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.

In this work we present a theoretical justification, based on a two-state model, of the recently observed close resemblance between the molecular first-order hyperpolarizability (${\mathrm{\beta }}^{\mathit{e}}$) obtained with traditional experimental or theoretical methods and the vibrational (or relaxation) contribution (${\mathrm{\beta }}^{\mathit{r}}$) for several classes of polyconjugated molecules. The vibrational hyperpolarizabilities have been evaluated according to a semiclassical model previously presented by the authors in which molecular polarizabilities are expressed in terms of vibrational (infrared and Raman) intensities. Here we prove that in the case of polyconjugated molecules, the analytic expressions of ${\mathrm{\beta }}^{\mathit{e}}$ and ${\mathrm{\beta }}^{\mathit{r}}$ are functions of the same physical parameters. This implies that ${\mathrm{\beta }}^{\mathit{e}}$ and ${\mathrm{\beta }}^{\mathit{r}}$ provide a measure of the same physical property. © 1996 The American Physical Society.