Probing Inter- and Intramolecular Interactions of Six New p-tert

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Langmuir 2002, 18, 8854-8861

Probing Inter- and Intramolecular Interactions of Six New p-tert-Butylcalix[4]arene-Based Bipyridyl Podands with Langmuir Monolayers Ange´line Van der Heyden,† Jean-Bernard Regnouf-de-Vains,‡ Piotr Warszyn´ski,§ Jean-Olivier Dalbavie,‡ Andrzej Z˙ ywocin´ski,| and Ewa Rogalska*,† Equipe de Physico-chimie des Colloı¨des, UMR 7565 CNRS/Universite´ Henri Poincare´ Nancy I, Faculte´ des Sciences, BP 239, 54506 Vandoeuvre-le` s-Nancy Cedex, France, GEVSM, UMR 7565 CNRS/Universite´ Henri Poincare´ Nancy I, Faculte´ de Pharmacie, 5, rue Albert Lebrun, BP 403, 54001 Nancy Cedex, France, Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek, 30-239 Krako´ w, Poland, and Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warszawa, Poland Received January 24, 2002. In Final Form: June 7, 2002 Six hydrophobic p-tert-butylcalix[4]arenes bearing at the lower rim two bipyridyl moieties and two alkyl or alkylaryl moieties or free hydroxyl groups in alternate positions were synthesized. The behavior of these molecules in Langmuir films was studied using surface pressure (Π) and surface potential (∆V) measurements performed as a function of film compression (compression isotherms) or as a function of time. For chosen calixarenes, Brewster angle microscopy in monolayers was utilized. The results obtained revealed that the calixarene lateral nonbipyridyl groups, different for each molecule, determined the intermolecular interactions in monolayers. On the contrary, since no significant differences could be observed in calixarene conformational reorganization, it was concluded that the bipyridyl groups, common for all molecules, dominated this process. Molecular mechanics simulations using HyperChem have supported the above interpretations.

1. Introduction The pioneering work of Gutsche and the resulting abundant literature related to the calixarenes have shown that these polyphenolic macrocycles play an important role in supramolecular chemistry.1 Because of host-guest properties calixarenes have found uses as phase-transfer agents, accelerants in instant adhesives, hydrolysis catalysts, molecular switches, and molecular capsules for substance delivery via “poly”calixarenes.2 The coordinative properties toward metal cations can be inferred on calixarenes by grafting chelating moieties on the aromatic crown. The ability to bind various metal ions selectively makes calixarene derivatives interesting extractants for heavy and noble metal ions3 and for rare earth metal ions,4 as well as selectors for chromatographic separations.5 The relation between the spatial organization of various calixarene-based heterocyclic podands and the capacity of complexing metal cations is at the center of many investigations.6 * Corresponding author. E-mail: [email protected]. † Equipe de Physico-chimie des Colloı¨des, Universite ´ Nancy I. ‡ GEVSM, Universite ´ Nancy I. § Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences. | Institute of Physical Chemistry, Polish Academy of Sciences. (1) (a) Gutsche, C. D. In Calixarenes; Stoddart, F. J., Ed.; Monographs in Supramolecular Chemistry; The Royal Society of Chemistry: Cambridge, 1989. (b) Calixarenes. A Versatile Class of Macrocyclic Compounds; Bo¨hmer, V., Vicens, J., Ed.; Kluwer Academic Publications: Dordrecht, 1991. (c) Gutsche, C. D. Calixarenes Revisited; Monographs in Supramolecular Chemistry; Royal Society of Chemistry: Cambridge, 1998. (d) Calixarenes 2001; Azfari, Z., Bo¨hmer, V., Harrowfield, J., Vicens, J., Ed.; Kluwer Academic Publishers: Dordrecht, 2001. (2) Arimura, T.; Matsumoto, S.; Teshima, O.; Nagasaki, T.; Shinkai, S. Tetrahedron Lett. 1991, 32, 5111-5114. (3) Yordanov, A. T.; Roudhill, D. M. Coord. Chem. Rev. 1998, 170, 93-124. (4) Ohto, K. Anal. Sci. 1995, 11, 893-902. (5) Glennon, J. D. J. Chromatogr. A 1996, 731, 47-55.

Recently, we have synthesized numerous metal chelating calixarenes with pronounced specificity for Cu(I) ions.6c,d,i-k These water-insoluble molecules were conceived as metal ion extractants in organic solvents.6n However, the polarity gradient induced along the main axis of the structures by the same four nitrogen atoms from the bipyridyl groups, which are involved in metal chelating, makes these calixarenes amphiphilic. As described in the literature, adjusting the amphiphilic nature of the calixarene molecule by chemical modification allows the formation of stable monolayers at the air/water interface7 and, subsequently, transfer of these monolayers on solid supports to prepare Langmuir-Blodgett films for sensing,8 membrane filtration9 or metal ion immobilization.10 While various calixarene derivatives can be readily assembled in stable Langmuir-Blodgett (LB) films, a better understanding and control of the molecule orientational and conformational changes during this process11 is obviously needed for a rationale engineering of different (6) (a) Wieser, C.; Dieleman C.; Matt, D. Coord. Chem. Rev. 1997, 165, 93-161. (b) Beer, P. D.; Martin, J. P.; Drew, M. G. B. Tetrahedron 1992, 48, 9917-9928. (c) Regnouf-de-Vains, J.-B.; Lamartine, R. Helv. Chim. Acta 1994, 77, 1817-1825. (d) Pellet-Rostaing, S.; Regnouf-deVains, J.-B.; Lamartine, R. Tetrahedron Lett. 1996, 37, 5889-5892. (e) Beer, P. D.; Chen, Z.; Goulden, A. J.; Grieve, A.; Hesek, D.; Szemes, F.; Wear, T. J. Chem. Soc., Chem. Commun. 1994, 1269-1271. (f) Grigg, R.; Holmes, J. M.; Jones, S. K.; Amilaprasadh, W. D. J. Chem. Soc., Chem. Commun. 1994, 185-187. (g) Ulrich, G.; Ziessel, R. Tetrahedron Lett. 1994, 35, 6299-6302. (h) Regnouf-de-Vains, J.-B.; Lamartine, R. Tetrahedron Lett. 1996, 37, 6311-6314. (i) Pellet-Rostaing, S.; Regnoufde-Vains, J.-B.; Lamartine, R.; Meallier, P.; Guittonneau S.; Fenet, B. Helv. Chim. Acta 1997, 80, 1229-1243. (j) Regnouf-de-Vains, J.-B.; Lamartine, R.; Fenet, B. Helv. Chim. Acta 1998, 81, 661-669. (k) PelletRostaing, S.; Regnouf-de-Vains, J.-B.; Lamartine, R.; Fenet, B. Inorg. Chem. Commun. 1999, 2, 44-47. (l) Molard, Y.; Bureau, C.; ParrotLopez, H.; Lamartine, R.; Regnouf-de-Vains, J.-B. Tetrahedron Lett. 1999, 40, 6383-6387. (m) Dalbavie, J.-O.; Regnouf-de-Vains, J.-B.; Lamartine, R.; Lecocq, S.; Perrin, M. Eur. J. Inorg. Chem. 2000, 4, 683-691. (n) Regnouf-de-Vains, J.-B.; Dalbavie, J.-O.; Lamartine, R.; Fenet, B. Tetrahedron Lett. 2001, 42, 2681-2684. (o) Psychogios, N.; Regnouf-de-Vains, J.-B. Tetrahedron Lett. 2001, 42, 2799-2800.

10.1021/la025575j CCC: $22.00 © 2002 American Chemical Society Published on Web 07/26/2002

p-tert-Butylcalix[4]arenes in Langmuir Films

Langmuir, Vol. 18, No. 23, 2002 8855

functional surfaces, such as sensors. With this objective in mind, we have selected a family of six structurally closely related ligands for monolayer studies. The ligands used in this work have two 2,2′-bipyridine subunits and two noncoordinating armssisobutyl (I), S-(+)-2-methylbutyl (II), 2-ethylbutyl (III), butyl (IV), benzyl (V), or H (VI)s grafted alternatively at the lower rim of the p-tert-butyl calix[4]arene platform (see Figure 1). The amphiphilic character of the six molecules is demonstrated in the fact that they all form stable Langmuir films at the air/water interface. The calixarene behavior in the films was studied using surface pressuremolecular area (Π-A) isotherms, surface potentialmolecular area (∆V-A) isotherms, film relaxation measurements (Π-t), Brewster angle microscopy (BAM), and molecular modeling. The results obtained gave us an insight into the inter- and intramolecular interactions of these molecules and showed that the calixarenes studied differ significantly in their interfacial behavior. Indeed, the relaxation, stability, and rigidity of the monolayers were shown to be strongly related to the small structural differences within the noncomplexing region of the molecules. We expect that these differences may be, however, crucial for a successful formation of metal complexes and their stability. Further studies aimed at constructing functional calixarene-based surfaces are in progress. 2. Experimental Section 2.1. Synthesis of Podands. The cone-conformed ligands II,6j V,6n and VI6c were prepared as previously described. The three alkyl derivatives I, III, and IV were prepared with yields of ∼60% by reacting VI with the corresponding alkyl bromide in dry DMF, using NaH as base. They gave satisfactory elemental, mass spectrometry, and NMR analyses. 13C NMR experiments confirmed that, as expected for this study, these three new compounds were in a cone conformation,12 with Ar-CH2-Ar resonance signals located at 31.51, 31.68, and 31.50 ppm, respectively. General. Melting points (°C, uncorrected) were determined with an Electrothermal 9100 capillary apparatus. 1H and 13C NMR spectra were recorded with a Bruker DRX 400 spectrometer (400 and 100.6 MHz, CDCl3, TMS as internal standard, chemical shifts in ppm, J in Hertz). Mass spectra (electrospray, ES) were recorded with a Platform Micromass apparatus at the Service Central d’Analyse du CNRS, Solaize. Infrared analysis was performed with a Mattson 5000 FT apparatus (ν in cm-1), and UV spectra were recorded with a Shimadzu UV 2401 PC or a SAFAS UVmc2 apparatus (λmax in nm,  in dm3 mol-1 cm-1). (7) (a) Zang, L. H.; et al. Langmuir 1999, 15, 1725-1730. (b) Merhi, G.; Munoz, M.; Coleman, A. W.; Barrat, G. Supramol. Chem. 1995, 5, 173-177. (c) Shahgaldian, P.; Coleman, A. W. Langmuir 2001, 17, 68516854. (d) Tyson, J. C.; Moore, J. L.; Hughes, K. D.; Collard, D. M. Langmuir 1997, 13, 2068-2073. (e) Lo Nostro, P.; Casanati, A.; Bossoletti, L.; Dei, L.; Baglioni, P. Colloids Surf., A 1996, 116, 203209. (f) Capuzzi, G.; Fratini, E.; Dei, L.; LoNostro, P.; Casnati, A.; Gilles, R.; Baglioni, P. Colloids Surf., A 2000, 167, 105-113. (g) Esker, A. R.; Zhang, L. H.; Olsen, C. E.; No, K.; Yu, H. Langmuir 1999, 15, 17161724. (h) Zhang, L. H.; Esker, A. R.; No, K.; Yu, H. Langmuir 1999, 15, 1725-1730. (8) (a) Yagi, K.; Khoo, S. B.; Sugawara, M.; Sakaki, T.; Shinkai, S.; Odashima, K.; Umezawa, Y. J. Electroanal. Chem. 1996, 401, 65-79. (b) Lavrik, N. V.; DeRossi, D.; Kazantseva, Z. I.; Nabok, A. V.; Nesterenko, B. A.; Piletsky, S. A.; Kalchenko, V. I.; Shivaniuk, A. N.; Markovskiy, L. N. Nanotechnology 1996, 7, 315-319. (9) Conner, M. D.; Janout, V.; Kudelka, I.; Dedek, P.; Zhu, J.; Regen, S. L. Langmuir 1993, 9, 2389-2397. (10) (a) Davis, F.; Stirling, C. J. M. Langmuir 1996, 12, 1892-1894. (b) Hassan, A. K.; Nabok, A. V.; Ray, A. K.; Davis, F.; Stirling, C. J. M. Thin Solid Films 1998, 329, 686-689. (11) Conner, M.; Kudelka, I.; Regen, S. L. Langmuir 1991, 7, 982987. (12) (a) de Mendoza, J.; Prados, J. P.; Nieto, P. M.; Sanchez, C. J. Org. Chem. 1991, 56, 3372-3376. (b) Magrans, J. O.; de Mendoza, J.; Pons, M.; Prados, P. J. Org. Chem. 1997, 62, 4518-4520.

Figure 1. Molecular structures of the six calixarenes studied. Elemental analyses were performed at the Service Central de Microanalyse, Ecole Supe´rieure de Chimie, Montpellier. Macherey-Nagel TLC plates were used for chromatography analysis (SiO2, Polygram SIL G/UV254, ref 805021). All commercially available products were used without further purification unless otherwise specified. 5,11,17,23-Tetra(tert-butyl)-25,27-di[(6-(6′-methyl-2,2′-bipyridine)-yl)methoxy]-26,28-di(1-(2-methyl)propoxy)calix[4]arene (I). Same procedure, from 0.2 g of 5,11,17,23-tetra(tert-butyl)-25,27-di[(6-(6′-methyl-2,2′-bipyridine)-yl)methoxy]-26,28-di(hydroxy)calix[4]arene (1.97 × 10-4 mol), 0.079 g of NaH (60% in oil, 1.97 × 10-3 mol), and 2-methylpropyl bromide (0.107 mL, 9.87 × 10-4 mol); reaction time 12 h; yield 0.125 g, 56%. White powder. Mp: 197-198 °C. 1H NMR: 0.775 (d, J ) 6.7, 12 H, OCH2CH(CH3)2); 0.95 (s, 18 H, Me3C); 1.28 (s, 18 H, Me3C); 1.91 (sext, J ) 6.7, 2 H, OCH2CH(CH3)2); 2.66 (s, 6 H, Mebpy); 3.09, 4.43 (“q”, AB, JAB

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) 12.6, 8 H, Ar-CH2-Ar); 3.40 (d, J ) 6.7, OCH2CH(CH3)2); 5.38 (s, 4 H, OCH2bpy); 6.62 (s, 4 H, Ar); 7.02 (s, 4 H, Ar); 7.16 (d, J ) 7.4, 2 H, bpy); 7.65 (t, J ) 7.6, 2 H, bpy); 7.85 (d, J ) 7.0, 2 H, bpy); 7.92 (t, J ) 7.6, 2 H, bpy); 8.10 (d, J ) 7.6, 2 H, bpy); 8.37 (d, J ) 7.3, 2 H, bpy). 13C NMR: 20.32 (OCH2CH(CH3)2); 25.10 (Mebpy); 29.27 (OCH2CH(CH3)2); 31.51 (Ar-CH2-Ar); 31.71, 32.09 (Me3C); 34.07, 34.39 (Me3C); 77.93 (OCH2bpy); 82.26 (OCH2CH(CH3)2); 118.77, 119.95, 123.40, 124.22, 125.20, 125.87, 137.33, 137.42 (C(3), C(3′), C(4), C(4′), C(5), C(5′) of bpy; C(m) of Ar); 132.63, 135.30, 144.16, 145.11, 153.66, 153.78, 155.66, 156.37, 158.04, 158.32 (C(2), C(2′), C(6), C(6′) of bpy; C(o),(p),(i) of Ar). MS (EI): 1124 ([M]+•); 1051 ([M - OCH2CH(CH3)2]+•); 942 ([M bpy]+•). Anal. Calc for C76H92O4N4 (1125.60): C, 81.10; H, 8.24; N, 4.98. Found: C, 81.12; H, 8.28; N, 4.84. 5,11,17,23-Tetra(tert-butyl)-25,27-di[(6-(6′-methyl-2,2′-bipyridine)-yl)methoxy]-26,28-di(1-(2-ethyl)butoxy)calix[4]arene (III). Same procedure, from 0.2 g of 5,11,17,23-tetra(tert-butyl)-25,27-di[(6-(6′-methyl-2,2′-bipyridine)-yl)methoxy]-26,28-di(hydroxy)calix[4]arene (1.97 × 10-4 mol), 0.079 g of NaH (60% in oil, 1.97 × 10-3 mol), and 2-ethylbutyl bromide (0.139 mL, 9.87 × 10-4 mol); reaction time 12 h; yield 0.130 g, 56%. White powder. Mp: 136-137 °C. 1H NMR: 0.673 (t, J ) 7.4, 12 H, OCH2CH(CH2CH3)2); 0.88 (s, 18 H, Me3C); 1.23 (sext, J ) 6.7, 8 H, OCH2CH(CH2CH3)2); 1.37 (s, 18 H, Me3C); 1.50 (quint, J ) 6.7, 2 H, OCH2CH(CH2CH3)2); 2.66 (s, 6 H, Mebpy); 3.12, 4.44 (“q”, AB, JAB ) 12.8, 8 H, Ar-CH2-Ar); 3.44 (d, J ) 6.7, OCH2CH(CH2CH3)2); 5.48 (s, 4 H, OCH2bpy); 6.53 (s, 4 H, Ar); 7.13 (s, 4 H, Ar); 7.16 (d, J ) 7.6, 2 H, bpy); 7.64 (t, J ) 7.6, 2 H, bpy); 7.92 (m, 4 H, bpy); 8.09 (d, J ) 7.8, 2 H, bpy); 8.37 (dd, J1 ) 6.7, J2 ) 2.5, 2 H, bpy). 13C NMR: 11.00 (OCH2CH(CH2CH3)2); 23.11 (OCH2CH(CH2CH3)2); 25.09 (Mebpy); 31.50 (Ar-CH2-Ar); 31.63, 32.18 (Me3C); 34.02, 34.45 (Me3C); 41.44 (OCH2CH(CH2CH3)2); 77.50, 78.87 (OCH2CH(CH2CH3)2, OCH2bpy); 118.76, 119.83, 122.61, 123.35, 123.94, 125.03, 126.07, 137.27, 137.47 (C(3), C(3′), C(4), C(4′), C(5), C(5′) of bpy; C(m) of Ar); 132.14, 135.79, 144.12, 145.13, 153.42, 153.99, 155.54, 156.41, 157.99, 158.65 (C(2), C(2′), C(6), C(6′) of bpy; C(o),(p),(i) of Ar). MS (EI): 1181 ([M]+•); 1079 ([M OCH2CH(CH2CH3)2]+•); 998 ([M - bpy]+•]). Anal. Calc for C80H100O4N4, 0.1 CH2Cl2 (1190.21): C, 80.83; H, 8.47; N, 4.70. Found: C, 80.84; H, 8.63; N, 4.46. 5,11,17,23-Tetra(tert-butyl)-25,27-di[(6-(6′-methyl-2,2′-bipyridine)-yl)methoxy]-26,28-di(1-butoxy)calix[4]arene (IV). 5,11,17,23-Tetra(tert-butyl)-25,27-di[(6-(6′-methyl-2,2′-bipyridine)-yl)methoxy]-26,28-di(hydroxy)calix[4]arene (0.2 g, 1.97 × 10-4 mol) and 0.079 g of NaH (60% in oil, 1.97 × 10-3 mol) were mixed in dry DMF (5 mL) under argon and at room temperature. After 1 h, 1-bromobutane (0.1 mL, 9.87 × 10-4 mol) was added and stirring was continued during ∼2 h, with TLC monitoring (Al2O3, CH2Cl2/hexane, 1/1). The DMF was evaporated to dryness under high vacuum, and the viscous residue was treated with H2O. The resulting white precipitate was collected and then chromatographed (Al2O3, CH2Cl2/hexane, 1/1) to give 3 (0.13 g, 60%). White powder. Mp:187 °C. 1H NMR: 0.65 (t, J ) 7.2, 6 H, OCH2CH2CH2CH3); 0.95 (m, 4 H, OCH2CH2CH2CH3); 0.95 (s, 18 H, Me3C); 1.24 (s, 18 H, Me3C); 1.73 (m, 4 H, OCH2CH2CH2CH3); 2.62 (s, 6 H, Mebpy); 3.81 (t, J ) 8.2, 4 H, OCH2CH2CH2CH3); 3.10, 4.47 (“q”, AB, JAB ) 12.6, 8 H, Ar-CH2-Ar); 5.01 (s, 4 H, CH2bpy); 6.61 (s, 4 H, Ar); 6.99 (s, 4 H, Ar); 7.12 (d, J ) 7.5, 2 H, bpy); 7.62 (t, J ) 7.7, 2 H, bpy); 7.68 (d, J ) 7.6, 2 H, bpy); 7.80 (t, J ) 7.7, bpy); 8.15 (d, J ) 7.7, 2 H, bpy); 8.37 (d, J ) 7.7, 2 H, bpy). 13C NMR: 14.08 (OCH2CH2CH2CH3); 18.95 (OCH2CH2CH2CH3); 24.68 (Mebpy); 31.68, 31.86 (Ar-CH2-Ar, OCH2CH2CH2CH3); 31.35, 31.70 (Me3C); 33.75, 34.00 (Me3C); 74.90 (OCH2CH2CH2CH3); 78.59 (OCH2bpy); 118.24, 119.82, 123.14, 123.26, 124.80, 125.30, 136.97, 137.03 (C(3), C(3′), C(4), C(4′), C(5), C(5′) of bpy; C(m) of Ar); 132.80, 135.03, 144.45, 152.63, 154.29, 155.70, 155.79, 157.39, 157.71 (C(2), C(2′), C(6), C(6′) of bpy; C(o),(p),(i) of Ar). ES-MS (positive mode; +80 V): 1125.8 ([3 + H]+). Anal. Calc for C76H92O4N4 (1125.85): C, 81.08; H, 8.24; O, 5.68; N, 5.00. Found: C, 81.19; H, 8.44; O, 5.44; N, 4.81. 2.2. Monolayer Experiments. Monolayer experiments were carried out with a KSV 5000 barostat (KSV, Helsinki). A Teflon trough (15 cm × 58 cm × 1 cm) with two hydrophilic Delrin barriers (symmetric compression) was used in all experiments. The system was equipped with an electrobalance and a platinum

Van der Heyden et al. Wilhelmy plate (perimeter 39.24 mm) as a surface pressure sensor, and with a surface potential measuring head with a vibrating electrode. A platinum plate (diameter 4 cm) immersed 4 mm below the water surface was used as a counter electrode. The apparatus was closed in a Plexiglas box, and temperature was kept constant at 18 °C. Before each utilization, the trough and the barriers were cleaned using cotton soaked in chloroform, gently brushed with ethanol and then with tap water, and finally rinsed with water purified by reverse osmosis (electrodeionized and osmozed water; Elix 3, Millipore, France). All solvents used for cleaning the trough and the barriers were of analytical grade. Aqueous subphases for monolayer experiments were prepared with water purified by reverse osmosis, which had the surface tension 72.75 mN/m at 20 °C. For all compounds studied, the compression isotherm experiments were performed using aqueous subphases of three different pH values: pure water, pH 5.7; 1 mM HCl solution, pH 3.0; sodium tetraborate buffer solution (0.0125 M Na2B4O7 × 10 H2O and 0.020 M NaOH), pH 10.0. Analytical grade sodium tetraborate decahydrate and sodium hydroxide were purchased from Fluka. Any residual surfaceactive impurities were removed before each experiment by sweeping and suction of the surface. Monolayers were spread, using calibrated solutions of calixarenes of a concentration of about 1 mg/mL, prepared with spectrophotometric grade chloroform (Aldrich, ACS). The stability of the surface potential (∆V) signal was checked before each experiment, after cleaning the subphase surface. After the ∆V signal had reached the constant value, it was zeroed, and the film was spread on the subphase. After the equilibration time of 20 min, the films were compressed at the rate 0.2 mm/min (0.1-0.2 Å2/(molecule min)). A PC computer and KSV software were used to control the experiments. 2.3. Molecular Mechanics Simulations. The molecular mechanics simulations of the six p-tert-butylcalix[4]arenes were carried out using the HyperChem molecular modeling package (version 6).13 The Amber force field14 supplied with the package was used with the Amber 1996 parameter set.15 Two procedures of assigning partial charges to atoms of calixarene molecules for molecular modeling were implemented. In the first one, the geometry of the molecule was preoptimized using the Amber force field with no partial charges. Then, the charges were assigned using the AM1 semiempirical method.16 Next, the geometry was again optimized using Amber. The last two steps of the procedure were repeated until the convergence was achieved (the convergence criterion for geometry optimization was 0.01 kcal/(Å mol)). A similar method of charge assignment was used recently for modeling of a sodium complex of a calixarene tetraester.17 The second procedure of assigning partial charges was based on the Partial Equalization of Orbital Electronegativity approach of Gasteiger and Marsili18 implemented in HyperChem. Since in this procedure the partial charges are assumed independent of the molecular geometry, charges were assigned once and then the molecule geometry was optimized. All calculations were performed using a distance dependent dielectric constant to simulate the effect of water as solvent.19 After charges were assigned, three runs of the molecular dynamic simulations were performed for each molecule to search for the possible existence of structures with lower energy than that of one obtained in the initial procedure. Each run consisted of heating the structure to 300 K, molecular dynamics simulations for 5 ps, and (13) HyperChem v.6 is available from HyperCube Inc., 1115 N. W. 4th Street, Gainesville, FL, 32601. (14) Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M., Jr.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P. A. J. Am. Chem. Soc. 1995, 117, 5179-5197. (15) The parameter set is available from http://www.amber.ucsf.edu/ amber. (16) (a) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 48994907. (b) Dewar, M. J. S.; McKee, M. L. J. Am. Chem. Soc. 1977, 99, 5231-5241. (c) Dewar, M. J. S.; Rzepa, H. J. Am. Chem. Soc. 1978, 100, 58-67. (17) Kane, P.; Fayne, D.; Diamond, D.; Bell, S. E. J.; McKervey, M. A. J. Mol. Model. 1998, 4, 259-267. (18) Gasteiger, J.; Marsili, M. Tetrahedron 1980, 36, 3219-3288. (19) Weiner, S. J.; Kollman, P. A.; Case, D. A.; Singh, U. C.; Ghio, C.; Alagona, G.; Profeta, S., Jr.; Weiner, P. J. Am. Chem. Soc. 1984, 106, 765-784.

p-tert-Butylcalix[4]arenes in Langmuir Films

Langmuir, Vol. 18, No. 23, 2002 8857

Table 1. Calixarene Molecular Parameters Obtained from Molecular Mechanics Simulations AM1 + AMBER (procedure I) sizea

crown (Å × Å)

11.79 × 5.49 11.81 × 5.50 11.77 × 5.50 11.67 × 5.52b 11.79 × 5.51 12.06 × 5.45b 11.67 × 5.55 11.46 × 5.62

I II III IV V VI a

GM + AMBER (procedure II)

molecular area at interface (Å2)

Feret diameter at interface (Å)

dipole moment (D)

135.5 152.6 145.5 138.7 133.5 127.6 142.0 120.9

13.2 13.9 13.6 13.4 13.0 12.7 13.5 12.4

-4.84 -4.43 -4.63 -4.83 -4.48 -4.70 -4.12 -4.01

sizea

crown (Å × Å)

molecular area at interface (Å2)

Feret diameter at interface (Å)

dipole moment (D)

135.1 147.2 143.6 137.6 133.5 125.4 142.1 120.1

13.2 13.7 13.5 13.3 13.0 12.6 13.5 12.2

-3.80 -2.79 -3.40 -3.61 -3.66 -3.74 -2.97 1.96

11.75 × 5.49 11.74 × 5.50 11.70 × 5.50 11.60 × 5.53b 11.65 × 5.53 11.96 × 5.46b 11.58 × 5.56 11.36 × 5.63

Distances between central carbon atoms of isobutyls 1,3 and 2,4. b Structures found after annealing. Table 2. Compression Isotherm and Film Relaxation Data isotherms collapse values A (Å2) Π (mN.m-1)

I II III IV V VI

139.4 152.0 138.4 113.0 134.7 114.1

16.9 13.6 12.2 24.8 24.7 23.8

relaxation measurements

Cs-1 (mN.m-1)

µeff/ (D)

Πmin (mN.m-1)

∆Πmax. (mN.m-1)

∆Π/∆t (mN.m-1) (900-1000 min)

147.0 126.5 131.9 316.7 454.7 153.2

1.15 ( 0.1 1.6 ( 0.2 1.0 ( 0.1 1.8 ( 0.2 1.0 ( 0.1 2.3 ( 0.1

9.33/7.44a 9.71 9.57 3.26 5.33 9.25

0.67/2.56b 0.29 0.43 6.74 4.67 0.75

0.000 76/0.001 46c 0.002 18 0.001 82 0.001 50 0.002 06 0.001 53

a Minimal surface pressure values obtained for the 1st/2nd Π decrease. b ∆Π c max values calculated for the 1st/2nd Π decrease. ∆Π/∆t calculated between 125 and 325 min/between 1000 and 1144 min (end of the kinetics).

then annealing to 0 K. The optimal structures obtained using these procedures for the six calixarenes studied are presented in Figure 1. The CPK space-filling molecular model1a,7e,20 was used to evaluate the molecular area at the interface. It was assumed that the air/water interface coincides with the plane of the phenol oxygens. The projections of the CPK model of the calixarenes studied were calculated, and the results concerning the crosssectional area, the Feret diameter (the diameter of the circle having the same area) of the molecule at the interface, and the dipole moments calculated on the basis of partial charges are collected in Table 1. They are discussed in the next section. For molecules II and IV, additional simulations were performed in which the structures were heated to 300 K, and then a single water molecule was placed in the gap between bipyridyls and the molecular dynamics simulation runs were performed on the whole system for 20 ps.

3. Results and Discussion The calixarenes used in this study have four tert-butyl groups grafted at the upper rim of the calixarene crown, two 6-methyl-2,2′-bipyridyl groups grafted via a 6′methylene linker at the calixarene lower rim, on phenol oxygens 1 and 3, as well as two other identical lateral moieties, grafted via phenol oxygens 2 and 4. The two lateral moieties present in different calixarenes are butyl, isobutyl, (S)-(+)-2-methylbutyl, 2-ethylbutyl, benzyl, and hydrogen. It can be seen (Figure 1) that because of the sequence of grafting of the lateral moieties, all six calixarenes belong to the group of symmetry C2. 3.1. Π-A Compression Isotherms. The calixarene isotherms were performed at the compression rate 0.2 mm/min. The compression isotherms of calixarene monolayers obtained on a pure water subphase at pH 5.7 are presented in Figure 2, and their characteristic parameters are collected in Table 2. The isotherms clearly show that the compounds studied differ significantly in their interfacial behavior, depending (20) (a) Cram, D. J.; Karbach, S.; Kim, Y.-H.; Baczynsky, L.; Marti, K.; Sampson R.; Kalleymeyn, J. W. J. Am. Chem. Soc. 1988, 110, 25542560. (b) Cram, D. J.; Karbach, S.; Kim, Y.-H.; Knobler, C. B.; Maverick, E. F.; Ericson, J. L. J. Am. Chem. Soc. 1988, 110, 2229-2237.

Figure 2. Surface pressure-area isotherms of calixarene I-VI monolayers on pure water.

on the structure of the lateral moieties. This observation indicates that the lateral moieties are involved in the intermolecular interactions in the monolayer. Consequently, it means that the calixarenes adopt a conformation at the interface in which the lateral moieties extend beyond the projection of the molecule on the water surface. Calixarenes can adopt such a conformation when the bipyridyl groups are brought together beneath the aromatic crown (see Figure 1). The latter would mean, however, that the bipyridyl groups are immersed in the water subphase. The hydrogen bonding of nitrogens with water molecules, which penetrate the gap between the bipyridyls, may facilitate such a situation. Indeed, the molecular mechanics results have shown that a single water molecule placed between the bipyridyls is present there in the simulation runs at 300 K. The compression of the film may be an additional force, pushing the bipyridyl groups to immerse in the water subphase and to fold together. On the basis of the compressibility (Cs) and the stability of the monolayer as seen by the values of the surface

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pressure at the point of the collapse of the film (Table 2), the calixarenes can be divided in two groups. In the first group three calixarenes bearing the branched alkyl chains (isobutyl (I), (S)-(+)-2-methylbutyl (II), 2-ethylbutyl (III)) show similarities in the interfacial behavior. The second group consists of molecules bearing the linear butyl chains (IV) and one with a benzyl moiety (V). The calixarene bearing free hydroxyl groups (VI) forms a film of a compressibility comparable with those of compounds I-III and a stability very close to those of compounds IV and V, and thus, it can be considered as an intermediary between the two precedent groups of compounds. The three branched-chain calixarenes form films of a relatively low stability, compared to the case of the other molecules, as indicated by the low surface pressures at the collapse point (Table 2). The least stable film is formed by calixarene III, followed by calixarene II, and calixarene I. This result might indicate that the lowering of the total number of carbon atoms in the branched lateral chains contributes to the stabilization of the film. Since the molecular areas of I and III at the collapse point are almost identical, within the limits of experimental error ((1 Å2), obviously the interactions between the lateral chains are such as to allow the same intermolecular distances in both cases. Such a situation is, indeed, possible because of the conformational flexibility of the 2-ethylbutyl groups, which, upon compression of the film, allows an upward orientation of the two terminal ethyl groups, relative to the water surface, the bonds C2-C3 and C2-C3′ being, of course, oriented outward from the calixarene crown (Figure 3). The upward orientation of the terminal ethyl groups (bonds C3-C4 and C3′-C4′) may create, however, unfavorable interactions with the corresponding aromatic ring of the calixarene crown. Consequently, the stability of the film would decrease, compared to that of the film of calixarene I, bearing small isobutyl groups. The fact that the most fluid (most compressible) among the three films is the one formed with calixarene II (Table 2) and that the molecular area of II is around 13 Å2 bigger than those of I and III indicates clearly that the dissymmetry of the calixarene II lateral chains contributes to the increase of the intermolecular distances, compared to the case of molecules I and III, bearing symmetric chains. This effect may be due more specifically to the fact that both (S)-(+)-2-methylbutyl groups in calixarene II are of the same absolute configuration and thus, in the film, the terminal methyl groups in one molecule encounter the terminal ethyl groups in another molecule. Thus, upon compression of the film, the terminal ethyl group in the lateral chain of calixarene II would not be pushed upward to the same extent, as in the case of the two ethyl groups in calixarene III (Figure 3). The interactions between the chiral lateral chains would thus lead to an increase of the molecular area, compared with those of molecules I and III. These conclusions are supported by a good agreement between the experimental molecular area values obtained at the collapse points for calixarenes I, II, and III (Table 2) and the respective molecular area values obtained from molecular modeling (Table 1). The latter values are 1012 Å2 higher for calixarene II than for I and III. The stability of the film of the podand II is slightly higher compared to that of the film of molecules III, possibly because of the absence of the unfavorable interactions between the terminal ethyl groups and the aromatic rings, but lower compared to that of the film of calixarene I, which may be because of a lower degree of ordering of molecules II in the film. The common feature of the films formed with the three branched-chain molecules is that the collapse of the film

Van der Heyden et al.

Figure 3. Side and top views of the six calixarenes in their most compact conformations; the bipyridyl chains are folded together and the lateral groups interact with the neighbor molecules.

is clearly visible and that it is followed by a plateau. The plateau is particularly long in the case of calixarenes II and III, showing that these molecules, forming the least stable monolayers, have a high propensity to form multilayers. The isotherms of the calixarenes with the n-butyl and benzyl moieties (calixarenes IV and V, respectively) are

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Figure 4. Brewster angle microscopy pictures for a calixarene IV monolayer during a lateral compression: (a-d) gas/liquid phase transition (Π ∼ 0.3 mN/m); (e) liquid phase (Π ∼ 15 mN/m); (f) collapse of the monolayer (Π ∼ 24 mN/m).

significantly different from the point of view of their compressibilities and stabilities, compared to those of the branched-chain molecules. Compounds IV and V form solid films, that is of a two/three times lower compressibility than the precedent ones and of a higher stability. Indeed, the surface pressure at the collapse of these films is approximately two times higher than that of calixarene III (Figure 2 and Table 2). While the molecular area measured at the collapse of the film with calixarene V is slightly lower than those of the branched-chain calixarenes, the molecular area of calixarene IV, bearing the n-butyl chain, is about 25 Å2 lower than those of calixarenes I and III. Also, the molecular simulation results indicate that the molecular area of calixarene IV at the interface is distinctly smaller than that for the calixarenes of the first group. However, the observed area per molecule at the collapse point is even lower than the calculated one. This could be understood if we assume that, upon compression of the film, the flexible n-butyl chains can be folded below the calixarene crown or even squeezed into the water subphase. In that case, the size of the calixarene crown would determine the limiting molecular area (Figure 3). This would indeed allow a decrease of the intermolecular distances and thus a decrease of the calixarene IV molecular area, compared to those of the branched-chain molecules, which are much less flexible. While the stability of the film formed with calixarene V is the same as that of calixarene IV, its molecular area is around 22 Å2 bigger. A similar difference in the molecular area between calixarenes IV and V was obtained from the molecular mechanics modeling (Table 1). Indeed, two neighboring molecules V with the lateral benzyl rings stacking together have bigger dimensions along the phenol oxygen-2,4 axis and can occupy more space on the water surface than two molecules IV but less than the branchedchain molecules I-III. The fact that calixarene V forms the most rigid and stable films (Figure 2) suggests that the aromatic ring stacking may contribute to the film stability as well. At this point it is worth remembering that the films studied in this work may be anisotropic because of the C2 symmetry of the calixarenes. The BAM images obtained with calixarene IV (Figure 4) support, indeed, this hypothesis. The structural calixarene features facilitating the anisotropic organization of the molecules in the film would contribute to its stability and rigidity.

The results obtained with calixarene V suggest that, because of π-π interactions, the benzyl rings could play such a role more successfully than the alkyl moieties. The common feature of calixarene IV and V films is that the collapse point is not clearly visible and, consequently, it was determined as the point of the isotherm deviation from the tangent traced to the most condensed phase. As a matter of fact, in the case of calixarene IV, the low compression rates were decisive for obtaining reliable surface pressure isotherms, with a collapse point value which concurred with the results obtained with the surface potential measurements and BAM (Figure 4). Calixarene VI, bearing free hydroxyl groups, has a molecular area at the collapse of the monolayer comparable with that of calixarene IV, within the limits of experimental error (Table 2). We concluded that, in the case of calixarene VI, the tert-butyl groups of the bipyridyltethered phenol rings determine the molecular area (Figure 3). One should not forget that, in the calixarene conformation in which the bipyridyl groups are immersed in water, the upper rims of the aromatic rings 1,3 with the tert-butyl groups are tilted outward. While different calixarenes may have dimensions along the phenol oxygen2,4 axis equal to, or bigger than, those in the axis 1,3, depending on the dimensions of the lateral chains in positions 2 and 4, the groups -OH are too small to protrude beyond the projection of the calixarene on the plane of water, as determined by the 1,3 phenol rings. Since the molecular areas at the collapse point for calixarenes IV and VI are very close, it suggests that the flexible n-butyl chain for calixarene IV has to be somehow folded below the phenol rings of the calixarene crown. On the other hand, the -OH groups can be engaged in a network of hydrogen bonds and thus participate in the intermolecular interactions between the neighboring molecules. A hydrogen bond may also be formed between the -OH groups and the nitrogen atoms of bipyridines, changing the conformation of the partially hydrophilic part of the molecule. To check the influence of a possible protonation of bipyridine nitrogens on the film parameters, the isotherms of all calixarenes were performed on subphases at different pH values (results not shown). The comparison of the isotherms obtained with different pH values has shown that it had no significant influence on the film properties.

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Figure 5. Surface potential-area isotherms of calixarenes I-VI on pure water. The arrows indicate the film collapse points.

It was observed, however, that all films were slightly more compressible at pH 10.0 than at pH 3.0 and 5.7. These results may indicate that with acidic subphases, contrary to the basic one, the bipyridine nitrogens are involved in a hydrogen bond network stabilizing the bipyridine moieties and thus making the monolayers more rigid. 3.2. Surface Potential Measurements. Figure 5 presents the dependence of surface potential versus area per molecule obtained for the compression rate 0.2 mm/ min. Most of the surface potential curves show a slow increase with the decrease of the molecular area after the liftoff point of the surface pressure isotherms. Before the liftoff point, high fluctuations of the measured surface potential are observed, which could be the result of a domain structure of the film at low surface pressure. After the collapse point is reached, they either level off or slowly decrease, which is an indication of the beginning of the formation of a multilayered structure. Taking into account that the surface potential can be expressed as

∆V )

µeff 1 0 A

where µeff is the effective molecular dipole moment at the interface,  is the dielectric constant at the interface, 0 is the vacuum dielectric permittivity, and A is the area per molecule, one can evaluate the ratio µeff/. This ratio in most cases cannot be separated, so one cannot find the effective molecular dipole moment without an additional assumption concerning the dielectric constants at the interface. Using the experimental data shown in Figure 5, we calculated the average values of µeff/ in the range between the liftoff and collapse points for all the calixarenes studied. The results are given in Table 2. Comparing these results with molecular dipole moments obtained from molecular simulations, one may argue that the effective dielectric constant at the interface when it is almost saturated with calixarene molecules is around 2-3, which is a quite reasonable value for a hydrocarbon layer of calixarene crowns. However, the problem of the surface potential at such an interface is much more complicated and requires further studies. 3.3. Relaxation Measurements. The film relaxation studies were realized on a subphase of pH 5.7 (Figure 6 and Table 2). The films were compressed at the same slow rate as that in the isotherm experiments, that is 0.2 mm/min, to the final surface pressure of 10 mN/m, and then the barriers were removed and variations of Π with time were recorded. With all calixarenes, an initial decrease of the

Figure 6. Relaxation of the calixarene I-VI monolayers after the initial compression to 10 mN/m.

surface pressure was observed. The Π decrease was, however, considerably more pronounced with calixarenes IV and V than with the other ones. Indeed, with molecules IV and V the maximal surface pressure decrease (∆Πmax.) was approximately 6.7 and 4.7 mN/m, respectively, while for the other molecules it was of the order of 0.3-0.7 mN/m (Table 2). The Π decrease reflects the film relaxation, and its amplitude is inversely proportional to the ability of the molecules to rearrange their organization in the film during the initial compression. Taking into account the fact that the monolayers of calixarenes IV and V have a more solidlike character (Figure 2 and Table 2) compared to that of the other molecules (the films of calixarenes IV and V have a compressibility which is about 2-3 times lower than those of calixarenes I, II, III, and VI), their movements during the film compression are hindered by strong intermolecular interactions. This reasoning is supported by the BAM studies, which have shown that, in the case of calixarene IV, condensed phase domains were formed (Figure 4) already at very low surface pressures, contrary to the case of the monolayers of calixarene II, where no domain formation was observed upon compression. The significant surface pressure decrease observed with calixarenes IV and V may be the result of the domain reorientation after the compression is stopped. While BAM studies were performed only for calixarenes II and IV, these molecules can be considered as representative for the two respective groups of molecules, that is, I, II, III, VI, and IV and V. Along this line, and by analogy to the case of calixarene II, no domain formation would be expected below 10 mN/m in the case of molecules I, III, and VI. The difference in ∆Πmax. values between calixarenes IV and V may be ascribed to the aromatic ring stacking effect, which facilitates an anisotropic organization of the calixarene V monolayer. The initial decrease of Π was followed by its increase at comparable rates for different calixarenes (Table 2). The case of calixarene I, for which this Π increase is followed by a second Π decrease, is discussed at the end of this section. Our proposal is that the Π increase is due to the calixarene conformational rearrangement, leading to the minimization of the surface free energy. Indeed, upon lateral compression of the film, the partly hydrophilic bipyridyl chains (a free 2,2′-bipyridyl is weakly soluble in water) may be pushed into the water. In this way, the bipyridyl moieties acquire a compact conformation (they may even interact among themselves via a hydrogen bond network established by the nitrogen atoms) and do not

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zation. The domain formation and growth would be triggered by the slow Π increase, as a consequence of the initial conformational calixarene reorganization. Along this line, in the films of calixarenes II, III, and VI, the formation of domains would not be expected at the surface pressure 10 mN/m, as no important relaxation of these films was observed. This hypothesis is supported by the BAM results, which showed that in the case of the calixarene II no domains were formed.

Figure 7. Side and top views of calixarene II with the bipirydyl groups (a) splayed at the air/water interface and (b) immersed in water. The molecular areas are 159 and 138 Å2, respectively.

interfere with the intermolecular interactions established via the lateral nonbipyridyl moieties. The driving force of the calixarene spontaneous conformational movements in the film would be the low hydrophilicity of the bipyridyl chains. The tendency of the bipyridyl groups to find the most favorable energetic state by positioning themselves at the air/water interface would induce a conformational movement of the whole molecule. Indeed, the modeling results have shown that calixarene II has an effective diameter at the interface of 13.7 Å in the most compact conformation, compared to 15.4 Å for the conformation where the bipyridyl groups are splayed at the air/water interface and interact with water via the nitrogen atoms (Figure 7). On the other hand, the A(t) measurements have shown that the calixarene II molecular area for a constant Π of 10 mN/m increased by 10 Å2 during 4000 min (0.0025 Å2/min) (results not shown). This means that it would take around 15 000 min (more than 10 days) for calixarene II to go from the most compact (147 Å2) to the splayed conformation (186 Å2), provided that the increase in molecular area is linear with time. While such longlasting experiments are not feasible and thus it is not possible to demonstrate experimentally the full conformational transformation of calixarenes, we believe that the observed Π(t) and A(t) increases reflect such changes. The behavior of calixarene I is markedly different from that of the other molecules in that the relaxation of its film is two-stage (Figure 6 and Table 2). The initial small decrease of Π is followed by a very slow Π increase until t of approximately 400 min. At this point, however, a second, abrupt drop of Π is observed, followed by the Π increase at a rate comparable with those of the other calixarenes. One can reason that the abrupt Π drop is a consequence of a structural evolution of the film. On the basis of the BAM and relaxation results obtained with calixarenes II and IV, we propose that the Π decrease after 400 min may reflect calixarene I domain reorgani-

4. Conclusions The results presented above allowed us an insight into intra- and intermolecular interactions of a family of six structurally related calixarenes. All observations taken together argue that the calixarenes adopt at the air/water interface an orientation and a conformation allowing intermolecular interactions of the small nonbipyridyl lateral moieties. We propose that, upon compression of the film, the partly hydrophilic bipyridyl groups are squeezed out from the interface into the water subphase. The ensuing change of the conformation of the calixarenes facilitates the intermolecular interactions of the nonbipyridyl substituents attached to the phenol rings. These interactions, which are different for every molecule, are decisive for the film properties, as exhibited in the isotherm characteristics. The differences in the film properties related to the lateral chain interactions are particularly well exemplified by the results of film relaxation. The resemblances between the calixarenes themselves and their films are, on the contrary, evidenced by the similar rate of the surface pressure increase of the relaxed films. The latter has been tentatively ascribed to the conformational movements of the bipyridyl groups, common for all calixarenes. We have shown that the most solidlike and stable monolayers are formed with calixarene V, bearing benzyl moieties. Taking into account the C2 symmetry of the calixarenes used in this study and, consequently, a possible anisotropic organization of the monolayers, it can be supposed that calixarene structural features facilitating the anisotropic organization of the molecules in the film contribute to its stability and rigidity. Accordingly, the nonflexible aromatic rings could play such a role more successfully than the alkyl moieties. This hypothesis will be verified using new p-tert-butylcalix[4]arene derivatives with bipyridyl groups and different aromatic lateral moieties. Acknowledgment. We thank Dr. Kurt-Dieter Wantke, Max-Planck-Institut fu¨r Kolloid- und Grenzfla¨chenforschung, Golm/Potsdam, Germany, for supplying us with HYPERCHEM software. Discussions with Dr. Gerald Brezesinski, Max-Planck-Institut fu¨r Kolloid- und Grenzfla¨chenforschung, Golm/Potsdam, Germany, and his help with BAM experiments are greatly appreciated. A.V.d.H. and J.-O.D. acknowledge the Ministe`re de l’Education Nationale, de l’Enseignement Supe´rieur et de la Recherche for their Ph.D. fellowships. We thank also Jeff Rice and Dr. C. Kowal for revising the English. LA025575J