Faradaic Phase Transition of Dibenzyl Viologen on ... - ACS Publications

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Faradaic Phase Transition of Dibenzyl Viologen on an HOPG Electrode Surface Studied by In Situ Electrochemical STM and Electroreflectance Spectroscopy Tomohiro Higashi,† Yasuhiro Shigemitsu,‡,§ and Takamasa Sagara||,* †

Department of Molecular Science, Graduate School of Science and Technology, Nagasaki University, Nagasaki 852-8521, Japan Affiliated Division of Eco-Material Science, Graduate School of Engineering, Nagasaki University, Nagasaki 852-8521, Japan § Industrial Technology Center of Nagasaki, Omura, Nagasaki 856-0026, Japan Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, Nagasaki 852-8521, Japan

)

‡

ABSTRACT: Phase transitions of an adsorption layer of dibenzyl viologen (dBV) as a typical diaryl viologen on a basal plane of a highly oriented pyrolytic graphite (HOPG) electrode are described using voltammetry, in situ electrochemical scanning tunneling microscopy (EC-STM), and electroreflectance (ER) spectroscopy. A monolayer redox process at less negative potential than the bulk redox process was found to be the firstorder faradaic phase transition between a gaslike adsorption layer of dication (dBV2+) and a 2D condensed monolayer of radical cation (dBV•+). Comparison of the results of cyclic voltammetry and potential step chronoamperometry was made with those of heptyl viologen (HV), which also undergoes a faradaic phase transition of the first order. It suggested that the contribution of intermolecular π
π interaction between benzyl groups of dBV to the phase transition is minor and apparently equivalent to interchain interaction between the heptyl chains of HV. In situ ECSTM images of the 2D condensed monolayer demonstrated stripe patterns of the rows of dBV•+ molecules forming 3-fold rotationally symmetric domains. The results of the ER measurements also revealed that the orientation of the longitudinal molecular axis of the bipyridinium moiety of dBV•+ molecules lying flat on the HOPG electrode surface, most likely with a side-on configuration.

I. INTRODUCTION Adsorption layers of organic molecules on electrode surfaces frequently exhibit sharp phase transitions triggered by the potential change.1
36 A better understanding of the potentialdriven phase transitions together with the molecular level adsorption structures would lead us to the development of devices that show drastic, sharp, and chemically reversible responses to the potential change at electrified interfaces. Viologens bearing long alkyl chains are among the molecules that exhibit faradaic phase transition,20
36 in many cases of the firstorder, on several electrode surfaces. The phase transition is represented by sharp spike peaks in cyclic voltammograms (CVs). At an HOPG electrode surface, for example, heptyl viologen (HV) undergoes a first-order faradaic phase transition from a gaslike adsorption layer of its oxidized form (HV2+) to an insoluble 2D condensed monolayer of the one-electron reduced monoradical monocation form (HV•+).20
23 Arihara and co-workers concluded, using their results of IR reflection spectroscopic measurements at an HOPG electrode, that 4,40 bibyridirium moieties of HV•+ assume side-on orientation with a parallel-to-surface orientation of all-trans alkyl chains.24 The molecular level image of the condensed phase of the reduced form of any viologen as the result of the first-order phase transition, however, has not been obtained yet on an HOPG electrode by STM r 2011 American Chemical Society

or AFM to our best knowledge. Apart from HOPG, Jiang and coworkers have recently observed a stripe pattern of adsorbed HV•+ on a chloride modified Cu(100) electrode surface by in situ STM.35 In their model, the main molecular axis of HV•+ is parallel to the surface in a side-on configuration, exhibiting the formation of π-stacked polymer chains. The first-order phase transition is featured by its bistable potential region, the width of which corresponds to the CV peak separation between anodic and cathodic spike-peaks at the lower limit of the potential sweep rate (ΔEp0).3,12,22 The predominant factor that determines the width of the bistable potential region is the strength of intermolecular interactions with the nearest neighbors. Arihara and co-workers reported that the longer alkyl side chains enhanced attractive intermolecular interaction in the condensed phase, resulting in a greater peak separation ΔEp at a constant potential sweep rate.24 Addition of a carboxylic acid group to the one end of the two alkyl side chains of HV resulted in an increase of ΔEp0 from 12 mV to 67 mV,33 indicating a great extent of additional contribution of the hydrogen-bonding to the intermolecular interaction. Furthermore, addition of the carboxylic Received: July 18, 2011 Revised: September 24, 2011 Published: September 28, 2011 13910

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Langmuir acid groups at the ends of both alkyl side chains resulted in ΔEp0 = 121 mV,33,34 almost doubled compared to the abovementioned value. Tsay and co-workers have observed stripe patterns of adsorbed dicarboxylated-HV•+ in a few phases on a chloride modified Cu(100) electrode surface by in situ EC-STM.36 Their model suggests that both face-to-face π
π stacking interaction of V•+ moieties between neighbors and hydrogen-bonding between the end groups of the alkyl side chains in a longitudinal direction are the origins of the formation of stripe patterns in the side-on configuration. These results point to the capability of a molecular design for tailoring the intermolecular interactions and thus the position and width of the bistable potential region. Another effective means to enhance the intermolecular interaction may be an additional introduction of π
π interaction in between viologen molecules, whereas this may also change the interaction with the HOPG substrate and induce steric hindrance. For example, aryl groups of a diaryl viologen are expected to interact with an HOPG electrode surface through the π
π interaction in addition to intermolecular π
π interaction. In the present work, we use dibenzyl viologen (dBV) as one of the most typical diaryl viologens. To evaluate the effect of aryl group addition upon the redox-induced phase change behavior, we examine whether dBV exhibits a first-order faradaic phase transition or not, and the molecular-level adsorption structure of dBV•+, if it forms a 2D condensed monolayer. We use the results of voltammetric, in situ electrochemical scanning tunneling microscopic (EC-STM), and electroreflectance (ER) measurements to describe the influence of the lateral intermolecular interaction and molecule
substrate interaction. The results of this study may provide us with an important roadmap for full understanding and prediction of adsorbed structures of redoxactive organic molecules that can undergo sharp phase changes.

II. EXPERIMENTAL SECTION Dibenzyl viologen (1,10 -dibenzyl-4,40 -bipyridinium dichloride: dBV2+ 2Cl
) purchased from Tokyo Kasei Kogyo Co., was recrystallized from acetone + ethanol, and dried in vacuo. Water was purified through a Milli-Q integral (Millipore) to a resistivity over 18 MΩ cm. All other chemicals were of the highest reagent grade commercially available and used as received. A plate of HOPG (Matsushita Electric Co., Panasonic graphite, PGX 05: size 12  12  3 mm thickness) was connected perpendicularly with a copper pipe. To expose a fresh basal plane, the surface of the HOPG was peeled off by the use of Scotch adhesive tape immediately before use. For the measurements of CVs, a potentiostat (HUSOU, HECS-9094) was coupled with a function generator (HUSOU, HECS-321B). Potential-step measurements were made by the use of a potentiostat equipped with a digital universal signal processing unit (HUSOU, HECS-326). A droplet of 0.1 mM dBV2+ 2Cl
+ 0.3 M KCl solution was pipetted to place it on an upward surface of a horizontally set HOPG electrode in a wetted Ar gas atmosphere. A tip of an Ag/AgCl/sat’d KCl reference electrode and an Au wire counter electrode were immersed in the droplet. This droplet configuration was used in voltammetric and potential step measurements unless otherwise stated. The active electrode area (A), which is the actual contact area of the HOPG surface to solution, was measured photographically. For in situ EC-STM, a scanning tunneling microscope (STA-330, SII nanotechnology) was employed with an SPI-3800N probe station. A tungsten tunneling tip was electrochemically etched and sealed with a transparent nail polish. All images were collected in the constant current mode with tunneling current of It. The bias potential (Ebias), which is the potential difference between tunneling tip (Etip) and HOPG electrode (Ework), was controlled by means of a bipotentiostat (EC-controller,

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SII nanotechnology). The specific tunneling conditions are given in the figure captions. Measurements of ER signals were carried out by both normal and oblique incidence of the monochromatic light at a hanging meniscus (H
M) configuration. This configuration was set by horizontally touching an HOPG electrode to an Ar gas/dBV solution interface from the Ar gas phase. The oblique incident angle of the light (θ) was variable in the range from 20
to 80
with respect to the HOPG surface normal. The details of the ER instrumentation and spectroelectrochemical cell were given in our previous publication.37 The key issues of the ER measurements with polarized incident light are as follows. The monochromated light from a grating monochromator was passed through a polarizer (Sigma Kouki Co., SPF-30C-32, extinction ratio 1/10 000) to irradiate the HOPG electrode surface with either p- or s-polarized light at various θ. The specularly reflected light was focused onto a photomultiplier (R928, Hamamatsu Photonics) through a depolarizer. The potential modulation used for the ER measurements is described as E ¼ Edc þ Eac ¼ Edc þ Eac exp ðjωtÞ

ð1Þ

where E is the electrode potential, Edc is the dc potential, √ Eac is the ac potential, ΔEac is the zero-to-peak ac amplitude, j =
1, ω = 2πf, which is the angular frequency (f is the frequency of the potential modulation), and t is the time. Phase-sensitive detection of the ac intensity of the reflected light Iac was made by using a lock-in amplifier (EG&G 5210). When the response Iac to Eac is linear, Iac is represented by a form of ΔIac exp [j(ωt
ϕ)] where ΔIac is the zero-to-peak amplitude of Iac and ϕ is the phase of Iac with respect to Eac. The ER signal (ΔR/R)ER was defined as ðΔR=RÞER ¼ Iac exp ð
jωtÞ=Idc

ð2Þ

where Idc is the time averaged dc intensity of the reflected light. An ER spectrum (ERS) is a plot of the both real (in phase to Eac) and imaginary (90
out of phase) parts of (ΔR/R)ER as a function of the wavelength of the incident light (λ). The p/s ratio is the intensity ratio of the ER signals for p- and s-polarized light. All of the electrochemical control was carried out by the use of an Ag/ AgCl/sat’d KCl reference electrode and a coiled Au wire counter electrode under Ar gas (>99.998%) atmosphere at room temperature (24 ( 2C
).

III. RESULT AND DISCUSSION Voltammetric Studies. Figure 1 shows typical CVs at an HOPG electrode in contact with 0.1 mM dBV2+ 2Cl
+ 0.3 M KCl solution. When sweeping the potential to more negative than
0.4 V, bulk reduction of soluble dBV2+ to insoluble dBV•+ Cl
deposit on the surface was observed as a diffusion-controlled cathodic peak at
0.59 V at a potential sweep rate (v) of 70 mV s
1 (not shown here). Oxidative dissolution of dBV•+ Cl
deposit was observed as an anodic response around
0.54 V. The spikelike peaks shown in Figure 1 occurred at far less-negative potentials than the above-mentioned bulk redox reaction. The CV peak charge was independent of both v and sweep direction. Its value corresponds to the amount of electroactive species of Γ = (2.5 ( 0.1)  10
10 mol cm
2, provided that the peaks are exclusively due to a one-electron redox couple of surface-confined dBV•+/2+ (vide infra). If the cathodic process was exclusively due to reductive adsorption of solution phase dBV2+ to a bare electrode surface, it should have taken ca. 1.5 s for adsorption of the amount of Γ to be completed, even if the Cottrell limiting diffusion condition was suddenly achieved at the onset of the peak, as being estimated using the diffusion coefficient. However, the experimental CV peak width is much less than 1 s 13911

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Figure 1. Cyclic voltammograms for an HOPG electrodes in the droplet configuration with 0.1 mM dBV2+ 2Cl
+ 0.3 M KCl solution: a, v = 5 mV s
1 and A = 0.79 cm2; b, v = 70 mV s
1 and A = 1.08 cm2.

(i.e., it is 5 100 mV at v = 100 mV s
1). As shown later, this phase transition is actually completed within a period of even 50 ms in potential step experiments. Therefore, the cathodic current is due to reduction of dBV2+ molecules, most of which were already present on the electrode surface or at least in the close proximity region at the beginning of reduction process. Because the double-layer charging contribution to the peak charge is negligible at a basal plane HOPG electrode,23,38
40 Γ can be equated to the amount of surface-confined dBV•+ molecules. If dBV•+ molecules of the amount of Γ form a monolayer on the electrode surface, the area occupied by one dBV•+ molecule is 0.66 nm2/molecule. Note that Faradaic phase transitions on metal electrode surfaces are often accompanied by nonfaradaic current involved in the voltammetric peak, which is due to, for example, a significant shift of the apparent potential of zero-charge (pzc). Estimation protocol of the nonfaradaic current due to the pzc shift was proposed by Kakiuchi and co-workers.41 The estimation requires pzc and interfacial differential capacity values for both positive and negative potential regions to the peak. The pzc of a bare HOPG electrode is known to be
0.2 V,42 whereas the pzc of the HOPG electrode covered with a dBV•+ adlayer is hardly obtainable because of its limited potential region. Semiquantitatively, if such a nonfaradaic charge component is involved in the peak area ruled with a flat background level, the peak charge should have depended on the magnitudes and differences of the above-mentioned two capacity values. In our results of nearly 80 experiments, even though the capacity levels were variant sample-to-sample, the peak charge was quite reproducible. This fact supports a negligibly small nonfaradaic charge component of the pzc jump originated from the changes of the adsorption amounts of organic molecules and water molecules as well as their dipole orientations, in addition to the ionic distribution change. Most likely, one of the reasons is the fact that an HOPG is a semimetal and possesses a depletion layer at the surface. Its low differential capacitance dominantly determines the interfacial potential drop.38
40 Other possible origins of additional nonfaradaic charge would be anionic adsorption/desorption and charge-unbalance adlayer formation. We found experimentally that the CV peak charge was unchanged in the KCl concentration

Figure 2. Plot of peak potentials (Ep) of spike response in the cyclic voltammogram (a) and plot of the logarithm of peak currents (b), both as a function of the logarithm of v. A = 0.79 cm2 for (a) and A = 0.64 cm2 for (b).

range from 30 mM to 0.5 M. Upon reduction, viologen dication molecules accept electrons from the electrode, and formally an equivalent amount of counteranions moves away from the electrode surface to the bulk solution. Therefore, these two factors have if any only minor contribution. Taken together, we can exclude the contribution from additional nonfaradaic current. Part a of Figure 2 shows the v-dependence of the spike-like peak potentials. Convergence of both anodic and cathodic peak potentials to constant values at v = 0.1 mV s
1 let us find the midpoint potential at the lower v limit (E1/2) at
280 mV and ΔEp0 of 12 mV. The cathodic peak potential exhibited a greater extent of shift over a wider v-range than the anodic counterpart, indicative of sluggish kinetics of the reduction process compared to the reoxidation. If one-electron redox process of dBV•+/2+ couple is the first-order phase transition, the value of ΔEp0 = 12 mV corresponds to the width of bistable potential region of the phase transition. This ΔEp0 value of dBV is comparable to that of HV,23 suggesting that the contribution of intermolecular π
π interaction between benzyl groups of dBV to the phase transition is minor and apparently equivalent to the interaction between heptyl chains of HV. As shown in part b of Figure 2, anodic and cathodic peak currents in CVs were proportional, respectively, to v0.6 or v0.7 in the v-range from 10 to 100 mV s
1. This dependence roughly meets a nucleation
growth
collision (NGC) model proposed by Camacho and his colleagues.43 Representative transients of current (i) obtained in the potential step chronoamperometry measurements are shown in Figure 3. In part a of Figure 3 (positive step), a current hump which accompanies a positive di/dt region around t = 1.5 ms was obvious, indicating the occurrence of an NGC type process, 13912

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Figure 3. Typical current transient curves as the results of potential step chronoamperometry for an HOPG electrode (A = 0.79 cm2) in the droplet configuration in contact with 0.1 mM dBV2+ 2Cl
+ 0.3 M KCl solution. Initial/final potentials were: a,
0.36 V/
0.20 V; b,
0.25 V/
0.36 V.

Figure 4. Plot of Qt as function of Ef for an HOPG electrode (A = 0.79 cm2) in the droplet configuration in contact with 0.1 mM dBV2+ 2Cl
+ 0.3 M KCl solution. Initial potentials for positive and negative step were respectively
0.20 and
0.40 V.

where t is the time passed after the step. In the negative step (part b of Figure 3), a small hump with a short time period of negative di/dt was observed around 6 ms. These characteristic current transient are the typical features of an NGC process. Integration of the current transients in Figure 3 using the procedures described elsewhere23 for various final potentials (Ef) enabled us to obtain the charges (Qt) associated with the phase transition. Figure 4 shows the plots of Qt values against Ef for both positive and negative steps. The equilibration time at the initial potential was always set at least for 30 s before each steps in order to reach steady states. Because the slope of Qt in the

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potential region far from the transition potential is negligibly small, the faradaic component dominates the change of Qt as the same as in the case of CV measurements. In other words, the additive nonfaradaic charges associated with adlayer condensation (negative potential step) and its dissolution (positive step) are negligible (vide supra). The smaller total charge obtained from the transient experiments than CV data (up to by 13%) was due to the overestimate of the surface area by ignoring the droplet shrink before the measurements. But this should not represent a serious problem, because the waveforms of Qt
t curves were never affected. In the positive step, 11 mV change of the Ef value from
284 mV to
273 mV gave a steep increase of Qt. The difference of Qt between
284 and
273 mV corresponded to ca. 81% of the total change of Qt (ΔQt) between fully oxidized and fully reduced states. The potential of Ef =
273 mV was close to the anodic spike-like peak potential in the CVs at v = 0.1 mV s
1. Note that in the case of HV, such a steep increase of the value of Qt occurred in a range of 3 mV.23 The decrease of the Qt value for the negative step corresponding to ca. 83% of ΔQt was recorded between Ef =
280 mV and
307 mV. This range of 27 mV of the steep change of Qt was wider than those of the positive step for dBV (11 mV) and the negative step for HV (7 mV).23 Note that the short decay time of current transients as being exemplified by Figure 3 indicates that the one-electron reduction process is mostly due to dBV2+ molecules that were already adsorbed on the electrode at the potential step rather than to the molecules diffused from the solution bulk phase (vide supra). The Ef potentials which take Qt values corresponding to ca. 50% of ΔQt are
276 mV and
288 mV respectively for positive and negative steps. We suppose that Ef corresponding to ca. 50% of ΔQt is the transition potentials, because the CV peak potentials at the infinitesimal sweep rate tend to converge actually to these potentials. If the one-electron redox process of the dBV•+/2+ couple is a first-order phase transition, the difference in the Ef potentials corresponding to ca. 50% of ΔQt between positive and negative potential steps, 12 mV, is the width of bistable potential region of the phase transition. This width of bistable potential region obtained from potential-step measurements is in good agreement with the value of ΔEp0 = 12 mV obtained from CV measurements. Although the voltammetric spike and steep Qt rise and fall look less sharp if compared to the phase transition of HV, the behavior of dBV phenomenologically meets the features of the first-order phase transition with NGC kinetics. We concluded that dBV undergoes a faradaic phase transition of the first-order on an HOPG electrode surface. In situ EC-STM Studies. A freshly prepared basal plane HOPG electrode surface was brought into contact with 0.1 mM dBV2+ 2Cl
+ 0.3 M KCl solution. When setting the HOPG electrode potential (Ework) at the more positive potentials than the cathodic transition potentials (Ect), we always obtained STM images of only the HOPG surface. Although a variety of tunneling current (It) and Ework (Ework > Ect) were used, the presence of any organic molecule firmly immobilized on the electrode surface was not observed. Recall that CV and potential step transient features pointed to the presence of dBV2+ molecules on the electrode surface at less negative potential than Ect. Therefore, it is likely that dBV2+ forms a gaslike adsorption layer on an HOPG electrode surface as in the case of HV2+.23 The adsorption of dBV2+ molecules on an HOPG electrode surface is likely to occur through van der Waals interaction, whereas intermolecular 13913

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Figure 5. In situ EC-STM image (It = 0.342 nA, Ebias = 0.20 V) of an HOPG electrode surface in contact with 0.1 mM dBV2+ 2Cl
+ 0.3 M KCl solution at Ework =
0.38 V at which the surface was fully covered by a condensed phase of dBV•+. The image areas were (a) 25 nm  25 nm, (b) 4.6 nm  4.6 nm, (c) 100 nm  100 nm, and (d) 50 nm  50 nm. Onto the high-resolution image (b), molecular structure models of dBV•+ was superimposed. In (c), schematic illustration for the proposed stacking model of dBV•+ in the condensed phase is shown. The 3-fold rotational symmetricity domain boundaries of dBV•+ found in (d) was focused in (e).

electrostatic repulsion and adsorption
desorption dynamic equilibrium prevent the molecules from being firmly immobilized and thus from being imaged by STM. After the STM observation at Ework > Ect, Ework was swept from 0.00 to
0.38 V at v = 70 mV s
1. We obtained a cathodic spikelike peak reproducing the wave shape of the curve in part b of Figure 1. Then, we held the potential at Ework =
0.38 V at which STM imaging was made. Figure 5 represents typical STM images thus obtained. The stripe pattern phase of the ordered dBV•+ molecules in part a of Figure 5 provided us with clear evidence of the formation of 2D condensed phase of dBV•+ on an HOPG electrode surface. An optimized molecular structure of dBV derived from DFT calculation has been reported by Pham and colleagues.44 In the present work, we also made DFT calculations to obtain a fully optimized structure of dBV in the ground state. We obtained, as shown in Figure 6, a staggered conformation of a dBV2+ molecule and a coplanar one for dBV•+. These structures almost reproduced previous ones by Pham and colleagues.44 The dihedral angle around the central C
C bond is 40.1
for dBV2+ and 2.34
dBV•+. Dihedral angle between benzyl ring and the neighboring ring of bipyridinium is 91.0
and 66.4
respectively for dBV2+ and dBV•+. On the basis of these calculations, an approximated molecular model in a real size of dBV•+ was superimposed to the images of part b of Figure 5 using the central C
C dihedral angle of 0
and benzyl-bipyridinium inter-ring angle of 67
as an approximation. A strong tendency of face-to-face π
π stacking of V•+ moieties is well known. In aqueous solution, this emerges as dimerization of V•+ molecules.45 This is also the case on a solid substrate. Using the results of in situ EC-STM measurements, Pham and colleagues have reported a stripe pattern of adsorbed dBV•+ molecules in a side-on configuration with their main molecular axis parallel to the surface, exhibiting the formation of π
π stacked polymer chains at a chloride modified Cu(100) electrode surface.44 On an HOPG electrode, Arihara and colleagues found that HV•+ molecules are oriented with the side-on

Figure 6. DFT-based optimized structures of dBV2+ and dBV•+ molecules in vacuo.

configuration by IR reflection spectroscopy.24 Taken together, it is quite likely that dBV•+ molecules assume side-on orientation adsorption structure with lateral π
π interaction rather than lay their bipyridinium moieties flat on the HOPG surface. These considerations are incorporated in the molecular ordering model in part b of Figure 5 where the longitudinal axis of dBV•+ molecules lies flat on the HOPG electrode surface with a side-on configuration to achieve π
π stacking between neighboring dBV•+ moieties. The model molecules are placed in the bright line rows so that the ends of the benzyl groups come just at the inner edge of a row. Then the width of bright line measured in parallel to the longitudinal axis of the viologen moiety is approximately 1.60 nm. 13914

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Figure 7. Electroreflectance spectrum of an HOPG electrode in the H-M configuration on 0.1 mM dBV2+ 2Cl
+ 0.3 M KCl solution. Conditions of the measurement were: Edc =
0.28 V, ΔEac = 0.07 V, and f = 14 Hz.

aqueous solution are quite similar in shape to each other. Therefore, we can interpret the ERS in Figure 7 as the reflection spectral difference feature between a colorless dBV2+ layercovered HOPG surface and an HOPG surface in direct contact with strongly light-absorbing dBV•+ layer. The ERS measurements using polarized light at various incidence angles (θ) were made for an HOPG electrode in an H
M configuration. The results were used to estimate the orientation of dBV•+ molecules in the condensed monolayer by the same method as that in our previous reports.37,52
55 The method was successfully used to estimate, for example, the orientation angle of viologen-thiol self-assembled monolayer (VT-SAM) on an Au electrode.53 For model-based working curves of p/s ratio (the intensity ratio of the ER signals for p- and s-polarized light) to be calculated as a function of θ, we used a stratified interfacial model with three phases, namely, underlying HOPG, dBV monolayer, and electrolyte solution. The colorless dBV2+ monolayer gives no contribution to the p/s ratio.52,53 We assumed that the electric moment of the dBV•+ responsible for its light absorption has a unique director angle ϕ with respect to the surface normal, while its azimuthal angle is 2D isotropic.52 The angle ϕ of director vector with respect to the surface normal represents the molecular orientation. The p/s ratio is given52,55 as p=s ¼

2E2p^ cos 2 ϕ þ E2p== sin 2 ϕ E2s sin 2 ϕ

ð3Þ

where E2p^ and E2p are the perpendicular-to-surface and parallelto-surface components at the position of the chromophore, respectively, of the mean square electric field of the surface standing wave of p-polarized light, and E2s is the parallel-tosurface component of the mean square electric field of the surface standing wave of s-polarized light there. A series of ERS were measured at various θ for an HOPG electrode in contact with 0.1 mM dBV2+ 2Cl
+ 0.3 M KCl solution. Their spectral curves were almost the same as that in Figure 7 regardless of θ. Figure 8 shows the plot of experimentally obtained p/s ratio at 546 nm as a function of θ. The broken lines in Figure 8 are the working curves representing eq 3 for three different presumed ϕ values. In the lower part of Figure 8, the mean square electric field amplitudes of the light at the position of )

In the same direction, the translatory distance between adjacent bright lines is 1.75 nm as shown in the image. The illustration for the proposed stacking model (part c of Figure 5) was made by the uses of the area of 0.66 nm2 occupied by one dBV•+ molecule and above-mentioned translatory distance of the stacking row (1.75 nm). The former area was obtained from the CV peak charge (Figure 1). This illustration enabled us to estimate the face-to-face distance between dBV•+ as being 0.38 nm. This value is typical for π
π stacking distances of aromatic systems. For example, the face-to-face π
π stacking distance for 2,20 -bipyridine phase on Au(111) is 0.37 nm15 and 0.36 nm on Au(100),16 that for a 4,40 -bipyridine phase on Au(111) is 0.38 nm,14 and HV•+ and dBV•+ stacking phase on chloride modified Cu(100) are 0.34 nm35 and 0.36 nm,44 respectively. We calculated the projection area expected for one dBV•+ molecule using the values reported for asymmetric viologens46 and our results of DFT calculation to obtain approximately 1.1 nm2 and 0.69 nm2 for flat-lying and side-on configuration, respectively. The area occupied by one molecule calculated from Γ obtained from CVs peak charge (Figure 1) is 0.66 nm2, which is a reasonable value to assume the side-on configuration rather than flat-lying, face-on configuration. Conversely, if the face-on configuration is assumed, experimental Γ is greater than the monolayer amount. It may be worthwhile to compare the ordering structure on an HOPG to that on an Au(111) electrode at which much better resolved STM images are usually obtainable. We found only one paper concerning adsorption structures of diaryl or dialkyl viologens without any anchor group to the electrode surface: Arihara and Kitamura analyzed the adsorption layer structure of HV•+ on an Au(111) electrode by employing in situ IR reflection absorption spectroscopy.29 They found two HV•+ molecules are stacked face-to-face to form a bilayer of HV•+ in which all the bipyridinium moieties of HV•+ are parallel to the Au(111) surface. Although the adsorption structures of HV•+ on an Au(111) electrode are largely different from dBV•+ on an HOPG electrode, in both the intermolecular π
π interaction between V•+ moieties plays a central role. The 3-fold rotational symmetricity domains of a 2D condensed monolayer were observed by the wide-range observation as shown in parts d and e of Figure 5. The domains have three distinct directions of the molecular rows, indicating dBV•+ molecular apparently arranged in line with one of the three possible main symmetry patterns of the underlying graphite lattice. A number of molecular ordering structures of organic molecules on an HOPG surface have been reported so far. Most of them have been obtained in organic solvent such as 1-phenyloctane, 1,2,3-trichlorobenzene and others.47
49 However, such images that enabled one to discuss the molecular alignments on an HOPG, especially under potential control in aqueous solution are few. Those molecular-level resolution images were reported by Tao and Shi for guanine50 and porphyrins.51 In the present work, in situ STM imaging sheds new light on the molecular level view of aqueous electrochemistry on an HOPG. Electroreflectance Studies. The ER signals of an HOPG electrode in the absence of dBV was zero plus noise in the range of λ from 350 to 800 nm around E1/2. Even in the presence of dBV in the solution phase, when Edc was far more positive potential than E1/2, the ER signal was zero. The ERS in Figure 7 was obtained at Edc = E1/2 in the presence of dBV using normal incidence. This ERS shows high resemblance to the ERS at HOPG/HV solution interface.23 In separate experiments, we confirmed that UV
vis absorption spectra of HV•+ and dBV•+ in

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simulated for dBV•+ molecules with 85
∼ 90
tilted longitudinal axis with respect to the surface normal. In summary, occurrence of a faradaic phase transition of the first order of dBV•+/2+ and the ordered structure of side-on adsorbed dBV•+ molecules on an HOPG electrode surface were elucidated. It is of paramount importance to describe a kinetic model for this phase transition using the results of time-resolved in situ measurements. Deeper analysis of potential step transients should also be useful to reveal the nucleation
growth processes and the effect of surface diffusion. We are currently underway of such a challenge in a droplet configuration.

’ AUTHOR INFORMATION Corresponding Author

*[email protected]. Figure 8. Plot of the p/s ratio of the ER signal at λ = 546 nm as a function of the incident angle, θ, at an HOPG electrode surface in the H
M configuration with 0.1 mM dBV2+ 2Cl
+ 0.3 M KCl solution. Three representative working curves of the plots of the p/s ratio of the ER signal vs the incident angle, θ, based on eq 3, are shown. The measured p/s ratio data are shown by the closed circles. The ER response to p- and s-polarized incident light were measured at Edc =
0.28 V, f = 4 Hz, and ΔEac = 0.05 V. Optical dielectric constant of the HOPG at 546 nm were, respectively, 5.898 and 11.068; real and imaginary parts of the refractive index of the monolayer were; respectively, 1.55 and 0.05, water, n1 = 1.333, and the film thickness of the thin organic film, d = 0.44 nm. •+

dBV are shown on an arbitrary scale. In these calculations, the wavelength dependent complex refractive indexes of the HOPG and the adsorption layer were taken, respectively, from our previous ellipsometric data54 and the previous article.23 Figure 8 demonstrates that the experimental p/s ratio fits well to the working curves calculated for dBV•+ molecules with 85
∼ 90
tilt of longitudinal axis with respect to the surface normal of the electrode surface. We can therefore conclude that the longitudinal molecular axis of the dBV•+ moiety is lying flat on the HOPG electrode surface being consistent to our STM results.

’ CONCLUSIONS The behavior of dBV on a basal plane HOPG electrode surface was investigated in depth to characterize its 2D faradaic phase transition. The results of electrochemical measurements enabled us to conclude that the monolayer redox process of dBV on an HOPG electrode is a faradaic phase transition of the first order. The separation between anodic and cathodic transition peak potentials at very slow potential sweep rates, which is a measure of the width of bistable potential region, was almost equal to that of HV. This fact indicates that the contribution of intermolecular π
π interaction between benzyl groups to the phase transition is minor while apparently equivalent to the heptyl chains of HV. We discussed adsorption structures of dBV on the electrode surface using the results of EC-STM and ER measurements. The EC-STM images revealed that dBV•+ molecules are adsorbed with their longitudinal molecular axis lying flat on the HOPG surface in a side-on configuration, and the arrays of molecules form stripe pattern domains through the intermolecular π
π interaction. The ERS represents reflection spectral difference features between colorless dBV2+ and strongly light-absorbing dBV•+ on the electrode surfaces, giving a similar ERS curve shape to that of HV. The experimental p/s ratio fit to the curves

’ ACKNOWLEDGMENT This work was financially supported in part by Nagasaki University president’s discretionary fund. The authors deeply thank Dr. Shinobu Uemura at Kumamoto University for her valuable advises in EC-STM measurements. ’ REFERENCES (1) Buess-Herman, C.; Bar
e, S.; Poelman, M.; Van krieken, M. Ordered Organic Adlayers at Electrode Surfaces. In Interfacial Electrochemistry: Theory, Experiment, and Applications; Wieckowski, A., Eds., Marcel Dekker: New York, 1999; pp 427
447. (2) Tao, N. J. Potential Controlled Ordering in Organic Monolayers at Electrode-Electrolyte Interfaces. In Imaging of Surfaces and Interfaces; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1999; pp 211
248. (3) Wandlowski, Th. Thermodynamics and Electrified Interfaces. In Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Eds.; Wiley-VCH: Weinheim, 2003; Vol. 1, pp 383
467. (4) Dretschkow, Th.; Wandlowski, Th. Structural Transitions in Organic Adlayers
A Molecular View. In Solid-Liquid Interfaces, Topics in Applied Physics; Wandelt, K., Thurgate, S., Eds.; Springer: Berlin, 2003; Vol. 85, pp 259
321. (5) Buess-Herman, C. Dynamics of Adsorption and Two-Dimensional Phase Transitions at Electrode Surfaces. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH, New York, 1992; Vol. 1, pp 77
118. (6) Lipkowski, J.; Stolberg, L. Molecular Adsorption at Gold and Silver Electrode. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J.; Ross, P. N., Eds.; VCH, New York, 1992, Vol. 1, pp 171
238. (7) Pohlmann, L.; Donner, C.; Baumg€artel, H. J. Phys. Chem. B 1997, 101, 10198–10204. (8) Nikitas, P. J. Electroanal. Chem. 1998, 446, 165–175. (9) Donner, C.; Kirste, St. Langmuir 2001, 17, 1630–1636. (10) Donner, C. J. Electroanal. Chem. 2003, 550
551, 209–218. (11) Prado, C.; Navarro, I.; Rueda, M.; Franc-ois, H.; Buess-Herman, C. J. Electroanal. Chem. 2001, 500, 356–364. (12) Retter, U.; Lohse, H. J. Electroanal. Chem. 1982, 134, 243–250. (13) Retter, U. J. Electroanal. Chem. 1984, 165, 221–230. (14) Mayer, D.; Dretschkow, Th.; Ataka, K.; Wandlowski, Th. J. Electroanal. Chem. 2002, 524
525, 20–35. (15) Dretschkow, Th.; Wandlowski, Th. Electrochim. Acta 1999, 45, 731–740. (16) Dretschkow, Th.; Wandlowski, Th. J. Electroanal. Chem. 1999, 467, 207–216. (17) Dretschkow, Th.; Lampner, D.; Wandlowski, Th. J. Electroanal. Chem. 1998, 458, 121–138. 13916

dx.doi.org/10.1021/la202746y |Langmuir 2011, 27, 13910–13917

Langmuir (18) Wandlowski, Th.; Dretschkow, Th. J. Electroanal. Chem. 1997, 427, 105–112. (19) Ta, T. C.; Kanda, V.; McDermott, M. T. J. Phys. Chem. B 1999, 103, 1295–1302. (20) Sagara, T.; Tanaka, S.; Miuchi, K.; Nakashima, N. J. Electroanal. Chem. 2002, 524
525, 68–76. (21) Tanaka, Y.; Sagara, T. J. Electroanal. Chem. 2008, 619
620, 65–74. (22) Sagara, T.; Miuchi, K. J. Electroanal. Chem. 2004, 567, 193–202. (23) Sagara, T.; Tanaka, S.; Fukuoka, Y.; Nakashima, N. Langmuir 2001, 17, 1620–1629. (24) Arihara, K.; Kitamura, F.; Nukanobu, K.; Ohsaka, T.; Tokuda, K. J. Electroanal. Chem. 1999, 473, 138–144. (25) Arihara, K.; Kitamura, F.; Ohsaka, T.; Tokuda, K. J. Electroanal. Chem. 2000, 488, 117–124. (26) Kitamura, F.; Ohsaka, T.; Tokuda, K. J. Electroanal. Chem. 1993, 347, 371–381. (27) Mill
an, J. I.; Rodríguez-Amaro, R.; Ruiz, J. J.; Camacho, L. Langmuir 1999, 15, 618–623. (28) Mill
an, J. I.; Rodríguez-Amaro, R.; Ruiz, J. J.; Camacho, L. J. Phys. Chem. B 1999, 103, 3669–3676. (29) Arihara, K.; Kitamura, F. J. Electroanal. Chem. 2003, 550
551, 149–159. (30) Arihara, K.; Ohsaka, T.; Kitamura, F. Phys. Chem. Chem. Phys. 2002, 4, 1002–1005. (31) Mill
an, J. I.; S
anchez-Maestre, M.; Camacho, L.; Ruiz, J. J.; Rodríguez-Amaro, R. Langmuir 1997, 13, 3860–3865. (32) Cieslinski, R.; Armstrong, N. R. J. Electroanal. Chem. 1984, 161, 59–73. (33) Tanaka, Y.; Sagara, T. Bull. Chem. Soc. Jpn. 2007, 80, 1511– 1517. (34) Sagara, T.; Fujihara, Y.; Tada, T. J. Electrochem. Soc. 2005, 152, 239–246. (35) Jiang, M.; Sak, E.; Gentz, K.; Krupski, A.; Wandelt, K. Chem. Phys. Chem. 2010, 11, 1542–1549. (36) Tsay, S.-L.; Tsay, J.-S.; Fu, T.-Y.; Broekmann, P.; Sagara, T.; Wandelt, K. Phys. Chem. Chem. Phys. 2010, 12, 14950–14959. (37) Sagara, T. UV-visible Reflectance Spectroscopy of Thin Organic Films at Electrode Surfaces. In Advances in Electrochemical Science and Engineering; Alkire, R. C., Kolb, D. M., Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: Weinheim, 2006; Vol. 9, pp 47
95. (38) Cline, K. K.; McDermott, M. T.; McReery, R. I. J. Phys. Chem. 1994, 98, 5314–5319. (39) Gerischer, H.; McIntyre, R.; Scherson, D.; Storck, W. J. Phys. Chem. 1987, 91, 1930–1935. (40) Gerischer, H. J. Phys. Chem. 1985, 89, 4249–4251. (41) Kakiuchi, T.; Usui, H.; Hobara, D.; Yamamoto, M. Langmuir 2002, 18, 5231–5238. (42) Randin, J.-P.; Yeager, E. J. Electroanal. Chem. 1972, 36, 257– 276. (43) Maestre, M. S.; Rodríguez-Amaro, R.; Mu~noz, E.; Ruiz, J. J.; Camacho, L. J. Electroanal. Chem. 1994, 373, 31–37. (44) Pham, D.-T.; Gentz, K.; Z€orlein, C.; Hai, N. T. M.; Tsay, S.-L.; Kirchner, B.; Kossmann, S.; Wandelt, K.; Broekmann, P. New J. Chem. 2006, 30, 1439–1451. (45) Monk, P. M. S. The Viologens: Physicochemical properties, synthesis, and applications of the salts of 4,40 -bipyridine; John Wiley & Sons: Chichester, England, 1989; pp 115
131. (46) Cotton, T. M.; Kim, J.-H.; Uphaus, R. A. Microchemical Journal 1990, 42, 44–71. (47) Tahara, K.; Lei, S.; Adisoejeso, J.; De Feyter, S.; Tobe, Y. Chem. Commun. 2010, 46, 8507–8525 and references therein. (48) Kampschulte, L.; Lackinger, M.; Maier, A.-K.; Kishore, R. S. K.; Griessl, S.; Schmittel, M.; Heckel, W. M. J. Phys. Chem. B 2006, 110, 10829–10836. (49) De Feyter, S.; De Schryver, F. C. Chem. Soc. Rev. 2003, 32, 139–150 and references therein. (50) Tao, N. J.; Shi, Z. J. Phys. Chem. 1994, 98, 1464–1471.

ARTICLE

(51) Tao, N. J. Phys. Rev. Lett. 1996, 76, 4066–4069. (52) Sagara, T.; Kaba, N.; Komatsu, M.; Uchida, M.; Nakashima, N. Electrochim. Acta 1998, 43, 2183–2193. (53) Sagara, T.; Maeda, H.; Yuan, Y.; Nakashima, N. Langmuir 1999, 15, 3823–3830. (54) Sagara, T.; Fukuda, M.; Nakashima, N. J. Phys. Chem. B 1998, 102, 521–527. (55) Sagara, T.; Kubo, Y.; Hiraishi, K. J. Phys. Chem. B 2006, 110, 16550–16558.

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