Polaron excitations in fullerenes : Theory as π - conjugated systems ( Review article in Prog . Theor . Phys . )

We review the recent theoretical treatment of fullerenes as π-conjugated systems. Polaronic properties due to the Jahn-Teller type effects are mainly discussed. (1) A Su-Schrieffer-Heeger type electron-phonon model is applied to fullerenes: C60 and C70, and is solved with the adiabatic approximation to phonons. When the system (C60 or C70) is doped with one or two electrons (or holes), the additional charges accumulate along almost an equatorial line of the molecule. The dimerization becomes the weakest along the same line. Two energy levels, the occupied state and the empty state, intrude largely in the gap. The intrusion is larger in C70 than in C60. These are “polarons” in doped fullerenes. It is also found that C60 and C70 are related mutually with respect to electronical structures as well as lattice geometries. (2) We apply the model to the fullerene epoxide C60O. It has the polaron-type lattice distortion around the oxygen, and also shows the energy level intrusion in the gap. (3) Optical properties of C60 are calculated and discussed. In the