A two-band model for donor/conjugated-bridge/acceptor systems

A simple model is introduced to describe the electronic structure of donor/conjugated-bridge/acceptor molecules. We apply a two-band model for the conjugated-bridge segment; these two (valence and conduction) bands are coupled with the frontier orbitals of the end groups (the donor and the acceptor, respectively). The ground state then consists of a hybridization of the valence band and the donor orbital. The excited state leading to charge transfer is expressed by a partially filled acceptor orbital hybridized with the conduction band. The partial density of states (PDOS) of the end groups is calculated by the Green's function method. The PDOS is determined by three factors: The unperturbed energy of the end groups, the unperturbed DOS of the bridge bands, and the coupling constants between the bridge bands and the end groups. Based on the results we obtain, a simple description of the intramolecular adiabatic charge transfer and the nonlinear optical response.