Structural Diversity of a Monodendron Molecule Self-Assembly in

Aug 18, 2009 - Structural Diversity of a Monodendron Molecule Self-Assembly in ... In contrast, a stable BIC linear adlayer is formed with the coadsor...
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J. Phys. Chem. C 2009, 113, 16193–16198

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Structural Diversity of a Monodendron Molecule Self-Assembly in Different Solvents Investigated by Scanning Tunneling Microscopy: From Dispersant to Counterpart Xu Zhang,† Qing Chen,† Guo-Jun Deng, Qing-Hua Fan, and Li-Jun Wan* Institute of Chemistry, Chinese Academy of Sciences (CAS), and Beijing National Laboratory for Molecular Sciences, Beijing 100190, People’s Republic of China ReceiVed: June 22, 2009; ReVised Manuscript ReceiVed: July 29, 2009

A monodendron molecule, 5-(benzyloxy)isophthalic acid derivative (BIC, C47H76O7), has been employed to fabricate self-assembly on graphite surface. The molecules were dissolved in different solvents of 1-phenyloctane, 1,2,4-trichlorobenzene (TCB), and 1-octanoic acid. The effects of these solvents on BIC self-assemblies were investigated by scanning tunneling microscopy (STM) at the liquid/graphite interface. It is found that 1-phenyloctane exerts as a dispersant without coadsorption in the self-assembly, although a structural transformation of BIC adlayer from lamella to hexamer can be seen with concentration decrease. TCB solvent coadsorbs with BIC molecules and affects the structure and stability of the BIC adlayer. In contrast, a stable BIC linear adlayer is formed with the coadsorption of 1-octanoic acid. The intermolecular hydrogen bonds between solvent molecule 1-octanoic acid and the BIC molecule are responsible for the formation of the linear structure. Furthermore, the effect of mixed solvents on BIC adlayer was also investigated. The results demonstrate that the solvent plays the important role of not only a simple dispersant but also a counterpart in forming the two-dimensional (2D) monodendron molecular self-assembly and provides an efficient approach to fabricating and controlling monodendron molecular nanostructure by changing solvent and molecular concentration. Introduction The formation of a molecular self-assembly or adlayer on a 2D solid surface is mainly dominated by molecular chemical structure and substrate where the molecule is staying,1-4 although external stimulation can also have an influence on the self-assembly structure.5-8 To achieve a desirable surface pattern, modifications of molecular chemical structure such as alkyl chain,9-12 functional group,13-15 and different substrates16,17 are well used. On the other hand, recent results show that solvent plays an important role in the self-assembly formation when a solution route was used to fabricate 2D self-assembly. It is known that the main function of solvent is as a dispersant, in which the solute molecules are dissolved and dispersed. At the liquid/solid interface, however, the solvent-solute interactions may become significant enough to regulate the self-assembly structure.18-28 For example, Flynn et al. investigated the solvent effect on the adsorption and mobility of triacontane and triacontanol molecules on highly oriented pyroltic graphite (HOPG) surface by STM. By choosing an alcoholic solvent phenylpentanol or an apolar solvent phenyloctane, the preferential adsorption of triacontance or triacontanol can be regulated.18,29 De Feyter et al. reported the effect of solvents on 2D self-assembly of a monodendron at the liquid/HOPG interface. The solvent-induced polymorphism can be attributed to the solubilizing nature of the solvent (solvent-solute interactions), specific solvent-solvent interactions, and the solvent dimension (length of alkyl chain).20 Our group reported the solvent effect on the assembly chirality of an achiral bent-core molecule, providing an important approach for controlling 2D * To whom correspondence should be addressed. Fax: (+86) 10-62558934. E-mail: [email protected]. † Also at the Graduate University of CAS, Beijing 100049, People’s Republic of China.

chiral structures.21 All of these results have demonstrated the influence of solvent on self-assembly. Therefore, it is necessary to further clarify the role of solvents in different 2D selfassembly systems. Herein, we report a detailed investigation on the role of solvents in the formation of the monodendron molecular adlayer on the HOPG surface. A monodendron molecule, 5-(benzyloxy)isophthalic acid derivative (BIC, C47H76O7),30,31 is employed to prepare assembly on HOPG surfaces. Monodendrons are welldefined and highly branched molecules with varying reactive groups and are of great interest as new materials.30,32-34 Three solvents, 1-phenyloctane, 1,2,4-trichiorobenzene (TCB), and 1-octanoic acid, are used to fabricate the self-assembly at the liquid/solid interface. The chemical structures of BIC and three solvents are shown in Table 1. As a result, it is found that solvent exerts its function in two ways: (1) as a dispersant without coadsorption with solute molecules and (2) as a counterpart participating in the assembly formation through solvent-solute hydrogen bonds. The solvent with bifunction can extend its role from a simple dispersant to a regulator for manufacturing molecular nanostructure, providing an efficient approach to fabricate and control new nanostructures of monodendron molecules. Experimental Section BIC used in this study was synthesized as described in a previous report.30 All the solvents were purchased from Aldrich with HPLC grade and used without further purification, including 1-phenyloctane, TCB, and 1-octanoic acid. The samples were prepared by depositing a droplet (∼2 µL) of solution containing monodendron BIC on a freshly cleaved atomically flat surface of HOPG (quality ZYB, Digital Instruments, Santa Barbara, CA).

10.1021/jp9058356 CCC: $40.75  2009 American Chemical Society Published on Web 08/18/2009

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TABLE 1: Chemical Structures of Monodendron BIC and Solvents Employed in This Research

STM measurements were performed on a Nanoscope IIIa SPM (Digital Instuments, Santa Barbara, CA) at the liquid/solid interface at room temperature. The tips were mechanically cut Pt/Ir wires (90/10). All the images were recorded with the constant current mode and shown without further processing. The specific tunneling conditions of each figure are given in the corresponding figure caption. HyperChem 7.0 (Hypercube Inc.) was employed to simulate hydrogen bonds and to build the 2D packing models of BIC in various solvents on HOPG surfaces. Results and Discussion A lamellar structure of BIC on the HOPG surface in air has been reported previously when using toluene as solvent.30 High resolution STM image showed a stable molecular adlayer. All molecules densely packed in a head-to-head configuration and all the alkyl chains packed tail-to-tail. However, BIC adlayers formed at the 1-phenyloctane, TCB, 1-octanoic acid/solid interface are different from that in the previous report. We have carefully investigated the effect of these solvents and mixed solvents on BIC adlayer. The roles of these solvents in the formation of the monodendron BIC adlayers are revealed. 1. Adlayer Structure in 1-Phenyloctane. 1.1. Lamellar Structure. In the large scale STM image shown in Figure 1a, it can be seen that the HOPG surface is covered with molecular adlayer from a 1-phenyloctane solution containing 5 × 10-4 M BIC molecule. The adlayer appears in a lamellar feature. Bright stripes and dark troughs can be clearly observed. Owing to the higher electronic density of aromatic cores compared with alkyl chains,30 the bright stripes are attributed to BIC aromatic cores, while the dark troughs are the BIC alkyl chains. More structural details are revealed by a high resolution STM image shown in Figure 1b. Individual BIC molecules can be clearly resolved in the image. Through a careful observation, no 1-phenyloctane molecules can be identified in the image. Each lamella is composed of two molecular rows. The molecular pairs in a lamella take a head-to-head configuration as illustrated in Figure 1b. The angle β between the lamellar axis and the direction of the molecular pair is measured to be 52((2)°. The width of the troughs extracted from the STM image is 1.9((0.2) nm, in accordance with the length of a hexadecane chain. Instead of a tail-to-tail arrangement, the alkyl chains are found to be interdigitally packed. The number of hexadecane chains observed in the image equals the number of BIC molecules, suggesting that one hexadecane chain in a BIC molecule may be oriented toward the solution phase.10,35 On the basis of the

STM observation, a structural model for the lamellar structure is proposed in Figure 1c. Hydrogen bonds are expected to exist in the neighboring carboxyl groups, as indicated by black dashed lines (optimized by HyperChem) in Figure 1c. A unit cell for the molecular adlayer is outlined in panels b and c of Figure 1 with the parameters of a ) 4.6((0.2) nm, b ) 1.1((0.2) nm, and R ) 86((2)°. The angle β in the model is also consistent with the result measured from the STM images. 1.2. Hexamer Structure. Diluting the sample to a concentration below 1 × 10-4 M BIC, a novel hexamer structure is observed in 1-phenyloctane on HOPG, as shown in Figure 2a. The flowerlike hexamers extend into the 2D surface with a hexagonal symmetry in the image. Such a structure can be imaged at the liquid/solid interface for several hours, but it can be easily disturbed by bias change. A high resolution STM image in Figure 2b reveals the details of the hexamer structure. Six BIC molecules can be clearly resolved in each hexamer, with all heads oriented toward the center of the hexamer. All

Figure 1. (a) Large scale STM image of BIC showing the lamellar structure on HOPG surface in 1-phenyloctane. Tunneling conditions: Vbias ) 579 mV, Iset ) 599 pA. (b) High resolution STM image of the lamellar structure in 1-phenyloctane. Tunneling conditions: Vbias ) 650 mV, Iset ) 500 pA. (c) Molecular model for the lamellar structure. The possible out-of-plane alkyl chains are not shown in the model. Hydrogen bonds are outlined by black dashed lines. A unit cell is indicated with the parameters of a ) 4.6((0.2) nm, b ) 1.1((0.2) nm, and R ) 86((2)°.

Diversity of a Monodendron Molecule Self-Assembly

Figure 2. (a) A large scale STM image of the hexamer structure in 1-phenyloctane. Tunneling conditions: Vbias ) -750 mV, Iset ) 400 pA. (b) High resolution STM image acquired in the tunneling conditions of Vbias ) -601 mV and Iset ) 400 pA. The inset in the upper right corner shows the details of the hexamer. (c) Structural model for the hexamer adlayer. Hydrogen bonds are outlined by black dashed lines in the enlarged inset.

hexadecane chains of BIC are found to be adsorbed on the HOPG surface. The chain length measured from the STM image is 1.9((0.2) nm, in good accordance with the length calculated by HyperChem. No solvent molecules are found in the STM image. The absence of 1-phenyloctane molecules in the image indicates that 1-phenyloctane acts as a dispersant in the assembly. Figure 2c is the structural model for the hexamer structure on the basis of STM results and HyperChem simulation. Hydrogen bonds are expected to exist in the neighboring carboxylic acid groups of BIC molecules and induce the formation of BIC hexamers and are indicated by black dashed lines in the enlarged inset in Figure 2c. The resulting hydrogen bonds pattern is similar to that in trimesic acid and isophthalic acid derivatives assembly on HOPG.22,36-39 A unit cell is given in Figure 2a, with the parameters of a ) 5.7((0.2) nm, b ) 5.7((0.2) nm, and R ) 60((2)°. The model is in good agreement with the observed results. The results show that the molecules can form different structures in the same solvent with the change of BIC molecular concentration, although the solvent would not coadsorb with BIC molecules. Further changing the molecular concentration does not result in any new structures other than lamellar and hexamer structures. 2. Adlayer Structure in 1,2,4-Trichlorobenzene (TCB). A lamellar structure (Figure S1, Supporting Information) similar to that in Figure 1 also can be observed on HOPG in TCB solvent containing BIC at concentrations higher than 5 × 10-4 M. Such a structure is ascribed to the dense packing of BIC molecules by hydrogen bonds existing in neighboring carboxyl acid groups and the interdigitated arrangement of alkyl chains. In this case, TCB molecules cannot take position in the adlayer and form coadsorption with BIC. With the decreasing of BIC concentration, a structure transformation from lamellar to hexamer is observed in TCB solution, as shown in Figure 3a. Although the overall shape of the structure is similar to that in Figure 2 obtained in 1-phenyloctane solvent, bright spots can be seen in the center of a hexamer. A careful observation and theoretical simulation result

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Figure 3. (a) A large scale STM image showing a hexamer structure of BIC in TCB. Tunneling conditions: Vbias ) -553 mV, Iset ) 666 pA. (b) High resolution STM image of the hexamer structure in TCB. Tunneling conditions: Vbias ) -682 mV, Iset ) 666 pA. (c) Structural model for the hexamer structure. Hydrogen bonds are indicated by black dashed lines in the enlarged inset.

indicate that the bright spots correspond to TCB molecules. Owing to the coadsorption, TCB molecules from solvent now occupy the central space of a hexamer and participate in the assembly formation. We find that this hexamer structure cannot be easily disturbed by STM tips and bias change, indicating that the hexamer is more stable than that in 1-phenyloctane due to the participation of TCB molecules. On the basis of the image feature, three kinds of hexamers with different orientations and shapes can be resolved, as illustrated by different colors in Figure 3a. A unit cell is also decided and shown in Figure 3a. Figure 3b is a high resolution STM image revealing details about the hexamer structure of BIC on HOPG. Six BIC molecules with their alkyl chains can be clearly resolved in each hexamer. The chain length is measured to be 1.9((0.2) nm, in agreement with that calculated by HyperChem. The alkyl chains are interdigitated with each other as shown in the model superimposed in the STM image of Figure 3b. The TCB molecule can be pinned down in the center of molecular rings on the HOPG surface due to its shape and size.26 The bright spots in the image of Figure 3b are assigned to TCB molecules trapped in the center of the hexamer. These trapped TCB molecules interact with BIC and stabilize hexamers on the HOPG surface. Intriguingly, not all centers of hexamers appear with the same contrasts as shown by black and white arrows in Figure 3b, implying that TCB molecules could escape from hexamer centers. Molecular simulation reveals that at most three TCB molecules can be pinned down in the center of the cyclic hexamer center. The bright spots are assigned to three TCB trapped in the center of the hexamer, while the dark spots indicated by white arrow may be due to less or no TCB molecule trapped. On the basis of the STM image, a structural model for the hexamer structure in TCB is proposed in Figure 3c. The three types of hexamer are outlined in Figure 3c by their geometric shapes and corresponding colors. Similar to the case in 1-phenyloctane, hydrogen bonds are expected to exist in the neighboring BIC molecules and are responsible for the formation

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Figure 5. (a) A large scale STM image of BIC linear assembly obtained at 5 × 10-4 M by using a mixture solution of 1-phenyloctane and 1-octanoic acid. Tunneling conditions: Vbias ) 889 mV, Iset ) 306 pA. (b) High resolution STM image obtained by using a mixed solvent. Tunneling conditions: Vbias ) 780 mV, Iset ) 338 pA.

Figure 4. (a) Large scale STM image of BIC adlayer in 1-octanoic acid. Tunneling conditions: Vbias ) 685 mV, Iset ) 363 pA. (b) High resolution STM image of the BIC adlayer. Tunneling conditions: Vbias ) 1.05 V, Iset ) 241 pA. (c) Structural model for the BIC adlayer. Possible hydrogen bonds are shown in the enlarged inset by black dashed lines. The arrows indicate 1-octanoic acid.

of hexamers in TCB. The details of intermolecular hydrogen bonds have been simulated as illustrated by black dashed lines in the enlarged inset in Figure 3c. The parameters of the unit cell in Figure 3a are measured to be a ) 9.9((0.2) nm, b ) 9.9((0.2) nm and R ) 60((2)°. 3. Adlayer Structure in 1-Octanoic Acid. After the STM examination of BIC self-assembled adlayers in 1-phenyloctane and TCB, the effect of solvent 1-octanoic acid on the BIC adlayer has been investigated. Figure 4a shows a large scale STM image obtained in a 5 × 10-4 M 1-octanoic acid solution on the HOPG surface. The structure is totally different from the images acquired in 1-phenyloctane and TCB solution. Such a structure can be consistently observed at various concentrations. The high resolution STM image in Figure 4b provides detailed information for such a structure. The adlayer is composed of bright elongated spots and alkyl chains. The bright elongated spots are attributed to BIC aromatic cores on the basis of the size as well as morphology of the molecule. From the molecular arrangement and intermolecular distance, it can be determined that every two molecules form a BIC dimer. According to the adlayer symmetry and periodic distribution of molecules, a unit cell is proposed and outlined in Figure 4b. A careful observation reveals that two alkyl chains in a BIC molecule are almost vertical to the aromatic core of the molecule in the self-assembled adlayer. The alkyl chains are interdigitally packed along direction b. Note that short chains could also be seen in the image in Figure 4b. The details are magnified in the inset of Figure 4b as indicated by white rounded rectangles. On the basis of the length and the shape, the short chains are attributed to 1-octanoic acid molecules coadsorbed with the BIC molecules. On the basis of the observation results and the above analysis, a structural model can be proposed for the BIC structure in 1-octanoic acid, as shown in Figure 4c. The lattice constants in the unit cell are determined to be a ) 6.7((0.2) nm, b ) 3.2((0.2) nm, and R ) 62((2)°. Hydrogen bonds in the structure are calculated and optimized, as illustrated in the enlarged inset in Figure 4c. A pair of hydrogen bonds are formed between the carboxyl groups of BIC molecules, while two pairs of hydrogen bonds are formed between the carboxyl groups of

BIC and 1-octanoic acid molecules. The solvent molecules play a significant counterpart role in the assembly, leading to a stable and featured hydrogen bonded dimeric assembly. 4. Adlayer Structure in a Mixed Solvent of 1-Phenyloctane and 1-Octanoic Acid. The 2D self-assembled adlayer of BIC has been examined by using a mixture solvent of 1-phenyloctane and 1-octanoic acid containing BIC molecules. A solution of 1-phenyloctane and 1-octanoic acid with a volume ratio of 1:1 was mixed and then utilized to dissolve BIC molecules. The self-assembled adlayers were fabricated by using the so-prepared solution and investigated at the Liquid-HOPG interface by STM, as shown in Figure 5a. A well-ordered linear structure can be seen in the image, similar to the BIC assembly in 1-octanoic acid in Figure 4. The structure can be observed in the mixed solvent at various concentrations. No other structures could be seen by STM. In the high resolution STM image in Figure 5b, 1-octanoic acid could be observed, as outlined by the white rounded rectangle. Obviously, 1-octanoic acid takes part in and regulates the assembly of BIC molecules in the mixed solution on HOPG. In a parallel experiment, we first prepared a BIC adlayer in 1-phenyloctane on the HOPG surface. After the adlayer the same as that in Figure 1 was imaged, a drop of 1-octanoic acid was directly added into the 1-phenyloctane solvent. The volume ratio of 1-phenyloctane and 1-octanoic acid was controlled to be 1:1. Figure 6a is an STM image obtained in the 1-phenyloctane solution containing BIC molecules in a high concentration on HOPG. The lamellar structure is clearly seen. Such a lamellar structure will transfer to a linear structure quickly after 1-octanoic acid was added. Figure 6b is an STM image showing the resulting linear structure, which is the same as the BIC assembly in 1-octanoic acid solvent. The linear structure is stable in the mixture solution for several hours. A similar structural transformation was also found at the low BIC concentration of 5 × 10-5 M. The BIC adlayer with hexamers was first fabricated on HOPG in 1-phenyloctane, as shown in Figure 6c. The structure is consistent with that in Figure 2. However, after 1-octanoic acid was added on HOPG, BIC hexamers could no longer be found. Figure 6d shows the resulting linear structure similar to the adlayer formed in 1-octanoic acid. It is clear that 1-octanoic acid exerts influence on the BIC assembly by breaking the original lamellar and hexamer structure, interacting with BIC molecules to form hydrogen bonds, and resulting in the linear adlayer structure. The self-assembled structures of BIC in different solvents are summarized in Table 2. The observed results have demonstrated that the self-assembly of monodendron BIC is influenced by solvents. The solvent plays the role as dispersant and

Diversity of a Monodendron Molecule Self-Assembly

J. Phys. Chem. C, Vol. 113, No. 36, 2009 16197 formation between the carboxyl group of BIC and 1-octanoic acid. It can be concluded that 1-octanoic acid exerts its influence by interacting with BIC molecules to form hydrogen bonds. The STM results in a mixed solution of 1-phenyloctane and 1-octanoic acid support the conclusion well. Conclusions

Figure 6. (a) An STM image of the BIC lamellar structure in 1-phenyloctane at high concentration. Tunneling conditions: Vbias ) 589 mV, Iset ) 475 pA. (b) An STM image showing the linear dimer structure acquired after a drop of 1-octanoic acid was directly added into the original sample used in Figure 6a. Tunneling conditions: Vbias ) 589 mV, Iset ) 475 pA. (c) A large scale STM image of the BIC hexamer structure in 1-phenyloctane at low concentration. Tunneling conditions: Vbias ) -560 mV, Iset ) 648 pA. (d) An STM image showing the linear structure acquired after a drop of 1-octanoic acid was added in the original sample used in panel c. Tunneling conditions: Vbias ) 551 mV, Iset ) 486 pA.

TABLE 2: Summarized Result of BIC Self-Assembled Structures in Different Solvents BIC concentration high 1-phenyloctane TCB 1-octanoic acid 1-phenyloctane + 1-octanoic acid

low

lamella (Figure 1) hexamer (Figure 2) lamella (Figure S1, hexamer (Figure 3) Supporting Information) dimer (Figure 4) dimer (Figures 5 and 6)

counterpart. In this case as dispersant, the solute molecules can be dissolved in solution and deposited on the solid surface. The solvent molecule could not coadsorb with solute molecules or participate in the formation of the molecular adlayer. As a counterpart, solvent molecules will coadsorb with solute molecules and take part in the formation of the molecular adlayer. The driving force for the coadsorption of solvents in the adlayer is the hydrogen bond. In the present research, it is impossible to form hydrogen bonds between 1-phenyloctane and BIC molecules. As a result, no 1-phenyloctane molecules coadsorb with BIC molecules. The structural transformation of BIC from lamella to hexamer is only due to the concentration decrease. The weak polar solvent 1-phenyloctane exerts as a dispersant without coadsorption in the self-assembly. A similar concentration induced structural transformation of BIC was also observed in TCB. However, it was found that TCB molecules could be pinned down to the center of hexamers on the surface, which is proposed to be induced by solute-solvent interactions as well as the size and shape of TCB molecules. On the other hand, the self-assembled structure of BIC is found to be dramatically affected by 1-octanoic acid. 1-Octanoic acid molecules act as a counterpart in the linear structure with the hydrogen bond

A self-assembled adlayer of monodendron molecule BIC on HOPG is successfully prepared. The structural relationship of the adlayer with solvents including 1-phenyloctane, TCB, and 1-octanoic acid is investigated by in situ STM at the liquid/ solid interface. STM results show that the BIC assembly is concentration dependent and solvent dependent. A structural transformation of BIC from lamella to hexamer was observed in 1-phenyloctane and TCB with BIC concentration change. 1-Phenyloctane acts as a dispersant, diluting the sample to form hexamers without coadsorption on the surface. TCB could be trapped in the hexamers, and influence the configuration and stability of the hexamer structure. Furthermore, 1-octanoic acid is able to coadsorb with BIC and form a stable self-assembly independent of the concentration, acting as a significant role of counterpart. STM measurement in a mixed solvent of 1-phenyloctane and 1-octanoic acid reveals that only the linear structure could be formed on the graphite surface, indicating that BIC assembly can be easily regulated by 1-octanoid acid. It could be concluded that the solvent exerts its function in two ways: (1) as a dispersant without coadsorption with solute molecules and (2) as a counterpart participating in the assembly formation through hydrogen bonding with BIC molecules. The role of solvent can be changed from a simple dispersant to a regulator in manufacturing 2D molecular nanostructure. The results provide a simple and powerful approach to control molecular self-assembly on solid surfaces, which is significant in realizing the fabrication of molecular nanostructure from passive adsorption to designed construction. Acknowledgment. This work was supported by the National NaturalScienceFoundationofChina(Nos.20821003,20821120291, and 20733004), the National Key Project on Basic Research (Nos. 2006CB806100 and 2006CBON0100), and the Chinese Academy of Sciences. Supporting Information Available: STM images of the lamellar structure of BIC at high concentration in TCB solvent. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139– 150. (2) Wan, L. J. Acc. Chem. Res. 2006, 39, 334–342. (3) Barth, J. V. Annu. ReV. Phys. Chem. 2007, 58, 375–407. (4) Li, S. S.; Northrop, B. H.; Yuan, Q. H.; Wan, L. J.; Stang, P. J. Acc. Chem. Res. 2009, 42, 249–259. (5) Miura, A.; De Feyter, S.; Abdel-Mottaleb, M. M. S.; Gesquiere, A.; Grim, P. C. M.; Moessner, G.; Sieffert, M.; Klapper, M.; Mullen, K.; De Schryver, F. C. Langmuir 2003, 19, 6474–6482. (6) Yoshimoto, S.; Suto, K.; Tada, A.; Kobayashi, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8020–8027. (7) Rohde, D.; Yan, C. J.; Yan, H. J.; Wan, L. J. Angew. Chem., Int. Ed. 2006, 45, 3996–4000. (8) Wang, D.; Chen, Q.; Wan, L. J. Phys. Chem. Chem. Phys. 2008, 10, 6467–6478. (9) Qiu, X. H.; Wang, C.; Zeng, Q. D.; Xu, B.; Yin, S. X.; Wang, H. N.; Xu, S. D.; Bai, C. L. J. Am. Chem. Soc. 2000, 122, 5550–5556. (10) Tahara, K.; Furukawa, S.; Uji-I, H.; Uchino, T.; Ichikawa, T.; Zhang, J.; Mamdouh, W.; Sonoda, M.; De Schryver, F. C.; De Feyter, S.; Tobe, Y. J. Am. Chem. Soc. 2006, 128, 16613–16625.

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