Generation Of Radical Cations Research Articles

Redox active surface films produced by electrooxidation of substituted indolysines

The electrochemical oxidation of substituted mono-and 3,3′-biindolysines is studied by methods of cyclic voltammetry and electron spin resonance (ESR) combined with in situ electrolysis. In acetonitrile, there occurs easy irreversible electrochemical oxidation and proceed processes of association of generated radical cations via unsubstituted positions of five-membered cycles. Polymeric products of oxidation of 2-arylindolysines, 3-indolysine-2-yl-quinoxalines, and 2,2′-diaryl-3,3′-biindolysines are obtained when cycling potential in the interval −0.3 V → +0.8 V → −1.3 V → −0.3 V (Fc/Fc+). The products deposit on the electrode with the formation of redox-active films that are capable of undergoing reversible oxidation with the formation of stable paramagnetic states registered by the ESR method.

The role of the position of the basic residue in the generation and fragmentation of peptide radical cations

Using simple di- and tripeptides GX, GGX, GXG, XG and XGG, the influence of the position of the basic residue, X (X = R, K and H), on the formation of peptide radical cations (M +) from [Cu II(tpy)M] 2+ complexes (where tpy = 2,2′:6′,2″-terpyridine) was probed. It was found that M + is formed with greatest abundance when the basic residue is at the C-terminus. For arginine containing peptides, this may be due to further fragmentation of GRG +, RG + and RGG + at the MS 2 stage. For lysine and histidine containing peptides, when the basic residue is not located at the C-terminus, competing fragmentation pathways that lead to peptide backbone cleavage are more facile than M + formation. In order to gain some insights into the binding modes of these peptides to [Cu II(tpy)] 2+, the formation and fragmentation of copper(II) complexes of tripeptides protected as their carboxy methyl/ethyl esters (M–OR′, R′ = Me/Et) were also probed. The products of the competing fragmentation pathways of [Cu II(tpy)M] 2+, as well as the formation and fragmentation of [Cu II(tpy)(M–OR′)] 2+, suggest that the unprotected peptides, M, mainly bind as zwitterions to [Cu II(tpy)] 2+. The fragmentation reactions of the radical cations (M +) were also studied. Radical driven side chain fragmentation reactions of M + are dependent on both the position of the residue as well as the identity of other residues present in the peptide radical cations. GR and RG, which undergo rearrangement to form a mixed anhydride in their protonated forms, do not undergo the same rearrangement in their radical cation forms.

Generation of carotenoid radical cation in the vicinity of a chlorophyll derivative bound to titanium oxide, upon excitation of the chlorophyll derivative to the Qy state, as identified by time-resolved absorption spectroscopy

Abstract Electron injection from a chlorophyll derivative (methyl 3-carboxy-3-devinylpyropheophorobide a , abbreviated as PPB a ) to TiO 2 nanoparticle took place in ≈30 fs following the decay of an excimer that was generated immediately after excitation to the Q y state (681 nm). Then, electron transfer from carotenoids (Cars) to PPB a + took place in ≈200–240 ps. The latter observation supports the electron-transfer mechanism proposed in a previous investigation, in which Cars were added as redox spacers to the PPB a -sensitized TiO 2 solar cells to enhance their performance (X.-F. Wang, J. Xiang, P. Wang, Y. Koyama, S. Yanagida, Y. Wada, K. Hamada, S. Sasaki, H. Tamiaki, Chem. Phys. Lett. 408 (2005) 409).

Reaction of aromatic amines with Cu(ClO 4) 2 in acetonitrile as a facile route to amine radical cation generation

This Letter describes a very simple and effective method for the generation and study of radical cations of aromatic amines. It is shown that simple mixing of micro molar solutions of aromatic amines with micro molar amounts of Cu(ClO 4) 2 in acetonitrile solution leads to formation of amine radical cations in good yields. The radical cations thus generated are unequivocally characterized by their absorption and electron spin resonance spectra. It is proposed that the radical cations are formed through the donation of an electron from the amines to Cu 2+.

Facile Generation of Tripeptide Radical Cations In Vacuo via Intramolecular Electron Transfer in Cu II Tripeptide Complexes Containing Sterically Encumbered Terpyridine Ligands

Molecular radical cations of tripeptides of the form glycylglycyl(residue X) (GGX*+) are produced by the collision-induced, intramolecular one-electron transfer of [Cu(II)(L)GGX]*2+ complexes (L = triamine ligand). We demonstrate, for the first time, the formation of molecular radical cations of all of the aliphatic, basic, aromatic, acidic, and some heteroatom-bearing GGX tripeptides, albeit inefficiently in some cases, by altering the structure of the auxiliary polyamine ligand bound to the copper atom. The design of the ligand allows exquisite control over the nature of the dissociation pathway. Steric hindrance of bulky groups in the ligand affects the binding of the peptide to the copper ion; this interaction is an important factor in determining whether the electron transfer pathway predominates.

Oxidation of 1-(4′-substituted phenyl)-2-(4-methylphenyl)diazene — Benzylic substitution vs. C–azo bond breaking

The low oxidation potential of 1-phenyl-2-(4-methylphenyl)diazene 4′-substituted with a dimethylamino or methoxy group (10 and 11) allows the easy single electron transfer with radical cation generation in the presence of FeCl3. At the same time, the stabilizing effect of the positive charge exerted by 4-dimethylaminophenyl > 4-methoxy groups on the intermediate carbocation favors the hydrogen atom elimination in comparison with proton loss. Whereas benzylic substitution towards di- and triarylmethane derivatives takes place starting from diazene 11, the oxidation of diazene 10 leads mainly to products of C–azo breaking. This difference is well-supported by the idea that the electron, in the first step of oxidation, is extracted from the azo and (or) from the amino nitrogen atom.Key words: diazenes, oxidation, single electron transfer, benzylic substitution, C–azo bond breaking.

Ship-in-a-bottle synthesis of triphenylamine inside faujasite supercages and generation of the triphenylamminium radical ion

The ship-in-a-bottle synthesis of triphenylamine encapsulated within basic X zeolite has been accomplished by reacting sodium diphenylamide with bromobenzene in the presence of a bifunctional palladium (Hartwig–Buchwald conditions). The presence of incarcerated triphenylamine was demonstrated by dissolving the zeolite with concentrated HF and analyzing the organic material in the dichloromethane extract. Laser flash photolysis (266 nm) gives rise to the generation of triphenylamminium radical cation detected as a transient species decaying in hundreds of microseconds. Upon repetitive cyclic voltammograms, zeolite encapsulated triphenylamine shows a reversible oxidation–reduction process. In contrast, in solution triphenylamine undergoes irreversible oxidation with the formation of coupling dimers.

Oxidation catalysis of unsaturated hydrocarbons with molecular oxygen via single-electron transfer in thermally treated H zeolites

An electron spin resonance investigation demonstrates that radical cations in dehydroxylated zeolites are intermediates of a catalytic oxidation reaction that consumes molecular oxygen and involves radical cation transients of unsaturated hydrocarbons. Radical cation generation has often been attributed to the action of Lewis acid sites, generated by a thermal treatment of a proton-exchanged zeolite. The present study of the conditions for radical cation generation and stabilization shows that the active centers are nonacidic, which means that separate special single-electron redox-active catalytic sites are present in the zeolite. Simultaneous monitoring by ESR of the organic radical cation, the superoxide radical anion, and molecular oxygen in the reaction of 2,5-dimethylhexa-2,4-diene with O 2 in dehydroxylated H-MOR revealed in one case more than 4000 cycles per active center, demonstrating their catalytic activity.

Evaluation of the efficiency of the photocatalytic one-electron oxidation reaction of aromatic compounds adsorbed on a TiO2 surface.

The TiO2 photocatalytic one-electron oxidation mechanism of aromatic sulfides with a methylene bridging group (-(CH2)n-, n=0-4) between the 4-(methylthio)phenyl chromophore and the carboxylate binding group on the surface of a TiO2 powder slurried in acetonitrile (MeCN) has been investigated by time-resolved diffuse reflectance (TDR) spectroscopy. The electronic coupling element (H(DA)) between the hole donor and acceptor, which was estimated from the spectroscopic characteristics of the charge transfer (CT) complexes of the substrates (S) and the TiO2 surface, exhibited an exponential decline with the increasing of the methylene number of S. The determined decay factor (beta) of 9 nm(-1) also supports the fact that the 4-(methylthio)phenyl chromophore is separated from the TiO2 surface. The efficiency of the one-electron oxidation of S adsorbed on the TiO2 surface, which was determined from the relationship between the amount of adsorbates and the concentration of the generated radical cations, significantly depended on the H(DA) value, but not on the oxidation potential of S determined in homogeneous solution.

Radical Cation of Pyrazine-di-N-oxide as a Mediator in Electrocatalytic Oxidation of Organic Compounds

The mechanism of oxidation of cyclohexanol, methanol, diethyl ether, triethyl orthoformate, and cyclohexane in the presence of a mediator—electrochemically generated radical cation of pyrazine-di-N-oxide (PyrDNO)—is studied on glassy carbon and platinum in a 0.1 M LiClO4 solution in acetonitrile employing cyclic voltammetry, ESR electrolysis, and gas chromatography. Effect of temperature, additives of acid and water, oxygen, and the nature of the substrate and solvent on the shape of cyclic voltammograms and intensity of ESR signal of PyrDNO is examined. ESR spectra for radical cations and anions of PyrDNO with g factors equal to, respectively, 2.0090 and 2.0031 are recorded. A mechanism for the overall two-electron catalytic oxidation of an organic substance, which involves a stage in which it complexes with the radical cation of PyrDNO, is suggested.

Radical Cation Generation from Singlet and Triplet Excited States of All‐trans‐Lycopene in Chloroform¶

ABSTRACTOn direct photoexcitation, subpicosecond time‐resolved absorption spectroscopy revealed that the 1Bu‐type singlet excited state of all‐trans‐lycopene in chloroform was about seven times more efficient than all‐trans‐β‐carotene in generating the radical cation. The time constant of radical cation generation from the 1Bu‐type state was found to be ∼0.14 ps, a value that was comparable for the two carotenoids. On anthracene‐sensitized triplet excitation, radical cation generation was found to be much less efficient for lycopene than for β‐carotene. A slow rising phase (20‐30 μs) in the bleaching of ground‐state absorption was common for both lycopene and β‐carotene in chloroform and was ascribed to an efficient secondary reaction with a solvent radical leading to the formation of carotenoid radical cations. The reverse ordering in the tendency of the excited states of different multiplicities for the two carotenoids to generate radical cations is discussed in relation to the two carotenoids as scavengers of free radicals.

Study of the ac conductivity of α, α′-dimethyl sexithiophene in pristine and doped states

The study of the electrical and charge transport properties in α, α′-dimethyl sexithiophene in neutral and doped states is addressed by means of dielectric spectroscopy. Doping treatment with iodine gives rise to the generation of radical cations, and also to its π-dimers and π-stacks associated forms. Quite different electric features have been found for the neutral and doped samples. The pristine material present a low conductivity and can be considered into the Mott model of optical phonon assisted hopping of charge carrier between localized states plus the energy difference between two adjacent sites; the charge carrier relaxation time and the polarization relaxation time are different and, moreover, they present different activation energies by which we can assume that both processes are different. The charge transport mechanism in the iodine-doped material has been found to be independent of the applied electric field frequency, thus the charge transport is consistent with the existence of a band-like motion within the planes into the micro-crystals, and a phonon-assisted thermally activated hopping process between the planes, with an activation energy of 0.17 eV.

Generation and absolute reactivity of an aryl enol radical cation in solution.

Laser flash photolysis of 1-bromo-1-(4-methoxyphenyl)acetone in acetonitrile leads to the formation of the alpha-acyl 4-methoxybenzyl radical that under acidic conditions rapidly protonates to give detectable amounts of the radical cation of the enol of 4-methoxyphenylacetone. This enol radical cation is relatively long-lived in acidic acetonitrile (tau approximately equal to 200 micros), which is on the same order of magnitude as the radical cations of other 4-methoxystyrene derivatives. Rate constants for deprotonation of the radical cation and the acid dissociation constant for the enol radical cation were also determined using time-resolved absorption spectroscopy. Deprotonation is rapid, taking place with a rate constant of 3.9 x 10(6) s(-1), but the enol radical cation is found to be only moderately acidic in acetonitrile having a pK(a) = 3.2. The lifetime of the enol radical cation was also found to be sensitive to the presence of oxygen and chloride. The sensitivity toward oxygen is explained by oxygen trapping the vinyloxy radical component of the enol radical cation/vinyloxy equilibrium, while chloride acts as a nucleophile to trap the enol radical cation.

Synthesis of Novel Steroid Backbones via Photoinduced Electron Transfer Initiated Domino Cyclizations of Silyl Enol Ethers

AbstractFor Abstract see ChemInform Abstract in Full Text.

Increased yields of radical cations by arene addition to irradiated 1,2-dichloroethane

Pulse radiolysis in chlorinated hydrocarbon liquids such as 1,2-dichloroethane is a versatile and effective method for the generation of solute radical cations. The addition of a large concentration of toluene or benzene to solutions of 1,2-dichloroethane is found to increase the yield of solute radical cations ( G=0.68 molecules 100 eV −1 in 1,2-dichloroethane (J. Phys. Chem. 83(15) (1979) 1944) by a factor of 2.5. The increased yield is found for solutes which have a potential of ∼1.1 V (vs. SCE) or below for the S + /S couple and is due to reaction of the chlorine atom:toluene (π-Cl ) complex with the solute. A similar species forms with benzene. π-Cl is formed with a yield of G=3.0, and arises principally as a result of geminate recombination of ions. It has an absorption in the visible with λ max 460 nm, ε max=1800 M −1 cm −1 and decays with an observed first-order rate constant k=1.12×10 6 s −1. The rate of reaction of the π-Cl • with added solutes ranges from 2.5 to 5×10 9 M −1 s −1. The other oxidant present in the 1,2-dichloroethane/toluene solutions is identified as the toluene cation dimer. This is formed from the 1,2-dichloroethane radical cation with bimolecular rate constant k=1.35×10 10 M −1 s −1 with a radiation chemical yield G=0.5. The rate of reaction of this species with the added solutes is diffusion controlled, k=1–2×10 10 M −1 s −1.

Matrix-Isolation in Cryogenic Water-Ices: Facile Generation, Storage, and Optical Spectroscopy of Aromatic Radical Cations

Radical cations of naphthalene and 4-methylpyrene have been generated for the first time in high conversion efficiencies in cryogenic water-ices at 15 K through vacuum ultraviolet photolysis. With these radical cations as probes it is shown that cryogenic water-ices at temperatures below 50 K are of good optical quality and inert matrices to isolate and study the electronic spectroscopic properties of neutral and ionic species in the wavelength region 250−900 nm. The spectral energies of guest-species in the cryogenic water-ices are closely comparable with those observed using rare-gas matrices, indicating similar host−guest interactions in rare-gas matrices and water-ices below 50 K. The radical cations are converted to the corresponding alcohols at temperatures higher than 50 K due to reactions between the host and ionized guest species. Thus, cryogenic water-ices are inert matrices that resemble and complement the rare-gas matrices in many aspects. Efficient ionization of organic molecules, such as PAHs studied here, in water-rich ices indicates that ionization-mediated processes play an important role in the evolution of cosmic ices that are exposed to ionizing radiation.

Radical Cation Generation from Singlet and Triplet Excited States of All-trans-Lycopene in Chloroform¶

On direct photoexcitation, subpicosecond time-resolved absorption spectroscopy revealed that the 1B(u)-type singlet excited state of all-trans-lycopene in chloroform was about seven times more efficient than all-trans-beta-carotene in generating the radical cation. The time constant of radical cation generation from the 1B(u)-type state was found to be approximately 0.14 ps, a value that was comparable for the two carotenoids. On anthracene-sensitized triplet excitation, radical cation generation was found to be much less efficient for lycopene than for beta-carotene. A slow rising phase (20-30 micros) in the bleaching of ground-state absorption was common for both lycopene and beta-carotene in chloroform and was ascribed to an efficient secondary reaction with a solvent radical leading to the formation of carotenoid radical cations. The reverse ordering in the tendency of the excited states of different multiplicities for the two carotenoids to generate radical cations is discussed in relation to the two carotenoids as scavengers of free radicals.

Radical cation generation from singlet and triplet excited states of all-trans-lycopene in chloroform.

On direct photoexcitation, subpicosecond time-resolved absorption spectroscopy revealed that the 1B(u)-type singlet excited state of all-trans-lycopene in chloroform was about seven times more efficient than all-trans-beta-carotene in generating the radical cation. The time constant of radical cation generation from the 1B(u)-type state was found to be approximately 0.14 ps, a value that was comparable for the two carotenoids. On anthracene-sensitized triplet excitation, radical cation generation was found to be much less efficient for lycopene than for beta-carotene. A slow rising phase (20-30 micros) in the bleaching of ground-state absorption was common for both lycopene and beta-carotene in chloroform and was ascribed to an efficient secondary reaction with a solvent radical leading to the formation of carotenoid radical cations. The reverse ordering in the tendency of the excited states of different multiplicities for the two carotenoids to generate radical cations is discussed in relation to the two carotenoids as scavengers of free radicals.

Synthesis of Novel Steroid Backbones via Photoinduced Electron Transfer Initiated Domino Cyclizations of Silyl Enol Ethers

Photoinduced electron transfer was used to activate silyl enol ethers. The generated radical cations of specifically designed precursors undergo ring closure reactions to yield novel steroid compounds. These domino reactions proceed with high stereo-selectivity. The synthesis of the complex silyl enol ethers, the stereo-selective domino cyclization process and the assignment of stereochemistry of the novel steroid compounds is presented.

Synthesis, cyclic voltammetric studies, and electrogenerated chemiluminescence of a new phenylquinoline-biphenothiazine donor-acceptor molecule.

We report the synthesis, electrochemistry, and luminescence of a novel ECL emitting compound containing two electron-accepting hexyl-phenylquinoline groups covalently attached to the 3,3'-positions of the electron-donating 10,10'-dimethylbiphenothiazine group. The optimized geometry as determined from semiempirical MNDO calculations shows that the two quinoline groups are twisted 82.5 degrees from the two phenothiazine rings, indicating a lack of electron delocalization among these groups. This unique geometry allows generation of localized radical cations and radical anions capable of generating ECL upon annihilation. However, the phenothiazine rings are twisted 46.5 degrees relative to each other, suggesting possible interactions between the two moieties. This is evident in the electrochemical behavior in which two closely spaced one-electron oxidations, rather than a single two-electron oxidation wave, were observed. The photophysical properties of BHQ-BPZ show strong resemblances to the parent compound, BPQ-PTZ, which contains a single phenothiazine moiety. In addition, the ECL spectrum produced via radical ion annihilation shows good agreement with the fluorescence emission of the compound.