Yann R. Leroux

Cyclic Conductance Switching in Networks of Redox-Active Molecular Junctions

Redox-active dithiolated tetrathiafulvalene derivatives (TTFdT) were inserted in two-dimensional nanoparticle arrays to build interlinked networks of molecular junctions. Upon oxidation of the TTFdT to the dication state, we observed a conductance increase of the networks by up to 1 order of magnitude. Successive oxidation and reduction cycles demonstrated a clear switching behavior of the molecular junction conductance. These results show the potential of interlinked nanoparticle arrays as chemical sensors.

Tunable electrochemical switch of the optical properties of metallic nanoparticles.

Control of the optical properties of oblate metallic nanoparticles (NP) is realized using an electrochemical switch consisting of a thin layer of conducting polymer (CP). Reversible modulation, moderate damping, and almost total quenching of the localized surface plasmon (LSP) resonance is achieved as a function of the thickness of the CP layer and the potential applied to the electrochemical systems, that is, the charge carrier density injected into the CP layer. These experimental results can be qualitatively reproduced using the single-particle model in the electrostatic approximation. We believe that combining an electroactive conducting polymer and NP will prove to be a general strategy for controlling the properties of various types of NP (fluorescent, magnetic, semiconducting) in many fields.

Giant Plasmon Resonance Shift Using Poly(3,4-ethylenedioxythiophene) Electrochemical Switching

Herein, we report the variation of localized surface plasmon resonance (LSPR) of gold nanoparticle (NP) arrays covered by poly(3,4-ethylenedioxythiophene) (PEDOT) as a function of the electronic state of the polymer. Giant shifts and fine-tuning of the LSPR of gold NPs surrounded by PEDOT/sodium docecyl sulfate have been achieved. The color variations of plasmonic/conducting polymer (CP) devices are given not only by changes of the optical properties of the CP upon doping but also by a close synergy of the optical properties of CP and NP. Such systems can considerably extend the field of CP-based electrochromic devices.

Active Plasmonic Devices with Anisotropic Optical Response: A Step Toward Active Polarizer

Control of the optical properties of metallic nanoparticles (NP) is realized using an electrochemical switch consisting of a thin layer of conducting polymer (CP). It is shown that the quenching of localized surface plasmon (LSP) sustained by oblate particles depends of the frequency of the LSP resonance. This effect is attributed to the variation of the CP dielectric function with wavelength. As a consequence, prolate arrays show total quenching of the LSP resonance along the major axis of the particles whereas modulation and moderate damping are observed along the minor axis. Combining electroactive conducting polymer and prolate NP makes it possible to design active plasmonic devices with anisotropic optical response upon CP switching. In the present case, such devices can be used as active filters or polarizers.

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.

High-yield formation of substituted tetracyanobutadienes from reaction of ynamides with tetracyanoethylene.

A high-yielding sequence of [2+2] cycloaddition-retroelectrocyclization of ynamides with tetracyanoethylene (TCNE) is described. The reaction provided tetracyanobutadiene (TCBD) species, which were characterized by various techniques. DFT and TD-DFT calculations were also performed to complement experimental findings.

Covalently Anchored Carboxyphenyl Monolayer via Aryldiazonium Ion Grafting: A Well-Defined Reactive Tether Layer for On-Surface Chemistry

Electrografting of aryl films to electrode surfaces from diazonium ion solutions is a widely used method for preparation of modified electrodes. In the absence of deliberate measures to limit film growth, the usual film structure is a loosely packed multilayer. For some applications, monolayer films are advantageous; our interest is in preparing well-defined monolayers of reactive tethers for further on-surface chemistry. Here, we describe the synthesis of an aryl diazonium salt with a protected carboxylic acid substituent. After electrografting to glassy carbon electrodes and subsequent deprotection, the layer is reacted with amine derivatives. Electrochemistry and atomic force microscopy are used to monitor the grafting, deprotection, and subsequent coupling steps. Attempts to follow the same procedures on gold surfaces suggest that the grafted layer is not stable in these reaction conditions.

Amine-Terminated Monolayers on Carbon: Preparation, Characterization, and Coupling Reactions

Aminophenyl and aminomethylphenyl monolayers have been electrografted to glassy carbon and pyrolyzed photoresist film from the corresponding diazonium ions using a protection-deprotection strategy based on Boc (tert-butyloxycarbonyl) and Fmoc (fluorenylmethyloxycarbonyl) groups. After grafting and then deprotecting films of Boc-NH-Ar, Fmoc-NH-Ar, and Fmoc-NH-CH2-Ar, depth profiling by atomic force microscopy confirmed that the resulting amine-terminated films were monolayers. In contrast, after deprotection, Boc-NH-CH2-Ar gave a multilayer film. Electroactive carboxylic acid derivatives were coupled to the monolayers through amide linkages. Electrochemical measurements revealed that the deprotected Fmoc-NH-CH2-Ar monolayer gave the highest surface concentration of coupled nitrophenyl and ferrocenyl groups and DFT calculations established that this monolayer has the highest theoretical surface concentration of those examined.

Atomic contacts via electrochemistry in water/cyclodextrin media: a step toward protected atomic contacts.

Atomic contacts are nanoscience devices proposed for applications such as single-atom switches in nanoelectronic circuits or one-molecule sensing devices. The conductance of such contacts varies in a stepwise fashion with a tendency to quantize near integer multiples of the conductance quantum (G0) but can also deviate significantly from integer values upon molecular adsorption. However, for sensing applications it is first necessary to coat the contact permanently to avoid nonspecific adsorption. Here, we show that marked differences are observed between atomic contacts generated in water, and in water/beta-CD. In this latter medium, atomic contacts with unusual properties can be generated. They have below 1 G0 conductance, low conductance fluctuation with time, and appear to be protected or partially protected from salicylate external molecular probes. Such contacts are not obtained in water, in water/glucose, or when beta-CD is added after 1 G0 contacts have been generated in water. These results indicate specific adsorption of beta-cyclodextrin on the atomic contacts and are compatible with the formation of encapsulated atomic contacts. However, direct independent structural evidence is still needed to confirm or infirm this interpretation.

Tunneling Dendrimers. Enhancing Charge Transport through Insulating Layer Using Redox Molecular Objects

Charge transport through an insulating layer was probed using ferrocenyl-terminated dendrimers and scanning electrochemical microscopy. Experiments show that the passage through the layer is considerably enhanced when the transferred charges are brought globally to the surface by the ferrocenyl dendrimer instead of a single ferrocene molecule. This result shows that charge tunneling through an insulator could be promoted by a purely molecular nano-object.