Photochemical reaction dynamics of molecules adsorbed into zeolites as revealed by time-resolved spectroscopic techniques.

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1. Flurescence Spectroscopic Investigation of the Interaction of aromatics with ZeolitesThe fluorescence spectroscopic results revealed a remarkable behavior of anthracene adsorbed into zeolites depending on the loading level and the state of hydration of the zeolites as well as the chemical composition of the zeolites. Anthracene molecules are suggested to be complexed with Na^+ ions in the zeolite frameworks of dehydrated NaX and NaY, and at high loading levels. they form dimers under the influence of Na^+ ion. Additionally, our fluorescence spectroscopic investigation revealed the formation of anthracene crystals at relatively high loadings in the presence of a large amount of water and other solvent molecules. Thus we found that the dynamic behavior of the guest aromatic molecules within the zeolite supercages are critically dependent upon the balance of interactions among the zeolite frameworks, guest species and coadsorbates. especially coadsorbed water.2. Effect of Coadsorbed So … More lventsWe found that at a sufficiently low loading level of anthracene the decay rate is dependent on the amount of water, methanol and pyridine but scarcely affected by n-hexane. 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 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 towards the zeolite frameworks. On the contrary, large quantities of the coadsorbed solvens are expected to interfere with the movement of anthracene in the cage networks because they are so crowded enough to fill the interconnecting windows between cages as well as a large portion of the cage volume.3. Molecular Diffusivity in ZeolitesWe present a new experimental technique based on photochemistry for acquiring diffusion constants of aromatic species adsorbed in zeolites. The technique basically observes the quenching kinetics of excited probes produced by the pulsed-laser irradiation. The probe molecules are expected either to deactivate or to undergo quenching by another molecule (a quencher) migrating within the cage networks. The quenching kinetics were analyzed by applying the CTRW (continuous time random walk) model. In addition to the estimation of the diffusion coefficient for anthracene and azulene in NaY, the present study showed the indication of the long-range reaction between molecules residing in the neighboring cages, reflecting open structure of the cage network. Less