Dendritic supramolecular assembly with multiple Ru(II) tris(bipyridine) units at the periphery: Synthesis, spectroscopic, and electrochemical study

A supramolecular assembly with eight peripheral ruthenium(II) tris(bipyridine), [RU(bpy)(3)](2+) units covalently linked to a carbosilane dendrimer platform has been synthesized. H-1 NMR and MALDI-TOF mass spectrometry confirm the target structure. Spectroscopic and electrochemical studies disclose that the identical [Ru(bpy)(3)](2+) units in this system interact neither in the ground state nor in the excited state. In acetonitrile solutions of identical molar concentration of [Ru(bpy)(3)](2+) units, both the dendrimer and reference monomeric [Ru(bpy)(2)(4-octoxy-2,2'-bipyridine](2+) exhibit identic al absorption and emission spectra. Cyclic voltammetry reveals that the dendrimer and monomer possess the same redox potentials of the metal-centered oxidation process [RU(bpy)(3)](2+/3+) and the first ligand-centered reduction process [RU(bpy)(3)](1+/2+). The abnormal redox peaks of the [RU(bpy)3](0/1+) and [RU(bpy)(3)](1-/0) transitions of the dendrimer are attributed to the accumulation of neutral dendrimer on the electrode. A preliminary study of the electrochemiluminescence (ECL) in tripropylamine-acetonitrile solution indicates that the ECL intensity of the dendrimer with eight [RU(bpy)(3)](2+) units is 5 times higher than that of the reference monomeric species. Therefore, the molar emission, generated either by photoexcitation or by electrochemical excitation, can be amplified by incorporating multiple luminophores into a multibranch platform without significantly changing the redox and photophysical properties. The possible use of supramolecular assemblies as labels for biodiagnostics is discussed.