The geometries, electronic structures and electronic spectra of stilbene derivatives and chromophores based on dithienothiophene are systematically studied by AM1 and INDO/CI methods. On the basis of correct UV-vis spectra, the position and strength o the two-photon absorption can be predicted. We compiled a program fo Sum-Over-States expression and calculated the nonlinear third-order optical susceptibilities and two photon absorption cross-sections by it. The influene of various molecular skeleton and different substitutional groups on two photon absorption cross-sections are discussed micromechanically.

Key words: ELECTRONIC SPECTROSCOPY, TWO PHOTON ABSORPTION, NON LINEAR OPTICS