Octopus molecules at the air-water interface. Mechanical control over

Langmuir 1991, 7,982-987

982

Octopus Molecules at the Air-Water Interface. Mechanical Control over Tentacle OrientationfJ Mark Conner, Ivo Kudelka,2and Steven L. R e g e n * Department of Chemistry and Zettlemoyer Center for Surface Studies, Lehigh University, Bethlehem, Pennsylvania 18015 Received October 24, 1990 Alkylation of 37,38,39,40,41,42-hexahydroxycalix[6]arenewith a series of brominated poly(ethy1ene glycol) monomethyl ethers [i.e., CHSO(CH2CH20),CH2CH2Br, where n = 0, 1, 2, and 31 yields octopus molecules la,1 b, IC, and Id,respectively. Examination of the monolayer properties of 1b and IC reveals that their polyether 'tentacles" lie at the air-water interface at low surface pressures and are forced down into the subphase when the film is compressed. Similar behavior has been observed for Id,but only over a saturated aqueous NaCl subphase. In the condensed state, calixarenes la-ld have their tentacles tucked beneath their aromatic core. For calix[6]arenela,the polyether moieties have been found to be too short to be moved back and forth between surfaceof water and the subphase. Introduction of bulky p-tert-butyl groups onto the upper rim of the calix[6]areneframework significantly reduces intermolecular polyether interactions. Qualitatively, similar behavior has been found with analogous calix[4]arene-based octopi. Introduction Molecules that contain a central hydrocarbon core, with polyether ligands extending outward from it, have been termed "octopus molecule^".^ Typical hydrocarbon frameworks that have been used to construct molecular octopi include benzene, naphthalene, biphenyl, and certain calix[r~]arenes.~-~ Two illustrative examples are shown in Chart I. Although considerable attention has focused on the conformational properties and binding characteristics of octopus molecules in solution and in the solid state, their monolayer behavior, at the air-water interface, remains undefined. We have become particularly interested in octopus molecules at the air-water interface, with the view that compression should provide a novel means for controlling the orientation of their tentacles. Our reasoning is based on the known monolayer behavior of poly(ethy1eneoxide) (PEO) and also of certain calix[nlarene-derived surfactants.lD-16Poly(ethy1ene oxide) is an amphiphilic M. Fowkes. (1) Supported by the Division of Basic Energy Sciences and the Department of Energy (DE-FG02-85ER-13403)and by Air Products and Chemicals, Inc., Allentown, PA. (2) On leave from the Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, Prague, Czechoslovakia. (3) Vogtle, F.; Weber, E. Angew. Chem., Znt. Ed., 1974, 23,814. (4) MacNicol, D.; Wilson, D. Chem. Commun. 1976. 494. (5) Fornasier, R.; Montanari, F.; Podda, G.; Tundo,'P. Tetrahedron Lett. 1976, 17, 1381. (6) Bocchi, V.; Foina, D.; Pochini, A.; Ungaro, R. Tetrahedron 1982, 38,373. (7) Ungaro, R.; Pochini, A.; Andreetti, G. D.; Ugozzoli, F. J. oflnclusion Phenom. 1985,3,409. (8) Taniguchi, H.; Nomura, E. Chem. Lett. 1988, 1773. (9) Molecules that closely resemble octopus molecules have also been reported: (a) Menger, F. M.; Takeshita, M.; Chow, J. F. J . Am. Chem. SOC.1981,103,5938. (b) Suckling, C. J. Chem. Commun. 1982,661. (c) Murakami, Y.; Nakano, A.; Miyata, R.; Matsuda, Y. J . Chem. SOC.,Perkin Trans. 1 1979, 1669. (d) Murakami, Y.; Nakano, A.; Akiyoshi, K.; Fukuya, K. J. Chem. SOC.,Perkin Trans. 1 1981,2800. (e) Freer, A. A.; Gall, J. H.; MacNicol, D. D. Chem. Commun. 1982,674. (f) Sabbatini, N.; Curadigli, M.; Mecati, A.; Balzani, V.; Ungaro, R.; Ghidini, E.; Casnati, A.; Pochini, A. Chem. Commun. 1990,878. (10) Shuler, R. L.; Zisman, W. A. J . Phys. Chem. 1970, 74, 1523. (11) Kawaquichi, M.; Komatsu, S.; Matsuzumi, M.; Takahashi, A. J . Colloid Interface Sci. 9184, 102, 356. (12) Sauer, B. B.; Yu, H.; Yazdanian, M.; Zografi, G.; Kim, M. W. Macromolecules 1989, 22, 2332. (13) Markowitz, Mi A.; Bielski, R.; Regen, S. L. J . Am. Chem. SOC. 1988, 1 IO, 7545. t Dedicated to the memory of Professor Frederick

/

pcW,h

\

7

callx[6]arene-based octopus

Chart I

benrene-based octopus

polymer that forms stable monolayers a t the air-water interface. When compressed to a surface pressure of ca. 10 dyn/cm, such monolayers yield a tightly packed film that lies flat on the water surface.lOJ1 Here, the oxygen atoms are presumed to be in intimate contact with water, and the hydrophobic methylene groups face outward into air. Recent ellipsometric studies provide strong evidence that segments of PEO are "pushed" into the aqueous subphase, when the film is compressed beyond its collapse point.12 In contrast, a variety of calix[4]arene- and calix[Glarene-based surfactants have been found to produce stable monolayers with relatively low compressibility. 13-16 In principle, one might except that by coupling polyether groups to one face of such calixarenes, the resulting surfactant should yield isotherms showing both an expanded and a condensed phase. The former would reflect the squeezing out the polyether segments from the air-water interface into water, while the latter would reflect much "harder" intermolecular hydrophobic interactions among the aromatic nuclei. In essence, compression should force the pendant polyether groups down into the subphase, while expansion should bring them back toward the surface of water (Scheme I). An intriguing extension of this hypothesis is that by adjusting the orientation of its tentacles, one might be able to fine-tune the interaction of an octopus with a guest ion or molecule in a manner that has not, heretofore, been possible (Chart 11). (14) Markowitz, M. A.; Janout, V.; Castner, D. G.; Regen, S. L. J. Am. Chem. SOC.1989,111,8192. (15) Nakamoto, Y.; Kallinowski, G.; Bohmer, V.; Vogt, W. Langmuir 1989,5, 1116. (16) Ishikawa, Y.; Kunitake, T.; Matsuda, T.; Otsuka, T.; Shinkai, S. Chem. Commun. 1989.736.

0743-7463/91/2407-0982$02.50/0 0 1991 American Chemical Society

eln

Langmuir, Vol. 7, No.5, 1991 983

Octopus Molecules at the Air- Water Interface

Scheme I

R

2a. n=6;m=l ; R=C(CH,), b, n=6;m=2; R=C(CH3),

C

A

chart I1

The primary goal of the work described in this paper was to examine the feasibility of controlling tentacle placement by adjusting the surface pressure of an appropriate monolayer. For this purpose, we have chosen to investigate calixarene-based octopi and not benzene-, naphthalene-, or biphenyl-derived ~ c t o p i . Our ~ ~ reason *~~ for this was 3-fold. First we expected that the calixarene framework would afford significantly greater intermolecular hydrophobic interactions and that the surface pressure-area isotherms of these octopi would more clearly express tentacle movement of the type illustrated in Scheme I. Second, the greater hydrophobicity of calixarene-based octopi should make them better-suited for forming stable monolayers due to their lower water solubility. Third, we reasoned that the existence of a cylindrical hydrophobic interior within the ocotopus could also be of potential benefit for complexing amphiphilic guests.19 The specific objectives of this work were (i) to search for well-defined two-component isotherms that clearly reflect orientational changes as depicted in Scheme I, (ii) to determine the minimum tentacle length whose vertical movement can be controlled via compressionexpansion cycles, and (iii) to examine the influence that calixarenerigidity and bulky p-tert-butyl groups have on tentacle orientation. With these objectives in mind, we have chosen to synthesize the conformationally flexible calix[61arene octopus molecules 1 and 2 and also the rigid calix[4]arene analogues 3 and 4.

Experimental Section General Methods. Unless stated otherwise,all reagents were obtained from commercial sources and used without further purification. Deionized water was purified by using a Millipore Mill-Q filtering system containing one carbon and two ionexchange stages. Chloroform and methanol used for chromatography were HPLC grade (Burdick and Jackson, Muskegon, MI). Anhydrous tetrahydrofuran (THF), anhydrous NJV-dimethylformamide (DMF), 2-bromoethyl methyl ether, and (17) Gutache, C. D. Calixarenes; The Royal Society of Chemistry, Thomas Graham HOW, Science Park Cambridge, 1989. (18) Guteche, C. D. In Synthesis of Macrocycles: The Design of Selective Complexing Agents; Izatt, R. M., Christeneen,J. J., Eda.; WileyInterecience: New York, 1987; p 93. (19) Shinkai, S.; Kawabata, H.; Matauda,T.; Kawaguchi, H.;Manabe, 0. Bull. Chem. SOC.Jpn. 1990,63, 1272.

3a. n=4;m=l; R=H b, n=4;m=2; R=H

4a, n=4;m=l ; R=C(CH~), b, n=4;m=2, R=C(CH,),

l-bromo-2-(2'-methoxyethoxy)ethane were purchased from Aldrich Chemical Co. and used directly. 3,6,9-Trioxa-l-decyl bromide were synthebromide and 3,6,9,12-tetraoxa-l-tridecyl sized by converting the appropriate poly(ethy1eneglycol) monomethyl ether to the corresponding tosylate, followed by displacement with bromide ion.20s1 Preparative thin layer chromatography was carried out by using UNIPLATE Silica gel GF 20 X 20 cm plates (Analtech) that were lo00 pm in thickness; analytical thin layer chromatography employed silica gel on polyester (Sigma). Silica gel that was for column chromatography was silica gel 70-230 mesh, 60A (Aldrich). Eluting solvents that were used for chromatography were as follows: solvent A, CHCls/acetone, 3/1 (v/v); solvent B, CHCls/acetone, 5/1(v/v); solvent C, CHC&/methanol, 10/ 1(v/v). Detection of compounds was made by UV and by iodine vapor. All lH NMR spectra were recorded with a Bruker 500 MHz instrument and CDCls as the solvent. Chemical shifts are reported relative to tetramethylsilane. Mass spectral analyses were performed a t the Midwest Center for Mass Spectrometry, University of Nebraska, Lincoln, NE. Melting points were measured in capillary tubes with an electrothermal melting point apparatus and are uncorrected. Melting points that were higher than 200 "C were determined in capillary tubes that were sealed under a nitrogen atmosphere. The following calixarenes were prepared by using procedures similar to those previously described in the literature: 5,11,17,23tetra-tert-butyl-25,26,27,28-tetrahydroxycalix[4 ]arene,22 25,26,27,28-tetrahydro~ycalix[4]arene,~ 5,11,17,23,29,35-hexatert-butyl-37,38,39,40,41,42-hexahydroxycalix[6]arene,23 and 37,38,39,40,4 1,42-hexahydroxycalix[61arene.23 37,38,39,40,41,42-Heztakis(3,6-dioxa-l-heptyloxy)calix[6 1arene (1b). To a suspension, formed from 0.36 g (15 mmol) of NaH (0.6 g of a 60% dispersion of NaH in mineral oil that was washed 3 X with 15 mL of hexane) plus 10 mL of anhydrous THF, was added 0.64 g (1.0 mmol) of 37,38,39,40,41,42hexahydroxycalix[6]arene dissolved in 30 mL of THF. The mixture was refluxed under a nitrogen atmosphere until hydrogen evolution ceased (typically 5 min). Anhydrous DMF (4 mL) was then added, followed by addition of 3.3 g (18 mmol) of 3,6-dioxal-heptyl bromide. The reaction mixture was heated to reflux under a nitrogen atmosphere for 24 h, cooled to room temperature, and quenched by the addition 5 mL of ice water, followed by 15mL of 1M HC1. After addition of70 mL of dichloromethane to the product mixture, the organic layer was separated, washed (threetimes) with 50 mL ofwater, dried over anhydrous Na2SO4, and concentrated under reduced pressure. Analysis of the crude product by TLC showedthe presence of three components having R,= 0.60,0.42, and 0.00 [solventA]. Recrystallization(twotimes) from methanol afforded 0.69g (56%) of l b as colorless crystals having mp 131-131.5 "C and R, = 0.42. lH NMH (CDCls, 500 MHz) 6 3.37 (s, 18 H, OCHs), 3.40-3.65 (m, 48 H, OCH2), 3.97 (br s, 12 H, ArCH2), 6.83 (t, 6 H, ArH), 7.01 (br s, 12 H, ArH). Anal. Calcd for C72H~O18:C, 69.20; H, 7.75. Found: C, 69.21; H, 7.77. MS exact mass calcd for C72H~O18(M + Li) = 1255.6758; found, 1255.6763; error = 0.4 ppm. 37,38,39,40,41,4tHexakis( 3,6,9-trioxa-1-decyloxy)calix[ 61arene (IC). A 0.086-g (0.15 mmol) sample of 37,38,39,40,41,42-hexahydroxycalix[6]arenewasalkylated with0.51 g (2.2 mmol) of 3,6,9-trioxa-l-decyl bromide to give a product mixture that contained five components having R,= 0.68,0.62,0.16-O.23,0.11, (20) Schou, 0.;Larsen, P. Acta Chem. Scad. 1981, B35,339. (21) Hyatt, J. A. J. Org. Chem. 1978,43, 1808. (22) Gutache, C. D.; Iqbal, M.; Stewart, D. J. Org. Chem. 1986,52,742. (23) Gutsche, C. D.; Lin, L. G. Tetrahedron 1986,42, 1633.

984 Langmuir, Vol. 7, No. 5, 1991 and 0.00 [solvent A]. After recrystallization (two times) using a hexane/benzene, 10/1 (v/v), 0.111 g (49%)of lo was obtained as colorless crystals having mp 87.5-88 OC and Rf = 0.11. 'H NMR (CDC13, 500 MHz) 6 3.35 (8, 18 H, OCH3), 3.40-3.75 (m, 72 H, OCHz), 3.97 (b s, 12 H, ArCHZ), 6.83 (t,6 H, ArH), 6.99 (b 8, 12 H, ArH). Anal. Calcd for CuH1200a: C, 66.64; H, 7.99. Found: C, 66.69; H, 7.93. MS exact mass calcd for CuH120024 (M+) = 1512.8172; found, 1512.8141; error = 1.9 ppm. 37,38,39,40,41,42-Hexakis(3,6,9,12-tetraoxa-l-t~decyloxy)calix[6]arene (ld). A 0.070-g (0.11 mmol) sample of 37,38,39,40,41,42-hexahydroxycalix[6]arenewas alkylated with 0.90 g (3.3 mmol) of 3,6,9,12-tetraoxa-l-tridecylbromide to give a product mixture containing four components having Rj = 0.60, 0.10, 0.034.07, and 0.00 [solvent A]. Preparative thin layer chromatographic purification [solvent C] gave bands having R/ values equaling 0.764.85, 0.40.54, 0.31-0.33, and 0.00.07. The component having Rf = 0.40.54 was extracted with 20 mL of CHC&/CHsOH,2/1 (v/v), and concentrated under reduced pressure. Recrystallization from hexane/benzene, 5/1 (v/v), afforded 6.4 mg (3%) of Id as colorless crystals having mp 40-42 OC; Rf = 0.07 (solvent A). 1H NMR (CDCl3,500 MHz) b 3.37 (8, 18H, OCH,), 3.41-3.69 (m, 96 H, OCHZ),3.96 (b s,12 H, ArCHd, 6.82 (t, 6 H, ArH), 6.99 (b 8, 12 H, ArH). Anal. Calcd for CwHl+,OW: C, 64.84; H, 8.16. Found: C, 63.65; H, 8.31. MS exact mass calcd for CMHluOm (M+) = 1776.9745; found, 1776.9757; error = 0.8 ppm. 37,38,39,40,41,42-Hexakis(3-oxa-l-butyloxy)calix[6]arene (la). A 0.27-g (0.41 mmol) sample of 37,38,39,40,41,42hexahydroxycalix[6]arene was alkylated with 0.97 g (7.0 mmol) of 3-oxa-1-butyl bromide to give a crude product that was subsequently purified by column chromatography [solventD] to give 0.13 g (31%) of la having an Rj = 0.63 [solvent B]. Recrystallization from acetone/CHCls (10/1, v / ~yielded ) colorless crystals having mp 216-217 OC. lH NMR (CDCl3,500 MHz) 6 2.80-3.70 (m,42 H,0CHp,0CH3),3.98 (brs,12 H,ArCH2),6.84 (t, 6 H, ArH), 7.02 (b s,12 H, ArH). MS exact mass calcd for CmH72012 (M + Li) = 991.5186; found, 991.5165; error = 2.0 ppm. Anal. Calcd for CmH72Ol2: C, 73.14; H, 7.37. Found: C, 72.99; H, 7.27.

Conner et al. tetrahydroxycalix[4]arene was alkylated with 2.8 g (15.3 mmol) of 3,6-dioxa-l-heptylbromide for 28 h, to give a product mixture that was purified by column chromatography (50 g of silica), eluting with solvent F. Four fractions which were isolated had the following Rj values [solvent B]: (i) 0.68; (ii) 0.53 plus 0.49; (iii) 0.49; (iv) 0.29. The cone isomer of 3a, which was isolated as a viscous oil, was found to be the component having Rf = 0.29 (0.17 g, 20%) as indicated by 1H NMR (CDC13,500MHz) 6 3.14 (d, 4 H, ArCH2), 3.37 (s,12 H, OCHS),3.52-3.55 (m, 8 H, OCHZ), 3.63-3.66 (m,8H,OCH2),3.89(t,8H,OCH2),4.13(t,8H,OCHp), 4.47 (d, 4 H, ArCH), 6.55-6.63 (m, 12 H, ArH). Anal. Calcd for C&Olz: C, 69.20; H, 7.75. Found, C, 70.52; 8.38. 25,26,27,28-Tetrakis(3,6,9-trioxa-l-decyloxy)cal~x[4]arene (3b). A 0.085-g (0.20 mmol) sample of 25,26,27,28tetrahydroxycalix[4]arene was alkylated with 0.68 g (3.0 mmol) of 3,6,9-trioxa-l-decyl bromide for 39 h to give a product mixture having components with Rf values [solvent B] equaling: 0.63, 0.57, 0.34, 0.23, 0.19, and 0.05. The mixture was then purified by preparative thin layer chromatography [solvent E] giving bands with Rf values equaling (i) 0.80.75, (ii) 0.62-0.54, (iii) 0.54-0.35, and (iv)0.00. Extraction of componentiiiwithCHC&/ CH3OH (2/1, v/v) yielded two components [solvent B] having Rf = 0.19, 0.05. Two consecutive preparative thin layer chromatographs [solvent E] afforded 40 mg (20%) of an oil (Rj = 0.05), whose lH NMR spectrum was consistent with the cone isomer of 3b. lH NMR (CDCls, 500 MHz) 6 3.13 (d, 4 H, ArCHz), 3.37 (s,12 H, OCH3),3.51-3.54 (m, 8H, OCHZ),3.60-3.67 (m, 24 H, OCHz), 3.87 (t,8 H, OCHa), 4.11 (t,8 H, OCHz), 4.47 (d, 4 H, ArCHp), 6.54-6.63 (m, 12 H, ArH). Anal. Calcd for CdmOle: C, 66.64; H, 7.99. Found: C, 65.33; H, 7.96.

5,11,17f3-Tetra-tert-butyl-25f6f7f8-tetrakis(3,6-doxal-heptyloxy)calix[4]arene (4a). A 0.324-g (0.5 mmol) sample of 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrahydroxy~~[4]arene was alkylated with 1.56 g (8.5 mmol) of 3,6-dioxa-l-heptyl bromide for 40h, affordinga mixture of products having &values (solvent F) equaling 0.68,0.56,0.49,0.26, and 0.00. Purification by preparative TLC (solvent B) afforded 156 mg (30%) of the cone isomer of 4a (Rj = 0.26, solvent F) having mp 97-99.5 OC. 'H NMR (CDCl3, 500 MHz) 6 1.07 (8, 36 H, CH3), 3.10 (d, 4 H, 5,11,17,23~,35-Hexa-tert-butyl-37,38,39,40,41,42-hexakis- ArCHZ), 3.38 ( 8 , 12 H, OCH3), 3.54-3.57 (m, 8 H, OCHp), 3.67(3,6-dioxa-l-heptyloxy)calix[6]arene(2a). By use of proce3.70 (m, 8 H, OCHZ),3.96 (t,8 H, OCHZ),4.12 (t, 8 H, OCHz), dures similar to that used for the preparation of lb, a0.49-g (0.50 mmol) sample of 5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,- 4.41 (d, 4 H, ArCHZ), 6.76 (8, 8 H, ArH). Anal. Calcd for CaHw012: C, 72.69; H, 9.15. Found: C, 72.52; H, 9.16. 41,42-hexahydroxycalix[6]arenewas alkylated with 1.69 g (9.2 5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis( 3,6,9-trimmol) of 3,6-dioxa-l-heptyl bromide to give a product which oxa-l-decyloxy)calix[4]arene (4b). A 0.130-g (0.2 mmol) showed four components having Rf values = 0.72,0.55,0.29,and sample of 5 , 1 1 , 1 7 , 2 3 - t e t r a - t e r t - b u t y l - 2 5 , ~ ~ 7 , ~ ~ ~ ~ y ~ ~ 0.00 [solvent A]. The reaction time used for this alkylation was [4]arene was alkylated with 0.68 g (3.0 mmol) of 3,6,9-trioxa-l53 h. Recrystallization (three times) from CHCl,/CH3OH, 1/10 decyl bromide for 45 h, affording a mixture of products having (v/v), afforded 0.17 g (25%) of an undesired tetraalkylated Rf [solvent B] equaling 0.61, 0.53, 0.41, 0.27, and 0.14-0.04. structure, as indicated by lH NMR. The mother liquor from the Purificationby preparativeTLC [solventA] afforded 98 mg (40%) first recrystallization was then concentrated and the residue was of the cone isomer of 4b (Rj = 0.144.04, solvent B)having mp recrystallized (three times) with CHC&/CH3OH (1/5, v/v), 65-67 OC. 'H NMR (CDC13,500 MHz) 6 1.07 (8,36 H, CHs),3.09 yielding 0.09 g (11%) of 2a having mp 175.5-176.5 OC; Rf = 0.29 (d, 4 H, ArCHz), 3.36 (s,12 H, OCH3),3.51-3.54 (m, 8 H, OCHZ), (solvent A). 1H NMR (CDCl3,500 MHz) 6 1.11(b s,54 H, CH,); 3.62-3.70 (m, 24 H, OCH,), 3.93 (t, 8 H, OCHz), 4.10 (t, 8 H, 2.90-4.62 (b m, 78 H, OCH3, OCH2, ArCHZ), 6.92 (b s, 12 H, OCHZ),4.41 (d, 4 H, ArCHp), 6.76 (s,8 H, ArH). Anal. Calcd ArH). Anal. Calcd for CwHl+,Ola: C, 72.69; H, 9.15. Found: for C72H112016: C, 70.10; H, 9.15. Found C, 69.92; H, 9.15. C, 72.59; H, 9.15. 5,11,17f3~9,35-Hexa-tert-buty1-37,38,39,40,41,42-he~kis- Surface Pressure-Area Isotherms. Surface pressurearea isotherms were recorded by using a MGW Lauda film balance, (3,6,9-trioxa-l-decyloxy)calix[6]arene(2b). A 0.195-g (0.20 mmol) sample of 5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,-maintained at 25 "C, and equipped with a computerized data acquisition station. Water (ca. 1 L) which was used as a sub41,42-hexahydroxycalix[6]arenewas alkylated for 48 h with 1.35 phase was purified via a Milli-Q filtration system and purged g (6.0 mmol) of 3,6,9-trioxa-l-decyl bromide to give a product with nitrogen for 15 min. Before addition to the film balance, that showed components with Rf values = 0.73,0.62,0.28,0.23, the surface of this degassed water was removed via aspiration in 0.19,0.07, and 0.00 [solvent A]. Recrystallization (three times) frommethanolafforded 59mg (19%)ofatetraalkylatedproduct, order to remove surface-active contaminants. All calisarene as indicated by 'H NMR. The mother liquor of the first solutionswere spread onto the aqueous subphase having a surface area of 600 cm2, using a 50-pL Hamilton syringe. Chloroform crystallization was then concentrated and the residue recrystallized (three times) from hexane/benzene (5/1, v/v) to give 15 (CDCU solutionsof calixarenes were prepared by direct weighing. mg (4%) of 2b having mp 129.5-132.5 O C (lit. mp 123-124 OC, Typical concentrations that were used were 2.0 mg/mL. The for 1/1 adduct with ethanol)" and Rf = 0.07 (solvent A). lH total quantity of calixarene that was used in each experiment NMR (CzDzCL, 500 MHz, 70 "C) 6 1.11(b ~ , 5 H, 4 CH3), 3.30-4.80 was ca. 2.8 X 10-8mol. Prior to compression,the spreadingsolvent (b m, 102 H, OCH3, OCH2, ArCHZ), 6.92 (e, 12 H, ArH). Anal. was allowed to evaporate for at least 30 min. Monolayers were Calcd for Cl&luO~: C, 70.10; H, 9.15. Found: C, 69.89; H, compressed under a nitrogen atmosphere at a rate of 60 cmz/ 9.08. min. Isotherms were normally recorded after two successive 25,26,27,28-Tetrakis(3,6-dioxa-l-heptyloxy)calix[41compression/expansion cycles. In the case of lb, the monolayer arene (3a). A 0.43-g (1.0 mmol) sample of 25,26,27,28isotherm was recorded on the first compression. Limiting areas

Langmuir, Vol. 7, No. 5,1991 985

Octopw Molecules at the Ail-Water Interface Scheme I1 R

calix[n]arene R= H, c(CH&

40

-#

R

>

CH30

were estimated by drawing a tangent from the condensed portion of the forcearea isotherm to 0 dyn/cm. Results and Discussion Octopus Synthesis. The target octopi (1-4) were synthesized via alkylation of the corresponding calix[n]arenes using a brominated form of the appropriate poly(ethylene glycol) monomethyl ether, i.e., CHsO(CH2CH20),CH&H2Br, where m = 0, 1,2, or 3 (Scheme 11). Each of the desired "cone" conformational isomers of the calix[l]arene series was isolated by thin-layer chromatography and identified by 500-MHz lH NMR spectroscopy (see Experimental Section). Monolayer Properties of Calixarenes la-d. Surface pressurearea isotherms that were measured for la-d, over a pure water subphase, are shown in Figure 1. Calixarene la, having the shortest tentacles, exhibited only a condensed phase with a limiting area and collapse pressure of 149 f 10 A2/molecule and 12 dyn/cm, respectively. Extending the length of the tentacles by one or two ethyleneoxy units resulted in well-defined two-component isotherms. The limiting area and collapse pressure for l b were 155 f 5 A2/molecule and 34 dyn/cm, respectively; for IC,they were 150 i 10 A2/molecule and 31 dyn/cm, respectively. Addition of a third ethyleneoxy group (i.e., ld) resulted in the loss of the condensed phase and considerable hysteresis. When Id was compressed over a saturated aqueous NaCl subphase, however, a twocomponent isotherm was produced which was very similar to that obtained for l b and ICover pure water, its limiting area and collapse pressure were 165f 10A2/moleculeand 34 dyn/cm, respectively. Space-filling (CPK)models predict a limiting area for l a 4 of 155A2/molecule if the pore axis of the calixarene lies perpendicular to the air-water interface and the polyether moieties are tucked beneath the aromatic core. If the polyethers were pushed up into air, along side the aromatic rings, then the expected limiting area would ca. 290 &/molecule. The fact that each calixarene within this series exhibits a limiting area of ca. 155 A2/molecule provides compelling evidence for an oriented monolayer of "tucked" octopi. The significant upward displacement of the expanded segment of the force-area curves, upon incremental extension of the polyether chains, clearly shows that substantial portions of the tentacles lie at the air-water interface at low surface pressure.24 Moreover, the very large areas that lb-ld occupy at low surface pressure are inconsistent with"c1oaed face" octopi (Scheme I, structure C) lying flat on the water surface; Le., the maximum areas that lb, IC,and Id could occupy for such a model are 190, 290, and 390 A2/molecule. The high areas that are observed at modest surface pressures can (24) Similar resulta have bean aeon with a homologous series of poly(oxyethylene) 1-dodecanob Lange, H. Vortrage Origi~lfassunglntern. Kongr. Grenzflochenaktiue Stoffe 3 1960, 1, 279.

AREA (Angstroms2/Molecule)

Figure 1. Surface pressurearea isotherms for monolayers of (- - -) la, (-) 1b, and (. * .) ic compressed over a pure water subphase; (- - -) Id was obtained by using a saturated NaCl subphase. Insert: surface pressure-area isotherm for Id over pure water.

,

only be accounted for by an "open face" model in which there is substantial "spreading out" of the pendant polyether groups (e.g., structures A and B, SchemeI). Because the calixarene tentacles may interdigitate with neighboring octopi, we cannot estimate, precisely, the degree to which the tentacles have spread out at the air-water interface. Despite this fact, the monolayer properties of lb-ld clearly show that one has considerable mechanical control over the orientation of the tentacles relative to the hydrophobic core. In principle, when the hydrated tentacles of a molecular octopus reach some critical length, the hydrophobichydrophilic balance should be shifted such that compression leads to dissolution into the subphase. The loss of the condensed phase of Id over pure water, together with the well-defined two-component isotherm that is observed over a saturated aqueous NaCl subphase is fully consistent with this expectation.25 In contrast, calixarene IC produced an isotherm over saturated aqueous NaCl that was similar to that found over pure water. Although we have no experimentalmeans which allows us to define the extent of polyether hydration over pure monolayers, it is highly probable that (i) the polyether units are extensively hydrated over pure water at low surface pressures, (ii) significant dehydration occurs on going from the gaseous to the condensed phase, and (iii) high concentrations of NaCl in the subphase reduce polyether hydration. One final a#pect of these isotherms which deserves comment is the "pseudocollapse" point that occurs at ca. 200 A2/molecule. Although this "collapse" persists, even when slow compressing speeds are employed (15 cm2/ min), we have found that by maintaininga constant surface area of 200 A2/molecule for lc, the surface pressure decreases to a limiting value after 5 min. This value corresponds to the surface pressure "minimum" that appears at 180A2/molecule(Figure 1). These observations clearly indicate that this phenomenon is kinetic and not thermodynamic in origin. We presently assign these pseudocollapse points to the "tucking under" of a last ethyleneoxy unit beneath the calixarene core. Monolayer Properties of Calixarenes 2a and 2b. In sharp contrast to lb, monolayers produced from the (25) High subphase concentrations of NaCl have previously been used toform stable monolayersfrom partially water-solublepolyethylene glycol derivatives of n-alcohols: (a) Lange, H.; Jeschke, P. In Nonionic Surfactants. Physical Chemistry; Schick, M., Ed.;Marcel Dekker, Inc.: New York, 1987; Vol. 23, p 5. (b) Schick, M. J. J. Colloid Sci. 1962,17,801. (c) Schick, M. J. J.Am. Oil Chem. SOC. 1963,40,680. (d) Brady, A. P. J. Colloid Sci. 1949, 417.

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986 Langmuir, Vol. 7, No. 5, 1991

El In

E

a

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AREA Angstroms2/Molecule

Figure 2. Surface pressure-area isotherms of (-1 (- - -1 2b over pure water.

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2a and

analogous calixarene 2a exhibit only a condensed phase. Calixarene 2b,possessing one additional ethyleneoxy unit, yielded an isotherm having both a compressible and a condensed phase (Figure 2). The limiting areas that are estimated for 2a and 2b are 210 and 220 A2/molecule, respectively. Space-filling models for 2a predict that, even if all of the pendant polyether groups were to lie flat on the water surface, the p-tert-butyl groups are sufficiently large, and the splay of the molecule sufficiently great, that nearest neighbor interactions should be dominated by the p-tert-butyl groups. Space-filling models further predict that the polyether moieties of 2b could "reach" beyond the p-tert-butyl groups, if they were in a fully extended conformation on the water surface. The observed surface pressure-area isotherms for these two calixarenes are fully consistent with these predictions. Although their limiting areas are in reasonable agreement with that which is expected for a perpendicular alignment of the pore axis with the air-water interface (i.e., 220 A2/molecule), it is not clear from these isotherms how much control one has over their tentacles. Similar to its "debutylated" analogue (IC), the monolayer properties of 2b over saturated aqueous NaCl were similar to that found over pure water. Monolayer Properties of Calixarenes 3a and 3b. Monolayers that were prepared from 3a over pure water produced a two-componentisotherm which was similar to that produced over a saturated NaCl suphase. In contrast, 3b yielded only an expanded isotherm over pure water with considerable hysteresis (data not shown). The latter calixarene appears to be too water-soluple for forming stable monolayers. In contrast, compression of 3b over a saturated aqueous NaCl subphase yielded a well-defined two-component isotherm (Figure 3). The greater watersolubility of 3b, as compared with its calix[6]arene analogues, is a likely result of its smaller oligomer size and/or conformational rigidity. The limiting areas that are estimate from the surface pressure-area isotherms for 3a and 3b are 85 f 10 and 120 f 10 A2/molecule, respectively; the corresponding collapse pressures are 31 and 32 dyn/cm. Although the limiting area for 3a is consistent with that which is expected for a condensed phase structure that has the pore axis of the calixarene lying perpendicular to the air-water interface (Le., the predicted limiting area is ca. 90 A2/molecule), the limiting area for 3b is somewhat greater in magnitude. The simplest explanation for this higher area of 3b is that there is partial coiling of the polyether groups and that the available space beneath the aromatic core is insufficient to significantly reduce intermolecular exposure. The significant upward displacement of the expanded segment of the force-area curve, upon extension of the

Figure 3. Surface pressure-area isotherms for monolayers of (-) 3a over water and (. 3b over a saturated NaCl subphase. Insert: surface pressure-area isotherm for 3b over pure a a)

water.

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AREA Angstroms2/Molacuk

Figure 4. Surface pressure-area isotherms of (-) (- - -) 4b over pure water.

4a and

polyether chains, plus the very large area which each molecule occupies at low surface pressure, also shows that substantial portions of the tentacles lie at the air-water interface. Thus, qualitatively, the tentacles of 3a and 3b can be controlled in a manner that is similar to that of their calix[6]arene counterparts. One noticeable difference, however, is that pseudocollapse points are not apparent. While we do not yet fully understand why pseudocollapse points are not observed for these octopi, we suscept that it is related, in some way, to the conformational rigidity of the calix[4]arene framework. Monolayer Properties of Calixarenes 4a and Ob. Surface pressure-area isotherms that were obtained for 4a and 4b over pure water are shown in Figure 4. Calixarene 4a exhibited a limiting area of 145 f 10 A2/ molecule and a collapse pressure of 32 dyn/cm. Extending the polyether chain by one ethyleneoxy unit resulted in a slightly more compressible calixarene 4b; its limiting area and collapse point were 162 f 10 &/molecule and 44dyn/cm. The isotherm produced for 4b over a saturated aqueous NaCl subphase was similar to that found over pure water (data not shown). Qualitatively, the monolayer characteristics of these calixarenes parallel those of their calix[6]arene analogues. The lower compressibility of 4a is a likely consequence of dominant intermolecular interaction among the p-tert-butyl groups. The higher compressibility of 4b can be accounted for if the polyether groups lie fully extended on the water surface. In such a case, one would expect them to be able to reach beyond the p-tert-butyl groups and to contribute to intermolecular interactions. Similar to 2a and 2b, it is not clear from

Octopus Molecules at the Ail- Water Interface these isotherms how much control one has over tentacle orientation of either 4a or 4b. Conclusions This study has demonstrated the feasibility of controlling the tentacle orientation of certain calixarene-based octopi at theair-water interface. For calix[6]arene, having pendant groups that consist of two or three ethyleneoxy units, monolayer compression forces the tentacles down into the subphase, while expansion returns them the surface of water. Similar control over a calix[6]arene, bearing pendant groups that consist of four ethyleneoxy units, is also possible, but only when the molecule is spread over a saturated aqueous NaCl subphase. When the tentacles are only one ethyleneoxy unit in length, the

Langmuir, Vol. 7, No.5, 1991 987 surface pressure of the film does not allow one to control their orientation. Introduction of bulky p-tert-butyl groupsonto the upper rim of the calix[6]arene framework, reduces the mechanical control over the tentacles. Qualitatively, similar behavior has been found with analogous calix [4]arene-based octopi. In principle, the ability to control the orientation of the tentacles of an octopus should allow one to fine-tune ita interaction with a guest ion or molecule. Studies, that are now in progress are aimed at exploring and exploiting this possibility. Supplementary Material Available: lH NMR (500 MHz) for all of the calixarene-based octopi (1-4, 12 pages). Ordering information is given on any current masthead page.