Theoretical Investigation of the Structure and Electronic Properties of Poly(dithieno[3,4-b:3′,4′-d]thiophene), a Small-Band-Gap Conjugated Polymer

We use a quantum chemical approach to investigate the chemical and electronic structure of poly(dithieno[3,4-b:3′,4′-d]thiophene) (pDTT′). In this compound, the monomer unit possesses four potential linking sites on α carbon positions. Semiempirical Austin Model 1 (AM1) calculations are performed on oligomer systems, considering different types of connection between the DTT′ units, in order to determine the most favorable chain conformation and the relative stabilities of the oligomer segments. The theoretical data indicate that, as a function of the connection mode, the pDTT′ sequences can either be totally planar and highly conjugated or strongly nonplanar, depending on the existence of steric interactions between sulfur and hydrogen atoms located on adjacent units. The electronic properties of the corresponding polymers are calculated with the valence effective Hamiltonian (VEH) method and compared to the results of optical spectroscopy measurements. Finally, we study the energetics of the pDTT′ chain on the basis of the energy per repeat unit (E_pru), and we compare the structure and electronic properties of this polymer to those of polyisothianaphthene.