Supramolecular dendrimers with a [Ru(bpy)3]2+ core and naphthyl peripheral units

We report the synthesis and the photophysical properties of first and second generation dendrimers built around a [Ru(bpy)3]2+ core (bpy = 2,2'- bipyridine) and bearing 12 and 24 naphthyl units, respectively, in the periphery. The metallodendrimers were obtained by complexation of ruthenium trichloride with bipyridine ligands carrying dendritic wedges in the 4,4'- positions. Since the chromophoric groups present in the dendritic complexes are separated by aliphatic connections, interchromophoric interactions are weak and the absorption spectra of the metallodendrimers are essentially equal to the summation of the spectra of the chromophoric groups which are present in their structures. The 'free' wedges show an intense emission band in the region of the naphthyl-type units. Such a band, however, is almost completely absent in the emission spectra of the metallodendrimers, which exhibit the visible emission band characteristic of their [Ru(bpy)3]2+- type unit, regardless of the excitation wavelength. These results show that a very efficient energy-transfer process takes place from the potentially fluorescent excited states of the aromatic units of the wedges to the metal- based dendritic core (antenna effect). We have also found that the dendrimer branches protect the Ru-bpy based core from dioxygen quenching.