CHEMPHYSCHEM 2002, No. 7 ¹ WILEY-VCH-Verlag GmbH, 69451 Weinheim, Germany, 2002 1439-4235/02/03/07 $ 20.00+.50/0 633

Observation of Functional GroupMediated Assembling of DendriticMolecules by STM

Peng Wu,[a] Qinghua Fan,[b] Qingdao Zeng,[a]

Chen Wang,[a] Guojun Deng,[b] and Chunli Bai*[a]

KEYWORDS:

adsorption ¥ dendrimers ¥ hydrogen bonds ¥ self-assembly ¥

Dendrimers, having cores with a few functional groups to whicha corresponding number of dendrons (dendritic wedges) areattached, are currently one of the most intensely studied classesof compounds. The high density of functional groups, theexistence of a usable ™surface∫, and various nanometer-scalestructures have made dendrimers interesting candidates for usein many applications.[1] The shape and size of a dendrimer isgoverned by the architecture of the dendrons,[2] the generation

makes us expect that CARS correlation spectroscopy willbecome a valuable tool.[24] An especially attractive area for suchapplications are studies of aggregation processes, which are ofcentral importance for many biological problems.

We thank Prof. Christoph Br‰uchle for continuous support.

[1] M. Eigen, R. Rigler, Proc. Natl. Acad. Sci. USA 1994, 91, 5740 ± 5747.[2] S. Maiti, U. Haupts, W. W. Webb, Proc. Natl. Acad. Sci. USA 1997, 94,

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Schrof, J. Phys. Chem. A 2001, 105, 3673 ± 3679.[13] A. Zumbusch, G. R. Holtom, X. S. Xie, Phys. Rev. Lett. 1999, 82, 4142 ±

4145.[14] Y. R. Shen, The Principles of Nonlinear Optics, John Wiley & Sons, New York,

NY, 1984.[15] M. D. Duncan, J. Reintjes, T. J. Manuccia, Opt. Lett. 1982, 7, 350 ± 352.[16] J.-X. Cheng, A. Volkmer, L. D. Book, X. S. Xie, J. Phys. Chem. B 2001, 105,

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FL, 1991.[18] E. Elson, D. Magde, Biopolymers 1974, 13, 1 ± 27.[19] R. Rigler, ‹. Mets, J. Widengren, P. Kask, Eur. Biophys. J. 1993, 23, 169 ±

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[22] K. Starchev, J. Zhang, J. Buffle, J. Coll. Interface Sci. 1998, 203,189 ± 196. For bead sizes comparable to the excitationvolume, the apparent beam size r0 increases to

����������������r2

0 � R2�

,where R is the radius of the particle. For beads 1 �m indiameter, we observe that two-photon autocorrelation andCARS autocorrelation curves are consistently steeper thanwould be expected from the model described here (datanot shown).

[23] J.-X. Cheng, L. D.Book, X. S. Xie, Opt. Lett. 2001, 105, 1341 ±1343.

[24] E. O. Potma, W. P. de Boeij, P. J. M. van Haastert, D. A. Wiers-ma, Proc. Natl. Acad. Sci. USA 2001, 98, 1577 ± 1582.

Received: March 25, 2002 [Z388]

[a] Prof. C. Bai, P. Wu, Dr. Q. Zeng, Prof. C. WangYouth Laboratory of Nanoscience and NanotechnologyCenter for Molecular ScienceInstitute of ChemistryThe Chinese Academy of SciencesBeijing 100080 (China)Fax: (86) 10-62557908E-mail : clbai@infoc3.icas.ac.cn

[b] Prof. Q. Fan, G. DengLMRSSCenter for Molecular ScienceInstitute of ChemistryThe Chinese Academy of SciencesBeijing 100080 (China)

Figure 4. Diffusion coefficients from the autocorrelation curves measured for the diffusionof 202 (�) and 528 nm (�) beads in sucrose/water solutions with different viscosities � (fitsas solid lines).

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number,[3] and its core's functionalities[4, 5] and multiplicities.[6] Ingeneral, increasing the number of dendrons changes the shapeof a dendrimer from flat tapered into conical. It has also beenfound that increasing the number of substituted alkyl chains onthe periphery of a dendron increases the size of the dendrimer.[3]

However, studies concerning the effect the functional group atthe focal point has upon the dendrimer's structure are scarce.

Fre¬chet-type dendrons are a type of monodendron with aspecial structure (Scheme 1).[7] As building blocks, they havebeen used in copolymer synthesis,[4, 8] and, by using X-raydiffraction analysis, the structure of such copolymers has beenrevealed.[9, 10] It has been noticed that the majority of X-ray

O O

X X

O O

X

O

O

O

O

X

[G-1]-X

[G-2]-X

Scheme 1. Fre¬chet-type dendrons ([G-1]-X, [G-2]-X).

characterizations of monodendron assemblies have been carriedout on alkyl periphery functionalized monodendrons.[11] Littledirect evidence concerning the assembled structure, especiallyfor the unsubstituted species such as Fre¬chet-type dendrons, hasbeen reported. Although it is known that substitution on theperiphery of dendrons appreciably impacts on the assembly'sarchitecture, it would be interesting to have the capability tostudy the fine structure of the assembly of monodendronswithout alkyl chains on their periphery. In 2000, using scanningtunneling microscopy (STM), Zhang and co-workers investigatedmonolayers of different sizes of Fre¬chet-type dendrons (with athiol group at the focal point), that had self-assembled on a goldsurface.[12] There was strong chemisorption between the den-dron thiol at the focal point and the gold surface, and a disklikearrangement–one of the main characteristic structures ofdendrimers (especially for low generation monodendrons inbulk)–was not observed.

This paper describes the characterization of the configurationof self-assembled, second-generation Fre¬chet-type ester (1) andacid (2) dendrons on an inert substrate (Scheme 2). Using STM,not only was the disk-shaped assembly of monodendronsdirectly identified, but a different arrangement of monoden-drons, induced by the functional groups at the focal point wasalso observed.

Scheme 2 shows the chemical structures of the monoden-drons (1 and 2) investigated. In the case of monodendron 1,large, uniform assemblies can clearly be recognized in the STMimage (Figure 1a). This is, we believe, the first STM observationof a disklike assembly of monodendrons on a highly orientedpyrolytic graphite (HOPG) surface. In the image, it can be seenthat the disklike units pack one by one, and a small dark areapresents in the unit. To study the structure in detail, a small-scaleimage was taken (Figure 1b). In one unit, two ladlelikemolecules, packed top-to-bottom were clearly observed. Asmeasured from the STM image, a unit cell with a�2.6� 0.2 nm,b�3.1� 0.1 nm, �� 56.3� 4� was outlined (Figure 1b). The

CH2Br

O

O

HO

HO

CO2CH3

O

O

CO2CH3

O

O

O

O

O

O

CO2H

O

O

O

O

[G-1]-CH2Br

8 h, 92.5%

1

>98%

2

4 h

Scheme 2. Synthesis of Fre¬chet-type dendrons ([G-2]-X, 1: X�COOCH3; 2 : X�COOH). Reagents and conditions: a) K2CO3, KI, [18]crown-6, acetone, refluxing; b) KOH,tetrahydrofuran/methanol, refluxing.

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Figure 1. a) STM image of a self-assembled monolayer of monodendron 1 on agraphite surface. Tunneling conditions were 1.03 nA, 720 mV (sample waspositive). The scan area was 55.1� 55.1 nm2. The Z axis was 0.6 nm. b) Highermagnification STM image of a self-assembled monolayer of 1. The unit cell isoutlined with a� 2.6� 0.2 nm, b� 3.1� 0.1 nm, �� 56.3� 4�. Tunneling con-ditions were 1.03 nA, 720 mV (sample was positive). The scan area was 22.6�22.6 nm2. The Z axis was 0.6 nm. c) The proposed packing model of 1. Blue spotsrepresent carbon atoms; oxygen atoms are indicated by red spots.

whole disk appeared slightly distorted along the long axis. Theproposed packing model is presented in Figure 1c.

In order to understand the influence the functional group atthe focal point has on the self-assembling process, we studied

the self-assembly of monodendron 2. A disklike structure (similarto that of 1) was observed (Figure 2a). However, the disklikeunits of 2 were packed more closely than those of 1, and theedge of a disk was not as clear as it was in 1. A unit cell with a�4.2�0.1 nm, b� 4.1� 0.1 nm, ��53.5�1� was outlined. In theassembly, two kinds of black regions were observed: one in thecenter of the disk, with a diameter of about 0.8� 0.1 nm; theother (which was surrounded by three disk units) in the inter-diskregion, with a diameter of about 1.3� 0.1 nm. The measuredvalues agree with those of a simplified model (see Figures 2cand 2d, and later in the text).

Figure 2b, a high-resolution image, presents detailed infor-mation. One disk is composed of many bright dots, whichrepresent aromatic rings with large electronic densities. Thebenzene groups are highly ordered in the bright area, as they arein the crystalline phase. As the diameter of one benzyl ring isabout 0.5 nm (from H to H), and the C�O bond length is 0.1 nm,the diameter of one bright dot is at least 0.6 nm. In one disk, thebright area can be taken for a cirque with external (parameter a)and internal diameters (inner dark region) of about 4.2 nm and0.8 nm, respectively. According to the ratio of areas, 47 phenylgroups are computed to be within one unit. As one monoden-dron contains seven phenyl groups, it can be concluded that onedisk is formed by six monodendrons. For the carboxyl group atthe focal point of monodendron 2, we considered every twoadjacent monodendrons to be connected by a hydrogen bond.Although it is not very easy to establish a big disk using onlyhydrogen bonding as the driving force, it is one of the most likelymechanisms. When six molecules are piled up according to therules for hydrogen bonds in a simplified model, the size of thecavity in the center of the model is quite consistent with thatobserved in the STM image (Figure 2c). The relatively uniformcontrast of the phenyl groups in the image is evidence that,when the molecules are adsorbed on the surface, the interactionbetween the adsorbate and substrate drives all the phenyl ringsparallel to the graphite substrate. With hydrogen bonds, six fan-shaped molecules form one disk, and three of these units formanother kind of interstitial spacing (Figure 2d).

To further illustrate the assembly, the Fourier transformspectrum (FTS) is shown in Figure 2e. In the center of thespectrum, six bright dots can clearly be seen, with a period of0.58�0.02 nm. Assuming that the aromatic rings are almost inone plane in the image, we measured the distances betweentwo neighboring rings connected by a covalent bond in thesimplified model (Figure 2c). The shortest distance between twoneighboring rings is 0.43 nm, whereas the longest one in themodel is 0.68 nm. The periodicity derived from the bright dots inFigure 2e lies between these two values.

When molecules assemble on the surface of a solid, mole-cule ± molecule interactions play an important role. It has beenfound that different substituted porphyrin molecules were ableto form various structures when they were adsorbed on a goldsurface. The structural variety can be ascribed to the differenceof noncovalent interactions between the individually adsorbedmolecules.[14] In our system, the physisorption between thebenzyl groups and the HOPG surface is similar for both samples.Thus, we can ascribe the different assembly structures to the

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different functional groups at the focal point. In the assembly ofmonodendron 2, with the existence of hydrogen bonding, themolecules are tightly connected, forming a close-packedstructure. In monodendron 1, the interactions between func-tional groups are weak, and the steric hindrance between methylgroups at the focal point is large. These two factors worktogether to reduce the number of molecules in one unit.

Using STM, we studied the self-assembled structures ofmonodendrons 1 and 2. For 2, a close-packed structure of diskswas observed, and the interstitial spacing was measured directlyfrom the image. It was also determined that one unit was formedby six monodendrons in the self-assembled structure of 2,whereas for 1, one unit was composed of only two monoden-drons. In a unit of monodendron 2, the carboxyl groups at the

Figure 2. a) STM image of a well ordered assembly of 2, at a large scale. Each circle represents one disk unit. The unit cell is outlined with a� 4.2� 0.1 nm, b� 4.1�0.1 nm, �� 53.5� 1�. Tunneling conditions were: 1.27 nA, 793.3 mV (sample was positive). The scan area was 41.37� 41.37 nm2. The Z axis was 1.0 nm. b) Highresolution STM image of self-assembled 2. The white circle contains one unit. The tunneling conditions were: 1.28 nA, 793.3 mV (sample was positive). The scan area was15.3� 15.3 nm2. The Z axis was 0.5 nm. c) Model of one unit cell containing six monodendrons. The inner diameter is 0.87 nm. The shortest distance between twoneighboring close-packed rings is 0.43 nm, and the longest is 0.68 nm. Light gray spots represent carbon atoms; oxygen atoms are indicated by dark gray spots. d) Aninter-disk region formed by three disk units. The space between them is 1.1 nm, and the diameter of one disk is 3.9 nm. Light gray spots represent carbon atoms; oxygenatoms are indicated by dark gray spots. e) Fourier transform spectrum of Figure 2 b. The periodicity corresponding to the six bright dots is 0.58� 0.02 nm.

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focal points were stabilized by hydrogen bonding. In mono-dendron 1, a different self-assembled structure appeared, andthis can be attributed to the much reduced interactions betweenthe functional groups at the focal points.

Experimental Section

Scheme 2 shows the synthesis of the second-generation ester (1) andacid (2) dendrons. Etherification of the bromide dendron [G-1]�CH2Br[13] with methyl 3,5-dihydroxybenzoate provided the esterdendron 1 in excellent yield. 1 was subsequently quantitativelyhydrolyzed to give the acid dendron 2. The detailed synthesis of thiskind of monodendron will be described elsewhere.

The samples were dissolved in a mixture (2/1 v/v) of dimethylsulf-oxide (HPLC grade, Aldrich) and toluene (HPLC grade, Aldrich), togive a concentration of less than one percent. A droplet of thesolution was deposited onto a freshly cleaved surface of highlyoriented pyrolytic graphite (HOPG; quality ZYB, Digital Instruments)and dried in air prior to imaging.

The experiments were performed on a Nanoscope IIIa SPM (DigitalInstruments, Santa Barbara, CA) under ambient conditions. The STMtips were mechanically formed Pt/Ir wires (90/10).

All the STM images were recorded in the constant current mode ofoperation. The specific tunneling conditions are given in the figurecaptions.

The authors thank the National Natural Science Foundation andthe Foundation of the Chinese Academy of Sciences for financial

support. Support from the National Key Project on Basic Research(grant G2000077501) is also acknowledged.

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Received: March 28, 2002 [Z394]