Electron transfer between carbon dots and tetranuclear Dawson-derived sandwich polyanions.

Among the photocatalysts which could be used for converting solar energy, polyoxometalates are often regarded as ideal candidates because of their remarkable performances in photocatalytic water splitting and photodegradation of pollutants. Nonetheless, these polyanions are only capable of absorbing UV light, unless coupled to a visible-light photosensitizer. Carbon nanodots are especially promising for this purpose because of their strong visible-light absorption, photostability, non-toxicity, and very low production costs. In this work we demonstrate the possibility of coupling carbon dots to polyoxometalates with different structures, by a simple self-assembly approach based on electrostatic interactions in solution phase. Our studies highlight an extremely efficient interaction between the two compounds, resulting in ultrafast photoinduced electron or energy transfer from carbon dots to the coupled polyoxometalates, depending on the structure of the latter, as revealed by a detailed study based on ultrafast transient absorption spectroscopy. The evidence herein provided shows how nanohybrids based on polyoxometalates photosensitized by carbon dots could find their place in photocatalytic applications, thanks to their remarkable efficiency and huge versatility.

Nanostructured Mixed Layers of Organic Materials Obtained by Nanosphere Lithography and Electrochemical Reduction of Aryldiazonium Salts.

In this work, we have combined nanosphere lithography with electrochemical reduction of aryldiazonium salts to elaborate nanostructured mixed layers of organic materials. The strategy consists first in the deposition of a close-packed hexagonal monolayer of microbeads used as a mask for the electroreduction of a first aryldiazonium salt. After removing the beads, an ultrathin organic layer with holes remains. Then, a second aryldiazonium salt is electrochemically reduced selectively inside the holes. The relative thickness of the two deposited materials can be changed, leading to mixed layers of different topographies. Moreover, using diazoniums with complementary redox properties, a modified bifunctional electrode acting as a filter for electron transfer with a low potential gap has been obtained. Such layers are similar to low-band-gap organic semiconductors that can be easily n or p doped. Despite this analogy, the oxidation and reduction of redox probes in solution on such nanostructured surfaces occur on completely separated areas of the mixed layer.

Substrate interaction dynamics and oxygen control in the active site of thymidylate synthase ThyX.

Thymidylate synthase ThyX, required for DNA synthesis in many pathogenic bacteria, is considered a promising antimicrobial target. It binds FAD and three substrates, producing dTMP (2'-deoxythymidine-5'-monophosphate) from dUMP (2'-deoxyuridine-5'-monophosphate). However, ThyX proteins also act as NADPH oxidase by reacting directly with O2. In the present study we investigated the dynamic interplay between the substrates and their role in competing with this wasteful and potentially harmful oxidase reaction in catalytically efficient ThyX from Paramecium bursaria Chlorella virus-1. dUMP binding accelerates the O2-insensitive half-reaction between NADPH and FAD by over four orders of magnitude to ~30 s-1. Thus, although dUMP does not have a direct role in FAD reduction, any turnover with molecular O2 requires its presence. Inversely, NADPH accommodation accelerates dUMP binding ~3-fold and apparently precedes dUMP binding under physiological conditions. In the oxidative half-reaction, excess CH2H4folate (N5,N10-methylene-5,6,7,8-tetrahydrofolate) was found to re-oxidize FADH2 within 1 ms, thus very efficiently competing with FADH2 oxidation by O2 (1.5 s-1 under aerobic conditions). The resulting reaction scheme points out how the interplay between the fast reactions with the native substrates, although not rate-limiting for overall catalysis, avoids NADPH oxidase activity in aerobic micro-organisms, including many pathogens. These observations also explain why ThyX proteins are also present in aerobic micro-organisms.

Multivalent-Ion versus Proton Insertion into Nanostructured Electrochromic WO3 from Mild Aqueous Electrolytes.

Mild aqueous electrolytes containing multivalent metal salts are currently scrutinized for the development of ecosustainable energy-related devices. However, the role of soluble multivalent metal ions in the electrochemical reactivity of transition metal oxides is a matter of debate, especially when they are performed in protic aqueous electrolytes. Here, we have compared, by means of (spectro)electrochemistry, the reversible electrochromic reduction of transparent nanostructured γ-WO3 thin films in mild aqueous electrolytes of various chemical composition and pH. This study reveals that reversible proton insertion is the only charge storage mechanism over a large pH range and that it is effective for aqueous electrolytes prepared from either organic (such as acetic acid) or inorganic (such as solvated multivalent cations) Bro̷nsted acids. By refuting charge storage mechanisms relying on the reversible insertion of multivalent metal ions, notably in aqueous electrolytes based on Al3+ ions or a mixture of Al3+ and Zn2+ ions, these fundamental results pave the way for the rational development of electrolytes and active materials for a range of aqueous-based devices, such as the emerging concept of an energy-saving smart window, which we also address in this study.

An original electrochemical pathway for the synthesis of porphyrin oligomers.

An electrosynthesis process has allowed the formation of four oligomers, containing three, four, or five macrocycles. This method is based on the nucleophilic attack of porphyrins substituted by several pendant pyridyl groups to the electrogenerated radical cation of zinc ß-octaethylporphyrin (ZnOEP), according to an ECEC processes. Thus, a control of the number of macrocycles and of the geometry of the oligomers can be performed. These new compounds have been characterized by HRMS as well as (1)H NMR, UV/Vis, and fluorescence spectroscopy, and electrochemistry. The results show a strong influence of the pyridinium spacers on the macrocycles.

Self-assembled molecular rafts at liquid|liquid interfaces for four-electron oxygen reduction.

The self-assembly of the oppositely charged water-soluble porphyrins, cobalt tetramethylpyridinium porphyrin (CoTMPyP(4+)) and cobalt tetrasulphonatophenyl porphyrin (CoTPPS(4-)), at the interface with an organic solvent to form molecular "rafts", provides an excellent catalyst to perform the interfacial four-electron reduction of oxygen by lipophilic electron donors such as tetrathiafulvalene (TTF). The catalytic activity and selectivity of the self-assembled catalyst toward the four-electron pathway was found to be as good as that of the Pacman type cofacial cobalt porphyrins. The assembly has been characterized by UV-visible spectroscopy, Surface Second Harmonic Generation, and Scanning Electron Microscopy. Density functional theory calculations confirm the possibility of formation of the catalytic CoTMPyP(4+)/ CoTPPS(4-) complex and its capability to bind oxygen.

Spectroelectrochemical characterization of small hemoproteins adsorbed within nanostructured mesoporous ITO electrodes.

3D nanostructured transparent indium tin oxide (ITO) electrodes prepared by glancing angle deposition (GLAD) were used for the spectroelectrochemical characterization of cytochrome c (Cyt c) and neuroglobin (Nb). These small hemoproteins, involved as electron-transfer partners in the prevention of apoptosis, are oppositely charged at physiological pH and can each be adsorbed within the ITO network under different pH conditions. The resulting modified electrodes were investigated by UV-visible absorption spectroscopy coupled with cyclic voltammetry. By using nondenaturating adsorption conditions, we demonstrate that both proteins are capable of direct electron transfer to the conductive ITO surface, sharing apparent standard potentials similar to those reported in solution. Preservation of the 3D protein structure upon adsorption was confirmed by resonance Raman (rR) spectroscopy. Analysis of the derivative cyclic voltabsorptograms (DCVA) monitored either in the Soret or the Q bands at scan rates up to 1 V s(-1) allowed us to investigate direct interfacial electron transfer kinetics. From the DCVA shape and scan rate dependences, we conclude that the interaction of Cyt c with the ITO surface is more specific than Nb, suggesting an oriented adsorption of Cyt c and a random adsorption of Nb on the ITO surface. At the same time, Cyt c appears more sensitive to the experimental adsorption conditions, and complete denaturation of Cyt c may occur as evidenced from cross-correlation of rR spectroscopy and spectroelectrochemistry.

Spectroscopic and electrochemical study of the interconversion and decomplexation of cobalt(II) sandwich polyoxometalates based on a Dawson-type anion.

The reaction of the trivacant Dawson polyoxometalate α-[P(2)W(15)O(56)](12-) and the divalent cations Co(2+) is known to form a symmetrically derived sandwich complex of formula ßß-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) [symbolized as ßß-Co(4)(P(2)W(15))(2)] at low pH (ca. pH 3). We have shown previously that, by a slight modification of the reaction conditions, trinuclear αß-[(NaOH(2))Co(3)(H(2)O)(P(2)W(15)O(56))(2)](17-) and dinuclear [(NaOH(2))(2)Co(2)(P(2)W(15)O(56))(2)](18-) complexes [symbolized as αß-NaCo(3)(P(2)W(15))(2) and Na(2)Co(2)(P(2)W(15))(2), respectively] can be synthesized as aqueous-soluble sodium salts. αß-NaCo(3)(P(2)W(15))(2) is a "lacunary" sandwich complex that can add a Co(2+) cation to form nearly quantitatively an unsymmetrical Dawson tetracobalt sandwich polyoxometalate, αß-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) [symbolized as αß-Co(4)(P(2)W(15))(2)]. Thus, for Co(4)(P(2)W(15))(2), the junctions between the trivacant {P(2)W(15)} subunits and the central tetrameric unit can be either both ß type or ß and α types. The interconversion between αß-Co(4)(P(2)W(15))(2) and ßß-Co(4)(P(2)W(15))(2) and the decomplexation process at low pH, leading to the formation of αß-NaCo(3)(P(2)W(15))(2) and/or Na(2)Co(2)(P(2)W(15))(2), have been followed in aqueous solution at various pH values by electrochemistry, UV-visible absorption spectroscopy, and (31)P NMR spectroscopy.

Interfacial self-assembly of water-soluble cationic porphyrins for the reduction of oxygen to water.

Meet at the border: Assembly of the water-soluble cobalt tetrakis(N-methylpyridinium-4-yl)porphyrin [CoTMPyP](4+) at soft interfaces is enhanced and stabilized by its interfacial interaction with the lipophilic anion (C(6)F(5))(4)B(-). The supramolecular structure thus formed provides excellent catalytic activity in the four-electron reduction of oxygen.

Formation and photocatalytic properties of nanocomposite films containing both tetracobalt Dawson-derived sandwich polyanions and tetracationic porphyrins.

Films based on electrostatic interactions between tetracationic zinc porphyrins, ZnOEP(py)(4)(4+) or ZnTMePyP(4+), and the tetracobalt Dawson-derived sandwich polyanion αßßα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) are formed by the so-called layer-by-layer method. These films have been characterized by UV-visible absorption spectroscopy, atomic force microscopy and electrochemistry. The composition of the film was measured by X-ray photoelectron spectrum (XPS). The XPS data confirm the presence of the expected elements. The photocatalytic properties of these films have been also studied for the reduction of silver and gold ions. Indeed, in these systems, porphyrins can be excited by visible light and then play the role of photosensitizers able to give electrons to POM known to be good catalysts. Silver nanowires and gold nanosheets have been obtained.

Photocurrents at polarized liquid|liquid interfaces enhanced by a gold nanoparticle film.

Photocurrent responses associated with the interfacial quenching of the photo-excited water-soluble zinc meso-tetra(4-carboxyphenyl)porphyrin (ZnTPPC) by ferrocene have been studied at a water|1,2-dichloroethane interface in the absence and in the presence of adsorbed gold nanoparticles. Upon addition of methanol, a mirror-like gold film is formed and an important enhancement of the photocurrent response can be observed. Intensity modulated photocurrent spectroscopy experiments (IMPS) have been performed, in order to deconvolute in the frequency domain the contribution from the competition between the recombination and the product separation arising after the electron transfer, and the attenuation associated with the resistance and interfacial capacitance (RC(int)) time constant of the cell.

Artificial photosynthesis at soft interfaces.

The concept of artificial photosynthesis at a polarised liquid membrane is presented. It includes two photosystems, one at each interface for the hydrogen and oxygen evolution respectively. Both reactions involve proton coupled electron transfer reactions, and some ultrafast steps at the photosensitization stage.

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-ß-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.

Synthesis and photocatalytic properties of mixed polyoxometalate-porphyrin copolymers obtained from Anderson-type polyoxomolybdates.

Hybrid polyoxometalate-porphyrin copolymeric films are obtained by the electro-oxidation of zinc octaethylporphyrin (ZnOEP) and zinc 5,15-dipyridinium octaethylporphyrin (5,15-ZnOEP(py)(2)(2+)) in the presence of the polyoxometalate [MnMo(6)O(18){(OCH(2))(3)CNHCO(4-C(5)H(4)N)}(2)](3-) (Py-POM-Py). These films allow the photocatalytic reduction of Ag(I)(2)SO(4) under visible irradiation in air in the presence of propan-2-ol at the 2D interface between water and the copolymeric films. The formation of metallic Ag(0) nanowires and triangular nanosheets is observed.

Highly Efficient Electron Transfer in a Carbon Dot-Polyoxometalate Nanohybrid.

Using solar radiation to fuel catalytic processes is often regarded as the solution to our energy needs. However, developing effective photocatalysts that are active under visible light has proven to be difficult, often due to the toxicity, instability, and high cost of suitable catalysts. We engineered a novel photoactive nanomaterial obtained by the spontaneous electrostatic coupling of carbon nanodots with [P2W18O62]6-, a molecular catalyst belonging to the class of polyoxometalates. While the former are used as photosensitizers, the latter was chosen for its ability to catalyze reductive reactions such as dye decomposition and water splitting. We find the electron transfer within the nanohybrid to be so efficient that a charge-separated state is formed within 120 fs from photon absorption. These results are a cornerstone in the engineering of a new class of nanodevices, which are nontoxic, are inexpensive, and can carry out solar-driven catalytic processes.

Highly Resolved Nanostructured PEDOT on Large Areas by Nanosphere Lithography and Electrodeposition.

Poly(ethylenedioxythiophene) (PEDOT) films were electrodeposited galvanostatically from an EDOT/sodium dodecyl sulfate solution in water, through a carboxylated polystyrene template monolayer self-assembled on ITO, after which the template was dissolved away in tetrahydrofuran. Analysis of the films by scanning electron microscopy and atomic force microscopy reveals large-area PEDOT honeycomb structures. The morphology of these structures was varied electrochemically, as the effective thickness and, surprisingly, the shape of the honeycomb arrangement depend on the polymerization time. Using nanospheres of 1 µm diameter and charge densities between 12 and 30 mC cm(-2) for electrodeposition generates PEDOT hexagons with very thin rectilinear walls 30-35 nm-thick and 800 nm-long, whereas at higher charge densities, circular bowls are created with 60 nm walls separating adjacent bowls; triangular areas as small as 0.02 µm(2) develop at the intersection of three nanospheres. These morphologies are specific to the use of carboxylated PS spheres and a water-based solution with a surfactant in the galvanostatic electrodeposition mode. Using smaller nanospheres, i.e. 500 nm in diameter, makes it possible to reach PEDOT hexagons with rectilinear walls as small as 11-17 nm-thick and 300 nm-long; circular bowls with 25-35 nm walls separating adjacent bowls and triangular areas as small as 0.003 µm(2) can also be generated. The wettabilities of the surfaces depend markedly on the pore depth of the PEDOT nanostructure, with contact angles going from 82° to 130° with increasing pore size. Finally these nanostructured PEDOT electrodes were used in Grätzel-type dye-sensitized solar cells (DSSCs) as Pt-free counter-electrodes, with an increase in the yield from 7.0 (bulk PEDOT) to 8.1%.

Synthesis, electrochemical and photophysical properties of covalently linked porphyrin-polyoxometalates.

Two covalently linked porphyrin-polyoxometalate hybrids have been prepared: an Anderson-type hexamolybdate [N(C(4)H(9))(4)](3)[MnMo(6)O(18){(OCH(2))(3)CNHCO(ZnTPP)}(2)] with two pendant zinc(II)-tetraphenylporphyrins, and a Dawson-type vanadotungstate [N(C(4)H(9))(4)](5)H[P(2)V(3)W(15)O(59){(OCH(2))(3)CNHCO(ZnTPP)}] with one porphyrin. Electrochemical studies show independent redox processes for the organic and inorganic parts at usual potentials. Photophysical studies reveal an electron transfer from the excited porphyrin to the Dawson polyoxometalate, but not to the Anderson polyoxometalate. Time resolved absorption spectroscopy allows the identification of the electron transfer pathways and the determination of the time constants.