Tuning Porphyrin Assembly and Electrochemical Catalytic Activity with

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Tuning Porphyrin Assembly and Electrochemical Catalytic Activity with Halogen Substituents Teppei Kawamoto, and Soichiro Yoshimoto Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03132 • Publication Date (Web): 08 Oct 2015 Downloaded from http://pubs.acs.org on October 15, 2015

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Tuning Porphyrin Assembly and Electrochemical Catalytic Activity with Halogen Substituents Teppei Kawamoto,† Soichiro Yoshimoto‡,§,*

†

Graduate School of Science and Technology, ‡ Priority Organization for Innovation and Excellence, 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.

*To whom correspondence should be addressed.

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ABSTRACT:

The

methoxyphenyl)porphyrin

adlayers cobalt(II)

of

three

(CoTMePP),

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metalloporphyrins—5,10,15,20-tetrakis(45,10,15,20-tetrakis(4-bromophenyl)porphyrin

cobalt(II) (CoTBrPP), and 5,10,15,20-tetrakis(4-iodophenyl)porphyrin cobalt(II) (CoTIPP)—on Au(111) were investigated at the solid-liquid interface under electrochemical conditions.

In situ

scanning tunneling microscopy (STM) was employed to investigate the adlayer structures of CoTMePP, CoTBrPP, and CoTIPP in HClO4 solution. Highly ordered adlayers of the three metalloporphyrins were formed on the Au(111) electrode surface by simple immersion into benzene solutions containing each compound. The adlayer structure of the three cobalt porphyrin derivatives was influenced by the functional group on the phenyl moieties. In particular, a characteristic molecular assembly of CoTIPP molecules was found on Au(111) due to the I···I interactions between CoTIPP molecules. The adlattice constants increased in the order –OCH3 < –Br < –I in the phenyl groups. The in situ STM observations showed that the CoTMePP adlayer changed during positive potential manipulation in 0.1 M HClO4, whereas these adlayers were stable in the potential range from 0.90 to 0 V versus the reversible hydrogen electrode. A dependence upon the functional groups among the three CoTPP derivatives was clearly found in the adlattice constants and O2 reduction potentials, revealing that the 2-D molecular assembly and electrochemical activity for dioxygen reduction of the tetraphenylporphyrin derivatives can be tuned by introducing functional groups at the 4-positions of the phenyl moieties, especially iodine substituents.


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Introduction Metalloporphyrins are involved in important biological processes such as dioxygen transport, photosynthesis, and catalysis. These characteristics have been exploited for use in biomimetic devices such as gas sensors, nanoporous catalytic materials, and organic solar cells.1–6 From the perspectives of both supramolecular and coordination chemistry, metalloporphyrins are excellent building blocks because various nanoarchitectures such as metal-organic frameworks and discrete cages with characteristic nanospaces can be easily formed by the selection of functional groups, central metal ions, and axial ligands.6,7 The “bottom-up” fabrication of characteristic molecular architectures with porphyrin derivatives is significant both for surface design and for controlling surface properties.8–11

The 2-D molecular

assembly and adlayer structures of porphyrins have been extensively studied on various metal surfaces under

ultra-high

vacuum

(UHV)

conditions8,10–16

and

in

solution.9,17–22

For

example,

tetraphenylporphyrin (TPP) derivatives are typical target porphyrins. The Hipps group reported the 2-D molecular assembly of several TPP complexes with different central metal ions on Au(111) under UHV.13,14 Subsequently, it was reported that identical adlayer structures of CoTPP, CuTPP, and ZnTPP were formed on Au(111) by simple immersion in a solution such as benzene.19,20

In addition,

characteristic molecular assemblies were accomplished by the interactions between functional groups such as cyano and carboxylic acid moieties through dipole–dipole interactions15,16 and/or hydrogen bonding.9,23,24 Recently, the formation of molecular wires and 2-D nanosheets consisting of organic molecules has become an important research objective.25–27 One attractive variation would involve the introduction of halogen substituents such as bromine and iodine into porphyrin derivatives, because these are expected to be molecular building blocks for ‘on-site’ thermal polymerization processes on surfaces via the Ullman reaction. For example, graphene nanoribbons can be fabricated by the 1-D polymerization of a dibromoanthracene derivative.26 Similarly, there have been many reports on the formation of 2-D


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porous covalent networks from starting materials such as 1,3,5-tri(p-bromophenyl)benzene28–31 and its derivatives.32

In the case of porphyrin derivatives, ‘on-site’ covalent bond formation of several

molecular architectures on Au(111) under UHV conditions was demonstrated by Grill’s group.33,34 Furthermore, characteristic nanoarchitectures by a stepwise method have been formed on Au(111) by the precise molecular design of the porphyrin molecular building block, as previously reported by Lin’s group.35,36 These systematic research efforts strongly encouraged us to understand and investigate the effects of halogen functional groups in porphyrin derivatives on 2-D molecular assembly on surfaces. It is important to not only develop molecular building blocks for the ‘on-site’ formation of 2-D nanosheets, but also control 2-D molecular assembly, because the introduction of halogen substituents allows us to change the electronic properties of a porphyrin due to their high polarizabilities. Indeed, it is known that the crystal structures of those porphyrin derivatives have unique lattices due to the specific halogen–halogen interactions between the substituent atoms.37

The formation of halogen–halogen

bonds is one of the key factors in creating new nanoarchitectures, especially in the fields of supramolecular chemistry38 and 2-D engineering.39 In terms of electrochemistry, halogen substitution would be expected to tune electrochemical properties such as the redox potential. However, to the best of our knowledge, a relationship between the 2-D adlayer and the electrochemical properties of porphyrin derivatives with halogen substituents has not been clarified. This offers a promising approach for not only the formation of precisely controlled nanoarchitectures, but also the design and tuning of electrocatalysts. In the present study, we focused on the molecular assembly and electrocatalytic activity for the dioxygen

reduction

of

5,10,15,20-tetrakis(4-methoxyphenyl)-porphyrin

5,10,15,20-tetrakis(4-bromophenyl)porphyrin

cobalt(II)

(CoTBrPP),

cobalt(II)

and

(CoTMePP),

5,10,15,20-tetrakis(4-

iodophenyl)porphyrin cobalt(II) (CoTIPP) on Au(111) (see Chart 1), to further explore and understand the role and effect of the substituent in the TPP derivative on a surface. Differences in the molecular packing arrangements and electrochemical stability among CoTMePP, CoTBrPP, and CoTIPP were


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clearly found in acidic solution at nanoscale by electrochemical–scanning tunneling microscopy (ECSTM).

Experimental Section CoTMePP, CoTBrPP, and CoTIPP were purchased from Strem Chemicals Inc. and Frontier Scientific Inc., and used without further purification. Perchloric acid (HClO4, Cica-Merck, ultrapure grade) and benzene (spectroscopy grade) were obtained from Kanto Chemical Co. Ltd. All electrolyte solutions were prepared with ultrapure water (Milli-Q Advantage A10; ≧18.2 MΩ cm). Au(111) single-crystal electrodes were prepared by the Clavilier method.40 Prior to each experiment, the Au substrate was annealed in a hydrogen flame and quenched in ultrapure water saturated with hydrogen to avoid contamination. The adlayers of CoTMePP, CoTBrPP, and CoTIPP were formed by immersing the electrode into a benzene solution of the metalloporphyrin (~50 µM) for 15 s at room temperature (rt, 15–20 °C). Then, the modified Au(111) electrode was thoroughly washed with ultrapure water.

Electrochemical

measurements were carried out in 0.1 M HClO4 under Ar at rt using an ALS/HCH model 704B electrochemical analyzer (ALS Co., Ltd.). For cyclic voltammetry, a Pt plate was used as the counter electrode, and a reversible hydrogen electrode (RHE) was used as the reference electrode. EC-STM measurements were performed in 0.1 M HClO4 by using a Nanoscope V system (Veeco Instruments Inc.). The STM images were obtained in the constant-current mode with a high-resolution scanner (HD-0.5I). Tungsten tips were etched in 1 M KOH. The tips were coated with nail polish to minimize residual faradaic currents.

Two Pt wires were used as the quasi-reference and counter

electrodes. All potential values are reported with respect to RHE.

Results and Discussion First, we examined the adlayer of the CoTPP derivative with methoxy substituents at the 4-position


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in each phenyl moiety. Figure 1 shows typical EC-STM images of the CoTMePP adlayer formed on Au(111). In a large-scan area, several triangle step lines can be seen, and the terraces are entirely covered with ordered CoTMePP arrays, as shown in Figure 1a. In addition, several bright islands are visible on the terraces. These are probably due to the aggregation of the CoTMePP molecules, because the sizes and numbers of the islands are dependent on the concentration and/or immersion time in the CoTMePP benzene solution. On the terraces, careful inspection reveals several distortions of the molecular rows, especially in the lower central part of the image. The typical domain size of a CoTMePP array is less than 20 nm. The high-resolution STM image acquired in a 15 × 15 nm2 area, shown in Figure 1b, reveals clear internal molecular structures and orientations. A careful inspection of this STM image allows us to distinguish individual CoTMePP molecules, clearly recognizable as ‘propeller-shaped’ images with additional spots at the four corners. The brightest spot at the center and the four additional spots in each CoTMePP molecule can be attributed to the Co ion and methoxyphenyl moieties, respectively. Some defects in which the central Co ions are missing can be seen as dark spots in the highly ordered domain. It is clear that all CoTMePP molecules possess the same orientation in each molecular row from the high-resolution STM image shown in Figure 1b. The intermolecular distance is 1.45 ± 0.05 nm. Molecular rows cross each other at an angle of approximately 90°, indicating that the adlayer structure of CoTMePP is almost the same as that previously reported for CoTPP.19 Therefore, we conclude that the formation of highly ordered arrays is determined by π–π interactions between the methoxyphenyl moieties in the CoTMePP molecules, as superimposed with model structures. Although we could not find clear evidence for the distortion of the molecular rows from the high-resolution STM image shown in Figure 1b, it is likely that the methoxy substituent in each phenyl group can adopt several orientations. The highly ordered CoTMePP adlayer was observed to be stable in the potential region between 0.90 and 0.10 V vs. RHE, whereas CoTMePP molecules are typically reduced from Co(III) to Co(II) at around 0.40 V. After confirming the stability of the highly ordered CoTMePP adlayer, the electrode potential was


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manipulated to more positive potentials. Figure 2 shows the potential-dependent STM images of the CoTMePP adlayer on Au(111) observed in 0.1 M HClO4. In Figure 2a, the terrace is covered with ordered CoTMePP arrays, with several islands that were formed via the aggregation of CoTMePP molecules at domain boundaries. These islands are indicated by dotted white circles, shown at the same location in each of the three STM images in Figure 2. When the electrode potential was stepped to 1.00 V from 0.95 V, a phase transition was observed in the CoTMePP adlayer. For example, the image in Figure 2b was taken 2 min after holding at 1.00 V. In particular, the positions of the CoTMePP molecules marked by the dotted circle are completely changed, i.e., the highly ordered adlayer of CoTMePP becomes smaller and disordered. After returning the potential to 0.90 V from 1.00 V, an ordered adlayer is again formed, as shown in Figure 2c. Notably, the CoTMePP molecular rows in the adlayer located at the central left are completely changed and aligned, having rotated by approximately 45°. Furthermore, careful inspection reveals many bright spots in the ordered domain. Because the oxidation of CoTMePP occurs at positive potentials, perchlorate or superoxide anion can coordinate to the central Co(III) as an axial ligand. In addition, the interaction between the CoTMePP molecule and Au substrate is strongly enhanced because the potential is much more positive than the potential of zero charge (pzc). In general, when the potential is held at levels more positive than the pzc, the Au substrate is positively charged and the lifting of “reconstruction” of the Au surface is caused by specific anion adsorption at the electrochemical interface.41 The disordering of the CoTMePP adlayer at 1.10 V might be caused by the adsorption of an anion such as perchlorate. Thus, the potential manipulation in the positive direction provides the motive force to control the formation of the CoTMePP adlayer. Figure 3 shows typical STM images of the highly ordered adlayers of CoTBrPP and CoTIPP formed on the Au(111) electrode surfaces in 0.1 M HClO4. In the large-scale image for CoTBrPP, the terraces are entirely covered with highly ordered molecular arrays, with domain sizes that are greater than those obtained in the CoTMePP adlayer. In addition, the aggregation of CoTBrPP molecules is hardly seen on the terraces. A careful inspection reveals that the highly ordered domains are composed of bright


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and darker spots on the terraces. As previously reported, the formation of similar large domains was observed in a nickel(II) meso-tetra(4-bromophenyl)porphine (NiTBrPP) adlayer on Au(111) under UHV.42 When tunneling currents exceeding 1 nA were applied, CoTBrPP molecules were desorbed from either the domain edges or defect sites. This suggests that the interaction between the CoTBrPP adlayer and the Au(111) electrode surface is not particularly strong. A close-up view of the CoTBrPP adlayer on Au(111) is shown in Figure 3b. The central cobalt ion and bromophenyl moieties can be assigned as the brightest spot and four additional spots, respectively. Similar bright-spot configurations were found on Au(111) and Ag(111) under UHV.42,43 Careful examination reveals that the central Co ion is missing in several CoTBrPP molecules. In the missing sites, TBrPP frameworks can be obviously distinguished from the CoTBrPP molecules, as indicated by the white dotted circles. The observed shape is very similar to that reported for a single molecule of H2TBrPP on Au(111).33 The packing arrangement of CoTBrPP is nearly the same as those of H2TBrPP33 and NiTBrPP42 obtained under UHV. The measured intermolecular distance is 1.54 ± 0.06 nm.

The adlattice of CoTBrPP is also

superimposed on the STM image shown in Figure 3b; the adlattice constant of CoTBrPP on Au(111) is slightly greater than those of CoTPP19 and CoTMePP described above. Based on the high-resolution STM image, a structural model was proposed (Figure 3c). Because a bromine substituent is larger than a hydrogen atom, π–π interactions through the phenyl groups are influenced by its incorporation. Therefore, the fact that CoTBrPP molecules are easily desorbed during scans with a higher tunneling current results from the weaker interaction between the CoTBrPP adlayer and Au(111) than that between the CoTPP adlayer and Au(111). In the adlayer structure of CoTBrPP, it is assumed that intermolecular bonding occurs through Br···Br interactions among nearest-neighbor CoTBrPP molecules rather than π–π interactions between the phenyl rings. In fact, from the proposed model, the Br···Br distance in nearest-neighbor CoTBrPP molecules is estimated to be approximately 4 Å. This value is consistent with the estimations for 2-D adlayers of other molecules containing bromine groups, as previously reported.32


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The STM image of the CoTIPP adlayer (50 × 50 nm2) is shown in Figure 3d. The terrace is completely covered with CoTIPP molecules, and a highly ordered CoTIPP adlayer is formed on the Au(111) surface. Although several defects can be seen on the terrace, the CoTIPP adlayer is composed of a “grid-like” structure. Long-range modulations are also visible as rows on the surface through the CoTIPP molecular adlayer. These rows indicate the change of the underlying Au atoms from a (1 × 1) to a (√3 × 22) lattice configuration, known as “reconstruction.”44 As previously reported, a similar reconstruction of Au(111) surfaces has been observed during the formation of various metal ioncoordinated tetraphenylporphyrin (MTPP) adlayers,19,20 even at potentials near the open circuit potential. This appearance of reconstructed rows suggests that the ability of the CoTIPP to donate to the Au(111) surface is stronger than those of CoTMePP and CoTBrPP because of the high polarizability of the iodine substituent. In the high-resolution STM image (15 × 15 nm2) shown in Figure 3e, differences in brightness can be observed, i.e., bright and dark molecular rows are alternately arranged in the ordered domain. In general, the central portion of a CoTPP appears as a bright spot because of the tunneling mediated by the half-filled dz2 orbital between the tip and Co ion in the CoTPP. On the other hand, a Cu ion in CuTPP appears as a dark spot: Cu has nine electrons in its d orbitals (d9), and the dz2 orbital is filled.14 Therefore, the brightest spots can be assigned to the central Co ion in each CoTIPP molecule. In fact, as indicated by the white arrows shown in Figure 3e, several defects occur in the molecular row consisting of the brightest spots, indicating that the central Co ions are missing, as described above. Remarkably, the central portion of each CoTIPP appears to be split into two or three bright spots, associated with the coordination of the anion ligands, such as the perchlorate anion in 0.1 M HClO4. These characteristics are sometimes observed when the imaging is carried out under low bias voltage and/or higher tunneling current conditions. In fact, the imaging conditions for the STM image shown in Figure 3e included a bias voltage of 0.18 V, whereas the bias voltage was 0.42 V for Figure 1b. A similar feature was reported for the CoTPP adlayer on Au(100)–(1 × 1).45 The central Co ion is in the Co(III) state in the potential range from 1.10 to 0.55 V; therefore, anionic species such as


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superoxide or perchlorate anions are coordinated to the Co(III) ion. Based on the cross-sectional profile, the intermolecular spacing was determined to be 1.61 ± 0.05 nm, which is greater than that of CoTBrPP. The adlattice was estimated to be nearly square, and is shown as superimposed with the white square in the STM image. Although we could not find a clear molecular shape for one CoTIPP molecule in the STM image, taking the intermolecular spacings and adlattice into consideration, a structural model was tentatively proposed as a “side-by-side” configuration through the iodine substituents, as shown in Figure 3f. The bright and dark parts of the STM image are tentatively assigned as the Co-ion-coordinated porphyrin framework and the four iodophenyl groups of the nearestneighbor CoTIPP molecules, respectively. From the crystal structure of ZnTIPP,37 it was reported that the I···I interaction is strong between TIPP molecules. Although a clearer molecular image is needed to determine the relationship between the adlattice and the molecular orientation of the iodophenyl moieties in CoTIPP, we conclude tentatively that the CoTIPP adlayer is formed due to the interactions between the iodine atoms in the CoTIPP molecules. The proposed model for the adlayer structure of CoTIPP is also supported by the explanation of the charge distribution and electrostatic potential, i.e., the high polarizability of the iodine atom in the I–C bond.46 I···I interactions play an important role in the characteristic adlayer formation. It is noteworthy that the highly ordered adlayers of both CoTBrPP and CoTIPP are stable, with no structural changes observed over the potential range from 0.95 to 0 V, whereas oxidative desorption takes place at potentials more positive than 1.10 V. A similar oxidative desorption was observed fro a Co octaethylporphyrin (CoOEP)-modified Au(111) electrode in 0.1 M HClO4.47 The oxidative desorption is probably caused by the electrochemical oxidation of the Au surface with the adsorption of OH species.48 To further explore the difference in molecular assembly (or electronic structure) among the three molecules, electrochemical dioxygen reduction reactions were performed on the three CoTPP derivative-modified Au(111) electrodes in 0.1 M HClO4 saturated with O2. Figure 4 shows typical linear sweep voltammograms for O2 reduction on the modified electrodes. At the CoTMePP- and


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CoTBrPP-modified Au(111) electrodes, clear electrochemical reduction of O2 at 0.58 V vs. RHE is observed (curves a and b). From the current density, we estimate that O2 molecules are reduced to H2O2 via two-electron reduction.9 The peak potentials and current densities obtained under these conditions are nearly equal to those on the CoTPP-modified Au(111) electrode.19 For the CoTIPP-modified Au(111) electrode, the reductive peak potential for the O2 reduction wave is observed at a slightly more positive potential (curve c): a clear catalytic current commences at 0.62 V. Interestingly, the positive shift for the O2 reduction peak potential is dependent upon the substituent at the 4-position in the phenyl group, indicating that this is probably related to the electronegativity of the iodine substituent. Finally, the adlattice constants and O2 reduction peak potentials are summarized in Table 1. The results of the present study suggest the possibility that the electronic structure, tuned by the introduction of electron-donating substituents, leads to a positive shift of the electrochemical reduction potential of O2.

Table 1. Adlattice constants and O2 reduction peak potentials of CoTMePP, CoTBrPP, and CoTIPP adlayers

adlayer

Lattice constant (nm)

O2 reductive peak potential (V)

CoTMePP CoTBrPP

1.45 ± 0.05 1.54 ± 0.06

0.31 0.32

CoTIPP

1.61 ± 0.05

0.38

Conclusion We succeeded in forming highly ordered adlayers of CoTMePP, CoTBrPP, and CoTIPP on Au(111). The molecular assembly of the cobalt porphyrin derivatives was dependent upon the size and interaction of the substituents at the 4-position of the phenyl moieties. Based on high-resolution STM images, the adlayer structure of CoTMePP on Au(111) was found to be similar to that of CoTPP. The CoTBrPP adlayer was also similar to the CoTMePP adlayer, although the adlattice was slightly larger than that of CoTMePP.

CoTIPP also exhibited a characteristic molecular assembly on Au(111),

implying the strong interaction between the iodine atoms (I···I) of neighboring CoTIPP molecules. The ACS Paragon Plus Environment

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onset and peak potentials of O2 reduction on the CoTIPP-modified Au(111) electrode were positively shifted in comparison with CoTMePP and CoTBrPP. Our results show that the 2-D molecular assembly and electrocatalytic activity of CoTPP derivatives can be adjusted by selecting the substituent at the 4position of the phenyl moieties.

Acknowledgment: This work was supported in part by a Grant-in-Aid for Scientific Research on Innovative Areas, “Coordination Programming,” Area 2107, No. 21108005, and Science Research (B) (No. 25286011) from MEXT, Japan.


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Figure captions: Chart 1. Chemical structures and CPK models (Co ions are omitted) of TMePP, TBrPP, and TIPP. Figure 1. (a) Large-scale (50 × 50 nm2) and (b) high-resolution (15 × 15 nm2) EC-STM images of CoTMePP adlayer on Au(111) in 0.05 M HClO4 obtained at (a) 0.70 and (b) 0.85 V vs. RHE. The tip potential and tunneling current were 0.40 V vs. RHE and 0.50 nA for (a) and 0.42 V vs. RHE and 1.00 nA for (b), respectively. Figure 2. Potential-dependent STM images of CoTMePP adlayer on Au(111) acquired at (a) 0.95, (b) 1.00, and (c) 0.90 V vs. RHE. The tip potential and tunneling current were 0.35 V and 1.0 nA, respectively. Figure 3. Large-scale (50 × 50 nm2) and high-resolution (15 × 15 nm2) EC-STM images of CoTBrPP adlayer (a, b) and CoTIPP adlayer (d, e) on Au(111) in 0.1 M HClO4 acquired at (a, b) 0.65 V, (d) 0.55 V, and (e) 0.58 V vs. RHE, respectively. The tip potentials and tunneling currents were (a) 0.40 V and 0.30 nA, (b) 0.38 V and 0.20 nA, (d) 0.40 V and 1.5 nA, and (e) 0.40 V and 1.0 nA, respectively. Figure 4. Linear sweep voltammograms for O2 reduction by (a) CoTMePP-, (b) CoTBrPP-, and (c) CoTIPP-adsorbed Au(111) electrodes in 0.1 M HClO4 saturated with O2. The scan rate was 100 mV s–1. Table 1. Adlattice constants and O2 reduction peak potentials of CoTMePP, CoTBrPP, and CoTIPP adlayers


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Chart 1. Chemical structures and CPK models (Co ions are omitted) of TMePP, TBrPP, and TIPP. 80x80mm (300 x 300 DPI)


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Figure 1. (a) Large-scale (50 × 50 nm2) and (b) high-resolution (15 × 15 nm2) EC-STM images of CoTMePP adlayer on Au(111) in 0.1 M HClO4 obtained at (a) 0.70 and (b) 0.85 V vs. RHE. The tip potential and tunneling current were 0.40 V vs. RHE and 0.50 nA for (a) and 0.42 V vs. RHE and 1.00 nA for (b), respectively. 85x45mm (300 x 300 DPI)


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Potential-dependent STM images of CoTMePP adlayer on Au(111) acquired at (a) 0.95, (b) 1.00, and (c) 0.90 V vs. RHE. The tip potential and tunneling current were 0.35 V and 1.0 nA, respectively. 155x55mm (300 x 300 DPI)


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Figure 3. Large-scale (50 × 50 nm2) and high-resolution (15 × 15 nm2) EC-STM images of CoTBrPP adlayer (a, b) and CoTIPP adlayer (d, e) on Au(111) in 0.1 M HClO4 acquired at (a, b) 0.65 V, (d) 0.55 V, and (e) 0.58 V vs. RHE, respectively. The tip potentials and tunneling currents were (a) 0.40 V and 0.30 nA, (b) 0.38 V and 0.20 nA, (d) 0.40 V and 1.5 nA, and (e) 0.40 V and 1.0 nA, respectively. 155x113mm (300 x 300 DPI)


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Figure 4. Linear sweep voltammograms for O2 reduction by (a) CoTMePP-, (b) CoTBrPP-, and (c) CoTIPPadsorbed Au(111) electrodes in 0.1 M HClO4 saturated with O2. The scan rate was 100 mV s–1. 62x46mm (300 x 300 DPI)


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TOC 70x51mm (300 x 300 DPI)


Tuning Porphyrin Assembly and Electrochemical Catalytic Activity with

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