DENDRITIC BIPYRIDINE LIGANDS AND THEIR TRIS(BIPYRIDINE)RUTHENIUM(II) CHELATES - SYNTHESES, ABSORPTION-SPECTRA, AND PHOTOPHYSICAL PROPERTIES

Several synthetic strategies have been explored to prepare dendrimers having the [Ru(bpy)(3)](2+) complex as their core (bpy = 2,2'-bipyridi ne). Dendritic ligands have been synthesized by attaching branches in the 4,4'-positions of bpy. The largest dendritic bipyridine ligand con tains 54 peripherical methylester units, Four Ru-II dendritic complexe s have been prepared, Their absorption and emission spectra are very s imilar to those of the unsubstituted parent Ru-II-bipyridine complexes , The large dendritic complexes, however, exhibit a more intense emiss ion and a longer excited-state lifetime than [Ru(bpy)(3)](2+) in aerat ed solutions, This is due to the shielding effect oi-the dendrimer bra nches on the Ru-bipyridine core, which limits the quenching effect of molecular oxygen, For the largest dendritic complex, which contains 54 peripherical methylester units, the excited-state lifetime in aerated acetonitrile solution is longer than 1 mu s, and the rate constant fo r dioxygen quenching is twelve times smaller than for [Ru(bpy)(3)](2+) .