Electrochemical Cleavage of Alkyl Carbon-Halogen Bonds at Carbon-Metal and Metal-Carbon Substrates: Catalysis and Surface Modification

International audienceThe reaction of ferrocene-tagged primary alkyl iodides (RI) in polar aprotic solvents (dimethylformamide, acetonitrile, and propylene carbonate) in the presence of tetraalkylammonium salts was considered at several kinds of solid electrodes: (i) glassy carbon doped with sub-nanomolar layers of transition metals, graphite, and graphene, and (ii) gold and platinum modified by deposits of natural graphite and graphene. Under such conditions, extensive grafting of alkyl groups onto carbon (as substrate or as modifier) occurred, monitored using the redox response of ferrocene. Details of different modes of C-I bond cleavage at such interfaces are discussed

Gold and gold-graphene used as cathodic interfaces for scission of carbon-halogen bonds. Application to the building of anthraquinone-Au electrodes

International audienceGold (smooth or covered with a thin layer of graphene) is an efficient free-radical scavenger when used as cathode material. This first work points out the immobilization of anthraquinone (AQ) in organic polar solvents containing tetraalkylammonium salts. It appears that Au and graphene, when negatively polarized (E > − 1.0 V vs. Ag/AgCl), may react towards 2-bromomethylantraquinone (AQ-CH2-Br) in different ways: with Au, a fast adsorption followed by one-electron transfer leads to a robust radical modification of the interface, whereas the presence of graphene permits the formation of a benzyl-type radical readily trapped by the graphene layer. Two different redox stable electrodes are thus produced. Additionally, stability of gold-graphene-AQ electrodes could be successfully tested in aqueous buffered solutions

New Gilman-type lithium cuprate from a copper(II) salt: synthesis and deprotonative cupration of aromatics

International audienceDeprotonative cupration of aromatics including heterocycles (anisole, 1,4-dimethoxybenzene, thiophene, furan, 2- fluoropyridine, 2-chloropyridine, 2-bromopyridine and 2,4-dimethoxypyrimidine) was realized in tetrahydrofuran at room temperature using the Gilman-type amido-cuprate (TMP)2CuLi in situ prepared from CuCl2*TMEDA through successive addition of 1 equivalent of butyllithium and 2 equivalents of LiTMP. The intermediate lithium (hetero)arylcuprates were evidenced by trapping with iodine, allyl bromide, methyl iodide and benzoyl chlorides, the latter giving the best results. Symmetrical dimers were also prepared from lithium azine and diazine cuprates using nitrobenzene as oxidative agent

Nucleophilic displacement versus electron transfer in the reactions of alkyl chlorosilanes with electrogenerated aromatic anion radicals

International audienceAnion radicals of a series of aromatic compounds (C6H5CN, C6H5COOEt, anthracene, 9,10-dimethyl-, 9,10-diphenyl- and 9-phenylanthracene, pyrene and naphthalene) react with trialkyl chlorosilanes R1R2R3SiCl (R1-3 = Me, Et; R1,2 = Me, R3 = t-Bu) in multiple ways, following classical bimolecular schemes. The ratio of one-electron transfer (ET) to a two-electron process (SN2-like nucleophilic attack of the reduced form of mediator on the chlorosilane, with k2 ≅ 102-108 M−1 s−1) is inversely related to the steric availability of Si for nucleophilic displacement reactions. The nucleophilic substitution pathway mainly results in mono- and disilylated aromatic products. Paralleling the electrochemical data with DFT calculations, the role of silicophilic solvent (DMF) in SN process was shown to be quite complex because of its involvement into coordination extension at silicon, dynamically modifying energetics of the process along the reaction coordinate. Although 2,2'-bipyridine also forms delocalized persistent anion radicals, they do not induce neither ET nor SN reactions in the same manner as aromatic mediators. Silicophilicity of 2,2'-bipyridine being superior to that of DMF, a R3SiCl·bipy complex of hypercoordinated silicon with electroactive ligand was formed instead, whose reduction requires about 1 V less negative potentials than bipyridine itself

Design of a Cooper pair box electrometer for application to solid-state and astroparticle physics

We describe the design and principle of operation of a fast and sensitive electrometer operated at millikelvin temperatures, which aims at replacing conventional semiconducting charge amplifiers in experiments needing low back-action or high sensitivity. The electrometer consists of a Cooper Pair box (CPB) coupled to a microwave resonator, which converts charge variations to resonance frequency shifts. We analyze the dependence of the sensitivity on the various parameters of the device, and derive their optimization. By exploiting the nonlinearities of this electrometer, and using conventional nanofabrication and measurement techniques, a charge sensitivity of a few $10^{- 7} e / \sqrt{Hz}$ can be achieved which outperforms existing single charge electrometers.Comment: 13 pages, 7 figure

Simultaneous control of emission localization and two-photon absorption efficiency in dissymmetrical chromophores

C. K's secondary address for this work: CNRS UMR6082 FOTON, INSA de Rennes, 20 avenue des Buttes de Coësmes, CS 70839, 35708 RENNES cedex 7, FranceWe thank E. Leroux for technical assistance in the synthesis, S. Soualmi in electrochemical mesaurements and M. H. V. Werts for help in the TPEF measurements.International audienceThe aim of the present work is to demonstrate that combined spatial tuning of fluorescence and two-photon absorption (TPA) properties of multipolar chromophores can be achieved by introduction of slight electronic chemical dissymmetry. In that perspective, two model series of structurally related chromophores have been designed and investigated. One is based on rod-like quadrupolar chromophores bearing either two identical or different electron-donating (D) end groups and the other on three-branched octupolar chromophores built from a trigonal donating moiety bearing identical or different acceptor (A) peripheral groups. The influence of the electronic dissymmetry is investigated by combined experimental and theoretical studies of the linear and nonlinear optical properties of dissymmetrical chromophores compared to their symmetrical counterparts. In both types of systems (i.e., quadrupoles and octupoles), experiments and theory reveal that excitation is essentially delocalized and that excitation involves synchronized charge redistribution (i.e., concerted intramolecular charge transfer) between the different D and A moieties within the multipolar structure. In contrast, the emission stems only from a particular dipolar subunit bearing the strongest D or A moiety due to fast excitation localization after excitation, prior to emission. Hence, control of emission characteristics (polarization and emission spectrum), can be achieved, in addition to localization, by controlled introduction of electronic dissymmetry (i.e., replacement of one of the D or A end-groups by a slightly stronger D′ or A′ unit). Interestingly, slight dissymmetrical functionalization of both quadrupolar and octupolar compounds does not lead to significant loss in TPA responses and can even be beneficial due to the spectral broadening and peak position tuning that it allows. This study thus reveals an original molecular engineering route allowing TPA enhancement in multipolar structures, due to concerted core-to-periphery or periphery-to-core intramolecular charge redistribution upon excitation, while providing for control of emission localization. Such a route could be extended to more intricate (dendritic) and multipolar (3D) systems

Covalent Grafting of Silatranes to Carbon Interfaces

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Facile cathodic surfacial carboxylation of glassy carbon by means of immobilized alkanoic acids chains

The reduction of ω-bromoalkanoic acids Br(CH2)nCOOH at glassy carbon cathodes in aprotic polar solvents (acetonitrile, N,N-dimethylformamide or propylene carbonate) in the presence of tetraalkylammonium salts at E<−1.7 V vs. Ag/AgCl results in a dense organic layer, covalently attached to carbon, which progressively covers the electrode surface. It was evidenced that the immobilization of alkyl chains occurs via the scission of C-Br bonds permitting an efficient coverage of the carbon surface with the layers built of alkyl links [CH2]n (3≤n≤11) terminated with COOH groups. The carboxyls immobilized this way were easily transformed into electroactive esters or into amides by reacting with phenols or amines, respectively. Thus grafted π-acceptor groups allowed estimating the carboxylation level as very high, ca. (1–5)×10−9 mol/cm2 when taking into account the starting surface area. In aqueous media, the immobilized films with long alkyl linkers visibly behave as hydrophobic layers. Keywords: Glassy carbon, Surface grafting, Carboxylation, Bromo-alkanoic acid

Novel method for grafting alkyl chains onto glassy carbon. Application to the easy immobilization of ferrocene used as redox probe.

International audiencePrimary alkyl iodides (RI) have been found to react with a cathodically charged glassy carbon surface at potentials more negative than -1.7 V vs Ag/AgCl. In aprotic solvents, this reaction results in grafting of the alkyl chains onto carbon. It is proposed that the process corresponds to the cathodic charge of graphitized and fullerenized zones present in carbon followed by a displacement reaction (analogous to a nucleophilic attack) toward alkyl iodides. This new mode of grafting is applied to the immobilization of ferrocene used as an electrochemical probe. The present work points out the reaction of ω-iodoalkylferrocenes and quantifies the level of grafting of alkyl chains via this promising method for modification of carbon surfaces. Coverage levels were found to be high, reaching the apparent surface concentrations of 8 × 10(-9) mol cm(-2). These large values are explained on the basis of swelling of the interface provoked by progressive charging of the carbon surface via insertion of tetraalkylammonium cations concomitantly with the substitution process. Alkylferrocene layers deposited onto carbon were found to be chemically and electrochemically stable