The photophysical properties of 2,4,6-triphenylpyrylium (TPP + ) and three para-substituted tritylium ions encapsulated within Y, β, and MCM-41 have been studied. It was found that TPP + adsorbed within MCM-41 or silica only emits fluorescence (λ max 470 nm), whereas when this cation is incorporated within HY and LaY, simultaneous emission of fluorescence and room-temperature phosphorescence (λ 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 µs 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.
Laser flash photolysis of a series of substituted styrenes embedded within the cavities of the large pore zeolite NaY leads to the formation of the corresponding styrene radical cation. The reactivity and spectra of these radical cations embedded within NaY are examined and compared to the reactivity of the same radical cations in solution. It is found that for the highly reactive parent styrene radical cation the zeolite framework provides a strong stabilizing effect. For the 4-methoxy-substituted styrene radical cation the zeolite framework plays less of a role in stabilizing the radical cation as compared to the reactivity of the same radical cation in acetonitrile solution. Rigorous analysis of the thermal stability of 4-methoxystyrene, 4-methylstyrene, and anethole in the zeolite micropores was carried out using two sources of NaY zeolite (Aldrich and The PQ Corporation). It was found that the thermal stability was surprisingly dependent on the source of the NaY zeolite. 4-Methoxystyrene, 4-methylstyrene, and anethole were thermally stable in NaY (Aldrich) but rapidly dimerized in NaY (PQ) upon incorporation with dichloromethane. We observed the formation of the same type of dimers not only for 4-methoxystyrene but also for 4-methylstyrene and anethole. In addition, 4-methoxystyrene was incorporated into a series of different acid zeolites (HZSM-5, HMordenite, HBeta, and HY) varying in the shape and size of their micropores where rapid thermal protonation occurs. Dimerization of the thermally formed 4-methoxyphenethyl cation with a neutral molecule of 4-methoxystyrene took place within all the acid zeolites examined. The generation of this secondary 1,3-bis(4-methoxyphenyl)-1-butylium ion was clearly observed in the medium pore ZSM-5. This carbocation was found to be thermally unstable in the acidic environment provided by the four acidic zeolites and underwent a proton and hydride transfer to form the more stable allylic 1,3-bis(4-methoxyphenyl)buten-1-ylium cation. In the large round cavities of HY a competing cyclization reaction took place which led to the formation of the 3-methyl-5-methoxy-1,4-methoxyphenylindanyl cation.
The mobility and location of pyrene within the cavities of the faujasite NaY have been examined using fluorescence and diffuse reflectance techniques. The photophysical properties of pyrene within the zeolite framework show that upon incorporation the pyrene molecules are initially distributed in the outer cavities of the zeolite granules. This leads to a high number of doubly occupied cavities and large excimer emission; this emission shows only 20-25 ps delay, suggesting that excimer-forming molecules are required to undergo only small intracavity motions. With time (days) the distribution of pyrene within the cavities of the zeolite equilibrates and monomer emission dominates the spectra. The time required for this equilibration to take place is shown to be highly dependent on sample preparation. In particular, water and hexane hinder pyrene redistribution, while this process is faster under nitrogen than in samples under vacuum. The detection of delayed fluorescence on the microsecond time scale on freshly prepared samples indicates that there is movement of the pyrene molecules located on the external surface of the zeolite after sample preparation; no delayed fluorescence is observed after 1-2 days.
The 9-(4-methoxyphenyl)xanthylium ion (AnX + ), a bulky cation that according to molecular modeling simulations cannot enter through the 7.4 Å pore opening of tridirectional large-pore zeolites Y and β, has been prepared by ship-in-a-bottle synthesis within the supercages of these two zeolites. The synthetic procedure involves two steps: (i) generation of xanthylium cation (X + ) adsorbed within the zeolite by treatment of xanthydrol onto the H + form of these solids; (ii) electrophilic attack of the cation to anisole within the framework of the zeolite. Carrying out the same procedure using toluene or benzene or using an alternative process involving the reaction of xanthone with anisole, led to the corresponding 9-arylxanthylium ions together with some unidentified adventitious material. Textural and acidic properties of these composites were tested using R-methylstyrene dimerization and were found to be very similar to the original HY or Hβ zeolites, thus indicating an internal location for AnX + . Entrapment within the zeolite framework restricts AnX + conformational mobility, thwarting the deactivation pathway occurring in solution. This has allowed for the first time characterization of its triplet excited state as a long-lived transient species (tens of microseconds), as well as the observation of room-temperature fluorescence in the nanosecond range.
4-Aminobenzophenone (ABP) has been incorporated within the internal voids of four acid zeolites (HY, H , HMor, and HZSM5) and the resulting composites characterized by diffuse reflectance, thermogravimetric analysis-differential scanning calorimetry, and FT-IR. Timeresolved diffuse reflectance of these composites showed that the photochemistry of ABP within these solids after laser excitation is extremely dependent on the zeolite microenvironment. The transient reflectance spectrum obtained for the ABP-HY composite closely resembles the transient absorption spectrum for ABP in polar solvents. However, the transient reflectance spectrum obtained for the ABP-HZSM5 composite shows one maximum at 530 nm, due to the T-T absorption of ABPH + triplet. A different situation was found for the ABP-H and ABP-HMor composites, where besides the reflectance at 530 nm, other bands were present. In addition, the zeolite framework also modifies how ABP behaves as a photosensitizer. The use of these composites as heterogeneous photosensitizers was explored by performing the photosensitized dimerization of 1,3-cyclohexadiene as a model reaction. Cyclobutane [2+2] dimers were the main products, indicating that an energy-transfer reaction is the predominant mechanism under these conditions.
The dynamics of the xanthyl radical in a series of alkali-metal cation exchanged (Li+, Na+, K+, Rb+, and Cs+) Y faujasites in the absence and presence of molecular oxygen are examined by using laser flash photolysis. Upon laser photolysis of xanthene-9-carboxylate incorporated within the supercages of NaY in the absence of oxygen, prompt formation of the xanthyl radical and the xanthylium cation is observed. The xanthyl radical is formed by photoionization of xanthene-9-carboxylate to the corresponding acyloxy radical that then rapidly decarboxylates. The prompt xanthylium cation is produced by photoionization of the xanthyl radical. This mechanism for the prompt xanthylium cation is supported by results from two-color laser and photoinduced electron transfer experiments. When oxygen is introduced into the sample, the radical is almost completely quenched. In addition, a slow growth of the xanthylium cation at 375 nm is observed. This growth is due to heterolytic cleavage of the peroxyl radical formed upon reaction of the xanthyl radical with molecular oxygen. The formation and resulting growth of the xanthylium cation in the presence of oxygen is found to be highly dependent on the zeolite counterion with significant carbocation formation occurring in both LiY and NaY, little carbocation formation in KY, and no carbocation formation in RbY and CsY. These are the first results showing how the oxidation of radicals to carbocations within zeolites can be controlled by simple alkali metal exchange.
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