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Langmuir and Langmuir-Blodgett Films of Metallosupramolecular Polyelectrolyte-Amphiphile Complexes Pit Lehmann,† Christian Symietz,† Gerald Brezesinski,† Henning Kraβ,†,‡ and Dirk G. Kurth*,†,§ Max Planck Institute of Colloids and Interfaces D-14424 Potsdam Germany, and The National Institute for Materials Science 1-1 Namiki, Tsukuba, Ibaraki 305-0044 Japan Received March 30, 2005 A detailed analysis of a metallosupramolecular polyelectrolyte-amphiphile complex (PAC) at the airwater interface is presented. Langmuir isotherms, Brewster angle microscopy, and X-ray reflectance and diffraction methods are employed to investigate the structure of the Langmuir monolayers. The PAC is self-assembled from 1,3-bis[4′-oxa-(2,2′:6′,2′′-terpyridinyl)]propane, iron acetate, and dihexadecyl phosphate (DHP). Spreading the PAC at the air-water interface results in a monolayer that consists of two strata. DHP forms a monolayer at the top of the interface, while the metallosupramolecular polyelectrolyte is immersed in the aqueous subphase. Both strata are coupled to each other through electrostatic interactions. The monolayers can be transferred onto solid substrates, resulting in well-ordered multilayers. Such multilayers are model systems for well-ordered metal ions in two dimensions.
Introduction Amphiphilic self-assembly is by far the most common mechanism for the construction of soft matter materials.1 The resulting materials exhibit significant technological value, as in microfluidic devices, electrical nanodevices,2 gene therapeutics,3 prosthesis,4 cellular engineering,5 and medical devices,6 as well as sensing, catalysis, and signal transduction.7 Self-assembly of polyelectrolytes and amphiphiles results in polyelectrolyte-amphiphile complexes (PACs), which are of fundamental importance in basic research and technological applications because of the ease of formation, the availability of amphiphilic molecules and polyelectrolytes, and the wide range of structures.8 Driven by cooperative electrostatic and hydrophobic forces, the polyelectrolytes spontaneously assemble with surfactants9 or lipids,10 giving rise to highly ordered architectures11 with interesting mechanical and dielectric properties. Such composite systems, containing biological or natural polyelectrolytes, can be used as drug delivery systems,12 pH-switchable systems,13 and templates for directing the structure of polymer architectures.14 The combination of amphiphilic molecules as structural com* To whom correspondence should be addressed. Phone: +49 (0)331/567-9211. Fax: +49 (0)331/567-9202. E-mail:
[email protected]. † Max Planck Institute of Colloids and Interfaces. ‡ Deceased August 2004. § National Insitute for Materials Science. (1) Tomalia, D. A.; Wang, Z. G.; Tirrell, M. Curr. Opin. Colloid Interface Sci. 1999, 4, 3. (2) Quake, S. R.; Scherer, A. Science 2000, 290, 1536. (3) Brown, M. D.; Schaetzlein, A. G.; Uchegbu, I. F. Int. J. Pharm. 2001, 229, 1. (4) Wolf, B. H.; Keitemeier, B. K.; Schmidt, A. E.; Richter, G. H.; Duncan, G. J. Prosthet. Dent. 2001, 85, 401. (5) Wilkinson, C. D.; Riehle, M.; Wood, M.; Gallagher, J.; Cutis, A. S. G. Mater. Sci. Eng., C 2002, 19, 263. (6) Otsuka, H.; Nagasaki, Y.; Kataoka, K. Curr. Opin. Colloid Interface Sci. 2001, 6, 3. (7) Mines, G. A.; Tzeng, B.-C.; Stevenson, K. J.; Li, J.; Hupp, J. T. Angew. Chem., Int. Ed. 2002, 41, 154. (8) Two reviews focused on the polyelectrolyte-surfactant complex: (a) Thu¨nemann, A. F. Prog. Polym. Sci. 2002, 27, 1473. (b) Faul, C. F. J.; Antonietti, M. Adv. Mater. 2003, 15, 673.
ponent and supramolecular modules as functional component appears very promising as a route to complex multifunctional materials. The resulting materials can be processed from solution or melt, possess good solubility properties, and form a wide range of liquid crystalline phases.15 Supramolecular modules emerge through self-assembly of molecular building blocks by weak and competing interactions.16 The building blocks primarily interact through molecular recognition processes involving metal ions,17 hydrogen bonding,18 electrostatics, or π-π interactions,19 giving rise to complex, discrete, or extended assemblies.20,21 The modular approach has the advantage that the individual components can be synthesized and (9) (a) Antonietti, M.; Conrad, J. Angew. Chem., Int. Ed. 1994, 33, 1869. (b) Antonietti, M.; Conrad, J.; Thu¨nemann A. F. Macromolecules 1994, 27, 6007. (c) Yeh, F.; Sokolov, E. L.; Walter, T.; Chu, B. Langmuir 1998, 14, 4350. (d) Antonietti, M.; Nesse, M.; Blum, G.; Kremer, F. Langmuir 1996, 12, 4436. (e) Tiitu, M.; Hiekkataipale, P.; Hartikainen, J.; Maekelae, T.; Ikkala, O. Macromolecules 2002, 35, 5212. (e) Hartikainen, J. Lahtinen, M. Torkkeli, M. Serimaa, R. Valkonen, J. Rissanen, K. Ikkala, O. Macromolecules 2001, 34, 7789. (f) Maeki-ontto, R. de Moel, K. Polushkin, E. G. van Ekenstein, A.; ten Brinke, G.; Ikkala, O. Adv. Mater. 2002, 14, 357. (10) (a) Antonietti, M.; Kaul, A.; Thu¨nemann, A. F. Langmuir 1995, 11, 2633. (b) Antonietti, M.; Wenzel, A.; Thu¨nemann, A. F. Langmuir 1996, 12, 2111. (11) Ober, C. K.; Wegner, G. Adv. Mater. 1997, 9, 17. (12) (a) Thu¨nemann, A. F. Beyermann, J. von Ferber, C. Loewen, H. Langmuir 2000, 16, 850. (b) Torchilin, V. P. J. Controlled Release 2001, 73, 137. (13) (a) Linhardt, J. G. Tirrel, D. A. Langmuir 2000, 16, 122. (b) General, S.; Thu¨nemann, A. F. Int. Pharm. J. 2001, 230, 11. (c) Dreja, M.; Lennartz, W. Macromolecules 1999, 32, 3528. (14) Faul, C. F. L.; Antonietti, M.; Sanderson, R. D.; Hentze, H.-P. Langmuir 2001, 17, 2031. (15) Ikkala, O.; Brinkel, G. 10 Science 2002, 295, 2407. (16) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, Germany, 1995. (17) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502. (18) (a) Whitesides, G. M.; Simanek, E. E.; Mathias, J. P.; Seto, C. T.; Chin, D. N.; Mammen, M.; Gordon, D. M. Acc. Chem. Res. 1995, 28, 37. (b) Rebek, J. Acc. Chem. Res. 1999, 32, 278. (19) Claessens, C. G.; Stoddart, J. F. J. Phys. Org. Chem. 1997, 10, 254. (20) See for examples: (a) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 2022. (b) Elemans, J. A. A. W.; Rowan, A. E.; Nolte, R. J. M. J. Mater. Chem. 2003, 13, 2661.
10.1021/la050841p CCC: $30.25 © 2005 American Chemical Society Published on Web 05/14/2005
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developed independently from each other before they are assembled as a whole to the supramolecular device or material. The modularity provides extensive control of structure and function from the molecular to macroscopic length scale. The use of structural and functional modules offers an exquisite degree of diversity, flexibility, and synthetic simplicity. While we witness an ever-increasing complexity of supramolecular systems,22 a central challenge in the area of materials science is the implementation of supramolecular devices as active functional units into well-defined material architectures.23 The development of methods to combine, position, and orient supramolecular modules in predictable ways is therefore a rapidly emerging research field.24 To handle and operate nanoscopic devices, it is advantageous to assemble them on surfaces.25 Metallosupramolecular modules made by metal ion directed selfassembly are of particular interest for the construction of technological devices because they possess a variety of photochemical, electrochemical, magnetic, and reactive properties.26 With the availability of metallosupramolecular polyelectrolytes (MEPEs) novel components are now available, which offer a facile entry to PACs with value-adding properties, such as magnetic ones.27 Metal ion induced self-assembly of ditopic bisterpyridine ligands results in formation of extended positively charged macromolecular assemblies.28 In a second independent step, self-assembly of MEPEs and suitable, oppositely charged amphiphiles provides the corresponding PACs.29 The neutral hydrophobic nature of PACs allows spreading them at the airwater interface, to study their structure and phase behavior under well-defined conditions, and to fabricate highly ordered multilayers on solid supports by Langmuir-Blodgett transfer.30 Such systems are interesting model systems for well-ordered arrays of metal ion in two dimensions. The surrounding amphiphilic mesophase offers the possibility to induce a phase transition, which may affect the properties of the metal ion complexes, e.g., in terms of electrochemical, photochemical, and magnetic structure-property relationships. Previously, (1,4-bis(2,2′: (21) See for example: (a) Rehahn, M. Acta Polym. 1998, 49, 201. (b) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J. H. K. K.; Lange, R. F. M.; Lowe, J. K. L.; Meijer, E. W. Science 1999, 278, 1601. (c) Safinya, C. R. Curr. Opin. Struct. Biol. 2001, 11, 440. (22) (a) Valdes, C.; Spitz, U. P.; Toledo, L. M.; Kubik, S. W.; Rebek, J. J. Am. Chem. Soc. 1995, 117, 12733. (b) Hasenknopf, B.; Lehn, J.-M.; Kneisel, B. O.; Baum, G.; Fenske, D. Angew. Chem., Int. Ed. Engl. 1996, 35, 1838. (c) Watton, S. P.; Fuhrmann, P.; Pence, L. E.; Caneschi, A.; Cornia, A.; Abbati, G. L.; Lippard, S. J. Angew. Chem., Int. Ed. 1998, 36, 2774. (d) Ashton, P. R.; Boyd, S. E.; Claessens, C. G.; Gillard, R. E.; Menzer, S.; Stoddart, J. F.; Tolley, M. S.; White, A. J. P.; Williams, D. J. Chem.sEur. J. 1997, 3, 788. (23) (a) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (b) Liu, S.; Kurth, D. G.; Bredenko¨tter, B.; Volkmer D. J. Am. Chem. Soc. 2002, 124, 12279. (c) Volkmer, D.; Bredenko¨tter, B.; Tellenbro¨ker, J.; Ko¨gerler, P.; Kurth, D. G.; Lehmann, P.; Schnablegger, H.; Schwahn, D.; Piepenbrink, M.; Krebs, B. J. Am. Chem. Soc. 2002, 124, 10489. (d) Krass, H.; Plummer, E. A.; Haider, J. M.; Barker, P. R.; Alcock, N. W.; Pikramenou, Z.; Hannon, M. J.; Kurth D. G. Angew. Chem., Int. Ed. 2001, 40, 3862. (24) Liu, S.; Volkmer, D.; Kurth D. G. Pure Appl. Chemistry, in press. (25) Kurth, D. G.; Severin, N.; Rabe, J. P. Angew. Chem., Int. Ed. 2002, 114, 3681. (26) See for example: (a) Long, N. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 21. (b) Gu¨tlich P, Garcia Y, Woike T. Coord. Chem. Rev. 2001, 219, 839. (c) Kurth D. G. Ann. N. Y. Acad. Sci. 2002, 960, 29. (27) Kurth, D. G.; Pietsch, U.; Bodenthin, Y.; Mo¨hwald, H. J. Am. Chem. Soc. 2005, 127, 3110. (28) Schu¨tte, M.; Kurth, D. G.; Linford, M. R.; Co¨lfen, H.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2891. (29) Meister, A.; Fo¨rster, G.; Thu¨nemann, A.; Kurth D. G. ChemPhysChem 2003, 4, 1095. (30) (a) Lehmann, P.; Kurth, D. G.; Brezesinski, G.; Symietz, C. Chem.sEur. J. 2001, 7, 1646. (b) Kurth, D. G.; Lehmann, P.; Schu¨tte M. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 5704.
Lehmann et al. Scheme 1. Self-Assembly of the PACa
a Metal ion coordination of the ditopic terpyridine ligand, 1, and Fe2+ leads to the positively charged MEPE. Amphiphilic self-assembly of MEPE and the oppositely charged DHP yields the corresponding PAC. The octahedral coordination geometry around the metal center is indicated through the wedges (schematic representation, not to scale).
6′,2′′-terpyridine-4′-yl)benzene), a rigid ditopic bisterpyridine ligand, was used to assemble the MEPE28 and the corresponding PAC. The liquid crystal structure29 and Langmuir and Langmuir-Blodgett multilayers30 were characterized in detail. The current study was undertaken to investigate the effect of ligand design on the structure and properties of metallosupramolecular PACs. In the present case, the MEPE is assembled from a bisterpyridine ligand (1,3-bis[4′-oxa(2,2′:6′,2′′-terpyridinyl)]propane) with a flexible propyl spacer. Results Preparation of PAC. Metal ion induced self-assembly of 1,3-bis[4′-oxa(2,2′:6′,2′′-terpyridinyl)]propane and iron acetate in acetic acid results in formation of the purplecolored MEPE. The color is attributed to the metal-toligand charge transfer (MLCT) band located at 560 nm, which is characteristic for iron(II) bisterpyridine complexes. In addition, the absorption bands of the pyridine groups also shift upon metal ion coordination from 240 and 277 nm to 244 and 271 nm. The occurrence of the MLCT band and the shift of the pyridine π-π* transitions confirm metal ion coordination and formation of MEPE.31 We note that MEPE is insoluble in most organic solvents, is somewhat soluble in water, and dissolves readily in solvents such as acetic acid. Amphiphilic self-assembly of solid MEPE and dihexadecyl phosphate (DHP) dissolved in CHCl3 results in formation of the PAC (Scheme 1). Investigations of the purple-colored PAC by UV-vis, IR, and NMR spectroscopy confirm the structural integrity of MEPE. Elemental analysis and 1H NMR spectroscopy suggest a stoichiometric composition with respect to the charge of the metal ion. On average, 2.0 ( 0.2 DHP molecules bind to a repeat unit, which is defined as a single ligand and one metal ion. The absence of signals from the acetate counterions in NMR and IR spectroscopy indicates a close to complete exchange of acetate counterions by DHPs. The PAC dissolves in common organic solvents, such as CHCl3, benzene, or toluene, but is insoluble in water or acetic acid. Most probably, DHP binds through electrostatic interactions to the metal ion centers of MEPE. The solubility indicates that the amphiphilic DHP molecules efficiently encapsulate the polar, hydrophilic metal centers. PAC Langmuir Monolayers. PAC spreads at the airwater interface and gives reproducible isotherms (Figure 1). The area per molecule is calculated with a composition (31) Constable, E. C. Prog. Inorg. Chem. 1994, 42, 67.
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Figure 1. Langmuir isotherm of PAC at the air-water interface.
of two DHPs per repeat unit and is plotted as area per DHP. The isotherm shows a change of the slope at a surface pressure of approximately 30 mN/m. The monolayer finally collapses at a surface pressure of 58 mN/m. At an area of approximately 210 Å2 the surface pressure begins to rise. This area corresponds approximately to the molecular dimensions of the repeat unit (approximately 220 Å2) if we neglect packing effects and assume a fully extended structure. The surface pressure steadily increases as the monolayer is compressed. We note that the compressibility of the PAC monolayer is significantly higher than that of neat DHP on water. The pressure increase in this region is, therefore, not attributed to DHP alone. Apparently amorphous (polyelectrolyte) domains are present in the monolayer, which are readily compressed. Further evidence for this hypothesis is provided in the section on X-ray measurements. At a molecular area of approximately 45 Å2 the surface pressure increases more rapidly, which indicates that the compressibility of the monolayer is reduced. Obviously, the compressibility of the PAC monolayer is now dominated by DHP. The molecular area at this point is still larger than that of two densely packed untilted alkyl chains. DHP on water forms a hexagonal lattice, and the chains are upright oriented at lateral pressures above 10 mN/m. The molecular area of densely packed DHP molecules is reached at much higher lateral pressures. Therefore, we conclude that either DHP is still tilted in the coupled system at 30 mN/m or the isotherm shows that the PAC monolayer contains many defects. The area-pressure behavior of PAC may be rationalized if we assume a stratified architecture of the monolayer. The DHP forms a stratum at the top interface, while the MEPE is submersed in the aqueous phase. The mismatch of the area of the polyelectrolyte and the amphiphile results initially in a heterogeneous monolayer consisting of amorphous domains with probably disordered DHP molecules and domains with ordered DHP (see the X-ray results). Such a heterogeneous monolayer can be easily compressed. At smaller areas the DHP molecules move closer together and begin to dominate the compressibility of the monolayer. At higher pressures, the surface area corresponds to the spatial requirements of tightly packed DHP molecules. The verification of this hypothesis and that the surface area is too small for the amphiphiles will be discussed in the next sections. Brewster Angle Microscopy. The morphology of the PAC monolayer is revealed by Brewster angle microscopy (BAM). Representative BAM images are shown in Figure 2. At zero surface pressure, the film is apparently in a gaseous/liquid condensed coexistence regime (images a and b) as indicated by the foamlike texture. The condensed domains grow and fuse upon compression, and as soon as the surface pressure increases the monolayer becomes
Figure 2. Representative BAM images of the PAC monolayer at the air-water interface (image area 500 × 500 µm). Images a and b are recorded at zero surface pressure at 250 (a) and 200 (b) Å2 per repeat unit, respectively. Images c and d are recorded at 1 and 30 mN/m, respectively.
more homogeneous. However, we note that wormlike structures become discernible (image c) and become more pronounced as the pressure increases (image d). The origin of this texture is not completely understood yet. One explanation could be the coexistence of regions with ordered and disordered DHP molecules. As noted above, one MEPE repeat unit needs approximately 220 Å2. Taking into account that two DHP molecules bind to one repeat unit, they require an area of only 80-100 Å2. The significant mismatch of the spatial requirements of MEPE and DHP may prohibit the formation of a completely homogeneous monolayer, giving rise to defect structures that show up as texture in BAM images. Perhaps some segments of MEPE may descend now partially into the subphase. X-ray Reflectivity and Grazing Incidence X-ray Diffraction. Details of the PAC monolayer architecture are revealed by X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GID). From the reflectance we can draw conclusions about the structure normal to the interface including the aqueous subphase. X-ray diffraction provides information on the packing of the alkyl chains. Figure 3 shows the X-ray reflectivity measured at different surface pressures. The reflectivity curve corresponding to a surface pressure of 10 mN/m is monotonically decreasing, and no distinct interference fringes are observed. At 20 mN/m two minima become visible. As discussed above, the monolayer is not homogeneous in this pressure region. Therefore, the reflectance curves cannot be fitted with a stratified box model. At a surface pressure of 30 mN/m the area per DHP shows that the monolayer must be homogeneous. Two pronounced minima are now detected. This reflectance curve can be fitted with a common box model (dotted line, Figure 4). The monolayer is divided into three boxes perpendicular to the interface, each one with a uniform thickness d and electron density Fel. To allow a smooth transition between each interface, an interfacial roughness τ is introduced. The best fit (solid line) of the experimental
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Table 1. Unit Cell Parameters a, b, and γ, Tilt Angle t, Cross-sectional Area A0, and Correlation Length ξ as Well as Peak Positions qxy and qz, Derived from Grazing Incidence X-ray Diffraction of PAC Langmuir Monolayers π (mN/m)
a (Å)
b (Å)
γ (deg)
T (deg)
A0 (Å2)
ξ1 (Å)
ξ2a (Å)
qxy (Å-1)
qz (Å-1)
0.5
4.91
4.87
120.2
11.1
20.3
67
40
4
4.88
4.82
120.4
10.9
20.0
78
32
10
4.84
4.82
120.2
5.2
20.1
100
35
20 30
4.80 4.80
4.80 4.80
120 120
0 0
20.0 20.0
59 63
1.481 1.492 1.493 1.511 1.501 1.509 1.511 1.51
0.251 0 0.247 0 0.118 0 0 0
a
In a hexagonal lattice ξ1 ) ξ2.
for DHP. The total thickness of the Langmuir monolayer amounts to 30.5 Å. The data strongly support a stratified architecture of the monolayer as schematically shown in Figure 5. DHP forms a stratum at the top of the Langmuir
Figure 3. Experimental X-ray reflectivity curves of a PAC monolayer (open circles) at different surface pressures (bottom, 10 mN/m; middle, 20 mN/m; top, 30 mN/m) and the corresponding fit (solid lines), the electron density profile of which is shown in Figure 4. The curves are shifted in the y direction for clarity.
Figure 4. Electron density profile of the PAC monolayer at 30 mN/m retrieved from X-ray reflectivity (see Figure 3). The dotted line shows the electron density and thickness of the box model. The solid line takes into account the interfacial roughness. The water subphase is at large positive z values. Boxes a, b, and c correspond to MEPE, phosphate headgroups, and alkyl chains, respectively. Details are provided in the text.
data is achieved with the electron density profile shown in Figure 4. Starting from the water subphase (Fel ) 0.334 Å-3), box a has an intermediate electron density (Fel ) 0.39 Å-3) and a thickness of 6.5 Å (τ ) 2.6 Å). The increased electron density is direct evidence for the presence of MEPE in this region of the monolayer. Also, the thickness (6.5 Å) compares well with the estimated average cross section (approximately 8 Å) of MEPE. Next, box b with a high electron density (Fel ) 0.48 Å-3) and a thickness of 3.5 Å (τ ) 2.7 Å) is attributed to the phosphate headgroup of DHP. Box c with a low electron density (Fel ) 0.29 Å-3) and a thickness of 20.5 Å (τ ) 3.1 Å) belongs to the alkyl chains of DHP, finally followed by the ambient medium (Fel ) 0). The thickness and the electron density of boxes b and c are in good agreement with the theoretical values
Figure 5. Schematic illustration of the stratified architecture of PAC at the air-water interface. The modular nature and the noncovalent bonding of the components permit the PAC to respond to the surface tension across the air-water interface by adapting the structure accordingly: DHP forms a closely packed monolayer, while MEPE is immersed into the subphase, electrostatically coupled to DHP (schematic representation, not to scale).
monolayer with the polar headgroups pointing into the water, while the MEPE stratum is immersed in the subphase, electrostatically coupled to the DHP layer. Details of the packing of the alkyl chains of DHP are further investigated with GID. The peak positions and unit cell parameters are summarized in Table 1. At surface pressures of 0.5, 4, and 10 mN/m we observe two diffraction peaks at different qxy and qz positions. The alkyl chains of DHP form a rectangular lattice with a tilt to the nearest neighbor (NN) direction. At surface pressures of 20 and 30 mN/m only one Bragg peak at qz ) 0 is observed. The cross sectional area A0 ) Axy cos(t), where t is the tilt angle of the alkyl chains and Axy the projected molecular area of one alkyl chain, does not change significantly as a function of the surface pressure (A0 ≈ 20 Å2). Therefore, 1/cos(t) is a linear function of the surface pressure. Extrapolation shows that the alkyl chains are in an upright position if the surface pressure exceeds 13.2 mN/m. They form a hexagonal lattice with a correlation length of ξ ≈ 60 Å. The correlation length is much smaller than for DHP on pure water (ξ ≈ 190 Å),30 which is attributed to the mismatch of the spatial requirements of DHP and MEPE. The scattering length l of the DHP alkyl chains is 22.4 Å. This value corresponds to an all-trans C16 alkyl chain including the ether linkage to the phosphate group. We note that the structure of the alkyl chains is almost identical to that of pristine DHP on pure water.30 The only difference is the slightly higher transition pressure into the LS phase (10.2 mN/m on water and 13.2 mN/m in the PAC monolayer). This indicates that the electrostatic interaction between MEPE and DHP has only little influence on the packing of the alkyl chains. In addition, we note that the total intensity of the Bragg peak increases by a factor of 1.5 if the monolayer is compressed from 20 to 30 mN/m. As shown above, there is no structural change in the monolayer in this pressure region. Therefore, the enhanced diffraction intensity is
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Kiessig fringes. The thickness for 11 and 21 layers is 308 and 572 Å, respectively. The average thickness amounts to 27.6 Å and is in excellent agreement with the ellipsometric data. Furthermore, the reflectivity curves show well-pronounced Bragg peaks at q ) 0.11, 0.22, 0.35, and 0.46 Å-1. These peaks are clear evidence for an internal structure of the LB films. The thickness of the lattice spacing is calculated to be 53.4 Å. This value corresponds to a double layer of PAC and is a characteristic feature of LB films with a Y-type structure as deposition of the film occurs during up and down stroke. Discussion and Conclusions Figure 6. Experimental X-ray reflectivity curves of PAC LB multilayers. The curves are shifted in the y direction for clarity. The thickness of the layers, calculated from the Kiessig fringes, is 308 and 572 Å for 11 and 21 layers, respectively. The average layer thickness is 27.6 Å. The lattice spacing of 53.4 Å corresponds to twice the average layer thickness, typical for LB films with y-type structure.
attributed to an increase in crystallinity of the Langmuir monolayer. This result confirms our previous assumption that the monolayer partially consists of amorphous domains that vanish as the surface pressures increases from 0 to 30 mN/m. Furthermore, we observed an additional Bragg peak at qxy ) 0.49 Å-1 (qz ) 0) at surface pressures exceeding 40 mN/m. However, the correlation length of this lattice is rather small (ξ ) 22 Å). The corresponding lattice spacing of 12.7 Å may correspond to the width of the coordinating unit in MEPE (12 Å) and could be an indication of partially ordered MEPE aggregates. Perhaps the flexibility of the spacer allows for regular packing at least over a short range. Langmuir-Blodgett (LB) Films. The PAC Langmuir monolayer is easily transferred onto solid supports, such as quartz or silicon wafers, with the LB technique. Repeated transfer leads to multilayer formation. Up to 21 layers were transferred, which generally provides a multilayer thick enough to be analyzed in detail. However, in principle it is possible to transfer many more layers. In each case, the transfer ratio is close to 1, indicating that the Langmuir monolayer is completely transferred on the solid substrate. Analysis by AFM imaging indicates a smooth surface morphology. Multilayer formation is investigated with UV-vis spectroscopy because of the strong absorbance of PAC. A linear growth of the characteristic bands at 244, 276, and 564 nm as a function of the number of transferred layers confirms regular multilayer formation as well as a continuous incorporation of MEPE into the film. The UV-vis bands in the LB films are shifted by 2-4 nm with respect to the solution spectrum, which is attributed to a change of the local polarity in the LB film. Regular film growth is independently confirmed with optical ellipsometry. The average thickness per layer is 27 ( 1 Å. This value is smaller than the thickness (30.5 Å) of the PAC Langmuir monolayer determined by X-ray reflectance. Most probably, transferinduced packing effects, including tilting of alkyl chains and interdigitation of adjacent MEPE layers, in the LB film account for the small discrepancy. We note that LB transfer does not result in anisotropic orientation that is an alignment of the MEPE backbone along the dipping direction. X-ray reflectivity curves of LB films consisting of 11 and 21 layers are shown in Figure 6. The occurrence of Kiessig fringes indicates a smooth layer structure. The thickness of the LB films is calculated by analyzing the
The previously reported concept30 of metallosupramolecular polyelectrolytes that form complexes with amphiphiles is extended here to another system. In the current case a flexible spacer is introduced in the ditopic bisterpyridine ligand. In contrast to the rigid phenyl spacer of the previous system, this modification leads to different properties. First, we notice that amphiphilic self-assembly of MEPE and DHP results in a stoichiometric complex on the basis of the charge of the central metal ion. This composition reflects the principal of electroneutrality of the resulting complex. The counterions of the metal ions are quantitatively exchanged by the amphiphiles. In contrast, the MEPE based on the bisterpyridine ligand linked by a rigid phenyl group gives a nonstoichiometric PAC with six DHPs per repeat unit. While both PACs form a stratified monolayer, where the amphiphiles form the top layer and where the MEPE is submersed in the aqueous phase, the presence of different amounts of amphiphiles has a profound effect on the interfacial properties. In the current case there is a significant mismatch of the steric requirements of the ligand and the amphiphiles. The steric requirements of the ligand exceed the area provided by the DHPs. As a result, the monolayer forms amorphous domains at large surface areas that are easily compressed. At smaller areas the repulsive steric interactions of the alkyl chains begin to dominate the surface pressure. In addition, the domains of ordered alkyl chains increase in size upon compression of the monolayer. Compared to neat DHP on water, the presence of MEPE in the monolayer has little influence on the alkyl chain packing. The flexible MEPE chain can apparently adapt to the charge pattern presented by the DHP layer. This is in marked contrast to the previously reported PAC. In this case, the occupied area of the six amphiphiles is large enough to accommodate the MEPE repeat units. As a result, DHP forms polycrystalline domains at all surface pressures. This explains the absence of any phase transition in the isotherm. The tight interaction of DHP and MEPE forces the alkyl chains into a hexagonal untilted phase at low surface pressures. At high surface pressures the alkyl lattice changes from hexagonal to rectangular. In neat DHP monolayers, the alkyl chains form a tilted rectangular lattice at low pressures, which changes into an untilted hexagonal lattice at high pressure. Both PACs can be transferred onto solid substrates, giving rise to ordered multilayers with Y-type architecture. However, the present PAC is not oriented during deposition; that is, the LB multilayers do not show optical anisotropy. Most likely the flexible linker in the ligands does not prevent disordering of the metallo units during transfer. As shown in the previous study, a rigid ligand results in anisotropic multilayers upon transfer. This study demonstrates the advantages of modular materials: By choosing appropriate modules (rigid versus flexible
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ligands), we can tailor the structure of the resulting assemblies from molecular (composition) to macroscopic (anisotropy) length scales. Experimental Section 1,3-Bis[4′-oxa(2,2′:6′,2′′-terpyridinyl)]propane. The ligand was prepared according to a literature procedure.32 UV-vis (CDCl3, nm): 271, 244. 1H NMR (400 MHz, CDCl3): δ 8.69 (d, 5.56 Hz, 4 H, H1), 8.60 (d, 7.32 Hz, 4 H, H4), 8.06 (s, 4 H, H5), 7.83 (td, 7.36 Hz, 2.00 Hz, 4 H, H3), 7.32 (t, 6.32 Hz, 4 H, H2), 4.48 (t, 6.04 Hz, 4 H, -OCH2-), 2.43 (quintuplet, 6.08 Hz, 2 H, -CH2-). Anal. Calcd for C33H26O2N6: C, 73.6; H, 4.9; N, 15.6. Found: C, 73.3; H, 5.3; N, 15.5. Mp: 193 °C. The MEPE was self-assembled from Fe(OAc)2 and 1,3-bis[4′-oxa(2,2′:6′,2′′-terpyridinyl)]propane according to a previously described procedure.30 DHP was purchased from Aldrich (99%) and used without further purification. The PAC was self-assembled by thoroughly mixing MEPE (26.1 mg, 3.66 × 10-5 mol) and DHP (40 mg, 7.33 × 10-5 mol) followed by extracting the solid with 50 mL of CHCl3 for 12 h. The organic solution was filtered and evaporated to dryness. Isolated yield: 56 mg (84%). This procedure results in higher yields of PAC compared to extraction of the aqueous phase as reported previously.30 UV-vis (CDCl3, nm): 560, 278, 246. 1H NMR (400 MHz, CDCl3): δ 0.88 (m, 6H, DHP), 1.27 (m, 52H, DHP), 1.70 (m, 4H, DHP), 2.45 (m, 2H, propyl), 3.96 (m, 4H, DHP), 4.51 (m, 4H, propyl), 7.33 (m, 4H, terpy), 7.86 (m, 4H, terpy), 8.08 (s, 4H, terpy), 8.61 (m, 4H, terpy), 8.71 (m, 4H, terpy). Anal. Calcd for [C33H26N6O2Fe(C32H67PO4)2]‚2H2O: C, 68.1; H, 9.6; N, 4.8. Found: C, 67.7; H, 9.4; N, 4.9. The isotherms and the LB transfer were carried out with a NIMA film balance (model TKB 2410A, Nima Technology, Coventry, England) and an R&K (Potsdam, Germany) dipping unit. PAC was dissolved in a 95:5 mixture of chloroform/methanol. The solution was spread onto distilled water (USF Seral water with a specific resistance exceeding 18.2 MΩ cm) at 20 °C. LB transfer was performed at a constant surface pressure of 35 mN/m and a dipping speed of 6 mm/min. Silicon (Wacker, Burghausen) and quartz wafer (Hellma Optik) were used as substrates. The transfer ratio was always 0.95 ( 0.05 for both directions. Brewster angle microscopy was done with a Brewster angle microscope (BAM 2, Nanofilm Technology GmbH, Go¨ttingen, Germany; 20 mW laser, wavelength 514 nm, resolution 3 µm) and an R&K film balance (Potsdam, Germany). Ellipsometric measurements were performed with null ellipsometry using a Multiskop (Optrel, Berlin, Germany; 2 mW HeNe laser, λ ) 632.8 nm, angle of incidence 70°). A refractive index of n ) 1.58-i0.017 was used for calculation of the thicknesses of the films. The synchrotron X-ray experiments at the air-water interface were performed at the undulator beam line BW1 at HASYLAB, DESY (Hamburg, Germany). The synchrotron beam was made monochromatic by Bragg reflection at a beryllium (002) crystal. The specular X-ray reflectivity (XR) was measured as a function of the vertical incidence angle, Ri, with the geometry Ri ) Rf ) R, where Rf is the vertical exit angle of the reflected X-rays. The vertical scattering vector component qz can be calculated by qz ) (4π/λ) sin(R).28 The background scattering from the subphase (32) Schubert, U. S.; Eschbaumer, C.; Hien, O.; Andres, P. R. Tetrahedron Lett. 2001, 42, 4705.
Lehmann et al. was measured at 2θxy ) 0.7° and subtracted from the signal measured at 2θxy ) 0. A polished glass block immersed in the trough reduced the depth of the liquid phase to ca. 0.3 mm and thereby suppressed mechanically excited capillary waves. The measured X-ray reflectivity, R(qz), is normalized by the Fresnel reflectivity, RF(qz), calculated for a sharp air/water interface.26 The extracted electron density profiles, F(z), are normalized by the electron density of the water subphase, Fwater ) 0.334 e/Å3. The electron density profile, F(z), is the laterally averaged structure, i.e., projected onto the z axis, which is normal to the interface. Although classical “slab” models have sometimes been found to be inadequate for detailed modeling of high-resolution XR data,33 the approach is useful here. Each PAC layer is described by three slabs representing, respectively, MEPE, the headgroup, and the hyrophobic part. A thickness, L, and an electron density, F, parametrize each slab. One common smearing, σ, is applied to smoothen the artificial sharp interface between adjacent slabs. The GID experiments were carried out with an angle of incidence of Ri ) 0.85Rc, with Rc being the critical angle of total external reflection of water (0.138°). A detailed description of the method is given elsewhere.33 The intensity of the scattered beam was measured with a position-sensitive detector as a function of the vertical component of the scattering vector qz ) (2π/λ) sin Rf, where Rf is the angle between the diffracted beam and the horizontal plane. The in-plane scattering component (with respect to the interface) qxy ) (4π/λ) sin θxy was detected by scanning over a 2θxy range along the horizon, where 2θxy is the angle between the incident and the diffracted beam projected onto the horizontal plane. The data were analyzed as follows: First, information about the order of the alkyl chains and the lattice type was taken from contour plots of the corrected X-ray intensities as a function of qxy and qz. Integration of the scattered intensity over the qz range reveals Bragg peaks. A Lorentzian profile was fitted to the measured intensity. The integration of the scattered intensity over the corresponding qxy range results in Bragg rods. A Gaussian profile was fitted to the Bragg intensity. The unit cell parameters of the scattering alkyl chains were calculated from the peak positions.33 The scattering length, l, of the molecules was calculated from the full width at half-maximum (fwhm) of the Bragg rod: l ) 2π/(fwhm). The positional correlation length ξ was estimated from the fwhm of the Bragg peak ∆int(qxy) (corrected for resolution effects of the detector) according to ξ ) 2/∆int(qxy). The X-ray reflectivity of the LB films on silicon wafers was measured with a commercial θ/2θ instrument (STOE & CIE GmbH, Darmstadt, Germany) using Cu KR radiation (λ ) 1.54 Å).
Acknowledgment. We thank Helmuth Mo¨hwald for inspiring discussions and Christa Stolle for the preparation of the compounds. Special thanks are due to HASYLAB at DESY, Hamburg, Germany, for beam time, and to Kristian Kjaer for his help in setting up the experiment. LA050841P (33) Als-Nielsen, J.; Mo¨hwald, H. In Handbook on Synchrotron Radiation; Ebashi, S., Koch, M., Rubinstein, E., Eds.; Elsevier: Amsterdam, 1991.
