Excited states of the C60 dimer

Recent investigations are reported on two different sorts of fullerene dimer. The first type is a simple cluster of two C60 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 إ 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 C60 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.