Frequency dispersions with chain length have been experimentally determined from Raman-scattering data published previously on a series of oligomers and polymers of paraphenylene, paraphenylene vinylene, thiophene, N-protected pyrrole, pyrrole, and furan. The dispersion behavior changes noticeably in the different series of compounds. Conformational flexibility and the confinement of π electrons within each aromatic ring are the two factors considered for the explanation of such an observation. The π-electron confinement is analyzed in terms of the effective conjugation coordinate theory which is related to the amplitude mode theory. Ab initio calculations performed on model compounds are used to support the experimental evidence of the competition between π-electron confinement within the rings and delocalization along the chain.

• Received 10 May 1994

DOI:https://doi.org/10.1103/PhysRevB.50.9815