Photophysics of ruthenium polypyridyl complexes formed with lacunary polyoxotungstates with iron addenda

Abstract

The interactions between luminophore [Ru(bpy)₃]²⁺, and the lacunary Dawson heteropolyanions, [P₂W₁₇O₆₁(FeOH₂)]⁷⁻, [P₂W₁₇O₆₁(FeBr)]⁶⁻ and [P₂W₁₇O₆₁]¹⁰⁻ were investigated using a combination of photophysics, optical and Raman spectroscopy. Extensive quenching of the excited state of [Ru(bpy)₃]²⁺ was observed in each case. Quenching is attributed to the formation of association complexes between [Ru(bpy)₃]²⁺ and the heteropolyanions in which the charge on the heteropolyanions is fully compensated for by the ruthenium polypyridyl species. The interaction appears to be principally electrostatic in nature producing [Ru(bpy)₃]_3.5[P₂W₁₇O₆₁(FeOH₂)], [Ru(bpy)₃]₃[P₂W₁₇O₆₁(FeBr)] and [Ru(bpy)₃]₅[P₂W₁₇O₆₁]¹⁰⁻. The association constants for formation of the clusters were obtained from photophysical studies and surprisingly, despite the electrostatic nature of the interaction, there was no correlation between the charge on the polyoxometallate and the association constant. In particular, the unsubstituted lacunary, [P₂W₁₇O₆₁]¹⁰⁻, showed considerably weaker association compared to the transition metal substituted lacunaries, in spite of its 10⁻ charge. Difference absorption spectroscopy revealed a new transition at ca. 480 nm for each of the cluster complexes. From resonance Raman spectroscopy the origin of this transition was found to involve the polyoxometallate. Unlike previously reported adducts, the cluster complexes formed were not luminescent. In all cases the cluster complexes exhibit remarkable photostability, with no photodecomposition or photo-induced ligand exchange reactions evident in acetonitrile, under conditions where [Ru(bpy)₃]²⁺ alone exhibits considerable photolability.