Fluorescence spectroscopic studies of anthracene adsorbed into zeolites:: From the detection of cation-π interaction to the observation of dimers and crystals

Anthracene adsorbed into NaY and NaX zeolites has been investigated mainly with a fluorescence spectroscopic technique as a function of sample loading both in the absence and in the presence of various amounts of coadsorbates. At low loadings, emission spectra characteristic of anthracene monomer were observed in dehydrated zeolites with a small red-shift and an appreciable broadening in the vibrational structure compared to those in solutions and siliceous Y (USY) zeolite. The distorted spectra are indicative of the interaction between anthracene and the zeolite frameworks, namely Nai ions; the fluorescence spectroscopic detection of the interaction, the cation-pi interaction in zeolites, is made for the first time. As the loading level increases, a broad and structureless band ascribable to the excimer of anthracene was detected at the expense of the monomer emission bands. The dimers formed inside the supercages are responsible for the occurrence of the excimer emission, and the anthracene dimer is unique to the zeolites NaY and NaX. In hydrated zeolites, on the other hand, the monomer emission spectrum gave narrow vibronic bands. This observation reinforces the assignment of the broadening of the monomer emission bands to the Na+-anthracene interaction because water possesses larger affinity toward the zeolite frameworks than does anthracene and can act to reduce the cation-pi interaction by the screening effect. The contribution of excimer emission decreases in the presence of water and other coadsorbed solvents (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 formed at the outer surfaces of zeolites through the process in which anthracene molecules are replaced by the solvents preferentially penetrating into the zeolite cavities. Dramatic changes in the adsorption state and association behavior of anthracene in zeolites were observed depending both on the loading level of anthracene and on the amount of coadsorbed water and other solvents.