Model exact study of DC-electric-field-induced second-harmonic-generation coefficients in polyene systems

Abstract

We report model exact electric-field-induced second-harmonic- (EFISH) generation coefficients of polyene systems, within the Pariser-Parr-Pople model. For a given unsubstituted polyene, we find that the EFISH generation coefficient is largest when the field is applied along the chain axis and depends nearly linearly on the field strength. The EFISH coefficients increase with increase in chain length according to a power law with an exponent of ⋍3.5 for ||E_x||=0.05 V/Å. The EFISH generation coefficients are sensitive to backbone stereochemistry and increase rapidly for twist angle θ up to about 75° but suddenly drop to very low values for θ=90°. In push-pull polyenes, the effect of the electric field on the SHG coefficients depends upon the direction of the field relative to the push-pull groups. The size dependence of the SHG coefficients in the presence of the external fields is weaker in these systems, although the behavior of the SHG coefficients with respect to backbone stereochemistry is very similar to that of the EFISH coefficients as well as that of the SHG coefficients in the absence of external fields.