Generation Of Radical Cations Research Articles

A Viologen-Perylenediimide Conjugate as an Efficient Base Sensor with Solvatochromic Property.

A viologen-perylenediimide conjugate, denoted PDEV, is prepared for efficient base sensing. The conjugate shows solvatochromic behavior as well. The base sensitivity of viologen is purposefully coupled with the emission property of perylenediimide (PDI) to lower the detection limit. PDEV shows base-sensing ability at the ppb level, which is at least three orders of magnitude lower than those of previously reported sensors. The probe is sensitive toward solvent polarity and generates different shades of colors according to the polarity of the medium (solvent). The photophysical properties show a linear correlation with the solvent polarity, and this makes it an efficient solvatochromic agent. On the other hand, the generation of viologen radical cations by bases affects the aggregation and consequently the absorption and emission behavior of the PDI core. The effect of bases can also be visualized, because the probe generates different colors in the presence of bases, both under normal and under UV light. Organic amines can be detected even in the crystalline state, since the dark red color of the PDEV crystals changes to purple in a reversible fashion on exposure to amine vapors. An easy and practical paper-based tool created by using the probe can efficiently be used to detect solvent polarity and presence of bases optically.

An M2 L4 Molecular Capsule with a Redox Switchable Polyradical Shell.

Preparation of molecular nanostructures with polyradical frameworks remains a significant challenge because of the limited synthetic accessibility which is entirely different from that of neutral and ionic ones. Herein we report the quantitative formation of a new M2 L4 molecular capsule from metal ions and dihydrophenazine-based ligands. The capsule has a spherical nanocavity (ca. 1 nm in diameter) enclosed by eight redox-active, dihydrophenazine panels. Electrochemical oxidation of the capsule leads to the generation of multiple radical cations on the shell framework. Moreover, a stable tetra(radical cation) capsule can be reversibly obtained by chemical as well as electrochemical oxidation.

An M2L4 Molecular Capsule with a Redox Switchable Polyradical Shell

AbstractPreparation of molecular nanostructures with polyradical frameworks remains a significant challenge because of the limited synthetic accessibility which is entirely different from that of neutral and ionic ones. Herein we report the quantitative formation of a new M2L4 molecular capsule from metal ions and dihydrophenazine‐based ligands. The capsule has a spherical nanocavity (ca. 1 nm in diameter) enclosed by eight redox‐active, dihydrophenazine panels. Electrochemical oxidation of the capsule leads to the generation of multiple radical cations on the shell framework. Moreover, a stable tetra(radical cation) capsule can be reversibly obtained by chemical as well as electrochemical oxidation.

Synthesis and Single-Molecule Conductances of Neutral and Cationic Indenofluorene-Extended Tetrathiafulvalenes: Kondo Effect Molecules.

Development of molecules that can switch between redox states with paired and unpaired electrons is important for molecular electronics and spintronics. In this work, a selection of redox-active indenofluorene-extended tetrathiafulvalenes (IF-TTFs) with thioacetate end groups was prepared from a readily obtainable dibromo-functionalized IF-TTF building block using palladium-catalyzed cross-coupling reactions, such as the Suzuki reaction. The end groups served as electrode anchoring groups for single-molecule conductance studies, and the molecules were subjected to mechanically controlled break-junction measurements with gold contacts and to low-bias charge transport measurements in gated three-terminal electromigration junctions. The neutral molecules showed clear conductance signatures, and somewhat surprisingly, we found that a meta–meta anchoring configuration gave a higher conductance than a para–meta configuration. We explain this behavior by “through-space” coupling between the gold electrode and the phenyl on which the anchoring group is attached. Upon charging the molecule in a gated junction, we found reproducibly a Kondo effect (zero-bias conductance) attributed to a net spin. Ready generation of radical cations was supported by cyclic voltammetry measurements, revealing stepwise formation of radical cation and dication species in solution. The first oxidation event was accompanied by association reactions as the appearance of the first oxidation peak was strongly concentration dependent.

Radiation-induced damage to cellular DNA: Chemical nature and mechanisms of lesion formation

This mini-review focuses on the recent identification of several novel radiation-induced single and tandem modifications in cellular DNA. For this purpose accurate high-performance electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS) was applied allowing their quantitative measurement and unambiguous characterization. Exposure of human cells to gamma rays led to the formation of several modified bases arising from the rearrangement of the pyrimidine ring of thymine, cytosine and 5-methylcytosine subsequent to initial addition of an hydroxyl radical (•OH) to the 5,6-ethylenic bond. In addition, 5-hydroxymethylcytosine, an novel epigenetic mark, and 5-formylcytosine, were found to be generated consecutively to •OH-mediated hydrogen abstraction from the methyl group of 5-methylcytosine. Relevant mechanistic information on one-oxidation reactions of cellular DNA was also gained from the detection of 5-hydroxycytosine and guanine-thymine intra-strand adducts whose formation is rationalized by the generation of related base radical cation. Attempts to search for the radiation-induced formation of purine 5′,8-cyclo-2′-deoxyribonucleosides were unsuccessful with the exception of trace amounts of (5′S)-5′,8-cyclo-2′-deoxyadenosine.

ChemInform Abstract: The Radical Cation Mediated Cleavage of Catharanthine Leading to the Vinblastine Type Alkaloids: Implications for Total Synthesis and Drug Design

AbstractReview: 67 refs.

Quantitative generation of cation radicals and dications using aromatic oxidants: effect of added electrolyte on the redox potentials of aromatic electron donors

Redox titrations using robust aromatic oxidants allow for a quantitative analysis of redox and optical properties of organic electron donors in their oxidized states. Unlike spectroelectrochemistry, redox titrations can be performed without added electrolyte in relatively nonpolar solvents, affording quick access to the redox and optical properties of a given electron donor without the need of a complex electrochemical setup. However, the redox potentials obtained by the two methods are not the same. To establish the direction and magnitude of this discrepancy, we have performed a systematic case study using a set of tetraarylethylene donors and a tetrasubstituted hydroquinone ether cation radical (THEO+•) as a stable aromatic oxidant. We show that redox potentials (especially second and higher oxidation potentials) measured by electrochemical methods are systematically lower compared with the redox potentials obtained by redox titrations in the absence of electrolyte, because of the enhanced stabilization of dications and polycations by electrolyte. We have also uncovered that the smaller cation radicals (e.g., a para‐hydroquinone or ortho‐hydroquinone ether cation radicals) are much more effectively stabilized when compared with the cation radicals in which charge is delocalized over larger area (e.g., tetraarylethylene cation radicals) in the presence of electrolyte because of increased ionic strength of the solution. Copyright © 2015 John Wiley & Sons, Ltd.

Strategies for generating peptide radical cations via ion/ion reactions.

Several approaches for the generation of peptide radical cations using ion/ion reactions coupled with either collision induced dissociation (CID) or ultraviolet photo dissociation (UVPD) are described here. Ion/ion reactions are used to generate electrostatic or covalent complexes comprised of a peptide and a radical reagent. The radical site of the reagent can be generated multiple ways. Reagents containing a carbon-iodine (C-I) bond are subjected to UVPD with 266-nm photons, which selectively cleaves the C-I bond homolytically. Alternatively, reagents containing azo functionalities are collisionally activated to yield radical sites on either side of the azo group. Both of these methods generate an initial radical site on the reagent, which then abstracts a hydrogen from the peptide while the peptide and reagent are held together by either electrostatic interactions or a covalent linkage. These methods are demonstrated via ion/ion reactions between the model peptide RARARAA (doubly protonated) and various distonic anionic radical reagents. The radical site abstracts a hydrogen atom from the peptide, while the charge site abstracts a proton. The net result is the conversion of a doubly protonated peptide to a peptide radical cation. The peptide radical cations have been fragmented via CID and the resulting product ion mass spectra are compared to the control CID spectrum of the singly protonated, even-electron species. This work is then extended to bradykinin, a more broadly studied peptide, for comparison with other radical peptide generation methods. The work presented here provides novel methods for generating peptide radical cations in the gas phase through ion/ion reaction complexes that do not require modification of the peptide in solution or generation of non-covalent complexes in the electrospray process.

Generation of long-lived methylviologen radical cation in the triplet-state mediated electron transfer in a β-cyclodextrin based supramolecular triad

A novel tris(bipyridyl)ruthenium–pyrene–methylviologen supramolecular triad was assembled through inclusion complexation of adamantane-linked Ru(II)–Py dyad in MV2+-linked β-cyclodextrin. Excitation of the Ru(II) chromophore populated its 3MLCT which upon energy transfer gave 3Py, which donates an electron to MV2+ to give Ru(II)–Py+–MV+. A second electron transfer then occurs from Ru(II) to Py+ to give the supramolecular Ru(III)–Py–MV+ charge separated state. Laser flash photolysis experiments confirmed formation of MV+ which exhibited 100μs lifetime. Steady state irradiation of the self-assembled system in the presence of sacrificial donor also led to formation of long-lived MV+.

SFRR-E Basic Science Lecture (Elsevier sponsored) - Oxidative degradation pathways of cellular DNA: product formation and mechanistic insights

More than 100 oxidized purine and pyrimidine nucleosides including hydroperoxides and diastereomers have been characterized so far in extensive model studies. However, much less information is currently available on the oxidatively generated base damage to cellular DNA at the exception however of the overwhelming modifications produced by singlet oxygen ((1)O2). This is mostly due to analytical difficulties that are now, at least, partly overcome with the advent of the accurate and sensitive high performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry (HPLC/ESI-MS/MS). Hydroxyl radical ((•)OH) and one-electron oxidants that may be either endogenously formed through oxidative metabolism, phagocytosis, inflammation and other pathological conditions are predominantly at the origin of oxidatively generated damage to cellular DNA. It is worth mentioning that exposure of cells to exogenous physical agents (UVA light, high intensity UV laser, ionizing radiation) and chemicals such as bromate may also induce oxidatively generated damage to DNA. Emphasis is placed in this presentation on the critical survey of the main recently available information concerning the formation of (1)O2, (•)OH and one-electron oxidant-mediated single and more complex DNA damage (tandem lesions, intra- and interstrand cross-links, DNA-protein cross-links) arising from one radical hit. Evidence was provided that (•)OH and one-electron oxidants, through the generation of neutral radicals and radical cations respectively from nucleobases, are able to induce partly common degradation pathways. In addition selective oxidative reactions giving rise to specific degradation products of (•)OH and one-electron oxidation reactions that can be used as representative biomarkers of these oxidants have been identified. Emphasis was recently placed on the detection of oxidatively generated damage to cytosine and 5-methylcytosine in human cells.

Hg(2+) -induced in situ generated radical cation of (S)-BINOL-based polymer for highly enantioselective recognition of phenylalaninol.

Phenylalaninol enantiomers are one of the most important chiral compounds due to its presence in biologically active molecules and pharmaceutical products. In this paper, a novel chiral fluorescence polymer sensor incorporating (S)-BINOL and oligomeric aniline via a nucleophilic addition-elimination reaction is designed and synthesized. Polymer sensor exhibits "turn-off" fluorescence quenching response upon the addition of Hg(2+) , and "turn-on" moderate fluorescence enhancement behavior towards phenylalaninol enantiomers. Meanwhile, this kind of (S)-BINOL-based polymer sensor can exhibit highly selective enantioselective recognition response towards (L)-phenylalaninol upon the addition of Hg(2+) and the value of ef can reach as high as 5.4, which can be attributed to the formation of in situ generated radical cation arisen from oligomeric aniline moiety by Hg(2+) induction.

Electron Transfer Reactions in the Chemistry of Di- and Tetrahydropyridines

The mechanism of electrochemical oxidation of 1,2,3,4-tetrahydropyridines in acetonitrile has been studied. A single reversible one-electron oxidation is registered in the accessible voltage range. The reversibility of the process is sensitive to the traces of oxygen in solution. The electrochemically generated radical cation of tetrahydropyridine may act as a mediator in an indirect oxidation of dihydropyridines if the difference in oxidation potentials between two compounds is less than 200 mV. During the indirect oxidation of 2,4,6-trimethyl-1,4-dihydropyridine-3,5-dicarboxylic acid diethyl ester to 3,5-bis(ethoxycarbonyl)-2,4,6-trimethylpyridinium perchlorate, some of the starting tetrahydropyridine is protonated, thus making it anodically inactive.

Near-Infrared Colorimetric and Fluorescent Cu2+ Sensors Based on Indoline–Benzothiadiazole Derivatives via Formation of Radical Cations

The donor-acceptor system of indoline-benzothiadiazole is established as the novel and reactive platform for generating amine radical cations with the interaction of Cu(2+), which has been successfully exploited as the building block to be highly sensitive and selective near infrared (NIR) colorimetric and fluorescent Cu(2+) sensors. Upon the addition of Cu(2+), an instantaneous red shift of absorption spectra as well as the quenched NIR fluorescence of the substrates is observed. The feasibility and validity of the radical cation generation are confirmed by cyclic voltammetry and electron paramagnetic resonance spectra. Moreover, the introduction of an aldehyde group extends the electron spin density and changes the charge distribution. Our system demonstrates the large scope and diversity in terms of activation mechanism, response time, and property control in the design of Cu(2+) sensors.

A comparative study of radical cations of thiourea, thiosemicarbazide, and diethylthiourea in aqueous sulphuric acid media employing pulse radiolysis technique

The generation of radical cations of thiourea [(NH2)2C=S], thiosemicarbazide [(NH2―NH)(NH2)C=S], and diethylthiourea [(C2H5―NH)2C=S] in aqueous sulphuric acid media of low to high acid strengths utilizing the pulse radiolysis technique is reported. In this system, the decay half‐life of radical cations of thiourea and its derivatives observed were longer (ranging from 45 to 100 µs at acid strength pH/H0 = −4.1), and the formation times could be altered with the change in acid strengths. By doing so, a wide range in half‐life of radical cations of thiosemicarbazide (0.7 to 100 µs) was observed in contrast to thiourea and diethylthiourea with the variation in acid strengths (pH/H0) from 0.7 to −4.1. This perhaps helps in studying the spectral and kinetic properties of transient radical cations comfortably. The general mechanism for the formation of radical cations of the aforementioned compounds in radiolysis has been revisited in which the electron transfer reactions are active. Copyright © 2013 John Wiley & Sons, Ltd.

A viologen-urea-based anion receptor: Colorimetric sensing of dicarboxylate anions

Abstract Compound 1 bearing urea and viologen groups has been designed and synthesized. It could be used as a colorimetric receptor for dicarboxylate anions due to the significant color changes of the solution upon the addition of dicarboxylates. More importantly, the color changes were related to the chain lengths of the dicarboxylates tested. UV–vis, 1 H NMR and HRMS studies demonstrated that receptor 1 utilized hydrogen bonds and electrostatic interactions to form 1:1 stoichiometry complexes with these anions. In addition, the generation of cation radical 1 + during the complexation process was also detected by EPR.

Core-Modified Naphthalenediimides Generate Persistent Radical Anion and Cation: New Panchromatic NIR Probes

The generation of the first persistent radical cation of naphthalenediimide with Cu(2+)/Fe(3+) under ambient conditions is reported. An alternate anionic trigger generates a persistent radical anion within the same motif. Steric protection and H-bonding enhances the half-life of radical cation by 290-fold. The radical anion and cation have orthogonal spin density, panchromatic and NIR optical bands, which can be applied as attractive multichannel probes.

Flow-injection methods for the determination of antioxidant activity based on free-radical processes

Flow-injection procedures are developed for the spectrophotometry determination of the antioxidant properties of low-weight biological molecules based on their interaction with free organic radicals, namely, the on-line generated radical cation from 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) and the longlived 2,2′-diphenyl-1-picrylhydrazyl radical. The optical and redox properties of these radicals in aqueous and water-ethanol media are studied with and without an antioxidant in the mixture. The applicability of the methods is demonstrated on a number of exogenous antioxidants: ascorbic and uric acid, glutathione, and others, trolox, a synthetic analogue of α-tocopherol, and species of plant origin: gallic, ferulic, and caffeic acids. The comparative evaluation of the total antioxidant activity of known anti-inflammatory medicines is performed. The throughput of the methods reported is 90–200 analyses per hour.

Influence of Phenolic Compounds on the Mechanisms of Pyrazinium Radical Generation in the Maillard Reaction

The generation of pyrazinium radical cations during the early stages of the Maillard reaction has been previously demonstrated. In this study, the effect of food phenolic compounds [4-methylcatechol (4-MeC), (+)-catechin (CAT), and (-)-epigallocatechin-3-gallate (EGCG)] on the fate of these intermediates in Maillard model systems was investigated. Aqueous solutions containing either glyoxal + alanine (GO-A) or glycolaldehyde + alanine (GA-A) were treated with a concentration gradient of each phenolic compound, and quantitative analysis of the resulting pyrazinium radicals in these models was performed using electron paramagnetic resonance (EPR) spectroscopy. CAT and EGCG were observed to affect pyrazinium radical generation rates, in some cases either enhancing or suppressing formation depending on concentration, whereas the simple catechol (4-MeC) had no such effect. A mechanistic study was carried out by LC-MS, which suggested that under some conditions, CAT and EGCG react with imine intermediates via their A-rings, thus influencing the formation of the enaminol radical precursor and, ultimately, pyrazinium radicals. To the authors' knowledge, this is the first study demonstrating imine trapping by phenolic compounds under Maillard conditions and how such phenolic quenching reactions can alter pyrazinium radical formation.

ChemInform Abstract: Electrochemical Reactions of Indolysines

AbstractReview: 118 refs.

Enhancement of Two-Photon Initiating Efficiency of a 4,4′-Diaminostyryl-2,2′-bipyridine Derivative Promoted by Complexation with Silver Ions

We report on the two-photon-induced polymerization (TPIP) ability of a new class of free radical two-photon initiator based on a cationic silver(I) complex incorporating 4,4′-diaminostyryl-2,2′-bipyridine (DES) derivatives as ligands. Coordination with Ag+ induces a strong increase of the charge transfer character at excited state, which enhances the two-photon absorption properties of the complex with respect to that of the free ligand. A comparative analysis of the photophysical properties of DES and DES2Ag+ shows that the presence of silver cation increases the efficiency for the generation of radical cations (DES•+), which can be used as hydrogen abstractor in free radical photopolymerization. We show that the addition of an aliphatic amine used as hydrogen donor also opens a parallel route for the regeneration of DES. The improvement of the two-photon polymerization efficiency is then evidenced by the fabrication of microstructures. We finally demonstrate that the use of DES2Ag+ as two-photon initiator offers new opportunities for the fabrication of functional nanostructures composed of metal–polymer nanocomposites.