Electronic first hyperpolarizability of polymethineimine chains with donor and acceptor groups

The static first hyperpolarizability of push-pull compounds based on the polymethineimine asymmetric unit cell has been investigated within a CHF/6-31G scheme. For push-pull pairs of small strength, the use of an asymmetric (polymethineimine) segment instead of a symmetric (polyacetylene) segment results in an increase of the first hyperpolarizability, whereas the opposite is true when using strong donor and acceptor groups. These effects have been explained as the consequence of the interplay between asymmetry and delocalization effects. We use the Hammet σ parameters in order to quantify the susceptibility with respect to the first hyperpolarizability of substituted polyacetylene and polymethineimine chains, Moreover the evolution with chain length of the first hyperpolarizability per unit cell of α,ω-nitro-amino polymethineimine was studied and compared to the non-substituted polymethineimine chains.