Conformational defects in Durham polyacetylene

Polyacetylene prepared by the Durham precursor route can be obtained in a disordered form by thermal conversion of a solution-cast film of the precursor polymer, or in a highly oriented form by stretching the precursor film during the transformation reaction. The oriented films show the properties that are characteristic of long straight-chain sequences, but the unstretched films show evidence for very short straight-chain lengths, of no more than 20 or 30 carbon atoms. We discuss the dispersion of the resonantly-enhanced Raman modes with excitation energy and show that within the amplitude-mode formalism of Horovitz the defects that terminate the straight sequences in unstretched material do not impose a preferred sense of bond alternation on the chain. Similarly, the defects introduced through chemical doping or photoexcitation show the characteristics of soliton-like states, with a single "mid-gap" electronic absorption feature which is at significantly higher energies than for stretched Durham or Shirakawa polyacetylene. The dopant-induced infra-red modes, however, are similar in frequency to those in stretched Durham and Shirakawa materials. © 1987.