Resonance Raman excitation profiles of polyacetylene/polyisoprene block co-polymers in toluene

Abstract

We have measured the resonance Raman spectra and excitation profiles of a soluble polyacetylene/polyisoprene block-copolymer in toluene solution. These are the first reported excitation profiles that have been referenced to an internal solvent standard, the toluene Raman bands. We find that this copolymer exhibits the same two bands in the CC stretch region, i.e., 1463 and 1519 cm −1,that have been attributed to a bimodal distribution in conjugation length for polyacetylene thin films. The 1519 cm −1 peak has been associated with short chain segments of polyacetylene and we assign it to an average chain length of N=11 CCs based on the frequency of the peak of its excitation profile. The excitation profile of the 1463 cm −1 mode peaks at 2.15 eV corresponding to a chain length of 31 CCs. Based on the relative integrated intensities of these two excitation profiles, our block copolymer contains a 35% mole fraction of long chain segments to chain segments. In contrast to previous results, we find that the 1294 cm −1 band, which is a mode with a mixture of CC stretch and CH deformation, is resonance enhanced, with an excitation profile that peaks at 2.6 eV, the same energy as the 1519 cm −1 mode's profile.