Photophysical properties of pyrene in zeolites 3. A direct time-resolved diffuse reflectance study in alkali cation-exchanged zeolites X and Y

Electron Trapping in Polar-Solvated Zeolites

Of current interest in our laboratory is the nature of photoinduced processes in the cavities of zeolites completely submerged in polar solvents, or polar-solvated zeolites (PSZ). The present study addresses the nature of electron trapping in PSZ with emphasis on the zeolites NaX and NaY. Free electrons were generated by two-photon, pulsed-laser excitation of either pyrene or naphthalene included in zeolite cavities. Trapped electrons were monitored by diffuse transmittance, transient absorption spectroscopy at visible wavelengths. In anhydrous alcohols, electron trapping by Na(4)(4+) ion clusters was observed in both NaX and NaY. The resulting trapped electrons decayed over the course of tens of milliseconds. No evidence for alcohol-solvated electrons was found. More varied results were observed in solvents containing water. In NaX submerged in CH(3)OH containing 5% or higher water, species having microsecond lifetimes characteristic of solvated electrons were observed. By contrast, a 2 h exposure of NaY to 95/5 CH(3)OH/H(2)O had no effect on electron trapping relative to anhydrous CH(3)OH. The difference between NaX and NaY was explained by how fast water migrates into the sodalite cage. Prolonged exposure to water at room temperature or exposure to water at elevated temperatures was necessary to place water in the sodalite cages of NaY and deactivate Na(4)(4+) as an electron trap. Additional studies in NaY revealed that solvent clusters eventually become lower energy traps than Na(4)(4+) as the water content in methanol increases. In acetonitrile-water mixtures, electron trapping by Na(4)(4+) was eliminated and no equivalent species characteristic of solvated electrons in methanol-water mixtures was observed. This result was explained by the formation of low energy solvated electrons which cannot be observed in the visible region of the spectrum. Measurements of the rate of O(2) quenching in anhydrous solvents revealed rate constants for the quenching of ion cluster trapped electrons that were 2-4 times higher than that for pyrene triplets. In NaX, the rate constant in methanol was 10(4) times smaller than that in cyclohexane, showing greater inhibition of O(2) reactivity in the medium of PSZ. The results of this study point out the conditions under which Na(4)(4+) is active as an electron trap in PSZ and that water must be present in the sodalite cage to produce solvated electrons in the supercage.

Long-lived charge-separation by retarding reverse flow of charge-balancing cation and zeolite-encapsulated Ru(bpy)(3)(2+) as photosensitized electron pump from zeolite framework to externally placed viologen

K(+)-exchanged, Ru(bpy)(3)(2+)-encapsulating zeolite-Y [K(+)-Ru(bpy)(3)(2+)Y] and N-[3-(dicyclohexylmethyl)oxypropyl-N'-methyl-4,4'-bipyridinium [DCH-MV(2+)] were prepared, and visible light-induced electron transfer from the zeolite-encapsulated Ru(II) complex to the size-excluded viologen was studied in acetonitrile. Addition of a series of crown ethers (CEs) into the heterogeneous solution leads to over a 10-fold increase in the yield of DCH-MV(*)(+), where the yield linearly increases as the formation constant of CE with K(+) [K(f)(K(+))(CE)] increases. The following two sequential events are attributed to be responsible for the above novel phenomenon. First, K(+) ions are liberated from the zeolite to solution during interfacial electron transfer from the photoexcited Ru(II) complexes to DCH-MV(2+). Second, the liberated K(+) ions form strong host-guest complexes with the added CE molecules, which leads to retardation of the reverse flow of the cations, hence the charge-balancing electrons, from the solution to the zeolite. Surprisingly, the yield of DCH-MV(*)(+) reaches more than approximately 50 times the amount of Ru(bpy)(3)(2+) situated in the outermost supercages, despite the absence of electron relay in the zeolite. This is attributed to photosensitized electron pumping from the zeolite framework to viologen by the outermost Ru(bpy)(3)(2+) ions. In support of the above conclusion, Ru(bpy)(3)(3+) does not accumulate in the zeolite host while DCH-MV(*)(+) accumulates in the supernatant solution. Consistent with the above, the independently prepared hexafluorophosphate salt of Ru(bpy)(3)(3+) is reduced to Ru(bpy)(3)(2+) in acetonitrile upon contact with Ru(bpy)(3)(2+)-free M(+)Y (M(+) = Li(+), Na(+), K(+), Rb(+), and Cs(+)), where the yield increases as the donor strength of the framework oxygen increases. Although small, thermal electron transfer also takes place from the zeolite framework to DCH-MV(2+), where the yield increases upon increasing the donor strength of the framework, concentration of DCH-MV(2+), temperature, and K(f)(K(+))(CE) (when K(+)Y is the zeolite host). The photoyield is always higher than the thermal yield by 4-30 times, confirming that the zeolite-encapsulated Ru(bpy)(3)(2+) serves as the photosensitized electron pump.