THEORETICAL-STUDY OF PYRROLE OLIGOMERS - ELECTRONIC EXCITATIONS, RELAXATION ENERGIES, AND NONLINEAR-OPTICAL PROPERTIES

We calculate the vertical-transition energies from the ground state to the lowest singlet excited states of pyrrole oligomers containing up to seven rings, by means of different configuration interaction techni ques. Special attention is paid to the evolution with chain length of the ordering of the lowest two excited states. The geometry-relaxation phenomena in the 1B and 2A states are modeled on the basis of the for mation of bipolaron-type defects. The chain-length dependence of the t hird-order polarizability gamma as well as the two-photon absorption a nd third-harmonic generation spectra are evaluated from a sum-over-sta tes approach.