Analysis of the sign reversal of the second-order molecular polarizability in polymethineimine chains

The computation of second-order polarizability tensor (β) components of polymethineimine oligomers was done with the semiempirical intermediate neglect of differential overlap (INDO) coupled either to finite-field real space or sum-over-states (SOS) approach. The sign reversal of the longitudinal β component was rationalized with chain length. Using a four-state SOS model, it was demonstrated that the shape of the β curve could be explained as resulting from the competition between two contributions of opposite signs. Those contributions were found to correspond to bond polarization and bond charge phenomena, in the framework of a real-space approach.