A comparison of the Raman dispersion in different polyacetylenes with aromatic ring substituents

Raman dispersion data for poly-2-ethynylpyridine, poly-4-ethynylpyridine, poly-β-naphthylacetylene, poly(2-pyridinium hydrochloride-2-pyridylacetylene) and poly(2-N-t-butylpyridiniumylacetilene)iodide were obtained and analysed using the Amplitude Mode Model (AMM).The slopes of the ∏n(ωnR/ωn0)2 versus ωL curves (D) ranged from 0.090 to 0.146 eV−1, substantially larger than the values reported for other photoluminescent conjugated polymers (up to 0.020 eV−1). This behaviour was interpreted as arising from the presence of a second Ag (3Ag) excited state close to the first one (2Ag).Polymers with charged substituents presented the larger D   values, as already observed for other conjugated polyacetylenes. It seems that the repulsion of the charged groups leads to flatter arrangements of the chains, increasing the conjugation length and consequently the slope of the ∏n(ωnR/ωn0)2 versus ωL curves.