Synthesis and photochemical properties of porous zirconium viologen phosphonate compounds

The authors recently reported stable, photoinduced charge separation in layered zirconium phosphonate viologen compounds (Zr(O[sub 3]PCH[sub 2]CH[sub 2](bipyridinium)CH[sub 2]CH[sub 2]PO[sub 3])X[sub 2] (X = Cl, Br, I), ZrPV (X)). Here, they report the synthesis and photochemical properties of porous versions of this material. Porosity can be built into the material by preparing a layered solid from a mixture of the large viologen bisphosphonic acid and a smaller phosphorus acid (i.e., Zr(O[sub 3]P-R)[sub 1](O[sub 3]PCH[sub 2]CH[sub 2](bipyridinium)CH[sub 2]CH[sub 2]PO[sub 3])[sub 0.5]C; R = O[sup [minus]], OH, H, alkyl). These materials were characterized by powder X-ray diffraction and IR and NMR spectroscopies. The resulting structures are microporous and undergo photoreduction of the viologen at a faster rate than in ZrPV (X) compounds. Evidence for the porosity in these mixed materials is seen in the air sensitivity of the photoreduced species. Photoreduction of the viologen groups is observed only under anaerobic conditions, and the photoreduced solid is bleached immediately and quantitatively in the air. In contrast the close-packed analogs (ZrPV(X)) can be photoreduced and are moderately stable in the air. Detailed photophysical and isotopic labeling studies suggest that the mechanism for photoreduction of viologen in these materials is similar to that proposed for ZrPV(X). 21 refs., 5 figs.