Article pubs.acs.org/Langmuir
Redox-Active π‑Conjugated Organometallic Monolayers: Pronounced Coulomb Blockade Characteristic at Room Temperature Chiao-Pei Chen, Wan-Rou Luo, Chen-Ni Chen, Shin-Mou Wu, Shuchen Hsieh, Chao-Ming Chiang, and Teng-Yuan Dong* Department of Chemistry, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung, Taiwan ABSTRACT: We report the electrical transport characteristics of a series of molecular wires, fc-CCC6H4SAc (fc = ferrocenyl; Ac = acetyl) and AcS-C6H4CC-(fc)n-CC C6H4SAc (n = 2, 3), consisting of multiple redox-active ferrocenyl centers. The self-assembled monolayers of these molecular wires on Au surfaces were comprehensively characterized by electrochemistry and conductive atomic force microscopy techniques. Characterization of the wires revealed that electron transport is made extremely efficient by the organometallic redox states. There is a strong electronic coupling between ferrocenyl moieties, and superior electrontransport ability exists through these semirigid molecular wires. Standard rate constants for the electron transfer between the electrode and the ferrocenyl moieties were measured for the monolayers by a potential-step chronoamperometry technique. The electron conduction through the molecular wires was estimated using the monolayers as a bridge from the Au(111) metal surface to the gold tip of a conductive atomic force microscope (CAFM). Using the CAFM, Coulomb blockade behavior arising from the capacitive charging of the multinuclear redox-active molecules was observed at room temperature. The conductance switching was mediated by the presence of various ferrocenyl redox states and each current step corresponded to a specific redox state.
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(Å−1) in the equation for the distance-dependent electrontransfer rate constant (eq 1). In eq 1, k (s−1) is the electrontransfer rate constant, d (Å) is the distance between the electrode and the metal center, and k° is the rate constant when the electrode and metal center are closest at a distance of d°. A small β value, therefore, indicates less impedance in the electron transport pathway through the molecule.
INTRODUCTION Functionalized molecular building blocks, such as molecular wires and switches, are essential structures for developing nanoscale electronic devices.1−5 This field is highly interdisciplinary, encompassing aspects of synthetic chemistry, physical measurements, device fabrication, and engineering. A molecular wire should consist of an extended π-conjugated rigid molecular chain that can promote strong electronic coupling between the two groups, such as redox centers or electrodes, attached to the chain ends. Many studies have investigated the electrical characteristics of organic molecular wires such as dithiolated alkyl chains,6 π-conjugated molecules,7−11 polypeptide chains, and DNA molecules bridged between two gold electrodes.12,13 Recently, several reports have described electron conduction through redox-active molecular wires of mononuclear and multinuclear transition metal complexes that were fabricated on an electrode surface by interfacial stepwise coordination methods.14−20 It is a convenient method to lengthen molecular wires onto the metallic surface because the reactions are programmable. Preparations of π-conjugated linear and branched M(tpy)2 (M = Fe, Co; tpy = 2,2′:6′,2″-terpyridine) oligomer wires using a stepwise coordination method have been described for the attachment of metal complexes onto the gold surface. The ability to conduct electrons over a long range along the M(tpy)2 oligomer wires was observed, and a strong electronic coupling between the redox-active M(tpy)2 metallic centers in a single molecular wire was found. The electronic coupling showed a small value for the attenuation factor β © 2013 American Chemical Society
k = k°exp{−β(d − d°)}
(1)
The present work on electronic conduction in molecular wires is based on insights gained from the electron transfer in mixed-valence biferrocenium molecules. In biferrocenium molecules, rapid intramolecular electron transfer processes were observed. Recently, we reported the electron transfer characteristics in a series of molecular wires containing redoxactive ferrocenylethynyl chains linked to two [M(η5-C5R5)P2] metallic centers ([(η5-C5R5)P2M-CC-(fc)n-CC-MP2(η5C5R5)] where fc = ferrocenyl and P2 = Ph2PCH2CH2PPh2).21,22 We found that the redox-active multinuclear ferrocenylethynyl spacers exhibit strong ferrocene-to-ferrocene electronic interactions along the semirigid step-like ferrocenyl molecular axis. The polyferrocenyl spacers themselves are insulators but can be converted into semiconducting materials by selective oxidation of ferrocenyl units to form mixed-valence polyferrocenium Received: September 23, 2012 Revised: January 19, 2013 Published: January 21, 2013 3106
dx.doi.org/10.1021/la303796r | Langmuir 2013, 29, 3106−3115
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Scheme 1. Nominal Adsorbate Structures of the Ferrocenyl-Ethynyl Monolayers of fcSAc, AcS-fc2-SAc (n = 2), AcS-fc3-SAc (n = 3), fcS, S-fc2-S (n = 2), and S-fc3-S (n = 3)
spacers. The electronic coupling through the ferrocenylethynyl chain can control the rate of electron transfer between the groups on the two chain ends. As illustrated in Scheme 1, we fabricated self-assembled monolayers (SAMs) of semirigid steplike ferrocenylethynyl molecules on Au(111) surfaces. Our design principle for the step-like ferrocenyl molecular wires fulfills the following criteria: (i) the wires contain redox-active π-conjugated semirigid ferrocenyl units to enhance their capability for electronic conduction along the molecular axis, and (ii) a modular synthetic approach allows for control of the length of the wire. We observed strong electronic coupling between ferrocenyl moieties and superior electron-transport ability through these semirigid molecular wires. This paper describes a synthetic pathway to, and the spectroscopic and heterogeneous electrochemical characterizations of, a family of spontaneously stable SAMs of ethynylferrocenyl molecules self-assembling onto a Au(111) surface. The monolayers were prepared using fc-CCC6H4SAc (1) and AcS-C6H4CC-(fc)n-CCC6H4SAc (2, n = 2; 3, n = 3) as precursors, where fc and Ac are ferrocenyl and acetyl moieties, respectively. Thiols or acetyl-protected thiol moieties were used as the Au-binding groups. We denote the fcCCC 6 H 4 SAc monolayers simply as fcSAc. For compounds 2 and 3, each molecule contains two or three ferrocenyl units. The monolayers of 2 and 3 are AcS-fc2-SAc and AcS-fc3-Sac, respectively. Compounds 1−3 were protected by thioacetate groups and were deprotected by adding diethylamine to the electrode coating solution to prepare monolayers of fcS, S-fc2-S, and S-fc3-S for the electrochemical studies. The electron transfer reactions of interest involve the Fe2+/3+ redox couple within monolayers of fcS, S-fc2-S, and Sfc3-S. Measurements of the standard electron transfer rate constants (kE(1/2)) in these monolayers have enabled us to gain a fundamental understanding of electron conduction between the electrode and the ferrocenyl moieties. Most importantly, we developed one-dimensional dithiolated ferrocenylethynyl molecules that function as wires. As shown in Scheme 2, multinuclear redox-active molecular wires of AcS-fcn-SAc and S-fcn-S (n = 2−3) are intended to form a bridge from the Au(111) metal surface to the gold tip of a conductive atomic force microscope (CAFM). Quantitative electron transfer characteristics and current vs potential (I−V) measurements (by CAFM) for these molecular wires are also described here.
Scheme 2. Apparatus Used for CAFM Measurements Depicting a Multinuclear Ferrocenyl Molecular Wire Attached to the Gold Tip
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EXPERIMENTAL SECTION
General Information. The preparations involving air-sensitive materials were carried out using standard Schlenk techniques under an inert atmosphere of N2. Chromatography was performed on neutral Al2O3 (act. III). Solvents were dried and purified using standard laboratory procedures. Dichloromethane was distilled over P2O5. Acetonitrile and DMF were dried over 4 Å molecular sieves. Tetrahydrofuran was distilled from sodium/benzophenone. Iodoethynylferrocene (1) was prepared by a modified literature procedure. The sample of S-4-(ferrocenylethynyl)phenyl ethanethioate (AcS-C6H4 CC-fc, 1) was prepared according to the literature procedure.23 The starting materials 1,1′-bis(ethynyl)biferrocene,24 1,1′-bis(ethynyl)triferrocene,24 and S-acetyl-4-iodothiophenol25 were also prepared according to the literature procedures. All other chemicals in this study were of reagent grade and were used as received. Preparation of AcS-C6H4CC-(fc)2-CCC6H4SAc (2). As shown in Scheme 3, 1,1′-bis(ethynyl)biferrocene (0.1 g, 0.24 mmol) and S-acetyl-4-iodothiophenol (0.14 g, 0.5 mmol) were dissolved in a deoxygenated solution of THF/diisopropylethylamine (1:1, 30 mL). Pd(PPh3)2Cl2 (10 mol %) and CuI (20 mol %) were then added to the reaction mixture. The mixture was stirred at 50 °C for 16 h. After cooling to room temperature, the solvent was removed under reduced pressure. The crude product was purified by chromatography on silica gel, eluting with hexane/CH2Cl2 (1:1). The yield for the reaction was approximately 46%. The physical properties are as follows. 1H NMR (CDCl3): δ 2.44 (s, 6H, CH3), 4.09 (s, 4H, fc-Cp), 4.22 (s, 4H, fc-Cp), 4.27 (s, 4H, fc-Cp), 4.41 (t, 4H, fc3107
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Scheme 3. Syntheses of Complexes 2 and 3 from the HCC-(fc)n-CCH Spacers
Cp), 7.34 (d, J = 7.8 Hz, 4H, Ph), 7.43 (d, J = 8.4 Hz, 4H, Ph). 13C NMR (CDCl3): δ 30.41 (s, CH3), 65.64 (s, fc-Cp), 68.39 (s, fc-Cp), 69.87 (s, fc-Cp), 70.30 (s, fc-Cp), 72.91 (s, fc-Cp), 84.38 (s, fc-Cp), 85.51, 90.48 (s, CC), 125.58, 127.02, 132.01, 134.28 (s, Ph), 193.90 (s, CO). MS (ESI): M+ at m/z 718. Anal. Calcd for C40H30O2S2Fe2: C, 66.87; H, 4.21. Found: C, 66.73; H, 4.66. Preparation of AcS-C6H4CC-(fc)3-CCC6H4SAc (3). This molecule was prepared by the same methods as AcS-C6H4C C-(fc)2-CCC6H4SAc. The yield was approximately 42%. The physical properties are as follows. 1H NMR (C6D6): δ 1.81 (s, 6H, CH3), 3.85 (t, J = 1.8 Hz, 4H, fc-Cp), 3.98 (t, J = 1.8 Hz, 4H, fc-Cp), 4.12 (t, J = 1.5 Hz, 4H, fc-Cp), 4.14 (t, J = 1.8 Hz, 4H, fc-Cp), 4.24 (t, J = 1.8 Hz, 4H, fc-Cp), 4.29 (t, J = 1.8 Hz, 4H, fc-Cp), 7.20 (d, J = 8.4 Hz, 4H, Ph), 7.38 (d, J = 8.4 Hz, 4H, Ph). 13C NMR (C6D6): δ 29.61 (s, CH3), 66.13 (s, fc-Cp), 68.16 (s, fc-Cp), 68.36 (s, fc-Cp), 69.32 (s, fc-Cp), 69.88 (s, fc-Cp), 70.55 (s, fc-Cp), 73.13 (s, fc-Cp), 83.43 (s, fcCp), 85.63 (s, fc-Cp), 86.11, 91.04 (s, CC), 125.72, 128.29, 132.10, 134.59 (s, Ph), 191.73 (s, CO). MS (MALDI-TOF): M+ at m/z 902. Anal. Calcd for C50H38O2S2Fe3: C, 66.54; H, 4.24. Found: C, 66.75; H, 4.60. Electrochemistry. Micro disk-shaped gold electrodes were purchased from CH Instruments (50 μm radius with >99.99% purity). The gold electrode was polished successively with 1, 0.3, and 0.05 μm alumina, followed by cleaning with 5% H2SO4 and water in a supersonic cleaner. The electrode was then rinsed with water, CH2Cl2, and ethanol. Compounds 1−3 were protected with thioacetate groups. The protected compounds were deprotected by adding ∼0.4 M of diethylamine to the electrode coating solution. Monolayers were prepared by immersing a freshly cleaned disk-shaped electrode in a CH2Cl2 coating solution containing the deprotected 1 mM ferrocene derivative under nitrogen at 50 °C for approximately 24 h. After preparing the monolayer, the electrode was rinsed with CH2Cl2 several times. Electrochemical measurements were performed using a CHI-600B system from CH Instruments Incorporation. The reference electrode was Ag/AgCl in a saturated KCl solution. The measurements for monolayers were performed in a standard three-compartment cell under N2 at 25 °C equipped with a Pt-coil counter electrode, the monolayer-protected gold working electrode and an Ag/AgCl reference electrode. Electrochemistry was performed in a CH2Cl2 solution containing 0.1 M (n-C4H9)4NPF6 electrolyte. Under these conditions, ferrocene showed a reversible redox couple at E1/2 = 0.46 V. The number of electroactive centers in the monolayers was calculated from the CV measurements. The area under the peak was integrated and divided by the scan rate to obtain the amount of charge passed to the ferrocene moieties. The surface charge densities (μC/ cm2) obtained from this procedure can be converted to ferrocene surface coverage by dividing by the electron charge. Potential step chronoamperometry measurements were also performed using the CHI-600B system. The potential was held more positive than the E1/2 of the ferrocenyl moiety to oxidize it to the ferrocenium ion. After oxidation, a potential step down to E1/2 was applied in order to observe the reduction current change with time. The negative overpotential measurements were carried out in a similar fashion. The electron-transfer rate constants, k, were estimated using the equation, I(t) = I0 exp(−kt). First-order rate constants were extracted from the slopes of the plots of ln I vs t. The k values increased with the magnitude of the overpotential. The intersection of the linear sections of the cathodic and anodic regions of the log k vs potential plots yielded the standard electron transfer rate constant (kE(1/2)) in the absence of overpotential effects.
CAFM Measurements. CAFM measurements were performed to probe the electronic properties of the monolayers of AcS-fcn-SAc and S-fcn-S. Au/glass (∼250 nm Au deposited on borosilicate glass (1.1 × 1.1 cm2)) were used for the CAFM measurements. Au/glass plates were annealed with a butane flame until they glowed dark red (∼1 min) in a dark room and then removed from the flame and allowed to cool down for approximately 30 s. This procedure was repeated three times, and then the substrates were blown with nitrogen for 3 min. This treatment gave an Au(111)-dominated surface. Monolayers were prepared by the immersion of an Au/glass plate in a dried CH2Cl2 solution of the ferrocenyl-ethynyl derivative for approximately 24 h at room temperature, after which the Au/glass plates were rinsed with dried CH2Cl2 several times. As illustrated in Scheme 2, the molecular electron junction was formed by bringing an AFM tip into contact with the SAM at a constant applied load of 2 nN across the measurement series. A gold-coated AFM tip with a radius of 30 nm was used in all experiments. The I−V characteristics were recorded by sweeping the tip voltage (±3.0 V) with the SAM-coated Au(111) substrate held at ground. Computational Details. Density functional theory calculations at the B3LYP level were performed to obtain the molecular structures of the AcS-C6H4CC-(fc)n-CCC6H4SAc molecules. The basis set used for C, H, S, and Fe atoms was 6-31g**. All calculations were performed in Gaussian 09. General Physical Methods. 1H NMR spectra were obtained on a Varian INOVA-500 MHz spectrometer or an INOVA-600 MHz spectrometer. Mass spectra were obtained with a VG-BLOTECHQUATTRO 5022 system and ESI-LCQ mass spectra were obtained with a Thermo Finnigan instrument. MALDI-TOF mass spectra were obtained with a Bruker microTOF-Q system.
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RESULTS AND DISCUSSION Precursors for the Monolayers. Precursors of 2 and 3 were isolated and purified by column chromatography (Scheme 3). Compounds were characterized by NMR techniques, ESIMS, and elemental analysis. Electrochemical Studies of fcS and S-fcn-S Monolayers. Electrochemical measurements were performed to evaluate the magnitude of standard electron transfer rate constants (kE(1/2)) that may occur upon the change of the linking group between the fc-CC- moiety and the electrode. We have found that acetyl-protected thiol provides an excellent method to resolve the stability problem of the precursors. However, the monolayers of fcSAc and AcS-fcn-SAc, synthesized using compounds 1−3 to generate the SAM directly without deprotection by exogenous base, are not suitable for study by electrochemistry. During the electrochemical measurements in the CH2Cl2 solution containing 0.1 M (n-C4H9)4NPF6 electrolyte, rapid desorption of the molecules that had been physically adsorbed to the gold microelectrode surface was observed. Therefore, the electron transfer characteristics were studied only in the cases of the monolayers of fcS and S-fcn-S. As shown in Figure 1a, the redox behavior of the monolayer fcS is dominated by the Fe2+/Fe3+ couple. The electrochemical parameters obtained from the CV (E1/2, half-wave potential; ΔEp, redox peak separation) are summarized in Table 1. The number of electroactive centers in the monolayers can be 3108
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Table 2. Electrochemical Data from Monolayers of fcS, Sfc2-S, and S-fc3-S in a CH2Cl2 Solution Containing 0.1 M (nC4H9)4NPF6 Electrolyte monolayer
Γavg (×1011)a
fcS S-fc2-S
12.4 10.2c
S-fc3-S
5.1c
kE(1/2) (s−1)b (2.1 (2.2 (3.1 (5.3 (4.9 (5.2
± ± ± ± ± ±
0.1) × 103 0.4) × 103 0.4) × 103 0.3)× 103 0.8) × 103 0.4) × 103
a
Average surface coverage (Γavg in mol/cm2) is calculated from the CV data in CH2Cl2 with scan rate of 1 V s−1. Γ = Qfc/nFA (A = 7.85 × 10−5 cm2). bkE(1/2) is the standard electron transfer rate constant. Each measurement was reproduced several times on a freshly prepared monolayer. cAverage surface coverage calculated from the first redox couple in the CV measurement in CH2Cl2 with scan rate of 1 V s−1.
polar ester group (fcCO2(CH2)n-SH) coadsorbed with CH3(CH2)nSH (n = 7, 9, 11), showed thermodynamically ideal surface electrochemistry. At low mole fractions, the oxidation and reduction peaks are symmetric with no splitting between the oxidation and reduction waves (ΔEp = 0). The lack of peak splitting indicates that the rate of electron transfer is rapid on the time scale of the electrochemical experiment. Chidsey suggested that the ferrocene groups were both homogeneous and noninteracting. At higher mole fractions of the electroactive thiol in the adsorption solution, the resulting cyclic voltammograms broaden, develop an asymmetry, and finally develop an additional set of peaks as the amount of ferrocene is increased. Chidsey suggested that the breakdown of the thermodynamically ideal behavior of this system is most likely due to a combination of the interactions between ferrocene sites and the inhomogeneity of those sites at higher surface concentrations. In comparison with Chidsey’s study, a more ideal electrochemical reaction was observed in the monolayer of fcS. The standard rate constant (kE(1/2)) for the electron transfer between the electrode and the ferrocenyl moiety at E1/2 was measured for the monolayer of fcS by a potential-step chronoamperometry (CA) technique.26 The CA analysis of fcS is also shown in Figure 1b−d. The kE(1/2) value is summarized in Table 2. In the case of slow kinetics, it has been reported that the splitting between the oxidation and reduction peaks in the cyclic voltammogram corresponds to the value of kE(1/2). The value of kE(1/2) can be conveniently determined by monitoring the peak separation of the CV wave. Furthermore, a zero peak-to-peak splitting (ΔEp) and a full width at halfmaximum (fwhm) of 90.6 mV are expected for a rapid one-
Figure 1. Electrochemical properties of the monolayer of fcS. (a) Cyclic voltammograms at different scan rates. (b) Sample current (I) vs time decay curve obtained by CA with an overpotential of −0.1 V with respect to E1/2. (c) Semilog plot of ln I vs time with linear regression fits to the linear portions as obtained by CA with an overpotential of −0.1 V with respect to E1/2. (d) Semilog plot of the measured decay rate constants vs positive and negative overpotentials.
calculated from the cyclic voltammograms. The area under the peak can be integrated and divided by the scan rate to obtain the amount of charge passed to the ferrocene moieties. The surface charge densities (μC/cm2) obtained by this procedure can be converted to ferrocene surface coverage by dividing by the electron charge. The cyclic voltammograms of the monolayer fcS exhibit symmetric waves with a small splitting between the oxidation and reduction peaks. Furthermore, the peak current of ipc is directly proportional to the scan rate (ν), rather than the ν1/2 dependence expected for a freely diffusing species. As shown in Table 1, the anodic and cathodic current maxima are separated by 25−107 mV, varying with scan rate from 1 V s−1 to 50 V s−1 in the monolayer of fcS. The full widths at half-maximum (fwhm) range from 106 to 164 mV. The average surface coverage (Γavg) of the Au electrode modified by the monolayer of fcS is 1.24 × 10−10 mol/cm2 (Table 2). Pioneering studies of electroactive SAMs of ferroceneterminated alkanethiols that were coadsorbed with unsubstituted n-alkanethiols on evaporated gold films were reported by Chidsey.26 Monolayers, containing low concentrations (mole fraction (χfc) ≤ 0.25) of alkanethiols linked to ferrocene by a
Table 1. CV Data of the Monolayers of fcS, S-fc2-S, and S-fc3-S in a CH2Cl2 Solution Containing 0.1 M (n-C4H9)4NPF6 Electrolyte ΔEp (mV)b monolayer
E1/2(V)
fcS S-fc2-S
0.63 0.54 0.84 0.42 0.76 0.96
S-fc3-S
a
1
2
5
10
15
20
30
50
25 44 44 23 22 28
23 33 41 19 24 32
33 31 50 30 31 41
36 62 61 24 36 41
46 63 60 32 43 45
66 76 76
83 85 95
107 122 123
a
All half-wave potentials are referenced to an Ag/AgCl electrode. Ferrocene shows a reversible one-electron oxidation wave at E1/2 = 0.46 V in the CH2Cl2/CH3CN (1:1) solution. bPeak to peak separations between the resolved reduction and oxidation waves at various scan rates (V s−1). 3109
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electron transfer. Our observation that the ΔEp is nonzero and is dependent on only the scan rate suggests that the electrontransfer process in our system is not limited by slow equilibrium kinetics. Slow kinetics would cause ΔEp to increase with an increasing scan rate. There are several reports of finite ΔEp values for surface-confined redox couples that are independent of scan rate. One example of such a system is when ferrocene carboxylic acids are adsorbed to a platinum surface.28 This nonkinetic behavior in the CV is due to a nonequilibrium behavior arising because some rate processes are slow on the time scale of the experiment, thus causing a finite ΔEp value to be observed.29 Potential step chronoamperometry allows us to probe the standard rate constants for electron transfer. For an ideal electrochemical reaction involving a surface-bound species, the Faradaic current following a potential step exhibits a single exponential decay in time: I(t ) = I 0 exp( −kt )
system involving oligo(phenylethynyl) bridges with variable length. In Craeger’s study, a standard electron transfer rate constant of 5 × 105 s−1 was obtained for the adsorbate with three phenylethynyl units attached to the gold electrode in an aqueous solution. As the length of the linker was increased to six phenylethynyl units, a standard electron transfer rate constant of 350 s−1 was obtained. As expected, the kE(1/2) values increase as the number of phenylethynyl units in the bridge linking the ferrocene to the electrode decreases. In comparison with Creager’s monolayer with three phenylethynyl units, the smaller standard electron-transfer rate constant obtained for the fcS system containing one phenylethynyl unit could be ascribed to the solvent effect, such as the reorganization of the solvent vibrations and environments to adapt to the electron transferred state. In the Creager’s study, the aqueous solution of 1 M HClO4 was used during the AC measurements. Electrochemical measurements were also performed to evaluate the magnitude of the standard electron transfer rate constants in aqueous solution. Unfortunately, rapid desorption of the adsorbates in the aqueous solution was observed during the CV measurements. However, a primary CA evaluation of the kE(1/2) value in the aqueous 1 M NaClO4 solution was made. The kE(1/2) value for the fcS system in the aqueous solution is greater than the upper time-scale limitation (∼105 s−1) of the CA technique. The CV and CA analyses of the monolayers of S-fc2-S and Sfc3-S are shown in Figures 2 and 3, respectively. Monolayers of
(2)
In eq 2, k is the rate constant for a given overpotential. Firstorder rate constants can be extracted from the slopes of the plots of ln I vs t. The k values in this study increased with the magnitude of the overpotential. The intersection of the linear portions of the cathodic and anodic regions of the plot of log k vs potential yields the standard electron transfer rate constant in the absence of overpotential effects.26 The CA analysis is a more direct measurement of the electron-transfer kinetics. The CA approach revealed that the current decayed exponentially in the monolayer of fcS. Consequently, the electron-transfer rate constants, k, were estimated using eq 2. For example, Figure 1b,c illustrates examples of the CA response observed for the monolayer of fcS. This response is typical of all cases in CH2Cl2 solutions. The linear semilog plot in Figure 1c shows that, on the millisecond time scale, only a single current decay can be resolved. The observation of a single rate constant suggests that the local microenvironments of the individual ferrocenyl redox centers interconvert rapidly on this time scale. It is possible that the short time scale for the interconversion of the ferrocenyl sites makes all redox sites that are actively undergoing electron transfer chemically identical. Under this condition, the measured rate constants would be representative of an average electron transfer process. Furthermore, an increased step potential relative to E1/2 (the half-wave potential given in Table 1) increases the free energy driving force for the electron transfer and causes increased electron-transfer rates to be observed. The semilog plot (Tafel plot) of the measured rate constants as a function of the applied electrode potentials is shown in Figure 1d. The symmetry of the data, the minimum at E1/2, and the linear slope near E1/2 for the monolayer of fcS indicate that specifying an overpotential with respect to the CV half-wave potential gives equal forward and backward rate constants. The value of kE(1/2) is (2.1 ± 0.1) × 103 s−1 for the monolayer of fcS in the CH2Cl2 solution. In comparison with the standard electron transfer rate constant (kE(1/2) = 1.25 s−1) measured in the aqueous solution of 1 M HClO4 by the CA technique for the monolayers prepared using ferrocene-terminated fcCO2(CH2)16-SH and unsubstituted CH3(CH2)16-SH as coadsorbates (χfc) = 0.1),26 we suggest that the stronger electronic conduction in the monolayer of fcS could be ascribed to the π-conjugated -CC(C6H4) spacer. Furthermore, we were able to make a comparison with an electrochemical study (AC voltammetry method) by Creager and co-workers27 on a similar ferrocene-based monolayer
Figure 2. Electrochemical properties of the monolayer of S-fc2-S. (a) Cyclic voltammograms at different scan rates. (b) Example of a current (I) vs time decay curve obtained by chronoamperometry with an overpotential of −0.03 V with respect to E1/2 (0.54 V). (c) Semilog plot of ln I vs time with linear regression fits to the linear portions obtained by chronoamperometry with an overpotential of −0.03 V with respect to E1/2 (0.54 V). (d) Semilog plot of the measured decay rate constants vs positive and negative overpotentials for each redox couple.
S-fc2-S and S-fc3-S show two-step and three-step one-electron redox processes with the formation of various redox states, respectively. As shown in Figure 2, the redox behavior in S-fc2S is dominated by two reversible Fe2+/Fe3+ redox couples (E1/2 at 0.54 and 0.84) vs Ag/AgCl. For the monolayer of S-fc3-S, three reversible ferrocene-based Fe2+/Fe3+ redox couples (E1/2 at 0.43, 0.76, 0.96 V) vs Ag/AgCl were observed. 3110
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that of biferrocene (0.30 V)30 indicates that the interactions between the Fe sites in our monolayers are all similar to that in biferrocene. The kE(1/2) values of the monolayers of S-fcn-S are given in Table 2. Interestingly, we found that the kE(1/2) measured for the first redox couple in the fcS ((2.1 ± 0.1) × 103 s−1), S-fc2-S ((2.2 ± 0.4) × 103 s−1), and S-fc3-S ((5.3 ± 0.3) × 103 s−1) monolayers increased as the number of ferrocenyl moiety increased. The slight asymmetry of the data (Figure 2d) for the monolayer S-fc2-S indicates that the forward and backward electron transfer rates at a given overpotential are not exactly equal. There are two and three ferrocenyl groups, respectively, at different distances removed from the electrode in the monolayers of S-fcn-S. The key questions that arise from the electrochemical observations are as follows: (1) Which ferrocenyl group is the first to be electrochemically oxidized? (2) By what mechanism do these oxidations occur? We suggest that there is an electron-hopping mechanism involving the intramolecular electron transfer process in the monolayers of Sfcn-S. The oxidation process in the S-fc2-S monolayer is initiated through control of the electrode potential, and the ferrocenyl moiety sitting closer to the electrode surface is oxidized first as illustrated schematically in Scheme 4 (mechanism A). Initially, the ferrocenyl site further from the electrode surface is still in the reduced Fe2+ state. Electrons can hop from the reduced ferrocenyl site to the Fe3+ site sitting closer to the electrode surface. Electron hopping occurs via a well-known electron self-exchange chemical process between the two ferrocenyl moieties.21,22 Theoretically, the magnitude of the electronic coupling between the ferrocenyl sites plays an important role in determining the possibility for the electronhopping mechanism. The ΔE1/2 value of S-fc2-S (0.30 V) indicates that there is a pronouncedly large interaction between the two Fe sites in the monolayer of S-fc2-S. We assume that the zero-point energy difference between the two vibronic states (the a and b states in mechanism A) in the potential energy diagram of [S-fc2-S]+ is almost equal to zero. This is reasonable in terms of the similar electronic effects of the free -CC(C6H4)SH and the Au-binding -CC(C6H4)S moieties. Considering the electron-donor effect of the ferrocenyl moiety on the stability of the Fe3+ state in the [S-fc3-S]+ monolayer, the monocation can exist as one of two energetically equivalent energy states (a′ and c′ states), or as the energetically nonequivalent b′ vibronic state in the potential energy diagram (mechanism B in Scheme 4). This electronhopping mechanism could significantly enhance the electronic conduction along the molecular axis. Furthermore, the asymmetry of the CA data (Figures 2d and 3d) for the monolayer S-fcn-S can be attributed to the various redox states. CAFM Measurements of the AcS-fcn-SAc and S-fcn-S Monolayers. CAFM measurements were performed to probe the molecular electron-junction characteristics of the AcS-fc2SAc and AcS-fc3-SAc monolayers. In the case of CAFM measurements for the acetyl-protected monolayers of AcS-fcnSAc, desorption of the molecules adsorbed physically to the gold microelectrode surface was not observed because of the lack of solvent effects, such as the chemical equilibrium between the molecules absorbed on the electrode and solvent molecules during the solution-state CV measurement. We observed highly nonlinear current vs potential (I−V) characteristics at room temperature (Figure 4). Clear Coulomb blockade behavior arising from the capacitive charging of small groups of molecules was seen at room temperature. The current increases
Figure 3. Electrochemical properties of the monolayer of S-fc3-S. (a) Cyclic voltammograms at different scan rates. (b) Sample current (I) vs time decay curve obtained by chronoamperometry with an overpotential of 0.03 V with respect to E1/2 (0.42 V). (c) Semilog plot of ln I vs time with linear regression fits to the linear portions obtained by chronoamperometry with an overpotential of 0.03 V with respect to E1/2 (0.42 V). (d) Semilog plot of the measured decay rate constants vs positive and negative overpotentials with respect to the E1/2 of 0.42 V. (e) Semilog plot of the measured decay rate constants vs positive and negative overpotentials with respect to the E1/2 of 0.76 V. (f) Semilog plot of the measured decay rate constants vs positive and negative overpotentials with respect to the E1/2 of 0.96 V.
The redox waves for the monolayers of S-fc2-S and S-fc3-S are slightly asymmetric. They also have a small splitting between the oxidation and reduction waves for Fe2+/Fe3+ redox couples that are all consistent with most of the current arising from reaction of an adsorbed layer of SAMs. As given in Table 2, the lower surface coverage for the monolayers of S-fc2-S (1.02 × 10−10 mol/cm2) and S-fc3-S (5.1 × 10−11 mol/cm2), compared with the monolayers with a single ferrocenyl moiety (1.24 × 10−10 mol/cm2), are attributed to the larger size of the multinuclear ferrocenyl moieties. It has been reported that the n-alkyl thiolate SAMs on Au(111) have a surface coverage of 7.5 × 10−10 mol/cm2.26c In addition to the result of larger size of the ferrocenyl moieties in the monolayers of S-fcn-S, the lower surface coverage in our case could indicate disorder short chain molecules. One of the interesting attributes of these monolayers is the magnitude of the interaction between the Fe sites that can be diagnosed from the magnitude of ΔE1/2 (difference of the halfwave potentials). Generally, a larger ΔE1/2 value is indicative of a stronger Fe−Fe interaction. A pronounced Fe−Fe interaction in the monolayers of S-fcn-S is evidenced directly from the observation of reversible Fe2+/Fe3+ redox couples. Comparing the ΔE1/2 values of S-fc2-S (0.30 V) and S-fc3-S (0.35 V) with 3111
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Scheme 4. Proposed Electron-Hopping Mechanisms for the S-fcn-S Monolayers
in steps as the bias voltage increases. In Figure 4a, there is a step-like current jump at a voltage of 0.65 V, leading to a pronounced current plateau between 0.97 and 1.86 V. Another current step was also observed at 1.86 V. The current steps correspond to the conductance peaks visible in Figure 4b near 0.65 and 1.86 V. Similar step-like current jumps at voltages of −1.55 and −2.70 V were also observed in the region of negative voltage. In comparison with the current plateaus observed in the positive voltage, current jumps in the negative voltage
region occurred at more negative potentials. We suggest that the structural asymmetry electrode in the wire junction formation process (i.e., molecules are adsorbed to one electrode, and the second electrode is brought into contact) results into the asymmetry I−V curve associated with the forward and reverse bias. This Coulomb blockade behavior suggests that charge is injected into the AcS-fc2-SAc monolayer. There are two redox-active ferrocenyl centers in AcS-fc2-SAc, leading to the observation of two current 3112
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Figure 4. CAFM measurements for the AcS-fcn-SAc monolayers. Each figure is an average of three trials. (a) Characteristic I−V response and (b) conductance histogram (dI/dV − V) of AcS-fc2SAc. (c) Characteristic I−V response and (d) conductance histogram (dI/dV − V) of AcS-fc3-SAc.
plateaus. Our findings strongly suggest that the conductance switching is mediated by the redox reactions of the monolayers, such as the formation of a singly oxidized mixed-valence [AcSfc2-SAc]+ (Fe(II)−Fe(III)) redox state at 0.65 V. Further charge is injected to give the doubly oxidized [AcS-fc2-SAc]2+ (Fe(III)−Fe(III)) redox state at 0.97 V. Similarly, three current plateaus were observed in the AcS-fc3-SAc monolayer as shown in Figure 4. Each current step corresponds to the injection of charge to produce a particular redox state. The CAFM measurements indicate a high degree of electronic communication between the ferrocenium (Fe(III)) and ferrocenyl (Fe(II)) centers connected by the diacetylthiolated phenylethynyl bridging ligand to the external electrodes. The average resistances of [AcS-fc2-SAc]+ (∼4.0 × 107 Ω) and [AcS-fc3-SAc]+ (∼5.6 × 107 Ω) are actually lower than those of aliphatic, polyolefinic and aromatic molecules. For example, 4.3 nm polyolefinic molecules give resistances of ∼1010 Ω.31−33 Multinuclear ferrocenyl spacers themselves are insulators but can be converted into semiconducting materials by selective oxidation of the ferrocenyl units to form mixed-valence multinuclear ferrocenium spacers in which a rapid intramolecular electron-hopping mechanism was observed. The redox-active multinuclear ferrocenylethynyl spacers exhibit strong ferrocenium-to-ferrocene electronic interaction along the semirigid step-like molecular axis. The resistances are slightly dependent on molecular length, in agreement with the proposed electron-hopping mechanism. Further investigation with atomic force microscopy for monolayer AcS-fc2-SAc shows clearly molecular groups with averaged height of 2.37 nm (Figure 5a). Furthermore, monolayer AcS-fc3-SAc shows clearly molecular groups with averaged height of 2.45 nm (Figure 5b). Each height is slightly less than the corresponding value (AcS-fc2-SAc (2.52 nm) or AcS-fc3-SAc (2.77 nm)) of the direct S-to-S distance in the unbound molecule with the addition of the Au−S bond length (0.24 nm).34 These values of molecular lengths were calculated by the density functional theory calculations at the B3LYP level. This finding also suggests that the molecular axis in the monolayer of AcS-fc2-SAc has a tilt angle of ∼20° (cos−1(2.37/ 2.52)) with respect to the normal of the gold surface. In the case of monolayer of AcS-fc3-SAc, molecular axis has a tilt angle of ∼28° (cos−1(2.45/2.77)). Very recently, Wöll and his co-workers reported an unexpected effect on the structure of
Figure 5. AFM images: (a) AcS-fc2-SAc and (b) AcS-fc3-SAc. Inset corresponds to the red line-scan.
the resulting SAMs when dodecyl thioacethioacetate (CH3(CH2)11SCOCH3, C12SAc) in ethanol was chose as the precursor on the formation of SAMs.35 The surface is dominated by the flat-lying phase of C12S with the molecules lying next to each other with alternating orientation. In the Wöll’s study, the leaving acetyl group probably reacts with ethanol in the formation of a kinetically stable flat-lying monolayer of C12S. In our case, dried CH2Cl2 solvent was used in the formation of AcS-fcn-SAc monolayers. The molecular electron-junction characteristics of the S-fc2-S and S-fc3-S monolayers were also examined by the CAFM technique. Nonlinear current vs potential (I−V) characteristics at room temperature (Figure 6) were also observed in the monolayers of S-fcn-S. However, two or three current plateaus of Coulomb blockade behavior arising from the capacitive charging of small groups of molecules observed in monolayers of AcS-fcn-SAc were not observed in the monolayers of S-fcn-S at room temperature. It is worth noting that Coulomb blockade behavior has been observed for wires with weak molecule− electrode coupling at low temperatures.20,36−40 For example, the electron transport in single-molecule transistors based on dithiolated phenylene-vinylene oligomers connected to external electrodes has been shown to exhibit such Coulomb charging effects at 4.2 K.41 In comparison with the acetyl-protected monolayers of AcS-fcn-SAc in which molecules are physically absorbed on the electrode surface, we suggest that there is a stronger chemical S-electrode bonding in the monolayers of Sfcn-S. As shown in Figure 6, an increase in current at the bias voltage values of 0.50 and 1.0 V were observed in the monolayers of S-fc2-S and S-fc3-S, respectively. The resistances of ∼4.1 × 109 and ∼3.5 × 1011 Ω were calculated in the ohmic region of −250 mV < V < +250 mV for the nonoxidized 3113
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Figure 6. CAFM measurements for the monolayers of S-fcn-S. Each figure is an average of three trials. (a) Characteristic I−V response and (b) conductance histogram (dI/dV − V) of S-fc2-S. (c) Characteristic I−V response and (d) conductance histogram (dI/dV − V) of S-fc3-S.
monolayers of S-fc2-S and S-fc3-S, respectively. The dependence of the resistance on the molecular length of the monolayers of S-fc2-S and S-fc3-S suggests that the oxidation state of the ferrocenyl moieties are Fe2+ in the ohmic region of −250 mV < V < +250 mV. Furthermore, the resistance of ∼2.0 × 108 Ω was calculated for the oxidized [S-fc2-S]n+ redox states in the ohmic region of −0.6 V < V < −0.8 V (Figure 6a). A resistance of ∼3.3 × 108 Ω was calculated for [S-fc3-S]n+ redox states in the ohmic region of −1.4 V < V < −1.8 V. As shown in Figure 7, monolayers of S-fc2-S and S-fc3-S show molecular groups with average heights of 1.36 and 1.74 nm, respectively. This result also suggests that the molecular axis in the monolayer of S-fc2-S has a tilt angle of ∼57° (cos−1(1.36/ 2.52)) with respect to the normal of the gold surface. In the case of monolayer of S-fc3-S, molecular axis has a tilt angle of ∼51° (cos−1(1.74/2.77)).
Figure 7. AFM images: (a) S-fc2-S and (b) S-fc3-S. Inset corresponds to the red line-scan.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +886-7-5252000 ext 3937. Fax: +886-7-5253908. E-mail:
[email protected].
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Notes
The authors declare no competing financial interest.
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CONCLUSION We have examined a series of molecular wires containing redoxactive ferrocenylethynyl chains by electrochemistry and CAFM techniques. These studies reveal that electron transport is made extremely efficient by the organometallic redox states. The monolayers of fcS, S-fc2-S and S-fc3-S show one-, two-, and three-step, respectively, one-electron redox processes with the formation of various redox states. The standard rate constants for electron transfer measured by potential step chronoamperometry for the first redox couple in the fcS, S-fc2-S, and S-fc3-S monolayers are (2.1 ± 0.1) × 103, (2.2 ± 0.4) × 103, and (5.3 ± 0.3) × 103 s−1, respectively. We suggest an electron-hopping mechanism involving the intramolecular electron transfer process in these monolayers. Clear Coulomb blockade behavior arising from the capacitive charging of molecules in the acetylprotected monolayers of AcS-fcn-SAc was seen at room temperature. We suggest that there is a weak molecule− electrode coupling in the series of AcS-fcn-SAc monolayers. Upon inspection of both the CV and CAFM data, we conclude that the charge injection is mediated by discrete redox states. The observation of pronounced Coulomb blockade behavior at room temperature suggests that these monolayers have potential applications in the fabrication of electronic switches.
ACKNOWLEDGMENTS This work was funded by the National Science Council (NSC98-2113-M-110-004-MY3), Taiwan, ROC, and Department of Chemistry and Center for Nanoscience and Nanotechnology at National Sun Yat-Sen University.
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REFERENCES
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