Structure of polyacetylene–iodine complexes

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Abstract
We confirm the existence of a 15 Å period in iodine‐doped polyacetylene and provide a new interpretation for this key feature as part of a general model for structural changes during iodine doping. The observed diffraction intensities for different samples suggest the existence of structures with two different types of dopant‐containing layers: layers obtained by complete replacement of polyacetylene chains by iodine columns (F layers) and layers obtained by replacement of every other polyacetylene chain by an iodine column (P layers). The F layers in the heavily doped complex alternate with dopant‐free layers of polyacetylene chains (U layers), corresponding to a (U F) n stacking sequence. The phase obtained at a lower dopant concentration, which provides the 15 Å spacing, is attributed to a (U P U F) n stacking sequence. At still lower dopant concentrations, one obtains a (U P) n stacking sequence. This model, along with published Raman, Mössbauer, and photoelectron spectroscopy data, suggests that the ratio of I− 5 to I− 3 increases in going from P layers to F layers. Intense and monotonically decreasing, diffuse x‐ray scattering suggests that vacancies of size ∼3 Å are present, probably in iodine columns. A diffuse reflection at 3.1 Å, observed in all iodine‐doped samples, is due to an average iodine–iodine distance in disordered columnar arrays. On the other hand, ordered arrays of iodine columns in oriented samples give rise to sharp meridional reflections. All ten observed reflections (down to 1.17 Å) in one sample could be indexed based on a 33.8 Å repeat corresponding to (–I− 3–I− 5–I− 3–) n arrays. The observed diffraction pattern was calculated from this model without using any freely adjustable parameters.