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 solution. Among the properties studied, particular attention is devoted to frequency and intensity dispersion of the Raman bands with increasing conjugation length and to the vibrational second order hyperpolarizability gamma(r). The results obtained are compared with those relative to polyenic systems. The behavior of the various classes of molecules studied is in some cases different both in absolute values and trends. This fact is discussed in order to clarify the influence of the topology of the rr-electron system on the properties of conjugated materials and to determine whether the presence of aromatic rings in the main chain can confine pi electrons and so reduce delocalization. Oligorylenes turn out to be the compounds with the largest vibrational gamma(r). The results also indicate that absolute Raman intensity shows strong intensity dispersion with conjugation length and can be used as a powerful tool in characterizing conjugated compounds. (C) 1997 American Institute of Physics.