FLUORESCENCE SPECTROSCOPIC STUDIES OF ANTHRACENE ADSORBED INTO ZEOLITES - FROM THE DETECTION OF CATION-PI INTERACTION TO THE OBSERVATION OFDIMERS AND CRYSTALS

Anthracene adsorbed into NaY and NaX zeolites has been investigated ma inly with a fluorescence spectroscopic technique as a function of samp le loading both in the absence and in the presence of various amounts of coadsorbates. At low loadings, emission spectra characteristic of a nthracene monomer were observed in dehydrated zeolites with a small re d-shift and an appreciable broadening in the vibrational structure com pared to those in solutions and siliceous Y (USY) zeolite. The distort ed spectra are indicative of the interaction between anthracene and th e zeolite frameworks, namely Nai ions; the fluorescence spectroscopic detection of the interaction, the cation-pi interaction in zeolites, i s made for the first time. As the loading level increases, a broad and structureless band ascribable to the excimer of anthracene was detect ed at the expense of the monomer emission bands. The dimers formed ins ide the supercages are responsible for the occurrence of the excimer e mission, and the anthracene dimer is unique to the zeolites NaY and Na X. In hydrated zeolites, on the other hand, the monomer emission spect rum gave narrow vibronic bands. This observation reinforces the assign ment of the broadening of the monomer emission bands to the Na+-anthra cene interaction because water possesses larger affinity toward the ze olite frameworks than does anthracene and can act to reduce the cation -pi interaction by the screening effect. The contribution of excimer e mission decreases in the presence of water and other coadsorbed solven ts (methanol, acetonitrile). Furthermore, the formation of anthracene crystals was observed in the presence of a large amount of coadsorbed water, methanol, or acetonitrile. The crystals are suggested to be for med at the outer surfaces of zeolites through the process in which ant hracene molecules are replaced by the solvents preferentially penetrat ing into the zeolite cavities. Dramatic changes in the adsorption stat e and association behavior of anthracene in zeolites were observed dep ending both on the loading level of anthracene and on the amount of co adsorbed water and other solvents.