Showing papers on "Radical ion published in 2017"
TL;DR: Several examples of type I and type II photosensitized oxidation reactions are provided to illustrate the complexity and the diversity of the degradation pathways of mostly relevant biomolecules upon one‐electron oxidation and singlet oxygen reactions.
Abstract: Here, 10 guidelines are presented for a standardized definition of type I and type II photosensitized oxidation reactions. Because of varied notions of reactions mediated by photosensitizers, a checklist of recommendations is provided for their definitions. Type I and type II photoreactions are oxygen-dependent and involve unstable species such as the initial formation of radical cation or neutral radicals from the substrates and/or singlet oxygen (1 O21 ∆g ) by energy transfer to molecular oxygen. In addition, superoxide anion radical (O2·-) can be generated by a charge-transfer reaction involving O2 or more likely indirectly as the result of O2 -mediated oxidation of the radical anion of type I photosensitizers. In subsequent reactions, O2·- may add and/or reduce a few highly oxidizing radicals that arise from the deprotonation of the radical cations of key biological targets. O2·- can also undergo dismutation into H2 O2 , the precursor of the highly reactive hydroxyl radical (·OH) that may induce delayed oxidation reactions in cells. In the second part, several examples of type I and type II photosensitized oxidation reactions are provided to illustrate the complexity and the diversity of the degradation pathways of mostly relevant biomolecules upon one-electron oxidation and singlet oxygen reactions.
461 citations
TL;DR: The mechanism of the recently reported photocontrolled cationic polymerization of vinyl ethers was investigated using a variety of catalysts and chain-transfer agents as well as diverse spectroscopic and electrochemical analytical techniques to reveal a complex activation step characterized by one-electron oxidation of the CTA.
Abstract: The mechanism of the recently reported photocontrolled cationic polymerization of vinyl ethers was investigated using a variety of catalysts and chain-transfer agents (CTAs) as well as diverse spectroscopic and electrochemical analytical techniques. Our study revealed a complex activation step characterized by one-electron oxidation of the CTA. This oxidation is followed by mesolytic cleavage of the resulting radical cation species, which leads to the generation of a reactive cation—this species initiates the polymerization of the vinyl ether monomer—and a dithiocarbamate radical that is likely in equilibrium with the corresponding thiuram disulfide dimer. Reversible addition–fragmentation type degenerative chain transfer contributes to the narrow dispersities and control over chain growth observed under these conditions. Finally, the deactivation step is contingent upon the oxidation of the reduced photocatalyst by the dithiocarbamate radical concomitant with the production of a dithiocarbamate anion tha...
107 citations
TL;DR: An unprecendented direct alkylation of unfunctionalized allylic/benzylic sp3 C–H bonds via photoredox induced radical cation deprotonation is disclosed.
Abstract: Despite notable recent efforts, a catalytic and convenient strategy for the direct alkylation of unactivated allylic or benzylic sp3 C–H bonds remains a formidable challenge facing the synthesis community. We herein report an unprecedented allylic/benzylic alkylation using only an organo-photoredox catalyst, which enables coupling of a broad scope of alkenes/arenes and electron-deficient alkenes in an atom- and redox-economic manner. A photoredox induced alkene/arene radical cation deprotonation is proposed to smoothly generate the key allylic and benzylic radical intermediates. It represents the first C–C bond formation via radical cation deprotonation under visible light conditions. The resulting products can be easily scaled up and directly converted to γ,δ-unsaturated or α,β-diaryl-acids, -esters, -amides, -pyrazoles, -isoxazoles, as well as lactones, which enables this mild and selective sp3 C–H alkylation to rapidly access complex bioactive molecules.
99 citations
TL;DR: In this article, a simple modification of N-ethylphenothiazine, which is only stable as a radical cation (not as a dication), was reported, and it was shown that introducing electron-donating methoxy groups to nitrogen leads to dramatically improved stability of the doubly oxidized (dication) state.
Abstract: Stable electron-donating organic compounds are of interest for numerous applications that require reversible electron-transfer reactions. Although many organic compounds are stable one-electron donors, removing a second electron from a small molecule to form its dication usually leads to rapid decomposition. For cost-effective electrochemical energy storage utilizing organic charge-storage species, the creation of high-capacity materials requires stabilizing more charge whilst keeping molecular weights low. Here we report the simple modification of N-ethylphenothiazine, which is only stable as a radical cation (not as a dication), and demonstrate that introducing electron-donating methoxy groups para to nitrogen leads to dramatically improved stability of the doubly oxidized (dication) state. Our results reveal that this derivative is more stable than an analogous compound with substituents that do not allow for further charge delocalization, rendering it a promising scaffold for developing atom-efficient, two-electron donors.
92 citations
TL;DR: In this article, the authors present the first instance of visible-light-induced activation of periodate (IO4−) into reactive iodine radicals via sensitized electron transfer from an organic dye, Rhodamine B (RhB), which not only leads to oxidative decolorization of RhB but also formation of reactive intermediates that degrade organic compounds.
Abstract: Inspired by the mechanism behind self-sensitized destruction of dyes on semiconductor photocatalysts, we herein present the first instance of visible-light-induced activation of periodate (IO4−) into reactive iodine radicals via sensitized electron transfer from an organic dye, Rhodamine B (RhB). The IO4− reduction not only leads to oxidative decolorization of RhB but also formation of reactive intermediates that degrade organic compounds. Electron transfer from the excited dye to IO4− was confirmed by detecting RhB radical cation (RhB+) and measuring its lifetime. The efficiency of organic compound degradation was found to significantly vary depending on the target substrate, i.e., phenol, bisphenol A, and 4-chlorophenol were rapidly decomposed, whereas benzoic acid, carbamazepine, 4-nitrophenol, and sulfamethoxazole exhibited moderate decomposition rate. Lines of evidence in addition to the substrate specificity, such as insignificant hydroxylation, non-stoichiometric dechlorination, and marginal quenching effects of organic/inorganic compounds (e.g., methanol, natural organic matters, and chloride ion), points toward the involvement of iodate radical (IO3). The dye-sensitized IO4− activation process was also found to be highly effective in inactivation of MS2 bacteriophage.
85 citations
TL;DR: The detection of transient amine radical cations involved in the intermolecular [3 + 2] annulation reaction of N-cyclopropylaniline and styrene 2 by electrospray ionization mass spectrometry (ESI-MS) in combination with online laser irradiation of the reaction mixture is presented.
Abstract: Visible-light-mediated photoredox reactions have recently emerged as a powerful means for organic synthesis and thus have generated significant interest from the organic chemistry community. Although the mechanisms of these reactions have been probed by a number of techniques such as NMR, fluorescence quenching, and laser flash photolysis and various degrees of success has been achieved, mechanistic ambiguity still exists (for instance, the involvement of the chain mechanism is still under debate) because of the lack of structural information about the proposed and short-lived intermediates. Herein, we present the detection of transient amine radical cations involved in the intermolecular [3 + 2] annulation reaction of N-cyclopropylaniline (CPA, 1) and styrene 2 by electrospray ionization mass spectrometry (ESI-MS) in combination with online laser irradiation of the reaction mixture. In particular, the reactive CPA radical cation 1+•, the reduced photocatalyst Ru(I)(bpz)3+, and the [3 + 2] annulation prod...
62 citations
TL;DR: The radical or radical ion-based fluorination reactions of organic compounds will be presented, and methodologies include fluorination processes accomplished through thermal or photoinduced radical/electron transfer methods.
Abstract: The radical or radical ion-based fluorination reactions of organic compounds will be presented. These methodologies include fluorination processes accomplished through thermal or photoinduced radical/electron transfer methods. In doing so, the fluorination reactions of diverse families of organic compounds such as aliphatic and aromatic substrates will be presented. Recently summarized or reviewed articles will be mentioned but not discussed.
61 citations
TL;DR: All features point to this type of molecular system as promising for ECL applications, including an intense greenish-blue ECL emission, easily observable even by naked eye, with quantum yield higher than the standard 9,10-diphenylanthracene.
Abstract: We describe the synthesis, computational analysis, photophysics, electrochemistry and electrochemiluminescence (ECL) of a series of compounds formed of two triphenylamines linked by a fluorene or spirobifluorene bridge. The phenylamine moieties were modified at the para-position of the two external rings by electron-withdrawing or electron-donating substituents. These modifications allowed for fine-tuning of the photoluminescence (PL) and ECL emission from blue to green, with an overall wavelength span of 73 (PL) and 67 (ECL) nm, respectively. For all compounds, we observed a very high PL quantum yield (79–89%) and formation of stable radical ions. The ECL properties were investigated by direct annihilation of the electrogenerated radical anion and radical cation. The radical-ion annihilation process is very efficient and causes an intense greenish-blue ECL emission, easily observable even by naked eye, with quantum yield higher than the standard 9,10-diphenylanthracene. The ECL spectra show one single ba...
61 citations
TL;DR: In this paper, the authors examined doubly phenoxazine-fused Ni(II) porphyrin and showed that they can be stored for over 6 months without any sign of deterioration.
Abstract: Oxidative fusion reactions of meso-phenoxazino Ni(II) porphyrin were found to be temperature dependent, giving rise to either a doubly phenylene-fused product at room temperature or a singly phenoxazine-fused product at 70 °C. The latter was further oxidized to a doubly phenoxazine-fused Ni(II) porphyrin, which was subsequently converted to the corresponding free base porphyrin and Zn(II) porphyrin. Compared to previously reported diphenylamine-fused porphyrins that displayed a molecular twist, doubly phenoxazine-fused porphyrins exhibited distinctly different properties owing to their highly planar structures, such as larger fluorescence quantum yields, formation of an offset face-to-face dimer both in solution and the solid state, and the generation of a mixed-valence π-radical cation dimer upon electrochemical oxidation. One-electron oxidation of the phenoxazine-fused Ni(II) porphyrin with Magic Blue gave the corresponding radical cation, which was certainly stable and could be isolated by separation over a silica gel column but slowly chlorinated at the reactive β-positions in the solid state. This finding led to us to examine β,β′-dichlorinated phenoxazine-fused and diphenylamine-fused Ni(II) porphyrins, which, upon treatment with Magic Blue, provided remarkably stable radical cations to an unprecedented level. It is actually possible to purify these radical cations by silica gel chromatography, and they can be stored for over 6 months without any sign of deterioration. Moreover, they exhibited no degradation even after the CH2Cl2 solution was washed with water. However, subtle structural differences (planar versus partly twisted) led to different crystal packing structures and solid-state magnetic properties.
51 citations
TL;DR: The last redox event is identified as being the turnover-limiting step of the overall process, and the redox properties of the carbazole unit can thus be rationally tuned to improve catalytic activity.
Abstract: A combination of electrochemical, spectroscopic, computational, and kinetic studies has been used to elucidate the key mechanistic aspects of the previously reported enantioselective iminium ion trapping of photochemically generated carbon-centered radicals. The process, which provides a direct way to forge quaternary stereocenters with high fidelity, relies on the interplay of two distinct catalytic cycles: the aminocatalytic electron-relay system, which triggers the stereoselective radical trap upon iminium ion formation, and the photoredox cycle, which generates radicals under mild conditions. Critical to reaction development was the use of a chiral amine catalyst, bearing a redox-active carbazole unit, which could rapidly reduce the highly reactive and unstable intermediate generated upon radical interception. The carbazole unit, however, is also involved in another step of the electron-relay mechanism: the transiently generated carbazole radical cation acts as an oxidant to return the photocatalyst i...
46 citations
TL;DR: This approach constitutes a novel oxidative "redox auxiliary" strategy that offers a practical means to circumvent a fundamental thermodynamic limitation facing photoredox reactions.
TL;DR: In this paper, a method for the synthesis of 1,2-diketones through the mild and metal-free catalytic photooxygenation of alkynes is described, using 9,10dicyanoanthracene as a catalytic sensitizer with/without biphenyl as a co-sensitizer.
Abstract: A method for the synthesis of 1,2-diketones through the mild and metal-free catalytic photooxygenation of alkynes is described. This reaction, using 9,10-dicyanoanthracene as a catalytic sensitizer with/without biphenyl as a co-sensitizer, readily furnished a variety of desired products upon visible-light irradiation. Mechanistic studies indicated that a radical cation process might have been involved. This report provides the first examples of the transformation of alkynes via acetenyl radical cation intermediates, which have thus far not been explored in organic synthesis.
22 Aug 2017
TL;DR: In this article, a density functional theory (DFT) was used to investigate the oxidative decomposition mechanism of fluoroethylene carbonate (FEC) used in high-voltage batteries.
Abstract: © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim The oxidative decomposition mechanism of fluoroethylene carbonate (FEC) used in high-voltage batteries is investigated by using density functional theory (DFT). Radical cation FEC•+ is formed from FEC by transferring one electron to electrode and the most likely decomposition products are CO2 and 2-fluoroacetaldehyde radical cation. Other possible products are CO, formaldehyde and formyl fluoride radical cations. These radical cations are surrounded by much FEC solvent and their radical center may attack the carbonyl carbon of FEC to form aldehyde and oligomers of alkyl carbonates, which is similar with the oxidative decomposition of EC. Then, our experimental result reveals that FEC-based electrolyte has rather high anodic stability. It can form a robust SEI film on the positive electrode surface, which can inhibit unwanted electrolyte solvent and LiPF6 salts decomposition, alleviate Mn/Ni dissolution and therefore, improve the coulombic efficiency and the cycling stability of high voltage LiNi0.5Mn1.5O4 positive electrodes. This work displays that FEC-based electrolyte systems have considerable potential replacement of the EC-based electrolyte for the applications in 5 V Li-ion batteries.
TL;DR: In this paper, a vacuum heat treatment of Nb2O5 allowed the ligand-to-metal charge transfer (LMCT) transition between its surface and adsorbed aromatic hydrocarbons, which corresponds to visible light (>390 nm) absorption.
Abstract: This study shows that a vacuum heat treatment of Nb2O5 permits the ligand-to-metal charge transfer (LMCT) transition between its surface and adsorbed aromatic hydrocarbons, which corresponds to visible light (>390 nm) absorption. Selective photooxidation of aromatic hydrocarbons to form the corresponding carbonyl compounds could effectively proceed under visible light irradiation by using this LMCT transition, and the catalytic activity is much superior to that of TiO2. Specifically, the LMCT bands of toluene-adsorbed Nb2O5 correlated well with the action spectrum for toluene photooxidation, suggesting that the reaction proceeded via the LMCT photoexcitation. We successfully detected toluene radical cation and its further oxidation product of benzylperoxy radical as intermediate species on toluene-adsorbed Nb2O5 through the LMCT transition. These findings suggest a novel reaction mechanism for the selective photooxidation of aromatic hydrocarbons via LMCT transition on Nb2O5.
TL;DR: The monohydration motif of Np+ differs qualitatively from that of the benzene cation in both structure and binding energy, indicating the strong influence of the multiple aromatic rings on the hydration of PAH+ cations.
Abstract: Polycyclic aromatic hydrocarbons (PAHs) are suggested to occur in interstellar media and ice grains. It is important to characterize hydrated PAHs and their cations to explore their stability in interstellar and biological media. Herein, the infrared photodissociation (IRPD) spectrum of the naphthalene+–H2O radical cation (Np+–H2O) recorded in the O–H and C–H stretch range is analysed by dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level to determine its structure and intermolecular bonding. Monohydration of Np+ in its 2Au ground electronic state leads to the formation of a bifurcated CH⋯O ionic hydrogen bond (H-bond), in which the lone pairs of H2O bind to two adjacent CH proton donors of the two aromatic rings. The frequency-dependent branching ratios observed for IRPD of cold Np+–H2O–Ar clusters allows the estimation of the dissociation energy of Np+–H2O as D0 ∼ 2800 ± 300 cm−1. The monohydration motif of Np+ differs qualitatively from that of the benzene cation in both structure and binding energy, indicating the strong influence of the multiple aromatic rings on the hydration of PAH+ cations. This difference is rationalized by natural bond orbital analysis of the ionic H-bond motif. Comparison with neutral Np–H2O reveals the large change in structure and bond strength of the hydrated PAHs upon ionization. While neutral Np–H2O is stabilized by weak π H-bonds (OH⋯π, π-stacking), strong cation–dipole forces favour a planar bifurcated CH⋯O ionic H-bond in Np+–H2O.
TL;DR: It is found that, in addition to the path leading to the well-known TA* photoproduct, an AT photo-dimerization path may be operative in duplexes, and the pathways leading to such species and to characterize their absorption spectra are mapped.
Abstract: There is increasing evidence that the direct absorption of photons with energies that are lower than the ionization potential of nucleobases may result in oxidative damage to DNA. The present work, which combines nanosecond transient absorption spectroscopy and quantum mechanical calculations, studies this process in alternating adenine–thymine duplexes (AT)n. We show that the one-photon ionization quantum yield of (AT)10 at 266 nm (4.66 eV) is (1.5 ± 0.3) × 10−3. According to our PCM/TD-DFT calculations carried out on model duplexes composed of two base pairs, (AT)1 and (TA)1, simultaneous base pairing and stacking does not induce important changes in the absorption spectra of the adenine radical cation and deprotonated radical. The adenine radicals, thus identified in the time-resolved spectra, disappear with a lifetime of 2.5 ms, giving rise to a reaction product that absorbs at 350 nm. In parallel, the fingerprint of reaction intermediates other than radicals, formed directly from singlet excited states and assigned to AT/TA dimers, is detected at shorter wavelengths. PCM/TD-DFT calculations are carried out to map the pathways leading to such species and to characterize their absorption spectra; we find that, in addition to the path leading to the well-known TA* photoproduct, an AT photo-dimerization path may be operative in duplexes.
TL;DR: It was shown that the N-peralkyl-substituted NHC-CAAC derived triazaalkenes can be used as an electron donor in the reduction of an aryldiazonium salt to the corresponding arene.
TL;DR: For the first time, a combination of chiroptical methods involving ECD, ORD, and VCD, supported by quantum mechanical predictions, enabled the elucidation of the absolute configuration of such open-shell helical species.
Abstract: A stable chiral hetero[4]helicene radical cation was synthesized and characterized by UV/Vis absorption and EPR spectroscopy, as well as X-ray crystallography. For the first time, a combination of chiroptical methods involving ECD, ORD, and VCD, supported by quantum mechanical predictions, enabled the elucidation of the absolute configuration of such open-shell helical species.
TL;DR: A charge-transfer complex self-assembled from an electron acceptor (NDI-EA: naphthalene diimide with appended diamine) and an electron donor (DAN: phosphonic acid-appended dialkoxynapthalene) in aqueous medium with flower-shaped assembly was visualized by scanning electron and transmission electron microscopy.
Abstract: A charge-transfer (CT) complex self-assembled from an electron acceptor (NDI-EA: naphthalene diimide with appended diamine) and an electron donor (DAN: phosphonic acid-appended dialkoxynapthalene) in aqueous medium. The aromatic core of the NDI and the structure of DAN1 were designed to optimize the dispersive interactions (π-π and van der Waals interactions) in the DAN1–NDI-EA self-assembly, while the amino groups of NDI also interact with the phosphonic acid of DAN1 via electrostatic forces. This arrangement prevented crystallization and favored the directional growth of 3D flower nanostructures. This molecular geometry that is necessary for charge transfer to occur was further evidenced by using a mismatching DAN2 structure. The flower-shaped assembly was visualized by scanning electron and transmission electron microscopy. The formation of the CT complex was determined by UV-vis and cyclic voltammetry and the photoinduced electron transfer to produce the radical ion pair was examined by femtosecond laser transient absorption spectroscopic measurements.
TL;DR: Gas-phase ion-molecule reactions showed that Cyt.+ undergoes hydrogen-atom abstraction from 1-propanethiol, radical recombination reactions with nitric oxide, and electron transfer from dimethyl disulfide, in agreement with a previous study.
Abstract: The radical cation of cytosine (Cyt.+ ) is generated by dissociative oxidation from a ternary CuII complex in the gas phase. The radical cation is characterized by infrared multiple photon dissociation (IRMPD) spectroscopy in the fingerprint region, UV/Vis photodissociation (UVPD) spectroscopy, ion-molecule reactions, and theoretical calculations (density functional theory and ab initio). The experimental IRMPD spectrum features diagnostic bands for two enol-amino and two keto-amino tautomers of Cyt.+ that are calculated to be among the lowest energy isomers, in agreement with a previous study. Although the UVPD action spectrum can also be matched to a combination of the four lowest energy tautomers, the presence of a nonclassical distonic radical cation cannot be ruled out. Its formation is, however, unlikely due to the high energy of this isomer and the respective ternary CuII complex. Gas-phase ion-molecule reactions showed that Cyt.+ undergoes hydrogen-atom abstraction from 1-propanethiol, radical recombination reactions with nitric oxide, and electron transfer from dimethyl disulfide.
TL;DR: This study highlights the preparation of novel HTMs in a simple, economic, and efficient manner using quinone-based chloranil/H+ reagent as the recyclable organic oxidant system to afford benzidines/naphthidines.
Abstract: A variety of arylamines are shown to undergo oxidative C–C bond formation using quinone-based chloranil/H+ reagent as the recyclable organic (metal-free) oxidant system to afford benzidines/naphthidines. Arylamines (3°/2°) designed with various substituents were employed to understand the steric as well as electronic preferences of oxidative dimerization, and a mechanism involving amine radical cation has been proposed. The tetraphenylbenzidine derivative obtained via oxidative C–C coupling has been further converted to blue-emissive hole-transporting material via a simple chemical transformation. This study highlights the preparation of novel HTMs in a simple, economic, and efficient manner.
TL;DR: Iron (III)-EDTA was a more effective activator than iron (II) and iron (III), and also provided a non-hydroxyl radical, non-sulfate radical degradation pathway.
TL;DR: 3-amino-perylene in SDS micelles, when combined with the bioavailable ascorbate as an extramicellar sacrificial donor, sustainably produces hydrated electrons through photoredox catalysis with green light, from a metal-free system, and at near-physiological pH.
Abstract: The hydrated electron represents a “super-reductant” in water, providing 2.9 eV of reductive power, which suffices to decompose nonactivated aliphatic halides. We show that 3-amino-perylene in SDS micelles, when combined with the bioavailable ascorbate as an extramicellar sacrificial donor, sustainably produces hydrated electrons through photoredox catalysis with green light, from a metal-free system, and at near-physiological pH. Photoionization of the amine with a 532 nm laser yields an extremely long-lived radical cation as the by-product, and a subsequent reaction of the latter with the sacrificial donor across the micelle/water interface regenerates the catalyst. The regeneration step involves parallel reactions between differently protonated forms, causing a bell-shaped pH dependence in basic medium. We have separated these processes kinetically. Employing this catalytic cycle for the laboratory-scale decomposition of chloroacetate, an accepted model compound for toxic and persistent halo-organic waste, gave turnover numbers of about 170. Even though both the substrate and the sacrificial donor compete for the hydrated electron, their consumption ratio is practically independent of the initial concentration ratio because the formal radical anion of the ascorbate undergoes secondary scavenging by the chloroacetate. In the course of the reaction, the initial hydrophobic catalyst is converted into a secondary species that is hydrophilic and still exhibits catalytic activity.
TL;DR: In this article, the presence of a stable hemi-bonded core (H2S∴SH2)+ in (H 2S)n+ (n = 3-6) in the gas phase is demonstrated by infrared spectroscopy combined with quantum chemical calculations.
Abstract: A two-center three-electron 2c–3e bond (hemi-bond) is a non-classical chemical bond, and its existence has been supposed in radical cation clusters with lone pairs. Though the nature of the hemi-bond and its role in the reactivity of radical cations have attracted great interest, spectroscopic observations of hemi-bonded structures have been very scarce. In the present study, the presence of a stable hemi-bonded core (H2S∴SH2)+ in (H2S)n+ (n = 3–6) in the gas phase is demonstrated by infrared spectroscopy combined with quantum chemical calculations. The spectral features of the free SH stretch of the ion core show that the hemi-bond motif of the ion core is maintained up to the completion of the first H-bonded solvation shell. All of the observed spectra are well reproduced by the minimum energy hemi-bonded isomers, and no sign of the proton-transferred ion core type H3S+–SH, which is estimated to have a much higher energy, is found. Spin density calculations show that the excess charge is almost equally delocalized over the two H2S molecules in the cluster for n = 3 to 6. This also indicates the hemi-bond nature of the (H2S∴SH2)+ ion core and the small impact of the formation of a solvation shell on the ion core.
TL;DR: In this article, a new redox conducting polymer, viz. poly[meso-N,N′-bis(salicylidene)-2,3-butanediaminonickel(II)], belonging to the Schiff base polymer family, was electrochemically synthesized.
Abstract: A new redox conducting polymer, viz. poly[meso-N,N′-bis(salicylidene)-2,3-butanediaminonickel(II)], poly[meso-Ni(II)-SaldMe], belonging to the Schiff base polymer family, was electrochemically synthesized. The charge transfer and polymerization mechanism were unraveled by simultaneous cyclic voltammetry (CV) and in situ UV–vis, FTIR-ATR, and ex situ low-temperature ESR spectroscopy. With the latter, a short-living paramagnetic transient form of electro-oxidized poly[meso-Ni(II)-SaldMe] was detected. This form was identified as the bisphenolic radical cation. In situ UV–vis and FTIR-ATR spectroelectrochemistry measurements revealed that the charge transfer of the polymer involved bisphenolic radical cation formation at the potential lower than 0.80 V vs Ag/Ag+ and then dication formation at the potential exceeding 0.80 V. The proposed mechanism of electropolymerization of meso-N,N′-bis(salicylidene)-2,3-butanediaminonickel(II), meso-Ni(II)-SaldMe, involves two steps. First, electro-oxidation of the monomer...
TL;DR: In this article, the effect of a radical cation on the activation barrier of different classes of photochromes (DHP, dithienylethene, dihydroazulene and fulgide) was analyzed at the same level of theory.
TL;DR: In this article, a triarylamine−Ru(bpy)32+−anthraquinone triad was excited with two temporally delayed laser pulses of different color and monitored the resulting photoproducts.
Abstract: Controlling light‐induced accumulation of electrons or holes is desirable in view of multi‐electron redox chemistry, for example for the formation of solar fuels or for photoredox catalysis in general. Excitation with multiple photons is usually required for electron or hole accumulation, and consequently pump‐pump‐probe spectroscopy becomes a valuable spectroscopic tool. In this work, we excited a triarylamine‐Ru(bpy)32+‐anthraquinone triad (bpy = 2,2′‐bipyridine) with two temporally delayed laser pulses of different color and monitored the resulting photoproducts. Absorption of the first photon by the Ru(bpy)32+ photosensitizer generated a triarylamine radical cation and an anthraquinone radical anion by intramolecular electron transfer. Subsequent selective excitation of either one of these two radical ion species then induced rapid reverse electron transfer to yield the triad in its initial (ground) state. This shows in direct manner that after absorption of a first photon and formation of the primary photoproducts, the absorption of a second photon can lead to unproductive electron transfer events that counteract further charge accumulation. In principle, this problem is avoidable by careful excitation wavelength selection in combination with good molecular design.
TL;DR: A visible-light-promoted synthesis of substituted naphthalenes via [4 + 2] annulation of amino-benzocyclobutenes with alkynes is reported, where the aniline group plays a dual role in which it not only directs the initial photooxidation to generate the amine radical cation but also serves as a leaving group to complete aromatization.
Abstract: We report a visible-light-promoted synthesis of substituted naphthalenes via [4 + 2] annulation of amino-benzocyclobutenes with alkynes Amino-benzocyclobutenes, which are conveniently synthesized by [2 + 2] cycloaddition of arynes with ketenes followed by reductive amination, undergo regioselective opening of the cyclobutenyl ring to reveal a presumably distonic radical cation upon photooxidation by an excited iridium complex The distonic radical cation undergoes the annulation with terminal and internal alkynes as well as diynes to afford structurally diverse naphthalenes The regiochemistry of the annulation follows the pattern displayed in the addition of nucleophilic carbon radicals to alkynes The aniline group plays a dual role in which it not only directs the initial photooxidation to generate the amine radical cation but also serves as a leaving group to complete aromatization
TL;DR: This first example of a gas-phase model system providing molecular-level details on the chemistry of an ionized DNA base pair paves the way toward a more complete understanding of molecular processes induced by radiation.
Abstract: The deleterious cellular effects of ionizing radiation are well-known, but the mechanisms causing DNA damage are poorly understood. The accepted molecular events involve initial oxidation and deprotonation at guanine sites, triggering hydrogen atom abstraction reactions from the sugar moieties, causing DNA strand breaks. Probing the chemistry of the initially formed radical cation has been challenging. Here, we generate, spectroscopically characterize, and examine the reactivity of the Watson–Crick nucleobase pair radical cation in the gas phase. We observe rich chemistry, including proton transfer between the bases and propagation of the radical site in deoxyguanosine from the base to the sugar, thus rupturing the sugar. This first example of a gas-phase model system providing molecular-level details on the chemistry of an ionized DNA base pair paves the way toward a more complete understanding of molecular processes induced by radiation. It also highlights the role of radical propagation in chemistry, b...
TL;DR: In this article, the formation of the prebiotically relevant molecule formamide under electron exposure of ammonia and carbon monoxide was studied at cryogenic temperatures of 30 −35 K. A resonant process centered around ∼9 eV and a threshold type increase of the yield above ∼12 eV were observed.
Abstract: The formation of the prebiotically relevant molecule formamide under electron exposure of ammonia and carbon monoxide was studied at cryogenic temperatures of 30–35 K. Postirradiation thermal desorption spectroscopy was used to study the energy dependence of the reaction. A resonant process centered around ∼9 eV and a threshold type increase of the yield above ∼12 eV were observed. On the basis of the absence of particular side products such as urea and ethanediamide and supported by quantum chemical calculations, reaction mechanisms related to the two observed energy regimes of formamide production are proposed. Below the ionization threshold, electron attachment to ammonia and the subsequent dissociation of the radical anion trigger the reaction sequence. At higher energies, electron impact ionization and addition of the formed radical cation to a neutral molecule ultimately lead to the formation of formamide.