The efficiency of photoinduced electron transfer is generally limited by the occurrence of deactivating back electron transfer, which competes with other reactive pathways of the generated radical ion pairs.' Different methodologies are being pursued to minimize this undesired process. Thus, laser flash photolysis techniques have shown that medium-and large-pore zeolites provide an appropriate microenvironment for this purpose, retarding dramatically the back electron transfer and increasing enormously the lifetime of the photogenerated ion pair.* This fact has been ascribed to the strong electrostatic fields experienced inside the voids of these solids, which must produce a high stabilization of the radical ions. In spite of these highly promising results, the above findings have not yet been exploited to increase the efficiency of photoinduced electron transfer processes from a preparative point of view.In heterogeneous reactions using microporous systems, impeded diffusion of the reagents and accessibility of the sites are limiting factors of paramount importance. In fact, the relatively small size of the micropores (typically 51.3 nm) has been claimed as the main drawback for an intensive use of zeolites as microscopic reactors.3 Therefore, it is interesting to determine the suitability of new zeotypes with extra large pore dimensions as hosts to perform photoinduced electron transfer reactions with bulky photosensitizers or substrates. Much effort is currently being devoted to the synthesis of this type of novel porous aluminosilicate, which should overcome most of the adsorption/diffusion limitations of conventional Y zeolites and constitute a significant breakthrough in the field of inclusion chemistry.3Recently we have reported that 2,4,6-triphenylpyrylium (TP+), a well-documented electron transfer photo~ensitizer,+~ can be encapsulated as framework charge-compensating cation within the zeolite Y supercages.* The resulting composite (TPY) is able to promote the isomerization of cis-stilbene (CS) to transstilbene (TS) through a radical cation mechanism (Scheme l).9 This well-documented isomerization cannot occur through an energy transfer mechanism between 2,4,6-triphenylpyrylium and cis-stilbene, due to the energy values of the triplets involved (ET = 53 and 60.3 kcal/mol, respectively), which would make this process energetically uphill.Although a higher contribution of in-cage isomerization was shown to be characteristic of the intrazeolite process, the overall chemical yield of trans-stilbene using the pyrylium salt imprisoned within zeolite Y was far from optimum, owing to the diffusional restrictions imposed on the reagents by the faujasite. ~~ _ _ (1) Fox, M. A,; Chanon, M. Photoinduced Electron Transfer, Parts A -D (2) Yoon, K. B. Denninge, V.; Finzel, R.; Kita, F.; Platsch, H.; Walter, H.; (6) Adam, W.; Walter, H.; Chen, G.-F.; Williams, F. J. Am. Chem. SOC. (7) Adam, W.; Sendelbach, J. J. Org. Chem. 1993, 58, 531C-5315. (8) Corma, A.; Fornes, V.; GarcEa, H.; Miranda, M. A.; Primo, J.; Sabater, (9...
