Interchain contributions to soliton properties in polyacetylene

Abstract

The energetic degeneracy of the phase-shifted trans polyacetylene chain segments on opposite sides of a soliton center is removed by interchain interactions, which are shown to have important consequences for soliton physics. This energy difference (ESL), the solition-lattice energy, has been calculated from available structural data as a sum of atom-centered, bond-centered, and electrostatic contributions. While ESL is small per C2H2 length of phase-shifted chain (roughly 4 cal/mol for P21/n and 14 cal/mol for P21/a), the resultant total energy cannot generally be ignored since it diverges with increasing separation between soliton and antisoliton or between soliton and chain segment end. Consequences of the interchain interactions are major changes in the equilibrium concentration of soliton pairs, localization of solitons in the proximity of a stationary defect which shifts bond alternation (for chains much longer than the soliton width), and a predicted temperature dependence of phase coherence in bond alternation. Analysis of experimental data provides upper limit estimates for the average chain conjugation length and a lower limit estimate for ESL (greater than 3 cal/mol), which is consistent with the energy calculations based on intermolecular potentials.