Estimation of π-π Electronic Couplings from Current Measurements.

The π-π interactions between organic molecules are among the most important parameters for optimizing the transport and optical properties of organic transistors, light-emitting diodes, and (bio-) molecular devices. Despite substantial theoretical progress, direct experimental measurement of the π-π electronic coupling energy parameter t has remained an old challenge due to molecular structural variability and the large number of parameters that affect the charge transport. Here, we propose a study of π-π interactions from electrochemical and current measurements on a large array of ferrocene-thiolated gold nanocrystals. We confirm the theoretical prediction that t can be assessed from a statistical analysis of current histograms. The extracted value of t ≈35 meV is in the expected range based on our density functional theory analysis. Furthermore, the t distribution is not necessarily Gaussian and could be used as an ultrasensitive technique to assess intermolecular distance fluctuation at the subangström level. The present work establishes a direct bridge between quantum chemistry, electrochemistry, organic electronics, and mesoscopic physics, all of which were used to discuss results and perspectives in a quantitative manner.

Evidence of monolayer formation via diazonium grafting with a radical scavenger: electrochemical, AFM and XPS monitoring.

This paper analyzes the impact of the use of a radical scavenger on organic films generated by aryldiazonium electrografting in terms of thickness, morphology and chemical composition. Glassy carbon (GC) and pyrolyzed photoresist films (PPFs) were modified by electrochemical reduction of 4-nitrobenzenediazonium salt in the presence of various amounts of 2,2-diphenyl-1-picrylhydrazyl (DPPH). The thicknesses of the organic films have been measured by atomic force microscopy (AFM) and the lower threshold values confirm that it is possible to reach a monolayer by radical trapping. X-ray photoelectron spectroscopy (XPS) highlights a decrease in the proportion of nitrophenyl groups grafted via azo bridges as the DPPH concentration decreases and the film thickness increases. A correlation of electrochemical, XPS and AFM data confirms that not all nitrophenyl groups are electroactive in films greater than 2 nm thick.

Evidence for induction of the ornithine transcarbamylase expression in Alzheimer's disease.

To more rapidly identify candidate genes located within chromosomal regions of interest defined by genome scan studies in Alzheimer's disease (AD), we have developed a customized microarray containing all the ORFs (n=2741) located within nine of these regions. Levels of gene expression were assessed in total RNA from brain tissue of 12 controls and 12 AD patients. Of all genes showing differential expression, we focused on the ornithine transcarbamylase (OTC) gene on Xp21.1., a key enzyme of the urea cycle which we found to be expressed in AD brains but not in controls, as confirmed by RT-PCR. We also detected mRNA expression of all the other urea cycle enzymes in AD brains. Immunochemistry experiments revealed that the OTC expression was strictly restricted to vascular endothelial cells in brain. Furthermore, OTC activity was 880% increased in the CSF of probable AD cases compared with controls. We analysed the association of the OTC -389 G/A and -241 A/G promoter polymorphisms with the risk of developing AD. We observed that rare haplotypes may be associated with the risk of AD through a possible modulation of the methylation of the OTC promoter. In conclusion, our results suggest the involvement of a new pathway in AD brains involving the urea cycle.

A study by spectroelectrochemical FTIR and density functional theory calculations of the reversible complexing ability of an electroactive tetrathiafulvalene crown.

We report on the study of the electrochemically targeted complexation/expulsion of a metal cation (Ba2+) by a crown ether tetra(thiomethyl)tetrathiafulvalene derivative (crown-TTM-TTF). Real time, in situ FTIR spectroelectrochemistry was used to obtain spectroscopic evidence of this electrochemically triggered phenomenon. Density functional theory calculations allowed the spectral information collected to be assigned. Both experimental and theoretical results clearly show that neutral crown-TTM-TTF complexes well Ba2+. Complexation is evidenced by a significant downshift of the frequency corresponding to the asymmetric stretching of the C-O-C ether groups. Concerning the cation crown-TTM-TTF, the spectroscopic signal of the complex form was difficult to identify, first because of the rather low value of the complexation constant and second because the vibration modes involving the oxygen atoms (which are the most affected by the complexation) were found by calculation to occur in the lower spectral region (<1000 cm(-1)), which is not accessible in our experimental conditions. In the case of the dication crown-TTM-TTF, it is now clear that the complex form does not exist, which means that the electrochemical formation of the dication necessarily involves the expulsion of the barium ion.

Differential modulation of Bordetella pertussis virulence genes as evidenced by DNA microarray analysis.

The production of most factors involved in Bordetella pertussis virulence is controlled by a two-component regulatory system termed BvgA/S. In the Bvg+ phase virulence-activated genes (vags) are expressed, and virulence-repressed genes (vrgs) are down-regulated. The expression of these genes can also be modulated by MgSO(4) or nicotinic acid. In this study we used microarrays to analyse the influence of BvgA/S or modulation on the expression of nearly 200 selected genes. With the exception of one vrg, all previously known vags and vrgs were correctly assigned as such, and the microarray analyses identified several new vags and vrgs, including genes coding for putative autotransporters, two-component systems, extracellular sigma factors, the adenylate cyclase accessory genes cyaBDE, and two genes coding for components of a type III secretion system. For most of the new vrgs and vags the results of the microarray analyses were confirmed by RT-PCR analysis and/or lacZfusions. The degree of regulation and modulation varied between genes, and showed a continuum from strongly BvgA/S-activated genes to strongly BvgA/S-repressed genes. The microarray analyses also led to the identification of a subset of vags and vrgs that are differentially regulated and modulated by MgSO(4) or nicotinic acid, indicating that these genes may be targets for multiple regulatory circuits. For example, the expression of bilA, a gene predicted to encode an intimin-like protein, was found to be activated by BvgA/S and up-modulated by nicotinic acid. Furthermore, surprisingly, in the strain analysed here, which produces only type 2 fimbriae, the fim3 gene was identified as a vrg, while fim2 was confirmed to be a vag.

A spectroelectrochemical study of the reduction of a Schiff base cryptand.

The electrochemical reduction of a bicyclic hexaimino Schiff base cryptand 1 (N[(CH2)2N-CH-meta-C6H4-CH=N(CH2)2]3N) and that of one of its strands 2 ((CH3)2CH-N=CH-meta-C6H4-CH=N-CH(CH3)2) has been studied by visible and near infrared in-situ spectroelectrochemical techniques. These results are in good agreement with those obtained using alkali metals, but in this case the effect of the formation of ion pairs is minimized through the use of tetrabutylammonium cations. It is confirmed that 1- and 1= have the same visible and near IR spectrum. The spectrum of the products of the electrochemical reduction of 2 is similar to those of 1- or 1=.

Tetrathiafulvalene crowns: redox-switchable ligands.

A series of redox-responsive ligands that associate the electroactive tetrathiafulvalene core with polyether subunits of various lengths has been synthesized. X-ray structures are provided for each of the free ligands. The requisite structural criteria for reaching switchable ligands are satisfied for the largest macrocycles, that is, planarity of the 1,1',3,3'-tetrathiafulvalene (TTF) pi system and correctly oriented coordinating atoms. The ability of these ligands to recognize various metal cations as a function of the cavity size has been investigated by various techniques (LSIMS, 1H NMR, and UV/Vis spectroscopy, cyclic voltammetry). These systems exhibit an unprecedented high coordination ability among TTF crown ethers. Their switchable ligating properties have been confirmed by cyclic voltammetry, and metal-cation complexation has been illustrated by X-ray structures of three of the corresponding metal complexes (Pb2+, Sr2+, and Ba2+). Solid-state structures of these complexes display original packing modes with channel-like arrangements.

S-position isomers of BEDT-TTF and EDT-TTF: synthesis and influence of outer sulfur atoms on the electrochemical properties and crystallographic network of related organic metals.

The synthesis and characterization of new modified tetrathiafulvalenes (TTF), the S-position isomers of BEDT-TTF and EDT-TTF, are described. The synthetic strategy presented in this work is based on an efficient and unprecedented two-step sequence for the conversion of a vicinal bis(hydroxymethyl) functionality into a disulfide ring. Different routes are discussed in terms of efficiency for the synthesis of the symmetric S-position isomer of BEDT-TTF and that of EDT-TTF. Their electrochemical properties are combined with data obtained from UV/Vis spectroscopy and orbital calculations, and the electronic influence of peripheral sulfur atoms on the neutral and oxidized species is discussed. The introduction of these outer sulfur atoms at the periphery of the TTF core gives rise to specific intermolecular S...S interactions in the corresponding organic materials. Crystallographic studies of radical cation salts synthesized upon electrocrystallization clearly showed that the network obtained is dictated by the outer sulfur atoms, which are responsible for a characteristic and unprecedented "windmill" array.

pi conjugation across the tetrathiafulvalene core: synthesis of extended tetrathiafulvalene derivatives and theoretical analysis of their unusual electrochemical properties

A series of extended tetrathiafulvalene (TTF) derivatives bearing one or two 1,4-dithiafulven-6-yl substitutents has been prepared. The new compounds present remarkable electrochemical singularities compared with other TTF derivatives, which are strongly affected by the nature of the substitution on the lateral heterocycle(s). This unusual electrochemical behaviour follows a square-scheme sequence and is attributed to structural changes upon oxidation of the pi-donating molecules. Digital simulations of the electrochemical data have been used to reach the values of the kinetic and thermodynamic constants involved in the square scheme. Theoretical calculations establish an important contribution of a highly delocalised resonant form involving a tetravalent sulphur in oxidised species, which could justify the occurrence of an electrochemical behaviour distinct from that of TTF. Finally, third-order susceptibilities chi 3 of two of these systems, for which electron-donating and electron-withdrawing substituents coexist and are conjugated through the TTF pi system, are given.