Introduction of [2]Catenanes into Langmuir Films and Langmuir

The formation of Langmuir films comprised of (i) dimyristoylphosphatidic acid and ..... Proceedings of the National Academy of Sciences 2002 99 (8), 4...
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Introduction of [2]Catenanes into Langmuir Films and Langmuir-Blodgett Multilayers. A Possible Strategy for Molecular Information Storage Materials Christopher L. Brown,† Ulrich Jonas,‡ Jon A. Preece,*,† Helmut Ringsdorf,‡ Markus Seitz,‡ and J. Fraser Stoddart§ School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., Department of Chemistry and Biochemistry, University of California at Los Angeles, 405 Hilgard Avenue, Los Angeles, California 90095-1569, and Institut fu¨ r Organische Chemie, Johannes Gutenberg-Universita¨ t, Becher Weg 18-20, D-55099 Mainz, Germany Received June 21, 1999. In Final Form: October 26, 1999

The formation of Langmuir films comprised of (i) dimyristoylphosphatidic acid and a [2]catenane composed of a bisparapheylene-34-crown-10 with its two π-electron-rich hydroquinone rings and the π-electrondeficient cyclophane bis(paraquat-p-phenylene) and (ii) dimyristoylphosphatidic acid and a [2]catenane composed of a macrocyclic polyether containing two hydroquinone rings and an azobenzene unit and the π-electron-deficient cyclophane bis(paraquat-p-phenylene), has been acheived. Utilizing Π-A isotherms and isochore measurements, it is possible to determine the optimum ratio of phospholipid to [2]catenane for good Langmuir film formation and to interpret these experimental findings in terms of intermolecular π-π interactions between the [2]catenane tetracations in the Langmuir films. They have been transferred via the Langmuir-Blodgett technique to hydrophobized quartz supports, and, through a combination of UV-vis spectroscopy and small-angle X-ray scattering (SAXS), it has been established that the Langmuir films are deposited onto the support without loss of the [2]catenane tetracations (UV-vis) and that the transfer results in a periodic layer structure (SAXS) commensuarte with the expected bilayer thickness of the phospholid and the [2]catenane. It is proposed that such films containing mechanically interlocked molecules, which have switchable characteristics, at least in the solution state, may be suitable candidates for spatially addressable information storage materials.

Introduction Supramolecular science is receiving a great deal of attention in the literature1 at the moment. There are several reasons for this interest. Here are three. First, noncovalent bonding interactions2 between molecules in biological systems3 are of paramount importance in controlling their forms and in expressing their functions. Second, supramolecular science addresses the important issue of how the weak interactions between molecules affect the bulk properties of materials.4 Third, supramo* To whom correspondence should be addressed. Tel: int. code +(121) 414 3528. Fax: int. code +(121) 414 4403. E-mail: [email protected]. †University of Birmingham. ‡Johannes Gutenberg-Universita ¨ t. § University of California at Los Angeles. (1) (a) Ringsdorf, H. Supramol. Sci. 1994, 1, 5-6. (b) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, 1995. (c) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Higgins, K. E.; Keser, M.; Amstutz, A. Nature 1997, 276, 384-385. (d) Baudoin, O.; Gonnet, F.; Teulada Fichou, M. P.; Vigneron, J. P.; Tabet, J. C., Lehn, J. M. Chem. Eur. J. 1999, 5, 2762-2771. (e) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem. Eur. J. 1999, 5, 2722-2729. (2) (a) Cram, D. J. (Nobel Lecture) Angew. Chem. 1988, 100, 10411052; Angew. Chem., Int. Ed. Engl. 1988, 27, 1009-1020. (b) Pedersen, C. J. (Nobel Lecture) Angew. Chem. 1988, 100, 1053-1059; Angew. Chem., Int. Ed. Engl. 1988, 27, 1021-1027. (c) Fyfe, M. C. T.; Stoddart, J. F. Acc. Chem. Res. 1997, 30, 393-401. (d) Kaifer, A. E. Acc. Chem. Res. 1999, 32, 62-71. (3) (a) DeFrees, S. A.; Kosch, W.; Way, W.; Paulson, J. C.; Sabesan, S.; Halcomb, R. L.; Huang, D.-H.; Ichikawa, Y.; Wong, C.-H. J. Am. Chem. Soc. 1995, 117, 66-79. (b) Beun, G. D. M.; van de Velde, C. J. H.; Fleuren, G. J. Immunol. Today 1994, 15, 11-15. (c) Swaminathan, C. P.; Surolia, N.; Surolia. A. J. Am. Chem. Soc. 1998, 120, 5153-5159, (d) He, J. A.; Samuleson, L.; Li, L.; Kumar, J.; Tripathy, S. K. Adv. Mater. 1999, 11, 435-448.

lecular science utilizes phenomena, such as self-assembly5 and self-organization,6 which are prevalent in nature, to construct unnatural molecules and supermolecules which mimic the actions found in living systems, such as substrate carrying,7 molecular sensing,8 and information storage.9 A class of complexes and compounds, which rely upon these natural phenomenon for their construction, are the (4) (a) Adam, D.; Schumacher, P.; Simmerer, J.; Ha¨ussling, L.; Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141-143. (b) Adam, D.; Schumacher, P.; Simmerer, J.; Ha¨ussling, L.; Paulus, W.; Siemenmeyer, K.; Etzbach, K.-H.; Ringsdorf, H.; Haarer, D. Adv. Mater. 1995, 7, 276-280. (c) Plesnivy, T.; Ringsdorf, H.; Schumacher, P.; Nu¨tz, U.; Diele, S. Liq. Cryst. 1995, 18, 185-190. (d) Tew, G. N.; Li, L.; Stupp, S. I. J. Am. Chem. Soc. 1998, 120, 56015602. (e) Rao, C. N. R. Bull. Mater. Sci. 1999, 22, 141-151. (5) (a) Lindsey, J. S. New J. Chem. 1991, 15, 153-180. (b) Philp, D.; Stoddart, J. F. Synlett 1991, 445-448. (c) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312-1319. (d) Amabilino, D. B.; Stoddart, J. F. Pure Appl. Chem. 1993, 65, 2351, 2359. (e) Branda, N.; Grozfeld, R. M.; Valdes, C.; Rebek, J., Jr. J. Am. Chem. Soc. 1995, 117, 85-88. (f) Fujita, M.; Ibukuro, F.; Yamaguchi, K.; Ogura, K. J. Am. Chem. Soc. 1995, 117, 4175-4176. (g) Whitesides, G.; Simanek, E. E.; Mathias, J. P.; Seto, C. T.; Chin, D. N.; Mammen, M.; Gordon, D. N. Acc. Chem. Res. 1995, 28, 37-44. (h) Kim, H. S.; Hartgerink, J. D.; Ghadiri, M. R. J. Am. Chem. Soc. 1998, 120, 4417-4424. (i) Emrick, T.; Frechet, J. M. J. Curr. Opin. Colloid Interface Sci. 1999, 4, 15-24. (j) Sijbesema, R. P.; Meijer, E. W. Curr. Opin. Colloid Interface Sci. 1999, 4, 24-42. (6) (a) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1990, 29, 13041319; Angew. Chem. 1990, 102, 1347-1363. (b) Terfort, A.; Whitesides, G. M. Adv. Mater. 1998, 10, 470-473. (c) Schubert, U. S.; Eschbaumer, C.; Weidhl, C. H.; Lehn, J. M. Abstracts of Papers of the American Chemical Society; American Chemical Society: Washington, DC, 1999; Vol 217, part 1, pp 561. (7) (a) Ohsetto, F.; Shinkai, S. Chem. Lett. 1993, 2045-2048. (b) Langer, R. Science 1990, 249, 1527-1533. (c) Doyle, D. A.; Cabral, J. M.; Pfuetzner, R. A.; Kuo, A.; Gulbis, J. M.; Cohen, S. L.; Chait, B. T.; MacKinnon, R. Science 1998, 280, 69-77.

10.1021/la990791m CCC: $19.00 © 2000 American Chemical Society Published on Web 12/04/1999

[2]Catenanes in Films and Multilayers

so-called pseudorotaxanes,10 rotaxanes,11 and catenanes.12 These supramolecular and molecular species, which incorporate π-electron-rich and -deficient aromatic units, constitute three kinds of (super)molecules which can exhibit high (super)structural order in the solid state, both inter- and intramolecularly.11,13 Also, by choosing appropriate recognition motifs in these (super)structures, they can show, in the solution state, mechanical switching properties. These molecular-level switches can be activated and driven by chemical,14 electrochemical,14 or photochemical15 stimuli. The marriage of these two phenomenasnamely, solid-state structural order and the solution-state mechanical switching propertiesscould constitute a significant step in the direction of fabricating materials which might act as information stores.16 This paper describes how two [2]catenanes (14+ and 24+) (Figure 1), which have been self-assembled at a molecular level, can be self-organized into supramolecular arrays. The secondsnamely 24+sof these [2]catenanes17 could, in principle, function as a molecular switch. This approach results in [2]catenane tetracations being organized in a layer-like manner on a solid support, in which the layers are separated by inherently softer and deformable thin layers comprised of the monosodium salt of the phospholipid 3H. The [2]catenane tetracations in these multilayers can be considered to reside in a “liquidcrystal”-like environment, wherein it is hoped to marry the superstructural order of the solid state with the dynamic processes of the solution state. The selforganization of the self-assembled [2]catenanes 14+ and 24+ is achieved by the fabrication of Langmuir films and (8) (a) Mukhopadhyay, S. B.; Hogart, C. Y. Adv. Mater. 1994, 6, 162164. (b) Iqbal, S.; Kremer, F. J. B.; Preece, J. A.; Ringsdorf, H.; Steinbeck, M.; Stoddart, J. F.; Shen, J.; Tinker, N. D. J. Mater. Chem. 1997, 7, 1147-1154. (c) Bohannon, T. M.; Caruso, P.-L.; Denzinger, S.; Fink, R.; Mo¨bius, D.; Paulus, W.; Preece, J. A.; Ringsdorf, H.; Schollmeyer, D. Langmuir 1999, 15, 174-184. (9) (a) Bissell, R. A.; de Silva, A. P.; Gunaratne, H. Q. M.; Lynch, P. L. M.; Maguire, G. E. M.; Sandanayake, K. R. A. Chem. Soc. Rev. 1992, 21, 187-195. (b) Naito, K. J. Mater. Chem. 1998, 8, 1379-1384. (c) Willner, I.; Willner, B. J. Mater. Chem. 1998, 8, 2543-2556. (10) Fitzmaurice, D.; Nagaraja Rao, S.; Preece, J. A.; Stoddart, J. F.; Wenger, S.; Zaccheroni, N. Angew. Chem. 1999, 111, 1220-1224; Angew. Chem., Int. Ed. 1999, 38, 1147-1150. (11) (a) Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Delgado, M.; Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philp, D.; Pietraszkiewicz, M.; Prodi, L.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Vicent, C.; Williams, D. J. J. Am. Chem. Soc. 1992, 114, 193-218. (b) Leigh, D. A.; Murphy, A. Chem. Ind. 1999, 178-183. (12) (a) Diederich, F.; Gomez-Lopez, M.; Nierengarten, J.-F.; Preece, J. A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem. 1997, 109, 16111614; Angew. Chem., Int. Ed. Engl. 1997, 36, 1448-1451. (13) Ashton, P. R.; Horn, T.; Menzer, S.; Preece, J. A.; Spencer, N.; Stoddart, J. F.; Williams, D. J. Synthesis 1997, 480-488. (b) Fyfe, M. C. T.; Stoddart, J. F. Coord. Chem. Rev. 1999, 183, 139-155. (14) (a) Bissell, R. A.; Co´rdova, E.; Kaifer, A. E.; Stoddart, J. F. Nature 1994, 369, 133-137. (b) Ashton, P. R.; Boyd, S. E.; Ballardini, R.; Balzani, V.; Credi, A.; Gandolfi, M. T.; Gomez-Lopez, M.; Iqbal, S.; Philp, D.; Preece, J. A.; Ricketts, H. G.; Stoddart, J. F.; Tolley, M. S.; Venturi, M.; Williams, D. J.; White, A. J. P. Chem. Eur. J. 1997, 2, 152-170. (c) de Silva, A. P.; Dixon, I. M.; Gunaratne, H. Q. M.; Gunnlausson, T.; Maxwell, P. R. S.; Rice, T. E. J. Am. Chem. Soc. 1999, 121, 1393-1394. (15) (a) Ballardini, R.; Balzani, V.; Gandolfi, M. T.; Prodi, L.; Venturi, M.; Philp, D.; Ricketts, H. G.; Stoddart, J. F. Angew. Chem. 1993, 105, 1362-1364; Angew. Chem., Int. Ed. Engl. 1993, 32, 1301-1303. (b) Koumara, N.; Zijlstra, R. W. J.; van Delden, R. A.; Harada, N.; Feringa, B. L. Nature 1999, 401, 152-155. (c) Armaroli, N.; Balzani, V.; Colin, J. P.; Gavina, P.; Sauvage, J. P.; Ventura, J. P. J. Am. Chem. Soc. 1999, 121, 4397-4408. (16) (a) Feynman, R. P. Sat. Rev. 1960, 43, 45-47. (b) Mo¨bius, D. Can. J. Phys. 1990, 68, 992-998. (c) Ahuja, R. C.; Caruso, P.-L.; Mo¨bius, D.; Philp, D.; Preece, J. A.; Ringsdorf, H.; Stoddart, J. F.; Wildburg, G. Thin Solid Films 1996, 284/285, 671-677. (d) Naito, K. J. Mater. Chem. 1998, 8, 1379-1384. (e) Saremi, S.; Teke, B. Adv. Mater. 1998, 10, 388-391. (17) Asakawa, M.; Ashton, P. R.; Balazani, V.; Brown, C. L.; Credi, A.; Matthews, O. A.; Newton, S. P.; Raymo, F. M.; Shipway, A. N.; Spencer, N.; Quick, A.; Stoddart, J. F.; White, A. J. W.; Williams, D. J. Chem. Eur. J. 1999, 5, 860-875.

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Figure 1. Chemical formulas and cartoon representations of the [2]catenanes 14+ and 24+ and cyclophane 44+ indicating the molecular areas when they are viewed from above the cavity of the tetracationic cyclophane and of the phospholipid 3- when it is viewed along its long axis.

Langmuir-Blodgett (LB) multilayers incorporating these tetracations by electrostatically anchoring them to the phospholipid anion 3- in an organic solution which is cospread at the air-water interface. Results and Discussion Monolayer Forming Ability. In a previous paper,18a it was established that the molar ratio of the tetracationic cyclophane 44+sone of the components of the [2]catenanessand the phospholipid 3‚Na is 1:4 in Langmuir films and in LB multilayers, irrespective of the ratio of the components present in the spreading solution. This ratio follows from the fact that (i) the four anionic charges of the phospholipid counterion exchange19 with the hexafluorosphosphate counterions of the tetracationic cyclophane and/or (ii) the molecular area requirement (1.6 nm2) of eight alkyl chains viewed in cross-section down the long axis are complementary to the area (1.7 nm2) of the tetracationic cyclophane viewed perpendicularly through its cavity. By considering the molecular area (2.3 nm2) of the [2]catenane 14+, it was established that, in order to fill the area above the catenane unit as viewed along an axis through the cavity of the tetracationic cyclophane moiety, approximately 10 alkyl chains were required, i.e., five phospoholipid molecules of 3‚Na. To investigate this area requirement even further, several (18) (a) Ahuja, R. C.; Caruso, P.-L.; Mo¨bius, D.; Wildberg, G.; Ringsdorf, H.; Philp, D.; Preece, J. A.; Stoddart, J. F. Langmuir 1993, 9, 1534-1544. The tetracationic cyclophane is one of the parent components of the [2]catenane 1‚4PF6 which has been self-assembled in high yield (70%) by appealing to the template direction resulting from the molecular recognition between the π-electron-accepting bipyridinium units in the cyclophane component and the π-electrondonating hydroquinone rings in the crown ether component. See: (b) Ashton, P. R.; Goodnow, T. T.; Kaifer, A. E.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Vicent, C.; Williams, D. J. Angew. Chem. 1989, 101, 1404-1408; Angew. Chem., Int. Ed. Engl. 1989, 28, 1396-1399.

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Figure 2. Surface pressure-area per molecule of the phospholipid isotherms of the phospholipid 3- by itself and the phospholipid 3-cospread with the [2]catenane 14+ in various molar ratios.

solutions were prepared containing 1.4PF6 and 3‚Na in molar ratios ranging 1:1, 1:3, 1:5, 1:7, and 1:9. It follows that solutions containing 1 and 3 molar equiv of the phospholipid experience a shortage of lipids, whereas the solutions with 1:7 and 1:9 ratios have a surplus of them, when considering the complementarity of molecular areas. The isotherms of these solutions are shown in Figure 2. Points to note are (i) the very expanded liquid analogous nature of the isotherms for the 1:1, 1:3, and 1:5 ratios, (ii) the short liquid-solid analogous coexistence region from 40 to 45 mN m-1 for the 1:3 and 1:5 ratios (the 1:1 system form a very ill-defined monolayer after 40 mN m-1), (iii) the solid analogous phase for the 1:3 and 1:5 ratios after the coexistence region, and (iv) the much less expanded isotherms in the case of the 1:7 and 1:9 ratios. Thus, when there is a shortage of 3- in the liquid analogous region of the isotherms, the monolayers are very expanded. This observation supports the view that the [2]catenane molecules are anchored to the phospholipid 3- and are dominating the packing of the monolayer. When there is surplus of 3-, the alkyl chains dominate the packing and the films are less expanded. However, at the transition to the solid analogous phase, all the [2]catenane-containing films approach the same area per molecule. This result supports the same conclusionswhich states that the monolayers containing the tetracationic cyclophane 44+ “squeezed out” any excess cyclophanes as the monolayer is compressed, resulting in a 1:4 ratio filmsthat was reached in an earlier paper.18a Isochores were measured by compressing the binary films rapidly to 40 mN-1 and then allowing the film to relax over a period of 100 min in order to obtain an idea (19) The process of counterion exchange can be achieved either via cospreading an anionic amphiphile with a cationic species [See: (a) Yonezawa, Y.; Mo¨bius, D.; Kuhn, H. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 1183.] or by adsorbing from the subphase a cationic species to the surface of a monolayer formed from an anionic amphiphiles [See: (b) Kirtstein, S.; Mo¨hwald, H.; Shimomura, M. Chem. Phys. Lett. 1989, 154, 308-310. (c) Mo¨bius, D.; Gru¨ninger, H. R. Bioelectrochem. Bioenerg. 1984, 12, 3701-3704. (d) Barraud, A.; Lesieur, P.; Richard, J.; RusudelTeisier, A.; Vandeyver, M. Thin Solid Films 1985, 133, 125-127.]. The pKa values of the two ionizable protons of 3H are 3 and 8, and the pH of the aqueous subphase is 5.6 as a result of CO2 uptake from the air. See: Tra¨uble, H.; Teubner, M.; Wooley, P.; Eibl, H.-J. Biophys. Chem. 1976, 4, 319-320. Thus, the monoanion, i.e., 3-, of 3‚Na is assumed to form on spreading. However, we are aware that the pKa values of phosphatidic acids in homogeneous solution cannot be directly related to the pKa values at the air-water interface and that the degree of deprotonation of the phosphatidic acid may be greater or less than 1 on average in the Langmuir films. However, it is possible to anchor and organize into Langmuir films and then into Langmuir-Blodgett multilayers, by electrostatic interactions with four dimyristoylphosphatidic acid anions (3-), the tetracationic cyclophane, and cyclobis(paraquat-p-phenylene) (44+); see ref 18a (Figure 1).

Figure 3. (a) Isochore measurements of cospread mixtures of the phospholipid 3- and the [2]catenane 14+ in various molar ratios, after initial film compression to 40 mN m-1, and (b) the film pressure of each cospread film after 100 min. Scheme 1. (a) Illustrating the Formation of a Langmuir Film Comprised of a 5:1 Molar Ratio of the Phospholipid 3- and [2]Catenane 24+ and (b) Illustrating the Formation of a Langmuir Film Comprised of an Excess of the Phospholipid 3Relative to the [2]Catenane 24+

of the extent of interaction between the [2]catenane molecules in the film. These isochores are illustrated in Figure 3a, together with the surface pressure at 80 min

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3-

Figure 4. (a) Isotherms formed from the phospholipid cospread mixtures of 3- and 24+ at 10, 20, and 30 °C and (b) isotherms of cospread mixtures of the phospholipids 3- and the [2]catenanes 14+ and 24+ in 1:5 and 1:6 molar ratios, respectively.

plotted against the molar ratio of the two-component films in Figure 3b. It is obvious that the pressure drop is least in the case of the 1:5 ratio. This result suggests that the film containing five DMPA molecules per [2]catenane molecule is the one in which the most order is present initially, since, on compression, there is no need to “squeeze out” any excess of [2]catenane tetracations. Additionally however, it suggests that there is added stabilization in this film relative to those films in which there is a shortage of DMPA molecules. On inspection of the 1:5 model in Scheme 1, it can be appreciated that the packing of the phospholipid monoanions and the [2]catenane tetracations are complementary. It follows that the donor-acceptor π-π interactions between the π-electron-rich hydroquinone rings and the π-electron-poor bipyridinium units can extend not only intramolecularly but also intermolecularly, in a manner analogous to that observed18c in 14+ in the solid state. This extension of the π-π interactions intermolecularly inhibits the relaxation of the film, and hence the surface pressure does not drop significantly with time in comparison with the two films in which there is a shortage of DMPA 3-. Conversely, if one considers the model in Scheme 1b, it is evident that, if there is an oversupply of DMPA, then, the [2]catenane molecules are separated from each other and so cannot stabilize the film with the intermolecular π-π interactions. As a consequence, the film relaxes. The incorporation into Langmuir films of [2]catenane tetracations which potentially have the ability to undergo a mechanical switching action would represent a sub-

Figure 5. (a) UV-vis absorption band for the charge-transfer band between the bipyridinium units and hydroquinone moieties in the [2]catenane 14+ in a Langmuir-Blodgett film formed from 16 bilayers of a cospread Langmuir film formed from the phospholipid 3- and 14+ in a 1:5 molar ratio and (b) a plot of the number of bilayers transferred to a hydrophobized quartz slide versus the bipyridinium absorption band at 265 nm.

Figure 6. Small-angle X-ray scattering results of LangmuirBlodgett films comprised of 10 bilayers formed from a Langmuir film comprised of the phospholipid 3- and the [2]catenane 14+ in a molar ratio of 1:5 and 10 bilayers of a Langmuir-Blodgett film comprised of the phospholipid 3- alone.

stantial advance in fabricating materials in which information could be stored at the molecular level, if such Langmuir films could be transferred to solid supports using the Langmuir-Blodgett technique. With this goal in mind, a photoactive azobenzene unit has been incorporated into the [2]catenane 24+.17 The isotherms for Langmuir films formed when 6 molar equiv of DMPA (3-) and 1 molar equiv of 24+ are illustrated in Figure 4a together with the

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Figure 7. (a) UV-vis absorption spectrum of LangmuirBlodgett films comprised of 5-40 bilayers of the phospholipid 3- and [2]catenane 24+ in a molar ratio of 6:1 and (b) plot of the UV-vis absorption at 265 nm of the Langmuir-Blodgett film against the number of bilayers.

isotherms of DMPA at 10, 20, and 30 °C. It is evident that the cospread mixtures are slightly more expanded than 3- alone and do not possess a distinct phase transition from the liquid-analogous to the solid-analogous film as does (Figure 4b) the 1:5 ratio incorporating the parent [2]catenane 14+. However, the cospread mixture does form a monomolecular film at the air-water interface. Langmuir-Blodgett Film Formation. The LB deposition of the Langmuir films formed between (i) 5 molar equiv of 3- and 1 molar equiv of 14+ (Scheme 2) and (ii) 6 molar equiv of 3- and 1 molar equiv of 24+ has been investigated. The deposition of the films was carried out at 40 mN m-1 from an aqueous subphase at 20 °C onto hydrophobized quartz slides and was followed (a) by measuring the UV-vis spectra of the film-coated quartz slides and (b) by small-angle X-ray scattering (SAXS) to obtain information about the periodicity, the overall layer thickness, and the regularity of the layers. The partial UV-vis spectra (Figure 5a) of 16 bilayers transferred to a quartz support of the 14+ system reveal the charge-transfer (CT) band that is associated with the π-π interaction between the π-electron-rich hydroquinone rings and the π-electrondeficient bipyridinium units. It is this CT band which gives these compounds their characteristic red color. Figure 5b records the linear increase in the UV-vis absorption of the bipyridinium units as the number of bilayers is increased. Thus, the complex Langmuir film is transferred intact in a regular manner. SAXS was performed on the LB film formed from 10 bilayers. Figure 6 reveals that for a 1:5 ratio, a Bragg peak is observable

Brown et al.

Figure 8. (a) Small-angle X-ray scattering result of the Langmuir-Blodgett film comprised of 40 bilayers of the phospholipid 3- and the [2]catenane 24+ in a 6:1 molar ratio and (b) small-angle X-ray scattering of the Kiessig fringe region, highlighting the ordered nature of the bilayer structure.

Figure 9. Plots of the actual (from SAXS measurements) and calculated bilayer thicknesses for the phospholipid 3- and the [2]catenane 24+ in a 6:1 molar ratio.

at an angle of 2θ ) 1.69 corresponding to a bilayer thickness of 5.22 nm, a result which agrees with the calculated bilayer thickness. Additionally, strong Kiessig fringes are observable which indicate an overall film thickness of 53 nm. For comparison, the SAXS result for a 10 bilayer film of 3- is shown in Figure 6 as well. In this case, the Bragg peak is observable at an angle of 2θ ) 1.68 corresponding to a bilayer thickness of 5.25 nm while the more well-defined Kiessig fringes afford an overall film thickness of 54 nm. Thus, the bilayer thicknesses and the

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Scheme 2. Schematic Representations of the Formation of the Langmuir Film and the Langmuir-Blodgett Bilayer Fabricated from the [2]Catenane 14+ and the Phospholipid 3- Spread at the Air-Water Interfacea

a

The π donors and the π acceptors of the [2]catenane are represented by D and A, respectively.

overall film thicknesses of both films are essentially the same, even although the two-component system has the [2]catenane tetracations sandwiched between the layers of DMPA monoanions. Similar experiments were performed with the [2]catenane 24+. From the UV-vis data illustrated in Figure 7, it is obvious that the absorption (λmax ) 265 nm) of the bipyridiunium unit increases linearly with the number of bilayers transferred. Thus, once again the electrostatic interactions between 3- and the 24+ are holding the [2]catenane tetracations to the monolayers as the LangmuirBlodgett transfer proceeds onto the quartz slides. Figure 8 displays the results for the SAXS study performed on the quartz substrate which had 40 bilayers of 24+ and 3(ratio 6:1) transferred to it. In Figure 8a, a strong firstorder Bragg peak can be observed at 2θ ) 1.71 corresponding to a bilayer thickness of 5.16 nm together with the less well-defined second-order Bragg peak. Figure 8b illustrates the series of well-defined Kiessig fringes translating to a overall thin film thickness of 206 nm. These data are consistent with the model proposed for the [2]catenane 14+. Figure 9 displays the bilayer thickness given from the first-order Bragg reflection for the LB films comprised of 5, 10, 20, and 40 bilayers of the [2]catenane 24+, together with the overall LB film thickness determined (i) from the Kiessig fringes and (ii) by multiplying the bilayer thickness by the number of bilayers transferred. It is evident that the bilayer thickness is constant (5.105.20 nm) for each film and that the overall film thickness increases in a periodic fashion with this value. Conclusion It has been demonstrated that, by utilizing the electrostatic interactions between the head groups of a phos-

pholipid anion and the tetracationic [2]catenanes 14+ and 24+, these complex charged species can be introduced into Langmuir films and subsequently into LangmuirBlodgett multilayers in an ordered fashion. Thus, the combination of like-like (of the 3- anions) and donoracceptor (of the [2]catenane tetracations) interactions leads to the establishment of highly complex supramolecular architectures dictated by the order present in the molecular structures of the [2]catenanes.11,13 The selforganization of Langmuir films and Langmuir-Blodgett multilayers, which incorporate self-assembled molecular entities with potentially controllable switching properties,20 constitutes one methodology for the construction,21,22 by a bottom-up approach,23 materials in which information might be able to be stored in arrays of these individual molecular entities.24 (20) We did not observe any cis-trans isomerisation of the azobenzene unit in the [2]catenane 24+ when incorporated into the LangmuirBlodgett multilayer structure, as is observed in solution.17 The explanation for this lack of cis-trans isomerisation is that the electronic charge-transfer interactions present in the [2]catenane 24+ between the azobenzene moiety and the bipyridinium units quench the isomerisation process.17 It may still be possible to exploit this methodology for incorporating switchable (supra)molecular structures into thin films and acheive the switching action. (For a demonstration of the use of azobenzene as a data-storage material, see: (a) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658-660.) However, the solution to this is not simple, and the challenge still remains! It should be noted that there are several examples in which azobenzenze units have been photoisomerized in Langmuir-Blodgett films. For selected examples see: (b) Goldenberg, L. M.; Bryce, M. R.; Wegener, S.; Petty, M. C.; Cresswell, J. P.; Lednev, I. K.; Hester, R. E.; Moore, J. N. J. Mater. Chem. 1997, 7, 2033-2037. (c) Maack, J.; Ahuja, R. C.; Mo¨bius, D.; Tachibana, H.; Matsumoto, H. Thin Solid Films 1994, 242, 122-126. (d) Nishiyama, K.; Fujihara, M. Chem. Lett. 1988, 1257-1260. (21) Cordroch, W.; Mo¨bius, D. Thin Solid Films 1992, 210/211, 135137.

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Experimental Section The [2]catenanes 1‚4PF6 and 2‚4PF6 and the tetracationic cyclophane 4‚4PF6 were obtained as described previously.11,13,17 The monosodium salt of dimyristoylphosphatidic acid (DMPA, 3‚Na) was used as supplied by Sigma. CHCl3 (50%), MeCN (25%), and MeOH (25%), used as a spreading solvent, were obtained from Baker Chemicals (HPLC grade). The water used in the (22) Ahuja, R. C.; Caruso, P.-L.; Mo¨bius, D.; Paulus, W.; Ringsdorf, H.; Wildburg, G. Angew. Chem. 1993, 105, 1082-1091; Angew. Chem., Int. Ed. Engl. 1993, 32, 1033-1036. (23) For a collection of papers dealing with bottom-up and top-down approaches to (molecular) devices, see: Philos. Trans. R. Soc. London, Ser. A 1995, 353, 279-293. These papers are a collective theme on the “perspectives on the limits of fabrication and measurement” compiled and edited by M. E. Welland and J. K. Gimzewski, and also see a special issue of the Jounal of Materials Chemistry (J. Mater. Chem. 1999, 9, 1853-2275) on Functional Organic Materials for Devices. (24) Recently, a combination of “top-down” and “bottom-up” approaches has provided, utilizing Langmuir-Blodgett films of the closely related rotaxane (see ref 11) molecular materials, electronically configurable molecular-based logic gates. See: Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391-394. Additionally, the pseudorotaxane architecture has been introduced into LB films. See: Lynch, D. E.; Hamilton, D. G.; Calos, N. J.; Wood, B.; Sanders, J. K. M. Langmuir 1999, 15, 5600-5605.

Brown et al. Langmuir troughs for the subphase was obtained from a Milli-Q filtration unit from the Millipore Corporation. The isotherm and isochore measurements were all recorded on a self-made trough with a Wilhelmy pressure pickup system. The spreading solutions contained 0.5-1.0 mg mL-1 of material. Langmuir-Blodgett transfer was achieved on a computer-controlled trough from KSV Instruments (KSV5000) at a film pressure of 40 mN m-1 and subphase temperature of 20 °C onto quartz slides which had been cleaned in a Nochromix/sulfuric acid solution overnight, rinsed thoroughly with water, dried, dipped in a solution of octadecyltrichlorosilane in CHCl3, and finally rinsed with CHCl3 (analytical grade). UV-vis spectra of the multilayers were recorded on a Firma Perkin-Elmer (Lambda 5) spectrometer. SAXS was performed on a Siemens θ-2θ goniometer using Cu KR radiation (λ ) 1.542 Å).

Acknowledgment. The research was supported in Germany in the first instance by a Royal Society Research Fellowship (to J.A.P.) and then subsequently by the Deutsche Forschungs Gemeinschaft through a European Science Exchange Fellowship (to J.A.P.) and the EPSRC in the U.K. (GR/L31838). LA990791M