Syntheses and properties of bimetallic chromophore-quencher assemblies containing ruthenium(II) and rhenium(I) centers

We have prepared a family of six new bimetallic complexes containing electron-donating trans-{(RuCl)-Cl-II(pdma)(2)}(+) [pdma = 1,2-phenylenebis(dimethylarsine)] centers linked to trans-[(RuCl)-Cl-II(pdma)(2,2'-bpy)(NO)] (bpy = bipyridyl) or fac-{Re-I(biq)(CO)(3)}(+)-based (biq = 2,2'-biquinolinyl) electron acceptors. The extended bridging units are based on 4,4'-bpy or bpe [E-1,2-bis(4-pyridyl)ethylene] ligands, and these assemblies are designed to display long-lived photoinduced charge-separated states. A number of new monometallic precursor complexes have also been synthesized and fully characterized. The electronic absorption spectra of the bimetallic species are dominated by intense, visible d(Ru-II) -> pi*(4,4'-bpy/bpe) metal-to-ligand charge-transfer (MLCT) bands and d(Re-I) -> pi*(biq) absorptions in the near-UV region. Cyclic voltammetric studies show both metal-based oxidation and ligand-based reduction processes and provide evidence that these assemblies contain thermodynamic driving forces for charge separation. Single-crystal X-ray structures have been determined for the monometallic complex salts fac-[Re-I(biq)(CO)(3)(MeCN)]CF3SO3 center dot CHCl3, fac-[Re-I(biq)(CO)(3)(4-BrCH2C5H4N)]PF6, fac-[Re-I(biq)(CO)(3)(4-HCO2CH2C5H4N)]PF6 center dot Me2CO, fac-[Re-I(biq)(CO)(3)(ISNE)]CF3SO3 (ISNE = ethylisonicotinate), and fac-[Re-I(biq)(CO)(3)(NC5H4CO2)](2)center dot HPF6 center dot 3MeCN. Ultrafast time-resolved infrared and Raman spectroscopic measurements will be employed to investigate the photoexcitation properties of the bimetallic species and of monometallic reference complexes.