Novel low bandgap polymers: polydicyanomethylene-cyclopentadithiophene and -dipyrrole

Results of quantum-chemical calculations are reported on the geometric and electronic structure of polydicyanomethylene-cyclopenta-dithiophene and its nitrogen analogue, polydicyanomethylene-cyclopenta-dipyrrole. Geometry optimizations are carried out by means of the semiempirical Austin Model 1 (AM1) technique. The introduction of the electron-withdrawing dicyanomethylene group between the thiophene or pyrrole rings of the unit cell induces major geometry modifications within each ring, resulting in the appearance of a geometric structure which can be viewed as a combination of aromatic and quinoid geometries. Based on the analysis of the bonding-antibonding electronic patterns appearing on the HOMO and LUMO levels of those polymers, as obtained from VEH band-structure calculations, it is shown that the small calculated bandgaps (0.59 and 0.37 eV for the thiophene and pyrrole derivatives, respectively) arise from HOMO and LUMO levels that are located between those of aromatic and quinoid polythiophene or polypyrrole chains.