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Formation of Langmuir Layers and Surface Modification Using New Upper-Rim Fully Tethered Bipyridinyl or Bithiazolyl Cyclodextrins and Their Fluorescen...
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Langmuir 2004, 20, 5338-5346

Formation of Langmuir Layers and Surface Modification Using New Upper-Rim Fully Tethered Bipyridinyl or Bithiazolyl Cyclodextrins and Their Fluorescent Metal Complexes Mounia Badis,† Ange´line Van der Heyden,† Romain Heck,‡ Alain Marsura,‡ Bernard Gauthier-Manuel,§ Andrzej Zywocinski,†,| and Ewa Rogalska*,† Equipe de Physico-chimie des Colloı¨des, UMR 7565 CNRS/Universite´ Henri Poincare´ Nancy 1, Faculte´ des Sciences, BP 239, 54506 Vandoeuvre-le` s-Nancy Cedex, France, GEVSM, UMR 7565 CNRS/Universite´ Henri Poincare´ Nancy 1, Faculte´ de Pharmacie, 5, rue Albert Lebrun, BP 403, 54001 Nancy Cedex, France, and Laboratoire de Physique et Me´ trologie des Oscillateurs LPMO/CNRS, 32, avenue de l’Observatoire, 25044 Besanc¸ on Cedex, France Received November 3, 2003. In Final Form: April 6, 2004 Seven new amphiphilic cyclodextrins bearing bipyridyl or bithiazolyl moieties at the narrow rim and free hydroxyl or methoxyl groups at the wide rim of the cyclooctaamylose crown were synthesized using a one step “phosphine imide” approach. These ligands form metal complexes that have fluorescence properties with potentials for optical applications. Here, the cyclodextrin derivatives were used as probes for evaluating the role of different moieties in the self-assembly process, providing crucial information in creating functional devices. The behavior of these molecules and of complexes with EuIII in some cases was studied in Langmuir films using surface pressure (Π) and surface potential (∆V) measurements performed as a function of film compression (compression isotherms). For chosen cyclodextrins, Brewster angle microscopy (BAM) in monolayers was performed. Films formed with derivatives 1, 3, 7, and 2compl were transferred on mica using the Langmuir-Blodgett technique. The properties of the films deposited on mica were analyzed with fluorimetry and, in the case of derivative 7, using fringe of equal chromatic order technique (FECO). The monolayer structure and the fluorescence properties of the Langmuir-Blodgett films indicate that the derivatives studied can be used for preparing cyclodextrin-based optical devices.

1. Introduction Cyclodextrins (Cd’s) constitute a class of oligosaccharides with six, seven, or eight D-glucose units linked by R-1,4-glucose bonds. It is well-known that Cd’s can accommodate various guest molecules in their hydrophobic cavity in aqueous solutions.1-4 Though the natural Cd’s are of great interest as molecular hosts, much of their utility in supramolecular chemistry derives from their structural modification. Indeed, the natural Cd’s may be viewed as molecular scaffolds on which functional groups and other substituents of increasing sophistication can be assembled with controlled geometry. Therefore, metallocyclodextrins,5 rotaxanes,6 and catenanes,7-9 or surface monolayers of these modified cyclodextrins,10,11 are now * To whom correspondence should be addressed. E-mail: [email protected]. † Equipe de Physico-chimie des Colloı¨des, Universite ´ Nancy 1. ‡ GEVSM, Universite ´ Nancy 1. § LPMO/CNRS, Besanc ¸ on. | On leave from the Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland. (1) Szejtli, J. Chem. Rev. 1998, 98, 1743. (2) Donze, C.; Coleman, A. W. J. Inclusion Phenom. 1995, 23, 11. (3) Eddaoudi, M.; Parrot-Lopez, H.; de Lamotte, S. F.; Ficheux, D.; Prognon, P.; Coleman, A. W. J. Chem. Soc., Perkin Trans. 2 1996, 1711. (4) Lo Nostro, P.; Santoni, I.; Bonini, M.; Baglioni, P. Langmuir 2003, 19, 2313. (5) Sandow, M.; Easton, C. J.; Lincoln, S. F. Aust. J. Chem. 1999, 52, 1151. (6) Lo Nostro, P.; Lopes, J. R.; Ninham, B. W.; Baglioni, P. J. Phys. Chem. B 2002, 106, 2166. (7) Nepogodiev, S. A.; Stoddart, J. F. Chem. Rev. 1998, 98, 1959. (8) Schalley, C. A.; Beizai, K.; Voegtle, F. Acc. Chem. Res. 2001, 34, 465. (9) Korybut-Daszkiewicz, B.; Wieckowska, A.; Bilewicz, R.; Domagala, S.; Wozniak, K. J. Am. Chem. Soc. 2001, 123, 9356.

accessible. The ligands used in this work have 2,2′bipyridyl or 2,2′-bithiazolyl subunits grafted at the cyclodextrin narrow rim via ureido groups,12 which confer their capacity to complex metal cations. Complexation of lanthanides and transition metals with these seven Cd derivatives was described recently.13 It was observed that the absorption-energy-transfer-emission (AETE) light conversion process (antennae effect) occurs in the lanthanide complexes of these upper-rim fully tethered cyclodextrins (URFT-Cd’s).14 We have also reported recently on a molecular redox switch based on the corresponding iron complexes.13 The ability to bind selectively various metal ions makes the cyclodextrin derivatives used in this study potential candidates for creating functional optical surfaces. To evaluate the suitability of different derivatives for such applications, we have undertaken a study of their behavior in Langmuir films formed at the air/water and air/NaCl aqueous solution interface. This approach attempted to elucidate the role of different moieties in the self-assembly process. The cyclodextrin behavior in the films was studied using surface pressure-molecular area (Π-A) isotherms, surface potential-molecular area (∆V-A) isotherms,15 and (10) Chmurski, K.; Jurczak, J.; Coleman, A. W. J. Coord. Chem. 1999, 47, 59. (11) Wazynska, M.; Temeriusz, A.; Chmurski, K.; Bilewicz, R.; Jurczak, J. Tetrahedron Lett. 2000, 41, 9119. (12) Wagner, M.; Engrand, P.; Regnouf-de-Vains, J.-B.; Marsura, A. Tetrahedron Lett. 2001, 42, 5207. (13) Heck, R.; Dumarcay, F.; Marsura, A. Chem. Eur. J. 2002, 8, 2438. (14) Charbonnier, F.; Humbert, T.; Marsura, A. Tetrahedron Lett. 1999, 40, 4047. (15) Tchoreloff, P. C.; Boissonnade, M. M.; Coleman, A. W.; Baszkin, A. Langmuir 1995, 11, 191.

10.1021/la036070b CCC: $27.50 © 2004 American Chemical Society Published on Web 05/26/2004

Amphiphilic Cyclodextrins with Bipyridyl or Bithiazolyl Moieties

Brewster angle microscopy (BAM). The monolayers formed with derivatives 1, 3, 7, and 2compl were transferred on mica slides for fluorimetry studies. FECO experiments were performed on the derivative 7 film, which is the most stable. The results obtained in this study show that the cyclodextrins differ significantly in their interfacial behavior. The stability and rigidity of the monolayers are strongly related to the cyclodextrin derivative structures. We expect the monolayer properties to be decisive for a successful formation of functional cyclodextrin-based optical devices. 2. Experimental Section 2.1. Synthesis of Cyclodextrins. Structures of all compounds were assigned with 1H and 13C NMR using a Bruker DRX-400 and a Bruker DRX-300 (MAS) spectrometer. FTIR spectra were recorded using a Perkin-Elmer 1600 and UV/Visible spectra were obtained with a Safas UV mc2. Fluorescence spectra were recorded on a Fluorolog II Spex-Jobin-Yvon spectrometer equipped with a 400 W xenon lamp. Mass spectra were recorded in FAB positive mode using a ZAB-SEQ mass spectrometer and a ThermoFinnigan Mat. MALDI-FTICR-MS; laser Nd:YAG (355 nm). The new compounds gave satisfactory spectroscopic data. In all experiments DMF was dried over CaSO4, filtered off, and flushed with argon to eliminate dimethylamine. The synthesis of the intermediates 5-monobromomethyl-5′methyl-2,2′-bipyridine, 5-azidomethyl-5′-methyl-2,2′-bipyridine, 5-aminomethyl-5′-methyl-2,2′-bipyridine, and URFT-cyclodextrins 1-7 has been described previously.12,13 {Hexakis-[6-deoxy-6-(5-methylene-ureido-5′-methyl-2,2′-bipyrHexafluorophosphate idinyl)]cyclomaltohexaose}europiumIII (1compl). EuCl3‚6H2O (0.00374 g; 1.02 × 10-5 mol) was added to a solution of hexakis-[6-deoxy-6-(5-methylene-ureido-5′-methyl2,2′-bipyridinyl]cyclomaltohexaose (0.00215 g; 9.28 × 10-6 mol) in MeOH (10 mL). The mixture was stirred at room temperature for 24 h, and then NH4PF6 (0.005 g; 3.06 × 10-5 mol) was added to the mixture which was stirred at room temperature 24 h more. The solution was then concentrated under reduced pressure and the resulting residue was treated with chloroform. The suspension was filtered off (0.2 µm microfilter); the filtrate was evaporated and dried over P2O5 under vacuum; the yield was 90%. {Hexakis-[2,3-di-O-methyl-6-deoxy-6-(5-methyleneureido-5′methyl-2,2′-bipyridinyl)]cyclomaltohexaose}europiumIII Hexafluorophosphate (2compl). EuCl3‚6H2O (0.00370 g; 9.95 × 10-6 mol) was added to a solution of hexakis-2,3-di-O-methyl-6-deoxy-6(5-methylene-ureido-5′-methyl-2,2′-bipyridinyl)]cyclomaltohexaose (0.0206 g; 8.29 × 10-6 mol) in MeOH (10 mL). The mixture was stirred at room temperature for 24 h, and then NH4PF6 (0.0045 g; 2.73 × 10-5 mol) was added to the mixture which was stirred at room temperature for an additional 24 h. The solution was then concentrated under reduced pressure and the resulting residue was treated with chloroform. The suspension was filtered off (0.2 µm microfilter) and the filtrate was evaporated over P2O5 under vacuum; the yield was 90%. {Heptakis-[6-deoxy-6-(5-methylene-ureido-5′-methyl-2,2′-bipyridinyl)]cyclomaltoheptaose}europiumIII Chloride (3compl). EuCl3‚ 6H2O (3.0 × 10-3 g; 8.14 × 10-6 mol) was added to a solution of 3 (20 × 10-3 g; 7.4 × 10-6 mol) in MeOH (10 mL). The mixture was stirred at room temperature for 72 h. The solution was then concentrated under reduced pressure. The product was precipitated from ether, filtered over a sintered glass, washed several times with ether, and dried over P2O5 under vacuum; the yield was 77%. IR (KBr): ν ) 1635 (CONH). UV/vis (MeOH): λmax () ) 291 nm (62 688 mol-1 dm3 cm-1). Fluorimetry: λmax (exc.) 291 nm, λmax (em.) 614 nm. 1H NMR (400 MHz, CD3OD): δ ) 9.00 (s, 1H, 4J ) 2.0 Hz, H6 bipy); 8.80 (s, 1H, 4J ) 1.4 Hz, H6′ bipy); 8.49 (d, 1H, 3J ) 8.3 Hz, 4J ) 2.3 Hz, H4 bipy); 8.09 (d, 1H, 3J ) 8.3 Hz, 4H bipy); 4.89-5.16 (m, 7H, H1); 4.28-3.33 (m, 44H, H2, H3, H4, H5, H6, CH2-bipy); 2.50 (s, 3H, CH3-bipy); 1.31 (S, 14H, OH). 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.

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Figure 1. Scheme of the process used for FECO experiments: (a) mica slide A covered partly with a protective mica slide B; (b) the mica slides covered with a Cd 7 monolayer using the Langmuir-Blodgett technique; (c) the mica slide A after the removal of the protective slide B; (d) the mica slide A covered with a thin layer of silver. The system was equipped with an electrobalance and a platinum 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 temperature was kept constant at 18 °C. For all compounds studied, the compression isotherm experiments were performed on a pure water subphase, pH 5.7 and on 5 M NaCl. The detailed procedure of the cleaning of the trough was published elsewhere.16 Langmuir-Blodgett films were prepared by a Z-type transfer17-19 of monolayers formed with the Cd derivatives 1, 3, 7, and 2compl on mica. The film of 1 was transferred from a 10-5 M Eu(Cl)3 subphase and the films of 3, 7, and 2compl were transferred from both a pure water and a 5 M NaCl subphase. The surface pressure with the four derivatives was, respectively, 5, 5, 9, and 20 mN/m held constant by feedback from a pressure monitor. With each derivative at least seven layers were transferred. The compression rate was maintained at 10 mm/min. The transfer ratio was close to 100%. The experimental setup used in the BAM experiments (BAM 2; NFT Go¨ttingen, Germany) was described previously.16 2.3. FECO Experiments. A mica slide (A) about 5 µm thick was cleaved from a thick Muscovite wafer (CLSS grade) in a laminar dust-free hood. A thin slide of a freshly cleaved mica (B) was immobilized with van der Waals forces on one side of the slide A, covering half of it, as shown in Figure 1. After a Cd derivative 7 monolayer was deposited on the mica using the Langmuir-Blodgett technique the slide B was removed, which allowed investigation of the geometric characteristics of the deposited film with multiple-beam interferometry. The fringes of equal chromatic order technique (FECO) developed by Tolansky20 allows a simultaneous measurement of the thickness and of the refractive index of the layers constituting the interferometer. The interferometer was realized by evaporating a thin layer (48 nm) of pure silver on each side of the mica slide. The fringes were obtained in the output plane of the spectrometer for the plate illuminated perpendicularly with a collimated beam of linearly polarized white light. A linear CCD array (4096 pixels, 7 µm width each) recorded the intensity of the light transmitted by the interferometer as a function of the wavelength. The measurement precision was 0.1 nm on the thickness and 0.01 on the refractive index. The superposition of three sharp interference fringes (order 26, 27, and 28) obtained with the monolayer-covered and the monolayer-free parts of the front side of the mica slide is presented in Figure 2. The presence of a cyclodextrin monolayer on the backside of the sheet A shifts the fringes toward higher wavelength values. (16) Cae¨l, V.; Van der Heyden, A.; Champmartin, D.; Barzyk, W.; Rubini, P.; Rogalska, E. Langmuir 2003, 19, 8697. (17) Kawabata, Y.; Matsumoto, M.; Nakamura, T.; Tanaka, M.; Manda, E.; Takahashi, H.; Tamura, S.; Tagaki, W.; Nakahara, H.; Fukuda, K. Thin Solid Films 1988, 159, 353. (18) Kobayashi, K.; Kajikawa, K.; Sasabe, H.; Knoll, W. Thin Solid Films 1999, 349, 244. (19) Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum Press: New York, 1990; p 425. (20) Tolansky, S. Multiple-Beam Interferometry of Surfaces and Films; Clarendon Press: Oxford, 1948.

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Figure 2. Superposition of three sharp interference fringes (order 26, 27, and 28) obtained with the mica slide A covered partly on the front side with the cyclodextrin monolayer. The quantitative analysis of the interference pattern was conducted by simulating the interference system using the formalism developed by Abeles.21 Each layer is described with three parameters: the thickness h and the two components (n and k) of the complex refractive index N ) n - ik of the material constituting the layer. The refractive index of the muscovite mica used here was measured previously using a similar technique22 and is equal to Nmica ) 1.5682 +740/ λ2 - 10-5i. The refractive index of silver is given in the literature.23 For the wavelength range 520 nm < λ < 580 nm used in the experiments it is Nsilver ) 0.12-3.04i. The slight variations of these parameters in the range of the wavelengths used are neglected in the first approximation. The remaining unknown parameters are the thickness of the mica (hm), the thickness of the cyclodextrine monolayer (hc), and the refractive index (nc) of the monolayer (the effect of the extinction coefficient of the monolayer is neglected). The analytical expressions of the transmissions of the lower and of the upper part of the slide A, covered with the Cd monolayer and Cd-free, respectively, were used to fit the data. Each fringe was independently fitted with an additional parameter I0 (the intensity of the incident light) to better describe the variations of the response with the wavelength. 2.4. Fluorimetry. Mica slides of dimensions 4 cm × 3 cm × 3 µm were covered on both sides with Z type Langmuir-Blodgett films. The films were examined first with a UV lamp at 312 nm and then the spectra were registered with a Spex Jobin-Yvon Fluorolog II photon-counting spectrofluorimeter equipped with a 400 W xenon lamp.

3. Results and Discussion For macromolecules of a complex structure the detailed knowledge of the mechanisms of intra- and intermolecular interactions leading to monolayer stabilization or to formation of 3-D aggregates is not readily available.24 In this work amphiphilic Cd derivatives 1-7 and europium complexes of derivatives 1, 2, and 3 (Figure 3) are used as molecular probes to get more insight into the relation between the molecular structure and the self-assembly process. The R-, β-, and γ-Cd derivatives bear hydrophobic bipyridyl or bithiazolyl moieties at the narrow rim of the glucopyranose cone, and free hydroxyl or methoxyl groups at its wide rim (Scheme 1). The intra- and intermolecular (21) Abeles, F. J. Phys. Radium 1952, 13, 240. (22) Gauthier-Manuel, B. Meas. Sci. Technol. 1998, 9, 485. (23) Lynch, D. W.; Hunter, W. R. In Handbook of Optical Constants of Solids; Palik, E. D., Ed.; Academic Press: New York, 1998; p 354. (24) Mazzaglia, A.; Ravoo, B. J.; Darcy, R.; Gambadauro, P.; Mallamace, F. Langmuir 2002, 18, 1945.

Figure 3. Molecular structures of the cyclodextrin ligands and complexes studied. Derivatives (a) 1, R ) OH, n ) 6, and 1compl formed with Eu(PF6)3; 2, R ) OMe, n ) 6, and 2compl formed with Eu(PF6)3; 3, R ) OH, n ) 7, and 3 compl formed with EuCl3; 4, R ) OMe, n ) 7; 5, R ) OH, n ) 8; 6, R ) OMe, n ) 8. (b) 7.

interactions in cyclodextrins 1-7 and in their complexes, as well as their interactions with the water subphase, are expected to be different,25-32 due to structural differences. The first difference concerns the number of the glucose units constituting the crown, and thus the number of -OH or -OMe and bipyridyl or bithiazol groups attached to it. The intramolecular hydrogen bonding existing between the secondary -OH groups in the β-Cd’s may lead to a decrease in the interactions with the water subphase compared to the R- and γ-Cd’s which miss such a hydrogen bonding network.1 Second, O-methylated derivatives, as more hydrophobic than their free -OH counterparts, are expected to interact less with the water subphase. This reasoning may not be true, however, with the β-Cd, as explained before. Third, the bithiazolyl moieties present in the derivative 7, as more hydrophobic than the bipyridyl moieties of the derivatives 1-6, can be expected to interact less with water (25) Coleman, A. W.; Nicolis, I.; Keller, N.; Dalbiez, J. P. J. Inclusion Phenom. Mol. Recognit. Chem. 1992, 13, 139. (26) Buchwald, P. J. Phys. Chem. B. 2002, 106, 6864. (27) Linert, W.; Margl, P.; Renz, F. Chem. Phys. 1992, 161, 327. (28) Steiner, T.; Saenger, W.; Lechner, R. E. Mol. Phys. 1991, 72, 1211. (29) Saenger, W. Annu. Rev. Biophys. Biophys. Chem. 1987, 16, 93. (30) Lesyng, B.; Saenger, W. Biochim. Biophys. Acta 1981, 678, 408. (31) Chacko, K. K.; Saenger, W. J. Am. Chem. Soc. 1981, 103, 1708. (32) Saenger, W. Nature (London, United Kingdom) 1979, 279, 343.

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Scheme 1. Cyclodextrin Derivatives 1-7 with the Aromatic Moieties in Compact Conformations Obtained Using a Molecular Modeling Program Chem 3D33

than the latter, and have a higher propensity to stick into the air, thus leading to film stabilization. Finally, it has to be noted that the Cd derivatives used in this study form complexes with trivalent metal cations via coordination by the urea groups. A possible modification of the conformation of the bipyridyl groups due to complexation and, consequently, modification of the film properties compared to the free ligands, was examined. The analysis, based on the compression isotherms, of the correlation between the molecule structure and its interfacial behavior and film properties is presented in the following paragraph. 3.1. Π-A Compression Isotherms. The cyclodextrin derivatives used in this study are soluble in pure chloroform (cyclodextrins 4, 6), in mixtures of chloroform and methanol (cyclodextrin 1, 6%; 2, 2%; 3, 6%; 5, 8% MeOH), or chloroform and DMSO (cyclodextrin 7, 32% DMSO). The stability of cyclodextrin films formed at the air/water interface was examined with all seven compounds by realizing film compression-decompression cycles. The isotherms obtained from consecutive compressions superposed, showing that the cyclodextrins are not soluble in water and that the presence of a water-miscible solvent in chloroform does not lead to the disappearance of cyclodextrin molecules from the water surface. The interpretation of the results obtained requires a comparison of the experimental and the calculated values of the cyclodextrin molecular areas. The external diameters of the cyclodextrin cones at their wide rims as calculated from CPK models are 14.6 ( 0.4, 15.4 ( 0.4, and 17.5 ( 0.4 Å for R-, β-, and γ-cyclodextrin, respectively. The corresponding wide rim areas are 158-177, 177-

196, and 229-252 Å2.34 The surface of the wide end of the R-cyclodextrin determines the minimal possible molecular area of the cyclodextrins in the monolayer, that is, at the point of its lowest compressibility, which corresponds to the collapse of the monolayer. With the 7-fold symmetry β-cyclodextrin and the 8-fold symmetry γ-cyclodextrin, the average minimal molecular areas of cyclodextrins in the monolayer at the collapse point should be higher than the corresponding wide rim areas. Indeed, formation of disordered dense structures can be expected in the case of β-cyclodextrins while in the case of γ-cyclodextrin one can expect that some interstitial space will be left void between the 8-glucopyranose crowns in the close-packed monolayer.35,36 3.1.1. Comparison of r-, β- and γ-Cd Derivatives. The compression isotherms of the R-, β-, and γ-cyclodextrins bearing free secondary hydroxyl groups are shown in Figure 4 and their characteristic parameters are collected in Table 1. The isotherms of the R-cyclodextrin 1 and of the γ-cyclodextrin 5 spread on pure water are superposable. The lift-off points of both isotherms are situated at around 100 Å2 (Figure 4, curves a and c). This value is considerably lower than the projections of the Rand γ-cyclodextrin annuli on the air/water interface, indicating that none of these two compounds forms a (33) http://www.cambridgesoft.com. The site was accessed on May 5, 2004. (34) Taneva, S.; Ariga, K.; Okahata, Y.; Tagaki, W. Langmuir 1989, 5, 111. (35) Easton, C. J.; Lincoln, S. F. Modified Cyclodextrins: Scaffolds And Templates For Supramolecular Chemistry; Imperial College Press: London, 1999. (36) Schalchli, A.; Benattar, J. J.; Tchoreloff, P.; Zhang, P.; Coleman, A. W. Langmuir 1993, 9, 1968.

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Badis et al. Table 1. Compression Isotherm Data

Water Acoll 1 1compl 2 2compl 3 3compl 4 5 6 7

[Å2]

/ / 214.0 72.7 114.9 90.3 301.7 / 155.5 195.6

Πcoll [mN/ m] / / 12.9 15.5 16.6 21.6 19.4 / 21.7 20.8

-1 [mN/

Cs

28.4 29.6 22.2 28.5 45.7 41.5 32.0 30.1 29.1 111.7

m]

5 M NaCl [Å2]

∆V [V]

µeff/ [D]

97.1 114.7 508.0 157.7 180.4 187.4 759.3 97.8 598.3 240.7

/ / 0.133 / 0.471 0.286 0.277 / 0.171 0.319

/ / 0.755 / 1.436 0.685 2.218 / 0.706 1.656

A0

Figure 4. Compression isotherms of the films formed with the free -OH derivatives 1, 3, and 5 on pure water (curves a, b, and c, respectively) and on 5 M NaCl (curves d, e, and f, respectively). The experiments were performed at 18 °C.

homogeneous monolayer.37 The isotherms of the film formed with the β-cyclodextrin 3 on pure water (Figure 4, curve b) show that this compound has a lower propensity to aggregation than cyclodextrins 1 and 5. We propose that in this compound, by analogy to native β-cyclodextrins, the interactions between the secondary hydroxyl groups leading to the formation of an intramolecular hydrogen network1,25-32 reduce the interactions between the cyclodextrin crown and the bulk water molecules. Thus, the β-cyclodextrin 3 may be less immerged in water than the compounds 1 and 5. Consequently, the bipyridyl groups grafted at its narrow rim may have less contact with the water surface than those of compounds 1 and 5. It can be supposed that the different organization of the aromatic groups in compounds 1 and 5 compared to 3 is responsible for their different propensity for aggregation. Indeed, the aggregation of cyclodextrins 1 and 5 may be induced by the π-π interactions between the aromatic bipyridyl groups38-41 splayed on the water surface. In compound 3, having the bipyridyl groups oriented more vertically into the air, the π-π interactions would, on the contrary, stabilize the film. The fact that the isotherms of the derivatives 1 and 5 are superposable indicates that the number of the bipyridyl groups is of a lesser importance for the film properties, compared to the structure of the hydrophilic crown. The isotherms of the same compounds spread on an aqueous NaCl subphase (Figure 4, curves d, e, and f) are (37) Alexandre, S.; Coleman, A. W.; Kasselouri, A.; Valleton, J. M. Thin Solid Films 1996, 284-285, 765. (38) Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int. Ed. Engl. 2003, 42, 1210. (39) Matthews, C. J.; Elsegood, M. R. J.; Bernardinelli, G.; Clegg, W.; Williams, A. F. Dalton Trans. 2004, 492. (40) Uji-i, H.; Yoshidome, M.; Hobley, J.; Hatanaka, K.; Fukumura, H. Phys. Chem. Chem. Phys. 2003, 5, 4231. (41) Roesky, H. W.; Andruh, M. Coord. Chem. Rev. 2003, 236, 91.

Acoll

[Å2]

73.5 74.4 240.0 195.7 102.6 86.3 393.8 142.1 367.2 219.6

Πcoll [mN/m]

Cs-1 [mN/m]

A0 [Å2]

29.4 22.2 23.1 16.8 21.0 21.4 32.8 19.6 28.6 31.0

41.7 35.8 68.4 55.4 44.7 37.8 114.3 41.1 143.7 179.6

250.0 193.8 362.8 282.1 194.4 197.9 553.5 272.0 458.3 265.6

shifted to higher molecular areas compared to a pure water subphase, indicating that in these conditions the aggregation is less favorable. This effect is particularly well seen with the compounds 1 and 5. The respective A0 values of 250 and 272 Å2 indicate formation of monomolecular films on an aqueous NaCl subphase at low surface pressures. This effect may be due to the expulsion of the molecules from the subphase in the presence of NaCl and to the increasing of the distance between the aromatic groups and the subphase. Consequently, the hydrogen bonds between water and the urea groups, and between water and the nitrogen atoms of the bipyridyl groups, may be abolished. Instead, formation of the intramolecular bifid hydrogen bonds between the carbonyl oxygen and the two amino hydrogens of the urea groups in neighboring arms can take place, stabilizing the orientation of the latter into the air. Such changes taken together should in turn favor the intermolecular π-π interactions between the aromatic arms, and, consequently, favor the stabilization of the film spread on the aqueous NaCl subphase, compared to the pure water subphase. Notably, the interfacial behavior of the derivative 5 is influenced to a much lesser degree by the presence of NaCl in the subphase than is the behavior of the derivatives 1 and 3. This observation corroborates our proposal that the intramolecular hydrogen bonding formed between the secondary hydroxyl groups in the β-derivative is decisive for its interactions with the aqueous subphase and, consequently, for the film properties. It can be concluded that while the role of the bipyridyl groups in the derivatives 1, 3, and 5 is decisive for 3-D aggregation versus monolayer formation, it is not related in a straightforward manner to their structure or number. Indeed, the role of these moieties can be switched from pro-aggregation to pro-monolayer stabilization. The driving force responsible for such role switching comes from the interactions established by the glucopyranose crown, according to its structure, with the aqueous subphase. 3.1.2. O-Methylated versus Free -OH Derivatives. The hypothesis presented in the precedent paragraph was further verified using molecules 2, 4, and 6 (Figure 5), which are the O-methylated analogues of compounds 1, 3, and 5. Compounds 2, 4, and 6 form monomolecular films on pure water (Figure 5, curves a, b, and c) at low surface pressures, as judged by the values of A0 (see Table 1) but aggregate upon further compression. However, on the aqueous NaCl subphase these compounds form monomolecular films as indicated both by the molecular area values at the collapse point, and by the A0 values. Contrary to the -OH derivatives, no striking difference between the behavior of the β versus R and γ derivatives could be observed with the O-methylated analogues. This fact supports our hypothesis that the intramolecular hydrogen bond system present in the glucose crown of the free hydroxyl Cd derivative 3 is decisive for the molecule

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Figure 5. Compression isotherms of the films formed with the O-methylated derivatives 2, 4, and 6 on pure water (curves a, b, and c, respectively) and on 5 M NaCl (curves d, e, and f, respectively). The experiments were performed at 18 °C.

organization at the water surface, such as to favor film stabilization. Since the intramolecular hydrogen bonding is abolished in the seven-member O-methylated derivative, and the interactions with the water subphase are diminished for all three O-methylated molecules compared to the free -OH derivatives, the film properties depend more directly on the number of the bipyridyl groups present in the molecules. The differences between the properties of the films obtained with the derivative 1 and its O-methylated counterpart 2 are further demonstrated using BAM42,43 (Figure 6). At very low surface pressures of around 0.7 mN/m the derivative 1 monolayer displays a twodimensional foam structure,44-47 corresponding to a gasliquid expanded phase coexistence. The liquid expanded phase (higher density appearing as the brighter region) formed an interconnected foam structure with bubbles of the gas phase (very low density appearing as the darker region). The O-methylated derivative 2 forms a uniform monolayer, with some rare aggregates appearing as concentric circles. It is reasonable to suppose that the O-methylated molecules 2, 4, and 6 are immerged in the water subphase less than their analogues having free secondary hydroxy groups. The interactions of the molecules 2, 4, and 6 with the subphase are expected to decrease even more in the presence of NaCl, as proposed before for the compounds 1, 3, and 5, favoring monolayer formation. Indeed, the compression isotherms obtained with the derivatives 2, 4, and 6 on the aqueous NaCl subphase confirm that such is the case. 3.1.3. Bithiazolyl versus Bipyridyl Cd Derivatives. The relation between the interactions of the cyclodextrins with the subphase and their tendency to aggregate versus monolayer formation were further checked using compound 7. This β-cyclodextrin has free hydroxy groups at its wider rim, resembling in this respect the derivative 3. At the narrow rim, however, the derivative 7 bears bithiazolyl moieties, contrary to the derivative 3 having (42) Munoz, M.; Deschenaux, R.; Coleman, A. W. J. Phys. Org. Chem. 1999, 12, 364. (43) Parazak, D. P.; Khan, A. R.; D’Souza, V. T.; Stine, K. J. Langmuir 1996, 12, 4046. (44) Rehage, H.; Achenbach, B.; Geest, M.; Siesler, H. W. Colloid Polym. Sci. 2001, 279, 597. (45) Castillo, R.; Ramos, S.; Ruiz-Garcia, J. J. Phys. Chem. 1996, 100, 15235. (46) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251. (47) Siegel, S.; Vollhardt, D. Colloids Surf., A 1996, 116, 195.

Figure 6. Comparison of the morphology of the films formed with (a) free OH-derivative 1 and (b) O-methylated derivative 2. Brewster angle microscopy images were taken at 0.7 mN/m. Both films were spread on water; temperature 18 °C. Scale: the length of the images is 560 µm, i.e., 1 cm equals 109.4 µm.

Figure 7. Compression isotherms of the film formed with the derivative 7 on pure water (curve a) and on a 5 M NaCl subphase (curve b). The isotherms of derivative 3 films spread on pure water (curve c) and on a 5 M NaCl subphase (curve d) are given for comparison. The experiments were performed at 18 °C.

bipyridyl groups. The compression isotherms of the films formed with the derivative 7 are shown in Figure 7 and their characteristic parameters are given in Table 1. The comparison of the molecular area values at the collapse point of the films (195.6 and 219.6 Å2, respectively on water and on aqueous NaCl) with the surface of the projection of a β-cyclodextrin crown on the water plane (186.2 Å2), leads to the conclusion that the cyclodextrin 7 forms monomolecular films on both subphases, possibly with a slight aggregation on the pure water subphase which is, however, overcome on the aqueous NaCl subphase. This observation suggests that the bithiazole moieties have a decisive role in stabilizing the monolayer. It has to be noted that the bithiazolyl moieties are expected to be significantly more hydrophobic compared to bipyridyl moieties and thus the film stabilizing effect of the

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Figure 8. Compression isotherms of the films formed with the EuIII complexes 1compl, 2compl, and 3compl on pure water (curves a, b, and c, respectively) and on an aqueous NaCl subphase (curves d, e, and f, respectively). The experiments were performed at 18 °C.

bithiazole groups escaping the water surface is more pronounced than the effect linked to bipyridyl groups in the derivative 3. 3.1.4. Complexes versus Free Ligands. The relation between the conformation adopted by Cd’s at the air/water interface and their capacity to form monolayers was further checked using Cd metal complexes.48 The complexes used contained the EuIII cation in the urea cavity. The isotherms shown in Figure 8 (complexes of 1, 2, and 3 spread on H2O and on NaCl subphases) indicate that, in general, the behavior of the complexes of Cd derivatives 1, 2, and 3, both on pure water and on the aqueous NaCl subphase, is not very different from the behavior of the corresponding free ligands (Figures 4 and 5, Table 1). However, in the case of the free ligand 3 and its complex, the A0 values are closer than in the case of the ligands 1 and 2 and their complexes. This result confirms our proposal that the intramolecular hydrogen bonding present in the β-derivatives is decisive for the film properties. The similarities observed with the isotherms of free ligands and their complexes suggest that that the complexation of EuIII in the urea cavity does not prevent conformational liberty of the bipyridyl arms and that the latter behave in a similar way in the complexes 1compl, 2compl, and 3compl, as they do in the respective free ligands. 3.1.5. Assessment of Different Effects. In conclusion to section 3.1 it can be said that the most advantageous set of interactions favoring monomolecular film formation is achieved with the derivative 7. This is obviously due both to the effect of free hydroxyl groups bonded by hydrogen interactions at the wide rim of the oligosaccharide crown and to the effect of the bithiazolyl groups, which are more hydrophobic than bipyridyl groups. The increased number of the bipyridyl groups in the γ-derivatives compared to the β-derivatives, and O-methylation of the hydroxyl groups, do not meaningfully contribute to film stabilization. 3.2. Surface Potential Measurements. The dependence of surface potential versus area per molecule obtained with free Cd ligands and the EuIII complexes on a pure water subphase is presented in Figure 9. While interpreting the results of surface potential of amphiphilic macromolecule films49 is intrinsically complex, these (48) Eddaoudi, M.; Coleman, A. W.; Junk, P. C. J. Inclusion Phenom. Mol. Recognit. Chem. 1996, 26, 133. (49) Van der Heyden, A.; Regnouf-de-Vains, J.-B.; Warszynski, P.; Dalbavie, J.-O.; Zywocinski, A.; Rogalska, E. Langmuir 2002, 18, 8854.

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Figure 9. Surface potential-area and surface pressure-area isotherms (upper and lower set of curves, respectively) for the films formed with compounds 1-7 on pure water.

measurements give some insight into the film properties. It can be noticed that the ∆V-A curves obtained with the free -OH and the O-methylated ligands form two distinct groups. Indeed, with the O-methylated derivatives 2, 4, and 6, the ∆V values increase only slightly with the decrease of the molecular area. In the case of the free -OH derivatives, the ∆V values increase sharply until the end of the measurement for the derivatives 1 and 5. For the derivative 3, the ∆V-A isotherm exhibits a large potential jump and reaches the maximal value before the collapse of the film. This observation tends to confirm a higher propensity of the free -OH R and γ ligands for aggregation compared to the β ligand. It is worth noticing that while the free ligand 1, 2, and 3 ∆V-A isotherms are significantly different, the curves obtained with the complexes 1compl, 2compl, and 3compl have similar profiles. This observation indicates that the complexed metal cations have a decisive impact on the orientation of the molecular dipole moments at the interface. In the case of derivative 7 high fluctuations of the measured surface potential are observed before the lift-off point, which could be the result of domain formation at low surface pressure. Taking into account that the surface potential can be expressed as

∆V)

µeff 1 0A

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 effective vertical component µ⊥ ) µeff/ of the total dipole moment of the molecule.15 This ratio in most cases cannot be separated, so the effective molecular dipole moment cannot be found without an additional assumption concerning the dielectric constants at the interface. Using the experimental data shown in Figure 9, the values of µ⊥ were calculated at the collapse points of the films (Table 1). 3.3. Preparation of Fluorescent Surfaces by Film Transfer. Films formed with Cd derivatives 1, 7, and 2compl were transferred on mica slides in a view to ascertain the possibility of preparing fluorescent surfaces. To elaborate the film deposition, the ligand 7, having an intrinsic fluorescence due to the presence of the bithiazolyl moieties and forming the most rigid monolayers, was used first. At least seven layers could be transferred on the

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disturb the fluorescence of this derivative. In the case of the fluorescent complex 2compl, 10 successive layers of the film were deposited on the mica substrate. The spectrum presented in Figure 10 B shows that the derivative 2compl is stable in an aqueous medium, since its fluorescent properties are maintained in the deposited film. The presence of NaCl does not interfere with the fluorescence of the 2compl film (spectrum not shown). This point is particularly important since it means that formation of functional Langmuir-Blodgett films using molecules which do not form stable monolayers on pure water but which do so on subphases saturated in NaCl is possible. The transfer of the film of the ligand 1 spread on Eu(Cl)3 solution aimed to find out if the complexation of EuIII takes place in situ, since free bipyridyl ligands, contrary to the bithiazolyl ligand, do not have any intrinsic fluorescence. As shown in Figure 10 C, the formation of a complex takes place in the aqueous medium, since a peak characteristic for the complexed EuIII is present at 618 nm. The films formed with ligands 1 and 3 transferred on the mica slides, dried in the air, and subsequently immerged in Eu(Cl)3 solution also gave rise to the characteristic fluorescence spectra, indicating formation of EuIII complexes (spectra not shown). 3.4. FECO. The fit of the measured transmittance with the analytical expression gives for the mica slide thickness, the cyclodextrin layer thickness, and refractive index, respectively, hm ) 5009.0 ( 0.1 nm, hc ) 5.7 ( 0.1 nm, and nc ) 1.80 ( 0.01. Figure 11 shows an example of the excellent agreement between the measured and the calculated transmittance. The obtained value of the thickness of the mica slide, which is a multiple of nanometers, is satisfactory, as mica is an aluminosilicate constituted of 1 nm thick stacking sheets. The value of the thickness of the cyclodextrine monolayer is in accord with the cyclodextrin molecular dimensions and confirms that the film is monomolecular. The high value of the refractive index indicates that the monolayer is compact. Important information on the monolayer roughness was also obtained. Indeed, the FECO method is very sensitive to the roughness of the surfaces,50 as the increase of the roughness induces enlargement of the fringe. Figure 11 shows the superposition of the two fringes of order 27, for the parts A and B of the plate, with the calculated transmittance. Since no enlargement occurs it can be concluded that the monolayer is perfectly smooth. 4. Conclusions Figure 10. Fluorescence spectra obtained with the mica slide covered with (a) the derivative 7 film transferred from the pure water subphase; (b) derivative 2compl film transferred from the pure water subphase; (c) the derivative 1 film transferred from the Eu(Cl)3 solution. The spectrum obtained with 2compl film transferred from NaCl solution is superposable with the film transferred from pure water. The analysis performed using an excitation wavelength λexc ) 295 nm gives rise to a structured emission of the EuIII ion via the AETE light conversion process at λem (5D0 f 7F2) ) 614 nm for the derivatives 1 and 2compl. For the derivative 7, λexc ) 338 nm gives rise to the emission of the bithiazolyl chromophors at λem ) 416 nm. All three slides examined with a low-pressure UV lamp at 312 nm showed a clearly visible pink fluorescence for the derivatives 1 and 2compl and a blue fluorescence for the derivative 7.

mica slide both from pure water and from the aqueous NaCl subphase. The fluorescence spectrum of the film transferred from the pure water subphase is shown in Figure 10 A. The fact that the spectra obtained with the film transferred from both subphases (the spectrum of the film transferred from the aqueous NaCl subphase is not shown) are superposable indicates that NaCl does not

These results allow one to conclude that the properties of the Langmuir films formed with a family of metalcomplexing bipyridyl and bithiazolyl Cd derivatives depend on the organization of the aromatic moieties at the air/water interface. We propose that the bipyridyl and bithiazolyl moieties stabilize the film via π-π interactions when oriented into the air, while favoring the formation of multilayers and aggregates when splayed on the water surface. Moreover, the organization of the aromatic groups is controlled by the interactions of the hydrophilic glucopyranose annuli with the aqueous subphase. The latter effect is well demonstrated with the hydroxylated bipyridyl- and bithiazolyl-β-Cd derivatives. In these molecules, the hydrogen bonding present in the glucopyranose annuli decreases the interactions with water, compared to the R and γ derivatives. The role of the aromatic groups switches from pro-aggregation to pro-film stabilization as a consequence of escaping the aqueous subphase. In the Cd (50) Levins, J. M.; Vanderlick, T. K. J. Colloid Interface Sci. 1993, 158, 223.

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Figure 11. Superposition of the calculated transmittance and of the two fringes of order 27.

molecules expulsed from the subphase, formation of the intramolecular hydrogen bonds between the urea groups may occur, contributing to the film stabilization. Fluorescent films of Cd metal complexes can be transferred on mica slides directly from the water subphase; alternatively, the complexes can be formed by immersion of the slides covered with the free ligand films in the metal salt solution. In summary, our results show that the metalcomplexing, bipyridyl or bithiazolyl Cd derivatives can be used for preparing fluorescent surfaces in a controlled way, using the Langmuir-Blodgett technique. Presently, γ-Cd dervatives with long terminal alkyl chains grafted in the complexing groups are under study to fine-tune the structure of the films and to amplify the antennae effect.

Acknowledgment. We thank Dr. Anthony W. Coleman for fruitful discussions. We thank Dr. Gerald Brezesinski, Max-Planck-Institut fu¨r Kolloid- und Grenzfla¨chenforschung, Golm/ Potsdam, Germany, for giving us access to the BAM device and Prof. M. Rogalski, Laboratoire de Thermodynamique et d’Analyze Chimique, Universite´ de Metz, for gravimetric analysis. A.V.d.H. and M.B. acknowledge for their PhD fellowships the Ministe`re de l’Education Nationale, de l’Enseignement Supe´rieur et de la Recherche, and the Re´gion Lorraine, respectively. We also thank Jeff Rice and Dr. C. Kowal for revising the English. LA036070B