SYNTHESIS AND PHOTOCHEMICAL PROPERTIES OF POROUS ZIRCONIUM VIOLOGEN PHOSPHONATE COMPOUNDS

We recently reported stable, photoinduced charge separation in layered zirconium phosphonate viologen compounds (Zr(O3PCH2CH2(bipyridinium)C H2CH2PO3)X2 (X = Cl, Br, 1), ZrPV(X)). Here we report the synthesis an d photochemical properties of porous versions of this material. Porosi ty can be built into the material by preparing a layered solid from a mixture of the large viologen bisphosphonic acid and a smaller phospho rus acid (i.e., Zr(O3P-R)1(O3PCH2CH2(bipyridinium)CH2CH2PO3)0.5C; R = O-, OH, H, alkyl). These materials were characterized by powder X-ray diffraction and IR and NMR spectroscopies. The resulting structures ar e microporous and undergo photoreduction of the viologen at a faster r ate than in ZrPV(X) compounds. Evidence for the porosity in these mixe d materials is seen in the air sensitivity of the photoreduced species . Photoreduction of the viologen groups is observed only under anaerob ic 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 mechanis m for photoreduction of viologen in these materials is similar to that proposed for ZrPV(X).