CIDEP studies of fullerene-derived radical adducts

Photolyses of solutions containing organomercury compounds (HgR2) in the pr esence of C-60 fullerene have been investigated by Fourier transform time-r esolved EPR (FT TR EPR) and continuous-wave EPR (CW EPR) techniques. By FT TR EPRI both electron-spin-polarized C-3(60) (A polarization) and electron- spin-polarized adducts (C60R)-C-. (E/A + E polarization) are observed. The CW EPR spectra of the (C60R)-C-. radicals under steady-state irradiation al so 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 C-60 to form (C60R)-C-.. The latter combine to form the dimers, [C60R](2), which are thermally stabl e and accumulate in the samples. in all of the reported experiments, a cert ain quantity of dimers is produced by photolysis before the EPR spectra are acquired. Ln the FT TR EPR experiments, laser excitation produces C-3(60) by excitation of C-60 and (C60R)-C-. by photocleavage of the dimers. The ob served E/A CIDEP patterns at short (<1 mu s) delays after the laser flash a re proposed to be a result of the creation of polarization through the radi cal-pair mechanism (RPM) resulting from the interactions of two (C60R)-C-. radicals (geminate or free) formed from the photocleavage of [C60R](2) dime rs. The additional E polarization observed at later times (>1 mu s) is prop osed to result from the interaction of C-3(60) With C60R radicals, creating E polarization through the radical-pair-triplet mechanism (RPTM). The pola rization observed in the CW EPR experiments is attributed to the maintenanc e of polarization through the radical lifetime because of the extremely lon g spin-lattice relaxation of the (C60R)-C-. radicals. The latter conclusion is consistent with the very small (50 mG) Line widths of the adduct radica ls. An upper limit for the bond energy of the [C60R](2) dimers of 226 kJ/mo l is established by the observation of the CIDEP of (C60R)-C-. radicals whe n 532-nm excitation is employed. The role of multiple adducts in the observ ed FT TR EPR spectra is discussed.