Hydrogen evolution from water by use of viologen polymers as electron transfer catalyst

The behavior of viologen polymer (P-V²⁺) as an electron transfer catalyst in the reaction of hydrogen generation was studied. In the photoirradiation system, which contains triethanolamine (TEA), Ru(bpy)³⁺₃, and P-V²⁺, the amount of hydrogen evolution was less than methyl viologen (MV²⁺); P-V²⁺, however, was more effective in sodium dithionite as the electron donor and showed higher initial rates than MV²⁺.     

Radical‐Cation Dimerization Overwhelms Inclusion in [n]Pseudorotaxanes

Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril ( CB7 ) in water is a well‐known phenomenon. Herein, two counter‐examples are presented. Two viologen‐containing thread molecules were designed, synthesized, and thoroughly characterized by ¹H DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square‐wave voltammetry, and chronocoulometry: BV ⁴⁺, which contains two viologen subunits, and HV ¹²⁺, which contains six. In both threads, the viologen subunits are covalently bonded to a hexavalent phosphazene core. The corresponding [3]‐ and [7]pseudorotaxanes that form on complexation with CB7 , that is, BV ⁴⁺?( CB 7)₂ and HV ¹²⁺?( CB 7)₆, were also analyzed. The properties of two monomeric control threads, namely, methyl viologen ( MV ²⁺) and benzyl methyl viologen ( BMV ²⁺), as well as their [2]pseudorotaxane complexes with CB7 ( MV ²⁺? CB7 and BMV ²⁺? CB7 ) were also investigated. As expected, the control pseudorotaxanes remained intact after one‐electron reduction of their viologen‐recognition stations. In contrast, analogous reduction of BV ⁴⁺?( CB 7)₂ and HV ¹²⁺?( CB 7)₆ led to host–guest decomplexation and release of the free threads BV ^{2( . +)} and HV ^{6( . +)}, respectively. ¹H DOSY NMR spectrometric and chronocoulometric measurements showed that BV ^{2( . +)} and HV ^{6( . +)} have larger diffusion coefficients than the corresponding [3]‐ and [7]pseudorotaxanes, and UV/Vis absorption studies provided evidence for intramolecular radical‐cation dimerization. These results demonstrate that radical‐cation dimerization, a relatively weak interaction, can be used as a driving force in novel molecular switches.     

Carbon‐Nanotube‐Mediated Electrochemical Transition in a Redox‐Active Supramolecular Hydrogel Derived from Viologen and an l‐Alanine‐Based Amphiphile

A two‐component hydrogelator (16‐A)₂‐V²⁺ , comprising an l ‐alanine‐based amphiphile ( 16‐A ) and a redox‐active viologen based partner ( V²⁺ ), is reported. The formation the hydrogel depended, not only on the acid‐to‐amine stoichiometric ratio, but on the choice of the l ‐amino acid group and also on the hydrocarbon chain length of the amphiphilic component. The redox responsive property and the electrochemical behavior of this two‐component system were further examined by step‐wise chemical and electrochemical reduction of the viologen nucleus (V²⁺/V⁺ and V⁺/V⁰). The half‐wave reduction potentials (E_1/2) associated with the viologen ring shifted to more negative values with increasing amine component. This indicates that higher extent of salt formation hinders reduction of the viologen moiety. Interestingly, the incorporation of single‐walled carbon nanotubes in the electrochemically irreversible hydrogel (16‐A)₂‐V²⁺ transformed it into a quasi‐reversible electrochemical system.     

Theoretical investigation of the alloxan–dialuric acid redox cycle

The redox cycle between alloxan, a mild oxidizing agent, and its reduction partner, dialuric acid, is investigated using density functional theory. It is found that the initial step is the one‐electron reduction of alloxan followed by protonation, yielding a stable neutral radical, AH^·. The radical can then accept another electron to form the dialuric acid anion. The formation of this anion is thermodynamically favored in both the gas phase and in solution. The radical may also undergo dimerization to alloxantin, followed by the transfer of a proton from one moiety to another, yielding alloxan and dialuric acid. This reduction is thermodynamically feasible in the gas phase, but not in aqueous solution. In the case of reduction of alloxan by glutathione at the physiological pH, computed redox potentials indicate that a two‐electron reduction is the favored course of reaction, yielding directly the dialuric acid anion, which then undergoes aerial oxidation to yield the superoxide radical. The redox cycling between alloxan and dialuric acid is responsible for the diabetogenic activity of alloxan, producing cytotoxic radicals on reoxidation of dialuric acid. © 2013 Wiley Periodicals, Inc.     

Preparation of Polymers with Viologen Moieties and Their Application to the Selective Reduction of Substituted Nitroarenes

Abstract

Polymers with viologen moieties were synthesized by using poly-chlorethyl vinyl ether (PCEVE) as mother supports. These polymers were used as electron-transfer catalysts (ETC) for the reduction of substituted nitroarenes under heterophase conditions (reductant: Na₂S₂O₄ in CH₂CI₂-H₂O). The experimental results show that the substituted nitroarenes were reduced selectively and efficiently to the corresponding aniline derivatives in the presence of viologen polymers. The catalytic active species of viologen were detected by ESR and electrochemical methods. It was found that the viologen cation radical (V⁺) acts as the active species during the viologen-mediated reduction of substituted nitroarenes.     

A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O2 in Water

Using light energy and O₂ for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O₂ by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)₃]²⁺, generating the highly oxidized chromophore and the powerful reductant methyl‐viologen radical MV^+.. MV^+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O₂ to O₂^.? in an aqueous medium to generate an active iron(III)‐(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways.     

Electrochemical method for producing globules of ultrasmall rhodium nanoparticles with poly(N-vinylpyrrolidone) bound to the surface of nanocellulose fibers

The study concerns the preparation of Rh nanoparticles (NPs) by room-temperature, methyl viologen (MV²⁺) mediated electrochemical reduction of RhCl₃ in the presence of poly(N-vinylpyrrolidone) (PVP, 40000 Da) and nanocellulose (NC, d = 57±36 nm) at a controlled potential of generation of MV^·+ radical cations in aqueous medium. When a theoretical amount of electricity is passed, the mediator is not consumed, while Rh^(III) is quantitatively reduced to Rh⁽⁰⁾. The reduction results in the Rh@PVP (141–191 nm in size) and Rh@PVP/NC nanocomposites dispersed in the solution bulk. The Rh@PVP/NC nanocomposite represents PVP globules (42±13 nm in size) with encapsulated ultrasmall spherical Rh NPs (1.3±0.4 nm in size) bound to the surface of NC fibers. The Rh nanocomposites exhibited high catalytic activity in the model reaction, viz., reduction of p-nitrophenol with sodium borohydride in aqueous medium. The catalytic activity increased in the presence of cetyltrimethylammonium chloride (CTAC).

     

Photo-induced electron transfer study of rhenium(I) bipyridyl complexes with covalently linked phenothiazine donor through different bridge

A novel rhenium(I) bipyridyl complex 1a, [(4,4’-di-COOEt-bpy)Re(CO)₃(py-NHCO-PTZ)PF₆] and a model 1b, [(4,4’-di-COOEt-bpy)Re(CO)₃(py-PTZ)PF₆] (bpy is 2, 2’-bipyridine, py-NHCO-PTZ is phenothiazine-(10-carbonyl amide) pyridine and py-PTZ is 10-(4-picolyl) phenothiazine) were synthesized. Their photo-induced electron transfer (ET) reaction with electron acceptor methyl viologen (MV²⁺) in acetonitrile was studied by nanosecond laser flash photolysis at room temperature. Photoexcitation of 1 in the presence of MV²⁺ led to ET from the Re moiety to MV²⁺ generating Re(II) and methyl viologen radical (MV^·+). Then Re(II) was reduced either by the charge recombination with MV^·+ or by intramolecular ET from the attached PTZ, regenerating the photosensitizer Re(I) and forming the PTZ radical at 510 nm. In the case of 1b, the absorption for PTZ radical can be observed distinctly accompanied intermolecular ET, whereas not much difference at 510 nm can be detected for 1a on the time scale of the experiments. This demonstrates that the linking bridge plays a key role on the intramolecular ET in complex 1.     

Electrochemical synthesis of nanocomposite of palladium nanoparticles with polymer viologen-containing nanocapsule

An efficient mediated electrosynthesis of the spherical (85 nm) nanocomposite material Pd@p(MVCA⁸⁺-co-St) was carried out in an aqueous medium. Ultrasmall palladium nanoparticles (3—8 nm) are stabilized in nanocapsules of water-soluble nanoparticles of the copolymer p(MVCA⁸⁺-co-St) consisting of tetraviologen calix[4]resorcinol (MVCA⁸⁺) with styrene (St). The role of the mediator is played by viologen units of a polymer nanoparticle at potentials of the MV²⁺/MV^?+ redox couple. The high catalytic activity of the nanocomposite material in the reduction of nitrophenol with sodium borohydride is shown.     

Spectroelectrochemical and spectrophotochemical properties of N-tetradecyl-N '-ethyl-viologen: Micellar effects

The results of Spectroelectrochemical studies in homogenous solutions have shown that below the cmc value the cation radical of N-tetradecyl-N '-ethyl viologen (TDEV) dimerizes. The TDEV and tetradecyltriethyl-ammonium bromide (TDEA) micelles were found to stabilize the cation radical TDEV^.+ and increase the rate constant for the reaction TDEV+TDEV²⁺ = TDEV^.+ as compared with the results obtained at concentrations below cmc.Based on the spectrophotochemical measurements for TDEV it was found that the quantum yield (Φ) of photoreduction in micellar evironment of TDEA was twice as large as Φ for reactions performed in homogenous solution. Moreover, in micellar solutions photoreduction of TDEV leads to a cation radical of reduced TDEV (TDEV⁺), but in homogenous solution to the dimer of TDEV [TDEV]₂. Therefore, the process of dimerization of TDEV^.+ cation radical is inhibited by micellar catalysis.     

Electron Transfer and Switching in Rigid [2]Rotaxanes Adsorbed on TiO2 Nanoparticles

Bistable [2]rotaxanes have been attached through a bulky tripodal linker to the surface of titanium dioxide nanoparticles and studied by cyclic voltammetry and spectroelectrochemical methods. The axle component in the [2]rotaxane contains two viologen sites, V₁ and V₂, interconnected by a rigid terphenylene bridge. In their parent dication states, V₁²⁺ and V₂²⁺ can both accommodate a crown ether ring, C, but are not equivalent in terms of their affinity towards C and have different electrochemical reduction potentials. The geometry and size of the tripodal linker help to maintain a perpendicular [2]rotaxane orientation at the surface and to avoid unwanted side‐to‐side interactions. When the rigid [2]rotaxane or its corresponding axle are adsorbed on a TiO₂ nanoparticle, viologen V₂²⁺ is reduced at significantly more negative potentials (?0.3 V) than in flexible analogues that contain aliphatic bridges between V₁ and V₂. These overpotentials are analysed in terms of electron‐transfer rates and a donor–bridge–acceptor (D–B–A) formalism, in which D is the doubly reduced viologen, V₁⁰, adjacent to the TiO₂ surface (TiO₂–V₁⁰), B is the terphenylene bridge and A is viologen V₂²⁺. We have also found that, in contrast with earlier findings in solution, no molecular shuttling occurs in rigid [2]rotaxane adsorbed at the surface. The observations were explained by the relative position of the viologen stations within the electrical double layer, screening of V₂²⁺ by the counterions and high capacity of the medium, which reduces the mobility of the crown ether. The results are useful in transposing of solution‐based molecular switches to the interface or in the design and understanding of the properties of systems comprising electroactive and/or interlocked molecules adsorbed at the nanostructured TiO₂ surface.     

Photochemistry of the iron(III) complex with pyruvic acid in aqueous solutions

Photochemistry of the 1: 1 Fe^pIII complex with pyruvic acid (PyrH) in aqueous solutions was studied by stationary photolysis and nanosecond laser flash photolysis with the excitation by the 3rd harmonics of an Nd:YAG laser. The quantum yield of [FeIIIPyr]2+ under the excitation at 355 nm is 1.0±0.1 and 0.46±0.05 in the absence and in the presence of dissolved oxygen, respectively. In experiments on laser flash photolysis, a weak intermediate absorption in the region 580–720 nm was found. The absorption was ascribed to the [FeII…MeC(O)COO•]^p2+ radical complex. Laser flash photolysis of [FePyr]^p2+ in the presence of methyl viologen dications (MV^p2+) resulted in the formation of the MV•+ radical cations. The proposed reaction mechanism includes the inner-sphere electron transfer in the light-excited complex accompanied by the formation of the [Fe^pII…MeC(O)COO•]^p2+ radical complex followed by its transformation into the reaction products.     

Kinetics of chloroplast-mediated photoxidation of diketogluonate

Abstract— Illuminated chloroplasts can mediate a photoxidation of diketogulonic acid (DKGA) with rates of oxygen uptake equivalent to rates of Hill reactions with ferricyanide or quinone. The photoxidation of DKGA is sensitive to dichlorophenyl dimethylurea (DCMU) and exhibits the drop in quantum yield at long wavelengths characteristic of photosystem II. Still, the reaction is only partially inactivated by heating chloroplasts at 50° for 10 min (which destroys oxygen evolution). The photoxidation is inhibited by copper and detergents; and is stimulated by added flavin (or methyl viologen) and manganous ions. A model system containing Mn³⁺ (as manganipyrophosphate) and DKGA, mimics the chloroplast system. Pre-illuminated chloroplast suspensions can be substituted for Mn³⁺ in the model dark reaction. It seems that a light-dependent oxidation of Mn²⁺ to Mn³⁺ by photosystem II is the essential contribution of the chloroplasts. Electrons from Mn²⁺ move through the electron transport system to ferricyanide or to photosystem I where, via flavin (or methyl viologen), oxygen is reduced to H₂O₂.     

Kinetics and mechanism of photolysis of the iron(III) complex with tartaric acid

The formation of MV^•+ radical cations was observed upon the laser flash photolysis of the iron(III) tartrate complex [Fe^IIITart]⁺ (1) in the presence of methyl viologen (MV²⁺). The rate constants of the reactions involving MV^•+ were measured. The intramolecular electron trans-fer to form Fe^II and escape of the organic radical to the solvent bulk upon the photolysis of 1 were proposed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 866–869, May, 2007.     

Reduction of mercury(II) by the carbon dioxide radical anion: a theoretical and experimental investigation

The laser flash photolysis technique (λ(exc) = 266 nm) was used to investigate the mechanism of the HgCl(2) reduction mediated by CO(2)(·-) radicals in the temperature range 291.7-308.0 K. For this purpose, the CO(2)(·-) radicals were generated by scavenging of sulfate radicals by formic acid. The absorbance of the reduced radical of methyl viologen, a competitive scavenger of CO(2)(·-), was monitored at 390 nm. Moreover, theoretical calculations, including solvent effects, were also performed within the framework of the density functional theory for various chemical species of Hg(I) and Hg(II) to aid in the modeling of the reaction of reduction of HgCl(2) by CO(2)(·-).     

Cyclotris(paraquat‐p‐phenylenes)

Reported here is the synthesis, solid‐state characterization, and redox properties of new triangular, threefold symmetric, viologen‐containing macrocycles. Cyclotris(paraquat‐p‐phenylene) ( CTPQT⁶⁺ ) and cyclotris(paraquat‐p‐1,4‐dimethoxyphenylene) ( MCTPQT⁶⁺ ) were prepared and their X‐ray single‐crystal (super)structures reveal intricate three‐dimensional packing. MCTPQT⁶⁺ results in nanometer‐sized channels, in contrast with its parent counterpart CTPQT⁶⁺ which crystallizes as a couple of polymorphs in the form of intercalated assemblies. In the solid state, MCTPQT^3(.+) exhibits stacks between the 1,4‐dimethoxyphenylene and bipyridinium radical cations, providing new opportunities for the manipulation and control of the recognition motif associated with viologen radical cations. These redox‐active cyclophanes demonstrate that geometry‐matching and weak intermolecular interactions are of paramount importance in dictating the formation of their intricate solid‐state superstructures.     

Well‐defined polymers containing high density of pendant viologen groups

Atom transfer radical polymerization (ATRP) of a viologen‐containing methacrylate, 1‐propyl‐1′‐[2‐(methacryloyloxy)ethyl]‐4,4′‐bipyridinium dihexafluorophosphate, is reported. To achieve good polymerization control, it was essential to use the viologen‐based monomer with a hexafluorophosphate instead of halide counterion, and 2,2′‐bipyridine as the ligand for the Cu‐based ATRP catalyst. The solubility of produced cationic polymers could be tuned by anion metathesis: the polymers with hexafluorophosphate counterions were soluble in organic solvents (e.g., acetone, DMF), and those with chloride counterions were water‐soluble. In aqueous solutions, the polymers (chloride salts) formed large aggregates, the sizes of which ranged from about 200 to about 400 nm (based on dynamic light scattering measurements) depending on the molecular weight. Upon addition of electrolytes (e.g., NaCl), the aggregates underwent dissociation. The apparent diffusion coefficients of the aggregates existing in aqueous solutions and the products of their electrolyte‐induced dissociation were measured by diffusion‐ordered NMR spectroscopy. The association–dissociation processes were also studied by fluorescence spectroscopy: the aqueous polymer solutions, which were originally fluorescent (λ _em = 402 nm at λ _ex = 350 nm), lost their fluorescence in the presence of NaCl. The addition of small amounts of the viologen‐containing polyelectrolytes to solutions of inorganic salts (NaCl) altered the crystal morphology of the salts due to interaction of the multiple charged pendant groups with small ions. In the presence of reducing agents, the pendant viologen groups were converted to viologen radical‐cations, which are prone to dimerize reversibly in aqueous solutions. Indeed, marked dimerization of viologen radical cations (with absorbance maxima at 520 and 870 nm) was observed in relatively dilute aqueous solutions (4 mg mL^?1) upon addition of reducing agents (hydrazine). © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 55 , 1173–1182     

Reduction of quinones by alloxantin or alloxan-derived radical

Quinones can be reduced apparently by water mediated by alloxantin (AT) or alloxan radical(A·) produced photochemically from alloxan monohydrate (A-hydrate).     

Catalysis of the reduction of dioxygen by poly(xylylviologen) coatings on graphite electrodes

At graphite electrodes coated with poly(xylylviologen), PXV, the reduction of dioxygen to hydrogen peroxide proceeds at potentials somewhat more positive than those where the reduction occurs at uncoated electrodes. Coated electrodes were used as rotating disks to evaluate the kinetics of the reduction. The data indicate that only one to two monolayers of the PXV coatings participate in the catalysis. Electrons can be delivered from the electrode surface to the catalytically active viologen sites rapidly enough for the catalytic current to be limited by the rate of the cross reaction between viologen radical cations and dioxygen. The rate constant for this heterogeneous reaction at the polymer—electrolyte interface was estimated to be ca. 10⁶M^?1s^?1.     

Photoinduced electron transfer catalysis of titania particles modified with a Ru(2,2′-bipyridyl)3 2+-grafted polymer by visible light

 The preparation and photocatalysis of colloidal titania modified with Ru(2,2′-bipyridyl)₃ ²⁺-grafted polyacrylate were investigated. Visible light irradiation of the Ru(2,2′-bipyridyl)₃ ²⁺-tethered titania in pH 7.0 buffer solution gave electron transfer to methyl viologen to form the cation radical via electron migration from the Ru(II) complex to the titania surface. Received: 16 September 1999 Accepted: 24 November 1999