Self-Assembled Polyphenylene Dendrimer Nanofibers on Highly

Besides forming globular clusters, all the polyphenylene dendrimers studied self-assemble into micrometer long nanofibers irrespective of their cores ...
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Langmuir 2002, 18, 8223-8230

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Self-Assembled Polyphenylene Dendrimer Nanofibers on Highly Oriented Pyrolytic Graphite Studied by Atomic Force Microscopy Daojun Liu,† S. De Feyter,† P. C. M. Grim,† T. Vosch,† D. Grebel-Koehler,‡ U.-M. Wiesler,‡ A. J. Berresheim,‡ K. Mu¨llen,‡ and F. C. De Schryver*,† Laboratory for Photochemistry and Spectroscopy, Department of Chemistry, Katholieke Universiteit Leuven (KULeuven), Celestijnenlaan 200F, B-3001 Heverlee, Belgium, and Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany Received May 7, 2002. In Final Form: July 29, 2002 Self-assembled nanostructures of various polyphenylene dendrimers, prepared by drop casting dendrimer solutions on the surface of highly oriented pyrolytic graphite (HOPG), have been investigated by noncontact atomic force microscopy (NCAFM). Besides forming globular clusters, all the polyphenylene dendrimers studied self-assemble into micrometer long nanofibers irrespective of their cores (e.g., tetraphenylmethane, biphenyl, or azobenzene). The effect of the dendrimer generation and the degree of branching on the formation of dendrimer nanofibers is studied.

Introduction The self-assembly of small molecules into larger submicron scale structural and functional elements through various intermolecular interactions, such as hydrogen bonds,1-4 inorganic metal-ligand coordinations,4-6 hydrophobic interactions (for example, the formation of micelles and lipid bilayers), aromatic π-stacking,7 electrostatic interactions,8 or van der Waals interactions, is an important goal of molecular nanoscience.9 Self-assembly of macromolecules with multiple functionalities has recently attracted much attention because of its capacity for the rapid construction of nanostructures with higher complexity and dimensions.10 For instance, block copolymers can self-assemble into nanoribbons with uniform height and width via intermolecular hydrogen bonding and π-π interactions.11 Nanofibers have also been constructed from peptide amphiphiles through combined intermolecular hydrophobic interactions and disulfide covalent bonds.12 * To whom correspondence should be addressed. Telephone: +3216-327405. Fax: +32-16-327989. E-mail: Frans.DeSchryver@chem. kuleuven.ac.be. † Katholieke Universiteit Leuven (KULeuven). ‡ Max-Planck-Institut fu ¨ r Polymerforschung. (1) Zimmerman, S. C.; Zeng, F.; Reichert, D. E. C.; Kolotuchin, S. V. Science 1996, 271, 1095. (2) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J. H. K. K.; Lange, R. F. M.; Lowe, J. K. L.; Meijer, E. W. Science 1997, 278, 1601. (3) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem., Int. Ed. 2001, 40, 988. (4) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. Rev. 2001, 101, 4071. (5) Fujita, M.; Umemoto, K.; Yoshizawa, M.; Fujita, N.; Kusukawa, T.; Biradha, K. Chem. Commun. 2001, 509. (6) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 2022. (7) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. Rev. 2001, 101, 1267. (8) Lee, G. S.; Lee, Y. J.; Yoon, K. B. J. Am. Chem. Soc. 2001, 123, 9769. (9) (a) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (b) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (10) Klok, H.-A.; Lecommandoux, S. Adv. Mater. 2001, 13, 1217. (11) (a) Zubarev, E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. J. Am. Chem. Soc. 2001, 123, 4105. (b) Zubarev, E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. Adv. Mater. 2002, 14, 198.

Dendrimers13-15 are a type of structurally regular and highly branched macromolecules which have the potential to carry on their branches multiple functionalities. Selfassembly of dendrimers with or without guest molecules at the ensemble16-21 as well as the single molecule level22 is of special interest because this creates a wide collection of novel structures and surfaces with new and promising properties. Self-assembly of dendrimer molecules on a solid surface to form a monolayer or multilayers through electrostatic interactions,23-25 polydentate interactions,26-29 or covalent bonding30 as well as their self-assembly into (12) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294, 1684. (13) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (14) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Molecules: Concepts, Synthesis, Perspectives; VCH: Weinheim, 1996. (15) Greyson, S. M.; Fre´chet, J. M. J. Chem. Rev. 2001, 101, 3819. (16) Tsukruk, V. V. Adv. Mater. 1998, 10, 253. (17) Smith, D. K.; Diederich, F. In Topics in Current Chemistry; Vo¨gtle, F., Eds.; Springer: Berlin, 2000; Vol. 210, p 183. (18) Emrick, T.; Fre´chet, J. M. J. Curr. Opin. Colloid Interface Sci. 1999, 4, 15. (19) Tully, D. C.; Fre´chet, J. M. J. Chem. Commun. 2001, 1229. (20) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (21) Tomalia, D. A.; Majoros, I. In Supramolecular Polymers; Liferri, A., Eds.; Marcel Dekker: New York, 2000; p 359. (22) Ko¨hn, F.; Hofkens, J.; Wiesler, U.-M.; Cotlet, M.; van der Auweraer, M.; Mu¨llen, K.; De Schryver, F. C. Chem. Eur. J. 2001, 7, 4126. (23) Watanabe, S.; Regen, S. L. J. Am. Chem. Soc. 1994, 116, 8855. (24) (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. (25) Yoon, H. C.; Kim, H. S. Anal. Chem. 2000, 72, 922. (26) (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. (27) Rahman, K. M. A.; Durning, C. J.; Turro, N. J.; Tomalia, D. A. Langmuir 2000, 16, 10154. (28) Zhang, H.; Grim, P. C. M.; Liu, D.; Vosch, T.; De Feyter, S.; Wiesler, U.-M.; Berresheim, A. J.; Mu¨llen, K.; Van Haesendonck, C.; Vandamme, N.; De Schryver, F. C. Langmuir 2002, 18, 1801. (29) Vossmeyer, T.; Guse, B.; Besnard, I.; Bauer, R. E.; Mu¨llen, K.; Yasuda, A. Adv. Mater. 2002, 14, 238. (30) (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.

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Self-Assembled Polyphenylene Dendrimer Nanofibers on HOPG

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Figure 1. Molecular structures of the polyphenylene dendrimers used in this study: (1) the third generation polyphenylene dendrimer with a biphenyl core; (2a, 3, and 4a) the second, third, and fourth generation polyphenylene dendrimers with a tetraphenylmethane core; (5a) the second generation polyphenylene dendrimer with a biphenyl core and a higher degree of branching; (6, 7, 8, and 9a) the first, second, third, and fourth generation polyphenylene dendrimers with an azobenzene core; (2b, 4b, 5b, and 9b) space-filling view of dendrimer 2a, 4a, 5a, and 9a built by a Merck Molecular Force Field (MMFF) method.

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three-dimensional structures such as core-shell tecto(dendrimers) through electrostatic interactions,31 has been reported. Percec and co-workers have described the selfassembly of monodendritic building blocks in the bulk into spherical, cylindrical, and more complex supramolecular and supramacromolecular dendrimers.32 Selfassembled nanostructures of dendronized polymers, a type of dendrimers with multifunctional polymer cores, both in the bulk and at solid and fluid interfaces have also been studied by Masuhara33 and Schlu¨ter.34 On the other hand, the supramolecular chemistry of dendrimers with other species allows them to be used as dendritic hosts for the encapsulation of guest molecules in a controllable manner35,36 and as ideal templates for the synthesis of uniform nanoparticles.37 In a previous paper,38 we reported the self-assembly of a biphenyl-core-based polyphenylene dendrimer 1 (structure shown in Figure 1) into micrometer long nanofibers. In this contribution, we investigate the self-assembly of polyphenylene dendrimers containing different cores, such as a biphenyl, a tetraphenylmethane, or an azobenzene unit, and are able to show that the formation of nanofibers is a common behavior for all dendrimers investigated.

Organics, New Jersey) were used as received. All the used solvents are of spectrophotometric grade. Sample Preparation. Assemblies of dendrimers were prepared on the surface of HOPG by drop casting. Briefly, a freshly cleaved HOPG substrate was put in a nearly closed glass container (20 cm × 20 cm × 8 cm) with preadded organic solvent (CH2Cl2, CHCl3, and toluene) in order to create a saturated solvent environment. Then five drops (ca. 150 µL) of dendrimer solution (see figure captions for concentrations of dendrimer solutions) were deposited on the surface of HOPG. The solvent evaporated slowly, typically within several hours, depending on the specific solvent. Atomic Force Microscopy (AFM). Assemblies of dendrimers were visualized with AFM. AFM was performed with a Discoverer TMX2010 AFM system (ThermoMicroscopes, San Francisco, CA) operating in noncontact mode using Si probes (ThermoMicroscopes, San Francisco, CA) 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 Modeling. Geometry optimization of the dendrimer molecules and dendrimer 2 dimers was performed in a vacuum by a molecular mechanics method (Merck Molecular Force Field)42 in Spartan (Wave Function Inc., Irvine, CA).

Experimental Section

Results and Discussion

Materials. The synthesis of the polyphenylene dendrimers used in this study (structures shown in Figure 1) has already been reported.39-41 These dendrimers exhibit good solubility in common organic solvents such as toluene and CH2Cl2 and thus can be characterized by matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF-MS). The perfect agreement between calculated and experimentally determined m/z ratios for these dendrimers as well as GPC analysis confirms their monodispersity. CH2Cl2 (Acros Organics, New Jersey), CHCl3 (Acros Organics, New Jersey), and toluene (Acros

(1) Formation of Nanofibers from Various Polyphenylene Dendrimers. In a previous paper,38 we reported the self-assembly of polyphenylene dendrimer 1 into micrometer long nanofibers by drop casting its solution in CH2Cl2 on a HOPG surface, as shown by a NCAFM image in Figure 2A. It was speculated that the dumbbell shape of dendrimer 1 molecules, due to their biphenyl core, assisted in a directional growth of dendrimer aggregates into nanofibers.38 The self-assembly of polyphenylene dendrimers based on different cores under analogous experimental conditions was investigated in this paper. Interestingly, a series of polyphenylene dendrimers with a tetraphenylmethane core (dendrimers 2-4 in Figure 1) also self-assembles into nanofibers on a HOPG surface. Parts B, E, and F of Figure 2 show representative AFM images of dendrimer nanofibers formed from dendrimers 2, 3, and 4, respectively. Nanofibers formed from these polyphenylene dendrimers have similar dimensions to those of nanofibers formed from dendrimer 1.38 The length of the nanofibers ranges from several tens of micrometers up to several hundreds of micrometers, while their height extends up to several tens of nanometers (Figure 2G). As can be seen from Figure 2E and F, the dendrimer also aggregates into globular clusters. It is important to note, however, that globular clusters of dendrimer molecules were seldomly visualized by AFM for the lower generation dendrimer 2. The clusters that can be seen in Figure 2B also consist of nanofibers. Figure 2C and D shows representative AFM images of such nanofiber clusters. The self-assembly of another series of polyphenylene dendrimers with an azobenzene core (dendrimers 6-9 in Figure 1) was also studied under similar experimental conditions. The molecular model of 9b shows that the trans-isomer azobenzene core leads to a more elongated dendrimer structure, as compared to those of dendrimers with other cores (2b, 4b, and 5b). The calculated aspect ratio for dendrimer 9 is about 1.8. The AFM measurements demonstrate that this series of dendrimers also selfassembles into nanofibers with similar dimensions to those of the dendrimer nanofibers described above. Representative AFM images of nanofibers formed from dendrimers

(31) (a) Uppuluri, S.; Swanson, D. R.; Piehler, L. T.; Li. J.; Hagnauer, G. L.; Tomalia, D. A. Adv. Mater. 2000, 12, 796. (b) Betley, T. A.; Hessler, J. A.; Mecke, A.; Manaszak Holl, M. M.; Orr, B. G.; Uppuluri, S.; Tomalia, D. A.; Baker, J. R., Jr. Langmuir 2002, 18, 3127. (32) (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. (i) Percec, V.; Holerca, M. N.; Uchida, S.; Cho, W. D.; Ungar, G.; Lee, Y.; Yeardley, D. J. P. Chem. Eur. J. 2002, 8, 1106. (33) Masuo, S.; Yoshikawa, H.; Asahi, T.; Masuhara, H. J. Phys. Chem. B 2001, 105, 2885. (34) Schlu¨ter, A. D.; Rabe, J. P. Angew. Chem., Int. Ed. 2000, 39, 864. (35) (a) Jansen, J. F. G. A.; de Brabander-van den Berg, E. M. M.; Meijer, E. W. Science 1994, 266, 1226. (b) Jansen, J. F. G. A.; Meijer, E. W.; de Brabander-van den Berg, E. M. M. J. Am. Chem. Soc. 1995, 117, 4417. (36) Yamamoto, K.; Higuchi, M.; Shiki, S.; Tsuruta, M.; Chiba, H. Nature 2002, 415, 509. (37) (a) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (b) Crooks, R. M.; Lemon, B. I., III; Sun, L.; Yeung, L. K.; Zhao, M. Top. Curr. Chem. 2001, 212, 81. (38) Liu, D.; Zhang, H.; Grim, P. C. M.; De Feyter, S.; Wiesler, U. M.; Berresheim, A. J.; Mu¨llen, K.; De Schryver, F. C. Langmuir 2002, 18, 2385. (39) Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. Rev. 1999, 99, 1747. (40) Wiesler, U. M.; Berresheim, A. J.; Morgenroth, F.; Lieser, G.; Mu¨llen, K. Macromolecules 2001, 34, 187. (41) Grebel-Koehler, D.; Liu, D.; De Schryver, F. C.; Mu¨llen, K. Submitted.

(42) Halgren, T. A. J. Comput. Chem. 1996, 17, 490.

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Figure 2. (A) NCAFM image (50 µm × 50 µm) of dendrimer nanofibers prepared by drop casting a 5.7 × 10-7 M solution of dendrimer 1 in CH2Cl2 on a HOPG surface. (B) NCAFM image (50 µm × 50 µm) of dendrimer nanofibers prepared by drop casting a 1.0 × 10-5 M solution of dendrimer 2 in CH2Cl2 on a HOPG surface. (C) NCAFM image (25 µm × 25 µm) of dendrimer nanofiber clusters prepared by drop casting a 1.0 × 10-5 M solution of dendrimer 2 in CH2Cl2 on a HOPG surface. (D) Smaller scale NCAFM image (3.5 µm × 3.5 µm) of part C. (E) NCAFM image (20 µm × 20 µm) of dendrimer nanofibers prepared by drop casting a 5.0 × 10-6 M solution of dendrimer 3 in CH2Cl2 on a HOPG surface. (F) NCAFM image (50 µm × 50 µm) of dendrimer nanofibers prepared by drop casting a 1.0 × 10-6 M solution of dendrimer 4 in CH2Cl2 on a HOPG surface. (G) Smaller scale NCAFM image (1.5 µm × 1.5 µm) of part F. The inset shows the topography profile along the dotted line indicated in the topography. (H) NCAFM image (20 µm × 20 µm) of a dendrimer nanofiber prepared by drop casting a 1.0 × 10-5 M solution of dendrimer 5 in CH2Cl2 on a HOPG surface.

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Figure 3. (A) NCAFM image (50 µm × 50 µm) of dendrimer nanofibers prepared by drop casting a 1.0 × 10-5 M dendrimer 6 solution in CH2Cl2 on a HOPG surface. (B) NCAFM image (30 µm × 30 µm) of a dendrimer nanofiber prepared by drop casting a 1.0 × 10-5 M dendrimer 7 solution in CH2Cl2 on a HOPG surface. (C) NCAFM image (15 µm × 15 µm) of a dendrimer nanofiber prepared by drop casting a 1.0 × 10-5 M dendrimer 8 solution in CH2Cl2 on a HOPG surface. (D) NCAFM image (20 µm × 20 µm) of dendrimer nanofibers prepared by drop casting a 1.0 × 10-5 M dendrimer 9 solution in CH2Cl2 on a HOPG surface.

6-9 are shown in parts A-D of Figure 3, respectively. The fact that polyphenylene dendrimers with different cores such as a tetraphenylmethane, a biphenyl, or an azobenzene unit self-assemble into nanofibers suggests that their formation is determined by the properties of the polyphenylene branches. Given that both micrometer long dendrimer nanofibers and globular clusters are formed from the third and fourth generation dendrimers 3 and 4, while nanofibers are the only self-assembled nanostructures observed for the second generation dendrimer 2, it is suggested that lower generation dendrimers favor the nanofiber formation. The effect of the degree of branching of dendrimers on the formation of nanofibers was investigated by comparing the self-assembly of dendrimer 5, which has a degree of branching of 4, with that of dendrimer 2, which has a degree of branching of 2. The different cores of these two dendrimers do not significantly influence their selfassembling behavior, as suggested by the similar selfassembling behaviors of dendrimer 1 and dendrimer 3. Simulations40 and the molecular model in Figure 1 show that the branches of polyphenylene dendrimers with a lower degree of branching (all dendrimers in Figure 1 except dendrimer 5) are rather separated even up to higher generations, whereas, for dendrimer 5, the dendrimer branches become spatially more crowded and the dendrimer structure resembles a closed spherical shape. The second generation dendrimer 5 can also self-assemble into micrometer long dendrimer nanofibers, but always in addition to the formation of globular clusters (Figure 2H). Comparing the self-assembling behavior of dendrimer 5 with that of dendrimer 2, both being of the same generation

but the latter having a lower degree of branching, suggests that a lower degree of branching and thus more separated polyphenylene branches facilitate the dendrimer nanofiber formation. In general, the fibers are not homogeneous; their width and height decrease from the middle to both ends. There is no indication that the dendrimer generation has an effect on the size of the fibers. Concentration (between 5.0 × 10-7 and 1.0 × 10-5 M) does not affect the fiber formation significantly, and similar observations were found when the dendrimers were deposited on HOPG from different organic solvents such as CH2Cl2, CHCl3, and toluene. We noticed that the speed of solvent evaporation affects the number of fibers formed (the faster the solvent evaporation, the less fibers). The effect of temperature has not been investigated. (2) Mechanistic Consideration of Dendrimer Nanofiber Formation. The driving force for the formation of polyphenylene dendrimer nanofibers is attributed to the π-π interactions among dendrimer molecules. The dendrimer molecules can adopt a preferential orientation with respect to each other and interdigitate in order to maximize intermolecular π-π interactions and van der Waals interactions upon aggregation, which then leads to a directional growth of dendrimer aggregates into nanofibers. Molecular mechanics calculations of a dimer of 2 show indeed a tendency for such interdigitation. Polyphe(43) 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. (44) Wind, M.; Wiesler, U. M.; Saalwa¨chter, K.; Mu¨llen, K.; Spiess, H. W. Adv. Mater. 2001, 13, 752-756.

Self-Assembled Polyphenylene Dendrimer Nanofibers on HOPG

Figure 4. (A) NCAFM image (6 µm × 6 µm) of two crossed dendrimer nanofibers prepared by drop casting a 1.0 × 10-6 M dendrimer 4 solution in CH2Cl2 on a HOPG surface. (B) Topography profiles along the dotted lines indicated in the topography image (A).

nylene dendrimers have already been characterized as shape-persistent macromolecules.43,44 We suggest that the rigidity of the polyphenylene dendrimer arms also plays a role in the nanofiber formation, as it guarantees a certain interbranch free volume, allowing the stacking of the dendrimer molecules and the subsequent growth of the dendrimer nanofibers. Lower generation dendrimer molecules with a lower degree of branching, whose branches are more separated and whose volume of the arms is smaller, might therefore have a higher tendency for the directional growth. This explains why a lower generation and a lower degree of branching of the dendrimers facilitate the formation of nanofibers.

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The self-assembly of dendrimer molecules is the result of the subtle balance of interactions among the dendrimer molecules, the substrate, and the solvent. Since the rigid dendrimer molecules can undergo only small conformational changes due to internal rotation, a full contact of the phenylene groups of the polyphenylene dendrimer molecules with the HOPG surface is impossible. Therefore, the π-π interactions among dendrimer molecules are expected to be stronger than those between the dendrimer molecules and the HOPG surface. The polyphenylene dendrimer molecules thus have a higher tendency to aggregate among themselves. The above considerations lead to the hypothesis that, in the course of the solvent evaporation, dendrimer aggregates (of indefinite size and aspect ratio) are formed in solution which subsequently deposit on the substrate surface and grow further till the end of the evaporation process. The observation of a crosspoint of two nanofibers, as shown in Figure 4, supports this idea. In Figure 4, the topography at the location of the crosspoint corresponds to the sum of the heights of two crossed nanofibers. The same phenomenon is also observed from Figure 2A, C, and D. Another pathway of the nanofiber formation might be that dendrimer molecules nucleate and grow into nanofibers directly on the substrate surface, as discussed previously.38 (3) Stability of Polyphenylene Dendrimer Nanofibers. The very high temporal and thermal stability of the polyphenylene dendrimer nanofibers was already reported for dendrimer 1 in our earlier work,38 and these observations remain valid for the other dendrimers investigated in this paper. The dendrimer nanofibers can be kept from dust under ambient conditions. They remain intact after being annealed at 200 °C for 2 h. The thermal stability of these nanofibers can be directly related to the high thermal stability of the dendrimer molecules themselves. These dendrimers show no glass transition temperature,40 and the thermal decomposition temperature of polyphenylene dendrimers was reported to be above 500 °C.37 However, the dendrimer nanofibers have a low mechanical stability, as demonstrated in Figure 5. The dendrimer nanofibers can be damaged or deformed by a line scan in contact mode AFM at a very low scanning rate even under a force load of 250 nN. The low mechanical stability of these nanofibers can be attributed to the rather weak noncovalent interactions among dendrimer molecules. It is expected that the mechanical stability of these polyphenylene nanofibers can be improved by a post-

Figure 5. NCAFM images (15 µm × 15 µm) of a dendrimer nanofiber prepared by drop casting a 5.7 × 10-7 M dendrimer 1 solution in CH2Cl2 on a HOPG surface: (A) original image; (B) image after line scans in a contact mode atomic force microscope at a scanning rate of about 40 nm/s under force loads of about 2500 nN (a), 250 nN (b), and 1500 nN (c), respectively.

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functionalization process which can covalently connect the adjacent phenylene units within nanofibers. Conclusions All polyphenylene dendrimers used in this study, irrespective of their generation and core (such as a biphenyl, a tetraphenylmethane, or an azobenzene unit), self-assemble into micrometer long nanofibers. The intermolecular π-π interactions and the shape-persistence of polyphenylene dendrimers are responsible for the nanofiber formation. It was demonstrated that lower generations as well as separated polyphenylene branches are favorable for the formation of polyphenylene dendrimer nanofibers. The dendrimer nanofibers have a high temporal and thermal stability but a low mechanical stability.

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The formation of micrometer long nanofibers from polyphenylene dendrimers is expected to add new aspects to the self-assembly of dendrimers. Acknowledgment. The authors thank the DWTC through IUAP-V-03, the FWO (Flemish Ministry of Education), the STWW through the IWT project “Molecular Nanotechnology”, the German Ministry of Education and Research, the Volkswagenstiftung, and ESF SMARTON for financial support. S.D.F. is a Postdoctoral Researcher of the Fund of Scientific ResearchsFlanders. The collaboration was made possible thanks to the TMR project SISITOMAS and a Max-Planck Research Award. LA020425U