The optical properties of two series of thiophene-based polymers are interpreted by torsion analysis. When approached by the calculated HOMO/LUMO energy gaps of model molecules, the wavelength shifts from solutions to films are reasonable in cyclohexyl-substituted series but not in hexyl-substituted series, indicating that solid packing force plays a more important role in the former than in the latter. The lengths of hexyl and cyclohexyl groups on bithiophene segment are the dominant factor that determines the distance of co-facial chain packing which affects the conformation of the polymer chain. In both series the photoluminescence (PL) efficiency increases as the equilibrium torsion angle decreases. (C) 2002 Published by Elsevier Science B.V.