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Showing papers on "Radical ion published in 2015"


Journal ArticleDOI
TL;DR: The photooxidizing capabilities of selected Cr(III) complexes for promoting radical cation cycloadditions are described, which have sufficiently long-lived excited states to oxidize electron-rich alkenes, thereby initiating [4+2] processes.
Abstract: The photooxidizing capabilities of selected CrIII complexes for promoting radical cation cycloadditions are described. These complexes have sufficiently long-lived excited states to oxidize electron-rich alkenes, thereby initiating [4+2] processes. These metal species augment the spectrum of catalysts explored in photoredox systems, as they feature unique properties that can result in differential reactivity from the more commonly employed ruthenium or iridium catalysts.

155 citations


Journal ArticleDOI
TL;DR: This diborene ranks amongst the most powerful neutral organic reductants known and readily reacted with MesBC4 Ph4 by SET to afford a stable boron-centered radical-anion radical-cation pair.
Abstract: Despite the fundamental importance of radical-anion radical-cation pairs in single-electron transfer (SET) reactions, such species are still very rare and transient in nature. Since diborenes have highly electron-rich BB double bonds, which makes them strong neutral reductants, we envisaged a possible realization of a boron-centered radical-anion radical-cation pair by SET from a diborene to a borole species, which are known to form stable radical anions upon one-electron reduction. However, since the reduction potentials of all know diborenes (E1/2=−1.05/−1.55 V) were not sufficiently negative to reduce MesBC4Ph4 (E1/2=−1.69 V), a suitable diborene, IiPr⋅(iPr)BB(iPr)⋅IiPr, was tailor-made to comply with these requirements. With a halfwave potential of E1/2=−1.95 V, this diborene ranks amongst the most powerful neutral organic reductants known and readily reacted with MesBC4Ph4 by SET to afford a stable boron-centered radical-anion radical-cation pair.

115 citations


Journal ArticleDOI
Dan-Wei Zhang1, Jia Tian, Lan Chen1, Liang Zhang1, Zhan-Ting Li1 
TL;DR: This Focus Review highlights the recent advance in its application as an emerging non-covalent force for the construction of various supramolecular structures.
Abstract: Aromatic stacking has played a significant role in the design of discrete supramolecular systems. However, for many years, the stacking of conjugated radical cations has been considered only as a novelty due to its weakness. In recent years, it has been demonstrated that the stacking of conjugated radical cations, particularly those formed by bipyridinium or tetrathiafulvalene, can be enhanced remarkably when they are incorporated into rationally designed, preorganized frameworks or entrapped into a confined space. This Focus Review highlights the recent advance in its application as an emerging non-covalent force for the construction of various supramolecular structures.

104 citations


Journal ArticleDOI
Jing Liu1, Fang Liu1, Yingzu Zhu1, Xingge Ma1, Xiaodong Jia1 
TL;DR: This method provides a new way to synthesize 2,4-diarylquinoline derivatives and reveals that a radical intermediate was involved in this catalytic oxidation of N-benzylanilines.

79 citations


Journal ArticleDOI
TL;DR: This study clearly shows that DESI-based electrochemical MS is capable of capturing electrochemically generated intermediates with half-lives on the order of microseconds, which is 4-5 orders of magnitude faster than previously reported electrochemical mass spectrometry techniques.
Abstract: The N,N-dimethylaniline (DMA) radical cation DMA(.+) , a long-sought transient intermediate, was detected by mass spectrometry (MS) during the electrochemical oxidation of DMA. This was accomplished by coupling desorption electrospray ionization (DESI) MS with a waterwheel working electrode setup to sample the surface of the working electrode during electrochemical analysis. This study clearly shows that DESI-based electrochemical MS is capable of capturing electrochemically generated intermediates with half-lives on the order of microseconds, which is 4-5 orders of magnitude faster than previously reported electrochemical mass spectrometry techniques.

72 citations


Journal ArticleDOI
TL;DR: The high durability with turnover number of 4320 for the hydroxylation of benzene to phenol with H2O2 was achieved using [Cu(tmpa)]2+ incorporated into Al-MCM-41 as an efficient and selective catalyst.

63 citations


Journal ArticleDOI
Lidan Wu1, Kunhui Liu1, Jialong Jie1, Di Song1, Hongmei Su1 
TL;DR: In this paper, the deprotonation behavior of guanine cation (G+•) in G-quadruplexes has been investigated by nanosecond laser flash photolysis.
Abstract: Although numerous studies have been devoted to the charge transfer through double-stranded DNA (dsDNA), one of the major problems that hinder their potential applications in molecular electronics is the fast deprotonation of guanine cation (G+•) to form a neutral radical that can cause the termination of hole transfer. It is thus of critical importance to explore other DNA structures, among which G-quadruplexes are an emerging topic. By nanosecond laser flash photolysis, we report here the direct observation and findings of the unusual deprotonation behavior (loss of amino proton N2–H instead of imino proton N1–H) and slower (1–2 orders of magnitude) deprotonation rate of G+• within G-quadruplexes, compared to the case in the free base dG or dsDNA. Four G-quadruplexes AG3(T2AG3)3, (G4T4G4)2, (TG4T)4, and G2T2G2TGTG2T2G2 (TBA) are measured systematically to examine the relationship of deprotonation with the hydrogen-bonding surroundings. Combined with in depth kinetic isotope experiments and pKa analysis, ...

54 citations


Journal ArticleDOI
TL;DR: Oxidative polymerization in ILs has been explored the most; although morphological changes were observed compared to the conventional methods, polymers were obtained in good yields and in some cases ILs were used as dopants to improve the desired properties.

53 citations


Journal ArticleDOI
TL;DR: These results reproduce acceptably well the imaging photoelectron-photoion coincidence spectra of naphthalene, in the photon-energy range 14.0-18.8 eV that was previously reported by this group.
Abstract: The fragmentation mechanisms of the naphthalene molecular ion to [M–C4H2]+•, [M–C2H2]+•, [M–H2]+•, and [M–H•]+ were obtained at the UB3LYP/6-311+G(3df,2p)//UB3LYP/6-31G(d) level of theory and were subsequently used to calculate the microcanonical rate constants, k(E)’s, for all the steps by the Rice-Ramsperger-Kassel-Marcus formalism. The pre-equilibrium and steady state approximations were applied on different regions of the potential energy profiles to obtain the fragmentation k(E)’s and calculate the relative abundances of the ions as a function of energy. These results reproduce acceptably well the imaging photoelectron-photoion coincidence spectra of naphthalene, in the photon-energy range 14.0–18.8 eV that was previously reported by our group. Prior to dissociation, the molecular ion rapidly equilibrates with a set of isomers that includes the Z- and E-phenylvinylacetylene (PVA) radical cations. The naphthalene ion is the predominant isomer below 10 eV internal energy, with the other isomers remaining at steady state concentrations. Later on, new steady-state intermediates are formed, such as the azulene and 1-phenyl-butatriene radical cations. The naphthalene ion does not eject an H atom directly but eliminates an H2 molecule in a two-step fragmentation. H• loss occurs instead from the 1-phenyl-butatriene ion. The PVA ions initiate the ejection of diacetylene (C4H2) to yield the benzene radical cation. Acetylene elimination yields the pentalene cation at low energies (where it can account for 45.9%–100.0% of the rate constant of this channel), in a three-step mechanism starting from the azulene ion. However, above 7.6 eV, the major [M–C2H2]+• structure is the phenylacetylene cation.

49 citations


Journal ArticleDOI
TL;DR: In this paper, a copper-catalyzed radical/radical C sp 3H/PH cross-coupling has been developed, which provides a radical/ radical cross coupling in a selective manner, in which aryl ketone o-acetyloximes and phosphine oxides were suitable for this transformation.
Abstract: The selective radical/radical cross-coupling of two different organic radicals is a great challenge due to the inherent activity of radicals. In this paper, a copper-catalyzed radical/radical C sp 3H/PH cross-coupling has been developed. It provides a radical/radical cross-coupling in a selective manner. This work offers a simple way toward β-ketophosphonates by oxidative coupling of aryl ketone o-acetyloximes with phosphine oxides using CuCl as catalyst and PCy3 as ligand in dioxane under N2 atmosphere at 130 °C for 5 h, and yields ranging from 47 % to 86 %. The preliminary mechanistic studies by electron paramagnetic resonance (EPR) showed that, 1) the reduction of ketone o-acetyloximes generates iminium radicals, which could isomerize to α-sp3-carbon radical species; 2) phosphorus radicals were generated from the oxidation of phosphine oxides. Various aryl ketone o-acetyloximes and phosphine oxides were suitable for this transformation.

48 citations


Journal ArticleDOI
TL;DR: A direct construction of quinoline-fused lactones and lactams was achieved by sp(3) C-H bond oxidation of N-aryl glycine esters and amides under catalytic radical cation salt-induced conditions, which provides a new synthetic approach to this class of heterocycles.
Abstract: A direct construction of quinoline-fused lactones and lactams was achieved by sp3 C–H bond oxidation of N-aryl glycine esters and amides under catalytic radical cation salt-induced conditions. These polycyclic products are formed in a single step from readily accessible starting materials, and this method provides a new synthetic approach to this class of heterocycles.

Journal ArticleDOI
TL;DR: In situ irradiation time-resolved electron paramagnetic resonance and Fourier transform infrared spectroscopies provided direct evidence for the generation of hydrated electrons and the indole radical cations, which supported the proposed degradation mechanism.
Abstract: A new photoreduction pathway for nitro-aromatic compounds (NACs) and the underlying degradation mechanism are described. 1,3-Dinitrobenzene was reduced to 3-nitroaniline by the widely distributed aromatic molecule indole; the reaction is facilitated by montmorillonite clay mineral under both simulated and natural sunlight irradiation. The novel chemical reaction is strongly affected by the type of exchangeable cation present on montmorillonite. The photoreduction reaction is initiated by the adsorption of 1,3-dinitrobenzene and indole in clay interlayers. Under light irradiation, the excited indole molecule generates a hydrated electron and the indole radical cation. The structural negative charge of montmorillonite plausibly stabilizes the radical cation hence preventing charge recombination. This promotes the release of reactive hydrated electrons for further reductive reactions. Similar results were observed for the photoreduction of nitrobenzene. In situ irradiation time-resolved electron paramagnetic...

Journal ArticleDOI
TL;DR: ESI can be expanded for the investigation of asphaltene and other polyaromatic systems beyond the polar constituents as non-polar hydrocarbons can be efficiently analyzed.
Abstract: Electrospray ionization (ESI) is the most common ionization method in atmospheric pressure ionization mass spectrometry because of its easy use and handling and because a diverse range of components can be effectively ionized from high to medium polarity. Usually, ESI is not employed for the analysis of non-polar hydrocarbons, but under some circumstances, they are effectively ionized. Polyaromatic hydrocarbons and aromatic heterocycles can form radical ions and protonated molecules after ESI, which were detected by Fourier transform ion cyclotron resonance mass spectrometry. The highly condensed aromatic structures are obtained from a heavy crude oil, and the results show class distribution from pure hydrocarbons up to more non-basic nitrogen-containing species. By using different solvent compositions [toluene/methanol (50/50 v/v), dichloromethane/methanol (50/50 v/v), dichloromethane/acetonitrile (50/50 v/v) and chloroform], the results show that the lack of proton donor agent helps to preserve the radical formation that was created at the metal/solution interface inside the electrospray capillary. The results demonstrate that with an appropriate selection of solvent and capillary voltage, the ratio between the detected radical ion and protonated molecule form can be manipulated. Therefore, ESI can be expanded for the investigation of asphaltene and other polyaromatic systems beyond the polar constituents as non-polar hydrocarbons can be efficiently analyzed. Copyright © 2015 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: Unstrained C–C activation via putative radical cation formation promotes a directed radical fluorination event using Selectfluor, catalytic 9-fluorenone, and light.
Abstract: Expanding the repertoire of controlled radical fluorination techniques, we present a photosensitized unstrained C–C bond activation/directed monofluorination method using Selectfluor and 9-fluorenone. The reaction is amenable to the opening of multiple 1-acetal-2-aryl substituted rings to yield ω-fluoro carboxylic acids, esters, alcohols, and ketones with relative ease. Initial mechanistic insight suggests radical ion intermediates.

Journal ArticleDOI
TL;DR: Heterogeneous semiconductor photoredox catalysis (SCPC), particularly with TiO2, is evolving to provide radically new synthetic applications and is described how photoactivated SCPCs can either interact with a precursor that donates an electron to the semiconductor thus generating a radical cation; or interact with an acceptor precursor that picks up an electron with production of a radical anion.
Abstract: Heterogeneous semiconductor photoredox catalysis (SCPC), particularly with TiO2, is evolving to provide radically new synthetic applications. In this review we describe how photoactivated SCPCs can either (i) interact with a precursor that donates an electron to the semiconductor thus generating a radical cation; or (ii) interact with an acceptor precursor that picks up an electron with production of a radical anion. The radical cations of appropriate donors convert to neutral radicals usually by loss of a proton. The most efficient donors for synthetic purposes contain adjacent functional groups such that the neutral radicals are resonance stabilized. Thus, ET from allylic alkenes and enol ethers generated allyl type radicals that reacted with 1,2-diazine or imine co-reactants to yield functionalized hydrazones or benzylanilines. SCPC with tertiary amines enabled electron-deficient alkenes to be alkylated and furoquinolinones to be accessed. Primary amines on their own led to self-reactions involving C–N coupling and, with terminal diamines, cyclic amines were produced. Carboxylic acids were particularly fruitful affording C-centered radicals that alkylated alkenes and took part in tandem addition cyclizations producing chromenopyrroles; decarboxylative homo-dimerizations were also observed. Acceptors initially yielding radical anions included nitroaromatics and aromatic iodides. The latter led to hydrodehalogenations and cyclizations with suitable precursors. Reductive SCPC also enabled electron-deficient alkenes and aromatic aldehydes to be hydrogenated without the need for hydrogen gas.

Journal ArticleDOI
TL;DR: In this paper, photo-curing kinetics and mechanism of the photopolymerization of dimethacrylates using three-component initiation systems consisting of 1-chloro-4-propoxy-9H-thioxanthen-9-one (CPXTO), diphenyl iodonium hexafluorophosphate (Ph2IPF6), and the aromatic N,N,3,5-tetramethyl aniline (TMA) or the aromatic-like N, N-dimethylbenzylamine or

Journal ArticleDOI
TL;DR: The reaction is selective (no nitrile byproduct is formed unlike other photochemical reactions involving aldoxime ethers) as well as regiospecific when using 2'-aryl groups with meta-substituents, making this reaction a useful alternative for preparing substituted phenanthridines.
Abstract: A series of 2′-arylbenzaldehyde oxime ethers were synthesized and shown to generate the corresponding phenanthridines upon irradiation in the presence of 9,10-dicyanoanthracene in acetonitrile. Mechanistic studies suggest that the oxidative cyclization reaction sequence is initiated by an electron transfer step followed by nucleophilic attack of the aryl ring onto the nitrogen of the oxime ether. A concave downward Hammett plot is presumably the result of a change in charge distribution in the radical cation species with strongly electron-donating substituents that yields a less electrophilic nitrogen atom and a decreased amount of cyclized product. The reaction is selective (no nitrile byproduct is formed unlike other photochemical reactions involving aldoxime ethers) as well as regiospecific when using 2′-aryl groups with meta-substituents, making this reaction a useful alternative for preparing substituted phenanthridines.

Journal ArticleDOI
TL;DR: The electron paramagnetic resonance data indicate that radicals in the PTMA assume a closest approach distance to each other when more than 60% of the backbone is populated with radical pendant groups, which is consistent with theoretical calculations and helps explain some experimentally determined electron-transport properties.
Abstract: The relationship between the polymer network and electronic transport properties for stable radical polymeric materials has come under investigation owing to their potential application in electronic devices. For the radical polymer poly(2,2,6,6-tetramethylpiperidine-4-yl-1-oxyl methacrylate), it is unclear whether the radical packing is optimal for charge transport partially because the relationship between radical packing and molecular structure is not well-understood. Using the paramagnetic nitroxide radical as a probe of the polymer and synthetic techniques to control the radical concentration on the methyl methacrylate backbone, we investigate the dependence of radical concentration on molecular structure. The electron paramagnetic resonance data indicate that radicals in the PTMA assume a closest approach distance to each other when more than 60% of the backbone is populated with radical pendant groups. Below 60% coverage, the polymer rearranges to accommodate larger radical–radical spacing. These f...

Journal ArticleDOI
TL;DR: Femtosecond time-resolved IR spectroscopy is used to investigate the excited-state dynamics of a dinucleotide containing an 8-oxoguanine anion at the 5'-end and neutral adenine at the 3'-end, and the quantum efficiency of this ultrafast charge shift reaction approaches unity.
Abstract: Femtosecond time-resolved IR spectroscopy is used to investigate the excited-state dynamics of a dinucleotide containing an 8-oxoguanine anion at the 5′-end and neutral adenine at the 3′-end. UV excitation of the dinucleotide transfers an electron from deprotonated 8-oxoguanine to its π-stacked neighbor adenine in less than 1 ps, generating a neutral 8-oxoguanine radical and an adenine radical anion. These species are identified by the excellent agreement between the experimental and calculated IR difference spectra. The quantum efficiency of this ultrafast charge shift reaction approaches unity. Back electron transfer from the adenine radical anion to the 8-oxguanine neutral radical occurs in 9 ps, or approximately 6 times faster than between the adenine radical anion and the 8-oxoguanine radical cation (Zhang, Y. et al. Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 11612–11617). The large asymmetry in forward and back electron transfer rates is fully rationalized by semiclassical nonadiabatic electron transf...

Journal ArticleDOI
TL;DR: In this paper, the reduction potential (Ered) of 1 was determined to be 1.23 V vs. SCE in acetonitrile based on analysis of the electron transfer equilibrium between 1 and a one-electron donor, [RuII(bpy)3]2+ (bpy = 2,2′-bipyridine).
Abstract: A mononuclear Cr(V)–oxo complex, [CrV(O)(6-COO−-tpa)](BF4)2 (1; 6-COO−-tpa = N,N-bis(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)amine) was prepared through the reaction of a Cr(III) precursor complex with iodosylbenzene as an oxidant. Characterization of 1 was achieved using ESI-MS spectrometry, electron paramagnetic resonance, UV-vis, and resonance Raman spectroscopies. The reduction potential (Ered) of 1 was determined to be 1.23 V vs. SCE in acetonitrile based on analysis of the electron-transfer (ET) equilibrium between 1 and a one-electron donor, [RuII(bpy)3]2+ (bpy = 2,2′-bipyridine). The reorganization energy (λ) of 1 was also determined to be 1.03 eV in ET reactions from phenol derivatives to 1 on the basis of the Marcus theory of ET. The smaller λ value in comparison with that of an Fe(IV)–oxo complex (2.37 eV) is caused by the small structural change during ET due to the dπ character of the electron-accepting LUMO of 1. When benzyl alcohol derivatives (R-BA) with different oxidation potentials were employed as substrates, corresponding aldehydes were obtained as the 2e−-oxidized products in moderate yields as determined from 1H NMR and GC-MS measurements. One-step UV-vis spectral changes were observed in the course of the oxidation reactions of BA derivatives by 1 and a kinetic isotope effect (KIE) was observed in the oxidation reactions for deuterated BA derivatives at the benzylic position as substrates. These results indicate that the rate-limiting step is a concerted proton-coupled electron transfer (PCET) from substrate to 1. In sharp contrast, in the oxidation of trimethoxy-BA (Eox = 1.22 V) by 1, trimethoxy-BA radical cation was observed by UV-vis spectroscopy. Thus, it was revealed that the mechanism of the oxidation reaction changed from one-step PCET to stepwise ET–proton transfer (ET/PT), depending on the redox potentials of R-BA.

Journal ArticleDOI
TL;DR: A notable strategy to accomplish direct radical/radical oxidative cross-coupling has been demonstrated, that is metal tuning a transient radical to a persistent radical intermediate followed by coupling with another transient radical.
Abstract: Highly selective radical/radical cross-coupling is paid more attention in bond formations. However, due to their intrinsic active properties, radical species are apt to achieve homo-coupling instead of cross-coupling, which makes the selective cross-coupling as a great challenge and almost untouched. Herein a notable strategy to accomplish direct radical/radical oxidative cross-coupling has been demonstrated, that is metal tuning a transient radical to a persistent radical intermediate followed by coupling with another transient radical. Here, a transient nitrogen-centered radical is tuned to a persistent radical complex by copper catalyst, followed by coupling with a transient allylic carbon-centered radical. Firstly, nitrogen-centered radical generated from N-methoxybenzamide stabilized by copper catalyst was successfully observed by EPR. Then DFT calculations revealed that a triplet diradical Cu(II) complex formed from the chelation N-methoxybenzamide nitrogen-centered radical to Cu(II) is a persistent radical species. Moreover, conceivable nitrogen-centered radical Cu(II) complex was observed by high-resolution electrospray ionization mass spectrometry (ESI-MS). Ultimately, various allylic amides derivatives were obtained in good yields by adopting this strategy, which might inspire a novel and promising landscape in radical chemistry.

Journal ArticleDOI
TL;DR: In this article, the authors summarized the recent advance in utilizing conjugated radical cation dimerization (CRCD) for constructing new unimolecular, such as foldamers, and supramolecular ordered structures.

Journal ArticleDOI
TL;DR: This work proposes that the adduct is formed via the same intermediate by combination of the radical ion pair, and assigns this intermediate to the radical anion which is protected from protonation by the protein.
Abstract: LOV domains are the light sensitive parts of phototropins and many other light-activated enzymes that regulate the response to blue light in plants and algae as well as some fungi and bacteria. Unlike all other biological photoreceptors known so far, the photocycle of LOV domains involves the excited triplet state of the chromophore. This chromophore is flavin mononucleotide (FMN) which forms a covalent adduct with a cysteine residue in the signaling state. Since the formation of this adduct from the triplet state involves breaking and forming of two bonds as well as a change from the triplet to the singlet spin state, various intermediates have been proposed, e.g. a protonated triplet state 3FMNH+, the radical anion 2FMN˙−, or the neutral semiquinone radical 2FMNH˙. We performed an extensive search for these intermediates by two-dimensional transient absorption (2D-TA) with a streak camera. However, no transient with a rate constant between the decay of fluorescence and the decay of the triplet state could be detected. Analysis of the decay associated difference spectra results in quantum yields for the formation of the adduct from the triplet of ΦA(LOV1) ≈ 0.75 and ΦA(LOV2) ≈ 0.80. This is lower than the values ΦA(LOV1) ≈ 0.95 and ΦA(LOV2) ≈ 0.99 calculated from the rate constants, giving indirect evidence of an intermediate that reacts either to form the adduct or to decay back to the ground state. Since there is no measurable delay between the decay of the triplet and the formation of the adduct, we conclude that this intermediate reacts much faster than it is formed. The LOV1-C57S mutant shows a weak and slowly decaying (τ > 100 μs) transient whose decay associated spectrum has bands at 375 and 500 nm, with a shoulder at 400 nm. This transient is insensitive to the pH change in the range 6.5–10.0 but increases on addition of β-mercaptoethanol as the reducing agent. We assign this intermediate to the radical anion which is protected from protonation by the protein. We propose that the adduct is formed via the same intermediate by combination of the radical ion pair.

Journal ArticleDOI
TL;DR: An effective Friedel-Crafts alkylation reaction of electron-rich aromatics with N-vinylamides, induced by electrochemically in situ-generated TBPA radical cation, has been developed; the resulting adducts are produced in good to excellent yields.
Abstract: An effective Friedel-Crafts alkylation reaction of electron-rich aromatics with N-vinylamides, induced by electrochemically in situ-generated TBPA radical cation, has been developed; the resulting adducts are produced in good to excellent yields. In the "ex-cell" type electrolysis, TBPA is transformed to its oxidized form in situ and subsequently employed as an electron transfer reagent to initiate a cationic chain reaction. An easily recoverable and reusable polymeric ionic liquid-carbon black (PIL-CB) composite was also utilized as a supporting electrolyte for the electrochemical generation of TBPA cation radical, without sacrificing efficiency or stability after four electrolyses. Cyclic voltammetry analysis and the results of control experiments demonstrate that the reaction of electron-rich aromatics and N-vinylamides occurs via a cationic chain reaction, which takes place though an oxidative activation of a C-H bond of electron-rich aromatics instead of oxidation of the N-vinylamide as previously assumed.

Journal ArticleDOI
TL;DR: The oxidation of guanine (G) is studied by using transient absorption and time-resolved resonance Raman spectroscopies combined with pulse radiolysis to demonstrate the pH dependence and the increase in the bonding order of pyrimidine (Pyr) and imidazole rings.
Abstract: The oxidation of guanine (G) is studied by using transient absorption and time-resolved resonance Raman spectroscopies combined with pulse radiolysis. The transient absorption spectral change demonstrates that the neutral radical of G (G•(−H+)), generated by the deprotonation of G radical cation (G•+), is rapidly converted to other G radical species. The formation of this species shows the pH dependence, suggesting that it is the G radical cation (G•+)′ formed from the protonation at the N7 of G•(−H+). On one hand, most Raman bands of (G•+)′ are up-shifted relative to those of G, indicating the increase in the bonding order of pyrimidine (Pyr) and imidazole rings. The (G•+)′ exhibits the characteristic CO stretching mode at ∼1266 cm–1 corresponding to a C–O single bond, indicating that the unpaired electron in (G•+)′ is localized on the oxygen of the Pyr ring.

Journal ArticleDOI
TL;DR: This study investigates the decomposition of negative and positive ions of 2-nitroimidazole and 4(5)-nitroIMidazoles using low- and high-energy Collision-Induced Dissociation (CID) and Electron- Induced Diss association (EID) by two different mass spectrometry techniques and is supported by quantum chemistry calculations.
Abstract: Nitroimidazoles are important compounds with chemotherapeutic applications as antibacterial drugs or as radiosensitizers in radiotherapy. Despite their use in biological applications, little is known about the fundamental properties of these compounds. Understanding the ionization reactions of these compounds is crucial in evaluating the radiosensitization potential and in developing new and more effective drugs. Thus, the present study investigates the decomposition of negative and positive ions of 2-nitroimidazole and 4(5)-nitroimidazole using low- and high-energy Collision-Induced Dissociation (CID) and Electron-Induced Dissociation (EID) by two different mass spectrometry techniques and is supported by quantum chemistry calculations. EID of [M+H](+) leads to more extensive fragmentation than CID and involves many radical cleavages including loss of H˙ leading to the formation of the radical cation, M˙(+). The stability (metastable decay) and the fragmentation (high-energy CID) of the radical cation M˙(+) have been probed in a crossed-beam experiment involving primary electron ionization of the neutral nitroimidazole. Thus, fragments in the EID spectra of [M+H](+) that come from further dissociation of radical cation M˙(+) have been highlighted. The loss of NO˙ radical from M˙(+) is associated with a high Kinetic Energy Release (KER) of 0.98 eV. EID of [M-H](-) also leads to additional fragments compared to CID, however, with much lower cross section. Only EID of [M+H](+) leads to a slight difference in the decomposition of 2-nitroimidazole and 4(5)-nitroimidazole.

Journal ArticleDOI
TL;DR: A polymer containing viologen radical cation monomer units is shown to reversibly switch between paramagnetic and diamagnetic states via non-covalent host-guest interactions or temperature control in water.

Journal ArticleDOI
TL;DR: In this paper, four arylene bisimides N-substituted with triarylamine and three core-functionalized with the same substituent were studied by cyclic voltammetry, UV-vis and EPR spectroelectrochemistry.
Abstract: Four arylene bisimides N-substituted with triarylamine and three bisimides core-functionalized with the same substituent were studied by cyclic voltammetry, UV-vis and EPR spectroelectrochemistry. All the investigated compounds showed ambipolar behaviour manifested by their quasi-reversible reduction to radical anions and quasi-reversible oxidation to radical cations. The presence of stable radical anions and radical cations was confirmed by EPR spectroelectrochemical experiments. Formation of the radical anions resulted in bleaching of the bisimide UV-vis bands with simultaneous hypsochromic shift of the charge transfer (CT) band and appearance of the radical anion peaks, the bands originating from the triarylamine remaining essentially unchanged. Electrochemical generation of radical cations resulted in turn in bleaching of the triarylamine band accompanied by a hypsochromic shift of the CT band and with the appearance of the radical cation bands at higher wavelengths, the bisimide bands remained essentially intact.

Journal ArticleDOI
TL;DR: Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components, and electron transfer was confirmed to be the quenching mechanism of corrole emission.
Abstract: Closely positioned donor-acceptor pairs facilitate electron- and energy-transfer events, relevant to light energy conversion. Here, a triad system TPACor-C60 , possessing a free-base corrole as central unit that linked the energy donor triphenylamine (TPA) at the meso position and an electron acceptor fullerene (C60) at the β-pyrrole position was newly synthesized, as were the component dyads TPA-Cor and Cor-C60. Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components. Steady-state fluorescence studies showed efficient energy transfer from (1) TPA* to the corrole and subsequent electron transfer from (1) corrole* to fullerene. Further studies involving femtosecond and nanosecond laser flash photolysis confirmed electron transfer to be the quenching mechanism of corrole emission, in which the electron-transfer products, the corrole radical cation (Cor(⋅+) in Cor-C60 and TPA-Cor(⋅+) in TPACor-C60) and fullerene radical anion (C60(⋅-)), could be spectrally characterized. Owing to the close proximity of the donor and acceptor entities in the dyad and triad, the rate of charge separation, kCS , was found to be about 10(11) s(-1), suggesting the occurrence of an ultrafast charge-separation process. Interestingly, although an order of magnitude slower than kCS , the rate of charge recombination, kCR , was also found to be rapid (kCR ≈10(10) s(-1)), and both processes followed the solvent polarity trend DMF>benzonitrile>THF>toluene. The charge-separated species relaxed directly to the ground state in polar solvents while in toluene, formation of (3) corrole* was observed, thus implying that the energy of the charge-separated state in a nonpolar solvent is higher than the energy of (3) corrole* being about 1.52 eV. That is, ultrafast formation of a high-energy charge-separated state in toluene has been achieved in these closely spaced corrole-fullerene donor-acceptor conjugates.

Journal ArticleDOI
TL;DR: In this paper, photoexcitation of the aryl chromophore of a 2-aryl substituted 1,3-dimethylbenzimidazolines (Ar-DMBIH) was investigated.