Philippe Hapiot

Electrochemical Reactivity in Room-Temperature Ionic Liquids

A review. Physicochem. properties of ionic liqs. are discussed. Chem. and electrochem. reactivity in ionic liqs. is described including electrode reactions, electrode reaction kinetics, electrosynthesis, etc.

Synthesis of new substituted tetrazines: electrochemical and spectroscopic properties

Original tetrazines substituted by heterocyclic rings have been prepared. Their syntheses, as well as their electrochemical and spectroscopic features, are described. Calculations have also been made on the cation radicals, anion radicals and neutral compounds and are in correct agreement with the experimental results. All compounds are electroactive, both in oxidation and reduction, and display two absorption bands in the UV and visible regions of the spectrum. Reduction potentials and maximum wavelengths are correlated with the electron-rich character of the heterocyclic substituent on the tetrazine ring. None of these compounds gives good quality polymers upon electro-oxidation, which was unexpected, especially for the bis(2-pyrrolyl) tetrazine. This latter result can be explained by the occurrence of a self-deprotonation equilibrium in the cation radical.

Evidence for OH radical production during electrocatalysis of oxygen reduction on Pt surfaces: consequences and application.

Multielectronic O(2) reduction reactions (ORR) at Pt surface (and at Au surface for comparison purpose) were examined both in water and in organic solvents using a strategy based on radical footprinting and scanning electrochemical microscopy (SECM). Experiments reveal a considerable and undocumented production of OH radicals when O(2) is reduced at a Pt electrode. These observations imply that the generally admitted description of ORR as simple competitive pathways between 2-electron (O(2) to H(2)O(2)) and 4-electron (O(2) to H(2)O) reductions is often inadequate and demonstrate the occurrence of another 3-electron pathway (O(2) to OH radical). This behavior is especially observable at neutral and basic pHs in water and in organic solvents like dimethylformamide or dichloromethane. In view of the high reactivity of OH radical versus organic or living materials, this observation could have important consequences in several practical situations (fuel cells, sensors, etc.) as far as O(2) reduction is concerned. This also appears as a simple way to locally produce highly reactive species as exemplified in the present work by the micropatterning of organic surfaces.

Electrochemical functionalization of nanotube films: growth of aryl chains on single-walled carbon nanotubes

Covalent exohedral derivatizations of HiPco nanotubes, through electrochemical reduction of aryldiazonium salts, is described. Four different aryldiazonium salts have been used under the same experimental conditions: 4-bromophenyl, 4-chloromethylphenyl, 4-sulfophenyl and 4-carboxyphenyl. Derivatized samples were characterized through X-ray photoelectron spectroscopy and micro-Raman diffusion. The evolution of the spectra (area of the D-band), as a function of the number of grafted groups, led us to the conclusion that the electrochemical reduction of aryldiazonium salts into radicals gives rise to the growth of aryl chains on the sidewalls of the nanotubes.

Conducting Ferrocene Monolayers on Nonconducting Surfaces

The redox activity of a ferrocenyl monolayer grafted on an n-type Si111 substrate was investigated by scanning electrochemical microscopy (SECM) in conditions where the substrate plays the role of an insulator. This approach permits the differentiation between the different possible electron-transfer and mass-transport pathways occurring at the interface. As an exciting result, the thin ferrocenyl monolayer behaves like a purely conducting material, highlighting very fast electron communication between immobilized ferrocenyl headgroups in a 2D-like charge-transport mechanism.

Redox-active organometallics: magnetic and electronic couplings through carbon-silicon hybrid molecular connectors.

Treatment of the triflate complex Cp*(dppe)FeOTf [12; Cp* = eta(5)-C(5)(CH(3))(5), dppe = 1,2-bis(diphenylphosphino)ethane, OTf = CF(3)SO(3)] with an excess of HC[triple bond]C-(Si(CH(3))(2))(x)-C[triple bond]CH (x = 2-4) in diethyl ether provides the binuclear bis(vinylidene) derivatives [Cp*(dppe)Fe=C=CH(Si(CH(3))(2))(x)CH=C=Fe(dppe)Cp*][OTf](2) (x = 2, 13; x = 3, 14; x = 4, 15), which were isolated as ochre solids and rapidly characterized by FT-IR, (1)H, (31)P, and (13)C NMR spectroscopies. The complexes 13-15 were reacted with potassium tert-butoxide to afford the bis(alkynediyl) complexes [Cp*(dppe)Fe-C[triple bond]C(Si(CH(3))(2))(x)C[triple bond]C-Fe(dppe)Cp*] (x = 2, 1; x = 3, 2; x = 4, 3), which were isolated as orange powders in yields ranging from 76 to 91%. The IR, cyclic voltammetry, and UV-vis data obtained for 1-3 and the X-ray crystal structures determined for 1 and 3 reveal the importance of the sigma-pi conjugation (hyperconjugation) between the Si-Si sigma bond and the adjacent C[triple bond]C pi-symmetric orbitals in the description of the electronic structure of the ground state of these complexes. When reacted at low temperature with 2 equiv of [(C(5)H(5))(2)Fe]X or AgX [X = BPh(4), B(3,5-(CF(3))(2)C(6)H(3))(4))], compounds 1-3 provide 1[X](2), 2[X](2), and 3[X](2), which can be isolated and stored below -20 degrees C. EPR spectroscopy and magnetization measurements established that the superexchange interaction propagates through the Si-Si bonds (J = -0.97(2) cm(-1) for 3[X](2)). UV-vis-near-IR spectra were obtained with an optically transparent thin-layer electrosynthetic (OTTLE) cell for 1-3[OTf](n) (n = 0-2). A band with a maximum that increases from 6400 cm(-1) (1[OTf]) to 8500 cm(-1) (3[OTf]) observed for the mixed-valence species was ascribed to intervalence charge transfer evidencing photodriven electron transfer through the carbon-silicon hybrid connectors with H(ab) parameters ranging from 64 to 285 cm(-1).

Tuning the Electronic Communication between Redox Centers Bound to Insulating Surfaces

Controlling communication: The electronic communication between ferrocenyl centers bound to insulating silicon surfaces can be efficiently controlled; scanning electrochemical microscopy (SECM) shows that both the surface coverage of the electroactive units and the nature of the redox mediator allow for this control. The lateral charge propagation can be precisely tuned from an extremely slow to a very fast process

Electrografting of calix[4]arenediazonium salts to form versatile robust platforms for spatially controlled surface functionalization

An essential issue in the development of materials presenting an accurately functionalized surface is to achieve control of layer structuring. Whereas the very popular method based on the spontaneous adsorption of alkanethiols on metal faces stability problems, the reductive electrografting of aryldiazonium salts yielding stable interface, struggles with the control of the formation and organization of monolayers. Here we report a general strategy for patterning surfaces using aryldiazonium surface chemistry. Calix[4]tetra-diazonium cations generated in situ from the corresponding tetra-anilines were electrografted on gold and carbon substrates. The well-preorganized macrocyclic structure of the calix[4]arene molecules allows the formation of densely packed monolayers. Through adequate decoration of the small rim of the calixarenes, functional molecules can then be introduced on the immobilized calixarene subunits, paving the way for an accurate spatial control of the chemical composition of a surface at molecular level.

Superoxide Protonation by Weak Acids in Imidazolium Based Ionic Liquids

The reactivity of the superoxide anion versus a series of substituted phenols was investigated in a common ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIm][TFSI]) and for comparison in dimethylformamide (+0.1 mol x L(-1) of Bu4NPF6 as supporting electrolyte). On the whole, the mechanism of the reduction of O2 in the presence of the different phenols was found to be very similar in [BMIm][TFSI] and in DMF: A 2-electron mechanism involving a succession of electrochemical and protonation steps. These steps are accompanied by the production of the corresponding phenolate that was identified through its oxidation potential. The reactivities of the phenols were observed to slightly differ in the two media. A qualitative analysis of the voltammogram allows a classification of the reactivities of the superoxide as a function of the phenols. As previously found in organic solvents, the protonation of superoxide by phenol is an uphill reaction that is rendered possible thanks to a subsequent irreversible electron transfer. Its pK(a) is estimated to be around 4-5 units lower than that of unsubstituted phenol.

Electronic Communication between Immobilized Ferrocenyl-Terminated Dendrimers

Redox-active ferrocenyl-terminated dendrimers have been used to build robust and active interfaces in which the inherent properties of the dendrimers are preserved. The procedure is based on the preparation of a polyarylcarboxylate layer through the controlled reduction of aryldiazonium salts followed by strong adsorption of the dendrimers on the insulating layer. Scanning electrochemical microscopy was used to probe the transport properties as lateral charge diffusion. The experiments demonstrate that charge-transfer communication between the dendrimers inside the layer is fast and efficient, depending on the dendrimer generation.