Sergio Roffia

ENHANCED ACCEPTOR CHARACTER IN FULLERENE DERIVATIVES. SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF FULLEROPYRROLIDINIUM SALTS

A family of N-methylpyrrolidinium fullerene iodide salts, possessing one (or two) solubilizing chain(s), has been synthesized. As evidenced by CV and steady-state voltammetry experiments, carried out in tetrahydrofuran (THF) solutions, these species show enhanced electron-accepting properties with respect to both the parent fulleropyrrolidines and C60. CV measurements performed at −60 °C and scan rates of 50 V/s, made possible by the use of ultramicroelectrodes, have allowed the observation, for the first time in fullerene derivatives, of six fullerene-centered reductions. Bulk electrolysis of these novel compounds, performed at the stage of the first reduction potential, generates a stable zwitterion, with both anion and cation located on the fullerene derivative.

Photoinduced Electron Transfer in a Tris(2,2′-bipyridine)-C60-ruthenium(II) Dyad: Evidence of Charge Recombination to a Fullerene Excited State

A novel fulleropyrrolidine covalently linked to a tris(2,2′-bipyridine)ruthenium(II) complex is presented (see picture). While electrochemical investigations suggest the absence of any ground-state interaction between ruthenium and fullerene chromophores, photoinduced optical absorption studies clearly show that electron transfer occurs to afford the Ru3+−C60 pair with characteristic fullerene radical anion band at λmax = 1040 nm.

Medium effects on the antioxidant activity of dipyridamole.

Very strong medium effects have been observed when testing the antioxidant activity of dipyridamole (DP) in different media such as benzene, tert-butanol, methanol solutions and egg yolk lecithin unilamellar and multilamellar vesicles. Actually, dipyridamole behaves as a very poor antioxidant in benzene while its ability to inhibit the lipid peroxidation reaction increases with increasing solvent polarity, being the highest in lipid vesicles. This behavior can not be rationalized on the basis of the classical chain breaking mechanism which operates in the case of phenolic and amine antioxidants and involving the transfer of a hydrogen atom to peroxyl radicals. An explanation is instead given in terms of an electron transfer reaction which leads to the oxidation of DP by the chain carrying peroxyl radical to give the dipyridamole cation radical, DP+*, and the peroxyl anion LOO-, and whose rate constant is expected to increase in strongly polar media. EPR and electrochemical data supporting this interpretation have been collected.

Electrochemistry and spectroelectrochemistry of ruthenium(II)-bipyridine building blocks. Different behaviour of the 2,3- and 2,5-bis(2-pyridyl)pyrazine bridging ligands

We report the results of an investigation, using electrochemical and spectroelectrochemical techniques, into redox properties of uncoordinated free bis-chelating 2,3- and 2,5-bis(2-pyridyl)pyrazine ligands (2,3- and 2,5-dpp) and of the complexes of the [Ru(2,3-dpp)n(bpy)3−n]2+ and [Ru(2,5-dpp)n(bpy)3−n]2+ families (bpy=2,2′-bipyridine), which are used as building blocks for obtaining polynuclear complexes. For comparison purposes, the electrochemical behaviour of the [Ru(2,3-dpp)(DCE-bpy)2]2+complex, where DCE-bpy is 5,5′-dicarboxyethyl-2,2′-bipyridine, has also been investigated. Correlations of the E1/2 values observed for the compounds examined (genetic diagrams) have allowed us to assign all the ligand-based reduction processes as well as to discuss electronic interactions. The localisation of the first three reduction processes for each complex has also been established on the basis of the spectroelectrochemical results. Theoretical calculations (AM1 semiempirical and ab-initio level) carried out for the 2,5-dpp and 2,3-dpp ligands show that, in the uncoordinated state, the former ligand does not exhibit any substantial conformation arrangement, whereas the latter has a stable conformation for a large (56°) dihedral angle between the pyridyl and pyrazine rings. The changes in conformation upon mono- and bis-coordination of 2,3-dpp can account for its peculiar electrochemical behaviour consisting in a change of the number of redox processes with varying coordination state.

Electrochemical behaviour of 4-nitroimidazole and 2-methyl-5-nitroimidazole: Autoprotonation of anion radical and redox-catalysed reduction of the supporting electrolyte cation

Abstract The electrochemical behaviour of the title imidazoles (HRNO 2 ) has been investigated in dimethylformamide and acetonitrile by polarography, cyclic voltammetry and controlled-potential electrolysis and coulometry. The above nitroimidazoles present similar behaviour, displaying two reduction waves paralleled by two reduction peaks in linear sweep voltammetry. The first peak is irreversible up to a sweep rate of 250 V s −1 , while the second one appears to be reversible even at low sweep rates. The experimental data in the first process are consistent with the hypothesis that the irreversibility of the first wave (peak) is due to a rapid decay of the primary anion radical HRNO 2 because of a fast protonation reaction by the starting nitroimidazole (a father-son type of reaction), with the formation of the conjugate base RNO 2 − and of the neutral radical HRNO 2 H. This last radical should give rise, as final reduction product, to the hydroxylamino derivative, the necessary protons being supplied by the starting HRNO 2 . The second process is attributable to the reduction of RNO 2 − to RNO 2 2 . As long as HRNO 2 has not been consumed, during controlled-potential electrolysis at second wave potentials, it is conceivable that the radical dianion RNO 2 2 is oxidized by the starting nitroimidazole to give its conjugate base together with the radical HRNO 2 , which follows the reaction path already described for the first wave. Once all the HRNO 2 has been consumed, the homogeneous reduction of the supporting electrolyte cation (C 2 H 5 ) 4 N + , catalysed by the redox system RNO 2 − /RNO 2 2 , takes place. Electrochemical and spectroscopic investigations on 1-methyl-4-nitro and 1-methyl-5-nitro derivatives of the above nitroimidazoles have provided both an indirect support to the above mechanism and information on the type of tautomer predominantly present in solution at equilibrium.

Synthesis, Electrochemistry, Langmuir-Blodgett Deposition and Photophysics of Metal-Coordinated Fullerene-Porphyrin Dyads

Abstract A novel fullerene–Ru–porphyrin conjugate ( 3b ) for Langmuir–Blodgett (LB) film deposition has been synthesized. The side polar chain that characterizes the complex is necessary to introduce enough amphiphilicity in these otherwise non-polar moieties. The complex can be deposited on solid substrates via the LB technique and the films obtained show charge separation upon light irradiation. In addition, a careful cyclic voltammetry study of the complex is reported.

Electrochemical Detection of C 60 in Solution: Is Tetrahydrofuran a Suitable Solvent for Fullerene Studies?

The electrochemistry of C 60 in tetrahydrofuran/tetrabutylammonium hexafluorophosphate (THF/TBAH) solutions was thoroughly investigated, and particular attention was devoted to possible interactions occurring between the fullerene and this medium and leading to irreversible modification of C 60 itself. Either untreated or carefully purified solvent was used in the experiments, showing that only in the latter case can reliable results be obtained, the occurrence of irreversible reactions between C 60 and an impurity present in traces (probably peroxides) having been monitored by both cyclic voltammetry (CV) and absorption spectroscopy.

Potential sweep voltammetry with uncompensated ohmic potential drop. Irreversible charge transfer

Summary Potential sweep voltammetric curves for a totally irreversible electrode process with uncompensated ohmic drop, when mass transfer takes place by semi-infinite linear diffusion, have been calculated. These curves have been used to derive operational diagrams that allow the evaluation of the electrochemical kinetic parameters. Experimental data obtained for the totally irreversible reduction of bromate ion at a mercury electrode have been examined in terms of the theory. The electrochemical kinetic parameters thus derived are in very good agreement with those reported in the literature.

A glutathione amperometric biosensor based on an amphiphilic fullerene redox mediator immobilised within an amphiphilic polypyrrole film

A novel biosensor for the amperometric detection of glutathione is proposed in which glutathione reductase is coupled to a pyrrolofullerene bis-adduct playing the role of redox mediator between electrode and enzyme. The adoption of a simple procedure for realizing the immobilization of the redox mediator, together with the enzyme, within a polypyrrole film cast onto a glassy carbon electrode, was made possible by the amphiphilic properties of the fullerene derivative. The biosensor showed a fast and reproducible response to glutathione and, because of its lower sensitivity, a wider linear range with respect to a similar device based on a diffusional redox mediator.

Switchable photoreduction pathways of antimony(V) tetraphenylporphyrin. A potential multielectron transfer photosensitizer

Photoreduction of [SbV(tpp)(OH)2]+(tpp = dianion of 5,10,15,20-tetraphenylporphyrin) yields either the metastable π-radical anion [SbV(tpp˙–)(OH)2] or the two-electron reduced[SbIII(tpp)]+ complex; product formation can be channelled in the desired direction by an appropriate choice of the reaction medium and dioxygen regenerates the starting compound.