Influence of Oxygen on the UV−Photocatalyzed Generation of Pyrene Cation Radicals in the Zeolites X and Y

Abstract

The photoinduced generation of pyrene cation radicals (Py+·) has been examined as a function of oxygen pressure in the internal cavities of the zeolites NaX and NaY. Excitation of pyrene was carried out at ultraviolet wavelengths by utilizing either a pulsed laser or a steady-state arc lamp. For lamp-irradiated samples, the generation of long-lived Py+· was monitored during the course of irradiation by visible-wavelength absorption spectroscopy. In dehydrated NaX (dNaX), irradiation of pyrene in the presence of 0.01−1 bar O2 produced an approximately 10-fold increase in the yield of Py+· relative to samples irradiated under evacuated conditions. By contrast, when pyrene was irradiated in dehydrated NaY (dNaY), no increase in Py+· yield was observed at any O2 pressure relative to evacuated conditions. However, in hydrated NaY, O2-induced increases in Py+· yield were observed, while no similar increases were observed in hydrated NaX. Following laser excitation of samples, the yield, stability, and reactivity of Py+· were monitored by transient absorption spectroscopy. The transient studies revealed differences in the yield of Py+· that were in accord with the lamp studies, but only when the effects of O2 on two-photon ionization were properly accounted for. An efficient back-reaction of Py+· with superoxide (O2 - •) is postulated in hydrated NaX, to account for the absence of Py+· in the lamp studies. A direct excited-state electron-transfer mechanism (type I) is favored for the O2-induced generation of Py+· in zeolites, and it is supported by results which rule out mechanisms involving singlet oxygen and the reaction of O2 with trapped electrons that would otherwise recombine with Py+·. Comparisons of the fluorescence spectrum and lifetime of pyrene in dNaX and dNaY show that pyrene encounters a microenviroment of unusually high polarity in dNaY. It is suggested that the highly polar interaction is related to the electrophilicity of dNaY and is characteristic of a type of charge-transfer interaction that draws electron density from pyrene and prevents the excited state from donating an electron to O2. Such an interaction does not occur in dNaX, which is known to be less electrophilic or more electronegative than dNaY. In hydrated NaY, the presence of water weakens the strongly polar interaction and allows the electron transfer to occur. This study illustrates an important difference between zeolites that vary in electronegativity and how photoinduced charge separation involving neutral molecules is affected by hydration of the zeolite.

Exploring the Photobehavior of Nanocaged Monomers and H- and J-Aggregates of a Proton-Transfer Dye within NaX and NaY Zeolites

“…53−55 Using pyrene as a probe, it has been suggested that NaX supercages are more basic and less polar than NaY ones. 56 We believe that the high trapping efficiency of NaY is due to its more polar character and more available space to HBA-4NP within its cages.…”

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

“…32,42,46,56 Using this value for estimating x, the in-depth variation of the concentration of guest species was obtained from CV data shown in Fig. 5 using eqn (16). The variation of I (= i p /v 1/2 ) with v for CV at LA@PL 25modified electrodes is shown in Fig.…”

“…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.…”