Geometrical and electronic structures of polyacetylene chlorinated via prolonged chemical doping

Geometrical and electronic structures of chlorinated polyacetylene, (CHCl)_x, a precursor of one-dimensional conjugated polyyne, have been investigated through experimental and theoretical approaches. Uniaxially oriented (CHCl)_x films prepared by means of prolonged FeCl₃-doping of mechanically stretched polyacetylene films have been subjected to analyses. Polarized infrared absorption spectra showed a dichroism to confirm an orientation of the (CHCl)_x films. On the basis of x-ray diffraction analysis, a fiber period of the (CHCl)_x has been determined to be 5.2 ± 0.1 Å. Ab initio crystal orbital calculations have been performed on structural models of infinite polymer systems for the (CHCl)_x. Energy gradient optimization afforded a relative stability of three different configurational structures of poly-1,2-dichloroethylene which is assumed to be a planar zigzag all-trans conformation with respect to carbon-carbon single bonds, i.e., di-syndiotactic (dst) > erythro-di-isotactic (e-dit) > threo-di-isotactic (t-dit) structures. In addition, the fiber period of the ist structure was calculated to be 5.2 Å, coinciding with the experimental value of the (CHCl)_x. In consequence, it has been demonstrated that the prolonged FeCl₃-doping of polyacetylene causes a stereospecific chlorination to yield a stereoregular dst structure of the (CHCl)_x. The present results imply that a usage of this sort of chlorination through the chemical doping should provide a novel reaction design, especially to control a stereospecificity of chlorination reaction.