PHOTOINDUCED ELECTRON-TRANSFER WITHIN ZEOLITE CAVITIES - CIS-STILBENE ISOMERIZATION PHOTOSENSITIZED BY 2,4,6-TRIPHENYLPYRYLIUM CATION IMPRISONED INSIDE ZEOLITE-Y

2,4,6-Triphenylpyrylium cation (TP+), a well-established photoinduced electron-transfer sensitizer, has been encapsulated in the Y zeolite as a charge-compensating cation by ship-in-a-bottle acid-catalyzed reaction of chalcone with acetophenone in isooctane at 383 K. Formation of TP+ was ascertained by FT-IR spectroscopy and diffuse reflectance UV-vis spectrophotometry of the canary yellow-colored solid. Estimation of the TP+ content by the reaction stoichiometry, thermogravimetric analysis, and pyridine titration of the remaining Bronsted sites indicates that approximately one-third of the latter are compensated by TP+ cations. FT-IR monitoring of the OH stretching bands of both original acidic HY and the TPY samples reveals that TP+ cations are mainly exchanging Bronsted sites of the supercavities, while pyridine adsorption shows that the rest of the cavities are still accessible to this base. A molecular modeling visualization of the TP+ imprisoned in the Y supercavity has been carried out by docking, using the Biosym Insight II package. TPY shows moderate activity as an electron- transfer photosensitizer, being able to promote the isomerization of cis-stilbene to trans-stilbene via the corresponding radical cations. The reaction is not perturbed by the presence of oxygen, as evidenced by the absence of byproducts arising from oxidative cleavage. This contrasts with the extensive photooxygenation produced by 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) under homogeneous conditions. A higher contribution of the in-cage isomerization, associated with a retardation of back electron transfer in the radical ion pairs, appears to be the most noticeable characteristic of the intrazeolite process.