Radical Cation Research Articles

Biaryl Anion Radical Formation by Potassium Metal Reduction of Aryl Isocyanates and Triaryl Isocyanurates.

The potassium metal reduction of aryl isocyanates (aryl = phenyl, p-tolyl, 3,5-dimethylphenyl, 4-biphenylyl, and 1-naphthyl) in THF with 18-crown-6 or in HMPA results in the formation of the corresponding triaryl isocyanurate anion radicals. Continued exposure to potassium results in loss of the isocyanurate anion radical and the eventual formation of the respective biaryl anion radical. The 1,1'-binaphthyl anion radical is found to undergo a cyclodehydrogenation reaction, which leads to formation of the perylene anion radical. When authentic triaryl isocyanurates are reduced with metal, the heterocyclic ring undergoes fragmentation with elimination of carbon monoxide to produce a triarylbiuret dianion. This ring opening reaction is initiated by the two-electron reduction of the neutral isocyanurate species. The biaryl anion radical is formed when the biuret dianion is reduced further with metal. A possible mechanism for biaryl formation involves a heterolytic cleavage of an aryl C-N bond and release of an aryl radical once the triarylbiuret dianion is further reduced. A subsequent intermolecular reaction between two aryl radicals forms the corresponding biaryl, which can then be reduced to the anion radical. Notably, when a mixture of two different triaryl isocyanurate compounds is reduced in solution, the corresponding mixed biaryl anion radical is generated.

Perylene diimide-hydroxyphenyl benzothiazole-based new class of radical anions/dianions: biochemical assay for glucose detection.

We report the design, synthesis and characterization of a perylene diimide-hydroxyphenyl benzothiazole (BT-PDI) dyad as a new class for the formation of radical anion (BT-PDI˙-) and dianion (BT-PDI2-) in aqueous medium using H2S. We demonstrate the applications of BT-PDI˙- for (i) the detection of H2O2; (ii) the detection of glucose in blood serum using a biochemical assay and (iii) the reduction of Ag+ to Ag0.

Vibronic coupling in the energetically six lowest electronic states of oxirane radical cation.

Multi-dimensional quantum mechanical simulations are carried out to understand the multi-state and multi-mode vibronic interactions in the first six low-lying viz., X̃2B1, Ã2A1, B̃2B2, C̃2A2, D̃2A1, and Ẽ2B1 electronic states of c-C2H4O·+. Vibronic coupling theory is applied to study interactions among electronic states using symmetry selection rules. A model 6 × 6 diabatic electronic Hamiltonian is constructed. The parameters of the diabatic Hamiltonian are estimated by performing extensive abinitio electronic structure calculations, using the EOM-IP-CCSD method. The nuclear dynamics calculations are performed with both time-independent and time-dependent quantum mechanical methods. The calculated vibronic structures of six electronic states are found to be in excellent agreement with the available experimental findings. Progressions found in the theoretical spectrum are assigned in terms of vibrational modes. It is found that extremely strong vibronic interactions among the X̃2B1-Ã2A1, B̃2B2-C̃2A2, and D̃2A1-Ẽ2B1 electronic states results into highly overlapping vibronic bands due to multiple multi-state conical intersections. The impact of associated nonadiabatic effects on the vibronic structure and dynamics of the mentioned electronic states is examined at length. Interesting comparison is made with the results obtained for the isomeric acetaldehyde radical cation.

Unimolecular isomerizations of C6H6•+ radical cations: a computational study.

Eighteen concerted isomerization reactions of various C6H6•+ radical cation (RC) species are studied and found to proceed via well-defined transition states, whose relative positions along the reaction pathway generally agree with Hammond's postulate. From the barrier heights, the rate coefficients of these reactions are estimated by using transition state theory, and the activation energies are computed. Through combination among themselves, these 18 isomerizations yielded 15 multi-step conversion routes of various C6H6•+ species to the lowest energy benzene radical cation isomer 1, which routes are compared. Use is made of DFT with the B3LYP and M06-2X functionals, along with the CBS-QB3 approach to arrive at better energies. From the barrier heights for each of the concerted reactions, canonical transition state theory was applied to evaluate rate coefficients k over the temperature range 200-500K. The Arrhenius activation energies were computed using the plot of ln k vs. 1/T.

Switchable protection and exposure of a sensitive squaraine dye within a redox active rotaxane

In nature, molecular environments in proteins can sterically protect and stabilize reactive species such as organic radicals through non-covalent interactions. Here, we report a near-infrared fluorescent rotaxane in which the stabilization of a chemically labile squaraine fluorophore by the coordination of a tetralactam macrocycle can be controlled chemically and electrochemically. The rotaxane can be switched between two co-conformations in which the wheel either stabilizes or exposes the fluorophore. Coordination by the wheel affects the squaraine’s stability across four redox states and renders the radical anion significantly more stable—by a factor of 6.7—than without protection by a mechanically bonded wheel. Furthermore, the fluorescence properties can be tuned by the redox reactions in a stepwise manner. Mechanically interlocked molecules provide an excellent scaffold to stabilize and selectively expose reactive species in a co-conformational switching process controlled by external stimuli.

Photoinduced Radical Approach for Desulfurative Alkylation of Cysteine Derivatives to Make Unnatural Amino Acids.

Unnatural amino acids (UAAs) are highly valuable molecules in organic synthesis, pharmaceutical sciences, and material science. Herein, we present a photocatalytic radical approach for desulfurative alkylation of cysteine derivatives with arenethiol as the hydrogen atom transfer catalyst for making UAAs and peptides. The formate salt, acting as the hydrogen atom donor, in situ generates the highly reductive CO2 radical anion species, which is the key to unlocking the C-S bond cleavage process with a simple benzoyl protecting group. No photocatalyst is required for the radical initiation and propagation, which makes such a visible-light-induced process mild, efficient, and sustainable.

Boron-bridged bis(tetrathiafulvalene) zwitterionic neutral radical conductors: substituent effects on intra- and intermolecular electronic interactions and physical properties

Abstract We have recently proposed a new molecular design for neutral radical molecular conductors, based on the chemical composition of D2X-type charge-transfer salts (D = π-electron donor molecule, X = monovalent anion). Namely, we connected two tetrathiafulvalene (TTF)-based π-electron skeletons having a charge of +0.5 via a boron anion B–, to obtain a purely organic zwitterionic neutral radical {[(PDT-TTF-Cat)2]+B–}• that shows unique electronic states and properties in the crystal due to the intra- and intermolecular interactions of the +0.5-charged π-skeletons. In order to further explore the structural and electronic features of this kind of zwitterionic neutral radical conductor, in this study we have examined chemical modifications to {[(PDT-TTF-Cat)2]+B–}•. As a result, an analogue molecule {[(BMT-TTF-Cat)2]+B–}•, in which propylenedithio (PDT) groups on the TTF terminals in {[(PDT-TTF-Cat)2]+B–}• are replaced by bis(methylthio) (BMT) ones, was successfully synthesized as air-stable single crystals. Interestingly, this chemical modification causes intramolecular charge disproportionation between the two TTF-based π-skeletons (i.e., the monovalent cation radical TTF+• and the neutral TTF0) coupled to the modulation of the intermolecular distances and interactions between these π-skeletons, leading to electrical and magnetic properties significantly different from those of the crystal of {[(PDT-TTF-Cat)2]+B–}•.

Self-Illuminating Copper-Luminol Coordination Polymers for Bioluminescence Imaging of Oxidative Damage.

Timely detection of reactive oxygen species (ROS) accumulated during inflammation is essential for an early disease diagnosis. Compared to fluorescence probes with limited sensitivity and accuracy, chemiluminescence (CL) imaging offers the potential for highly sensitive molecular visualization of ROS by minimizing background interferences. However, the development of bright and easily manufacturable CL probes for ROS imaging remains challenging. In this study, a novel chemiluminescent nanoprobe named Cu-Lum@NPs for ROS imaging in inflammation was synthesized by using a one-step solvothermal method. The Cu-Lum@NPs, which are composed of coordination polymers containing copper ions and luminol (Lum), demonstrate intrinsic peroxidase-like activity that relies on Cu(I) as the catalytic active center to initiate the Fenton reaction. This catalytic process facilitates the decomposition of hydrogen peroxide (H2O2) into hydroxyl radicals (•OH) and superoxide anion radicals (O2•-), leading to the oxidation of Lum and inducing strong luminescence. Cu-Lum@NPs, displaying nanozyme characteristics, were observed to accelerate and enhance the ROS-responsive luminescence (10-1600-fold in solution and over 100-fold in neutrophils) and notably extend persistent luminescence. The Cu-Lum@NPs allowed for CL imaging of endogenous ROS in living cells and animals with an outstanding signal-to-noise ratio exceeding 96 and facilitated oxidative damage luminescence imaging for tissue-specific detection. The study presents Cu-Lum@NPs, a highly sensitive and easily manufacturable chemiluminescent nanoprobe for ROS imaging both in vitro and in vivo, exhibiting enhanced luminescence and prolonged persistence for ROS-related disease detection.

Does the Mesolytic C−C Bond Cleavage Occur in the Radical Anions of Diarylethanes?

AbstractWe have been studying mesolytic cleavage of X−Y σ‐bond (X−Y=CH2−O, CH2−S, S−S and Se−Se) tethering aryl (Ar) moieties such as phenyl, biphenyl, α‐ and β‐naphthyl groups. In the present study, possibility of mesolytic cleavage of carbon‐carbon bonds in radical anions of diarylethanes (Ar1CCAr2) was investigated by pulsed electron radiolysis in N,N‐dimethylformamide (DMF) and 2‐methyltetrahydrofuran (MTHF). No mesolytic C−C bond cleavage in the radical anions occurred in both solvents whereas in MTHF, formation of H‐adducts of the diarylethanes was seen in the transient absorption spectral changes. Based on the DFT calculations for the bond dissociation energy of the C−C bond and the singly occupied molecular orbital distributions on Ar1CCAr2 radical anions, we rationalize the absence of the mesolysis in comparison with experimental facts previously reported for Ar1X‐YAr2 radical anions having X−Y σ‐bond.

Characterization of Glycylglycyltyrosine Radical Cations: Insights into β-Radical Formation and N–Cα Peptide Bond Dissociation at the Tyrosine Residue

We investigated the dissociation and characterization of radical cations of glycylglycyltyrosine [GGY]•+, focusing on β-radical-induced N–Cα peptide bond cleavage reactions at the tyrosyl residue. By combining density functional theory (DFT) calculations with experimental studies utilizing low-energy collision-induced dissociation (CID) mass spectrometry and deuterium labeling on the two β-hydrogen atoms of the tyrosyl residue in [GGY]•+, we elucidated the intricacies of the β-radical structure and its origin. Unlike tryptophan-containing [GGW]•+, which forms canonical π-radical precursors, infrared multiphoton dissociation (IRMPD) spectroscopy results reveal that the β-radical [GGYβ•]+ isomerizes from the phenoxy-radical [GGYo•]+, with the radical localized on the β-carbon of the tyrosyl residue and the phenolic oxygen atom, respectively. The isomerization barriers from [GGYo•]+ to [GGYβ•]+ are < 109 kJ mol-1.

Photoredox-catalyzed intramolecular nucleophilic amidation of alkenes with β-lactams.

The direct nucleophilic addition of amides to unfunctionalized alkenes via photoredox catalysis represents a facile approach towards functionalized alkylamides. Unfortunately, the scarce nucleophilicity of amides and competitive side reactions limit the utility of this approach. Herein, we report an intramolecular photoredox cyclization of alkenes with β-lactams in the presence of an acridinium photocatalyst. The approach uses an intramolecular nucleophilic addition of the β-lactam nitrogen atom to the radical cation photogenerated in the linked alkene moiety, followed by hydrogen transfer from the hydrogen atom transfer (HAT) catalyst. This process was used to successfully prepare 2-alkylated clavam derivatives.

Porous Bimetallic Ti-MOFs for Photocatalytic Oxidation of Amines in Air.

A family of microporous titanium-containing metal-organic frameworks (denoted as M2Ti-CPCDC, M = Mn, Co, Ni) has been synthesized by using a bimetallic [M2Ti(μ3-O)(COO)6] cluster and a tritopic carbazole-based organic ligand H3CPCDC. M2Ti-CPCDC are stable and display permanent porosity for N2 and CO2 uptake, ranking among the most porous titanium-based metal-organic frameworks. M2Ti-CPCDC crystals exhibit n-type semiconductor behavior. Further catalytic studies demonstrate that all M2Ti-CPCDC materials are applicable for triggering photo-oxidative reactions of amines in air. More specifically, amines with electron-donating groups afford the aldehydes as the main products, while amines bearing electron-withdrawing groups give rise to imines as the main product. Among them, Mn2Ti-CPCDC exhibit the best photocatalytic activity, with conversion of benzylamine up to 99% and selectivity of 99%. Mn2Ti-CPCDC could be recycled in at least three runs while retaining crystallinity and catalytic activity. The reaction mechanism indicates that photoinduced hole (h+), superoxide radical anion (O2·-), and singlet oxygen (1O2) are the main active species involved in the photo-oxidation process.

Fluorescence detection of superoxide anion with CsPbBr3 perovskite quantum dots in aqueous media

Among reactive oxygen species, the superoxide radical anion (O2−) is of particular interest due to its involvement in oxidative stress-related diseases and environmental pollutants. Through solution-phase strategy mediated by ligands, colloidal spherical and water-stable perovskite quantum dots (PQDs) of cesium lead bromide (CsPbBr3) were fabricated by functionalizing with D-tartaric acid (D-TA) (namely, CsPbBr3@D-TA PQDs), exhibiting exceptional photoluminescence quantum yield of 29.88 % and intense emission peak at 522 nm when excited at 380 nm. The as-fabricated CsPbBr3@D-TA PQDs showed strong durability and displayed bright green fluorescence when exposed to 365 nm UV radiation. The emission peak of CsPbBr3 PQDs at 522 nm was completely quenched by O2−, showing wider linear range from 0.125 to 25 µM with a detection limit of 39.82 nM. Moreover, CsPbBr3@D-TA PQDs were also applied for bio-imaging of yeast cells. The CsPbBr3@D-TA PQDs-based fluorescence approach holds great potential for sensing of O2− in real samples.

Novel sulphur-selective method for simultaneous determination of thiram and asomate based on carbon dots and its application in fruits

Thiram (THI) and asomate (ASO) are frequently detected agrochemicals in food samples due to widespread use as fungicides. However, their detection suffers from matrix interference, or the necessity of derivatization before measurement. Integrating HPLC with a post-column carbon dots (CDs)–enhanced chemiluminescence (CL) system makes it possible to simultaneously separate and selectively detect ASO and THI in fruit samples free of derivatization and deep purification. The studies on mechanisms demonstrated that CD intermediates of anionic radical were responsible for the remarkable CL enhancement of ASO or THI. The CL intensity is directly proportional to the fungicide concentration, with the ranges of linearity for ASO and THI being 5–250 μg·L−1 (R2 = 0.9993) and 4–200 μg·L−1 (R2 = 0.9986), respectively, and the corresponding limits of detection (S/N = 3) being 1.61 and 1.33 μg·L−1, respectively. The developed method enables the exploration of the novelly sulphur-targeted CL application based on carbon nanomaterials.

Synergistic adsorption–photocatalytic degradation of the emerging contaminant hydroxybenzotriazole by a 3D sponge-like easy separation polypyrrole/TiO2 composite

Herein, a 3D sponge-like polypyrrole/TiO2 (Ppy-TiO2) composite aerogel was developed for the first time to remove hydroxybenzotriazole (HOBt) from water. Mesoporous TiO2 was prepared via a sol–gel method, and then the Ppy-TiO2 composite hydrogel was prepared by oxidative polymerization and converted to aerogel by freeze-drying. The morphological, compositional, and surface characteristics of the prepared materials were detailly characterized. The characterization studies revealed that pure anatase mesoporous TiO2 nanoparticles were successfully prepared and incorporated into amorphous 3D Ppy with a porous chain-like network structure. Coupling Ppy and TiO2 extended the light absorption to the visible region and decreased the electron/hole recombination rate. The performance studies revealed that the Ppy-TiO2 composite has higher adsorption and photocatalytic activities than the sum of the individual components. Optimum performance was obtained at pH 5.3 using 0.25 g/L of the Ppy-TiO2 composite with a Ppy: TiO2 mass ratio of 1:1. The intermolecular hydrogen bonding was pivotal in the adsorption process which was multilayer. The degradation of HOBt occurs primarily by holes, then superoxide anion radicals. The total organic carbon (TOC) analysis showed a 90% reduction in carbon content after 30 min of treatment. The toxicity study indicated that the photocatalytic process decreased the toxicity of the HOBt solution. The synergism between adsorption and photocatalysis, easy separation, and reusability promote the application of Ppy-TiO2 composite aerogel for water treatment.

Electron paramagnetic resonance study of the paramagnetic centers in gamma-irradiated 2-nitrobenzoic acid single crystal

2-nitrobenzoic acid (C7H5NO4) (2NBA) single crystals were made paramagnetic using 60Co-gamma rays. An attempt was made to detect the resonance form formed in 2-nitrobenzoic acid single crystal with Electron Paramagnetic Resonance (EPR) Spectroscopy. EPR spectra were taken at 125 K in three different axes of the crystal. As a result of EPR analysis of gamma irradiated 2NBA single crystal, it was understood that 2NBA anion radical was formed. The principal values of the hyperfine structure constants, the principal values of the g-tensor and their direction cosines were calculated.

Differences in the Reaction Mechanisms of Chlorine Atom and Hydroxyl Radical with Organic Compounds: From Thermodynamics to Kinetics.

Hydroxyl radical (HO•) and chlorine atom (Cl•) are common reactive species in aqueous environments. However, the intrinsic difference in their reactions with organic compounds has not been revealed. This study compared the reaction mechanisms of HO• and Cl• with 13 aromatic and 11 aliphatic compounds by quantum chemical calculation and laser flash photolysis. Both HO• and Cl• can spontaneously react with aromatic compounds via radical adduct formation (RAF), hydrogen atom transfer (HAT), and single electron transfer (SET) pathways. The SET reactions of Cl• were more thermodynamically favorable than HO•, but contrary results were obtained for HAT reactions. According to the free energy of activation (ΔGaq‡), the dominant oxidation mechanisms of aromatic compounds were RAF and SET by HO• and SET by Cl•. The important role of SET in the HO• reactions with aromatic compounds was further verified by accurately calculating the solvation free energy of HO•/HO- and experimentally tracking the radical cations, which were generally neglected in previous studies. Meanwhile, the ΔGaq‡ value of each reaction pathway of Cl• was lower than that of HO•, resulting in higher rate constants of Cl• with aromatic compounds than HO•. For saturated aliphatic compounds, HAT was found to be the only mechanism accounting for their transformation by HO• and Cl•. This study proposed general rules for the reaction mechanisms of HO• and Cl• and unraveled their differences in the aspects of thermodynamics and kinetics, providing fundamental information for understanding contaminant transformation in processes involving HO• and Cl•.

Functional Properties and Sensory Quality of Kombucha Analogs Based on Herbal Infusions

Traditionally, kombucha is produced by the fermentation of black or green tea infusions with the use of SCOBY (Symbiotic Culture of Bacteria and Yeasts). However, SCOBY exhibits the ability to ferment other substrates as well, which can be used to create novel products with new sensory and health-promoting properties. This paper investigates the antioxidant activity, chemical composition, and sensory properties of mint, nettle, and blackcurrant leaf-based kombucha analogs. It has been demonstrated that the fermentation process with SCOBY significantly influenced (p ≤ 0.05) sugar, organic acids, and mineral contents, with the increase in iron, magnesium, and calcium amounts in all tested herbal kombucha. The study shows that the type of herb infusion has a significant influence on the parameters associated with antioxidant potential. The fermentation with SCOBY resulted in an increase in antioxidant activity as measured by the superoxide anion radical (O2•−) inhibition of all three tested herbal infusions, with the greatest changes observed in nettle kombucha. Herbal kombucha was characterized by significantly increased total phenolic content as determined by Folin’s reagent and a changed phenolic compound profile by LC-MS/MS (liquid chromatography with tandem mass spectrometry) in comparison to nonfermented infusions. Very high sensory scores were achieved for fermented mint and blackcurrant-based kombucha.

Preliminary Study to Assess the Antioxidant and Antibacterial Activity of Extracts from the Leaves of Morelia Senegalensis (A Rich)

frican populations rely heavily on traditional medicine, which uses plants to treat human and animal diseases. However, scientific validation of this empirical knowledge concerning the efficacy, toxicity and dosage of the proposed treatments is urgently needed. The aim of this study was to evaluate the antioxidant and antibacterial activity of Morelia senegalensis leaf extracts on the 2,2-diphenyl-1-picrylhydrawyl (DPPH) radical and the (2,2’-azino-bis-(3-ethylbenzothiazoline-6-sulfonic) acid) ABTS cation radical and on four reference bacterial strains (E. coli 25922, E. coli 35218, E. faecalis 29212 and S. aureus 29213) using protocols already described in the literature. The phytochemical screening test revealed the presence of flavonoids, alkaloids, tannins, saponins, reducing compounds and sterols in Morelia senegalensis leaves. All the extracts showed antioxidant activity to varying degrees. The most active ethyl acetate (AE) extract had half-maximum inhibition concentration (IC50 )of 0.33 and 0.62 mg/mL for the ABTS and DPPH tests respectively, followed by DCM and EB extracts with IC50 of 0.86 and 0.88 mg/mL for the ABTS test respectively, while the vitamin C used as a reference was more active with IC50 of 0.04 and 0.06 mg/mL. Antibacterial activity showed inhibition diameters ranging from 7 to 16 mm depending on the concentration and strain considered. The ethyl acetate fraction was the most active on S aureus 29213 with an inhibition diameter of 16 mm at 50 mg/mL. These interesting results were confirmed by the Minimal Inhibitory Concentration inhibitory (MIC) and Minimal Bacterial Concentration (MBC) studies, which showed that all the extracts had a bactericidal effect on all the studied strains. Morelia senegalensis plant has shown promising preliminary results in the treatment of bacterial infections. Further studies are needed to isolate and characterize the secondary metabolites contained in the leaves of this plant.

Nonadiabatic Photodynamics of Amantadine and1-Cyanoadamantane Cations.

Cations of diamondoids and derivatives thereof have recently become the subject of experimental, spectroscopic studies due to their potential role in astrochemistry. In particular, their electronic spectra and photoinduced dynamics trigger great interest. Here, we report on computational investigations of two nitrogen-containing derivatives of the adamantane cation (Ada+, ), the amantadine cation (Ama+, and the 1-cyanoadamantane cation (Ada-CN+, C10H15CN+). Specifically, we study electronic (vibrationally resolved) spectra and nonadiabatic molecular dynamics (modeled using the surface hopping approach based on semiempirical electronic structure theory) of these radical cations. The internal conversion time constants as well as reactive relaxation outcomes (cage-opening and hydrogen loss) are compared for the two derivatives and also with the case of Ada+ [29] Remarkably, we find a longer ground-state recovery time for Ada-CN+ than for Ama+ (for the same excitation energy window), despite a smaller excitation energy for the former. Thus, a static energy gap law cannot be used to rationalize nonadiabatic dynamics and excited state lifetimes in this case: Dynamics and details of the couplings between several states play a decisive role.