Theoretical Analysis of the Geometric and Electronic Structure of Small-Band-Gap Polythiophenes: Poly(5,5′-bithiophene methine) and Its Derivatives

Results of geometry optimizations and band-structure calculations are reported on poly(5,5′-bithiophene methine), a small-band-gap conjugated polymer made of alternating aromatic and quinoid thiophene rings. Derivative are also investigated in which two methyl groups are added to, or a benene ring fused on, positions 3 and 4 of either the aromatic or quinoid rings of the backbone. It is shown that the small band gap calculated for poly(5,5′-bithiophene methine), 1 eV, arises from a mixing of the highest occupied molecular orbital and lowest unoccupied molecular orbital wavefunctions of polythiophene chains possessing, on the one hand, totally aromatic and, on the other hand, totally quinoid geometric structures. In the substituted derivatives, the results indicate a weak influence of the methyl groups on the geometric and electronic structure of poly(5,5′-bithiophene methine); much more pronounced modifications are obtained for the benzene-substituted compounds, leading to a polymer with a calculated 0.5-eV band gap when benzene rings are fused along the 3—4 bonds of the poly(5,5′-bithiophene methine) quinoid rings.