José Carrasco

Faradaic impedance behaviour of oxidized and reduced poly (2,5-di-(-2-thienyl)-thiophene) films

Abstract Poly(2,5-di-(2-thienyl)-thiophene) films with thickness less than 1.32 μm were electrogenerated on Pt from 5 mM and 20 mM monomer concentrations in 0.1 M LiClO 4 + acetonitrile at constant current. Cyclic voltammograms recorded between 0 and 1.10 V vs. SSCE in the electrolyte showed a redox couple associated to the reduced and oxidized states. The polymer retains its reduced form until 0.6 V, its oxidized form from 0.8 V and a mixture of both states in the potential region 0.6 to 0.8 V. Faradaic impedance measurements for both states were performed at a constant potential between 0.3 and 1.0 V after reducing the electrogenerated polymer at 0 V. A progressive decrease in impedance was always found with increasing the film thickness and more positive potential. An equivalent circuit involving two complex elements is proposed to simulate adequately the Bode plots. Nyquist diagrams and complex plane admittance diagrams obtained. The film resistance is very high for all polymers, suggesting that the mass transport of ClO 4 − ions through the polymer is the rate-determining of the oxidation-doping process. The film capacitance is low for the reduced form and gradually increases when the polymer is oxidized. Deposits obtained from a 20 mM monomer solutions have higher charge-storage capacity than those generated from a 5 mM one, particularly for the oxidized polymer where large amounts of ClO 4 − counterions are accumulated.

Oxidized and reduced poly(2,5-di-(-2-thienyl)-pyrrole): solubilities, electrodissolution and molar mass

Abstract Oxidized poly(2,5-di-(-2-thienyl)-pyrrole was synthesized electrochemically from a 5 to 20 mM monomeric solution in 0.1 M LiClO 4 + acetonitrile (ACN). The electrochemical behaviour of films was studied in 0.1 M LiClO 4 aqueous solution, where an insoluble reduced form was found. Solubilities of both oxidized and reduced states were determined in DMSO, acetone and ACN. Their densities and electrical conductivities were also obtained. The solubility of the reduced form is about 15 times higher in DMSO and ACN than that of the oxidized form. The oxidized polymer is insoluble in ACN solutions of perchlorate salts. The high solubility of the reduced state in this medium allows a quantitative electrodissolution of the polymer following the reduction process. Results from electrochemical experiments, ultramicrogravimetry and elemental analysis indicate an average presence of 0.65 ClO 4 − ions per monomeric unit incorporated in the oxidized polymer. Both states contain soluble oligomers with from two to six monomeric units, as detected by mass spectrometry (fast atom bombardment). The molecular mass of these oligomers indicates a loss of two protons per monomer incorporated in the polymer, in agreement with an electropolymerization mechanism involving polycondensation of radicals.

Electropolymerization of 2,5-di-(-2-thienyl)-pyrrole in ethanolic medium. Effect of solution stirring on doping with perchlorate and chloride ions

Abstract The electrochemical behavior of 2,5-di-(-2-thienyl)-pyrrole (SNS) on Pt has been studied from a 10 mM monomer solution in 0.2 M LiClO 4 +ethanol or in 0.2 M LiCl+ethanol by cyclic voltammetry, chronopotentiometry and chronoamperometry. The monomer exhibits two similar consecutive oxidation processes. Uniform, adherent and electroactive films of dark-blue poly(SNS) doped with ClO 4 − or with Cl − are obtained at low potentials related to the first process. Reproducible film weights are found at 0.700 V versus Ag|AgCl during 360 s. The increase in transport rate of reactants by stirring the solution with a magnetic bar at 150 rpm accelerates the SNS electropolymerization, allowing to collect much more polymer weight than from the quiescent solution. The productivity of poly(SNS) doped with Cl − determined by ‘ex situ’ ultramicrogravimetry increases notably under stirring, although its percentage in Cl − is similar to that found under quiescent conditions. This is ascribed to the production of a major proportion of longer linear molecules in polymer, consistent with its higher conductivity when it is synthesized under stirring. This effect is not so clear for the poly(SNS) doped with ClO 4 − due to the little influence of stirring upon its productivity and conductivity. The detection of short linear oligomers in the soluble fractions of polymers in thioglycerol by mass spectrometry-fast atom bombardment allows to propose a radical polycondensation as initial electropolymerization mechanism.

Electrochemical oxidation of 2,5-di-(-2-thienyl)-pyrrole in acetonitrile: cathodic stripping of the electrogenerated conducting polymer

Abstract The electrochemical behaviour of 2,5-di-(-2-thienyl)-pyrrole was studied by cyclic voltammetry, chronoamperometry and chronopotentiometry. The monomer oxidation—polymerization takes place on platinum at potentials more positive than 0.3 V vs SCE. At higher potentials, the oxidation processes in the polymer layer seem to control the generation of the polymer and the polymer properties. Polymer films electrogenerated at potentials lower than 0.7 V are insoluble in LiClO 4 + acetonitrile solutions. When the polymer film is reduced electrochemically, it becomes soluble. The solubility is a faradaic process: the weight of electrochemically solubilized polymer is proportional to the cathodic electrical charge consumed. The linearity of the process was proved using different cathodic current densities. The polymer oxidation at potentials more positive than 1 V promotes the insolubility of the polymer, probably by a crosslinking process. The reduction of the polymer at potentials below 0.2 V promotes stripping of the polymer from the electrode. The polymer structure was studied by Fourier transform IR reflectance. The UV—Vis study of the oxidized and reduced polymer in solution led to the band structure and showed the self-reduction of the dissolved polymer.

Electrical and Electrochemical Properties of Reduced, Oxidized, and Overoxidized Poly[2,5-di(-2-thienyl)-pyrrole] Films in Water/Acetonitrile Mixture

Poly[2,5-di(-2-thienyl)-pyrrole] [poly(SNS)] films up to 1.29 μm thick were anodically synthesized from a 5 mM monomer and 0.1 M LiClO 4 + acetonitrile solution at 0.50 mA cm -2 . These deposits with a large proportion of electroactive polarons compensated with ClO 4 - dissolve upon reduction in this medium. Insoluble reduced films were synthesized in a water + acetonitrile (3:2 v/v) mixture with 0.1 M LiClO 4 at -200 mV, having a small proportion of electroinactive charges trapped in polymer. Their cyclic voltammograms display an initial redox pair related to the reversible doping/undoping process with electroactive polarons, which can be continuously cycled from -200 to 340 mV. A further oxidation peak involves generation of bipolarons, while two subsequent overoxidation processes lead to polymer reticulation, without charge accumulation. Reduced films are then progressively oxidized from ca. 150 to ca. 600 mV and overoxidized between 660 and 1400 mV. The electrical conductivity rises upon oxidation, remaining almost constant during overoxidation. The impedance of reduced and oxidized forms decays with increasing film thickness and potential although upon overoxidation, it increases as potential is shifted in the positive direction. All impedance measurements can he adequately simulated by a simple equivalent circuit. The calculated film capacitance for reduced and overoxidized forms is very low, and increasing values are found only as the polymer is gradually oxidized from ca. 150 to 500 mV. The high capacitances found for oxidized poly(SNS) indicate that it is adequate for charge-storage devices, although its use should be limited to potentials only involving its reversible doping/undoping cycling.

Anodic electrosynthesis and cathodic electrodissolution of poly(2,5-di-(2-thienyl)pyrrole). A new way of processibility

Abstract Poly(2,5-di-(2-thienyl)pyrrole) was electrogenerated at constant current from acetonitrile solutions. The weight of the obtained polymer film was followed by ‘ex situ’ ultramicrogravimetry. One electron was consumed to incorporate each new monomeric unit to the oxidized polymer film. The polymerization process takes place through α-α′ positions preferentially, as was deduced from FT-IR experiments. The electrogenerated polymer film is soluble in methanol, acetone, THF and other organic solvents, partially soluble in acetonitrile and insoluble in LiClO4 acetonitrile solutions. The electrochemically reduced polymer is soluble in LiClO4 acetonitrile solution. The weight of solved polymer is proportional to the electrical charge consumed during the reduction process. The solubility is completed when one electron is injected to the polymer every 3.3 monomeric units present in the polymer. The solution of the most oxidized polymeric state evolves to more reduced states as was followed by UV-Vis spectroscopy.

Electrogeneration and solubilities of oxidized poly(2,5-di-(-2-thienyl) -thiophene)

Solutions of 2,5-di-(-2-thienyl)-thiophene (SSS) were studied by cyclic voltammetry, chronoamperometry and chronopotentiometry in 0.1 M LiClO4 + acetonitrile solution. Very reproducible weights of polymer were electrogenerated by galvanostatic experiments. Studies performed at different current densities, ranging from 0.2 to 1 mA cm−2, and for different concentrations of SSS, ranging from 5 to 20 mM, prove the faradaic nature of the electropolymerization process. The productivity of the current was 2.7 × 10−3 mgmC−1 and 1.15 electrons were consumed to incorporate one molecule of SSS into the oxidized polymer. Density, conductivity and solubilities of the oxidized poly(SSS) in DMF, CH2Cl2, CHCl3, acetone and acetonitrile were determined. UV-vis measurements of saturated solutions in such solvents indicate that the dimer is the most important soluble component. The elemental analysis shows the presence of 0.38 ClO4− counterions and 0.20 ionic couples of Li+ClO4− per monomeric unit in the oxidized polymer.

Study of the processability of the poly(SNS) in different media

Abstract Electrogeneration and electrodissolution processes of poly(2,5-di-(-2-thienyl)-pyrrole), “[poly(SNS)]”, films in acetonitrile medium was studied by chronoamperometry, chronopotentiometry and “ex situ” microgravimetry. The influence of several tetraalkyammonium salts, as electrolytes, on the processability of this material was stated.

Electrochemical processing of the conducting polymer poly(SNS)

Oxidized poly(2,5-di-(2-thienyl)-pyrrole) [poly(SNS)] films have been electrosynthesized on Pt from a 5 mM monomer and 0.1 M LiClO 4 + acetonitrile solution at constant anodic current density. The electrogenerated films follow a faradaic electrodissolution when subjected to a cathodic current in different LiClO 4 + acetonitrile media. giving solutions containing the reduced polymer. Deposits of oxidized poly(SNS) have been further obtained on a Pt electrode by a flow of an anodic constant current through a solution of the reduced and solved poly(SNS) in LiClO 4 + acetonitrile. These electrodeposited films are partially soluble and electrosoluble. The yield of the electrodeposition process is lower than that of the further electrodissolution process. This fact is due to the presence of parallel reactions such as crosslinking and discharge of residual contaminants during the electrodeposition process. A careful purification, i.e. removal of the contaminants. closes the gap between the productivity of the consecutive electrodeposition and electrodissolution processes: 72% of the electrodeposited polymer is subsequently electrodissolved, showing that both processes are close to technological reversibility. This is the first time that a process envisaging such a possible electrochemical processability. where a polymer mimics the electrodissolution and electrodeposition of metals, is described.

Oxidized and reduced oligo(2,5-di-(-2-thienyl)-furan : solubilities and related properties

Abstract Insoluble films of oxidized oligo(2,5-di-(-2-thienyl)-furan) [oligo(SOS)] were synthesized on Pt from a 5 mM monomer and 0.2 M LiClO 4 + acetonitrile + 0.25% (v/v) water solution at 0.4 mA cm −2 . Insoluble films of reduced oligo(SOS) were farther obtained by reduction in the background electrolyte at 0 V vs. Ag/AgCl. The solubilities of both forms in several solvents were determined. Dimer, trimer, tetramer and pentamer present in solution were detected by mass spectroscopy-fast atom bombardment. The band diagrams for the electronic transitions of both states were established from UV-Vis spectra of their saturated solutions.