Role of ring torsion angle in polyaniline: Electronic structure and defect states

Abstract

The role of phenyl-ring torsion angle in determining the nature of the ground and charged-defect states of polyaniline is explored. The coupling of the transfer integral between nitrogen atom and phenyl-ring constituents of the polyaniline chain to the dihedral angle of the rings competes with the substantial steric repulsion between adjacent rings in determining the conformation of these systems. The ring conformation of the leucoemeraldine-base (LB) form of polyaniline is described by a novel ring-torsion-angle order parameter. The anharmonicity of the interring steric potential leads to a temperature-dependent mean order parameter, and thus to the prediction of thermochromic effects consistent with experiment. Furthermore, changes in optical spectra accompanying derivatization of the rings can be understood by the response of the substituted polymer to modifications of the steric potential. The existence of two degenerate ring-torsion-angle phases in LB implies that both polaronic and solitonic ring-angle-alternation defect states may be relevant in describing the charged states in polyaniline. As these defects involve substantial changes in ring torsion angle, they are expected to possess large kinetic mass, in agreement with photoinduced absorption experiments on polyaniline. The anticipated Peierls ground state of the oxidized pernigraniline-base form of polyaniline can be regarded in part as a ring-torsion-angle dimerized state; consequently, the charge states of this material are also expected to be massive defects in the ring-rotational order. The importance of ring rotations in other ring-containing electronic polymers, such as poly(paraphenylene sulfide), is discussed.