Marcus Theory and Electrochemical Impedance ... - ACS Publications

Letter pubs.acs.org/JPCL

Rapid Electron Transport Phenomenon in the Bis(terpyridine) Metal Complex Wire: Marcus Theory and Electrochemical Impedance Spectroscopy Study Hiroaki Maeda, Ryota Sakamoto,* and Hiroshi Nishihara* Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Downloaded by STOCKHOLM UNIV on September 14, 2015 | http://pubs.acs.org Publication Date (Web): September 11, 2015 | doi: 10.1021/acs.jpclett.5b01725

S Supporting Information *

ABSTRACT: The authors reported previously that bis(terpyiridne)iron(II) complex oligomer wires possess outstanding long-range intrawire electron transport ability. Here, molecular arrays of gold-electrode−bis(terpyridine)iron(II)−ferrocene are constructed by stepwise coordination as simple models of the oligomer wire system. The fast electron transfer between the terminal ferrocene and the gold electrode through the bis(terpyiridne)iron(II) complex unit is studied by potential step chronoamperometry (PSCA) and electrochemical impedance spectroscopy (EIS). Tafel plots derived from PSCA are analyzed based on Marcus theory. The plots reveal greater first-order electron transfer rate constant, weaker electronic coupling between the terminal ferrocene and the gold electrode, and smaller reorganization energy than shown by a conventional ferrocenylalkanethiol self-assembled monolayer. The electron transfer rate constants estimated by EIS agree with the PSCA results.

F

features a step−terrace structure with a smooth texture (Figure 2a). On the other hand, Au−A1FeT1 and Au−A2FeT2 display uniform but slightly rugged surface, which is resulted from homogeneous surface modification (Figure 2b,c). The cyclic voltammogram of Au−A1FeT1 (Figure 3a) features reversible redox waves at 0.19 and 0.83 V vs Ag+/Ag, which are derived respectively from the Fc+/Fc and [Fe(tpy)2]3+/2+ redox couples. Similarly, Au−A2FeT2 displays reversible redox waves at 0.29 and 0.90 V vs Ag+/Ag (Figure 3d). The proportional increases in the anodic and cathodic peak currents with the scan rate reveals that the ferrocene-terminated Fe(tpy)2 array is confined on the gold electrode (Figure 3b,c,e,f). The surface coverage of the terminal ferrocene (ΓFc) was estimated to be (4.0 ± 1.6) × 10−11 mol cm−2 for Au−A1FeT1 and (8.5 ± 1.6) × 10−11 mol cm−2 for Au−A2FeT2, by integrating its anodic current (see Supporting Information for detail). Next, the two samples were subjected to PSCA with a potential step set such that electron transfer occurred between the terminal ferrocene and the gold electrode. During the electron transfer, the valence of the Fe(tpy)2 unit remained at +2. In this scheme, the decay of current (I) with time (t) is expressed as follows:13

uture molecular electronic devices may rely on molecular wires with smooth and rapid charge transport to achieve high performance and low energy use. Recent energetic research has established that various types of molecular wire show supreme electron transport ability. For example, their electron transfer rates change little with distance; that is, they show very small attenuation factors β.1−5 We found that oligomer wires of bis(terpyidine)metal complex [M(tpy)2] show one of the smallest β values (0.002 ± 0.001 Å−1) of any molecular wire series, because the [M(tpy)2] unit serves as a sound electron hopping site, promoting intrawire electron transfer.6−8 Thorough quantitative investigation of charge transport is important to the design and realization of superior electron transport systems. Given this, here we prepare two types of goldelectrode−Fe(tpy)2−ferrocene molecular arrays as the simplest model of our [M(tpy)2] oligomer wire system, and then analyze electron transfer through the Fe(tpy)2 unit between the terminal ferrocene and the gold electrode using potential-step chronoamperometry (PSCA) and electrochemical impedance spectroscopy (EIS). Tafel plots derived from the PSCA result are reproduced using Marcus theory9,10 to extract parameters relating to the electron transfer, such as the first-order rate constant at zero overpotential (ket0). The EIS results are used to propose an equivalent circuit suitable for the Fe(tpy)2 system to fit the observed impedance spectra, such that various parameters are again extracted. The Fe(tpy)2−ferrocene arrays, Au−A1FeT1 and Au− 2 A FeT2, were constructed by stepwise coordination (Figure 1, the formation process is described in detail in the Supporting Information) on gold(111) surfaces.6−8,11,12 Figure 2 shows topographical images of gold substrates before and after the construction of the molecular arrays. A bare gold(111) substrate © XXXX American Chemical Society

I = I0exp( −kett )

(1)

where I0 is the current value at t = 0, and ket is the first-order electron transfer rate constant. The equation indicates that the linear slope of a ln I−t plot corresponds to − ket. The ket values collected at various overpotentials (η) are assembled in the Tafel Received: August 7, 2015 Accepted: September 9, 2015

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DOI: 10.1021/acs.jpclett.5b01725 J. Phys. Chem. Lett. 2015, 6, 3821−3826

Letter


The Journal of Physical Chemistry Letters

Figure 1. Preparation process of Fe(tpy)2-ferrocene arrays, Au−A1FeT1 and Au−A2FeT2.

Figure 2. STM topographical images acquired with a sample bias of +0.3 V under an ambient condition. (a) Bare gold(111), (b) Au−A1FeT1, and (c) Au−A2FeT2.

plots of Figure 4. The ket value at η = 0, ket0, is then extracted employing Marcus theory9,10

where ν is the electronic coupling between the electrode and the terminal ferrocene, ρ is the density of states of the metal electrode, λ is the reorganization energy, and kB is the Boltzmann constant. The fitting result is overlaid in Figure 4, and values of νρ, λ, and ket0 are listed in Table 1. Reference data for a selfassembled monolayer (SAM) of ferrocene-terminated hexadecanethiol (Au−SC16O2CFc) are also listed.10 The ket0 values of Au−A1FeT1 and Au−A2FeT2 were estimated to be respectively (3.0 ± 0.9) × 102 s−1 and (1.4 ± 0.2) × 102 s−1, 2 orders of magnitude greater than that of Au− SC16O2CFc (2.5 s−1), thereby proving the Fe(tpy)2 wires’ good electron transport ability. Note that the distance between the terminal ferrocene and the gold electrode is greater in Au− A1FeT1 and Au−A2FeT2 than in Au−SC16O2CFc. Nonetheless, the νρ values of Au−A1FeT1 and Au−A2FeT2 are 1 order of magnitude smaller than that of Au−SC16O2CFc. Given the fact that ρ is supposed to be almost constant at similar applied

ket = k f + kb ∞

∫ ∫

= υρkBT

⎧ exp⎨− ⎩

( )⎡⎢⎣x − kBT 4λ

λ + eη ⎤ ⎦ kBT ⎥

1 + exp(x)

2⎫

⎬ ⎭

d

−∞

x + υρkBT

∞

⎧ exp⎨− ⎩

( )⎡⎢⎣x − kBT 4λ

λ − eη ⎤ ⎦ kBT ⎥

1 + exp(x)

2⎫

⎬ ⎭

dx

−∞

(2) 3822


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Figure 3. Cyclic voltammograms of (a) Au−A1FeT1 and (d) Au−A2FeT2, peak current-scan rate dependence for the Fc+/Fc redox couple in (b) Au− A1FeT1 and (e) Au−A2FeT2, and that for the [Fe(tpy)2]3+/2+ redox couple in (c) Au−A1FeT1 and (f) Au−A2FeT2.

potentials, the electronic coupling between the electrode and the terminal ferrocene, ν, should be weaker in the Fe(tpy)2 system than in Au−SC16O2CFc. The smoother electron transfer in Au− A1FeT1 and Au−A2FeT2 with weaker electronic coupling, compared with Au−SC16O2CFc, stems from their different electron transfer modes. The alkyl spacer in Au−SC16O2CFc provides electron transfer based on the superexchange mechanism, which requires electronic coupling between the redox site and the electrode mediated by the spacer.14 In contrast, electron transfer in the Fe(tpy)2 system is dominated by the intrawire electron hopping mechanism,6−8 where electronic coupling is insignificant. Note that λ of Au−A1FeT1 and Au− A2FeT2 is 1 order of magnitude smaller than that of Au−

SC16O2CFc, and may contribute to the smooth electron transport. In addition, the weak dependence of ket on the overpotential (Figure 4) indicates sequential hopping electron transport, as discussed elsewhere regarding ferrocene-terminated peptide-chains on gold electrodes.15−18 Let us briefly compare Au−A1FeT1 and Au−A2FeT2. In the entire η range, Au−A1FeT1 exhibits greater ket values than Au− A2FeT2, which is consistent with our previous experimental finding of the π-conjugated azobenzene bridge affording smoother electron transport in bis(terpyridine)metal complex wires.11,12 Au−A1FeT1 exhibits a νρ value nearly two times greater than that of Au−A2FeT2. The better electronic coupling 3823


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Figure 4. Tafel plots for Au−A1FeT1 (blue diamonds) and Au−A2FeT2 (orange diamonds). The green dashed lines show the theoretically reproduced curves.

in Au−A1FeT1 is also mediated by the π-conjugated azobenzenebridged anchor ligand A1. The two wires were subjected to EIS (Figure 5), and an equivalent circuit (Figure 6) was employed to reproduce the electron transport via the Fe(tpy)2 unit between the terminal ferrocene and the gold electrode. The simulated spectra are overlaid in Figure 5, and the parameters used for the fitting are listed in Table 2. The ket0 values may be extracted from the parameters using the following equation:19

ket0 =

RT n F R et ΓFcA 2 2

Figure 5. Representative electrochemical impedance spectra of (a,b) Au−A1FeT1 and (c,d) Au−A2FeT2: log Z (impedance)-log f (applied potential frequency) curves for (a) Au−A1FeT1 and (c) Au−A2FeT2; Phase-log f (frequency) on alternating potential frequency for (b) Au− A1FeT1 and (d) Au−A2FeT2. Green solid lines correspond to the fitting curves.

(3)

where R is the gas constant, T is temperature, n is the stoichiometric number of electrons involved in the electron transfer, F is the Faraday constant, and A is the electrode area. The two wires show differences in the admittance of the constant phase element (Y0) and the resistance of the electrical double layer (Rdl). Y0 should be proportional to the coverage of the terminal ferrocene, ΓFc. Therefore, the greater ΓFc in Au− A2FeT2 than in Au−A1FeT1 gives the former a greater Y0 value. The small Rdl for Au−A2FeT2 stems from the shorter length of its molecular compared with Au−A1FeT1. The ket0 values are estimated to be (2.1 ± 0.8) × 102 s−1 for Au−A1FeT1 and (0.89 ± 0.30) × 102 s−1 for Au−A2FeT2, in good agreement with those estimated from PSCA (Table 1). According to the PSCA analysis based on Marcus theory, the reorganization energy λ of ferrocene at the terminus is ca. 0.1 eV, which is much smaller than that of ferrocene-terminated alkyl chain SAMs (ca. 0.80−0.90 eV)9,10,20 and may contribute to the rapid electron transport. An additional factor promoting the rapid electron transfer stems from the Rdl element in the equivalent circuit, which represents the penetration of counterions into the monolayers,19,21 although a typical equivalent circuit for surface-confined redox-active SAMs does not contain the element (i.e., Rdl = ∞).22−24 Finite Rdl

Figure 6. Equivalent circuit used to reproduce the EIS shown in Figure 4. Impedance of an constant phase element (CPE), ZCPE, is expressed as ZCPE = (jω)−n/Y0.

elements are essential for the fitting of the EIS for Au−A1FeT1 and Au−A2FeT2, which both should possess large enough spaces for counterions to enter and leave smoothly. Such spaces may be realized by the bulky, cationic, and robust Fe(tpy)2 complex unit, which prevents the accumulation and assembly of the molecular array. This is in contrast to the alkyl- or peptide-chain SAMs that are flexible and slender, where hydrophobic interactions and hydrogen bonding allow them to form dense films. Densely packed molecular wires prevent ion penetration and correspond to infinite Rdl values, such that the Rdl element is ignored.

Table 1. νρ, λ, and ket0 Values for the Reduction and Oxidation of the Terminal Ferrocene Unit reduction sample 1

1

Au−A FeT Au−A2FeT2 Au−SC16O2CFca a

3 −1

νρ /10 s

−1

eV

7.0 ± 1.3 3.8 ± 0.6 67.3

oxidation 3 −1

λ/eV

νρ/10 s

0.10 ± 0.03 0.12 ± 0.01 0.85

−1

eV

6.1 ± 1.3 3.2 ± 0.5 67.3

λ/eV

ket0/102 s−1

0.08 ± 0.02 0.10 ± 0.01 0.85

3.0 ± 0.9 1.4 ± 0.2 0.025

From ref 10. 3824


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The Journal of Physical Chemistry Letters Table 2. Parameters Employed for the EIS Fitting and Estimated ket0 Values sample 1

1


Au−A FeT Au−A2FeT2

Rsol/Ω

Ret/Ω

Y0/μS

n

Rdl/MΩ

Cdl/μF

ket0/102 s−1

162 ± 3 182 ± 8

145 ± 25 145 ± 42

12 ± 2 23 ± 4

0.95 ± 0.02 0.94 ± 0.01

5.0 ± 0.5 3.1 ± 0.7

0.029 ± 0.009 0.033 ± 0.011

2.1 ± 0.8 0.89 ± 0.30

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In conclusion, two types of ferrocene-terminated Fe(tpy)2 complex molecular arrays (Au−A1FeT1 and Au−A2FeT2) were constructed on gold electrode surfaces. Parameters related to the intrawire electron transfer between their terminal ferrocene and the gold electrode were quantified by applying Marcus theory to Tafel plots from PSCA and also by EIS. Au−A1FeT1 gave a greater first-order electron transfer rate constant (ket0) and greater electron coupling between the terminal ferrocenyl moiety and the gold electrode (ν) than did Au−A2FeT2, owing to its πconjugated azobenzene linker ligand A1. In comparison with a redox-molecule-terminated alkyl SAM, Au−A1FeT1, and Au− A2FeT2 featured greater ket0, weaker ν, and smaller reorganization energy λ, reflecting the sequential electron hopping mechanism. ket0 values estimated by EIS agree with the PSCA results. The employed equivalent circuit contains a contribution by the electrical double layer resistance (Rdl), affording smooth electron transport. The results demonstrate the excellent electron transport ability of the bis(terpyridine)metal complex wires.

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(1) Hong, W.; Li, H.; Liu, S.-X.; Fu, Y.; Li, J.; Kaliginedi, V.; Decurtins, S.; Wandlowski, T. Trimethylsilyl-Terminated Oligo(phenylene ethynylene)s: An Approach to Single-Molecule Junctions with Covalent Au−C σ-Bonds. J. Am. Chem. Soc. 2012, 134, 19425−19431. (2) Li, Q.; Yao, C.; Wang, X.; Wang, F. Enhancing Molecular Conductance of Oligo(p-phenylene ethynylene)s by Incorporating Ferrocene into Their Backbones. J. Phys. Chem. C 2012, 116, 17853− 17861. (3) Li, Z.; Park, T.-H.; Rawson, J.; Therien, M. J.; Borguet, E. QuasiOhmic Single Molecule Charge Transport through Highly Conjugated meso-to-meso Ethyne-Bridged Porphyrin Wires. Nano Lett. 2012, 12, 2722−2727. (4) Sikes, H. D.; Smalley, J. F.; Dudek, S. P.; Cook, A. R.; Newton, M. D.; Chidsey, C. E. D.; Feldberg, S. W. Rapid Electron Tunneling Through Oligophenylenevinylene Bridges. Science 2001, 291, 1519− 1523. (5) Giacalone, F.; Segura, J. L.; Martin, N.; Guldi, D. M. Exceptionally Small Attenuation Factors in Molecular Wires. J. Am. Chem. Soc. 2004, 126, 5340−5341. (6) Nishimori, Y.; Kanaizuka, K.; Kurita, T.; Nagatsu, T.; Segawa, Y.; Toshimitsu, F.; Muratsugu, S.; Utsuno, M.; Kume, S.; Murata, M.; et al. Superior Electron-Transport Ability of π-Conjugated Redox Molecular Wires Prepared by the Stepwise Coordination Method on a Surface. Chem. - Asian J. 2009, 4, 1361−1367. (7) Sakamoto, R.; Katagiri, S.; Maeda, H.; Nishimori, Y.; Miyashita, S.; Nishihara, H. Electron Transport Dynamics in Redox-MoleculeTerminated Branched Oligomer Wires on Au(111). J. Am. Chem. Soc. 2015, 137, 734−741. (8) Sakamoto, R.; Wu, K.-H.; Matsuoka, R.; Maeda, H.; Nishihara, H. π-Conjugated Bis(terpyridine)metal Complex Molecular Wires. Chem. Soc. Rev. 2015, DOI: 10.1039/C5CS00081E. (9) Sek, S.; Palys, B.; Bilewicz, R. Contribution of Intermolecular Interactions to Electron Transfer through Monolayers of Alkanethiols Containing Amide Groups. J. Phys. Chem. B 2002, 106, 5907−5914. (10) Chidsey, C. E. D. Free Energy and Temperature Dependence of Electron Transfer at the Metal-Electrolyte Interface. Science 1991, 251, 919−922. (11) Kurita, T.; Nishimori, Y.; Toshimitsu, F.; Muratsugu, S.; Kume, S.; Nishihara, H. Surface Junction Effects on the Electron Conduction of Molecular Wires. J. Am. Chem. Soc. 2010, 132, 4524−4525. (12) Katagiri, S.; Sakamoto, R.; Maeda, H.; Nishimori, Y.; Kurita, T.; Nishihara, H. Terminal Redox-Site Effect on the Long-Range Electron Conduction of Fe(tpy)2 Oligomer Wires on a Gold Electrode. Chem. Eur. J. 2013, 19, 5088−5096. (13) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (14) Yamamoto, H.; Waldeck, D. H. Effect of Tilt-Angle on Electron Tunneling through Organic Monolayer Films. J. Phys. Chem. B 2002, 106, 7469−7473. (15) Morita, T.; Kimura, S. Long-Range Electron Transfer over 4 nm Governed by an Inelastic Hopping Mechanism in Self-Assembled Monolayers of Helical Peptides. J. Am. Chem. Soc. 2003, 125, 8732− 8733. (16) Takeda, K.; Morita, T.; Kimura, S. Effects of Monolayer Structures on Long-Range Electron Transfer in Helical Peptide Monolayer. J. Phys. Chem. B 2008, 112, 12840−12850. (17) Brooksby, P. A.; Anderson, K. H.; Downard, A. J.; Abell, A. D. Electrochemistry of Ferrocenoyl β-Peptide Monolayers on Gold. Langmuir 2010, 26, 1334−1339.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b01725.

■

REFERENCES

Experimental details, chronoamperograms and ln I−t plots for Au−A1FeT1 and Au−A2FeT2. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present communication is supported by Grants-in-Aid from MEXT of Japan (Nos. 25107510, 26708005, 26107510, 26620039, 15H00862, 15K13654, areas 2406 [All Nippon Artificial Photosynthesis Project for Living Earth], 2506 [Science of Atomic Layers], and 2509 [Molecular Architectonics]). R.S. is grateful to Ogasawara Foundation for the Promotion of Science & Engineering, Noguchi Institute, Tokuyama Science Foundation, Asahi Glass Foundation, The Murata Science Foundation, Iketani Science and Technology Foundation, The Japan Prize Foundation, Kao Foundation for Arts and Sciences, Japan Association for Chemical Innovation, The MIKIYA Science and Technology Foundation, Yazaki Memorial Foundation for Science and Technology, Shorai Foundation for Science and Technology, The Hitachi Global Foundation, Kumagai Foundation for Science and Technology, Foundation for Interaction in Science & Technology (FIST), and The Foundation for The Promotion of Ion Engineering for financial supports. 3825


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The Journal of Physical Chemistry Letters (18) Okamoto, S.; Morita, T.; Kimura, S. Electron Transfer through a Self-Assembled Monolayer of a Double-Helix Peptide with Linking the Terminals by Ferrocene. Langmuir 2009, 25, 3297−3304. (19) Abhayawardhana, A. D.; Sutherland, T. C. Heterogeneous Proton-Coupled Electron Transfer of an Aminoanthraquinone SelfAssembled Monolayer. J. Phys. Chem. C 2009, 113, 4915−4924. (20) Weber, K.; Creager, S. E. Voltammetry of Redox-Active Groups Irreversibly Adsorbed onto Electrodes. Treatment Using the Marcus Relation between Rate and Overpotential. Anal. Chem. 1994, 66, 3164− 3172. (21) Boubour, E.; Lennox, R. B. Potential-Induced Defects in nAlkanethiol Self-Assembled Monolayers Monitored by Impedance Spectroscopy. J. Phys. Chem. B 2000, 104, 9004−9010. (22) Creager, S. E.; Wooster, T. T. A New Way of Using ac Voltammetry To Study Redox Kinetics in Electroactive Monolayers. Anal. Chem. 1998, 70, 4257−4263. (23) Arikuma, Y.; Takeda, K.; Morita, T.; Ohmae, M.; Kimura, S. Linker Effects on Monolayer Formation and Long-Range Electron Transfer in Helical Peptide Monolayers. J. Phys. Chem. B 2009, 113, 6256−6266. (24) Eckermann, A. L.; Feld, D. J.; Shaw, J. A.; Meade, T. J. Electrochemistry of redox-active self-assembled monolayers. Coord. Chem. Rev. 2010, 254, 1769−1802.

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Marcus Theory and Electrochemical Impedance ... - ACS Publications

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