Hypermobility of aromatic guest species in zeolite as observed from anthracene triplet self-quenching within NaY in the presence of critical amounts of coadsorbed solvents

The decay kinetics was investigated of tripler-state anthracene adsorbed into the cavities of a faujasite zeolite, NaY, both in the absence and in the presence of various quantities of water and other coadsorbed inert solvent molecules. It was found that at a sufficiently low loading level of anthracene the decay rate is dependent on the amount of water, methanol, and pyridine but is scarcely affected by n-hexane. When the decay rates are plotted versus the amount of solvents introduced, convex curves were obtained with a peak at ca. 0.1 cm(3).g(-1), which is approximately a quarter of the total pore volume of NaY (0.378 cm(3).g(-1)). The enhanced triplet decay rate was found to be ascribable to a self-quenching event, i.e., the triplet quenching by ground-state anthracene. Thus a picture was given that anthracene molecules are highly mobile when the zeolite cavities are loaded with the coadsorbed solvents in quantities sufficient to shield the cationic sites at which anthracene is supposed to be adsorbed. The kinetic analysis of the triplet quenching process revealed that the inter-cage mobility in the presence of 0.1 cm(3).g(-1) of water is more than 100 times higher than that in the presence Of 0.3 cm(3).g(-1) of water. The high mobility of the guest species is presumably caused by the liberation of adsorption interaction of anthracene with the zeolite walls by the action of small quantities of the solvent molecules with strong affinity toward the zeolite frameworks. By contrast, large quantities of the coadsorbed solvents are expected to interfere with the movement of anthracene in the cage networks because they are crowded enough to fill the interconnecting windows between cages as well as a large portion of the cage volume. The dramatic effect of coadsorbed water and other solvents on the diffusional motion of anthracene within the zeolite supercage networks depending on the quantity adsorbed is described.