Electronic structures of polymers containing carbon multiple bond and disilane units in their backbone

The electronic structures of poly(ethynylenedisilanylene) FED (Si2H4C2)(x) and both cis- and trans-poly(vinylenedisilanylene) PVD (Si2H4C2H2)(x) are s tudied by means of the ab initio crystal orbital calculations with 6-31G* a nd 3-21G basis sets. PED in its antiperiplanar form shows sigma/pi conjugat ion; however, for PVD, of the three possible conformers studied only shew c an be classified as sigma/pi. In fact, the anticonformer of PVD, the one us ually found drawn in the literature, displays only sigma conjugation and sh ould be unstable, according to vibrational frequency analysis. Eclipsed and shew structures are minima for PVD; the eclipsed conformer displays pi con jugation, but the skew conformer is true sigma/pi and the global minimum fo r PVD. cis-Configurational isomers for PVD are higher in energy than the tr ans-isomers. Analyses of the ionization potential and the effective hole ma ss suggest that PED is delocalized, but the properties of PVD are highly de pendent on conformation: anti PVD and eclipsed PVD are localized, but skew PVD is predicted to have a lower ionization potential than PED. Overall, th e electronic delocalization of PVD and PED is inferior to that of either pu re polysilane or pure polyacetylene, because the nature of sigma/pi conjuga tion is not very effective.