NONLINEAR-OPTICAL AND VIBRATIONAL PROPERTIES OF CONJUGATED POLYAROMATIC MOLECULES

Raman spectra of oligo-p-phenylenes, oligorylenes, and oligoacenes of different chain lengths have been obtained in the solid state and in s olution. Among the properties studied, particular attention is devoted to frequency and intensity dispersion of the Raman bands with increas ing conjugation length and to the vibrational second order hyperpolari zability gamma(r). The results obtained are compared with those relati ve to polyenic systems. The behavior of the various classes of molecul es studied is in some cases different both in absolute values and tren ds. This fact is discussed in order to clarify the influence of the to pology of the rr-electron system on the properties of conjugated mater ials and to determine whether the presence of aromatic rings in the ma in chain can confine pi electrons and so reduce delocalization. Oligor ylenes turn out to be the compounds with the largest vibrational gamma (r). The results also indicate that absolute Raman intensity shows str ong intensity dispersion with conjugation length and can be used as a powerful tool in characterizing conjugated compounds. (C) 1997 America n Institute of Physics.