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Electron Transfer (electron + transfer)
Kinds of Electron Transfer Terms modified by Electron Transfer Selected AbstractsHeterogeneous Electron Transfer and Oxygen Reduction Reaction at Nanostructured Iron(II) Phthalocyanine and Its MWCNTs NanocompositesELECTROANALYSIS, Issue 9 2010Solomon Abstract Electron transfer and oxygen reduction dynamics at nanostructured iron(II) phthalocyanine/multi-walled carbon nanotubes composite supported on an edge plane pyrolytic graphite electrode (EPPGE-MWCNT-nanoFePc) platform have been reported. All the electrodes showed the category 3 diffusional behaviour according to the Davies,Compton theoretical framework. Both MWCNTs and MWCNT-nanoFePc showed huge current responses compared to the other electrodes, suggesting the redox processes of trapped redox species within the porous layers of MWCNTs. Electron transfer process is much easier at the EPPGE-MWCNT and EPPGE-MWCNT-nanoFePc compared to the other electrodes. The best response for oxygen reduction reaction was at the EPPGE-MWCNT-nanoFePc, yielding a 4-electron process. [source] Direct Electron Transfer and Electrocatalysis of Hemoglobin on Chitosan-TiO2 Nanorods-Glass Carbon ElectrodeELECTROANALYSIS, Issue 20 2008Xiaoling Xiao Abstract The direct electron transfer between hemoglobin (Hb) and the glassy carbon electrode (GC) can be readily achieved via a high biocompatible composite system based on biopolymer chitosan (CHT) and TiO2 nanorods (TiO2 -NRs). TiO2 -NRs greatly promote the electron transfer between Hb and GC, which contribute to the higher redox peaks. UV-vis spectra result indicated the Hb entrapped in the composite film well keep its native structure. The immobilized Hb remains its bioelectrocatalytical activity to the reduction of H2O2 with a lower detection limit. A novel, sensitive, reproducible and stable electrochemical biosensing platform of H2O2 based on Hb-TiO2 -CHT electrode is explored. [source] Oligonucleotide Duplexes with Tethered Photoreactive Ruthenium(II) Complexes: Influence of the Ligands and Their Linker on the Photoinduced Electron Transfer and Crosslinking Processes of the Two StrandsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 4 2009Stéphanie Deroo Abstract The photoreactivity of new RuII -oligonucleotide conjugates is investigated in the presence of their complementary strands. The goal is to determine the origins of different effects of parameters that control the photocrosslinking process of the two strands. Therefore, two RuII compounds, either [Ru(tap)3]2+or [Ru(tap)2phen]2+ (tap = 1,4,5,8-tetraazaphenanthrene, phen = 1,10-phenanthroline) with different oxidation powers, were tethered with different linkers to either the 5,- or 3,-phosphate end of the probe strand before hybridization with the complementary strand. These systems were studied by time-resolved emission spectroscopy, UV/Vis absorption experiments, PAGE and MS (ESI) analyses. The best yields of photocrosslinking (45,%) obtained with [Ru(tap)3]2+ tethered to the 3,-position are due to (i) a higher oxidation power of the complex and (ii) its attachment at the 3,-position. Indeed, this tethering favours the interaction of the Ru compound with the duplex and, therefore, inhibits its photodechelation. This work allows better design of sequence-specific DNA photodamaging agents prior to biological applications.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Synthesis and Characterization of Radical Cations Derived from Mono- and Biferrocenyl-Substituted 2-Aza-1,3-butadienes: A Study of the Influence of an Asymmetric and Oxidizable Bridge on Intramolecular Electron TransferEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 12 2005Vega Lloveras Abstract The synthesis and study of structural and electronic properties of mono-ferrocenyl ,-conjugated complexes 5a,d, whose electronic characteristics have been systematically varied by introducing an electron-donating or electron-withdrawing substituent either at the 1-position or at the 4-position of the 2-aza-1,3-butadiene moiety linked to the ferrocenyl unit, are presented. The structural and electronic properties of the homobimetallic complex 5f, with two ferrocene units linked through the asymmetric and oxidizable 2-aza-1,3-butadiene bridge, is also reported. The crystal structures of complexes 5b, 5d, and 5f show a large degree of conjugation in this family of compounds. Complexes 5 show a rich electrochemical behavior due both to the oxidation of ferrocenyl units and the 2-aza-1,3-butadiene bridge, as revealed by cyclic voltammetry. Radical cations 5+· were prepared from 5 by coulometric oxidations following their generation by absorption spectroscopy. The electronic properties of all reported neutral and oxidized ,-conjugated complexes have been investigated by means of UV/Vis,near-IR, EPR and 57Fe Mössbauer spectroscopy. The detailed study of mono-oxidized species 5a+·,5f+· has permitted the determination of the influence of an asymmetric bridge with an electroactive character on the intramolecular electron transfer (IET) phenomenon, thus demonstrating that the 2-aza-1,3-butadiene bridge promotes the IET between the two metallic units of 5f+· through two different pathways. The experimental data and conclusions are supported by DFT computations (B3LYP/3-21G*) and time-dependent DFT methods. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Multilayer Hybrid Films Consisting of Alternating Graphene and Titania Nanosheets with Ultrafast Electron Transfer and Photoconversion PropertiesADVANCED FUNCTIONAL MATERIALS, Issue 22 2009Kiran Kumar Manga Abstract Alternating graphene (G) and titania (Ti0.91O2) multilayered nanosheets are fabricated using layer-by-layer electrostatic deposition followed by UV irradiation. Successful assemblies of graphene oxide (GO) and titania nanosheets in sequence with polyethylenimine as a linker is confirmed by UV,vis absorption and X-ray diffraction. Photocatalytic reduction of GO into G can be achieved upon UV irradiation. Ultrafast photocatalytic electron transfer between the titania and graphene is demonstrated using femtosecond transient absorption spectroscopy. Efficient exciton dissociation at the interfaces coupled with cross-surface charge percolation allows efficient photocurrent conversion in the multilayered Ti0.91O2/G films. [source] Proton-Coupled Electron Transfer of Unsaturated Fatty Acids and Mechanistic Insight into LipoxygenaseHELVETICA CHIMICA ACTA, Issue 10 2006Shunichi Fukuzumi Abstract A proton-coupled electron transfer (PCET) process plays an important role in the initial step of lipoxygenases to produce lipid radicals which can be oxygenated by reaction with O2 to yield the hydroperoxides stereoselectively. The EPR spectroscopic detection of free lipid radicals and the oxygenated radicals (peroxyl radicals) together with the analysis of the EPR spectra has revealed the origin of the stereo- and regiochemistry of the reaction between O2 and linoleyl (=,(2Z)-10-carboxy-1-[(1Z)-hept-1-enyl]dec-2-enyl) radical in lipoxygenases. The direct determination of the absolute rates of H-atom-transfer reactions from a series of unsaturated fatty acids to the cumylperoxyl (=,(1-methyl-1-phenylethyl)dioxy) radical by use of time-resolved EPR at low temperatures together with detailed kinetic investigations on both photoinduced and thermal electron-transfer oxidation of unsaturated fatty acids provides the solid energetic basis for the postulated PCET process in lipoxygenases. A strong interaction between linoleic acid (=,(9Z,12Z)-octadeca-9,12-dienoic acid) and the reactive center of the lipoxygenases (FeIIIOH) is suggested to be involved to make a PCET process to occur efficiently, when an inner-sphere electron transfer from linoleic acid to the FeIII state is strongly coupled with the proton transfer to the OH group. [source] Photoinduced Electron Transfer in Zinc Phthalocyanine Loaded on Single-Walled Carbon Nanohorns in Aqueous SolutionADVANCED MATERIALS, Issue 43 2009Atula S. D. Sandanayaka Notable electronic communication within ZnPc-SWNHox nanoensembles, where ZnPc is zinc phthalocyanine and SWNHox is an oxidized single-walled nanohorn, in both the ground and excited states is revealed by steady-state absorption and fluorescence spectroscopy measurements. The details of electron transfer reported here with time-resolved absorption and fluorescence measurements may broaden the use of SWNHox nanoensembles in photochemistry and photobiology. [source] Gas Sensors: Room-Temperature Gas Sensing Based on Electron Transfer between Discrete Tin Oxide Nanocrystals and Multiwalled Carbon Nanotubes (Adv. Mater.ADVANCED MATERIALS, Issue 24 200924/2009) Hybrid nanostructures consisting of multiwalled carbon nanotubes (CNTs) uniformly coated with SnO2 nanocrystals can be used as a novel gas sensing platform, exhibiting high sensitivity to low-concentration gases (NO2, H2, and CO) at room temperature, report Junhong Chen and co-workers on p. 2487. The hybrid nanomaterial provides a new opportunity to engineer sensing devices through electronic transfer between the nanocrystals and the CNT. [source] Solution-Processable Near-IR Photodetectors Based on Electron Transfer from PbS Nanocrystals to Fullerene DerivativesADVANCED MATERIALS, Issue 6 2009Krisztina Szendrei Nanocrystal/fullerene derivative inorganic,organic hybrid photodetectors exhibiting high detectivity for near-IR wavelengths and a linear power dependence are produced. The ultrafast electron transfer from the PbS crystals to the fullerene opens a new route to obtaining efficient photodetectors that are appealing, cost-effective alternatives to the currently available technology. [source] Redox Reactions and Electron Transfer Across the Red Cell MembraneIUBMB LIFE, Issue 7 2003Eleanor Kennett Abstract Plasma membrane electron transport systems appear to be ubiquitous. These systems are implicated in hormone signal transduction, cell growth and differentiation events as well as protection from oxidative stress. The red blood cell is constantly exposed to oxidative stress; protection against the reactive species generated during this process may be the main role of its membrane electron transport systems. Membrane redox activity has been studied for over three-quarters of a century, and yet many questions remain regarding its identity and likely roles: are electron transfers by distinct and specific mechanisms; what are the physiological donors and acceptors; and how do these systems affect metabolism? Current evidence suggests that the human erythrocyte membrane contains a number of distinct electron transfer systems, some of which, at least, involve membrane proteins, and NADH or ascorbate as electron donors. The activity of these systems appears to be closely related to the metabolic state of the cell, suggesting that mediation of reducing equivalents across the plasma membrane allows redox buffering of each environment, intra- and extracellular, by the other. We have decided to study this from a new perspective, NMR spectroscopy the area of our own technical expertise, hence this review is slanted towards this more recent analysis. IUBMB Life, 55: 375-385, 2003 [source] Conducting Polymer Enzyme Alloys: Electromaterials Exhibiting Direct Electron TransferMACROMOLECULAR RAPID COMMUNICATIONS, Issue 14 2010Brianna C. Thompson Abstract Glucose oxidase (GOx) is an important enzyme with great potential application for enzymatic sensing of glucose, in implantable biofuel cells for powering of medical devices in vivo and for large-scale biofuel cells for distributed energy generation. For these applications, immobilisation of GOx and direct transfer of electrons from the enzyme to an electrode material is required. This paper describes synthesis of conducting polymer (CP) structures in which GOx has been entrained such that direct electron transfer is possible between GOx and the CP. CP/enzyme composites prepared by other means show no evidence of such "wiring". These materials therefore show promise for mediator-less electronic connection of GOx into easily produced electrodes for biosensing or biofuel cell applications. [source] Removal of the PsaF Polypeptide Biases Electron Transfer in Favor of the PsaB Branch of Cofactors in Triton X-100 Photosystem I Complexes from Synechococcus sp.PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2008PCC 700 Continuous wave (CW) and transient electron paramagnetic resonance studies have implied that when PsaF is removed genetically, the double reduction of A1A is facile, the lifetime of A1A, is shorter and the ratio of fast to slow kinetic phases increases in PS I complexes isolated with Triton X-100 (Van der Est, A., A. I. Valieva, Y. E. Kandrashkin, G. Shen, D. A. Bryant and J. H. Golbeck [2004] Biochemistry43, 1264,1275). Changes in the lifetimes of A1A, and A1B, are characteristic of mutants involving the quinone binding sites, but changes in the relative amplitudes of A1A, and A1B, are characteristic of mutants involving the primary electron acceptors, A0A and A0B. Here, we measured the fast and slow phases of electron transfer from A1B, and A1A, to FX in psaF and psaE psaF null mutants using time-resolved CW and pump-probe optical absorption spectroscopy. The lifetime of the fast kinetic phase was found to be unaltered, but the lifetime of the slow kinetic phase was shorter in the psaF null mutant and even more so in the psaE psaF null mutant. Concomitantly, the amplitude of the fast kinetic phase increased by a factor of 1.8 and 2.0 in the psaF and psaE psaF null mutants, respectively, at the expense of the slow kinetic phase. The change in ratio of the fast to slow kinetic phases is explained as either a redirection of electron transfer through A1B at the expense of A1A, or a shortening of the lifetime of A1A, to become identical to that of A1B,. The constant lifetime and the characteristics of the near-UV spectrum of the fast kinetic phase favor the former explanation. A unified hypothesis is presented of a displacement of the A-jk(1) ,-helix and switchback loop, which would weaken the H-bond from Leu722 to A1A, accounting for the acceleration of the slow kinetic phase, as well as weaken the H-bond from Tyr696 to A0A, accounting for the bias of electron transfer in favor of the PsaB branch of cofactors. [source] Involvement of Electron Transfer in the Photoreaction of Zebrafish Cryptochrome-DASH,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2008Kazunori Zikihara Photoreaction of a blue-light photoreceptor Cryptochrome-DASH (Cry-DASH), a new member of the Cryptochrome family, from zebrafish was studied by UV,visible absorption spectroscopy in aqueous solutions at 293 K. Zebrafish Cry-DASH binds two chromophores, a flavin adenine dinucleotide (FAD) and a N5,N10-methenyl-5,6,7,8-tetrahydrofolate (MTHF) noncovalently. The bound FAD exists in the oxidized form (FADox) in the dark. Blue light converts FADox to the neutral radical form (FADH?). Formed FADH? is transformed to the fully reduced form FADH2 (or FADH,) by successive light irradiation, or reverts to FADox. FADH2 (or FADH,) reverts to FADH? or possibly to FADox directly. The effect of dithiothreitol suggests a possible electron transfer between FAD in zebrafish Cry-DASH and reductants in the external medium. This is the first report on the photoreaction pathway and kinetics of a vertebrate Cry-DASH family protein. [source] Cationic Dye,sensitized Degradation of Sodium Hyaluronate Through Photoinduced Electron Transfer in the Upper Excited State,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 3 2001Masanobu Kojima ABSTRACT The formation of ground-state complexes of methylene blue (MB) and thionine (TN) with sodium hyaluronate (NaHA) was clearly observed by means of absorption spectra in aqueous solution. Irradiation of the complexes using 313 nm light caused significant degradation of NaHA under oxygen and argon. However, the use of visible light over 400 nm, which gives the lowest excited singlet state of the cationic dyes, caused no degradation. MB and TN were more efficient sensitizers for the degradation of NaHA than rose bengal (RB), although RB is a more efficient singlet oxygen (1O2) sensitizer than the cationic dyes. Under similar conditions the polysaccharides with carboxyl groups, such as alginic acid and polygalacturonic acid, also photodecomposed. However, the polysaccharides without carboxyl groups, such as pullulan and methyl cellulose, did not. The irradiation of the polysaccharides in the presence of powdered titanium dioxide as a photocatalyst to generate the hydroxyl radical (·OH) in aerated aqueous solution caused the fragmentation of all the polymers. It was confirmed that methyl viologen, an electron-accepting sensitizer, formed a charge-transfer complex with NaHA, the irradiation of which caused the efficient degradation of NaHA. In the presence of ,- and ,-cyclodextrins the MB- and TN-sensitized photodegradation of NaHA was markedly suppressed. This was probably due to the formation of the inclusion complexes comprising the cationic dyes and the cyclodextrins. On the basis of the results obtained we propose that the cationic dye,sensitized degradation of NaHA involves a photoinduced electron-transfer process between the upper excited dyes and the ground-state NaHA and that ·OH and 1O2 do not participate in the degradation. [source] Reversible Thermally and Photoinduced Electron Transfer in a Cyano-Bridged {Fe2Co2} Square Complex,ANGEWANDTE CHEMIE, Issue 22 2010Yuanzhu Zhang Dr. Spektroskopische, (photo)magnetische und Strukturstudien belegen reversible thermische und lichtinduzierte Änderungen der optischen und magnetischen Eigenschaften für den abgebildeten {Fe2Co2}-Cyanometallat-Vierkernkomplex. Die beobachtete Bistabilität ist das Ergebnis eines intramolekularen Elektronentransfers, wie er auch in dreidimensionalen Co/Fe-Preußischblau-Derivaten auftritt. [source] Unambiguous Observation of Electron Transfer from a Zeolite Framework to Organic Molecules,ANGEWANDTE CHEMIE, Issue 36 2009Lu Li Wer bekommt das Elektron? Sauerstoffatome, die zwei Siliciumatome in einem protonierten Zeolithen verbrücken, können durch Reaktion mit einem Alkylbromid oxidiert werden, wodurch ein Zeolithgerüst mit paramagnetischen Zentren entsteht (siehe Strukturen und EPR-Spektrum; rot O, blau Si, violett Al, grün C, gelb Br, weiß H). Ein solcher Elektronentransfer vom Zeolithgerüst kann die Produktbildung in säurekatalysierten Reaktionen mit Zeolithkatalysatoren erklären. [source] Highly Diastereoselective Ionic/Radical Domino Reactions: Single Electron Transfer Induced Cyclization of Bis-sulfoxides.CHEMINFORM, Issue 47 2007Jean-Philippe Goddard Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Electron Transfer in Uranyl(VI)-Uranyl(V) Complexes in Solution.CHEMINFORM, Issue 42 2004Timofei Privalov Abstract For Abstract see ChemInform Abstract in Full Text. [source] ChemInform Abstract: Parallel (Face-to-Face) versus Perpendicular (Edge-to-Face) Alignment of Electron Donors and Acceptors in Fullerene Porphyrin Dyads: The Importance of Orientation in Electron Transfer.CHEMINFORM, Issue 3 2002Dirk M. Guldi Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Highly Stereoselective Carbon-Functionalization of Electron-Deficient Arylalkenes by Use of Organosilicon Compounds via Photoinduced Electron Transfer.CHEMINFORM, Issue 27 2001Tomoo Hayamizu Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Diastereoselective Tandem Addition,Cyclization Reactions of Unsaturated Tertiary Amines Initiated by Photochemical Electron Transfer (PET).CHEMINFORM, Issue 17 2001Samuel Bertrand Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Systematic Investigation of Photoinduced Electron Transfer Controlled by Internal Charge Transfer and Its Consequences for Selective PdCl2 CoordinationCHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2010Thomas Schwarze Abstract Fluoroionophores of fluorophore,spacer,receptor format were prepared for detection of PdCl2 by fluorescence enhancement. The fluorescent probes 1,13 consist of a fluorophore group, an alkyl spacer and a dithiomaleonitrile PdCl2 receptor. First, varying the length of the alkylene spacer (compounds 1,3) revealed a dominant through-space pathway for oxidative photoinduced electron transfer (PET) in CH2 -bridged dithiomaleonitrile fluoroionophores. Second, fluorescent probes 4,9 containing two anthracene or pyrene fragments connected through CH2 bridges to the dithiomaleonitrile unit were synthesized. Modulation of the oxidation potential (EOx) through electron-withdrawing or -donating groups on the anthracene moiety regulates the thermodynamic driving force for oxidative PET (,GPET) in bis(anthrylmethylthio)maleonitriles and therefore the fluorescence quantum yields (,f), too. The new concept was confirmed and transferred to pyrenyl ligands, and fluorescence enhancements (FE) greater than 3.2 in the presence of PdCl2 were achieved by 7 and 8 (FE=5.4 and 5.2). Finally, for comparison, monofluorophore ligands 10,13 were synthesized. [source] Distinct Oxygenation Difference between Manganese(IV) Hydroxo and Oxo Moieties: Electron Transfer versus Concerted Oxygen TransferCHEMISTRY - A EUROPEAN JOURNAL, Issue 43 2009Aihua Xu And the winner is? Through oxygenation of triarylphosphines by the MnIVOH and MnIVO functional groups in the corresponding [MnIV(Me2EBC)] complexes (Me2EBC: 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane), it was found that the oxygenation by the MnIVOH group in [MnIV(Me2EBC)(OH)(H2O)]3+ proceeds by an electron-transfer process, whereas a concerted oxygen transfer occurs in the case of oxygenation by the MnIVO group in [MnIV(Me2EBC)(O)2] (see scheme). [source] Carbon Nanotube,Acridine Nanohybrids: Spectroscopic Characterization of Photoinduced Electron TransferCHEMISTRY - A EUROPEAN JOURNAL, Issue 15 2009Nicolas Mackiewicz Dr. Abstract Change happens: Acridine,carbon nanotube nanohybrids were built (see figure) and their photoinduced electron-transfer properties investigated, showing variable behaviour depending on the acridine partner. Single-walled carbon nanotubes (NT) were covalently functionalized with either 9-phenyl acridine (PhA) or 10-methyl-9-phenyl acridinium (PhMeA+). Absorption and fluorescence properties of acridine derivatives tethered to the nanotubes were studied in homogeneous dispersions. Exciplex emission was observed for NT functionalized with 9-phenylacridine. This phenomenon was attributed to an "intramolecular" interaction between excited phenyl acridine and carbon nanotubes. Interestingly, reverse photoinduced electron transfer from the nanotube to 10-methyl-9-phenylacridinium was detected for the NT-PhMeA+ nanohybrid. This electron transfer led to a strong quenching of the acridinium fluorescence and to the formation of a metastable acridine radical. Evidence for the formation of this radical was obtained by ESR studies. [source] Single-Step Electron Transfer on the Nanometer Scale: Ultra-Fast Charge Shift in Strongly Coupled Zinc Porphyrin,Gold Porphyrin DyadsCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2008Jérôme Fortage Dr. Abstract The synthesis, electrochemical properties, and photoinduced electron transfer processes of a series of three novel zinc(II),gold(III) bisporphyrin dyads (ZnPSAuP+) are described. The systems studied consist of two trisaryl porphyrins connected directly in the meso position via an alkyne unit to tert -(phenylenethynylene) or penta(phenylenethynylene) spacers. In these dyads, the estimated center to center interporphyrin separation distance varies from 32 to 45,Å. The absorption, emission, and electrochemical data indicate that there are strong electronic interactions between the linked elements, thanks to the direct attachment of the spacer on the porphyrin ring through the alkyne unit. At room temperature in toluene, light excitation of the zinc porphyrin results in almost quantitative formation of the charge shifted state .+ZnPSAuP., whose lifetime is in the order of hundreds of picoseconds. In this solvent, the charge-separated state decays to the ground state through the intermediate population of the zinc porphyrin triplet excited state. Excitation of the gold porphyrin leads instead to rapid energy transfer to the triplet ZnP. In dichloromethane the charge shift reactions are even faster, with time constants down to 2,ps, and may be induced also by excitation of the gold porphyrin. In this latter solvent, the longest charge-shifted lifetime (,=2.3,ns) was obtained with the penta-(phenylenethynylene) spacer. The charge shift reactions are discussed in terms of bridge-mediated super-exchange mechanisms as electron or hole transfer. These new bis-porphyrin arrays, with strong electronic coupling, represent interesting molecular systems in which extremely fast and efficient long-range photoinduced charge shift occurs over a long distance. The rate constants are two to three orders of magnitude larger than for corresponding ZnPAuP+ dyads linked via meso -phenyl groups to oligo-phenyleneethynylene spacers. This study demonstrates the critical impact of the attachment position of the spacer on the porphyrin on the electron transfer rate, and this strategy can represent a useful approach to develop molecular photonic devices for long-range charge separations. [source] A Radical-Anion Chain Mechanism Initiated by Dissociative Electron Transfer to a Bicyclic Endoperoxide: Insight into the Fragmentation Chemistry of Neutral Biradicals and Distonic Radical AnionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2008David Abstract The electron-transfer (ET) reduction of two diphenyl-substituted bicyclic endoperoxides was studied in N,N -dimethylformamide by heterogeneous electrochemical techniques. The study provides insight into the structural parameters that affect the reduction mechanism of the OO bond and dictate the reactivity of distonic radical anions, in addition to evaluating previously unknown thermochemical parameters. Notably, the standard reduction potentials and the bond dissociation energies (BDEs) were evaluated to be ,0.55±0.15,V and 20±3,kcal,mol,1, respectively, the last representing some of the lowest BDEs ever reported. The endoperoxides react by concerted dissociative electron transfer (DET) reduction of the OO bond yielding a distonic radical-anion intermediate. The reduction of 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1) results in the quantitative formation of 1,4-diphenylcyclohex-2-ene- cis -1,4-diol by an overall two-electron mechanism. In contrast, ET to 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]octane (2) yields 1,4-diphenylcyclohexane- cis -1,4-diol as the major product; however, in competition with the second ET from the electrode, the distonic radical anion undergoes a ,-scission fragmentation yielding 1,4-diphenyl-1,4-butanedione radical anion and ethylene in a mechanism involving less than one electron. These observations are rationalized by an unprecedented catalytic radical-anion chain mechanism, the first ever reported for a bicyclic endoperoxide. The product ratios and the efficiency of the catalytic mechanism are dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for ,-scission fragmentation is presented, and provides insight into why the fragmentation chemistry of distonic radical anions is different from analogous neutral biradicals. [source] Selective Inclusion of Electron-Donating Molecules into Porphyrin Nanochannels Derived from the Self-Assembly of Saddle-Distorted, Protonated Porphyrins and Photoinduced Electron Transfer from Guest Molecules to Porphyrin DicationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 31 2007Takahiko Kojima Prof. Abstract A doubly protonated hydrochloride salt of a saddle-distorted dodecaphenylporphyrin (H2DPP), [H4DPPP]Cl2, forms a porphyrin nanochannel (PNC). X-ray crystallography was used to determine the structure of the molecule, which revealed the inclusion of guest molecules within the PNC. Electron-donating molecules, such as p -hydroquinone and p -xylene, were selectively included within the PNC in sharp contrast to electron acceptors, such as the corresponding quinones, which were not encapsulated. This result indicates that the PNC can recognize the electronic character and steric hindrance of the guest molecules during the course of inclusion. ESR measurements (photoirradiation at ,>340,nm at room temperature) of the PNC that contains p -hydroquinone, catechol, and tetrafluorohydroquinone guest molecules gave well-resolved signals, which were assigned to cation radicals formed without deprotonation based on results from computer simulations of the ESR spectra and density functional theory (DFT) calculations. The radicals are derived from photoinduced electron transfer from the guest molecules to the singlet state of H4DPP2+. Transient absorption spectroscopy by femtosecond laser flash photolysis allowed us to observe the formation of 1(H4DPP2+)*, which is converted to H4DPP+. by electron transfer from the guest molecules to 1(H4DPP2+)*, followed by fast disproportionation of H4DPP+., and charge recombination to give diamagnetic species and the triplet excited state 3(H4DPP2+)*, respectively. [source] Electron Transfer to Sulfides and Disulfides: Intrinsic Barriers and Relationship between Heterogeneous and Homogeneous Electron-Transfer KineticsCHEMISTRY - A EUROPEAN JOURNAL, Issue 28 2007Belèn Meneses Abstract The electron-acceptor properties of series of related sulfides and disulfides were investigated in N,N -dimethylformamide with homogeneous (redox catalysis) and/or heterogeneous (cyclic voltammetry and convolution analysis) electrochemical techniques. The electron-transfer rate constants were determined as a function of the reaction free energy and the corresponding intrinsic barriers were determined. The dependence of relevant thermodynamic and kinetic parameters on substituents was assessed. The kinetic data were also analyzed in relation to corresponding data pertaining to reduction of diaryl disulfides. All investigated reductions take place by stepwise dissociative electron transfer (DET) which causes cleavage of the CalkylS or SS bond. A generalized picture of how the intrinsic electron-transfer barrier depends on molecular features, ring substituents, and the presence of spacers between the frangible bond and aromatic groups was established. The reduction mechanism was found to undergo a progressive (and now predictable) transition between common stepwise DET and DET proceeding through formation of loose radical anions. The intrinsic barriers were compared with available results for ET to several classes of dissociative- and nondissociative-type acceptors, and this led to verification that the heterogeneous and the homogeneous data correlate as predicted by the Hush theory. [source] Photoinduced Electron Transfer in ,-Cyclodextrin-Based Supramolecular Dyads: A Free-Energy-Dependence StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 25 2006Bijitha Balan Abstract Photoinduced electron transfer (PET) between ,-cyclodextrin-appended pyrene (PYCD) and a few acceptor molecules was studied in aqueous solutions. The pyrene moiety in PYCD is located above the narrower rim of the ,-CD and is fully exposed to water. The acceptors are monocyclic organic molecules and, upon dissolution in water in the presence of PYCD, a fraction of the donor,acceptor systems is present as supramolecular dyads and the remaining fraction as free molecules. Free-energy-dependence studies showed that electron transfer in the supramolecular dyads follows the Marcus equation. The donor,acceptor coupling and the reorganization energy were determined from fits of the data to the Marcus equation. The electronic coupling was found to be similar to those reported for hydrogen-bonded systems. It appears that the actual ,out values are somewhat lower than values calculated with the continuum model. The experimental design has also allowed, for the first time, a visual demonstration of the inverted region on the basis of the raw fluorescence lifetime data. [source] Dynamic Chemical Devices: Photoinduced Electron Transfer and Its Ion-Triggered Switching in Nanomechanical Butterfly-Type Bis(porphyrin)terpyridines,CHEMISTRY - A EUROPEAN JOURNAL, Issue 7 2006Myriam Linke-Schaetzel Dr. Abstract A series of butterfly-type molecular constructs has been prepared in good yield by using a double Stille coupling synthetic protocol. They are composed of a terpyridine (terpy) scaffold and two wings composed of appended porphyrins that are capable of switching from an extended W geometry to a compact U geometry upon cation coordination of the terpy unit. The porphyrin moieties exist in the constructs either as free bases or they can be sequentially metallated, thus giving rise to wings of different "colours". Stationary and time-resolved emission studies of the HZn, ZnAu and Zn2Au constructs show that the electronic properties are strongly dependent on the geometry. In the extended W conformation an energy-transfer process is seen from the free base to the Zn-metallated porphyrin. In the U conformation in Zn2Au the donor luminescence resulting from the singlet excited state of the Zn wing is strongly, quenched not only due to the heavy atom effect but also due to a fast electron-transfer process to the ground state of the Au wing. Furthermore, the binding of (,,,)-diamine substrates to the ZnII,porphyrin sites can also influence the conformation of the system. For the Zn2Zn construct, single-crystal diffraction experiments with synchrotron radiation allowed the structure to be solved by direct methods and fully refined; it shows the expected U conformation. The central Zn atom is six-coordinate, whereby the zinc atom is coordinated by the ,3 -terpy ligand as well by monodentate and semi-chelating acetate anions. The structure is made rigid by hydrogen bonds involving the aqua ligands on the outer Zn centres and acetate oxygen atoms. The present system thus represents a double-trigger-modulated optomechanical switching device with selective substrate binding for either metal atoms or tailored ligands. Both energy- and electron-transfer processes can be controlled opening a means of improving the on/off ratio in future constructs. Une série d'architectures moléculaires, présentant une forme de papillon, a été préparée avec de bons rendements en utilisant une méthodologie synthétique comprenant comme étape clé un double couplage de Stille. Ces architectures sont composées d'un squelette terpyridine (terpy), qui compose le torse du papillon, sur lequel deux ailes porphyriniques ont été greffées; la géométrie peut varier entre une conformation étendue en forme de W, et une conformation compacte en forme de U, dû à la complexation du torse terpy par un cation de taille et coordination adéquate. Les ailes porphyriniques se présentent soit comme bases libres ou, peuvent être métallées successivement avec différentes métaux, pouvant ainsi avoir différentes "couleurs". Des études de fluorescence non seulement stationnaire mais aussi résolues dans le temps sur les architectures HZn, ZnAu et Zn2Au, montrent que les propriétés électroniques sont très dépendantes de la géométrie adoptée. Dans la conformation W étendue, nous avons mis en évidence un processus de transfert d'énergie de la porphyrine base libre vers la porphyrine metalée avec du Zn. Dans la conformation U de Zn2Au la luminescence de l'aile donneuse, à cause de l'état excité singulet, est fortement éteinte, non seulement à cause de l'effet d'atome lourd, mais aussi à cause d'un processus de transfert d'électron vers l'état fondamental de l'aile metalée avec de l'or. De plus, par compléxation avec des (,,,)-diamines des atomes de ZnIIdans les tetrapyrroles porphyrinques, la conformation induite du système peut être profondément influencée. Pour le composé Zn2Zn, que nous avons pu obtenir en monocristaux, des expériences de diffraction en utilisant comme source lumineuse un synchrotron, ont fourni des données qui ont permis de résoudre la structure par des méthodes directes et de la raffiner pour montrer la conformation en U attendue. L'atome central de Zn a une coordination de six dont trois provenant du ligand ,3 -terpy, et les autres de deux anions acetates, un monodenté et l'autre semi-chelatizé. La structure est ligotée par des multiples liaisons hydrogène entre des ligands aqueux sur les atomes de Zn externes et les atomes d'oxygène des anions acetates. Le système présenté est un double effecteur qui montre une commutation opto-mechanique dirigée soit par des ions métalliques, soit par d'autres ligands de taille adaptée. En même temps, les processus de transfert d'énergie et d'électrons peuvent être commuté en laissant de la place pour améliorer le rapport "on/off" dans de futures architectures. Abstract in Romanian:O serie de arhitecturi moleculare având o form, de fluture au fost preparate cu randamente bune utilizând o metodologie sintetic, cuprinzând ca etap, cheie un dublu cuplaj Stille. Aceste architecturi sunt compuse dintr-un schelet terpiridinic (terpy) alc,tuind trupul fluturelui, la care au fost implementate dou, aripi porfirinice care sunt capabile sa varieze între o conforma,ie extins, în forma de liter, W ,i o conformai,ie compact, în form, de litera U, datorit, complexarii trupului terpy printr-un cation de talie ,i coordina,ie adecvat,. Aripile porfirinice se prezintã fie ca base libere sau, datorita unei metalari succesive cu diferite metale, pot avea diferite "culori". Studii de fluorescen,a atât sta,ionar, cât ,i rezolvat, în timp, asupra arhitecturilor HZn, ZnAu ,i Zn2Au, arat, c, proprit,,ile electronice sunt foarte dependente de geometria adoptat,. In conforma,ia W extins,, am putut pune în eviden,, un proces de transfer de energie dinspre baza liber, c,tre porfirina metalat, cu Zn. In conforma,ia U a Zn2Au luminescen,a aripei donoare, datorate st,rii singulet excitate, este puternic stins, nu numai datorit, efectului de atom greu, cât ,i a unui proces de transfer de electron c,tre starea fundamental, a aripei metalat, cu aur. Pe deasupra, complexând (,,,)-diamine c,tre atomii de ZnIIîntre tetrapirolii porfirinici, sunt induse profunde influen,e asupra conforma,iei sistemului. Pentru compusul Zn2Zn, care a putut fi crescut monocristalin, experimente de difrac,ie folosind ca surs, luminoas, un sincrotron au condus la un set de date care a permis ca structura sa fie elucidat, prin metode directe ,i rafinat,, ar,tând conforma,ia în U a,teptat,. Atomul central de Zn are o coordina,ie de sase unde pe lang, lignadul ,3 -terpy, atomul de Zn este coordinat de c,tre doi anioni acetat, unul monodentat, iar cel,lalt semi-chelatizant. Structura este br,zdat, de leg,turi de hidrogen care implic, liganzii apo,i situa,i pe centrii de Zn exteriori, ,i atomii de oxigen din ionii acetat. Sistemul prezentat este un dutblu efector, ar,tând comutare opto-mecanic,, datorat, fie lig,rii prin ioni metalici, fie prin al,i liganzi perfect croitori,i. In acel,i timp, atât procese de transfer de energie, cât ,i de electroni, pot fi perfect comutate, l,sând loc pentru imbun,t,,irea raportului "on/off" în arhitecturi viitoare. [source] | |||||||||||||||||||||||||||||||||||||||||||