The configuration change from the acetylenic form =(RC-C=C-CR' =)(x) (x = 1,2,3.5) to the butatrienic form -(RC=C=C=CR'-)(x) (x = 1,2,3), considered as model systems for the two alternative structures of polydiacetylene chains, has been investigated through complete active space self-consistent field and second-order perturbation approach CASSCF/CASPT2 calculations. The character and energetics of the low-lying excited states of both structures are reported. The excited states properties of the oligomers are compared to those of the three-membered ring forms -(C=C-CH=CH-)(3) and -(CH=C=C=CH-)(3). A qualitative interpretation of the femtosecond time-resolved molecular dynamics of the polydiacetylene backbone is proposed on the basis of wave packet propagations on associated potential energy curves connecting the electronic ground and excited states of the two structures in the cyclic form.