EFFICIENT PHOTOINDUCED CHARGE SEPARATION IN LAYERED ZIRCONIUM VIOLOGEN PHOSPHONATE COMPOUNDS

Stable photoinduced charge separation has been observed in layered zir conium 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 w hich is stable in air. Studies of these microcrystalline solids as wel l as related thin films are presented. Transparent thin films of ZrPV( Cl) were grown directly onto fused silica substrates from aqueous solu tion. Photoreduction of viologen in these thin-film samples is very ef ficient (quantum yield = 0.15), showing simple isobestic behavior in t he electronic spectra. Contrary to the bulk solids, the photoreduced t hin films are very air sensitive. Photophysical data is presented, whi ch shows that the primary photoprocess involves a viologen-centered ex citation, followed by oxidation of a halide anion, Subsequent reaction s lead to the formation of a stable charge-separated state. Significan t deuterium isotope effects (k(H)/k(D) = 3) for selectively labeled co mpounds 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 inv olved in the formation of the ultimate charge-separated state. These s tructural rearrangements are most likely responsible for the longevity of the photoreduced states in ZrPV(Cl). Evidence is provided which sh ows that the photoreduction of viologen in ZrPV(Br) has a significant reversible component as well.