Third order hyperpolarizabilities of cyanines: measurements and calculations of the chain length dependence

Summary form only given. Cyanines with long chains exhibit very high third order hyperpolarizabilities, which have been connected to the bond equalization in the /spl Pi/-chain. However a sign reversal of hyperpolarizabilities of short chains has been explained by bond alternation. To correlate measured optical nonlinearities with molecular structures, e.g., chain-lengths dependences of homologous series, it is necessary to take into account the dispersion of /spl gamma/ which is strongly affected by structural changes. Therefore, we determined for the first time the dispersion of the real and of the imaginary parts of the third order (electronic) hyperpolarizability in solutions of the whole homologeous series of bis-(dimethylamino)n-methinium dyes.