Ali-Adib, Dr. M. Bardasova, and Professor P. Hodge for X-ray measurements, Mr. A. M. Leeson and Dr. R. Short for XPS measurements, and Dr. T. Richardson for help with LB depositions. LA980078H
