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.