Analysis of the sign reversal of the second-order molecular polarizabilityin polymethineimine chains

The second-order polarizability tensor (beta) components of polymethineimin e oligomers have been computed with the semiempirical intermediate neglect of differential overlap (INDO) coupled either to sum-over-states (SOS) or f inite-field real space approach. Our goal is to rationalize the sign revers al of the longitudinal beta component with chain length. The INDO results a re shown to compare well with the values obtained from previous ab initio c alculations. Using a four-state SOS model that reproduces correctly the evo lution of beta with chain length, we demonstrate that the shape of the beta curve can be explained as resulting from the competition between two contr ibutions of opposite signs. In the framework of a real-space approach, thos e contributions are found to correspond to bond charge and bond polarizatio n phenomena. (C) 2001 American Institute of Physics.