Towards organic polymers with very small intrinsic band gaps. I. Electronic structure of polyisothianaphthene and derivatives

We present valence effective Hamiltonian (VEH) band-structure calculations on polyisothianaphthene and some of its simple derivatives. Our goal is to rationalize the lowering of the band gap by about 1 eV which is experimentally observed in polyisothianaphthene (E_g=∼1 eV) relative to the parent polymer, polythiophene (E_g=∼2 eV). We show that the smaller band gap found in polyisothianaphthene can be understood on the basis of a relationship which can be established between the band gap energy and the importance of the effective quinoid contributions to the electronic structure. Our theoretical results also indicate that simple dimethyl, dimethoxy, or dicyano substitutions are not expected to affect the size of the band gap in any significant way.