Zeolite photochemistry : Investigation of cation-π interaction and diffusion of guest species

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1. A kinetic study on energy transfer between guest aromatic species and rare earth charge-compensating cations within zeolites :The real-time observation is made of energy transfer (ET) between guest aromatic species, benzophenone and naphthalene, and a rare-earth charge compensating cation, Tb^<3+> within a faujasite zeolite Y by the measurement of a time-dependent rise and decay of emission from the ^5D_4 excited-state of Tb^<3+> on excitation of the guest molecules with a pulsed-laser light. Confinement of the guest species within the cages leads to a greatly enhanced emission intensity relative to solution systems, thus facilitating the ET by enforcing the formation of excited-state complexes. Besides, the kinetic behavior of the emission reveals that while benzophenone molecules are immobilized at the protonic sites within the cages, naphthalene molecules are still mobile leading to a remarkable observation of the loading-dependent rise and decay of the Tb^<3+> emission. Consiste … More nt with the assumption of the excited-state complex, a direct excitation of the absorption band of ^7F_6→^5D_4 band (488 nm) of Tb^<3+> results in a single-exponential emission decay with a lifetime of 2.4 ms both in the absence and in the presence of benzophenone or naphthalene.2. Visual and spectroscopic demonstration of intercrystalline migration and resultant photochemical reactions of aromatic molecules adsorbed in zeolites :We applied a fluorescence microscopic method to investigate the intercrystalline migration of aromatic molecules adsorbed in faujasite zeolites. Photophysical processes such as intersystem crossing and energy transfer assisted by particular charge compensating cations, and photochemical reactions such as charge transfer (CT) and triplet-triplet energy transfer between guest species incorporated in the zeolites were exploited as indicator reactions to afford luminescence color characteristic to an individual zeolite particle. Two types of migration mechanism were evidenced assisted by sample heating at temperatures between 60 and 120℃ : a through space diffusional transfer mode between separated zeolite crystals and a molecular injection process from a loaded crystal to another unloaded crystal, both in contact. A preferential direction of guest migration was found to exist for a few cases : for instance, aromatics such as naphthalene, phenanthrene and chrysene migrate from sodium form of zeolite X (Na^+-X) to thallium-exchanged zeolite X (Tl^+-X). On the other hand, the migration-assisted formation of CT complexes between electron-donating arenes such as phenanthrene and chrysene, and electron-accepting 1,2,4,5-tetracyanobenzene, both incorporated into separate zeolite Na^+-X crystals, takes place as a result of the migration of the donors. Less