Unexpected High Second-Order Nonlinear Optical Activity of Metal Complexes with Three-Branched Hexadentate 2,2'-Bipyridine Ligands.

Two hexadentate bipyridine ligands and their RuII and NiII complexes were prepared. The helical alignment of the three electron-donor-π-bridge-electron-acceptor (d-π-A) single-strands with bundle architecture in cooperation with the metal center can strongly enhance the nonlinear optical (NLO) properties. The complexation of the novel cage-type hexadentate ligands with a paramagnetic NiII -core almost doubles the ßHRS values compared with the corresponding diamagnetic RuII complexes. The hyper-Rayleigh scattering (HRS) was performed with a highly sensitive setup for simultaneous discrimination between multi-photon fluorescence and the molecular first hyperpolarizability.

2,2'-Bipyridine-based dendritic structured compounds for second harmonic generation.

Parallel alignment of dipolar electron-donor-π-bridge-electron-acceptor entities can strongly enhance their nonlinear optical properties. This favorable arrangement can be in principle achieved by linking these units covalently or through metal coordination. Four dipolar single-strand chromophores decorated with a 5-electron-donor-5'-electron-acceptor-modified 2,2'-bipyridine functionality were synthesized. For two of these chromophores triple-stranded dendritic structures were successfully formed. All of the compounds were characterized with respect to their linear and nonlinear optical properties. For the aldehyde derivatives an enhancement of the first hyperpolarizability of 4.5 rather than 3 was obtained when going from single to triple strands. Theoretical calculations with density functional theory suggest that interstrand transitions contribute to the optical properties of the dendritic structures.