Polaron Excitations in Fullerenes: Theory as π-Conjugated Systems

We review the recent theoretical treatment of fullerenes as pi-conjugatedsystems. Polaronic properties due to the Jahn-Teller type effects are mainlydiscussed. (1) A Su-Schrieffer-Heeger type electron-phonon model is applied tofullerenes: C_60 and C_70, and is solved with the adiabatic approximation tophonons. When the system (C_60 or C_70) is doped with one or two electrons (orholes), the additional charges accumulate along almost an equatorial line ofthe molecule. The dimerization becomes the weakest along the same line. Twoenergy levels, the occupied state and the empty state, intrude largely in thegap. The intrusion is larger in C_70 than in C_60. These are ``polarons'' indoped fullerenes. It is also found that C_60 and C_70 are related mutually withrespect to electronical structures as well as lattice geometries. (2) We applythe model to the fullerene epoxide C_60O. It has the polaron-type latticedistortion around the oxygen, and also shows the energy level intrusion in thegap. (3) Optical properties of C_60 are calculated and discussed. In theabsorption of the doped molecule, a new peak structure is present owing to thepolaronic distortion. In the luminescence of the neutral C_60, the spacingbetween H_g(8)-phonon side-band peaks and the relative intensities agree wellwith experiments. In the dispersion of the third harmonic generation, themagnitudes of |chi^(3)| agree with those of experiments at the resonance of thelowest allowed transition as well as in the region away from the resonance.Comment: Review article in Prog. Theor. Phys., 14pages, 7 figure