Abstract
We report a time-resolved fluorescence study of three water-soluble polythiophene derivatives with homologous oligo(ethylene oxide) side-chains that are distinct in their lengths. Their excited-state relaxation dynamics was found to exhibit a strong dependence on both side-chain length and aqueous solution temperature. With lengthening side-chain, the isotropic fluorescence decays become not only remarkably slow but also increasingly complex. Except for the polymer with the shortest side-chain, significant acceleration of the isotropic decay was observed when the solution temperature was raised to a characteristic point and beyond. The corresponding time-resolved fluorescence anisotropies also show striking changes, manifest by a substantial increase of initial values from about 0.2 to 0.4 and the appearance of a pronounced fast decay component with an estimated lifetime of 36 ps. Both the anisotropic and isotropic decays are attributed to conformational relaxation of the polymer backbones, but different mechanisms are proposed for the effects induced by side-chain length and sample solution temperature.
