Photoinduced Color Change and Photomechanical Effect of Naphthalene Diimides Bearing Alkylamine Moieties in the Solid State

Photoinduced color change of naphthalene diimides (NDIs) bearing alkylamine moieties has been observed in the solid state. The color change is attributed to the generation of a NDI radical‐anion species, which may be formed through a photoinduced electron‐transfer process from the alkylamine moiety to the NDI. The photosensitivity of NDIs is highly dependent on the structures of the alkylamine moieties. Crystallographic analysis, kinetic analysis, UV/Vis/NIR spectroscopic measurements, and analysis of the photoproduct suggested that a radical anion was formed through an irreversible process initiated by proton abstraction between an amine radical cation and the neutral amine moiety. The radical anions formed stacks including mixed‐valence stacks and radical‐anion stacks, as shown by the broad absorption bands in near‐IR spectra. These photosensitive NDIs also showed crystal bending upon photoirradiation, which may be associated with a change in the intermolecular distance of the NDI stacks by the formation of monomeric radical anions, mixed‐valence stacks, and radical‐anion stacks.     

On the Oxidation of the Three‐Dimensional Aromatics [B12X12]2− (X=F,Cl, Br,I)

The perhalogenated closo‐dodecaborate dianions [B₁₂X₁₂]^2? (X=H, F, Cl, Br, I) are three‐dimensional counterparts to the two‐dimensional aromatics C₆X₆ (X=H, F, Cl, Br, I). Whereas oxidation of the parent compounds [B₁₂H₁₂]^2? and benzene does not lead to isolable radicals, the perhalogenated analogues can be oxidized by chemical or electrochemical methods to give stable radicals. The chemical oxidation of the closo‐dodecaborate dianions [B₁₂X₁₂]^2? with the strong oxidizer AsF₅ in liquid sulfur dioxide (lSO₂) yielded the corresponding radical anions [B₁₂X₁₂] ^{? ?} (X=F, Cl, Br). The presence of radical ions was proven by EPR and UV/Vis spectroscopy and supported by quantum chemical calculations. Use of an excess amount of the oxidizing agent allowed the synthesis of the neutral perhalogenated hypercloso‐boranes B₁₂X₁₂ (X=Cl, Br). These compounds were characterized by single‐crystal X‐ray diffraction of dark blue B₁₂Cl₁₂ and [Na(SO₂)₆][B₁₂Br₁₂] ? B₁₂Br₁₂. Sublimation of the crude reaction products that contained B₁₂X₁₂ (X=Cl, Br) resulted in pure dark blue B₁₂Cl₁₂ or decomposition to red B₉Br₉, respectively. The energetics of the oxidation processes in the gas phase were calculated by DFT methods at the PBE0/def2‐TZVPP level of theory. They revealed the trend of increasing ionization potentials of the [B₁₂X₁₂]^2? dianions by going from fluorine to bromine as halogen substituent. The oxidation of all [B₁₂X₁₂]^2? dianions was also studied in the gas phase by mass spectrometry in an ion trap. The electrochemical oxidation of the closo‐dodecaborate dianions [B₁₂X₁₂]^2? (X=F, Cl, Br, I) by cyclic and Osteryoung square‐wave voltammetry in liquid sulfur dioxide or acetonitrile showed very good agreement with quantum chemical calculations in the gas phase. For [B₁₂X₁₂]^2? (X=F, Cl, Br) the first and second oxidation processes are detected. Whereas the first process is quasi‐reversible (with oxidation potentials in the range between +1.68 and +2.29 V (lSO₂, versus ferrocene/ferrocenium (Fc^0/+))), the second process is irreversible (with oxidation potentials ranging from +2.63 to +2.71 V (lSO₂, versus Fc^0/+)). [B₁₂I₁₂]^2? showed a complex oxidation behavior in cyclic voltammetry experiments, presumably owing to decomposition of the cluster anion under release of iodide, which also explains the failure to isolate the respective radical by chemical oxidation.     

Radical Cation of Pyrazine-di-N-oxide as a Mediator in Electrocatalytic Oxidation of Organic Compounds

The mechanism of oxidation of cyclohexanol, methanol, diethyl ether, triethyl orthoformate, and cyclohexane in the presence of a mediator—electrochemically generated radical cation of pyrazine-di-N-oxide (PyrDNO)—is studied on glassy carbon and platinum in a 0.1 M LiClO₄ solution in acetonitrile employing cyclic voltammetry, ESR electrolysis, and gas chromatography. Effect of temperature, additives of acid and water, oxygen, and the nature of the substrate and solvent on the shape of cyclic voltammograms and intensity of ESR signal of PyrDNO is examined. ESR spectra for radical cations and anions of PyrDNO with g factors equal to, respectively, 2.0090 and 2.0031 are recorded. A mechanism for the overall two-electron catalytic oxidation of an organic substance, which involves a stage in which it complexes with the radical cation of PyrDNO, is suggested.     

Cationic Titanocene(III) Complexes for Catalysis in Single‐Electron Steps

By exploiting solvent and anion effects, [Cp₂Ti]⁺ complexes for atom‐economical catalysis in single‐electron steps were developed and applied for the first time. These complexes constitute remarkably stable and active catalysts for radical arylations. The reaction kinetics and catalyst composition were studied by cyclic voltammetry and in situ IR spectroscopy.     

Aqueous Oxidation of Sulfonamide Antibiotics: Aromatic Nucleophilic Substitution of an Aniline Radical Cation

Sulfonamide antibiotics are an important class of organic micropollutants in the aquatic environment. For several, sulfur dioxide extrusion products have been previously reported upon photochemical or dark oxidation. Using quantum chemical modeling calculations and transient absorption spectroscopy, it is shown that single‐electron oxidation from sulfadiazine produces the corresponding aniline radical cation. Density functional theory calculations indicate that this intermediate can exist in four protonation states. One species exhibits a low barrier for an intramolecular nucleophilic attack at the para position of the oxidized aniline ring, in which a pyrimidine nitrogen acts as a nucleophile. This attack can lead to a rearranged structure, which exhibits the same connectivity as the SO₂‐extruded oxidation product that was previously observed in the aquatic environment and characterized by NMR spectroscopy. We report a detailed reaction mechanism for this intramolecular aromatic nucleophilic substitution, and we discuss the possibility of this reaction pathway for other sulfonamide drugs.     

Electrochemical transformations of cycloheptatriene derivatives

Redox reactions of a series of substituted cycloheptatrienes are studied on a platinum electrode in acetonitrile using the cyclic voltammetry method. It was found that heptaphenyl-substituted cycloheptatrienes are irreversibly oxidized in two stages in the range of high anodic potentials to form unstable radical cations and dications. As opposed to the unsubstituted and monosubstituted cycloheptatrienes, they are not reduced at the potentials higher than ?2 V (SCE). A new stable radical anion was found in the reduction of N-cycloheptatrienylphthalimide. Its spin density distribution is studied by ESR spectroscopy.     

Element–Element Bond Formation upon Oxidation and Reduction

The redox chemistry of [(Cp′′′Co)₂(μ,η²:η²‐E₂)₂] (E=P ( 1 ), As ( 2 ); Cp′′′=1,2,4‐tri(tert‐butyl)cyclopentadienyl) was investigated. Both compounds can be oxidized and reduced twice. That way, the monocations [(Cp′′′Co)₂(μ,η⁴:η⁴‐E₄)][X] (E=P, X=BF₄ ( 3 a ), [FAl] ( 3 b ); E=As, X=BF₄ ( 4 a ), [FAl] ( 4 b )), the dications [(Cp′′′Co)₂(μ,η⁴:η⁴‐E₄)][TEF]₂ (E=P ( 5 ), As ( 6 )), and the monoanions [K(18‐c‐6)(dme)₂][(Cp′′′Co)₂(μ,η⁴:η⁴‐E₄)] (E=P ( 7 ), As ( 8 )) were isolated. Further reduction of 7 leads to the dianionic complex [K(18‐c‐6)(dme)₂][K(18‐c‐6)][(Cp′′′Co)₂(μ,η³:η³‐P₄)] ( 9 ), in which the cyclo‐P₄ ligand has rearranged to a chain‐like P₄ ligand. Further reduction of 8 can be achieved with an excess of potassium under the formation of [K(dme)₄][(Cp′′′Co)₂(μ,η³:η³‐As₃)] ( 10 ) and the elimination of an As₁ unit. Compound 10 represents the first example of an allylic As₃ ligand incorporated into a triple‐decker complex.     

2－乙基蒽醌电化学还原过程的研究

以Ｐｔ、Ｐｄ及几种Ｐｄ－离子注入材料为电极使用ＣＶ和ＥＳＲ法研究了２ＥＡＱ在ＤＭＳＯ中的电化学还原作用。阐明了两个电子迁移步骤所具有的准可逆性质及其后续均相化学反应。提出了电化学还原作用的历程,计算了有关步骤的动力学参数。对体系中水含量的影响进行了测量,讨论了质子给予体的作用。通过实验结果的对比,提出了在２ＥＡＱ还原中以Ｐｄ－离子注入材料代替金属Ｐｄ催化剂的可能性。     

Study of electrochemical oxidation of nickel catecholate complexes with bis(diphenylphosphino)ethane by cyclic voltammetry and ESR

One-and two-electron electrochemical oxidation of the (dppe)Ni(Cat) complexes (dppe is bis(diphenylphosphino)ethane, Cat is the sterically hindered catechol dianion) was studied. The transfer of the first electron proceeds reversibly to give paramagnetic species; parameters of their EPR spectra attest to a square planar geometry of one-electron oxidation products. The transfer of the second electron is irreversible because of co-proportionation of radical cations involving the initial complexes and the generated dications. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 99–102, January, 2007.     

Flavin Derivatives with Tailored Redox Properties: Synthesis,Characterization, and Electrochemical Behavior

This study establishes structure–property relationships for four synthetic flavin molecules as bioinspired redox mediators in electro‐ and photocatalysis applications. The studied flavin compounds were disubstituted with polar substituents at the N1 and N3 positions (alloxazine) or at the N3 and N10 positions (isoalloxazines). The electrochemical behavior of one such synthetic flavin analogue was examined in detail in aqueous solutions of varying pH in the range from 1 to 10. Cyclic voltammetry, used in conjunction with hydrodynamic (rotating disk electrode) voltammetry, showed quasi‐reversible behavior consistent with freely diffusing molecules and an overall global 2e^?, 2H⁺ proton‐coupled electron transfer scheme. UV/Vis spectroelectrochemical data was also employed to study the pH‐dependent electrochemical behavior of this derivative. Substituent effects on the redox behavior were compared and contrasted for all the four compounds, and visualized within a scatter plot framework to afford comparison with prior knowledge on mostly natural flavins in aqueous media. Finally, a preliminary assessment of one of the synthetic flavins was performed of its electrocatalytic activity toward dioxygen reduction as a prelude to further (quantitative) studies of both freely diffusing and tethered molecules on various electrode surfaces.     

Two-step electrochemically directed synthesis of Pr4N(TCNQ)n (n=1, 2): preparation, structure, and properties of a magnetically isolated dimer and a quasi-one-dimensional chain

Solid-state electrochemistry of a tetracyanoquinodimethane (TCNQ)-modified electrode in contact with a tetrapropylammonium cation (Pr(4)N(+)) electrolyte showed two electron-transfer steps to give Pr(4)N(TCNQ)(2) (1) and Pr(4)N(TCNQ) (2) rather than the traditional one-electron step to directly give Pr(4)N(TCNQ). Two thermodynamically stable Pr(4)N(+)-TCNQ stoichiometries, 1 and 2, were synthesized and characterized. The degree of charge transfer (ρ) calculated from the crystal structure is -0.5 for the TCNQ moieties in 1 and -1.0 for those in 2. Raman spectra for Pr(4)N(TCNQ)(2) show only one resonance for the extracyclic C=C stretching at 1423 cm(-1), which lies approximately midway between that of TCNQ at 1454 cm(-1) and TCNQ(-) at 1380 cm(-1). Both the magnetic susceptibility and EPR spectra are temperature-dependent, with a magnetic moment close to that for one unpaired electron per (TCNQ)(2) unit in 1, whereas 2 is almost diamagnetic. Pressed discs of both complexes show conductivity (1-2×10(-5) S cm(-1)) in the semiconductor range. For 1, the position of zero current for the steady-state voltammograms implies 50% of TCNQ(-) and 50% TCNQ(0) is present in solution, thereby supporting a dissociation of (TCNQ)(2)(-) in solution, but is indicative of only TCNQ(-) being present for 2.     

Quantitative conformational study of redox-active [2]rotaxanes, part 1: Methodology and application to a model [2]rotaxane

This paper reports a novel methodology for the conformational analysis of [2]rotaxanes. It combines NMR spectroscopic (COSY, NOESY and the recently reported paramagnetic line-broadening and suppression technique) and electrochemical techniques to enable a quantitative analysis of the co-conformations of interlocked molecules and the conformations of their components. This methodology was used to study a model [2]rotaxane in solution. This [2]rotaxane consists of an axle that incorporates an electron-poor, doubly positively charged viologen that threads an electron-rich crown ether. It has been shown that the axle of the [2]rotaxane in its dicationic state adopts a folded conformation in solution and the crown ether is localised at the viologen moiety. Following a one-electron reduction of viologen, the paramagnetic radical cation of the [2]rotaxane retains its folded conformation in solution. The data also demonstrate that in the radical cation the crown ether remains localised at the viologen, despite its reduced affinity for the singly reduced viologen. The combined quantitative NMR spectroscopic and electrochemical characterisation of the electromechanical function of the model [2]rotaxane in solution provides an important reference point for the study of switching in structurally related bistable [2]rotaxanes, which is the subject of the second part of this work.     

Reduction of Transient Histidine Radicals by Tyrosine: Influence of the Protonation State of Reactants

The role of tyrosine radicals as mediators of electron transfer reactions in enzymes is well established, as is the involvement of histidine as a binding partner. But how environmental factors affect these reactions remains poorly explored. In the study presented here, kinetic data on the influence of the protonation state of the reactants on the reduction of transient histidine radicals by tyrosine were obtained in neutral and basic aqueous solution (pH 6–12) using time-resolved chemically induced dynamic nuclear polarization (CIDNP). The histidine radicals were generated in the photo-induced reaction with the photosensitizer 3,3′,4,4′-tetracarboxy benzophenone. From model simulations of the detected CIDNP kinetics, pH dependent second-order rate constants of the reduction of histidine radicals were obtained for four possible combinations of the amino acids and their N-acetyl derivatives, and also for the systems histidine-phenylalanine dipeptide/N-acetyl tyrosine, and N-acetyl histidine/tyrosine-glutamine dipeptide. The pH dependences of the rate constant of the reduction reaction are explained accounting for the protonation states of reactants, and also protonation state of the equilibrium form of the product - reduced form of histidine radical, which is histidine with neutral or a positively charged imidazole.