The geometric and electronic structure of [3‐(4‐Octylphenyl)]Thiophenes (POPT) and [3‐(4‐octylphenoxy)thiophenes] (POPOT) oligomers, ranging in size up to the octamer (240 atoms), have been calculated with density functional theory (B3LYP/
). The knowledge of the geometries for these polymers is of utmost importance in order to understand their optical properties in different phase (solution and condensed phase). The calculations indicate that, in opposition to the first polymer where the introduction of the pendant phenyl disturb the planarity for the backbone of the conjugated segment, the second polymer has a nice organisation where the conjugated chain still planar even the presence of octylphenyl groups. The optical spectra are calculated with TDDFT and Zindo approaches on the basis of the geometries provided by DFT, and compared to experimental data.