Nuclear magnetic resonance (NMR) spectroscopy and time-resolved diffuse reflectance (TRDR) have been combined to study the effects of constrained rotational motion on the rates of photoinduced electron-transfer reactions within zeolites. By synthesizing tris(2,2′-bipyridine) ruthenium (II) [Ru(bpy) 3 2+ ] within the large cages of zeolites Y and EMT, it was possible to directly investigate the effect of zeolite cage size on molecular motion of Ru(bpy) 3 2+ and the influence of the cage size on the rate of intrazeolitic electron transfer. Deuterium solid-state NMR shows that zeolite Y imposes restraint on molecular rotation of Ru(bpy) 3 2+ , while zeolite EMT allows for motion. Temperature-dependent studies show the ability to freeze or induce motion of Ru-(bpy) 3 2+ within zeolite EMT while motion within zeolite Y remains unaffected, even at elevated temperatures. For zeolite EMT, an increase in the rate of the photoinduced forward and back electron transfer from Ru-(bpy) 3 2+ to methyl viologen was noted and correlated with access, rotational motion, and favorable orbital overlap. The overall photochemical charge separation efficiencies for the intrazeolitic Ru(bpy) 3 2+ -bipyridinium reactions were similar for both zeolites. IntroductionOne of the major obstacles in the development of artificial photosynthetic systems is that the thermal back-electron transfer reaction negates the utilization of photochemically created reactive redox species. 1 Thus, considerable effort is being made to understand the features that control electron-transfer dynamics. Toward that end, many donor-acceptor systems have been examined, and assembly of molecular units on microheterogeneous supports is being pursued to influence electron-transfer dynamics. 2 By virtue of the nature of microheterogeneous supports, the motion of molecules is modified and it is of interest to examine how electron transfer is influenced by such immobilization. Zeolites, because of the molecular size of their cavities, are an excellent host system to investigate immobilization phenomena.Zeolite Y consists of a regular network of sodalite cages in a tetrahedral arrangement resulting in 13 Å supercages with 7 Å windows (Figure 1a). 3 This is large enough to accommodate the luminescent molecule trisbipyridine ruthenium (II) [Ru-(bpy) 3 2+ ] which has a molecular diameter of 12 Å and can be assembled within the supercages by using a "ship-in-a-bottle" synthetic procedure. 4 These intrazeolitic ruthenium complexes can then be surrounded by bipyridinium ions such as methyl viologen (MV 2+ ) using the inherent ion-exchange properties of the zeolite. It has been shown that zeolite Y can promote longlived, Ru(bpy) 3 3+ -bipyridinium radical cation charge-separated states. 5,6 Molecular modeling of Ru(bpy) 3 2+ entrapped within zeolite Y indicated a significant number of contacts between the metal complex and the walls of the zeolite that were within the van der Waals radii of the atoms. 6 It was proposed that these contacts inhibit the rotational motion of the i...