Photophysical and photochemical processes of an unsymmetrical fullerene dimer, C121

Photophysical and photochemical processes of C-121, an unsymmetrical fullerene dimer, have been investigated by using laser flash photolysis mainly. The triplet excited state of C-121 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 mus. The singlet and triplet excited states of C-121 are considered to be localized on one fullerene moiety of the two-fullerene cages. The one- and two-electron reductions of C-121 were observed in the presence of tetrakis(dimethylamino)ethylene in the ground state. The dianion of C-121 showed an absorption band in shorter-wavelength side due to the interaction of the transition dipole of anion radical on each fullerene cage of C-121. Photoinduced one-electron reduction of C-121 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 C-121. On the other hand, one-electron oxidation of the triplet excited state of C-121 was confirmed in the presence of tetracyanoethylene as an electron acceptor in a moderately polar solvent such as o-dichlorobenzene.