Dendrimers with a photoactive and redox-active [Ru(bpy)(3)](2+)-type core:Photophysical properties, electrochemical behaviour, and excited-state electron-transfer reactions

We report the synthesis of six new dendrimers built around a [Ru(bpy)(3)](2 +)-type core (bpy = 2,2'-bipyridine) and bearing up to 24 4'-tert-butylphen yloxy of 48 benzyl units in the periphery. The metallodendrimers were obtai ned by complexion of ruthenium trichloride of Ru(bpy)(2)Cl-2 with bipyridin e ligands carrying dendritic wedges in the 4,4'-positions. The absorption s pectra and luminescence properties (spectra and lifetimes at 77 and 298 K; quantum yields at 298 K) of the six novel compounds are reported. All of th em show the characteristic luminescence of the [Ru(bpy)(3)](2+)-type core u nit. The dendritic branches protect the luminescent excited state of the co re by dioxygen quenching. For the three compounds containing the 4'-tert-bu tylphenyloxy peripheral units, the electrochemical behavior and the excited -state quenching via electron transfer were also studied. The electrochemic al experiments have evidence an oxidation and three reduction one-electron processes centered in the [Ru(bpy)(3)](2+)-type core and two multielectron oxidation processes involving the dioxybenzene-and oxybenzene-type units of the dendritic branches. The core of the largest dendrimer shows an electro chemical behavior typical of encapsulated electroactive units. The reaction of the luminescent excited state of the [Ru(bpy)(3)](2+)-type core with th ree electron-transfer quenchers(namely, methyl violgen dication, tetrathraf ulvalene, and anthraquinone-2,6-disulfonate anion) was found to take place by a dynamic mechanism in all cases. The quenching rate constants, obtained by Stern-Volmer kinetic analysis, are compared with those found for the si mple [Ru(bpy)(3)](2+) complex. The results show that, for each quencher, th e value of the rate constant decreases with increasing number and size of t he dendritic branches. For the second-generation dendrimer containing 24 4' -tert-butylphenyloxy units at the periphery, the rate constant of the react ion with methyl viologen is more than 1 order of magnitude small than that of the "naked" [Ru(bpy)(3)](2+) complex. All the experiments were performed in acetonitrile solution, except for luminescence experiments at 77 K wher e butyronitrile was used.