Nonlinear excitations in conjugated polymers: Characterization of ring torsional solitons in leucoemeraldine

The formation of solitons involving a kink in the ring-torsion alternation pattern of leucoemeraldine base is discussed on the basis of quantum-chemical calculations. We obtain that these solitons present a modified functional tanh-like evolution which is markedly different from that in bond-length solitons and is a result of a compromise between steric interaction and conjugation effects. The ring-torsional solitons optimally extend over about three rings. They do not cause the appearance of electronic states within the band gap but contribute to widen it. The continuum-limit model previously used to describe them is found to be inadequate.