ZEOLITE HOST-GUEST INTERACTIONS - OPTICAL SPECTROSCOPIC PROPERTIES OF TRIS(BIPYRIDINE)RUTHENIUM(II) IN ZEOLITE-Y CAGES

Zeolite cages and cavities provide an attractive medium for carrying out novel chemical reactions. The steric and electrostatic constraints on the reactants can modify their reactivity. In this paper, we have studied the molecule Ru(bpy)32+ within the supercages of the faujasitic zeolite Y. The choice of this molecule is dictated by several factors. Its size is ∼12.1 Å and therefore fits securely in the ∼13-Å supercages of zeolite Y. This also ensures that the molecule cannot escape through the ∼7-Å ring openings of the supercage. The thermal stability of the Ru(bpy)32+ molecule has made it possible to examine the influence of the zeolite under dehydrated conditions and surrounded by nonpolar hexane molecules. The Ru(bpy)32+ molecule can also exist in a relatively long lived excited metal-to-ligand charge-transfer state. We have studied the ground and excited states of encapsulated Ru(bpy)32+ by a combination of various techniques, including diffuse reflectance absorption spectroscopy, emission spectroscopy, Raman spectroscopy, time-resolved emission and Raman spectroscopy, and measurement of lifetimes. X-ray photoelectron spectroscopy and X-ray fluorescence spectroscopy were used to study the environment around ruthenium as well as its loading in the zeolite. These studies indicate that the influence of the zeolite, in both its hydrated and dehydrated forms, has only a minimal effect on the ground-state structure of the encapsulated Ru(bpy)32+. The excited state of the molecule in the hydrated zeolite Y is also similar to that in aqueous solution. However, dehydration influences the structure of the excited state markedly. We have proposed that this arises principally from two effects, both originating from the dipolar nature of the excited state. These include the lack of stabilization of the polar excited state by solvent molecules and the electrostatic interactions between the negative aluminosilicate framework and the excited state. © 1990 American Chemical Society.