A model exact calculation of SHG coefficients in polysubstituted push—pull polyenes

Abstract

We report the model exact second-harmonic generation coefficients (β exact) in polysubstituted all-trans push—pull polyenes up to ten carbon atoms. The Pariser—Parr—Pople (PPP) model with standard molecular parameters has been used in the calculations. The two-state charge-transfer model in these systems is inadequate and an exact calculation is necessary to obtain reliable SHG coefficients. We find that for a fixed geometry, β is enhanced if all the electron withdrawing substituents (pull) are successively placed from one end of the polyene chain and all the electron pushing groups (push) from the other. A twisted conformation of the polyene backbone leads to large SHG coefficients at the YAG laser frequency since the system approaches two-photon resonance. For a given chain length, the SHG coefficients increase with increase in the number of the push and pull groups.