The effect of the confinement of π electrons in one– and two–dimensional domains is illustrated with several examples ranging from linear polyene chains to planar molecules with honeycomb structure. Theoretical computations and specific Raman experiments on molecular materials demonstrate that a molecular approach provides a unified key to the interpretation of the Raman response both of linear polyconjugated polymers (polyacetylene) and of nanostructured graphitic materials.

A valence force field based on Hückel’s theory has been developed, which allows us to establish a close correlation between phonons of graphite and the normal modes of small polycyclic aromatic hydrocarbons (such as coronene and hexabenzocoronene). The results show that in these systems two kinds of motions dominate the Raman spectrum: the Я mode and the “breathing” A mode. These modes are the equivalent, in a finite domain, of the $E_{2g}$ phonon of graphite at the Γ point and the $A^{\prime }$ phonon at the K point of the first Brillouin zone. This study provides a useful basis for the understanding of the Raman spectra of any material containing ${\mathrm{sp}}^2$ carbon domains.

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.

A common valence force field for polycyclic aromatic hydrocarbons (PAHs) and graphite has been used in order to explain their Raman spectra from a unified viewpoint. On this basis the correlation observed between experimental spectra has been explained and rationalized. Quantum chemical density functional theory calculations of Raman intensities of small PAHs have been also performed, supporting the conclusions obtained from the dynamical analysis. The results obtained are useful for the characterization of materials containing graphitic domains and provide some new insight on the nature of the $D$ peak in disordered graphite.