Photoinduced electron transfer in nanostructured assemblies of layered semiconducting oxide and methylviologen: Effect of the location of acceptor molecules

We have synthesized nanostructured assemblies of layered hexaniobate, which is a semiconducting oxide possessing intercalating capability and photocatalytic activity, and a redox-active bipyridinium species (methylviologen, MV2+) by exfoliation–restacking and intercalation techniques. The exfoliated–restacked assembly was prepared through delamination of the niobate using triethanolammonium ions (TEAH+) as an exfoliating reagent and subsequent drop-cast of the exfoliated suspension. UV-irradiation of the niobate–MV2+ assemblies caused photoinduced electron transfer from the niobate layers to the MV2+ species to produce methylviologen radical cations (MV+). The difference in the assembling technique yielded different MV+ species: dimers for the exfoliated–restacked assembly but monomers for the intercalation compound. XRD measurements showed a disordered layered structure of the exfoliated–restacked assembly, as evidenced by broad diffraction peaks. These observations suggest that the MV2+ ions are not strictly incorporated in the interlayer spaces of the niobate but present in a relatively mobile environment with weak binding to the oxide layers; this rationalizes the dimerization of the MV+ cations. On the other hand, ordered stacking of the intercalated assembly evidenced by sharp XRD peaks allows immobilization of the MV2+ cations in the interlayer spaces of the niobate. The disordered environment of the exfoliated–restacked assembly is ascribed to the hydration of the TEAH+ ions introduced to the assembly as the exfoliating reagent.