Towards organic polymers with small intrinsic band gaps. III. Geometric and electronic structures of the base and salt forms of polypyrrylenemethine

We present semiempirical modified neglect of diatomic overlap (MNDO) geometry optimizations and valence effective Hamiltonian (VEH) pseudopotential band structure calculations on the base and salt (protonated) forms of polypyrrylenemethine. This compound is a conjugated polymer based on 2H-pyrrole rings which are bridged by methine-type carbons. In the base (neutral) form, the band gap is calculated to be 1.47 eV. The presence of imine-type nitrogens results in an electronic structure for polypyrrylenemethine which is significantly different from that of polypyrrole despite the presence of six π electrons per unit cell. After protonation of the imine nitrogens, we find that the band gap for the salt form decreases to 0.78 eV, i.e., a value half as big as that of the neutral form. This strong band gap reduction originates from the geometry modification imposed by the protonation process, which leads to a situation where the pyrrole rings alternate between aromatic-type and quinoid-type geometries. Comparison is made with, on the one hand, the oxidation of polybipyrrolemethenylene, and on the other, the protonation of emeraldine base.