RELAXATION DYNAMICS OF PHOTOINDUCED EXCITON BLEACHING IN POLYMERS

Presented in this paper are systematic studies of photoinduced exciton bleaching dynamics in the polymers with nondegenerate ground state, s uch as polydiacetylene, polythiophene and polyaniline. From a three-le vel model, the photoexcitation and relaxation of the exciton bleaching in the polymers are simulated. The results show that the exciton blea ching 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 relaxa tion of the exciton to the ground state is dominant, the polymers exhi bit an ultrafast initial bleaching relaxation; when the relaxation of the exciton to the self-trapped exciton state is dominant, the polymer s exhibit a slower initial bleaching relaxation than that in the forme r 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 rela xation dynamics of the photoinduced exciton bleaching in the emeraldin e base form of polyaniline. Fitting the experimental data to the theor etical model indicates that the fast component arises from the relaxat ion of excitons to the ground state (similar to 100 fs) and the slow c omponent arises from the relaxation of self-trapped excitons to the gr ound state (similar to 30 ps).