Electrocatalytic Hydrogen Evolution from Molybdenum Sulfide-Polymer Composite Films on Carbon Electrodes.

The design of more efficient catalytic electrodes remains an important objective for the development of water splitting electrolyzers. In this context a structured composite cathode material has been synthesized by electrodeposition of molybdenum sulfide (MoSx) into a poly(pyrrole-alkylammonium) matrix, previously coated onto carbon electrodes by oxidative electropolymerization of a pyrrole-alkylammonium monomer. The composite material showed an efficient electrocatalytic activity toward proton reduction and the hydrogen evolution reaction (HER). Data from Tafel plots have demonstrated that the electron transfer rate in the composite films is fast, in agreement with the high catalytic activity of this cathode material. Bulk electrolysis of acidic water at carbon foam electrodes modified with the composite have shown that the cathodes display a high catalytic activity and a reasonable operational stability, largely exceeding that of regular amorphous MoSx electrodeposited on naked carbon foam. The enhanced catalytic performances of the composite electrode material were attributed to the structuration of the composite, which led to a homogeneous distribution of the catalyst on the carbon foam network, as shown by SEM characterizations.

Polypyrrole-Ru(2,2'-bipyridine)3(2+)/MoSx structured composite film as a photocathode for the hydrogen evolution reaction.

The development of photoelectrochemical devices for solar light-driven water splitting and H2 production requires new strategies for the fabrication of materials that combine the necessary photoredox and catalytic properties, to allow the hydrogen evolution reaction (HER) to take place at a low overvoltage under visible light irradiation. We report the first example of a structured composite, synthesized by electrodeposition of MoSx cocatalyst into a photosensitive Ru complex film deposited onto carbon electrodes by electropolymerization of a pyrrole-functionalized Ru(II)(2,2'-bipyridine)3(2+). Composite films show efficient photocatalytic activity for HER. Our study highlights the great simplicity of this versatile electrochemical procedure to synthesize photocathodes.

Iridium oxide-polymer nanocomposite electrode materials for water oxidation.

Nanocomposite anode materials for water oxidation have been readily synthesized by electrodeposition of iridium oxide nanoparticles into poly(pyrrole-alkylammonium) films, previously deposited onto carbon electrodes by oxidative electropolymerization of a pyrrole-alkylammonium monomer. The nanocomposite films were characterized by electrochemistry, transmission electron microscopy, and atomic force microscopy. They showed an efficient electrocatalytic activity toward the oxygen evolution reaction. Data from Tafel plots have demonstrated that the catalytic activity of the iridium oxide nanoparticles is maintained following their inclusion in the polymer matrix. Bulk electrolysis of water at carbon foam modified electrodes have shown that the iridium oxide-polymer composite presents a higher catalytic activity and a better operational stability than regular oxide films.

Long-range electronic connection in picket-fence like ferrocene-porphyrin derivatives.

The effects of a direct connection between ferrocene and porphyrin units have been thoroughly investigated by electrochemical and spectroscopic methods. These data not only reveal that substitution of the porphyrin macrocycle by one, two, three or four ferrocenyl groups strongly affects the electronic properties of the porphyrin and ferrocenyl moieties, they also clearly demonstrate that the metallocene centres are "connected" through the porphyrin-based electronic network. The dynamic properties of selected ferrocene-porphyrin conjugates have been investigated by VT NMR and metadynamic calculations. 1,3-Dithiolanyl protecting groups have been introduced on the upper rings of the ferrocene fragments to allow a straightforward and easy access to redox active picket-fence porphyrins. X-ray diffraction analyses of the zinc(II) 5-[1'-[2-(1,3-dithiolanyl)]ferrocenyl]-10,15,20-tri(p-tolyl)porphyrin and 5,15-bis[1'-[2-(1,3-dithiolanyl)]ferrocenyl]-10,20-bis(p-tolyl)porphyrin complexes reveal the existence of S-Zn bonds involved in supramolecular arrays. The solid state analysis of the trans-5,15-di-(1'-(formyl)ferrocenyl)-10,20-di-(p-tolyl)-porphyrinatozinc(II) complex, obtained by deprotection of the dithiolane substituted analog, is conversely found in the crystal lattice as a monomer exhibiting a hexacoordinated zinc metal centre.

Multilayer assemblies of polyelectrolyte-gold nanoparticles for the electrocatalytic oxidation and detection of arsenic(III).

Multilayers of poly(diallyldimethylammonium chloride) (PDDA) and citrate capped Au nanoparticles (AuNPs) anchored on sodium 3-mercapto-1-propanesulfonate modified gold electrode by electrostatic layer-by-layer assembly (LbL) technique are shown to be an excellent architecture for the direct electrochemical oxidation of As(III) species. The growth of successive layers in the proposed LbL architecture is followed by atomic force microscopy, UV-vis spectroscopy, quartz crystal microbalance with energy dissipation, and electrochemistry. The first bilayer is found to show rather different physico-chemical characteristics as compared to the subsequent bilayers, and this is attributed to the difference in the adsorption environments. The analytical utility of the architecture with five bilayers is exploited for arsenic sensing via the direct electrocatalytic oxidation of As(III), and the detection limit is found to be well below the WHO guidelines of 10ppb. When the non-redox active PDDA is replaced by the redox-active Os(2,2'-bipyridine)(2)Cl-poly(4-vinylpyridine) polyelectrolyte (PVPOs) in the LbL assembly, the performance is found to be inferior, demonstrating that the redox activity of the polyelectrolyte is futile as far as the direct electro-oxidation of As(III) is concerned.

Electrochemical synthesis of a thiophene-containing cyclo[9]pyrrole.

A new member of the cyclo[n]pyrrole class of expanded porphyrins could be prepared from the corresponding thiophene-containing terpyrrole precursor through use of a mild electrochemical oxidative procedure. The isolated macrocycle, featuring nine heterocyclic subunits directly connected through their α,α'-positions, is the largest cyclo[n]pyrrole derivative reported to date (see figure).

Electrochemical syntheses of cyclo[n]pyrrole.

Cyclo[8]pyrrole was obtained efficiently when 3,3',4,4'-tetraethylbipyrrole was subjected to bulk electrolysis, with yields spanning a range from close to 0 % with tetra-n-butylammonium fluoride as the electrolyte, to almost 70 % when tetra-n-butylammonium hydrogensulfate was used for this purpose. These observations are consistent with the conclusion that the reaction is controlled by anion-related factors such as a specific templating effect. Note that cyclo[8]pyrrole was the only detectable macrocyclic product obtained from 3,3',4,4'-tetraethylbipyrrole under the conditions of the electrochemical oxidation. When similar electrolyses were performed by using 3,4-diethylpyrrole as the starting material, two products could be isolated, which were identified as being the cyclo[7]pyrrole and cyclo[8]pyrrole, respectively. Detailed analyses of the oxidized forms of cyclo[7]pyrrole and cyclo[8]pyrrole revealed that under the conditions of the electrolysis these latter species are not stable.

Soluble heterometallic coordination polymers based on a bis-terpyridine-functionalized dioxocyclam ligand.

Soluble homo- and heterometallic coordination polymers containing transition metal cations (Cu(2+), Fe(2+), Co(2+), and Ni(2+) ions) were prepared in a two-step procedure using a polytopic bis(terpyridine)dioxocyclam ligand 1H(2) (dioxocyclam = 1,4,8,11-tetraazacyclotetradecane-5,7-dione). These supramolecular systems incorporate two different metal complexes, the metal cations being located both between two terpyridine units and in the macrocyclic framework. The characterization of these soluble architectures was investigated by cyclic voltammetry, mass spectrometry, viscosimetry, and UV-vis absorption and electron paramagnetic resonance (EPR) spectroscopies. Our results clearly indicate the formation of well-organized heterometallic polymers in which two different metal ions alternate in the self-assembled structure. These investigations furthermore brought to light an original acid-controlled disassembling process of the homometallic copper(II) polymer into dinuclear complexes.

Off-line coupled electrocatalytic oxidation and liquid phase polymer based retention (EO-LPR) techniques to remove arsenic from aqueous solutions.

Electrochemistry and membrane ultrafiltration methods (electro-oxidation and liquid phase polymer based retention technique, LPR, respectively) were off-line coupled to remove As(III) inorganic species from aqueous solutions. Our main objective was to achieve an efficient extraction of arsenic species by associating a polymer-assisted liquid phase retention procedure, based on the As(V) adsorption properties of cationic water-soluble polymers, with an electrocatalytic oxidation process of As(III) into its more easily removable analogue As(V). The electrocatalytic oxidation of As(III) to As(V) was performed in the presence of different water-soluble poly(quaternary ammonium) salts acting also as supporting electrolyte, i.e. poly(vinylbenzyl)trimethyl ammonium chloride, P(ClVBTA), poly[3-(methacryloylamine)propyl]trimethyl ammonium chloride, P(ClMPTA), and poly(4-vinyl-1-methylpyridinium bromide), P(BrVMP). After complete electrocatalytic conversion of As(III) into As(V), the mixtures were introduced into an LPR cell to remove the As(V)-polymer adducts. Using P(ClMPTA), P(ClVBTA), or P(BrVMP) ammonium salts in a 20:1 polymer:As(III) mol ratio at pH 8, complete (100%) retention of the arsenic was achieved. Moreover, the As(V) retention efficiency turned out to be directly related to the net charge consumed during the electrochemical conversion of As(III) to As(V).

The aza-analogues of 1,4-naphthoquinones are potent substrates and inhibitors of plasmodial thioredoxin and glutathione reductases and of human erythrocyte glutathione reductase.

Various aza-analogues of 1,4-naphthoquinone and menadione were prepared and tested as inhibitors and substrates of the plasmodial thioredoxin and glutathione reductases as well as the human glutathione reductase. The replacement of one to two carbons at the phenyl ring of the 1,4-naphthoquinone core by one to two nitrogen atoms led to an increased oxidant character of the molecules in accordance with both the redox potential values and the substrate efficiencies. Compared to the 1,4-naphthoquinone and menadione, the quinoline-5,8-dione 1 and both quinoxaline-5,8-diones 5 and 6 behaved as the most efficient subversive substrates of the three NADPH-dependent disulfide reductases tested. Modulation of these parameters was observed by alkylation of the aza-naphthoquinone core.

Acid-base-driven interconversion between a mononuclear complex and supramolecular coordination polymers in a terpyridine-functionalized dioxocyclam ligand.

A polytopic cyclam-bis-terpyridine ligand has been used to accomplish an acid-base-triggered formation of either a mononuclear neutral complex or metallopolymers with Cu(2+) ions. A controlled interconversion between these two forms was achieved through the reversible displacement of a Cu(2+) cation from the macrocycle to the terpyridine units.

Electrochemically driven intramolecular ring closure of a linear hexapyrrole.

[24]hexaphyrin(1.0.1.0.0.0) was electrochemically synthesized in good yield starting from a linear hexapyrrolic precursor. This straightforward approach provides an efficient "green" alternative to the reported chemical synthesis, which requires the use of trifluoroacetic acid as the solvent and toxic chromium(VI) salts as the oxidant.

Redox-active biomolecular architectures and self-assembled monolayers based on a cyclodecapeptide regioselectively addressable functional template.

A nanometer scale redox active biomolecular architecture has been successfully synthesized through an efficient chemoselective oxime based coupling between ferrocenyl groups and a regioselectively addressable cyclodecapeptide. This molecular tool exhibits electronic, structural, and chemical properties driven by the biomimetic recognition activity of the polypeptide skeleton associated to the well-defined electrochemical activity of metallocenyl probes. Biomolecular materials obtained by confinement of the redox cyclopeptide in self-assembled monolayers on gold surfaces shows efficient through-bond electron transfer from the ferrocenes to the electrode surface via the peptidic backbone, as well as markedly improved sensing properties toward anionic species in organic electrolyte, as compared to those observed in homogeneous solution.

Electrochemical synthesis of cyclo[8]pyrrole.

The utility of an electrochemical oxidative strategy in the synthesis of expanded porphyrins is demonstrated by the preparation of cyclo[8]pyrrole in high yield via a bipyrrole C(2)-C(2)' coupling process.

Calix[4]phyrin based redox architectures: towards new molecular tools for electrochemical sensing.

New redox active molecular macrocyclic architectures characterized by a direct connection between dipyrrin, tripyrrin and ferrocenyl fragments have been synthesized and characterized. Contrarily to fully conjugated porphyrins, in which four pyrrole moieties contribute to the overall aromatic pi-electronic system and behave as a unique electroactive species, calixphyrins can be regarded as an assembly of independent redox active pyrrole and conjugated oligopyrrole fragments linked through sp(3) hybridized meso carbon atoms. The disruption of the conjugation pathway not only multiplies the number of redox centres throughout the molecule but also leads to a large variety of molecular architectures with specific physico-chemical properties. These novel ferrocene containing hybrid macrocycles exhibit especially attractive electronic and structural features suited for use as molecular sensing tools. An efficient voltammetric sensing of exogenic electron rich anionic species could especially be performed using a metallo-calix[4]phyrin-(1.1.1.1) through the displacement of the labile axial binding site, the perturbation of the Fc(0/+) redox couple being directly related to complexed species features.

Redox-triggered molecular movement in a multicomponent metal complex in solution and in the solid state.

The Cu(I) and Cu(II) complexes of the new 1,8-diferro-cenylmethyl-4,11-dimethyl-1,4,8,11-tetraazacyclotetra-decane ligand (denoted L) have been isolated and characterized by X-ray structure determination and electrochemical studies. The Cu(I) complex presents an unprecedented stability toward dioxygen. The two complexes adopt two energetically distinct and stable geometries, which differ mainly by the relative positioning of the substituents above or below the cyclam plane. Triggered by a copper-centered electron transfer, a fast and reversible motion of the noncoordinating subunits is obtained in homogeneous solution and in the solid state.

Redox sensing of anions in pure aqueous environment by ferrocene-containing 4,4'-bipyridinium-based receptors and polymer films.

Selective voltammetric sensing of ATP(2-) anions in water was achieved using ferrocene-viologen based redox active receptors and related polymer films.

Self-assembly of a ferrocene-substituted porphyrin capable of electrochemically sensing neutral molecules via a "tail on-tail off" process.

The supramolecular assembly of a ferrocene-porphyrin conjugate allowed ferrocene-based electrochemical sensing of the metalloporphyrin axial coordination via a "tail on-tail off" binding process.

Thermically and electrochemically induced isomerization of a (bis(ferrocene)-cyclam)copper(II) complex.

The new bis(ferrocene)-cyclam macrocycle 1,8-bis(ferrocenylmethyl)-1,4,8,11-tetraazacyclotetradecane, denoted L, has been synthesized. Two Cu(II) complexes with L have been isolated and characterized from X-ray structure determination and electrochemical studies. These two LCu(II) complexes correspond to the type I (ferrocenyl subunits in the same side of the cyclam plane) and type III (ferrocenyl subunits above and below the cyclam plane) isomers. The type I LCu(II) complex was synthesized from L and a Cu(2+) salt, while the type III isomer was obtained by oxidation in air or by comproportionation of the Cu(I) complex. The interconversion between type I and type III LCu(II) complexes is negligible in acetonitrile and slow in dimethyl sulfoxide but fast via an electrochemical reduction-reoxidation cycle. According to UV-vis and electrochemical characterizations, the type III isomer is thermodynamically more stable and the type I isomer is kinetically favored. A type III LNi(II) complex was also isolated and characterized by X-ray diffraction analysis and from electrochemical studies.