ELECTRONIC FIRST HYPERPOLARIZABILITY OF POLYMETHINEIMINE CHAINS WITH DONOR AND ACCEPTOR GROUPS

The static first hyperpolarizability of push-pull compounds based on t he 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 (polya cetylene) segment results in an increase of the first hyperpolarizabil ity, whereas the opposite is true when using strong donor and acceptor groups. These effects have been explained as the consequence of the i nterplay between asymmetry and delocalization effects. We use the Hamm et sigma parameters in order to quantify the susceptibility with respe ct to the first hyperpolarizability of substituted polyacetylene and p olymethineimine chains. Moreover the evolution with chain length of th e first hyperpolarizability per unit cell of alpha,omega-nitro-amino p olymethineimine was studied and compared to the non-substituted polyme thineimine chains.