Intrazeolite photochemistry .15. Influence of aging, inert gases, and water on the mobility of pyrene molecules on the faujasite NaY

The mobility and location of pyrene within the cavities of the faujasite NaY have been examined using fluorescence and diffuse reflectance techniques. The photophysical properties of pyrene within the zeolite framework show that upon incorporation the pyrene molecules are initially distributed in the outer cavities of the zeolite granules, This leads to a high number of doubly occupied cavities and large excimer emission; this emission shows only 20-25 ps delay, suggesting that excimer-forming molecules are required to undergo only small intracavity motions. With time (days) the distribution of pyrene within the cavities of the zeolite equilibrates and monomer emission dominates the spectra. The time required for this equilibration to take place is shown to be highly dependent on sample preparation. In particular, water and hexane hinder pyrene redistribution, while this process is faster under nitrogen than in samples under vacuum. The detection of delayed fluorescence on the microsecond time scale on freshly prepared samples indicates that there is movement of the pyrene molecules located on the external surface of the zeolite after sample preparation; no delayed fluorescence is observed after 1-2 days.