Theoretical interpretation of the electroabsorption spectra of polyacene crystals. II. Charge-transfer states

In recent articles, a two-dimensional analog of the Merrifield model of the coupling between the Frenkel and charge transfer (CT) excitations of a molecular crystal was successfully applied for the calculation of the electroabsorption (EA) spectra of polyacene crystals. Presently, the results of those calculations are rationalized in terms of a simplified two-state model, designed specifically for interpretational purposes. Particular attention is given to the CT manifold where symmetry considerations rule out the existence of eigenstates endowed with nonzero dipole moment, seemingly contradicting the existing experimental evidence of second-derivative EA signals. The apparent contradiction is resolved by analyzing EA contributions from pairs of zero-field eigenstates coupled by the modulating field. The new approach provides a bridge to the conventional interpretation rooted in the localized model of CT excitations, and allows to determine the limits of validity of the traditional analysis of EA spectra.