Reversible Optical Switching of Polyoxovanadates and Their Communication via Photoexcited States

The 2-bit Lindqvist-type polyoxometalate (POM) [V₆O₁₃((OCH₂)₃CCH₂N₃)₂]^2- with a diamagnetic {V₆O₁₉} core and azide termini shows six fully oxidized V^V centers in solution as well as the solid state, according to ⁵¹V NMR spectroscopy. Under UV irradiation, it exhibits reversible switching between its ground S₀ state and the energetically higher lying states in acetonitrile and water solutions. TD-DFT calculations demonstrate that this process is mainly initialized by excitation from the S₀ to S₉ state. Pulse radiolysis transient absorption spectroscopy experiments with a solvated electron point out photochemically induced charge disproportionation of V^V into V^IV and electron communication between the POM molecules via their excited states. The existence of this unique POM-to-POM electron communication is also indicated by X-ray photoelectron spectroscopy (XPS) studies on gold-metalized silicon wafers (Au//SiO₂//Si) under ambient conditions. The amount of reduced vanadium centers in the "confined" environment increases substantially after beam irradiation with soft X-rays compared to non-irradiated samples. The excited state of one POM anion seems to give rise to subsequent electron transfer from another POM anion. However, this reaction is prohibited as soon as the relaxed T₁ state of the POM is reached.