Photoinduced Electron Transfer between Ruthenium-bipyridyl Complex and Methylviologen in Suspensions of Smectite Clays

We examined photoinduced electron transfer (PET) in multicomponent aqueous suspensions composed of tris­(2,2′-bipyridine)­ruthenium­(II) (Ru­(bpy)₃²⁺, photocatalyst), methylviologen (1,1′-dimethyl-4,4′-bipyridinium dication, MV²⁺, electron acceptor), and ethylenediamine tetraacetate (EDTA, sacrificial electron donor) together with particles of smectite-type clays although previous studies indicated inhibition of the electron transfer from Ru­(bpy)₃²⁺ to MV²⁺ in the presence of clay particles. Clays with different lateral particle sizes were compared: hectorite (Hect) and saponite (Sapo) with small particle sizes (∼30 nm) and fluorohectorite (FH) and montmorillonite (Mont) with large particle sizes (>0.1 μm). Clay particles were flocculated and were settled in many cases after the addition of Ru­(bpy)₃²⁺, MV²⁺, and EDTA species, and the Ru­(bpy)₃²⁺ and MV²⁺ cations were almost all adsorbed on the clay particles. When Hect and Sapo were used, reduction of MV²⁺ was observed on the aggregated clay particles upon visible light irradiation indicating the occurrence of PET from Ru­(bpy)₃²⁺ to MV²⁺. However, the reaction was not observed for the samples where the clay particles were not settled. When FH and Mont were used, PET was not observed irrespective of the flocculation of clay particles. These results demonstrated that PET from Ru­(bpy)₃²⁺ to MV²⁺ in the presence of clay particles is possible when the clay particles with small sizes are appropriately aggregated to allow interparticle electron hopping