Showing papers on "Radical ion published in 2007"
TL;DR: L-tyrosine and L-Dopa had an effect on DPPH radical scavenging, ABTS radical scavenming, superoxide anion radical scaventing, H2O2 scavenging), total ferric ions reducing power and metal chelating on ferrous ions activities.
Abstract: Phenolic compounds are interesting because of their antioxidant properties. In the present study, the antioxidant properties of L-tyrosine as a monophenolic and L-Dopa as a diphenolic amino acid were investigated by using different antioxidant assays: (i) 1,1-diphenyl-2-picryl-hydrazyl free radical (DPPH*) scavenging; (ii) 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation decolorization assay; (iii) total antioxidant activity by ferric thiocyanate method; (iv) ferric ions (Fe3+) reducing power; (v) superoxide anion radical (O2*-) scavenging; (vi) hydrogen peroxide (H2O2) scavenging, and (vii) ferrous ions (Fe2+) chelating activities. Butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), alpha-tocopherol and trolox, a water-soluble analogue of tocopherol, were used as the reference antioxidant compounds. At the same concentration (20 microg/mL), L-tyrosine and L-Dopa showed 30.6 and 67.9% inhibition of lipid peroxidation of linoleic acid emulsion, respectively. On the other hand, BHA, BHT, alpha-tocopherol and trolox indicated inhibitions of 74.4, 71.2, 54.7 and 20.1% on the peroxidation of linoleic acid emulsion, respectively, at the above-mentioned concentration. In addition, L-tyrosine and L-Dopa had an effect on DPPH radical scavenging, ABTS radical scavenging, superoxide anion radical scavenging, H2O2 scavenging, total ferric ions reducing power and metal chelating on ferrous ions activities.
327 citations
TL;DR: The covalent connection of the electron acceptor C 60 to p-quinonoid pi-extended tetrathiafulvalenes (exTTFs) has allowed for the preparation of new photo- and electroactive conjugates able to act as artificial photosynthetic systems and active molecular materials in organic photovoltaics.
Abstract: The covalent connection of the electron acceptor C60 to p-quinonoid π-extended tetrathiafulvalenes (exTTFs) has allowed for the preparation of new photo- and electroactive conjugates able to act as artificial photosynthetic systems and active molecular materials in organic photovoltaics. The gain of aromaticity undergone by the π-extended TTF unit in the oxidation process results in highly stabilized radical ion pairs, namely, C60•–/exTTF•+. Lifetimes for such charge-separated states, ranging from a few nanoseconds to hundreds of microseconds, have been achieved by rationally modifying the nature of the chemical spacers. These long-lived radical pairs are called to play an important role for the conversion of sunlight into chemical or electrical power.
324 citations
Journal Article•
TL;DR: It is concluded that minor complexes which bind zeaxanthin are sites of charge transfer quenching in vivo and that they can assume Non-quenching and Quenching conformations, the equilibrium LHC(N) ⇆LHC(Q) of which is modulated by the transthylakoid pH gradient, the PsbS protein, and protein-protein interactions.
Abstract: Author(s): Avenson, Thomas H. | Abstract: Previous work on intact thylakoid membranes showed that transient formation of a zeaxanthin radical cation was correlated with regulation of photosynthetic light-harvesting via energy-dependent quenching. A molecular mechanism for such quenching was proposed to involve charge transfer within a chlorophyll-zeaxanthin heterodimer. Using near infrared (880-1100 nm) transient absorption spectroscopy, we demonstrate that carotenoid (mainly zeaxanthin) radical cation generation occurs solely in isolated minor light-harvesting complexes that bind zeaxanthin, consistent with the engagement of charge transfer quenching therein. We estimated that less than 0.5percent of the isolated minor complexes undergo charge transfer quenching in vitro, whereas the fraction of minor complexes estimated to be engaged in charge transfer quenching in isolated thylakoids was more than 80 times higher. We conclude that minor complexes which bind zeaxanthin are sites of charge transfer quenching in vivo and that they can assume Non-quenching and Quenching conformations, the equilibrium LHC(N) LHC(Q) of which is modulated by the transthylakoid pH gradient, the PsbS protein, and protein-protein interactions.
193 citations
TL;DR: The binding behaviors and thermodynamic origins of p-sulfonatocalix[4]arene (C4AS) and C5AS with methyl viologen (MV2+) have been investigated by the methods of isothermal titration calorimetry, NMR, and cyclic voltammetry, showing that the binding abilities are dramatically pH-controlled.
Abstract: The binding behaviors and thermodynamic origins of p-sulfonatocalix[4]arene (C4AS) and p-sulfonatocalix[5]arene (C5AS) with methyl viologen (MV2+) have been investigated by the methods of isothermal titration calorimetry, NMR, and cyclic voltammetry, showing that the binding abilities of C4AS and C5AS and their host selectivity are dramatically pH-controlled, which is closely discussed from the viewpoint of thermodynamics. Moreover, the radical form of MV+• can also be effectively included by C4AS and C5AS.
114 citations
TL;DR: In this article, the mechanism of photo-induced step polymerization of thiophene by using diphenyliodonium and triphenylsulphonium (Ph3S+) ions as photoinitiators was investigated.
Abstract: Laser flash photolysis and EPR studies were performed to elucidate the mechanism of photoinduced step polymerization of thiophene by using diphenyliodonium (Ph2I+) and triphenylsulphonium (Ph3S+) ions as photoinitiators. Photoexcitation of these ions generated phenyliodinium (PhI•+) and diphenylsulphinium (Ph2S•+) radical cations, which were readily quenched by thiophene with rate constants of kq = 1.26 × 1010 and 1.7 × 105 M-1 s-1, respectively. The transient absorption spectra of the corresponding thiophene radical cations were not directly detectable because of the spectral overlap with the precursor salts. However, the related electron-transfer reaction was confirmed by quenching of the PhI•+ radical cation with bithiophene to form the radical cation of bithiophene, which absorb strongly at 420 nm. EPR studies also confirmed the proposed electron-transfer mechanism through the direct detection of the radical cation of thiophene.
91 citations
TL;DR: Characteristic side-chain losses are observed for several amino acid residues, which are useful to predict their presence in peptide/protein ions and allow facile distinction of these two isomeric residues.
Abstract: A series of c- and z•-type product ions formed via gas-phase electron-transfer ion/ion reactions between protonated polypeptides with azobenzene radical anions are subjected to ion trap collision activation in a linear ion trap. Fragment ions including a-, b-, y-type and ammonia-loss ions are typically observed in collision induced dissociation (CID) of c ions, showing almost identical CID patterns as those of the C-terminal amidated peptides consisting of the same sequences. Collisional activation of z• species mainly gives rise to side-chain losses and peptide backbone cleavages resulting in a-, b-, c-, x-, y-, and z-type ions. Most of the fragmentation pathways of z• species upon ion trap CID can be accounted for by radical driven processes. The side-chain losses from z• species are different from the small losses observed from the charge-reduced peptide molecular species in electron-transfer dissociation (ETD), which indicates rearrangement of the radical species. Characteristic side-chain losses are ...
84 citations
TL;DR: In this article, the electrochemical oxidation of pure quinone has been investigated in acetonitrile solutions under acidic and basic conditions, and it is shown that the oxidation wave observed in typical experiments of quinones in the presence of proton donors is attributed to hydroquinone associated with the conjugated base of the proton donor.
80 citations
TL;DR: A regeneration of beta-carotene, effective in the liposome lipid phase as antioxidant, from the cation radical by deprotonated forms of puerarin was demonstrated in 9:1 chloroform/methanol using laser flash photolysis.
Abstract: The 4'-hydroxyl group of puerarin, a C-glycoside of the isoflavonoid daidzein, was shown, using 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) radical cation and stopped-flow spectroscopy and by comparison with the 7-propylpuerarin (A ring derivative) and 4'-propylpuerarin (B ring derivative), to be a more efficient radical scavenger as compared to the 7-hydroxyl group by a factor of 2, a difference increasing upon deprotonation. The difference in radical scavenging agreed with the oxidation potentials (cyclic voltammetry in acetonitrile, 0.1 M Bu4NBF4 at 25 degrees C): E/mV=862+/-3 for puerarin, 905+/-10 for 7-propylpuerarin, and 1064+/-2 for 4'-propylpuerarin relative to ferrocene/ferricenium. In aqueous solution, the reduction potential was shown to decrease for increasing pH, and deprotonation of the 4'-hydroxyl group increased radical scavenging more than deprotonation of the 7-hydroxyl group. The 7-hydroxyl was found to be more acidic (pKa1=7.20+/-0.01 in puerarin and pKa=7.23+/-0.01 in 4'-propylpuerarin) than the 4'-hydroxyl group (pKa2=9.84+/-0.08 in puerarin and pKa=9.51+/-0.02 in 7-propylpuerarin); aqueous solution, ionic strength of 0.1, and 25 degrees C. In phosphatidyl choline liposome of pH 7.4, puerarin and beta-carotene each showed a modest antioxidant activity measured as prolongation of the lag phase for formation of conjugate dienes and using the water-soluble radical initiator APPH with effects of puerarin and beta-carotene being additive. For the lipophilic initiator AMVN, the antioxidative effect decreased for puerarin and increased for beta-carotene as compared to APPH and showed a clear synergism. A regeneration of beta-carotene, effective in the liposome lipid phase as antioxidant, from the cation radical by deprotonated forms of puerarin was demonstrated in 9:1 chloroform/methanol using laser flash photolysis with k2=2.7x10(4) L mol-1 s-1 for the bimolecular process between the cation radical and the puerarin dianion.
80 citations
TL;DR: In this paper, the radiolytic behavior of N,N,N′, N′-tetraoctyldiglycolamide, N, N´-dimethyl-N, n′-dioctyl-2-(3′-oxapentadecyl)malonamide, and N, n´hexanamide in n-dodecane was investigated by irradiation with γ-rays and electron pulses
79 citations
TL;DR: Five diamines with thiophene-based bridges with strong coupling between the two amine redox centres, stronger than that observed in species with phenylene-based bridging groups of comparable length have been synthesised.
Abstract: Five diamines with thiophene-based bridges--(E)-1,2-bis{5-[bis(4-butoxyphenyl)amino]-2-thienyl}ethylene (1), 5,5'-bis[bis(4-methoxyphenyl)amino]-2,2'-bithiophene (2), 2,6-bis[bis(4-butoxyphenyl)amino]dithieno[3,2-b:2',3'-d]thiophene (3), N-(4-tert-butylphenyl)-2,6-bis[bis(4-methoxyphenyl)amino]dithieno[3,2-b:2',3'-d]pyrrole (4 a) and N-tert-butyl-2,6-bis[bis(4-methoxyphenyl)amino]dithieno[3,2-b:2',3'-d]pyrrole (4 b)--have been synthesised. The syntheses make use of the palladium(0)-catalysed coupling of brominated thiophene species with diarylamines, in some cases accelerated by microwave irradiation. The molecules all undergo facile oxidation, 4 b being the most readily oxidised at about -0.4 V versus ferrocenium/ferrocene, and solutions of the corresponding radical cations were generated by addition of tris(4-bromophenyl)aminium hexachloroantimonate to the neutral species. The near-IR spectra of the radical cations show absorptions characteristic of symmetrical delocalised species (that is, class III mixed-valence species); analysis of these absorptions in the framework of Hush theory indicates strong coupling between the two amine redox centres, stronger than that observed in species with phenylene-based bridging groups of comparable length. The strong coupling can be attributed to high-lying orbitals of the thiophene-based bridging units. ESR spectroscopy indicates that the coupling constant to the amino nitrogen atoms is somewhat reduced relative to that in a stilbene-bridged analogue. The neutral species and the corresponding radical cations have been studied with the aid of density functional theory and time-dependent density functional theory. The DFT-calculated ESR parameters are in good agreement with experiment, while calculated spin densities suggest increased bridge character to the oxidation in these species relative to that in comparable species with phenylene-based bridges.
79 citations
TL;DR: Spectroscopic and electrochemical measurements show that encapsulation by cucurbituril increases the thermodynamic and kinetic stability of the radical cation of the threaded oligoaniline.
Abstract: A cucurbituril-encapsulated oligoaniline rotaxane is synthesized in high yield by reductive amination in aqueous acid. Spectroscopic and electrochemical measurements show that encapsulation by cucurbit[7]uril increases the thermodynamic and kinetic stability of the radical cation of the threaded oligoaniline. This radical cation has a stoichiometry similar to that of the conducting emeraldine salt form of polyaniline, which implies that cucurbituril insulation of polyaniline might drastically alter the oxidation potential of the threaded conjugated polymer chain.
TL;DR: The reaction of a N-acyliminium ion pool with benzylsilane proceeds by a chain mechanism involving oxidative C−Si bond cleavage and a radical/cation/radical cation crossover mechanism involving single electron transfer has been proposed.
Abstract: The reaction of a N-acyliminium ion pool with benzylsilane proceeds by a chain mechanism involving oxidative C−Si bond cleavage. A radical/cation/radical cation crossover mechanism involving single electron transfer has been proposed. The effective binary system consisting of a stoichiometric amount of a benzylsilane and a catalytic amount of a more easily oxidized benzylstannane has been developed.
TL;DR: The mechanism of qE involving the simple replacement of Vio with Zea in LHC II needs to be reconsidered and generated and characterized carotenoid radical cations by means of resonant two color, two photon ionization (R2C2PI) spectroscopy.
Abstract: Nonphotochemical quenching (NPQ) is a fundamental mechanism in photosynthesis which protects plants against excess excitation energy and is of crucial importance for their survival and fitness. Recently, carotenoid radical cation (Car•+) formation has been discovered to be a key step for the feedback deexcitation quenching mechanism (qE), a component of NPQ, of which the molecular mechanism and location is still unknown. We have generated and characterized carotenoid radical cations by means of resonant two color, two photon ionization (R2C2PI) spectroscopy. The Car•+ bands have maxima located at 830 nm (violaxanthin), 880 nm (lutein), 900 nm (zeaxanthin), and 920 nm (β-carotene). The positions of these maxima depend strongly on solution conditions, the number of conjugated CC bonds, and molecular structure. Furthermore, R2C2PI measurements on the light-harvesting complex of photosystem II (LHC II) samples with or without zeaxanthin (Zea) reveal the violaxanthin (Vio) radical cation (Vio•+) band at 909 nm...
TL;DR: It is demonstrated that a hydrogen-bonding motif is a successful self-assembly method to build SWNTs bearing donor-acceptor nanohybrids, which are useful for light-energy harvesting and photovoltaic applications.
Abstract: An ammonium ion-crown ether interaction has been successfully used to construct porphyrin-single-walled carbon nanotube (SWNT) donor-acceptor hybrids. The [18]crown-6 to alkyl ammonium ion binding strategy resulted in porphyrin-SWNT nanohybrids that are stable and soluble in DMF. The porphyrin-SWNT hybrids were characterized by spectroscopic, TEM, and electrochemical techniques. Both steady-state and time-resolved emission studies revealed efficient quenching of the singlet excited state of the porphyrins and free-energy calculations suggested that electron-transfer quenching occurred. Nanosecond transient absorption spectral results supported the charge-separation quenching process. Charge-stabilization was also observed for the nanohybrids in which the lifetime of the radical ion pairs was around 100 ns. The present nanohybrids were also used to reduce the hexyl viologen dication (HV 2+ ) and to oxidize 1-benzyl-1,4-dihydronicotinamide in solution in an electron-pooling experiment. Accumulation of the radical cation (HV .+ ) was observed in high yields, which provided additional proof for the occurrence of photoinduced charge separation. The present study demonstrates that a hydrogen-bonding motif is a successful self-assembly method to build SWNTs bearing donor-acceptor nanohybrids, which are useful for light-energy harvesting and photovoltaic applications.
TL;DR: In this paper, the ion formation mechanism in electrospray MS is reviewed, with special focus on the electrochemical red/ox reactions responsible for the formation of radical molecular ions.
TL;DR: It is proposed that the extensive charge delocalization in P680+ mainly causes the decrease in the redox potential of P680/P680 in isolated RC complexes, which explains the well-known phenomenon that YZ is not oxidized by P 680+ in RC complexes.
Abstract: The structure and the electronic properties of P680 and its radical cation in photosystem II (PSII) were studied by means of Fourier transform infrared spectroscopy (FTIR). Light-induced P680+/P680 FTIR difference spectra in the mid- and near-IR regions were measured using PSII membranes from spinach, core complexes from Thermosynechococcus elongatus, and reaction center (RC) complexes (D1-D2-Cytb559) from spinach. The spectral features of the former two preparations were very similar, indicating that the structures of P680 and its radical cation are virtually identical between membranes and cores and between plants and cyanobacteria. In sharp contrast, the spectrum of the RC complexes exhibited significantly different features. A positive doublet at ∼1724 and ∼1710 cm-1 due to the 131-keto CO stretches of P680+ in the membrane and core preparations were changed to a prominent single peak at 1712 cm-1 in the RC complexes. This observation was interpreted to indicate that a positive charge on P680+ was ext...
TL;DR: The interaction of emeraldine base polyaniline (EB) with Cu(II), Fe(III), and Zn(II) in 1-methyl-2-pyrrolidinone (NMP) solution was monitored using electronic UV−vis−NIR, resonance Raman, and electron paramagnetic resonance (EPR) spectroscopies as discussed by the authors.
Abstract: The interaction of emeraldine base polyaniline (EB) with Cu(II), Fe(III), and Zn(II) in 1-methyl-2-pyrrolidinone (NMP) solution was monitored using electronic UV−vis−NIR, resonance Raman, and electron paramagnetic resonance (EPR) spectroscopies. The films prepared from these solutions were also spectroscopically characterized. It was demonstrated that the nature of the products (semiquinone and quinone segments) formed from the interaction of EB and metal ions is strongly dependent on the nature of the cation, the metal/EB molar ratio, and the concentration of the components. The presence of semiquinone segments (radical cation) in EB solutions with Cu(II) and Fe(III) was undoubtedly confirmed by the observation of an electronic absorption band at ca. 900 nm, a characteristic Raman band at ca. 1330 cm-1 (νC-N•+), and also an EPR signal at g = 2.006. The influences of metal/EB molar ratio and metal ion concentration on the formed species were investigated in Cu(II) and Fe(III) solutions, and it was verifie...
TL;DR: Femtosecond transient absorption spectroscopy shows that laser excitation of both ZnTPP-PDI and (ZnTPP -PDI(4) results in quantitative formation of Zn TPP(+*)-PDI(-*) radical ion pairs in a few picoseconds, suggesting that the PDI (-*) radicals interact strongly with adjacent PDI molecules within the columnar stack.
Abstract: A bichromophoric electron donor–acceptor molecule composed of a zinc tetraphenylporphyrin (ZnTPP) surrounded by four perylene-3,4:9,10-bis(dicarboximide)
(PDI) chromophores (ZnTPP-PDI4) was synthesized. The properties of this molecule were compared to a reference molecule having ZnTPP covalently bound to a single PDI (ZnTPP-PDI). In toluene, ZnTPP-PDI4 self-assembles into monodisperse aggregates of five molecules arranged in a columnar stack, (ZnTPP-PDI4)5. The monodisperse nature of this assembly contrasts sharply with previously reported ZnTPP-PDI4 derivatives having 1,7-bis(3,5-di-t-butylphenoxy) groups (ZnTPP-PPDI4). The size and structure of this assembly in solution was determined by small angle X-ray scattering (SAXS) using a high flux synchrotron X-ray source. The ZnTPP-PDI reference molecule does not aggregate. Femtosecond transient absorption spectroscopy shows that laser excitation of both ZnTPP-PDI and (ZnTPP-PDI4)5 results in quantitative formation of ZnTPP+˙-PDI−˙ radical ion pairs in a few picoseconds. The transient absorption spectra of (ZnTPP-PDI4)5 suggest that the PDI−˙ radicals interact strongly with adjacent PDI molecules within the columnar stack. Charge recombination occurs more slowly within (ZnTPP-PDI4)5
(τ
= 4.8 ns) than it does in ZnTPP-PDI (τ
= 3.0 ns) producing mostly ground state as well as a modest yield of the lowest triplet state of PDI (3*PDI). Formation of 3*PDI occurs by rapid spin–orbit induced intersystem crossing (SO-ISC) directly from the singlet radical ion pair as evidenced by the electron spin polarization pattern exhibited by its time-resolved electron paramagnetic resonance spectrum.
TL;DR: An overview of the current literature on the balance between the formation of molecular ions and of protonated forms of organic compounds in electrospray ionization mass spectrometry (ESI-MS) is presented in this article.
Abstract: In electrospray ionization mass spectrometry (ESI-MS), charge formation is proposed to occur as a result of acid-base reactions or a coordination with metal cations. However, the increasing number of papers have reported the formation of radical ions (molecular ions) as a result of loss of one or two electrons. In this paper, we present an overview of the current literature on the balance between the formation of molecular ions and of protonated forms of organic compounds in ESI-MS. Correlations between the radical formation process and the ionization energy, calculated by molecular orbital methods or obtained by eletrochemical techniques, were applied to explain this apparently unexpected behavior. The use of high level theoretical methods especially designed to produce accurate thermochemical data is also reported.
TL;DR: Evidence for the bonded exciplex intermediate comes from studies of steric and Coulombic effects on the quenching rate constants and from extensive DFT computations that clearly show a curve crossing between the ground state and the low-energy bonded exciterx state.
Abstract: Charge-transfer quenching of the singlet excited states of cyanoaromatic electron acceptors by pyridine is characterized by a driving force dependence that resembles those of conventional electron-transfer reactions, except that a plot of the log of the quenching rate constants versus the free energy of electron transfer is displaced toward the endothermic region by 0.5−0.8 eV. Specifically, the reactions with pyridine display rapid quenching when conventional electron transfer is highly endothermic. As an example, the rate constant for quenching of the excited dicyanoanthracene is 3.5 × 109 M-1 s-1, even though formation of a conventional radical ion pair, A•-D•+, is endothermic by ∼0.6 eV. No long-lived radical ions or exciplex intermediates can be detected on the picosecond to microsecond time scale. Instead, the reactions are proposed to proceed via formation of a previously undescribed, short-lived charge-transfer intermediate we call a “bonded exciplex”, A-−D+. The bonded exciplex can be formally th...
TL;DR: In this paper, the authors demonstrate that planar s-trans and s-cis conformations are energy minima, separated by an extremely weak energy gap and the energy barrier corresponding to the equilibrium between the two forms has been also estimated from DFT calculations, as well as their relative stability in the radical cation state of the TTF.
Abstract: Racemic and enantiopure ethylenedithio-tetrathiafulvalene-thiomethyl-oxazoline (EDT-TTF-(SMe)-OX) derivatives have been synthesized. Single-crystal X-ray diffraction analyses reveal the establishment of O⋯S nonbonded interactions, unprecedented in the TTF series, characterized by short S⋯O distances and linear O⋯SMe motifs. Theoretical calculations at the DFT/B3LYP level on a model molecule demonstrate that both possible planar s-trans (O⋯S interaction) and s-cis (N⋯S interaction) conformations are energy minima, separated by an extremely weak energy gap. The energy barrier corresponding to the equilibrium between the two forms has been also estimated from DFT calculations, as well as their relative stability in the radical cation state of the TTF. According to the latter, there is a slight tendency towards the enhancement of the N⋯S interactions. Following the predictions of the theoretical calculations, the coexistence of both O⋯S and N⋯S nonbonded interactions is observed in two mixed valence radical cation salts of the racemic TTF with the dianionic cluster Mo6Cl14, prepared upon electrocrystallization. Interestingly, the ratio TTF : dianion is finely tuned by the choice of the electrocrystallization solvent. Physical measurements such as electrical conductivity and thermoelectric power on single crystals combined with magnetic susceptibility data in one case and extended Huckel tight-binding calculations demonstrate and rationalize the semiconducting behavior of both mixed valence salts. This study demonstrates that intramolecular N⋯S and O⋯S interactions can efficiently modulate and direct in the TTF series the occurrence of original solid-state structures provided with physical properties.
TL;DR: Neighboring group participation was investigated in the *OH-induced oxidation of S-methylglutathione in aqueous solutions, and additional insight into the details of the association of sulfur radical cations with the oxygen atoms of the peptide bonds was gained by comparing the behavior of the S- methylglUTathione (S thereforeO+-bonded five-membered ring) with the peptides gamma-Glu-Met-Gly.
Abstract: Neighboring group participation was investigated in the •OH-induced oxidation of S-methylglutathione in aqueous solutions. Nanosecond pulse radiolysis was used to obtain the spectra of the reaction intermediates and their kinetics. Depending on the pH, and the concentration of S-methylglutathione, pulse irradiation leads to different transients. The transients observed were an intramolecularly bonded [>S∴NH2]+ intermediate, intermolecularly S∴S-bonded radical cation, α-(alkylthio)alkyl radicals, α-amino-alkyl-type radical, and an intramolecularly (S∴O)+-bonded intermediate. The latter radical is of particular note in that it supports recent observations of sulfur radical cations complexed with the oxygen atoms of peptide bonds and thus has biological and medical implications. This (S∴O)+-bonded intermediate had an absorption maximum at 390 nm, and we estimated its formation rate to be ≥6 × 107 s-1. It is in equilibrium with the intermolecularly S∴S-bonded radical cation, and they decay together on the tim...
TL;DR: Observations indicate that the reactions involve disproportionation of the iron(IV)-oxo species 2 to give an iron(III) species and a more highly oxidized iron species, presumed to be an iron-oxo porphyrin radical cation, that is the true oxidant in the reactions.
Abstract: The kinetics of the reactions of three porphyrin-iron(IV)-oxo derivatives with alkenes and benzylic alcohols were measured. The iron-oxo systems studied were 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin-iron(IV)-oxo (2a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin-iron(IV)-oxo (2b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin-iron(IV)-oxo (2c). Species 2 were stable for hours at room temperature as dilute solutions in acetonitrile and reacted hundreds to thousands of times faster in the presence of high concentrations of substrates. Typical second-order rate constants determined from pseudo-first-order kinetic studies are 1-2 x 10(-2) M(-1) s(-1) for reactions with styrene and 3 x 10(-2) M(-1) s(-1) for reactions with benzyl alcohol. The reactivity order for the iron-oxo species was 2a > 2b > 2c, which is inverted from that expected on the basis of the electron demand of the porphyrin macrocycles, and the oxidation reaction was suppressed when excess porphyrin-iron(III) complex was added to reaction mixtures. These observations indicate that the reactions involve disproportionation of the iron(IV)-oxo species 2 to give an iron(III) species and a more highly oxidized iron species, presumed to be an iron(IV)-oxo porphyrin radical cation, that is the true oxidant in the reactions. Analyses of the kinetics of oxidations of a series of para-substituted benzylic alcohols with Hammett sigma+ -substituent constants and with a dual-parameter method developed by Jiang (Jiang, X. K. Acc. Chem. Res. 1997, 30, 283) indicated that considerable positive charge developed on the benzylic carbons in the oxidation reactions, as expected for electrophilic oxidants, and also that substantial radical character developed on the benzyl carbon in the transition states.
TL;DR: Heptacene, generated in inert gas matrices by photobisdecarbonylation of a bridged alpha-diketone precursor, undergoes ionization into radical anion and radical cation upon UV irradiation.
TL;DR: The ability of intramolecular anodic olefin coupling reactions to form new carbon−carbon bonds has been shown to depend on the polarization of the intermediate radical cation rather than how electron-rich it is.
Abstract: The ability of intramolecular anodic olefin coupling reactions to form new carbon−carbon bonds has been shown to depend on the polarization of the intermediate radical cation rather than how electron-rich it is. A series of substrates was studied that allowed for a direct comparison of these two parameters. The successful cyclizations led to the formation of highly functionalized bicyclic molecules containing four contiguous stereogenic atoms, one of which was tetrasubstituted. For the first time, an ene diol ether derivative was shown to be compatible with the cyclization reaction.
TL;DR: In this article, the cyclic voltammograms of the chalcogenophene derivatives containing aryl fragments showed well-defined reversible both anodic and cathodic steps derived from the unusually stable 5π CHG radical cations and 7πCHG radical anions.
TL;DR: In this article, a poly(isocyanide) containing tetrathiafulvalene (TTF) moieties in the side chains has been synthesized and characterized in its neutral state and has been oxidized to generate mixed valence states which display charge mobility in solution.
Abstract: The poly(isocyanide) skeleton is shown to be a good support for assemblies of molecular units which permit p-type charge transport. A poly(isocyanide) containing tetrathiafulvalene (TTF) moieties in the side chains has been synthesized and characterized in its neutral state and has been oxidized to generate mixed valence states which display charge mobility in solution. UV−vis−NIR, EPR, and Raman spectroscopies were used to study the electronic properties of the polymer in its doped states, which were generated chemically with different oxidants. A broad charge-transfer band at 2100 nm characteristic of mixed valence agglomerations of neutral and cation radical TTFs was shown at lower doping levels, while evidence of aggregates between radical cation and the dicationic form of the heterocyclic system were seen at higher degrees of oxidation. The neutral macromolecule, the first mixed valence state, and the cation radical system can be reversibly interconverted by alternate oxidation with Fe(ClO4)3 and sub...
TL;DR: Different dehalogenation mechanisms for the reductive dechlorination of chloropropenes and halogenated ethenes are supported, supported by mass spectrometric analysis of assays and the formation of side products.
Abstract: The reductive dehalogenation of chlorinated propenes was studied with the tetrachloroethene reductive dehalogenase purified from Sulfurospirillum multivorans to obtain indications for a radical mechanism of this reaction. When reduced methyl viologen (MV), which is a radical cation, was applied as electron donor for the reduction of different chloropropenes, a significant part of MV could not be rereduced with Ti(III) citrate, indicating that a part of the MV was consumed in a side reaction. Mass spectrometric analysis of assays with MV as electron donor revealed the formation of side products, the masses of which might account for the formation of adducts from a chloropropenyl radical and reduced methyl viologen. With Ti(III) citrate as sole electron donor, 2,3-dichloropropene was reduced and as a side product, 2,5-dichloro-1,5-hexadiene was formed demonstrating that the reductive dechlorination of 2,3-dichloropropene proceeds via a radical reaction mechanism. The results support different dehalogenation mechanisms forthe reductive dechlorination of chloropropenes and halogenated ethenes.
TL;DR: Novel conjugated azometHines consisting of 1 to 5 thiophenes and up to 4 azomethine bonds prepared from a stable diaminothiophene are presented and it is confirmed the thermodynamically stable E isomer was formed uniquely and that the thiopenes are coplanar adopting an antiparallel arrangement.
Abstract: Novel conjugated azomethines consisting of 1 to 5 thiophenes and up to 4 azomethine bonds prepared from a stable diaminothiophene are presented. The effect of the number of thiophene and azomethines bonds on the photophysics and electrochemistry was examined. A high degree of conjugation was confirmed by bathochromic shifts upward of 120 and 210 nm for the absorbance and fluorescence, respectively, relative to the diaminothiophene precursor. Acid doping with methanesulfonic acid resulted in further bathochromic shifts along with lowering of the HOMO-LUMO energy gaps to 1.3 eV. Moreover, the compounds are extremely stable as evidenced by the absence of decomposition products under acid conditions. The resulting heteroatomic covalent bonds are furthermore reductively and hydrolytically resistant. Increasing the degree of conjugation shifts the nonradiative mode of singlet excited state energy dissipation from internal conversion (IC) to intersystem crossing (ISC). The resulting triplet manifold produced by ISC was efficiently deactivated by intramolecular self-quenching from the azomethine bond leading to a nonemissive triplet. Cyclic voltammetry revealed unprecedented reversible radical cation formation of the azomethines. Both one-electron oxidations and reductions were found by electrochemical measurements demonstrating the azomethines' capacity to be mutually p- and n-doped. One of the azomethines exhibited reversible electrochromic behavior with the electrochemically generated radical cation absorbing in the NIR at 1630 and 792 nm. X-ray crystallography confirmed the thermodynamically stable E isomer was formed uniquely and that the thiophenes are coplanar adopting an antiparallel arrangement.
TL;DR: It was found that the charge recombination between TPEB*+ and T PEB*- occurred to give 1TPEB* as the emissive species, but not the excimers because of the large repulsion between substituents caused by the rotation around C-C single bonds of TPEBs.
Abstract: Emission from charge recombination between radical cations and anions of various tetrakis(phenylethynyl)benzenes (TPEBs) was measured during pulse radiolysis in benzene (Bz). The formation of TPEB in the singlet excited state (1TPEB*) can be attributed to the charge recombination between TPEB*+ and TPEB*-, which are initially generated from the radiolytic reaction in Bz. This mechanism is reasonably explained by the relationship between the annihilation enthalpy change (-DeltaH degrees) for the charge recombination of TPEB*+ and TPEB*- and excitation energy of 1TPEB*. It was found that the charge recombination between TPEB*+ and TPEB*- occurred to give 1TPEB* as the emissive species, but not the excimers because of the large repulsion between substituents caused by the rotation around C-C single bonds of TPEBs. Since donor-acceptor-substituted TPEBs possess three types of charge-transfer pathways (linear-conjugated, cross-conjugated, and "bent" conjugated pathways between the donor and acceptor substituents through the ethynyl linkage), the emission spectra of 1TPEBs* with intramolecular charge transfer (ICT) character depend on the substitution pattern and the various kinds of donor and acceptor groups during pulse radiolysis in Bz.