Visual observation of contact-induced intercrystalline migration of aromatic species adsorbed in zeolites by fluorescence microscopy

Intercrystalline migration and a migration-assisted chemical reaction of adsorbed aromatic species between zeolite particles in physical contact were visualized by fluorescence microscopy coupled with a particle manipulation technique. The luminescence color characteristics of particular zeolite particles originating from the specific photochemistry of adsorbed species was exploited to follow the migration of the molecules. Two examples are shown that are relevant to the visualization of the time-dependent migration process: A one guest-two sets of zeolite crystals system: chrysene (Chry)-loaded zeolite Na+-X (the sodium form of zeolite X) crystals were placed in contact with unloaded Tl+-X (thallium-exchanged X) crystals and allowed to stand at room temperature. Initially, the blue fluorescence of Chry was detected only from the Na+-X particles, but later, the development of green phosphorescence emission was discernible from the Tl+-X, which suggests that Chry migrated from the Na+-X to the Tl+-X crystals. A two guest-species systems: Electron-donating Chry-loaded Na+-X crystals were placed in contact with electron-accepting 7,2,4,5-tetracyanobenzene (TCNB)-loaded Na+-X or Na+-Y crystals. With time, the former system (Chry/Na+-X and TCNB/Na+-X) gave rise to the emission of Chry-TCNB charge-transfer complexes resulting mainly from the migration of Chry while the latter system (Chry/Na+-X and TCNB/Na+-Y) afforded the same emission resulting largely from the migration of TCNB. The present investigation reveals that there is a certain direction for guest migration depending on the zeolite host and the nature of host-guest or guest-guest interaction.