Long-lived Radical Cations Research Articles

Ag3PO4 enables the generation of long-lived radical cations for visible light-driven [2 + 2] and [4 + 2] pericyclic reactions

Photocatalytic redox reactions are important for synthesizing fine chemicals from olefins, but the limited lifetime of radical cation intermediates severely restricts semiconductor photocatalysis efficiency. Here, we report that Ag3PO4 can efficiently catalyze intramolecular and intermolecular [2 + 2] and Diels-Alder cycloadditions under visible-light irradiation. The approach is additive-free, catalyst-recyclable. Mechanistic studies indicate that visible-light irradiation on Ag3PO4 generates holes with high oxidation power, which oxidize aromatic alkene adsorbates into radical cations. In photoreduced Ag3PO4, the conduction band electron (eCB−) has low reduction power due to the delocalization among the Ag+-lattices, while the particle surfaces have a strong electrostatic interaction with the radical cations, which considerably stabilize the radical cations against recombination with eCB−. The radical cation on the particle’s surfaces has a lifetime of more than 2 ms, 75 times longer than homogeneous systems. Our findings highlight the effectiveness of inorganic semiconductors for challenging radical cation-mediated synthesis driven by sunlight.

Absorption of Low-Energy UV Radiation by Human Telomere G-Quadruplexes Generates Long-Lived Guanine Radical Cations.

Telomeres, which are involved in cell division, carcinogenesis, and aging and constitute important therapeutic targets, are prone to oxidative damage. This propensity has been correlated with the presence of guanine-rich sequences, capable of forming four-stranded DNA structures (G-quadruplexes). Here, we present the first study on oxidative damage of human telomere G-quadruplexes without mediation of external molecules. Our investigation has been performed for G-quadruplexes formed by folding of GGG(TTAGGG)3 single strands in buffered solutions containing Na+ cations (TEL21/Na+). Associating nanosecond time-resolved spectroscopy and quantum mechanical calculations (TD-DFT), it focuses on the primary species, ejected electrons and guanine radicals, generated upon absorption of UV radiation directly by TEL21/Na+. We show that, at 266 nm, corresponding to an energy significantly lower than the guanine ionization potential, the one-photon ionization quantum yield is 4.5 × 10-3. This value is comparable to that of cyclobutane thymine dimers (the major UV-induced lesions) in genomic DNA; the quantum yield of these dimers in TEL21/Na+ is found to be (1.1 ± 0.1) × 10-3. The fate of guanine radicals, generated in equivalent concentration with that of ejected electrons, is followed over 5 orders of magnitude of time. Such a quantitative approach reveals that an important part of radical cation population survives up to a few milliseconds, whereas radical cations produced by chemical oxidants in various DNA systems are known to deprotonate, at most, within a few microseconds. Under the same experimental conditions, neither one-photon ionization nor long-lived radical cations are detected for the telomere repeat TTAGGG in single-stranded configuration, showing that secondary structure plays a key role in these processes. Finally, two types of deprotonated radicals are identified: on the one hand, (G-H2)• radicals, stable at early times, and on the other hand, (G-H1)• radicals, appearing within a few milliseconds and decaying with a time constant of ∼50 ms.

What Makes Oxidized N-Acylanthranilamides Stable?

Oligoamides composed of anthranilic acid derivatives present a promising choice for mediating long-range charge transfer and controlling its directionality. Hole hopping, modulated by the anthranilamide (Aa) permanent dipoles, provides a plausible means for such rectified long-range charge transduction. All aliphatic and most aromatic amides, however, decompose upon oxidation, rendering them unacceptable for hole-hopping pathways. We, therefore, employ electrochemical and computational analysis to examine how to suppress oxidative degradation and stabilize the radical cations of N-acylated Aa derivatives. Our findings reveal two requirements for attaining long-lived radical cations of these aromatic amides: (1) keeping the reduction potentials for oxidizing the Aa residues under about 1.4 V vs SCE and (2) adding an electron-donating group para to the N-terminal amide of the aromatic ring, which prevents the electron spin density of the radical cation from extending over the C-terminal amide. These findings provide essential information for the design of hole-transfer amides.

Site-specific hydrogen exchange and hydrogen transfer processes preceding the fragmentation of long-lived radical cations of ethyl dihydrocinnamate and related arylalkanoates

An electron ionisation study on the fragmentation of metastable molecular radical cations of ethyl 3-phenylpropanoate (ethyl dihydrocinnamate) and related arylalkanoic acid esters was performed by mass-analysed ion kinetic energy (MIKE) spectrometry. Six deuterium-labelled isotopomers of ethyl dihydrocinnamate were synthesised and studied by MIKE spectrometry. The fragmentation leading to ions C7H7O+ (m/z 107) involving migration of the alkoxycarbonyl group was also observed in the 70-eV mass spectra of related alkyl dihydrocinnamates, but it was found to be a high-energy process that does not compete at low energies in metastable molecular ions. Instead, the metastable ions of ethyl dihydrocinnamate undergo competing losses of carbon monoxide, ethanol and the combined loss of these neutral fragments, giving ionised styrene, C8H8•+ (m/z 104). A highly specific H/D interchange involving the four hydrogen atoms at the benzylic α- and ortho-positions was found to precede the losses of ethanol and [ethanol+CO]. This represents another striking case of complete 4H – scrambling that enables the molecular ion to fully equilibrate the interchanging hydrogen atoms prior to fragmentation. A mechanism rationalising these observations and extending previously suggested mechanisms is proposed involving a series of distonic ions and the intermediacy of an ion/neutral complex. The metastable ions of the related esters exhibit in part similar fragmentation behaviour, but the McLafferty reaction turns out to be more favourable with higher alkyl dihydrocinnamates. For example, n-propyl 3-phenylpropanoate and isopropyl 3-phenylpropanoate react through highly distinct fragmentation channels.

Long‐Lived Radical Cations of Monocyclic Arenes at Room Temperature Obtained by NbF5 Acting as an Oxidizing Agent and Counterion Precursor

Salt of the earth: Radical cation salts of monocyclic arenes, including benzene, have been obtained in a reaction where niobium pentafluoride behaves both as an oxidizing agent (conversion into NbF4) and fluoride acceptor (to afford the counterion [Nb2F11]−, see picture). Anion–π-electron density interactions, as revealed by experimental and computational studies, are crucial in providing unprecedented inertness to the radical cations.

Ultrafast solvation dynamics of 12′-apo- β-carotenoic-12′-acid in [C 6mim] +[Tf 2N] −

Intra- and intermolecular relaxation pathways of 12′-apo- β-carotenoic-12′-acid, an apocarotenoid with terminal carbonyl substitution, were identified after photoexcitation using ultrafast pump-supercontinuum probe spectroscopy in the 350–750 nm range. The apocarotenoid was dissolved in 1- n-hexyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)amide and excited to the S 2 state at 482 nm. Internal conversion occurred within 170 fs to a single S 1/ICT state with intramolecular charge transfer character. This state exhibits characteristic excited state absorption band-shifts due to time-dependent solvation, which appears to be a general feature of carbonyl apocarotenoids with polarity-dependent S 1/ICT lifetimes. We also demonstrate that higher pump beam energies open a route for the clean preparation of long-lived radical cations by resonant one-color two-photon excitation using S 2 as the intermediate state. The radical cations are easily identified by a characteristic absorption peak in the region 700–750 nm and an absorption tail towards shorter wavelengths.

Gas-Phase Fragmentation of Long-Lived Cysteine Radical Cations Formed Via NO Loss from Protonated S-Nitrosocysteine

In this work, we describe two different methods for generating protonated S-nitrosocysteine in the gas phase. The first method involves a gas-phase reaction of protonated cysteine with t-butylnitrite, while the second method uses a solution-based transnitrosylation reaction of cysteine with S-nitrosoglutathione followed by transfer of the resulting S-nitrosocysteine into the gas phase by electrospray ionization mass spectrometry (ESI-MS). Independent of the way it was formed, protonated S-nitrosocysteine readily fragments via bond homolysis to form a long-lived radical cation of cysteine (Cys(*+)), which fragments under collision-induced dissociation (CID) conditions via losses in the following relative abundance order: *COOH CH(2)S >> *CH(2)SH approximately = H(2)S. Deuterium labeling experiments were performed to study the mechanisms leading to these pathways. DFT calculations were also used to probe aspects of the fragmentation of protonated S-nitrosocysteine and the radical cation of cysteine. NO loss is found to be the lowest energy channel for the former ion, while the initially formed distonic Cys(*+) with a sulfur radical site undergoes proton and/or H atom transfer reactions that precede the losses of CH(2)S, *COOH, *CH(2)SH, and H(2)S.

Long-Lived Radical Cations as Model Compounds for the Reactive One-Electron Oxidation Product of Vitamin E

Heterocyclic compounds with structures similar to vitamin E, but without the hydroxyl hydrogen atom, were synthesized and their electrochemical behavior examined in acetonitrile solutions and as solids in aqueous solutions of varying pH by attaching the compounds to the surface of a glassy carbon electrode. Compound 1, containing a fully methylated aromatic ring was found to be the most long-lived following one-electron oxidation, with its radical cation (1+*) surviving in acidic aqueous solutions and able to be isolated as a salt, 1+*(SbF6-), when reacted with NOSbF6 in CH3CN. Electrochemical, UV-vis and FTIR experiments on 1+*, in addition to the results from theoretical calculations, indicated that the electrochemical, electronic and structural properties of 1+* are very similar to those of the radical cation of vitamin E.

Photoinduced Charge Separation in Titania Nanotubes

The photocatalytic one-electron oxidation reaction of an aromatic compound during UV light irradiation of titania nanotubes and nanoparticles was investigated using time-resolved diffuse reflectance spectroscopy. Remarkably long-lived radical cations of the aromatic compound and trapped electrons were observed for the nanotubes when compared to those for nanoparticles. The influences of the morphology on the one-electron oxidation process of an aromatic compound adsorbed on the surface were discussed in terms of the charge recombination dynamics between the radical cation and electrons in TiO2.

Highly efficient two-photon initiated polymerization in solvent by using a novel two-photon chromophore and co-initiators

Highly efficient two-photon initiated polymerization in solvent was carried out in a three-component system, which consisted of a novel D-π-A-π-D type chromophore (1 or 2, respectively), the co-initiator, 2, 6-diisopropyl-N,N-dimethylaniline (DDA) and diphenyliodoinum salt, Ph2I+PF6−(PIP). We measured the quasi two-photon polymerization rates (Rp) based on scanning speed and the width of the written protruding line. Polymerization rates of the chromophore 1 and 2 were found to be 113 µm3 s−1 and 157 µm3 s−1 in the case of the laser power 4.5 mW at 820 nm (pulse width: 85 fs, repetition rate 82 MHz ) respectively. In order to estimate the mechanism of the photochemical reaction, the change of UV absorption of the three-component system, T–T transient absorption and the cyclic voltammetry (CV) of the novel chromophores were measured. Correlated with the CV, we interpreted that highly efficient electron transfer between DDA and the chromophore contributed to the long-lived radical cations of the chromophore. As a result, the generation of both DDA radicals and phenyl radicals was ascribed to high sensitivity in TPIP.

Transients of the Oxidation of Pyrimidines with SO4•-: Structure and Reactivity of the Resulting Radicals

Depending on the methylation pattern, the reaction of SO4•- with various methylated pyrimidines yields radicals deprotonated at N(1) and C(5)−OH or C(6)−OH adduct radicals and probably allyl radicals by subsequent deprotonation at the C(5)-methyl group of the thymines. All of these radicals are derived from the initial SO4•- adducts on the pyrimidines that have lifetimes of several microseconds in the case of N(1)-methylated thymines. These transients could be taken for long-lived pyrimidine radical cations. However, spectral and kinetic comparison with the pyrimidine radical cations generated by electron transfer in nonpolar solvents (butyl chloride, acetone) reveals the adduct nature of these transients. In the course of sulfate adduct decay, pyrimidine radical cations could be formed as nondetectable short-lived (τ < 20ns) intermediates rapidly reacting to the above-mentioned radical products. The reactivity of the SO4•- adducts, as well as that of the other pyrimidinyl radicals observed, was characterized by the subsequent oxidation of triphenylamine and di- or trimethoxybenzene, which supports the mechanistic interpretation given above. Hence there is no indication of long-lived pyrimidine radical cations under the conditions of the SO4•- oxidation of pyrimidines in aqueous solution.

Photoinduced Electron Transfer Reactions within Zeolites: Detection of Radical Cations and Dimerization of Arylalkenes1

Photosensitized electron transfer reactions between excited singlet acceptors and arylalkenes included within NaX zeolites have been studied using a combination of product studies, fluorescence spectroscopy, and diffuse reflectance laser flash photolysis. Steady-state and time-resolved fluorescence quenching of cyanoaromatic and ionic sensitizers by arylalkenes demonstrates that singlet quenching occurs predominantly by a static process. Diffuse reflectance flash photolysis studies indicate that quenching of singlet cyanoaromatic sensitizers by trans-anethole and 4-vinylanisole occurs via electron transfer and yields relatively long-lived radical cations. Signals due to trapped electrons (Na43+) are also observed, suggesting that photoionization of the cyanoaromatic sensitizer occurs in competition with electron transfer quenching of the excited singlet by the alkene. The long lifetimes of the radical cations indicate the utility of the zeolite environment for controlling the energy-wasting back electron ...

Generation, entrapment and reactivity of long-lived organic carbocations and radical cations within a supramolecular assembly: Ca Y zeolite

Diarylethenes spontaneously form the corresponding radical cations and carbocations upon inclusion within activated Ca Y zeolite; oxygen plays an important role in the generation of the radical cations.

Methyl propionate radical cation

Examination of the reactions of the long-lived (>0.5-s) radical cations of CD3CH2COOCH3 and CH3CH2COOCD3 indicates that the long-lived, nondecomposing methyl propionate radical cation CH3CH2C(O)OCH 3 (+·) isomerizes to its enol form CH3CH=C(OH)OCH 3 (+·) (ΔH isomerization ≃ -32 kcal/mol) via two different pathways in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. A 1,4-shift of a β-hydrogen of the acid moiety to the carbonyl oxygen yields the distonic ion (·)CH2CH2C(+) (OH)OCH3 that then rearranges to CH3CH=C(OH)OCH 3 (+·) probably by consecutive 1,5- and 1,4-hydrogen shifts. This process is in competition with a 1,4-hydrogen transfer from the alcohol moiety to form another distonic ion, CH3CH2C(+)(OH)OCH 2 (·) , that can undergo a 1,4-hydrogen shift to form CH3CH=C(OH)OCH 3 (+·) . Ab initio molecular orbital calculations carried out at the UMP2/6-31G** + ZPVE level of theory show that the two distonic ions lie more than 16 kcal/mol lower in energy than CH3CH2C(O)OCH 3 (+·) . Hence, the first step of both rearrangement processes has a great driving force. The 1,4-hydrogen shift that involves the acid moiety is 3 kcal/mol more exothermic (ΔH isomerization=-16 kcal/mol) and is associated with a 4-kcal/mol lower barrier (10 kcal/mol) than the shift that involves the alcohol moiety. Indeed, experimental findings suggest that the hydrogen shift from the acid moiety is likely to be the favored channel.

The Long-Lived Radical Cations of Simple Carbon Esters Isomerize to the Lowest-Energy Structure

The Long-Lived Radical Cations of Simple Carbon Esters Isomerize to the Lowest-Energy Structure

ChemInform Abstract: Long‐Lived Radical Cations of Simple Organophosphates Isomerize Spontaneously to Distonic Structures in the Gas Phase.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Long-lived radical cations of simple organophosphates isomerize spontaneously to distonic structures in the gas phase

The structures and relative stabilities of the long-lived radical cations formed upon removal of an electron from simple organophosphorus esters have been examined in the gas phase. Different mass spectrometric methods were used to investigate the relative heights of the energy barriers for fragmentation and isomerization. Reference ions were generated by dissociative ionization of suitable precursor ions

Electron acceptor behavior of 9-phenylxanthenium carbocation singlet

The fluorescence of 9-phenylxanthenium carbocation is efficiently quenched by aromatics, predominantly by a charge transfer mechanism in which the carbocation acts as an acceptor. The electron transfer nature of the quenching is established by laser flash photolytic observation of long-lived aromatic radical cations and 9-phenylxanthenyl radical in microsecond time regimes.

Long-lived trialkylamine radical cations containing .alpha.-carbon-hydrogen bonds in acyclic alkyl groups

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTLong-lived trialkylamine radical cations containing .alpha.-carbon-hydrogen bonds in acyclic alkyl groupsStephen F. Nelsen and Glen T. CunkleCite this: J. Org. Chem. 1985, 50, 20, 3701–3705Publication Date (Print):October 1, 1985Publication History Published online1 May 2002Published inissue 1 October 1985https://doi.org/10.1021/jo00220a005RIGHTS & PERMISSIONSArticle Views138Altmetric-Citations12LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (643 KB) Get e-Alerts Get e-Alerts

ChemInform Abstract: LONG‐LIVED RADICAL CATIONS FROM MESOCYCLIC DITHIOETHERS

Chemischer InformationsdienstVolume 7, Issue 31 Preparative Organic Chemistry ChemInform Abstract: LONG-LIVED RADICAL CATIONS FROM MESOCYCLIC DITHIOETHERS W. K. MUSKER, W. K. MUSKERSearch for more papers by this authorT. L. WOLFORD, T. L. WOLFORDSearch for more papers by this author W. K. MUSKER, W. K. MUSKERSearch for more papers by this authorT. L. WOLFORD, T. L. WOLFORDSearch for more papers by this author First published: August 3, 1976 https://doi.org/10.1002/chin.197631127Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume7, Issue31August 3, 1976 RelatedInformation