Geometric and electronic structures of polydicyanomethylene-cyclopenta-dicyclopentadiene, a conjugated polymer possessing a very small intrinsic bandgap energy

Summary form only given. We present a detailed theoretical analysis of the geometric and electronic structure of polydicyanomethylene-cyclopenta-dicyclopentadiene, a polymer made by the succession of bicyclopentadiene units in which the two rings are linked by a dicyanomethylene moiety. The geometry optimization is carried out by means of the semiempirical AMl technique. The results indicate that the introduction of the withdrawing group between the two cyclopentadiene rings of the unit-cell does not significantly affect the geometric structure of the polymer. Based on the analysis of the bonding-antibonding electronic patterns appearing on the HOMO and LUMO levels of the polymer, as obtained from VEH band-structure calculations, it is shown that the small calculated bandgap, 0.17 eV, is mainly due to a strong stabilizing interaction occuring between the LUMO of the conjugated skeleton and the LUMO of the withdrawing group.