Conformational defects in Durham polyacetylene: photo-induced IR absorption

Abstract

The authors report measurements of photo-induced absorption (PA) in the energy range 400-4500 cm^-1 in samples of stretch-oriented and unstretched Durham route polyacetylene. Stretch-oriented samples show spectra very similar to those for Shirakawa polyacetylene, with the electronic 'midgap' absorption peaking at 3600 cm^-1 (0.45 eV) and vibrational modes peaking at 520, 1287 and 1365 cm^-1. Whilst the oriented films show that characteristics of long-chain polyacetylene, the unoriented samples are very disordered, with only short sections of straight polyene. Similar PA features are seen, but shifted to higher frequencies, with the electronic 'midgap' absorption peak at 0.55 eV. The relative strength of the vibrational model is reduced by a factor of two, indicating an increase in soliton mass by this factor. They consider that the chain bends, etc., that terminate the straight-chain sections increase the effective electron-phonon coupling which raises the pi - pi * gap 2 Delta ₀ by 30% and the soliton mass ( varies as Delta ₀³) by 100%. The 'midgap' absorption is at an energy lower than Delta ₀ by an effective electron correlation energy (of about 0.5 eV) and the authors find, in accord with recent calculations, that this varies linearly with Delta ₀.