8547
2005, 109, 8547-8550 Published on Web 04/16/2005
Dependence of Molecular Recognition of Fullerene Derivative on the Adlayer Structure of Zinc Octaethylporphyrin Formed on Au(100) Surface Soichiro Yoshimoto,† Yosuke Honda,† Yasujiro Murata,‡ Michihisa Murata,‡ Koichi Komatsu,‡ Osamu Ito,§ and Kingo Itaya*,†,| Department of Applied Chemistry, Graduate School of Engineering, Tohoku UniVersity, 6-6-04 Aoba, Sendai 980-8579, Japan, Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan, Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Aoba-ku, Sendai 980-8577, Japan, and Core Research EVolutional Science and Technology organized by Japan Science and Technology Agency (CREST-JST), Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ReceiVed: March 4, 2005; In Final Form: March 15, 2005
Adlayers of ZnOEP were prepared on reconstructed Au(100)-(hex) and unreconstructed Au(100)-(1 × 1) surfaces by immersing into a benzene solution containing ZnOEP molecules, and the adlayer structures were characterized by scanning tunneling microscopy (STM). A hexagonally arranged ZnOEP array was formed on an Au(100)-(hex) surface, whereas a rectangularly arranged ZnOEP array was found on an Au(100)-(1 × 1) surface. The adlayer structure of ZnOEP was dependent upon the underlying Au atomic arrangements. Furthermore, an investigation of the spuramolecular assembly for these modified surfaces was carried out by using an open-cage C60 derivative (opened C60). A supramolecular assembled adlayer with a 1:1 composition of opened C60/ZnOEP was formed on Au(100)-(hex), whereas aggregates of opened C60 were found on the ZnOEP-modified Au(100)-(1 × 1) surface. Electrochemical responses of opened C60 were significantly influenced by underlying ZnOEP arrays. This finding suggests that precise control of underlying ZnOEP adlayers with the Au atomic structure is important to recognize the opened C60 on them.
Introduction Porphyrin-fullerene supramolecular assembly has been studied extensively because of the interest in its application to photoinduced energy process and electron transfer processes.1-5 Especially, fullerenes are considered to be suitable building blocks for three-dimensional molecular architectures owing to their strong π-electron accepting ability.1 Recently, supramolecular assemblies on metal surfaces are being explored to control surface properties.6-11 We recently succeeded in forming a 1:1 fullerene-porphyrin supramolecular assembly of a C60 derivative on a highly ordered ZnOEP array formed on an Au(111) surface.7-9 Such a supramolecular assembly produced through donor-acceptor interaction would be useful for the design and organization of functional organic molecules on electrode surfaces. To further explore and understand the role of substrate in the formation of molecular assemblies, precise control of the adlayer structure is of importance both in building nanoarrays and in designing surfaces for the control of properties of fullerenes using donor-acceptor interaction in supramolecular assembly systems. For example, the C60 assembly was found * To whom correspondence should be addressed. E-mail: itaya@ atom.che.tohoku.ac.jp (K. Itaya). Phone/Fax: +81-22-214-5380. † Department of Applied Chemistry, Graduate School of Engineering, Tohoku University. ‡ Kyoto University. § Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. | Core Research Evolutional Science and Technology organized by Japan Science and Technology Agency (CREST-JST).
10.1021/jp051112l CCC: $30.25
CHART 1: Schematic Illustration of Reconstructed (Au(100)-(hex)) and Unreconstructed (Au(100)-(1 × 1)) Atomic Arrangements of Au(100)
to be strongly influenced by the underlying coronene- and perylene-modified Au(111) surfaces.10 In the present paper, we focus our discussion on the adlayers formed on the Au(100) plane, because an Au(100) substrate can have either of two different atomic structures (i.e., reconstructed (5 × 20) (or (hex)) or unreconstructed (1 × 1) structures; see Chart 1), which is controllable by electrochemical and annealing treatments. The formation of supramolecular assembled film of open-cage C60 derivative (opened C60; see Chart 2) was characterized by using cyclic voltammetry (CV) and scanning tunneling microscopy (STM) in acidic solutions, which showed that the film structure depended upon the adlayer structure of ZnOEP on the Au(100) surface. Experimental Section Opened C60 was synthesized by using the procedure described in the literature.12 ZnOEP was purchased from Aldrich and used without further purification. Benzene was obtained from Kanto © 2005 American Chemical Society
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Letters
CHART 2: Schematic Illustrations of Opened C60 and ZnOEP
Chemical Co. (spectroscopy grade). Au(100) single-crystal electrodes were prepared by the Clavilier method.13 The reconstructed Au(100)-(hex) surface was obtained after annealing in a hydrogen flame and cooling slowly in air for 3 min. The unreconstructed Au(100)-(1 × 1) surface was prepared by immersing into pure 0.1 M HClO4 for 20 min to remove Au islands on the terrace after annealing in a hydrogen flame and quenching in ultrapure water (Milli-Q SP-TOC; g18.2 M cm) saturated with hydrogen. In general, the Au(100)(1 × 1) surface is known to form many Au islands because of the lifting of reconstruction, which occurs immediately after quenching in ultrapure water.14,15 To remove the small Au islands, the Au atoms were allowed to diffuse by holding the electrode potential at 1.0 V for more than 20 min in 0.1 M HClO4.16 Adlayers of opened C60/ZnOEP were formed by immersing an Au(100) electrode successively into ca. 100 µM ZnOEP for 10-30 s and into a 50 µM benzene solution of opened C60 for 20-60 s.7 The opened C60/ZnOEP-adsorbed Au(100) was then rinsed with ultrapure water, and it was transferred into an electrochemical STM cell, which was then filled with 0.05 M H2SO4 or 0.1 M HClO4 (Cica-Merck, ultrapure grade). The structure of the ZnOEP adlayer was independent of the composition of the electrolyte solution (i.e., identical structures were obtained either in 0.05 M H2SO4 or in 0.1 M HClO4). CV was carried out at 20 °C using a potentiostat (HOKUTO HAB-151, Tokyo) with the hanging meniscus method in a three-compartment electrochemical cell in N2 atmosphere. Electrochemical STM measurements were performed in either 0.05 M H2SO4 or 0.1 M HClO4 by using a Nanoscope E (Digital Instruments, Santa Barbara, CA) with a tungsten tip etched in 1 M KOH. To minimize residual faradaic currents, the tips were coated with nail polish. STM images were recorded in the constant-current mode with a highresolution scanner (HD-0.5I). All potential values are referred to the reversible hydrogen electrode (RHE). Results and Discussion Figure 1a shows typical STM images of a ZnOEP adlayer on Au(100)-(hex) surface acquired at 0.75 V in 0.05 M H2SO4. Highly ordered domains were formed on the atomically flat terrace, and several molecular rows can be seen in a large area measuring 50 × 50 nm2. In each domain, some straight lines were regularly found. Because the formation of small Au islands by the lifting of reconstruction was not found on the terrace, such a smooth surface indicates that the reconstructed Au(100)-(hex) surface is entirely retained at this potential. As reported by He et al., an ordered hexadecane layer on Au(111) in HClO4 stabilized the reconstructed Au(111)-(22 × x3) surface, suggesting that the adsorption of insoluble alkane molecules screens water molecules and electrolyte ions from the gold surface.17 As one of the possible reasons the recon-
Figure 1. Large-scale (50 × 50 nm2) and high-resolution (15 × 15 nm2) STM images of ZnOEP arrays on Au(100)-(hex) in 0.05 M H2SO4 acquired at 0.75 V vs RHE (a and b) and on Au(100)-(1 × 1) in 0.1 M HClO4 acquired at 0.7 V vs RHE (d and e). Corresponding structural models are shown in (c) and (f), respectively. Tip potentials and tunneling currents were 0.46 V and 1.5 nA for panels a and b and 0.2 V and 0.73 nA for panels d and e, respectively.
structed surface is stable, the adsorption of ZnOEP induces a negative charge on the Au(100) surface so that the formation of a hydrophobically ordered ZnOEP layer may block the adsorption of certain species such as specific anions. A highresolution STM image of a highly ordered ZnOEP array on Au(100)-(hex) is shown in Figure 1b. The center of each ZnOEP molecule was observed as a dark spot, which was surrounded by small additional spots corresponding to eight ethyl groups, as depicted by black circles. ZnOEP molecules were observed to be hexagonally organized on the Au(100)-(hex) surface. As indicated by the white arrows in Figure 1b, individual ZnOEP molecules in these molecular rows were slightly rotated in every four or five molecular rows. Careful inspection revealed that each molecule in the two molecular rows pointed out by the arrows was rotated by about 5° with respect to the individual ZnOEP molecules in the other molecular rows. In the structural model shown in Figure 1c, the ZnOEP molecular rows marked by white arrows in Figure 1b correspond to the molecular rows colored in brighter reddish purple. Thus, this slightly rotated molecular orientation is reflected as remarkable molecular rows in the large-scale STM image shown in Figure 1a. From the cross-sectional profiles, the intermolecular distances in Figure 1b were measured to be 1.60 ( 0.07, 1.68 ( 0.07, and 1.45 ( 0.05 nm for the directions indicated by red, blue, and green arrows, respectively. The surface concentration was 9.1 × 10-11 mol‚cm-2. The packing arrangement of ZnOEP on Au(100)(hex) was different from that obtained on the Au(111) surface,7
Letters suggesting that interaction between ZnOEP molecules and reconstructed Au substrate depends on the crystallographic orientation of Au. In the case of Au(111), it is seen that the ZnOEP molecules were alternately arranged with a different molecular orientation.7-9 The adlayer structure of ZnOEP on Au(111) was identical to that of not only NiOEP in ultrahigh vacuum (UHV),18 but also to those of CoOEP, CuOEP, and FeClOEP in solution.9 When the modification was carried out on Au(100)-(1 × 1) surface, the STM image showed a completely different adlayer structure of ZnOEP as seen in Figure 1d. The domain size for the ZnOEP array formed on Au(100)-(1 × 1) was much smaller than that on the Au(100)(hex) surface. The difference in domain size might be due to the difference in the strength of interaction between ZnOEP and the Au(100)-(1 × 1) surface. Careful inspection revealed that two different domains (i.e., hexagonally and rectangularly arranged ZnOEP arrays) were formed on the Au(100)-(1 × 1) surface. In particular, a rectangularly arranged ZnOEP array was predominantly found on the Au(100)-(1 × 1) surface. A closeup view is shown in Figure 1e. Although several phase boundaries were present, the ZnOEP molecules were almost rectangularly arranged in the ordered region. Intermolecular distances between the nearest-neighbor molecules were measured to be 1.45 and 1.63 nm. Thus, the adlayer consisted of rectangular unit cells, as shown in the structural model in Figure 1f. The concentration of ZnOEP on the Au(100)-(1 × 1) surface was estimated to be 7.2 × 10-11 mol‚cm-2, which is smaller than that on the Au(100)-(hex) surface. Figure 2a,b shows typical STM images of an adlayer of opened C60 formed on ZnOEP-modified Au(111) and Au(100)(hex) in 0.05 M H2SO4. Highly ordered arrays consisting of bright, round spots were observed over the entire surface of both the ZnOEP-modified Au(111) and Au(100)-(hex) surfaces. As reported in our previous paper, the STM image shown in Figure 2a reveals a 1:1 supramolecular assembly consisting of opened C60 and a ZnOEP array on Au(111).7 In Figure 2b, some defects were found on the modified surface, as indicated by arrows. The corrugation height of each spot was approximately 0.35 nm. Bonifazi et al. reported that C60 molecules on a zinc porphyrin derivative array formed on an Ag(100) surface located precisely on top of the 3-cyanophenyl substituents in a zinc porphyrin derivative in a UHV environment.11 According to their paper, each protrusion height appeared to be 0.44 nm from the porphyrin layer. Therefore, each bright spot in Figure 2b is attributed to opened C60. Intermolecular distances between the nearest-neighbor molecules of opened C60 formed on Au(100)(hex) were found to be 1.47 ( 0.05, 1.61 ( 0.07, and 1.67 ( 0.07 nm, respectively, which are in good agreement with those obtained from Figure 1a. Note that the top layer of opened C60 was easily removed by the operation of scanning at tunneling currents higher than 1.0 nA, indicating that the interaction between opened C60 and ZnOEP is very weak. A similar tipinduced motion of C60 molecules on Au(111) was reported by Guo et al.19 In contrast, a completely disordered structure was observed for the adlayer of opened C60 on the Au(100)-(1 × 1) surface. Although several individual molecules of opened C60 could be distinguished under the present conditions, the ZnOEPmodified Au(100)-(1 × 1) surface was covered largely with aggregates of opened C60, as can be seen in Figure 2c. This difference in STM image is clearly reflected in cyclic voltammograms for the modified electrodes. As described in our previous paper, opened C60 was proposed to undergo a twoelectron redox reaction between >CdO and >C•sOH for two carbonyl groups in each molecule of opened C60 on the highly
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Figure 2. Typical high-resolution (20 × 20 nm2) STM images of the adlayer of opened C60 formed on ZnOEP-modified Au(111) (a), Au(100)-(hex) (b), and Au(100)-(1 × 1) (c), all acquired in 0.05 M H2SO4. Tip potentials and tunneling currents were 0.175 V and 0.425 nA (a), 0.46 V and 0.625 nA (b), and 0.43 V and 0.2 nA (c), respectively. Corresponding cyclic voltammograms of opened C60/ ZnOEP-adsorbed Au(111), Au(100)-(hex), and Au(100)-(1 × 1) electrodes in 0.05 M H2SO4 are shown in (d), (e), and (f), respectively. The scan rate was 20 mV s-1.
ordered ZnOEP array on an Au(111) electrode.7 A pair of characteristic redox peaks clearly appeared at 0 and 0.85 V during cathodic and anodic scans, respectively. The voltammetric responses for the opened C60 on the ZnOEP-modified Au(100)-(hex) surface were similar to that on the ZnOEPmodified Au(111) surface as shown in Figure 2e. This result suggests that the carbonyl groups of opened C60 were oriented toward the solution phase on the ZnOEP-modified Au(100)(hex) surface because of the formation of a 1:1 supramolecular assembly with the highly ordered ZnOEP array. On the basis of the electronic charge calculated from the reductive peak area, the amount of transferred electronic charge was estimated to be ca. 16.2 µC‚cm-2, leading to the surface concentration of (8.4 ( 0.7) × 10-11 mol‚cm-2. For the opened C60 on the ZnOEP-modified Au(100)-(1 × 1) electrode, a pair of broad reduction and reoxidation peaks were observed during the scan. Redox peak currents were smaller than those obtained at the Au(100)-(hex) surface, suggesting that the molecular orientation of opened C60 was random on the ZnOEP-modified Au(100)-(1 × 1) surface. On the basis of the STM images obtained in the present study, we propose the models illustrated in Figure 3. A 1:1 supramolecular assembled layer of opened C60 is formed on the ZnOEP adlayer on the Au(100)-(hex) surface, whereas such highly
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Letters of opened C60. This finding suggests that precise control of underlying ZnOEP adlayers is important in supramolecular recognition between opened C60 and ZnOEP arrays on Au. Acknowledgment. This work was supported in part by CREST-JST, and by the Ministry of Education, Culture, Sports, Science and Technology, a Grant-in-Aid for Young Scientists (B) (no. 16750106) and the Center of Excellence Project, Giant Molecules and Complex Systems, 2005. The authors acknowledge the assistance provided by Dr Y. Okinaka in writing this manuscript. References and Notes
Figure 3. Proposed models of opened C60/ZnOEP on reconstructed Au(100)-(hex) (a) and on unreconstructed Au(100)-(1 × 1) surface (b).
ordered arrays of molecules of opened C60 do not form on the ZnOEP-modified Au(100)-(1 × 1) surface. In the ZnOEP array on the Au(100)-(1 × 1) surface, there are gaps surrounded by ZnOEP molecules, which indicates that the attractive force between opened C60 and the substrate might be locally stronger. The potential of zero charge (pzc) is also different from the pzc values of unmodified Au(100)-(hex) and Au(100)(1 × 1). As reported by Kolb and co-workers, the values of pzc of Au(100)-(hex) and Au(100)-(1 × 1) are 0.30 and 0.08 V vs saturated calomel electrode (SCE), respectively, in 0.01 M HClO4 solution.15,20 Therefore, the difference in molecular recognition between opened C60 and ZnOEP might be explainable in terms of the difference in surface charge of the ZnOEPmodified Au surfaces. Although further investigations are needed, the difference in surface charges between reconstructed Au(100)-(hex) and unreconstructed Au(100)-(1 × 1) appears to result in the difference in the attractive force between opened C60 and ZnOEP. Conclusion Two different highly ordered adlayers were formed on Au(100)-(hex) and Au(100)-(1 × 1) surfaces. On these surfaces, the difference in molecular recognitions of opened C60 by ZnOEP on Au(100)-(hex) and on Au(100)-(1 × 1) was clearly observed in STM images and electrochemical responses
(1) Balzani, V. Ed. Electron Transfer in Chemistry; WILEY-VCH: New York, 2001; Vol. 3. (2) Olmstead, M. M.; Costa, D. A.; Maitra, K.; Noll, B. C.; Phillips, S. L.; Van Calcar, P. M.; Balch, A. L. J. Am. Chem. Soc. 1999, 121, 70907091. (3) Da Ros, T.; Prato, M.; Guldi, D. M.; Ruzzi, M.; Pasimeni, L. Chem.sEur. J. 2001, 7, 816-827. (4) Guldi, D. M. Chem. Soc. ReV. 2002, 31, 22-36. (5) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem. Photobiol., C 2004, 5, 79-104. (6) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139150. (7) Yoshimoto, S.; Tsutsumi, E.; Honda, Y.; Murata, Y.; Murata, M.; Komatsu, K.; Ito, O.; Itaya, K. Angew. Chem., Int. Ed. 2004, 43, 30443047. (8) Yoshimoto, S.; Tsutsumi, E.; Honda, Y.; Ito, O.; Itaya, K. Chem. Lett. 2004, 33, 914-915. (9) Yoshimoto, S.; Saito, A.; Tsutsumi, E.; D’Souza, F.; Ito, O.; Itaya, K. Langmuir 2004, 20, 11046-11052. (10) Yoshimoto, S.; Tsutsumi, E.; Fujii, O.; Narita, R.; Itaya, K. Chem. Commun. 2005, 1188-1190. (11) Bonifazi, D.; Spillmann, H.; Kiebele, A.; de Wild, M.; Seiler, P.; Cheng, F.; Jung, T.; Diederich, F. Angew. Chem., Int. Ed. 2004, 43, 47594763. (12) Murata, Y.; Murata, M.; Komatsu, K. Chem.sEur. J. 2003, 9, 1600-1609. (13) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. J. Electroanal. Chem. 1980, 107, 205-209. (14) Magnussen, O. M.; Hotlos, J.; Behm, R. J.; Batina, N.; Kolb, D. M. Surf. Sci. 1993, 296, 310-332. (15) Dakkouri, A. S.; Kolb, D. M. Reconstruction of Gold Surfaces. Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; Chapter 10. (16) Yoshimoto, S.; Suto, K.; Tada, A.; Kobayashi, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8020-8027. (17) He, Y.; Ye, T.; Borguet, E. J. Phys. Chem. B 2002, 106, 1126411271. (18) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2002, 106, 996-1003. (19) Guo, S.; Fogarty, D. P.; Nagel, P. M.; Kandel, S. A. J. Phys. Chem. B 2004, 108, 14074-14081. (20) Kolb, D. M.; Schneider, J. Electrochim. Acta 1986, 31, 929936.
