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Formation, Characterization, and Properties of Nanostructured [Ru(bpy)3]2+-C60 Langmuir-Blodgett Films in Situ at the Air-Water Interface and ex Situ on Substrates Dirk M. Guldi* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556
Michele Maggini and Simonetta Mondini Dipartimento di Chimica Organica, Universita` di Padova,Via Marzolo 1, 35131 Padova, Italy
Fre´de´ric Gue´rin and Janos H. Fendler Center for Advanced Materials Processing and Department of Chemistry, Clarkson University, Potsdam, New York 13699-5814 Received June 25, 1999. In Final Form: September 17, 1999 The amphiphilic nature of a [Ru(bpy)3]2+-C60 dyad, namely, a hydrophobic fulleropyrrolidine core covalently linked to a hydrophilic [Ru(bpy)3]2+ chromophore, through a trioxyethylene spacer, secures a controllable two-dimensional crystal growth of the dyad molecules at the air-water interface. The existence of a truly monolayered structure was confirmed by means of surface pressure (Π) versus surface area (A) isotherms and as well as by Brewster angle microscopy. A limiting area of 117 Å2/molecule for dyad 2 is in satisfactory agreement with that reported for pristine C60 (93 Å2/molecule). Brewster angle microscopy reveals, upon compression of the monolayer to the point of collapse, a phase transition. In particular, the dyad monolayer transforms into threadlike fibers, which align perpendicular to the compression direction. Two-dimensional AFM images suggest that the dyad fibers are composed of close-packed several hundred µm long and 1.0 ( 0.2 µm wide clusters, which in turn consist of nanosized dyad clusters, with diameters of 100 ( 20 nm.
Introduction Photoactive molecular systems, in which a donor and an acceptor moiety are covalently linked, are particularly appealing as novel materials for the conversion of solar energy into electric current.1 In essence, in such systems, a rapid photoinduced electron transfer (ET) generating a charge-separated radical pair, thus mimicking a key step in natural photosynthesis, should follow photoexcitation of the donor chromophore. Recently, fullerene C60 began to play an important role as a new three-dimensional acceptor moiety.2 The favorable reduction behavior of C60 has been studied under a variety of conditions, e.g., in intra- and intermolecular ET events. Driven by the low reorganization energy, ET to C60 proceeds rapidly, relative to comparable two-dimensional electron acceptors.3 More (1) (a) Fox, M. A.; Chanon, M. Photoinduced Electron Transfer; Elsevier: Amsterdam, 1988. (b) Balzani, V. Supramolecular Photochemistry, D. Reidel Publ.: Dordrecht, 1987. (c) Gust, D.; Moore, T. A. Photoinduced Electron-Transfer III; Mattay, J., Ed.; Springer: Berlin, 1991. (d) Carter, F. L. Molecular Electronic Devices; Dekker: New York, 1987. (2) Fullerenes and Related Structures. Topics in Current Chemistry, Vol. 199; Hirsch, A., Vol. Ed.; Springer-Verlag: Berlin, 1999. (3) (a) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (b) Imahori, H.; Sakata, Y. Adv. Mater. 1997, 9, 537 and references therein. (c) Prato, M. J. Mater. Chem. 1997, 7, 1097. (d) Liddell, P. A.; Kuciauskas, D.; Sumida, J. P.; Nash, B.; Nguyen, D.; Moore, A. L.; Moore T. A.; Gust, D. J. Am. Chem. Soc. 1997, 119, 1400. (e) Sun, Y.; Drovetskaya, T.; Bolskar, R. D.; Bau, R.; Boyd, P. D. W.; Reed, C. A. J. Org. Chem. 1997, 62, 3642. (f) Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1997, 119, 974. (g) Baran, P. S.; Monaco, R. R.; Khan, A. U.; Schuster, D. I.; Wilson, S. R. J. Am. Chem. Soc. 1997, 119, 8363. (h) Martı´n, N.; Sa´nchez, L.; Illescas, B.; Pe´rez, I. Chem. Rev. 1998, 98, 2527. (i) Prato M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519.
importantly, the low reorganization energy leads to a significant decrease in the rate of back ET and is beneficial for the stabilization of long-lived charge separated states.3b The energy of excited charge transfer states of 1.97 eV and a lifetime up to 1 µs are favorable attributes that promote the utilization of tris(2,2′-bipyridyl)ruthenium(II) complexes as light harvesting donor molecules in donoracceptor (D-A) assemblies.4 Thus, the design of [Ru(bpy)3]2+-C60 dyads has been recently pursued as a strategy for fullerene containing photoactive materials. We have investigated, for instance, a series of D-A dyads in which the spacer, separating the metal complex from the C60 core, has been varied from a simple flexible triethyleneglycol chain or a rigid androstane moiety to a more complex peptide backbone.5 Rapid, solvent-dependent quenching of the ruthenium metal-to-ligand-charge-transfer (MLCT) excited state was noted in these dyads, regardless of the spacer. With the assistance of time-resolved flash pho(4) (a) Photochemistry and Photophysics of Coordination Compounds; Yersin, H., Vogler, A., Eds.; Springer: Berlin, 1987. (b) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (c) Collin, J. P.; Guillerez, S.; Sauvage, J. P.; Barigelletti, F.; Flamigni, L.; De Cola, L.; Balzani, V. Coord. Chem. Rev. 1991, 111, 291. (d) Charge-Transfer Photochemistry; Horvath, O., Stevenson, K. L., Eds.; VCH: Weinheim, 1993. (e) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem. Rev. 1996, 96, 759. (5) (a) Maggini, M.; Dono` A.; Scorrano, G.; Prato, M. J. Chem. Soc., Chem. Commun. 1995, 845. (b) Sariciftci, N. S.; Wudl, F.; Heeger, A. J.; Maggini, M.; Scorrano, G.; Prato, M.; Bourassa, J.; Ford, P. C. Chem. Phys. Lett. 1995, 247, 210. (c) Maggini, M.; Guldi, D. M.; Mondini, S.; Scorrano, G.; Paolucci, F.; Ceroni, P.; Roffia, S. Chem. Eur. J. 1998, 4, 1992. (d) Armspach, D.; Constable, E. C.; Diederich, F.; Housecroft, C. E.; Nierengarten, J.-F. Chem. Eur. J. 1998, 4, 723. (e) Polese, A.; Mondini, S.; Bianco, A.; Toniolo, C.; Scorrano, G.; Guldi, D. M.; Maggini, M. J. Am. Chem. Soc. 1999, 121, 3456.
10.1021/la990834z CCC: $19.00 © 2000 American Chemical Society Published on Web 11/17/1999
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tolysis, e.g., pico- and nanosecond, the efficient formation of the charge separated (CS) state, [Ru(bpy)3]3+-C60•-, has been unequivocally identified. Furthermore, lifetimes on the order of a few hundred nanoseconds are worth mentioning, particularly in light of potential applications.5 Well-ordered monolayer films are of great interest because of the valuable insight they provide regarding their potential application to important technologies related to surface modifications.6 The construction of nanoscale fullerene composites is subject to an interdisciplinary interest focusing on the development of devices employable for electron or energy storage.7 Vacuum deposition, Langmuir-Blodgett film formation, and selfassembly techniques are a number of extensively pursued techniques to fabricate two-dimensional ordered fullerene films.8-10 The optical and electrical properties of C60 films are strongly affected by the deposition conditions. The (6) Nanostructures and Nanostructured Films; Fendler, J., Ed.; WileyVCH: Weinheim, 1998. (7) (a) Rosseinsky, M. J. J. Mater. Chem. 1995, 5, 1497. (b) Mirkin, C. A.; Caldwell, W. B. Tetrahedron 1996, 52, 5113. (c) Prato, M. J. Mater. Chem. 1997, 7, 1097. (d) Guldi, D. M. In Nanostructures and Nanostructured Films; Fendler, J., Ed.; Wiley-VCH: Weinheim, 1998, p 119. (8) (a) Wilson, R. J.; Meijer, G.; Bethune, D. S.; Johnson, R. D.; Chambliss, D. D.; deVries, M. S.; Hunziker, H. E.; Wendt, H. R. Nature 1990, 348, 621. (b) Wragg, J.; Chamberlain, J. E.; White, H. W.; Kra¨tschmer, W.; Huffman, D. R. Nature 1990, 348, 623. (c) Li, Y. Z.; Patrin, J. C.; Chander, M.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E. Science 1991, 252, 547. (d) Tanigaki, K.; Ebbesen, T. W.; Saito, S.; Mizuki, J.; Tsai, J. S.; Kubo, Y.; Kuroshima, S. Nature 1991, 352, 222-223. (e) Altman, E. I.; Colton, R. J. Surf. Sci. 1992, 279, 49. (f) Weaver, J. H. Acc. Chem. Res. 1992, 25, 143. (g) Hamza, A. V.; Balooch, M. Chem. Phys. Lett. 1993, 201, 404. (h) Hebard, A. F.; Zhou, O.; Zhong, Q.; Fleming, R. M.; Haddon, R. C. Thin Solid Films 1995, 257, 147. (9) (a) Williams, G.; Pearson, C.; Bryce, M. R.; Petty, M. C. Thin Solid Films 1992, 209, 150. (b) Milliken, J.; Dominguez, D. D.; Nelson, H. H.; Barger, W. R. Chem. Mater. 1992, 4, 252. (c) Jehoulet, C.; Obeng, Y. S.; Kim, Y.-T.; Zhou, F.; Bard, A. J. J. Am. Chem. Soc. 1992, 114, 4237. (d) Iwahashi, M.; Kikuchi, K.; Achiba, Y.; Ikemoto, I.; Araki, T.; Mochida, T.; Yokoi, S.-I.; Tanaka, A.; Iriyama, K. Langmuir 1992, 8, 2980. (e) Nakamura, T.; Tachibana, H.; Yumara, M.; Matsumoto, M.; Azumi, R.; Tanaka, M.; Kawabata, Y. Langmuir 1992, 8, 4. (f) Wang, P.; Metzger, R. M.; Bandow, S.; Maruyama, Y. J. Phys. Chem. 1993, 97, 2926. (10) (a) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993, 115, 1193. (b) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945. (c) Shi, X.; Caldwell, W. B.; Chen, K.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 11598.
Guldi et al. Scheme 1
strong π-π interaction and the resulting tendency to form aggregates preclude, however, the formation of stable monolayers at the air-water interface. Only the controlled functionalization of the fullerene core by appropriate moieties that guarantee the adequate hydrophobichydrophilic balance has been demonstrated to provide a viable alternative to control the supramolecular structures formed upon their spreading on the water surface.11 In this paper we will demonstrate, for the first time, the controlled deposition of a [Ru(bpy)3]2+-C60 dyad (2, Chart 1) via Langmuir-Blodgett processing in stacked multilayered composites that display strong absorption in the visible region. Experimental Section Materials. Dyad 2 was prepared by coordinating ligand 4 to ruthenium, using Ru(bpy)2Cl2 in refluxing 1,2-dichloroethane in the presence of excess NH4PF6. Ligand 4 was, in turn, obtained via azomethine ylide cycloaddition to C60 as outlined in Scheme 1. Details on the synthesis of 2, glycine 3, and ligand 4 will be reported elsewhere. Model N-methylfulleropyrrolidine 1 was prepared as reported in the literature.12 Dihexadecyl phosphate (DHP, Aldrich, 99%) in the acid form, dimethyldioctadecylammonium bromide (DODAB, Fluka, 99%), toluene (Aldrich, 99.8%), dichloromethane (Aldrich, 99.9%), and chloroform (Fisher, HPLC grade) were used as received. Ultrapure water (pH 5.6 and resistivity 18 MΩ cm-1) from a Millipore Milli-Q columns system provided with a Milli-pak filter (0.22-µm pore size) at the outlet was used for the subphase preparation. LB Films. Langmuir monolayers were prepared by evenly spreading a calculated amount of toluene or CH2Cl2 solutions of dyad 2 (0.1-0.2 mM) on water in a commercial Lauda Model P (11) (a) Goldenberg, L. M.; Williams, G.; Bryce, M. R.; Monkman, A. P.; Petty, M. C.; Hirsch, A.; Soi, A. J. Chem. Soc., Chem. Commun. 1993, 1310. (b) Isaacs, L.; Ehrsig, A.; Diederich, F. Helv. Chim. Acta 1993, 76, 1231. (c) Diederich, F.; Jonas, U.; Gramlich, V.; Herrmann, A.; Ringsdorf, H.; Thilgen, C. Helv. Chim. Acta 1993, 76, 2445. (d) Maggini, M.; Karlsson, A.; Pasimeni, L.; Scorrano, G.; Prato, M.; Valli, L. Tetrahedron Lett. 1994, 35, 2985. (e) Maggini, M.; Pasimeni, L.; Prato, M.; Scorrano, G.; Valli, L. Langmuir 1994, 10, 4164. (f) Hawker, C. J.; Saville, P. M.; White, J. W. J. Org. Chem. 1994, 59, 3503. (g) Guldi, D. M.; Tian, Y.; Fendler, J. H.; Hungerbu¨hler, H.; Asmus, K.-D. J. Phys. Chem. 1995, 99, 17673. (h) Jonas, U.; Cardullo, F.; Belik, P.; Diederich, F.; Gu¨gel, A.; Harth, E.; Herrmann, A.; Isaacs, L.; Mu¨llen, K.; Ringsdorf, H.; Thilgen, C.; Uhlmann, P.; Vasella, A.; Waldraff, C. A. A.; Walter, M. Chem. Eur. J. 1995, 1, 243. (i) Leigh, D. A.; Moody, A. E.; Wade, F. A.; King, T. A.; West, D.; Bahra, G. S. Langmuir 1995, 11, 2334. (k) Kawai, T.; Scheib, S.; Cava, M. P.; Metzger, R. M. Langmuir 1997, 13, 5627. (12) Maggini, M.; Scoranno, G.; Prato, M. J. Am. Chem. Soc. 1993, 115, 9798.
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Figure 1. Surface pressure (Π) vs surface area (A) isotherm of 1 on water. The line drawn to Π ) 0 indicates the collapse area of the condensed multilayer (32 Å2/molecule, solid line). film balance at ambient temperature (21 ( 2 °C). Prior to spreading the solution, the surface of the water subphase was cleaned by repeated compression, aspiration, and expansion cycles. The surface was deemed clean when the surface pressure increase was less than 0.2 mN/m upon compression to a 20th of the original area and when this surface pressure increase remained the same after aging for several hours. After allowing the solvent to evaporate for 15 min, the spread monolayer was then compressed at a constant barrier speed of 5 A2/mol min and Π-A isotherms of monolayers were recorded simultaneously. LB films were prepared by compressing the monolayer to a surface pressure of 30-35 mN/m followed by a 5-min incubation period. Either quartz or indium-tin oxide (ITO) substrates were then immersed and subsequently extracted from the subphase at a rate of 1 cm/min. Good transfer ratios of 0.9-1.2 were obtained in all depositions. Absorption Spectra. Absorption spectra were taken on a Hewlett-Packard 8452A diode array spectrophotometer. Brewster Angle Microscopy. In situ examination of Langmuir monolayer surfaces by Brewster angle microscopy (BAM) was carried out in a home-constructed system equipped with a rectangular (95 × 350 mm) Teflon trough and a Wilhelmy-type surface-pressure sensor. The beam of a p-polarized argon ion laser (λ ) 488 mm, 5 mW) was directed to the monolayer surface at the Brewster angle (ca. 54°) by two mirrors mounted on precision xyz-rotators. The bottom-reflected light was absorbed by a piece of black Teflon placed at the bottom of the trough. The reflected light from the monolayer was then collected by a lens set (f ) 2.4 cm) and focused on a CCD (charge-coupled device) camera (MTI CCD 72). The images were real-time videotaped during compression or decompression, then frame-grabbed, and printed. All structures appeared to be compressed in the vertical direction. Optical Microscopy. The optical microscopic observations of the transferred Langmuir monolayers were performed with a confocal microscope (Olympus BHMJ) equipped with attachments for epifluorescence microscopy. Optical images were captured by a CCD color camera (NEC NC-8) and then printed. Atomic Force Microscopy. AFM was performed on a commercial Topometrix TMX-2000 SPM system in noncontact mode with a 2.6-µm and 130-µm Explorer scanners. All measurements were conducted at room temperature under ambient conditions. Si3N4 tips with a spring constant of 36-44 N/m were used as received. The resonance frequency of the cantilever was ∼170 kHz. Images were acquired on at least three different areas of each sample at a scanning rate of 2 µm/s and 50 µm/s to the small and big scanners, respectively.
Results and Discussion Film Formation and Characterization. Π-A Isotherms. The Π-A behavior of model fulleropyrrolidine 1 is shown in Figure 1. The Π-A isotherm evidences that this fullerene derivative forms a quite unstable film at low surface pressure. Upon external compression, the film collapses before transforming finally into a stable surface layer on water with a collapse pressure of nearly 60 mN/ m. High slopes in the Π-A isotherms are indicative of the presence of incompressible and condensed two-dimensional phases. Extrapolation of the linear parts of the isotherms, due to the solid phase of the surface layer, to zero surface pressure (Π0) enables the determination of the surface area covered by a single fullerene moiety. In the case of derivative 1, a derived value of 32 Å2/molecule14 is, however, smaller than the one estimated by a hexagonal space-filling monolayer model for pristine C60 (93 Å2/ molecule13).This leads to the hypothesis that stacking of several monolayers occurs at the air-water interface rather than the existence of a stable fullerene monolayer. This phenomenon is common for pristine C60 and for some fullerene derivatives.9 In contrast, the Π-A isotherm (Figure 2) of dyad 2 displays a liquidlike region below 15 mN/m followed by a condensed region above this pressure (>15 mN/m), indicating a phase transition. The formation of the liquidlike phase is probably governed by very weak recognition forces among the hydrophobic fullerene cores that are assembled on the aqueous phase. The surface area value of 117 Å2 obtained for dyad 2 by extrapolation from the condensed phase to zero surface pressure (Π0) is in satisfactory agreement with that reported previously for C60 (93 Å2/molecule).13 This, in turn, correlates well with a monolayered two-dimensional fullerene growth on the air-water interface and indicates the formation of a true monolayer. A possible alignment of dyad 2 in this monolayered structure implies that the polar spacer (triethylene glycol chain) and the hydrophilic ruthenium (13) Obeng, Y. S.; Bard, A. J. J. Am. Chem. Soc. 1991, 113, 6279. (14) An AΠf0 value of about 20 Å2/molecule, concentration and temperature independent, was already reported for derivative 1 (see ref 11f).
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Figure 2. Surface pressure (Π) vs surface area (A) isotherm of dyad 2 on water. The line drawn to Π ) 0 indicates the collapse area of the condensed monolayer (117 Å2/molecule, solid line).
Figure 3. Surface pressure (Π) vs surface area (A) isotherm of (a) DHP (5.0 × 10-3 M) (please note that the surface area is not corrected for the DHP concentration), (b) dyad 2 (1.0 × 10-5 M), and (c) mixed DHP (5.0 × 10-3 M) dyad 2 (1.0 × 10-5 M) composite spread evenly onto water (please note the surface area is only corrected for dyad (2) concentration).
complex are immersed in the aqueous subphase. On the other hand, the hydrophobic fullerene core is expected to remain on top of the aqueous phase and, therefore, anchor the dyad at the air-water interface. It should be noted that the spreading behavior of 2 prevailed independent of its concentration in the investigated range of 1-50 µM. At higher concentrations (> 20 µM), the estimated area occupied by a fullerene moiety increased slightly to ca. 145 Å2/molecule. This increase is due to the interactions of the fullerene molecules in the liquidlike phase. Based on the above findings, significant three-dimensional fullerene aggregation can be ruled out. A possible rationale
for this exclusive two-dimensional crystal growth evolves from the amphiphilic nature of the two component ruthenium-fullerene donor-acceptor system. Addition of surfactants that bear alternatively a negatively (DHP) or positively charged headgroup (DODAB) to a solution of dyad 2 in dichloromethane resulted in similar Π-A isotherms than noticed in experiments performed in the absence of a surfactant molecule (Figure 3). However, strikingly increased surface areas at similar surface pressures contrast the earlier experiments. To clarify the nature of the shifted area, a systematic variation of the surfactant concentrations was deemed important.
Nanostructured [Ru(bpy)3]2+-C60 LB Films
The extrapolated surface areas decreased gradually upon lowering the surfactant concentration, approaching the value determined without a surfactant. On the other hand, increasing the surfactant content led to further enlarged surface areas. Both the [Ru(bpy)3]2+-C60 dyad and surfactant molecules self-assemble individually into monolayered structures at the air-water interface. Therefore, the present result points to a heterogeneous blending of the two components. Also, the complementary BAM measurements substantiate an exclusive two-dimensional film growth. This suggests that under experimental conditions, which guarantee a large excess of surfactant molecules, the dyad molecules are embedded into small domains of the surfactant film and exclude formation of two-dimensional fullerene arrays. Brewster Angle Microscopy. Important information on the spreading behavior of reference compound 1 and dyad 2 was obtained by means of Brewster angle microscopy. This surface characterization technique was employed to further corroborate the Π-A isotherm by visualizing the surface coverage at the air-water interface during the external compression and expansion cycles. In a typical experiment, the number of substrate layers are recorded as a function of light intensity. Images taken immediately after spreading of 1 on the water surface reveal the spontaneous formation of heterogeneously shaped, relatively bright islands (Figure 4a). More, importantly, these domains have noticeably different light intensities, which suggests the coexistence of monolayered and multilayered domains. The bright islands were pushed together by lateral compression of the film (Figures 4b and 4c). During the compression no apparent changes in the brightness of the domains were noted. Therefore, the fullerene film formation is dominated by uneven packing of monolayers and multilayers. Also the absence of appreciable phase transition is in accord with the observed Π-A isotherm (see Figure 1) for 1. Quite different BAM images were observed upon spreading and compressing the [Ru(bpy)3]2+-C60 dyad 2 on the water surface. The homogeneous distribution of light intensity demonstrates the uniform nature of the freshly spread film. Domains, formed at even low surface pressure (Figures 5a and 5b), enlarged gradually upon compression and their brightness increased until the film collapsed (Figures 5c and 5d). It is interesting to note that the collapsed film gives rise to a periodic texture perpendicular to the compression direction. External expansion of the film, prior to the collapse, still revealed uniformly structured monolayer domains (Figures 5e and 5f). In summary, BAM further corroborated the successful formation of a true monolayer for 2, while N-methylfullerpyrrolidine, 1, coexists in mono- and multilayered structures. A simple picture helps to illustrate the monolayer formation. The hydrophilic headgroups enhance the interaction with the aqueous subphase and, in turn, allow a two-dimensional fixation of the fullerene core at the air-water interface. The pyrrolidine functionality in reference 1, on the other hand, is an insufficient inhibitor for the 3-dimensional fullerene aggregation. Atomic Force Microscopy. The texture observed in the BAM experiments led us to complement the surface characterization of the [Ru(bpy)3]2+-C60 LB films by means of atomic force microscopy (AFM). The films formed from dyad 2 monolayers upon compression to the collapse pressure were therefore transferred onto a glass slide. Prior to the AFM investigation the topography of the transferred LB films was probed with a confocal microscope. Typical optical images with a resolution of 150 ×
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Figure 4. Brewster angle microscopic (BAM) images of 1, (a) prior to compression, (b) compressed on the water surface in a Langmuir balance to Π ) 25 mN/m, and (c) to Π ) 50 mN/m. The length of each image is 200 µm.
110 µm taken on a quartz plate are shown in Figures 6 a-c. Uniformly oriented rods, several hundred micrometers in length, are evident that align in a parallel manner. This suggests a quite dramatic phase transition of the monolayered dyad molecules at the air-water interface. The two-dimensional 130 × 130 µm AFM image of a monolayer of [Ru(bpy)3]2+-C60, shown in Figure 7a, reveals threadlike wires on the surface that align perpendicularly to the compression direction, resembling the results of the optical microscope experiments. The observed periodically oriented fibers could be reproduced at a variety of different scan orientations. Their homogeneity is evident in the two-dimensional 75 × 75 µm image scanned perpendicular to the fibers which have a thickness of 4.0 ( 0.5 µm (Figure 7b). To reveal the topography of the
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Figure 5. Brewster angle microscopic (BAM) images of dyad 2, (a) prior to compression, (b) compressed on the water surface in a Langmuir balance to Π ) 25 mN/m, (c) Π ) 50 mN/m, (d) Π ) 60 mN/m, the collapse pressure, (e) subsequent expansion to Π ) 20 mN/m, and (f) Π ) 0 mN/m. The length of each image is 200 µm.
wires, individual dyad fibers were scanned with a 20 × 20 µm resolution. In the corresponding AFM image (Figure 7c) the dyad fibers appear to be composed of close-packed, assembled clusters. While the outside diameter of a wire was measured to average about 1.0 ( 0.2 µm, the individual cluster size varies between 0.53 ( 0.05 µm and 0.61 ( 0.05 µm. Further enlargement to a 2.6 × 2.6 µm area was carried out in order to gain more detailed information on the cluster composition. The marked patterns in the resolved AFM images of the individual dyad clusters in Figure 7d should be recognized. From a corresponding line scan a mean diameter of 100 ( 20 nm has been deduced as a molecular dimension for the particle size. This value is, however, discrepant with the diameter of a single fullerene
molecule (ca. 1.0 nm). Consequently, the particles that assemble macroscopically to the wire structures, several hundred micrometers in length, are composed, on a microscopic scale, by nanosized dyad clusters. The dyad monolayers may undergo, upon collapse, a spiral-type, possibly cooperative, transformation to form fibrous wires whose structures are discernible in the supramolecularly resolved AFM images. The observation of dyad clusters, even on the nanometer scale, can be understood in terms of a core-shell ensemble. The fact that the fibers assemble only on the water surface suggests a hydrophobic cluster surface. In this view, the hydrophilic [Ru(bpy)3]2+ complexes, located in the core, are surrounded by a shell of hydrophobic fullerene moieties which point toward the aqueous phase.
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needlelike structures and the observation of spherical hollow particles of (C59N)216. It should be mentioned that with respect to the silicon alkoxyde functionality and the triethylene glycol spacer, substitution of the [Ru(bpy)3]2+ complex by either coumarin or fluorescein chromophores failed to evidence similar macroscopic structures. This underlines the key function of the [Ru(bpy)3]2+ complex in the formation of the observed supramolecular wires. Transfer to Solid Substrates. Monolayers prepared from dyad 2 were successfully transferred to quartz substrates via repeated dipping and withdrawal of the substrate in the aqueous subphase. In particular, the hydrophilic substrate was placed into the aqueous subphase prior to the spreading of the organic dyad (2) solution. After compressing the dyad film, the barrier was stopped and kept at a controlled surface pressure (Π ) 30 mN/m) while the substrate was withdrawn from the subphase to deposit the film. The initial withdrawing step of the hydrophilic substrate from the aqueous phase is dominated by attaching the hydrophilic [Ru(bpy)3]2+ complex, rather than the hydrophilic fullerene core, to the surface. As a consequence, the removed substrate is now coated with a layer of hydrophobic fullerene moieties. In the next step, e.g., immersing of the substrate into the aqueous trough, stacking of the hydrophobic fullerene cores governs the assembly of another dyad layer. Absorption spectroscopy is a convenient method to monitor the successive stacking of individually deposited monolayers of 2. For example, Figure 8 shows an expansion of the visible range, depicting the [Ru(bpy)3]2+ MLCT transition, of the transferred LB films on quartz. From Figure 8, a repeatable absorption pattern could be recorded that illustrates the layer by layer transfer of LB films. The insert displays a plot of absorbance at 460 nm as a function of monolayers of dyad 2. The linear dependence is a meaningful criterion that corroborates the satisfactory stacking of the dyads in the deposited LB films. Significant broadening of the MLCT absorption suggests, nevertheless, the existence of strong aggregation forces among the hydrophobic fullerene cores. This is in line with previous observations made with various functionalized fullerene derivatives in polar environments and may impact the electrochemical performance of this film. Conclusion Figure 6. Optical microscopic images (a-c) of dyad 2 transferred from the water surface to a quartz plate at Π ) 70 mN/m (image size 150 × 110 µm). The images, scanned at different positions of the transferred film, depict bundles of fibers (a), a few isolate fiber rods (b), and homogeneously cut edges of fiber bundles (c).
The fiber formation was found to be irreversible; repeated compression and expansion (in the Π ) 0-70 mN/m range) did not destroy the fibers. Conversely no fiber formation could be observed if the initial compression stopped in the liquidlike region, e.g., at low surface pressures Π < 20 mN/m. The formation of fibrous wires of dyad 2 is reminiscent of that recently described for the transformation of a monolayered fullerene film (e,e,e-C60[C(COOEt)2]3)15 into (15) Guldi, D. M.; Tian, Y.; Fendler, J. H.; Hungerbu¨hler, H.; Asmus, K.-D. J. Phys. Chem. 1996, 100, 2753. (16) Prassides, K.; Keshavarz, K. M.; Beer, E.; Bellavia, C.; Gonzalez, R.; Murata, Y.; Wudl, F.; Cheetham, A. K.; Zhang, J. P. Chem. Mater. 1996, 8, 2405.
In conclusion, we have demonstrated the controllable two-dimensional crystal growth of a [Ru(bpy)3]2+-C60 dyad (2), containing a hydrophobic fulleropyrrolidine core covalently linked to a hydrophilic [Ru(bpy)3]2+ chromophore, at the air-water interface. The existence of a truly monolayered structure was confirmed by means of surface pressure (Π) versus surface area (A) isotherms and as well as by Brewster angle microscopy. Interestingly, Brewster angle microscopy reveals that the dyad monolayer transforms, upon compression of the monolayer to the point of collapse, into threadlike fibers. Twodimensional AFM images confirm the formation of dyad fibers which are composed of close-packed several hundred micrometers long and 1.0 ( 0.2 µm wide clusters. The latter consist, in turn, of nanosized dyad clusters, with diameters of 100 ( 20 nm. Further characterization, including near-field scanning optical microscopy (NSOM), of these fascinating supramolecular self-organizing assemblies constitutes an important aspect of our current research.
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Figure 7. Two-dimensional AFM images of dyad 2 transferred from the water surface to a quartz plate at Π ) 70 mN/m and scanned perpendicular to the fibers. (a) 130 × 130 µm, (b) 75 × 75 µm, (c) 20 × 20 µm, and (d) 2.6 × 2.6 µm.
Figure 8. UV-Visible absorption spectra of Langmuir-Blodgett films of dyad 2 on an optical quartz plate with 1-, 3-, 5-, 7-, 9-, 11-, and 13-monolayers (from bottom to top). The absorbance at 460 nm is plotted as a function of number of layer in the insert.
Acknowledgment. This work was supported by the Office of Basic Energy Sciences of the Department of Energy. This is document NDRL 4132 from the Notre Dame Radiation Laboratory. Part of this work was
supported by CNR through CMRO (legge 95/95) and by MURST (contract no. 9803194198). D.G. and M.M. thank NATO for a travel grant (CRG 960099). LA990834Z
