Specific Effects of Room Temperature Ionic Liquids on Cleavage Reactivity: Example of the Carbon−Halogen Bond Breaking in Aromatic Radical Anions

Specific solvation effects of ionic liquids have been evidenced on the chemical reactivity of radical anions with three different ionic liquids (1-butyl-3-methylimidazolium, trimethylbutylammonium, and triethylbutylammonium cations associated with the same anion (bis(trifluoromethylsulfonyl)imide). Large modifications depending on the localization of the negative charge in the radical anions and, to a less extent, on the nature of the ionic liquids cations are reported. When the charge is spread out over the entire molecule as in the 9-chloroanthracene radical anion, an acceleration of the carbon−halogen bond cleavage when passing from acetonitrile to the ionic liquid is observed. On the contrary, in the case of 4-chlorobenzophenone radical anion where the negative charge is more localized on the oxygen atom of the carbonyl group, a large decrease of the C−Cl cleavage rate occurs in relation with a positive shift of the reduction standard potentials. These effects can be explained by specific ion-pair associations between the radical anion and the cation of the ionic liquid that stabilizes the unpaired electron in the π* orbital of the aromatic system and thus decreases its presence in the σ* bond breaking. The experimental results can be rationalized using Marcus-type formalism (Savéant's model describing the dynamics of electron transfers and bond cleavage) and agree well with the calculated ion-pair stabilization energies estimated with density functional theory (B3LYP). Besides the decrease of the cleavage rate, the ion pairing favors the dimerization between two radical anions that prevails over the cleavage reaction, leading to a different mechanism

Variations of Diffusion Coefficients of Redox Active Molecules in Room Temperature Ionic Liquids upon Electron Transfer

In ionic liquids, the diffusion coefficients of a redox couple vary considerably between the neutral and radical ion forms of the molecule. For a reduction, the inequality of the diffusion coefficients is characterized by the ratio γ = Dred/Dox, where Dred and Dox are the diffusion coefficients of the electrogenerated radical anion and of the corresponding neutral molecule, respectively. In this work, measurements of γ have been performed by scanning electrochemical microscopy (SECM) in transient feedback mode, in three different room temperature ionic liquids (RTILs) sharing the same anion and with a series of nitro-derivative compounds taken as a test family. The smallest γ ratios were determined in an imidazolium-based RTIL and with the charge of the radical anion localized on the nitro group. Conversely, γ tends to unity when the radical anion is fully delocalized or when the nitro group is sterically protected by bulky substituents. The γ ratios, standard potentials of the redox couple measured in RTILs, and those observed in a classical organic solvent were compared for the investigated family of compounds. The stabilization energies approximately follow the γ ratios in a given RTIL but change considerably between ionic liquids with the nature of the cation

Charge Transfer between Electroactive Species Immobilized on Carbon Surfaces by Aryl Diazonium Reduction. SECM Investigations

Electron transfers in modified polyaryl multilayers containing redox active molecules (ferrocenyl moieties) have been investigated by scanning electrochemical microscopy (SECM) in feedback mode. The modified surfaces were prepared by the electro-reduction of aryl diazonium salts that provides anchoring layers for the immobilization of the electroactive groups. Two types of anchoring films were prepared, the first with aminophenyl and the second with phenylcarboxylic acid groups, allowing us to vary the oxidation level of the electroactive film. Determination of the apparent electron transfer rates between the modified surface and a series of redox mediators displaying increasing standard potentials permits the analysis of different processes involved in the charge transfer, namely, the permeation of the organic molecules (the mediator) and the conduction mechanism. In addition to the first oxidation of the immobilized molecules by the mediator at the solution−film interface, the global oxidation kinetics involves the conduction by charge transfer between grafted ferrocenes and the reverse charge transfer reaction from the film to mediator. This last step that is required to maintain the charge balance could become a kinetic limit for the highest driving force. For the most oxidizing mediator, analyses also suggest that the aromatic layer participates in the charge transfer

Reactivity of Platinum Metal with Organic Radical Anions from Metal to Negative Oxidation States

The reaction of platinum metal with an organic molecular radical anion leads to the formation of iono-metallic phases where Pt exists under negative oxidation states. This puzzling transformation of a “noncorrodible metal” was examined using localized electrochemical techniques in dimethylformamide containing different tetra-alkylammonium salts chosen as test systems. Our experiments demonstrate that the platinum metal is locally reduced as soon as the Pt faces relatively moderate reducing conditions, for example, when the Pt is used as a negative electrode or when the metal is in the presence of a reducing agent such as an organic radical anion. Scanning electrochemical microscopy (SECM) analysis, current−distance curves, and transient mode responses provide detailed descriptions of the reactivity of Pt to form negative oxidation states (the key step is the reaction of the metal with a molecular reducing agent), of the insulating nature of the “reduced” solid phases of the thermodynamics and kinetics conditions of the Pt conversion. The passage from the conductor to insulator states controlled the spatial development of the reaction that always remains in competition with the other “natural” roles of a metallic electrode. Formally, the phenomena can be treated by analogy with the C. Amatore's model previously developed for the mediated reduction of the poly(tetrafluoroethylene). Consequences of this general reactivity of Pt are discussed in view of a wide utilization of this metal in reductive conditions and the possible applications of such processes in the micropatterning of metallic surfaces

Controlling the Stepwise Closing of Identical DTE Photochromic Units with Electrochemical and Optical Stimuli

The full or stepwise controlled closing of identical photochromic dithienylethene units in the same molecule was addressed with a combination of electrochemical and optical stimuli in a trimetallic carbon-rich ruthenium complex

Electron Transfer Kinetics in a Deep Eutectic Solvent

Electron transfer (ET) kinetic rate constants ks in Ethaline (1:2 choline chloride + ethylene glycol) have been measured for two common redox couples (ferrocene/ferrocenium and ferrocyanide/ferricyanide) on a glassy carbon electrode and compared with ET kinetics in ionic liquids and classical organic solvents in the same conditions (acetonitrile and water). Particular care has been taken to treat ohmic drop in DES. For both couples, we found that ET rate constants are just a little lower than those measured in classical solvents (around 50% or less). These results contrast with ET rates in ionic liquids where electron transfers are considerably slower (100 times lower). Data are discussed as a function of the solvent relaxation time using Marcus Theory for an adiabatic electron transfer

Self-Assembled Monolayers of Redox-Active 4d–4f Heterobimetallic Complexes

In this work, we report the preparation of functional interfaces incorporating heterobimetallic systems consisting in the association of an electroactive carbon-rich ruthenium organometallic unit and a luminescent lanthanide ion (Ln = Eu3+ and Yb3+). The organometallic systems are functionalized with a terminal hexylthiol group for subsequent gold surface modification. The formation of self-assembled monolayers (SAMs) with these complex molecular architectures are thoroughly demonstrated by employing a combination of different techniques, including infrared reflection absorption spectroscopy, ellipsometry, contact angle, and cyclic voltammetry measurements. The immobilized heterobimetallic systems show fast electron-transfer kinetics and, hence, are capable of fast electrochemical response. In addition, the characteristic electrochemical signals of the SAMs were found to be sensitive to the presence of lanthanide centers at the bipyridyl terminal units. A positive shift of the potential of the redox signal is readily observed for lanthanide complexes compared to the bare organometallic ligand. This effect is equally observed for preformed complexes and on-surface complexation. Thus, an efficient ligating recruitment of europium and ytterbium cations at gold-modified electrodes is demonstrated, allowing for an easy electrochemical detection of the lanthanide ions along with an alternative preparative method of SAMs incorporating lanthanide cations compared to the immobilization of the preformed complex

Flexible Strategy for Immobilizing Redox-Active Compounds Using in Situ Generation of Diazonium Salts. Investigations of the Blocking and Catalytic Properties of the Layers

A versatile two-step method is developed to covalently immobilize redox-active molecules onto carbon surfaces. First, a robust anchoring platform is grafted onto surfaces by electrochemical reduction of aryl diazonium salts in situ generated. Depending on the nature of the layer termini, −COOH or −NH2, a further chemical coupling involving ferrocenemethylamine or ferrocene carboxylic acid derivatives leads to the covalent binding of ferrocene centers. The chemical strategy using acyl chloride activation is efficient and flexible, since it can be applied either to surface-reactive end groups or to reactive species in solution. Cyclic voltammetry analyses point to the covalent binding of ferrocene units restricted to the upper layers of the underlying aryl films, while AFM measurements show a lost of compactness of the layers after the chemical attachment of ferrocene centers. The preparation conditions of the anchoring layers were found to determine the interfacial properties of the resulted ferrocenyl-modified electrodes. The ferrocene units promoted effective redox mediation providing that the free redox probes are adequately chosen (i.e., vs size/formal potential) and the underlying layers exhibit strong blocking properties. For anchoring films with weaker blocking effect, the coexistence of two distinct phenomena, redox mediation and ET at pinholes could be evidenced

Diarylethene-Containing Carbon-Rich Ruthenium Organometallics: Tuning of Electrochromism

The association of a dithienylethene (DTE) system with ruthenium carbon-rich systems allows reaching sophisticated and efficient light- and electro-triggered multifunctional switches R-[Ru]-CC-DTE-CC-[Ru]-R, featuring multicolor electrochromism and electrochemical cyclization at remarkably low voltage. The spin density on the DTE ligand and the energetic stabilization of the system upon oxidation could be manipulated to influence the closing event, owing to the noninnocent behavior of carbon-rich ligands in the redox processes. A combination of spectroscopic (UV–vis–NIR–IR and EPR) and electrochemical studies, with the help of quantum chemical calculations, demonstrates that one can control and get a deeper understanding of the electrochemical ring closure with a slight modification of ligands remote from the DTE unit. This electrochemical cyclization was established to occur in the second oxidized state (EEC mechanism), and the kinetic rate constant in solution was measured. Importantly, these complexes provide an unprecedented experimental means to directly probe the remarkable efficiency of electronic (spin) delocalization between two <i>trans</i> carbon-rich ligands through a metal atom, in full agreement with the theoretical predictions. In addition, when no cyclization occurs upon oxidation, we could achieve a redox-triggered magnetic switch

One-Pot Electrografting of Mixed Monolayers with Controlled Composition

Surface functionalization with ultrathin layers exhibiting a highly robust interface is of paramount importance for designing materials with tailored properties or operating functions, without modifying drastically the material’s bulk structures. A fine-tuning of the surface composition obtained, for instance from binary mixed layers, is also a key issue for developing high value-added applications like efficient sensors. Herein, binary mixtures of calix[4]­arene-tetra-diazonium salts generated in situ from their corresponding calix[4]­tetra-anilines are electrografted to form covalently bound monolayers onto substrates for yielding versatile functionalizable molecular platforms. Wettability studies, X-ray photoelectron spectroscopy analyses, and scanning electrochemical microscopy show the formation of homogeneous mixed monolayers. The distribution of the two calixarenes on the surface is directed by their relative molar fraction in the deposition solution. The strategy allows the control of the composition of mixed monolayers in a one-step approach. Postfunctionalization of the mixed layers with ferrocene centers is performed to exemplify the benefit of a dilution procedure when functional groups are introduced at the calix[4]­arene small rim. This study highlights the potential of diazonium salt electrografting as a competitive alternative to chemisorption strategies such as self-assembled monolayers of alkyl thiols in the field of surface functionalization