Formation of Supramolecular Nanobelt Arrays Consisting of Cobalt (II

Since “picket-fence” porphyrin has bulky functional groups and a unique O2 coordination ability, it is attractive as a material to study for under...
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Langmuir 2007, 23, 809-816

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Formation of Supramolecular Nanobelt Arrays Consisting of Cobalt(II) “Picket-Fence” Porphyrin on Au Surfaces Soichiro Yoshimoto,*,†,§ Kazuhiro Sato,† Shoko Sugawara,† Yu Chen,‡,| Osamu Ito,‡ Takahiro Sawaguchi,§ Osamu Niwa,§ and Kingo Itaya*,† Department of Applied Chemistry, Graduate School of Engineering, Tohoku UniVersity, 6-6-07 Aoba, Sendai 980-8579, Japan, Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Aoba-ku, Sendai 980-8577, Japan, and National Institute of AdVanced Industrial Science and Technology (AIST), Central 6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan ReceiVed June 16, 2006. In Final Form: August 22, 2006 Adlayers of cobalt(II) 5,10,15,20-tetrakis(R,R,R,R-2-pivalamidophenyl)porphyrin (CoTpivPP) were prepared by immersing either Au(111) or Au(100) substrate in a benzene solution containing CoTpivPP molecules, and they were investigated in 0.1 M HClO4 and 0.1 M H2SO4 by cyclic voltammetry and in situ scanning tunneling microscopy (STM). The adlayer structure and electrochemical properties of CoTpivPP are compared to those of 5,10,15,20tetraphenyl-21H,23H-porphine cobalt(II) (CoTPP). Characteristic nanobelt arrays consisting of CoTpivPP molecules were produced on both Au(111) and Au(100) surfaces. The stability of the nanobelt arrays was controlled by manipulating the electrode potential. On the other hand, the formation of nanobelt arrays consisting of O2-adducted CoTpivPP molecules depended upon the crystallographic orientation of Au. The state of O2 trapped in the cavity of CoTpivPP was distinctly observed in STM images as a bright spot in the nanobelt array formed on reconstructed Au(100)-(hex) surface, but not on Au(111) surface. This result suggests that the arrangement of underlying Au atoms plays an important role in the formation of nanobelt arrays with the sixth ligand coordination.

Introduction Porphyrin assembly is now recognized as an important material in key technologies such as photoelectronics and fuel cells.1-6 Especially in the field of electrochemistry, thin films of metalloporphyrin and metallophthalocyanine derivatives have been intensively studied because of the interest in electrocatalytic reactions such as the reduction of O2 for developing efficient fuel cells. Such reactions have been investigated mainly at graphite electrodes.5-7 However, very little attention has been paid so far to the adlayer structure of those porphyrins; thus, the relationship between the adlayer structure and the electrocatalytic activity is still unclear at the molecular level. The characterization of ordered adlayers of porphyrin and phthalocyanine molecules on metal * To whom correspondence should be addressed. E-mail: [email protected] (S. Yoshimoto); [email protected] (K. Itaya). Phone/Fax: +81-29-861-6167/6177 (S. Yoshimoto). † Graduate School of Engineering, Tohoku University. ‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. § AIST. | Present address: Lab for Advanced Materials, Department of Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China. (1) Balzani, V. Electron Transfer in Chemistry; Wiley-VCH: New York, 2001; Vol. 3. (2) Guldi, D. M. Chem. Soc. ReV. 2002, 31, 22. (3) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem. Photobiol., C 2004, 5, 79. (4) Boyd, P. D. W.; Reed, C. A. Acc. Chem. Res. 2005, 38, 235. (5) (a) Collman, J. P.; Marrocco, M.; Denisevich, P.; Konai, Y.; Koval, C.; Anson, F. C. J. Electroanal. Chem. Interfacial Electrochem. 1979, 101, 117. (b) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F. C. J. Am. Chem. Soc. 1980, 102, 6027. (c) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 1537, and references therein. (6) (a) Zagal, J.; Sen, R. K.; Yeager, E. J. Electroanal. Chem. Interfacial Electrochem. 1977, 83, 207. (b) Yeager, E. Electrochim. Acta 1984, 29, 1527, and references therein. (7) (a) Collman, J. P.; Fu, L. Acc. Chem. Res. 1999, 32, 455. (b) Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu, L. Chem. ReV. 2004, 104, 561.

surfaces in solution is therefore important for understanding the O2 reduction mechanism. It is also known that porphyrins and related derivatives constitute active centers, “hemes”, in metalloproteins, and it is necessary to understand their role and electrochemical properties in relation to the phenomenon such as dioxygen storage.7 Especially, metal-coordinated “picket-fence” porphyrins (MTpivPP) were synthesized by Collman’s group,8 and they were found to serve as a model for understanding the process of dioxygen storage by myoglobin and hemoglobin. The O2 binding affinity of MTpivPP) where M represents iron or cobalt ion, i.e., FeTpivPP or CoTpivPP (Chart 1), has been demonstrated to be much greater than that of native myoglobin. The binding affinity of O2 is controlled by the coordination of axial ligand to the vacant sixth coordination site. Electrochemical properties such as the electroreduction of O2 and the coordination of imidazole derivatives were examined on a CoTpivPP-coated graphite electrode by Anson’s group.9 On the other hand, supramolecular assemblies of organic molecules at metal surfaces have been explored to control surface properties.10-14 Porphyrins are considered to be promising building blocks for molecular architectures. The mode of formation of molecular assemblies such as dipole-dipole interaction, hydrogen bonding, metal-ligand coordination, and fullerene-porphyrin supramolecular assembly can be controlled (8) (a) Collman, J. P.; Gagne, R. R.; Halbert, T. R.; Marchon, J. C.; Reed, C. A. J. Am. Chem. Soc. 1973, 95, 7868. (b) Collman, J. P.; Gagne, R. R.; Reed, C. A.; Halbert, T. R.; Lang, G.; Robinson, W. T. J. Am. Chem. Soc. 1975, 97, 1427. (9) (a) Steiger, B.; Anson, F. C. Inorg. Chem. 2000, 39, 4579. (b) Zou, S.; Clegg, R. S.; Anson, F. C. Langmuir 2002, 18, 3241. (10) (a) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139. (b) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290. (11) Yoshimoto, S. Bull. Chem. Soc. Jpn. 2006, 79, 1167. (12) (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) Yoshimoto, S.; Inukai, J.; Tada, A.; Abe, T.; Morimoto, T.; Osuka, A.; Furuta, H.; Itaya, K. J. Phys. Chem. B 2004, 108, 1948. (d) Yoshimoto, S.; Tada, A.; Itaya, K. J. Phys. Chem. B 2004, 108, 5171.

10.1021/la061733l CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2006

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Chart 1. Chemical Structure of Cobalt(II) 5,10,15,20-tetrakis(r,r,r,r-2-pivalamidophenyl)porphyrin (Co(II)TpivPP)

by changing functional groups, underlying layers, or substrates.15-20 The inclusion of guest entities such as metal ions and small organic molecules into host systems for exploiting the formation of noncovalent bonds at metal surfaces has been attempted on the basis of the concept of supramolecular chemistry. Since “picket-fence” porphyrin has bulky functional groups and a unique O2 coordination ability, it is attractive as a material to study for understanding the assembly of “picket-fence” porphyrin at the molecular level not only for the supramolecular design of nanoarchitectures but also for the elucidation of the host-guest mechanism of O2 storage in biological systems. In the present work, not only to understand the biological function of molecular oxygen storage but also to explore the possibility of supramolecular design at electrode surfaces, we demonstrate, using in situ STM, that the manipulation of electrochemical potential allows us to control the coordination of O2 to central Co ion in a highly ordered cobalt(II) “picketfence” porphyrin (CoTpivPP) nanobelt array formed on Au electrode surfaces. The adlayers and electrochemical properties of CoTpivPP are compared to those of 5,10,15,20-tetraphenyl21H,23H-porphine cobalt(II) (CoTPP) whose framework is identical to that of CoTpivPP. (13) (a) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature (London) 2003, 424, 1029. (b) Theobald, J. A.; Oxtoby, N. S.; Champness, N. R.; Beton, P. H.; Dennis, T. J. S. Langmuir 2005, 21, 2038. (14) (a) 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. (b) Clair, S.; Pons, S.; Brune, H.; Kern, K.; Barth, J. V. Angew. Chem., Int. Ed. 2005, 44, 7294. (15) (a) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature (London) 2001, 413, 619. (b) Yokoyama, T.; Kamikado, T.; Yokoyama, S.; Mashiko, S. J. Chem. Phys. 2004, 121, 11993. (16) (a) Suto, K.; Yoshimoto, S.; Itaya, K. J. Am. Chem. Soc. 2003, 125, 14976. (b) S. Yoshimoto, S.; Higa, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8540. (c) Yoshimoto, S.; Yokoo, N.; Fukuda, T.; Kobayashi, N.; Itaya, K. Chem. Commun. 2006, 500. (d) Suto, K.; Yoshimoto, S.; Itaya, K. Langmuir 2006, 22, 10766. (17) (a) Hipps, K. W.; Scudiero, L.; Barlow, D. E.; Cooke, M. P., Jr. J. Am. Chem. Soc. 2002, 124, 2126. (b) Scudiero, L.; Hipps, K. W.; Barlow, D. E. J. Phys. Chem. B 2003, 107, 2903. (c) Barlow, D. E.; Scudiero, L.; Hipps, K. W. Langmuir 2004, 20, 4413. (18) (a) Elemans, J. A. A. W.; Lensen, M. C.; Gerritsen, J. W.; Kerpen, H.; Speller, S.; Nolte, R. J. M.; Rowan, A. E. AdV. Mater. 2003, 15, 2070. (b) Lensen, M. C.; van Dingenen, S. J. T.; Elemans, J. A. A. W.; Dijkstra, H. P.; van Klink, G. P. M.; van Koten, G.; Gerritsen, J. W.; Speller, S.; Nolte, R. J. M.; Rowan, A. E. Chem. Commun. 2004, 762. (c) van Gerven, P. C. M.; Elemans, J. A. A. W.; Gerritsen, J. W.; Speller, S.; Nolte, R. J. M.; Rowan, A. E. Chem. Commun. 2005, 3535. (19) (a) Yoshimoto, S.; Tsutsumi, E.; Honda, Y.; Ito, O.; Itaya, K. Chem. Lett. 2004, 33, 914. (b) Yoshimoto, S.; Tsutsumi, E.; Honda, Y.; Murata, Y.; Murata, M.; Komatsu, K.; Ito, O.; Itaya, K. Angew. Chem., Int. Ed. 2004, 43, 3044. (c) Yoshimoto, S.; Honda, Y.; Murata, Y.; Murata, M.; Komatsu, K.; Ito, O.; Itaya, K. J. Phys. Chem. B 2005, 109, 8547. (d) Yoshimoto, S.; Saito, A.; Tsutsumi, E.; D’Souza, F.; Ito, O.; Itaya, K. Langmuir 2004, 20, 11046. (e) Yoshimoto, S.; Sugawara, S.; Itaya, K. Electrochemistry 2006, 74, 175. (20) (a) Bonifazi, D.; Spillmann, H.; Kiebele, A.; de Wild, M.; Seiler, P.; Cheng, F.; Jung, T.; Diederich, F. Angew. Chem., Int. Ed. 2004, 43, 4759. (b) Spillmann, H.; Kiebele, A.; Sto¨hr, M.; Jung, T. A.; Bonifazi, D.; Cheng, F.; Diederich, F. AdV. Mater. 2006, 18, 275.

Figure 1. Typical cyclic voltammograms of bare Au(111) (dotted line), CoTPP-modified (blue line) and CoTpivPP-modified Au(111) (red line) electrodes in (a) 0.1 M HClO4 under N2 atmosphere and (b) 0.1 M HClO4 saturated with O2. The scan rate was 50 mV s-1.

Experimental Section Cobalt(II) 5,10,15,20-tetrakis(R,R,R,R-2-pivalamidophenyl)porphyrin (CoTpivPP) was synthesized by using the method described in the literature.8b The compound of 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) (CoTPP) was purchased from Aldrich and used without further purification. Benzene was obtained from Kanto Chemical Co. (Spectroscopy Grade). Au(111) and Au(100) singlecrystal electrodes were prepared by the Clavilier method.21 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 CoTpivPP were formed by immersing a Au electrode into a 10 µM CoTpivPP-benzene solution for 10-20 s, after annealing the Au electrode in a hydrogen flame and cooling in air (on a clean bench to avoid contamination). The CoTpivPPadsorbed Au(111) or Au(100) was then rinsed with ultrapure water, and it was transferred into an electrochemical STM cell and filled with either 0.1 M HClO4 or 0.1 M H2SO4 (Cica-Merck, ultrapure grade) prepared with ultrapure water (Milli-Q SP-TOC; g18.2 MΩ cm). 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 or O2 atmosphere. Electrochemical STM measurements were performed by using a Nanoscope E system (Digital Instruments, Santa Barbara, CA) with a tungsten tip etched in 1 M KOH. To minimize the residual faradaic current, 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).11,12

Results and Discussion Figure 1a shows typical cyclic voltammogram (CV) of a welldefined Au(111) (dotted line) and that of CoTpivPP directly attached to the Au(111) electrodes (red solid line) in 0.1 M HClO4. Both CVs were recorded at the scan rate of 50 mV s-1. The voltammogram for bare Au(111) in the double-layer potential region is identical to that reported previously,22 indicating that (21) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (22) (a) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. Electrochim. Acta 1986, 31, 1051. (b) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429.

Arrays of Co(II) Porphyrin on Au Surfaces

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a well-defined Au(111) surface was exposed to the solution. The blue line in Figure 1a is the CV obtained in the first scan of a CoTPP-adsorbed Au(111) electrode. The open circuit potential (OCP) of the CoTPP-adsorbed Au(111) electrode was around 0.70-0.85 V, and the potential scan was made in the negative direction from the OCP. The effect of the adsorption of CoTPP was clearly observed in the double-layer charging current. The decrease in double-layer charging current in Figure 1a (blue line) suggests that the Au(111) surface was covered with hydrophobic CoTPP molecules. A reductive peak was seen at 0.32 V during the cathodic scan. The increase in cathodic current commencing at -0.05 V is due to the H2 evolution reaction. An oxidation peak broader than that of the cathodic peak was seen in the potential range between 0.30 and 0.70 V during the anodic scan. Murray’s group performed electrochemical investigations of a self-assembled monolayer of a CoTPP derivative having four thiol moieties, 5,10,15,20-tetrakis(o-(2-mercaptoethoxy)phenyl)porphyrin (Co(o-TMEPP)) on evaporated Au films.23 The Co(o-TMEPP)-modified Au electrode exhibited the redox wave of Co(III/II) couple at ca. 0.15 V vs SCE in 1 M HClO4 at the scan rate of 50 mV s-1. The Co(III/II) couple was identified on a transparent Au electrode by spectroelectrochemical measurement combined with UV-visible spectra.23b The CV profile obtained in the present study was similar to that of the previous study. Furthermore, even in a nominally deaerated acid solution, Murray and co-workers found that a small amount of residual oxygen fully oxidized the Co(II) (o-TMEPP) monolayer formed on Au within 10 s.23b In our case, the oxidation to Co(III) probably took place on the CoTPP-adsorbed Au(111) electrode at or near the OCP in 0.1 M HClO4, because the electrode was transferred into the electrochemical cell after rinsing with undeaerated ultrapure water. The cathodic peak observed at 0.32 V in Figure 1a, therefore, is attributed to the reduction of Co(III) to Co(II). On the other hand, in the presence of CoTpivPP, no peak was observed in the potential range between -0.05 and 0.90 V, suggesting that the central Co ion in each TpivPP cavity was in the state of Co(II). On the other hand, the O2 reduction at the CoTpivPP-modified Au(111) electrode was examined in O2-saturated 0.1 M HClO4, and the CV obtained was compared to those of bare and CoTPPmodified Au(111) electrodes. On the bare Au(111) electrode, the cathodic current for the reduction of O2 began to appear at ca. 0.55 V, and it increased gradually as the potential was scanned from 0.50 to -0.10 V. For porphyrin-modified Au(111) electrodes, a slight difference was observed in electrocatalytic activity for the reduction of O2 between CoTPP- and CoTpivPP-modified Au(111) electrodes. On the CoTPP-modified Au(111) electrode, an enhancement of O2 reduction current was observed at 0.32 V. For the CoTpivPP-adsorbed Au(111) electrode, the catalytic O2 reduction current commenced at ca. 0.65 V during the cathodic scan, exhibiting a clear electrocatalytic O2 reduction peak at 0.30 V. At potentials more negative than 0.30 V, the reductive current remained almost constant, but a slight increase was observed in the potential range between 0.10 and -0.15 V. However, the current density for the CoTpivPP-modified Au(111) electrode was smaller than that for the CoTPP-modified Au(111) electrode. A similar difference in electrocatalytic activity for O2 was observed at modified, edged-plane graphite electrodes.9a From the current density (ca. 0.4 mA cm-2) and the analysis using the rotating disk electrode (RDE) method, it is assumed

that the two-electron reduction of O2 to H2O2 occurred on the CoTPP-modified Au(111) electrode, as reported in our previous paper.12a It is likely that a similar electrocatalytic reaction took place on the CoTpivPP-modified Au(111) electrode. The observed current density for O2 reduction is consistent with the result reported by Porter’s group on water-insoluble cobalt porphyrins having thiol moieties chemisorbed onto Au surfaces.24 Figure 2 shows typical STM images of the CoTPP adlayer on Au(111). In the image for the large area of 50 × 50 nm2 in Figure 2a, it is seen that atomically flat terraces are extended clearly with steps of monatomic height. The atomically flat terraces of the Au(111) surface are completely covered with several domains consisting of highly ordered CoTPP molecules. Each CoTPP molecule is clearly recognized as a bright spot not only on the atomically flat terraces but also near the steps. As reported in our previous paper, the highly ordered domain of CoTPP was maintained even at potentials close to the hydrogen evolution potential.12a To clarify structural details of the CoTPP array on a Au(111) surface, a close-up view is shown in Figure 2b. Each CoTPP molecule is recognized in Figure 2b as a propeller-shaped image with the brightest spot at the center and four additional bright spots at the corners of each CoTPP molecule. The brightest spot at the center and the four additional spots can be attributed, respectively, to the cobalt ion and the phenyl moieties in the CoTPP molecule with flat-lying orientation. The intermolecular distance was found to be 1.41 ( 0.03 nm. Molecular rows cross each other at an angle of approximately 85°. It is clear that all CoTPP molecules possess the same orientation. Similar bright central spots have been reported also for CoPc25a and CoTPP25b molecules in UHV. It is known that the Co(II) ions create the brightest spots in the image because of a strong tunneling current passing through the partially filled dz2 orbital.25 In acidic solution, however, the central Co ion is in the state of Co(III) formed by

(23) (a) Hutchison, J. E.; Postlethwaite, T. A.; Murray, R. W. Langmuir 1993, 9, 3277. (b) Postlethwaite, T. A.; Hutchison, J. E.; Hathcock, K. W.; Murray, R. W. Langmuir 1995, 11, 4109. (c) Hutchison, J. E.; Postlethwaite, T. A.; Chen, C.-H.; Hathcock, K. W.; Ingram, R. S.; Ou, W.; Linton, R. W.; Murray, R. W.; Tyvoll, D. A.; Chng, L. L.; Collman, J. P. Langmuir 1997, 13, 2143.

(24) Zak, J.; Yuan, P.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772. (25) (a) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197. (b) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Am. Chem. Soc. 2001, 123, 4073.

Figure 2. (a) Large-scale (50 × 50 nm2) and (b) high-resolution (9 × 9 nm2) STM images of the CoTPP adlayer on Au(111) surface in 0.1 M HClO4, acquired at 0.85 V vs RHE. Potential of the tip and tunneling currents were 0.35 V vs RHE and 1.0 nA for (a) and 1.25 nA for (b), respectively. (c) A proposed model of O2--coordinated Co(III)TPP complex in the highly ordered CoTPP array on Au(111) in an acidic solution.

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electrochemical oxidation (Figure 1b). The cobalt ions might be observed as dark spots because an electron is lost within the dz2 orbital upon oxidation of Co(II) to Co(III) at 0.85 V. However, Co(III) porphyrin complexes are known to be unstable.26 In view of the fact that the CoTPP-modified Au(111) electrode was thoroughly rinsed with ultrapure water containing O2, it is conceivable that dioxygen species such as superoxide ion, O2-, became coordinated with Co(III) to form the more stable Co(III)CTPP-O2- complex (see Figure 2c).26 A species resembling Co(III)-O2- was recently reported to form on the electrodeposited tetrakis-4-sulfonatephenylporphyrin cobalt(II) (CoTSPP) adlayer by Camacho’s group.27 In fact, as shown in our recent paper on 5-(4-carboxyphenyl)-10,15,20-tri(phenyl)porphyrin cobalt(II) (CoCTPP) array, a slight difference in height was found between STM images recorded at different potentials, i.e., the corrugation height was approximately 0.4 nm at 0.70 V, whereas it was 0.3 nm at 0.20 V, indicating the coordination of oxygen species to the central Co(III) ion.16c The strong π-donating axial ligand led to smaller Co hyperfine couplings, and the π-electrons were localized on the oxygen moiety, which possibly provided a new tunneling pathway. The possibility of the formation of the Co(III)-O2- complex is also known for highly ordered cobalt(II) porphine (CoP),12c cobalt(II) octaethylporphyrin (CoOEP),12c cobalt(II) phthalocyanine (CoPc),12b and crown ether-substituted CoPc28 arrays on Au(111). It is noteworthy that an identical adlayer of CoTPP was found on the reconstructed Au(100)(hex) surface.29 Figure 3 shows typical STM images of CoTpivPP adlayers on Au(111) acquired at 0.85 V (near the open circuit potential) in 0.1 M HClO4. In the STM image of the large area of 125 × 125 nm2 shown in Figure 3a, the atomically flat terraces of the Au(111) surface are seen to be covered with several characteristic nanobelt arrays. The dark areas between the ordered belt arrays are covered with disordered CoTpivPP molecules. Nanobelt arrays consisting of CoTpivPP molecules are likely to be located on the reconstructed Au(111) surface, whereas disordered CoTpivPP molecules are seen on the unreconstructed (1 × 1) surface. These two different situations appear to have resulted from the unsymmetrical chemical structure of CoTpivPP. In the higherresolution STM image of Figure 3b, several bright spots are seen in the nanobelt arrays. Further details of the internal structure, orientation, and packing arrangement of the CoTpivPP adlayer are seen in the close-up view of the area of 9 × 9 nm2 in Figure 3c, in which individual CoTpivPP molecules are recognized to be square-shaped with four bright spots at the corners. Careful inspection reveals the presence of three small protrusions separated from each other in each bright spot, suggesting that they are tert-butyl groups in each CoTpivPP molecule, as marked by black circles in Figure 3c. A similar molecular image of tert-butyl groups was also found in the p-tert-butylcalix[4]arenedithiolate adlayer formed on Au(100)-(1 × 1) as reported in our previous paper.30 It is clear that all CoTpivPP molecules are oriented in the same direction on Au(111). Interestingly, the central area of each CoTpivPP molecule was found to be dark. (26) (a) Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier/NorthHolland 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 223-321. (27) Pe´rez-Morales, M.; de Miguel, G.; Mun˜oz, E.; Martı´n-Romero, M. T.; Camacho, L. Electrochem. Commun. 2006, 8, 638. (28) (a) Yoshimoto, S.; Suto, K.; Itaya, K.; Kobayashi, N. Chem. Commun. 2003, 2174. (b) Yoshimoto, S.; Suto, K.; Tada, A.; Kobayashi, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8020. (29) Yoshimoto, S.; Tada, A.; Suto, K.; Yau, S.-L.; Itaya, K. Langmuir 2004, 20, 3159. (30) Yoshimoto, S.; Abe, M.; Itaya, K.; Narumi, F.; Sashikata, K.; Nishiyama, K.; Taniguchi, I. Langmuir 2003, 19, 8130.

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Figure 3. (a) Large-scale (125 × 125 nm2), (b) middle-scale (40 × 40 nm2), and (c) high-resolution (9 × 9 nm2) STM images of highly ordered CoTpivPP array formed on Au(111) acquired at 0.85 V vs RHE in 0.1 M HClO4. Tip potentials and tunneling currents were 0.45 V vs RHE and 0.80 nA for panel (a), 0.30 V vs RHE and 0.75 nA for panels (b) and (c), respectively. (d) A proposed model of CoTpivPP array formed on Au surface.

As described in the literature, the central Co ion in each CoTPP molecule appeared bright in the STM image both in ultrahigh vacuum25 and in aqueous solution.12a In the case of CoTpivPP, it is difficult to detect the central Co ion because of the bulkiness of the “picket-fence”. However, as seen in the middle portion of Figure 3c, the framework of a CoTpivPP molecule is brighter in appearance. This is probably due to the coordination of O2 or related species to the central Co ion. From the cross-sectional profile, the intermolecular distances between CoTpivPP molecules aligned in the direction of the red arrow and that in the direction of the blue arrow were both measured to be 1.44 ( 0.05 nm. One CoTpivPP molecule is superimposed by a black square with four black circles in Figure 3c. As depicted in Figure 3d, CoTpivPP molecules are oriented with a nearly square packing arrangement, similarly to the cases of CoTPP, CuTPP, and ZnTPP on Au(111) and Au(100)-(hex) reported in our previous papers.12a,29,31 It should be noted that the CoTpivPP nanobelt arrays disappeared at potentials more negative than 0.60 V, indicating that CoTpivPP molecules were highly mobile on the Au(111) electrode surface. In fact, as shown in Figure 1b, the difference in the potential where the enhancement of O2 reduction current appeared at the CoTPP- and CoTpivPP-modified Au(111) electrodes might be attributable to the clear difference between the stabilities of CoTPP and CoTpivPP adlayers on the Au surface resulting, for example, from the face-to-face dimer formation. The dimer formation, which promotes the reduction of O2, might be caused by the high mobility of the species on the electrode surface. Such an enhancement of electrochemical activity was also observed with the iron(III) chloride octaethylporphyrin (FeClOEP) adlayer on Au(111).12d Similar nanobelt arrays consisting of CoTpivPP molecules on Au(111) were also observed in 0.1 M H2SO4, (31) Yoshimoto, S.; Tsutsumi, E.; Suto, K.; Honda, Y.; Itaya, K. Chem. Phys. 2005, 319, 147.

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Figure 4. Potential-dependent (75 × 75 nm2) STM images of highly ordered CoTpivPP nanobelt array on Au(111) in 0.1 M H2SO4. The images were acquired at (a) 0.88 V, (b) 0.95 V, (c) 1.00 V, and (d) 1.08 V vs RHE. Tip potential and tunneling current were 0.31 V vs RHE and 0.18 nA, respectively. Cyclic voltammograms of clean (dotted line) and CoTpivPP-modified (red line) Au(111) electrodes in 0.5 M H2SO4 are shown at the center. They were recorded at a scan rate of 50 mV s-1.

indicating that the adlayer structure of CoTpivPP is identical in HClO4 and in H2SO4. However, in 0.1 M H2SO4, the nanobelt arrays were found to be strongly dependent upon electrode potential. The nanobelt arrays were stable in the potential range between 0.70 and 0.90 V. At potentials more negative than 0.70 V, the nanobelt arrays disappeared because of the high mobility of CoTpivPP molecules on the Au surface, whereas they were destroyed when the potential was changed to a value more positive than 0.90 V. The precise manipulation of potential allowed us to fabricate partial nanobelt arrays, as shown in Figure 4. The width of the central nanobelt array formed at 0.88 V became narrow, and that part of the array which is marked by the white arrow sign in Figure 4a disappeared at 0.95 V, as shown in Figure 4b. Furthermore, at 1.00 V, nanobelt arrays became much smaller in size (see Figure 4c). Finally, Figure 4d shows that the ordered nanobelt arrays were completely absent on the Au surface at 1.08 V. This phenomenon might be due either to the oxidative desorption of CoTpivPP molecules from the Au surface or to the formation of a sulfate/bisulfate network adlayer by the displacement of the CoTpivPP molecules. In general, it is well-known that sulfate (or bisulfate) anions form a network with water molecules or hydronium cations with the so-called x3 × x7 structure.32-37 The decomposition of nanobelt arrays by the positive shift of potential might be caused by the displacement (32) (a) Magnussen, O. M.; Hagebo¨ck, J.; Hotlos, J.; Bhem, R. J. Faraday Discuss. 1992, 94, 329. (b) Magnussen, O. M. Chem. ReV. 2002, 102, 679. (33) Itaya, K. Prog. Surf. Sci. 1998, 58, 121.

of the sulfate (x3 × x7) network structure. In the CV profile drawn by the dotted gray line in Figure 4, the broad anodic peaks observed in 0.5 M H2SO4 at 0.52 and 0.70 V are attributed to the anion-induced reconstruction from (x3 × 22) to (1 × 1) and the adsorption of sulfate anions, respectively. The pair of spikes observed at 1.00 V is due to the order-disorder phase transition of adsorbed sulfate anions.32 A characteristic spike indicating the formation of a (x3 × x7) network structure was found at the CoTpivPP-modified Au(111) electrode at 1.06 V. The CV profile of this electrode was similar to that of the clean Au(111) electrode. Actually, we observed the (x3 × x7) network structure at 1.10 V (see Supporting Information, Figure S1), although a much clearer STM image of the (x3 × x7) network structure should have been obtainable in the clean environment as reported in our recent paper.37 It should be noted that no ordered nanobelt arrays were formed on the Au surface when the potential was returned to a value between 0.90 and 0.70 V. When the potential was held at a value more positive than 0.90 V, the CoTpivPP molecules were probably removed from the Au(111) surface as a result of their electrochemical oxidation. (34) Edens, G. J.; Gao, X.; Weaver, M. J. J. Electroanal. Chem. 1994, 375, 357. (35) Cuesta, A.; Kleinert, M.; Kolb, D. M. Phys. Chem. Chem. Phys. 2000, 2, 5684. (36) Dakkouri A. S.; Kolb, D. M. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; Chapter 10. (37) Sato, K.; Yoshimoto, S.; Inukai, J.; Itaya, K. Electrochem. Commun. 2006, 8, 725.

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Figure 5. (a) Large-scale (125 × 125 nm2) and (b) middle-scale (40 × 40 nm2) STM images of highly ordered CoTpivPP array formed on Au(100)-(hex) acquired at 0.80 V vs RHE in 0.1 M HClO4. Tip potentials and tunneling currents were 0.32 V vs RHE and 0.27 nA for panel (a), and 0.32 V vs RHE and 0.18 nA for panel (b), respectively.

In contrast, highly ordered CoTpivPP arrays were formed on the reconstructed Au(100)-(hex) surface, but not on the unreconstructed Au(100)-(1 × 1) surface. As shown in Figure 5a, several islands were formed in the gaps between nanobelt arrays consisting of CoTpivPP molecules, indicating that the lifting of reconstruction, i.e., the phase transition of “hex to 1 × 1”, occurred partially during the modification. The size of the nanobelt array consisting of CoTpivPP molecules was larger than that formed on Au(111), although the molecular packing arrangement of CoTpivPP on Au(100)-(hex) was almost identical to that on Au(111). On Au(100)-(hex), a nanobelt array was typically composed of 15 to 20 CoTpivPP molecules, as shown in Figure 5b. As reported in our previous paper, CoTPP molecules form highly ordered arrays on the Au(100)-(hex) surface.29 In view of the fact that the molecular size of CoTpivPP is almost the same as that of CoTPP, the formation of the highly ordered arrays of CoTpivPP is attributable to the match between the size and symmetry (and hence intermolecular distance) of those molecules on one hand and the periodicity of reconstructed rows of Au(100) on the other.11 It should be noted that nanobelt arrays were not formed on the metal-free TpivPP (H2TpivPP)-modified Au(111) and Au(100)-(hex) surfaces in 0.1 M HClO4. H2TpivPP molecules also formed highly ordered arrays on both Au(111) and Au(100) surfaces, but those surfaces were completely covered with highly ordered H2TpivPP molecules (see Supporting Information, Figure S2). The formation of characteristic nanobelt arrays appears to depend on the central metal ion in MTpivPP. To investigate the origin of the bright spots, the adlayers of CoTpivPP were prepared on both Au(111) and Au(100)-(hex) surfaces after bubbling O2 gas into a benzene solution containing CoTpivPP molecules for at least 3 h. Figure 6a,b shows typical STM images obtained at Au(111) and Au(100)-(hex), respectively. Remarkably, on the Au(111) surface, no ordered arrays were seen, whereas highly ordered CoTpivPP nanobelt arrays formed on the reconstructed Au(100)-(hex) surface. It was found that the formation of the adlayer of CoTpivPP containing an O2 molecule is dependent on the structure of underlying Au lattice. One possible reason for this dependence is that the potential of zero charge (pzc) of the latter electrode is different from the pzc values of unmodified Au(111) and Au(100). As reported by Kolb and co-workers, the values of pzc of Au(111)-(22 × x3) and Au(100)-(hex) are 0.32 and 0.30 V vs SCE, respectively, in 0.01 M HClO4 (note that the pzc values for Au(111)-(1 × 1) and Au(100)-(1 × 1) are 0.24 and 0.08 V vs SCE, respectively).38 Therefore, the subtle difference in the attractive force between (38) Kolb, D. M.; Schneider, J. Electrochim. Acta 1986, 31, 929.

Figure 6. Large-scale (125 × 125 nm2) STM images of CoTpivPP adlayers prepared by immersion of (a) Au(111) and (b) Au(100)(hex) into an O2-bubbled benzene solution of CoTpivPP. The STM images were acquired in 0.1 M HClO4 at (a) 0.85 V and (b) 0.80 V vs RHE. Tip potentials and tunneling currents were 0.35 V vs RHE and 0.75 nA for (a), and 0.35 V vs RHE and 0.23 nA for (b), respectively. Corresponding cyclic voltammograms of CoTpivPP-adsorbed Au(111) and Au(100)-(hex) electrodes in 0.1 M HClO4 under N2 atmosphere are shown in (c) and (d), respectively. The scan rate was 50 mV s-1. The dotted lines in (c) and (d) are CVs of the CoTpivPP-adsorbed electrode prepared under Ar atmosphere.

O2-adducted CoTpivPP and Au substrate appears to be tentatively explainable in terms of the difference in surface charge of the Au surfaces. The number of bright spots was found to increase in the nanobelt array consisting of CoTpivPP molecules on Au(100)-(hex) at 0.80 V, suggesting that these bright spots resulted from the coordination of O2 or related species to the central Co ion. This result indicates that O2-adducted CoTpivPP molecules significantly influence the formation of the highly ordered arrays and that the molecular assembly of O2-adducted CoTpivPP molecules depends on the arrangement of underlying Au atoms. As reported by Anson’s group, O2-adducted CoTpivPP is stabilized by the coordination of imidazole moieties.9b,39 In view of the report that oxygen is bound to CoTPP on N donating ligands such as self-assembled monolayers (SAMs) of 4-mercaptopyridine (4-PyS-) through axial coordination with the assistance of axial ligands after the oxidation of Co(II) to Co(III),40 the central Co ion might be in the form of Co(III) in O2-adducted CoTpivPP. Especially, because reconstructed rows of Au(100) exhibit a height difference larger than those of Au(111), the electronic charge distribution might be locally different, i.e., the formation of electron-donating surface is possible as in 4-PyS-SAMs. Therefore, the reconstructed surface of Au(100) stabilizes O2adducted CoTpivPP through the sixth axial ligand, suggesting that the interaction between the substrate and the vacant sixth coordination site on the central metal can significantly change the O2 binding affinity. This difference in STM image corresponds to the difference in CV between CoTpivPP-modified Au(111) and CoTpivPP-modified Au(100)-(hex) electrodes, as shown in Figure 6c,d. The peak expected to be due to O2 reduction from (39) Steiger, B.; Baskin, J. S.; Anson, F. C.; Zewail, A. H. Angew. Chem., Int. Ed. 2000, 39, 257. (40) Kalyuzhny, G.; Vaskevich, A.; Ashkenasy, G.; Shanzer, A.; Rubinstein, I. J. Phys. Chem. B 2000, 104, 8238.

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Figure 7. Time-dependent (a-c) and potential-dependent (d-f) STM images (60 × 60 nm2) of highly ordered CoTpivPP nanobelt array formed on Au(100)-(hex) at the same location. The images were obtained at (a-c) 0.80 V, (d) 0.78 V, (e) 0.73 V, and (f) 0.68 V vs RHE in 0.1 M HClO4. Tip potential and tunneling current were 0.25 V vs RHE and 0.23 nA, respectively. Panels (b) and (c) were taken, respectively, 2 and 3 min after panel (a) was recorded.

the “picket-fence” cavity was found on the Au(100)-(hex) electrode at approximately 0.35 V, whereas a much smaller current was observed at 0.25 V on the Au(111) electrode. These results indicate that the bright spots are due to the coordination of O2 or related species to the Co ion in the cavity of each CoTpivPP molecule. To understand further details on the bright spots, timedependent STM images were recorded at the same location. Careful comparisons among parts a-c of Figure 7 reveal that the positions of the bright spots changed with time. For example, it can be easily seen that the arrangements and the number of bright spots in the white dotted circle changed with time. This exchange reaction appears to proceed very rapidly, as was confirmed even when the tip was scanned at high rates. Furthermore, potential-dependent STM images are shown in Figure 7d-f. A slight negative shift in potential caused the number of bright spots in the nanobelt array to decrease, e.g., the number of bright spots at 0.78 V was less than that at 0.80 V. When the electrode potential was held at 0.73 V (Figure 7e), there was a further decrease in the number of bright spots in the CoTpivPP nanobelt array. Bright spots gradually decreased in number with exchanges of the positions in the nanobelt array upon changing the potential further in the negative direction (Figure 7f). Finally, bright spots were no longer observed in the nanobelt array at potentials more negative than 0.65 V. These results show that the reaction of O2 release and the electronic state of adducting O2 or related species can be controlled in the CoTpivPP nanobelt array by manipulating the potential. One possible cause of this potential effect is the exchange reaction of trapped O2 molecule or related species in the nanobelt array, as depicted in Figure 8a. However, Anson’s group reported that the coordination of O2 to CoII ion is slower for CoIITpivPP than for CoIITPP.9a In addition, the CVs in Figure 6d show that the electrochemical reduction

Figure 8. Proposed models explaining the mobility of bright spots in the nanobelt array consisting of CoTpivPP molecules on the reconstructed Au(100)-(hex) surface in 0.1 M HClO4. (a) Exchange reaction of O2 and (b) rapid location exchange of activated O2 by electron donation from the Au surface.

of O2 does not proceed in the potential range between 0.80 and 0.60 V. The O2 molecule or related species might be trapped in the cavity of most of the CoIITpivPP molecules in the nanobelt array, although it is not visible in the STM image. Thus, the appearance of the bright spots could be explained alternatively in terms of the rapid donation of electrons from the Au substrate to the nanobelt array consisting of O2-adducted CoTpivPP, i.e., O2-trapped CoTpivPP molecules are activated by electron donation from the Au substrate (Figure 8b). Although further investigations are needed, it is most likely that those bright spots observed in

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the STM images are due to O2-adducted CoTpivPP molecules. We further examined the potential dependence of the CoTpivPP adlayer on Au(100)-(hex). No bright spots were found in the one-dimensional molecular belt array consisting of CoTpivPP in the potential range between 0.65 and 0.20 V, suggesting that the O2 molecules included in CoTpivPP cavities were completely released by their electrochemical reduction. It was found that the number of bright spots did not increase at least within 20 min after the potential was returned to 0.80 V. Because O2 molecules must be reduced at 0.20 V according to the CV profile shown in Figure 1b, the trapped O2 molecule in each CoTpivPP molecule is completely released into the solution. The released O2 molecules probably diffused into the solution phase. These results support the contention that the axial coordination of O2 or related species to the organized adlayer consisting of CoTpivPP molecules in the aqueous solution is different from that in the benzene solution, i.e., it is difficult for the released O2 molecules to return to CoTpivPP cavities in the aqueous solution. In contrast, the STM image became unclear at potentials more negative than 0.15 V, indicating that CoTpivPP molecules became mobile on the Au(100)-(hex) surface during the O2 reduction. Although the nanobelt array was observed at 0.20 V at tunneling currents lower than 0.30 nA, the adlayer disappeared at tunneling currents higher than 0.50 nA. This result indicates that the interaction between CoTpivPP molecule and Au(100)-(hex) surface becomes weaker as the potential is shifted to the negative direction.

Conclusions The adlayers of CoTpivPP on Au(111) and Au(100)-(hex) prepared from a solution phase contain characteristic nanobelt arrays, whereas CoTPP formed highly ordered two-dimensional arrays. The adlayer structure of CoTpivPP in the nanobelt array was identical to that of CoTPP. The molecules of O2-adducted CoTpivPP were clearly different, i.e., the formation of a nanobelt

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array consisting of O2-adducted CoTpivPP molecules depended upon the crystallographic orientation of Au. The state of molecular oxygen trapped in the cavity of CoTpivPP was distinctly observed in STM images as a bright spot in the nanobelt array formed on reconstructed Au(100)-(hex) surface, but not on the Au(111) surface. These results suggest that the arrangement of underlying Au atoms plays an important role as the sixth ligand coordination sites assisting the formation of O2-adducted CoTpivPP. The bright spots observed in the STM images are attributed to O2-adducted CoTpivPP molecules, although a further investigation using a technique such as surface plasmon resonance is needed to understand these results more fully. Potential manipulation made it possible to perform precise control of the electrochemical O2 reduction in the cavity of each CoTpivPP molecule. The nanobelt array consisting of CoTpivPP may be useful for O2 storage and/or for the design of sensors at the nanoscale. Acknowledgment. This work was supported in part by the Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency (JST), and by the Ministry of Education, Culture, Sports, Science and Technology, a Grant-in-Aid for Young Scientists (B) (no. 16750106/18750132) and the Center of Excellence (COE) Project, Giant Molecules and Complex Systems, 2006. The authors acknowledge Dr. Y. Okinaka for his assistance in writing this manuscript. Note Added after ASAP Publication. There was an error in the caption to Figure 6 in the version posted ASAP on November 29, 2006. The corrected version posted ASAP on December 5, 2006. Supporting Information Available: Additional experimental data. This material is available free of charge via the Internet at http://pubs.acs.org. LA061733L