Showing papers on "Radical ion published in 2013"

Toward Designed Singlet Fission: Solution Photophysics of Two Indirectly Coupled Covalent Dimers of 1,3-Diphenylisobenzofuran

Abstract: In order to identify optimal conditions for singlet fission, we are examining the photophysics of 1,3-diphenylisobenzofuran (1) dimers covalently coupled in various ways. In the two dimers studied presently, the coupling is weak. The subunits are linked via the para position of one of the phenyl substituents, in one case (2) through a CH2 linker and in the other (3) directly, but with methyl substituents in ortho positions forcing a nearly perpendicular twist between the two joint phenyl rings. The measurements are accompanied with density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Although in neat solid state, 1 undergoes singlet fission with a rate constant higher than 10(11) s(-1); in nonpolar solutions of 2 and 3, the triplet formation rate constant is less than 10(6) s(-1) and fluorescence is the only significant event following electronic excitation. In polar solvents, fluorescence is weaker because the initial excited singlet state S1 equilibrates by sub-nanosecond charge transfer with a nonemissive dipolar species in which a radical cation of 1 is attached to a radical anion of 1. Most of this charge transfer species decays to S0, and some is converted into triplet T1 with a rate constant near 10(8) s(-1). Experimental uncertainties prevent an accurate determination of the number of T1 excitations that result when a single S1 excitation changes into triplet excitation. It would be one if the charge-transfer species undergoes ordinary intersystem crossing and two if it undergoes the second step of two-step singlet fission. The triplet yield maximizes below room temperature to a value of roughly 9% for 3 and 4% for 2. Above ∼360 K, some of the S1 molecules of 3 are converted into an isomeric charge-transfer species with a shorter lifetime, possibly with a twisted intramolecular charge transfer (TICT) structure. This is not observed in 2.

One-Electron Oxidation of an Organic Molecule by B(C6F5)3; Isolation and Structures of Stable Non-para-substituted Triarylamine Cation Radical and Bis(triarylamine) Dication Diradicaloid

Abstract: The methylene-bridged triphenylamine 2 has been oxidized to planar radical cation 2•+ by B(C6F5)3 or Ag+. Further reaction of 2•+[Al(ORF)4]− and 2 with trace amounts of silver salt resulted in dication 32+, providing a rare example of structurally characterized bis(triarylamine) “bipolarons”. 32+ can be directly prepared by the reaction of 3 with 2 equiv of Ag+. X-ray structural analysis together with theoretical calculation shows that 32+ has singlet diradical character and is analogous to Chichibabin’s hydrocarbons.

Isolation and Characterization of the Cycloparaphenylene Radical Cation and Dication

Abstract: Charged nanobelts: The radical cation and the dication of [8]cycloparaphenylene ([8]CPP) were prepared and isolated as hexahaloantimonate salts by the one- or two-electron chemical oxidation of [8]CPP with NOSbF6 or SbCl5 . ESR spectroscopy of CPP(.+) and single-crystal X-ray analysis of CPP(2+) demonstrated that the spin and charge were equally and fully delocalized over the para-phenylene rings.

Blending through-space and through-bond π-π-coupling in [2,2']-paracyclophane-oligophenylenevinylene molecular wires.

Abstract: A series of ZnP-pCp-oPPV-C60 conjugates covalently connected through [2,2′]-paracyclophane-oligophenylenevinylene (pCp-oPPV) bridges containing one, two, and three [2,2′]-paracyclophanes (pCps) has been prepared in multistep synthetic procedures involving Horner–Wadsworth–Emmons olefination reactions and/or Heck type Pd-catalyzed reactions. Molecular modeling suggests that charge transfer is effectively mediated by the pCp-oPPVs through a predominant hole-transfer mechanism. Photophysical investigation supports molecular modeling and reveals two major trends. On one hand, C60 excitation of 1, 2, and 3 leads exclusively to charge transfer between pCp and C60 to afford a ZnP-(pCp-oPPV)•+-C60•– radical ion pair state without giving rise to a subsequent charge shift to yield the ZnP•+-pCp-oPPV-C60•– radical ion pair state. On the other hand, ZnP excitation of 1, 2, and 3 results in a rather slow charge transfer between ZnP and C60, after which the ZnP•+-pCp-oPPV-C60•– radical ion pair state evolves. In temper...

Stable tetraaryldiphosphine radical cation and dication.

Abstract: Salts containing tetraaryldiphosphine radical cation 1•+ and dication 12+ have been isolated and structurally characterized. Radical 1•+ has a relaxed pyramidal geometry, while dication 12+ prefers a planar, olefin-like geometry with a two-electron π bond. The alteration of the geometries of the tetraaryldiphosphine upon oxidation is rationalized by the nature of the bonding. The EPR spectrum showed that the spin density of radical 1•+ is mainly localized on phosphorus atoms, which is supported by theoretical calculation.

Catalytic sp3 C-H oxidation of peptides and their analogues by radical cation salts: from glycine amides to quinolines.

Abstract: A catalytic α-sp3 C–H oxidation of peptides and glycine amides was achieved under radical cation salt catalysis in the presence of O2, producing a series of substituted quinolines. The scope of this reaction shows good functional group tolerance and high efficiency of the oxidative functionalization.

Single-step synthesis of polypyrrole nanowires by cathodic electropolymerization

Abstract: Polypyrrole nanowires are successfully fabricated with a one-step process by cathodic electropolymerization from an aqueous solution without templates and chemical additives The method utilizes electrochemically generated NO+ to oxidize the neutral pyrrole monomers, making it possible to use oxidizable metal substrates, such as Cu and Ni The synthesized nanowires are directly deposited on the substrate in the form of a thin film consisting of fine polypyrrole nanowires with a nanoporous and interconnected network structure The growth kinetics of the polypyrrole nanowires was investigated by analyzing the effects of the chemical composition of the electrolyte and the synthesis time on the formation of polypyrrole nanowires It was found that the polymerization process of pyrrole is very sensitive to the reactivity of radical cations For a radical cation with high reactivity, the polypyrrole nanospheres are synthesized near the electrode in the solution In contrast, for a radical cation with sufficiently low reactivity, the polypyrrole nanowires are grown on the priorly deposited polypyrrole nanospheres

Photophysical and theoretical investigations of the [8]cycloparaphenylene radical cation and its charge-resonance dimer

Abstract: Treatment of [8]cycloparaphenylene (CPP) with the oxidant triethyloxonium hexachloroantimonate afforded an isolable radical cation of the parent carbon nanohoop. The photophysical properties of [8]CPP˙+SbCl6− were investigated, showing the presence of two absorptions at 535 nm and 1115 nm. Time-dependent density functional theory (DFT) calculations were used to examine these optical absorptions, revealing a delocalized, quinoidal carbon nanohoop. Upon mixing with neutral [8]cycloparaphenylene, the formation of an unusually strong charge-resonance complex ([8]CPP2)˙+ was observed. Spectroscopic and computational studies were indicative of extensive intermolecular charge delocalization between the two carbon nanohoops as well.

Near-infrared colorimetric and fluorescent Cu(2+) sensors based on indoline-benzothiadiazole derivatives via formation of radical cations.

Abstract: The donor–acceptor system of indoline–benzothiadiazole is established as the novel and reactive platform for generating amine radical cations with the interaction of Cu2+, which has been successfully exploited as the building block to be highly sensitive and selective near infrared (NIR) colorimetric and fluorescent Cu2+ sensors. Upon the addition of Cu2+, 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 Cu2+ sensors.

Oxidative Cyclization Reactions: Controlling the Course of a Radical Cation‐Derived Reaction with the Use of a Second Nucleophile

Abstract: The oxidative generation of reactive radical cation intermediates can serve as a powerful tool for the construction of new ring systems.[1,2] For example, substrates with electronrich olefins can be oxidized to generate radical cations that trigger cyclizations with a variety of electron-rich groups.[3] Enol ethers, vinylsulfides, ketene derivatives, electron-rich aryl rings, and styrenes have all been oxidized to form radical cations, whereas enol ethers, allyl and vinylsilanes, aryl rings, styrenes, alcohols, amides, sulfonamides, and amines have all been used to trap the radical cation. The reactions have led to the synthesis of fused and bicyclic ring skeletons and are often compatible with the formation of tetrasubstituted carbons. In addition, they have served to help us gain a better understanding of radical cation intermediates.[4]

Reaction selectivity in an ionized water dimer: nonadiabatic ab initio dynamics simulations.

Abstract: We study dynamical processes following water dimer ionization. The nonadiabatic dynamical simulations of the water dimer radical cation are performed using a surface hopping technique and a Complete Active Space-Self Consistent Field (CASSCF) method for the description of electronic structure. The main goal of this study is to find out whether a state-dependent reactivity is observed for the water dimer radical cation. We provide a detailed mapping of the potential energy surfaces (PESs) in the relevant coordinates for different electronic states. Dynamical patterns are discussed on the basis of static PES cuts and available experimental data. As a product of the reaction, we observed either proton transferred structure (H3O(+)···OH˙) or various dissociated structures (H3O(+) + OH˙, H2O˙(+) + H2O, H˙ + OH˙ + H2O˙(+)). The relative yields are controlled by the populated electronic state of the radical cation. The proton transfer upon the HOMO electron ionization is an ultrafast process, taking less than 100 fs, in cases of higher energy ionization the dynamical processes occur on longer timescales (200-300 fs). We also discuss the implications of our simulations for the efficiency of the recently identified intermolecular coulomb decay (ICD) process in the water dimer.

Properties and Reactivity of Gaseous Distonic Radical Ions with Aryl Radical Sites

Abstract: The reactivity of carbon-centered distonic radical ions has been of interest for decades. The existence of distonic radical ions was first postulated by Gross and McLafferty in the early 1970’s.1,2 In 1978, Bouma, MacLeod and Radom reported experimental results that supported theoretical predictions of the existence of a stable ring-opened ethylene oxide distonic radical cation.3 Ions with spatially separated charge and radical sites were coined as “distonic ions” by Yates, Bouma, and Radom4 in 1984. They later refined this definition5 to correspond to radical ions generated by ionization of a zwitterion, ylide, or diradical. Eberlin and co-workers later introduced the term “distonoid”, meaning distonic like, to encompass any radical ion that displays distonic “character” (i.e., ions with a high degree of discrete (non-mandatory) charge-spin separation) and is over-looked as a result of the strict distonic ion definition.6 However, the “distonoid” classification is not commonly used by the scientific community. Currently, the term “distonic” is widely accepted and used to denote ions with formally separated charge and radical sites even if they do not fall into the formal definition.7 According to the conventional valence bond description, the charge and radical sites are on adjacent atoms in α-distonic ions while they are separated by one and two atoms in β- and γ-distonic ions, respectively. A vast amount of experimental and theoretical studies were dedicated to distonic ions from the 1980’s to 1990’s, which have been previously reviewed separately by Hammerum and Kenttamaa.8,9,10 The focus of this review is on gaseous ions with one or more aryl radical sites, a subgroup of distonic radical cations. The interest in these distonic ions was initially sparked by the limited knowledge on the reactivity of neutral phenyl radicals and their diradical counterparts in spite of the vast amount of research dedicated to these reactive intermediates.11–70 Many such mono- and diradicals have been investigated as they are thought to play a vital role in numerous fields, including combustion,11–13 polymerization,14–16 atmospheric chemistry,17–19 interstellar chemistry,20 organic synthesis8 and the biological activity of certain drugs.21–33 In the 1990’s, the formation of such aromatic diradicals in naturally occurring anti-tumor antibiotics was associated with their DNA-cleaving ability.21–33 The two radical sites are thought to abstract a hydrogen atom from each strand of double stranded DNA, thus causing irreversible DNA cleavage. Since then, theoretical and experimental research on aryl mono- and diradicals has boomed. An area of special interest has been the mechanistic understanding of hydrogen atom abstraction by these radicals from small organic and biological molecules both in solution34–45 and in the gas phase.46–70 However, the ability to predict the rates of such seemingly simple reactions has proven challenging due to a poor understanding of the nature of the transition states for these reactions. Further, the examination of the chemical properties of neutral radicals is a challenge due to the difficulty to cleanly generate them both in solution and in the gas phase. In order to address the above difficulties, studies were carried out in the early 1990’s on distonic radical cations’ ion-molecule reactions inside mass spectrometers as ions can be easily manipulated in this environment.46–70 Distonic radical cations that have a phenyl radical site spatially separated from a chemically inert charge site were found to almost exclusively undergo radical reactions at the radical site(s) in the gas phase46–70 and lately also in solution.45 Hence, examination of these distonic ions will provide information on the properties of phenyl mono- and diradicals. A special benefit of using mass spectrometry to study above species is that the desired charged radical can be isolated before examining its reactivity. Hence, the precursors to any products formed in these gas-phase experiments are known, which is not always true for solution experiments wherein highly reactive molecules cannot be isolated. The chemical properties of many aryl mono-, di- and triradicals have been successfully examined in mass spectrometers by using this ‘distonic ion approach’.49,50,52 The results obtained in these studies provide valuable information on the relative reactivities of mono- and polyradicals, which would otherwise not be available. This paper reviews the current knowledge of the properties and reactivity of distonic radical ions with aryl radical sites and the mechanisms of these reactions. Distonic phenyl radical ions generated within peptides are not included due to space limitations and also since these radicals are usually generated as precursors to less reactive nonaromatic peptide radicals that are the true interest of the researchers. However, this is an important and exciting new field of distonic ion research that should be reviewed separately.

Time-resolved magnetic field effects distinguish loose ion pairs from exciplexes.

Abstract: We describe the experimental investigation of time-resolved magnetic field effects in exciplex-forming organic donor–acceptor systems. In these systems, the photoexcited acceptor state is predominantly deactivated by bimolecular electron transfer reactions (yielding radical ion pairs) or by direct exciplex formation. The delayed fluorescence emitted by the exciplex is magnetosensitive if the reaction pathway involves loose radical ion pair states. This magnetic field effect results from the coherent interconversion between the electronic singlet and triplet radical ion pair states as described by the radical pair mechanism. By monitoring the changes in the exciplex luminescence intensity when applying external magnetic fields, details of the reaction mechanism can be elucidated. In this work we present results obtained with the fluorophore-quencher pair 9,10-dimethylanthracene/N,N-dimethylaniline (DMA) in solvents of systematically varied permittivity. A simple theoretical model is introduced that allows ...

Background Emission of Electrogenerated Chemiluminescence during Oxidation of Tri-n-propylamine from the Dimeric 1Δg State of O2

Abstract: The background electrogenerated chemiluminescence (ECL) emission observed only upon electrochemical oxidation of tri-n-propylamine (TPrAH) on a platinum electrode is a limiting factor in ECL analytical techniques and is poorly understood. We studied this reaction in aerated acetonitrile (MeCN) solution with TPrAH oxidized at a constant potential at the Pt surface and observed ECL spectra with an emission band at 630 nm, which is characteristic of the emission of the dimeric 1Δg state of O2. No ECL emission was observed when the same solution was deaerated. This background ECL emission is attributed to the reaction between dissolved oxygen and two different products of TPrAH oxidation: the TPrAH• radical that reduces O2 to the superoxide ion and the TPrAH•+ radical cation that oxidizes this species to singlet O2.

Proton transfer or hemibonding? The structure and stability of radical cation clusters

Abstract: The basin hopping search algorithm in conjunction with second-order Moller–Plesset perturbation theory is used to determine the lowest energy structures of the radical cation clusters (NH3)n˙+, (H2O)n˙+, (HF)n˙+, (PH3)n˙+, (H2S)n˙+ and (HCl)n˙+, where n = 2–4. The energies of the most stable structures are subsequently evaluated using coupled cluster theory in conjunction with the aug-cc-pVTZ basis set. These cationic clusters can adopt two distinct structural types, with some clusters showing an unusual type of bonding, often referred to as hemibonding, while other clusters undergo proton transfer to give an ion and radical. It is found that proton transfer based structures are preferred by the (NH3)n˙+, (H2O)n˙+ and (HF)n˙+ clusters while hemibonded structures are favoured by (PH3)n˙+, (H2S)n˙+ and (HCl)n˙+. These trends can be attributed to the relative strengths of the molecules and molecular cations as Bronsted bases and acids, respectively, and the strength of the interaction between the ion and radical in the ion–radical clusters.

Photocatalytic monofluorination of benzene by fluoride via photoinduced electron transfer with 3-cyano-1-methylquinolinium.

Abstract: The photocatalytic fluorination of benzene occurs under photoirradiation of an oxygen-saturated acetonitrile (MeCN) of the 3-cyano-1-methylquinolinium ion (QuCN(+)) containing benzene and tetraethylammonium fluoride tetrahydrofluoride (TEAF·4HF) with a xenon lamp (500 W) attached to a colored-glass filter (λ < 290 nm) to yield fluorobenzene and hydrogen peroxide. The quantum yield of formation of fluorobenzene was 6%. Nanosecond laser flash photolysis measurements were performed to elucidate the mechanistic details for photocatalytic fluorination. Transient absorption spectra taken after the nanosecond laser excitation at 355 nm of a degassed MeCN solution of QuCN(+) and benzene exhibited absorption bands due to QuCN(•) (λmax = 500 nm) and the benzene dimer radical cation (λmax = 900 nm), which were generated by photoinduced electron transfer from benzene to the singlet excited state of QuCN(+). The decay rate of the transient absorption band due to the benzene dimer radical cation was accelerated by the addition of TEAF·4HF. The observed rate constant increased with increasing concentration of TEAF·4HF. The rate constant of the electrophilic addition of fluoride to the benzene radical cation was determined to be 9.4 × 10(9) M(-1) s(-1). Thus, the photocatalytic reaction is initiated by intermolecular photoinduced electron transfer from benzene to the single excited state of QuCN(+). The benzene radical cation formed by photoinduced electron transfer reacts with the fluoride anion to yield the F-adducted radical. However, QuCN(•) can reduce O2 to O2(•-), and this is followed by the protonation of O2(•-) to afford HO2(•). The hydrogen abstraction of HO2(•) from the F-adduct radical affords fluorobenzene and H2O2 as the final products.

Ultrafast electron transfer in a porphyrin-amino naphthalene diimide dyad

Abstract: Photo-induced electron transfer in a covalently linked zinc(II) tetraphenylporphyrin-amino naphthalene diimide dyad (ZnTPP-ANDI) is reported. The fluorescence of ZnTPP-ANDI is strongly quenched in both toluene and benzonitrile solvents compared to emission from ZnTPP. Ultrafast pump-probe spectroscopy has identified transient absorptions attributable to the ZnTPP•+ radical cation and the ANDI•− radical anion. It is shown that electron transfer (ET) can occur directly upon excitation to the S2 state of the porphyrin followed by rapid charge recombination to form S1 that subsequently undergoes a further slower ET to ANDI. The kinetics of charge separation (CS) and charge recombination (CR) for the latter ET process are strongly solvent dependent with a dramatically accelerated charge recombination rate (kCR) in the more polar solvent benzonitrile (kCR = 1.59 × 1011 s−1) compared to toluene (kCR = 8 × 109 s−1), in which inter-system crossing (ISC) from the CS state to form the lowest porphyrin triplet state is the dominating decay pathway (kCR/ISC = 3.46 × 1010 s−1). The implications of the results for designing molecular systems to potentially utilise ET following S2 excitation are discussed.

Oxidation of sulfonamide antibiotics: Aromatic nucleophilic substitution of an aniline radical cation

Abstract: Reference EPFL-CONF-190964View record in Web of Science Record created on 2013-12-09, modified on 2016-08-09

Dissociative photoionization of glycerol and its dimer occurs predominantly via a ternary hydrogen-bridged ion-molecule complex.

Abstract: The photoionization and dissociative photoionization of glycerol are studied experimentally and theoretically. Time-of-flight mass spectrometry combined with vacuum ultraviolet synchrotron radiation ranging from 8 to 15 eV is used to investigate the nature of the major fragments and their corresponding appearance energies. Deuterium (1,1,2,3,3-D5) and (13)C (2-(13)C) labeling is employed to narrow down the possible dissociation mechanisms leading to the major fragment ions (C3H(x)O2(+), C2H(x)O2(+), C2H(x)O(+), CH(x)O(+)). We find that the primary fragmentation of the glycerol radical cation (m/z 92) occurs only via two routes. The first channel proceeds via a six-membered hydrogen-transfer transition state, leading to a common stable ternary intermediate, comprised of neutral water, neutral formaldehyde, and a vinyl alcohol radical cation, which exhibits a binding energy of ≈42 kcal/mol and a very short (1.4 A) hydrogen bond. Fragmentation of this intermediate gives rise to experimentally observed m/z 74, 62, 44, and 45. Fragments m/z 74 and 62 both consist of hydrogen-bridged ion-molecule complexes with binding energy >25 kcal/mol, whereas the m/z 44 species lacks such stabilization. This explains why water- or formaldehyde-loss products are observed first. The second primary fragmentation route arises from cleaving the elongated C-C bond. Also for this channel, intermediates comprised of hydrogen-bridged ion-molecule complexes exhibiting binding energies >24 kcal/mol are observed. Energy decomposition analysis reveals that electrostatic and charge-transfer interactions are equally important in hydrogen-bridged ion-molecule complexes. Furthermore, the dissociative photoionization of the glycerol dimer is investigated and compared to the main pathways for the monomeric species. To a first approximation, the glycerol dimer radical cation can be described as a monomeric glycerol radical cation in the presence of a spectator glycerol, thus giving rise to a dissociation pattern similar to that of the monomer.

Effects of Alkoxy Groups on Arene Rings of Lignin β-O-4 Model Compounds on the Efficiencies of Single Electron Transfer-Promoted Photochemical and Enzymatic C-C Bond Cleavage Reactions

Abstract: To gain information about how alkoxy substitution in arene rings of β-O-4 structural units within lignin governs the efficiencies/rates of radical cation C1-C2 bond cleavage reactions, single electron transfer (SET) photochemical and lignin peroxidase-catalyzed oxidation reactions of dimeric/tetrameric model compounds have been explored. The results show that the radical cations derived from less alkoxy-substituted dimeric β-O-4 models undergo more rapid C1-C2 bond cleavage than those of more alkoxy-substituted analogues. These findings gained support from the results of DFT calculations, which demonstrate that C1-C2 bond dissociation energies of β-O-4 radical cations decrease as the degree of alkoxy substitution decreases. In SET reactions of tetrameric compounds consisting of two β-O-4 units, containing different degrees of alkoxy substitution, regioselective radical cation C-C bond cleavage was observed to occur in one case at the C1-C2 bond in the less alkoxy-substituted β-O-4 moiety. However, regioselective C1-C2 cleavage in the more alkoxy-substituted β-O-4 moiety was observed in another case, suggesting that other factors might participate in controlling this process. These observations show that lignins containing greater proportions of less rather than more alkoxylated rings as part of β-O-4 units would be more efficiently cleaved by SET mechanisms.

Light-induced conversion of Trp to Gly and Gly hydroperoxide in IgG1.

Abstract: The exposure of IgG1 in aqueous solution to light with λ = 254 nm or λ > 295 nm yields products consistent with Trp radical cation formation followed by αC–βC cleavage of the Trp side chain The resulting glycyl radicals either are reduced to Gly or add oxygen prior to reduction to Gly hydroperoxide Photoirradiation at λ = 254 nm targets Trp at positions 191 (light chain), 309 and 377 (heavy chain) while photoirradiation at λ > 295 nm targets Trp at position 309 (heavy chain) Mechanistically, the formation of Trp radical cations likely proceeds via photoinduced electron or hydrogen transfer to disulfide bonds, yielding thiyl radicals and thiols, where thiols may serve as reductants for the intermediary glycyl or glycylperoxyl radicals

Oxygenation and chlorination of aromatic hydrocarbons with hydrochloric acid photosensitized by 9-mesityl-10-methylacridinium under visible light irradiation

Abstract: Efficient photocatalytic oxygenation of toluene occurs under visible light irradiation of 9-mesityl-10-methylacridinium (Acr+–Mes) in oxygen-saturated acetonitrile containing toluene and aqueous hydrochloric acid with a xenon lamp for 15 h. The oxygenated products, benzoic acid (70 %) and benzaldehyde (30 %), were formed after the photoirradiation. The photocatalytic reaction is initiated by intramolecular photoinduced electron transfer from the mesitylene moiety to the singlet excited state of the Acr+ moiety of Acr+–Mes, which affords the electron-transfer state, Acr•–Mes•+. The Mes•+ moiety can oxidize chloride ion (Cl−) by electron transfer to produce chlorine radical (Cl•), whereas the Acr• moiety can reduce O2 to O 2 •− . The Cl• radical produced abstracts a hydrogen from toluene to afford benzyl radical in competition with the bimolecular radical coupling of Cl•. The benzyl radical reacts with O2 rapidly to afford the peroxyl radical, leading to the oxygenated product, benzaldehyde. Benzaldehyde is readily further photooxygenated to yield benzoic acid with Acr•–Mes•+. In the case of an aromatic compound with electron-donating substituents, 1,3,5-trimethoxybenzene, photocatalytic chlorination occurred efficiently under the same photoirradiation conditions to yield a monochloro-substituted compound, 2,4,6-trimethoxychlorobenzene.

A metallofullerene electron donor that powers an efficient spin flip in a linear electron donor-acceptor conjugate.

Abstract: The dream target of artificial photosynthesis is the realization of long-lived radical ion pair states that power catalytic centers and, consequently, the production of solar fuels. Notably, magnetic field effects, especially internal magnetic field effects, are rarely employed in this context. Here, we report on a linear Lu3N@Ih-C80–PDI electron donor–acceptor conjugate, in which the presence of the Lu3N cluster exerts an appreciable electron nuclear hyperfine coupling on the charge transfer dynamics. As such, a fairly efficient radical ion pair intersystem crossing converts the initially formed singlet radical ion pair state, 1[(Lu3N@Ih-C80)•+–PDI•–], to the corresponding triplet radical ion pair state, 3[(Lu3N@Ih-C80)•+–PDI•–]. Most notably, the radical ion pair state lifetime of the latter is nearly 1000 times longer than that of the former.

Anodic oxidation of disulfides: detection and reactions of disulfide radical cations.

Abstract: The anodic oxidation of five diaryldisulfides have been studied in a dichloromethane/NBu4B(C6F5)4 electrolyte. Cyclic voltammetry scans of (p-RC6H4)2S2 (R = Me, 1a; R = F, 1b; R = OMe, 1c) show modest chemical reversibility for the 1(0/+) couple (E1/2 values vs ferrocene: 1.04 V for 1a, 1.21 V for 1b, 0.92 V for 1c), providing the first voltammetric evidence for the radical cation Ar2S2(+). A dimer dication, Ar4S4(2+), is proposed as an intermediate in the formation of the electrolysis product, the trisulfide Ar3S3(+). The chemical reversibility of the one-electron oxidations of Ar2S2 vanishes in PF6(-)-containing electrolytes. The radical cations of the more sterically constrained ortho-substituted analogues dimesityldisulfide (2a, E1/2 = 1.01 V) and bis(2,4,6-triisopropylphenyl)disulfide (2b, E1/2 = 0.98 V) show less tendency to dimerize. In all cases except 2b, the bulk electrolysis product is R3S3(+), consistent with earlier literature reports. A mechanism is proposed in which the trisulfide is formed by reaction of the dimer dication Ar4S4(2+) with neutral Ar2S2 to afford the trisulfide in a net 2/3 e(-) process. Oxidation of Ar2S2, either anodically or by a strong one-electron oxidant, in the presence of cyclohexene gives an efficient synthetic route to 1,2-substituted cyclohexyldisulfides.

Electrochemistry and electrogenerated chemiluminescence of three phenanthrene derivatives, enhancement of radical stability, and electrogenerated chemiluminescence efficiency by substituent groups

Abstract: The electrochemistry and electrogenerated chemiluminescence (ECL) of three phenanthrene derivatives, 3,6-diphenyl-9,10-bis-(4-tert-butylphenyl)phenanthrene (TphP, T1), 3,6-di(naphthalen-2-yl)-9,10-bis(4-tert-butylphenyl)phenanthrene (TnaP, T2), and 3,6-di(pyrene-1-yl)-9,10-bis(4-tert-butylphenyl)phenanthrene (TpyP, T3), are investigated in an acetonitrile:benzene (v:v = 1:1) solvent. Cyclic voltammetry (CV) of the three derivatives shows reversible reduction waves and less chemically reversible oxidation waves at low scan rates. The CV character becomes more reversible, and the stability of the radical cations increases as the conjugation of the substituent groups appended to the phenanthrene increases. This finding indicates that the radical ion stabilities in phenanthrene derivatives are drastically improved by increasing the conjugation of the substituent groups; thus, electrochemically stable radical ions can be obtained by introducing more conjugated groups to the phenanthrene center. Additionally, E...