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

Tunable Electrochemical Switches Based on Ultrathin Organic Films

We have performed electroreduction of bisthienyl aminobenzene and thienyl aminobenzene diazonium salts on glassy carbon and polycrystalline gold and generated ultrathin organic films. This method allows covalent bonding between the corresponding aromatic radical and the electrode. Electrochemical behaviors of these modified electrodes toward reversible outer-sphere redox species show unusual properties. No current is observed in the potential range where redox probe reactions on bare electrodes usually occur. The organic layer totally blocks the electrode in such a potential window. Above a threshold voltage, the current dramatically increases and an “irreversible” wave is observed. These results reveal that ultrathin organic layers constitute a reversible conductance switch. Moreover, the threshold voltage of this organic layer can be tuned by nature of the π-conjugated molecules grafted onto the electrode. Such junctions based on conjugated oligomers with a well-defined metal/oligomer interface retaining reversible on/off switching capabilities controlled by the redox state of the oligomer will be of interest as active interconnects for molecular electronics or in redox molecular actuators

Bilayer Molecular Electronics: All-Carbon Electronic Junctions Containing Molecular Bilayers Made with “Click” Chemistry

Bilayer molecular junctions were fabricated by using the alkyne/azide “click” reaction on a carbon substrate, followed by deposition of a carbon top contact in a crossbar configuration. The click reaction on an alkyne layer formed by diazonium reduction permitted incorporation of a range of molecules into the resulting bilayer, including alkane, aromatic, and redox-active molecules, with high yield (>90%) and good reproducibility. Detailed characterization of the current–voltage behavior of bilayer molecular junctions indicated that charge transport is consistent with tunneling, but that the effective barrier does not strongly vary with molecular structure for the series of molecules studied

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 polarizer

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

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