Zeolite-induced solvation effects on excited-state properties of Ru(bpy)32+:: Implications for intrazeolitic photochemical quenching reactions

Photosensitized electron-transfer reactions between tris(2,2 ' -bipyridine)ruthenium(II) (Ru(bpy)(3)(2+)) and bipyridinium ions have been extensively studied. Entrapment of this donor-acceptor system in zeolite Y retards the back electron transfer as compared to the forward electron transfer and appears to be an effective system for the photogeneration of long-lived charge-separated species. In this paper, we evaluate how the properties of the photoexcited zeolite-entrapped Ru(bpy)(3)(2+) change as it is surrounded by bipyridinium ions. However, most bipyridinium ions quench the excited state of Ru(bpy)(3)(2+) and the characteristic properties of the excited state of Ru(bpy)(3)(2+) cannot be monitored. So, we have used tetraethylammonium (TEA) to surround the zeolite-entrapped Ru(bpy)(3)(2+), which helps mimic the effect of the presence of large organic cations, while avoiding the problem of quenching. Upon exchange of TEA, a blue shift in the fluorescence emission of 21 nm, an increase in the emission intensity by a factor of 2.7, and an increase in the lifetime of excited Ru(bpy)(3)(2+) by a factor of 2 are observed. We propose that these effects are a result of the intrazeolitic bulk-like uncomplexed water being displaced by the TEA ions. The remaining water molecules are held tightly by the framework and the sodium cations, creating an environment typical of a frozen medium. Similar effects should occur when the intrazeolitic Ru(bpy)(3)(2+) is surrounded by bipyridinium ions. Indeed, very unexpected quenching data and emission spectra are observed for Ru(bpy)(3)(2+) in zeolite Y in the presence of the quencher N,N'-dimethyl-2,2 ' -bipyridinium ion. This quencher has a large reduction potential (-0.72 V), thereby inefficiently quenching Ru(bpy)(3)(2+)* and making it possible to observe the emission spectrum. The implications of this study are that in the Ru(bpy)(3)(2+) -bipyridinium system in zeolite Y, the photoexcited reactant Ru(bpy)(3)(2+)* has a longer lifetime and should promote the extent of the forward electron-transfer reaction to the bipyridinium ions in neighboring cages.