Excited state properties of bis-tetraazaphenanthrene-Ru(II) diad complexes with a ferrocenyl unit

The excited state properties of the diad complexes Ru(tap)(2)(ppFc)(2+) and Ru(bpy)(2)(ppFc)(2+) (tap, 1,4,5,8-tetraazaphenanthrene; bpy, 2,2'-bipyridine), bearing a reducing ferrocenyl (Fc) centre on a derivatized (pyridine)pyrazole (pp) ligand, were compared with those of the corresponding complexes without an Fc unit. For these latter complexes, the existence of intermolecular photoelectron transfer with added ferrocene was clearly demonstrated. However, direct evidence for intramolecular photoelectron transfer in Ru(bpy)(2)(ppFc)(2+) and Ru(tap)(2)(ppFc)(2+) could not be obtained unambiguously. Although the absence of luminescence in these diad complexes indicated the existence of this intramolecular process, laser flash photolysis did not allow the detection of a reduced complex entity in most experimental conditions. Only flash photolysis of Ru(bpy)(2)(ppFc)(2+) in water generated a long-lived intermediate with the characteristic spectral features of the radical anion bpy(.-). Indirect arguments, such as the formation of a product on illumination of both diad complexes, were in favour of a photoreaction induced by intramolecular electron transfer.