Synthesis, characterization, and photochromic properties of hybrid organic-inorganic materials based on molybdate, DABCO, and piperazine

Prompted by our interest in new photochromic organic-inorganic hybrid materials, the reactivity of Mo7O24]6- toward a structure-directing reagent diamine such as 1,4-diazabicyclo2.2.2]octane (DABCO) and piperazine (pipz) has been investigated, and three new molybdenum(VI)-containing compounds, namely, (H2DABCO)3Mo7O24].4H2O (1), (H2DABCO)Mo3O10].H2O (2), and (H2DABCO)2(NH4)2Mo8O27].4H2O (3), have been synthesized and characterized. New synthetic routes to achieve the known compounds (H2DABCO)2(H2pipz)Mo8O27] (4), (H2pipz)3Mo8O27] (5), and (H2DABCO)2Mo8O26].4H2O (6) are also reported. All of these compounds contain different poly(oxomolybdate) clusters, i.e., discrete Mo7O24]6- blocks in 1, infinite polymeric chains 1/infinityMo3O10]2- in 2, 1/infinityMo8O27]6- in 3-5, and 1/infinityMo8O26]4- in 6, associated in a tridimensional assembly by hydrogen bonds with H2DABCO2+ and/or H2pipz2+ cations. Interconversion pathways and chemical factors affecting the stabilization of the different species are highlighted and discussed. At the opposite of 6, compounds 1-5 show photochromic behavior under UV excitation. Namely, compounds 1-5 shift from white or pale yellow to pale pink, reddish brown, or purple under UV illumination depending on the chemical nature of the mineral framework, with the kinetics of the color change being dictated by the nature of the organic component and by the organic-inorganic interface.