Geometrical and electronic structures of new pi-conjugated pyrrole- and furan-nonheterocycle copolymers

The geometrical and electronic structures of new hypothetical pyrrole- and furan-nonheterocycle copolymers were theoretically investigated. Nonheterocycles include cyclopentadiene, silole, oxocyclopentadiene, and thiocyclopentadiene, whose homopolymers are of the quinoid forms in their ground states. AM1 band calculations show that in the ground states the aromatic forms of the copolymers containing cyclopentadiene or silole rings are more stable than the quinoid types, whereas the aromatic forms of the oxocyclopentadiene- or thiocyclopentadiene-containing copolymers are less stable. Modified extended Huckel band calculations predict that bandgaps of the copolymers in the ground states are smaller than those of the corresponding homopolymers. The absorption peaks of pi-pi* band transitions are estimated to appear at 1.68 eV for poly(pyrrolylenesilolylene) and 1.67 eV for poly(furylenesilolylene). Small bandgaps of the copolymers come from the fact that the copolymers consist of different types of cyclic units. Decomposition of the bandgaps reveals that the electronic effect of the heteroatoms (NH and O) in the copolymers is less than half of the effect found in polypyrrole and polyfuran. The small electronic effect decreases bandgaps of the aromatic forms and increases bandgaps of the quinoid types, in comparison to the average of the gaps of the corresponding homopolymers.