Influence of Exchanged Alkali Metal Cations within Zeolite Y Cages on Spectroscopic and Photooxidation Properties of the Incorporated Tris(2,2′-bipyridine)ruthenium(II) Complexes

Tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)(3)(2+)) complexes contained within a series of zeolite Y cages with various extraframework alkali metal cations (Li+, Na+, K+, Rb+, and Cs+) were successfully synthesized. These complexes were characterized to determine the effect of the host zeolite microenvironment on the physicochemical properties of the guest molecules. Formation of the Ru(bpy)(3)(2+) complexes was ascertained by X-ray diffraction (XRD), diffuse reflectance UV-vis spectroscopy, IR, and Ru K-edge X-ray absorption fine structure (XAFS) measurements. The steric constraints induced by increasing the size of the alkali metal cations led to a decrease in both Brunauer-Emmett-Teller (BET) surface area and average Ru-N distances. The intensity of the photolumineseence spectra of Ru(bpy)(3)(2+) complexes associated with zeolites increased in the presence of lighter alkali metal cations. This result was found to correlate with the increased turnover number (TON) for the photoinduced oxidation of styrene derivatives under visible-light irradiation (lambda > 430 nm) and in the presence of molecular oxygen (O-2). Scavenging experiments demonstrated that a superoxide anion (O-2(center dot-)) was the main reactive species. This O-2(center dot-) Species was produced by an excited (MLCT)-M-3 state of Ru(bPY)3 reacting with 02 via photoinduced electron transfer. This suggests that the electronic configuration of the lowest triplet state of Ru(bpy)(3)(2+) can be enhanced with the aid of light alkali metal cations, which predominantly determine the intensity of photoluminescence and the photooxidation rate.