Second harmonic generation coefficients in push-pull polyenes: a model exact study

Abstract

The correlated Pariser-Parr-Pople (PPP) model for interacting p electrons is employed for calculating the model exact SHG coefficients (β^exact) of push-pull polyenes with 4 to 10 carbon atoms in the all-trans configuration. These calculations are restricted to di-substituted polyenes with an electron donating (push) and an electron withdrawing (pull) groups. The effects of the push-pull strengths, the locations of the push and the pull groups in the chain, the twist around the polyene backbone, and the length of the chain, on the SHG response of these molecules are studied. These results are compared with that obtained (β^CT) from a simple two-state model where the excited state is a charge transfer state. The similarities and the differences in these two quantities (β^exact and β^CT) are discussed. Based on these results we present guide lines for synthesizing molecules with larger SHG coefficients.