Electrochemical Redox Control of Ferrocene Using a Supramolecular

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Electrochemical Redox Control of Ferrocene Using a Supramolecular Assembly of Ferrocene-Linked C60 Derivative and Metallooctaethylporphyrin Array on a Au(111) Electrode Soichiro Yoshimoto,† Akira Saito,† Eishi Tsutsumi,† Francis D’Souza,‡ Osamu Ito,§ and Kingo Itaya*,†,| Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-04 Aoba, Sendai 980-8579, Japan, Department of Chemistry, Wichita State University, 1845 Fairmount, Wichita, Kansas 67260-0051, 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 September 3, 2004 Supramolecular assembled layers of ferrocene-linked C60 derivative (C60Fc) and various metal ions coordinated to octaethylporphyrin (MOEP) were formed on the surface of a Au(111) single-crystal electrode by immersing the Au substrate successively into a benzene solution containing MOEP and one containing C60Fc molecules. The MOEPs used were zinc(II) (ZnOEP), cobalt(II) (CoOEP), copper(II) (CuOEP), and iron(III) chloride (FeClOEP) of OEP (2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine). The molecules of C60Fc directly attached to the Au(111) electrode showed poorly defined electrochemical redox response, whereas a clear electrochemical redox reaction of the ferrocene group in the C60Fc molecule was observed at 0.78 V versus reversible hydrogen electrode on ZnOEP, CoOEP, and CuOEP adlayers, but not on the FeClOEP adlayer. Adlattices of the underlying layer and the top layer of C60Fc were determined by in situ scanning tunneling microscopy. Adlayer structures of MOEP were independent of the central metal ion; that is, MOEP molecules were arranged hexagonally with two different orientations. Highly ordered C60Fc arrays were formed with 1:1 composition on the ZnOEP-, CoOEP-, and CuOEP-modified Au(111) surface, whereas a disordered structure of C60Fc was found on the FeClOEP-modified Au(111) surface. The presence of Cl ligand was found to prevent the formation of supramolecularly assembled layers with C60Fc molecules, resulting in an ill-defined unclear electrochemical response of the Fc group. The well-defined electrochemical response of the Fc group in C60Fc was clearly due to the control of orientation of C60Fc molecules.

Introduction Porphyrins provide an extremely versatile synthetic base for a variety of material applications in many disciplines of chemistry and physics, such as optoelectronics, electrochemistry, catalysis, data storage, and solar cells.1 Especially, the porphyrin-fullerene supramolecular assembly has been studied extensively because of its relevance to photoinduced energy and electron-transfer processes.1-3 Fullerenes are considered to be building blocks suitable for three-dimensional molecular architecture due to their strong π-electron accepting ability.2,4,5 Several interesting studies on supramolecular assembly using the fullerene-porphyrin system were reported by several groups.6-12 For example, one-dimensional aggregation, or a supramolecular polymer, was constructed * To whom correspondence should be addressed. Phone/Fax: +81-22-214-5380. E-mail: [email protected]. † Department of Applied Chemistry, Tohoku University. ‡ Department of Chemistry, Wichita State University. § Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. | Japan Science and Technology Agency. (1) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: New York, 2001; Vol. 3. (2) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (3) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem. Photobiol., C 2004, 5, 79. (4) Buckminsterfullerenes; Billups, W. E., Ciufolini, M. A., Eds.; VCH: New York, 1993. (5) Ishii, T.; Aizawa, N.; Kanehama, R.; Yamashita, M.; Sugiura, K.-i.; Miyasaka, H. Coord. Chem. Rev. 2002, 226, 113.

from a 1:2 complex of fullerene-porphyrin derivative.11 Supramolecular peapods consisting of fullerene and zinc bisporphyrin were reported by Aida and co-workers.12 In both cases, fiber formation resulting from a strong porphyrin-C60 interaction was observed in the presence of fullerenes by scanning electron microscopy (SEM). Supramolecular assembly on metal surfaces has also been explored to control surface properties and to construct molecular architectures by the layer-by-layer growth technique. To prepare composite thin films with a high quality of supramolecular assembly by using donoracceptor and metallosupramolecular array systems and to develop new functional electrodes or devices, the control of molecular orientation at the nanoscale is of importance. For example, a two-dimensional nanoporous network was formed on Cu(100) by the metal-organic coordination between terephthalic acid (TPA) or 4,1′,4′,1′′-terphenyl1,4′′-dicarboxylic acid (TDA) and Fe in ultrahigh vacuum (6) 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, 7090. (7) Da Ros, T.; Prato, M.; Guldi, D. M.; Ruzzi, M.; Pasimeni, L. Chem.s Eur. J. 2001, 7, 816. (8) Sun, D.; Tham, F. S.; Reed, C. A.; Chaker, L.; Burgess, M.; Boyd, D. W. J. Am. Chem. Soc. 2000, 122, 10704. (9) El-Khouly, M. E.; Rogers, L. M.; Zandler, M. E.; Gadde, S.; Fujitsuka, M.; Ito, O.; D’Souza, F. ChemPhysChem 2003, 4, 474. (10) Schuster, D. I.; Cheng, P.; Jarowski, P. D.; Guldi, D. M. J. Am. Chem. Soc. 2004, 126, 7257. (11) Shirakawa, M.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2003, 125, 9902. (12) Yamaguchi, T.; Ishii, N.; Tashiro, K.; Aida, T. J. Am. Chem. Soc. 2003, 125, 13934.

10.1021/la047795y CCC: $27.50 © 2004 American Chemical Society Published on Web 11/04/2004

Electrochemical Redox Control of Ferrocene Chart 1. Chemical Formulas of C60Fc and Metallooctaethylporphyrin (MOEP)

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by using a simple method for the construction of a 1:1 supramolecular assembled film of FcC60 and metallooctaethylporphyrin (MOEP) on a Au(111) electrode. Highly ordered arrays of FcC60 as the second layer on the welldefined MOEP adlayer on Au(111) can be directly observed in 0.1 M HClO4 by electrochemical measurement and scanning tunneling microscopy (STM). The central metal ion in OEP was found to influence the formation of fullerene-porphyrin supramolecular assembly on the Au surface. Experimental Section

(UHV).13 Such network arrays composed of metal atoms and organic molecules could selectively recognize C60 molecules as guest molecules. We recently succeeded in controlling the electrochemical response of the quinone group in the open-cage C60 derivative on the highly ordered ZnOEP array formed on a Au(111) surface by the fullerene-porphyrin supramolecular assembly.14a Molecules of C60 also could be assembled on the ZnOEPmodified Au(111) surface.14b This finding shows that it is possible to construct spontaneously not only molecular architectures but also photoinduced optical devices. Recently, studies on fullerenes linked to multiple redoxand/or photoactive molecular entities have been carried out extensively because they have potential for constructing supramolecular electronic devices and artificial light energy-generating systems.15,16 Especially, because ferrocene is a typical redox species, it is a promising material for an electrochemical switching or sensing device. In the field of electrochemistry, self-assembled monolayer (SAM) systems using thiols terminated with an electroactive ferrocene group are most widely employed.17-24 However, the electrochemical properties of ferrocene-linked C60 derivative (C60Fc; Chart 1) fixed on an electrode have not been clarified yet. In the present paper, we propose a unique approach for controlling the electrochemical redox reaction of ferrocene (13) 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. (14) (a) Yoshimoto, S.; Tsutsumi, E.; Honda, Y.; Murata, Y.; Murata, M.; Komatsu, K.; Ito, O.; Itaya, K. Angew. Chem., Int. Ed. 2004, 43, 3044. (b) Yoshimoto, S.; Tsutsumi, E.; Honda, Y.; Ito, O.; Itaya, K. Chem. Lett. 2004, 33, 914. (15) (a) Sawamura, M.; Kuninobu, Y.; Toganoh, M.; Matsuo, Y.; Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2002, 124, 9354. (b) Toganoh, M.; Matsuo, Y.; Nakamura, E. Angew. Chem., Int. Ed. 2003, 42, 3530. (16) (a) D’Souza, F.; Zandler, M. E.; Smith, P. M.; Deviprasad, G. R.; Arkady, K.; Fujitsuka, M.; Ito, O. J. Phys. Chem. A 2002, 106, 649. (b) Zandler, M. E.; Smith, P. M.; Fujitsuka, M.; Ito, O.; D’Souza, F. J. Org. Chem. 2002, 67, 9122. (c) Fujitsuka, M.; Tsuboya, N.; Hamasaki, R.; Ito, M.; Onodera, S.; Ito, O.; Yamamoto, Y. J. Phys. Chem. A 2003, 107, 1452. (17) (a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (b) Chidsey, C. E. D. Science 1991, 251, 919. (18) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (19) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (20) (a) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385. (b) Shimazu, K.; Ye, S.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 375, 409. (c) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (21) Nishiyama, K.; Ueda, A.; Tanoue, S.; Koga, T.; Taniguchi, I. Chem. Lett. 2000, 930. (22) Imahori, H.; Fukuzumi, S. Adv. Funct. Mater. 2004, 14, 525. (23) (a) Imahori, H.; Tamaki, K.; Araki, Y.; Sekiguchi, Y.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2002, 124, 5165. (b) Kesti, T. J.; Tkachenko, N. V.; Vehmanen, V.; Yamada, H.; Imahori, H.; Fukuzumi, S.; Lemmetyinen, H. J. Am. Chem. Soc. 2002, 124, 8067. (c) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S.-K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129. (24) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205.

2-(Ferrocenyl)fulleropyrrolidine (C60Fc) was synthesized by using the procedure described in the literature.16a The chemicals 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine zinc(II) (ZnOEP), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine cobalt(II) (CoOEP), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine copper(II) (CuOEP), and 2,3,7,8,12,13,17,18-octaethyl21H,23H-porphine iron(III) chloride (FeClOEP) were purchased from Aldrich and used without further purification. Benzene was obtained from Kanto Chemical Co. (Spectroscopy Grade). The Au(111) single-crystal electrode was prepared by the Clavilier method.24 The aligned gold single-crystal bead was cut and successively polished with finer grades of alumina paste, and the electrode was then annealed at ca. 950 °C for at least 12 h in an electric furnace to remove mechanical damage. Adlayers of C60Fc on MOEP arrays were formed by immersing a Au(111) electrode successively into a ca. 100 µM MOEP (M ) Zn, Co, Cu, Fe) benzene solution for less than 10 s and then into a 50 µM C60Fc benzene solution for 10-60 s, after the Au(111) substrate was annealed in hydrogen flame and quenched into ultrapure water (Milli-Q SP-TOC; g18.2 MΩ cm) saturated with hydrogen.25 The C60Fc/MOEP-adsorbed Au(111) was then rinsed with ultrapure water, and it was transferred into an electrochemical STM cell filled with 0.1 M HClO4 (Cica-Merck, ultrapure grade). Cyclic voltammetry 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 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 high-resolution scanner (HD-0.5I). All potential values are referred to the reversible hydrogen electrode (RHE).

Results and Discussion Figure 1a shows typical cyclic voltammograms (CVs) of a well-defined, bare Au(111) electrode (dotted line) and a Au(111) electrode with directly attached C60Fc (red solid line) in 0.1 M HClO4. They were recorded at a scan rate of 50 mV s-1. The voltammogram for the bare Au(111) in the double-layer potential region was identical to that reported previously,25 indicating that a well-defined Au(111) surface was exposed to the solution. For the Au(111) electrode directly modified with C60Fc, a broad redox peak was observed in the potential range between 0.7 and 1.05 V. The anodic current commencing at 0.95 V is likely to be due to the oxidative desorption of C60Fc molecules from the Au surface, because such an anodic current was also observed at C60- and C120-modified Au(111) electrodes.26 The redox current of the Fc moiety might be involved in such oxidative current of the C60 moiety. The dotted line in Figure 1b shows a CV of a ZnOEP-modified Au(111) electrode in 0.1 M HClO4. No noticeable peak was observed in the potential range between 0 and 0.7 V. After (25) (a) Yoshimoto, S.; Inukai, J.; Tada, A.; Abe, T.; Morimoto, T.; Osuka, A.; Furuta, H.; Itaya, K. J. Phys. Chem. B 2004, 108, 1948. (b) Yoshimoto, S.; Tada, A.; Itaya, K. J. Phys. Chem. B 2004, 108, 5171. (26) Yoshimoto, S.; Narita, R.; Tsutsumi, E.; Matsumoto, M.; Itaya, K.; Ito, O.; Fujiwara, K.; Murata, Y.; Komatsu, K. Langmuir, 2002, 18, 8518.

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Figure 1. Cyclic voltammograms of (a) bare Au(111) (dotted line) and C60Fc-adsorbed Au(111) (red solid line) and (b) ZnOEPadsorbed (dotted line) and C60Fc/ZnOEP-adsorbed (red solid line) Au(111) electrodes in pure 0.1 M HClO4. The scan rate was 50 mV s-1.

Figure 2. (a) Large-scale (30 × 30 nm2) and (b) high-resolution (10 × 10 nm2) STM images of a C60Fc adlayer formed directly on Au(111), acquired at 0.8 V vs RHE in 0.1 M HClO4. Tip potentials and tunneling currents were 0.44 V and 5.3 nA for panel a and 0.44 V and 2.0 nA for panel b, respectively.

subsequent immersion of the electrode into a ca. 50 µM C60Fc benzene solution for 1 min, a decrease in doublelayer charging current was observed, and a pair of characteristic redox peaks clearly appeared at 0.78 V during cathodic and anodic scans, respectively (red line in Figure 1b). Also, the increase of anodic current commencing at 0.95 V was suppressed by the adsorption of ZnOEP. This result suggests that C60Fc was attached on the ZnOEP-modified Au(111) surface. The redox potential of the ferrocene moiety reported by several groups17-21 indicates that the CV profile drawn with a solid red line in Figure 1b is associated with the electrochemical redox reaction of Fc to Fc+ in each molecule of C60Fc. From the electronic charge calculated from the oxidative peak area, the amount of electronic charge transferred was estimated to be ca. 8.0 µC cm-2. If a oneelectron redox reaction occurs on the C60Fc/ZnOEPmodified Au(111) electrode, the value corresponds to the surface concentration of (8.1 ( 0.3) × 10-11 mol cm-2. The peak separation was almost immeasurable in Figure 1b, indicating that the electron-transfer process is very rapid. Figure 2a shows a typical STM image of an adlayer of C60Fc formed directly on clean Au(111) in 0.1 M HClO4. This STM image shows a disordered structure of C60Fc directly attached to the surface of Au(111), although

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individual molecules of C60Fc can be discriminated as bright spots. This feature indicates that the C60Fc molecules were randomly oriented on the Au(111) surface. It is considered that the interaction between C60Fc and the Au surface is very strong, as the Fc and C60 moieties in a C60Fc molecule are linked with the pyrolle moiety with a N atom in the cyclic ring. These characteristics of the modified Au(111) surface are also seen in the CV profile shown in Figure 1a. A similar STM image was obtained with the open-cage C60 derivative directly adsorbed on a Au(111) surface.14a We rarely could find small ordered domains of C60Fc molecules when only a freshly prepared benzene solution containing C60Fc molecules was used for the modification. A high-resolution STM image of highly ordered arrays of C60Fc molecules is shown in Figure 2b. This image shows two or three protrusions in each bright spot, indicating the presence of Fc groups. Molecules of C60Fc are aligned to the close-packed direction indicated by the blue arrow with an intermolecular distance of 1.05 ( 0.05 nm. The average distance between the nearest neighbor rows indicated by the red arrow was found to be 2.03 ( 0.08 nm. The unit cell is superimposed as a parallelogram. According to the paper by Klusek’s group, the adlayer of a similar molecule, ferrocene-cycloadducted C60 derivative (C60ONCFn), was observed on highly oriented pyrolytic graphite (HOPG).27 An ordered array of C60ONCFn was observed as a structure consisting of single molecular chains. They assigned the observed large spots to fullerenes and the small ones to Fn groups. In our case, however, because small bright protrusions are seen in each bright spot, we assigned the Fc group in each C60Fc molecule to face the solution phase. On the contrary, highly ordered arrays consisting of bright round spots were found on the ZnOEP-modified Au(111) as shown in Figure 3a. Careful inspection revealed that each molecule has a dumbbell shape, suggesting that there is a difference in either electronic structure or molecular orientation of the C60Fc molecule. Such a feature has never been found in the supramolecularly assembled array of C60 molecules on the ZnOEP-modified Au(111) surface,14b although STM images are known to depend on tunneling conditions. The CV and STM images suggest that the electrochemical response of the ferrocene moiety is clearly enhanced on the highly ordered ZnOEP-modified Au(111) surface, but not on the unmodified Au(111) surface. Further details of the internal structure, orientation, and packing arrangement of the supramolecular assembled adlayer of C60Fc/ZnOEP on the Au(111) surface are revealed in the high-resolution STM image of an area of 8 × 8 nm2 shown in Figure 3b. Two or three protrusions were clearly observed in each bright spot, indicating the presence of Fc groups facing the solution phase in the same manner as was found with the open-cage C60 derivative array on the ZnOEP-modified Au(111) surface. The protrusions in each C60Fc molecule were independently oriented on the ZnOEP-modified Au(111) surface. From a cross-sectional profile, intermolecular distances between the nearest neighbor C60Fc molecules were found to be either 1.64 ( 0.07 or 1.41 ( 0.05 nm for directions along each molecular row, which are clearly different from the values obtained from Figure 2b. The surface concentration was estimated to be 8.0 × 10-11 mol cm-2, which is in good agreement with that calculated from Figure 1b. The corrugation height of C60Fc was greater than that of the ZnOEP layer on Au(111). Morphological details of each molecule are clearly displayed in Figure 3b. (27) Byszewski, P.; Klusek, Z.; Pierzgalski, S.; Datta, S.; Kowalska, E.; Poplawska, M. J. Electron Spectrosc. Relat. Phenom. 2003, 130, 25.

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Figure 3. (a) Large-scale (30 × 30 nm2) and (b) high-resolution (8 × 8 nm2) STM images of highly ordered C60Fc arrays on ZnOEP-modified Au(111), acquired at 0.75 V vs RHE in 0.1 M HClO4. Tip potentials and tunneling currents were 0.46 V and 0.35 nA, respectively.

When the tunneling current was changed in the middle of the scanning operation, the underlying ZnOEP layer on Au(111) was clearly visible, as shown in Figure 4a. The scan was made from the lower end of the image. Images of individual C60Fc molecules immediately disappeared upon changing the tunneling current, showing that the molecules were aggregated on the highly ordered ZnOEP adlayer. Actually, the top layer of C60Fc was easily removed by the operation of scanning at tunneling currents higher than 1.0 nA. Such an aggregation influenced by the scanning operation was also observed at the supramolecular C60-ZnOEP interface on the Au(111) surface.14b As described in our previous paper, the formation of opencage C60 derivative and the highly ordered ZnOEP adlayer with 1:1 supramolecular assembly was confirmed by stepping the tunneling current from 30 pA to 2.0 nA in the middle of scanning.14a These results indicate that the interaction between fullerenes and ZnOEP is very weak on the Au(111) surface. A close-up view of the underlying ZnOEP layer after several scans is shown in the highresolution STM image in Figure 4b. Careful inspection of Figure 4b allows one to distinguish between two different orientations of ZnOEP molecules in the molecular rows. Each ZnOEP molecule can be recognized as a circle with eight additional spots at the corners corresponding to eight ethyl groups. The adlayer structure of ZnOEP on Au(111) was identical to that of CoOEP or FeClOEP on Au(111) reported in our previous papers.25 Although the unit cell is superimposed in Figure 4b as a parallelogram with the lengths of 1.67 and 2.82 nm because of two different orientations of ZnOEP molecules, intermolecular spacings between the molecules in the rows in Figure 3b were measured to be 1.67 ( 0.07 and 1.42 ( 0.05 nm, which are in good agreement with the values observed in Figure 3a.

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A structural model is proposed in Figure 4c. The fact that the intermolecular distances between C60Fc molecules are nearly equal to the distance between ZnOEP molecules indicates that each C60Fc is located on the center above each ZnOEP molecule. This structure was consistently observed in the potential range between 0.75 and 0 V. Note that the ZnOEP adlayer remained unchanged upon the adsorption of C60Fc. Potential-dependent STM images are shown in Figure 5. Before the oxidation of C60Fc, well-defined arrays of C60Fc were observed at 0.7 V. When the electrode potential was modulated in the anodic direction, for example, at 0.9 V, the STM image gradually became unclear. As shown in Figure 5b, some bright clusters covered the highly ordered arrays of C60Fc, and the surface became slightly rough, that is, dark areas enlarged with the bright clusters. At more positive potentials, for example, 1.05 V, the surface was covered with clusters and mists, and areas of the dark areas were enlarged further. According to several papers on SAMs of ferrocenyl alkanethiolate on gold investigated by Fourier transform infrared (FT-IR) spectroscopy,20b,20c electrochemical quartz crystal microbalance (EQCM),20a,20b and Fourier transform surfaceenhanced Raman spectroscopy (FT-SERS),21 structural changes were caused by the redox reaction because of the incorporation of ClO4-. In our system, although molecular reorientation might also have taken place because of the oxidation of Fc to Fc+ in C60Fc, no such details were observed in the present experiment. Highly ordered arrays clearly reappeared on the terrace when the potential was returned to 0.75 V, where Fc was present in the reduced form. On the contrary, the identical adlayer structure of C60Fc on the highly ordered ZnOEP-modified Au(111) surface was seen in the potential region between 0.8 and 0 V. As shown in our previous papers, highly ordered supramolecular assembled layers consisting of either C60 or open-cage C60 derivative and ZnOEP on a Au(111) surface were also observed even at 0 V versus RHE,14 suggesting that either hydrogen evolution reaction or desorption of ZnOEP is blocked with fullerene thin films. To elucidate details of the effect of the central metal ion in OEP, OEPs with other metals such as CoOEP, CuOEP, and FeClOEP were investigated as an underlying adsorbed layer. Figure 6 shows cyclic voltammograms of C60Fc on CoOEP-, CuOEP-, and FeClOEP-modified Au(111) electrodes. It is seen from panels a and b in Figure 6 that the double-layer charging currents of CoOEP and CuOEP on Au(111) electrodes (dotted lines) were suppressed by the adsorption of C60Fc molecules. Especially, Figure 6a shows that the reduction of Co(III) to Co(II) of the CoOEP layer observed at 0.32 V disappeared, suggesting that the CoOEP layer was entirely covered with C60Fc molecules. Judging from the oxidative peak area, the electronic charges consumed during the oxidation were calculated to be 6.1 and 4.8 µC cm-2 for CoOEP and CuOEP layers, respectively. The electronic charge of CuOEP consumed by the oxidation reaction of Fc to Fc+ was estimated to be slightly lower than that of CoOEP. Furthermore, when FeClOEP was used as an underlying adsorbed layer, a clearly different CV was obtained. As shown in Figure 6c, the redox peak currents observed at 0.79 V were much smaller than those of ZnOEP-, CoOEP-, and CuOEPmodified Au(111) electrodes, and another reductive peak appeared at 0.41 V corresponding to the reduction of Fe(III) to Fe(II) in FeClOEP. This result suggests that the surface was not fully covered with C60Fc molecules. To understand details of the interfacial phenomena, STM measurements were carried out in 0.1 M HClO4 for each case. Highly ordered arrays of each MOEP monolayer

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Figure 4. (a) Composite STM image (30 × 30 nm2) and (b) high-resolution STM image (8 × 8 nm2) of the underlying layer of a C60Fc/ZnOEP supramolecular assembled layer on Au(111) in 0.1 M HClO4 acquired at 0.75 V. Tip potentials and tunneling currents were 0.35 V and 0.25 nA (lower half) and 1.0 nA (upper half) for panel a and 0.46 V and 1.8 nA for panel b, respectively. (c) Structural model for the C60Fc/ZnOEP supramolecular assembled adlayer.

Figure 5. Large-scale STM images (70 × 70 nm2) of a C60Fc array on the ZnOEP-modified Au(111) surface acquired at (a) 0.75 V, (b) 0.85 V, and (c) 1.05 V versus RHE in 0.1 M HClO4. Tip potentials and tunneling current were 0.46 V and 0.55 nA, respectively.

on the Au(111) surface were clearly observed in the highresolution STM images shown in Figure 7a-c. Hexagonal molecular arrangements were formed by the same procedure for both CoOEP and CuOEP (see the highresolution STM images shown in Figure 7a,b). Both molecular arrays are two-dimensionally well organized. Each molecule is seen as a propeller-shaped image with the brightest spot at the center and eight additional bright spots at the corners of each molecule. Only one difference was found in STM images between CuOEP and CoOEP (or FeClOEP): that is, the center spot of each CuOEP appeared as a dark spot, whereas that of CoOEP (or FeClOEP) was bright. This difference can be explained in terms of the difference in the mode of the dz2 orbital. The adlattices were identical to each other. As reported by Hipps’ group in UHV28 and by our group in solution,29 the center of each CoPc molecule appears as the brightest spot in the STM image because of the tunneling being mediated by a half-filled dz2 orbital between the Au surface

and the tip. However, at 0.8 V the central Co ion is in the state of Co(III) formed by the electrochemical oxidation (Figure 6a). Because the dz2 orbital should be empty in this oxidation state, another mechanism must be considered to explain the tunneling at the center of the Co(III)OEP molecule. It is generally known that Co(III) porphyrin is unstable, and to make it more stable, the addition of an axial ligand, such as O2 like a superoxide anion, O2-, is necessary.30a Furthermore, water molecules (28) (a) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197. (b) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem. 1996, 100, 11207. (c) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391. (d) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 11899. (e) Scudiero, L.; Barlow, D. E.; Mazur, U.; Hipps, K. W. J. Am. Chem. Soc. 2001, 123, 4073. (29) (a) Yoshimoto, S.; Tada, A.; Suto, K.; Narita, R.; Itaya, K. Langmuir 2003, 19, 672. (b) Yoshimoto, S.; Tada, A.; Suto, K.; Itaya, K. J. Phys. Chem. B 2003, 107, 5836. (c) Suto, K.; Yoshimoto, S.; Itaya, K. J. Am. Chem. Soc. 2003, 125, 14976. (d) Yoshimoto, S.; Suto, K.; Tada, A.; Kobayashi, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8020. (e) Yoshimoto, S.; Higa, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8540.

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Figure 6. Cyclic voltammograms of C60Fc on (a) CoOEP-, (b) CuOEP-, and (c) FeClOEP-modified Au(111) (red solid line) in pure 0.1 M HClO4. The dotted lines show voltammograms of the underlying MOEP-modified Au(111) electrode. The scan rate was 50 mV s-1.

might coordinate with Co(III)Pc in an aqueous solution, because complexes such as [CoTPP(H2O)2]ClO4 or [CoOEP(H2O)2]ClO4 are also known.30b Since our STM measurements were carried out in solution containing air, it is feasible that O2 molecules dissolved in solution were eventually attached to Co(III) ions to stabilize the Co(III)OEP molecule. It is known that the stronger σ-donating axial ligands lead to the smaller Co hyperfine coupling, promoting the localization of π-electrons on the oxygen.30a This π orbital might act as a new tunneling pathway, which could increase the tunneling current at the central position of Co(III)OEP. This accounts for the observation that the central cobalt ion in each CoOEP molecule appeared bright in the image, whereas the copper ion in each CuOEP molecule appeared dark. The center of each FeClOEP molecule was also observed as a bright spot. At this potential, the central Fe ion is in the state of Fe(III) because of the coordination with Cl- anion; that is, the dz2 orbital should be empty. Chloride may act as a new tunneling pathway, which could increase the tunneling current at the central position of Fe(III)Cl.25b Figure 7d-f shows typical STM images of the C60Fc arrays formed on CoOEP-, CuOEP-, and FeClOEPmodified Au(111) surfaces, respectively. As can be seen in Figure 7d,e, C60Fc molecules were hexagonally arranged. Adlattices of C60Fc on the CoOEP and CuOEP adlayers were almost identical to that on the ZnOEP adlayer. The STM images shown in Figure 7d,e indicated that C60Fc molecules entirely covered both surfaces. The difference in the electronic charge found on the CVs in Figure 6a,b might be attributable to the difference in the electronic state of the central metal ion between Co(II) and Cu(II) ions. These results suggest that the central metal ions significantly affect the electron-transfer reac(30) (a) Porphyrins and metalloporphyrins; Smith, K. M., Ed.; Elsevier/North-Holland Biomedical Press: Amsterdam, 1976. (b) Brothers, P. J. In Advances in Organometallic Chemistry; Stone, F. G. A., West, R., Eds.; Academic Press: San Diego, 2000; Vol. 46, pp 223321.

Figure 7. High-resolution STM images (15 × 15 nm2) of (a) CoOEP, (b) CuOEP, and (c) FeClOEP underlying layers on a Au(111) surface and (d-f) large-scale (30 × 30 nm2 for panels d and e and 50 × 50 nm2 for panel f) STM images of C60Fc arrays on (d) CoOEP-, (e) CuOEP-, and (f) FeClOEP-modified Au(111) acquired at 0.75 V vs RHE in 0.1 M HClO4. Tip potentials and tunneling currents were 0.46 V and 0.35 nA for panel a, 0.46 V and 0.36 nA for panel b, and 0.46 V and 0.38 nA for panel c, respectively.

tion between the Fc moiety in C60Fc and MOEP molecules. Such metal ion dependence was reported in the investigation on affinity between the metalloporphyrin cyclic dimer and fullerene. According to the previous paper by Zheng et al., the association constant, Kassoc, values for the metalloporphyrin cyclic dimer of Co and Zn ions were greater than 107 M-1, whereas the other three host molecules with other metal ions such as Ni, Cu, and Ag were inferior to the above host molecules (Kassoc < 107 M-1).31 Figure 7f shows a typical large-scale STM image of a C60Fc array on the FeClOEP-modified Au(111) surface. The scan area was 50 × 50 nm2. Although some ordered areas are visible, the entire surface was rough. After the potential was modulated to values more negative than 0.4 V, a disordered structure of C60Fc appeared on the surface. This is attributed to the mobility of underlying FeClOEP molecules. As described in our recent paper, FeClOEP molecules are highly mobile due to the reduction of central iron(III) to iron(II) at potentials more negative than 0.4 V,25b although FeClOEP molecules also formed a hexagonally packed adlayer similar to adlayers of ZnOEP, CoOEP, and CuOEP. Thus, the coordination of (31) Zheng, J. Y.; Tashiro, K.; Hirabayashi, Y.; Kinbara, K.; Saigo, K.; Aida, T.; Sakamoto, S.; Yamaguchi, K. Angew. Chem., Int. Ed. 2001, 40, 1858.

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Cl is likely to affect significantly the fullerene and porphyrin supramolecular assembly on metal surfaces. Conclusions By immersing a Au(111) substrate successively into a benzene solution containing ZnOEP and one containing C60Fc, a well-defined electrochemical response of C60Fc was obtained at 0.78 V versus RHE in 0.1 M HClO4 aqueous solution as a result of the formation of a supramolecular assembly of C60Fc and ZnOEP. In situ STM observation revealed that highly ordered C60Fc arrays with 1:1 composition formed on the ZnOEPmodified Au(111) surface in the reduced form, whereas surface morphology showed a structural change at the potentials of the oxidized form. Electrochemical responses of the ferrocene group of the C60Fc molecule were clearly observed on the ZnOEP-, CoOEP-, and CuOEP-modified

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Au(111) surfaces, whereas C60Fc molecules on the FeClOEP-modified Au(111) surface hardly exhibited any electrochemical response because of the coordination of Cl on the Fe ion. The effect of the central metal ion in OEP on the formation of supramolecular assemblies with C60Fc molecules was clearly demonstrated in this study. 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 the Encouragement of Young Scientists (No. 16750106), and the Center of Excellence (COE) Project, Giant Molecules and Complex Systems, 2004. The authors acknowledge Dr. Y. Okinaka for his assistance in writing the manuscript. LA047795Y