An Original Electrochemical Method for Assembling Multilayers of

Article pubs.acs.org/Langmuir

An Original Electrochemical Method for Assembling Multilayers of Terpyridine-Based Metallic Complexes on a Gold Surface Sébastien Liatard,†,‡ Jérôme Chauvin,*,† Franck Balestro,‡ Damien Jouvenot,† Frédérique Loiseau,† and Alain Deronzier† †

Département de Chimie Moléculaire, UMR-5250, Laboratoire de Chimie Inorganique Rédox, Institut de Chimie Moléculaire de Grenoble FR- CNRS-2607, Université Joseph Fourier Grenoble 1/CNRS, BP-53, 38041 Grenoble Cedex 9, France ‡ Institut Néel, CNRS et Université Joseph Fourier, BP 166, F-38042 Grenoble Cedex 9, France S Supporting Information *

ABSTRACT: A new method based on the electrochemical oxidation of thiols was used to easily generate multilayer assemblies of coordination complexes on a gold surface. For this purpose, two complexes bearing two anchoring groups for surface attachment have been prepared: [Ru(tpySH)2]2+ (1) and [Fe(tpySH)2]2+ (2) (tpySH = 4′-(2-(p-phenoxy)ethanethiol)-2,2′:6′,2″terpyridine). Cyclic voltammetry of 1 in CH3CN exhibits two successive oxidation processes. The first is irreversible and attributed to the oxidation of the thiol substituents, whereas the second is reversible and corresponds to the 1 e− metal-centered oxidation. In the case of 2 both processes are superimposed. Monolayers of 1 or 2 have been formed on gold electrodes by spontaneous adsorption from micromolar solutions of the complexes in CH3CN. SAMs (self-assembled monolayers) exhibit redox behavior similar to the complexes in solution. The high surface coverage value obtained (Γ = 6 × 10−10 and 4 × 10−10 mol cm−2 for 1 and 2, respectively) is consistent with a vertical orientation for the complexes; thus, one thiol is bound to the gold electrode, with the second unreacted thiol moiety exposed to the outer surface. Successive cyclic voltammetry induced a layer-by-layer nanostructural growth at the surface of the SAMs, and this is presumably due to the electrochemical formation of disulfide bonds, where the thiol moieties play a double role of both an anchoring group and an electroactive coupling agent. The conditions of the deposition are studied in detail. Modified electrodes containing both 1 and 2 alternatively can be easily prepared following this new approach. The film proved to be stable, displaying a similar current/voltage response for more than 10 repeating cycles in oxidation up to 0.97 V vs Ag/AgNO3 (10−2 M).

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chemiluminescence7 with thiol-terminated [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) derivatives. Self-assembled monolayers (SAMs) are easily obtained from a micromolar solution of a complex bearing thiol groups. However, multilayer assemblies are usually prepared following stepwise coordination reactions.8 This procedure consists of the direct coordination of a metallic salt onto a ligand-functionalized SAM, followed by the coordination of a ditopic ligand where the two coordination sites are on opposite sides (like 1,4-bisterpyridylbenzene).9 Alternating heterometallic assemblies containing [Ru(tpy)2]2+ (tpy = 2,2′:6,2″-terpyridine) and Fe(II)10 or Co(II) complexes11 have been obtained using this procedure. In this case the ditopic ligand includes the Ru(II) complex as a bridge between the two free coordination sites. However, this chemical approach is limited to metallic centers for which coordination readily occurs under mild conditions.12 More recently, pure organic multilayer assemblies were obtained from aromatic and alkyl dithiol compounds, where

INTRODUCTION Molecular ordered thin films, from monolayer to multilayer assemblies, show considerable promise for the development of micro- and nanotechnology. In particular, those incorporating metallic complexes offering multiple redox or spin states may play an important role for molecular electronic devices,1 for biotechnological and chemical sensors,2 or in the field of molecular catalysis and solar energy conversion.3 For example, in the field of nanoelectronics, by incorporating redox-active metallic centers into an organic backbone, molecular wires delineated to transport charge between electrodes have notably been obtained showing efficient charge conduction over long distances.4 Films containing ruthenium(II) polypyridinic complexes are good candidates for these purposes, thanks to their electrochemical and light-responsive properties. Several techniques are available to obtain thin and regular assemblies of polypyridinic Ru(II) complexes on a surface. Among them, the self-assembling method of anchoring a ligand substituted by an alkyltrichlorosilane on SiO2 or by thiols on a gold surface proved to be the most effective.5 The latter process offers the advantage of grafting compounds onto a conductive surface, thus allowing the successful generation of a photocurrent6 or © 2012 American Chemical Society

Received: April 26, 2012 Revised: June 26, 2012 Published: June 28, 2012 10916

dx.doi.org/10.1021/la301709d | Langmuir 2012, 28, 10916−10924

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peak potential; E1/2 = (Epa + Epc)/2; ΔEp = Epa − Epc) was performed using a CHI-620b potentiostat (CH Instruments). Controlled potential electrolysis experiments were performed using a SP300 Biologic potentiostat equipped with a digital coulometer. A standard three-electrode electrochemical cell was used. Potentials were referenced to an Ag/AgNO3 (10−2 M) reference electrode in CH3CN + 0.1 M TBAPF6. Potentials referred to that system can be converted to the ferrocene/ferricinium couple by subtracting 87 mV, to the SCE by adding 298 mV, or to the NHE reference electrode by adding 548 mV. Vitreous carbon (VC) (diameter = 3 mm), platinum (diameter = 2 mm), or gold (diameter = 2 mm) working electrodes (CH Instruments) were polished with diamond paste (Mecaprex Presi, 1 μm) before each recording. Electrolyses were conducted at 25 °C using a cylinder-shaped platinum gauze or a reticulated vitreous carbon foam 45 ppi (Electrosynthesis Co.). Formation of SAM on Gold. Gold disk electrode was polished thoroughly with diamond paste. A blank reduction cycle from 0 V down to −2 V was run in CH3CN + 0.1 M TBAPF6 with the gold electrode as a working electrode before any adsorption. This reduction cycle was essential to molecular grafting. The gold electrode was then immersed in a 5 × 10−4 M solution of 1 or 2 in CH3CN for 15−20 h. Before electrochemical experiments, the modified electrode was rinsed with acetonitrile and transferred to a solution of 0.1 M TBAPF6 in acetonitrile. Absorbance and Emission. Absorption and emission spectra were obtained using respectively a Cary 300 UV−visible spectrophotometer (Varian) and a Cary Eclipse fluorimeter. Emission lifetime measurements were performed after irradiation at λ = 400 nm using a setup already described.20 Atomic Force Microscopy (AFM). AFM images were recorded on a Veeco dimension 3100 atomic force microscope. The probes used were RTSP probe and the DNP-20 probe for tapping mode and contact mode, respectively. For these experiments 1 was electropolymerized on a gold layer deposited on a Si/SiO2 wafer. The resulting electrodes were rinsed and images were recorded under air with a scan rate of 0.9 Hz. The scan sizes were 3 μm2 and 500 nm2 for the tapping and contact mode, respectively.

only one thiol group is bound to the surface and the unreacted thiol group is exposed to the solution. Residual oxygen13 or the addition of an oxidant14 in solution leads then to a multilayer construction by oxidative S−S coupling between dithiol derivatives. The disulfide bond is confirmed by surface enhanced Raman spectroscopy15 and can be suppressed after a reductive agent (tri-n-butylphosphine) is added to the solution.16 Following this method, biomolecules were successfully immobilized on a thiol-functionalized surface by applying a potential to induce oxidative coupling between the thiolmodified surface and the thiols of cysteine residues of a peptide.17 To the best of our knowledge, this electroinduced coupling reaction has never been reported in the formation of nanostructured multilayer assemblies of coordination compounds. This process could certainly be beneficial for technological applications. Fabrication of well-defined nanometer scale spaced metallic electrodes to connect to large molecules is still an experimentally difficult task, resulting in expensive and often irreproducible devices.4b,18 An interesting approach would be the utilization of an electroinduced coupling reaction to grow molecular wire directly between two metallic electrodes separated by a nanogap obtained by lithography techniques.19 For this purpose, two new bis(4′-(4-(2-mercaptoethoxy)phenyl)-2,2′:6′,2″-terpyridine) ruthenium(II) (1) and iron(II) (2) complexes (Scheme 1) have been synthesized and fully Scheme 1. Molecular Structure of Complexes 1 and 2

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RESULTS AND DISCUSSIONS Synthesis and Characterization. Complexes 1 and 2 (Scheme 1) were synthesized in four steps each. Details of the synthesis and characterization (NMR and ESI) of 1, 2, and all the intermediate species are presented in the Supporting Information. Complex 1 was synthesized by following a metalcentered strategy that consisted of the formation of a bisterpyridine Ru(II) complex that bears the appropriate functional groups for further addition of a thioacetate as a first step. The thioesters were then deprotected in acidic conditions under argon. This strategy presents the advantage of easier purification procedures. In the case of 2, where the lability of iron bisterpyridine complex is an issue, a ligandcentered strategy has been adopted. The deprotected 4′-(4-(2mercaptoethoxy)phenyl)-2,2′:6′,2″-terpyridine was synthesized and then complexed to Fe2+ as a sulfate salt under argon. UV−vis absorption spectra of 1 and 2 are given in Figure 1. Absorption bands in the UV region are ascribed to ligandlocalized transitions, mainly the π→π * transitions of the terpyridine fragments, whereas the band in the visible region is assigned to a metal-to-ligand charge transfer (MLCT) transition. The bands appear at wavelengths similar to those of [Ru(tpy)2]2+ 21 or [Fe(tpy)2]2+.22 Complex 1 is poorly luminescent at room temperature in deoxygenated CH3CN with an emission maximum at 656 nm and a luminescence lifetime of 0.7 ns. At 77 K in a butyronitrile rigid matrix, the luminescence lifetime is increased to 12 μs, as expected.23

characterized. These complexes contain two thiol groups, one of which allows easy attachment to the gold surface. The second can then be used for film growth, as oxidized sulfides show a high reactivity toward dimerization with free thiols in solution. Hence, the electrochemical behavior of complexes 1 and 2 was explored in detail. Iterative cyclic voltammetry has clearly given evidence for the formation of the expected multilayer assemblies on a gold surface. This technique was therefore extended to the formation of an organized bimetallic assembly of 1 and 2.

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EXPERIMENTAL SECTION

Materials and General. Acetonitrile (CH3CN, Rathburn, HPLC grade) and tetra-nbutylammonium hexafluorophosphate (TBAPF6, Aldrich) were used as received and stored under argon in a dry glovebox (Jaram). Ruthenium(II) and iron(II) bis(4′-(4-(2mercaptoethoxy)phenyl)-2,2′:6′,2″-terpyridine) hexafluorophosphate, respectively denoted 1 and 2, were synthesized following the procedures described in the Supporting Information. Electrochemistry. All electrochemical measurements were run under an argon atmosphere in a dry glovebox at room temperature. Cyclic voltammetry (CV) (Epa, anodic peak potential; Epc, cathodic 10917


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is absent in the case of Ru(II) bis-thioacetyl protected parent complex 1′ (see Supporting Information, Figure S1). This irreversible oxidation is associated with an irreversible reduction process, observed at a very negative cathodic potential around −1.0 V on a Pt working electrode. The latter process is not detected using a vitreous carbon electrode (Supporting Information, Figure S2). This feature is typical of a proton reduction process that is catalyzed on a Pt electrode. The oxidation of the thiol moieties forms a radical cation species that is extremely unstable and is coupled to a proton release mechanism. The redox processes can be summarized with the eqs 1a−1c.

Figure 1. UV−visible absorption spectra of 1 (solid line) and 2 (dotted line) in CH3CN.

[Ru(II)(tpySH)2 ]2 + → [Ru(II)(tpyS•+H)2 ]4 + + 2e− (1a)

General Electrochemical Properties. The electrochemical behavior of 1 and 2 in CH3CN + 0.1 M TBAPF6 was first investigated by cyclic voltammetry on vitreous carbon and Pt electrodes to avoid any spontaneous and rapid adsorption process. The electrochemical data are summarized in Table 1,

[Ru(II)(tpyS•+H)2 ]4 + → [Ru(II)(tpyS•)2 ]2 + + 2H+ (1b)

2e− + 2H+ → H 2

Table 1. Redox Potentials of 1 and 2 in CH3CN + 0.1 M TBAPF6 under Ara complexes

MIII/MII E1/2/V (ΔEp/mV)

1, M = Ru 2, M = Fe

0.93 (51) 0.76 (58)

1, M = Ru 2, M = Fe

0.97 (58) 0.78 (20)

1, M = Ru 2, M = Fe

0.94 (49) 0.79 (10)

thiol oxidation Epa/V In Solutionb 0.73 0.76c In SAM − − In Multilayer − −

(1c)

After oxidation, the thiol radicals may form different products. Depending on the concentration of water and/or oxygen in the medium, disulfide bonds may be obtained by the coupling of two thiol radicals or thiosulfinates or thiosulfonates,24 leading to soluble polymers. However, in our experimental conditions, i.e., in a dry, inert atmosphere glovebox, the thiosulfinate and thiosulfonate formation is avoided as much as possible. In the cathodic part, 1 exhibits two ligand-based reduction processes. The second one at −1.81 V is partly irreversible and associated with a reoxidation process at −0.54 V. This is probably a consequence of the superimposition of an irreversible reduction process of the thiol moieties with the second terpyridine-based reduction. Indeed, for the protected complex 1′ the ligand-based reduction appears perfectly reversible (Supporting Information, Figure S1). Cyclic voltammetry is similar for 2, except that, in the anodic area, the irreversible oxidation of the thiol moieties is hidden within the Fe2+/3+ reversible peak. In the cathodic part, the thiols are evidenced by a new irreversible reduction peak that emerges at −1.87 V, after the two terpyridine reversible reduction processes.20 Owing to the specific interaction of thiol with a gold (or to a much lesser extent with a platinum) metallic surface, selfassembled monolayers (SAMs) of the complexes can be obtained. For that purpose, a gold electrode has been soaked for 15 to 20 h in a solution of 1 or 2 (5 × 10−4 M) in CH3CN. Since the monolayer is redox active, it can easily be detected by cyclic voltammetry. After washing and transferring the electrode to a pure CH3CN + 0.1 M TBAPF6 solution, the redox properties of the SAM of 1 adsorbed on gold are recorded (Figure 3). The metal-centered oxidation and the two ligand-centered reductions appear at similar potentials to those of the free complexes in solution (Table 1). The variation of the oxidation current peak intensity is linear with the scan rate according to the expected behavior of an adsorbed layer (Figure 3 inset). The intensity of the redox response of the SAM remains constant after several electrochemical cycles over the entire potential window (+1.2, −2.1 V), indicating that no desorption occurs after oxidation or reduction of the complex on the voltammogram time scale. A surface coverage Γ of 6 × 10−10 mol cm−2 has been determined according to eq 2

tpy-based reduction E1/2/V (ΔEp/mV) −1.54 (64); −1.81 (184) −1.53 (92); −1.70 (73) −1.51 (8); −1.82 (40) −1.53 (20); −1.75 (80) −1.55 (25); −1.87 (53) −1.50 (24); −1.67 (24)

Potentials are referred to Ag/AgNO3 (10−2 M). ν = 100 mV s−1. bPt working electrode (diameter = 2 mm). cThe potential cannot be accurately determined due to the proximity of the FeIII/FeII redox system. a

and the voltammograms of the two complexes are compared in Figure 2. In the anodic part, 1 features two successive oxidation processes. The most anodic one associated on the reversed scan with a cathodic peak (ΔEp = 51 mV) is reversible and corresponds to the Ru2+/3+ system. The peak at 0.73 V is irreversible in contrast with the oxidation of the metallic center and assigned to the oxidation of the thiol moieties. Such a peak

Figure 2. Cyclic voltammograms in CH3CN + 0.1 M TBAPF6 at a Pt disk electrode (diameter = 2 mm) of 5 × 10−4 M 1 (top) and 5 × 10−4 M 2 (bottom). Scan rate ν = 100 mV s−1. 10918


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of the Ru2+/3+ couple (Figure 4). This behavior is typical of the accumulation of redox-active species deposited on the electrode

Figure 3. Cyclic voltammogram in CH3CN + 0.1 M TBAPF6 of a SAM of 1 on a gold electrode (diameter = 2 mm), scan rate ν = 50 mV s−1. Inset: linear plot of the oxidation current peak for the Ru2+/3+ system in a SAM of 1 on gold vs scan rate.

Γ=

Q nFA

Figure 4. Evolution of the signal during oxidative electropolymerization of 5 × 10−4 M 1 in CH3CN + 0.1 M TBAPF6 by repeated potential scans (0, +0.97 V) at a gold electrode (diameter = 2 mm), ν = 100 mV s−1. Inset: modified electrode after transfer into a clean electrolyte (CH3CN + 0.1 M TBAPF6).

(2)

where Q is the charge required to oxidize the metallic center determined from the area under the oxidation peak of M2+/3+ (for that purpose, the double layer charge was not included), n the number of electrons transferred (n = 1), F the Faraday’s constant, and A the area of the electrode. In the case of 1, the monolayers obtained appear markedly denser than for a SAM of bisthiolated [Ru(bpy)3]2+, where the two thiol groups are borne by the same bipyridine ligand ([Ru(bpy)2(bpy(SH)2)]2+, Γ = 8 × 10−11 mol cm−2).25 The difference results from the nature of the ligand and the geometry of the involved complexes. The more rigid phenylterpyridine ligands in 1 offer more linear geometry for the resulting complex, and the two anchoring groups are on opposite sides in contrast to the substitution of one bipyridine by two mercaptomethyl groups in the case of [Ru(bpy)2(bpy(SH)2)]2+. This mainly leads to a different organization of the complexes on the gold surface, with a higher concentration of vertical edge-on complexes for 1 compared to [Ru(bpy)2(bpy(SH)2)]2+, where the two thiol groups can both be bound to the surface, resulting in a horizontal orientation of the complexes and thus a lower surface density. On the other hand, a surface coverage saturation of 2 × 10−11 mol cm−2 was obtained for a SAM of bis-octanethiolated terpyridine Ru(II) complex,26 whereas a full coverage rate of 6 × 10−10 mol cm−2 (equal to the one we found for 1) was determined with free terpyridine on gold.27 These comparisons suggest that a long and flexible spacer between the terpyridines and the two thiol ends also favors a horizontal orientation of the complexes on the surface. For the SAM of 1, due to the shorter linker, the surface density is in accordance with a close-to-full monolayer on gold, where only one of the two available thiols per complex interacts with the surface. SAMs are also obtained from 2 under the same experimental conditions as for 1. The surface coverage is however slightly smaller, Γ = 4 × 10−10 mol cm−2, and the electrochemical potential values are similar to those of the free complex in solution (Table 1). Electroformation of Multilayer Assemblies. Additional experiments were performed using a standard three-electrode electrochemical cell with a gold working electrode. Cyclic voltammetry experiments performed in a solution of 1 led to a similar signal than the one obtained in Figure 2. However, repeated potential scans in oxidation, over the range from 0 to +0.97 V, resulted in a continuous increase of the peak current

as a consequence of oxidation.28 This phenomenon is wellknown for polymeric films containing metallic complexes, as for instance polypyrrole functionalized by [Ru(bpy)3]2+ 29 or [Ru(tpy)2]2+ subunits.30 Such polymers are formed by the oxidation-induced coupling of the pyrrole moieties. For 1, the presence of the thiol groups capable of forming disulfide bonds upon oxidation likely triggers the polymerization on the surface of the electrode. Such modified electrodes, after washing, are transferred into a clean electrolyte solution and exhibit the regular electroactivity of the immobilized redox systems of 1 (Figure 4, inset). The quantity of complexes present on the electrode is estimated at 3 × 10−8 mol cm−2 after 70 cycles according to eq 2. This value, 50 times higher than the maximum surface coverage number of previously described SAMs (Γ = 6 × 10−10 mol cm−2), indicates an efficient polymerization of 1 at the surface of the electrode during the iterative cycles. When the iterative cycling conditions for the deposition process are extended to the entire potential window (+1.06, −2 V), accumulation of 1 is still observed (Figure 5). In the cathodic part, the cyclic voltammogram shows, in addition to

Figure 5. Evolution of the signal during oxidative electropolymerization of 5 × 10−4 M 1 in CH3CN + 0.1 M TBAPF6 by repeated potential scans (1.06, −2.0 V) at a gold electrode (diameter = 2 mm), ν = 100 mV s−1 (top). Modified electrode after transfer into a clean electrolyte (CH3CN + 0.1 M TBAPF6) (bottom). 10919


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the redox signal of 1, the presence of an irreversible peak. It has been shown previously that thiol groups are able to desorb and readsorb onto the gold surface during a reduction cycle.31 We therefore attribute the irreversible system consisting of a reduction peak superimposed with the reductions of the terpyridine ligands and a reoxidation peak at around −0.6 V to the desorption and readsorption of the thiol moieties. The cyclic voltammogram obtained for the resulting electrode after transfer to a clean electrolyte solution (Figure 5, bottom) indicates that the metal-centered oxidation (E1/2 = 0.94 V, ΔEp = 49 mV) and the two ligand-based reductions (E1/2 = −1.55 V, ΔEp = 25 mV; E1/2 = −1.87 V, ΔEp = 53 mV) of 1 in the modified electrode appear at potentials close to those of a SAM on gold. In addition to these systems, a couple of prepeaks are observed at the base of the Ru2+/3+ oxidation and of the first ligand reduction. These prepeaks are the consequence of the formation of a resistive film on the electrode surface. For films having initially large resistivity, it is well-established that prepeaks emerge in the cyclic voltammogram due to the flow of charge previously delayed during the scan.32 This behavior illustrates that the conductivity of the adsorbed film on the electrode is ensured by electron hopping between adjacent metallic centers. Complementary electrochemical experiments were performed in order to gain insight into the structure of the adsorbed film and the mechanism of formation. First of all, the same experiments using vitreous carbon or Pt working electrodes did not lead to an increase of the redox peaks during repeated potential scans. This suggests that the specific interaction of thiolated compounds with a gold surface is a necessary condition for the formation of an electrodeposited film. Indeed if the adsorbed film is simply due to a deposition of insoluble species after oxidation of the thiol groups into disulfides, then the film should grow on any kind of electrode. The electroformation of the adsorbed film is therefore initiated by the rapid formation of an adsorbed monolayer of 1 on gold after the first oxidation cycle, resulting in a thiol-modified surface. Formation of SAMs from alkylthiols on a gold substrate at positive potentials has already been found to produce monolayers much faster than regular adsorption processes (i.e., without applied potential).33 The film growth upon successive cycles is likely to be the consequence of the oxidative formation of a sulfur−sulfur bond between pending thiols on the surface and thiols in solution. Moreover, the regular stacking of the layers rules out an intralayer S−S bond formation between neighbors, implying an efficient interlayer S−S linking reaction during the successive oxidation cycles. A similar behavior has already been observed for a series of alkanedithiols13b,15,34 but was never reported for the formation of covalent multilayers of coordination complexes. A series of experiments were performed to determine the role of the Ru2+/3+ system on the film growth. The upper potential limit in the repeating cyclic scan deposition process (25 cycles) was changed either after the thiol irreversible oxidation peak (0.80 V) for the first experiment or after the Ru2+/3+ oxidation peak (0.97 V) for the second. The resulting modified electrodes were then transferred into a clean electrolyte solution. Figure 6 shows the presence of the Ru2+/3+ system on both the modified electrodes with a more intense redox response in the second experiment. The surface concentration after 25 cycles is estimated to be 4 × 10−9 and 1.25 × 10−8 mol cm−2, respectively, for the first and second deposition process. The increase by a factor of 3 in the

Figure 6. Cyclic voltammograms in CH3CN + 0.1 M TBAPF6 of modified gold electrodes by multilayer assemblies of 1, ν = 100 mV s−1. Modified electrodes are obtained after 25 anodic scans from 0 to +0.81 V (full line) and +0.97 V (dashed line), in a 5 × 10−4 M solution of 1 in CH3CN + 0.1 M TBAPF6.

deposition efficiency when the upper limit is extended to 0.97 V can be attributed to both a longer oxidation time and a catalytic effect of the metallic centers, where the generated Ru3+ species rapidly oxidize the thiol moieties.32b In a second series of experiments, the efficiency of the film growth of 1 on a gold surface upon cycling (20 cycles) was explored at several stages of an exhaustive electrolysis performed at 0.80 V on a reticulated vitreous carbon foam to confirm the role of the thiol moieties for the deposition process. Four different modified gold electrodes were produced, using the conditions previously described, by anodic cycling in a solution of 1 (1.4 × 10−6 mol in 10 mL of CH3CN + 0.1 M TBAPF6) (i) before electrolysis, (ii) after consumption of 0.13 C (1 e− per complex), (iii) of 0.21 C (1.6 e− per complex), and (iv) of 0.28 C (2.2 e− per complex), at the end of the electrolysis. Cyclic voltammograms of the resulting modified electrodes show that the complex in solution gradually loses its ability to adsorb on the surface after the selective electrolysis of the thiol moieties (Figure 7). This proves that the resulting disulfides or the higher oxidized forms of 1 do not bind to gold after repetitive oxidative cycles. This confirms the necessity of the presence of thiol functions for the film deposition and growing processes. It has already been reported that dialkyl disulfide adsorption process occurs preferentially under cathodic conditions in contrast to the deposition of alkylthiols

Figure 7. Cyclic voltammograms in CH3CN + 0.1 M TBAPF6 of modified gold electrode by multilayer assemblies of 1, ν = 100 mV s−1. Modified electrodes are obtained before electrolysis of a solution of 1.4 × 10−4 M 1 in CH3CN + 0.1 M TBAPF6 (a), after transfer of 1 e− per complex (b), after transfer of 1.6 e− per complex (c), and after transfer of 2.2 e− per complex (d). 10920


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that proceeds readily under anodic conditions.35 Our series of experiments support then the hypothesis of a two-step mechanism for the electroformation of the multilayer assembly. First, a monolayer of dithiol complexes is attached to a gold surface under anodic conditions by the covalent bonding of only one of the two sulfur moieties available per complex. In a second step an oxidative S−S coupling occurs between the first layer and complexes still in solution, whereas the disulfide species generated in solution do not adsorb at the electrode. In order to better understand the architecture of the deposition, the growth of the multilayer assemblies during repeated anodic cycles (0, +0.97 V) in a 5 × 10−4 M solution of 1 was followed by AFM in the tapping mode. For that purpose the gold electrodes were fabricated on a Si/SiO2 wafer using standard microlithography techniques. After 30, 50, and 100 anodic cycles a small square of the modified electrode was scratched with the tip of the AFM in contact mode. To determine the thickness of the deposited film, the difference of altitude between the free gold surface and the multilayer assemblies was measured in the tapping mode (Figure 8). Thus,

probably prevent this phenomenon, as it would offer a more rigid linker to the surface.37 An alternative approach could consist of the incorporation of a short molecular spacer between complexes in the SAM.38 A similar process can lead to the construction of multilayer assemblies of 2. Figure 9 shows the typical growth of the

Figure 9. Evolution of the signal during oxidative electropolymerization of 5 × 10−4 M 2 in CH3CN + 0.1 M TBAPF6 by repeated potential scans at a gold electrode (diameter = 2 mm), ν = 100 mV s−1. Inset: Modified electrode after transfer into a clean electrolyte (CH3CN + 0.1 M TBAPF6) and 10 sweeps from 0 to 0.97 V.

current peak with the iterative anodic cycle (0, +0.97 V) in a solution of 2 (5 × 10−4 M). After 40 cycles the resulting modified electrode is thoroughly rinsed and transferred to a clean electrolyte solution. The cyclic voltammogram confirms the presence of an adsorbed film on the electrode. However, this film is less stable than the film formed with 1. It appeared that after 10 cycles between 0 and 0.97 V in free electrolyte, the resulting electrode (Figure 9 inset) shows a stable current/ voltage response corresponding to only 2 × 10−9 mol cm−2, whereas a value of 2 × 10−8 mol cm−2 was found for the initial cycle. The loss of stability of the modified electrode is probably due to the formation, during iterative scans, of further oxidized sulfur species, which may desorb. Using quartz crystal microbalance coupled electrochemical analysis, Chon and Paik proved for instance that thiol adsorbed on a gold surface desorbs at higher potentials by oxidation to sulfate or sulfonate.39 For 1, however, the presence of the Ru2+/3+ redox system at higher potential seems to protect disulfide moieties toward further oxidation, and the multilayer assemblies appear slightly more stable at higher potential. Heterobinuclear Assemblies of 1 and 2. The process of fabrication of the multilayer assemblies was extended to the formation of heterobinuclear layers on gold surfaces. The initial step consisted of soaking a gold electrode for 20 h in a solution of 2 (5 × 10−4 M) in CH3CN. The electrode is then rinsed and transferred into a clean electrolyte. Its cyclic voltammogram in the anodic part (Figure 10, top) confirms the formation of an adsorbed monolayer of compound 2. The Γ value (4 × 10−10 mol cm−2) confirms that the surface of the electrode is saturated by the complex. This SAM was then dipped in a solution of 1 (5 × 10−4 M) in CH3CN + 0.1 M TBAPF6, after which three successive anodic scans (0, +0.97 V) induced the deposition of 1. The electrode is rinsed and after transfer in a clean electrolyte, the voltammogram shows in the anodic part the presence of two oxidation systems (E1/2 = 0.79 V, ΔEp = 12 mV; E1/2 = 0.93 V, ΔEp = 30 mV) assigned respectively to the

Figure 8. AFM images of the multilayer assemblies of 1 before and after scratching (top). Dependence of the film thickness with the number of repeating anodic cycles (bottom).

the thickness of the multilayer assemblies increased up to 32 nm for 100 voltammetric scans. This value can be compared to the length of the complex, estimated to 2.5 nm by MOPAC calculation,36 which suggests a vertical assembly of more than 12 molecules after 100 scans. The thickness of the film is thinner than expected taking into account the surface coverage value determined by eq 2. For instance, 25 cycles of electrodeposition gives a coverage value (1.25 × 10−8 mol cm−2) corresponding to 20 times that of the surface density of a SAM (see above in the text), whereas the thickness determined by AFM (11 nm) would only correspond to a vertical stacking of four molecules after 30 cycles. One reason for the difference between these two experimental results could be that the film does not in fact grow vertically but lies on the surface after a few number of scans. A bi- or tripodic anchoring group would 10921


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powerfull tool for the generation of electro- and photoactive nanowires between nanogap electrodes. Application of heterobimetallic bilayers of 1 and 2 as photoelectrode is currently under investigation.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis procedure and characterization (1H NMR, ESI-MS) of compounds 1 and 2, cyclic voltammogram of protected 1′ on platinum working electrode, and cyclic voltammogram of 1 on vitreous carbon electrode. This material is available free of charge via the Internet at http://pubs.acs.org

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Figure 10. Cyclic voltammogram in CH3CN + 0.1 M TBAPF6 of modified gold electrode by a SAM of 2 (top) and a bilayer of 2 and 1 (bottom). ν = 100 mVs−1.

AUTHOR INFORMATION

Corresponding Author

*Fax: (+33) 476514267. E-mail: jerome.chauvin@ujf-grenoble. fr.

Fe2+/3+ and Ru2+/3+ centers. The intensities of the oxidation peaks in these conditions are in accordance with a 1:1 ratio for the two metallic centers (Figure 10, bottom). The redox potential systems of the bimetallic assembly are well-separated and correspond to the addition of the redox system of the two SAMs of 1 and 2. This hence confirms that the two metallic centers are totally independent redox species. The film proved to be reasonably stable, displaying a similar current/voltage response for more than 10 cycles in oxidation (0, +0.97 V). It should be noted that such a construction of a heterobimetallic system containing both Fe(II) and Ru(II) bisterpyridine fragments would have been more difficult to obtain by classical molecular chemistry. Indeed, in solution, a FeII/RuII bisterpyridine dyad bearing thiol terminated groups for surface anchoring such as [HS-(tpyFetpy)-(tpyRutpy)-SH]4+ would rearrange rapidly because of the lability of the [Fe(tpy)2]2+ moiety. This would lead to a statistical distribution of possible isomers (i.e., [HS-(tpyFetpy)-SH]2+ and [HS-(tpyRutpy)(tpyFetpy)-(tpyRutpy)-SH)]6+.40 In addition, the synthesized dyad could be attached to the surface by either the ruthenium or iron center, which would further complicate the electron transport studies.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.L. thanks the CNRS for an “ATIP jeunes chercheurs” funding. S.L. thanks the University Joseph Fourier of Grenoble for a Ph.D. grant. F.B. thanks the ERC Advanced Grant MolNanoSpin No. 226558. The chemistry platform NanoBio campus in Grenoble is acknowledged for luminescence lifetime measurement facilities.

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REFERENCES

(1) Due to the large literature on the subject, we selected only few examples: (a) Man, K.; Wong, H.; Chan, W.; Djurisic, A.; Beach, E.; Rozeveld, S. Use of a Ruthenium-Containing Conjugated Polymer as a Photosensitizer in Photovoltaic Devices Fabricated by a Layer-byLayer Deposition Process. Langmuir 2006, 22, 3368−3375. (b) Seo, K.; Konchenko, A.; Lee, J.; Bang, G.; Lee, H. Molecular Conductance Switch-On of Single Ruthenium Complex Molecules. J. Am. Chem. Soc. 2008, 130, 2553−2559. (c) Zhao, W.; Tong, B.; Pan, Y.; Shen, J.; Zhi, J.; Shi, J.; Dong, Y. Fabrication, Electrochemical, and Optoelectronic Properties of Layer-by-Layer Films Based on (Phthalocyaninato)ruthenium(II) and Triruthenium Dodecacarbonyl Bridged by 4,4′Bipyridine as Ligand. Langmuir 2009, 25, 11796−11801. (d) 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. (e) Terada, K.-I.; Nakamura, H.; Kanaizuka, K.; Haga, M.-A.; Asai, Y.; Ishida, T. LongRange Electron Transport of Ruthenium-Centered Multilayer Films via a Stepping-Stone Mechanism. ACS Nano 2012, 6, 1988−1999. (2) see, for instance: (a) Madoz, J.; Kuznetov, B.; Medrano, F.; Garcia, J.; Fernandez, V. Functionalization of Gold Surfaces for Specific and Reversible Attachment of a Fused β-Galactosidase and Choline-Receptor Protein. J. Am. Chem. Soc. 1997, 119, 1043−1051. (b) Collman, J.; Devaraj, N.; Decréau, R.; Yang, Y.; Yan, Y.-L.; Ebina, W.; Eberspracher, T.; Chidsey, C. A Cytochrome c Oxidase Model Catalyzes Oxygen to Water Reduction Under Rate-Limiting Electron Flux. Science 2007, 315, 1565−1568. (3) see for instance: (a) Abdelrazzaq, F.; Kwong, R.; Thompson, M. Photocurrent Generation in Multilayer Organic−Inorganic Thin Films with Cascade Energy Architectures. J. Am. Chem. Soc. 2002, 124, 4796−4803. (b) Cheng, L.; Cox, J. Nanocomposite Multilayer Film of a Ruthenium Metallodendrimer and a Dawson-Type Polyoxometalate as a Bifunctional Electrocatalyst. Chem. Mater. 2002, 14, 6−8. (c) Concepcion, J.; Jurss, J.; Brennaman, M.; Hoertz, P.; Patrocinio, A.; Iha, M.; Neyde, Y.; Templeton, J.; Meyer, T. Making Oxygen with Ruthenium Complexes. Acc. Chem. Res. 2009, 42, 1954−1965. (d) Yongblood, W.; Lee, S.-H.; Kobayashi, Y.; Hernandez-Pagan, E.; Hoertz, P.; Moore, T.; Moore, A.; Gust, D.; Mallouk, T. Photoassisted

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CONCLUSION In this study, self-assembled monolayers of thiol-functionalized bisterpyridine Ru(II) and Fe(II) complexes on gold substrate were successfully prepared from micromolar solutions. The surface coverage values (Γ = 6 × 10−10 mol cm−2) obtained from the area under the redox peak suggest a vertical orientation of the complexes featuring one terminal thiol adsorbed onto the gold surface while the other thiol extremity is exposed to the solution and therefore available for oxidative S−S coupling. We proved, by iterative cycling in a solution of 1, the multilayer construction of a film by S−S linking chemistry. Molecular depositions of 1.25 × 10−8 and 3 × 10−8 mol cm−2 were obtained by cycling the potential between 0 and 0.97 V, 25 and 75 times, respectively, enlightening this efficient polymerization process. The process has been successfully extended to obtain organized bimetallic electro- and photoactive nanostructures by the electrocoupling between a SAM of 2 and 1 in solution. The light harvesting and electron/energy transfer properties of such structures could be exploited in nanodevices. Notably, the thiol extremity of the structure can be used to anchor gold nanoparticules on the top of the assembly. This original method to produce multilayer assemblies can also be a 10922


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An Original Electrochemical Method for Assembling Multilayers of

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