Charge-transfer interactions—their manifestations in electroabsorption spectra

The paper presents an overview of fundamental concepts necessary for interpretation of electroabsorption (EA) spectra. A single Frenkel exciton transition is discussed in terms of classical electrodynamics, with parameters derived from the quantum mechanical description of the electric field effect on intramolecular excitations. It is shown that the resultant EA signal follows the first derivative of the corresponding absorption spectrum, irrespective of the exciton oscillator strength. It is argued that a consistent description of charge transfer (CT) bands, dependent on intermolecular overlap, is possible only on quantum mechanical grounds. The magnitude of the off-diagonal CT interactions, limited by intermolecular overlap, determines the shape of the CT state EA signal, which may either follow the second derivative of the absorption band, or be resolved into a pair of first derivatives, one simple and the other inverted. The off-diagonal CT interactions, mixing the CT states with the Frenkel states, also influence the total intensity of CT absorption and electroabsorption, and the amplitude of the EA signal of Frenkel excitons.