Interruption of conjugations of polyacetylene chains

The interruption of conjugations in polyacetylene, (CH)ₓ, is studied with the use of a tight-binding model Hamiltonian capable of describing various chain defects in (CH)ₓ. The effect of twisting, local impurities, cis segments, chain bending, and attached side groups on electronic transition energies and intensities is investigated. Rotations around ``double'' C?C bonds of the conjugated (CH)ₓ chain lead to the formation of a soliton-antisoliton pair. To study the effect of sp³-type defects or crosslinks in polyacetylene, all-valence-electron model calculations are performed for smaller model systems. It is found that rotations around single C---C bonds, carbonyl side groups, and sp³-type chain defects lead to a partial interruption of the conjugation. To account for experimental findings from, e.g., resonance Raman scattering, such types of defects must be present in a relatively high concentration which is possible only if certain domains with a very high density of defects exist in polyacetylene while keeping the average defect concentration at a low value.