Electronic properties of the base and protonated forms of polypyrrylenemethine, a small bandgap conjugated polymer

We present MNDO geometry optimization results and VEH band structure calculations on the base and protonated forms of polypyrrylenemethine (PPM), a conjugated polymer based on oxidized pyrrole rings bridged by a methine-type carbon. The 1.47 eV bandgap calculated for neutral PPM decreases to 0.78 eV after protonation of the nitrogens. This is a consequence of the geometry modifications imposed by the protonation process which leads to a situation where the pyrrole rings alternate between aromatic and quinoid geometries. It is important pointing out that both the neutral and protonated forms of PPM have been recently synthesized.