Sulfur Radical Cations

In-situ formatting donor-acceptor polymer with giant dipole moment and ultrafast exciton separation

Donor-acceptor semiconducting polymers present countless opportunities for application in photocatalysis. Previous studies have showcased their advantages through direct bottom-up methods. Unfortunately, these approaches often involve harsh reaction conditions, overlooking the impact of uncontrolled polymerization degrees on photocatalysis. Besides, the mechanism behind the separation of electron-hole pairs (excitons) in donor-acceptor polymers remains elusive. This study presents a post-synthetic method involving the light-induced transformation of the building blocks of hyper-cross-linked polymers from donor-carbon-donor to donor-carbon-acceptor states, resulting in a polymer with a substantial intramolecular dipole moment. Thus, excitons are efficiently separated in the transformed polymer. The utility of this strategy is exemplified by the enhanced photocatalytic hydrogen peroxide synthesis. Encouragingly, our observations reveal the formation of intramolecular charge transfer states using time-resolved techniques, confirming transient exciton behavior involving separation and relaxation. This light-induced method not only guides the development of highly efficient donor-acceptor polymer photocatalysts but also applies to various fields, including organic solar cells, light-emitting diodes, and sensors.

Synthesis and Single-Electron Oxidation of Bulky Bis(m-terphenyl)chalcogenides: The Quest for Kinetically Stabilized Radical Cations

Sterically encumbered bis(m-terphenyl)chalcogenides, (2,6-Mes₂ C₆ H₃ )₂ E (E=S, Se, Te) were obtained by the reaction of the chalcogen tetrafluorides, EF₄ , with three equivalents of m-terphenyl lithium, 2,6-Mes₂ C₆ H₃ Li. The single-electron oxidation of (2,6-Mes₂ C₆ H₃ )₂ Te using XeF₂ /K[B(C₆ F₅ )₄ ] afforded the radical cation [(2,6-Mes₂ C₆ H₃ )₂ Te][B(C₆ F₅ )₄ ] that was isolated and fully characterized. The electrochemical oxidation of the lighter homologs (2,6-Mes₂ C₆ H₃ )₂ E (E=S, Se) was irreversible and impaired by rapid decomposition.

Evaluation of Hydroxyl Radical Reactivity by Thioether Group Proximity in Model Peptide Backbone: Methionine versus S-Methyl-Cysteine

Hydroxyl radicals (HO^•) have long been regarded as a major source of cellular damage. The reaction of HO^• with methionine residues (Met) in peptides and proteins is a complex multistep process. Although the reaction mechanism has been intensively studied, some essential parts remain unsolved. In the present study we examined the reaction of HO^• generated by ionizing radiation in aqueous solutions under anoxic conditions with two compounds representing the simplest model peptide backbone CH₃C(O)NHCHXC(O)NHCH₃, where X = CH₂CH₂SCH₃ or CH₂SCH₃, i.e., the Met derivative in comparison with the cysteine-methylated derivative. We performed the identification and quantification of transient species by pulse radiolysis and final products by LC-MS and high-resolution MS/MS after γ-radiolysis. The results allowed us to draw for each compound a mechanistic scheme. The fate of the initial one-electron oxidation at the sulfur atom depends on its distance from the peptide backbone and involves transient species of five-membered and/or six-membered ring formations with different heteroatoms present in the backbone as well as quite different rates of deprotonation in forming α-(alkylthio)alkyl radicals.

Competition Between Cα -S and Cα -Cβ Bond Cleavage in β-Hydroxysulfoxides Cation Radicals Generated by Photoinduced Electron Transfer†

A kinetic and product study of the 3‐cyano‐N‐methyl‐quinolinium photoinduced monoelectronic oxidation of a series of β‐hydroxysulfoxides has been carried out to investigate the competition between C_α‐S and C_α‐C_β bond cleavage within the corresponding cation radicals. Laser flash photolysis experiments unequivocally established the formation of sulfoxide cation radicals showing their absorption band (λ_max ≈ 520 nm) and that of 3‐CN‐NMQ^• (λ_max ≈ 390 nm). Steady‐state photolysis experiments suggest that, in contrast to what previously observed for alkyl phenyl sulfoxide cation radicals that exclusively undergo C_α‐S bond cleavage, the presence of a β‐hydroxy group makes, in some cases, the C_α‐C_β scission competitive. The factors governing this competition seem to depend on the relative stability of the fragments formed from the two bond scissions. Substitution of the β‐OH group with ‐OMe did not dramatically change the reactivity pattern of the cation radicals thus suggesting that the observed favorable effect of the hydroxy group on the C_α‐C_β bond cleavage mainly resides on its capability to stabilize the carbocation formed upon this scission.

N-Terminal Decarboxylation as a Probe for Intramolecular Contact Formation in γ-Glu-(Pro)n-Met Peptides

The kinetics of intramolecular-contact formation between remote functional groups in peptides with restricted conformational flexibility were examined using designed peptides with variable-length proline bridges. As probes for this motion, free radicals were produced using the ^•OH-induced oxidation at the C-terminal methionine residue of γ-Glu-(Pro)_n-Met peptides (n = 0-3). The progress of the radicals' motion along the proline bridges was monitored as the radicals underwent reactions along the peptides' backbones. Of particular interest was the reaction between the sulfur atom located in the side chain of the oxidized Met residue and the unprotonated amino group of the glutamic acid moiety. Interactions between them were probed by the radiation-chemical yields (expressed as G values) of the formation of C-centered, α-aminoalkyl radicals (αN) on the Glu residue. These radicals were monitored directly or via their reaction with p-nitroacetophenone (PNAP) to generate the optically detected PNAP^•- radical anions. The yields of these αN radicals were found to be linearly dependent on the number of Pro residues. A constant decrease by 0.09 μM J^-1 per spacing Pro residue of the radiation-chemical yields of G(αN) was observed. Previous reports support the conclusion that the αN radicals in these cases would have to result from (S∴N)⁺-bonded cyclic radical cations that arose as a result from direct contact between the ends of the peptides. Furthermore, by analogy with the rate constants for the formation of intramolecularly (S∴S)⁺-bonded radical cations in Met-(Pro)_n-Met peptides ( J. Phys. Chem. B 2016, 120, 9732), the rate constants for the formation of intramolecularly (S∴N)⁺-bonded radical cations are activated to the same extent for all of the γ-Glu-(Pro)_n-Met peptides. Thus, the continuous decrease of G(αN) with the number of Pro residues (from 0 to 3) suggests that the formation of a contact between the S-atom in the C-terminal Met residue and the N-atom of a deprotonated N-terminal amino group of Glu is controlled in peptides with 0 to 3 Pro residues by the relative diffusion of the S^•+ and unoxidized N-atom. The overall rate constants of cyclization to form the (S∴N)-bonded radical cations were estimated to be 3.8 × 10⁶, 1.8 × 10⁶, and 8.1 × 10⁵ s^-1 for peptides with n = 0, 1, and 2 Pro residues, respectively. If activation is the same for all of the peptides, then these rate constants are a direct indication for the end-to-end dynamics along the chain.

Tuning Structures and Properties for Developing Novel Chemical Tools toward Distinct Pathogenic Elements in Alzheimer's Disease

Multiple pathogenic factors [e.g., amyloid-β (Aβ), metal ions, metal-bound Aβ (metal-Aβ), reactive oxygen species (ROS)] are found in the brain of patients with Alzheimer's disease (AD). In order to elucidate the roles of pathological elements in AD, chemical tools able to regulate their activities would be valuable. Due to the complicated link among multiple pathological factors, however, it has been challenging to invent such chemical tools. Herein, we report novel small molecules as chemical tools toward modulation of single or multiple target(s), designed via a rational structure-property-directed strategy. The chemical properties (e.g., oxidation potentials) of our molecules and their coverage of reactivities toward the pathological targets were successfully differentiated through a minor structural variation [i.e., replacement of one nitrogen (N) or sulfur (S) donor atom in the framework]. Among our compounds (1-3), 1 with the lowest oxidation potential is able to noticeably modify the aggregation of both metal-free Aβ and metal-Aβ, as well as scavenge free radicals. Compound 2 with the moderate oxidation potential significantly alters the aggregation of Cu(II)-Aβ₄₂. The hardly oxidizable compound, 3, relative to 1 and 2, indicates no noticeable interactions with all pathogenic factors, including metal-free Aβ, metal-Aβ, and free radicals. Overall, our studies demonstrate that the design of small molecules as chemical tools able to control distinct pathological components could be achieved via fine-tuning of structures and properties.

Electron Transfer Mechanism in the Oxidation of Aryl 1-Methyl-1-phenylethyl Sulfides Promoted by Nonheme Iron(IV)-Oxo Complexes: The Rate of the Oxygen Rebound Process

The oxidation of aryl 1-methyl-1-phenylethyl sulfides promoted by the nonheme iron(IV)-oxo complexes [(N4Py)Fe^IV═O]²⁺ and [(Bn-TPEN)Fe^IV═O]²⁺ occurs by an electron transfer-oxygen rebound (ET-OT) mechanism leading to aryl 1-methyl-1-phenylethyl sulfoxides accompanied by products derived from C_α-S fragmentation of sulfide radical cations (2-phenyl-2-propanol and diaryl disulfides). For the first time, the rate constants for the oxygen rebound process (k_OT), which are in the range of <0.8 × 10⁴ to 3.5 × 10⁴ s^-1, were determined from the fragmentation rate constants of the radical cations (k_f) and the S oxidation/fragmentation product ratios.

Fate of thianthrene in biological systems

1. Thianthrene is a sulfur-containing tricyclic molecule distributed widely within the macrostructure of hydrocarbon fossil fuels. Identified nearly 150 years ago, its chemistry has been widely explored leading to insights into reaction mechanisms and radical ion formation. 2. It has been claimed to have therapeutic application in the treatment of dermal infections and to interfere with enzyme and nucleic acid function, but appears to have little toxicity. 3. Following its oral administration to the rat, the majority remained within the gastrointestinal tract. After three days, about 88% was detected in the combined excreta with the remainder still within the animal. It is readily taken up into fish from the surrounding aqueous environment and has been placed within the "bioaccumulative category" to be regarded with concern. 4. Mammalian metabolism appeared to be restricted to ring carbon oxidation and subsequent glucuronic acid conjugation. Small amounts of sulfoxide and disulfoxide were also formed. No ring degradation was evident. Microorganisms similarly undertook aromatic ring hydroxylation but were able also to rupture the ring system by attacking the carbon-sulfur linkages and thereby degrading the molecule.

Oxidation of Aryl Diphenylmethyl Sulfides Promoted by a Nonheme Iron(IV)-Oxo Complex: Evidence for an Electron Transfer-Oxygen Transfer Mechanism

The oxidation of a series of aryl diphenylmethyl sulfides (4-X-C6H4SCH(C6H5)2, where X = OCH3 (1), X = CH3 (2), X = H (3), and X = CF3 (4)) promoted by the nonheme iron(IV)-oxo complex [(N4Py)Fe(IV)═O](2+) occurs by an electron transfer-oxygen transfer (ET-OT) mechanism as supported by the observation of products (diphenylmethanol, benzophenone, and diaryl disulfides) deriving from α-C-S and α-C-H fragmentation of radical cations 1(+•)-4(+•), formed besides the S-oxidation products (aryl diphenylmethyl sulfoxides). The fragmentation/S-oxidation product ratios regularly increase through a decrease in the electron-donating power of the aryl substituents, that is, by increasing the fragmentation rate constants of the radical cations as indicated by a laser flash photolysis (LFP) study of the photochemical oxidation of 1-4 carried out in the presence of N-methoxyphenanthridinium hexafluorophosphate (MeOP(+)PF6(-)).

Odd-electron-bonded sulfur radical cations: X-ray structural evidence of a sulfur-sulfur three-electron σ-bond

The one-electron oxidations of 1,8-chalcogen naphthalenes Nap(SPh)2 (1) and Nap(SPh)(SePh) (2) lead to the formation of persistent radical cations 1(•+) and 2(•+) in solution. EPR spectra, UV-vis absorptions, and DFT calculations show a three-electron σ-bond in both cations. The former cation remains stable in the solid state, while the latter dimerizes upon crystallization and returns to being radical cations upon dissolution. This work provides conclusive structural evidence of a sulfur-sulfur three-electron σ-bond (in 1(•+)) and a rare example of a persistent heteroatomic three-electron σ-bond (in 2(•+)).

Heterogeneous photocatalytic reactions of sulfur aromatic compounds

Sulfur aromatic compounds, such as mono-, di-, tri-, and tetraalkyl-substituted thiophene, benzothiophenes, dibenzothiophenes, are the molecular components of many fossils (petroleum, oil shale, tar sands, bitumen). Structural units of natural, cross-linked heteroaromatic polymers present in brown coals, turf, and soil are similar to those of sulfur aromatic compounds. Many sulfur aromatic compounds are found in the streams of petroleum refining and upgrading (naphthas, gas oils) and in the consumer products (gasoline, diesel, jet fuels, heating fuels). Besides fossils, the structural fragments of sulfur aromatic compounds are present in molecules of certain organic semiconductors, pesticides, small molecule drugs, and in certain biomolecules present in human body (pheomelanin pigments). Photocatalysis is the frontier area of physical chemistry that studies chemical reactions initiated by absorption of photons by photocatalysts, that is, upon electronic rather than thermal activation, under "green" ambient conditions. This review provides systematization and critical review of the fundamental chemical and physicochemical information on heterogeneous photocatalysis of sulfur aromatic compounds accumulated in the last 20-30 years. Specifically, the following topics are covered: physicochemical properties of sulfur aromatic compounds, major classes of heterogeneous photocatalysts, mechanisms and reactive intermediates of photocatalytic reactions of sulfur aromatic compounds, and the selectivity of these reactions. Quantum chemical calculations of properties and structures of sulfur aromatic compounds, their reactive intermediates, and the structure of adsorption complexes formed on the surface of the photocatalysts are also discussed.

Neighboring amide participation in thioether oxidation: relevance to biological oxidation

To investigate neighboring amide participation in thioether oxidation, which may be relevant to brain oxidative stress accompanying beta-amyloid peptide aggregation, conformationally constrained methylthionorbornyl derivatives with amido moieties were synthesized and characterized, including an X-ray crystallographic study of one of them. Electrochemical oxidation of these compounds, studied by cyclic voltammetry, revealed that their oxidation peak potentials were less positive for those compounds in which neighboring group participation was geometrically possible. Pulse radiolysis studies provided evidence for bond formation between the amide moiety and sulfur on one-electron oxidation in cases where the moieties are juxtaposed. Furthermore, molecular constraints in spiro analogues revealed that S-O bonds are formed on one-electron oxidation. DFT calculations suggest that isomeric sigma*(SO) radicals are formed in these systems.

Photosensitized oxidation of sulfides: discriminating between the singlet-oxygen mechanism and electron transfer involving superoxide anion or molecular oxygen

The oxidation of diethyl and diphenyl sulfide photosensitized by dicyanoanthracene (DCA), N-methylquinolinium tetrafluoroborate (NMQ(+)), and triphenylpyrylium tetrafluoroborate (TPP(+)) has been explored by steady-state and laser flash photolysis studies in acetonitrile, methanol, and 1,2-dichloroethane. In the Et(2)S/DCA system sulfide-enhanced intersystem crossing leads to generation of (1)O(2), which eventually gives the sulfoxide via a persulfoxide; this mechanism plays no role with Ph(2)S, though enhanced formation of (3)DCA has been demonstrated. In all other cases an electron-transfer (ET) mechanism is involved. Electron-transfer sulfoxidation occurs with efficiency essentially independent of the sulfide structure, is subject to quenching by benzoquinone, and does not lead to Ph(2)SO cooxidation. Formation of the radical cations R(2)S(*+) has been assessed by flash photolysis (medium-dependent yield, dichloroethane>>CH(3)CN>CH(3)OH) and confirmed by quenching with 1,4-dimethoxybenzene. Electron-transfer oxidations occur both when the superoxide anion is generated by the reduced sensitizer (DCA(*-), NMQ(*)) and when this is not the case (TPP(*)). Although it is possible that different mechanisms operate with different ET sensitizers, a plausible unitary mechanism can be proposed. This considers that reaction between R(2)S(*+) and O(2)(*-) mainly involves back electron transfer, whereas sulfoxidation results primarily from the reaction of the sulfide radical cation with molecular oxygen. Calculations indeed show that the initially formed fleeting complex RS(2)(+)...O-O(*) adds to a sulfide molecule and gives strongly stabilized R(2)S-O(*)-(+)O-SR(2) via an accessible transition state. This intermediate gives the sulfoxide, probably via a radical cation chain path. This mechanism explains the larger scope of ET sulfoxidation with respect to the singlet-oxygen process.

Rates of C−S Bond Cleavage in tert-Alkyl Phenyl Sulfide Radical Cations

[reaction: see text] Radical cations of tert-alkyl phenyl sulfides 1-4 have been generated photochemically in MeCN in the presence of the N-methoxyphenanthridinium cation (MeOP(+)), and the rates of C-S bond cleavage have been determined by laser flash photolysis.

Effect of pH on One-Electron Oxidation Chemistry of Organoselenium Compounds in Aqueous Solutions

Pulse radiolysis coupled with absorption detection has been employed to study one-electron oxidation of selenomethionine (SeM), selenocystine (SeCys), methyl selenocysteine (MeSeCys), and selenourea (SeU) in aqueous solutions. Hydroxyl radicals (*OH) in the pH range from 1 to 7 and specific one-electron oxidants Cl2*- (pH 1) and Br2*- (pH 7) have been used to carry out the oxidation reactions. The bimolecular rate constants for these reactions were reported to be in the range of 2 x 10(9) to 10 x 10(9) M(-1) s(-1). Reactions of oxidizing radicals with all these compounds produced selenium-centered radical cations. The structure and stability of the radical cation were found to depend mainly on the substituent and pH. SeM, at pH 7, produced a monomer radical cation (lambdamax approximately 380 nm), while at pH 1, a dimer radical cation was formed by the interaction between oxidized and parent SeM (lambdamax approximately 480 nm). Similarly, SeCys, at pH 7, on one-electron oxidation, produced a monomer radical cation (lambdamax approximately 460 nm), while at pH 1, the reaction produced a transient species with (lambdamax approximately 560 nm), which is also a monomer radical cation. MeSeCys on one-electron oxidation in the pH range from 1 to 7 produced monomer radical cations (lambdamax approximately 350 nm), while at pH < 0, the reaction produced dimer radical cations (lambdamax approximately 460 nm). SeU at all the pH ranges produced dimer radical cations (lambdamax approximately 410 nm). The association constants of the dimer radical cations of SeM, MeSeCys, and SeU were determined by following absorption changes at lambdamax as a function of concentration. From these studies it is concluded that formation of monomer and dimer radical cations mainly depends on the substitution, pH, and the heteroatoms like N and O. The availability of a lone pair on an N or O atom at the beta or gamma position results in monomer radical cations having intramolecular stabilization. When such a lone pair is not available, the monomer radical cation is converted into a dimer radical cation which acquires intermolecular stabilization by the other selenium atom. The pH dependency confirms the role of protonation on stabilization. The oxidation chemistry of these selenium compounds is compared with that of their sulfur analogues.

Sulfur Radical Cations. Kinetic and Product Study of the Photoinduced Fragmentation Reactions of (Phenylsulfanylalkyl)trimethylsilanes and Phenylsulfanylacetic Acid Radical Cations

Laser and steady-state photolysis, sensitized by NMQ+, of PhSCH(R)X 1-4 (R = H, Ph; X =SiMe3, CO2H) was carried out in CH3CN. The formation of 1+*-4+* was clearly shown. All radical cations undergo a fast first-order fragmentation reaction involving C-Si bond cleavage with 1+* and 2+* and C-C bond cleavage with 3+* and 4+*. The desilylation reaction of 1+* and 2+* was nucleophilically assisted, and the decarboxylation rates of 3+* and 4+* increased in the presence of H2O. A deuterium kinetic isotope effect of 2.0 was observed when H2O was replaced by D2O. Pyridines too were found to accelerate the decarboxylation rate of 3+* and 4+*. The rate increase, however, was not a linear function of the base concentration, but a plateau was reached. A fast and reversible formation of a H-bonded complex between the radical cation and the base is suggested, which undergoes C-C bond cleavage. It is probable that the H-bond complex undergoes first a rate determining proton-coupled electron transfer forming a carboxyl radical that then loses CO2. The steady-state photolysis study showed that PhSCH3 was the exclusive product formed from 1 and 3 whereas [PhS(Ph)CH-]2 was the only product with 3 and 4.

Conformationally Induced Electrostatic Stabilization of Persulfoxides: A New Suggestion for Inhibition of Physical Quenching of Singlet Oxygen by Remote Functional Groups

1,5-Dithiacyclooctane is shown to chemically react more efficiently and to remove singlet oxygen from solution more rapidly than either thiane or 1,4-dithiane. These unusual characteristics of the 1,5-dithiacyclooctane reaction were explored using ab initio quantum chemical methods. A large number of persulfoxides, thiadioxiranes, and hydroperoxy sulfonium ylides were located and their structures analyzed. The unusual efficiency of the reaction was attributed to a conformational change that electrostatically stabilized the persulfoxide and increased the potential energy barrier for physical quenching.

Persistent ? radical cations: self-association and its steric control in the condensed phase

Pi Radical cations, which are highly reactive in general, can be made persistently stable by appropriate structural modification with heteroatoms, pi-conjugated systems, and alkyl substituents. Many of these pi radical cations undergo self-association in the condensed phase. The steric control of such self-association of stabilized pi radical cations is the subject of the present article. Such an association can result in the formation of pi- and/or sigma-dimers. The pi-dimerization in particular is now considered as an important intermolecular interaction for model studies of a charge-transport phenomenon in positively doped conducting polymers. On the other hand, the intermolecular interactions can be suppressed when the pi-system is modified with sterically demanding structural units, for example, by annelation with bicycloalkene frameworks. This structural modification not only brings about unusual stabilization of the radical cations but provides valuable information on the electronic structure/properties of the positively charged pi-systems in a segregated state.