Intrazeolite photochemistry. II. Evidence for site inhomogeneity from studies of aromatic ketone phosphorescence

The luminescent properties of several aromatic ketones included in the hydrophobic zeolite Silicalite have been examined. Acetophenone, benzophenone, and β-phenylpropiophenone show readily detectable phosphorescence; by contrast, valerophenone does not luminesce, but undergoes the Norrish Type II reaction. Thus, irradiated samples of valerophenone in Silicalite show phosphorescence due to the accumulation of acetophenone. In the case of β-phenylpropiophenone the triplet lifetime is ca. 100 000 times longer than the solution value, suggesting severe conformational restrictions. Co-inclusion of acetophenone with various substrates and oxygen quenching studies indicate that Silicalite has at least two distinct classes of inclusion sites. In one of them energy transfer processes are rapid and efficient, suggesting a cooperative effect in the inclusion of ketones in these regions of the Silicalite framework.