Transformation of Self-Assembled Structure by the Addition of Active

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Transformation of Self-Assembled Structure by the Addition of Active Reactant Yibao Li,† Junhua Wan,‡ Ke Deng,† Xiaona Han,§ Shengbin Lei,|| Yanlian Yang,† Qiyu Zheng,*,§ Qingdao Zeng,*,† and Chen Wang*,† †

)

National Center for Nanoscience and Technology, No.11, North First Street, Zhongguancun, Haidian District, Beijing, 100190, P. R. China ‡ Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou, 310012, P. R. China § Beijing National Laboratory for Molecular Sciences, CAS Key Laboraotry of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China Key Laboratory of Microsystems and Microstructures Manufacturing, Ministry of Education & The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin, 150080, P. R. China

bS Supporting Information ABSTRACT: We have designed and synthesized one molecular building block, an aromatic trialdehyde derivative with three aldehyde groups, to form a potentially active structure. The scanning tunneling microscopy (STM) images show that the trialdehyde derivative forms lamellar structures. By the addition of active reactant, 5-aminoisophthalic acid, it is found that the structure of the adlayer can be transformed from lamellar to hexagonal structure as was expected. The structural formation of molecular networks is attributed to the condensation reaction between aldehyde and amine. Density functional theory (DFT) calculations reveal the observation of structural transformation at the solid/liquid interface due to the more stable adsorption of the triimine product than that of the trialdehyde derivative.

1. INTRODUCTION Self-assembly leads to the successful formation of molecular nanostructures through noncovalent interactions, such as van der Waals interactions, hydrogen bonding, and metal ligand coordination.1,2 With the development in the fabrication supramolecular assemblies and molecular nanostructures, the rational design and structural control of the structure of two-dimensional (2D) molecular assemblies have attracted a great deal of attention.3
5 Several attempts to control the self-assembled structure by external stimuli have recently been reported. The changes of the selfassembled structures can be induced by light,6
9 temperature,10
14 concentration,15 the addition of appropriate guest molecules,16 and so on. An example of light-sensitive structures has been obtained with the binary system TCDB/4NN-Macrocycle.9 After the irradiation of UV light, 4NN-Macrocycle undergoes photoisomerization from the trans
trans
trans
trans (t,t,t,t) to trans
cis
trans
cis (t,c,t,c) and trans
trans
trans
cis (t,t,t,c) photoisomers, which can be observed by STM at the same time on the HOPG surface. Temperature-induced phase transitions were reported by Marie et al. for hexakis-(n-dodecyl)-peri-hexabenzocoronene (HBCC12).13 As the temperature increased from 20 to 50 °C, three irreversible phase transitions of HBC-C12 self-assemblies were observed by STM at the liquid/solid interface. In addition, a “concentration-in-control” concept has appeared to explain surface patterns on the concentration dependence.15 By adjusting the molecular concentration in solution, the ratio of the two self-assemblies can r 2011 American Chemical Society

be controlled such that either a regular porous honeycomb network (at low concentrations) or a dense-packed linear network (at high concentrations) is formed. These results indicate that the environment factors can be used to design nanostructure formation. In fact, the chemical structure of the molecule plays important roles as well. The structural changes have been observed by metal ligand coordination in monolayers at the HOPG/1-phenyloctane interface in real time.17 Kikkawa et al. found that uncomplexed bipyridine derivatives existed in a bent form in the monolayer, and the alkyl chains were interdigitated; however, Pd-complexed derivatives were in a straight form, and the alkyl chains were not interdigitated. At the same time, an intermediate state was successfully observed during metal coordination. In ultrahigh-vacuum (UHV) conditions,18
23 several remarkable examples have been reported by covalently connecting large numbers of molecules to form new 2D networks. For example, Linderoth and co-workers18 reported the formation of a bisimine by coadsorption of a bis(hydroxybenzaldehyde) and octylamine on a Au(111) surface maintained at room temperature. The condensation reaction was confirmed by X-ray photoelectron spectroscopy (XPS) and by comparison of the STM images with those of the reaction product obtained from solution and Received: October 12, 2010 Revised: March 8, 2011 Published: March 23, 2011 6540

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Figure 1. (a) Condensation reaction between the trialdehyde and 5-aminoisophthalic acid. (b) Schematic illustration of the transformation from trialdehyde to triimine. The triangles and the spheres denote building blocks and reactive side groups, respectively.

subsequently deposited onto the surface. Another typical example of radical covalent coupling of (4-bromophenyl) porphyrin derivatives was recently reported by Grill et al.22 The reactions rely on the thermal dissociation of bromine
carbon(phenyl) bonds to give radical fragments that are prone to connect. The mono- or polyradical species diffused on the substrate and then reacted with each other to form either 1D or 2D arrays of coupled porphyrins, depending upon the number and positions of the bromine atoms in the precursor porphyrins. Thus, it is possible to change self-assembling structures by the monomer building blocks with the reactive side groups through chemical reaction with other molecules at room temperature. In this work, we investigated the surface assembly of an aromatic trialdehyde derivative and its structural transformation from lamellar to hexagonal structure induced by the addition of an active reactant, 5-aminoisophthalic acid, at solid/liquid.

2. EXPERIMENTAL SECTION 2.1. Synthesis of 1,3,5-Tri(40 -formylphenyl)benzene. The trialdehyde was synthesized according to the reported method.24 A 2.5 M nBuLi in hexane (11 mL, 27.6 mmol) was added to the solution of 1,3,5-tri(40 -bromophenyl)benzene (1.5 g, 2.76 mmol) in dry THF (150 mL) with a syringe at
78 °C (CAUTION!). After being stirred for 6 h, dry DMF (5 mL) was added, and the reactant

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was heated slowly to room temperature and stirred for 12 h. Then 3 M HCl (40 mL) was added to quench the reaction, and the mixture was stirred for 6 h. After removing the solvent under the vacuum, the product was extracted with CH2Cl2, washed with brine and water, and dried over Na2SO4. The pure product was obtained by column chromatography as the pale yellow powder (300 mg). 1H NMR: 10.11 (s, 3H, CHO), 8.03 (d, J = 8 Hz, 6H, ArH), 7.91 (s, 3H, ArH), 7.88 (d, J = 8 Hz, 6H, ArH). 2.2. Ex Situ Synthesis of Triimine. The mixture of 1,3,5tri(40 -formylphenyl)benzene (70 mg 0.22 mmol), 5-amino-isophthalic acid (0.6 g, 3.3 mmol), and anhydrous MgSO4 (1 g) in dry THF was refluxed under argon for 2 days to yield the product as a yellow powder in 92% yield, which is quite unstable and easily decomposes. 1H NMR: 8.86 (s, 3H, CHdN), 8.36 (s, 3H, ArH), 8.15 (s, 3H, ArH), 8.14 (s, 12H, ArH), 8.02 (s, 6H, ArH). 2.3. STM Characterization. The STM measurements were performed with a Nanoscope IIIa (Veeco Metrology, USA) with mechanically formed Pt/Ir (80/20) tips. The solvent used in this work was 1-octanoic acid and THF (HPLC grade, Aldrich). For STM imaging, the solution containing trialdehyde (1 μL with a concentration of 10
4 mol/L) was first deposited on freshly cleaved highly oriented pyrolytic graphite (HOPG) (Grade ZYB, Veeco Metrology, USA) substrate to form a submonolayer structure. Subsequently, the solution containing 5-aminoisophthalic acid (3 μL with a concentration of 10
4 mol/L) was added to the HOPG surface. After 24 h, we used STM to investigate the result at the liquid
solid interface. All images were recorded in constant current mode. The specific tunneling conditions are given in the corresponding figure captions. 2.4. Theoretical Calculation. All theoretical calculations were made using density functional theory (DFT) provided by DMol3 code.25 The Perdew and Wang parametrization26 of the local exchange-correlation energy is applied in local spin density approximation (LSDA) to describe exchange and correlation. We expanded the all-electron spin-unrestricted Kohn
Sham wave functions in a local atomic orbital basis. For the large system, the numerical basis set was applied. All calculations are all-electron ones and performed with the medium mesh. Selfconsistent field procedure was done with a convergence criterion of 10
5 au on the energy and electron density.

3. RESULTS AND DISCUSSION The design of suitable molecules for structural transformation requires the building block with reactive side groups that can be activated, without breaking the other bonds since the reaction between an aldehyde and an amine easily takes place.18,27 For this reason, an aldehyde derivative is chosen. This molecule is chemically stable and is viable for efficient connection through condensation reaction. Figure 1a shows the predicted condensation reaction between one aromatic trialdehyde and three aromatic amines. The tentative model (as shown in Figure 1b) presents a description of the designed structural transformation. The triangles and the spheres denote building block and reactive side groups, respectively. The spheres could be considered as the reactive side groups, connected by other reactive molecules. The imaging and characterization of the molecular nanostructures were carried out with an STM at room temperature. The star-shaped trialdehyde (Figure 1a) consists of a central benzene ring connected to three benzaldehydes separated by 120° angles. A typical STM image of a pure trialdehyde self-assembled adlayer at the 1-octanoic acid/graphite interface is shown in Figure 2a. 6541

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Figure 2. (a) Self-assembled structure by the trialdehyde (V =
732.4 mV, I = 322.6 pA, 11.2 nm  11.2 nm). (b) Proposed structural model of trialdehyde. (c) High-resolution STM images of the assembly structure formed by the triimine prepared in situ at room temperature at the 1-octanoic acid/graphite interface (V =
879.6 mV, I = 214.6 pA, 12.8 nm  12.8 nm). (d) Proposed structural model of the molecular network of triimine.

The molecules align into lamellar structures. The high-resolution image indicates that the lamellae are composed by pairs of bright triangles. The distance between each apex and the center of the triangle is measured to be about 0.6 nm. Therefore, the triangles are attributed to individual molecules of trialdehyde. The distance between these two molecules in the dimer is measured to be 1.7 ( 0.1 nm, in good agreement with that of an optimized molecular dimer of trialdehyde connected by a weak intermolecular CH 3 3 3 OdC hydrogen bond. According to the STM images, the corresponding molecular model is proposed in Figure 2b. The parameters for the unit cell (a = 1.7 ( 0.2 nm, b = 2.5 ( 0.1 nm, and R = 60 ( 1.0°) are consistent with the experimental data. Upon addition of 5-aminoisophthalic acid (3 μL with a concentration of 10
4 mol/L) to the already formed lamellar structures at the solid/liquid interface, after 24 h, we used STM to investigate the results. This procedure resulted in an open network connected by hydrogen bonds between carboxyl groups (Figure 2c), different from the structures formed by the trialdehyde. With close inspection of the high-resolution STM image (Figure 2c), the backbones of the product appear as the same bright triangle feature as trialdehyde, but its size is much larger than that of the trialdehyde. The distance between each two apexes of the triangle is measured to be 1.2 ( 0.1 nm and 2.2 ( 0.2 nm for trialdehyde and triimine, respectively. As observed in the high-resolution STM image, two different types of cavities are observed in the network. The comparatively larger cavity with 6-fold symmetry is composed of six benzene rings from six triimines with hydrogen bonds between the carboxylic groups, which is similar to that of the network formed by trimesic acid.28
35 The other kind of cavity is formed from two triimine molecules. The structural model derived from STM observations is presented in Figure 2d, which indicates that the two-dimensional networks were formed by hydrogen-bonding interactions between carboxylic groups. The parameters for the

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Figure 3. Series of STM sequence images of a 70.0 nm  70.0 nm region illustrating the structural transformation recorded with V = 935.3 mV and I = 223.3 pA. The time of the first image is arbitrarily set to zero (a).

unit cell are measured to be a = b = (3.8 ( 0.2) nm and R = (60 ( 1.0)°, respectively. The structural transformation of molecular networks may be attributed to the condensation reaction between aldehyde and amine. To verify the proposed mechanism, an in situ STM experiment was performed to investigate the transformation process after the addition of 5-aminoisophthalic acid. Upon continued imaging in the same region, the image sequence shown in Figure 3a
d visualized the process of structural transformation. In the initial frame (Figure 3a), the molecules of pure trialdehyde align into lamellar structures. In the second frame (Figure 3b), the lamellar structures disappear, and the structure becomes irregular after adding 5-aminoisophthalic acid. In the third frame (Figure 3c), the open network structures appear after 12 h. In the last frame (Figure 3d) some defects and many boundaries of domains still exist after 24 h. Although each ordered domain is not large, their open network structures are very clear and cover nearly all the surface in our observations. To further confirm the reaction, the triimine was synthesized ex situ by conventional solution-phase techniques and characterized by 1H NMR and IR (see Supporting Information Figures S(2
3)). The ex situ synthesized triimine was subsequently deposited on HOPG surface. Adsorption of the ex situ reaction product indeed resulted in open network structures (see Supporting Information Figure S4) identical to the structure in Figure 3d. Other evidence is that the hexagonal structure is different from that formed by the 5-aminoisophthalic acid. Figure 4a shows an STM image at the 1-octanoic acid/graphite interface after adsorption of 5-aminoisophthalic acid. In this image, each 5-aminoisophthalic acid molecule looks like a small bright circle with a diameter of about 0.6 nm. Careful observation also reveals that it forms a close packing structure through zigzag chains by intermolecular hydrogen bonds between carboxyl groups. The parameters for the unit cell are measured to be a = 0.8 ( 0.2 nm, b = 1.6 ( 0.1 nm, and the angle R = 90 ( 1°, respectively, which is consistent with the molecular model (in Figure 4b). 6542

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Figure 4. (a) Lamellar structure formed by 5-aminoisophthalic acid (V =
856.7 mV, I = 345.6 pA, 12.2 nm  12.2 nm). (b) Proposed packing molecular network structural model of 5-aminoisophthalic acid.

Thus, we could conclude that the structural transformation of molecular networks should be attributed to the condensation reaction between aldehyde and amine. As is well-known, the solid/liquid interface provides an excellent environment for the rearrangement of structures due to the dynamics related to molecular adsorption
desorption processes. Structural transformation of molecular networks has been reported changing from a linear nonporous structure to a honeycomb porous network as a response to the addition of guest molecules at the solid/liquid interface.16 Although in the UHV study all reported reactions occurred on the surface, in the present case it is uncertain that the reaction occurred directly on the surface or occurred in the solution. Surprisingly, other structures of the product are not clearly resolved in the present study. If the reaction takes place in the solution, it is possible that only one or two aldehyde groups have reacted. At the solid
liquid interface, the relative surface coverage and stability of different phases can strongly depend on concentration.15,36,37 Since the transformation process involved many different molecules, it is possible that a concentration effect also exists for the self-assembly of different structures. This result has not been observed by adjusting the different ratio of the two reactant molecules. During the transformation at the solid/liquid interface, complex processes and interactions (such as solvent
adsorbate, solvent
substrate, adsorbate
substrate, and adsorbate
adsorbate interactions) exist.37 Thus, there exists an equilibrium between reaction, the adsorption and desorption at the solid/liquid interface. In view of the total system energy, the selfassembly adlayer is most frequently favored for stronger intermolecular interaction. Density-functional theory (DFT) calculations show that the total interaction energy for the triimine product (170.26 kcal/ mol) is much larger than those of reactants (50.59 and 57.53 kcal/mol for trialdehyde and 5-aminoisophthalic acid, respectively). Therefore, for the current system, we only observe the assembling structure of the triimine product. In addition, the solvent plays an important role in the transformational process. In this system, two potential factors of the solvent are presented. One is the hydrophobic effect between the triimine molecule and octanonic acid, and the other may be the activator effect of the solvent. It has been shown as an effective method to stabilize the molecular networks with carboxyl-terminated building units by tuning the hydrophobicity of the solvents.38 It was also postulated that coadsorption of solvent molecules may also contribute to stabilizing the porous networks. Such a solvent hydrophobic effect was also illustrated in an earlier study on the selective adsorption and assembling of the triacontane/triacontanol mixtures.39 The hydrophobic effect of these solvents will increase

the desorption rate at the solution/solid interface, thus favoring the formation of a porous structure with low surface density. For the latter effect, the solvent/octanonic acid might have a contribution to activation of reactant. In general, imine formation in solution involves dehydration of a hemiaminal intermediate, which is a reaction catalyzed by the solvent acting as a combined proton donor/acceptor, such as acetic acid.27 Although the octanonic acid is a weaker acid, it may have a catalysis influence in the coupling reaction.

4. CONCLUSION We report the structural transformation by covalently interlinking an aldehyde and an amine at the solid/liquid interface under ambient conditions. The scanning tunneling microscopy (STM) images show that the trialdehyde derivative forms lamellar structures. After the addition of 5-aminoisophthalic acid, it is found that the structure of the adlayer can be transformed from lamella to hexagon open networks. The structural formation of molecular networks is attributed to condensation reaction between aldehyde and amine. Density-functional theory (DFT) calculations also indicate the possible observation of structural transformation at the solid/liquid interface. ’ ASSOCIATED CONTENT

bS

1

H NMR spectrum for trialdehyde and triimine prepared ex situ, IR spectrum for triimine prepared ex situ, and STM images of the assembly structure formed by the triimine prepared ex situ. This material is available free of charge via the Internet at http://pubs.acs.org. Supporting Information.

’ AUTHOR INFORMATION Corresponding Author

*Qingdao Zeng. E-mail: [email protected]. Tel.: þ86 10 82545548. Fax: þ86 10 82545548. Chen Wang. E-mail: wangch@ nanoctr.cn. Tel.: þ86 10 8254 5561. Fax: þ86 10 6265 6765. Qiyu Zheng. E-mail: [email protected]. Tel.: þ86 10 62652811. Fax: þ86 10 62554449.

’ ACKNOWLEDGMENT This work was supported by National Basic Research Program of China (Grant Nos. 2006CB932100, 2007CB936503, 2011CB932300) and the Knowledge Innovation Program of the 6543

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The Journal of Physical Chemistry C Chinese Academy of Sciences (Grant No. KJCX2-YW-M04). The National Natural Science Foundation of China (Grant No. 20673029, 50602007, 21073048), the start-up funding of HIT, New Century Excellent Talents in University (NCET), and Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (2008DP173016) are also gratefully acknowledged for financial support.

’ REFERENCES (1) Gimzewski, J. K.; Joachim, C. Science 1999, 283, 1683. (2) De Feyter, S.; De Schryver, F. C. Chem. Soc. Rev. 2003, 32, 139. (3) Theobald, J. A.; Oxtoby, N. S.; Philips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (4) Stepanow, S.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Delvigne, E.; Lin, N.; Deng, X.; Cai, C.; Barth, J. V.; Kern, K. Nat. Mater. 2004, 3, 229. (5) Elemans, J. A. A. W.; Lei, S. B.; De Feyter, S. Angew. Chem., Int. Ed. 2009, 48, 7298. (6) Heinz, R.; Stabel, A.; Rabe, J. P.; Wegner, G.; De Schryver, F. C.; Corens, D.; Dehaen, W.; S€uling, C. Angew. Chem., Int. Ed. 1994, 33, 2080. (7) Vanoppen, P.; Grim, P. C. M.; R€ucker, M.; De Feyter, S.; Moessner, G.; Valiyaveettil, S.; M€ullen, K.; De Schryver, F. C. J. Phys. Chem. 1996, 100, 19636. (8) Miura, A.; De Feyter, S.; Abdel-Mottaleb, M. M. S.; Gesquiere, A.; Grim, P. C. M.; Moessner, G.; Sieffert, M.; Klapper, M.; M€ullen, K.; De Schryver, F. C. Langmuir 2003, 19, 6474. (9) Shen, Y. T.; Guan, L.; Zhu, X. Y.; Zeng, Q. D.; Wang, C. J. Am. Chem. Soc. 2009, 131, 6174. (10) Li, C. J.; Zeng, Q. D.; Liu, Y. H.; Wan, L. J.; Wang, C.; Wang, C. R.; Bai, C. L. ChemPhysChem 2003, 4, 857. (11) Klappenberger, F.; Weber-Bargioni, A.; Auw€arter, W.; Marschall, M.; Schiffrin, A.; Barth, J. V. J. Chem. Phys. 2008, 129, 214702–1. (12) Treossi, E.; Liscio, A.; Feng, X.; Palermo, V.; M€ullen, K.; Samor, P. Small 2009, 5, 112. (13) Marie, C.; Silly, F.; Tortech, L.; M€ullen, K.; Fichou, D. ACS Nano 2010, 4, 1288. (14) Gutzler, R.; Sirtl, T.; Dienstmaier, J. F.; Mahata, K.; Heckl, W. M.; Schmittel, M.; Lackinger, M. J. Am. Chem. Soc. 2010, 132, 5084. (15) Lei, S. B.; Tahara, K.; De Schryver, F. C.; Auweraer, M. V.; Tobe, Y.; De Feyter, S. Angew. Chem., Int. Ed. 2008, 47, 2964. (16) Furukawa, S.; Tahara, K.; De Schryver, F. C.; Auweraer, M. V.; Tobe, Y.; De Feyter, S. Angew. Chem., Int. Ed. 2007, 46, 2831. (17) Kikkawa, Y.; Koyama, E.; Tsuzuki, S.; Fujiwara, K.; Miyake, K.; Tokuhisa, H.; Kanesato, M. Langmuir 2006, 22, 6910. (18) Weigelt, S.; Busse, C.; Bombis, C.; Knudsen, M. M.; Gothelf, K. V.; Læsgaard, E.; Besenbacher, F.; Linderoth, T. R. Angew. Chem., Int. Ed. 2008, 47, 4406. (19) Treier, M.; Richardson, N. V.; Fasel, R. J. Am. Chem. Soc. 2008, 130, 14054. (20) Zwaneveld, N. A. A.; Pawlak, R.; Abel, M.; Catalin, D.; Gigmes, D.; Bertin, D.; Porte, L. J. Am. Chem. Soc. 2008, 130, 6678. (21) Matena, M.; Riehm, T.; St€ohr, M.; Jung, T. A.; Gade, L. H. Angew. Chem., Int. Ed. 2008, 47, 2414. (22) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nat. Nanotechnol. 2007, 2, 687. (23) Gutzler, R.; Walch, H.; Eder, G.; Kloft, S.; Heckl, W. M.; Lackinger, M. Chem. Commun. 2009, 29, 4456. (24) Weber, E.; Hecker, M.; Koepp, E.; Orlia, W. J. Chem. Soc., Perkin Trans. 1988, 125, 1251. (25) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244. (26) Becke, A. J. Chem. Phys. 1988, 88, 2547. (27) Cordes, E. H.; Jencks, W. P. J. Am. Chem. Soc. 1962, 84, 832. (28) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M. Single Mol. 2002, 3, 25.

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(29) Nath, K. G.; Ivasenko, O.; Miwa, J. A.; Dung, H.; Wuest, J. D.; Nanci, A.; Perepichka, D. F.; Rosei, F. J. Am. Chem. Soc. 2006, 128, 4212. (30) Lu, J.; Zeng, Q.; Wang, C.; Zheng, Q.; Wan, L.; Bai, C. J. Mater. Chem. 2002, 12, 2856. (31) Dimitriev, A.; Lin, N.; Weckesser, J.; Barth, J. V.; Kern, K. J. Phys. Chem. B 2002, 106, 6907. (32) Ishikawa, Y.; Ohira, A.; Sakata, M.; Hirayama, C.; Kunitake, M. Chem. Commun. 2002, 2652. (33) Su, G. J.; Zhang, H. M.; Wan, L. J.; Bai, C. L.; Wandlowski, T. J. Phys. Chem. B 2004, 108, 1931. (34) Yan, H. J.; Lu, J.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2004, 108, 11251. (35) Li, M.; Deng, K.; Lei, S. B.; Yang, Y. L.; Wang, T. S.; Shen, Y. T.; Wang, C. R.; Zeng, Q. D.; Wang, C. Angew. Chem., Int. Ed. 2008, 47, 6717. (36) Kampschulte, L.; Werblowsky, T. L.; Kishore, R. S. K.; Schmittel, M.; Heckl, W. M.; Lackinger, M. J. Am. Chem. Soc. 2008, 130, 8502. (37) Tahara, K.; Okuhata, S.; Adisoejoso, J.; Lei, S.; Fujita, T.; De Feyter, S.; Tobe, Y. J. Am. Chem. Soc. 2009, 131, 17583. (38) Li, Y. B.; Ma, Z.; Qi, G. Cc; Yang, Y. L.; Zeng, Q. D.; Fan, X. L.; Wang, C.; Huang, Wei. J. Phys. Chem. C 2008, 112, 8649. (39) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608.

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