Electron Trapping in Polar-Solvated Zeolites

Abstract

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.

Ruthenium(II) Tris(2,2′-bipyridyl) Complex Incorporated in UiO-67 as Photoredox Catalyst

“…Thus, the transient signal recorded when irradiating the UiO-67­(Zr) at 266 nm should be correlated to the photogeneration of the oxidized bpdc ligand together with the solvated electron, most probably due to the presence of adsorbed H 2 O on the MOF. It is well known in literature that the solvated electron in porous materials such as zeolites lives in the time scale of micro- to milliseconds and the maximum absorption wavelength of its TAS depends on the chemical composition of the porous material in which it is photogenerated. − The TAS of the oxidized bpdc ligand, which is characterized by a continuous absorption in all the spectral range with higher absorption intensity at short wavelengths, is similar to the TAS recorded at long times, 500 μs, for the Rudcbpy-UiO-67­(Zr) MOF upon 532 nm laser excitation (see comparative Figure S16). Analysis of the transient spectra of Rudcbpy-UiO-67­(Zr) was made considering that it should be the sum of the absorption spectra of electrons and holes in Rudcbpy-UiO-67­(Zr).…”

“…6,10,18,45 The nature of the positive hole photogenerated after the PET process could be assigned, at first wavelength of its TAS depends on the chemical composition of the porous material in which it is photogenerated. [49][50][51] The TAS of the oxidized bpdc ligand, which is characterized by a continuous absorption in all the spectral range with higher absorption intensity at short wavelengths, is similar to the TAS recorded at long times, 500 µs, for the Rudcbpy-UiO-67(Zr)…”

Optical Spectroscopy and Microscopy Studies on the Spatial Distribution and Reaction Dynamics in Zeolites

“…In-depth studies have been carried out for photoinduced electron transfer, electron trapping, and charge separation. [13][14][15][16][17][18][19][20] In addition, optical microscopy is emerging as an appropriate research tool with the advent of single-molecule spectroscopy 21 and nanomaterials research. 22 Various optical methods including confocal and dark-field techniques can be used to study zeolite systems.…”

“…The transient absorption spectroscopy has been a tool for intensive research in zeolite photochemistry. In-depth studies have been carried out for photoinduced electron transfer, electron trapping, and charge separation. − …”

Determination of the depth profile distribution of guest species in microporous materials using the voltammetry of immobilized particles methodology: application to lapachol attachment to palygorskite and kaolinite

“…7 On the other hand, the distribution of pharmacologically active guests in nanostructured materials is relevant for determining the kinetics and performance of drug delivery processes. [8][9][10] As recently studied [11][12][13] and reviewed 14 by Hashimoto, methods reported for this purpose include diffuse reflectance transient absorption spectroscopy, 14 photoinduced electron transfer and charge separation, [15][16][17][18][19][20][21][22] confocal optical microscopy, 23 microscope FTIR mapping, 24 and multidimensional NMR correlation spectroscopy, 25 among others. 14 The influence of charge compensating ions to control the distribution and conformation of the guest species within the channels of ion-permeable microporous materials should be noted, 7,14 and also that species distribution can influence photochemical 26,27 and mechanical 28 properties of these kinds of materials.…”