Abstract: On the basis of AMIand INDO/CImethods, according to the sum-over-states expression, we devised a program for calculating nonlinear second-order optical susceptibilities β_ijk and performedrelevant study of molecular design, i.e., on the basis of(这里有图片19930622-821-1.gif) we examined theeffect of different substituents introduced into the phenyl and quinonoid rings, on the second-order optical nonlinearity.It was concluded that introducing a donor into the phenyl ring leads to the increase of β and introducing an acceptor into the phenyl ring leads to a reduction of β; in contrast with this, in troducing a donor in the quinonoid ring leads to the reduction of β and introducing an acceptor in quinonoid ring leads to the increas of β.The regularity summarized from above results has been explained micromechanically.Finally, two molecules having larger nonlinear second-order optical susceptibilities have been designed.

Key words: Molecular design, NLO second-order susceptibility, Excited states