CIDEP Studies of Fullerene-Derived Radical Adducts

Photolyses of solutions containing organomercury compounds (HgR2) in the presence of C60 fullerene have been investigated by Fourier transform time-resolved EPR (FT TR EPR) and continuous-wave EPR (CW EPR) techniques. By FT TR EPR, both electron-spin-polarized 3C60 (A polarization) and electron-spin-polarized adducts •C60R (E/A + E polarization) are observed. The CW EPR spectra of the •C60R radicals under steady-state irradiation also exhibit some electron-spin polarization. The chemically induced dynamic electron polarization (CIDEP) in the FT TR EPR experiments is explained by the following series of steps. Photolysis initially causes cleavage of the organomercury compounds into radicals that add to C60 to form •C60R. The latter combine to form the dimers, [C60R]2, which are thermally stable and accumulate in the samples. In all of the reported experiments, a certain quantity of dimers is produced by photolysis before the EPR spectra are acquired. In the FT TR EPR experiments, laser excitation produces 3C60 by excitation of C60 and •C60R by photocleavage of the dimers. The observed E/A CIDEP patterns at short (1 μs) is proposed to result from the interaction of 3C60 with •C60R radicals, creating E polarization through the radical-pair-triplet mechanism (RPTM). The polarization observed in the CW EPR experiments is attributed to the maintenance of polarization through the radical lifetime because of the extremely long spin−lattice relaxation of the •C60R radicals. The latter conclusion is consistent with the very small (50 mG) line widths of the adduct radicals. An upper limit for the bond energy of the [C60R]2 dimers of 226 kJ/mol is established by the observation of the CIDEP of •C60R radicals when 532-nm excitation is employed. The role of multiple adducts in the observed FT TR EPR spectra is discussed.