2068
Langmuir 1997, 13, 2068-2073
Amphiphilic Cup-Shaped [(4-Alkylphenyl)azo]-Substituted Calixarenes: Self-Assembly and Host-Guest Chemistry at the Air-Water Interface J. Cameron Tyson, Jeffery L. Moore, Kenneth D. Hughes, and David M. Collard* School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400. Received April 3, 1996X Chromophoric, amphiphilic [(4-alkylphenyl)azo]-substituted calixarenes 1-4 form stable monolayers at the air-water interface of a Langmuir trough. Monolayer limiting areas and collapse pressures are controlled with changes in pH and electrolyte concentration of the subphase. At pH ) 8.2 or higher, deprotonation of the hydroxyls of the calixarene produces more polar headgroups, which result in larger mean molecular areas and increases in film collapse pressures. Similar effects result from increasing the concentration of electrolyte in the subphase. Effects of potential guests on monolayer assembly of [(4-alkylphenyl)azo]-substituted calixarenes at the air-water interface were investigated. The limiting area of tetrakis[(4-octylphenyl)azo]calix[4]arene decreases in the presence of N,N,N-trimethylanilinium iodide while no change is detected for tetramethylammonium iodide or N,N,N-trimethyl-N-(2-naphthyl)ammonium iodide. Interactions of the amphiphilic calixarenes and potential ammonium guests are discussed.
Introduction The modification of surfaces with artificial receptors capable of selectively binding analytes in contacting solutions is of interest for the development of sensors1 and chromatographic stationary phases.2 The design and fabrication of sensors based on the host-guest chemistry of immobilized cup-shaped molecular hosts (i.e., cyclodextrins, resorcinarenes, and calixarenes) includes studies of modified electrodes,3 quartz crystal microbalances,4 and modified fiber optics.5 Calixarenes are macrocycles prepared by the basecatalyzed condensation of 4-alkylphenols with formaldehyde.6 The cup-shaped macrocycles possess a hydrophobic cavity capable of binding organic molecules. Calixarenes bind small organic molecules in the solid state.6 However, there are fewer examples of complexation in solution7 or at interfaces.8 The binding properties of calixarenes and their solubility in organic or aqueous solutions can be tailored by modification of the upper (arene) and lower X
(hydroxyl) rims. Functionalized calixarenes have received wide attention for a variety of applications, e.g., recovery of uranium, accelerators for instant adhesives, ionophoric electronic devices, and phase transfer agents.9 Chemical modification of the upper or lower rim with alkyl groups affords amphiphilic calixarenes.10 Initial studies of the self-assembly of tert-butyl calixarene, in which alkyl groups are located on the upper rim, and the lower rim bears polar hydroxyl groups, indicate that these molecules form stable monolayers at the air-water interface.11 Regen and co-workers have transferred monolayers of amphiphilic calixarenes with hydrophobic tails and polar mercurated headgroups to polymeric substrates to study permselectivity of perforated monolayers.12 Other studies of monolayer assemblies of functionalized calixarenes indicate selective binding of metal ions at the air-water interface.13 The response of chromophoric and fluorogenic calixarenes to the presence of molecular guests has been described.14 Calixarenes have been rendered chromophoric through reactions with diazonium salts to
Abstract published in Advance ACS Abstracts, March 15, 1997.
(1) Janata, J. Anal. Chem. 1992, 64, 196R. (2) (a) Friebe, S.; Gebauer, S.; Krueger, G. J.; Goermar, G.; Krueger, J. J. Chromatogr. Sci. 1995, 33, 281. (b) Glennon, J. D.; O’Conner, K.; Srijaranai, S.; Manley, K.; Harris, S. J.; McKervey, M. A. Anal. Lett. 1993, 26, 153. (c) Ko¨nig, W. A.; Krebber, R.; Wenz, G. J. High Resolut. Chromatogr. 1989, 12, 641. (d) Wutte, A.; Gu¨bitz, G.; Friebe, S.; Krauss, G. J. J. Chromatogr. 1994, 677, 186. (e) Armstrong, D. W.; Tang, Y.; Ward, T.; Nichols, M. Anal. Chem. 1993, 65, 1114. (3) (a) Cadogan, A.; Gao, Z.; Lewenstam, A.; Ivaska, A.; Diamond, D. Anal. Chem. 1992, 64, 2496. (b) Wang, J.; Liu, J. Anal. Chim. Acta 1994, 294, 201. (c) Sakaki, T.; Harada, T.; Deng, G.; Kawabata, H.; Kawahara, Y.; Shinkai, S. J. Inclusion Phenom. Mol. Recognit. Chem. 1992, 14, 285. (d) Malinowska, E.; Brzozka, Z.; Kasiura, K.; Egberink, R. J. M.; Reinhoudt, D. N. Anal. Chim. Acta 1994, 298, 253. (e) Cunningham, K.; Svehla, G.; Harris, S. J.; McKervey, M. A. Analyst 1993, 118, 341. (f) Odashima, K.; Yagi, K.; Tohda, K.; Umezawa, Y. Anal. Chem. 1993, 65, 1074. (4) (a) Kutner, W. Electrochim. Acta 1992, 37, 1109. (b) Schiebaum, K. D.; Weiss, T.; Go¨pel, W.; Van Velzen, T.; Engbersen, J. F. J.; Reinhoudt, D. N.; Go¨pel, W. Science 1994, 265, 1413. (5) (a) Vo-Dinh, T.; Alarie, J. P. Talanta 1991, 38, 529. (b) Litwiler, K. S.; Catena, G. C.; Bright, F. V. Anal. Chim. Acta 1990, 237, 485. (6) For comprehensive reviews of calixarene chemistry: (a) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713. (b) Gutsche, C. D. Calixarenes; Stoddart, F. J., Ed.; Monographs in Supramolecular Chemistry; The Royal Society of Chemistry: Cambridge, 1989; and references therein. (c) Bo¨hmer, V.; Vicens, J. Calixarenes. A Versatile Class of Macrocyclic Compounds; Kluwer Academic Publications: Dordrecht, 1991.
S0743-7463(96)00316-2 CCC: $14.00
(7) (a) Gutsche, C. D.; Bauer, L. J. J. Am. Chem. Soc. 1985, 107, 6063. (b) Gutsche, C. D.; Alam, I. Tetrahedron 1988, 44, 4689. (c) Gutsche, C. D.; Alam, I. J. Org. Chem. 1990, 55, 4487. (d) Shinkai, S.; Araki, K.; Kubota, M.; Arimura, T.; Matsuda, T. J. Org. Chem. 1991, 56, 295. (e) Shinkai, S.; Mori, S.; Koreishi, H.; Tsubaki, T.; Manabe, O. J. Am. Chem. Soc. 1986, 108, 2409. (f) Shinkai, S.; Kawabata, H.; Matsuda, T.; Kawaguchi, H.; Manabe, O. Bull. Chem. Soc. Jpn. 1990, 63, 1272. (g) Gutsche, C. D.; Iqbal, M.; Alam, I. J. Am. Chem. Soc. 1987, 109, 4314. (h) Shinkai, S.; Araki, K.; Matsua, T.; Manabe, O. Bull. Chem. Soc. Jpn. 1989, 62, 3856. (8) Kaifer, A. E.; Gokel, G. W.; Zhang, L.; Godinez, L.; Tianbao, L. Angew. Chem., Int. Ed. Engl. 1995, 34, 235. (9) Perrin, R.; Lamartine, R.; Perrin, M. Pure Appl. Chem. 1993, 65, 1549. (10) Nakamoto, Y.; Kallinowski, G.; Bo¨hmer, V.; Vogt, W. Langmuir 1989, 5, 1116. (11) Markowitz, M.; Bielski, R.; Regen, S. L. J. Am. Chem. Soc. 1988, 110, 276. (12) (a) Markowitz, M.; Bielski, R.; Regen, S. L. J. Am. Chem. Soc. 1988, 110, 7545. (b) Markowitz, M.; Janout, V.; Castner, D.; Regen, S. L. J. Am. Chem. Soc. 1989, 111, 8192. (c) Conner, M.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1993, 115, 1178. (d) Dedek, P.; Janout, V.; Regen, S. L. J. Org. Chem. 1993, 58, 6553. (e) Conner, M.; Janout, V.; Kudelka, I.; Dedek, P.; Zhu, J.; Regen, S. L. Langmuir 1993, 9, 2389. (f) Dedek, P.; Webber, A.; Janout, V.; Hendel, R. A.; Regen, S. L. Langmuir 1994, 10, 3943. (g) Woongki, L.; Hendel, R. A.; Dedek, P.; Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1995, 117, 6793. (13) Ishikawa, Y.; Kunitake, T.; Matsuda, T.; Otsuka, T.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1989, 736.
© 1997 American Chemical Society
[(4-Alkylphenyl)azo]-Substituted Calixarenes
generate azo compounds. Previous reports of chromophoric phenylazocalixarenes include a description of the autocatalytic nature of azo bond formation,15 the selectivity for binding to metals,16 spectral properties of the azophenol-quinine-hydrazone tautomerization,17 and detection of amines with a chromophoric calix[8]arene.18 An understanding of host packing and host-guest interactions at the molecular level is critical to the design and fabrication of sensors based on analyte-receptor binding. To this end, we have designed and synthesized amphiphilic, chromophoric [(4-alkylphenyl)azo]-substituted calixarenes that may be aligned at the air-water interface with controllable packing densities. These compounds serve as models for investigating host-host interactions and guest-host interactions at the air-water interface. The amphiphilic host molecules developed are calixarenes functionalized with azo linkages. These macrocycles possess a large hydrophobic cavity suitable for binding of organic guests and chromophoric linkages which can be monitored for responses to host-guest interactions. Reaction of calix[n]arenes with (4-alkylphenyl)diazonium salts affords the corresponding [(4-alkylphenyl)azo]substituted calix[n]arenes 1-4. Monolayer formation by
amphiphiles 1-4 at the air-water interface on a Langmuir trough was studied as a function of pH and electrolyte concentration of the subphase. Both limiting area and (14) (a) McCarrick, M.; Wu, B.; Harris, S. J.; Diamond, D.; Barrett, G.; McKervey, M. A. J. Chem. Soc., Perkin Trans. 2 1993, 1963. (b) Grigg, R.; Holmes, J. M.; Jones, S. R.; Norbert, W. D. J. A. J. Chem. Soc., Chem. Commun. 1994, 185. (c) Jin, T.; Ichikawa, K.; Koyoma, T. J. Chem. Soc., Chem. Commun. 1992, 499. (d) Aoki, I.; Sakaki, T.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1992, 730. (e) Perez-Jimenez, C.; Harris, S. J.; Diamond, D. J. Chem. Soc., Chem. Commun. 1993, 480. (15) (a) Shinkai, S.; Araki, K.; Shibata, J.; Manabe, O. J. Chem. Soc., Perkin Trans. 1 1989, 195. (b) Shinkai, S.; Araki, K.; Shibata, J.; Tsugawa, D.; Manabe, O. J. Chem. Soc., Perkin Trans. 1 1990, 3333. (16) (a) Nomura, E.; Taniguchi, H.; Tamura, S. Chem. Lett. 1989, 1125. (b) Shimizu, H.; Iwamoto, K.; Fujimoto, K.; Shinkai, S. Chem. Lett. 1991, 2147. (c) Nomura, E.; Tanguchi, H.; Otsuji, Y. Bull. Chem Soc. Jpn. 1993, 66, 3797. (17) Shinkai, S.; Araki, K.; Shibata, J.; Tsugawa, D.; Manabe, O. Chem. Lett. 1989, 931. (18) Chawla, H.; Srinivas, K. J. Chem. Soc., Chem. Commun. 1994, 2593.
Langmuir, Vol. 13, No. 7, 1997 2069
collapse pressure can be controlled by selection of subphase composition. The exposure of monolayers to ammonium salts prior to spreading on the subphase and after compression of a monolayer was investigated. These investigations provide evidence that the calixarene cavity may be used to selectively bind ammonium derivatives and that this binding affects the mean molecular area and collapse pressure of the monolayer. Experimental Section General Methods. All reagents were obtained from commercial sources and used without further purification unless stated otherwise. Deionized water was obtained from a Barnstead filtering system containing four ion-exchange columns. Buffered solutions were prepared from sodium hydroxide and monobasic potassium dihydrogen phosphate. Adjustments to pH of buffered solutions were made with 80% phosphoric acid or 0.1 M aqueous sodium hydroxide. Isotherm investigations and host-guest binding experiments were conducted with a NIMA Technology System 2022 L-D2-S2 circular LangmuirBlodgett trough controlled by an Apple Macintosh IIci computer running programs developed with LabVIEW software (National Instruments).19 Surface pressures were measured using a NIMA 601S film balance. Chloroform solutions of amphiphilic calixarenes 1-4 (1-2 mg/mL) were spread with a Hamilton 50 µL syringe on the subphase of the trough and allowed to equilibrate for 15 min before compression. Compression of the films was performed at a rate of 15 cm2/min. Measurement of pH of the subphase was made with a combination pH electrode (Fisher Scientific). Injection experiments were performed at a constant pressure of 10 mN/m for all films. Chloroform solutions containing amphiphiles were spread on the subphase for 15-20 min to allow for solvent evaporation and equilibration. The monolayer was compressed at 15 cm2/min until the target pressure of 10 mN/m was reached. The monolayer was maintained at this target pressure until equilibrium was reached, as determined by a stable barrier position (approximately 2 h). Phosphate-buffered solutions containing ammonium salts 6-8 were injected into the subphase beneath the monolayer to produce a 1 mM subphase solution. Barrier position changes over time were monitored at constant pressure. A decrease in barrier position indicated a decrease in the equilibrium mean molecular area, whereas an increase in barrier position indicated an increase in the equilibrium mean molecular area. Synthesis. 5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrahydroxycalix[4]arene20 and 5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,41,42-hexahydroxycalix[6]arene were prepared by reaction of tert-butylphenol with formaldehyde according to the method of Gutsche.21 25,26,27,28-Tetrahydroxycalix[4]arene and 37,38,39,40,41,42-hexahydroxycalix[6]arene were prepared by treatment of tert-butylcalixarenes with aluminum chloride according to the method of Ungaro.22 Column chromatography was performed on silica gel (40 mesh, 60 Å, Baker). Thin layer chromatography was performed on 5 × 5 cm2 plates of silica gel (0.2 mm thick, 60 F254) on an aluminum support (EM Separations). All 1H NMR spectra were recorded with Varian Gemini 300 MHz instrument in CDCl3, DMSO-d6, or D2O as the solvent. Chemical shifts are reported relative to tetramethylsilane for the organic solvents or sodium 3-(trimethylsilyl)-1-propanesulfonate for aqueous solutions. 13C spectra were obtained at 75.5 MHz. IR analysis was performed on a Nicolet 520 FTIR spectrometer. Elemental analysis was performed in duplicate by Atlantic Microlab Inc. (Atlanta, GA). Representative syntheses are described here. The characterization of other homologs and macrocycle ring sizes is provided in the Supporting Information. (19) (a) Hughes, K. D.; Moore, J. L.; De Bry, B. A. Appl. Spectrosc. 1995, 49, 386. (b) Hughes, K. D.; Moore, J. L. Instrum. Newsl. 1996, 17, A-4. (20) Nomenclature for calixarenes used here follows the rules outlined in ref 6b. (21) (a) Gutsche, C. D.; Iqbal, M. Org. Synth. 1990, 68, 234. (b) Gutsche, C. D.; Dhawan, B.; Leonis, M.; Stewart, D. Org. Synth. 1990, 68, 238. (22) Bocchi, V.; Pochini, A.; Foina, D.; Ungaro, R.; Andretti, G. D. Tetrahedron 1982, 38, 373.
2070 Langmuir, Vol. 13, No. 7, 1997 5,11,17,23-Tetrakis[(4-butylphenyl)azo]-25,26,27,28tetrahydroxycalix[4]arene (1). A solution of sodium nitrite (258 mg, 3.74 mmol) in water (1 mL) was added to a solution of 4-butylaniline (504 mg, 3.38 mmol) and concentrated HCl (422 µL, 5.11 mmol) in water/methanol (3 mL/1 mL), and the mixture was stirred for 5 min. A solution of calix[4]arene (239 mg, 563 µmol) in a THF/pyridine mixture (3 mL/275 µL) was added dropwise to the solution of diazonium salt. A red precipitate formed immediately. The solution was stirred for 10 min, and chloroform (25 mL) was added to dissolve the red precipitate. The organic layer was separated and washed with 5% aqueous HCl (2 × 25 mL) and water (2 × 25 mL). The chloroform was removed under reduced pressure to give a red solid. Column chromatography (CHCl3:silica gel) gave 5,11,17,23-tetrakis[(4butylphenyl)azo]-25,26,27,28-tetrahydroxycalix[4]arene as a crystalline orange solid (138 mg, 23%). Rf ) 0.2 (CHCl3). Mp 275277 °C dec. 1H NMR (CDCl3) δ 0.83 (t, J ) 7 Hz, 12H, CH3), 1.27 (sextet, J ) 7 Hz, 8H, C3-CH2), 1.52 (p, J ) 7 Hz, 8H, C2-CH2), 2.56 (t, J ) 7 Hz, 8H, C1-CH2), 3.78 (d, J ) 12 Hz, 4H, bridging CH), 4.35 (d, J ) 12 Hz, 4H, bridging CH), 7.18 (d, J ) 8 Hz, 8H, ArH), 7.67 (d, J ) 8 Hz, 8H, ArH), 7.70 (s, 8H, calixarene ArH), 10.18 (s, 4H, ArOH). 13C NMR (CDCl3) 151.8, 151.7, 148.6, 146.7, 129.6, 128.9, 124.9, 123.2, 35.8, 33.7, 32.2, 22.5, 14.1. IR (KBr) 3300 (OH stretch), 2927 (aliph CsH stretch), 1600 cm-1 (arene CdC stretch). UV-vis (CHCl3) λmax 336 nm, ) 8 × 104 M-1 cm-1. Elem Anal. (C68H72N8O4‚0.5H2O) Calcd: C, 76.00; H, 6.87; N, 10.43. Found: C, 75.92; H, 6.86; N, 10.22. 4-[(4-Octylphenyl)azo]phenol (5). The procedure above was followed to couple the diazonium salt of 4-octylaniline to phenol in aqueous sodium hydroxide solution to afford 4-[(4octylphenyl)azo]phenol as a crystalline orange solid (122.0 mg, 30%). Rf ) 0.1 (CHCl3). Mp 79-80 °C. 1H NMR (CDCl3) δ 0.89 (t, J ) 7 Hz, 3H, CH3), 1.2-1.3 (m, J ) 7 Hz, 10H, (CH2)5), 1.64 (p, J ) 7 Hz, 2H, C2-CH2), 2.66 (t, J ) 7 Hz, 2H, C1-CH2), 5.56 (br s, 1H, OH), 6.91 (d, J ) 8 Hz, 2H, ArH), 7.30 (d, J ) 8 Hz, 2H, ArH), 7.80 (d, J ) 8 Hz, 2H, ArH), 7.85 (d, J ) 8 Hz, 2H, ArH). 13C NMR (CDCl3) 159.8, 152.6, 148.9, 147.8, 130.7, 126.4, 124.2, 117.4, 37.2, 33.2, 32.6, 30.7, 30.6, 30.5, 23.9, 15.3. IR (KBr) 3368 (OH stretch), 3045 (ar CsH stretch), 2927 (aliph CsH stretch), 1611 cm-1 (arene CdC stretch). N,N,N-Trimethyl-N-(2-naphthyl)ammonium Iodide (8). The procedure of Rodionov23 for the methylation of 2-aminonaphthalene was followed with modifications. Methyl iodide (18 mL, 289 mmol) was added dropwise to a solution of 2-aminonaphthalene (5.08 g, 35.5 mmol) in methanol/chloroform (15 mL/3 mL) at room temperature. The solution was heated to 60 °C under nitrogen for 2 h. TLC (ethyl acetate/petroleum ether (1:5 v/v)) indicated a mixture of alkylated products. The solvent was removed under reduced pressure to give a brown solid. The solid was dissolved in chloroform (200 mL), and the solution was treated with 5% aqueous sodium hydroxide (2 × 200 mL) and water (2 × 200 mL). The organic phase was dried with magnesium sulfate, and the chloroform was removed under reduced pressure to give a gray solid. The solid was dissolved with methanol (15 mL), and the solution was treated with methyl iodide (18 mL, 289 mmol) at 60 °C under nitrogen for 2 h. The solution was cooled to room temperature and filtered to give crude product. Recrystallization from methanol afforded N,N,Ntrimethyl-N-(2-naphthyl)ammonium iodide as a colorless crystalline solid (3.23 g, 29%). Mp 193-194 °C (lit. mp 193 °C). 1H NMR (D2O) 3.75 (s, 9H, (CH3)3), 7.7-8.4 (m, 7H, ArH). IR (KBr) 3006 (ar C-H stretch), 1611 cm-1 (arene CdC stretch).
Tyson et al.
Figure 1. Surface pressure-area isotherms for compression of 1-4 on a pH ) 8.2 (50 mM phosphate buffer) subphase. See Experimental Section for further details.
Synthesis of Tetrakis[(4-alkylphenyl)azo]calixarenes (1-4). Azocalixarenes 1-4 were synthesized by treatment of the calix[n]arene with the diazonium salts of the appropriate 4-alkylaniline. Column chromatography using chloroform as the eluent affords 1 and 3 as orange solids and 2 and 4 as viscous orange liquids. All compounds gave appropriate spectral data and combustion analyses. As previously reported for other azo-substituted calixarenes,15b 1-4 were isolated with water of hydration.
Calix[4]arenes can exist in four possible conformations: cone, partial cone, 1,2-alternate, and 1,3-alternate.6 1H NMR indicates that phenylazocalix[4]arenes 1-2 adopt a cone conformation in which the subunits of the macrocycle are aligned with the hydroxyl groups localized on one face of the calixarene. The cone conformation of calix[4]arenes is stabilized by intramolecular hydrogen bonding. The C4v symmetry of the cone conformation of 1-2 leads to a simple NMR spectrum which includes a distinct set of doublets with a geminal coupling constant of 12 Hz for the protons on the methylene bridge between phenolic subunits.6 Calix[6]arenes adopt several conformations in solution as a result of the increased flexibilty of these larger macrocycles. Cone, winged, and hinged conformations are common.6 The winged conformation of the calix[6]arene consists of two opposing subunits displaced outward from the cone conformation. The hinged conformation consists of two sets of three contiguous subunits oriented on opposite sides of the macrocycle. A mixture of cone and either winged or hinged conformations was observed in CDCl3 in the 1H NMR spectrum of the phenylazocalix[6]arenes 3-4. The cone conformation was confirmed by the appearance of a distinct set of doublets for the bridging methylene units and a pair of doublets and a singlet in the aromatic region. The 1H NMR spectra of the other component consists of two pairs of doublets (corresponding to a total of 16 and 8 protons, respectively) and three singlets (each corresponding to 4 protons) in the aromatic region. The winged or hinged conformations of the calix[6]arenes possess similar symmetry (C2v and C2h, respectively), and their 1H NMR spectra are similar. Thus, macrocycles 3-4 exist as a mixture of cone and either winged or hinged conformations in solution. Peak integration provides the ratio of cone isomer to winged or hinged isomers as 4:5 for the cyclic hexamers 3-4. Monolayer Characteristics of 1-4. Calix[n]arenes functionalized with azo linkages on the upper rim of the macrocycle and hydroxyl groups on the lower rim give rise to amphiphilic chromophoric macrocycles 1-4. Although amphiphiles containing hydroxyl headgroups do not generally form stable monolayer films at the airwater interface,24 the hydroxyl groups of 1-4 provide a polar face for the calixarene. In addition, the acidity of the hydroxyl groups is enhanced by the electronwithdrawing nature of the azo linkages in the 4-position. Macrocycles 1-4 were spread from chloroform solution on aqueous phosphate buffered subphases (pH ) 8.2) on a Langmuir trough. Isotherms for the pair of homologs of each macrocycle (cyclic tetramer and hexamer) are shown in Figure 1. The pressure-area isotherms are
(23) Rodionov, V. M.; Vvedenskii, V. E. J. Chem. Ind. 1930, 7, 11; Chem. Abstr. 1931, 25, 45456.
(24) Hann, R. A. In Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum Press: New York, 1991; p 43.
Results and Discussion
[(4-Alkylphenyl)azo]-Substituted Calixarenes
Langmuir, Vol. 13, No. 7, 1997 2071
Table 1. Monolayer Properties of [(4-Alkylphenyl)azo]-Substituted Calixarenes 1-4 on a pH 8.2 Phosphate-Buffered (50 mM) Subphase molecule
limiting areaa (Å2/molecule)
collapse pressure (mN/m)
collapse area (Å2/molecule)
1 2 3 4
122 ( 2 126 ( 3 209 ( 4 232 ( 8
14 ( 2 25 ( 2 16 ( 2 23 ( 1
98 ( 3 96 ( 2 162 ( 6 166 ( 5
a Limiting areas at a pressure of 0 mN/m were determined by linear least square regression analysis of the solid region of the isotherm.
consistent with the assembly of calixarenes in a cone conformation on the subphase with strong intramolecular hydrogen bonds between the polar hydroxyl groups on the lower rim and organization of the hydrophobic (alkylphenyl)azo groups. The limiting areas for (phenylazo)calix[n]arenes are approximately 120 and 220 Å2/ molecule for the cyclic tetramers and cyclic hexamers, respectively. The limiting areas, listed in Table 1, are consistent with predictions based on CPK models for a hexagonal closest packed arrangement of these molecules at the air-water interface. Small variations are observed in the limiting areas for the cyclic tetramers (1-2), while larger differences are apparent for the cyclic hexamers (3-4). These differences may be assigned to the packing orientations of the different homologs. The compression of 1-4 to form stable monolayers is controlled by the size of the macrocycle, the length of the hydrophobic chain attached to the upper rim, and the intramolecular interactions of the hydroxyl groups. The steep onset of the isotherm of 1 indicates that a rigid film is formed at the interface. In lower homologs (i.e., 1), the side chains radiate out from the cone formed by the calixarenes and are not capable of interacting with adjacent chains (intramolecularly). They may interact with chains of other calixarenes (intermolecularly) upon compression. Increasing the alkyl chain length to octyl increases the limiting area. This results from the larger volume occupied by the longer alkyl chains. In addition to the slight increase in area, the stronger intermolecular interactions of the longer hydrophobic chains on the upper rim improves the film stability (i.e., the collapse pressure increases). Similiar trends are observed for the cyclic hexamers (3-4) although the magnitude of the differences in limiting area is much greater. Pressure onset during compression of monolayers of 4 begins at approximately 320 Å2/ molecule, whereas homolog 3 undergoes a phase change at approximately 230 Å2/molecule (Figure 1). The larger volume occupied by the longer alkyl chains in conjuction with the flexibility of the calix[6]arene macrocycle gives rise to differences in pressure onset between 3 and 4. As higher pressures are reached, packing density is dictated by the size of the rigid calixarene macrocycle such that 3 and 4 give similar limiting areas. Once again, the stronger intermolecular interaction of the longer hydrophobic chains on the upper rim gives rise to increased film stability. Effects of pH on Monolayers of 2. The effect of pH on the monolayer assembly of 2 was studied using phosphate-buffered (50 mM) subphases over a pH range of 5.5 to 10.5 (Figure 2). An increase in film stability (i.e., an increase in collapse pressure) was observed on basic subphases. Only small differences in the surface pressure-area isotherm for 2 were observed for pH values greater than 8.2 (Table 2). The increase in limiting area and film stability is related to pertubations in the inter- and intramolecular interac-
Figure 2. Surface pressure-area isotherm for compression of 2 on phosphate-buffered subphases over a pH range of 5.5 to 10.5: b ) buffered subphase; u ) unbuffered subphase. Table 2. Monolayer Properties of Tetrakis[(4-octylphenyl)azo]calix[4]arene (2) as a Function of Subphase pH pH
limiting area (Å2/molecule)
collapse pressure (mN/m)
collapse area (Å2/molecule)
5.5a 5.5b 7.0 8.2 9.5 10.5
102 ( 1 112 ( 1 119 ( 4 126 ( 3 123 ( 5 128 ( 3
8(1 10 ( 1 17 ( 1 25 ( 2 30 ( 1 24 ( 3
91 ( 1 96 ( 1 96 ( 2 96 ( 2 93 ( 1 98 ( 2
a
Unbuffered subphase. b Phosphate-buffered subphase (50 mM).
tions of the phenol headgroup with changes in pH. The polar hydroxyl-bearing face of 2 is rendered acidic by the electron-withdrawing effects of the azo substituents. Monolayer films of 2, formed at pH 5.5 have low collapse pressures (approximately 10 mN/m). At this pH, the hydroxyl groups of the calixarene are protonated and intramolecular hydrogen-bonding occurs. This translates into a poor headgroup for the calixarene, thus giving monolayers with low collapse pressures. As the pH increases, deprotonation of hydroxyls on the calixarene occurs. At pH 8.2, deprotonation of two of the four hydroxyl groups of the calixarene is possible.25 This increase in pH results in loss of intramolecular hydrogen bonding, which holds the units of this macrocycle in a rigid cone conformation. This produces a macrocycle with a more polar headgroup. Increases in headgroup polarity give rise to larger mean molecular areas due to electrostatic effects associated with packing of these calixarenes. Increases in collapse pressure result from this more polar (i.e., better) headgroup at high pH. Further studies of the effects of pH on the intramolecular hydrogen bonding were carried out on compressed monolayers held at constant pressure. When monolayers of 2 were compressed on a pH ) 8.2 phosphate buffer (5 mM) to a pressure of 10 mN/m and held at this pressure for 2 h, injection of 80% phosphoric acid into the subphase lowered the pH of the subphase and resulted in a contraction in the mean molecular area. Over the range of pH 8.2 to 5.5 the trough barrier moved inward by 8 Å2/molecule in order to maintain a constant pressure of 10 mN/m. The contraction of the film of 2, as the pH of the subphase decreases, indicates that protonation of the phenoxide groups results in a lower molecular area. Protonation of the phenoxide headgroups decreases intermolecular electrostatic repulsion and reestablishes intramolecular hydrogen bonding of the macrocycle. This (25) On the basis of the pKa values reported by Shinkai for the tetrakis[(4-nitrophenyl)azo]calixarene, a dianion is most probably formed from the [(4-alkylphenyl)azo]calix[4]arene on the subphase of the Langmuir trough at pH 8.2; see ref 17.
2072 Langmuir, Vol. 13, No. 7, 1997
Figure 3. Surface pressure-area isotherms for compression of 2 on pH ) 8.2 phosphate-buffered subphases at varying buffer concentrations: 50, 5, and 0.5 mM.
Figure 4. Surface pressure-area isotherms for compression of 2 on a pH ) 8.2 phosphate-buffered (50 mM) subphase (A) and in the presence of 1 mM 6 (B).
is consistent with the effect of subphase pH on the appearance of isotherms shown in Figure 2. Comparison of the isotherms of 2 at different sodium phosphate buffer concentrations suggests that electrolyte concentration affects intramolecular hydrogen bonding and thus monolayer assembly. The effect of phosphate concentration on the isotherm of 2 was studied at 50, 5, and 0.5 mM at pH 8.2 (Figure 3). The limiting area increased by 14 Å2/molecule over this range of buffer concentrations and by 10 Å2/molecule compared to that for the unbuffered monolayer. Increasing electrolyte concentration provides counterions needed to stabilize the deprotonated phenoxide ions of the calixarene. Thus, higher buffer concentration leads to stable monolayer formation. Effect of Alkylammonium Salts on the Assembly of Monolayers of 2. Calixarenes7 and (phenylazo)calixarenes26 are known to bind ammonium ions in solution and to form complexes in the solid state.27 The monolayer assembly of 2 was investigated with various alkyl- and arylammonium salts in the subphase. When monolayers of 2 were spread on a pH ) 8.2 phosphate buffer (50 mM) subphase containing 1 mM trimethylanilinium iodide 6 and then compressed, the limiting area for the surface pressure-area isotherm decreased on average by 8 Å2/ molecule relative to the isotherm when no trimethylanilinium iodide was present in the subphase (Figure 4). No change in the film stability (i.e., collapse pressure) was observed. When monolayers of 2 were spread on a pH ) 8.2 phosphate-buffered (50 mM) subphase containing 1 mM N,N,N-trimethyl-N-(2-naphthyl)ammonium iodide (26) Tyson, J. C.; Collard, D. M.; Hughes, K. D. Unpublished results. Upfield shifts are observed in the 1H NMR for the water-soluble homolog [(4-carboxyphenyl)azo]calix[4]arene. (27) (a) Harrowfield, J. M.; Ogden, M. I.; Richmond, W. R.; Skelton, B. W.; White, A. H. J. Chem. Soc., Perkin Trans. 2 1993, 2183. (b) Harrowfield, J. M.; Richmond, W. R.; Sobolev, A. N.; White, A. H. J. Chem. Soc., Perkin Trans. 2 1994, 5.
Tyson et al.
(7) and then compressed, no change in limiting area was observed, but the monolayer showed a decrease in film stability (25 to 14 mN/m, not shown). This decrease in film stability with a larger ammonium guest results from different binding of the larger naphthyl ammonium salt compared to the phenyl analog. No change in monolayer formation of 2 on a pH ) 8.2 phosphate buffer (50 mM) was observed when 1 mM tetramethylammonium iodide (8) was present in the subphase. Injections of alkyl- and arylammonium salts into the subphase (to produce 1 mM solutions in 50 mM phosphate buffer, pH ) 8.2), after formation of monolayers of 2, were performed. Monolayers of 2 were compressed to a pressure of 10 mN/m and held at this pressure for 2 h prior to injection of ammonium salts into the subphase. No change was observed in the mean molecular area of the film of 2 when 6, 7, or 8 was injected into the subphase beneath the compressed monolayers. The importance of the cup-shaped structure of the (phenylazo)calixarenes on the interaction with ammonium guests is illustrated by experiments (data not shown) using monolayers of 4-[(4′-octylphenyl)azo]phenol (5) as a model for the monomeric subunit of 2. This model compound forms stable monolayers on a pH ) 8.2 phosphate buffer (50 mM) with a limiting area of 32 ( 1 Å2/molecule. No changes were observed for the limiting area or the film stability of 5 when the subphase contained 1 mM 6. From these results, it is clear that ammonium derivatives interact with monolayers of 2 at the air-water interface and alter the characteristics of the film. Perturbations in monolayer characteristics of 2 are observed for arylammonium salts when present in the subphase before monolayer formation. These results also indicate different binding of phenyl and naphthyl derivatives (6 and 7, respectively). Changes in the intramolecular hydrogen bonding of 2 with pH and electrolyte concentration yield significant differences in molecular packing. In each instance, the mean molecular area increases as intramolecular hydrogen bonding is disrupted, and the cup-shaped cavity of 2 is perturbed. Monolayer formation of 2 on a pH 8.2 phosphate-buffered subphase containing 7 or 8 yields no change in mean molecular area, which indicates that there is no interaction with the intramolecular hydrogen bonding of 2. However, the presence of trimethylanilinium iodide in the subphase does effect the mean molecular area of 2, indicating interaction with the phenoxide polar headgroups and alteration of intramolecular hydrogen bonding. Three possible mechanisms may be proposed for this interaction: Trimethylanilinium iodide may intercalate into the monolayer film with the phenyl substituent located between molecules of 2; alternatively, the charged nitrogen group could interact with the phenoxides with the phenyl substituent located in the subphase; or the phenyl substituent located in the cavity of 2. The first two mechanisms may be discarded for the following reasons. Intercalation of trimethylanilinium iodide into the monolayer film would increase the mean molecular area observed during isotherm generation, which is not observed. The intercalation mechanism does not provide a rationale for molecular level selectivity between phenyl or napthyl derivatives. Further support for elimination of the second mechanism can be made from the observation that injection of trimethylanilinium iodide under a compressed monolayer film does not yield any change in the monolayer pressure. The analogous experiment with phosphoric acid injection under the monolayer did yield the expected shift (decrease) in mean molecular area corresponding to changes in phenol hydrogen bonding. The last mechanism provides a plausible explanation for
[(4-Alkylphenyl)azo]-Substituted Calixarenes
the decrease in mean molecular area observed when trimethylanilinium iodide is present in the subphase. In this mechanism the phenyl subsituent of 6 is bound within the cavity of 2. Placement of the arene within the cavity provides for interaction between the positively charged nitrogen and the phenoxide group(s) of 2, generating a more cylindrical geometry and a smaller mean molecular area. Initial spectroscopic investigation indicates a shift (approximately 10 nm) in the electronic absorption of the calixarenes upon binding of ammonium derivatives.28 These experiments are being pursued and should provide evidence for the orientations of both species. In conclusion, the molecular level packing of amphiphilic calixarene derivatives at the air-water interface is controlled by the pH and electrolyte concentration of the subphase. Initial guest-host binding experiments yielded evidence that these amphiphilic calixarenes bind ammonium derivatives by different mechanisms. These (28) Moore, J. L.; Tyson, J. C.; Collard, D. M.; Hughes, K. D. Unpublished results.
Langmuir, Vol. 13, No. 7, 1997 2073
results provide strong evidence that selective monolayer organic films can be designed and synthesized for chemical sensing applications. Acknowledgment. We thank Beth A. Haywood for performing preliminary experiments and the College of Sciences at the Georgia Institute of Technology for partial financial support for this research. D.M.C. gratefully acknowledges the Henry and Camille Dreyfus Foundation (New Faculty Award), Research Corporation (Cottrell Scholarship), and the National Science Foundation (CAREER Award) for presentation of junior faculty awards. K.D.H. acknowledges support in the form of the National Science Foundation Environmental Traineeship Grant #564. Supporting Information Available: NMR and IR spectral data of tetrakis[(alkylphenyl)azo]calix[n]arenes 2-4 (2 pages). Ordering information is given on any current masthead page. LA960316D
