Article pubs.acs.org/JPCC
Electrochemically Controlled 2D Assembly of Paddle-Wheel Diruthenium Complexes on the Au(111) Surface and Identification of Their Redox States Soichiro Yoshimoto,*,†,§ Kouhei Sakata,‡ Rempei Kuwahara,∥,# Keita Kuroiwa,∥,# Nobuo Kimizuka,∥,⊥ and Masashi Kunitake‡,⊥ †
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 ∥ Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ⊥ JST-CREST, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *
ABSTRACT: The 2D molecular assemblies of chloride-coordinated mixedvalence diruthenium complexes, each possessing phenyl, naphtyl, or anthracenyl moieties, were examined on an Au(111) at electrochemical interface. In situ scanning tunneling microscopy images revealed a clear dependence of the molecular assembly on both the nature of the aryl functional groups and on the redox state of the dinuclear ruthenium complex, either chloride-coordinated RuII/RuIII or noncoordinated RuII/RuII. At potentials where the RuII/RuIII and RuII/RuII redox states were in equilibrium, two distinct redox states were clearly identified at the single-molecular level. We found that manipulating both the electrochemical potential and the aryl functional group substitution was important for controlling the 2D molecular assembly of a chloride-coordinated diruthenium complex on an Au(111) surface.
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dinuclear Fe−Ru complex has been developed.22,23 Although it provided stable electrochemical redox waves when bound to a gold surface through a thiol linker,23 their redox states have not been identified. The bimetallic compounds flatly lying on surfaces are less attractive candidates for QCA cell because such a molecular orientation limits recording of electronic information in the highest density. Meanwhile, paddle-wheeltype mixed valence complexes with two perpendicularly stacked ruthenium centers are considered to be an ideal system for developing high-density molecular memories because they show multiple redox states (RuII/RuII and RuII/RuIII-X, X = halogen ion) that are expected to allow writing and reading electronic signal in a minimum molecular cross-sectional area. Moreover, the change in mixed valence structure is accompanied by dissociation and binding of axial halogen ligands, which should facilitate STM identification of these electronic structures in two-dimensionally aligned systems. Therefore, it is of significant importance to develop 2D selfassemblies of paddle-wheel diruthenium complexes and to visualize their redox structures at a single-molecular level.
wo-dimensional self-assembly of single-molecule electronic device has been attracting considerable interest as a potential route to fabricate next-generation molecular electronics.1−4 The design of multiple-redox molecules and control on the peripheral intermolecular interactions as well as those between aligned molecules and solid substrates play pivotal roles to control 2D nanostructures and patterns.5−7 For example, 2D arrays of double- and triple-decker-type rare earth metal-porphyrins and phthalocyanines have been extensively studied to control their molecular rotation, switching, and magnetic properties on surfaces.8−13 In contrast with porphyrin and phthalocyanine complexes, mixed valence-state complexes, as exemplified by 1D halogen-bridged mixed-valence compounds, are of interest in many disciplines14−16 because they exhibit novel electric, magnetic, and optical properties due to their unique oxidation and spin states. String-like 1D coordinated polymer chains made from halogen-bridged diruthenium complexes were observed on a highly oriented pyrolytic graphite surface by using scanning tunneling microscopy (STM) and atomic force microscopy (AFM).17,18 The significance of such mixed valence complexes lies in their application for the molecular computing system,19,20 that is, quantum-dot cellular automata (QCA) cell.19−24 On the basis of the concept of QCA cell that allows writing and reading molecular memory in the nanoscale size region, a hetero© 2012 American Chemical Society
Received: June 17, 2012 Revised: July 25, 2012 Published: July 30, 2012 17729
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Herein, three analogous diruthenium (RuII/RuIII) compounds, 1, 2, and 3, were synthesized with varying vinyl aryl moieties (Chart 1) to fabricate a redox state of mixed-valence Chart 1. Chemical Structures of Chloride-Coordinated Paddle-Wheel Diruthenium Complexes Vinyl Ph 1, Vinyl Np 2, and Vinyl Anth 3
diruthenium complexes on a Au(111) electrode surface. The resultant Au(111) adlayers were characterized at singlemolecular level by electrochemical (EC−) STM.
Figure 1. Cyclic voltammograms of RuII/RuIIICl complex 1- (red line), 2- (blue line), and 3- (black line) modified Au(111) electrodes recorded at scan rates of (a) 20 and (b) 10 V s−1, both in 0.1 M HClO4.
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EXPERIMENTAL SECTION Mixed-valence RuII−RuIII(Cl) complexes were synthesized on the basis of previous literature (see Supporting Information). Au(111) single-crystal electrodes were prepared by Clavilier’s method.25 Modifications of the RuII/RuIII(Cl) complexes were carried out by immersing an Au(111) electrode into ∼10 μM methanolic solution for 10 s to 10 min. Electrochemical measurements were carried out in 0.1 M HClO4 at room temperature from 20 to 25 °C using a ALS/HCH model 650C electrochemical analyzer. A Pt plate was used as the counter electrode, and a reversible hydrogen electrode (RHE) was used as the reference electrode for cyclic voltammetry. Electrochemical STM measurements were performed in 0.1 M HClO4 using a Nanoscope E system (Digital Instruments, Santa Barbara) with a tungsten tip (0.25 mm diameter) etched in 1 M KOH. Tips were coated with transparent nail polish to minimize faradaic current. STM images were obtained in the constant-current mode with a high-resolution scanner (HD0.5I). All potentials are versus the RHE for CV and in situ STM.
identical to those obtained for the 1-modified Au(111) electrode, ∼100 mV. However, when the scans were carried out at a much higher rate (10 V s−1), the CV profiles demonstrated a clear dependence on the aryl group (Figure 1b). A wide peak separation, 150 mV, and slightly lower electronic charge were observed in the case of the 1-modified Au(111) electrode, but the 2- and 3-modified Au(111) electrodes showed nearly identical peak separations to those obtained at 20 mV s−1, indicating a rapid electron-transfer reaction. The peak separation and electronic charges estimated from the reductive peak in each CV are summarized in Table 1. The results indicate differences in how each of the three modified metal complexes interacts with the Au(111) surface. Table 1. Electronic Charge Consumed during the Cathodic Scan and Peak Separation Obtained at 10 V s−1
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QC/μC ΔE/mV
RESULTS AND DISCUSSION Figure 1 shows typical cyclic voltammograms (CVs) of three RuII/RuIII 1-, 2-, and 3-modified Au(111) electrodes obtained in pure 0.1 M HClO4. As shown in Figure 1, characteristic CV profiles were observed in the potential region between 0.75 and 0.10 V. The redox couple for complex 1 appeared at 0.32 V. The redox currents were proportional to the scan rate, indicating that molecules of complex 1 adsorbed on Au(111). The amount of transferred electronic charge can be estimated by integration of the peak area in the 20 mV s−1 CV profile (Figure 1a), and the average value was found to be 1.12 × 10−5 C cm−2. This value leads to a surface excess of 1.16 × 10−10 mol cm−2, assuming that a single-electron reaction occurs on the 1modified Au(111) electrode. As previously reported by the Collman group,26 the RuII/RuIII redox couple for a ruthenium tetraphenylporphyrin (RuTPP)-attached SAM electrode was observed at ∼0 V versus SCE in 0.5 M HClO4/0.5 M NH4PF6.26 The RuIII/RuII redox couple observed in our system seems to be consistent with that obtained for the RuTPPmodified SAM electrode. Therefore, we can assume that complex 1 is being reduced from RuII/RuIII to RuII/RuII. The 2and 3-modified Au(111) electrodes were investigated in the same manner. The observed peak separations were nearly
1
2
3
0.85 ± 0.10 150 ± 5
0.35 ± 0.05 80 ± 5
0.12 ± 0.02 95 ± 5
To understand each adlayer on Au(111), EC-STM investigations were carried out in 0.1 M HClO4. Figure 2 shows potential-dependent STM images of an adlayer of complex 1 on Au(111). At open circuit potentials (OCPs) at or near 0.80 V, only a disordered adlayer with some aggregations was found on the terrace (Figure 2a). When the potential was lowered to +0.55 V, several ordered portions could be partially observed on the terrace. A careful inspection revealed that these small, ordered portions of adlayer 1 formed exclusively several belt-type arrays with the width of 4−6 nm (Figure 2b). These belt-type arrays are seemingly formed along the reconstructed rows of Au(111) because their widths coincide with each other. In general, potential-induced surface reconstruction of Au(111) in electrolyte solution gives noncontiguous and irregular rows27 and not regularly arranged herringbone type reconstruction,28 as observed in this case. Similar molecular assemblies on Au(111) were found for several metal complexes such as copper(II) phthalocyanine (CuPc)29 and cobalt(II) “picketfence” porphyrin30 in 0.1 M HClO4. These ordered arrays thus began forming at potentials greater than the redox potential of complex 1, indicating that potential manipulation is effective for 17730
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nearest intermolecular distance between neighboring complex 1 molecules was measured to be approximately 1.3 to 1.4 nm. The adlattice of adlayer 1, superimposed within the white square in Figure 2d, is assigned as a square-type adlattice with intermolecular distances of 1.35 nm. The adlattice of adlayer 1 is similar to those reported for CoII, CuII, and ZnII tetraphenyl porphyrin (CoTPP,31 CuTPP,31 and ZnTPP32) 2D assemblies on Au(111), all of which have similar molecular sizes to complex 1. On the basis of the lattice constant estimated from the intermolecular distance, the surface excess of complex 1 was estimated to be 9.1 × 10−11 mol cm−2, which is consistent with that obtained from the electronic charge in Figure 1a. When the potential was increased to 0.75 V, the ordered arrays of adlayer 1 disappeared, and the disordered arrays were reformed (Figure 2e). As a result, the structural phase transition of adlayer 1 is reversible for the electrochemical redox of RuII/RuIII. In the present study, we also examined a RuII/RuIII complex with phenyl groups in place of the vinylphenyl moieties, but only disordered arrays were formed on the terrace (see Figure s1 of the Supporting Information). This observation implies that the vinyl linker is a required to form a highly ordered RuII/RuIII surface array. The vinyl linker affects the intermolecular distance and thus plays an important role in the interactions between molecules of complex 1 on the Au(111) surface. The observed molecular packing arrangement is attributed to πstacking interactions between intermolecular vinylphenyl groups in complex 1, as depicted in Figure 2f. Careful inspection of the high-resolution STM image revealed differing brightness levels of the central spot in each complex 1 molecule when the potential was increased above +0.15 V and held at +0.22 V. Figure 3a−c shows timedependent STM images observing the same area, and the molecules of complex 1 appeared as both bright and dark spots in the ordered domain. The insert shows a close-up view, where the individual molecules of complex 1 can be clearly observed as four-lobed shapes. Both RuII/RuIII and RuII/RuII redox states can coexist in this potential region, as can be observed in the CV profile shown in Figure 1a. On the basis of the crosssectional profile along the red arrow in Figure 3a, the average height amplitude difference was found to be 0.4 nm, as indicated in Figure 3d. As previously reported by Ikeda et al., a similar height amplitude was found in mixed self-assembled layers of free-base porphyrin for both chloride- and pyridinecoordinated RhIII porphyrin derivatives on HOPG;33 this observation was due to the axial ligand coordination of chloride and pyridine. Another possibility for this phenomenon is “flipflop” type inversion of the molecular configuration. Actually, for example, Wee’s group recently reported that Cl-coordinated AlPc can adsorb with the Cl ligand facing to the metal surface (Cl-down orientation) by drastically changing bias voltage from +3.3 to −2.6 V in UHV, which showed a different contrast from the Cl-up orientation.34 Berndt’s group demonstrated pushing and pulling the central Sn ion in nonplanar SnPc molecule on Ag(111) using high bias voltage in UHV.35,36 Both cases showed contrast changes of central part of Pc framework in the STM image by applying high bias voltages. Under our condition, the STM images were taken under low bias voltage (∼ 0.15 V), and it is impossible to apply such high bias voltages because of the decomposition of water in the electrolyte solution. Therefore, the difference in the brightness at the central part of each 1 complex is reasonably assigned as an exchange reaction of Cl−, not molecular orientation change of complex 1. The STM image taken 1 min later showed a drastic
Figure 2. Potential-dependent STM images of Au(111) adlayer 1 obtained at (a) 0.80 V, (b) 0.55 V, and (c) 0.15 V versus RHE in 0.1 M HClO4. Tip potential and tunneling currents were (a) 0.22 V versus RHE and 0.14 nA, (b) 0.20 V versus RHE and 0.13 nA, and (c) 0.35 V versus RHE and 0.25 nA. High-resolution STM images of adlayer 1 obtained at (d) 0.15 and (e) 0.75 V (increased stepwise from 0.15 V). Tip potential and tunneling currents were (d) 0.35 V versus RHE and 0.25 nA and (e) 0.42 V versus RHE and 0.30 nA. A proposed structural model of Au(111) adlayer 1 observed at (f) 0.15 V versus RHE.
controlling the interaction between a molecule of complex 1 and the Au(111) surface. In particular, the complex 1 is likely to prefer to position on reconstructed rows of Au(111). Lowering the potential further (+0.15 V) resulted in an increased presence of ordered domains. Several domains were found in an area on the order of 50 × 50 nm2. A close-up view of the highly ordered array of adlayer 1 is shown in Figure 2d. Squarely arranged molecules of complex 1 were clearly observed in this domain. Each complex 1 molecule was ovular with a central dark spot. Because molecular shape is dependent on the tip sharpness and tunneling current, several molecular resolutions of adlayer 1 were observed (Figure 3). As seen in Figure 2d, the two vinyl phenyl groups are thought to be visualized as the two enhanced bright spots. If the chloridebound RuII/RuIII pair is electrochemically reduced during the cathodic scan, then the central Ru ions should lose Cl− and exist in the noncoordinated RuII/RuII redox state. The central part of each individual molecule of complex 1 was visualized as a dark spot. On the basis of the cross-sectional profile, the 17731
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Figure 3. Time-dependent STM images (15 × 15 nm2) of Au(111) adlayer 1 obtained at 0.22 V versus RHE in 0.1 M HClO4. Tip potential and tunneling currents were 0.42 V versus RHE and 0.30 nA. STM images b and c were taken 1 and 5 min, respectively, after image a was recorded. A close-up view is shown at the right side of image c (inset). A cross-sectional profile of a molecular row along the red arrow (d). Models of Cl coordination and noncoordination during the redox process of complex 1 in the highly ordered Au(111) adlayer (e).
change, shown in Figure 3b. The number of bright spots in the row of molecules indicated by the blue arrow notably decreased. Furthermore, the STM image shown in Figure 3c was recorded 4 min after the image in Figure 3b and indicated another increase in the number of bright spots. The prominent change in brightness over time can be observed directly by paying particular attention to the position marked by white dotted circles (Figures 3a−c). Chloride ion exchange took place in 2D ordered adlayer 1 during the scan, suggesting that Cl− has a potential-controlled weak interaction with the diruthenium complex, depicted in the models shown in Figure 3e. This chloride ion exchange is accomplished by equilibrating between the oxidized and reduced forms of complex 1. Following these observations, we conclude that the observation of both bright and dark spots suggests the simultaneous presence of both chloride-coordinated RuII/RuIII and noncoordinated RuII/RuII redox states. The coexistence of these two redox states results from a rapid internal electron exchange between the RuII/RuIII and RuII/RuII complexes in the highly ordered adlayer of complex 1. The similar time-dependent exchange reaction was found on the O2-traped cobalt(II) “picket-fence” porphyrin nanobelt array on a reconstructed Au(100) surface.30 It is noteworthy that 1D polymeric chain type structure has never been seen at potentials where the RuII/ RuIIICl oxidation state is stable. Although the ordered adlayer 1 with the square adlattice could be formed at potentials more negative than redox potential of 1, it is impossible to construct and to keep chloride-bridged 1D chain structures under the potential where the RuII/RuII state is because of the release of Cl−. If the highly ordered arrays of 1 are stably formed in the potential range from OCP to 0.60 V where the RuII/RuIIICl state is, then chloride-bridged polymeric structures would be constructed three-dimensionally onto the Au(111) surface. A similar adlayer structure for complex 2 was found, even when the potential was at or near OCP. Figure 4a shows a typical STM image of adlayer 2 obtained at 0.80 V, where several ordered domains were found on the terrace. When the
Figure 4. Potential-dependent STM images (50 × 50 nm2) of Au(111) adlayer 2 (a,b) and Au(111) adlayer 3 (c,d) in 0.1 M HClO4 obtained at (a) 0.80, (b) 0.15, (c) 0.70, and (d) 0.15 V versus RHE. The tip potentials and the tunneling currents for each panel were (a) 0.42 V and 0.25 nA, (b) 0.34 V and 0.65 nA, and (c,d) 0.43 V and 0.35 nA.
potential was decreased to 0.15 V, highly ordered domains were still observed. The structure of adlayer 2 (Figure 4b) is identical to that of adlayer 1. However, the domain size was smaller than that obtained for adlayer 1, indicating that the interactions between the molecules of complex 2 and the Au(111) surface are slightly stronger than those observed for complex 1. In contrast with complexes 1 and 2, complex 3 did not form a highly ordered adlayer even when the potential was lowered to 17732
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potentials less than OCP (Figure 4c,d). Bright spots were observed on the terrace, but this observation was due to the strong interactions between complex 3 molecules and the Au(111) surface. The surface diffusion of complex 3 is extremely restricted by too strong adhesion. Compared with 1 and 2, the stronger interaction between 3 and the Au(111) substrates was proven by prolonged observation at 0.15 V versus RHE.
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CONCLUSIONS We demonstrated that the formation of an ordered array of mixed-valence RuII/RuIII complexes on an Au(111) electrochemical interface is strongly dependent on both aryl substitution and electrochemical potential manipulation. A visual difference between the RuII/RuIII and RuII/RuII redox states was found unambiguously at the single-molecular level by EC-STM. Axial chloride ligands show exchange at potentials where both the RuII/RuIII and RuII/RuII redox states coexist. The information obtained in this study provides us with a basis for precisely forming highly ordered and stable arrays of diruthenium complexes that materialize the concept of selfassembling QCA cell.
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ASSOCIATED CONTENT
S Supporting Information *
Detail of the synthesis of compounds 1, 2, and 3 and in situ STM images of adlayer 4 (RuII/RuIII complex with phenyl groups, not vinyl phenyl groups). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Present Address #
Department of Nanoscience, Faculty of Engineering, Sojo University, 4-22-1 Ikeda, Nishi-ku, Kumamoto 860-0082, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency (JST) and the JST program ‘Special Coordination Funds for Promoting Science and Technology’.
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