Temperature Dependence of Fullerene Electroabsorption Spectra: Model Calculations

As demonstrated in recent experiments, the electroabsorption spectra of fullerene thin films exhibit rather strong temperature dependence in the energy regions where the signal is attributed to the eigenstates of CT parentage. This is difficult to rationalize in terms of a purely electrostatic model where the CT excitons are viewed as localized charge pairs. In the present paper, a simple dimer model is used to account for the main features of the off-diagonal CT interaction that gives rise to charge delocalization. The results demonstrate that the strong temperature dependence of the EA signal may be explained by disorder-modulated orientational dependence of charge-transfer integrals, and highlight the role of the off-diagonal CT coupling in organic molecular crystals.