Relaxation dynamics of photoinduced exciton bleaching in polymers

Abstract

Presented in this paper are systematic studies of photoinduced exciton bleaching dynamics in the polymers with nondegenerate ground state, such as polydiacetylene, polythiophene and polyaniline. From a three-level model, the photoexcitation and relaxation of the exciton bleaching in the polymers are simulated. The results show that the exciton bleaching decay is composed of two components, the fast component and the slow component. For the fast component, the speed of exciton bleaching decay depends on the way in which the excitons relax. When the relaxation of the exciton to the ground state is dominant, the polymers exhibit an ultrafast initial bleaching relaxation; when the relaxation of the exciton to the self-trapped exciton state is dominant, the polymers exhibit a slower initial bleaching relaxation than that in the former case. For the slow component, the exciton bleaching decay is due to the relaxation of the self-trapped excitons to the ground state. Using femtosecond time-resolved pump-probe technology, we measured the relaxation dynamics of the photoinduced exciton bleaching in the emeraldine base form of polyaniline. Fitting the experimental data to the theoretical model indicates that the fast component arises from the relaxation of excitons to the ground state (similar 100 fs) and the slow component arises from the relaxation of self-trapped excitons to the ground state (similar 30 ps).