Understanding the conformational stability and electronic structures of modified polymers based on polythiophene

TLDR: In this article, a conformational study of polythieno(3,4-b)thiophene with various fused fragments was performed to ascertain the relative stability of the aromatic vs quinoid forms as well as the torsional potentials.

Abstract: : Conformations and electronic structures of polymers based on polythiophene with various fused fragments were theoretically investigated. Thioethylenic, thiodimethylenic, and ethylenic fragments were explored, yielding polythieno(3,4-b)thiophene, polythieno(3,4-c)thiophene, and poly(3- thiabicyclo(3.2.0)-1,4,6-heptatriene), respectively. A conformational study, using the method of Partial Retention of Diatomic Differential Overlap, was performed to ascertain the relative stability of the aromatic vs. quinoid forms as well as the torsional potentials of the aromatic forms. The electronic structures of the polymers were obtained through modified extended Huckel band calculations. It was found that the stability of a conformer is mainly governed by the electronic effects associated with a given fragment. The more stable conformer of a polymer has a lower highest occupied crystal orbital level, a larger band gap, and a smaller highest valence band-width. The symmetries and the relative energy levels of the frontier orbitals of a fragment compared to those of a parent polymer play important roles in determining the electronic effects of a fragment and, in turn, the stability of a conformer. It is expected that polythieno(3,4-b)thiophene should be a very promising conducting polymer whose band gap is predicted to be comparable to that of polyacetylene.