Spectroscopic studies on doped polyacetylene and β-carotene

Abstract

In order to elucidate the structural details of doped polyacetylene (a highly conducting organic polymer), the optical absorption, Raman, and infrared spectra of not only trans‐(CH)_x doped with iodine, AsF₅, and SO₃ but also β‐carotene doped with iodine and SO₃ were studied. The infrared spectra of two kinds of isotopically substituted polyacetylenes (CD)_x and (¹³CH)_x doped with iodine were also observed. Analysis of the experimental results shows that upon doping each of the four vibrational branches (ν₁–ν₄) in the 1600–900 cm⁻¹ region of a polyene chain splits into two groups, namely, the higher frequency group and the lower frequency one. The former group consists of the ’’gerade’’ vibrations of polyene parts which are not directly attacked by dopants but are perturbed along the chain, whereas the latter is made up of the ’’ungerade’’ vibrations of the positively charged polyene part with the doped site at its center. The Raman bands in the higher‐frequency group of ν₁ (mainly the C=C stretching mode) observed with various laser lines give definite information on the segments of conjugated trans double bonds existing in doped polyacetylene. The Raman spectra are also useful for clarifying the structures of dopants. In the infrared spectra of doped trans‐(CD)_x and (¹³CH)_x the bands appearing on doping all showed respective isotope shifts which confirmed the view that these bands are of vibrational origin. In many respects doped β‐carotene proved to be a useful model of doped trans‐polyacetylene.