Intrazeolite Photochemistry. 15. Influence of Aging, Inert Gases, and Water on the Mobility of Pyrene Molecules on the Faujasite NaY

Abstract

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.

Visual Observation of Contact‐Induced Intercrystalline Migration of Aromatic Species Adsorbed in Zeolites by Fluorescence Microscopy

“…We checked the stability of the dehydrated and sealed samples for more than a month by means of fluorescence spectroscopy; no change was discernible. Although the relocation of guest molecules within zeolite crystals kept in an unsealed container over a few months has been described previously for pyrene, [16] the fluorescence spectrum of which is very sensitive to the monomerdimer equilibrium dependent on the degree of hydration, we could not make this type of investigation with Chry because of the insensitive nature of Chry fluorescence to such an equilibrium.…”

“…For some particular cases, photophysical and photochemical techniques have been found useful for monitoring the migration of large aromatic molecules: A diffuse-reflectance transient-absorption study of the quenching kinetics of photoexcited triplet-state anthracene through a triplet-triplet energy transfer mechanism by a few acceptors, azulene, ferrocene, and anthracene in the ground state permitted the estimation of diffusion coefficients of these molecules in zeolite Na + -Y. [14,15] Additionally, time-dependent emission spectroscopic studies of a monomer-dimer equilibrium [16,17] for aromatic species such as pyrene, naphthalene and anthracene, and of singlet-singlet energy transfer [18] between oxonine and pyronine in zeolites have been applied to monitor the intracrystalline relocation of these molecules.…”

Supramolecular photochemistry in zeolites: From catalysts to sunscreens

“…Guest exit from the particle interior has also been observed in similar systems by fluorescence microscopy [58,59] Sample aging can also be an important factor in terms of guest distribution. The emission spectra recorded immediately after preparation of pyrene encapsulated in zeolite Y indicates the presence of predominantly the excimer (excited dimer) form, despite the fact that the calculated occupancy is well below a value indicating doubly occupied cavities [55]. Note that the molecular dimensions of pyrene are close to the size of the pore aperture.…”

“…There is also the possibility of more unexpected effects, such as the "lubricating" effect observed for nitrogen, where large aromatic molecules are observed to be M. N. CHRÉTIEN more highly mobile in nitrogen-purged samples as compared to evacuated ones. This lubricating effect was attributed to occupation of cation binding sites by nitrogen, thereby reducing the interaction between the cation and the guest molecule [55].…”

“…When the sample is aged over a period of one month, the emission spectrum shifts from primarily excimer to monomer in nature. Aging of the sample results in a redistribution of pyrene until an equilibrium distribution, with only singly occupied cavities, is achieved [55].…”