Tony Breton

Spontaneous Grafting of Nitrophenyl Groups on Carbon: Effect of Radical Scavenger on Organic Layer Formation

The effect of a radical scavenger (DPPH: 2,2-diphenyl-1-picrylhydrazyl) on the spontaneous covalent grafting of nitrophenyl functionalities on a vitreous carbon substrate using the 4-nitrobenzene diazonium cation has been studied by electrochemical measurements and X-ray photoelectron spectroscopy. The addition of micromolar concentrations of DPPH to the diazonium solution efficiently limits the multilayer formation and leads to monolayer surface coverage. Control of polyaryl layer formation via the capture of the reactive nitrophenyl radical was also found to increase the proportion of nitrophenyl groups grafted to the surface via azo bridges. This work validates the recently reported strategy using a radical scavenger to prevent the formation of a polyaryl layer without interfering with direct surface grafting.

Effects of aromatic spacers on the properties of organic field effect transistors based on π-extended tetrathiafulvalene derivatives

OFETs built from extended TTF derivatives containing N-methylpyrrole, furan, thiophene and benzene with meta or para linkages as spacers are presented. The aromatic spacer plays a crucial role in the characteristics and stability of the OFETs. The less conjugated derivative with a meta-benzene moiety as spacer has the best mobility, with a higher on/off ratio and strongly enhanced stability in air.

Self-assembled monolayers of mono-tetrathiafulvalene calix[4]pyrroles and their electrochemical sensing of chloride.

Since the first reports more than a half-century ago, interest in self-assembled monolayers (SAMs) on metal surfaces has increased almost exponentially. At present, such supported SAMs are used for numerous applications, ranging from metallurgy—wherein the surface properties, such as conductivity, wettability, corrosion resistance, and etching resistance, are modified—to the preparation of sensors and materials for use in separations. In fact, the use of SAMs is now recognized as offering a convenient, flexible, and simple way to tailor the interfacial properties of metals. This is because the spontaneous organization and absorption of molecules on metal, metal oxide, or semiconducting surfaces often creates highly ordered systems with well-defined dimensions on the nanoscale. During the last decade, increasing attention has been dedicated to the design and elaboration of SAMs that integrate a molecular or ionic receptor unit with a transducer element so as to create sensing devices for environmental or biologic purposes. If a redox-active element is incorporated into the monolayer or the guest, the recognition event can be followed by cyclic voltammetry (CV). In other cases, the monitoring can be completed by using impedance spectroscopic measurements. Although, cation (generally metallic ions) recognition using SAMs is well documented, and several systems that exhibit both excellent selectivity and sensitivity have been described in the literature, much less work on anion recognition by using SAMs has been reported, perhaps reflecting the fact that anions have variable sizes and shapes and exhibit strong solvation in most solvents. The first anion recognition SAM systems were reported by Astruc et al. and involved the use of ferrocene derivatives for dihydrogenophosphate anion detection. More recently, Echegoyen et al. described SAMs for use in acetate anion recognition. Although these systems are very selective, their sensitivity is relatively low and the limits of detection do not reach the sub-millimolar scale. Calix[4]pyrrole and its derivatives have been extensively studied in the search for chemosensors capable of recognizing specific chemical species, with a number of optically active calix[4]pyrrole-based sensors having now been reported. Some of us have recently described the first fully successful examples of electrochemically active sensors based on calix[4]pyrroles; these were prepared by attaching one, two, or four redox-active tetrathiafulvalene (TTF) units directly to the calix[4]pyrrole platform so as to produce receptors with enhanced binding affinities toward anions as compared to the parent meso-octamethyl calix[4]pyrrole. In the case of the mono-TTF calix[4]pyrrole, we observed selectivity between the chloride anion (Ka= 2900m ) and the bromide anion (Ka=96m ) in CH2Cl2 solution. This success has led us to consider that TTF calix[4]pyrroles could be used to create anion-sensing SAMs. These kinds of receptors are attractive in this regard because they incorporate within one molecular framework both pyrrolebased anion recognition and TTF-derived transducer subACHTUNGTRENNUNGunits. We wish to report here that such SAMs may be prepared and that they are effective for chloride anion recognition at the sub-millimolar level, thus providing a sensitivity that differs dramatically from previously reported systems. [a] L. G. Jensen, Dr. K. A. Nielsen, Prof. J. O. Jeppesen Department of Physics and Chemistry University of Southern Denmark Campusvej 55, DK-5230, Odense M (Denmark) Fax: (+45)6615-8780 E-mail : joj@ifk.sdu.dk Homepage: http://www.jojgroup.sdu.dk [b] Dr. T. Breton, Prof. Dr. E. Levillain, Dr. L. Sanguinet Universit d’Angers—CNRS 2 boulevard Lavoisier 49045, Angers Cedex (France) E-mail : eric.levillain@iniv-angers.fr lionel.sanguinet@univ-angers.fr [c] Prof. J. L. Sessler Department of Chemistry and Biochemistry The University of Texas at Austin 1 University Station, A5300, Austin, TX 78712-0165 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200901394.

Phase segregation on electroactive self-assembled monolayers: a numerical approach for describing lateral interactions between redox centers.

A numerical method is proposed in order to differentiate a random distribution from a phase segregation of redox centers on (mixed) SAMs. This approach is compared to Lavirons interactions model and voltammetric data of nitroxylalkanethiolate SAMs.

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

Since the pioneering work by Nuzzo and Allara in 1983, selfassembled monolayers (SAMs) of alkanethiols have gained much attention in the interfacial electrochemistry and other research fields. 3] During the last decade, 5] increasing attention has been dedicated to the design and elaboration of redox-responsive SAMs. Employing redox SAM-modified electrodes for catalysis, recognition and sensing have resulted in some elegant examples that incorporate sophisticated receptors on the electrode surfaces. 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-Tetramethylpiperidine-1-oxyl) and its derivatives (under their oxoammonium form) have been extensively studied in the search for organic synthesis as a redox mediator, mostly, for the oxidation of primary alcohols. The electrochemical oxidation of TEMPO is known to be a stable and reversible one-electron process in both aqueous and non-aqueous electrolytes. 13] Despite the wide electrochemical applications of nitroxyl radical, rare works have been devoted to design and elaborate of redox-responsive TEMPO SAMs. In 1997, Fuchigami et al. reported the first preparation of stable self-assembled TEMPO-modified electrodes in acetonitrile : the electrochemical stability was only reached with highly diluted mixed SAMs of nitroxyl radical derivative and hexadecanethiol. In 1999, Kashiwagi et al. reported electrocatalysis attempts of amines with a mixed SAM of chiral nitroxyl radical derivative and hexadecanethiol : as mentioned by authors, results could be applied to the determination of optical purity of chiral amines. After a ten-years pause, Finklea et al. 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 1 m H2SO4. Herein, we raised the challenge of a design of redox-responsive TEMPO SAMs to be active, stable and providing electrocatalytic activities in both aqueous and non-aqueous solvents. To reach this goal, we synthesized the three nitroxyl radical derivates 1 a, 1 b and 1 c shown in Scheme 1. The synthesis of 1 a, 1 b and 1 c were carried out as outlined in Scheme 1. w-thioACHTUNGTRENNUNGacetyl carboxylic acids with different carbon chains length 2 a–c were synthesized from the corresponding w-bromocarboxylic acids by nucleophilic displacement with potassium thioACHTUNGTRENNUNGacetate in DMF. 4-aminoTEMPO was coupled to w-thioacetyl carboxylic acids 2 a-c via the active ester method, using diACHTUNGTRENNUNGcyclohexylcarbodiimide (DCC) and 1-hydroxybenzotriazole (HOBT) in methylene chloride (CH2Cl2), to give the TEMPO derivatives 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) of 3 a, 3 b or 3 c exhibit a reversible one-electron process in CH2Cl2 and CH3CN, close to 0.56 V and 0.41 V (vs Ag/AgNO3 in 0.1 m Bu4NPF6) respectively. The kinetics for the formation of SAMs, prepared on Au substrate 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, it is possible to establish that deposition kiScheme 1. Synthesis of nitroxyl radical derivatives 1 a, 1 b and 1 c.

Self-assembled monolayer-assisted mass spectrometry

This article deals with the use of self-assembled monolayers (SAMs) for the formation and characterization of gaseous ions in mass spectrometry (MS). The first part reviews the different results reported in the literature concerning the use of SAMs for surface induced dissociation (SID) of produced ions into a mass spectrometer. In SID/MS, the ion collision at a given kinetic energy allows the accumulation of internal energy for reaching the activation energy of fragmentation reactions. The different chemical structures of SAMs in SID/MS are described and their influence on the amount of the kinetic energy (Ekin) converted into internal energies (Eint) is reported. The second part is dedicated to the implication of SAMs in the laser desorption–ionization (LDI) methods allowing gas-phase ion formation and highlights the specifications required for the SAMs elaboration in the LDI/MS application field. The matrix-free LDI method is more particularly described. The results obtained with the so-called DIAMS technique (desorption–ionization on self-assembled monolayer surface) are reported and the organization and stability of SAMs are pointed out to obtain reliable results in LDI/MS.

Controlling Grafting from Aryldiazonium Salts: A Review of Methods for the Preparation of Monolayers

Surface modification by grafting from aryldiazonium salts has been widely studied and applied to many substrates as a simple and versatile method for preparing strongly adherent organic coatings. Unless special precautions or conditions are used, the usual film structure is a loosely packed disordered multilayer; however, over the past decade, attention has been paid to establishing strategies for grafting just a monolayer of modifiers to the surface. To date, four general approaches to monolayer preparation have emerged: use of aryldiazonium ions with cleavable protection groups; use of aryldiazonium ions with steric constraints; grafting in the presence of a radical scavenger; and grafting from ionic liquids. This review describes these approaches, illustrates some of their applications, and highlights the advantages and disadvantages of each.

A facile route to steady redox-modulated nitroxide spin-labeled surfaces based on diazonium chemistry

A TEMPO derivative was covalently grafted onto carbon and gold surfaces via the diazonium chemistry. The acid-dependent redox properties of the nitroxyl group were exploited to elaborate electro-switchable magnetic surfaces. ESR characterization demonstrated the reversible and permanent magnetic character of the material.

Electrochemical transduction on self-assembled monolayers: are covalent links essential?

Electrochemical transduction without covalent links between redox and complexant units in a complexing self-assembled monolayer has been established. The results demonstrate that transduction depends on the crown ether/ferrocene ratio and appears to be tunable.

Very Strong Binding for a Neutral Calix[4]pyrrole Receptor Displaying Positive Allosteric Binding

The dual-analyte responsive behavior of tetraTTF-calix[4]pyrrole receptor 1 has been shown to complex electron-deficient planar guests in a 2:1 fashion by adopting a so-called 1,3-alternate conformation. However, stronger 1:1 complexes have been demonstrated with tetraalkylammonium halide salts that defer receptor 1 to its cone conformation. Herein, we report the complexation of an electron-deficient planar guest, 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA, 2) that champions the complexation with 1, resulting in a high association constant Ka = 3 × 1010 M-2. The tetrathiafulvalene (TTF) subunits in the tetraTTF-calix[4]pyrrole receptor 1 present a near perfect shape and electronic complementarity to the NTCDA guest, which was confirmed by X-ray crystal structure analysis, DFT calculations, and electron density surface mapping. Moreover, the complexation of these species results in the formation of a charge transfer complex (22⊂1) as visualized by a readily apparent color change from yellow to brown.