EPR STUDY OF FULLERENE RADICALS GENERATED IN PHOTOSENSITIZED TIO2 SUSPENSIONS

A UV (300-400 nm) photoactivated TiO2 suspension in a (1:1) mixture of toluene and methanol at 295 K was found to be a very effective system for generating fullerene radicals in high concentrations with good reproducibility. The primary generated radical A is characterized by a single-line EPR spectrum (g = 2.0000 and peak-to-peak width (pp) = 0.09 mT). After an induction period, the formation of an additional paramagnetic product B with g = 2.0006 and pp = 0.042 mT is observed. Higher ratios of fullerene:TiO2 and shorter irradiation periods favor the formation of radical A. Both radicals A and B were also generated via in-situ cathodic reduction and in both systems (TiO2 and electrolytic) were found to be similar with respect to g factor, line width, and saturation behavior. Using fullerene enriched to 7% in C-13, the line width of A increased from 0.09 to 0.17 mT. Lowering the temperature, line A was narrowed from pp(300 K) = 0.095 to pp(180 K) = 0.032 mT and at 150 K became asymmetric. At 100 K the observed spectrum is well described by an axially symmetric g matrix with g** - g* = 0.0008. Analogous VIS/NLR measurements with an intensive line at 1075 nm confirm that in the EPR investigations radical A is a monoanion of C-60.