OPTICAL NONLINEARITIES IN SYMMETRICAL CYANINE DYES AND RELATED SYSTEMS

First- and third-order dynamic polarizabilities and hyperpolarizabilities of aza-cyanine and symmetric cyanine dyes have been computed exactly within the correlated Pariser-Parr-Pople model defined over the pi framework. The exact nonlinearities of the model have been compared with those of polyenic systems. It is found that for the same chain length the cyanine dyes have the largest linear (alpha(ij)) and third-order nonlinear (gamma(ijkl)) coefficients among the carbon backbone systems. Azacyanine dyes have intermediate polarizabilities, and the polymethineimines have the lowest. This is explained by the presence of terminal nitrogen-atom conjugation in the cyanine dyes, resulting in the lowering of optical gaps, which in turn brings the system close to resonance and results in larger linear and nonlinear coefficients. Chain length and frequency dependencies of alpha and gamma of these systems are also reported.