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 techniques. 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 formation of bipolaron-type defects. The chain-length dependence of the third-order polarizability γ as well as the two-photon absorption and third-harmonic generation spectra are evaluated from a sum-over-states approach.