Showing papers by "Antonio Toffoletti published in 1997"

Spin polarization in fullerene derivatives containing a nitroxide group. Observation of the intermediate photoexcited quartet state in radical triplet pair interaction

Abstract: A series of C60 fullerene derivatives containing a nitroxide group has been photoexcited by short LASER pulses in the microwave cavity of a cw-EPR spectrometer. Strongly spin polarized signals have been observed, in glassy matrix as well as in liquid solution, for both the ground electronic state and the excited quartet state. In the quartet state the excitation resides in the fullerene part and the molecule constitutes a triplet-radical pair with the partner covalently linked. The absorptive or emissive character of the transitions is explained in terms of the mechanism of radicaltriplet interaction producing spin polarization. Opposite initial sign and polarization patterns are observed for molecules with different spacer between nitroxide and fullerene. The time evolution of the relevant sublevel populations is fitted by a kinetic model taking into account quartet decay constants, quartet and doublet spin-lattice relaxation rates and branching ratios.

Conformational dynamics of triradical nitroxides as studied by EPR

Abstract: The EPR spectra of a flexible nitroxide triradical are recorded in a large temperature range. A detailed analysis of the spectral features is performed and the spectra are compared with computer simulations. An excellent agreement with the experimental spectra is obtained by using a dynamic model based on the conformation interconversion of different molecular structures, characterized by the electron exchange interactions between pairs of free radicals. The energy barrier for the interconversion process and the temperature variation of the constant for the equilibrium among the conformations have been obtained. The results show that EPR spectra of nitroxide triradicals spin labelled polymers could be of valuable use for studying subtle details of molecular dynamics.

Spin Polarization and Photoinduced Electron Transfer between Ferrocene and Fullerene Derivatives Containing a Nitroxide Group.

Abstract: Fulleropyrroliddine derivatives containing a TEMPO group (FPNOs) are examined in toluene solution, between 200 and 270 K, by TR-EPR spectroscopy after LASER excitation. Signals of the ground and excited states are observed and their time profiles are recorded. In presence of ferrocene the time pattern of the polarization is modified, showing the occurrence of intermolecular electron transfer from the donor ferrocene to the excited FPNOs. The rate constantk E. is obtained from the simulation of the EPR signal time evolution of the ground and excited states.