Backward Charge Transfer in Conjugated Polymers

It has been known that the static polarizability of a polymer chain with a biexciton is negative.In order to understand this peculiar fact, this paper studies thedynamical process of the charge transfer in the polymer chaininduced by an external electric field E during formingthe biexciton. The time dependence of the charge distribution inthe chain reveals that the charge transfer is backward: thepositive charge shifts in the opposite direction of the externalelectric field. Such a backward charge transfer (BCT) produces anopposite dipole, which makes the polarization negative. The effectof electron interaction on the BCT is illustrated.