Third-order optical nonlinearities of organometallics: influence of dendritic geometry on the nonlinear properties and electrochromic switching of nonlinear absorption

We have investigated a large number of organometallic structures possessing high second and third order optical nonlinearities. Most third-order NLO experiments were performed with 100 femtosecond light pulses at 800 nm. While investigating structure-property relationships we note an enhancement of the real and imaginary parts of the cubic hyperpolarizability in structures of trigonal symmetries, and, in particular, of dendritic geometries with multipolar charge distribution, due to the presence of charge acceptor groups. Two-photon absorption cross sections achievable in these structures are comparable with the best values reported in the literature. We also show that, for some of the ruthenium alkynyl complexes investigated in this study, it is possible to perform electrochemical switching between two forms of the compound. The nonlinear properties of the two forms were investigated by the technique of Z-scan carried out in situ in an electrochemical cell. The neutral form is essentially nonabsorbing in the infrared but shows two-photon absorption at 800 nm. Upon oxidation the complex becomes infrared absorbing and the oxidized form exhibits an absorption saturation effect at 800 nm. This electrochromic switching of both the linear absorption and the sign of the imaginary part of the third-order susceptibility is found to be reversible and is, therefore, of potential application interest.