Alkyl Viologen Research Articles

Voltammetric and spectroscopic study of the adsorption of alkyl viologens on a HOPG electrode

Adsorption states of N,N′-dialkyl-4,4′-bipyridinium ions (viologens) and their cation radicals on a highly oriented pyrolytic graphite (HOPG) electrode surface were investigated by cyclic voltammetry and in situ infrared reflection-absorption spectroscopy (IRAS). A pair of spike-like anodic and cathodic waves was observed in the potential region well positive of the first reduction wave for the solution species. The peak potentials were found to depend on their alkyl chain length, and were shifted in the positive direction with the increase in the number of carbon atoms in the alkyl chains. Each pair is ascribed to a one-electron faradaic process of adsorbed viologens. It was indicated from IRAS measurements that the adsorbed cation radicals oriented with their longer molecular axis parallel to the HOPG surface, while the bipyridine ring planes were perpendicular to the surface (side-on configuration).

Spin pairing (‘dimerisation’) of the viologen radical cation: kinetics and equilibria

Viologen radical cations dimerise (spin pair) in aqueous solution, causing the colour of the viologen dye to alternate between blue (monomer) and dimer (red). Equilibrium constants K of viologen dimerisation have been determined using UV–vis spectroscopy for alkyl viologen radical-cation species in halide solutions. Equilibrium constants of dimerisation were obtained by deconvolution of optical spectra. Values of ΔG°dim have a crude linear dependence on n, the length of the alkyl substituent chain, for n=2 to 8. Rate constants of spin-pairing kdim (i.e. dimerisation of the radical cation) have been determined. Bjerrum plots indicate that the rate-limiting reaction during dimerisation involves anions: it is postulated that the dimerisation reaction is two-step, with dimer formation occurring after the formation of an association pair comprising a radical cation and a counter anion. Recent comproportionation results are used to analyse the structure of the dimer and the role of anions during dimerisation. The possibility that dimer comprises anions placed between the planes of the two bipyridilium radicals is also discussed.

Intra-polymer photosensitized charge separation using a series of ruthenium(II) complex and viologen-containing polymers

Spectroscopic and thermal properties of a series of ruthenium(II) complex and viologen-containing polymers based on poly( 1-vinylimidazole) and partially quaternized poly( 1-vinylimidazole) and photosensitized charge separation using these polymers have been investigated. These polymers have the structures in which bis(2,2-bipyridine)rutneuium(II) complexes are coordinated with the imidazolyl residues and the alkyl viologen is covalently linked to the imidazolyl residue through a hexylene spacer on the same polymer backbone. These polymers have two maximum wavelengths in the luminescence spectra. 560 nm and 65() nm, which correspond to emissions of the ruthenium( II) complex residue compartmentalized by viologen residues and the alkyl side chains exposed to the bulk solution, respectively. The luminescence intensity at 560 nm decreases with decreasing viologen content. For photosensitized charge separation, the rates of viologen radical formation depend linearly on the initial concentration of the ruthenium(II) complex and viologen residue, indicating that the reactions proceed through an intra-polymer process.

Enhanced cytotoxicity of alkyl viologens and N,N'-diamino analogs toward cultured murine leukemia L1210 cells under vortex-stirring with a high molecular weight polyacrylic acid.

Alkyl viologens showed cytotoxicity when incubated with cultured murine leukemia L1210 cells for 48 h, whereas they were not cytotoxic when briefly incubated for 10 min. Under the permeabilizing conditions achieved by vortex-stirring with a high molecular weight polyacrylic acid (A-119), appreciable cytotoxicity was shown even after a 10-min exposure to any of the alkyl and amino viologens examined. Acetylamino viologen showed no cytotoxicity regardless of the presence or absence of A-119. This permeabilizing procedure was demonstrated to be applicable to internalize water-soluble positively charged viologen molecules into the cell.

Ultrathin self-assembled polymer films on solid surfaces 4. Electrochemical analysis of film microstructure on gold electrodes

Bound monolayer films of alkyldisulfide-grafted polyacrylates on gold electrodes were studied via electrochemical cyclic voltammetric methods. Ferricyanide, ruthenium hexamine, and alkyl viologen redox probes in aqueous electrolyte were used to reveal the response of coated ultrathin films on electrodes to different probe molecules, film defects, and surface coverages. Surface microstructure of these films assembled on gold electrodes probed by cyclic voltammetry (CV) produced electron-transfer blocking efficiencies very dependent upon the redox probe utilized. Current—voltage profiles using ferricyanide indicated efficient blocking of the electrochemistry mediated by aqueous redox probes while films probed with ruthenium hexamine exhibited a leakier structure. Poor redox blocking efficiencies were observed on these same coated electrodes using methyl and alkyl viologen redox probes, exhibiting a current spike corresponding to surface adsorption of reduced viologen species, indicating incomplete electrode surface coverage by the polymer monolayer films. Morphological changes in coated films upon exposure to organic solvent at various temperatures were also examined using CV analysis. Despite structural defects resulting from relatively loose-packed polymer film architecture, ellipsometric and X-ray photoelectron spectroscopy measurements indicate that a molecular or microscopic rearrangement of film components, rather than partial removal of coated films, is responsible for the observed changes in barrier properties.

Solubility, diffusion coefficient and ionic conductivity of alkyl viologens in poly(ethylene oxide) and its derivatives

A series of alkyl viologens RV (R denotes ethyl, butyl, hexyl, heptyl, and dodecyl) was dissolved in poly(ethylene oxide) (PEO) oligomers (average molar masses of 200, 300, 400, 600 and 1000 g mol −1). The solubility of RV in PEO oligomers decreased with increasing alkyl chain length of RV and the molar mass of PEO. Cyclic voltammograms of RV in PEO containing 0.50 M LiClO 4 clearly show two redox waves. The ionic conductivity of PEO oligomers containing RV decreased with increasing alkyl chain length, suggesting the migration of RV itself in the PEO oligomers. Potential step chronoamperometry was used to obtain the apparent diffusion coefficient of RV in the PEO oligomers. The ionic conductivity has a linear relationship with the apparent diffusion coefficient regardless of the RV species, the PEO molar mass and the temperature. RV was shown to act as a redox mediator in PEO oligomers as long as the ionic conductivity of the PEO was high. Poly(oligo(oxyethylene) methacrylate) (PMEO) was used as a solid solvent for a series of alkyl viologens. Since PMEO is an excellent ion-conducting polymer, RV was confirmed to be an effective redox mediator in this PMEO. It was concluded in this study that ionic conductivity in the polymer matrix could be used as an effective parameter for prediction of the diffusion coefficient of charged organic molecules.

An electron spin resonance (ESR) and simultaneous electrochemical and electron spin resonance (SEESR) spectroscopic study of motion, stability and potential controlled release of radical guests from the β-cyclodextrin inclusion polymer

The influence of molecular inclusion and separation of radical guests inside the amorphous β-cyclodextrin host polymer (β-CDP) matrices on the motion and stability as well as controlled potential release of radicals was studied by electron spin resonance spectroscopy (ESR) and simultaneous electrochemistry and electron spin resonance (SEESR) spectroscopy. A pronounced restriction of rotational motion was observed for the included stable protonated form of the 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) free radical and a partial restriction of motion of the NN, N', N'-tetramethyl-1,4-phenylenediamine (TMPD .+) radical (generated ex situ by controlled potential electrolysis), while virtually no restriction was found in the case of the methyl (MV .+) and heptyl viologen (HV .+) monocation radicals as well as of the 2-nitrotoluene anion radical (2NT .− ). The MV .+, HV .+ and 2NT .− unstable radicals were electrochemically generated inside the β-CDP film coat at a Pt flag electrode. The rate of the open-circuit decay of the included unstable radicals was markedly decreased as compared with their decay at the bare electrode. It was also found that the extent of inclusion of alkyl viologens was governed by their ionic charge, i.e. the higher the charge of the ion the weaker its inclusion.

Photoreduction of Alkyl Polyviologens and Their Low Molecular Analogs in PVA Matrix

Abstract The photoreduction behavior of alkyl viologen compounds in a PVA matrix was investigated. The initial photoreduction of alkyl viologens and related polyviologens follows second-order reaction kinetics, and the respective rate constants, which vary only slightly with alkyl chain length, were determined. While the polymer effect was not remarkable in a PVA matrix, the association behavior of radical cations was similar to that found in a 2-propanol aqueous solution.

Electrochromic properties of alkylviologen-cyclodextrin systems

Electrochromic properties of methyl, ethyl, n-hexyl and n-heptyl viologens, and α-, β-, γ-cyclodextrin systems were studied using cyclic voltammetry and absorption spectroscopy. Colourations of red-violet (λ max = 530 or 560 nm), blue (λ max = 600nm) and light purple (λ max = 560 and 600 nm) could be obtained by changing the length of the alkyl chain of viologens or the radius of cyclodextrin. The intermolecular interaction between viologen radicals is weakened when the radicals form inclusion complexes so the blue colour inherent in the monomer of viologen radical appears. The red-violet colour is ascribed to the dimer of the reduced viologen. With the addition of γ-cyclodextrin, viologens were found to be included as dimer. The light purple colour is ascribed to the mixed colour of the monomer and the dimer. Furthermore, since the ratio of monomer and dimer depends upon the concentration of cyclodextrin, any colour between the red-violet of the dimer and the blue of the monomer can be obtained by adjusting the concentration of cyclodextrin. The redox reaction of an inclusion complex was found to depend upon the length of the alkyl viologen and the size of the cavity in cyclodextrin.

Photoreduction of alkyl viologens by zinc tetraphenylporphyrin in organic solvent-water systems

Light-induced redox reactions between zinc meso-tetraphenylporphyrin (ZnTPP) and alkyl viologens (C n V 2+) in organic solvent-surfactant-water solutions have been investigated as functions of (i) organic solvent content of solutions, (ii) surfactant charge, and (iii) alkyl viologen hydrophobicity. The best system appears to be an oil-in-water emulsion containing 5 × 10 −5 M ZnTPP and 10 −3 M C 10V 2+ stabilized by cationic surfactants (hexadecyltri-methylammonium bromide or chloride) with 6% N, N'-dimethylformamide (DMF), N, N'-dimethylacetamide (DMAC) or dimethyl sulfoxide (DMSO) in water.

Charge transfer between two immiscible electrolyte solutions Transfer of tris(2,2'-bipyridine)ruthenium(ii) and alkyl viologen dications across the water/nitrobenzene, water/dichloromethane and water/dichloroethane interfaces

Charge transfer between two immiscible electrolyte solutions Transfer of tris(2,2'-bipyridine)ruthenium(ii) and alkyl viologen dications across the water/nitrobenzene, water/dichloromethane and water/dichloroethane interfaces

Charge transfer between two immiscible electrolyte solutions: Transfer of tris(2,2′-bipyridine)ruthenium(II) and alkyl viologen dications across the water/nitrobenzene, water/dichloromethane and water/dichloroethane interfaces

Charge transfer between two immiscible electrolyte solutions: Transfer of tris(2,2′-bipyridine)ruthenium(II) and alkyl viologen dications across the water/nitrobenzene, water/dichloromethane and water/dichloroethane interfaces