Tuning the Photochromic Properties of Molybdenum Bisphosphonate Polyoxometalates

Seven hybrid organic inorganic bisphosphonate molybdenum(VI) polyoxometalate complexes with the general formula [(Mo3O8)(4)(O3PC(CmH2mNRR'R '')(O)PO3)(4)](8-) (m = 3; R, R', and R '' = H or CH3) and [(Mo3O8)(2)(O)-(O3PC(CmH2mNRR'R '')(O)PO3)(2)](6-) (m = 3 or 4; R, R', and R '' = H or CH3) have been synthesized and their structures solved using single-crystal X-ray diffraction. These compounds are made of a {Mo-12} or a {Mo-6} inorganic core functionalized by various alkylammonium bisphosphonates, with these ligands differing by the length of their alkyl chains and the number of methyl groups grafted on the N atom. The nature of the counter-cations (Na+ K+, Rb+, Cs+, and/or NH4+) constituting these materials has also been modulated. P-31 NMR spectroscopic studies in aqueous media have shown that all the dodecanuclear complexes reported here are stable in solution, whereas for the hexanuclear compounds, a dynamic equilibrium between two isomers has been evidenced, and the corresponding standard thermodynamic parameters determined for one of them. The electrochemical properties of six representative compounds of this family have been investigated. It has been found that the Mo6+/Mo5+ reduction potential is similar for all the polyoxometalates studied. Besides, it is shown that electrochemical cycling is an efficient method for the deposition of these compounds on a surface. The photochromic properties of all the complexes reported herein have been studied in the solid state. Under irradiation in the near ultraviolet (UV), the {Mon} systems shift from white to reddish-brown, while the {Mo-6} compounds develop a purple coloration. The coloration kinetics has been systematically quantified and the optical band gaps, the salient coloration kinetic parameters and the coloration kinetic half-life times have been determined. This has evidenced that several of these materials develop very strong and rapid UV-induced color changes, with remarkable coloration contrasts. Finally, the optical properties of these systems are discussed in light of several salient parameters as the POM topology, the nature of the grafted bisphosphonate ligand, and the design of the hydrogen-bonding network at the organic-inorganic interface.