Nanostructured Monolayers on Carbon Substrates Prepared by Electrografting of Protected Aryldiazonium Salts

The electrogeneration of aryl radicals from protected diazonium salts combined with protection–deprotection steps was evaluated to design functional monolayers on carbon substrates with a well-controlled organization at the nanometric scale. The structure of the obtained monolayer is adjusted by varying the size of the protecting group that is introduced on the precursors (trimethylsilyl, triethylsilyl, and tri­(isopropyl)­silyl were tested in the present study). After deprotection, a robust ethynylaryl monolayer is obtained whatever the substituent that serves as a platform to attach other functional groups by a specific “click chemistry” coupling step. Electrochemical and structural analyses show that the organization of the attached monolayer is totally governed by the size of the protecting group that leaves a footprint after removal but maintains a total availability of the immobilized functional groups. Properties of the monolayer (charge transfer, permeation of molecules through the layer, density of functional groups) were examined in combination with the performances for postfunctionalization taken with an alkyl-ferrocene derivative as an example of the immobilized species

Electron Transfer-Induced Conformational Changes of Highly Hindered Aromatic Compounds. The Case of Hexakis(alkylsulfonyl)benzenes

The monoelectronic reduction of hexakis(alkylsulfonyl)benzenes (alkyl = methyl 1a, ethyl 1b, butyl 1c, iso-propyl 1d, and iso-butyl 1e) to the corresponding radical anion in dimethylformamide involves two widely separated redox systems, except in the case of 1e that shows a single perfectly reversible system. Electrochemical data supported by calculations of molecular modeling (DFT and PM3 methods) show the existence of a four-member square scheme for which the neutral and radical anion species can both exist under chair and boat conformations. The relative stability of the conformers was found to be strongly dependent on the nature of the alkyl substituents. Generally, the most stable neutral forms adopt a chairlike geometry, and the radical anions adopt a boatlike geometry. For the most hindered compound 1e, the steric contribution of the (iso-butylsulfonyl) substituents becomes so strong that the conformational changes are considerably slowed, resulting in a frozen chair conformation

Redox Chemistry of Bipyrroles: Further Insights into the Oxidative Polymerization Mechanism of Pyrrole and Oligopyrroles

The oxidation of 2,2‘-bipyrrole, 5-methyl-2,2‘-bipyrrole, and 5,5‘-dimethyl-2,2‘-bipyrrole has been investigated by means of electrochemistry, flash photolysis, and pulse radiolysis. The bipyrrole cation radical was found to give polypyrrole or oligopyrrole under electrochemical and chemical oxidation and also under UV-light irradiation of the solution in the presence of CCl4 as an electron acceptor. The cation radicals have been characterized by their optical absorption spectra, and their decay processes have been followed. In all processes (chemical, electrochemical, and photochemical), the first step involves the reaction between two cation radicals. The cation radical does not react on starting bipyrrole nor on pyrrole monomer. Depending on pH, the cation radical can deprotonate to form a neutral radical. It was found that only the cation radicals, but not the neutral radicals, produce higher oligomers, which explains the inhibition of polymerization by strong bases

Investigating the Dynamics of Carbanion Protonation by Means of Laser Flash Electron Photoinjection from an Electrode

The investigation of protonation/deprotonation at carbon is traditionally limited to molecules where acidity has been boosted by introduction of an electron-withdrawing group or by removal of an electron. These restrictions can be removed by application of the laser flash electron photoinjection technique. A thin layer of radicals is initially formed upon reduction of an appropriate substrate by the photoinjected electrons. The time-resolved current−potential responses for the reduction of the radicals thus generated are sensitive to the rate of the protonation of the ensuing carbanion by purposely added acids. The second-order rate constant may then be extracted from the half-wave potential versus time data with satisfactory accuracy in a wide range of values that extends up to the diffusion limit. The method is demonstrated with the example of diphenylmethyl and benzyl carbanions. Several observations may be derived from these first illustrating experiments. There is a large kinetic isotope effect. Proton transfer is intrinsically slow, showing that this property is not the result of the presence of an electron-withdrawing group. The intrinsic barrier is larger in the benzyl case than in the diphenylmethyl case. Unusual temperature effects (negative activation enthalpy) are observed at least in some cases, calling for systematic investigation in future studies

Chemically Irreversible Redox Mediator for SECM Kinetics Investigations: Determination of the Absolute Tip–Sample Distance

The use of a chemically irreversible redox probe in scanning electrochemical microscopy (SECM) was evaluated for the determination of the absolute tip–substrate distance. This data is required for a quantitative use of the method in the analysis of functional surfaces with an unknown redox response. Associated with the relevant model curves, the electrochemical response allows an easy positioning of the tip versus the substrate that is independent of the nature of the materials under investigation. The irreversible oxidation of polyaromatic compounds was found to be well adapted for such investigations in organic media. Anthracene oxidation in acetonitrile was chosen as a demonstrative example for evaluating the errors and limits of the procedure. Interest in the procedure was exemplified for the local investigations of surfaces modified by redox entities. This permits discrimination between the different processes occurring at the sample surface as the permeability of the probe through the layer or the charge transfer pathways. It was possible to observe small differences with simple kinetic models (irreversible charge transfer) that are related to permeation: charge transport steps through a permeable redox layer

Electrochemical Investigations on Liquid-State Polymerizing Systems: Case of Sol−Gel Polymerization of Transition Metal Alkoxides

The polymerization kinetics of zirconium propoxide (and in a few cases of titanium butoxide) were studied in various conditions by following the diffusion of an electroactive probe attached to the metal center. Two different probes were considered, implying the complexation of the metal alkoxides with bidentate ligands functionalized by ferrocene moeties, respectively, a substituted acetylacetone (acac) ligand, and a stronger complexing ligand as a salicylate ligand. The polymerization kinetics can be followed with good accuracy on a real time scale by investigating the variation of the diffusion coefficients of the bounded ferrocenes. This variation can be related to the mass increase of the oligomers formed during the hydrolysis/condensation process. Electrochemical probing with the functionalized acac ligand shows that some decomplexation of the ligand occurs during the polymerization process. On the contrary, the salicylate ligand was irreversibly linked to the metal centers for the alkoxide precursor and also for the larger formed oxopolymers, allowing an accurate measurement of the mass variation of the electroactive species. The electrochemical results have been compared with information provided by other techniques. The influence of the electron self-exchange reaction between species with different diffusion coefficients on the measured diffusion coefficient was considered in the global analysis

SECM Investigations of Immobilized Porphyrins Films

Electronic properties of electrogenerated Zn-porphyrin layers linked by an electroactive linker and immobilized on a semitransparent ITO electrode were investigated by steady-state SECM in unbiased conditions in view of the numerous possible applications of such surface. This SECM strategy took advantage of the variations of the charge transfer kinetics of the organic redox couple (the mediator used in SECM) on ITO surface with the standard potential of the mediator. After preliminary characterization of nonmodified ITO, analysis of the SECM approach curves recorded with a series of redox mediators allows the characterizations of both film permeability and charge transport inside the organic film in conditions close to a “real optoelectronic device”. Two types of porphyrin films were considered. In the first one, the film was produced by electropolymerization of a modified zinc-β-octaethylporphyrin in which the bipyridinium pendant substituent is first introduced. The second type of film was prepared directly from an in situ electropolymerization method in which the Zn porphyrin is simply oxidized in the presence of 4,4′-bipyridine. Experiments show the occurrence of efficient charge transport inside both films after initial reduction of the electroactive linker. However, the first preparation method leads to films with stronger blocking character versus organic molecules and higher charge injection rates

Use of Catechol As Selective Redox Mediator in Scanning Electrochemical Microscopy Investigations

The use of catechols, and more specifically of dopamine, as a specific redox mediator for scanning electrochemical microscopy (SECM) investigations was evaluated in the challenging situation of an ultrathin layer deposited on a conductive substrate (carbon materials). Experiments show that dopamine is a well-adapted redox system for SECM in feedback mode and in unbiased conditions. Used as a redox mediator, catechol permits the investigations of modified surfaces without an electrical connection of the sample thanks to fast charge transfer kinetics but with a surface selectivity that does not exist in classical outer-sphere redox mediators. The interest of catechol in SECM as a sensitive redox mediator is exemplified by monitoring several modification steps of an ultrathin (<1 nm) hierarchically porous organic monolayer deposited on carbon substrates. For quantitative analysis, the SECM approach curves using dopamine could simply be characterized with an irreversible electron transfer kinetics model in a large range of pH

Scanning Electrochemical Microscopy Investigations of Monolayers Bound to p-Type Silicon Substrates

p-Si type electrodes modified with different organic monolayers were investigated by reaction with radical anion and cation electrogenerated at a microelectrode operating in the configuration of a scanning electrochemical microscope. The method proves to be a convenient tool for investigating both the quality and the redox properties of the layer as previously demonstrated on metallic electrodes especially when the sample cannot be electrically connected. Approach curves recorded with the different mediators were used to investigate the electron-transfer rates across alkyl monolayers bound to p-type silicon substrates. Preliminary results indicate that the interfacial electron transfer occurs via electron tunneling through the organic layer as generally described for SAMs grafted on gold electrodes