PHOTOPHYSICAL PROBES FOR MONITORING THE ELECTRIC-FIELD MICROPOLARITY WITHIN THE FAUJASITE SUPERCAGE

The micropolarity of the supercages of zeolites is a reflection of the electric field provided by the cations present within them. The notion of polarity within the cages of X and Y zeolite does not have precisely the same meaning as in solution. In this study, pyrene, pyrenealdehyde, and para-dimethylaminobenzonitrile have been used as photophysical probes to monitor the polarity of the zeolite interior. With pyrenealdehyde and para-dimethylaminobenzonitrile the supercage wherein the probe is expected to reside is more polar or possesses a higher electric field when the cation is Li or Na. The polarity/electric field of the supercage decreases with cation size (Li > Na > K > Rb > Cs) and the supercage is more polar in the case of X zeolites than in Y zeolites. The results with pyrene are generally consistent with those with the other two probes. However, pyrene is unable to sense the difference between Li, Na and other cations.