Laser flash photolysis study on photochemical generation of radical cations of fullerenes C-60, C-70, and C-76

Photochemical oxidation processes of fullerenes C-60, C-70, and C-76 were examined with a nanosecond laser flash photolysis by observing the transient absorption spectra in the near-IR region. Using tetracyanoethylene as an electron acceptor for fullerenes in the triplet states, the transient absorption bands ascribable to their radical cations were observed. In the solution containing tetracyanoquinodimethane or chloranil as an acceptor, on the other hand, generation of the radical cation was not observed. The difference in these acceptor abilities can be explained on the basis of the free energy changes for the reactions. Photosensitized oxidation of the fullerenes in the presence of cosensitizer was also examined. In the case of C-70, oxidation occurred effectively by this reaction; the radical cation of C-70 appeared immediately after the laser irradiation. C-76 also showed transient absorption attributed to its radical cation, while the generation rate was smaller than those of C-60 and C-70.