Anderson-Type Polyoxometalates Functionalized by Tetrathiafulvalene Groups: Synthesis, Electrochemical Studies, and NLO Properties

Three polyoxometalates (POMs) functionalized by tetrathiafulvalene (TTF) molecules have been synthesized by a coupling reaction between the Anderson-type POMs [MnMo6O18-{(OCH2)(3)CNH2}(2)](3-) or [AlMo6O18(OH)(3){(OCH2)(3)CNH2}](3-) and the TTF carboxylic acid derivative (MeS)(3)TTF(S-CH2-CO2H). The monofunctionalized TTF-AlMo6 POM contains one TTF group covalently grafted on an Al Anderson platform. The symmetrical TTF-MnMo6-TTF POM possesses two TTF groups grafted on each side of a Mn Anderson derivative while the asymmetrical TTF-MnMo6-SP POM contains a TTF and a spiropyran groups. These three trianionic species have been characterized by elemental analysis, H-1 and C-13 NMR, FT-IR spectroscopy, ESI-MS spectrometry, and single-crystal X-ray diffraction (for TTF-MnMo6-TTF). In the solid state, the grafted TTF molecules of TTF-MnMo6-TTF POMs interact via S center dot center dot center dot S and pi center dot center dot center dot pi interactions and form chains. The electrochemical properties of the complexes reflect the contributions of both the inorganic POM and the TTF moieties. Despite adsorption of the oxidized hybrid species on the Pt grid working electrode, UV-vis-NIR spectroelectrochemical investigations evidence peaks characteristic of the oxidation of the TTF units. Finally, hyper-Rayleigh scattering (HRS) measurements show that the three novel TTF derivatives exhibit beta values between 20 and 37 x 10(-30) esu. Moreover it is observed that the oxidation of the TTF moieties by Fe3+ ions increases the NLO response. These values are in the order of magnitude of that found for the well-known 4-dimethylamino-N-methyl-4-stilbazolium (DAS(+)) cation (beta = 60 x 10(-30) esu).