Viologen Films Research Articles

Substituent-Adjusted Electrochromic Behavior of Symmetric Viologens.

As a promising electrochromic material, viologens have attracted increasing attention due to their high redox activity and adjustable electrochromic capability. In order to investigate the effect of alkyl substituents on electrochromic behavior, four alkyl-substituted viologens and a benzyl-substituted viologen were synthesized, namely 1,1′-dioctyl-4,4′-bipyridinium dibromide (OV), 1,1′-didekyl-4,4′-bipyridinium dibromide (DeV), 1,1′-didodecyl-4,4′-bipyridinium dibromide (DoV), 1,1′-dihexadecyl-4,4′-bipyridinium dibromide (HV), and 1,1′-dibenzyl-4,4′-bipyridinium dibromide (BV). The different photophysical and electrochemical properties of these viologens were attributed to their deviation in spatial structure caused by different substituents. Compared with benzyl-substituted BV, a slight blueshift occurred for the absorption peaks of alkyl-substituted viologens from 262 to 257 nm with the increase in alkyl chain length. Moreover, the first redox couple increased positively, and the dimerization of the compound decreased gradually, accompanied by the decrease in optical contrast and distinct chromatic difference. A comparison of chromatic and optical contrasts indicated that OV had the longest coloring response time (RTc), while it was shortest for HV. The bleaching response time (RTb) of viologen films gradually decreased with the alkyl chain length, and the OV film had the shortest RTb. Furthermore, when increasing the length of the alkyl chain, the cycling stabilities of alkyl viologens increased gradually. In addition, the OV film exhibited the best contrast after 200 continuous cycles.

Octadecyl-viologen Photooxidation in Surface Films: Macroscopic Contraction of Langmuir Monolayer by UV Irradiation.

The effects of UV radiation on a viologen derivative, octadecylviologen (OV), in Langmuir monolayers at the air-aqueous solution interface and in Langmuir-Blodgett (LB) films have been investigated. Langmuir monolayers suffer a sharp contraction after UV irradiation, clearly visible by the drop in surface pressure or the loss of surface area observed in the surface pressure-area isotherms. The UV-vis reflection measurements reveal a deep change in the OV monolayer caused by a photochemical reaction, which suggests the pyridones formation as photoreaction products. LB films (Z type), before and after being irradiated with UV light, have been studied by using UV-vis absorption and infrared and X-ray photoelectron spectroscopy. The results confirm that after the photodegradation of the viologen films, the presence of oxygen results in the appearance of pyridones as reaction products. This article demonstrates that, in the absence of catalysts, the photooxidation of viologen surface films occurs only under a particular molecular organization imposed by the air-water interface.

Electrochemical synthesis and characterization of branched viologen derivatives

In this work two three-branched cyanopyridine based monomers were synthesized and used as starting materials for the preparation of viologen derivatives with branched structures. The film synthesis was performed using reductive electropolymerization in aqueous solution. Both electrochemical and spectroelectrochemical characterization proved that the viologen films undergo a well-defined and reversible two step redox reaction, which is the typical performance for viologen materials. Furthermore, FTIR and SEM were utilized to characterize their structures and morphologies. SEM studies showed that the viologen films have a highly porous structure. Additionally, discharging experiments confirmed that the viologen films show different intercalation behavior as size of electrolyte anion had various effects on the redox potential. Finally, it has been proven that these two viologen derivative materials have the potential to be utilized as a supporting material with intrinsic redox activity and with excellent properties for hosting especially negatively charged particles, molecules and macromolecules. The promising properties of these materials make them good candidates in electronics and also in organic solar cells when loaded with large molecules such as fullerenes. In the organic rechargeable battery applications they can be used as anode-active materials as they exhibit high charging-discharging capacity at negative potentials.

Preparation, Characterization of Styrene-Functional Hyperbranched Viologen, and Its Application in Optical Writing and Electrical Erasing Device

Styrene-functional hyperbranched viologen (SHBV2+) was successfully synthesized and characterized in detail by FTIR, 1H NMR, and XPS spectra, respectively. Subsequently, it was cured by visible light to obtain cross-linked hyperbranched viologen film. It is noteworthy that cross-linked SHBV2+ film possessed better photochromic and electrochromic performances. After UV illumination for 60 s, the cross-linked SHBV2+ film can swiftly change its color from pale yellow to deep bluish violet, whereas the absorbance of SHBV2+ film at 610 nm did not change significantly and was still retained about 0.423 after 20 coloring/bleaching cycles. The single-type SHBV2+ EC device can undergo reversible color change in response to the alternate external voltages of −2.0 V and 2.0 V, respectively, while the changes in optical transmission and charge density of EC device at 550 nm did not deteriorate significantly and still remained about 48.2% and 13.85 mC·cm−2 after 10 coloring/bleaching cycles. Furthermore, single-type SHBV2+ PEC device possessed better optical writing and electrical erasing performances. After 10 repeated writing/erasing cycles, the contrast did not decrease significantly and still remained about 48.0%, while the electrical erasing time did not deteriorate significantly and remained about 22.0 s. This device can realize rapid storage, modulation, and erasing of optical information.

Immobilized Viologen Polymers for Carbohydrate Fuel Cells

Viologen, organic catalyst, must be immobilized on the anode of a direct-carbohydrate fuel cell to achieve a recyclable and efficient fuel cell. Novel viologen monomers, called 1-(3-acrylamidopropyl)-(4, 4'-bipyridin)-1-ium (AAPB), are synthesized and immobilized on a gold electrode surface by electroreductive polymerizaiton. The polymeric viologen film is examined and visibly seen under SEM. Cyclic voltammograms of these monomers have been run and the results are reported here.

Electrochemical synthesis and characterization of copolyviologen films

Two cyanopyridine-based monomers were synthesized and utilized for the cathodic electropolymerization of polyviologen (BPV) and copolyviologen (CoPV) films onto glassy carbon (GC) electrodes. One of the monomer material is branched (BCP) whereas the other one has a linear structure (LCP) the former bearing three the latter two polymerizable cyanopyridinium groups. The electrochemical synthesis of the polyviologen and copolyviologen films were made in aqueous solutions and the film properties were characterized by electrochemical and spectroscopic techniques. Atomic force microscopy was used in order to study the surface morphologies of the films. The BPV and CoPV films undergo a two-step reduction reaction and show redox properties typical for viologen derivatives in monomer free electrolyte solutions. In comparison to the polyviologen film, electrochemically synthesized from the branched monomer (BCP), the redox property of the copolymer was changed. The results from in situ UV–vis spectra show that the copolyviologen film has good stability and undergoes reversible electron transfer reactions successfully. The structure of the copolyviologen film was characterized by FTIR spectroscopy, confirming successful polymerization of both monomer materials. The copolymerization therefore opens up a direct electrochemical method to form viologen films with large cavities. This feature gives a potential to apply the partly conjugated copolyviologen film with redox properties as a conducting host material for immobilization of macromolecules.

Photocurrents from the Direct Irradiation of a Donor–Acceptor Complex Contained in a Thin Film on Indium Tin Oxide

We describe the performance of a photoelectrochemical cell based on an electron donor–acceptor (EDA) complex formed by the association of tetraphenylborate anion with a viologen dication siloxane film immobilized on an indium tin oxide (ITO) electrode. Visible irradiation of a weak absorbance band of the EDA complex leads to simultaneous photoreduction of the viologen and photooxidation of the tetraphenylborate, forming a viologen monocation radical and tetraphenylborate radical. Spontaneous decomposition of the latter to products incapable of further reaction inhibits reverse electron transfer, permitting harvesting of the redox equivalents stored as the viologen monocation radical at the electrode. Results of statistically designed two-level factorial experiments indicate that the photocurrent increases with light intensity (L) and viologen coverage (P), but decreases with viologen siloxane film age (A). Although the solution BPh4– concentration (F) does not significantly affect the photocurrent response under our experimental conditions, the nature of the electrolyte anion (C) does. Although significant interactions among the L, P, A, and C variables exist, our analyses provide a model that (1) describes the viologen film aging; (2) adequately predicts cell photocurrent responses as functions of levels of the L, P, and, to a somewhat lesser extent, A variables; and (3) identifies areas for improvement and optimization of cell performance.

Electrical energy from carbohydrate oxidation during viologen-catalyzed O2-oxidation: Mechanistic insights

Catalytic oxidation of carbohydrates by oxygen using various viologens under mild conditions was recently shown to occur at high efficiency with formate and carbonate formed as products. However, high efficiency was only achieved at viologen/carbohydrate ratios >10. The use of such high catalyst ratios is undesirable and a mechanistic study was initiated to better understand how catalytic efficiency is influenced by catalyst ratio. The results suggest that the dipositive viologen reacts with the enediol form of the carbohydrate, initiating carbohydrate oxidation with subsequent reduction of the viologen to the radical cation form. The latter is then oxidized by air or by current collecting electrodes when the reaction is conducted in a fuel cell. If the viologen/carbohydrate ratio is low, electron transfer from the carbohydrate to the oxidized form of the viologen becomes limiting and the carbohydrate undergoing oxidation rearranges into unreactive intermediates such as carboxylic acids, and alcohols. However, at high ratios, excess viologen more efficiently oxidizes the carbohydrate and minimizes formation of unreactive intermediates. Viologen polymers were more efficient than an equivalent concentration of monomers, suggesting that the higher localized concentration in polymeric viologen acts to efficiently oxidize carbohydrates and simulates high viologen/carbohydrate ratios. The use of viologen films on electrodes or conducting viologen polymers should facilitate use of carbohydrates for electricity production in fuel cells.

Fast Nolinear Optical Mechanisms in Bi-Layered Cells Composed by Lyotropic Ionic Liquid Crystals with Dye and Viologen Films

Mechanisms of the fast optical nonlinearity are studied in two types of cells based on lyotropic ionic liquid crystals (LILC) of metal alkanoates. They are (I) bi-layer cell, which consists of a thin dye film covered by LILC, and (II) LILC with electrochromic impurity (viologen). Applying dc electric voltage to the cell II leads to adsorption of viologen redox products with formation of a thin nanosized layer on the cathode. The dynamic holographic technique with pulsed laser radiation of the nanosecond duration was used to investigate nonlinear optical properties in both types of cells. The mechanism of photoconversion of viologen redox products under strong laser radiation is proposed to explain grating recording in the cell II.

Surface redox chemistry of adsorbed viologens on Cu(100)

The surface redox-chemistry of adsorbed viologens is studied by means of cyclic voltammetry (CV) in combination with in situ scanning tunneling microscopy (STM). 1,1′-Dibenzyl-4,4′-bipyridinium molecules (DBV2+) adsorb on a chloride modified Cu(100) electrode surface under formation of a laterally well ordered 2D array of supramolecular cavitand ensembles. Each cavitand consists at least of 4 individual DBV2+ sub-units which are arranged in a certain circular manner making this supramolecular cavitand chiral. Both possible enantiomeric forms are found in two mirror domains at the surface. Reducing the di-cationic DBV2+(ads) species to the corresponding radical mono-cation DBV•+(ads) causes a phase transition from the pre-existing DBV2+(ads) cavitand phase to a stripe pattern following a nucleation and growth mechanism. DBV•+(ads) species are adsorbed with their main molecular axis parallel to the surface in a side-on adsorption geometry. Enhanced intermolecular π–π-interactions are identified as the main driving force for the formation of 1D oligomer and polymer chains as the characteristic structural motif of the DBV•+(ads) phase. These structural motifs are generally independent of the electronic and structural substrate properties. Chloride desorption through the viologen film is discussed as the reason for an order–disorder transition within the viologen film at even more negative potentials.

Photoinduced and Thermal-Activated Doping of Polyaniline

The reaction of 50% oxidized polyaniline (emeraldine base, EB) with alkyl halide and viologen in the dry state was studied using X-ray photoelectron spectroscopy, UV−visible absorption spectroscopy, and sheet resistance measurements. To facilitate this study, a low-density polyethylene substrate was first surface graft copolymerized with vinylbenzyl chloride or viologen moieties. A coating of polyaniline (PANI) was then deposited on the top of the film. Under UV irradiation or heat treatment at 65 °C or higher, the PANI reacted with the vinylbenzyl chloride and/or viologen, resulting in its conversion from the insulating base state to the conducting state. The conversion was more readily achieved with viologen, and a significant portion of the imine nitrogen of PANI was converted to positively charged nitrogen. The photoinduced method was more effective than the thermal-activated process in achieving a conducting PANI film. The most conductive film was obtained by irradiating the PANI−viologen film for 30 min or more using a 1 kW Hg lamp, and the conductivity achievable was about 1 order of magnitude lower than that of the conventional acid-protonated PANI film.

Reversible color changes induced by photosensitized charge separation in partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complex and viologen films

The partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complex and viologen films (RuQPImC n VC 16 films: n=4 and 6) have been prepared, and the reversible color change reaction in these films has been investigated. The metallopolymers consist of bis(2,2′-bipyridine)ruthenium(II) complex coordinated with two imidazolyl residues and the viologen having a long alkyl group linked covalently to the imidazolyl residue through the alkyl spacer on the partially quaternized poly(1-vinylimidazole). Upon visible light irradiation ( λ>440 nm) of these metallopolymer films in air, the color changed from orange to blue, and reverted back to orange upon standing in the dark. The color change reaction was reversible with the responses being more rapid from the orange to blue relative to the opposite cycle. UV–vis absorption spectra showed that the blue color arose from the viologen radicals, and the absorption spectra of the ruthenium(II) complex residue did not change during light irradiation, indicating that the viologen radicals were generated through the photosensitized charge separation reaction. FTIR and UV spectra suggested that the adsorbed water in these polymer films through hydrogen bonding and electrostatic attraction acted as an electron donor for the ruthenium(III) complex residue, and the quaternized imidazolium residues acted as the scavenger molecules for one-electron oxidized water.

Combined Spectroscopic Ellipsometry and Voltammetry of Tetradecylmethyl Viologen Films on Gold

Dynamic measurements of changes in composition and conformation during adsorption oftetradecylmethyl viologen (C 14 MV) on a gold electrode were made using spectroscopic ellipsometry. Film thickness measurements indicated that a monolayer is initially formed in which the viologen species is in the upright extended position and that conversion to a tilted conformation occurs during equilibration to the final monolayer film. Subsequent reduction of the film as electrode potential was scanned from -0.2 to -0.9 V resulted in linear growth to a multilayer film in which dications (C 14 MV 2+ ) were reduced to radical (C 14 MV .+ ) and neutral (C 14 MV 0 ) species. A gradual increase of the film charge-to-thickness ratio and of the film index of refraction during reduction indicated densification of the deposit. The pattern in the variation of film extinction coefficient with electrode potential indicated that radical dimers were the dominant species in the potential range corresponding to the first reduction step. During the reverse potential scan, the radical film detached due to faster reoxidation of the layers adjacent to the electrode surface.

Redox photochromism of viologen in organized solid state

N,N′-Diphenylbipyridinium(viologen) having alkylbenzene sulfonate with long alkyl chains (C 6-C 15) was synthesized by three reaction sequences. The viologens with alkyl chains of C 10-C 15 exhibit a liquid crystal phase and display smectic-A type. Upon irradiation with near-UV light, viologen films change from pale yellow to blue-green, whose absorption maxima shift remarkably to red and the shapes differ with the length of the alkyl chain of the counter anions. The colour induced by light is maintained above 95% at 30° C and can be bleached at higher temperature. This has made possible, for the first time, the colour development by light and bleaching by heat of transparent anisotropic thin films.

The electrochemical properties of oligomeric viologen films immobilised by polyacids

A polyelectrolyte salt, which is insoluble in water, was obtained by exchanging the bromide counterions with high molecular weight polymeric anions (poly(p-styrenesulphonic) and polyacrylic) in the polycondensate of 1,4-dibromobutane and 4,4′-dipyridine. Films of this redox active polymer show good oxidation-reduction reversibility, retained even after 2000 cycles. The electrochemical accessibility of the redox centres depends on the thickness of the polymer films and the composition of the solution. In the thin films ( < 1 μm) nearly 100% of the redox centres are active. Atomic force microscopy studies show that these films change their volume during redox cycling due to cations and solvent movement.

“In-situ” Quartz Crystal Microbalance Responses and Electrochemistry Regarding Langmuir–Blodgett Film of Viologen Polyion Complex on the Electrode Surface

Abstract The electrochemical behavior and the response of the quartz crystal microbalance(QCM) for the Langmuir–Blodgett(LB) viologen film have been examined. LB monolayers employed are prepared by electrostatically fixing the viologen monolayer containing the alkyl chain moiety on the poly(potassium 1-sulfatoethylene). The number of monolayers accumulated on the polycrystalline Au electrode of a piezoelectric quartz crystal was controlled by changing the general vertical dipping times. The QCM frequency response for the first reduction scan of viologen dication (V2+) to monocation radical (V+•) shows extremely large frequency increase, qualitatively indicative of mass loss, viscoelasticity decrease, or mechanical stress decrease during the reduction. A frequency increase during the second reduction scan is also observed, although not as large as for the first scan. The frequencies increase continuously with increasing number of scans and the steady-state responses can only be obtained after many repetitive potential scans. The results of the conductance spectra measurements of the quartz crystal/LB film composite resonator are interpreted as break-in and/or an increase of aggregation of the orientated structure during potential scans. The cyclic voltammetric wave obtained in the potential range 0 to −0.7 V vs. sodium chloride saturated calomel electrode has the characteristics of a simple, one electron transfer between V2+ and V+•, being chemically reversible process. The subsequent cyclic voltammogram responses obtained after the first cycle of potential scan are nearly identical for some hours, in contrast to the responses of the QCM. It is shown that structural change of the organized film dose not have a significant influence on the shape of the electrochemical response but does have an effect on the frequency response of the QCM.

Voltammetric, chronoamperometric and chronoabsorptometric studies of the nucleation of n-heptyl viologen films on SnO2, silane-modified SnO2 and ion-beam-treated ito-metallized polymer films

Nucleation of the first monolayers of the n-heptyl viologen-bromide or-biphthalate films on SnO 2 electrodes was found to be accelerated by the addition of one molecular layer of dimethyldiethoxy (dmde)-silane to the SnO 2 surface. Partitioning of the reactant viologen into the non-polar surface layer and lowering of the surface tension against nucleation site growth in the silane layer may both be responsible for the enhanced nucleation rates. Similar acceleration of the nucleation of n-HV + was found on ion-beam-etched indium-tin oxide-metallized polymer electrodes (MPOTE). A second nucleation site with a much lower, and distinguishable rate of nucleation, was produced on this type of electrode surface. Chronoamperometry and chronoabsorptometry were used to confirm that the nucleation of n-heptyl viologen cation-radical salts follows an instantaneous rather than progressive nucleation mechanism, on any of the surfaces studied.

Voltammetric, chronoamperometric and chronoabsorptometric studies of the nucleation of n-heptyl viologen films on SnO 2, silane-modified SnO 2 and ion-beam-treated ito-metallized polymer films

Nucleation of the first monolayers of the n-heptyl viologen-bromide or-biphthalate films on SnO 2 electrodes was found to be accelerated by the addition of one molecular layer of dimethyldiethoxy (dmde)-silane to the SnO 2 surface. Partitioning of the reactant viologen into the non-polar surface layer and lowering of the surface tension against nucleation site growth in the silane layer may both be responsible for the enhanced nucleation rates. Similar acceleration of the nucleation of n-HV + was found on ion-beam-etched indium-tin oxide-metallized polymer electrodes (MPOTE). A second nucleation site with a much lower, and distinguishable rate of nucleation, was produced on this type of electrode surface. Chronoamperometry and chronoabsorptometry were used to confirm that the nucleation of n-heptyl viologen cation-radical salts follows an instantaneous rather than progressive nucleation mechanism, on any of the surfaces studied.

Solid-state Properties of Evaporated Methyl Viologen

Abstract The methyl viologen film evaporated at about 250 °C has a violet color, and its electronic absorption spectrum resembles that of the enzymatically reduced methyl viologen. The evaporated film becomes colorless upon the introduction of air. By means of the photoemission spectroscopical method, a very low ionization potential, 3.6 eV, was obtained for the evaporated methyl viologen film. The character of the evaporated methyl viologen is a free-radical one, and its specific resistivity is very low as compared with other organic radicals.