Characterization of Persistent Intermediates Generated upon Inclusion of 1,1-Diarylethylenes within CaY Zeolite:  Spectroscopy and Product Studies

Abstract

Inclusion of 1,1-diarylethylenes within activated CaY results in the formation of colored samples that are stable for prolonged periods. For example, diphenylethylene generates a green color within CaY. Two reactive intermediates have been shown to be responsible for this color. The species with the short wavelength (λ max 410 nm) absorption has been established to be the diphenylmethyl cation. The identity of the species with the long wavelength ((λ max 620 nm) absorption has been debated for three decades. We establish that the diarylethylene radical cation is not responsible for the long wavelength absorption. Laser flash photolysis studies show that a number of diarylethylene radical cations have a characteristic absorption in the 400 nm region in both solution and in NaY zeolite. The absence of this absorption for diarylethylene-CaY samples unequivocally eliminates the possibility that these radical cations are responsible for color formation. This is substantiated by the fact that a number of 2-methyl-substituted 1,1-diarylethylenes generate only the short wavelength cation absorption upon inclusion in CaY; radical cation formation would be expected to be equally facile for the methyl-substituted derivatives. Our results suggest that an allylic dimer cation is the most likely source of the long wavelength ((λ max 620 nm) absorption within CaY.

Wavelength Dependent Oxygen Mediated Electron- Transfer Reactions within M+Y Zeolites: Photo Oxidation and Reduction of 1,1-Diarylethylenes

“…The trapped electron is quenched by oxygen presumably with the formation of superoxide anion. The 400 nm signal is assigned to the radical cation of 5 , in good agreement with spectra obtained by photosensitized electron transfer in both solution and zeolite . Note that the weak signal above 500 nm in the presence of oxygen has contributions from both a small amount of residual Na 4 3+ and a second weak absorption band of the radical cation.…”

“…The spectra shown in Figure do not provide any evidence for the protonation of 5 to give a diarylethyl cation which should have absorption at ∼500 nm . However, as noted in previous work, additional signals in the 500 nm region that we attribute to a dimer radical cation are evident for higher loadings of 5 …”

“…In the second set of experiments, formation of radical cations from 1,1-diarylethylenes upon laser excitation was probed through transient diffuse reflectance laser studies. We had recently used photosensitized electron transfer to generate and characterize the radical cations of a series of 1,1-diarylethylenes in solution and in zeolites . Several of the olefins had also been shown to give detectable yields of radical cation upon direct 266 or 308 nm excitation.…”

Hyperfine and Host−Guest Interactions of the Mu-Cyclohexadienyl Radical in NaY Zeolite

“…While most studies of this nature in zeolites have been of radical cations, − there are also several reports of ESR studies of neutral free radicals in zeolites, − ,− some involving stable free radicals. ,, Radicals are often formed in geminate pairs by photolysis of selected organic precursors, and recombination reactions can be fast, though mitigated by the restricted zeolite geometry at lower temperatures. ,, Stable nitroxide radicals have also been observed on the external surfaces of two MFI zeolites 12 (silicalite and ZSM-5). An important class of neutral organic free radicals are those formed directly by H-atom addition to unsaturated bonds, which might generally be expected from Bronsted active OH centers in zeolites via specific hydrogen exchange reactions. , Of particular interest here is H-atom transfer to benzene, leading to the classic cyclohexadienyl radical, C 6 H 7 , as a basis for more complex aromatic systems in faujasites. ,,,,, The only report 13 of the direct observation of C 6 H 7 in a zeolite, formed by H-atom addition, is from the radiolysis of HZSM-5. However no studies of its dynamics were reported and, to our knowledge, there have been no studies reported at all of this radical in faujasites, where radical recombination is more facile .…”

Stereocontrol within Confined Spaces: Enantioselective Photooxidation of Enecarbamates Inside Zeolite Supercages

“…b The enecarbamates 1 were loaded as a 50:50 mixture of 3‘( R/S ) diastereomers, one molecule per 15 supercages (low loading level of enecarbamates ∼3 mg was kept to establish the concept. Higher loading levels (gram scale, possible in preparative scale irradations)) …”