Reversible photodimerization of coumarin-modified Wells-Dawson anions

Organo-modified photo-sensitive polyoxometalates (POMs) are a unique class of functional molecular materials. Here we show how the light-driven dimerization in organo-functionalized POMs can be achieved. To this end, the vanadium-substituted Wells-Dawson cluster [P2V3W15O62](9-) was modified with a coumarin-based derivative to give the new organic-inorganic hybrid (nBu(4)N)(4)H-2[C12H9O4NHC(CH2O)(3)P2W15V3O59]. Upon photo-irradiation, the compound undergoes photodimerization which was characterized using various spectroscopic methods including FT-IR, UV-vis, DLS (dynamic light scattering), H-1 NMR, COSY NMR and ESI-MS. These results are in good agreement with the formation of dimer species. In addition, XPS and EPR analyses of the photo-irradiation product indicate that V5+ is partially reduced to V4+, while W6+ is not affected, suggesting that the coupled photo-dimerization and selective redox-switching might become possible.