Raman and infrared studies on trans-copoly(acetylene+acetylene-d ₂): Vibrational frequency dispersion and the origin of doping-induced absorptions

Raman spectra of trans copolymers of acetylene and acetylene-d₂ have been measured with various isotopic mixing ratios. From the spectral changes associated with varying isotopic ratio, some criteria have been derived concerning the slopes of vibrational frequency dispersion curves near the zone center and the frequency ranges of dispersion for both (C₂H ₂)_n, and (C₂D₂)_n. Comparison of the criteria with the calculated dispersion curves reported so far shows that the force field for the infinite trans polyene chains should be reexamined. Infrared spectra of iodine-doped copolymers have also been investigated. Upon isotopic mixing, the infrared bands characteristic of doped polyacetylene behave quite differently from the Raman bands of pristine polyacetylene. This finding indicates that the doping-induced infrared absorptions cannot be correlated simply with the Raman-active vibrations in the unperturbed polyene chain. The behavior of the infrared bands is qualitatively accounted for by assuming a charged domain consisting of several or more consecutive monomer units for which the force field is severely affected by the charge transfer reaction with a dopant.