Reactions of Photoexcited Triplet States of Zinc Porphyrin with Transient Radicals in Aqueous Solutions

The technique of simultaneous pulse radiolysis and photolysis, PRAP, has been utilized to study the reactions of various radicals with ground state ZnTPPS and the triplet state ZnTPPS^T in aqueous solutions. The radicals H and OH add to both states with k ∼ 1 × 10¹⁰ M⁻¹ s⁻¹. The CH₂C(CH₃)₂OH radical from t-BuOH is relatively inert toward ZnTPPS but reacts rapidly (k = 1.8 × 10⁹ M⁻¹ s⁻¹) with ZnTPPS^T to form an adduct. Electron transfer reactions are found to be about an order of magnitude faster with the triplet than with the ground state. The (CH₃)₂COH radical reduces both ZnTPPS (k = 1 × 10⁸ M⁻¹ s⁻¹) and ZnTPPS^T (k = 3 × 10⁹ M⁻¹ s⁻¹) to the anion radical (ZnTPPS)⁻. The radical Br⁻₂ oxidizes both states to the cation radical (ZnTPPS)⁺ with k = 8 × 10⁸ M⁻¹ s⁻¹ for the ground state and 5 × 10⁹ M⁻¹ s⁻¹ for the triplet. The transient cation Cd⁺ reduces both states with a diffusion-controlled rate (k = 1 × 10¹⁰ M⁻¹ s⁻¹) to produce the anion radical. The above mechanisms of radical addition and electron transfer are also supported by the product spectra.     

Tetrathiafulvalene. VIII. Ethylenverbrückte polymere Tetrathiafulvalene

Tetrathiafulvalenes. VIII. Ethylene-bridged Polymer Tetrathiafulvalenes Syntheses and properties of ethylene-bridged polymer tetrathiafulvalenes are described. These polymers react with bromine, iodine or tetracyanoquinodimethane to form the radical cation salts, whose electrical conductivity are estimated by means of powder compactions. The polymer radical cation salts have a higher conductivity than the polymer TTF (σ_RT max. 10⁻⁵ Ω⁻¹cm⁻¹). The differences in the conductivity between polymer TTF-cationhalides and polymer TTF-TCNQ-salts are small.     

Lipid peroxidation and oxidation of several compounds by H2O2 activated metmyoglobin

Activated metmyoglobin (MetMb) by H₂O₂ initiates oxidation of microsomal unsaturated fatty acids, β-carotene and methional but not formate. Lipid peroxidation by activated MetMb was not inhibited by catalase. The activated species which initiaes lipid peroxidation appears to be a porphyrin cation radical, P _∸ ⁺ Fe^IV=O, and not a hydroxyl radical.     

The Role of pH in the Mechanism of .OH Radical Induced Oxidation of Nicotine

The ^.OH radical induced oxidation of nicotine was studied using pulse radiolysis techniques from pH 1 to 13.6. Theoretical calculations were used to help interpret the experimental results. The bond dissociation enthalpies of all of the C H bonds of nicotine were determined using DFT calculations, coupled with the isodesmic reaction. From time-dependent density functional response theory, estimates were obtained of the location of the dominant transient absorption bands (λ_max), their intensities (electronic oscillator strength, f), and the electronic composition of these transitions. OH radicals as well as other potent oxidants reacted with free nicotine through separated, concerted electron proton transfer, leading mostly to the formation of an alpha-aminoalkyl radical located on the C_2′ carbon of the aliphatic ring ( A_2′ ). Protonated nicotine underwent hydrogen atom abstraction at the C_2′ and N_1′ positions, resulting in the formation of the conjugate acid of A_2′ ( A_2′H⁺ ) and the alkylamine radical cation ( N⁺ ), respectively. Doubly protonated nicotine underwent the same reaction pathways, leading to two corresponding conjugate acid species, protonated at the aromatic nitrogen position: PyrH⁺A_2′H⁺ and PyrH⁺N⁺ _. All these radicals interconverted between each other through hydrolytic reactions. The radical A_2′ and its conjugate acid PyrH⁺A_2′ absorbed 10 times stronger than the N⁺ species, based on calculations of f. From the growth of the transient absorption of A_2′ (λ_max=330 nm, ε=8080 M ⁻¹ cm⁻¹), second-order rate constants were determined: k_(OH+Nic)=6.7×10⁹ M ⁻¹ s ⁻¹, k_(OH+NicH)=1.0×10⁹ M ⁻¹ s⁻¹. The alpha-aminoalkyl radicals decayed by disproportionation to form iminium cations 1 – 5 , which contributed to an increase in the specific conductivity of the basic solutions of nicotine following electron pulse irradiation.     

Electrochemical study of tris(4-bromophenyl)amine and 2,2,6,6-tetramethylpiperidine-1-oxyl in room-temperature ionic liquids

The electrochemical behavior of tris(4-bromophenyl)amine (TBPA) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was comparatively studied in room temperature ionic liquids (RTILs) containing 1-butyl-3-methylimidazolium cation [BMIm]⁺ and the anions BF₄⁻, PF₆⁻and CF₃SO₃⁻. TEMPO showed a well-defined electrochemical reversibility with anodic to cathodic peak current ratio (I_a/I_c) equals to unity in all RTILs, at a glassy carbon electrode. In contrast, I_a/I_c ratio greater than unity was observed at all scan rates employed (10-1000 mV s⁻¹) in cyclic voltammograms (CV) of TBPA in RTILs. Controlled-potential bulk electrolysis of TBPA in RTILs yielded a dark blue colored solution of monocation TBPA⁺, which is stable under nitrogen. Diffusion coefficient of TBPA⁺ cation was determined in all three ILs by chronoamperometry and found to be at least one order of magnitude less than the calculated value of TBPA. This effect, in conjunction with the deviation of the I_a/I_c ratio from unity could be ascribed to ion-pairing interaction between the TBPA⁺ cation and the anion of the RTILs.     

A radical ion-pair complex exhibiting strong C–H…O H-bonding interaction,moderately broaden near-IR absorbance and complicated EPR spectrum

An ion-pair complex of [Ni(mnt)₂]^2? with p-N-benzylpyridinium α-nitronyl nitroxide radical cations (p-BzPYNN) in acetonitrile shows a moderate and broad absorbance in near-IR region. In the crystal of this complex, two radical cations form a dimer via strong H-bonding interaction; such kinds of dimers are connected into H-bonding chain by [Ni(mnt)₂]^2? dianions through weak H-bonding interactions. The neighboring H-bonding chains are arranged into supramolecular sheet via π…π stacking interactions between the five-membered chelate-ring of anion and the superimposed pyridyl rings of cations. The polycrystalline EPR spectrum exhibits two isotropic EPR signals. Based on the analyses of crystal structure, variable temperature magnetic susceptibility and electronic spectrum, the stronger EPR signal is assigned to the radical cation, the weaker one probably arises from a trace amount of [Ni(mnt)₂]^1? species.     

Electrochemical properties of poly(decaviologen) in polymer media

Poly(decaviologen) (DV²⁺, 2ClO ₄ ^– ) has been prepared and used in the characterization of two redox couples; dication/cation radical ((DV²⁺, 2 ClO ₄ ^– )/(DV⁺, ClO ₄ ^– )) and cation radical/decaviologen ((DV⁺, ClO ₄ ^– )/(DV^o)) in a polyethylene oxide-LiClO₄ medium by linear potential sweep voltammetry. The (DC²⁺, 2 ClO ₄ ^– )/(DV⁺, ClO ₄ ^– ) couple has been used to determine the transport number of lithium in poly(ethylene oxide)–LiClO₄ or LiCF₃SO₃ complex media.     

Aspects regarding the grafting of some lignosulfonates with acrylamide under a magnetic field

The kinetics of the graft polymerization of acrylamide onto some lignosulfonate products in a continuous external magnetic field was studied. The registered magnetokinetic effects were correlated and associated with the conditions of the classic reaction. The grafting process of acrylamide onto the macromolecular chains depending on the cation (Ca²⁺, Cr²⁺, Fe²⁺, Fe³⁺ and NH₄⁺) present in the lignosulfonate structure and the magnetic field influence were corroborated and explained through the radical pairs mechanism. The augmentation of the graft polymerization rate in magnetic field and conversion was ascribed to the singlet-triplet transitions in the radical pairs as a result of the presence of the magnetic field.     

Evidence for the photochemical formation of a high-valent 2A2u antimony porphyrin π-radical cation

Irradiation of [Sb^V(tpp)(OH)₂]⁺ (tpp=dianion of 5,10,15,20-tetraphenylporphyrin) at 298 K in argon saturated dichloromethane under steady state conditions results in the formation of the highly oxidising π-radical cation [Sb^V(tpp)(OH)₂]²⁺ with spectral characteristics corresponding to a ²A_2u electronic ground state.     

Fourier Transform EPR Measurement of Homogeneous Electron Transfer Rates

Fourier-Transform Electron Paramagnetic Resonance has measured the rates of homogeneous electron transfer reactions involving duroquinone radical anions. The radicals are generated by laser photolysis of duroquinone in methanol solution containing 10% triethylamine. The duroquinone concentration was varied over a factor of 1000. The rate constant for electron transfer between the radical anion and the neutral duroquinone is 1.5×10⁸M⁻¹s⁻¹ and the intrinsic spin relaxation rates, T⁻¹₁ and T⁻¹₂ are 0.32 MHz and 0.44 MHz, respectively. Two-Dimensional magnetization transfer spectra show that the reaction is a homogeneous electron transfer reaction. The electron transfer rates are measured by two novel pulse sequences designed for more efficient data acquisition in samples with no unresolved inhomogeneous broadening. These two sequences result in a more rapid, accurate determination of the second-order chemical rate constant since the second-order rate constant is obtained directly and is not derived from a single measurement of a pseudo-first-order rate. The transient duroquinone radical anions studied here appear to have the same T₁, T₂ and electron transfer rates as stable duroquinone radical anions in alkaline solutions.     

Reaction of Aryl Iodides with (PCP)Pd(II)—Alkyl and Aryl Complexes: Mechanistic Aspects of Carbon—Carbon Bond Formation

The reaction of the PCP-type complex Pd(Me){2,6-(ⁱPr₂PCH₂)₂C₆H₃}( 3 ) with phenyl iodide results in the formation of Pd(I){2,6-(ⁱPr₂PCH₂)₂C₆H₃} ( 5 ), methyl iodide, toluene, and biphenyl. Formation of Pd(Ph){2,6-(ⁱPr₂PCH₂)₂C₆H₃}( 4 ) is observed during the reaction by ³¹P NMR. Reaction of 4 with aryl iodides results in the formation of 5 and Ph–Ph, Ph–Ar, and Ar–Ar, products indicative of a radical reaction. Under pseudo-first-order conditions, the rates of the reactions follow the order p-OMe > p-Me > H > p-NO₂ > m-Cl. The reaction is likely to involve electron transfer from 4 to the aryl iodide followed by fast decomposition of a postulated radical cation [Pd(Ph){2,6-(ⁱPr₂PCH₂)₂C₆H₃}]^+. ( 4 ^+.) to give a phenyl radical and [Pd{2,6-(ⁱPr₂PCH₂)₂C₆H₃}]⁺ ( 6 ⁺). Facile decomposition of the aryl iodide radical anion generates an aryl radical and I⁻. Recombination of aryl radicals gives rise to mixed biaryls, and 6 ⁺ combines with I⁻ to give 5 .     

Photochemistry of trichloro(phenylazo)bis(triphenylphosphine)ruthenium(II) induced by Ru to phenyldiazonium MLCT excitation

The complex trichloro(phenylazo)bis(triphenylphosphine)ruthenium(II) shows a longest-wavelength absorption at λ_max = 520 nm which is assigned to a Ru(II) → phenyldiazonium⁺ MLCT transition. MLCT excitation leads to the oxidation of the metal and reduction of the diazonium cation which decays to nitrogen and phenyl radical.     

Photoinitiated degradation of polymers by metal salts-recent developments

This paper reexamines the role of transition metal salts (mainly CuCl₂ and FeCl₃) in the photooxidative degradation of polymers. Metal salts belong to external impurities originated in polymers from synthesis, processing, storage, contacts with metal corrosion, etching reactions, etc. Transition metal salts in the presence of water become very reactive species. In such metal chlorides as CuCl₂ and FeCl₃ UV and/or visible light irradiation causes an electron-transfer from Cl^? to metal cation. The produced chlorine radical can abstract hydrogen atom from a polymer molecule giving polymer alkyl radical. which is easily oxidized in air. This process is accompanied with a chain scission reaction which is responsible for the degradation of a polymer. Mechanisms of these reactions are discussed in detail.     

Electrochemical aspects of display technology based on nanostructured titanium dioxide with attached viologen chromophores

Progress in recent years in the field of electrochromic displays based on viologen modified high-surface area TiO₂ electrodes (Vio²⁺/TiO₂) has moved the technology towards commercialisation. Viologen molecules (Vio²⁺), derivatised with phosphonic acid attachment groups can be chemisorbed on nanostructured TiO₂ layers of thickness 2-10 μm. Characterisation by cyclic voltammetry, spectroelectrochemistry and impedance spectroscopy demonstrates that colourless Vio²⁺/TiO₂ is reversibly reduced to the strongly coloured cation radical species Vio⁺/TiO₂. This system can constitute the working electrode of an electrochromic display with a capacitive doped SnO₂ electrode as counter electrode, the latter coated by an electrochemically inert white light-reflecting layer. Such a device is stable upon repeated colouration-bleaching cycles with a bleached-to-coloured state contrast ratio exceeding 5. Multicolour displays can be achieved by patterning different electrochromophores onto different areas of one working electrode.     

Laser flash photolysis of poly(silylenes) in dilute solution. Generation of ionic species

Radical cations are generated via monophotonic processes upon irradiation of poly(methylphenyl silylene) (PMPSi), poly(biphenylmethyl silylene) (PBMSi), poly(dihexyl silylene) (PDHeSi), and poly(hexylmethyl silylene) (PHeMSi) in dilute tetrahydrofuran solution with 15- or 20-ns flashes of 266- or 347-nm light, respectively. This was inferred from electrical conductivity measurements. Typical quantum yields (ion) are as follows: for PBMSi, 4×10² (_inc=266nm) and 3×10³ (_inc=347nm); and for PDHeSi, 8×10³ (_inc=266nm) and 9×10⁴ (_inc=347nm). In all cases, a significant portion of the transient absorption spectrum recorded at the end of the flash is attributed to the spectrum of the radical cation. The latter strongly overlaps the spectra of other intermediates (presumably silyl radical and silylene).     

Electrode reaction of 2,2,6,6-tetramethylpiperdinenitroxide cation at platinum in acetonitrile containing anhydrous hydrogen chloride

Electrode reaction of 2,2,6,6-tetramethylpiperidine-nitroxide (TMPNO) cation at platinum in acetonitrile containing anhydrous hydrogen chloride was investigated by means of linear sweep voltammetry, electric conductivity measurement and esr spectrum, TMPNO cation is reversibly reduced to TMPNO radical which undergoes an irreversible protonation to cation radical with a rate constant of approximately 5·9 × 10³l/mol/s.     

Anodic chlorination of substituted benzenes - the choice of the chlorination reagents

Some substituted benzenes ArH were anodically chlorinated in acetonitrile LiClO₄ on a platinum electrode in the presence of either Cl^? anion or a chlorinated Lewis acid (such as AlCl₃). The overvoltage on the oxidation of Cl^? caused by a strong complexation by some Lewis acid allows consideration that the anodic reaction passes through the radical cation of the aromatic substrate. In such conditions, polychlorinations are possible and in the specific case of toluene, it is shown that the reaction favors the parachlorotoluene formation. The influence of different parameters (for example, the ratio of the concentrations AlCl₃/ArH, the nature of the Lewis acid) in such a reaction are discussed.     

Etude electrochimique de la N, N-dimethyl-mesidine en milieu organique. Oxydation electrochimique en presence de phosphonate de diethyle

The electrochimical oxidation of the N, N-dimethylmesidine in acetonitrile (0.1 M LiClO₄) provides a cation radical which fortly deprotonates on a N-methyl. The corresponding neutral radical either dimerises or is oxidized to the iminium cation. The dimer is later on oxidized and gives either trimer and tetramer or the corresponding iminium cation. The iminium cations are characterized by their products of hydrolysis of their adducts with the diethyl phosphonate. The mechanism of formation of characterized products are discussed using electrochemical, chemical yields and the voltamperometric curves evolution.     

Influence CO2 partial pressure and the supporting electrolyte cation on the product distribution in CO2 electroreduction

This paper reports a study of the influence of CO₂ partial pressure and the cation of the supporting electrolyte on the electrochemical reduction of CO₂ at copper electrodes in aqueous solution at 25°C. Both current efficiency (CE) and the rates of formation of the reduction products diminish linearly with decreasing CO₂ pressure, while evolution of hydrogen increases. The product distribution was greatly influenced by the supporting electrolyte cation. The CE for the formation of C₂H₄ increased with cation in the order Cs⁺ K⁺ > Na⁺ > Li⁺, apparently in a manner related to the size of the cation radius. The non-metallic cation NH⁺ ₄ allowed only hydrogen evolution with a CE of about 92%.     

Tetrathiafulvalene. VII [1]. Arylenverbrückte polymere Tetrathiafulvalene

Tetrathiafulvalenes. VII. Arylene-bridged Polymeric Tetrathiafulvalenes Synthesis and properties of phenylene — and biphenylbridged polymeric tetrathiafulvalenes, which are substituted by two methyl-groups in the 3- and 6-position, are described. These polymers react with bromine, iodine or tetracyanoquinodimethane to form the radical cation salts, whose electrical conducticity are estimated by means of powder compactions. The differences in the conductivity of the TCNQ-salts and radical cation halides are very small. The phenylene-bridged polymer-salts show a higher conductivity than the bi-phenylylene-bridged ones up to values of 10⁻³Ω⁻¹ cm⁻¹. Surprisingly the monomer salts have a lower conductivity than the polymer salts.