Preliminary note

A poly(pyrrole-nickel(II)pyridiniumporphyrin)-modified electrode

New conducting polymers were obtained by anodic oxidation of various unmetallated or metalloporphyrins bearing fluorene or 9,9′-spirobifluorene groups in meso positions. The anodic behaviour of monomers and polymers is presented. The physicochemical properties (IR, UV–Vis spectroscopy and spectroelectrochemistry, scanning electron microscopy, electronic microanalysis) of these new materials are then discussed.

Meso-tetra-(4-sulfonatophenyl)-porphin (TPPS₄) has been incorporated into polypyrrole (PPy) films as a dopant-anion during electropolymerisation from aqueous solutions. The addition of TPPS₄ to the polymerisation solution increases the rate of electropolymerisation substantially, changes the morphology markedly, and increases the redox-activity of the PPy films formed. This catalytic effect of the TPPS₄ on the rate of electropolymerisation was established in the case of non-metallized porphyrin; it means that the effect is not caused by the influence of redox-active metal-ions inserted into the porphin centre. Insertion of the Co²⁺ ions into the porphin centres was carried out from acetate solutions with high efficiency (70%) even in the case of thick (3–10 μm) TPPS₄ doped PPy films. Well-developed waves connected with the Co^2+/3+ transition taking place in meso-tetra-(4-sulfonatophenyl)-porphyrinato cobalt (II) (CoTPPS₄) were established. The voltammograms measured in an air atmosphere show that the electrocatalytic reduction of molecular oxygen starts at the same potential as the wave of Co redox-transition. The PPy films have been characterised by cyclic voltammetry, SEM, AFM, and X-ray microanalysis.

The Os²⁺ functionalised pyrrole monomer, osmium-bis-N,N′-(2,2′-bipyridyl)-N-(pyridine-4-yl-methyl- (8-pyrrole-1-yl-octyl)-amine)chloride, 1, has been synthesised and characterised by spectroscopic (UV–Vis, ¹H NMR, IR spectroscopy) techniques and cyclic voltammetry. Solution phase studies of 1 gave a redox couple associated with the Os^3+/2+ and an irreversible wave associated with the oxidation of the pyrrole moiety. Attempts to form stable polymeric films of 1 on vitreous carbon electrodes, by homopolymerisation, proved unsuccessful. However electroactive films of 1 were obtained by the electrooxidation of 1 through the Os^3+/2+ couple in low dielectric media such as toluene + acetonitrile mixtures. The efficiency of film deposition and properties depend upon the supporting electrolyte and the solvent employed. Films formed in mixtures of acetonitrile and low dielectric constant solvents (dichloromethane and toluene) are electrochemically active and exhibit a redox couple associated with the Os^3+/2+ system. The voltammetric behaviour of these solid-state films was investigated in both aqueous and non-aqueous solvents with a variety of supporting electrolytes. Upon redox switching between the Os^3+/2+ redox states the solid-state charge-transfer processes are coupled to the insertion/expulsion of anions from/to the solution phase. Scanning electron microscopy reveals that films formed in the presence of acetonitrile exhibit different morphology from films formed in acetonitrile-free solutions. Slow and fast linear sweep voltammograms have been employed to provide an absolute determination of the fixed site concentration as 1.5 M and an apparent diffusion coefficient of 5.2 × 10⁻¹² cm² s⁻¹ in 0.3 M KCl. Electroactive films of 1 were then investigated for their electrocatalytic ability towards the oxidation of ascorbic acid in acidic medium. The anodic oxidation peak current was linearly dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range 0.1–2 × 10⁻³ M of ascorbic acid with a correlation coefficient of 0.9993. The detection limit (3σ) was found to be 5.5 × 10⁻⁵ M.

The electrooxidation of 8-(3-acetylimino-6-methyl-2,4-dioxopyran)-1-amino-naphthalene Schiff base in acidic aqueous solution containing 0.1 M H₂SO₄ yielded a polymer film on glassy carbon electrode. The electrooxidative polymerization process was studied by potential sweep electrolysis and controlled potential electrolysis. The film showed a reversible well-defined redox system in aqueous acidic solutions. The factors that affected the electroactivity of the film, such as scan repetition, number of cycles used for the film formation, concentration of sulphuric acid in the electrolytic medium and scan rate (ν) were studied in detail. The modified electrode was shown to decrease the overpotential for oxidation of some redox species, such as catechol, hydroquinone. In addition to the decrease in overpotential, there was also an increase in the current for the oxidation process. The electrode was stable chemically and electrochemically both in aqueous and organic solutions, making it excellent candidate for sensing and/or electrocatalytic applications.

New Cu (II), Mn (III) and Fe (III) deuteroporphyrins functionalized by two electropolymerizable pyrrole groups have been synthesized. The electrooxidation of these metallodeuteroporphyrins has allowed the formation of conducting polypyrrole films exhibiting electrochemical behavior identical to that of the corresponding monomers. In addition, the efficient immobilization of metallodeuteroporphyrins was corroborated by spectrophotometric measurements. Moreover, the potential electrocatalytic properties of Mn (III)-based polypyrrole films in the presence of dioxygen and benzoic anhydride have been illustrated by cyclic voltammetry. Furthermore, the recognition properties of Fe (III)-based polypyrrole films have been highlighted with cyanide in organic electrolyte. Preliminary experiments have also demonstrated the feasibility of such recognition processes in aqueous media.

SiO₂/TiO₂ binary oxides were initially prepared by the sol–gel process and further reacted with antimony (V) in aqueous solution. Calcined and uncalcined SiO₂/TiO₂/Sb₂O₅ composites with different compositions were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy techniques. Large ion exchange capacities were determined for alkaline metal ions: Li⁺, between 0.91 and 1.15 mmol g⁻¹; Na⁺, between 1.32 and 1.70 mmol g⁻¹; K⁺, between 1.65 and 1.95 mmol g⁻¹. The proton exchange capacities increased for samples with larger amount of Sb₂O₅ in the composites. The thermal treatment of the samples, i.e., comparing the exchange capacities of uncalcined samples with that of the calcined ones (773 K), did not show significant change.