Intermolecular charge transfer excitons in C70 as compared with C60 films

Electronic excited states specific to solid fullerenes have been identified at approximately 2.35 and 2.75eV for C60 films and at 1.85 and 2.30eV for C70 films by several spectroscopies including UV-visible absorption and electroabsorption, luminescence and its interaction with external electric field as well as steady state photoconductivity. The solid state specific features have been assigned to intermolecular charge transfer (CT) excitons. The evolution of the CT excitons can be related to the electronic structure (0-0 electronic transition at 1.85 and 1.83eV for C60 and C70 films, respectively), the Coulomb potential for electrons (exciton binding energy 0.50 and 0.03eV for C60 and C70 films, respectively) and the molecular symmetry. In solid fullerenes not only intramolecular electronic processes (Frenkel exciton) but also intermolecular electronic processes (CT exciton) govern the excitation relaxation and photocarrier generation mechanisms.