Radical Cation Research Articles

Multifaceted effects of ring fusion on the stability of charged dialkoxyarene redoxmers

Due to their almost unlimited scalability, redox flow batteries can make versatile and affordable energy storage systems. Redox active materials (redoxmers) in these batteries largely define their electrochemical performance, including the life span of the battery that depends on the stability of charged redoxmers. In this study, we examine the effects of expanding the π-system in the arene rings on the chemical stability of dialkoxyarene redoxmers that are used to store positive charge in RFBs. When 1,4-dimethoxybenzene is π-extended to 1,4-dimethoxynaphthalene, a lower redox potential, improved kinetic stability, and longer cycling life are observed. However, when an additional ring is fused to make 9,10-dimethoxyanthracene, the radical cation undergoes rapid O-dealkylation possibly due to increased steric strain that drives methoxy out of the arene plane thus breaking the π-conjugation with O 2p orbitals. On the other hand, the planar structure of 1,4-dimethoxynaphthalene may facilitate second-order reactions of radical cations leading to their neutralization in the bulk. Our study suggests that extending the π-system changes reactivity in multiple (sometimes, opposite) ways, so lowering the oxidation potential through π-conjugation to improve redoxmer stability should be pursued with caution.

Coherence mapping to identify the intermediates of multi-channel dissociative ionization

Identifying the short-lived intermediates and reaction mechanisms of multi-channel radical cation fragmentation processes remains a current and important challenge to understanding and predicting mass spectra. We find that coherent oscillations in the femtosecond time-dependent yields of several product ions following ultrafast strong-field ionization represent spectroscopic signatures that elucidate their mechanism of formation and identify the intermediate(s) they originate from. Experiments on endo-dicyclopentadiene show that vibrational frequencies from various intermediates are mapped onto their resulting products. Aided by ab initio methods, we identify the vibrational modes of both the cleaved and intact molecular ion intermediates. These results confirm stepwise and concerted fragmentation pathways of the dicyclopentadiene ion. This study highlights the power of tracking the femtosecond dynamics of all product ions simultaneously and sheds further light onto one of the fundamental reaction mechanisms in mass spectrometry, the retro-Diels Alder reaction.

Nanogold Foundry Involving High‐Shear‐Mediated Photocontact Electrification in Water

Controlling the size and morphology of gold nanoparticles occurs in the absence of added reducing agents or other excipients such as surfactants, on UV irradiation (λ 254 nm) of aqueous auric acid (H[AuCl4]) in a thin film of liquid in a vortex fluidic device (VFD) within a rapidly rotating tilted quartz tube. This involves contact electrification (CE), which occurs at the solid−liquid interface with the oxidation of water photoinduced, forming the hydroxyl radical, OH•. In air, the redox couple is reduction of 3O2 to the superoxide radical anion, O2−•, which then reduces Au3+ to elemental gold, as does other reactive oxygen species present, competing with CE reduction of Au3+. The resulting nanogold structures effectively mold the different high‐shear topological fluid flows in the VFD, being isolated as ultrathin 2D sheets, prisms, hierarchical structures comprising nanoparticles embedded within these sheets, and rosette and tubular structures, depending on the VFD processing parameters and the concentration of auric acid. Processing under a nitrogen atmosphere while UV irradiated affords mainly 2D gold through the above reduction of Au3+. The findings establish a paradigm for VFD processing in water involving photo‐CE, generating hydrogen peroxide and hydrogen gas with the surfaces of the gold nanoparticles pristine.

Heterogeneous-Structure-Based AuNBs@TiO2 Nano-Photosensitizers for Computed Tomography Imaging Guided NIR-II Photodynamic Therapy and Cancer Metastatic Prevention.

Photodynamic therapy (PDT) is a minimally invasive cancer treatment that, despite its significant attention, faces limitations in penetration depth, which restrict its effectiveness. Herein, it is found that gold nanobipyramid (AuNBs) coated with TiO2 can form a core-shell heterogeneous structure (AuNBs@TiO2) with strong absorption at second near infrared (NIR-II) region. A substantial quantity of reactive oxygen species (ROS), including singlet oxygen (1O2), superoxide anion radicals, and hydroxyl radicals, can be rapidly generated when subjecting the AuNBs@TiO2 aqueous suspension to 1064nm laser irradiation. The quantum yield for sensitization of 1O2 by AuNBs@TiO2 is 0.36 at 1064nm light excitation. In addition, the Au element as high-Z atoms in the nanosystem can improve the ability of computed tomographic (CT) imaging. As compared to commercial iohexol, the AuNBs@TiO2 nanoparticle exhibits significantly better CT imaging effect, which can be used to guide PDT. In addition, the nano-photosensitizer shows a remarkable therapeutic effect against established solid tumors and prevents tumor metastasis and potentiates immune checkpoint blockade therapy. More importantly, here the great potentials of AuNBs@TiO2 are highlighted as a theranostic platform for CT-guided cancer photodynamic immunotherapy.

Paper-in-Tip Bipolar Electrospray Mass Spectrometry for Real-Time Chemical Reaction Monitoring.

Capturing short-lived intermediates at the molecular level is key to understanding the mechanism and dynamics of chemical reactions. Here, we have developed a paper-in-tip bipolar electrolytic electrospray mass spectrometry platform, in which a piece of triangular conductive paper incorporated into a plastic pipette tip serves not only as an electrospray emitter but also as a bipolar electrode (BPE), thus triggering both electrospray and electrolysis simultaneously upon application of a high voltage. The bipolar electrolysis induces a pair of redox reactions on both sides of BPE, enabling both electro-oxidation and electro-reduction processes regardless of the positive or negative ion mode, thus facilitating access to complementary structural information for mechanism elucidation. Our method enables real-time monitoring of transient intermediates (such as N,N-dimethylaniline radical cation, dopamine o-quinone (DAQ) and sulfenic acid with half-lives ranging from microseconds to minutes) and transient processes (such as DAQ cyclization with a rate constant of 0.15 s-1). This platform also provides key insights into electrocatalytic reactions such as Fe (III)-catalyzed dopamine oxidation to quinone species at physiological pH for neuromelanin formation.

Paper‐in‐Tip Bipolar Electrospray Mass Spectrometry for Real‐Time Chemical Reaction Monitoring

Capturing short‐lived intermediates at the molecular level is key to understanding the mechanism and dynamics of chemical reactions. Here, we have developed a paper‐in‐tip bipolar electrolytic electrospray mass spectrometry platform, in which a piece of triangular conductive paper incorporated into a plastic pipette tip serves not only as an electrospray emitter but also as a bipolar electrode (BPE), thus triggering both electrospray and electrolysis simultaneously upon application of a high voltage. The bipolar electrolysis induces a pair of redox reactions on both sides of BPE, enabling both electro‐oxidation and electro‐reduction processes regardless of the positive or negative ion mode, thus facilitating access to complementary structural information for mechanism elucidation. Our method enables real‐time monitoring of transient intermediates (such as N,N‐dimethylaniline radical cation, dopamine o‐quinone (DAQ) and sulfenic acid with half‐lives ranging from microseconds to minutes) and transient processes (such as DAQ cyclization with a rate constant of 0.15 s−1). This platform also provides key insights into electrocatalytic reactions such as Fe (III)‐catalyzed dopamine oxidation to quinone species at physiological pH for neuromelanin formation.

Helically Chiral π-Expanded Azocines Through Regioselective Beckmann Rearrangement and Their Charged States.

We report a synthetic approach to π-expanded [6]helicenes incorporating tropone and azocine units in combination with a 5-membered ring, which exhibit intriguing structural, electronic, and chiroptical properties. The regioselective Beckmann rearrangement allows the isolation of helical scaffolds containing 8-membered lactam, azocine, and amine units. As shown by X-ray crystallographic analysis, the incorporation of tropone or azocine units leads to highly distorted [6]helicene moieties, with distinct packing motifs in the solid state. The compounds exhibit promising optoelectronic properties with considerable photoluminescence quantum yields and tunable emission wavelengths depending on the relative position of the nitrogen center within the polycyclic framework. Separation of the enantiomers by chiral high-performance liquid chromatography (HPLC) allowed characterization of their chiroptical properties by circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopy. The azocine compounds feature manifold redox chemistry, allowing for the characterization of the corresponding radical anions and cations as well as the dications and dianions, with near-infrared (NIR) absorption bands extending beyond 3000 nm. Detailed theoretical studies provided insights into the aromaticity evolution upon reduction and oxidation, suggesting that the steric strain prevents the azocine unit from undergoing aromatization, while the indene moiety dominates the observed redox chemistry.

Helically Chiral π‐Expanded Azocines Through Regioselective Beckmann Rearrangement and Their Charged States

We report a synthetic approach to p‐expanded [6]helicenes incorporating tropone and azocine units in combination with a 5‐membered ring, which exhibit intriguing structural, electronic, and chiroptical properties. The regioselective Beckmann rearrangement allows the isolation of helical scaffolds containing 8‐membered lactam, azocine, and amine units. As shown by X‐ray crystallographic analysis, the incorporation of tropone or azocine units leads to highly distorted [6]helicene moieties, with distinct packing motifs in the solid state. The compounds exhibit promising optoelectronic properties with considerable photoluminescence quantum yields and tunable emission wavelengths depending on the relative position of the nitrogen center within the polycyclic framework. Separation of the enantiomers by chiral high‐performance liquid chromatography (HPLC) allowed characterization of their chiroptical properties by circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopy. The azocine compounds feature manifold redox chemistry, allowing for the characterization of the corresponding radical anions and cations as well as the dications and dianions, with near‐infrared (NIR) absorption bands extending beyond 3000 nm. Detailed theoretical studies provided insights into the aromaticity evolution upon reduction and oxidation, suggesting that the steric strain prevents the azocine unit from undergoing aromatization, while the indene moiety dominates the observed redox chemistry.

Effects of radical scavengers for reactive oxygen species on vitamin K-induced phototoxicity under UVA irradiation

Vitamin K possesses efficacy as a topical dermatological agent. However, vitamin K is phototoxic and susceptible to photodegradation. Herein, we investigated the mechanisms underlying the phototoxicity of phylloquinone (PK, vitamin K1) and menaquinone-4 (MK-4, vitamin K2) under ultraviolet A (UVA) irradiation using various reactive oxygen species (ROS) scavengers. This resulted in the production of superoxide anion radicals via type I and singlet oxygen via type II photodynamic reactions, which were quenched by the ROS scavengers: superoxide dismutase and sodium azide (NaN3). In HaCaT cells, MK-4 and PK induced the production of intracellular ROS, particularly hydrogen peroxide, in response to UVA irradiation. Furthermore, the addition of catalase successfully decreased maximum ROS levels by approximately 30%. NaN3 and catalase decreased the maximum reduction in cell viability induced by UVA-irradiated PK and MK-4 in cell viability by approximately 2–7-fold. Additionally, ROS scavengers had no effect on the photodegradation of PK or MK-4 at 373 nm. Therefore, the phototoxicities of PK and MK-4 were attributed to the generation of singlet oxygen and hydrogen peroxide, underscoring the importance of photoshielding in circumventing phototoxicity.

Hydrogen and electricity cogeneration driven by the salinity gradient from actual brine and river water using reverse electrodialysis

Reverse electrodialysis (RED) is a promising method for harvesting the salinity gradient energy (SGE) between the brine and river water. Many investigations are carried out by using the artificially brine and fresh water, bringing difficulties in reflecting the practical RED performance under the complex influences of feed solutions. This work explores the performance of the RED system for hydrogen and electricity co-generation that is driven by salinity gradient between the actual concentrated brine and natural river water, and compared with the simulative brines and river waters under six testing schemes. The results show both the working current and solutions influence the system performances, including the output voltage, power density, hydrogen production, and energy conversion efficiency. Besides, the effects of salinity gradient of feed solutions are greater than that of the presence of trace multivalent ions and anionic radical in brine. The effective monovalent ion ionic strength is vital. The experimental maximum hydrogen production and power density are 82.12 mL·h−1 and 0.124 W·m−2 respectively at the current is 0.2 A, and the total power reached 0.32 W with the energy conversion efficiency of 25%. The research offers recommendations for harnessing SGE from actual brines and freshwater to produce hydrogen.

Multimodular Wide-Band Capturing Nanohybrids: Role of Carbon Nanotubes in Slowing Charge Recombination in Supramolecular C60-BisstyrylBODIPY-(Zinc Porphyrin)2 Donor-Acceptor Molecular Cleft.

The importance of diameter-sorted single-wall carbon nanotubes (SWCNTs) noncovalently bound to a donor-acceptor molecular cleft, 1, in prolonging the lifetime of charge-separated states is successfully demonstrated. For this, using a multistep synthetic procedure, a wide-band capturing, multimodular, C60-bisstyrylBODIPY-(zinc porphyrin)2, molecular cleft 1, was newly synthesized and shown to bind diameter-sorted SWCNTs. The molecular cleft and its supramolecular assemblies were characterized by a suite of physicochemical techniques. Free-energy calculations suggested that both the (6,5) and (7,6) SWCNTs bound to 1 act as hole acceptors during the photoinduced sequential electron transfer events. Consequently, selective excitation of 1 in 1:SWCNT hybrids revealed a two-step electron transfer, leading to the formation of charge-separated states. Due to the distant separation of the cation and anion radical species within the supramolecules, improved lifetimes of the charge-separated states could be achieved. The present supramolecular strategy of improving charge separation involving SWCNTs and donor-acceptor molecular clefts highlights the potential application of these hybrid materials for various light energy harvesting and optoelectronic applications.

Astro-electrochemistry of NH3 clusters and ice: e− trapping, stability, and electron transfer

ABSTRACT Quantifying electron trapping and transfer to small molecules is crucial for interfacial chemistry. In an astrochemical context, we study how NH3 clusters in both crystalline and amorphous forms can capture low-energy electrons to form ammoniated electrons. Electron affinities, vertical detachment energies, and vertical attachment energies were computed via ab initio static and dynamics simulations, (DFT, DLPNO-CCSD(T);AIMD), for (NH3)n clusters (n = 4, 5, 6, 8, 14, 23, and 38). Our results indicate that the clusters could trap and stabilize the unpaired electron which is always externally localized on the clusters. Interactions of the ammoniated electron clusters with astrochemically relevant molecules indicate that electron transfer to water and methanol are feasible, forming the radical anions (H2O)−· and (CH3OH)−·. The trapping of electrons by both crystalline and amorphous NH3 ices, and subsequent transfer to small molecules, highlights ‘astro-electrochemical’ reactions, and has implications for both astrochemistry as well as terrestrial cluster science.

Enhanced mitigation of membrane fouling and degradation of perfluorooctanoic acid in the treatment of natural surface water through the application of dielectric barrier discharge/sulfite

The present study employed a dielectric barrier discharge (DBD)/sulfite system to mitigate membrane fouling and enhance the degradation of perfluorooctanoic acid (PFOA) in surface water. Results showed that the membrane flux was remarkably enhanced by 93.6%, and the degradation of PFOA showed an impressive increase of 87.9%. The synergistic redox effect of free radicals, including hydroxyl radical (•OH), sulfate radical (SO4∙-), electron (e-/eaq-), su-peroxide anion radicals (∙O2-), and other radical, played a crucial role in mitigating membrane fouling, accounting for 36.26%, 30.10%, 18.69%, 6.75%, and 8.20%, respectively. The correlation analysis revealed that effective removal of UV254 and TOC emerged as the most crucial factors positively contributing to mitigating membrane fouling. Additionally, SO4∙-, •OH, and eaq- were identified as the primary active substances responsible for attacking PFOA, accounting for 41.30%, 25.20%, and 23.70%, respectively. The degradation pathway of PFOA was proposed through analysis using density functional theory and mass spectrometry, reducing the biotoxicity of its degradation products. This study presents a suitable strategy for simultaneous mitigation of membrane fouling and enhancement of PFOA degradation, which holds great potential for advancing the field of water treatment towards practical applications.

Iodinated lutetium (III) phthalocyanine: Synthesis, photochemical and nonlinear optical properties

New octaiodo-substituted lutetium (III) phthalocyanine was prepared using a six-step synthetic procedure starting from commercially available o-xylene. The amide synthesis and dehydration step were optimized. The influence of the nature of peripheral halogen atoms on optical and photochemical properties was studied. For the complex under study, the possibility of participation in photoreactions of types I and II with the formation of two different forms of reactive oxygen species was investigated: superoxide anion radical and singlet oxygen respectively. Competition between two photoreactions was demonstrated. The Z-scan method revealed a pronounced nonlinear optical response of the iodinated complex, caused by the action of heavy atoms of peripheral iodine groups.

Unraveling the synergistic and coupling effects between catalyst, oxygen, and organics for highly efficient mineralization of refractorily aromatic rings

Self-producing of H2O2 and their activation into •OH over a pronged graphene-like carbon nitride (Fe@CN/WO3) for efficiently removing organics from water were investigated. Experimental and theoretical analyses revealed an S-scheme with an inner electric field as strong as 1.11 V in separating e-h+, and a strong synergy of photocatalysis and Fenton process. Fe@CN/WO3 exhibited 15 and 11 times the H2O2-activation and hydroxyl radical (•OH)-generation rates than those of pristine CN, respectively, and their performance in degrading organics is dynamically comparable to some heterogeneous Fenton. This work reveals a most reasonable process for decontaminating organics under solar empowerment and atmospheric oxygenation specified as: i) photoelectrons reduced O2 to superoxide anion radical (•O2−) and cycled ≡Fe3+/≡Fe2+; ii) •O2− deprotonated functional groups of TC-HCl, facilitating two-electron O2 reduction into H2O2; iii) ≡Fe2+ efficiently activated the produced H2O2 into •OH; iv) •OH compensated for the weakness of •O2− in disassembling refractorily aromatic rings, helping to achieve unparalleled efficiency in mineralizing organic contaminants. Particularly, success is unraveling the photo-self Fenton process between catalyst-oxygen-organics.

Photocatalytic properties of BaTiO3/Fe2O3 under visible and ultraviolet irradiation

Nanosized composite materials based on BaTiO3 with 0.5 % and 1 % Fe2O3 were prepared by the sol-gel method and subsequent heat treatment. This work showed that increasing the content of iron(III) oxide in a heterostructured material affects the bleaching of the dye rhodamine B upon irradiation with visible and ultraviolet light. The highest photocatalytic activity was observed on the solid surface of the samples in an air environment under both visible and UV irradiation. The BaTiO3/0.5 % Fe2O3 sample was found to be most effective in an air environment when irradiated with an artificial visible light source. The increase in efficiency could be due to the best separation of charge carriers at the interface between the barium titanate and iron(III) oxide structures and the highest concentration of the resulting radical anions. An increase in the mass fraction of Fe2O3 led to a decrease in the efficiency of the nanoscale composite photocatalyst, which was due to a greater recombination of charge carriers.

Mechanism of Anti-Trypanosoma cruzi Action of Gold(I) Compounds: A Theoretical and Experimental Approach

In the search for a more effective chemotherapy for the treatment of Chagas’ disease, caused by Trypanosoma cruzi parasite, the use of gold compounds may be a promising approach. In this work, four gold(I) compounds [AuCl(HL)], (HL = bioactive 5-nitrofuryl containing thiosemicarbazones) were studied. The compounds were theoretically characterized, showing identical chemical structures with the metal ion located in a linear coordination environment and the thiosemicarbazones acting as monodentate ligands. Cyclic voltammetry and Electron Spin Resonance (ESR) studies demonstrated that the complexes could generate the nitro anion radical (NO2−) by reduction of the nitro moiety. The compounds were evaluated in vitro on the trypomastigote form of T. cruzi and human cells of endothelial morphology. The gold compounds studied showed activity in the micromolar range against T. cruzi. The most active compounds (IC50 of around 10 μM) showed an enhancement of the antiparasitic activity compared with their respective bioactive ligands and moderate selectivity. To get insight into the anti-chagasic mechanism of action, the intracellular free radical production capacity of the gold compounds was assessed by ESR and fluorescence measurements. DMPO (5,5-dimethyl-1-pirroline-N-oxide) spin adducts related to the bioreduction of the complexes and redox cycling processes were characterized. The potential oxidative stress mechanism against T. cruzi was confirmed.

Treatment of emerging pyrrolizidine alkaloids in drinking water by UV/persulfate process: Kinetics, energy efficiency and degradation pathway

Pyrrolizidine alkaloids (PAs), recognized as a group of toxic secondary metabolites, have raised growing concerns in recent years due to their potential high toxicity. However, studies on the degradation of PAs in drinking water treatments have yet to be conducted. Herein, the treatment of Heliotrine (HEL) and N-oxide (HELNO), two dominant PAs detected in the treated drinking water, are analyzed for the first time. The UV/peroxydisulfate (UV/PDS) process was demonstrated to be efficient in the degradation of PAs, achieving almost complete removal of HEL and HELNO in 30 min. SO4•− plays a dominant role in the system, with second-order rate constants of 2.3 × 109 M−1 s−1 and 1.0 × 109 M−1 s−1 for HEL and HELNO, respectively. Based on the steady-state kinetic models, the effect of influence factors, including PDS dosage, initial pollutant concentration, pH, dissolved organic matter (DOM), and chloride (Cl−), were evaluated. Acidic conditions were proven to be favorable for PA degradations, primarily contributed to the minimized quenching effect of OH− and phosphate on SO4•−. The inhibitory effect of DOM on radical quenching significantly overweighted the promotion effect of the generated excited triplet DOM (3DOM*), resulting in a diminished degradation efficiency. The pronounced inhibition of Cl− could be attributed to the conversion of SO4•− to OH•, involving the formation of reactive chlorine species. The UV/PDS treatment was suggested economical in the removal of PAs as a post-filtration and before-chlorination treatment through electrical energy per order (EE/O) calculation. Finally, reasonable degradation pathways of HEL and HELNO reacted with SO4•− were proposed, involving the initial one-electron oxidation of HEL/HELNO by SO4•− radical, and further reactions (e.g., hydroxylation, pyrrolidine ring-cleavage and self-coupling) of the formed N-centered cation radicals.

Positive cooperativity in binding: Spectroscopic characterization and dissociation energy of the anisole…(CH4)2 complex

We have previously reported on the spectroscopy and binding energy of the anisole…methane complex, which exhibits a dual mode of binding that involves both CH/O and CH/π interactions. In this work, we seek to examine cooperativity in binding through a study of the isolated anisole-(methane)2 complex, using a combination of experiments that include mass-selected two-color resonant two-photon ionization spectroscopy (2CR2PI), two-color appearance potential (2CAP) measurements, and velocity mapped ion imaging (VMI) augmented with a complementary theoretical characterization. Using 2CAP and VMI, we derive the dissociation energies of the complex in ground (S0), excited (S1), and cation radical (D0) states. The experimental values from the two methods are in excellent agreement and are compared with selected theoretical values calculated using DFT and ab initio methods. The data show that the dissociation energy increases by some 10 % for the second methane relative to the first, with this trend being consistent across all three electronic states, indicating a cooperative binding effect where the initial solvation turns on binding of a second methane onto the opposite face. This lies in contrast to the aniline-(methane)2 complex, where recent studies have shown a negativity cooperativity, and these trends are examined.

Progress of photocatalytic oxidation-adsorption synergistic removal of organic arsenic in water.

Photocatalytic oxidation-adsorption synergistic treatment of organic arsenic pollutants is a promising wastewater treatment technology, which not only degrades organic arsenic pollutants by photocatalytic degradation but also removes the generated inorganic arsenic by adsorption. This paper compares the results of photocatalytic oxidation-adsorption co-treatment of organic arsenic pollutants such as monomethylarsonic acid, dimethylarsinic acid, phenylarsonic acid, p-arsanilic acid, and 3-nitro-4-hydroxyphenylarsonic acid on titanium dioxide, goethite, zinc oxide, and copper oxide. It examines the influence of the morphology of organic arsenic molecules, pH, coexisting ions, and the role of natural organic matter. The photocatalytic oxidation-adsorption co-treatment mechanism is investigated, comparing the hydroxyl radical oxidation mechanism, the hydroxyl radical and superoxide anion radical cooxidation mechanism, and the hydroxyl radical and hole cooxidation mechanism. Finally, the future prospects of metal oxide photocatalytic materials and the development of robust and efficient technologies for removing organic arsenic are envisioned.