The photochemistry of polyoxovanadates has been studied using electrochemistry, X–ray crystallography, gas chromatography, IR and electron paramagnetic resonance (EPR) spectroscopy. These materials show potential as photoresponsive catalysts that can operate in an aqueous environment. The photoreduction reaction of V(V) to V(IV) in the respective tartrates is presented and discussed. Additionally, it is possible to use light to switch between different oxidation states through the presence of a mixed valence [V(V)/V(IV)] intermediatespecies. Irradiation at 365 nm (175 mW/cm² LED source) causes solutions of V(V)tartrate to change from orange yellow through green to brownish purple, developing a characteristic three–band absorption spectrum with maxima corresponding to a V(IV) photoproduct. The crystal structures as characterized by X–ray crystallography reveals that the initial V(V) species is tetranuclear Na₅ [V₄O₈(2R,3R)-tart) (2S,3S)-tart)] and the final photoproduct is dinuclear Na₄ [(VO)₂(2R,3R)-tart) (2S,3S)-tart)]. EPR spectroscopy confirms that reduction from diamagnetic V(V) to paramagnetic V(IV) takes place, with the growth of an 8-line V(IV) hyperfine pattern over time, with the corresponding color change. This suggests that the odd electron is located on a mixed valence intermediate. Gas chromatography and IR spectroscopy showed that CO₂is produced during photolysis of this system, indicating catalytic oxidation of the hydroxy acid. The process is reversible, suggesting that novel macroscopic photoresponsive effects in molecules and materials may emerge from this photochemistry. We now expect to develop interesting macroscopic effects with the light responsive properties of Vanadium at the molecular level in cellulose-agarose hydrogels.

Malcolm Forbes

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