Maurizio Carano

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.

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.

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.

Photophysical, electrochemical, and mesomorphic properties of a liquid-crystalline (60)fullerene-peralkylated ferrocene dyad

Two fullerene–peralkylated ferrocene derivatives were synthesized: (1) a liquid-crystalline dyad (compound 1) was obtained by introduction of nonamethyl ferrocene into a liquid-crystalline fullerene derivative and (2) a reference compound (compound 2) was synthesized by attachment of nonamethyl ferrocene to a fulleropyrrolidine. The liquid-crystalline dyad displayed an enantiotropic smectic A phase from 57 to 155 °C. Oxidation and reduction processes were investigated by cyclic voltammetry, and were in agreement with the electrochemical characteristics of the redox-active units (peralkylated ferrocene, fullerene, dendrimer). Photoinduced electron transfer from ferrocene derivative to fullerene was identified.

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.

Electrochemical properties of a liquid-crystalline mixed fullerene-ferrocene material and related species

The electrochemical properties of a fullerene–ferrocene liquid crystal are reported. The electrochemical study was carried out under strictly aprotic conditions: a total of seven reduction peaks were detected in the negative potential region, and two oxidation peaks in the positive potential one. Comparison of the data with those obtained from suitable model compounds (i.e., analogous fullerene- or ferrocene-free materials ) has allowed the assignment of all reduction and oxidation steps. The CV study evidenced the occurrence of a fast follow-up chemical reaction coupled to the third (at 25 °C) or fourth (at −60 °C) reduction of either 1 or 3. Such a reaction, presumably related to the cleavage of one of the cyclopropane bonds, is quantitatively reversed upon re-oxidation. Conversely, under bulk electrolysis conditions (second reduction), the species undergo the retro-Bingel reaction, i.e. the irreversible removal of the bis(alkoxycarbonyl)methano adducts to give the parent C60.

Novel fulleropyrrolidinium-based materials

The cyclic voltammetry of a series of soluble N-methylfulleropyrrolidinium salts shows that these materials are better electron-acceptors than unmodified C60. The stability of mono-reduced, zwitterionic, and doubly-reduced anionic species, produced either electrochemically or chemically, has been checked by absorption spectroscopy. Langmuir–Blodgett (LB) layers of the C60 derivatives mixed with tricosanoic acid have been built up on different substrates. The optical properties of the films have been characterized using absorption spectroscopy and their structure was investigated by X-ray diffraction.

Extensive redox series in dinuclear and dendritic Ru(II) complexes

Abstract The electrochemical behavior of a dinuclear and a hexanuclear dendritic ruthenium(II) bipyridine complexes with 2,3-bis(2-pyridyl)pyrazine (2,3-dpp) as bridging ligands has been investigated in highly purified liquid SO 2 or N , N -dimethylformamide (DMF) solutions. The compounds have general formula [Ru n (bpy) n +2 (2,3-dpp) n −1 ] 2 n + , where n =2 or 6; bpy is 2,2′-bipyridine. The wide anodic and cathodic potential windows explored in liquid SO 2 and DMF, at low temperature, (up to ca. 4.3 and −3.1 V vs. SCE, respectively) have allowed the observation of several metal-centered oxidations and ligand-centered oxidations and reductions, leading to the most extensive redox series so far reported, comprising up to 26 reversible ligand-centered reductions and 14 reversible metal- or ligand-centered oxidations, for a total of 40 redox processes in the hexanuclear complex. The redox standard potentials for overlapping processes in multielectron waves have been obtained from the analysis of the voltammetric curves and their digital simulation, and the localization of all the redox processes and an evaluation of the mutual interactions between the redox centers have been obtained. These results allowed to clarify the important role played by bridging ligands in mediating the interactions between the equivalent redox sites. Furthermore, the localization of the redox sites and the determination of relative standard potentials allowed the mapping of MLCT excited states within the dendrimeric structure, through the correlation between the absorption spectroscopic data and the relevant voltammetric data.

Electrochemical properties of soluble fullerene derivatives

Abstract The family of fullerene derivatives obtained by 1,3-dipolar cycloaddition of azomethine ylides to C 60 is colloquially termed fulleropyrrolidines. In this paper, we report the synthesis and the electrochemical properties of a series of fulleropyrrolidine derivatives. All compounds are characterized by the presence of a solubilizing mTEG group. In addition, a derivative containing a thiophene moiety, covalently attached to the pyrrolidine ring has been studied. Cyclic voltammetry measurements, performed also at low temperatures, allowed the observation of five fullerene-centered reductions for all the species. Adsorption phenomena, however, were found associated to the last reduction processes in the thiophene–fulleropyrrolidine conjugate. Moreover, thanks to the increased solubility of the latter species in polar solvents (acetonitrile–dichloromethane mixtures) its anodic polymerization was carried out, leading to a conducting film still retaining the redox properties of the fullerene derivative.