Dendrimers with a Photoactive and Redox-Active [Ru(bpy)₃]²⁺-Type Core: Photophysical Properties, Electrochemical Behavior, 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 or 48 benzyl units in the periphery. The metallodendrimers were obtained by complexation of ruthenium trichloride or Ru(bpy)2Cl2 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 evidenced 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 viologen dication, tetrathiafulvalene, 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 smaller 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.