On the basis of ZINDO methods, according to the sum-over-states (SOS) expression, a program for the calculation of nonlinear second-order optical susceptibilities βijk has been devised. The structures and nonlinear second-order optical properties of phenothiazine derivatives substituted at nitrogen by different substituents have been studied. The conclusion is that electron donating groups substituted at N facilitate the nonlinear second-order optical susceptibility, whereas electron with drawing groups substituted at N decrease the nonlinear second-order optical susceptibility. Extending the conjugated area increases the nonlinear second-orer optical susceptibility. The calculated results have been explained micromechanically.

Key words: OPTICAL PROPERTIES, NON LINEAR OPTICS, PHENOTHIAZINE P