Fullerene derivatives in poly(methyl methacrylate): An EPR and zero-field ODMR study of their photoexcited triplet states

Samples of two fullerene derivatives, namely, C60C2H4N(CH3) and C60C2H4N(Ph(2)C(6)H(4)-4-OCH3) in poly(methyl methacrylate) (PMMA), were prepared either by solvent evaporation from CHCl3 solutions or by thermal polymerization of methyl methacrylate monomers containing the fulleropyrrolidines. In the first case PMMA behaves as a neutral polymeric matrix, while in the second the material swells by absorption of the solvent and its grass transition temperature is shifted up by 8 degrees C with respect to undoped material. Both pieces of evidence are typical of cross-linking between the polymer chains that involves fulleropyrrolidines. EPR and ODMR spectroscopies are applied to the study of the cross-linked species. It is found that its excited triplet state is characterized by unusually large electron dipolar splitting (\D\ = 248 x 10(-4) cm(-1)). Calculations of D have been carried out using a model that considers C-3(60) and the fullerene derivatives as a collection of fully localized double bonds, The experimental negative sign of D has been reproduced for C-3(60), and a positive sign of D is expected for cross-linked fulleropyrrolidine affected by cross-links in the equatorial region. Such a sign reversal can explain the different spin polarization patterns exhibited by their EPR spectra.