Presence Of Electron Donor Research Articles

Electron Transfer from the Porphyrin S2 State in a Zinc Porphyrin-Rhenium Bipyridyl Dyad having Carbon Dioxide Reduction Activity

fac-Re(bpy)(CO)3Cl (bpy = 2,2′-bipyridine) and similar complexes are known to act as photocatalysts for the reduction of CO2 to CO in the presence of electron donors. However, these complexes do not have significant absorption in the visible region. In a separate publication, we have reported the successful combination of fac-Re(bpy)(CO)4Cl with a zinc porphyrin that has strong absorption in the visible region to produce the ZnTMP-Re(bpy)(NHAc) dyad(5-{4-[N-4′-(rhenium(I)tricarbonylchloride-4-acetylamino-2,2′-bipyridyl)-carbamoyl]phenyl}-10,15,20-trimesityl-porphyrinatozinc(II)). In that paper we reported the photochemical reduction of CO2 using excitation of the porphyrin upper excited singlet state, S2 state (visible region), in the dyad. The very wide visible light region is able to be utilized by the sensitizer. In the present work, as regards the detailed molecular mechanism, we have demonstrated the direct electron transfer from the Zn porphyrin S2 state to the Re complex in the dyad by ultrafast tr...

Nanostructured thin films based on phthalocyanines: electrochromic displays and sensors

The group of the University of Valladolid is a multidisciplinary team formed by chemists, physicists and engineers. The activities of the group are focused to the study of the physicochemical properties of nanostructured Langmuir-Blodgett thin films based on phthalocyanines and their applications. Films of a variety of phthalocyanine molecules including several metallophthalocyanines, lanthanide double decker phthalocyanines and heteroleptic derivatives have been prepared. Their spectroelectrochemical properties have been described in detail and compared with those observed in disordered casted films or microcrystalline evaporated films. The group has dedicated special attention to films based on rare earth double decker compounds due to their unique semiconducting, optical and electrochemical properties. A rich electrochromism has been demonstrated in thin films of this family of compounds. The reversibility is improved in nanostructured Langmuir-Blodgett films. This has permitted development of an electrochromic display that can change its color from blue to green and finally to red. At the present moment, our main objective is the design of sensors able to detect gases and liquids. It has been demonstrated that thin film assemblies based on rare earth bisphthalocyanines modify their conductivity and their optical properties in the presence of electron donor or electron acceptors gases. Changes are also observed when the devices are exposed to Volatile Organic Compounds such as esters, alcohols or aldehydes which are responsible of odors in foods and beverages. Liquid sensors have also been developed. Their working principle is based in the fact that the rich electrochemical properties of phthalocyanine thin films are extremely sensitive to the nature of the electrolytic solution. Arrays of phthalocyanines have been used to construct an electronic nose able to discriminate odors from a variety of foods and beverages. Similarly, phthalocyanines have also been used to construct an electronic tongue based on voltammetric sensors. This is one of the main contributions of the group to the field of sensors.

The photochemical behaviour of naphthoylenebenzimidazolone dyes in 1-methyl-2-pyrrolidone

The photochemical behaviour of selected naphthoylenebenzimidazolone dyes is discussed in terms of the photobleaching of the dyes in the presence of electron donors (phenylthioacetic acid, N-phenylglycine, ethyl 4- N, N-dimethylaminobenzoate) as well as an electron acceptor (1-methoxy-4-phenylpyridinium tetrafluoroborate) that are used in photoredox systems for light-induced polymerization. The fluorescence quenching of the dyes in the presence of the electron donors and the electron acceptor was also examined.

Anaerobic mineralization of pentachlorophenol (PCP) by combining PCP‐dechlorinating and phenol‐degrading cultures

The dechlorination and mineralization of pentachlorophenol (PCP) was investigated by simultaneously or sequentially combining two different anaerobic microbial populations, a PCP-dechlorinating culture capable of the reductive dechlorination of PCP to phenol and phenol- degrading cultures able to mineralize phenol under sulfate- or iron-reducing conditions. In the simultaneously combined mixture, PCP (about 35 microM) was mostly dechlorinated to phenol after incubation for 17 days under sulfate-reducing conditions or for 22 days under iron-reducing conditions. Thereafter, the complete removal of phenol occurred within 40 days under both conditions. In the sequentially combined mixture, most of the phenol, the end product of PCP dechlorination, was degraded within 12 days of inoculation with the phenol degrader, without a lag phase, under both sulfate- and iron-reducing conditions. In a radioactivity experiment, [14C-U]-PCP was mineralized to 14CO2 and 14CH4 by the combined anaerobic microbial activities. Analysis of electron donor and acceptor utilization and of the production and consumption of H2, CO2, and CH4 suggested that the dechlorinating and degrading microorganisms compete with other microorganisms to perform PCP dechlorination and part of the phenol degradation in complex anoxic environments in the presence of electron donors and acceptors. The presence of a small amount of autoclaved soil slurry in the medium was possibly another advantageous factor in the successful dechlorination and mineralization of PCP by the combined mixtures. This anaerobic-anaerobic combination technology holds great promise as a cost-effective strategy for complete PCP bioremediation in situ.

Synthesis and Properties of Functionalized 4 nm Scale Molecular Wires with Thiolated Termini for Self-Assembly onto Metal Surfaces

We report the synthesis of new oligo(aryleneethynylene) molecular wires of ca. 4 nm length scale by palladium-catalyzed Sonogashira cross-coupling methodology. Key structural features are the presence of electron donor 9-(1,3-dithiol-2-ylidene)fluorene (compounds 13 and 14) and electron acceptor 9-[di(4-pyridyl)methylene]fluorene units (compound 16) at the core of the molecules. Terminal thiolate substituents are protected as cyanoethylsulfanyl (13 and 16) or thioacetate derivatives (14). The molecules display well-defined redox processes in solution electrochemical studies. The optical properties in solution are similar to those of the fluorenone analog 6: the strongest absorptions for 6, 13 and 16 are in the region lambda(max) = 387-393 nm, with 13 showing an additional shoulder at 415 nm which is not present for 6 and 16; this shoulder is assigned to a HOMO-LUMO transition from the dithiole to the fluorene unit. Molecules 6, 13, 14 and 16 form self-assembled monolayers on gold substrates which exhibit essentially symmetrical current-voltage (I-V) characteristics when contacted by a gold scanning tunelling microscope (STM) tip. The effects of the chemical modifications at the central unit of 6, 14 and 16 on the HOMO-LUMO levels and electron transport through the molecules in vacuum have been computed by an ab initio approach.

Photocatalytic hydrogen generation over alkaline-earth titanates in the presence of electron donors

The efficiency of alkaline-earth titanate-based compounds (Ca, Sr, Ba) for catalysts in photocatalytic hydrogen generation has been investigated. In this report, we have shown that the addition of organic donors (such as formic acid, acetic acid, methanol, 2-propanol and formaldehyde) enhanced the efficiency of the studied process. The systematic study has shown that the most efficient organic donor in regards to its hydrogen generation efficiency is formic acid. Of the catalysts explored, the highest photocatalytic activity was shown by SrTiO 3:TiO 2. Additionally, the effects of photocatalyst quantity and formic acid concentration on hydrogen evolution have been investigated.

Electrochemical Rectification by Redox-Labeled Bioconjugates: Molecular Building Blocks for the Construction of Biodiodes

In the present work, we describe the properties of a bifunctional redox-labeled bioconjugate at electrode surfaces mediating the electron transfer across the electrode-electrolyte interface. We show that the assembly of ferrocene-labeled streptavidin on biotinylated electrodes results in a reproducible unidirectional current flow in the presence of electron donors in solution. Such rectifying films were built up by spontaneous binding of tetrameric streptavidin molecules to biotin centers immobilized on the electrode surface. Due to the high affinity of biotin to streptavidin, such bifunctional films completely bind any biotinylated compounds. The charge transport between donors in solution and the Au electrode is mediated by the ferrocene moieties, allowing us to develop a molecular rectifier. Our experimental results suggest that such redox-labeled proteins with a high binding capacity constitute a promising alternative to organic compounds used in molecular electronics.

Photochemical reaction of cyclohexyl phenyl ketone within lyotropic liquid crystals

Lyotropic liquid crystals (LCs) formed by sodium dodecyl sulfate (SDS), n-pentanol, and H 2O at room temperature in their tertiary phase diagram have been explored as a confined medium for the typical photochemical reaction of cyclohexyl phenyl ketone ( 1), which can lead to either intramolecular hydrogen abstraction product 2 or intermolecular reduction product 3 in isotropic solutions upon irradiation. Studies on the product distributions of ketone 1 in the absence and presence of electron donors in this work demonstrate that LC not only restricts the movement of the substrates and intermediates but also encapsulates the substrates and electron donors together during photoirradiation, thereby giving rise to the formation of intermolecular hydrogen abstraction product 3 with high efficiency. A comparison of the same reaction in SDS micelle reveals that LC provides much better constraint than the micelles. The solution-like LC can be used as a microreactor to direct the reaction pathway of ketone 1 by controlling the viscosity and close contact between substrates and electron donors.

Molecular oxygen (a substrate of the cyclooxygenase reaction) in the kinetic mechanism of the bifunctional enzyme prostaglandin-H-synthase

Prostaglandin-H-synthase is a bifunctional enzyme catalyzing conversion of arachidonic acid into prostaglandin H2 as a result of cyclooxygenase and peroxidase reactions. The dependence of the rate of the cyclooxygenase reaction on oxygen concentration in the absence and in the presence of electron donor was determined. A two-dimensional kinetic scheme accounting for independent proceeding and mutual influence of the cyclooxygenase and peroxidase reactions and also for hierarchy of the rates of these reactions was used as a model. In the context of this model, it was shown that there are irreversible stages in the mechanism of the cyclooxygenase reaction between points of substrate donation (between donation of arachidonic acid and the first oxygen molecule and also between donation of two oxygen molecules).

Photonics of dimers of cyanine dyes

The results of study on the properties of dimers of thiamonomethine-and thiatrimethinecyanines (thiacarbocyanines) in the ground and electronically excited states in aqueous solutions are presented. Dimers of cyanine dyes have the sandwiched structure with near-parallel alignment of the polymethine chains of the monomers in the dimer. The formation of dimers is manifested by two absorption bands of different intensities due to splitting of the S* level of the monomers upon their resonance interaction. Dimers of thiacarbocyanines are characterized by a low fluorescence quantum yield φfl as compared to monomers; however, φfl of dimers of thiamonomethinecyanines are markedly higher than that of monomers. Dimers of cyanine dyes are also characterized by a relatively high quantum yield of intersystem crossing to the triplet state. In the triplet-triplet absorption spectra, two bands of different intensities are revealed, which are due to the splitting of the higher triplet level of the monomers that form the dimer. In the presence of electron donors (ascorbic acid, hydroquinone) and/or acceptors (p-benzoquinone, p-nitroacetophenone, methylviologen), the triplet state of dimers is quenched as a result of electron transfer yielding radical products. Dimers in the triplet state can serve as photosensitizers of redox reactions.

Effects of electron donors and acceptors on anaerobic reduction of azo dyes by Shewanella decolorationis S12

Shewanella decolorationis S12 was able to reduce various azo dyes in a defined medium with formate, lactate, and pyruvate or H(2) as electron donors under anaerobic conditions. Purified membranous, periplasmic, and cytoplasmic fractions from strain S12 analyzed, respectively, only membranous fraction was capable of reducing azo dye in the presence of electron donor, indicating that the enzyme system for anaerobic azoreduction was located on cellular membrane. Respiratory inhibitor Cu(2+), dicumarol, stigmatellin, and metyrapone inhibited anaerobic azoreduction by purified membrane fraction, suggesting that the bacterial anaerobic azoreduction by strain S12 was a biochemical process that oxidizes the electron donors and transfers the electrons to the acceptors through a multicompound system related to electron transport chain. Dehydrogenases, cytochromes, and menaquinones were essential electron transport components for the azoreduction. The electron transport process for azoreduction was almost fully inhibited by O(2), 6 mM of NO3-, and 0.9 mM of NO2-, but not by 10 mM of Fe(3+). The inhibition may be a result from the competition for electrons from electron donors. These findings impact on the understanding of the mechanism of bacterial anaerobic azoreduction and have implication for improving treatment methods of wastewater contaminated by azo dyes.

Photocatalysis and solar hydrogen production

Abstract Photocatalytic water splitting is a challenging reaction because it is an ultimate solution to energy and environmental issues. Recently, many new powdered photocatalysts for water splitting have been developed. For example, a NiO (0.2 wt %)/NaTaO3:La (2 %) photocatalyst with a 4.1-eV band gap showed high activity for water splitting into H2 and O2 with an apparent quantum yield of 56 % at 270 nm. Overall water splitting under visible light irradiation has been achieved by construction of a Z-scheme photocatalysis system employing visible-light-driven photocatalysts, Ru/SrTiO3:Rh and BiVO4 for H2 and O2 evolution, and an Fe3+/Fe2+ redox couple as an electron relay. Moreover, highly efficient sulfide photocatalysts for solar hydrogen production in the presence of electron donors were developed by making solid solutions of ZnS with AgInS2 and CuInS2 of narrow band gap semiconductors. Thus, the database of powdered photocatalysts for water splitting has become plentiful.

Qualitative (sunlight) and quantitative (xenon light source) aspects of the photochemistry of [S2W18O62]4− in the presence of electron donors

The photoelectrochemical properties of the polyoxotungstate anion [S2W18O62]4− dissolved in acetonitrite have been surveyed. Qualitative studies revealed that the rate of oxidation of the electron donor in the presence of sunlight correlated well with the voltammetric peak potential for oxidation of the electron donor at a glassy carbon electrode. Quantitative studies on the photooxidation of tetrahydrofuran, dimethylformamide, triphenylphosphine and ferrocene, as detected at a platinum channel electrode in the presence of a xenon light source, were assigned to a CE mechanism. Simulated fits for the proposed mechanism provide estimates of the reaction rates and data are compared with those obtained previously on the basis of a CECE scheme for the molybdenum analogue.

A Theoretical Elucidation on the Solvent‐dependent Photosensitive Behaviors of C60

In this paper, the solvent-dependent photosensitive behaviors of fullerene (C(60)) were investigated in polar and nonpolar solvents by time-dependent density functional theory (TD-DFT) calculation. Based on the calculated physicochemical parameters on triplet state, it is revealed that excited-state C(60) only generates (1)O(2) via energy transfer in benzene, but can give birth to O(2)(.-) and (1)O(2) in water via energy transfer and electron transfer, respectively. Considering the fact that electron transfer is more favorable compared with energy transfer in polar biological systems, especially with the presence of electron donors, the O(2)(.-)-generating process will get predominant in physiological systems. These results account well for the experimental observations that O(2)(.-) and (.)OH are primarily responsible for the photoinduced DNA cleavage by C(60) under physiological conditions, whereas (1)O(2) plays a critical role in nonpolar solvents.

Limitations to Natural Bioremediation of Perchlorate in a Contaminated Site

ABSTRACT Perchlorate (ClO4 −) has been detected in many drinking water supplies in the United States, including the Las Vegas Wash and Lake Mead, Nevada. These locations are highly contaminated and contribute perchlorate to Lake Mead and the Colorado River system. Essential elements for perchlorate bioremediation at these locations were examined, including the presence of perchlorate-reducing bacteria (PRB), sufficient electron donors, occurrence of competing electron acceptors, and ability of PRB to utilize a variety of electron donors. Enumeration of PRB was performed anoxically using most probable number (MPN). Values ranged from ≤20 to 230 PRB/100 ml or ≤20 to ≥ 1.6× 105 PRB/g for Lake Mead water samples and Las Vegas Wash sediments, respectively. 16S rRNA sequences revealed that isolates were γ -proteobacteria, Aeromonas, Dechlorosoma, Rahnella and Shewanella. A screening of potential electron donors using BIOLOGTM demonstrated that all isolates were capable of metabolic versatility. Measurements of total organic carbon (TOC), nitrate and dissolved oxygen (DO) indicated limited presence of electron donor at all sites, whereas the electron acceptors varied throughout the Wash and Lake Mead. The persistence of perchlorate in the sites is attributed to lack of available electron donor and/or the presence of competing electron acceptors. A location has been identified where perchlorate biodegradation could be implemented thereby halting the transport of perchlorate to Lake Mead and the Colorado River.

Low Frequency Impedance Studies on Covalently Modified Glassy Carbon Paste

Cyclic voltammetry and impedance studies have been carried out on covalently coupled horseradish peroxidase, hemin, reconstituted horseradish peroxidase and blank glassy carbon paste electrodes. The cyclic voltammogram of native enzyme paste showed a clear oxidation peak at −50 mV and an increase in the oxidation currents in presence of the substrate was observed. In order to increase the electron transfer kinetics the redox center (hemin) of the enzyme was directly bound onto the electrode followed by reconstitution with apoperoxidase. Hemin bound paste showed a reduction peak at −275 mV and an oxidation peak at ca. −50 mV. Upon reconstitution with apoperoxidase a shift of 175 mV in the reduction peak and ca. 100 mV in the oxidation peak was observed. We have also studied the effect of electron donors like hydroquinone and p-aminobenzoic acid on the modified electrodes. It was observed that the native enzyme showed a clear increase in the electron transfer in the presence of electron donor whereas with the reconstituted enzyme paste the effect of the electron donor was not evident. Low frequency electrochemical impedance spectroscopy was carried out in the frequency range of 10 mHz to 1 kHz in order to study the kinetics of slow processes occurring at the electrode surface. The impedance data of the system in the supporting electrolyte was simulated to an equivalent circuit model, the modified Randles cell circuit for an irreversibly adsorbed species. The data in the presence of substrate was modeled to modified Randles cell circuit with Warburg impedance. The models show a reasonably good fit in the frequency region studied. The unmodified blank paste electrode showed more or less a constant phase angle behavior. From the simulated data the parameters values of the circuit used were obtained which were used to calculate the kinetic parameters.

Photocatalytic transformation of rhodamine B and its effect on hydrogen evolution over Pt/TiO 2 in the presence of electron donors

In the presence of electron donors, oxalic acid, ethanol and disodium ethylene-diamine tetraacetate (EDTA), the photocatalytic transformation of rhodamine B (Rh B) and its effect on photocatalytic hydrogen evolution over Pt/TiO 2 have been examined. UV–Vis and fast atom bombardment mass spectrometry (FABMS) evidences demonstrate that two reductive reaction paths (photocatalytic hydrogenation and additive combination of Rh B with ethanol radical) take place over Pt/TiO 2 in the reaction system. The adsorption behavior of Rh B on Pt/TiO 2 with coexisting electron donors is a key factor for the transformation. As a result of the hydrogenation and the addition of ethanol radical, the rates of photocatalytic hydrogen generation by the donors decrease notably. A possible reaction mechanism is discussed.

Slow photochemical transformations of single dye molecules in polymer environment at room temperature

Wide-field epi-fluorescence microscopic images of single dye molecules in thin films have been studied with 1 s time resolution using a CCD camera. The fluorophores at a concentration of 10 −7 M were doped in a 12 nm thin polystyrene film layered by spin coating on a surface of fused silica. Terrylene, N, N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide and a heterocyclic derivative of the latter were chosen as dyes. The solvents, polymers and the quartz surface were purified carefully in order to get rid of uncontrolled fluorescence. The time dependence of fluorescence intensity for about 100 molecules in each sample has been followed at an excitation intensity of 500 W cm −2. Both the constant or one-step bleaching patterns of fluorescence intensity were found to be rather an exception than a rule for many systems. Most of the molecules exhibit multiple on–off switching, fluctuating or creeping behavior. The population shelving on the T 1 level and the rotational diffusion of the transition dipole cannot serve as universal explanation of such fluorescence intensity dynamics. The fluctuations depend on the composition of the gas atmosphere (air, N 2 or neat O 2), and the presence of electron donors ( N, N, N′N′-tetramethyl- p-phenylenediamine) or acceptors (C 2Cl 6). It has to be surmised that additional phenomena, such as photoisomerisation may be involved in the blinking effect if it persists in nitrogen atmosphere.

Bioaccumulation of palladium by Desulfovibrio desulfuricans

AbstractPalladium uptake by resting cell suspensions of Desulfovibrio desulfuricans NCIMB 8307 was studied without or with electron donor (formate), which gave metal uptake attributable to biosorption of Pd(II) and bioreduction of Pd(II) to Pd(0), respectively. The maximum biosorption capacity of palladium (at pH 2) was up to 196 mg Pd g−1 dry cells (1.85 mmol g−1; approx 20% of the dry weight). Biosorption was to 85% of the maximum in less than 10 min and the biomass was saturated within 30 min. Biosorption of Pd(II) was greater from the chloro‐ than the ammine complex and was inhibited in the presence of excess chloride ion. Bioreductive accumulation of Pd(II) from Pd(NH3)42+ was achieved in the presence of electron donor (formate) but was also inhibited by excess Cl−. Up to 100% of Pd(II) reduction to Pd(0) was achieved within 5 min anaerobically at pH 7 and 30 min at pH 3. Pd(0) was localized on the biomass surface using electron microscopy and was characterized using energy dispersive X‐ray microanalysis (EDAX) and X‐ray diffraction analysis (XRD). Biosorption was Pd‐specific with respect to Pt and Rh using test solutions and acid (aqua regia) leachates from spent automotive catalysts. The total Pd removed from the latter was only 15%, attributable to the inhibitory effect of residual chloride ion from the acidic extractant. Pd biorecovery is limited by the need for an improved extraction technology to minimize the formation of PdCl42− in solution rather than by constraints of the Pd‐accumulating biomass.© 2002 Society of Chemical Industry

Photosensitization of N,N-Dimethylaniline by N,N-Dimethyl-4-nitroaniline as a New Bimolecular Photoinitiation System for Polymerization

Photophisics, photochemical and polymerization activity of N,N-dimethyl-4-nitroaniline (DMNA) alone and in the presence of N,N-dimethyl-4 (DMNA) as coinitiator were analyzed to elucidate the influence of the bifunctional character of this compound on the efficiency and mechanism of generating inititing radicals. The presence of electron donor and acceptor groups in the same moleci=ule determines the capability of DMNA to initiate the polymerozation with higher efficiency and lower initiator consumption as compared with conventional bifunctional ketones. Detailed studies of the spectroscopy of both reactants were carried out providing experimental evidence of photosensitization of DMA throught excitation energy transfer from DMNA. The consumption rate of both reactants reveals that up to 165 molecules of DMA are consumed for each DMNA molecule phoreduced in spit of the low photoreduction quantum yield of DMNA induced by DMA. The photosensitization of DMA, working concurrently to the photoreduction reaction, enhances the polymerization efficiency as compared with other nitroaromatic initiators.