The spectroscopic and photophysical properties of [Ru(bpy)3]2[[Mo18O54(SO3)2], where bpy is 2,2′-bipyridyl and [Mo18O54(SO3)2]4− is either the α or β-sulfite containing polyoxomolybdate isomer, have been measured and compared with those for the well known but structurally distinct sulfate analogue, α-[Mo18O54(SO4)2]4−. Electronic difference spectroscopy revealed the presence of new spectral features around 480 nm, although they are weak in comparison with the [Ru(bpy)3]2[Mo18O54(SO4)2] analogue. Surprisingly, Stern–Volmer plots of [Ru(bpy)3]2+ luminescence quenching by the polyoxometallate revealed the presence of both static and dynamic quenching for both α and β-[Mo18O54(SO3)2]4−. The association constant inferred for the ion cluster [Ru(bpy)3]2α-[Mo18O54(SO4)2] is K = 5.9 ± 0.56 × 106 and that for [Ru(bpy)3]2β-[Mo18O54(SO4)2] is K = 1.0 ± 0.09 × 107. Unlike the sulfate polyoxometalates, both sulfite polyoxometalate–ruthenium adducts are non-luminescent. Despite the strong electrostatic association in the adducts resonance Raman and photoelectrochemical studies suggests that unlike the sulfato polyoxometalate analogue there is no sensitization of the polyoxometalate photochemistry by the ruthenium centre for the sulfite anions. In addition, the adducts exhibit photochemical lability in acetonitrile, attributable to decomposition of the ruthenium complex, which has not been observed for other [Ru(bpy)3]2+ -polyoxometalate adducts. These observations suggest that less electronic communication exists between the [Ru(bpy)3]2+ and the sulfite polyoxoanions relative to their sulfate polyoxoanion counterparts, despite their structural and electronic analogy. The main distinction between sulfate and sulfite polyoxometalates lies in their reversible reduction potentials, which are more positive by approximately 100 mV for the sulfite anions. This suggests that the capacity for [Ru(bpy)3]2+ or analogues to sensitize photoreduction in the adducts of polyoxometalates requires very sensitive redox tuning.
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