Self-Assembly of Polyphenylene Dendrimers into Micrometer Long

surface the dendrimers only aggregate into globular clusters. Two possibilities for the development of dendrimer nanofibers are proposed. Introduction...
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Langmuir 2002, 18, 2385-2391

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Self-Assembly of Polyphenylene Dendrimers into Micrometer Long Nanofibers: An Atomic Force Microscopy Study Daojun Liu,† Hua Zhang,† P. C. M. Grim,† S. De Feyter,† U.-M. Wiesler,‡ A. J. Berresheim,‡ K. Mu¨llen,‡ and F. C. De Schryver*,† Laboratory for Molecular Dynamics and Spectroscopy, Department of Chemistry, Katholieke Universiteit Leuven (KULeuven), Celestijnenlaan 200F, B-3001 Heverlee, Belgium. Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany Received August 10, 2001. In Final Form: November 18, 2001 Individual polyphenylene dendrimer 1 and their self-assembled nanostructures, prepared by spincoating and solvent casting on various substrates such as mica, silanized mica, and highly oriented pyrolytic graphite (HOPG), have been investigated by noncontact atomic force microscopy. Besides globular clusters and monolayers, polyphenylene dendrimer 1 self-organizes into micrometer long nanofibers on a HOPG surface. Fibrillar nanostructures have also been visualized on a silanized mica surface, while on a mica surface the dendrimers only aggregate into globular clusters. Two possibilities for the development of dendrimer nanofibers are proposed.

Introduction Dendrimers, highly branched and structurally regular molecules, have attracted much attention for the past two decades because of their special properties and potential applications in life and material science.1-5 Among the dendrimer studies, two of the most rapidly expanding fields are the functionalization of dendrimers and the supramolecular chemistry of dendrimers. Functionalization of dendrimers either at the periphery or at the core will give them special properties.6-12 Assembly of dendrimers,13-18 among themselves or with other guest molecules, creates a wide collection of novel structures and surfaces with new and promising properties. * To whom correspondence should be addressed. Telephone: +3216-327405 Fax: +32-16-327989 E-mail: Frans.DeSchryver@ chem.kuleuven.ac.be. † Laboratory of Molecular Dynamics and Spectroscopy, Katholieke Universiteit Leuven. ‡ Max-Planck-Institut fu ¨ r Polymerforschung. (1) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Molecules: Concepts, Synthesis, Perspectives; VCH: Weinheim, 1996. (2) (a) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (b) Tomalia, D. A. Adv. Mater. 1994, 6, 529. (3) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665. (4) Fisher, M.; Vo¨gtle, F. Angew. Chem., Int. Ed. 1999, 38, 885. (5) Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. Rev. 1999, 99, 1747. (6) (a) Fre´chet, J. M. J. Science 1994, 263, 1710. (b) Adronov, A.; Fre´chet, J. M. J. Chem. Commun. 2000, 1701. (c) Grayson, S. M.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2000, 122, 10335. (7) Smith, D. K.; Diederich, F. Chem. Eur. J. 1998, 4, 1353. (8) Higashi, N.; Koga, T.; Niwa, M. Adv. Mater. 2000, 12, 1373. (9) Archut, A.; Vo¨gtle, F. Chem. Soc. Rev. 1998, 27, 233. (10) Sebastian, R. M.; Caminade, A. M.; Majoral, J. P.; Levillain, E.; Huchet, L.; Roncali, J. Chem. Commun. 2000, 507. (11) Nierengarten, J. F. Chem. Eur. J. 2000, 6, 3667. (12) Noble, C. O.; McCarley, R. L. J. Am. Chem. Soc. 2000, 122, 6518. (13) Tsukruk, V. V. Adv. Mater. 1998, 10, 253. (14) Smith, D. K.; Diederich, F. In Topics in Current Chemistry; Vo¨gtle, F., Eds.; Springer: Berlin, 2000; Vol. 210, p 183. (15) Emrick, T.; Fre´chet, J. M. J. Curr. Opin. Colloid Interface Sci. 1999, 4, 15. (16) Tully, D. C.; Fre´chet, J. M. J. Chem. Commun. 2001, 1229. (17) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (18) Tomalia, D. A.; Majoros, I. In Supramolecular Polymers; Liferri, A., Eds.; Marcel Dekker: 2000, p 359.

The self-assembly of dendrimers in two dimensions on a solid substrate has been studied by different approaches. By electrostatic deposition techniques, polyamidoamine (PAMAM) dendrimers were used to fabricate self-assembled monolayers and multilayers.19-21 PAMAM dendrimers can also self-assemble into monolayers on a Au substrate via multiple gold/amine polydentate interactions.22 Perhaps the most remarkable breakthrough concerning the dendrimeric self-assembled monolayers is that reported by Fre´chet et al.23 They have demonstrated that monolayers of dendritic polymers can be prepared by covalent attachment to a silicon wafer surface. These ultrathin dendrimer films can serve as effective resists for high-resolution lithography using a scanning probe microscope.23 The dendrimers or dendritic polymers can also selfassemble into three-dimensional structures. Tomalia and co-workers24 have reported the self-assembly of amineand carboxylic acid-terminated PAMAM dendrimers by using charge neutralization as the organizing force to form supramolecular core-shell dendrimer assemblies. The subsequent amidation at the amine-carboxylic acid contact points was used for a rapid construction of nanoarchitectures of higher complexity and dimensions. Percec and co-workers25 have reported the self-assembly of monodendritic building blocks in bulk into spherical, cylindrical, and more complex supramolecular and supramacromolecular dendrimers. Stupp et al.26 reported that dendron rodcoil molecules self-assembled into well-defined nano(19) Watanabe, S.; Regen, S. L. J. Am. Chem. Soc. 1994, 116, 8855. (20) (a) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (b) Bliznyuk, V. N.; Rinderspacher, F.; Tsukruk, V. V. Polymer 1998, 39, 5249. (21) Yoon, H. C.; Kim, H. S. Anal. Chem. 2000, 72, 922. (22) (a) Tokuhisa, H.; Zhao, M.; Baker, L. A.; Phan, V. T.; Dermody, D. L.; Garcia, M. E.; Peez, R. F.; Crooks, R. M.; Mayer, T. M. J. Am. Chem. Soc. 1998, 120, 4492. (b) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323. (c) Lackowshi, W. M.; Campbell, J. K.; Edwards, G.; Chechik, V.; Crooks, R. M. Langmuir 1999, 15, 7632. (23) (a) Tully, D. C.; Wilder, K.; Fre´chet J. M. J.; Trimble, A. R.; Quate, C. F. Adv. Mater. 1999, 11, 314. (b) Tully, D. C.; Trimble, A. R.; Fre´chet J. M. J. Chem. Mater. 1999, 11, 2892. (24) Uppuluri, S.; Swanson, D. R.; Piehler, L. T.; Li. J.; Hagnauer, G. L.; Tomalia, D. A. Adv. Mater. 2000, 12, 796.

10.1021/la011270d CCC: $22.00 © 2002 American Chemical Society Published on Web 02/22/2002

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Figure 1. (A) Molecular structure of polyphenylene dendrimer 1. (B) Space-filling view of a dendrimer 1 molecule built by a Merck Molecular Force Field (MMFF) method.

ribbons with uniform width and thickness. Masuhara et al.27 also demonstrated that wire-type dendrimers could self-assemble into doughnut-like structures. In the present study, we investigated the self-assembly of a biphenyl-core based polyphenylene dendrimer 1 (structure shown in Figure 1A). In contrast to many other reported dendrimers which flatten on a surface because of their high flexibility and/or interactions with the substrate, polyphenylene dendrimers are highly shapepersistent and hence do not flatten.28,29 An AFM was used to examine the self-assembled nanostructures since it is a powerful tool to visualize the surface topography and to examine surface properties at the nanoscale.30,31 Individual polyphenylene dendrimer 1 molecules as well as their selfassembled nanostructures, prepared by spin-coating and (25) (a) Hudson, S. D.; Jung, H. T.; Percec, V.; Cho, W. D.; Johansson, G.; Ungar, G.; Balagurusamy, V. S. K. Science 1997, 278, 449. (b) Percec, V.; Ahn, C. H.; Ungar, G.; Yeardley, D. J. P.; Mo¨ller, M.; Sheiko, S. S. Nature 1998, 391, 161. (c) Percec, V.; Ahn, C. H.; Cho, W. D.; Jamieson, A. M.; Kim, J.; Leman, T.; Schmidt, M.; Gerle, M.; Mo¨ller, M.; Prokhorova, S. A.; Sheiko, S. S.; Cheng, S. Z. D.; Zhang, A.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 1998, 120, 8619. (d) Percec, V.; Cho, W. D.; Ungar, G.; Yeardley, D. J. P. Angew. Chem., Int. Ed. 2000, 39, 1597. (e) Percec, V.; Cho, W. D.; Mo¨ller, M.; Prokhorova, S. A.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 2000, 122, 4249. (f) Yeardley, D. J. P.; Ungar, G.; Percec, V.; Holerca, M. N.; Johansson, G. J. Am. Chem. Soc. 2000, 122, 1684. (g) Percec, V.; Cho, W. D.; Ungar, G. J. Am. Chem. Soc. 2000, 122, 10273. (h) Percec, V.; Cho, W. D.; Ungar, G. Yeardley, D. J. P. J. Am. Chem. Soc. 2001, 123, 1302. (26) Zubarev, E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. J. Am. Chem. Soc. 2001, 123, 4105. (27) Masuo, S.; Yoshikawa, H.; Asahi, T.; Masuhara, H. J. Phys. Chem. B 2001, 105, 2885. (28) Zhang, H.; Grim, P. C. M.; Foubert, P.; Vosch, T.; Vanoppen, P.; Wiesler, U.-M.; Berresheim, A. J.; Mu¨llen, K.; De Schryver, F. C. Langmuir 2000, 16, 9009. (29) Morgenroth, F.; Ku¨bel, C.; Mu¨llen, K. J. Mater. Chem. 1997, 7, 1207. (30) Takano, H.; Kenseth, J. R.; Wong, S. S.; O’Brien, J. C.; Porter, M. D. Chem. Rev. 1999, 99, 2845. (31) Lillehei, P. T.; Bottomley, L. A. Anal. Chem. 2000, 72, 189R.

Figure 2. (A) NCAFM image (5 µm × 5 µm) of individual dendrimer 1 molecules on mica, spin-coated at a concentration of 1.1 × 10-8 M in CH2Cl2. (B) Smaller scale NCAFM image (1 µm × 1 µm) of individual dendrimer 1 molecules on mica. The inset shows the topography profile across the center of an individual dendrimer molecule presented in topography.

solvent casting on various substrates such as mica, silanized mica and HOPG, have been visualized using AFM. Interestingly, dendrimer 1 can self-assemble into micrometer long nanofibers. So far, intertwined dendritic microfibrils from PAMAM dendrimers terminated with carboxylate groups have been prepared by solvent casting on a silicon substrate.32 Butt et al.33 also reported the formation of nanorods from functionalized polyphenylene dendrimers. However, to the best of our knowledge, this is the first report of the formation of discrete long nanofibers consisting of dendrimer molecules. Experimental Section Materials. The synthesis of the biphenyl-core-based polyphenylene dendrimer 1 used in this study has already been reported.5,34 Dendrimer 1 exhibited good solubility in common organic solvents such as toluene and CH2Cl2 and was characterized by matrix-assisted laser desorption ionization time-of-flight mass spectroscopy. The perfect agreement between calculated and experimentally de(32) Evenson, S. A.; Badyal, J. P. S. Adv. Mater. 1997, 9, 1097. (33) (a) Loi, S.; Wiesler, U. M.; Butt, H. J.; Mu¨llen, K. Chem. Commun. 2000, 1169. (b) Loi, S.; Wiesler, U.-M.; Butt, H. J.; Mu¨llen, K. Macromolecules 2001, 34, 3661. (34) (a) Morgenroth, F.; Reuther, E.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1997, 36, 631. (b) Morgenroth, F.; Berresheim, A. J.; Wagner, M.; Mu¨llen, K. Chem. Commun. 1998, 1139.

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Figure 3. (A) NCAFM image (5 µm × 5 µm) of globular aggregates of dendrimer 1 molecules on mica, spin-coated at a concentration of 5.7 × 10-8 M in CH2Cl2. (B) Topography profile along the dotted line indicated in the topography image (A). (C) NCAFM image (50 µm × 50 µm) of globular aggregates of dendrimer 1 molecules on mica, solvent casted at a concentration of 5.7 × 10-8 M in CH2Cl2 according to method 2. (D) Topography profile along the dotted line indicated in the topography image (C).

termined m/z ratios confirms its monodispersity. Dendrimer solutions were filtered prior to use. Octadecyltrichlorosilane (95%, Acros Organics, New Jersey, USA), CH2Cl2 (Acros Organics, New Jersey, USA), toluene (Acros Organics, New Jersey, USA), acetone (Aldrich Chemical Co. Inc., Milwaukee, USA), and ethanol (BDH Laboratory Suppliers, Poole, England) were used as received. All the used solvents are spectrophotometric grade. Sample Preparation. Assemblies of dendrimer 1 were prepared on the surface of mica, silanized mica, and HOPG by spin-coating and solvent casting. Mica and HOPG were freshly cleaved immediately before use. Silanized mica was prepared as follows. Freshly cleaved mica was immediately immersed in a 5.0 × 10-4 M octadecyltrichlorosilane solution in toluene (maximum water content, 0.03%) and kept for 24 h under ambient conditions. The film-covered substrate was cleaned by rinsing with toluene, acetone, and ethanol and finally blow-dried with nitrogen. Spin-Coating. A solution of dendrimer 1 in CH2Cl2 at a concentration between 1.1 × 10-9 and 5.7 × 10-8 M was spin-coated at 2000 rpm on freshly cleaved mica for 20 s. Samples were visualized with AFM immediately after preparation. Solvent Casting. Assemblies of dendrimer 1 were also prepared by solvent casting. To control the rate of solvent evaporation, two preparation methods were employed. Method 1: A freshly cleaved HOPG substrate (1.2 cm × 1.2 cm) was immersed in a 2 mL solution of dendrimer 1 (5.7 × 10-8 M) in a flat-bottomed vessel (volume 20 mL) from which the solvent could slowly evaporate over several days. Method 2: A freshly cleaved mica, HOPG or silanized

mica substrate was put in a nearly closed glass container (20 cm × 20 cm × 8 cm) with pre-added CH2Cl2 in order to create a saturated solvent environment. Five drops of dendrimer 1 solution in CH2Cl2 (5.7 × 10-8 M, 5.7 × 10-7 M, or 5.7 × 10-6 M) were deposited on the surface of the substrates. In this case, the solvent evaporated more rapidly and disappeared in a time span of a few hours. AFM. Atomic force microscopy was performed with a Discoverer TMX2010 AFM system (ThermoMicroscopes, San Francisco, USA) operating in noncontact mode using Si probes (ThermoMicroscopes, San Francisco, USA) with a spring constant of 34-47 N/m and a resonance frequency of 174-191 kHz. A calibration silicon grating (TGZ01, pitch 3 µm, ∆z ) 26 ( 1 nm, MicroMasch, Tallinn, Estonia) was used to calibrate the piezo scanner. Measurements were done under ambient conditions. Image analysis was performed with Topometrix SPMLab 5.0. Molecular Force Field.35 The molecular model of dendrimer 1 was built in a vacuum by a Merck Molecular Force Field (MMFF) method in Spartan (Wave function Inc., Irvine, USA) (shown in Figure 1B). The MMFF (from Merck Pharmaceuticals) is limited in scope to organic systems and biopolymers (including some charged species), but allows the calculation of molecular geometry conformation. Results and Discussion 1. Individual Molecules of Polyphenylene Dendrimer 1. Figure 2A shows a NCAFM image of dendrimer 1 by spin-coating a 1.1 × 10-8 M solution in CH2Cl2 on a (35) Halgren, T. A. J. Comput. Chem. 1996, 17, 490.

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Figure 4. (A) NCAFM image (20 µm × 20 µm) of a dendrimer 1 monolayer prepared by solvent casting on HOPG surface. The HOPG substrate was immersed in 2 mL 5.7 × 10-8 M dendrimer solution in CH2Cl2 in a vessel by method 1. (B) A smaller scale NCAFM image (4.7 µm × 4.7 µm) of a dendrimer 1 monolayer. (C) Topography profile along the dotted line indicated in the topography image (B).

mica surface. Many separated and randomly deposited globular spots can be observed, which appear to be substantially uniform in size, i.e., they are essentially monodisperse. The height of the spots, 4.1 ( 0.2 nm, can be obtained from Figure 2B, a smaller scale image of a few individual dendrimer molecules. The height is in good agreement with the value obtained by MMFF calculation of dendrimer 1, which gives a height of 3.9-4.3 nm. The dendrimer 1 molecule has a biphenyl core and a dumbbell like shape. The height of the molecule varies within the range of 3.9-4.3 nm, depending on which branches of the dendrimer are in contact with the substrate surface.

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The good agreement between the measured height and the calculated value suggests that the observed spots in the AFM images are individual dendrimer molecules. When the dendrimer solution was diluted ten times to 1.1 × 10-9 M and spin-coated on mica, much less separate spots were observed in the AFM image (image not shown), but again with an identical height, which further indicates that we visualized individual dendrimer molecules. The width of individual dendrimers, 50-55 nm, is much bigger than their height values because of the convolution effect of the AFM tip. In view of their shape-persistence,29 these polyphenylene dendrimers, in contrast to many others reported in the literature,36,37 do not flatten on the substrate surface. The coincidence of the experimental height and the theoretical size for another polyphenylenebased dendrimer molecule was reported in our previous work.28 2. Self-Assembly of Polyphenylene Dendrimer 1. Samples for investigating the self-assembly of dendrimers 1 were prepared by spin-coating and solvent casting of a dendrimer solution on the surface of mica, silanized mica, and HOPG. Three kinds of morphologies of dendrimer assemblies have been observed by AFM, namely, globular dendrimer aggregates, discontinuous dendrimer monolayers, and dendrimer nanofibers. 2.1 Self-Assembled Globular Dendrimer Aggregates. As discussed above, individual dendrimer molecules can be obtained by spin-coating a very dilute dendrimer solution (1.1 × 10-8 M) on a mica surface. However, when the concentration of dendrimer solution was increased to 5.7 × 10-8 M, single dendrimer molecules were no longer observed and well-dispersed globular dendrimer aggregates were obtained instead, as shown in Figure 3A. The height of the resulting aggregates is 6.3 ( 0.6 nm (Figure 3B). When a dendrimer solution of the same concentration (5.7 × 10-8 M) was casted on the surface of mica according to method 2 (see Experimental Section), much larger aggregates were observed. Figure 3C is a representative AFM image of these globular dendrimer aggregates. The height of these aggregates is 47 ( 10 nm (Figure 3D). Note the difference in the lateral size scale between Figure 3A and 3C. By comparing the images in Figure 3, it can be seen that the globular dendrimer aggregates with a lower density and, at the same time, larger dimensions were formed on mica by solvent casting relative to those prepared by spin-coating. Furthermore, the dendrimer aggregates prepared by solvent casting have a broader distribution. This is not surprising because the solvent evaporates more rapidly by spin-coating and the dendrimer molecules do not have enough time to move along the mica surface to aggregate into bigger clusters. Spincoating is a well-known nonequilibrium method to prepare well-dispersed particles and uniform polymer films. The driving force for dendrimer molecules to aggregate into globular structures can be related to their dewetting properties on the mica surface. Although on a mica surface polyphenylene dendrimers only self-assemble into globular aggregates, similar structures were also observed on a HOPG surface, especially when the solvent was evaporated under ambient conditions. The formation of globular aggregates on a HOPG surface will be shown in section 2.3. Since polyphenylene dendrimers can also form monolayers on a HOPG surface, as will be discussed later, the main reason for the formation of the dendrimer globular aggregates on (36) Li, J.; Piehler, L. T.; Qin, D.; Baker, J. R.; Tomalia, D. A.; Meier, D. J. Langmuir 2000, 16, 5613. (37) Mansfield, M. L. Polymer 1996, 37, 3835.

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Figure 5. (A) NCAFM image (50 µm × 50 µm) of dendrimer nanofibers prepared by solvent casting a 5.7 × 10-8 M dendrimer 1 solution in CH2Cl2 on a HOPG surface by method 2. The globular dendrimer aggregates and monolayer are visualized simultaneously with the dendrimer nanofibers. (B) Topography profile along the dotted line indicated in the topography image (A). (C) NCAFM image (49 µm × 49 µm) of dendrimer nanofibers prepared by solvent casting a 5.7 × 10-7 M dendrimer 1 solution in CH2Cl2 on HOPG by method 2. More dendrimer nanofibers are visualized in this image. (D) Topography profile along the dotted line indicated in the topography image (C).

HOPG, unlike on a mica surface, is the rapid evaporation of the solvent. Using the rapid solvent casting method by blowing a strong flow of air onto a cast solution of dendrimers on mica, Sano et al.38 have prepared welldispersed PAMAM dendrimer nanodots. 2.2 Self-Assembled Dendrimer Monolayers. Using method 1 of solvent casting (see Experimental Section), the polyphenylene dendrimers (5.7 × 10-8 M) can form a film on the surface of HOPG. Figure 4A and 4B show a discontinuous dendrimer film on a different scale, with holes and defects as dark areas in the layer and dendrimer aggregates as bright islands. The thickness of the film is 5.0 ( 0.3 nm (Figure 4C), which suggests that the film is a monolayer. The thickness of the monolayer is somewhat higher than the height of individual dendrimer molecules (4.1 ( 0.2 nm), which might be explained as a result of overlapping of dendrimer arms to give an optimal packing. The driving force for the formation of dendrimer monolayers on HOPG can be attributed to π-π interactions of polyphenylene dendrimer molecules with the HOPG surface. The phenyl groups have a high affinity toward the HOPG surface owing to such interactions.39,40 However, the rather rigid dendrimer molecules can (38) Sano, M.; Okamura, J.; Ikeda, A.; Shinkai, S. Langmuir 2001, 17, 1807. (39) (a) Vernov, A.; Steele, W. A. Langmuir 1991, 7, 2817. (b) Vernov, A.; Steele, W. A. Langmuir 1991, 7, 3110.

undergo only small conformational changes due to internal rotation, and therefore a complete contact of the phenylene groups with the substrate surface seems impossible. This may be the reason these polyphenylene dendrimer monolayers show a large number of defects. One way to obtain a higher degree of surface coverage is to use dendrimers that are derivatized with alkyl groups at the periphery. Indeed, polyphenylene dendrimers with derivatized alkyl groups can form well-organized monolayers on the surface of HOPG because of the strong adsorption of alkyl chains on the HOPG surface.33 2.3 Self-Assembled Dendrimer Nanofibers. When the dendrimer solution of 5.7 × 10-8 M was casted on HOPG according to method 2, a very remarkable nanostructure that we observed with AFM, besides the dendrimer globular aggregates and dendrimer monolayers, was the dendrimer nanofiber. Figure 5A shows an AFM image of a part of dendrimer nanofibers on a HOPG surface as well as globular dendrimer aggregates and a dendrimer monolayer. The nanofibers have various dimensions depending on the concentration of dendrimer solution, the substrate, and the rate of solvent evaporation. Typically, when HOPG is used as a substrate, the length of the nanofibers extends up to several hundreds of micrometers (we have observed nanofibers with a length (40) Matties, M. A.; Hentschke, R. Langmuir 1996, 12, 2495.

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Figure 6. NCAFM image (13 µm × 13 µm) of dendrimer nanofibers prepared by solvent casting a 5.7 × 10-6 M dendrimer 1 solution in CH2Cl2 on silanized mica by method 2.

of 400 µm), the width up to several hundreds of nanometers and the height up to about 100 nm. The nanofibers are not homogeneous, their width and height decrease from the middle to both ends. They are not evenly distributed on the substrate surface, more dendrimer nanofibers can be visualized in some areas, as shown in Figure 5C. The dendrimer nanofibers are stable and can be kept at ambient temperature in a dust-free atmosphere. They also have a high thermal stability, considering the fact that the nanofibers are still intact after being annealed at 120 °C for 2 h. The influence of the substrate and the rate of solvent evaporation on the formation of dendrimer nanofibers were investigated. As we have observed, on a mica surface the polyphenylene dendrimers can only assemble into globular aggregates. If, however, the surface of mica is silanized with octadecyltrichlorosilane, dendrimer nanofibers are observed, as evidenced in Figure 6. The dark holes shown in Figure 6 are defects in the silane film. High-quality self-assembled monolayers of alkyltrichlorosilane derivatives are difficult to produce, mainly because of the need to carefully control the amount of water in solution.41-43 The AFM image in Figure 6 shows that dendrimer nanofibers can deform or break, which might be the result of the lateral flow of the solvent at the end of the evaporation. The effect of the substrate indicates that hydrophobic surfaces facilitate the formation of dendrimer nanofibers. Figure 7 shows two AFM images of samples prepared by solvent casting a 5.7 × 10-7 M dendrimer solution on HOPG with a different rate of solvent evaporation. In Figure 7A, the sample was prepared by method 2 (evaporation time of a few hours), while Figure 7B was obtained from the sample prepared by solvent casting the dendrimer solution under ambient conditions. In the latter case, the solvent evaporated within several minutes. It can be seen from Figure 7B that, even though the solvent evaporated very rapidly, dendrimer nanofibers were also formed, but in the presence of more and bigger globular dendrimer aggregates. The influence of the rate of solvent evaporation clearly demonstrates that evaporation of the solvent at a controlled rate is favorable for the formation of dendrimer nanofibers. (41) Ulman, A. Chem. Rev. 1996, 96, 1533. (42) (a) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H.; Leitner, T.; Resch, R.; Friedbacher, G. J. Phys. Chem. B 1998, 102, 7190. (b) Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H.; Basnar, B.; Vallant, M.; Friedbacher, G. Langmuir 1999, 15, 1899. (43) Lambert, A. G.; Neivandt, D. J.; McAloney, R. A.; Davies, P. B. Langmuir 2000, 16, 8377.

Figure 7. (A) NCAFM image (50 µm × 50 µm) of dendrimer nanofibers prepared by solvent casting a 5.7 × 10-7 M dendrimer 1 solution in CH2Cl2 on HOPG by method 2. (B) NCAFM image (20 µm × 20 µm) of dendrimer nanofibers prepared by solvent casting a 5.7 × 10-7 M dendrimer 1 solution in CH2Cl2 on HOPG under ambient conditions.

Since a dendrimer nanofiber is a novel self-assembled structure, it is worthwhile to speculate on its formation mechanism. The polyphenylene dendrimer used in this study has a biphenyl core and has a dumb-bell like shape5 and hence has anisotropic growth potential. With the progress of the solvent evaporation, the dendrimer solution becomes highly concentrated and the dendrimer molecules nucleate on the surface owing to the intermolecular π-π interactions, resulting in an oriented growth into nanofibers. Taking into consideration the observed phenomena, two possibilities for the development of dendrimer nanofibers can be proposed. Either the dendrimer aggregates preform in solution, deposit, and subsequently grow into nanofibers on the surface of the substrate, or the dendrimer molecules nucleate and grow into nanofibers directly on the substrate surface. Further study on the mechanism of nanofiber formation is in progress. Conclusions Individual polyphenylene dendrimer molecules, prepared by spin-coating a very dilute dendrimer solution in CH2Cl2 on a mica surface, have been visualized with NCAFM. The measured height by NCAFM is in good agreement with the dendrimer molecule size as calculated

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by Merck Molecular Force Field method. On a mica surface, the dendrimer molecules can only aggregate into globular clusters, while they can also form monolayers on a HOPG surface and nanofibers on a HOPG and silanized mica surface. Hydrophobic surfaces facilitate the formation of dendrimer nanofibers. Two possibilities for the development of dendrimer nanofibers are proposed. Dendrimer aggregates preform in solution, deposit, and grow into nanofibers on the surface of substrate, or dendrimer molecules could nucleate and grow into nanofibers directly on the substrate surface. Dendrimer nanofibers have a long-term stability and also a high thermal stability. Such novel structures are expected to add new aspects to the

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self-assembly of dendrimers and find applications in materials science. Acknowledgment. The authors thank the DWTC through IUAP-IV-11, the FWO (Flemish Ministry of Education), the STWW through the IWT project “Molecular Nanotechnology”, and ESF SMARTON for financial support. S. De Feyter is a Postdoctoral Researcher of the Fund of Scientific Research - Flanders. The authors also thank T. Vosch for the MMFF calculation of the dendrimer molecules. The collaboration was made possible thanks to the TMR project SISITOMAS. LA011270D