Modifications of the optical properties of poly [2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] induced by fluorination of the vinylene units are investigated by means of time dependent density functional theory (TD-DFT) calculations and spectroscopic measurements in solution. The energy of the main absorption peak is blue-shifted by more than 0.8 eV in the fluorinated polymers. TD-DFT excitation energies for non-fluorinated and fluorinated oligomer structures of increasing number of monomers, employing fully relaxed geometries, are compared to the experimental absorption energies of the polymers. We found that the measured large blue-shift induced by the fluorination of the vinylene units is not caused by the electron-withdrawing effect of the fluorine substituents but it is related to a steric effect. The inter-monomer torsional angle of the fluorinated structures increases above 50 degrees, while in the non-fluorinated systems it is below 20 degrees. Further insight into the origin of the large blue-shift of the excitation energies is gained by a detailed analysis of the torsional potentials of non-fluorinated and fluorinated dihydroxystilbene. While for planar geometries the energy gap increases due to fluorination, it decreases for highly distorted geometries. In addition, we found that the torsional potential of dihydroxystilbene is rather flat, meaning that different isomers might, e.g., in the solid state, coexist.