EFFICIENT PHOTOINDUCED CHARGE SEPARATION IN LAYERED ZIRCONIUM VIOLOGEN PHOSPHONATE COMPOUNDS

Stable photoinduced charge separation has been observed in layered zirconium viologen phosphonate compounds, i.e. Zr(O3PCH2CH2(bipyridinium)CH2CH2PO3)X2 (X = Cl, Br, I), ZrPV(X). Photolysis of these crystalline solids results in the formation of the blue viologen radical cation which is stable in air. Studies of these microcrystalline solids as well as related thin films are presented. Transparent thin films of ZrPV(Cl) were grown directly onto fused silica substrates from aqueous solution. Photoreduction of viologen in these thin-film samples is very efficient (quantum yield = 0.15), showing simple isobestic behavior in the electronic spectra. Contrary to the bulk solids, the photoreduced thin films are very air sensitive. Photophysical data is presented, which shows that the primary photoprocess involves a viologen-centered excitation, followed by oxidation of a halide anion, Subsequent reactions lead to the formation of a stable charge-separated state. Significant deuterium isotope effects (k(H)/k(D) = 3) for selectively labeled compounds suggest that the oxidized halide ions abstract hydrogen atoms from the methylene groups of ZrPV(Cl). NMR studies of these materials show that a number of different structural rearrangements are then involved in the formation of the ultimate charge-separated state. These structural rearrangements are most likely responsible for the longevity of the photoreduced states in ZrPV(Cl). Evidence is provided which shows that the photoreduction of viologen in ZrPV(Br) has a significant reversible component as well.