Vibrational spectroscopic study on partly hydrogenated trans-polyacetylenes

Raman and infrared spectra have been studied on variously hydrogenated trans-polyacetylenes which are prepared by exposing Na-doped trans-polyacetylene films to ethanol. Raman spectra have been analyzed by reference to the relations between the number of trans-conjugated double bonds (N_C=C) and the ν₁ and ν₃ frequencies. N_C=C's of increased short segments are correlated with the distribution of dopants. Information on (CH₂)_k groups and very short (CH=CH) _n groups including isolated (CH=CH) is obtained from infrared spectra and hydrogenation processes are proposed. Upon iodine doping the hydrogenated films exhibit three infrared bands which are similar to the well-known three-band pattern of doped polyacetylene, while they give poor electrical conductivities at the maximum iodine doping. Hence, the three bands of doped polyacetylene do not straightforwardly correlate with the overall conductivity. The infrared results strongly indicate that a relatively short conjugated segment with methylene groups at both terminals can be doped as well as a long polyene and the structure including the force field of the doped part in the segment is similar to that in the long polyene. Importance of intermolecular packing of the doped parts in realizing a high electrical conductivity is pointed out.