Sensitization of photo-reduction of the polyoxometalate anions [S2M18O62]4− (M = Mo, W) in the visible spectral region by the [Ru(bpy)3]2+ cation

Abstract

Voltammetric, photo-physical and photo-electrochemical properties of the Dawson polyoxometalate anions α-[S₂M₁₈O₆₂]⁴⁻ (M = Mo, W) are presented, both in the presence and absence of a series of [Ru^IIL_n]^+/2+ cations [L_n = (bpy)₃, (bpy)₂(Im)₂, (bpy)₂(dpq), (bpy)₂(box) and (biq)₂(box)]. Electrochemical processes for both the anion and Ru^II/III couples were detected in solutions of the salts [Ru^IIL_n]₂[S₂M₁₈O₆₂] in dimethylformamide (0.1 M Bu₄NPF₆) by both cyclic and hydrodynamic voltammetries. Responses were also detected when the solid salts were adhered to the surface of a glassy carbon electrode in contact with an electrolyte in which they are insoluble (CH₃CN; 0.1M Bu₄NPF₆). Photolysis experiments were performed on solutions of the salts [R₄N]₄[S₂M₁₈O₆₂] (R = n-butyl or n-hexyl) and [Ru^IIL_n]₂[S₂M₁₈O₆₂] at 355 and 420 nm in dimethylformamide and acetonitrile in the presence and absence of benzyl alcohol (10% v/v). When associated with [Ru(bpy)₃]²⁺, the molybdate anion exhibited a large increase in the quantum yield for photo-reduction at 420 nm. The quantum yield for the tungstate analogue was lower but the experiments again provided clear evidence for sensitization of the photo-reduction reaction in the visible spectral region. The origin of this sensitization is ascribed to the new optical transition observed around 480 nm in static ion clusters {[Ru(bpy)₃][S₂M₁₈O₆₂]}²⁻ and {[Ru(bpy)₃]₂[S₂M₁₈O₆₂]} present in solution. Measurable photocurrents resulted from irradiation of solutions of the anions with white light in the presence of the electron donor dimethylformamide. Evidence is also presented for possible quencher–fluorophore interactions in the presence of certain [Ru^IIL_n]⁺ cations.