Photophysical and photochemical processes of C121, an unsymmetrical fullerene dimer, have been investigated by using laser flash photolysis mainly. The triplet excited state of C121 showed a transient absorption band at 750 nm, whose deactivation process was governed by mixed-order kinetics of first and second order, indicating the contribution of triplet-triplet annihilation process. The intrinsic lifetime of the triplet excited state was estimated to be 13 μs. The singlet and triplet excited states of C121 are considered to be localized on one fullerene moiety of the two-fullerene cages. The one- and two-electron reductions of C121 were observed in the presence of tetrakis(dimethylamino)ethylene in the ground state. The dianion of C121 showed an absorption band in shorter-wavelength side due to the interaction of the transition dipole of anion radical on each fullerene cage of C121. Photoinduced one-electron reduction of C121 was confirmed in the presence of N,N,N′,N′-tetramethyl-1,4-phenylenediamine as a donor. The reaction rate constant was close to the diffusion limiting rate because of the sufficiently negative free energy change of the reduction process via the triplet excited C121. On the other hand, one-electron oxidation of the triplet excited state of C121 was confirmed in the presence of tetracyanoethylene as an electron acceptor in a moderately polar solvent such as o-dichlorobenzene.
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