Nature of optical transitions in conjugated oligomers. I. Theoretical characterization of neutral and doped oligo(phenylenevinylene)s

The nature of the main optical transitions taking place in oligo(phenylenevinylene)s is analyzed in both the neutral and oxidized states. Geometry optimizations are first carried out with the Hartree-Fock semiempirical Austin Model 1(AM1) method; on the basis of the resulting geometries, the transition energies and their intensities are determined by means of the Hartree-Fock semiempirical intermediate neglect of differential overlap (INDO) method combined with a single configuration-interaction (SCI) technique. The major aspect of the results is to show that two subgap absorption peaks are induced by the generation of polarons (radical cations), while a single absorption feature is expected when bipolarons (dications) are formed, in agreement with the experimental absorption spectra of the oligomers; this situation is, however, in marked contrast with that in the polymer. The possible formation of diamagnetic π dimers is also discussed.