No abstract

The photophysical properties of 2,4,6-triphenylpyrylium (TPP + ) and three para-substituted tritylium ions encapsulated within Y, β, and MCM-41 have been studied. It was found that TPP + adsorbed within MCM-41 or silica only emits fluorescence (λ max 470 nm), whereas when this cation is incorporated within HY and LaY, simultaneous emission of fluorescence and room-temperature phosphorescence (λ max 560 nm) was observed. The fluorescence decay consists of two consecutive first-order processes and is dominated by the fast (0.2-0.7 ns) component. In addition to the prompt fluorescence, delayed emission observable 40 µs after excitation was also detected for the three TPP + samples. Weak fluorescence was observed for the series of tritylium ions embedded within zeolites. The characteristic T-T absorption spectrum of the TPP + triplet excited state has been detected using time-resolved diffuse reflectance. Depending on the zeolite, shifts in the reflectance maximum and changes in the extinction coefficient of the long-wavelength band have been noted. Similar transient spectra have also been obtained for the tritylium samples, which also show long-wavelength bands that are attributed to the corresponding triplet excited state.

Adsorption of thianthrene (Th) within a series of acid zeolites with medium, large, and extra large pore size including novel zeotypes MCM-22 and MCM-41 led as a general process to the generation of Th radical cation (Th''). This species was unambiguously characterized by UV/vis diffuse reflectance, EPR, and IR spectroscopies. In the case of Y and p hosts, dimeric aggregates could be observed. Vapor phase incorporation of Th at 473 K under Ar atmosphere was found to be the most clean and convenient adsorption procedure. Ambient moisture was the main factor responsible for aging of Th'-composites. This process was totally reversible. This result suggest that reversible complexation of Th'+ with H20 is the process ocurring at initial stages. On the other hand, oxygen is not necessary for the formation of Th". The Bronsted and Lewis population of the acid sites were characterized by IR spectroscopy, following the pyridine adsorption-desorption method. Bronsted zeolites prepared by removal of the extraframework Al are active to generate Th". Neutralization of Bronsted sites by Na+ exchange fully deactivated the solids despite of the presence of residual Lewis sites. Th oxidation must occur predominantly on the zeolite Bronsted sites.