Photophysics and Photosensitization Behavior of Microcrystalline {Pt(bpy)2}2{Pt2(pop)4}nH2O

The insoluble double salt complex [Pt(bpy)₂]₂[Pt₂(pop)₄] ·nH₂O ("PBPop"; bpy=2,2′- bipyridine and pop=H₂P₂O₅^2-) is prepared by mixing an aqueous solution of the monomers. In the solid state, the initial orange complex has ca. 4 waters of hydration and a variable diffuse reflectance maximum in the region of 510-530 nm which imparts an orange color. The complex dehydrates readily and irreversibly to a yellow form with ca. 2 waters of hydration whose diffuse reflectance spectrum shows no maximum at wavelengths greater than 500 nm. Both orange and yellow forms exhibit the same emission spectrum, with a maximum at 615 nm and a halfwidth of 3100 cm^-1 at room temperature. The emission of yellow PBPop is extremely intense, with a room temperature lifetime of 45 ns and an estimated radiative rate constant of 70 000 s^-1. The emission maximum does not shift with decreasing temperature, but the halfwidth narrows to 2500 cm^-1 at 40 K. In the case of orange PBPop, the emission intensity is very weak. The emission is attributed to either a chromophore or a trap state common to both orange and yellow forms. Powder suspensions of both forms of PBPop are active as photosensitizers for generating H₂ from aqueous solutions containing ethylenediaminetetraacetate (EDTA^2-) as an electron donor and a colloidal Pt catalyst. Visible light excitation of the yellow double salt (λ_ex>400 nm) leads to large turnover numbers ([H₂]/[PBPop]>100), indicating that H₂ is evolved catalytically. The photoproduction of H₂ is also observed in the absence of the Pt catalyst; however, the rate is a factor of 15 lower. The yellow form of PBPop is considerably more active than the orange form. In the presence of the latter complex, a relative long induction period accompanies H₂ formation. Both the yield and the rate of H₂ production with PBPop display a marked dependence on the excitation wavelength in the region 400-515 nm. Although PBPop is relatively stable in acidic EDTA^2- solution (pH 4.75), it undergoes dark decomposition in alkaline triethanolamine solution (pH 9.5). Also, during UV illumination (λ_ex>345 nm), photodegradation of PBPop particles occurs along with H₂ evolution. The basis of these and other phenomena are discussed.