Radical anions of mono- and bis-fulleropyrrolidines: An EPR study

The radical anions of three C-60 derivatives (N-methyl-3,4-fulleropyrrolidine, N-methyl-2-(3,6,9-trioxadecyl)-3,4-fulleropyrrolidine, and bis-N-methyl-3,4-fulleropyrrolidine are respectively MFP-, tegMFP(-), and bisMFP(-)) have been investigated by continuous wave (cw) and pulsed EPR in methyltetrahydrofuran and tetrahydrofuran solutions. The EPR spectra for MFP- and tegMFP(-) show a 1:1:1 triplet, due to the hyperfine coupling with the pyrrolidine nitrogen. The EPR spectrum of bisMFP- shows a superimposition of two signals, each due to a 1:2:3:2:1 quintet. Each quintet is due to the hyperfine coupling with the two pyrrolidine nitrogen atoms. The two signals are attributed to two different conformers of the radical. A direct transfer of spin density from the C-60(-) Sphere to the nitrogen atoms is discussed as arising from the bent conformation of the pyrrolidine rings. All the spectra show several weaker satellite lines, due to the radical anions containing a C-13 atom. From their analysis a C-2 nu symmetry for the spin distribution is obtained in the case of MFP- and tegMFP(-). The EPR line widths of these latter radicals at room temperature are larger than those expected on the basis of spin relaxation due to rotational diffusion in solution, and the line widths decrease upon decreasing the temperature. Pulsed EPR measurements of T-1 in liquid and frozen solutions show that the EPR line widths are determined by the anomalously short lifetimes of the spin states. The temperature dependence of T-1 obeys to the Arrhenius law, giving an activation energy of 7 kJ/mol. The latter results are compared with those obtained in the past for the lone C-60(-). They are attributed to the closeness of the electronic levels, due to the weak perturbation of the pyrrolidine ring on the symmetrical electronic distribution of C-60 On the other hand, the spin relaxation properties of bisMFP(-) are shown those expected for a nonsymmetrical radical. The comparison of cw- and pulsed-EPR spectra of MFP- in frozen solutions of MeTHF and THF indicates the presence of aggregates of interacting radicals in THF.