REDUCTIVE QUENCHING OF NOVEL MIXED-LIGAND TRIS(BIPYRIDYL)RUTHENIUM(II) COMPLEXES IN AQUEOUS-SOLUTION AND INERT COLLOIDAL SUSPENSIONS

The reductive quenching of the lowest energy electronically excited state of four mixed-ligand tris(bipyridyl)ruthenium(II) complexes by anionic ascorbate and N-phenylglycine electron donors was investigated in aqueous solution at pH greater than or equal to 5.0. The central ruthenium(II) metal atom is coordinated to either two 2,2'-bipyridine (bpy) or two 4,4'-dicarboxy-2,2'-bipyridine (dcbpy) ligands to render the complexes a net 2+ ([Ru(bpy)(2)(L)](2+)) or 2- ([Ru(dcbpy)(2)(L)](2-)) charge. Reductive quenching was found to proceed with a larger bimolecular rate constant (k(q) = 1.2-7.4 x 10(8) M(-1) s(-1)) than that previously measured for the quenching of the parent Ru(bpy)(3)(2+) by ascorbate. Moderate yields (phi(ce), = 0.20-0.40) of the reduced metal complex were formed in the process. The reductive quenching of the dianionic complexes in homogeneous solution was compared with that in suspensions (pH 5.0) of inert colloidal alumina-coated silica particles, where both:the anionic donor and the chromophore are coadsorbed to the cationic particles. The apparent bimolecular rate constants were found to be enhanced by more than 1 order of magnitude in these colloidal systems, with a concomitant decrease in the yield of redox products escaping from the solvent cage.