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 of 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 μs, the rate constant for dioxygen quenching is twelve times smaller than for [Ru(bpy)3]2+.