From Amphiphilic Organic Ligands to Metal-Coordinated Complexes

Mar 3, 2007 - Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, ... for the coordination from organic ligands to resulted co...
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J. Phys. Chem. C 2007, 111, 4667-4672

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From Amphiphilic Organic Ligands to Metal-Coordinated Complexes: Structural Difference in Their Self-Organizations Studied by STM Shan-Shan Li,†,‡ Zhi-Yong Yang,†,‡ Cun-Ji Yan,†,‡ Hui-Juan Yan,† Li-Jun Wan,*,† Pei-Zhi Guo,†,‡ and Ming-Hua Liu*,† Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100080, China, and CAS Graduate School, Beijing, China ReceiVed: October 28, 2006; In Final Form: January 30, 2007

Amphiphilic organic ligands, 5-octadecyloxy-2-(2-pyridylazo)phenol (PARC18) and 1,10-bis[3′-hydroxy-4′(2′′-pyridylazo)phenoloxy]decane (PAR)2C10, are coordinated with Cu(II) ions, resulting in two complexes. Scanning tunneling microscopy (STM) images provide direct structural evidence for the coordination from organic ligands to resulted complexes. The appearance and conformation differences in ligands and corresponding complexes can be clearly seen in STM images recorded from their self-organizations on highly oriented pyrolytic graphite (HOPG). The adlayer structures of PARC18-Cu and (PAR)2C10-Cu complexes are significantly different from those of their corresponding ligands. A phase separation is found in the adlayer formed with the PARC18 ligand and its complex. The results demonstrate that STM is a powerful tool in coordination chemistry in analyzing ligand and coordinated complex. The coordination from ligands to complexes with metal ions would be a facile approach to surface modification and functional two-dimensional (2D) assembly fabrication.

Introduction With the development of modern coordination chemistry, various metal-ligand complexes with elaborate structures have been synthesized.1-4 These complexes usually possess special geometry and size and present interesting electronic, optical, and magnetic properties5-8 that may not be accessible from purely organic systems. The study on these complexes is an important issue in fundamental research and industrial application such as surface modification. Using coordinated complexes, various adlayers were prepared on a solid surface. The structural details of the adlayers were intensively investigated by STM, which is a powerful and useful tool in monitoring the adlayer formation, structure, molecular arrangement, and conformation.9 For example, Kurth et al. prepared and observed straight chains of a supramolecular coordination complex on HOPG using long alkyl chains as a template.10 Semenov et al.11 and Ziener et al.12 investigated the [2 × 2] grid-type Zn(II) and Co(II) complexes on HOPG and found that the orientation of the assembly could be tuned by making subtle changes in the complexes. Zell et al. reported the structure controlled self-assembly of Cu(II) salicylic aldehyde and aldimine derivative complexes.13 With STM, the orderings of the self-assembled monolayers of the complexes was found to be gradually changed by ligand substitution and resulted in a new nanostructure at graphite/ liquid interface. Mao et al. reported the dissociation of a coordination complex precursor-[(dptap-Ag)2](NO3)2.14 They found an ordered silver adlayer due to the surface-induced dissociation of the complex on Au(111) and Au(100) surfaces. Wan and Stang et al. reported the self-organizations of selfassembled supramolecular rectangle, square, and three-dimen* To whom correspondence should be addressed. Tel. and Fax: +8610-62558934. E-mail: [email protected]. † Beijing National Laboratory for Molecular Sciences. ‡ CAS Graduate School.

sional cage on HOPG and Au (111) surfaces.15,16 The orientation of a rectangular adlayer was found to be dependent on the adsorbed surfaces of HOPG and Au(111), demonstrating the importance of intermolecular and molecule/substrate interactions in the formation of supramolecular architecture. As a result of the intensive studies, it is known that when complex molecules adsorb on a solid surface, they would spontaneously self-organize into assembly with defined structures. Now, the self-assembling or the self-organizing has been recognized as one of the powerful and well-established techniques in chemistry, material science, and “bottom-up” strategy in nanotechnology.17-19 On the other hand, to date, the researches have mainly focused on the study of the selforganization with coordinated complexes. However, complex is composed of ligand. Therefore, to obtain a functional adlayer, understanding the adsorptions and adlayer structures of a ligand and its corresponding complex is of interest in coordination chemistry and surface modification. Recently, the adsorption of linear-spacer-bridged ligands, bis(pyrrol-2-yl-methyleneamine)s, BPMB and BPMmB, and their Zn(II) coordinated complexes, BPMB/Zn(II) and BPMmB/Zn(II), was investigated on Au(111) surface.20 Both ligands with different spacer bridges and their Zn(II) complexes adsorbed on Au(111) surface and self-organized into highly ordered 2D arrays. Two complexes, BPMB/Zn and BPMmB/Zn, appeared in helix and triangular conformations, respectively, consistent with their chemical structures. Although the metal-ligand complex is composed of ligand, the assembled structures and adlayer symmetries of the ligand and the complex are totally different. PAR (4-(2-pyridylazo)resorcinol) molecule and its derivatives are widely used as ligands because they can easily form colored complexes with transition-metal ions such as Cu(II), Bi(II), Cd(II), Fe(III), Co(II), Ni(II), and Zn(II).21 PAR molecules also show high color sensitivity to metal ions.22,23 Therefore, the molecules can be used as chromatographic spray reagents for

10.1021/jp0670790 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/03/2007

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Figure 1. Chemical structures and models of ligands of PARC18 and (PAR)2C10 and their corresponding complexes with Cu(II).

detecting metal ions in food and biological samples and for developing ion sensors.24 In the present study, we employ two PAR derivatives, PARC18-an amphiphilic organic ligand and (PAR)2C10-an amphiphilic organic ligand with two PAR groups, to prepare complexes coordinated with Cu(II) ions. The molecular conformation, arrangement, and adlayer structure of the ligands and their corresponding complexes are investigated by STM to reveal if the complex synthesis is successful and what the selforganization difference in ligands and complexes is. STM is employed in coordination chemistry. The chemical structure and models of the ligands and their complexes with Cu(II) are shown in Figure 1. On the basis of the results reported by Kurahashi et al.,25 the complexes from PARC18 and (PAR)2C10 coordinated with Cu(II) should form the structures as shown in Figure 1. PARC18 molecule coordinates with Cu(II) through the phenolic oxygen atom, the azo nitrogen adjacent to the benzene ring, and the pyridyl nitrogen atom to form two five-membered chelate rings. The (PAR)2C10 ligand is the same coordination behavior as that of PARC18. The solutions containing two pairs of ligands and complexes are prepared and deposited on HOPG, respectively. It is found that all ligands and resulting complexes form ordered self-organizations on HOPG surface. PARC18 and (PAR)2C10 appear in linear conformation, consistent with their chemical structures. High-resolution STM images demonstrate a good coordination from ligands to complexes, indicating a successful synthesis. The adlayer structures of PARC18-Cu and (PAR)2C10-Cu are significantly different from those of their ligands. The results demonstrate that STM is a powerful tool in analyzing ligands and coordinated complexes and provides direct structural information for determining the differences between the adlayers with ligands and coordinated complexes. Experimental Section Ligand PARC18 and (PAR)2C10 were synthesized as described in the literature.26,27 They were dissolved in ethanol (Acros Organics, U.S.A.) with a concentration of less than 1 mM. The

Figure 2. Color difference in three samples of (A) PARC18 ligand, (B) PARC18-Cu complex, and (C) mixture of PARC18 and PARC18Cu.

complexes were synthesized by mixing the ligands with Cu(ClO4)2 by a specialized ratio in the same solvent. For PARC18Cu, the ratio is 1:1 (PARC18 and Cu(ClO4)2), whereas for (PAR)2C10-Cu, the ratio is 1:2. A high concentration mixture solution was also prepared by PARC18 with Cu(ClO4)2 (5:1) to see the color sensitivity. The colors of ligand, complex, and high-concentration mixture solution are totally different. Figure 2 is an example of the color change. Three samples of PARC18 ligand, PARC18-Cu complex, and their mixture show different colors of light green, purple, and orange. The various adlayers were prepared by the method reported in the literature.28 Briefly, a drop of solution containing the ligand and the complex was deposited onto a freshly cleaved HOPG surface and dried in air prior to STM imaging. The STM experiment was performed on a Nano III SPM (Digital Instrument Inc.) in ambient condition. STM tips were mechanically formed by cutting Pt/Ir wire (90/10). All of the STM images were recorded using the constant-current mode. The specific tunneling conditions were given in the corresponding figure captions. All images are raw data without further processing, such as Fourier transformation.

Amphiphilic Organic Ligands

Figure 3. (a) Large-scale and (b) high-resolution STM images of PARC18 adsorbed on HOPG. The imaging condition is E ) 519 mV, I ) 648 pA. (c) Proposed structural model for the ordered adlayer of PARC18 on HOPG.

Results and Discussion PARC18 Ligand. The molecular conformation and adsorption of the ligands were investigated on an HOPG surface. Figure 3a is a typical STM image of the PARC18 ligands. From this image, it is clear that a uniform and well-ordered monolayer of PARC18 has been prepared. It can be seen that the molecular rows extend on the HOPG surface. The domain size is more than 100 nm. Bright and dark stripes alternately appear in this image and form a lamellar structure. The structural details of the ligand adlayer are revealed by a high-resolution STM image. Figure 3b is a high-resolution STM image acquired on the PARC18 ligand adlayer. The average distance between the two bright stripes is measured to be d ) 3.6 ( 0.1 nm. The bright stripes are seen to be composed of bright spots with a diameter of 0.3 ( 0.1 nm, whereas the dark stripes are composed of alkyl chains. The contrast difference is from the electronic density difference between aromatic cores and alkyl chains.29,30 The individual alkyl chain is clearly seen in the dark stripes with a parallel arrangement. The length of an alkyl chain is measured to be 2.1 ( 0.1 nm, consistent with the length of the octadecane chain. In the arrangement, we find that the alkyl chains form a “V” type shape in neighboring stripes. Intriguingly, one can see that each alkyl chain is connected with two bright spots, corresponding to a PARC18 molecule from its chemical structure. The two bright spots are attributed to the two aromatic rings in a PARC18 ligand marked in the image by red solid spots. The intermolecular distance and molecular conformation demonstrate a flat-lying orientation of PARC18 on HOPG surface with its

J. Phys. Chem. C, Vol. 111, No. 12, 2007 4669 aromatic cores parallel to the substrate. The important factors in determining the final arrangement of a self-assembly are the interactions of molecule-molecule and molecule-substrate. By the interdigitations of the aromatic core and the alkyl chains, a close-packed self-organization of PARC18 ligands is realized. Meanwhile, H-bondings between aromatic cores of the neighboring PARC18 molecules are proposed in the adlayer.17,19 In this way, a stable monolayer with a sufficiently low molecular lateral mobility is assembled, which is a prerequisite for recording a high-quality STM image. A unit cell for the adlayer is outlined in Figure 3b. By a careful measurement, the lattice parameters are determined from the STM image to be a ) 7.2 ( 0.1 nm, b ) 1.3 ( 0.1 nm, and θ ) 80 ( 2°. On the basis of the STM observation, a structural model for the molecular orientation and packing arrangement in the adlayer is illustrated in Figure 3c. The bright stripes in the STM image are corresponding to aromatic cores, whereas the dark stripes are from alkyl chains. The model is in a good agreement with the results of STM images. An expanded inset in Figure 3c shows the existence of the hydrogen bondings. The hydrogen bondings indicated by black dashed lines can be easily realized in the model from the hydrogen atoms in pyridine rings to the oxygen atoms in phenol rings between the neighboring ligands in a distance of about 0.2-0.3 nm. PARC18-Cu Complex. As a comparison with the PARC18 ligand adlayer, we prepared a PARC18 complex by the addition of Cu(ClO4)2. The resulting PARC18-Cu molecules were prepared as an adlayer on HOPG. Figure 4a is a large-scale STM image recorded on the complex adlayer. Similar to that of the PARC18 adlayer, a well-ordered organization is observed, consisting of alternative bright and dark stripes with a lamellar structure. In the large-scale STM image, two domains can be seen. The domain boundaries cross each other in an angle of 60 ( 2°. Although the lamellar feature is consistent with that in the PARC18 ligand adlayer, a careful observation find that the interstripe distance in the present adlayer is different from that in the PARC18 adlayer increased from d ) 3.6 nm to d′ ) 4.6 nm as indicated in Figure 4b. Figure 4b is a high-resolution STM image from the complex adlayer. The molecular conformation and packing arrangement are clearly seen. An obvious difference in the image exists in the bright stripes. The width of the bright stripes is larger than that in Figure 3b, although the stripes are still composed of bright spots. The coordinated complex should be responsible for the difference. On the other hand, in Figure 4b, we see that each bright stripe forms bright lines. There are four bright spots in a line. Between the bright stripes, dark stripes originating from alkyl chains are seen. As a tentative consideration, we illustrate the molecular model of the PARC18-Cu complexes on the STM image in Figure 4b. The molecular size is drawn in the scale to the underlying STM image. It is found that the conformation and the size of the complex are in a good agreement with the STM image. It can be seen that the aromatic cores in the complex molecules take a parallel arrangement instead of the interdigitated one in Figure 3b. Four bright spots marked in red solid spots from the aromatic “heads” of two PARC18-Cu molecules form a bright line. In dark stripes, the alkyl chains interdigitate each other as the same as that in Figure 3b. Therefore, the distance difference in the ligand and the complex should be attributed to the coordination with Cu(II) ions in the head part of the complex. Although we could not see the exact positions of the ions in the STM image, from the observed results, we can deduce that the ligands are now coordinated by Cu(II) ions and form PARC18-Cu complexes.

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Figure 4. (a) Large-scale and (b) high-resolution STM images of PARC18-Cu adsorbed on HOPG. The imaging condition is E ) 558 mV, I ) 364 pA. (c) Proposed structural model for the ordered adlayer of PARC18-Cu on HOPG.

Figure 5. (a) Large-scale and (b) high-resolution STM images of (PAR)2C10 adsorbed on HOPG. The imaging condition is E ) 745 mV, I ) 693 pA. (c) Proposed structural model for the ordered adlayer of (PAR)2C10 on HOPG.

A unit cell for the adlayer is outlined in Figure 4b. The lattice parameters are determined from the STM image to be a ) 4.6 ( 0.1 nm, b ) 1.2 ( 0.1 nm, and θ ) 86 ( 2°. Figure 4c is a proposed structural model for the complex adlayer. In this model, the alkyl chains form a parallel arrangement within the neighboring dark stripes instead of a “V” shape in Figure 3b. The PAR headgroups and H2O molecules will coordinate with Cu(II) ions, and a (ClO4)- ion is used to achieve a electrostatic equilibrium.31 The structures result from the equilibrium of the forces, including interaction between alkyl chains and graphite and repulsive interaction in headgroups. The equal (ClO4)- ions with (PAR)2C10-Cu+ ions stabilize the headgroups against the electrostatic repulsion. In addition, the electrostatic attraction between the positive headgroups and the mobile electrons in the conducting graphite surface may also help to stabilize the headgroups.32 It can be seen from the described results that the difference exists in ligand PARC18 and complex PARC18-Cu when they form adlayers. Both molecules form stripe structures with the same arrangement in the parts of alkyl chains. However, owing to the chemical structure, the interaction in the parts of aromatic cores is different, resulting in a different adlayer structure with the increased distance between molecular stripes. The coordination of ligands with ions can be demonstrated from the observed STM results. (PAR)2C10 Ligand. A featural difference in chemical structures between PARC18 and (PAR)2C10 is two PAR groups in

the (PAR)2C10 molecule. The effect of the special structure on the molecular adsorption was investigated by STM. After depositing the solution containing (PAR)2C10 molecules, an adlayer was expected on the HOPG surface. Figure 5a is a largescale STM image recorded on the adlayer of (PAR)2C10 ligands. In an area of 100 nm × 100 nm, (PAR)2C10 ligands self-organize into an assembly. Several domains can be seen in the image with regular molecular rows. The details of the (PAR)2C10 ligand adlayer are revealed by a high-resolution STM image in Figure 5b. It is clear that the bright stripes consist of bright spots. Within a stripe in direction A, there are four bright spots, although they are not in a straight line. In the high-resolution STM image, among the four spots we find that two spots form a pair. A visible gap exists between the neighboring pairs. The bright stripes extend in direction B with a repeated distance of b ) 0.7 ( 0.1 nm. A careful observation can find the existence of alkyl chains between the bright spots. From the chemical structure and the feature of two PAR groups, a pair of bright spots corresponds to an aromatic “head” of a (PAR)2C10 molecule. The two “heads” are connected by an alkyl chain. The structural model of a (PAR)2C10 molecule is illustrated in Figure 5b. The length of the alkyl chain is measured to be 1.1 ( 0.1 nm, consistent with the size in its chemical structure. In the adlayer, all (PAR)2C10 molecules are in a “head-to-head” configuration with a flat-lying orientation. A unit cell for the adlayer is outlined in Figure 5b. The lattice parameters are determined from the STM image to be a ) 3.1

Amphiphilic Organic Ligands

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Figure 7. STM image of a mixture of PARC18 and PARC18-Cu adsorbed on HOPG. The imaging condition is E ) 707 mV, I ) 462 pA. (Insets) High-resolution STM images from A and B domains.

Figure 6. (a) Large-scale and (b) high-resolution STM images of (PAR)2C10-Cu adsorbed on HOPG. The imaging condition is E ) 630 mV, I ) 466 pA. (c) Proposed structural model for the ordered adlayer of (PAR)2C10-Cu on HOPG.

( 0.1 nm, b ) 0.7 ( 0.1 nm, and θ ) 85 ( 2°. Figure 5c is a proposed model for the molecular adlayer. The results in the STM observation demonstrate the effect of chemical structure on the adlayer structure. Although the substrate is the same, the feature of two PAR groups mainly dominates the adsorption of (PAR)2C10. The (PAR)2C10 molecule has two PAR headgroups. Owing to the electrostatic repulsion, it is difficult to form an interdigitated arrangement between the neighboring molecules. Furthermore, the possible hydrogen bondings between PAR headgroups in the neighboring molecules should be very weak. On the other hand, the structure of two PAR groups with more π electrons would have a strong interaction with the substrate, resulting in a stable adlayer. (PAR)2C10-Cu complex. As a similar procedure to PARC18Cu, we synthesized (PAR)2C10-Cu complex with Cu(ClO4)2. A complex adlayer was prepared on HOPG. The STM experiment was conducted on the adlayer to observe the molecular structure. Figure 6a is a typical large-scale STM image of the complexes adlayer on HOPG. It can be seen from this image that the complexes adsorb on the substrate surface and form an ordered adlayer. The clear difference in the image is the disappearance of the stripe feature in the (PAR)2C10 ligand, the PARC18 ligand, and its Cu(II) complex adlayers. Figure 6b is a high-resolution STM image from the (PAR)2C10-Cu adlayer. Although the adlayer is composed of bright spots, the spots are distorted and elongated like an “S” shape. The length of the spots along its distorted axis is measured to be 2.4 ( 0.1 nm, shorter than the chemical structure

of (PAR)2C10-Cu complex. The alkyl chains in the complex cannot be corresponded in the image. In Figure 5b, (PAR)2C10 clearly shows its conformation. Both the alkyl chains and the PAR headgroups are well recognized. From the chemical structure of (PAR)2C10-Cu, the PAR headgroups and H2O molecules will coordinate with Cu(II) ions, and (ClO4)- ions are used to achieve a electrostatic equilibrium.31 After the coordination, the molecular conformation will change from a near planar one to a nonplanar one because of the change of alkyl chains from an all-trans zigzag packing to a gauche conformation.27 Therefore, the appearance of the complex will be different from those in a near planar conformation. The electronic density is high in the headgroups. The nonplanar conformation of the complexes should be responsible for the bright “S” shaped spots. From the observation, it is reasonable to conclude that the molecules adsorb on HOPG with a nonplanar conformation, in which the alkyl chain takes a gauche conformation. On the basis of the observed result and analysis, we propose that one bright “S” shaped spot corresponds to a (PAR)2C10-Cu molecule. The molecular model is illustrated in the image of Figure 6b. From Figure 6b, it is seen that regular rows extending along direction A and B. The two directions cross each other at an angle of 90 ( 2 °. The intermolecular distances in direction A and B are a ) 2.0 ( 0.1 nm and b ) 2.0 ( 0.1 nm, respectively. A unit cell is determined as outlined in Figure 6b. A structural model for the adlayer is proposed in Figure 6c. It can be seen that the molecules align along direction A and B. Owing to a nonplanar conformation, the alkyl chains would be bent on the HOPG surface, resulting in a shorter size in the STM image compared with its chemical structure. Mixed Adlayers of PARC18 and PARC18-Cu. To understand the interaction between ligands and their complexes in a molecular adlayer, we investigated the co-adsorption of the two molecules. After mixing the two molecules together, we deposited the solution on HOPG as the same procedure used in this experiment. Figure 7 is a large-scale STM image of the mixed adlayer with PARC18 and PARC18-Cu on HOPG. There are several domains in an area of 80 nm × 80 nm, indicated by A and B. The stripe feature can be seen in this image. The width of the stripe structure is different in the domains. However, the width of the stripe is the same in one domain. The origin of the stripe structures with different widths is revealed by highresolution STM images shown in the insets of Figure 7. In domain A, the structure is from the PARC18 ligands with a narrow stripe space, whereas from the PARC18-Cu complexes in domain B with a large stripe space. The results indicate that the molecules spontaneously separate onto HOPG surface and form pure ligand or complex domain. In their pure domains, the molecular arrangement is the same as observed in Figure 3

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TABLE 1: Unit Cell Parameters of the Adlayers with Ligands and Corresponding Complexes ligand PARC18

complex PARC18-Cu

ligand (PAR)2C10

complex (PAR)2C10-Cu

7.2 1.3 80

4.6 1.2 86

3.1 0.7 85

2.0 2.0 90

a (nm) b (nm) θ (deg)

and 4, respectively. Owing to the different interactions between molecule and molecule, molecule and substrate, there are three adsorption models as to a binary system adsorbed on a solid surface:9,15 (1) Phase separation, if the interaction with the substrate is of little difference for two compounds. (2) Preferential adsorption, if the interaction with the substrate is quite different for two compounds. (3) Ordered assembly with two compounds, if there exists a strong interaction with each other. In the present study, both molecules adsorb on HOPG. There is no strong interaction between the two molecules. As a result, the phase separation is found to form separated domains. On the basis of STM observation, the adlayer structures of ligands and complexes are well decided. Table 1 is a summary of the lattice parameters for the four adlayers. From the experimental results, it can be seen that ligands and their corresponding complexes can adsorb on an HOPG surface and form long-range ordered adlayers. The adsorption is due to the intermolecular reaction and the molecule/substrate reaction, which result in a stable 2D organization. The molecules studied in the present experiment consist of alkyl chains and PAR groups. According to the results in the literature,9,17,30 the molecules with long alkyl chains could easily form an ordered adlayer on HOPG. Therefore, the interactions of alkyl chains as well as PAR groups play an important role in the adlayer formation. However, owing to the difference in chemical structure of the ligand and the complex such as the molecular size and the conformation, the adlayer structure formed by the ligand and the corresponding complex should be different. Although the stripe feature is similar in PARC18 ligand and its Cu(II)-coordinated complex adlayer, the lattice parameters in the two adlayers are dependent on molecular chemical structures. In the present systems, the lattice parameters in ligands and their complex adlayers are mainly dominated by molecular dimensions. The quantitative estimation in structure and in-situ coordination from ligands to complexes would be helpful to predict new structures. Conclusion In summary, we have successfully prepared the well-ordered self-organized adlayers with ligands of PARC18 and (PAR)2C10 and their Cu(II)-coordinated complexes on an HOPG surface. High-resolution STM images reveal the adlayer structure and the molecular arrangement. The chemical structures of these ligands and complexes are responsible for the formation of the structurally defined adlayers. The appearance and conformation differences in ligands and their corresponding complexes can be clearly seen in STM images of their self-organizations. The adlayer structures of PARC18-Cu and (PAR)2C10-Cu complexes are significantly different from those of the respective

ligands. Phase separation is found between the ligand and its corresponding complex. The experiment demonstrates that STM is a powerful tool in coordination chemistry in investigating a ligand and its coordinated complex. The results are significant in fabricating functional surfaces for applications in the fields such as electronics, sensors, and catalysis. The present study on the relationship between ligand and corresponding complex adlayers is the first step for surface modification. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20575070, 20673121, and 20121301), National Key Project on Basic Research (Grant No. 2006CB806100), and the Chinese Academy of Sciences. References and Notes (1) Piguet, C.; Bernardinelli, G.; Hopfgartner, G. Chem. ReV. 1997, 97, 2005. (2) Swiegers, G. F.; Malefetse, T. J. Chem. ReV. 2000, 100, 3483. (3) Seidel, S. R.; Stang, P. J. Acc. Chem. Res. 2002, 35, 972. (4) Mu¨ller, I. M.; Mo¨ller, D.; Schalley, C. A. Angew. Chem., Int. Ed. 2005, 44, 480. (5) Schwab, P. F. H.; Levin, M. D.; Michl, J. Chem. ReV. 1999, 99, 1863. (6) Dinolfo, P. H.; Hupp, J. T. Chem. Mater. 2001, 13, 3113. (7) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 2022. (8) Cotton, F. A.; Lin, C.; Murillo, C. A. Acc. Chem. Res. 2001, 34, 759. (9) Wan, L. J. Acc. Chem. Res. 2006, 39, 334. (10) Kurth, D. G.; Severin, N.; Rabe, J. P. Angew. Chem., Int. Ed. 2002, 41, 3681. (11) Semenov, A.; Spatz, J. P.; Mo¨ller, M.; Lehn, J.-M.; Sell, B.; Schubert, D.; Weidl, C. H.; Schubert, U. S. Angew. Chem., Int. Ed. 1999, 38, 2547. (12) Ziener, U.; Lehn, J. M.; Mourran, A.; Mo¨ller, M. Chem.-Eur. J. 2002, 8, 951. (13) Zell, P.; Mo¨gele, F.; Ziener, U.; Rieger, B. Chem. Commun. 2005, 1294. (14) Xu, X. M.; Zhong, H. P.; Zhang, H. M.; Mo, Y. R.; Xie, Z. X.; Mao, B. W. Chem. Phys. Lett. 2004, 386, 254. (15) Gong, J. R.; Wan, L. J.; Yuan, Q. H.; Bai, C. L.; Jude, H.; Stang, P. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 971. (16) Yuan, Q. H.; Wan, L. J.; Jude, H.; Stang, P. J. J. Am. Chem. Soc. 2005, 127, 16279. (17) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290. (18) Pan, G. B.; Liu, J. M.; Zhang, H. M.; Wan, L. J.; Zheng, Q. Y.; Bai, C. L. Angew. Chem., Int. Ed. 2003, 42, 2747. (19) Lehn, J. M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH: Weinheim, 1995. (20) Yuan, Q. H.; Wan, L. J. Chem.-Eur. J. 2006, 12, 2808. (21) Anderson, R. G.; Nickless, G. Analyst (London) 1967, 92, 207. (22) Kallistratos, G.; Pfau, A.; Ossowski, B. Anal. Chim. Acta, 1960, 22, 195. (23) Iwamoto, T. Bull. Chem. Soc. Jpn. 1961, 34, 605. (24) Fredrikson, M.; Carlsson, N. G.; Almgren, A.; Sandberg, A. S. J. Agric. Food Chem. 2002, 50, 59. (25) Kurahashi, M. Bull. Chem. Soc. Jpn. 1976, 49, 2927. (26) Liu, M. H.; Ushida, K.; Nakahara, H.; Kira, A. AdV. Mater. 1997, 9, 1099. (27) Guo, P. Z.; Liu, M. H.; Nakahara, H.; Ushida, K. ChemPhysChem 2006, 7, 385. (28) Gong, J. R.; Wan, L. J. J. Phys. Chem. B 2005, 109, 18733. (29) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (30) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139. (31) Ooi, S.; Carter, D.; Fernando, Q. Chem. Commun. 1967, 1301. (32) Sakai, H.; Nakamura, H.; Kozawa, K.; Abe, M. Langmuir 2001, 17, 1817.