Stable Radical Anions Research Articles

Novel electrochemiluminescent materials for sensor applications.

Electrochemiluminescence (ECL) uses redox reactions to generate light at an electrode surface, and is gaining increasing attention for biosensor development due to its high sensitivity and excellent signal-to-noise ratio. ECL studies of monodisperse oligofluorene-truxenes (T4 series) have been reported previously, showing the production of stable radical cations and radical anions, generating blue ECL. The compound in this study differs from the original structures, in that there are 2,1,3-benzothiadazole (BT) units inserted between the first and second fluorene units of the quarterfluorenyl arms. It was therefore anticipated that the incorporation of these highly luminescent and ECL-active compounds into sensor development would lead to significant decreases in detection limits. In this contribution, we report on the impact of incorporating these novel complexes into sensor devices on the ECL efficiency, as well as the ability of these to improve the detection sensitivity and decrease the limit of detection using the reagent-free detection of model analytes. The real world impact of these compounds is elucidated through the comparison with more standard ECL materials such as ruthenium-based compounds. The potential for multiple applications is to be examined within this contribution.

Thiadiazole dioxide-fused picene: acceptor ability, anion radical formation, and n-type charge transport characteristics

Picene, which is an organic p-channel semiconductor and a component of superconductors, was derivatized with a thiadiazole-dioxide moiety to afford a novel acceptor molecule (PTDAO₂). PTDAO₂ can form stable anion radical species in solution and as a crystal, and its neutral crystalline thin film exhibits n-type transistor characteristics.

Analytical determination of nimesulide and ofloxacin in pharmaceutical preparations using square-wave voltammetry

The electrochemical behaviour and analytical detection procedure for nimesulide (NIM) and ofloxacin (OFX) and their assay in commercial formulations were evaluated using square-wave voltammetry (SWV) combined with a hanging mercury drop electrode (HMDE). All experimental and voltammetric conditions were previously optimized to obtain the best analytical signal in terms of intensities and profile of the reduction peaks. For NIM, the peak currents were related to the one-electron reduction of a nitro group to a stable radical anion, which is followed by a one-electron transfer and a protonation step with a consequent formation of a nitrosoanion. The voltammetric results indicated that the mechanism of OFX involved the transfer of two electrons and two protons in a totally irreversible reduction related to the conversion of a ketone group to an alcohol group. Analytical parameters such as linearity range, equations of the analytical curves, correlation coefficients, detection and quantification limits, recovery efficiency, and relative standard deviation for repeatability and reproducibility experiments were compared to similar results obtained by the use of UV-Vis spectrophotometry, and the results showed that the voltammetric procedure using HMDE is suitable to determine pharmaceutical compounds in complex samples. The applicability of the proposed procedure was tested on pharmaceutical formulations of NIM and OFX by observing the stability, specificity, recovery and precision of the procedure in tablets, oral solution and ophthalmic solution.

Cathodic and Photocatalytic Reduction of Nitroquinolones Investigated by In Situ EPR/UV-Vis Spectroelectrochemistry and EPR spectroscopy

The generation of paramagnetic intermediates upon photoinduced reduction of substituted nitroquinolones 1–6 in dimethylsulfoxide/methanol titania suspensions was investigated by in situ EPR spectroscopy. The assignment of the primary photogenerated paramagnetic signals was based on the results of cyclic voltammetry, amperostatic in situ spectroelectrochemistry and in situ EPR/UV-Vis spectroelectrochemistry in aprotic dimethylsulfoxide and dimethylsulfoxide/methanol mixed solvent. The primary reduction step in the cathodically- or in the photocatalytically-induced electron transfer process represents the formation of radical monoanion, the stability of which is crucially influenced by the 1-ethyl substitution at the nitrogen of the 4-pyridone ring of quinolone. 1-Ethyl 6-nitroquinolones typically form stable radical anions with well-resolved EPR spectra, with detailed interpretation of hyperfine coupling constants (hfcc) supported by theoretical calculations. On the other hand, the radical anions of nitroquinolones with amino hydrogen at nitrogen of the enaminone system (N–C=C–C=O) convert rapidly to diamagnetic s-dimer dianions, reduced in the second reversible reduction step to paramagnetic s-dimer radical trianions. The EPR spectra obtained upon prolonged irradiation of 1-ethyl nitroquinolones in titania suspensions were assigned to the R-NO•H intermediates produced via nitro group reduction. Experiments with deuterated methanol unambiguously confirmed the photoinduced reduction of the nitro group, including the interaction with hydrogen from the hydroxyl group of methanol. The generation of reactive radicals formed via methanol and dimethylsulfoxide oxidation in irradiated titania suspensions was investigated by EPR spin trapping technique. TiO2. Keywords: Cathodic reduction, DFT calculations, EPR spectroscopy, Nitroquinolones, Photoinduced reduction, TiO2

Photoelectron spectroscopic and density functional theoretical studies of the 2′-deoxycytidine homodimer radical anion

The intact (parent) 2'-deoxycytidine homodimer anion, (dC)2 (●-), was generated in the gas phase (in vacuo) using an infrared desorption∕photoemission source and its photoelectron spectrum was recorded using a pulsed, magnetic bottle photoelectron spectrometer. The photoelectron spectrum (PES) revealed a broad peak with the maximum at an electron binding energy between 1.6 and 1.9 eV and with a threshold at ∼1.2 eV. The relative energies and vertical detachment energies of possible anion radicals were calculated at the B3LYP/6-31++G(∗∗) level of theory. The most stable anion radicals are the complexes involving combinations of the sugar[middle dot][middle dot][middle dot]base and base[middle dot][middle dot][middle dot]base interactions. The calculated adiabatic electron affinities and vertical detachment energies of the most stable (dC)2 (●-) anions agree with the experimental values. In contrast with previous experimental-computational studies on the anionic complexes involving nucleobases with various proton-donors, the electron-induced proton transferred structures of (dC)2 (●-) are not responsible for the shape of PES.

Synthesis of Zwitterionic Cobaltocenium Borate and Borata-alkene Derivatives from a Borole-Radical Anion

Chemical single-electron reduction of 1-mesityl-2,3,4,5-tetraphenylborole (3) gave a stable radical anion [CoCp*2 ][3] as shown in earlier investigations. Herein, we present the reaction of [CoCp*2 ][3] with the 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO), a common radical trap. Instead of radical recombination, the reaction proceeds through a redox pathway involving oxidation of the borole radical anion combined with reduction of TEMPO. This electron-transfer process is accompanied by a deprotonation reaction of the cobaltocenium counterion by the base TEMPO(-) to give TEMPO-H and a neutral cobalt(I) fulvene complex (7). The latter was not observed directly during the reaction, because it instantaneously reacts as a nucleophile attacking at the boron center of the in situ generated borole 3 to give the borate 6. However, 7 was synthesized independently by deprotonation of [CoCp*2 ][PF6 ]. In addition, the obtained zwitterionic cobaltocenium borate 6 undergoes a photolytic rearrangement to form the borata-alkene derivative 9 that thermally transforms to the chiral cobaltocenium borate 12. Our investigations are based on spectroscopic evidence, X-ray crystallography, elemental analysis, as well as DFT calculations.

Bipolar properties of polythiophene derivatives with 1,3,5-triazine units

Bipolar properties of model polythiophene derivatives with 1,3,5-triazine units are investigated. The conjugation length is controlled by the meta substitution. In situ spectroelectrochemical UV–Vis–NIR and ESR measurement recorded during electrochemical reduction and oxidation are presented. Spectroelectrochemistry recorded during reduction of polymers in the solid state is especially interesting. Most literature data of spectroelectrochemistry of solid state polymers has been obtained during oxidation which is due to poor stability of radical anions in reduced polymers. Cathodic reduction leads to the formation of stable radical anions in a certain potential range. Exceeding this range leads to overreduction. A similar effect is observed during electrooxidation. Obtained results indicate that not only oligomers with 1,3,5-triazine units but also respective polymers are worth considering during studies on new bipolar polymer materials to broad range of optoelectronic and photovoltaic applications.

Electrochemical reduction and quantification of menadione in sodium dodecyl sulfate micellar media

Menadione cyclic voltammograms show a pair of redox steps on glassy carbon electrode in 0.1 M H3PO4 with potential separation of 343 mV. Cationic, nonionic, and anionic surfactants micellar media significantly decrease the menadione peak potential separation. Statistically significant increase of menadione reduction current (3- and 4.4-fold) has been observed in Triton X 100 and sodium dodecyl sulfate (SDS) micellar media, respectively. Electrochemical reduction of menadione in 9 mM SDS micellar media is reversible diffusion-controlled one-electron process corresponding to formation of relatively stable semiquinone anion radical. The linear dynamic ranges of menadione determination are 7–560 and 600–2,550 μM with the limits of detection and quantification of 1.66 and 5.53 μM, respectively. The current concentration sensitivity is (8.6 ± 0.2) × 103 μA μM−1. The voltammetric method for the determination of menadione in pharmaceutical “Aekol” based on preliminary extraction with ethanol has been developed.

Single Electron Transfer-Driven Multi-Dimensional Signal Read-out Function of TCNQ as an “Off-the-Shelf” Detector for Cyanide

Herein we report the first applications of TCNQ as a rapid and highly sensitive off-the-shelf cyanide detector. As a proof-of-concept, we have applied a kinetically selective single-electron transfer (SET) from cyanide to deep-lying LUMO orbitals of TCNQ to generate a persistently stable radical anion (TCNQ(•-)), under ambient condition. In contrast to the known cyanide sensors that operate with limited signal outputs, TCNQ(•-) offers a unique multiple signaling platform. The signal readability is facilitated through multichannel absorption in the UV-vis-NIR region and scattering-based spectroscopic methods like Raman spectroscopy and hyper Rayleigh scattering techniques. Particularly notable is the application of the intense 840 nm NIR absorption band to detect cyanide. This can be useful for avoiding background interference in the UV-vis region predominant in biological samples. We also demonstrate the fabrication of a practical electronic device with TCNQ as a detector. The device generates multiorder enhancement in current with cyanide because of the formation of the conductive TCNQ(•-).

The equilibrium and kinetics of intermolecular hydrogen bonding in nitroarene anion radical-alcohol system: 1,3-dinitro-benzene anion radical - R-OH (R═CH3 -, C2 H5 -, (CH3 )2 CH-)

To explore the possibility of hydrogen bonding of a stable anion radical with DNA – component sugar, hormones, steroid, and so on (through hydroxyl group), as a first step, the possibility of hydrogen bonding of 1,3-dinitrobenzene anion radical (1,3-DNB−•) with aliphatic alcohols was studied. It was found that 1,3-DNB−• anion radical undergoes hydrogen bonding with alcohols: methanol, ethanol, and 2-proponal. The hydrogen-bonding equilibrium constant Keq and the (hydrogen-bonding) rate constants k2 were evaluated through the use of linear scan and cyclic voltammetry theory and techniques. The Keq was found to be in the range of 1.4–6.0 m−1, whereas the rate constants k2 were found to be in the range of 1.5–3.6 m−1 s−1, depending upon the hydrogen-bonding agent and the equation used for the calculation of the rate constants. The hydrogen-bonding number n was found to be around 0.5 or 1.0. The implication of this study in, for example, the replication of DNA, the prevention of the formation of super oxide, and so on is discussed. Copyright © 2012 John Wiley & Sons, Ltd.

Photoelectron Spectroscopy and Computational Modeling of Thymidine Homodimer Anions

The intact thymidine homodimer anion (dT(2)(-)) was generated in the gas phase using an infrared desorption/photoemission source and recorded by a pulsed photoelectron spectrometer. The photoelectron spectrum (PES) revealed a broad signal with the maximum at electron binding energy ∼2.0 eV and the threshold value at 1.1 eV. The relative energies and vertical detachment energies of the possible anion structures were calculated at the B3LYP/6-31++G(d,p) level. Here we report that the most stable anion radical homodimer geometries observed in the PES are the anionic nucleoside coordinated by the O8 atom of thymine to the deoxyribose of the second neutral nucleoside. Unlike previous experimental-computational studies on anionic complexes involving nucleobases with proton donors, the electron-induced proton-transferred structures are not responsible for the shape of the PES of dT(2)(-).

Synthesis and Electrochemistry of Organometallic Cobaltadithiaazulenes

Reaction of tropolone or hinokitiol with phosphorus pentasulfide (P(2)S(5)) directly gives the sulfurized precursor [PS(2)(SST)](2) or [PS(2)(SSH)](2) (SST = dithiotropolonato or SSH = dithiohinokitiolato). The resulting [PS(2)(SST)](2) or [PS(2)(SSH)](2) is further reacted with [CpCoI(2)(CO)] (Cp = η(5)-cyclopentadienyl) to form the organometallic [CpCo(I)(SST)] (1) or [CpCo(I)(SSH)] (2), respectively. 1 and 2 have a cobaltadithiaazulene ring containing one cobalt and two sulfur atoms in the five-membered ring of azulene. Although X-ray structure analysis of 1 reveals the iodide-coordinated structure, 1 becomes the iodide-free complex [CpCo(SST)](+) (4(+)) in solution. Electrochemical studies of 4(+) by CV and spectroelectrochemical measurements (ESR, UV-vis-NIR) in solution are carried out. 4(+) is stepwise reduced by 2e(-) to form the stable neutral radical (4(•)) and unstable anion (4(-)). It is proposed that the anion 4(-) undergoes dimerization to afford the dimer (6(2-)) by anion radical coupling at the 5 or 7 position in the seven-membered ring of the cobaltadithiaazulene, since the similar anion radical coupling of a reduced azulene has been reported. Electrochemical reoxidation of 6(2-) slowly undergoes monomerization, giving the original monomer 4(•). DFT calculation of 4(+) explains that there is a delocalized lowest unoccupied molecular orbital (LUMO) in the whole molecule, and that of radical 4(•) has a delocalized singly occupied molecular orbital (SOMO). In these CpCo-SST (or SSH) complexes, there could be metal/ligand electron transfer since the SST (or SSH) ligand is potentially redox active. The spin density distribution of 4(-) obtained by the DFT method supports the mechanism of the anion radical coupling at the 5 or 7 position in the seven-membered ring.

ChemInform Abstract: Persistent and Stable Silyl Radicals

AbstractReview: 106 refs.

Electron Attachment to the Cytosine-Centered DNA Single Strands: Does Base Stacking Matter?

Electron attachment to the trimer of nucleotide, dGpdCpdG, has been investigated by a quantum mechanical approach at a reliable level of theory. The study of the electron attached dGpdCpdG species demonstrates that cytosine contained DNA single strands have a strong tendency to capture low-energy electrons and to form electronically stable cytosine-centered radical anions. The comparative study of the model molecules pdCpdG and dGpdCp reveals that base stacking has little contribution to the adiabatic electron affinity (AEA) of cytosine in DNA single strands. Additionally, the base-base stacking does not affect the vertical detachment energy (VDE) of the cytosine-centered radicals. Intrastrand H-bonding is found to be critical in increasing the values of the AEA and VDE. However, base-base stacking is revealed to be important in enlarging the vertical electron affinity (VEA) of cytosine. The electron attachment to the cytosine moiety intensifies the intrastrand H-bonding between the neighboring G and C bases. This process disrupts the base-base stacking interaction in the radical anion of dGpdCpdG.

Low Energy Electron Attachment to the Adenosine Site of DNA

To evaluate the role of adenosine in low energy electron (LEE) induced DNA strand breaks, theoretical investigations of the LEE attachment induced C-O σ bonds and N-glycosidic bond breaking of 2'-deoxyadenosine-3',5'-diphosphate (3',5'-dAMP) were performed at the B3LYP/DZP++ level of theory. The results indicate that, although adenine-rich oligonucleotides are capable of capturing the near 0 eV electron to form the electronically stable radical anions in the gas phase, it is unlikely to undergo either C-O σ bond cleavage or the glycosidic processes due to the low electron detachment energy (VDE) of 3',5'-dADP(-) unit (0.26 eV). Instead, these radical anions should directly yield an electron detachment product in the gas phase. In the presence of polarizable surroundings, due to the large increase of the electron detachment energy (VDE increases to 1.59 eV), the adenine-centered radical anions could directly lead to strand breaks in the adenine-rich DNA single strands through either σ bond or N-glycosidic bond breaking. The values of activation energy for rupture of the C(5')-O(5') σ bond (22.5 kcal/mol), the N-glycosidic bond (20.2 kcal/mol), and the C(3')-O(3') σ bond (13.2 kcal/mol) indicate that C(3')-O(3') σ bond breaking should dominate. Moreover, along with the previous research, the predicted ratio of the activation energy barrier of the C(5')-O(5') σ bond breakage in 3',5'-dXDP(-) (X = G, A, T) partly explains the observation in the femtosecond time-resolved laser spectroscopic experiment that the C(5')-O(5') σ bond breaking only occurs in dGMP(-) and dTMP(-) not in dAMP(-). This study completes the series of LEE-induced DNA single strand breaking investigations for the four basic DNA units 3',5'-dXDP (X = G, A, T, C). The obtained results are vital for elucidating the experimental observations. Combined with the previous studies, the information revealed in this study is crucial for understanding the mechanisms of the interactions between the LEE and the DNA stands.

Communication: Stabilization of radical anions with weakly bound electron in condensed media: A case study of diacetonyl radical anion

The radical anion resulting from electron capture by diacetonyl molecule has been characterized by EPR and optical absorption spectroscopy in glassy ether matrices at 77 K. In non-polar alkane glasses this species was not observed under the same conditions, which confirms the crucial role of matrix interactions in stabilizing this species. Calculations at the MP2 level show the vertical detachment energy to increase gradually from roughly zero for a bare anion to ∼1 eV for the complex involving six ether molecules.

Pharmacophore Modeling and Density Functional Theory Analysis for A Series of Nitroimidazole Compounds with Antitubercular Activity

In an attempt to highlight structural features required for potent antitubercular activity, five pharmacophoric features were developed for PA-824 and its analogs. The generated pharmacophore indicated importance of a nitro group, three hydrogen bond acceptor features, and a distal aromatic ring for potent activity. The model based on pharmacophore alignment has good correlation coefficient for the training set (r(2) = 0.81, SD = 0.31, F = 122.9, N = 152), which was evaluated using a test set (Q(2) = 0.77, root-mean-square error = 0.35, Pearson-R = 0.88, N = 49). Structure-activity relationship investigation further revealed that hydrophobic substitutions at the para-position of distal aromatic ring could lead to more potent analogs. The most active and inactive compounds were further studied using density functional theory at B3LYP/3-21*G level. The calculated electrostatic profile indicated that these compounds possess maximum negative potential in the vicinity of nitro group extending laterally to the imidazole ring. Furthermore, the calculated electron affinity values indicate the stability of radical anions, which could form upon one electron reduction in the biological system, thus, indicating the electron acceptor capacity of these compounds. Results of this study are expected to be useful in the design of novel potent nitroimidazoles as antitubercular agents.

Bond elongation in the anion radical of coordinated tetrazine ligands: A crystallographic, spectroscopic and computational study of a reduced {Re(CO)3Cl} complex

The binuclear complex [(μ-Me2BPTZ)(Re(CO)3Cl)2] (1), where Me2BPTZ=3,6-(5-methyl-pyridyl)-1,2,4,5-tetrazine, can be reduced by addition of bis(η5-pentamethylcyclopentadienyl) iron(II) (decamethylferrocene, Fc∗), to obtain a stable radical anion form 1−. A single-crystal X-ray diffraction study of the radical anion (1−)(Fc∗+) was conducted and compared with a computational model of the same compound in the neutral and reduced states. As such, this work presents the first structural analysis of a reduced diimine ligand that is coordinated to {Re(CO)3Cl} moieties. Bond-length changes within the tetrazine ring system were consistent with previously reported examples of tetrazine radicals and with calculated structures that show clear elongation of the azo-type NN bond. Consistently atomic charge calculations indicate that the extra electron in the radical anion resides largely at the tetrazine core. A negligible change in the Re–Cl bond length is observed and computed.

Assembly of DNA Triangles Mediated by Perylene Bisimide Caps

Perylene bisimides (PBI) have been synthetically incorporated as caps onto a Y-shaped DNA triple strand. These PBI caps serve as "sticky" ends in the spontaneous assembly of larger DNA ensembles, linking the triangular DNA through stacking interactions. This, in turn, yields a hypsochromic shift in the absorption and a red shift in the fluorescence as characteristic optical readouts. This assembly occurs spontaneously without any enzymatic ligation process and without the use of overhanging DNA as sticky ends. Instead, dimerizations of the PBI chromophores in the assembly are controlled by the DNA as a structural scaffold. Thereby, the PBI-driven assembly is fully reversible. Due to the fact that PBI dimerization does not occur in the single strand, the aggregates can be destroyed by thermal dehybridization of the DNA scaffold and reassembled by reannealing of the DNA construct. In view of the fact that PBI forms stable radical anions, the presented DNA architectures are not only interesting optical biomaterials, but are also promising materials for molecular electronics with DNA.

Photoinduced formation of reversible dye radicals and their impact on super-resolution imaging

Radical ions of organic dyes are highly reactive species and have been studied for decades by transient absorption spectroscopy and pulse radiolysis experiments in oxygen-depleted solution. Here we show by continuous wave EPR, absorption, and fluorescence experiments that the triplet state of rhodamine dyes can be photoreduced by thiols to form stable radical anions in aqueous solution with a lifetime of up to several hours. Our data demonstrate that reduction of the triplet state and photoinduced oxidation of reactive intermediates by oxygen represents a general mechanism for reversible photoswitching of dyes in aqueous thiol-containing solutions highlighting the key role of molecular oxygen for super-resolution fluorescence imaging. Since cells contain the thiol glutathione at millimolar concentrations and reactive oxygen species are formed as side products our findings are of consequence for live cell fluorescence microscopy.