Photophysics and photosensitization behavior of microcrystalline [Pt(bpy) 2] 2[Pt 2(pop) 4]· nH 2O

Abstract

The insoluble double salt complex [Pt(bpy) 2] 2[Pt 2(pop) 4]· nH 2O (“PBPop”; bpy=2,2′-bipyridine and pop=H 2P 2O 5 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 H 2 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 ([H 2]/[PBPop]>100), indicating that H 2 is evolved catalytically. The photoproduction of H 2 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 H 2 formation. Both the yield and the rate of H 2 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 H 2 evolution. The basis of these and other phenomena are discussed.