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 measurements in thin films of several phenylene-based π-conjugated materials. We compare results for unsubstituted poly(phenylene vinylene) (PPV), polyfluorene, and a substituted five-ring oligomer of PPV. We observe universal spectral features in all materials, implying a common origin for the photoexcited species in all members of this family. In each material, we observe two photoinduced absorption (PA) features in the visible region in addition to a stimulated emission (SE) feature. The first PA feature shows strong dependence of the dynamics on excitation intensity, and correlates precisely with the SE feature, proving that this feature is due to the primary emissive exciton. The second PA feature shows dynamics independent of intensity. In the oligomers, this feature grows quadratically with respect to the first PA feature, as a result of nonlinear interactions of the primary excitons. In the polymers, however, the second feature grows linearly with intensity, and indicates an ultrafast decay channel to nonemissive interchain species due to defects.