Excited states of the C-60 dimer

Recent investigations are reported on two different sorts of fullerene dimer. The first type is a simple cluster of two C-60 molecules, i.e., a van der Waals complex. The second one is a covalently bound system. The nature of the electronic excitations has been studied for these model systems. It is found that for the van der Waals complex the low-lying excitations remain localized on one monomer and, thus, they are precursors of the Frenkel-type excitons in the crystal. At significantly higher energies, one obtains so-called charge transfer type excitations, which are precursors of the Wannier excitons. The picture of different excitations resembles that of a very weakly bound system, e.g., two benzene molecules at 5 Angstrom from each other. For the covalent dimer, we considered the binding situation where there are two single bonds between the monomers. The extent of the conjugation between the two C-60 molecules is monitored by the change in the excitation energies. The lowest-lying transitions were observed at higher energies than those in the monomer. This feature is quite unusual for a system exhibiting at least partial conjugation, and it is in agreement with recent band structure calculations and hybridization studies in the fullerene polymer.