Charge-transfer excitons in the dielectric theory of molecular crystals

The dielectric theory of molecular crystals is augmented by introducing a nonlocal charge-transfer polarizability defined for a pair of equivalent molecules. This entails using an expanded crystal unit cell. Analysis of the simplest scalar model crystal with one molecule per primitive unit cell confirms that the expanded cell still yields the correct Frenkel exciton level in the absence of charge transfer. Expressions derived for the coupled exciton levels that arise from Frenkel and charge-transfer excitons are equivalent to those derived by Hamiltonian approaches. An alternative derivation using the sum-over-states expression is given for the excited-state polarizability changes due to the coupling between the excitons. Extension of the dielectric theory allows charge-transfer to be incorporated readily in applications such as crystal energetics and dynamics. (C) 1997 Elsevier Science B.V. All rights reserved.