The pore structure of zeolite Y consists of 13 Å supercages connected through 7 Å windows. This study deals with the intrazeolitic photoelectron transfer from trisbipyridyl ruthenium (II) (Ru(bpy) 3 2+ ) synthesized within zeolite Y supercages to ion-exchanged bipyridinium molecules in neighboring supercages. Three N,N′-dialkyl-2,2′-bipyridinium ions and a 4,4′-bipyridinium ion with reduction potentials varying from -0.37 to -0.65 V have been studied. For the 2,2′-bipyridinium salts (members of the diquat family), two, three, and four CH 2 bridging units abbreviated as 2DQ 2+ , 3DQ 2+ , and 4DQ 2+ , respectively, have been examined. The fourth viologen is 1,1′-dimethyl-4,4′-bipyridinium, commonly known as methyl viologen and abbreviated here as MV 2+ . Because of the limitations of the time-resolved diffuse reflectance instrument, only a lower limit of the forward electron transfer rate constant from Ru(bpy) 3 2+ * to the bipyridinium ion was obtained and is >10 7 s -1 . The back electron transfer from the photogenerated bipyridinium radical ions to Ru(bpy) 3 3+ was monitored at different intrazeolitic bipyridinium concentrations. At low loadings of bipyridinium ions (1 per 10-15 supercages), the transient signal between 360 and 390 nm has contributions from both Ru(bpy) 3 2+ * and the bipyridinium radical ions, since the bipyridinium ion concentration was not high enough to quench all of the Ru(bpy) 3 2+ *. The decay of this signal (360-390 nm) could be fitted to the sum of two exponentials, representing the disappearance of unquenched Ru(bpy) 3 2+ * and the back electron transfer from the bipyridinium radical ions to Ru(bpy) 3 3+ . The rate constants for the back electron transfer for 2DQ 2+ , MV 2+ , 3DQ 2+ , and 4DQ 2+ were found to be 4.0 × 10 4 , 1.7 × 10 4 , 1.1 × 10 4 , and 7.3 × 10 3 s -1 . The decrease in the electron transfer rates with increasing driving force for the reaction indicates that the electron transfer is occurring in the Marcus inverted region. At high loadings of the bipyridinium ions (1.2-1.7 molecules/supercage), the bipyridinium radical ions were considerably longer lived, and a simple exponential decay no longer described the loss of the bipyridinium radical signal. A model that allows for electron exchange processes between bipyridinium ions to compete with the back electron transfer was necessary. This kinetic model allowed us to extract the back electron transfer rate at high loadings along with the rate constants for electron hopping and a second-order electron (bipyridinium radical)/hole (Ru(bpy) 3 3+ ) recombination process. For the series 2DQ 2+ , MV 2+ , 3DQ 2+ , and 4DQ 2+ with high loading, the back electron transfer rate constants were 2.5 × 10 5 , 9 × 10 4 , 1.8 × 10 5 , and 1.2 × 10 5 s -1 , higher than the low loading samples. In the high loading case, longer lived charge separation was observed because of the presence of a route for charge propagation by electron hopping via the densely packed viologen molecules.Practical solar energy conversion processes, such as the p...

A series of zeolite Y samples having different contents of entrapped Ru(bpy)3 2+ (Z−Ru(bpy)3) has been prepared. In contrast to the case in homogeneous media, in Y-zeolite the entrapped molecules cannot diffuse and the range of possible intermolecular distances between the complexes is restricted to discrete values. Because of the specific arrangement of interacting species, simplified models can be used in the study of the intermolecular interactions. The results of the present study indicate that intermolecular interactions between the entrapped complexes have a profound effect on the intensity of the electronic emission and kinetics of the excited-state depopulation. Analysis of excited-state lifetime results show that, although isolated Z−*Ru(bpy)3 molecules decay to the ground state with rates comparable to that observed in aqueous solution, the decay of the excited molecules, which have another Ru(bpy)3 2+ or *Ru(bpy)3 2+ complex entrapped in an adjacent zeolite supercage, is much more rapid. In the case of excited-state−ground-state interaction, this effect has been attributed to the nonradiative decay enhancement, and in the case of the excited-state−excited-state interaction, it has been attributed to triplet−triplet annihilation via electron transfer.

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Preparation of suspensions of Ru(bpy) 3 2+ -zeolite Y has made it possible to use conventional optical transmission spectroscopic methods to examine the entrapped Ru(bpy) 3 2+ species within the zeolite. To prepare the suspensions, the surface hydroxyl groups of nanocrystalline zeolite Y were silylated using noctadecyltrichlorosilane. The hydrocarbon sheath on the surface of the zeolites prevented agglomeration of the particles and made the surface hydrophobic enough for dispersion in toluene. The spectroscopic properties of the entrapped Ru(bpy) 3 2+ were measured via transmission techniques. For low levels of zeolite in the suspension (1 mg/10 mL), the scattering was low, and the absorption and fluorescence spectra of Ru(bpy) 3 2+ -entrapped zeolite Y were quantitatively identical to the solution spectra with comparable Ru(bpy) 3 2+concentrations. This suggests that all of the Ru(bpy) 3 2+ inside the zeolite is being sampled and the extinction coefficient of the complex is not altered upon incorporation inside the zeolite. The nature of the quenching mechanism in the Ru(bpy) 3 2+-viologen-zeolite Y system was studied by steady-state fluorescence and lifetime measurements, and it was determined to be mostly due to static quenching. Intrazeolitic electron transfer from the photogenerated viologen radical to Ru(bpy) 3 3+ was followed by flash photolysis, and information about the dynamics of the electron-transfer process was obtained. Comparison of the electron-transfer and quenching data with previous studies on zeolite samples examined by diffuse reflectance methods led to the conclusion that, in reflectance measurements, mostly the surface of the zeolite is sampled. The suspensions, on the other hand, provide a method to examine molecules inside the crystal. Overall, this study demonstrates that zeolite-entrapped complexes can be examined using transmission spectroscopic techniques by using nanocrystalline zeolites with modified surfaces.