Raman excitation profiles of conjugated segments in solution

We report on optical absorption and Raman scattering of trans-polyacetylene-polynorbornene diblock copolymers of varying block lengths in solution. Raman spectroscopy on the polyacetylene-type copolymer revealed well-separated peaks within the usually broad C=C stretching vibration band. These peaks show multimodal excitation profiles, and they are assigned to segments with 7, 9, 11, and 13 conjugated double bonds. The Raman frequencies of these peaks shift up to 5 ${\mathrm{cm}}^{\mathrm{-}1}$ with excitation energy, which decreases with conjugation length. The overall dispersion of the vibration band with excitation energy is explained by the varying resonance conditions for the available conjugation lengths. Extrapolating the Raman frequencies towards infinite conjugation length, we estimate a limiting value of about 1440 ${\mathrm{cm}}^{\mathrm{-}1}$ for the C=C stretching vibration. The dispersion of the gap energies with conjugation lengths gives a limiting value of about 1.50 eV for solutions in benzene or amorphous films. The molar distribution of conjugation lengths is calculated under the assumption that the contribution of a given conjugation length to the Raman spectrum is proportional to its concentration multiplied by an exponential function of the conjugation length ${\mathit{n}}^{\mathrm{\alpha }}$, with α taken from the literature.