Conjugated polymers have attracted tremendous attention recently as advanced materials for photonic applications. Therefore, the nature of the photoexcited states in this class of material is of theoretical and practical interest. In this thesis work, femtosecond time-resolved absorption and luminescence techniques were used to study the photoexcitation and relaxation processes in a variety of conjugated polymers which includes poly(benzimidazobenzophenanthroline ladder) (BBL) and poly(p-phenylene benzobisthiazole) (PBZT), BBL/PBZT blends, and the rod-coil copolymers poly((benzobisthiazole)-(1,4-(2-hydroxy)phenylene))-co-poly(benzobisthiazoledecamethylene) (HPBT-co-PBTC10).;Femtosecond pump-probe transient transmission difference measurements were performed on a series of pure BBL and PBZT and blends of BBL/PBZT of various compositions. In pure BBL and PBZT samples, transient photoinduced bleaching of the $pi{-}pisp*$ transition accompanied with photoinduced absorption in the optical gap were observed. The photoinduced bleaching and absorption recover with a fast sub-picosecond component and a slow picosecond component. The experimental results are well accounted for by the self-trapped exciton model.;In BBL/PBZT blends, when the probe was above the optical gap (620 nm), bleaching signals were observed in all samples. The bleaching recovery shows a substantial slow down in samples with lower BBL concentrations. When the probe photon energy was below the gap, an initial gain signal was observed in samples with 60% and 75% BBL concentrations. A significant photoinduced absorption enhancement was observed in samples of other concentrations. These experimental results suggest a strong coupling of the electronic structure of the components of the blends, and is explained by the excited state charged transfer between the two components.;Intramolecular proton transfer in the excited electronic state of 5% HPBT-co-PBTC10 copolymer is investigated on the femtosecond time scale. Femtosecond fluorescence up-conversion measurement has found that the rise time of the Stokes shifted fluorescence of the excited keto state at 540 nm was less than 300 fs, indicating that proton transfer occurs on a subpicosecond time scale. However, the femtosecond pump and probe experiment found that the optical gain did not occur instantly. Instead, an initial transient absorption was observed and followed by an optical gain. This induced absorption is thought to be due to the vibrational hot enol tautomers of the chromophores with longer conjugated lengths, whereas the optical gain is attributed to the stimulated emission of the keto tautomer states formed by excited state proton transfer.
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