Abstract: Calculations of the nonlinear second-order optical susceptlbilities(β_ijk)for sub- stituted tl1iophene derivative;with quinoidlike conformation are reported.These systetems possess small dipole moment;and large differences between dipole mo- ments of ground and first-excited states.Geometry optimizations of the molecules investigated were carried out using AM1 method.The calculations were performed using INDO/CImethod comboned with a sum-over-states expression for β_jik.The calculated results sbw that the second-order susceptibility is a function of the na- ture and location of substituents and is larger for disubstituted molecules than monosubstituted molecules.Bipolymeric thiophenemetmne with NH₂/NO₂ groups was calctilated to have a β_μ of 79.920 × 10^-30 esu.It was found that the NH₂ and NO₂ groups in above disubstituted molecules are pull-pull groups in ground states,but are usual push-pull groups in the first excited states.

Key words: Nonlinear second-order optical susceptibility, INDO/CI, Substituted thiophene derivatives