Recent photo- and radiation chemical studies of sulfur radical cations

Elucidating the N-N and C-N Bond-breaking Mechanism in the Photoinduced Formation of Nitrile Imine

In this study, we revealed the significance of chemical bonding for the photochemically induced mechanism of 2-phenyl tetrazole derivatives generating nitrile imines. The correlated electron localization function shows that the formation of imine nitrile involves two key bond events: (i) the heterolytic C-N breakage taking place in the T₁ state and (ii) the homolytic N-N rupture occurring in the T₂ excited state. In particular, a cation-radical specie results from the C-N cleavage, whereas the N-N rupture creates a biradical resonant form of imine nitrile. Additionally, we noticed that the substantial pair delocalization of the C-C-N bonded structure could play a significant role in the conversion of the biradical imine nitrile into both the propargylic and allenic forms via the T₁ →S₀ deactivation.

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.

On the origins of the mechanistic variants in the thermal reactions of Sx+ (x = 1-3) with benzene

The S-π interaction between sulfur atom(s) and aromatic ring prevails in chemical and biochemical processes. The thermal gas-phase reactions of the S_n⁺ (n = 1-3) ions with benzene have been explored by using Quadrupole-Ion Trap (Q-IT) mass spectrometry complemented by quantum chemical calculations. Charge transfer was found to be the only reaction channel for S₂⁺/C₆H₆, while both charge transfer and bond activation are available for the S⁺/C₆H₆ and S₃⁺/C₆H₆ couples. Upon interrogating the associated electronic origins, multiple factors were found to matter for these processes. In contrast to the σ-type two-center three-electron (2c-3e) S-π hemibond as reported previously, unusual S-π hemibonds were addressed for the S_n⁺/C₆H₆ couples, i.e. the 2c-3e π(S061Eπ) and the three-center three-electron (3c-3e) σ(S₂061Eπ) hemibonds. Such S-π interaction was found to be responsible for the charge transfer processes in S⁺/C₆H₆ and S₂⁺/C₆H₆, but uninvolved in any transformation for S₃⁺/C₆H₆.

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.

Gamma Radiation-Induced Oxidation, Doping, and Etching of Two-Dimensional MoS2 Crystals

Two-dimensional (2D) MoS2 is a promising material for future electronic and optoelectronic applications. 2D MoS2 devices have been shown to perform reliably under irradiation conditions relevant for a low Earth orbit. However, a systematic investigation of the stability of 2D MoS2 crystals under high-dose gamma irradiation is still missing. In this work, absorbed doses of up to 1000 kGy are administered to 2D MoS2. Radiation damage is monitored via optical microscopy and Raman, photoluminescence, and X-ray photoelectron spectroscopy techniques. After irradiation with 500 kGy dose, p-doping of the monolayer MoS2 is observed and attributed to the adsorption of O2 onto created vacancies. Extensive oxidation of the MoS2 crystal is attributed to reactions involving the products of adsorbate radiolysis. Edge-selective radiolytic etching of the uppermost layer in 2D MoS2 is attributed to the high reactivity of active edge sites. After irradiation with 1000 kGy, the monolayer MoS2 crystals appear to be completely etched. This holistic study reveals the previously unreported effects of high-dose gamma irradiation on the physical and chemical properties of 2D MoS2. Consequently, it demonstrates that radiation shielding, adsorbate concentrations, and required device lifetimes must be carefully considered, if devices incorporating 2D MoS2 are intended for use in high-dose radiation environments.

Direct structural and mechanistic insights into fast bimolecular chemical reactions in solution through a coupled XAS/UV–Vis multivariate statistical analysis

A combined multivariate and theoretical analysis of coupled XAS/UV–Vis data was proven to be an innovative method to obtain direct structural and mechanistic evidence for bimolecular reactions in solution involving organic substrates.

Light-triggered dual-modality drug release of self-assembled prodrug-nanoparticles for synergistic photodynamic and hypoxia-activated therapy

Photodynamic therapy (PDT) leads to tumor hypoxia which could be utilized for the activation of hypoxia-activated prodrugs (HAPs). However, conventional photosensitizer-loaded nanoformulations suffer from an aggregation-caused quenching (ACQ) effect, which limits the efficiency of PDT and synergistic therapy. Herein, prodrug-nanoparticles (NPs) are prepared by the self-assembly of heterodimeric prodrugs composed of pyropheophorbide a (PPa), hypoxia-activated prodrug PR104A, and a thioether or thioketal linkage. In addition, a novel dual-modality drug release pattern is proposed on the basis of the structural states of prodrug-NPs. Under light irradiation, PR104A is released via photoinduced electron transfer (PET) due to the aggregation state of prodrugs. With the disassembly of prodrug-NPs, the ACQ effect is relieved, and PPa produces singlet oxygen which further promotes the reactive oxygen species (ROS)-sensitive release of PR104A. Such prodrug-NPs turn the disadvantage of the ACQ effect to facilitate drug release, demonstrating high-efficiency synergy in combination with PDT and hypoxia-activated therapy.

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.

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(-)).

Structures of 4-substituted thioanisole radical cations studied by time-resolved resonance Raman spectroscopy during pulse radiolysis and theoretical calculations

The structures of 4-substituted thioanisole radical cations were studied by time-resolved resonance Raman spectroscopy during pulse radiolysis and DFT calculation, indicating importance of charge and spin distributions toward the dimerization.

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(•+)).