Raman dispersion in a substituted polyacetylene

Raman spectra of poly(2-pyridinium hydrochloride-2-pyridylacetylene) (P2EPH) were obtained using several excitation lines. A dependence of the frequencies of the polymer backbone modes with the laser line energy was clearly noticed. The observed dispersion (0.05 cm−1 nm−1) is significantly smaller than the one reported for polyacetylene (0.3 cm−1 nm−1). The Raman dispersion was analysed using the conjugation length and the amplitude mode models. Using the amplitude mode theory, the relationship between ∏n(ωnR/ωn0)2 and ωL is non-linear, suggesting a smaller conjugation length in P2EPH than in polyacetylene and/or the presence of a second Ag excited state close to the first one.