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'-bipyridine). Dendritic ligands have been synthesized by attaching branches in the 4,4'-positions of bpy. The largest dendritic bipyridine ligand contains 54 peripherical methylester units, Four Ru-II dendritic complexes have been prepared, Their absorption and emission spectra are very similar to those of the unsubstituted parent Ru-II-bipyridine complexes, The large dendritic complexes, however, exhibit a more intense emission and a longer excited-state lifetime than [Ru(bpy)(3)](2+) in aerated solutions, This is due to the shielding effect oi-the dendrimer branches 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 for dioxygen quenching is twelve times smaller than for [Ru(bpy)(3)](2+).