Intrazeolite photochemistry .13. Photophysical properties of bulky 2,4,6-triphenylpyrylium and tritylium cations within large- and extra-large-pore zeolites

The photophysical properties of 2,4,6-triphenylpyrylium (TPP+) and three para-substituted tritylium ions encapsulated within Y, beta, and MCM-41 have been studied. It was found that TPP+ adsorbed within MCM-41 or silica only emits fluorescence (lambda(max) 470 nm), whereas when this cation is incorporated within HY and LaY, simultaneous emission of fluorescence and room-temperature phosphorescence (lambda(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 mu s 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.