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 (11) [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.