Signatures of excitons and polaron pairs in the femtosecond excited-state absorption spectra of phenylene-based conjugated polymers and oligomers

We present results of broadband femtosecond transient absorption measuremen ts in thin films of several phenylene-based pi-conjugated materials. We com pare results for unsubstituted poly(phenylene vinylene) (PPV), polyflourene , and a substituted five-ring oligomer of PPV. We observe universal spectra l features in all materials, implying a Common origin for the photoexcited species in all members of this family. In each material, we observe two pho toinduced absorption (PA) features in the visible region in addition to a s timulated emission (SE) feature. The first PA feature shows strong dependen ce of the dynamics on excitation intensity, and correlates precisely with t he SE feature, proving that this feature is due to the primary emissive exc iton. The second PA feature shows dynamics independent of intensity. In the oligomers, this feature grows quadratically with respect to the first PA f eature, as a result of nonlinear interactions of the primary excitons. In t he polymers, however, the second feature grows linearly with intensity, and indicates an ultrafast decay channel to nonemissive interchain species due to defects.