Highly Efficient Photoinduced Electron Transfer with 2,4,6-Triphenylpyrylium Cation Incorporated inside Extra Large Pore Zeotype MCM-41

Abstract

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...

Electro- and magneto-electrochemistry of zeolite Y- and MCM-41-associated bipyrylium ion

“…BTP@Y and BTP@MCM exhibit similar diffuse reflectance UV-Vis and IR spectra, described in detail elsewhere [33]. MCM-41 silicate was obtained by hydrothermal crystallization at 80°C for 3 days using TEOS and Al 2 O 3 as sources and cetyltrimethylammonium bromide as the structure-directing agent as previously reported [36,37].…”

“…To interpret these results it should be considered that the main effect resulting from the application of a parallel magnetic field results in the alteration of the hydrodynamic regime of ions in the solution phase, as described in the literature [34][35][36][37][38][39][40][41][42]. This magnetoconvective effect must produce a peak current enhancement for electron transfer processes involving species in the solution phase.…”

“…Since MCM-41 has a channel diameter of 30 Å [36,37], the incorporation of BTP 2+ should not be impeded by size. Since electron transfer processes in zeolite-associated species involves the ingress/issue of charge-compensating electrolyte cations on/to the zeolite pore and channel system, size exclusion-effects have been studied in comparing the electrochemistry of zeolite-modified electrodes in Et 4 N + + MeCN and Bu 4 N + + MeCN.…”

Silicon oxide based materials for controlled release in orthopedic procedures

“…Long chain cationic surfactants were used as the template or pore forming agent [19]. Depending on starting materials and synthesis conditions, several mesoporous silica oxides with an ordered structure in the form of a hexagon (denoted as MCM-41) [20][21][22][23][24][25][26][27], a cube (denoted as MCM-48) [21][22][23][24][25][26][27] or a lamella (denoted as MCM-50) [19,28,29] were formed. A more useful form of mesoporous silica nanoparticles for biomedical applications was first synthesized by Grṻn et al [30].…”

Silica-Based Mesoporous Nanospheres

“…[31][32][33][34] In 1992, Mobil researchers disclosed the first family of highly ordered mesoporous molecular sieves, M41S (pore size in the range 2-10 nm), in which long chain cationic surfactants were used as the template or pore forming agents during the hydrothermal sol-gel synthesis. 35 Depending on starting materials and synthesis conditions, different mesoporous silica oxides with ordered structures in the form of hexagonal (denoted as MCM-41), 18,20,[35][36][37][38][39][40][41][42] cubic (denoted as MCM-48), [36][37][38][39][40][41][42] and lamella (denoted as MCM-50), 20,43,44 were formed (see Figure 1). Since then, many synthetic routes and strategies have been developed to yield a wide variety of materials of various frameworks with different chemical compositions and pore structures.…”