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Real Space Visualization of the Disklike Assembly Structure of Dendritic Molecules on Graphite Peng Wu, Qinghua Fan, Guojun Deng, Qingdao Zeng, Chen Wang,* and Chunli Bai* Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China Received October 29, 2001. In Final Form: February 20, 2002 The self-assembly of monodendrons with peripherally attached alkyl substituents has been studied by scanning tunneling microscopy. The disklike assembly, a characteristic structure of low generation dendrimers, has been directly observed. The subunits of disk structure can be well resolved as well as the ordered alkyl parts.
Introduction The self-assembly of organic molecules into monolayers or multilayers on a substrate surface has attracted intense studies in the past decade due to their important roles in many interfacial phenomena such as wetting, lubrication, adhesion, and molecular and biological recognition.1 The understanding of their structure and properties is critical for applications in fields such as tribology,2 sensors,3-5 molecular electronic devices,6 and nanofabrications.7 Scanning tunneling microscopy (STM) is a powerful tool in the study of self-assembled monolayers with the merits of real space investigation and atomic resolution. From linear alkanes8 to planar phthalocyanines, porphyrin,9-11 and aromatic molecules,12 STM has been applied successfully in studying the structures of many organic molecules. Dendrimers are highly branched macromolecules consisting of a multifunctional core from which successive branched repeat units extend radially outward.13 Recently, increasing studies on the self-assembly of dendrimers at interfaces have been reported. Prokhorova et al.14 studied monodendron-jacked linear polystyrenes by scanning force microscopy on mica. The single wormlike molecules were visualized, and the ordering was found to be dependent on the branching density and interplay between intramo* To whom correspondence should be addressed. Fax: (86)-1062557908. E-mail:
[email protected], wangch@ infoc3.icas.ac.cn. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wybbem, K. J.; Yu, H. Langmuir 1987, 3, 932. (2) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101 (19), 3800. (3) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (4) Wang, J.; Frostman, L. M.; Ward, M. D. J. Phys. Chem. 1992, 96, 5224. (5) Chidsey, C. E. D. Science 1991, 251, 919. (6) Vondrak, T.; Cramer, C. J.; Zhu, X. Y. J. Phys. Chem. B 1999, 103 (42), 8915. (7) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (8) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (9) Chiang, S. Chem. Rev. 1997, 97, 1083. (10) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197. (11) Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K.; Tang, H.; Joachim, C. Science 1996, 271, 181. (12) Strohmaier, R.; Petersen, J.; Gompf, B.; Eisenmenger, W. Surf. Sci. 1998, 418, 91. (13) Grayson, S. M.; Fre´chet, J. M. J. Chem. Rev. 2001, 101, 3819. (14) Prokhorova, S. A.; Sheiko, S. S.; Ahn, C.-H.; Percec, V.; Mo¨ller, M. Macromolecules 1999, 32, 2653.
lecular and surface interactions of the dendritic coat. By atomic force microscopy (AFM), Ponomarenko et al.15 investigated carbosilane liquid crystalline dendrimer films of different thicknesses. Zhang et al.16 studied the stiffness and adhesion properties of individual polyphenylene dendrimers. Using STM, Zhang et al. imaged the selfassembly of different sizes of Fre´chet type dendrons on gold17 and the self-assembly of the second Fre´chet type monodendrons with a different tailored structure was studied recently.18 It has been known by X-ray diffraction results19 that there exist two characteristic types of dendrimer assembly structures, namely, the spherical structure and the disklike structure. Spherical structure is prevalent for higher generation dendrimers and has been directly visualized in AFM studies.15,20 On the other hand, the disklike structure which is the preferential one for low generation dendrimers has not been well characterized. The studies on the characteristics of the assembling process of low generation dendrimers should provide complementary information for understanding the interactions between dendritic molecules. In 2000, Prokhorova et al.21 realized that when linear polymers with monodendron side groups were adsorbed on highly oriented pyrolytic graphite (HOPG), the positional and orientational order of the adsorbed molecules was determined by the specific interaction of the alkyl substituents of the monodendrons with the surface. The results illustrated that the monodendron structure is a most important factor in the molecular arrangement in the monolayer. In this paper, using STM we studied, on the HOPG surface, the self-assembly of monodendrons with alkyl substituents on the periphery, using the first generation (15) Ponomarenko, S. A.; Boiko, N. I.; Shibaev, V. P.; Magonov, S. N. Langmuir 2000, 16, 5487. (16) Zhang, H.; Grim, P. C. M.; Foubert, P.; Vosch, T.; Vanoppen, D.; Wiesler, U.-M.; Berresheim, A. J.; Mu¨llen, K.; De Schryver, F. C. Langmuir 2000, 16, 9009. (17) Zhang, L.; Huo, F. W.; Wang, Z. Q.; Wu, L. X.; Zhang, X. Langmuir 2000, 16, 3813. (18) Zhang, L.; Zou, B.; Dong, B.; Huo, F. W.; Zhang, X.; Chi, L. F.; Jiang, L. Chem. Commun. 2001, 1906. (19) Percec, V.; Cho, W.-D.; Ungar, G.; Yeardley, D. J. P. Angew. Chem., Int. Ed. 2000, 39, 1598. (20) Li, J.; Swanson, D. R.; Qin, D.; Brothers, H. M.; Piehler, L. T.; Tomalia, D.; Meier, D. J. Langumir 1999, 15, 7347. (21) Prokhorova, S. A.; Sheiko, S. S.; Mourran, A.; Azumi, R.; Beginn, U.; Zipp, G.; Ahn, C.-H.; Holerca, M. N.; Percec, V.; Mo¨ller, M. Langmuir 2000, 16, 6862.
10.1021/la0116138 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/01/2002
Structure of Dendritic Molecules on Graphite
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Scheme 1. Synthesis of the Second-Generation 3,5-Bis(1-decyloxyl)benzyl Ether Modified Fre´ chet Type Monodendron (2) and Molecular Structure of Monodendron 3,4,5-Tridecyloxybenzoic Acid (1) and Monodendron 2
of monodendron 1, 3,4,5-tridecyloxybenzoic acid, and the second generation of modified Fre´chet type acid monodendron 2 as examples. For the first time, the disklike units were directly observed and the fine structure of the monolayer was obtained. Experimental Section The first generation of monodendron 1 was synthesized according to the published method.22 Scheme 1 describes the synthesis of the second generation of modified Fre´chet type acid monodendron 2. Etherification of the bromide dendron with methyl 3,5-dihydroxybenzoate provided the ester monodendron followed by hydrolysis to give the acid monodendron 2 in high reaction yield. The detailed synthesis of this kind of monodendrons will be described elsewhere. The samples were dissolved in toluene (HPLC grade, Aldrich) with a concentration of less than 1%. A droplet of the solution was deposited onto a freshly cleaved surface of HOPG (quality ZYB, Digital Instruments) and dried in air prior to STM imaging. The experiment was performed on a Nanoscope IIIa SPM (Digital Instruments, Santa Barbara, CA) in ambient conditions. STM tips were mechanically formed Pt/Ir wire (90/10). All of the STM images were recorded using the constant current mode of operation. The specific tunneling conditions are given in the figure captions.
Results and Discussion Scheme 1 shows the molecular structure of monodendrons 1 and 2. Figure 1a shows the large uniform and close-packed monodendron assembly of molecule 1. The bright disklike units display a small dark hole in the center. As indicated by the markers, the distance between two units in one direction is 3.4 ( 0.1 nm while in another orientation the distance is 4.6 ( 0.1 nm. This directional periodicity asymmetry could be attributed to the effects of the underlying graphite lattice as discussed later. A high-resolution image is presented in Figure 1b. Within each disklike unit, four bright dots are clearly discernible. With the measured diameter of 0.50 ( 0.05 nm, each bright spot is considered to represent an aromatic ring. Surrounding the four bright dots, alkyl chains are grouped with a measured angle of 110°, in contrast to the generally expected right-angled orientation for tetramer formations. This deviation from 90° may be attributed to the effect of (22) Borisch, K.; Diele, S.; Go¨ring, P.; Kresse, H.; Tschierske, C. J. Mater. Chem. 1998, 8, 529.
the HOPG lattice which has a 3-fold symmetry. The length of the alkyl chains is measured to be 1.3 ( 0.1 nm, which is consistent with the length of a decane molecule in its all-trans conformation. The alkyl chains pack closely, and the periodicity along the ordered alkyls is measured to be about 2.8 nm. Within this distance, six parallel alkyl chains can be arranged on the surface with an average distance between the chain skeletons of 4.6 Å. Considering the structure of monodendron 1, the formation of the tetramer unit is believed to be due to the existence of hydrogen bonds among the carboxyl groups at the focal points of molecules. The proposed model in Figure 1b fits well with the image. Figure 2a shows the observed assembly of the second generation of monodendron 2. In the image, the disklike arrangement exists but is not as orderly as that in the assembly of monodendron 1. From Figure 2b, some bright spots representing benzyl rings can be recognized. It can be derived that there are three molecules in one disk. The aromatic groups of the three molecules pack closely to form a nearly complete disk with the alkyl chains extending outwardly. The measured angle between every two adjacent alkyl chain groups is approximately 120°, which is commensurate with the lattice of HOPG. The proposed molecular assembly model, as one of the possible patterns, is presented in Figure 2b. The disklike arrangement of monodendrons on HOPG illustrated above conforms to the typical packing pattern of dendrimers with low generation in bulk. Due to the strong interaction between flexible alkyl chains and HOPG, all alkyl chains of both monodendrons 1 and 2 are parallel to the surface and form ordered structures rather than random entanglements as reported for dendritic molecules assembled in bulk.19 This adsorption effect is considered essential in forming a 2D assembly of dendritic molecules at a solid support surface. The close-packed arrangement of alkyl chains increases the desorption energy and stabilizes the assembly.23 While the formation of hydrogen bonds at the focal points among molecules may be the driving force for the formation of the observed tetramer and trimer structure of monodendrons 1 and 2, together with the steric hindrance effect which will be (23) Hentschke, R.; Schurmann, B. L.; Rabe, J. P. J. Chem. Phys. 1992, 96, 6213.
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Figure 1. (a) STM image of a well-ordered assembly of 1 in large scale. The distances between two units are 3.4 ( 0.1 and 4.6 ( 0.1 nm in different orientations. The tunneling condition is 1.18 nA and 880.0 mV (sample is positive). The scan area is 66.33 × 66.33 nm2. (b) STM image of the assembly of 1 in high resolution. The angle between two adjacent alkyl groups is 110°. A schematic representation of the adsorbed molecules is given as an overlayer. A blue spot represents a carbon atom, while an oxygen atom is indicated by a red spot. The tunneling condition is 1.14 nA and 880.0 mV (sample is positive). The scan area is 14.60 × 14.60 nm2.
more pronounced for higher generation dendrimers, the final structure of the molecular assembly is determined by the joint effect of both molecular-molecular and molecular-substrate interactions. Conclusion In summary, the monolayers of two low generation dendrimers with alkyl substituents on the periphery have
Wu et al.
Figure 2. (a) STM image of the assembly of monodendron 2 in large scale. The tunneling condition is 1.08 nA and 818.7 mV (sample is positive). The scan area is 63.61 × 63.61 nm2. (b) Higher resolution image of monodendron 2. The tunneling condition is 1.28 nA and 768.8 mV (sample is positive). The scan area is 18.30 × 18.30 nm2. Overlayer: the proposed molecular model. A blue spot represents a carbon atom, while an oxygen atom is indicated by a red spot.
been studied. From the high-resolution image, the fine structure of individual disks was obtained. The 2D crystallization of alkyl chains on the graphite surface and the hydrogen bonds at the focal points are the driving forces for the formation of ordered structures.
Acknowledgment. The authors thank the National Natural Science Foundation and the Foundation of the Chinese Academy of Sciences for financial support. Support from the National Key Project on Basic Research (Grant G2000077501) is also acknowledged. LA0116138
