Effect of the Formation of Highly Ordered Platinum(II

(7-9, 17-19) These reports have demonstrated that the stability of the first adlayer is an important factor in the design of surfaces for influencing ...
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Effect of the Formation of Highly Ordered Platinum(II) Octaethylporphyrin Adlayer on the Surface Reconstruction of Gold and Supramolecular Assembly of Fullerenes Soichiro Yoshimoto,*,†,§ Satoi Yasunishi,‡ and Teppei Kawamoto‡ †

Priority Organization for Innovation and Excellence and ‡Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan § Kumamoto Institute for Photo-Electro Organics (Phoenics), 3-11-38 Higashi-machi, Higashi-ku, Kumamoto 862−0901, Japan S Supporting Information *

ABSTRACT: The structures of 2D molecular assemblies of platinum(II) octaethylporphyrin (PtOEP) on Au(111) and Au(100) surfaces were examined using in situ scanning tunneling microscopy (STM) in 0.05 M HClO4 solutions. The in situ STM observations revealed that the reconstructed rows of Au(111) and Au(100) were stabilized through the formation of highly ordered arrays by adsorption of PtOEP in a wide potential range of the electric double-layer region and that the PtOEP overlayer expanded the stable potential range for the reconstructed phase. Furthermore, the supramolecular assembly of C60 and C84 on a highly ordered PtOEP adlayer formed on Au(111) surfaces was investigated. STM images revealed site-selective supramolecular assembly of fullerenes, especially C84, which exhibited a low coverage on the PtOEP adlayer. C60 molecules exhibited a full coverage and formed highly ordered adlayers on PtOEP, whereas C84 exhibited disordered arrays. The results of this study showed that the strong π-electron donation by the PtOEP molecules stabilized the reconstructed rows of single-crystal Au planes and that the PtOEP adlayer acted as a strong electron-donating layer, aiding the formation of supramolecularly assembled C60 molecules.

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INTRODUCTION Metalloporphyrins have been recognized as important materials with applications in a number of particularly significant fields, such as photoelectronics, catalytic systems, gas-sensing systems, and supramolecular architecture systems.1−6 Supramolecular assemblies using porphyrin derivatives have been investigated with the aim of potentially fabricating precisely controlled molecular wires and polymeric nanosheets.7−9 In particular, the supramolecular assembly of porphyrins and fullerenes is a subject attracting considerable interest.2,3,10 Special nanoarchitectures consisting of two-dimensionally assembled fullerenes on surfaces have been investigated for applications such as the preparation of nanopatterns and the incorporation of C60 through donor−acceptor interactions in the field of surface science.8,11−16 The assembly of such structures has been studied using scanning probe microscopy (SPM) techniques including scanning tunneling microscopy (STM) and atomic force microscopy (AFM).7−9,17−19 These reports have demonstrated that the stability of the first adlayer is an important factor in the design of surfaces for influencing the host−guest selectivity for fullerenes. Transition-metal ions such as Fe2+, Co2+, Cu2+, Ni2+, and Zn2+ are generally utilized in the assembly of metal ion-coordinated porphyrins.17,18 We have previously investigated the spontaneous assembly of highly ordered, waterinsoluble cobalt(II) tetraphenylporphyrin (CoTPP) and cobalt(II) octaethylporphyrin (CoOEP) molecular arrays on Au(111) surfaces using benzene solutions.20,21 Furthermore, the © 2014 American Chemical Society

structures of the CoTPP and CoOEP adlayers on the Au(111) surfaces in HClO4 solutions were determined to be identical to those previously obtained under ultrahigh vacuum (UHV) conditions.22,23 Subsequently, the structural investigation of the CoTPP and CoOEP adlayers was extended to Au(100) surfaces.24,25 The adlayer structures of CoOEP revealed two different molecular arrangements on reconstructed and unreconstructed Au(100) surfaces;25 therefore, it was concluded that the control and the fabrication of characteristic porphyrin molecular assemblies is dependent on the crystallographic orientation of Au. This offers a promising approach for the formation of precisely controlled nanoarchitectures. Porphyrins containing precious metal ions such as Pt2+ are also attractive materials for oxygen sensors, organic field effect transistors (FET), and electroluminescent light-emitting devices.26−28 However, to the best of our knowledge, there are no reports in the literature on two-dimensional (2D) molecular adlayers of platinum(II) octaethylporphyrin (PtOEP) and the supramolecular assembly of fullerene− platinum porphyrins. Supramolecular assembly through π−π or donor−acceptor interactions would be useful for energy and chemical conversion on electrode surfaces. From the standpoint Received: September 15, 2014 Revised: November 25, 2014 Published: November 25, 2014 29880

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of interfacial and supramolecular coordination programming,29,30 investigation on metalloporphyrin supramolecular assemblies should be extended to precious metal ions that contain electrons in the f orbital. In addition, it is interesting to understand the effect of π-electron donation by metalloporphyrins on the surface reconstruction of gold from the point of view of electrochemical surface science.31−33 It is known that the reconstruction phenomenon exhibited by single-crystal gold electrode surfaces is dependent on the concentration of the electrolyte solution, the type of anions in the solution, and the electrode potential.34−36 In the present study, we focus on the electron-donating ability of PtOEP from the standpoint of interfacial molecular assembly. Electrochemical scanning tunneling microscopy (ECSTM) was employed to examine the formation of PtOEP adlayers on Au(111) and Au(100) and the supramolecular assembly of C60 and C84 molecules on the PtOEP adlayer on Au(111) surfaces.

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Figure 1. Large-scale (125 × 125 nm2) STM images of (a) CuOEP and (b) PtOEP adlayers on Au(111) in 0.05 M HClO4 at (a) 0.85 V and (b) 0.80 V vs RHE. The tip potential was 0.45 V, and the tunneling currents were 1.25 and 0.45 nA for panels a and b, respectively.

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RESULTS AND DISCUSSION Figure 1 shows typical STM images of CuOEP and PtOEP adlayers on Au(111) surfaces, observed in 0.05 M HClO4 solutions. Long-range modulations are visible as rows on the surface in the STM images for both the adlayers. These rows, separated by a spacing ranging from 6.8 to 9.2 nm on the terrace over a large scan area, indicate the change of the underlying Au atoms from a (1 × 1) to a (√3 × 22) lattice configuration. This kind of a surface change in the topmost layer is called reconstruction. It is well-known that, in general, a thermally annealed Au(111) surface exhibits so-called “herringbone” structures under UHV conditions.39 Indeed, we were able to see a herringbone type reconstruction in the case of the CuOEP molecular adlayer in Figure 1a. In general, electrochemically induced reconstruction of bare Au(111) surfaces is observed at potentials more negative than the potential of zero charge (pzc). 36 Remarkably, in the present case, the reconstructed rows could be seen even at potentials close to the open circuit potential (OCP), which is nearly 0.85 V more positive than the pzc. This result suggests that the reconstruction took place as a result of the adsorption and formation of highly ordered arrays of CuOEP and PtOEP. As previously reported, a similar reconstruction of Au(111) surfaces has been observed in the cases of the formation of various metal ion-coordinated OEPs and tetraphenyl porphyrin adlayers,20−23,40−44 even at potentials near the OCP. Interestingly, in the case of PtOEP, we often found longrange straight rows throughout the PtOEP molecular adlayer, although herringbone type reconstruction was also observed in the region marked by a white dotted circle in Figure 1b. The appearance of long-range straight reconstructed rows is associated with the strong π-electron donation ability of PtOEP to the Au(111) surface. Because the electrons in Pt occupy not only the d orbital but also the f orbital, the ability of PtOEP to donate to the Au(111) surface is much stronger than those of FeClOEP40 and CoOEP.21 To investigate the structural details of PtOEP, higherresolution STM images were obtained and are shown in Figure 2. In the middle scan area shown in Figure 2a, the elbows of the reconstructed rows of Au(111) could still be clearly seen through the molecular adlayer of PtOEP. To obtain the structural details of the PtOEP adlayer, a high-resolution STM image was recorded at 0.80 V vs RHE. Figure 2b shows a molecular resolution STM image acquired in a 15 × 15 nm2

EXPERIMENTAL SECTION

PtOEP and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine copper(II) (CuOEP) were purchased from Aldrich and used without further purification. Fullerenes C60 (99.99%) and C84 (99%) were purchased from MTR Corp. and BuckyUSA, respectively, and used without further purification. Perchloric acid (HClO4, Cica-Merck, ultrapure grade) and benzene (spectroscopy grade) were obtained from Kanto Chemical Co. Ltd. Au(111) and Au(100) single-crystal electrodes were prepared by Clavilier’s method.37 The Au substrates were first annealed in a hydrogen flame and cooled in a clean bench for 3 min to avoid contamination, and the cleaned substrates were immersed in approximately 100 μM benzene solutions of either PtOEP or CuOEP for 10−20 s. Then, the PtOEP-modified Au(111) and Au(100) samples were dried and transferred into an electrochemical cell filled with 0.05 M HClO4 or an electrochemical STM cell. For the preparation of the supramolecular assembled fullerene layers, the PtOEP-modified Au(111) substrates were further immersed in either a 0.5−1.0 μM C60 solution in benzene for less than 5 s or a benzene solution saturated with C84 for duration of 10 s to 5 min. The concentration of C84 in benzene was estimated to be less than 1 μM (we could visually confirm some C84 particles). PtOEP layers fully covered with C60 were prepared by immersing a PtOEP-modified Au(111) electrode into an approximately 10 μM C60 solution in benzene for 10−20 s. The Au(111) substrates with C60 or C84 arrays on the PtOEP adlayer produced using the above method were finally rinsed with ultrapure water. Note that benzene was selected over toluene as a solvent because the adsorption of toluene on Au(111) is stronger than that of benzene.38 Electrochemical STM measurements were performed in 0.05 M HClO4 using a Nanoscope E system (Digital Instruments, Santa Barbara) with a tungsten tip (0.25 mm diameter) etched in 1 M KOH. The tips were coated with a transparent nail polish to minimize faradaic current. The STM images were obtained in the constant-current mode with a high-resolution scanner (HD-0.5I). All the values of potentials in the results of in situ STM measurements are reported against the reversible hydrogen electrode (RHE). 29881

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Figure 2. STM images of a single-component PtOEP adlayer on (a, b) Au(111) and (d, e) a reconstructed Au(100)−(hex) surface in 0.05 M HClO4 observed at (a, b) 0.80 V, (d) 0.75 V, and (e) 0.78 V vs RHE. The tip potential and the tunneling current were 0.35 V and 0.45 nA for panels a and b, 0.45 V and 0.98 nA for panel d, and 0.94 nA for panel e. The diagrams in the right-hand panels present a proposed model structure of PtOEP adlayers formed on (c) Au(111) and (f) Au(100)−(hex).

Table 1. Dependence on the Central Metal Ion in OEP Framework for the Molecular Packing Arrangement on Au SingleCrystal Surfacesa Co2+

orientation Au(111) Au(100)− (hex) a

21

alternate every fourth or fifth molecular row25

Cu2+

Zn2+

44

alternate every fourth or fifth molecular row25

12,44

alternate every fourth or fifth molecular row45

Pt2+ b

alternate alternate or every second molecular rowb

All MOEP adlayers were prepared by immersing a freshly annealed Au substrate into ca. 100 μM MOEP/benzene solution for 10 s. bPresent work.

on a reconstructed Au(100)−(hex) surface. Figure 2d shows typical STM images of PtOEP adlayers formed on Au(100)− (hex) surfaces. The PtOEP adlayers were formed by immersing the Au(100)−(hex) surfaces into 0.1 mM PtOEP solutions in benzene for 10 s. As seen in Figure 2d, the terrace was completely covered with highly ordered PtOEP molecular arrays. The homogeneous molecular adlayer formation suggests that a reconstructed Au(100) surface existed under the PtOEP adlayer over an area of 50 × 50 nm2. The PtOEP molecules were hexagonally arranged on the Au(100)−(hex) surface. A high-resolution STM image of the PtOEP adlayer on a Au(100)−(hex) surface is shown in Figure 2e. While this image looks complicated on a cursory glance, careful inspection revealed eight ethyl groups belonging to one PtOEP molecule in each molecular row. To interpret the STM image easily, models drawn with white and blue lines are superimposed in Figure 2e. Alternate PtOEP molecules possess different orientations in the molecular rows marked by green and black arrows. Each PtOEP molecule in the molecular rows indicated by the arrows was alternately rotated by approximately 5° with respect to each other. The intermolecular distance between the adjacent molecules along the green and black arrows was found to be 1.55 ± 0.05 nm. The structural model in Figure 2f shows that the row of PtOEP molecules marked with two arrows in Figure 2e corresponds to the two molecular rows in different colors. Thus, the large-scale STM image in Figure 2d shows that the slight rotation in the PtOEP

area, revealing clear internal molecular structures and the molecular orientations in the ordered domain. A careful inspection of the STM image allowed us to distinguish between two different orientations for each PtOEP molecule in the molecular rows. In other words, the orientations of the PtOEP molecules in the molecular rows marked by the red and blue arrows were slightly rotated with respect to each other. Each PtOEP molecule could be recognized as a central bright region with eight spots at the corners corresponding to eight ethyl groups. The nearest-neighbor distance along the arrow was 1.41 ± 0.05 nm, whereas the intermolecular distance in the molecular rows along the red and blue arrows was 1.61 ± 0.06 nm. Each unit cell included two PtOEP molecules, leading to a surface concentration of 8.7 × 10−11 mol cm−2. The adlattice is superimposed on the STM image in Figure 2b, and a structural model is proposed in Figure 2c. As denoted by the two different colors, the PtOEP molecules in each molecular row were alternately arranged with a slight rotation with respect to each other. We can determine from the values of the intermolecular distances that the adlattice of PtOEP is identical to the adlattices of CoOEP,21 NiOEP,23 ZnOEP,12,13 and FeClOEP40 on Au(111) surfaces, reported previously. Thus, the formation of a highly ordered adlayer of PtOEP can be determined by the chemical framework of OEP. To further understand the formation of a highly ordered PtOEP molecular adlayer and the stabilization of the reconstructed rows, a PtOEP adlayer was similarly prepared 29882

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appeared at every second molecular row. Although the orientation of each PtOEP molecule in every row could not be recognized from this STM image, it could be seen that the packing arrangement of the PtOEP molecules was different from that observed in Figure 2e, suggesting that the molecular packing arrangement of PtOEP depended on the direction of the reconstructed rows. When the potential was held at values more positive than 1.1 V vs RHE, the highly ordered PtOEP adlayer transformed into a disordered adlayer and the reconstructed rows disappeared. This result indicates the occurrence of a phase transition in the PtOEP adlayer (the so-called “lifting of reconstruction”), with a change in the electrochemical potential.34,35 The presence of the PtOEP overlayer expanded the stable potential range for the reconstructed phase. A similar structural change in the adlayer was observed in the case of the CoOEP adlayer on a Au(100)− (hex) surface under the same electrochemical conditions in HClO4.25 Therefore, it may be concluded that the formation of a highly ordered PtOEP adlayer contributes to the stabilization of the underlying reconstructed atomic structure on both Au(111) and Au(100) surfaces. To understand the strong electron-donating ability of the PtOEP adlayer on Au, supramolecular assemblies of fullerenes (C60 and C84) on the PtOEP adlayers were examined by immersing PtOEP-modified Au(111) substrates in benzene solutions containing either C60 or C84. Figure 4a,b shows typical STM images of a C60 supramolecular assembly with a low coverage of the PtOEP adlayer on a Au(111) surface, prepared by immersion of the PtOEP-modified Au(111) substrate in an approximately 1 μM C60 benzene solution for 5 s. It is estimated that the concentration of C60 in benzene is nearly equal to that of benzene solution saturated with C84. In the STM image shown in Figure 4a, several bright clusters or spots can be seen on a terrace covered with highly ordered PtOEP arrays. Furthermore, the reconstructed rows of Au(111) can also be seen through the PtOEP adlayer. Interestingly, the clusters composed of C60 were located in the darker regions between the brighter rows throughout the PtOEP adlayer. Under the preparation conditions used, the C60 molecules adsorbed onto the PtOEP adlayer and the C60 molecules appear to adsorb preferentially onto the valley sites of Au(111). From the higher magnification STM image shown in Figure 4b, it appears that each bright spot was located centrally in a PtOEP molecule. As indicated in the cross-sectional profile in Figure 4c marked with arrow I, the height of the bright spot was measured to be approximately 0.2 nm, corresponding to one C60 molecule. Interestingly, individual C84 molecules were separated from each other and aligned with the reconstructed rows of Au(111) near the hcp sites, as shown in Figure 4d. In the close-up view shown in Figure 4e, different shapes and sizes of the bright spots were observed on the PtOEP adlayers. As indicated in the cross-sectional profile in Figure 4f, which corresponds to arrow II in Figure 4e, the average variation in height was 0.25 nm, which is slightly higher than that for C60. On the basis of the intermolecular distance between the bright spots, it can be concluded that each C84 molecule is located in the central part of PtOEP. C84 includes two major isomers, namely D2(22)-C84 and D2d(23)-C84.46 The differences in the sizes and shapes of the bright spots can be tentatively explained either by the type of isomers present or by the different molecular orientations. It may be noted that the C60 and C84 molecules were observed on the highly ordered PtOEP adlayers

molecular orientation manifested as significantly darker molecular rows. The adlayer structure of PtOEP was slightly different from those of ZnOEP45 and CoOEP25 on Au(100)− (hex) surfaces. While similar packing arrangements were observed in the cases of ZnOEP and CoOEP, each CoOEP and ZnOEP molecule was rotated slightly in every fourth or fifth molecular row, respectively. The packing arrangement of the OEP adlayer was dependent on the number of electrons in the d and f orbitals of the central metal ion. The correlation between the packing arrangement on the reconstructed Au(100)−(hex) surface and the central metal ion in OEP framework is summarized in Table 1. Interestingly, the reconstructed rows of Au(100) were partially visible on the terrace, indicating that the highly ordered arrays must have formed on the reconstructed Au(100) surface. In fact, we were able to observe the underlying reconstructed rows of Au(100) by changing the electrode potential and tunneling current used in the STM measurements. Figure 3a shows the STM images obtained with a high

Figure 3. Large-scale (50 × 50 nm2) and high-resolution (15 × 15 nm2) STM images of PtOEP adlayers on a reconstructed Au(100)− (hex) surface in 0.05 M HClO4 at substrate potentials and tunneling currents of (a) 0.59 V vs RHE and 5 nA, (b) 0.55 V vs RHE and 65 nA, (c) 0.70 V vs RHE and 1.0 nA, and (d) 0.75 V vs RHE and 1.5 nA. The tip potential was 0.45 V vs RHE.

tunneling current of 5 nA and a substrate potential of 0.6 V. Under these conditions, the reconstructed rows were clearly observed because the PtOEP adlayer was completely invisible with the higher tunneling current. As shown in Figure 3b, the reconstructed atomic rows were identical to the (5 × 20) structure, reported previously by Magnussen et al.34,36 The reconstructed rows of Au(100) were aligned to the atomic row, in the same direction, indicating that the highly ordered PtOEP adlayer was precisely formed on the reconstructed Au(100)− (hex) surface. When the potential of the substrate was returned to 0.85 V and the tunneling current was decreased to 1 nA, the PtOEP adlayer was formed again on the reconstructed rows (see Figure 3c). By inspecting the STM images, we can determine that each row of PtOEP molecules was aligned at approximately 15° with respect to the reconstructed atomic row direction. A close-up view of Figure 3c is shown in Figure 3d. In the region under scrutiny, bright and dark rows periodically 29883

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Figure 4. (a) Large-scale (75 × 75 nm2) and (b) high-resolution (20 × 20 nm2) STM images of C60 molecules formed on PtOEP adlayers on Au(111) surfaces in 0.05 M HClO4, obtained at 0.80 V vs RHE. The tip potential was 0.43 V, and the tunneling currents were 0.30 and 0.23 nA for panels a and b, respectively. (c) A cross-sectional profile along the direction of arrow I shown in panel b and a proposed model of a supramolecularly assembled layer of C60 on a PtOEP adlayer. (d) Large-scale (75 × 75 nm2) and (e) high-resolution (30 × 30 nm2) STM images of individual C84 molecules on PtOEP adlayers on Au(111) surfaces in 0.05 M HClO4, obtained at 0.85 V vs RHE. The tip potential and tunneling currents were 0.43 V and 0.12 nA, respectively. (f) A cross-sectional profile along arrow II shown in panel e and a proposed model of a supramolecularly assembled layer of C84 on a PtOEP adlayer.

in the potential range of −0.10 to 1.00 V (see Figure s2 in Supporting Information). When the Au surface was modified with a longer immersion time, completely different observations were made for C60 and C84. Figure 5 shows typical STM images of C60 and C84 arrays formed on PtOEP adlayers on Au(111) surfaces. As shown in Figure 5a, a homogeneous growth over an area of 100 × 100 nm2 was found on the PtOEP adlayer, where the terrace was completely covered with highly ordered C60 molecules. A highresolution STM image of the supramolecular assembly of C60 molecules on the PtOEP adlayer on a Au(111) surface is shown in Figure 5b. The nearest-neighbor distances between the C60 molecules were identical to the nearest-neighbor distances in the PtOEP adlayer on Au(111), indicating a precise 1:1 supramolecular assembly of C60 and PtOEP molecules on Au(111). The values ranging from 1.4 to 1.6 nm are greater than those of C60 directly attached Au(111), 1.0 nm.13 It may be noted that the PtOEP adlayer was not replaced with C60, as seen in the time-dependent (see Figure s3 in Supporting Information) and tunneling current switched STM images. In particular, the tunneling current switched STM image revealed both the topmost and underlying layers in one image. When the tunneling current was stepped from 0.43 to 5.0 nA during the scan, the STM image dramatically changed, as shown in Figure 5c. The C60 adlayer immediately disappeared and the underlying PtOEP layer on Au(111) was visible, indicating that each C60 molecule was precisely formed on the highly ordered PtOEP adlayer with a 1:1 supramolecular assembly. In fact, the top layer of C60 disappeared upon scanning at tunneling currents higher than 5.0 nA and the underlying PtOEP layer was visible, as seen in the lower part of Figure 5c. Similar STM images were also found in the C60 and its derivatives supramolecularly assembled ZnOEP adlayers.12,13,44 Furthermore, the C60 arrays on the PtOEP adlayer were more stable for tunneling currents up to 1 nA in a bias voltage range of −0.3 to

Figure 5. (a) Large-scale (75 × 75 nm2) and (b) high-resolution (20 × 20 nm2) STM images of supramolecularly assembled C60 layer on PtOEP adlayer in 0.05 M HClO4 observed at 0.85 V vs RHE. The tip potential and the tunneling currents were 0.35 V and 0.43 nA, respectively. (c) A composite STM image of supramolecularly assembled C60 layer on PtOEP adlayer observed at 0.78 V vs RHE. The tip potential was 0.35 V vs RHE. The image was obtained by changing the tunneling current from 0.43 to 5.0 nA.

−0.4 V, compared to ZnOEP adlayers.12,13 It is noteworthy that the bias condition is limited because the examination was carried out in an electrolyte aqueous solution under electrochemical conditions. Therefore, it is difficult to investigate whether PtOEP adlayer is replaced with C60 molecules at 29884

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istic and selective adsorption on the highly ordered PtOEP adlayer on Au(111) and were clearly characterized at the molecular level. The adsorption of C60 on a highly ordered PtOEP adlayer with a high coverage provided a high level of stability required for the formation of an interfacial supramolecular assembly. The formation of a highly ordered PtOEP adlayer plays an important role in the stabilization of the reconstructed Au surface and the supramolecular assembly of C60 because of the strong π-electron donation ability of PtOEP.

single-molecular level by using scanning tunneling spectroscopy. However, when a fullerene derivative containing a ferrocene group was used as a second adlayer, the electrochemical response of the ferrocene moiety was clearly changed by the comparison to the case of that directly attached Au(111).44 The previous results indicate that OEP adlayer plays an important role in controlling the molecular orientation of the fullerene derivative. Therefore, it is concluded that the displacement of PtOEP with the adsorption of C60 does not take place. In contrast to the supramolecularly assembled C60 adlayer, Figure 6a shows a C84 adlayer on PtOEP obtained by the

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and other in situ STM images. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +81-96-3423948. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for Scientific Research on Innovative Areas “Coordination Programming” Area 2107, 21108005 from MEXT, Japan.

Figure 6. STM images of C84 on PtOEP adlayers by immersing a PtOEP-modified Au(111) substrate into C84-saturated benzene solution for (a) 5 min and (b) 10 min. The STM images were obtained in 0.05 M HClO4 at (a) 0.78 V and (b) 0.76 V vs RHE. The tip potential and the tunneling current were 0.40 V and 0.20 nA for (a) and 0.37 V and 0.65 nA (b), respectively.

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REFERENCES

(1) Electron Transfer in Chemistry; Balzani, V., Ed.; WILEY-VCH: New York, 2001; Vol. 3. (2) Guldi, D. M. Fullereneporphyrin Architectures; Photosynthetic Antenna and Reaction Center Models. Chem. Soc. Rev. 2002, 31, 22− 36. (3) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. Intermolecular and Supramolecular Photoinduced Electron Transfer Processes of Fullereneporphyrin/Phthalocyanine Systems. J. Photochem. Photobiol., C 2004, 5, 79−104. (4) Yeager, E. Electrocatalysis for O2 Reduction. Electrochim. Acta 1984, 29, 1527−1537. (5) Anson, F. C.; Shi, C.; Steiger, B. Novel Multinuclear Catalysts for the Electroreduction of Dioxygen Directly to Water. Acc. Chem. Res. 1997, 30, 437−444. (6) Durot, S.; Taesch, J.; Heitz, V. Multiporphyrinic Cages: Architectures and Functions. Chem. Rev. (Washington, DC, U.S.) 2014, 114, 8542−8578. (7) Kudernac, T.; Lei, S.; Elemans, J. A. A. W.; De Feyter, S. Twodimensional Supramolecular Self-Assembly: Nanoporous Networks on Surfaces. Chem. Soc. Rev. 2009, 38, 402−421. (8) Mohnani, S.; Bonifazi, D. Supramolecular Architectures of Porphyrins on Surfaces: The Structural Evolution from 1D to 2D to 3D to Devices. Coord. Chem. Rev. 2010, 254, 2342−2362. (9) Colson, J. W.; Dichtel, W. R. Rationally Synthesized Twodimensional Polymers. Nat. Chem. 2013, 5, 453−465. (10) Bonifazi, D.; Enger, O.; Diederich, F. Supramolecular [60]fullerene Chemistry on Surfaces. Chem. Soc. Rev. 2007, 36, 390−414. (11) Theobald, J. A.; Oxtoby, N. S.; Champness, N. R.; Beton, P. H.; Dennis, T. J. S. Growth Induced Reordering of Fullerene Clusters Trapped in a Two-Dimensional Supramolecular Network. Langmuir 2005, 21, 2038−2041. (12) Yoshimoto, S.; Tsutsumi, E.; Honda, Y.; Murata, Y.; Murata, M.; Komatsu, K.; Ito, O.; Itaya, K. Controlled Molecular Orientation in an Adlayer of a Supramolecular Assembly Consisting of an Open-Cage C60 Derivative and ZnII Octaethylporphyrin on Au(111). Angew. Chem., Int. Ed. 2004, 43, 3044−3047.

further modification of a PtOEP-covered Au(111) substrate by immersion in a C84 saturated benzene solution for 5 min. Unfortunately, there were no highly ordered domains observed and we were unable to obtain an adlayer fully covered with C84 molecules. An examination of the STM image shown in Figure 6a shows that it is likely that a disordered adlayer of C84 molecules formed on the PtOEP adlayer. In addition, careful inspection of the image revealed that the underlying PtOEP adlayer was still visible. Because the solubility of C84 in benzene is quite low (less than 1 μM), much longer modification is necessary to prepare a surface fully covered with C84. When the modification was carried out by a longer immersion time, for example, 10 min, the terrace was almost covered with C84 molecules, as shown in Figure 6b. We could obtain small domains consisting of several bright spots by careful inspection. However, the nearest intermolecular distance was approximately 1 nm, suggesting the closely packed C84 adlayer directly formed on Au(111). Thus, it was difficult to judge whether the displacement occurred under the modification condition. If the solubility of C84 is the same as that of C60, the supramolecular assembly of C84 onto the PtOEP adlayer may be kinetically controlled. To overcome the difficulty presented by the low solubility of C84 in benzene, alternative solvents such as toluene are considered to be effective for the supramolecular assembly of C84 on PtOEP adlayer.

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CONCLUSION Molecular adlayers of PtOEP formed on Au(111) and Au(100) were examined using electrochemical STM. The formation of long-range reconstructed rows of Au(111) and Au(100) was driven by the formation of highly ordered arrays of PtOEP. Supramolecular assemblies of C60 and C84 exhibited character29885

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp509312d | J. Phys. Chem. C 2014, 118, 29880−29886