Staging in polyacetylene–iodine conductors

Abstract

Evidence is presented for the existence of highly conducting polyacetylene complexes with structures related to high‐stage graphite, as well as structures related to first‐stage graphite. X‐ray diffraction measurements on polyacetylene–iodine complexes indicate equatorial lines at 7.7–8.0 and 13.8–14.3 Å. The shorter spacing arises in part from a structure in which iodine‐rich planes alternate with planes of polyacetylene chains. The longer spacing, which disappears upon atmospheric exposure, is consistent with a structure analogous to third‐stage graphite in which dopant‐rich planes are separated by three close‐packed planes of polyacetylene chains. The third‐stage complex can be viewed as a perturbation of the structure of undoped polyacetylene, with the region between dopant layers consisting essentially of a one unit cell thickness of the parent polymer structure. Packing calculations for this model, in which a linear column of anions (I3 − and/or I5 −) displaces either every chain or every other chain in the dopant‐rich layer, provide an interlayer spacing which is equal to that observed. Evidence consistent with third‐stage structures (with both fractional occupation and complete occupation of the dopant plane) is also found by reexamination of published sorption data, which provides slope changes at close to the calculated limiting compositions for these structures [(CHI0.056) x and (CHI0.13) x ]. However, a first‐stage structure with alternating dopant arrays and polymer chains in the dopant plane [for which (CHI0.13) x is calculated] provides a better explanation for the second slope change, as well as for the composition obtained under dynamic vacuum, (CHI0.14) x . These results for iodine complexes are compared with those derived for the group VA halide complexes of polyacetylene.