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

2004-04-15T00:00:00Z (GMT) by Eric H. Ellison
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.