Nitroxyl Radical Self-Assembled Monolayers on Gold: Versatile Electroactive Centers in Aqueous and Organic Media

DOI: 10.1002/cphc.200900448

Nitroxyl Radical Self-Assembled Monolayers on Gold: Versatile Electroactive Centers in Aqueous and Organic Media

Olivier AlvÞque,^[a] Fawzia Seladji,^[b]Christelle Gautier,^[a]Marylne Dias,^[a]Tony Breton,*^[a]and Eric Levillain*^[a]

Dedicated to Professor E. Laviron

Since the pioneering work by Nuzzo and Allara in 1983,^[1]self- assembled monolayers (SAMs) of alkanethiols have gained much attention in the interfacial electrochemistry and other re- search fields.^{[2, 3]}During the last decade,^{[4, 5]}increasing attention has been dedicated to the design and elaboration of redox-re- sponsive SAMs. Employing redox SAM-modified electrodes for catalysis, recognition and sensing have resulted in some ele- gant examples that incorporate sophisticated receptors on the electrode surfaces.^[5] However, to the best of our knowledge, no redox-responsive SAM was designed to be active and stable in both aqueous (on wide range of pH) and organic media.

TEMPO (2,2,6,6-Tetramethylpi- peridine-1-oxyl) and its deriva- tives (under their oxoammonium form) have been extensively studied in the search for organic synthesis as a redox mediator, mostly, for the oxidation of pri- mary alcohols.^[6–8] The electro- chemical oxidation of TEMPO is known to be a stable and rever- sible one-electron process in both aqueous^[9–11]and non-aque- ous electrolytes.^{[12, 13]}Despite the wide electrochemical applica- tions of nitroxyl radical, rare works have been devoted to

design and elaborate of redox-responsive TEMPO SAMs.^[14–16]In 1997, Fuchigami et al.^[14] reported the first preparation of stable self-assembled TEMPO-modified electrodes in acetoni- trile: the electrochemical stability was only reached with highly diluted mixed SAMs of nitroxyl radical derivative and hexade- canethiol. In 1999, Kashiwagi et al.^[15] reported electrocatalysis attempts of amines with a mixed SAM of chiral nitroxyl radical derivative and hexadecanethiol: as mentioned by authors, re-

sults could be applied to the determination of optical purity of chiral amines. After a ten-years pause, Finklea et al.^[15] worked on Au-SACHTUNGTRENNUNG(CH₂)₁₀C(O)N(H)-TEMPO SAM to the first estimation of the standard rate constant and the reorganization energies of TEMPO/TEMPO⁺in 1mH₂SO₄.

Herein, we raised the challenge of a design of redox-respon- sive TEMPO SAMs to be active, stable and providing electroca- talytic activities in both aqueous and non-aqueous solvents. To reach this goal, we synthesized the three nitroxyl radical deri- vates1 a,1 band1 cshown in Scheme 1. The synthesis of1 a, 1 b and1 c were carried out as outlined in Scheme 1.w-thio-

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acetyl carboxylic acids with different carbon chains length 2 a–c were synthesized from the corresponding w-bromocar- boxylic acids by nucleophilic displacement with potassium thio-

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acetate in DMF.^[17]4-aminoTEMPO was coupled tow-thioacetyl carboxylic acids 2 a-c via the active ester method, using di-

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cyclohexylcarbodiimide (DCC) and 1-hydroxybenzotriazole (HOBT) in methylene chloride (CH₂Cl₂), to give the TEMPO de- rivatives 3 a–c in good yields (77 to 84 %). The thioacetates 3 a–c were subsequently deprotected to thiols 1 a–c, under basic conditions, using CsOH.H₂O in a mixture of THF and MeOH (yields: 50 to 91 %).

Cyclic voltammograms (CVs)^[18] of3 a,3 bor3 cexhibit a re- versible one-electron process in CH₂Cl₂ and CH₃CN, close to 0.56 V and 0.41 V (vs Ag/AgNO₃in 0.1m Bu₄NPF₆) respectively.

The kinetics for the formation of SAMs, prepared on Au sub- strate^[19] from 1 mm solution of 1 a, 1 b or 1 c in CH₂Cl₂ or CH₃CN, were followed by using quartz crystal microbalance (QCM) measurements. By employing, the Langmuir adsorption isotherm model,^[20]it is possible to establish that deposition ki- Scheme 1.Synthesis of nitroxyl radical derivatives1 a,1 band1 c.

[a]Dr. O. AlvÞque, Dr. C. Gautier, Dr. M. Dias, Dr. T. Breton, Dr. E. Levillain Laboratoire CIMA—Universit d’Angers—CNRS

2 boulevard Lavoisier 49045, Angers cedex (France) Fax: (+33) 241 735 405

E-mail: tony.breton@univ-angers.fr eric.levillain@univ-angers.fr [b]Dr. F. Seladji

Laboratoire de chimie et d’lectrochimie des complexes organomtalliques Universit des Sciences et de la technologie Mohammed Boudiaf BP 1505—El’M’naouer ORAN (Algrie)

Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200900448.

ChemPhysChem2009, 10, 2401 – 2404 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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netics are chain-length (n) dependent: k_s increase with n and are smaller than 0.1 s ¹ (see Supporting Information Figure SI- 1).^[3] A maximum coverage, close to ~4.5 10 ¹⁰mol cm ² in both solvents, is reached within a few minutes (t_{99 %} <3 min for1 a).

In non-aqueous solvents (i.e. DCM or CH₃CN; Bu₄NPF₆0.1m), the electrochemical properties of SAM prepared from 1 a, 1 b or1 cwere very similar to those reported above for3 a,3 bor 3 c in solution (vide supra), but the shape of voltammetric waves and the linear dependency between peak intensities and scan rates were characteristic of surface-confined redox species (Figure 1). CVs parameters are quasi chain-length (n) in- dependent (Table 1) and surface coverages, deduced by inte- gration of the voltammetric signal, are in agreement with QCM

experiments in both solvents. Interestingly, shapes of voltam- metric waves are solvent and supporting electrolyte depend- ent: sharp (FWHM~36 mV) in 0.1mBu₄NPF₆/CH₂Cl₂and broad (FWHM ~117 mV) in 0.1m Bu₄NPF₆/CH₃CN (Figure 1 and Table 1).

These reproducible opposite values of full width at half max- imum (FWHM) deviate from the expected value (~89/n mV at 293 K) of an “ideal system”, based on a Langmuir isotherm (i.e.

all adsorption sites are equivalents and there are no interac-

tions between immobilized electroactive centers). The interfa- cial electron transfer process and the interaction between the immobilized functional moieties have been widely studied to clarify the “non ideality” of experimental data^[21–30]but it still re- mains an open-ended question.

However, from a Frumkin type isotherm (i.e. all adsorption sites are equivalents and there are interactions between immo- bilized electroactive centers), Laviron^[31] developed in the sev- enties a lateral interaction model to account for the sharpness or broadening and shifts of apparent formal potentials of ex- perimental CVs. Based on diagnostic criteria reviewed in ref. [31e], strong attractive interactions between immobilized oxidized centers lead to a sharp shape (i.e. in CH₂Cl₂) and, medium or weak repulsive interactions to a broad shape (i.e. in CH₃CN) (Figure 1 and Figure 2). Taken in concert, these results suggest that the local ionic environment (coulombic repulsion

and solvatation) of charged redox centers plays a significant role in the electrochemical behaviour of the electroactive SAMs.

In aqueous media, a drastic preparation^[32] of SAMs is re- quired to reach a stable and reproducible redox activity

(FigACHTUNGTRENNUNGure SI-2). As in solution,^{[12, 13]} apparent redox potentials

(Eapp (at pH 7)~0.71 V vs Ag/AgCl/KCl_sat) and shape of SAMs pre-

pared from 1 a, 1 b and1 c are not pH dependent (Figure 1).

Between pH 0 and pH 10.5, voltammetric waves (Figure SI-3) are quasi chain-length (n) independent and close to an “ideal system” (i.e. based on a Langmuir isotherm). Surface coverages, deduced by integration of the voltammetric signal, are compa- rable to the ones assessed in non-aqueous solvents (Table 1).

Beyond pH 10.5, the electrochemical behaviour of SAMs is un- stable due to the proximity of the water oxidation.

Because the structure of monolayer limits the accessibility of interfacial reactive sites,^[3] the control of the structure-electro- activity relationship is decisive for enhancing interfacial reac- Figure 1.Left: (c) CVs of SAM prepared from1 bin 0.1mBu4NPF6/CH2Cl2

and in Bu₄NPF₆/CH₃CN; Right : (c) CVs of SAM prepared from1 bin HClO4/NaClO4/H2O; It is noteworthy that CVs are superimposable upon one another from pH 2 to pH 6.6. (a): CVs calculated from theoretical models based on a Frumkin type isotherm).^[31]All CVs were performed at 0.1 V. s¹.

Table 1.CV parameters of SAMs prepared from1 b.

Medium At 0.1 V s¹ CH2Cl2 MeCN H2O

0.1m Bu4NPF6

G[mol cm ²] 4.7 10¹⁰ 4.5 10¹⁰ –

FWHM [mV] 36 117

E_p[V] 0.52^[a] 0.49^[a]

0.1m NaClO4

G[mol cm ²] –

4.6 10¹⁰ 5.0 10¹⁰

FWHM [mV] 87 96

Ep[V] 0.43^[a] 0.71^[b]

[a] vs Ag/AgNO3(0.01m), [b] vs Ag/AgCl/KClsat

Figure 2.Left: CVs of SAM prepared from different1 c:decanethiol ratios, leading to 4.6, 3.8, 2.9, 2.1, 1.5 and 0.9 10 ¹⁰mol cm²; Right: CVs calculated from theoretical models based on a Frumkin type isotherm. In addition to the usual electrochemical parameters, calculations were performed with r0=1.0 (corresponding to a strong interaction between oxidized centers) and only one variable changes at a time, the surface coverage (q=1.00, 0.74, 0.62, 0.52 and 0.38).^[31]All CVs were performed in 0.1mBu4NPF6/CH2Cl2

at 0.1 V s ¹.

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tions. An approach to improve the problems arising from inter- molecular interactions is to dilute the electroactive species in the monolayer by mixing alkanethiols.^[33]Indeed, electrochemi- cal behaviours of mixed SAMs, prepared from1 a,1 bor1 c:al- kanethiol mixtures,^[34]fit with the interaction model developed by Laviron: the full width at half maximum and the apparent redox potential are linearly dependent on the surface coverage (Figure SI-4), and the simulations are in qualitative (i.e. shape) and quantitative (i.e. intensities) agreement with CVs of mixed SAMs (Figure 2). The excellent and rare^[35]agreement observed between theory and experiments provides evidence of a random distribution of electroactive centers on gold.

This outlook is established by electrocatalytic activities of SAMs towards benzyl-alcohol oxidation in CH₂Cl₂ and in H₂O (pH 10). First, SAMs prepared from 1 a, 1 b or 1 c are stable under electrocatalytic conditions because catalytic currents are persistent under repetitive potential scans in aqueous and non aqueous media (Figure 3 and Figure SI-5). Second, mixed SAMs

(up to a limiting1 a,1 bor1 c:alkanethiol ratio) increase abso- lute catalytic current in spite of few immobilized electroactive centers (e.g. despite five-fold lower electroative sites, the cata- lytic current is 50 % higher on mixed SAMs compared to un- mixed ones) and improved the stability of electrocatalytic pro-

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cess (Figure 3). These results also provide support for the claim^[36] that electroactive centers are random distributed on gold. As expected,^[37]oxidations of benzyl-alcohol led to ben- zaldehyde.

In summary, nitroxyl radical SAMs present a noteworthy electrochemical stability in both aqueous (from pH 0 to pH 10.5) and organic media. The control of the surface cover- age associated with the catalytic properties of oxoammonium cation provide a clear and convincing illustration of potentiali-

ties of nitroxyl radical SAMs. Further work is aimed at develop- ing mixed SAMs designed to a redox-responsive material.

Acknowledgements

This work was supported by the Centre National de la Recherche Scientifique (CNRS—France), the “Agence Nationale de la Recher- che” (ANR—France), and the “Rgion des Pays de la Loire”

(France). We gratefully thank Pr. David Rondeau for MS experi- ments and helpful discussions.

Keywords: cyclic voltammetry·electrocatalysis·monolayers· quartz crystal microbalance·self-assembly

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Received: June 10, 2009 Published online on July 28, 2009

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