Electronic properties of polyacetylene prepared by the Durham ‘photoisomer’ route

Abstract

We report here the properties of polyacetylene prepared via a precursor polymer, modified from the standard Durham precursor by photoisomerization of the monomer to 3,6-bis(trifluoromethyl)pentacyclo-[6.2.0.0 2,4,O 3,6,O 5,7]dec-9-ene. This modification gives a precursor with improved thermal stability, in that conversion to polyacetylene only occurs above 60°C. The conversion is highly exothermic, but the method is suitable for the formation of thin films. There is evidence from infra-red and X-ray photoelectron spectroscopy measurements for retention of fluorine in the thermally treated polymer, and we consider that this is probably due to isomerization of some of the side groups on the precursor polymer, rather than their elimination. Polyacetylene produced in the form of thin unoriented films is highly disordered, with a coherence length interchain order determined from X-ray scattering of only 1.8 nm. The electronic and vibrational structure of these films is much affected by the disorder, with the peak in the π-π∗ optical absorption band at 2.65 eV, and correspondingly high Raman-active vibrational modes. We compare also the properties of polymers prepared with different degrees of polymerization, achieved by addition of a chain transfer agent during polymerization of the monomer to form the precursor polymer. There is some evidence from the Raman spectra that the lower molecular weight precursors produce better ordered polyacetylene.