Evolution of the bipolaronic structure in going from one- to two-dimensional pi model systems.

Long neutral and doubly charged oligodiacetylenes (with a maximum of 30 repeat units) as well as some of their aggregates have been studied in a semiempirical framework. By analyzing the plots of the one-electron ground-state density matrices, we have shown that the 12-units long oligomer is the borderline between two different behaviors of the electronic structure of the isolated charged systems. For oligomers shorter than this limit the bipolaronic structure is the dominant one, while for larger oligomers the polaron-pair structure is preferred. The major conclusion of this work concerns the effects of the interchain interactions in bipolaron systems, which are usually neglected in theoretical works. It is predicted that as a consequence of the latter in the bulk system a bipolaron generated on a single polymer chain will soon split into two polarons on adjacent chains.