EPR STUDIES ASSOCIATED WITH THE ELECTROCHEMICAL REDUCTION OF C-60 ANDSUPRAMOLECULAR COMPLEXES OF C-60 IN TOLUENE-ACETONITRILE SOLVENT MIXTURES

Electron paramagnetic resonance (EPR) studies at 295 K of the radical anion of buckminsterfullerene (C-60(.-)) generated electrochemically b y bulk-controlled potential electrolysis of neutral C-60 in mixed tolu ene-acetonitrile solvents (5-20% v/v acetonitrile with 0.1-0.2 M n-Hex (4)NPF(6) or n-Bu4NPF6 as the supporting electrolyte) have shown that at least three different forms of C-60(.-) exist in solution. The radi cal anions have different g values and can thus be detected by EPR spe ctroscopy in the form of single line signals with different line width s. The familiar species with the broad line width signal (Delta H-pp = 6 mT) was present in a high percentage (ca. >95%) along with two spec ies with narrow line width (Delta H-pp = 0.16 and 0.07 mT) signals tha t were present between 1 and 5% of the total concentration of C-60(.-) . The concentrations of the species with narrow line width EPR signals increased with increasing time so that over a 20-h period their EPR s ignal intensity approximately doubled. The purity of the toluene was f ound to be very important in determining the number of species detecte d with sharp EPR signals. Although two species were always detected wi th sharp EPR signals, when lower purity toluene (<99%) was used, the n umber of species with narrow line width signals increased to >2. The i ncrease in the sharp EPR signals with increasing time is not inconsist ent with a slow reaction between C-60(.-) and low levels of impurities in the solvent to form lower symmetry paramagnetic species, the narro w line width signals being very similar to those reported for substitu ted fullerene radicals. EPR and cyclic voltammetry experiments were co nducted on C-60 in the presence of several complexing agents, cyclotri veratrylene (CTV), the symmetrical tris-allyl substituted analogue of CTV [CTV(allyl)(3)], and p-benzyl-calix[5]arene. The intensity of the narrow and broad Line width EPR signals decreased when C-60(.-) was el ectrochemically generated in the presence of the complexing agents, in dicating that the host-guest pi-pi interactions were sufficiently stro ng to alter the EPR signals of C-60(.-). Cyclic voltammetry experiment s performed on C-60 in the presence of the complexing agents showed th at the first four reduction processes of C-60 split into two new proce sses upon complexation, with the time allowed for the host-guest react ion being critical in determining the voltammetric behavior.