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-butylphenyloxy of 48 benzyl units in the periphery. The metallodendrimers were obtained by complexion of ruthenium trichloride of Ru(bpy)(2)Cl-2 with bipyridine ligands carrying dendritic wedges in the 4,4'-positions. The absorption spectra 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 them show the characteristic luminescence of the [Ru(bpy)(3)](2+)-type core unit. The dendritic branches protect the luminescent excited state of the core by dioxygen quenching. For the three compounds containing the 4'-tert-butylphenyloxy peripheral units, the electrochemical behavior and the excited-state quenching via electron transfer were also studied. The electrochemical 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 electrochemical behavior typical of encapsulated electroactive units. The reaction of the luminescent excited state of the [Ru(bpy)(3)](2+)-type core with three electron-transfer quenchers(namely, methyl violgen dication, tetrathrafulvalene, 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 simple [Ru(bpy)(3)](2+) complex. The results show that, for each quencher, the value of the rate constant decreases with increasing number and size of the dendritic branches. For the second-generation dendrimer containing 24 4'-tert-butylphenyloxy units at the periphery, the rate constant of the reaction 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 where butyronitrile was used.