COMPARISON OF GEOMETRIES AND ELECTRONIC-STRUCTURES OF POLYACETYLENE, POLYBOROLE, POLYCYCLOPENTADIENE, POLYPYRROLE, POLYFURAN, POLYSILOLE, POLYPHOSPHOLE, POLYTHIOPHENE, POLYSELENOPHENE AND POLYTELLUROPHENE

Geometries of monomers through hexamers of cylopentadiene, pyrrole, fu ran, silole, phosphole, thiophene, selenophene and tellurophene, and m onomers through nonamers of borole were optimized employing density fu nctional theory with a slightly modified B3P86 hybrid functional. Band gaps and bandwidths were obtained by extrapolating the appropriate ene rgy levels of trimers through hexamers (hexamers through nonamers for borole) to infinity. Bandgaps increase with increasing pi-donor streng ths of the heteroatom. Tn general, second period heteroatoms lead to l arger bandgaps than their higher period analogs. Polyborole is predict ed to have a very small or no energy gap between the occupied and the unoccupied pi-levels. Due to its electron deficient nature polyborole differs significantly from the other polymers. It has a quinoid struct ure and a large electron affinity. The bandgaps of heterocycles with w eak donors (CH2, SiH2 and PH) are close to that of polyacetylene. For polyphosphole this is due to the pyramidal geometry at the phosphonous which prevents interaction of the phosphorus lone pair with the pi-sy stem. The bandgap of polypyrrole is the largest of all polymers studie d. This can be attributed to the large pi-donor strength of nitrogen. Polythiophene has the third largest bandgap. The valence bandwidths di ffer considerably for the various polymers since the avoided crossing between the flat HOMO - 1 band and the wide HOMO band occurs at differ ent positions. The widths of the wide HOMO bands are similar for all s ystems studied. All of the polymers studied have strongly delocalized pi-systems. (C) 1998 Elsevier Science S.A. All rights reserved.