THE INTERPLAY BETWEEN GEOMETRIC AND ELECTRONIC-STRUCTURES IN POLYISOTHIANAPHTHENE, POLYISONAPHTHOTHIOPHENE, POLYTHIENO(3,4-B)PYRAZINE, AND POLYTHIENO(3,4-B)QUINOXALINE

PRDDO and ab initio molecular orbital calculations are employed to show that polyisothianaphthene and polyisonaphthothiophene are nonplanar polymers in their aromatic forms, due to steric interactions between the sulfur and hydrogen atoms from adjacent monomeric units; however, the planar quinoid form of polyisothianaphthene is calculated to be ~2 kcal/mol more stable than the aromatic nonplanar form. A simple procedure is introduced for extrapolating the conformational energetics calculated from oligomers to the infinite polymeric system. Similar calculations show that polythieno(3,4-b)pyrazine and polythieno(3,4-b)quinoxaline are perfectly planar systems in both the aromatic and quinoid forms. The extended Hückel technique is used to evaluate the electronic properties of both the aromatic and quinoid forms of these polymers. The band gaps obtained for the quinoid structure of these polymers are in good agreement with the experimental and other theoretical findings. These results have important implications for the design of new conducting polymers. © 1990, American Chemical Society. All rights reserved.